Ultra-high strength steel sheet and method of manufacturing the same

Abstract

According to one embodiment of the present invention, in weight percent, it contains carbon (C): 0.15% to 0.5%, silicon (Si): 0.8% to 2.0%, manganese (Mn): 1.5% to 4.0%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.005% to 1.0%, boron (B): 0.0003% to 0.005%, chromium (Cr) + nickel (Ni) + copper (Cu) + molybdenum (Mo): 0.005% to 0.5%, titanium (Ti) + niobium (Nb) + vanadium (V) + zirconium (Zr): 0.005% to 0.1%, and the remainder is iron (Fe) and other unavoidable The present invention provides an ultra-high strength steel plate and a method for manufacturing the same, characterized by containing impurities and satisfying the following formula (1). Formula (1): 0.1 ≤ HmA2 × dA2 ≤ 10.0 (wherein, A2 is defined as the region up to the point where the ratio of the region in which the nanohardness value measured at the same depth is 5 GPa or less is 30% or more and less than 80%, by measuring hardness at 3 µm intervals through a nano-indentation test of 90 µm or more in the depth direction and 30 µm or more in the length direction from the surface of the steel plate, and HmA2 is the average nanohardness of the A2 region, Hm(k) is the nanohardness corresponding to measurement position k within the A2 region, and N represents the number of variables in which a nano-indentation test is performed within the A2 region.)

Description

Technology Field

[0001] The technical concept of the present invention relates to steel plates, and more specifically, to ultra-high strength steel plates with excellent bendability and a method for manufacturing the same. Background Technology

[0002] High-strength steel for automotive steel sheets is being developed to meet the two demands of vehicle lightweighting driven by stricter energy, resource, and environmental regulations, and ensuring crash safety driven by stricter safety regulations. Since automotive steel sheets are mostly formed by press processing, excellent press formability is required, and high ductility is essential to achieve this. In other words, for automotive steel sheets, possessing high ductility as high-tensile steel is of paramount importance. However, as strength and elongation have a trade-off relationship, difficulties in forming parts arise as material strength gradually increases; consequently, various studies are underway to develop high-strength steel with superior formability.

[0003] Regarding technologies capable of securing both high strength and high elongation, TRIP (Transformation Induced Plasticity) steel utilizes the phenomenon of residual austenite transforming into martensite during plastic deformation. It offers the advantage of leveraging both the excellent formability of austenite and the hard properties of martensite after forming. In the case of such ultra-high-strength steel, the austenite phase formed during the heating stage transforms into martensite or bainite during cooling and reheating, or some remains as austenite, resulting in the formation of a composite phase. Such structural changes can affect the mechanical properties of the steel.

[0004] The bending properties of ultra-high strength steel are determined by the difference in hardness between the phases of the matrix structure of the steel. In particular, in ultra-high strength steel, bending properties deteriorate when the hard martensite phase and the hard ferrite phase are mixed. This is because the fine boundary between the two phases can cause stress concentration during bending. Therefore, to improve the bending properties of ultra-high strength steel, it is possible to reduce the difference in hardness between the phases of the structure by forming a single-phase ferrite structure layer on the surface of the steel plate where initial cracks occur. Conventionally, decarburization control technology was proposed by controlling heat treatment conditions such as dew point and temperature within the annealing furnace; however, there was a problem in that the effect of improving bendability was significantly reduced when the high-strength martensite or retained austenite structure was mixed within the decarburization layer in proportions exceeding a certain level. The problem to be solved

[0005] The technical problem that the technical concept of the present invention aims to solve is to provide an ultra-high strength steel plate that simultaneously satisfies strength, elongation, and bendability, and a method for manufacturing the same.

[0006] However, these tasks are exemplary, and the technical concept of the present invention is not limited thereto. means of solving the problem

[0007] According to one aspect of the present invention, an ultra-high strength steel plate is provided.

[0008] The above ultra-high strength steel sheet contains, in weight percent, carbon (C): 0.15% to 0.5%, silicon (Si): 0.8% to 2.0%, manganese (Mn): 1.5% to 4.0%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.005% to 1.0%, boron (B): 0.0003% to 0.005%, chromium (Cr) + nickel (Ni) + copper (Cu) + molybdenum (Mo): 0.005% to 0.5%, titanium (Ti) + niobium (Nb) + vanadium (V) + zirconium (Zr): 0.005% to 0.1%, and the remainder is iron (Fe) and other It is characterized by containing unavoidable impurities and satisfying the following formula (1).

[0009] Equation (1): 0.1≤Hm A2 ×d A2 ≤10.0

[0010]

[0011] (Here, A2 is defined as the area up to the position where the proportion of the region with a nanohardness value of 5 GPa or less measured at the same depth is 30% or more and less than 80%, obtained by measuring hardness at 3 µm intervals through a nano-indentation test of at least 90 µm in the depth direction and at least 30 µm in the length direction from the surface of the steel plate, and Hm A2 ε₀ is the average nanohardness of region A2, Hm(k) is the nanohardness corresponding to measurement position k within region A2, N represents the number of variables for which nanoindentation tests were performed within region A2, and the thickness t(A1+A2) corresponds to the depth of the decarburized layer.

[0012] According to one embodiment, the steel plate may be characterized by satisfying the following formula (2).

[0013] Equation (2): 30㎛ ≤ t (A1+A2) ≤ 50㎛

[0014] (Here, A1 refers to the region where the proportion of the area with a nanohardness value of 5 GPa or less is 80% or more, and the thickness t(A1+A2) corresponds to the depth of the decarburization layer.)

[0015] According to one embodiment, the method comprises one or more plating layers formed on the steel plate, wherein the plating layer may be any one of a hot-dip galvanized layer, a hot-dip aluminum plating layer, an aluminum alloy plating layer, an electro-galvanized layer, an electro-galvanized layer, and an alloyed hot-dip galvanized layer.

[0016] According to one embodiment, the steel plate may have a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a bendability (R / t) of 2.0 or less.

[0017] According to one embodiment, the A1 region is formed on the surface layer of the base iron in contact with the zinc plating layer and is a ferrite decarburized layer having a single-phase ferrite microstructure, and the A2 region may be a transition decarburized layer comprising a complex structure of ferrite, martensite, and retained austenite.

[0018] According to one embodiment, the decarburization layer and the transition decarburization layer may be formed on top of a matrix structure comprising a martensite and residual austenite composite structure.

[0019] According to another aspect of the present invention, a method for manufacturing an ultra-high strength steel plate is provided.

[0020] The above method for manufacturing an ultra-high strength steel sheet comprises: a step of annealing a cold-rolled steel sheet in an annealing furnace; a step of first cooling the annealed steel sheet to a temperature of 650°C or higher and less than 800°C at an average cooling rate of 3 to 20°C / s; and a step of second cooling the first cooled steel sheet to a temperature of Ms or lower at an average cooling rate of 20°C / s or higher; wherein the step of performing the annealing heat treatment is characterized in that the moisture concentration in the annealing furnace is 8500 ppm or higher.

[0021] According to one embodiment, the annealing heat treatment step may be characterized in that the dew point application section is maintained until the end of the first cooling.

[0022] According to one embodiment, the method is further characterized by performing a step of spraying humidifying gas onto the steel plate after the annealing heat treatment and before the first cooling.

[0023] According to one embodiment, the annealing heat treatment and primary cooling may be performed at a temperature corresponding to the austenite and ferrite phases, and may include a step of inducing a decarburization reaction on the surface of the steel plate to transform the austenite present in the surface layer of the steel plate into ferrite.

[0024] According to one embodiment, the annealing heat treatment step can be performed under a hydrogen concentration of 3% to 20%.

[0025] According to one embodiment, the step of manufacturing the cold-rolled steel sheet may include: a step of hot-rolling a steel slab so that the exit side temperature is 850 to 970°C and coiling it at a temperature of 650°C or lower to form a hot-rolled steel sheet; and a step of cold-rolling the hot-rolled steel sheet with a reduction rate of 40 to 60%.

[0026] According to one embodiment, the cold-rolled steel sheet contains, in weight percent, carbon (C): 0.15% to 0.5%, silicon (Si): 0.8% to 2.0%, manganese (Mn): 1.5% to 4.0%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.005% to 1.0%, boron (B): 0.0003% to 0.005%, chromium (Cr)+nickel (Ni)+copper (Cu)+molybdenum (Mo): 0.005% to 0.5%, titanium (Ti)+niobium (Nb)+vanadium (V)+zirconium (Zr): 0.005% to 0.1%, and the remainder is It may contain iron (Fe) and other unavoidable impurities.

[0027] According to one embodiment, the method may further include the step of reheating the steel plate to 360°C to 520°C at an average speed of 10°C / s or more after the second cooling step and maintaining it for 30 seconds to 500 seconds; and the step of performing a plating treatment on the steel plate. Effects of the invention

[0028] According to the technical concept of the present invention, by extending the high dew point application range to the slow cooling termination range to provide sufficient time for carbon present within the decarburization transition layer to diffuse, the hard phase remaining within the decarburization transition layer can be reduced, thereby minimizing the hardness variation between phases. Through this, a high-strength steel sheet can be manufactured having a yield strength of 880 MPa or more, a tensile strength of 1180 MPa or more, a maximum elongation of 14% or more, and a bending (R / t) of 2.0 or less. The effects of the present invention described above are illustrative and the scope of the present invention is not limited by these effects. Brief explanation of the drawing

[0029] FIGS. 1 and FIGS. 2 are schematic diagrams showing the structure of a steel plate according to one embodiment of the present invention. FIG. 3 is a flowchart illustrating a method for manufacturing an ultra-high strength steel plate according to one embodiment of the present invention. FIGS. 4 and 5 are graphs illustrating the subsequent heat treatment (annealing, cooling, reheating) steps applied to a cold-rolled steel sheet in a method for manufacturing an ultra-high strength steel sheet according to one embodiment of the present invention. FIG. 6 is a diagram showing the bending characteristics of the transition decarburization layer of a substrate steel according to one embodiment and a comparative example of the present invention. Specific details for implementing the invention

[0030] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical concept of the present invention to those skilled in the art. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings.

[0031] In this specification and claims, the content or concentration of a specific component means weight percent unless otherwise specifically stated.

[0032] The technical concept of the present invention is to provide an ultra-high strength steel plate with excellent bendability, having a yield strength of 880 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a bendability (limit bending radius / plate thickness: R / t) of 2.0 or less, and a method for manufacturing such a steel plate.

[0033] The following describes the alloy content and suitable heat treatment conditions for securing the characteristics such as tensile strength, elongation, and bendability targeted in the present invention. In this invention, it was discovered that in order to secure bendability, the difference in hardness between phases within the decarburized layer must be reduced by controlling the time of the annealing section applying a high dew point; therefore, the alloy content and heat treatment process conditions for obtaining such a microstructure are presented as follows.

[0034] Hereinafter, ultra-high strength steel sheets or galvanized steel sheets according to the technical concept of the present invention will be described in detail.

[0035] An ultra-high strength steel sheet according to one embodiment of the present invention comprises a base steel and a zinc plating layer formed on the base steel, wherein the base steel comprises, in weight%, carbon (C): 0.15% to 0.5%, silicon (Si): 0.8% to 2.0%, manganese (Mn): 1.5% to 4.0%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.005% to 1.0%, boron (B): 0.0003% to 0.005%, chromium (Cr)+nickel (Ni)+copper (Cu)+molybdenum (Mo): 0.005% to 0.5%, titanium (Ti)+niobium (Nb)+vanadium (V)+zirconium (Zr): It contains 0.005% to 0.1%, and the remainder consists of iron (Fe) and other unavoidable impurities.

[0036] The role and content of each component included in the ultra-high strength steel plate according to the present invention are described below. In this case, the content of each component element refers to a weight percent relative to the total base steel.

[0037] Carbon (C): 0.15% to 0.5%

[0038] It is added to ensure the strength of the steel, particularly increasing the strength of the martensite structure. Additionally, sufficient carbon content is required because partitioning stabilizes the austenite, thereby ensuring elongation through the TRIP effect. The carbon content ranges from 0.15% to 0.5% by weight. If the carbon content is less than 0.15%, process costs may increase, and if it exceeds 0.5%, there may be disadvantages such as reduced weldability and hydrogen embrittlement.

[0039] Silicon (Si): 0.8% to 2.0%

[0040] Silicon acts as a ferrite-stabilizing element, delaying the formation of carbides in ferrite and martensite during partitioning and providing solid solution strengthening effects. In particular, since delaying the formation of carbides in martensite is essential for securing austenite stability, it is desirable to add silicon in an amount of 0.8 to 2.0 wt%. If the amount is less than 0.8 wt%, the effect of inhibiting carbide formation is insufficient, making it difficult to sufficiently secure the stability of the residual austenite; if the amount exceeds 2.0 wt%, oxides such as Mn2SiO4 are formed during the manufacturing process, which impairs plating performance and may increase carbon equivalents, thereby reducing weldability.

[0041] Manganese (Mn): 1.5% to 4.0%

[0042] Manganese has a solid solution strengthening effect and increases hardenability, thereby delaying the formation of ferrite and bainite during cooling. It is preferable to add manganese in an amount of 1.5 to 4.0 weight%. If the amount is less than 1.5%, the effect is insufficient, making it difficult to ensure hardenability; if it exceeds 4.0%, the transformation of bainite may be excessively delayed, and machinability may be reduced due to the formation of inclusions such as MnS or segregation, and weldability may be reduced by increasing the carbon equivalent.

[0043] Phosphorus (P): 0.05% or less (excluding 0%)

[0044] Phosphorus is an impurity included in the manufacturing process of steel, and it is desirable to limit it to 0.01% or less. Although the addition of phosphorus can help improve strength through solid solution strengthening, adding more than 0.05% may result in low-temperature brittleness.

[0045] Sulfur (S): 0.02% or less (excluding 0%)

[0046] Sulfur is an impurity included in the steel manufacturing process and it is desirable to limit it to 0.02% or less. Sulfur is limited to 0.02% or less because it forms non-metallic inclusions such as FeS and MnS, which reduce bendability, toughness, and weldability.

[0047] Aluminum (Al): 0.005% to 1.0%

[0048] Aluminum is used as a deoxidizer and, like Si, can help suppress the formation of carbides. It is preferable to add aluminum in an amount of 0.005 to 1.0 wt%. If the amount is less than 0.005 wt%, the deoxidation effect may be insufficient, and if it is added in an amount exceeding 1.0 wt%, AlN may be formed during slab manufacturing, which may cause cracks during casting or hot rolling.

[0049] Nitrogen (N): 0.01% or less (excluding 0%)

[0050] Nitrogen can help stabilize austenite, but it can react with Al to form AlN, which can cause cracks during continuous casting, so it must be limited to 0.01 mass% or less.

[0051] Boron (B): 0.0003% to 0.005%

[0052] Boron can be included as an element that can replace silicon. The boron can improve hardenability and strengthen grain boundaries in extremely small amounts, thereby improving strength.

[0053] Chromium (Cr) + Nickel (Ni) + Copper (Cu) + Molybdenum (Mo): 0.005% to 0.5%

[0054] Chromium and molybdenum have solid solution strengthening effects and contribute to strength improvement by increasing hardenability; they also interact with C and Mn to refine the martensite and bainite structures and contribute to the stabilization of retained austenite. Nickel and copper stabilize austenite and can also help increase the hardenability of steel.

[0055] It is preferable that the sum of chromium, nickel, copper, and molybdenum be added in an amount of 0.005 to 0.5 weight percent, and if added in an amount exceeding 0.5 percent, the transformation of bainite may be excessively delayed and the manufacturing cost of the steel may increase, which is undesirable.

[0056] Titanium (Ti) + Niobium (Nb) + Vanadium (V) + Zirconium (Zr): 0.005% to 0.1%

[0057] Niobium, titanium, and vanadium are precipitate-forming elements that can increase strength through precipitation strengthening and have a grain refinement effect. Zirconium precipitates as ZrO, which acts as a nucleus for the precipitation of austenite; thus, it is an element that increases the equiaxedness of austenite and refines the austenite grains.

[0058] It is preferable that the sum of niobium, titanium, vanadium, and zirconium be added in an amount of 0.005 to 0.1%. If added in excess of 0.1% by weight, the manufacturing cost of the steel may increase significantly, and the rolling load may increase significantly due to excessive precipitation during rolling, which may lead to a decrease in elongation, so it should be limited.

[0059] The remaining component of the above ultra-high strength steel plate is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the conventional steelmaking process, they cannot be excluded. As these impurities are known to any skilled technician in the conventional manufacturing process, all details thereof are not specifically mentioned in this specification.

[0060] The steel plate having the above alloy composition satisfies the following formula (1) and optionally satisfies the following formula (2).

[0061] Equation (1): 0.1≤Hm A2 ×d A2 ≤10.0

[0062]

[0063] Equation (2): 30㎛ ≤ t (A1+A2) ≤50㎛

[0064] (Here, A1 is defined as the area up to where the proportion of the region where the nanohardness value measured at the same depth is 5 GPa or less is 80% or more, by measuring hardness at 3 µm intervals through a nanoindentation test of 90 µm or more in the depth direction and 30 µm or more in the length direction from the surface of the steel plate, and A2 is defined as the area up to where the proportion of the region with a nanohardness value of 5 GPa or less is 30% or more and less than 80%, and Hm A2 ε₀ is the average nanohardness of region A2, Hm(k) is the nanohardness corresponding to measurement position k within region A2, N represents the number of variables for which nanoindentation tests were performed within region A2, and the thickness t(A1+A2) corresponds to the depth of the decarburized layer.

[0065] Hm (Martens hardness) represents the combined hardness determined by the depth-loading method using a nanoindenter. The depth-loading method using a nanoindenter allows for the evaluation of the hardness characteristics of a steel plate by inserting a fine-sized indenter into the surface and simultaneously measuring the depth and load.

[0066] In the above equations (1) and (2), the region where the fraction of the Hm ≤ 5Gpa region is 80% or more is defined as A1, the region where the fraction of the Hm ≤ 5Gpa region is 30% or more or less than 80% is defined as A2, and the region where the fraction of the Hm ≤ 5Gpa region is less than 30% is defined as A3.

[0067] The above Equation (1) is an indicator for verifying the hardness difference between phases in the A2 region. If the range of Equation (1) is satisfied, the low-temperature phase fraction in the A2 region is reduced, and the hardness difference between phases can be minimized. At this time, d A2 is the standard deviation of hardness in the A2 region, and the average nanohardness (Hm A2 It can be obtained by dividing the square of the difference between the nanohardness (Hm(k)) and the measurement location by the number of variables.

[0068] The above equation (2) means that in order to secure the bending characteristics of the steel plate, it is necessary to secure the thickness of the decarburization layer formed on the surface of the steel plate in the range of 30 to 50 μm.

[0069] Referring to the structure of the cold-rolled steel sheet shown in FIG. 1, the base steel is composed of a matrix structure (A3), a transition decarburization layer (A2), and a ferrite decarburization layer (A1).

[0070] Alternatively, referring to the structure of the cold-rolled steel sheet illustrated in FIG. 2, a plating layer may be formed on the base steel. The plating layer may be one or more selected from a hot-dip galvanized layer, a hot-dip aluminum plating layer, an aluminum alloy plating layer, an electro-galvanized layer, an electro-galvanized layer, and an alloyed hot-dip galvanized layer.

[0071] In the above Figure 1 or Figure 2, region A3 is a matrix structure comprising a complex structure of martensite and retained austenite, and the depth of regions A1 and A2 is defined as a soft decarburized layer formed on the matrix structure.

[0072] The above A1 region is formed on the surface layer of the base iron in contact with the zinc plating layer and is a ferrite decarburization layer having a single-phase ferrite microstructure. The above A2 region is formed between the A1 region and the A3 region and is a transition decarburization layer comprising a complex structure of ferrite, martensite, and retained austenite.

[0073] The ultra-high strength steel sheet of the present invention is characterized by allowing carbon in the A2 region of the decarburization layer to sufficiently escape to the outside, and ultimately reducing the low-temperature phase fraction in the A2 region to minimize the difference in hardness between phases. If the decarburization reaction does not occur sufficiently in the A2 region, a mixed structure containing martensite, bainite, and pearlite may be formed in addition to the desired ferrite, which causes an excessive difference in hardness between phases and reduces the effect of improving the bending characteristics of the steel sheet through the formation of the decarburization layer.

[0074] Therefore, in order to secure the bending characteristics of the steel plate according to the present invention, it is desirable to satisfy Equation (1) and Equation (2).

[0075] The ultra-high strength steel sheet of the present invention can be realized as a cold-rolled high strength steel sheet with excellent bendability, having a yield strength (YP) of 880 MPa or more, a tensile strength (TS) of 1180 MPa or more, an elongation (El) of 14% or more, and a bendability (R / t) of 2.0 or less.

[0076] Hereinafter, with reference to the attached drawings, a method for manufacturing an ultra-high strength steel plate having the composition range described above, which follows the technical concept of the present invention, will be explained.

[0077] Method for manufacturing steel plates

[0078] FIGS. 3 and 4 are flowcharts illustrating a method for manufacturing an ultra-high strength steel sheet or a plated steel sheet according to an embodiment of the present invention.

[0079] Referring to FIG. 3, a method for manufacturing an ultra-high strength steel plate according to one embodiment of the present invention comprises the steps of: providing a steel material (S10); hot rolling the steel material to form a hot-rolled steel plate (S20); cold rolling the hot-rolled steel plate to form a cold-rolled steel plate (S30); annealing the cold-rolled steel plate (S40); cooling the annealed steel plate first and secondarily (S50, S60); and reheating the cooled steel plate (S70).

[0080] Alternatively, referring to FIG. 4, after step S70, a step (S80) of performing a zinc plating treatment on the steel plate may be additionally performed.

[0081] Below, a method for manufacturing an ultra-high strength steel sheet or a plated steel sheet according to one embodiment of the present invention will be examined in detail.

[0082] Steel supply step (S10)

[0083] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a slab. The slab in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0084] The composition of the above molten steel is, in weight percent, carbon (C): 0.15% to 0.5%, silicon (Si): 0.8% to 2.0%, manganese (Mn): 1.5% to 4.0%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.005% to 1.0%, chromium (Cr)+nickel (Ni)+copper (Cu)+molybdenum (Mo): 0.005% to 0.5%, titanium (Ti)+niobium (Nb)+vanadium (V)+zirconium (Zr): 0.005% to 0.1%, and the remainder is iron (Fe) and other unavoidable impurities.

[0085] Hot rolling step (S20)

[0086] A step (S20) of forming a hot-rolled steel plate by applying a hot rolling process to the above steel material is performed.

[0087] Since the above steel is a high-alloy steel, it is necessary to minimize edge breakage and rolling load to ensure mass production, so the rolling finish temperature and coiling temperature can be set to a high temperature range.

[0088] The above steel is reheated to a temperature of Ac3 or higher, for example, at a slab reheating temperature (SRT) in the range of 1150°C to 1300°C. Through this reheating, the resolution of segregated components and precipitates may occur. If the reheating temperature is below 1150°C, a problem may arise where the hot rolling load increases rapidly. If the reheating temperature exceeds 1300°C, charging and discharging from the furnace may be difficult due to slab bending, and it may be difficult to secure the strength of the final product steel sheet due to the coarsening of the initial austenite grains. The reheating temperature may vary depending on the steel.

[0089] Subsequently, the reheated steel is hot-rolled, and hot-rolling can be performed at a Finish Delivery Temperature (FDT) of, for example, 850°C to 970°C. If the Finish Delivery Temperature exceeds 970°C, there is a risk that the quality of the steel sheet will deteriorate due to the formation of surface scale on the steel sheet. In addition, if the Finish Delivery Temperature is less than 850°C, it may cause an increase in rolling load and a decrease in productivity. The Finish Delivery Temperature may vary depending on the steel.

[0090] Next, the hot-rolled steel is cooled at a cooling rate of 10 to 30°C / s and then coiled at a coiling temperature (CT) in the range of, for example, 400°C to 650°C. The coiling temperature may vary depending on the steel. If the coiling temperature exceeds 650°C, an undesirable internal oxide layer may form on the hot-rolled steel sheet or the coiled hot-rolled coil. Since the internal oxidation of the coiled hot-rolled coil varies, it may be difficult to uniformly control the thickness of the internal oxide layer. If the coiling temperature is below 400°C, an undesirable low-temperature structure may be formed.

[0091] Cold rolling step (S30)

[0092] A step (S30) of forming a cold-rolled steel sheet by applying a cold rolling process to the above hot-rolled steel sheet is performed.

[0093] Meanwhile, in a method for manufacturing an ultra-high strength steel plate with excellent weldability according to one embodiment of the present invention, a softening heat treatment step and a pickling step may be performed sequentially first before performing a cold rolling process.

[0094] In the cold-rolled steel sheet forming step (S30), cold rolling can be performed with, for example, an average reduction rate of 40% to 60% and a reduction force of, for example, 700 ton to 1800 ton, and a cold-rolled steel sheet can be manufactured accordingly. The microstructure of the cold-rolled steel sheet has an elongated shape of the microstructure of the hot-rolled steel sheet, and the microstructure of the steel sheet produced as a final product in subsequent heat treatment is determined.

[0095] FIG. 5 is a graph illustrating the subsequent heat treatment (annealing, cooling, reheating) steps applied to a cold-rolled steel sheet in a method for manufacturing an ultra-high strength steel sheet with excellent weldability according to one embodiment of the present invention.

[0096] Annealing heat treatment step (S40)

[0097] Referring to FIG. 5, the cold-rolled steel sheet is subjected to annealing heat treatment in a continuous annealing furnace with a conventional slow cooling section. The annealing heat treatment can be performed at a temperature corresponding to the austenite and ferrite phases. By proceeding with the heat treatment in the phases, an appropriate fraction of ferrite is secured, thereby realizing ideal ferrite, martensite, and retained austenite within the final microstructure, and the target material of the steel sheet can be obtained.

[0098] In the section where the steel sheet is heated to the annealing temperature, a mixture of air and fuel can be combusted on the surface of the steel sheet to heat it. In the heating section, the cold-rolled steel sheet can be preheated using an annealing furnace, then rapidly heated at a heating rate of 10°C / s to 20°C / s, and then the rapidly heated cold-rolled steel sheet can be heated using an annealing furnace in a reducing gas atmosphere containing nitrogen and hydrogen. The hydrogen concentration may be in the range of 3% to 20%.

[0099] To ensure optimal decarburization conditions during annealing, it is necessary to control the moisture concentration and annealing time.

[0100] It is preferable that the moisture concentration inside the annealing furnace be 8,500 ppm or higher, and at the said moisture concentration, the dew point inside the furnace can be controlled to a high dew point of 0°C. The said moisture concentration can be controlled by spraying water or humidifying gas onto the cold-rolled steel sheet inside the annealing furnace. For reference, the dew point inside the furnace refers to the temperature of the dew point according to the water vapor pressure inside the furnace where the annealing heat treatment is performed.

[0101] Referring to FIG. 5, the dew point application period is defined as from the time of reaching the recrystallization temperature until the end of the first cooling (S3). Conventionally, a high dew point was applied during the annealing temperature holding time (S1) or from the time of reaching the recrystallization temperature until the end of annealing (S2). However, in such cases, there was a problem in that the effect of improving bendability was significantly reduced because there was often a high proportion of martensite or retained austenite structure with high strength mixed within the decarburized layer.

[0102] Accordingly, in the present invention, the dew point application range is extended until the end of the first cooling stage so that the moisture concentration is maintained at 8500 ppm or higher, thereby ensuring that sufficient decarburization occurs within the decarburization layer. To achieve this, a humidifying member can be additionally installed at the location of the annealing furnace where the annealing temperature holding time ends to apply high dew point heat treatment.

[0103] 1st and 2nd cooling steps (S50, S60)

[0104] The annealed cold-rolled steel sheet is cooled in multiple stages. Specifically, the annealed steel sheet can be cooled in a first stage (S50) and in a second stage (S60).

[0105] The step (S50) of first cooling the above steel plate to a temperature of 650°C or higher and 800°C or lower at an average cooling rate of 3 to 20°C / s is a slow cooling step, and by attempting to secure a certain amount of ferrite within the final microstructure during the heat treatment process, the plasticity of the final microstructure can be secured. If the cooling end temperature of the slow cooling is less than 650°C, ferrite transformation may occur in an undesirable amount, and consequently, the strength may be reduced.

[0106] In the present invention, the dew point application period is defined from the time of reaching the recrystallization temperature until the end of the first cooling (S3), so that the high dew point is extended until the end of the first cooling, thereby providing sufficient time for carbon present in the decarburized layer to diffuse. Accordingly, the problem of high-strength martensite or retained austenite structures being mixed in at a certain proportion or higher within the decarburized layer can be resolved. Through this, the bendability of the steel sheet can be improved.

[0107] Continuing, the steel plate is cooled at an average cooling rate of 20℃ / s or more, preferably 50℃ / s or more, to the martensite transformation start temperature (M s The second cooling step (S60) below is a rapid cooling step. In the rapid cooling step (S60), by controlling the rapid cooling end temperature, the austenite in the microstructure after slow cooling can be transformed into martensite, thereby facilitating the securing of the final material. Additionally, an average cooling rate of 20℃ / s or more is required to suppress phase transformation that may occur during the rapid cooling process.

[0108] Subsequently, the secondarily cooled cold-rolled steel sheet can be maintained at the second cooling termination temperature of 200°C or higher and less than 300°C for a time ranging, for example, from 1 second to 100 seconds. During this holding time after rapid cooling, temperature homogenization of the steel may initially proceed. The second cooling termination temperature is the martensite transformation start temperature (M s ) and martensite transformation completion temperature (M fIt can be a temperature between ).

[0109] Subsequently, while maintaining isothermally at the above secondary cooling end temperature, some of the residual austenite may transform into lower bainite, etc.

[0110] Reheating step (S70)

[0111] The above multi-stage cooled cold-rolled steel sheet can be reheated at a heating rate of, for example, 10℃ / s or more, and partitioning heat treatment can be performed by maintaining it at, for example, a temperature of 360℃ to 520℃ for a time of 30 seconds to 500 seconds. The partitioning heat treatment temperature is the martensite transformation start temperature (M s It may be higher than ). The purpose of the above reheating process is to secure strength and elongation through carbon concentration in the residual austenite and martensite tempering during the process, and finally to maintain the final microstructure composition.

[0112] The aforementioned multi-stage cooling steps (S50, S60) and reheating steps (S70) correspond to the Q&P (Quenching and Partitioning) heat treatment steps developed to simultaneously secure high strength and high ductility of the steel material. This technology suppresses the formation of carbon carbide precipitates emitted from the martensite structure during quenching and allows carbon to diffuse into the retained austenite structure through partitioning. By re-diffusion of carbon, the retained austenite structure is stabilized even at room temperature, and ultimately, it is possible to secure high ductility through the retained austenite structure and high strength through the martensite structure.

[0113] zinc plating step (S80)

[0114] As shown in FIG. 4, the annealed cold-rolled steel sheet can be immersed in a plating bath to selectively perform plating treatment, and the plating bath may be one or more selected from a molten zinc plating bath, a molten aluminum plating bath, an aluminum alloy plating bath, an electro-zinc plating bath, an electro-zinc alloy plating bath, and an alloyed molten zinc plating bath.

[0115] As one embodiment, a step (GI) of forming a hot-dip galvanized steel sheet by immersing in a hot-dip galvanizing bath and performing hot-dip galvanizing can be performed, and a step (GA) of forming an alloyed hot-dip galvanized steel sheet by alloying the cold-rolled steel sheet and the hot-dip galvanized steel sheet formed with the hot-dip galvanizing layer can be further performed.

[0116] In the case of the plating material, after the above partitioning heat treatment step is performed, it can enter the plating bath as is without cooling to room temperature.

[0117] The entry temperature of the plating bath may be, for example, 460°C, and the composition of the plating bath may be a zinc plating bath containing 0.11 to 0.22 weight% of aluminum and saturated Fe. Alternatively, the plating bath may be a Zn-Mg-Al plating bath. Meanwhile, the alloying heat treatment temperature may be 450 to 600°C.

[0118] Experimental example

[0119] Preferred experimental examples are presented below to aid in understanding the present invention. However, the following experimental examples are intended only to aid in understanding the present invention, and the present invention is not limited by the following experimental examples.

[0120] Table 1 is a table showing the composition of ultra-high strength cold-rolled steel sheets according to experimental examples of the present invention, and Table 2 is a table showing the process conditions of ultra-high strength cold-rolled steel sheets according to experimental examples of the present invention.

[0121] ingredient C Si Mn P S Al N B Cr+Ni+Cu+Mo Ti+Nb+V+Zr Content 0.15 2.0 2.5 0.01 0.003 0.03 0.001 0.001 0.01 0.01

[0122] Hot rolling process Reheating temperature: 1200℃, Finish rolling temperature: 900℃, Coiling temperature: 600℃ Heat treatment process Annealing temperature: 850℃, Slow cooling temperature: 700℃, Quick cooling temperature: 250℃, Reheating temperature: 460℃

[0123] A steel having the composition (unit: weight%) of Table 1 above is prepared, and a hot-rolled steel sheet is prepared by undergoing a predetermined hot-rolling process. The remainder consists of iron (Fe) and other unavoidable impurities. Both the example and the comparative example have the same alloy composition. Referring to Table 2, the hot-rolling process applied process conditions of a reheating temperature: 1200°C, a finishing rolling temperature: 900°C, and a coiling temperature: 600°C. After the cold-rolling process, process conditions of an annealing temperature: 850°C, a slow cooling temperature: 700°C, a rapid cooling temperature: 250°C, and a reheating temperature: 460°C were applied.

[0124] In the experimental examples of the present invention, hot-dip galvanized (GI) steel sheets were formed by applying the composition of Table 1 and the process conditions of Table 2 described above, and then a tensile test was performed and the nanohardness distribution was measured by inserting an indenter into the surface. In addition, for bendability, the specimen was processed into a V-bend, and the radius (R) inside the bending section was varied to observe whether cracks occurred on the surface. The final radius at which no cracks occurred was expressed as the bendability R value of the steel, and this was divided by the thickness (t). The results are shown in Table 3.

[0125] division Annealing section with high dew point application Moisture concentration (ppm) YP(MPa) TS(MPa) El(%) Nanohardness distribution (%) in the A2 region of the substrate steel Hm A2 (GPa) d A2 Hm A2 ×d A2 t(A1+A2)(㎛) Flexibility (R / t) ≤5GPa ≤10GPa ≤15GPa ≤20GPa Comparative Example 1 - 0 942 1506 15.3 - - - - - 0 - 0 4.0 Comparative Example 2 S1 2600 921 1476 14.3 37.1 28.4 29.9 4.6 6.81 4.30 29.3 21 3.7 Comparative Example 3 S2 2600 913 1467 15.2 38.4 29.3 28.4 3.9 6.41 3.98 25.5 20 3.7 Comparative Example 4 S3 2600 911 1460 14.5 41.6 29.0 25.1 4.3 6.33 3.63 23.0 22 3.5 Comparative Example 5 S1 8500 903 1471 13.9 60.3 25.7 11.3 2.7 5.89 2.63 15.5 31 2.5 Comparative Example 6 S2 8500 895 1465 14.6 60.7 26.7 9.8 2.8 5.78 1.87 10.8 33 2.2 Example 1 S3 8500 895 1458 15.1 79.4 16.6 3.3 0.7 3.86 1.35 5.2 33 1.6

[0126] FIG. 6 is a diagram showing the bending characteristics of the transition decarburization layer of a substrate steel according to an embodiment and a comparative example of the present invention. FIG. 6 (a) shows the result of Example 1 of Table 3, and (b) shows the result of Comparative Example 6.

[0127] Referring to Table 3 and Figure 6, in the case of the example where the dew point annealing section is applied to the S3 region of Figure 5, the R / t value is 1.6, confirming that the bending characteristics of the steel plate targeted by the present invention are satisfied. On the other hand, in the case of Comparative Example 6, where the dew point annealing section is applied to the S2 region, the R / t value is 2.2, confirming that the effect of improving bending characteristics is insufficient.

[0128] Referring to Table 3, Comparative Example 1 is an example excluding the moisture concentration control process, and the bending characteristics were poor because no decarburization reaction occurred.

[0129] Comparative Examples 2 to 4 are examples in which the moisture concentration inside the annealing furnace was controlled to a low level of 2600 ppm, and the thickness of t(A1+A2) corresponding to the decarburization layer was only about 20 μm, so the condition of R / t ≤ 2.0 was not satisfied. This indicates that the carbon present inside the decarburization transition layer did not diffuse sufficiently, resulting in a thinner thickness of the decarburization layer, and consequently, the minimum radius at which cracks do not occur increased.

[0130] In addition, Hm corresponding to the difference in longitude between phases in area A2 A2 ×d A2 It can be seen that the high value indicates a high fraction of the low-temperature phase in the A2 region, which degrades the bending characteristics of the steel sheet. Through this, it can be understood that sufficient moisture concentration within the annealing furnace must be ensured to reduce the interphase hardness difference in the A2 region and thereby improve the bending characteristics of the steel sheet. Comparative Examples 5 and 6 are examples where the annealing section with the applied dew point was short; although the thickness of t(A1+A2) corresponding to the decarburized layer is 30㎛ or more, a low-temperature phase with a high nanohardness value remains within the A2 region of the base steel, and Hm corresponding to the interphase hardness difference A ×d A2 The value was not sufficiently high (10 or more), so the bending characteristics were not sufficiently secured.

[0131] On the other hand, in Example 1, as the high dew point annealing section was extended, most of the structure in the A2 region of the base iron changed to ferrite, and thus the interphase hardness variation was significantly reduced, resulting in a sufficient increase in bending characteristics. That is, unlike Comparative Examples 1 to 6, Example 1 satisfies both Equation (1) and Equation (2) of the present invention, thereby confirming that it has the best bending characteristics.

[0132] According to the technical concept of the present invention as described above, by extending the high dew point application section to the slow cooling termination section to provide sufficient time for carbon present in the decarburization transition layer to diffuse, the hard phase remaining in the decarburization transition layer can be reduced, and accordingly, the hardness variation between phases can be minimized. Through this, a high-strength steel sheet or galvanized steel sheet can be manufactured having a yield strength of 880 MPa or more, a tensile strength of 1180 MPa or more, a maximum elongation of 14% or more, and a bendability (R / t) of 2.0 or less.

[0133] Meanwhile, although it is obvious that the technical concept of the present invention can be applied to transformation-induced plasticity (TRIP) steel, it is evident that the technical concept of the present invention can be widely applied to various steel plates and is not limited to application only to the TRIP steel plates.

[0134] It will be obvious to those skilled in the art that the technical concept of the present invention described above is not limited to the aforementioned embodiments and attached drawings, and that various substitutions, modifications, and changes are possible within the scope of the technical concept of the present invention.

Claims

Claim 1 In weight percent, it contains carbon (C): 0.15% to 0.5%, silicon (Si): 0.8% to 2.0%, manganese (Mn): 1.5% to 4.0%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.005% to 1.0%, boron (B): 0.0003% to 0.005%, chromium (Cr)+nickel (Ni)+copper (Cu)+molybdenum (Mo): 0.005% to 0.5%, titanium (Ti)+niobium (Nb)+vanadium (V)+zirconium (Zr): 0.005% to 0.1%, and the remainder comprises iron (Fe) and other unavoidable impurities, and the following formula (1) and an ultra-high strength steel plate characterized by satisfying the following equation (2). Equation (1): 0.1≤Hm A2 ×d A2 ≤10.0 Equation (2): 30㎛ ≤ t (A1+A2) ≤ 50㎛(wherein, A1 is defined as the region up to where the proportion of the area where the nanohardness value measured at the same depth is 5 GPa or less is 80% or more, by measuring hardness at 3㎛ intervals through a nano-indentation test of 90㎛ or more in the depth direction and 30㎛ or more in the length direction from the steel plate surface, and A2 is defined as the region up to where the proportion is 30% or more and less than 80%, and Hm A2 ε is the average nanohardness of region A2, Hm(k) is the nanohardness corresponding to measurement position k within region A2, N represents the number of variables for which nanoindentation tests were performed within region A2, the decarburization layer is composed of regions A1 and A2, and the thickness t(A1+A2) corresponds to the depth of the decarburization layer. Claim 2 delete Claim 3 An ultra-high strength steel plate according to claim 1, comprising one or more plating layers formed on the steel plate, wherein the plating layer is any one of a hot-dip galvanized layer, a hot-dip aluminum plating layer, an aluminum alloy plating layer, an electro-galvanized layer, an electro-galvanized layer, and an alloyed hot-dip galvanized layer. Claim 4 In claim 1, an ultra-high strength steel plate having a yield strength (YP) of 880 MPa or more, a tensile strength (TS) of 1180 MPa or more, an elongation (El) of 14% or more, and a bendability (R / t) of 2.0 or less. Claim 5 In claim 1, the ultra-high strength steel plate, wherein the A1 region is formed on the surface layer of the steel plate and is a ferrite decarburized layer having a single-phase ferrite microstructure, and the A2 region is a transition decarburized layer comprising a ferrite, martensite, and retained austenite composite structure. Claim 6 delete Claim 7 In claim 5, the ferrite decarburization layer or the transition decarburization layer is formed on top of a matrix structure comprising a martensite and residual austenite composite structure, an ultra-high strength steel plate. Claim 8 Cold-rolled, containing, in weight percent, carbon (C): 0.15% to 0.5%, silicon (Si): 0.8% to 2.0%, manganese (Mn): 1.5% to 4.0%, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.02% or less (excluding 0%), nitrogen (N): 0.01% or less (excluding 0%), aluminum (Al): 0.005% to 1.0%, boron (B): 0.0003% to 0.005%, chromium (Cr) + nickel (Ni) + copper (Cu) + molybdenum (Mo): 0.005% to 0.5%, titanium (Ti) + niobium (Nb) + vanadium (V) + zirconium (Zr): 0.005% to 0.1%, and the remainder comprising iron (Fe) and other unavoidable impurities. A step of annealing a steel plate in an annealing furnace; a step of first cooling the annealed steel plate to a temperature of 650°C or higher and less than 800°C at an average cooling rate of 3 to 20°C / s; a step of secondarily cooling the firstly cooled steel plate to a temperature below Ms at an average cooling rate of 20°C / s or higher; after the second cooling, a step of reheating the steel plate to 360°C to 520°C at an average rate of 10°C / s or higher and maintaining it for 30 seconds to 500 seconds; and a step of performing a plating treatment on the steel plate; wherein the cold-rolled steel plate comprises a step of hot-rolling a steel slab so that the exit side temperature becomes 850 to 970°C and coiling it at a temperature of 650°C or lower to form a hot-rolled steel plate; A method for manufacturing an ultra-high strength steel sheet, wherein the method comprises the step of cold rolling the hot-rolled steel sheet with a reduction rate of 40 to 60%, wherein the annealing heat treatment step is performed under conditions where the moisture concentration in the annealing furnace is 8500 ppm or more by volume fraction and the hydrogen concentration is 3% to 20% by volume fraction, and the dew point application section is maintained from the time of reaching the recrystallization temperature until the end of the first cooling. Claim 9 A method for manufacturing an ultra-high strength steel plate according to claim 8, characterized by further performing the step of spraying humidifying gas onto the steel plate after the annealing heat treatment and before the first cooling. Claim 10 A method for manufacturing an ultra-high strength steel sheet according to claim 8, wherein the annealing heat treatment step and the first cooling step are performed at a temperature corresponding to the austenite and ferrite ideal region, and the method comprises a step of inducing a decarburization reaction on the surface of the steel sheet to transform the austenite present in the surface layer of the steel sheet into ferrite. Claim 11 delete Claim 12 delete Claim 13 delete Claim 14 delete

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