Steel sheet and manufacturing method therefor

Abstract

One aspect of the present invention provides an ultra-high strength steel sheet having excellent formability due to having high elongation and hole expandability, and a manufacturing method therefor.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more specifically, to a steel plate and a method for manufacturing the same that can be preferably applied to automobile collisions and structural members, etc.

[0002] The development of high-strength steel sheets has been continuously pursued to reduce vehicle weight and enhance safety. Recently, the importance of ultra-high-strength steel with a tensile strength of 1,500 MPa or higher has been growing to improve the driving range of electric vehicles and protect batteries. However, since existing MART steels lack formability, the development of ultra-high-strength steel sheets for cold forming that also possess formability is expected to have significant economic value. To improve the formability of steel and increase elongation, a widely used method involves introducing retained austenite to utilize the TRIP (Transformation Induced Plasticity) phenomenon. However, in the case of such TRIP steel sheets, the addition of Si and Al is required to introduce retained austenite, and a larger amount of retained austenite can be obtained when accompanied by bainite transformation. Nevertheless, because bainite transforms at relatively high temperatures, its tensile strength (TS) is low, and its yield strength (YS) is also relatively low for use as ultra-high-strength steel.

[0003] Therefore, the recent trend is to adopt the Quenching and Partitioning process to increase the strength of steel plates while utilizing the TRIP phenomenon. In the case of so-called Q&P steel, the main structure of the matrix is ​​tempered martensite, which has excellent yield strength and hole extension ratio (HER), and if residual austenite is actively formed, an appropriate level of elongation can also be obtained.

[0004] Meanwhile, the importance of automotive safety is increasing day by day from the perspective of passenger protection. Although it is believed that the risk of vehicle collisions can be fundamentally eliminated through advancements in autonomous driving technology, there is a risk that the severity of accidents may actually increase during the transitional period leading up to technological maturity due to passengers' lack of awareness of the accident situation; consequently, regulations on vehicle crash tests have recently been further strengthened. It is known that the bending properties of steel are crucial for reducing the risk of cracking in vehicle structural members during a collision. If the steel exhibits excellent bending properties, the material can fold rather than crack, absorbing more collision energy, while the remaining parts withstand the impact, thereby inducing the collapse of a stable structure.

[0005] However, applying the Q&P process alone is insufficient to obtain excellent bending formability, and additional ideas are needed to secure excellent bending formability along with high strength of 1.3 GPa or more.

[0006] [Prior Art Literature]

[0007] [Patent Literature]

[0008] (Patent Document 1) Japanese Patent Publication No. JP 2005-272954

[0009] According to one embodiment of the present invention, an ultra-high strength steel sheet having excellent formability and excellent hydrogen embrittlement resistance due to high elongation and hole expansion, and a method for manufacturing the same can be provided.

[0010] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0011] A steel sheet according to one embodiment of the present invention comprises, in weight percent, C: 0.10~0.30%, Si: 0.50~2.50%, Mn: 1.0~3.0%, Cr: 0.01~1.20%, P: 0.001~0.100%, S: 0.010% or less, Sol.Al: 0.0010~0.1000%, N: 0.001~0.010%, Mo: 0.02~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.120%, and Nb: 0.010~0.050%, and the remainder may consist of Fe and other unavoidable impurities, and the microstructure of the center of the steel sheet may have a matrix structure of tempered martensite, and from the surface of the steel sheet in the direction of the thickness of the steel sheet The average circular equivalent diameter of Ti-Mo precipitates measured at the 1 / 4T point is 120–300 nm, and the average circular equivalent diameter of Ti-Mo precipitates measured at the surface layer of the steel plate may be 20–120 nm.

[0012] The steel plate described above may have a carbon equivalent (Ceq) of 0.46 or less, derived by the following relationship 1.

[0013] [Relationship 1] Ceq = [C]+[Si] / 30+[Mn] / 20+2[P]+4[S]

[0014] In the above equation, [C], [Si], [Mn], [P], and [S] represent the weight percent of each element.

[0015] The microstructure of the core of the steel plate described above may include, in area %, one or more of tempered martensite: 70.0~95.0%, retained austenite: 3.0~15.0%, fresh martensite: 10.0% or less, and ferrite: 5.0% or less.

[0016] The steel plate described above may have an internal oxide layer formed on the surface layer of the steel plate.

[0017] The surface layer of the steel plate described above may have a decarburization rate of 30% or more.

[0018] The steel plate described above may have a yield strength of 1000 MPa or more, a tensile strength of 1470 MPa or more, an elongation of 10% or more, a hole expansion factor (HER) of 25% or more, and a bendability (R / t) of 2.0 or less.

[0019] A method for manufacturing the above-described steel sheet according to another embodiment of the present invention comprises the steps of: preparing a cold-rolled steel sheet comprising, in weight percent, C: 0.10~0.30%, Si: 0.50~2.50%, Mn: 1.0~3.0%, Cr: 0.01~1.20%, P: 0.001~0.100%, S: 0.010% or less, Sol.Al: 0.0010~0.1000%, N: 0.001~0.010%, Mo: 0.02~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.120%, and Nb: 0.010~0.050%, and the remainder being Fe and other unavoidable impurities; and continuously annealing the cold-rolled steel sheet at a temperature of Ac3℃ or higher and 900℃ or lower. The method includes the step of cooling the cold-rolled steel sheet at a cooling rate of 5 to 60°C / s to a cooling end temperature of 150°C to Ms(°C); and the step of reheating the cold-rolled steel sheet to a temperature of 200°C to Bs and over-aging treatment for 6 to 40 minutes, wherein the continuous annealing can be performed in an atmosphere comprising 0.1 to 30 volume% hydrogen and H2O with a dew point of -35 to 20°C, with the remainder being nitrogen and impurity gas.

[0020] The above-described method for manufacturing a steel plate may additionally include a step of temper rolling the cold-rolled steel plate with an elongation of 0.010 to 1.0% after over-aging treatment.

[0021] The above-described cold-rolled steel sheet comprises, in weight%, C: 0.10~0.30%, Si: 0.50~2.50%, Mn: 1.0~3.0%, Cr: 0.01~1.20%, P: 0.001~0.100%, S: 0.010% or less, Sol.Al: 0.0010~0.1000%, N: 0.001~0.010%, Mo: 0.02~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.120%, and Nb: 0.010~0.050%, and the remainder being Fe and other unavoidable impurities; a step of heating a steel slab to a temperature range of 1000~1350℃; It can be obtained through the steps of: finishing hot rolling the steel slab in a temperature range of Ar3 to 1000℃ to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet in a temperature range of 450 to 650℃; and cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet.

[0022] The present invention can provide an ultra-high strength cold-rolled steel sheet with excellent formability due to high elongation and hole expansion, and a method for manufacturing the same.

[0023] Furthermore, the present invention can provide an ultra-high strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and a method for manufacturing the same.

[0024] Figure 1a is a transmission electron microscope image observed at the 1 / 4t thickness reference point, and Figures 1b and 1c are EDS mapping results observed at the 1 / 4t thickness reference point.

[0025] Figure 2a is a transmission electron microscope image observed on the surface layer of a steel plate, and Figures 2b and 2c are EDS mapping results observed on the surface layer of a steel plate.

[0026] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0027] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0028] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0029] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0030] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0031] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0032] It should be noted that, although not essential, the technical solution according to each aspect of the present invention may be usefully applied to other aspects of the technical solution. Furthermore, the composition and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0033] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described.

[0034] C: 0.10~0.30%

[0035] Carbon (C) is a very important element added to strengthen the transformation structure. C promotes high strength and facilitates the formation of martensite in the transformed structure steel. As the C content increases, the amount of martensite in the steel increases. However, if the C content exceeds 0.30%, although the strength of the martensite increases, the strength difference with ferrite, which has a low carbon concentration, widens. Since this strength difference easily induces fracture at the interphase interface when stress is applied, the elongation flangeability is reduced. In addition, weldability is reduced, leading to welding defects during part processing at the customer's site. On the other hand, if the C content is less than 0.10%, it is difficult to secure the strength of the tempered martensite intended for this invention. Therefore, it is desirable for the C content to have a range of 0.10 to 0.30%. It is more preferable for the lower limit of the C content to be 0.12%, and even more preferable for it to be 0.14%. The upper limit of the above C content is more preferably 0.28%, and more preferably 0.26%.

[0036] Si: 0.50~2.50%

[0037] The above-mentioned Si, like carbon, plays a role in improving the strength of steel through solid solution strengthening. To this end, the present invention may add 0.50% or more of the above-mentioned Si. However, the above-mentioned silicon (Si) promotes ferrite transformation and increases the carbon content in the untransformed austenite, thereby forming a composite structure of ferrite and martensite, which hinders the increase in strength of martensite. Furthermore, regarding surface characteristics, it not only causes surface scale defects but can also reduce chemical treatment properties. Accordingly, in the present invention, the content of the above-mentioned Si can be controlled to 2.50% or less. It is more preferable that the content of the above-mentioned Si be 2.30% or less, and even more preferable that it be 2.10% or less.

[0038] Mn: 1.0~3.0%

[0039] Manganese (Mn) is an element that refines grains without compromising ductility and strengthens steel by completely precipitating sulfur in the steel as MnS, thereby preventing hot brittleness caused by the formation of FeS. Additionally, it lowers the critical cooling rate required to obtain the martensite phase, making it easier to form martensite. If the Mn content is less than 1.0%, it may be difficult to sufficiently secure the aforementioned effects. If the Mn content exceeds 3.0%, there is a high probability that problems such as weldability and hot rolling performance will occur. Therefore, it is desirable for the Mn content to be in the range of 1.0% to 3.0%. The lower limit of the Mn content is more preferably 1.20%, and more preferably 1.50%. The upper limit of the Mn content is more preferably 2.90%, and more preferably 2.70%.

[0040] Cr: 0.01~1.20%

[0041] Chromium (Cr) is a component added to improve the hardenability of steel and ensure high strength, and it is an effective element for forming martensite, a low-temperature transformation phase. If the Cr content is less than 0.01%, it is difficult to sufficiently obtain the aforementioned effects. If the Cr content exceeds 1.20%, not only are the effects saturated, but cold rolling performance may also deteriorate as the strength of the hot-rolled steel sheet increases excessively. Therefore, it is desirable for the Cr content to be in the range of 0.01% to 1.20%. It is more preferable for the lower limit of the Cr content to be 0.05%, and even more preferable for it to be 0.10%. It is more preferable for the upper limit of the Cr content to be 1.00%, and even more preferable for it to be 0.80%.

[0042] P: 0.001~0.100%

[0043] Phosphorus (P) is a substitutional alloying element with the greatest solid solution strengthening effect, playing a role in improving in-plane anisotropy and enhancing strength. If the P content is less than 0.001%, it may be difficult to sufficiently secure the aforementioned effects, and it may also cause problems with manufacturing costs. If the P content exceeds 0.100%, press formability is reduced, and brittleness of the steel may occur. Therefore, it is desirable for the P content to be in the range of 0.001% to 0.100%. It is more desirable for the lower limit of the P content to be 0.005%, and more desirable for it to be 0.008%. It is more desirable for the upper limit of the P content to be 0.080%, and more desirable for it to be 0.060%.

[0044] S: 0.010% or less

[0045] Sulfur (S) is an impurity element in steel that impairs the ductility and weldability of steel sheets. Since there is a high possibility that the ductility and weldability of steel sheets will be impaired when the S content exceeds 0.010%, it is preferable in the present invention to limit the S content to 0.010% or less. It is more preferable that the S content be 0.005% or less, and even more preferable that it be 0.003% or less.

[0046] Sol.Al: 0.0010~0.1000%

[0047] The above-mentioned soluble aluminum (Sol.Al) is an element effective not only for deoxidizing by combining with oxygen in the steel but also for improving the hardenability of martensite by distributing carbon within the ferrite to the austenite. If the content of Sol.Al is less than 0.0010%, it may be difficult to sufficiently secure the above effect. If the content of Sol.Al exceeds 0.1000%, not only is the above effect saturated, but manufacturing costs may also increase. Therefore, the content of Sol.Al may have a range of 0.0010% to 0.1000%. It is more preferable that the lower limit of the Sol.Al content be 0.0015%, and more preferable that it be 0.0020%. It is more preferable that the upper limit of the Sol.Al content be 0.0900%, and more preferable that it be 0.0800%.

[0048] N: 0.001~0.010%

[0049] Nitrogen (N) is a component that acts effectively to stabilize austenite. If the content of N is less than 0.001%, it is difficult to sufficiently obtain the aforementioned effect. If the content of N exceeds 0.010%, the risk of cracking during continuous casting increases significantly due to AlN formation, etc. Therefore, it is desirable for the content of N to be in the range of 0.001 to 0.010%. It is more desirable for the lower limit of the N content to be 0.002%, and more desirable for it to be 0.003%. It is more desirable for the upper limit of the N content to be 0.009%, and more desirable for it to be 0.008%.

[0050] Mo: 0.02~0.30%

[0051] Molybdenum (Mo) is an element advantageous for securing strength through improved hardenability and the formation of Mo-based precipitates. If the Mo content is less than 0.02%, it may be difficult to sufficiently secure the above effects. If the Mo content exceeds 0.30%, coarse carbides may form, which may result in a disadvantage of reduced elongation. Therefore, it is desirable for the Mo content to be in the range of 0.02 to 0.30%. It is more desirable for the lower limit of the Mo content to be 0.03%, and more desirable for it to be 0.04%. It is more desirable for the upper limit of the Mo content to be 0.25%, and more desirable for it to be 0.23%.

[0052] B: 0.0010~0.0050%

[0053] Boron (B) is an element that delays the transformation of austenite into pearlite during the cooling process after annealing, inhibits the formation of ferrite, and promotes the formation of martensite. If the content of B is less than 0.0010%, it may be difficult to sufficiently secure the above effects. If the content of B exceeds 0.0050%, there may be a disadvantage in that manufacturing costs increase due to an excess of ferroalloy. Therefore, it is desirable for the content of B to have a range of 0.0010% to 0.0050%. It is more desirable for the lower limit of the B content to be 0.0012%, and more desirable for it to be 0.0014%. It is more desirable for the upper limit of the B content to be 0.0045%, and more desirable for it to be 0.0040%.

[0054] Ti: 0.010~0.120%

[0055] Titanium (Ti) is an element effective for increasing the strength of steel sheets and for grain refinement by forming nano-precipitates through bonding with carbon. These nano-precipitates also play a role in reducing the hardness difference between phases by strengthening the matrix structure. If the Ti content is less than 0.010%, it may be difficult to sufficiently secure the above effects. If the Ti content exceeds 0.120%, ductility may decrease due to the formation of coarse precipitates. Therefore, it is desirable for the Ti content to be in the range of 0.010 to 0.120%. It is more desirable for the lower limit of the Ti content to be 0.020%, and even more desirable for it to be 0.030%. It is more desirable for the upper limit of the Ti content to be 0.110%, and even more desirable for it to be 0.105%.

[0056] Nb: 0.010~0.050%

[0057] Niobium (Nb) is an element effective for increasing the strength of steel sheets and for grain refinement by forming nano-precipitates through bonding with carbon. These nano-precipitates also play a role in reducing the hardness difference between phases by strengthening the matrix structure. If the Nb content is less than 0.010%, it may be difficult to sufficiently secure the above effects. If the Nb content exceeds 0.050%, ductility may decrease due to the formation of coarse precipitates. Therefore, it is desirable for the Nb content to be in the range of 0.010 to 0.050%. It is more preferable for the lower limit of the Nb content to be 0.015%, and even more preferable for it to be 0.020%. It is more preferable for the upper limit of the Nb content to be 0.045%, and even more preferable for it to be 0.040%.

[0058] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0059] A steel plate according to one embodiment of the present invention may have a carbon equivalent (Ceq) derived by the following relationship 1 of 0.46 or less.

[0060] [Relationship 1] Ceq = [C]+[Si] / 30+[Mn] / 20+2[P]+4[S]

[0061] In the above equation 1, [C], [Si], [Mn], [P], and [S] represent the weight percent of each element.

[0062] In order to manufacture high-strength steel, the present invention allows for the addition of appropriate amounts of hardenability-enhancing elements such as high content of C, Si, Mn, P, and S. In particular, when manufacturing high-strength steel through air cooling after rolling and QP (Quenching & Partitioning) heat treatment, adding a large amount of alloying elements not only results in excessively high manufacturing costs but can also significantly reduce weldability due to the high carbon equivalent.

[0063] Accordingly, the inventors found that high strength and excellent weldability can be secured simultaneously by making the Ceq derived by the above relationship 1 0.46 or less.

[0064] The present invention does not separately limit the lower limit of the above Ceq. However, as an example, the above Ceq may be 0.40 or higher.

[0065] Meanwhile, the microstructure of the core of the steel plate of the present invention may have a tempered martensite matrix structure.

[0066] As a more specific example, the microstructure of the core of the steel sheet of the present invention preferably comprises, in area %, one or more of tempered martensite: 70~95%, retained austenite: 3~15%, fresh martensite: 10% or less, and ferrite: 5% or less.

[0067] The above-mentioned tempered martensite is a structure advantageous for securing tensile strength. If the fraction of the above-mentioned tempered martensite is less than 70%, the tensile strength targeted by the present invention cannot be obtained. If the fraction of the above-mentioned tempered martensite exceeds 95%, the tensile strength may become excessively high.

[0068] The above-mentioned retained austenite is a structure advantageous for securing elongation. If the fraction of the above-mentioned retained austenite is less than 3%, the elongation targeted by the present invention cannot be obtained. If the fraction of the above-mentioned retained austenite exceeds 15%, the tensile strength targeted by the present invention cannot be obtained.

[0069] The above fresh martensite is a structure that is disadvantageous for ensuring formability. If the fraction of the above fresh martensite exceeds 10%, the tensile strength may become excessively high. The above ferrite is a structure that is disadvantageous for ensuring formability. If the fraction of the above ferrite exceeds 5%, the difference in hardness between phases may increase, thereby reducing hole expansion.

[0070] A steel sheet according to one embodiment of the present invention can improve elongation along with high strength characteristics by appropriately distributing Ti-Mo fine precipitates on the steel sheet.

[0071] In particular, the present invention found that by distributing relatively fine Ti-Mo precipitates in the surface layer of the steel plate and distributing relatively coarse Ti-Mo precipitates at the 1 / 4T point in the thickness direction of the steel plate compared to the surface layer, superior bendability and hydrogen embrittlement resistance can be secured along with high strength and high elongation characteristics throughout the entire thickness of the steel plate. At this time, the Ti-Mo precipitates may refer to precipitates containing 25.00 wt% or more of Ti and 5.00 wt% or more of Mo when observed through EDS mapping. In addition, the surface layer of the steel plate may refer to the portion extending from the surface of the steel plate to 30 μm in the thickness direction of the steel plate.

[0072] That is, in one embodiment of the present invention, the average circular equivalent diameter of Ti-Mo precipitates measured at a point 1 / 4T from the steel plate surface in the steel plate thickness direction may be 120 to 300 nm, and the average circular equivalent diameter of Ti-Mo precipitates measured at the steel plate surface layer may be 20 to 120 nm.

[0073] If the average circular equivalent diameter of the Ti-Mo precipitates at the 1 / 4T thickness point is less than 120 nm, it may be difficult to secure high strength properties due to a lack of fine precipitates at the 1 / 4T thickness point. On the other hand, if the average circular equivalent diameter of the Ti-Mo precipitates exceeds 300 nm, the elongation may decrease as coarse precipitates are formed at the 1 / 4T thickness point. As another example, the average circular equivalent diameter of the Ti-Mo precipitates at the 1 / 4T thickness point may be 150 to 280 nm or 180 to 250 nm.

[0074] In addition, if the average circular equivalent diameter of Ti-Mo precipitates in the surface layer of the steel plate is less than 20 nm, it may be difficult to secure high strength characteristics in the surface layer. On the other hand, if the average circular equivalent diameter of Ti-Mo precipitates exceeds 120 nm, the elongation in the surface layer may decrease. As another example, the average circular equivalent diameter of Ti-Mo precipitates in the surface layer of the steel plate may be 30 to 115 nm or 40 to 110 nm.

[0075] When measuring the average circle equivalent diameter of the Ti-Mo precipitates described above, the minimum diameter of the Ti-Mo precipitates subject to observation may be 20 nm.

[0076] A steel plate according to one embodiment of the present invention may have an internal oxide layer formed on the surface layer of the steel plate.

[0077] In other words, the present invention can improve weldability by forming the aforementioned internal oxide layer on the surface layer. Furthermore, the present invention can improve bendability through the internal oxide layer, and as a result, prevent cracking from occurring in a hydrogen environment.

[0078] At this time, the decarburization rate of the surface layer of the steel plate may be 30% or more.

[0079] As described below, if the dew point temperature inside the heat treatment furnace is properly controlled, internal oxides are formed on the surface layer of the steel plate, and at the same time, a reaction occurs in which carbon in the steel reacts with oxygen adsorbed on the surface of the steel plate to gasify into CO or CO2, and a C depletion region is formed on the surface layer of the base material. When such a decarburization reaction occurs, spot welding LME crack resistance and bendability are improved. However, if the decarburization rate is less than 30%, the decarburization reaction is insufficient, and LME crack resistance or bendability may be inferior. Furthermore, as described above, the present invention can secure the aforementioned decarburization rate to ensure excellent bendability, and as a result, hydrogen embrittlement resistance can also be improved.

[0080] The steel plate of the present invention described above has excellent formability due to its high strength, elongation, and hole expansion properties.

[0081] More specifically, the cold-rolled steel sheet of the present invention described above may have a yield strength of 1100 MPa or more, a tensile strength of 1470 MPa or more, an elongation of 10% or more, a hole expansion ratio (HER) of 25% or more, and a bendability (R / t) of 2.0 or less. In addition, a steel sheet according to another embodiment of the present invention may have excellent weldability and hydrogen embrittlement resistance in addition to the characteristics described above.

[0082] Next, a method for manufacturing a steel plate according to one embodiment of the present invention will be described.

[0083] First, a method for manufacturing a steel sheet according to one embodiment of the present invention can prepare a cold-rolled steel sheet comprising C: 0.10~0.30%, Si: 0.50~2.50%, Mn: 1.0~3.0%, Cr: 0.01~1.20%, P: 0.001~0.100%, S: 0.010% or less, Sol.Al: 0.0010~0.1000%, N: 0.001~0.010%, Mo: 0.02~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.120% and Nb: 0.010~0.050%, with the remainder being Fe and other unavoidable impurities.

[0084] Since the method for manufacturing the above cold-rolled steel sheet can be applied in the same way as methods commonly used in the relevant technical field, the present invention does not separately limit the method for manufacturing the above cold-rolled steel sheet.

[0085] However, as an example, the above cold-rolled steel sheet may be obtained through the steps of: heating a slab having the alloy composition described above to a temperature range of 1000 to 1350°C; finishing hot-rolling the steel slab at a temperature range of Ar3 to 1000°C to produce a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature range of 450 to 650°C; and cold-rolling the coiled hot-rolled steel sheet to produce a cold-rolled steel sheet.

[0086] Below, a method for manufacturing the above cold-rolled steel sheet according to one example is described.

[0087] First, the present invention can heat a steel slab having the alloy components described above to a temperature range of 1000 to 1350°C.

[0088] If the heating temperature of the above steel slab is less than 1000℃, there is a possibility that it will be hot-rolled in a region below the finishing hot-rolling temperature range. If the heating temperature of the above steel slab exceeds 1350℃, there is a possibility that it will reach the melting point of the steel and melt. Therefore, it is preferable to heat the above steel slab at 1000~1350℃.

[0089] After heating as described above, the present invention can produce a hot-rolled steel sheet by finishing hot rolling the steel slab in a temperature range of Ar3 to 1000℃.

[0090] If the above finishing hot rolling temperature is below Ar3℃, there is a high possibility that the resistance to hot deformation will increase rapidly, and there is a possibility that problems may occur during the manufacturing process. If the above finishing hot rolling temperature exceeds 1000℃, not only may an excessively thick oxide scale be formed, but there is also a high possibility that the microstructure of the steel sheet will become coarsened. Therefore, the above finishing hot rolling can be performed at Ar3~1000℃. It is more preferable that the lower limit of the above finishing hot rolling temperature be 800℃, and more preferable that it be 850℃. It is more preferable that the upper limit of the exit side temperature of the finishing rolling mill during the above finishing hot rolling be 980℃, and more preferable that it be 970℃. Meanwhile, the above Ar3 can be calculated through the following [Equation 1].

[0091] [Equation 2]

[0092] Ar3(℃) = 910 - 203√C - 30Mn + 44.7Si - 11Cr + 31.5Mo

[0093] Next, the present invention can wind the hot-rolled steel sheet at a temperature range of 450 to 650°C.

[0094] If the above coiling temperature is below 450℃, excessive martensite is generated, leading to an excessive increase in the strength of the hot-rolled steel sheet, which may cause problems such as shape defects due to the load during cold rolling. If the above coiling temperature exceeds 650℃, pickling performance may be reduced due to an increase in surface scale. Therefore, it is desirable for the above coiling temperature to have a range of 450 to 650℃. It is more desirable for the lower limit of the above coiling temperature to be 465℃, and more desirable for it to be 480℃. It is more desirable for the upper limit of the above coiling temperature to be 630℃, and more desirable for it to be 600℃.

[0095] As described above, after winding, the present invention can manufacture a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet.

[0096] In the present invention, the cold rolling conditions are not specifically limited, and any conditions used in the relevant technical field may be utilized. However, as an example, the cold rolling may be performed with a cold reduction rate of 20 to 90%. If the cold reduction rate is less than 20%, it may be difficult to secure the target thickness precision, and it may also be difficult to correct the shape of the steel sheet. If the cold reduction rate exceeds 90%, cracks may occur at the edge of the steel sheet, and the cold rolling load may become excessively large in terms of productivity. Therefore, the cold reduction rate may be 20 to 90%. More preferably, it has a cold reduction rate of 40 to 70%.

[0097] Then, the method for manufacturing a steel sheet according to one embodiment of the present invention can continuously anneale the cold-rolled steel sheet manufactured as described above at a temperature of Ac3°C or higher and 900°C or lower.

[0098] If the temperature during the continuous annealing is below Ac3℃, a large amount of ferrite is generated, making it difficult to secure the yield strength and tensile strength targeted by the present invention. In the present invention, if the temperature during the continuous annealing exceeds 900℃, the grain size of the austenite increases, which may increase the packet size of the martensite formed upon cooling. Therefore, it is preferable that the temperature during the continuous annealing be in the range of Ac3 to 900℃. It is more preferable that the lower limit of the temperature during the continuous annealing be (Ac3+20)℃, and more preferable that it be (Ac3+40)℃. It is more preferable that the upper limit of the temperature during the continuous annealing be 890℃, and more preferable that it be 880℃. Meanwhile, Ac3 can be calculated using the following [Equation 2].

[0099] [Equation 2]

[0100] Ac3(℃)=900-206C+26.2Si-25Mn-12.3Cr+9.12Mo+50.2Nb+148Ti-131B

[0101] In particular, when forming an internal oxide layer on the surface layer of a steel plate according to a method for manufacturing a steel plate according to another embodiment of the present invention, the present invention may perform the continuous annealing in an atmosphere comprising 0.1 to 30 volume% hydrogen and H2O with a dew point of -35 to 20, with the remainder being nitrogen and impurity gas. Since the impurity gas is sufficiently known to a person skilled in the art, it is not separately specified in this specification. However, the impurity gas is a gas that follows due to the process and equipment, and as an example, there may be trace amounts of O₂, Ar, CO, CO₂, hydrocarbons, and other trace gases that are unavoidably included due to air leakage, combustion gases, or decomposition of surface residues.

[0102] If the above dew point temperature is higher than -35°C, the oxygen partial pressure inside the furnace becomes higher than that of a normal reducing annealing atmosphere, which may lead to internal oxidation. As internal oxidation occurs in this way, decarburization due to the oxidation of C also occurs, and the refinement of Ti-Mo oxides in the surface layer is possible, thereby improving weldability and bendability. It is desirable to limit the above dew point temperature to a maximum of +20°C. If the dew point exceeds +20°C, the critical oxygen level for Fe oxidation is reached, resulting in Fe surface oxidation rather than internal oxidation, and thus the effect of internal oxidation cannot be obtained. Furthermore, it may be difficult to properly distribute Ti-Mo oxides at the 1 / 4T point relative to the steel plate and in the surface layer of the steel plate. As another example, the above dew point temperature may be -20 to +20°C.

[0103] After continuous annealing as described above, a method for manufacturing a steel sheet according to one embodiment of the present invention may include a step of cooling the cold-rolled steel sheet to 150°C to Ms(°C).

[0104] This cooling is intended to transform austenite into martensite, secure the shape of the coil in the width and length directions, and ensure a high yield ratio and hole expandability. If the cooling end temperature is less than 150°C during the above cooling, the yield strength and tensile strength may increase simultaneously and ductility may be significantly reduced due to an excessive increase in martensite during overaging heat treatment. In particular, shape degradation due to rapid cooling is expected to occur, leading to deterioration in workability during the machining of automotive parts. If the cooling end temperature exceeds Ms(°C) during the above cooling, the austenite generated during continuous annealing fails to transform into martensite, and high-temperature transformation phases such as bainite and granular bainite are formed, which may cause a rapid decrease in yield strength. The occurrence of such a structure can lead to a decrease in hole expandability along with a decrease in the yield ratio. Therefore, it is desirable for the secondary cooling end temperature to be in the range of 150°C to Ms. The lower limit of the above cooling end temperature is more preferably 170℃, and more preferably 200℃. Meanwhile, the above Ms can be calculated using the following [Equation 3].

[0105] [Equation 3]

[0106] Ms(℃)= 539 - 423C - 30.4Mn - 7.5Si + 30Al - 12.1Cr - 7.5Mo

[0107] Meanwhile, the above cooling may be performed at a cooling rate of 5 to 60°C / s. If the above secondary cooling rate is less than 5°C / s, high-temperature phases such as upper bainite are incorporated during cooling, making it impossible to obtain the target tempered martensite fraction and high strength. On the other hand, if the above cooling rate exceeds 60°C / s, there may be a disadvantage in that the shape of the product deteriorates. Therefore, it is desirable for the above cooling rate to have a range of 5 to 60°C / s. More preferably, the above cooling rate may be limited to 7°C to 40°C / s.

[0108] Subsequently, the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may include a step of overaging treatment in which the cold-rolled steel sheet cooled according to the above-described conditions is reheated at a temperature of 200°C to Bs and maintained for 6 to 40 minutes. Through the above process, interphase carbon distribution and additional phase transformation necessary for stabilizing the residual austenite are obtained. The reheating is intended for interphase carbon distribution necessary for stabilizing the residual austenite. If the reheating temperature is less than 200°C, there may be disadvantages such as excessively high strength and poor formability. If the reheating temperature exceeds Bs, it is difficult to obtain the strength intended for the present invention. Therefore, it is preferable that the reheating temperature has a range of 200°C to Bs. It is more preferable that the lower limit of the reheating temperature be 210°C, and even more preferable that it be 250°C. Meanwhile, Bs can be calculated by [Equation 4] below.

[0109] [Equation 4]

[0110] Bs(℃)=830-270C-90Mn-37Ni-70Cr-83Mo

[0111] Meanwhile, in the present invention, if the holding time is less than 6 minutes, the total amount of phase transformation is insufficient at the end of the holding stage, so the residual austenite fraction increases and a large amount of fresh martensite is generated, which may lead to poor bending properties.

[0112] Furthermore, the present invention may additionally include a step of temper rolling a cold-rolled steel sheet, which has been over-aged as described above, with an elongation of 0.010 to 1.0%, if necessary. Typically, when temper rolling a transformed structure steel, an increase in yield strength of 50 MPa or more occurs with almost no increase in tensile strength. If the elongation is less than 0.010%, it may become very difficult to control the shape of the ultra-high strength steel as in the present invention. If the elongation exceeds 1.0%, the operability may become significantly unstable due to the high-elongation process.

[0113] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0114] (Example)

[0115] A slab having the alloy composition listed in Table 1 below was heated to a temperature range of 1000–1350°C and then finished hot-rolled at a temperature range of Ar3–1000°C to produce a hot-rolled steel sheet. Next, it was coiled at a temperature range of 450–650°C and cold-rolled with a cold reduction rate of 20–90% to produce a cold-rolled steel sheet. The cold-rolled steel sheet produced in this manner was subjected to continuous annealing, cooling, and over-aging treatment processes under the conditions listed in Table 2 below, and then temper-rolled with an elongation of 0.010–1.0%. Meanwhile, the conditions listed in Table 2 below were based on the surface temperature of the steel sheet.

[0116] The microstructure and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 3 and 4 below.

[0117] Each measurement method is explained in detail below.

[0118] (Microstructure)

[0119] The types and fractions of the microstructure were observed at the center of the steel plate (1 / 4t (t: thickness of the steel)) using a scanning electron microscope (SEM) and an optical microscope (OM), and the fractions of each phase were analyzed 20 times through image analysis to calculate the average value.

[0120] The decarburization rate of the steel plate surface was expressed by measuring the profile of C on the GDOES (Glow Discharge Optical Emission Spectroscopy) depth profile up to about 50 μm in the depth direction of the base steel plate, and then measuring the fraction of the depleted area relative to the total area on the said profile.

[0121] (Ti-Mo precipitates)

[0122] First, specimens were taken from the surface of each steel plate at a point 1 / 4T in the thickness direction, and Ti-Mo precipitates were observed using a Transmission Electron Microscope (TEM). Next, after distinguishing them from other carbonitrides through EDS mapping, Ti-Mo precipitates with a diameter of less than 20 nm were filtered out as noise. At this time, when observed via EDS mapping, precipitates containing 25.00 wt% or more of Ti and 5.00 wt% or more of Mo were classified as Ti-Mo precipitates. Subsequently, the average circle equivalent diameter of the above Ti-Mo precipitates was observed and is shown in Table 3 below.

[0123] In addition, if a decarburized layer is present, the decarburized layer is removed by grinding, and Ti-Mo precipitates on the surface layer of the steel plate are observed. The average circle equivalent diameter and number density of the Ti-Mo precipitates on the surface layer of the steel plate are observed according to the same procedure as described above and are shown together in Table 3 below.

[0124] (Mechanical properties)

[0125] Yield strength, tensile strength, yield ratio, and total elongation were measured by processing cold-rolled steel sheets into specimens of JIS No. 5 and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0126] The bending workability (R / t) was determined by processing a cold-rolled steel sheet into a specimen with a width of 100 mm × a length of 30 mm, performing a 90° bending test at a test speed of 800 mm / min, checking for the occurrence of cracks in the bending section using a stereoscopic microscope, and calculating the minimum bending radius (R value of the die) at which no cracks occurred by dividing it by the thickness (mm) of the specimen.

[0127] Hole expandability (HER) was measured by forming a 10 mm Ψ punching hole (die inner diameter 10.3 mm, clearance 12.5%) by applying a conical punch with an apex angle of 60° to the punching hole at a pressure of 20 mm / min in a direction where the burr of the punching hole becomes outward.

[0128] Hole Expandability (HER) (%) = {(D - D0) / D0} Х 100

[0129] (Note: D: hole diameter when the crack penetrates the steel plate (mm), D0: initial hole diameter (mm).)

[0130] To evaluate weldability, the occurrence of Liquid Metal Embrittlement (LME) cracks was measured. The occurrence of LME cracks was evaluated under resistance spot welding conditions. Spot welding was performed by overlapping two metal plates, using a current in the range of 7.5–8.0 kA and setting the clearance to 3 mm. The test was conducted with hold times divided into 0, 30, 40, and 50 cycles, and the occurrence of cracks was checked based on the holding time and current intensity. Weldability was evaluated as good when no cracks occurred at all throughout the entire cycle.

[0131] To evaluate hydrogen embrittlement resistance, the 4-point bending test method was used. This is a method for evaluating the susceptibility to delayed fracture of a material. The specimens were rectangular, measuring 30 mm × 115 mm, and cut by shear processing. Subsequently, heat treatment was performed at 150 ± 5°C for 10 minutes under a stress level of 1 times the yield strength. Afterward, the specimens were immersed in an HCl solution to inject hydrogen; the pH of the solution was maintained at 1 and 1.5 for 72 and 168 hours, respectively, and the solution was replaced at 24-hour intervals. If no cracks occurred, the hydrogen embrittlement resistance was indicated as "good," and if cracks occurred in the specimen, the hydrogen embrittlement resistance was indicated as "poor."

[0132] Steel Type Steel Composition (Weight%) CSI MnCrMoBAlTiNbPSNCeq 10.240 1.97 2.340.28 0.18 0.00220.02000.090 0.0200.006 0.0020.004 0.442 0.232 1.312.48 0.25 0.210.0014 0.00210.0200.0200.009 0.003 0.004 0.4330.243 1.49 2.55 0.430.120.00210.00210.0200.0200.011 0.0040.0050.4640.2381.512.340.050.100.00180.00180.0200.0200.0110.0040.0050.4450.3301.402.690.530.120.00220.00300.0200.0200.0110.0030.0050.5560.2450.402.450.210.070.00170.02500.0200.0200.0120.0030.0050.42*The remaining components are Fe and unavoidable impurities.

[0133] Classification Steel Grade Ac3(°C Ms(°C Bs(°C Continuous Annealing Cooling and Aging Continuous Annealing Temperature(°C Dew Point Temperature(°C Cooling Termination Temperature(°C Average Cooling Rate(°C Overaging Temperature(°C Overaging Holding Time(sec)) Example 1 1856362520876-4025031324540 Example 2 2827361509870-4024031312582 Example 3 3825353495863-4025531308492 Example 4 4836366543867-4026431299583 Example 5 1856362520875627530305632 Example 6 282736 1509866325531299611 Example 73825353495859524831297596 Example 84836366543864525531306511 Example 95800310452865-4223832309586 Example 106801358523868-4122432288608 Example 115800310452862523832309586

[0134] Classification Microstructure Fraction (Area %) Ti-Mo Precipitate Decarburization Rate (%) Surface Layer 1 / 4T Point FTMF MRA Average Diameter (nm) Average Diameter (nm) Example 1 2.08 4.7 2.31 1.017 32230 Example 2 2.88 6.22.48 6.146 1880 Example 3 2.38 7.51.88 6.415 41980 Example 4 4.38 4.73.47 620 22450 Example 5 2.18 6.81.89.375 2113 3.2 Example 6 3.48 4.62 .89.25619438.9 Example 71.389.61.57.68420340.3 Example 81.890.01.86.410223642.8 Example 92.683.44.49.61522150 Example 101.785.611.51.21532340 Example 112.184.64.58.81221143

[0135] Classification YS(MPa)TS(MPa)El(%)HER(%)R min / t Weldability Hydrogen Embrittlement Example 1 1110 15 23 11.7 3 12.1 Poor Poor Example 2 10 9 5 15 12 10.9 2 9 2.5 Poor Poor Example 3 11 32 14 9 6 11.2 2 8 2.8 Poor Poor Example 4 10 9 31 50 6 10.6 3 0 2.8 Poor Poor Example 5 11 42 14 8 3 12.3 3 2 1.1 Good Good Example 6 11 11 150211.6290.7 Good Good Example 7 1083150111.2281.1 Good Good Example 8 1132149611.3290.7 Good Good Example 9 1082148211.5312.8 Poor Poor Example 10 108614228.6332.5 Poor Poor Example 11 1056147810.7281.7 Poor Good

[0136] As shown in Table 1-4 above, in the case of Examples 5-8, which satisfy the alloy composition and steel microstructure of the present invention, the yield strength was 1000 MPa or higher, the tensile strength was 1470 MPa or higher, the elongation was 10% or higher, the hole expansion factor (HER) was 25% or higher, and the bendability (R / t) was 2.0 or lower; at the same time, weldability and hydrogen embrittlement resistance were also good. On the other hand, Examples 1-4, 9, and 10 did not satisfy the atmosphere conditions during continuous annealing, and therefore did not satisfy the average circle equivalent diameter range of Ti-Mo precipitates at the 1 / 4T thickness point and / or the surface layer of the steel plate as presented in the present invention. Accordingly, the bendability, weldability, and hydrogen embrittlement resistance were inferior.

[0137] Finally, Example 11, in which Ceq exceeds 0.46 as the C content exceeds the range proposed in the present invention, had poor weldability despite the dew point temperature being -35°C or higher during continuous annealing.

[0138] Meanwhile, Figures 1 and 2 below are transmission electron microscope images and EDS mapping results observed at a point 1 / 4T in the thickness direction from the surface of the steel plate of Example 5 and at the surface layer of the steel plate.

[0139] More specifically, Figure 1a below is a transmission electron microscope image observed at a thickness reference point of 1 / 4T, and Figures 1b and 1c are EDS mapping results observed at a thickness reference point of 1 / 4T.

[0140] In addition, Figure 2a below is a transmission electron microscope image observed on the surface layer of the steel plate, and Figures 2b and 2c are EDS mapping results observed on the surface layer of the steel plate.

[0141] Looking at the aforementioned Figures 1 and 2, it can be seen that in Example 5, finer Ti-Mo precipitates are formed on the surface layer of the steel plate compared to the point at 1 / 4T based on the steel plate thickness.

[0142] The present invention can provide a steel plate having high strength characteristics and excellent elongation simultaneously across the entire thickness of the steel plate by appropriately distributing fine Ti-Mo precipitates and coarse Ti-Mo precipitates in this way.

Claims

1. In wt%, comprising C: 0.10~0.30%, Si: 0.50~2.50%, Mn: 1.0~3.0%, Cr: 0.01~1.20%, P: 0.001~0.100%, S: 0.010% or less, Sol.Al: 0.0010~0.1000%, N: 0.001~0.010%, Mo: 0.02~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.120% and Nb: 0.010~0.050%, and the remainder being Fe and other unavoidable impurities, The microstructure of the center of the steel plate has a matrix structure of tempered martensite, and The average circular equivalent diameter of Ti-Mo precipitates measured at a point 1 / 4T in the thickness direction from the steel plate surface is 120–300 nm, and A steel plate having an average circular equivalent diameter of Ti-Mo precipitates measured in the surface layer of the steel plate, ranging from 20 to 120 nm.

2. In Paragraph 1, A steel plate having a carbon equivalent (Ceq) of 0.46 or less derived by the following relationship 1. [Relationship 1] Ceq = [C]+[Si] / 30+[Mn] / 20+2[P]+4[S] In the above equation, [C], [Si], [Mn], [P], and [S] represent the weight percent of each element.

3. In Paragraph 1, A steel plate having a microstructure in the center of the above steel plate in area % comprising one or more of tempered martensite: 70.0~95.0%, retained austenite: 3.0~15.0%, fresh martensite: 10.0% or less, and ferrite: 5.0% or less.

4. In Paragraph 1, A steel plate having an internal oxide layer formed on the surface layer of the steel plate.

5. In Paragraph 1, The above steel plate surface layer is a steel plate having a decarburization rate of 30% or more.

6. In Paragraph 1, A steel plate having a yield strength of 1000 MPa or more, a tensile strength of 1470 MPa or more, an elongation of 10% or more, a hole expansion factor (HER) of 25% or more, and a bendability (R / t) of 2.0 or less.

7. A step of preparing a cold-rolled steel sheet comprising, in wt%, C: 0.10~0.30%, Si: 0.50~2.50%, Mn: 1.0~3.0%, Cr: 0.01~1.20%, P: 0.001~0.100%, S: 0.010% or less, Sol.Al: 0.0010~0.1000%, N: 0.001~0.010%, Mo: 0.02~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.120% and Nb: 0.010~0.050%, and the remainder being Fe and other unavoidable impurities; A step of continuously annealing the above cold-rolled steel sheet at a temperature of Ac3℃ or higher and 900℃ or lower; A step of cooling the above cold-rolled steel sheet to a cooling end temperature of 150℃~Ms(℃) at a cooling rate of 5~60℃ / s; and The method includes the step of reheating the above cold-rolled steel sheet to a temperature of 200℃ to Bs(℃) and over-aging it for 6 to 40 minutes. A method for manufacturing a steel sheet, wherein the above continuous annealing is performed in an atmosphere containing 0.1 to 30 volume% hydrogen and H2O with a dew point of -35 to 20°C, and the remainder being nitrogen and impurity gases.

8. In Paragraph 7, A method for manufacturing a steel sheet, further comprising the step of temper rolling the cold-rolled steel sheet with an elongation of 0.010 to 1.0% after over-aging treatment.

9. In Paragraph 7, The above cold-rolled steel sheet is, A step of heating a steel slab comprising, in weight%, C: 0.10~0.30%, Si: 0.50~2.50%, Mn: 1.0~3.0%, Cr: 0.01~1.20%, P: 0.001~0.100%, S: 0.010% or less, Sol.Al: 0.0010~0.1000%, N: 0.001~0.010%, Mo: 0.02~0.30%, B: 0.0010~0.0050%, Ti: 0.010~0.120% and Nb: 0.010~0.050%, with the remainder being Fe and other unavoidable impurities, to a temperature range of 1000~1350℃; A step of manufacturing a hot-rolled steel sheet by finishing hot-rolling the above steel slab in a temperature range of Ar3 to 1000℃; A step of winding the above hot-rolled steel sheet at a temperature range of 450 to 650°C; and A method for manufacturing a steel plate obtained through the step of cold-rolling the above hot-rolled steel plate to produce a cold-rolled steel plate.

Images