High-strength thick steel plate with excellent hole-expanding properties and ductility, and method for manufacturing the same.

Abstract

The present invention provides a high-strength, thick steel sheet and a method for manufacturing the same, which have a low yield ratio and high strength, as well as excellent formability such as hole expansion due to improved ductility, for use as a material for automotive structural components and the like. [Solution] A high-strength thick steel sheet is provided, containing, by weight %, carbon: 0.05-0.12%, manganese: 2.0-3.0%, silicon: 0.5% or less (excluding 0%), chromium: 1.0% or less (excluding 0%), niobium: 0.1% or less (excluding 0%), titanium: 0.1% or less (excluding 0%), boron: 0.003% or less (excluding 0%), aluminum: 0.02-0.05%, phosphorus: 0.05% or less (excluding 0%), sulfur: 0.01% or less (excluding 0%), nitrogen: 0.01% or less (excluding 0%), iron and other unavoidable impurities, and having a microstructure that includes 10-30% ferrite by area fraction, 10-25% recrystallized ferrite bridges, 20-30% bainite and the remainder martensite.

Description

Technical Field

[0005] ,

[0001] The present invention relates to steel suitable as a material for automobiles, and more specifically, to a high-strength thick steel sheet excellent in hole expansion property and ductility and a method for manufacturing the same.

Background Art

[0002] Recently, in the field of the automobile industry, due to environmental regulations on CO2 emissions and regulations on energy use, the use of high-strength steel is required to improve fuel efficiency or durability.

[0003] In particular, as regulations on the impact stability of automobiles expand, members, seat rails, pillars, etc. for improving the impact resistance of the vehicle body are made of high-strength steel with excellent strength as materials for structural members such as (pillar).

[0004] Such automobile parts have complex shapes according to stability and design, and are mainly manufactured by forming with a press die, so high-level formability is required together with high strength. However, while the higher the strength of steel, the more advantageous the characteristics for absorbing impact energy, generally

[0005] when the strength increases, the elongation rate decreases and the formability deteriorates. Furthermore when the yield strength is excessively high, the inflow of the material in the die decreases during forming, resulting in a decrease in formability and an increase in manufacturing cost. In addition, since there are many forming parts that expand after machining holes in automobile parts, hole expandability (HER) is required for smooth

[0006] forming. (Hole Expandability, HER) is required for forming. However, high-strength steel has poor hole-expanding properties, so defects such as cracks can occur during forming. There is a problem that this occurs. In this way, when the hole-expanding ability decreases, the parts will be damaged during a car collision. Cracks can easily form in the molded parts, making them prone to breakage and potentially jeopardizing passenger safety. Yes. Also, as standards for passenger safety become higher, some automakers The use of thick materials in the core to ensure rigidity is also steadily increasing.

[0007] On the other hand, typical high-strength steels used in automotive materials include duplex steel (Dual Phase Steel (DP steel), Transformation-Induced Plastic Steel Induced Plasticity Steel, TRIP Steel), Composite Structure Steel (C Complex Phase Steel (CP steel), Ferrite-Bainite Steel (Fe Examples include rrite (Bainite steel, FB steel), etc.

[0008] DP steel, which is an ultra-high-strength steel, has a low yield ratio of approximately 0.5 to 0.6. It has the advantages of being easy to work with and having a high elongation rate second only to TRIP steel. Therefore, Door outer, seat rail, seat belt, suspension, arm, wheel derailleur In reality, it is being applied to schools and similar institutions.

[0009] TRIP steel has a yield ratio in the range of 0.57 to 0.67, resulting in excellent formability (high It has the characteristic of exhibiting ductility, and therefore, members, loops, seat belts, bumper rails It is suitable for parts that require high moldability, such as those made of rubber.

[0010] CP steel, with its low yield ratio, high elongation, and bendability, is suitable for side panels, underbodies, and other applications. Applied to reinforcement materials, FB steel has excellent hole-expanding properties, and is mainly used in suspension lowers. It is applied to arms, wheel discs, etc.

[0011] Among these, DP steel mainly consists of ferrite with excellent ductility and a hard phase (martensite) with high strength. It is composed of the bainite phase and a small amount of residual austenite. Such DP steel has low yield strength and high tensile strength, therefore, the yield ratio (Yield Ratio) Low io (YR), high work hardening rate, high ductility, continuous yield behavior, room temperature aging resistance, and bake hardening. It possesses excellent properties such as quality. Furthermore, it exhibits superior fractions of each phase, recrystallization degree, and distribution uniformity. By controlling these factors, it is possible to manufacture high-strength steel with high hole-expanding properties.

[0012] However, in order to secure ultra-high strength of 980 MPa or more, The fraction of hard phases, such as martensite, must be increased. However, in this case, the yield strength increases, and defects such as cracks may occur during press forming. There is a problem in that this occurs.

[0013] Generally, DP steel for automobiles is produced by steelmaking and continuous casting processes to create slabs, and then this slab After obtaining hot-rolled coils by performing [heating - rough rolling - finish hot rolling], the material undergoes an annealing process. It is then manufactured as a final product.

[0014] Here, the annealing process is mainly performed during the manufacturing of cold-rolled steel sheets, and cold-rolled steel sheets are hot-rolled The material is acid-washed to remove surface scale, and then cold-rolled at room temperature with a constant reduction ratio. Afterward, it is manufactured through an annealing process and, if necessary, a further temper rolling process.

[0015] The cold-rolled steel sheet (cold-rolled material) obtained by cold rolling is in a very hardened state by itself and is not suitable for manufacturing parts that require workability. Therefore, as a subsequent process, in a continuous annealing furnace it can be softened by heat treatment to improve workability.

[0016] As an example, in the annealing process, the steel sheet (cold-rolled material) is heated to about 650 - 850 °C in a heating furnace and then maintained for a certain period of time, so that the hardness can be lowered and the workability can be improved by recrystallization and phase transformation phenomena.

[0017] For the steel sheet that has not undergone the annealing process, the hardness, especially the surface hardness, is high and the workability is insufficient. In contrast for the steel sheet that has undergone the annealing process, by having a recrystallized structure, the hardness, yield point, and tensile strength are low and the workability can be improved.

[0018] As a typical method for lowering the yield strength of DP steel, during continuous annealing, by completely recrystallizing ferrite in the heating process to produce an equiaxed crystal morphology, in subsequent processes, when austenite is generated and grows, it becomes an equiaxed crystal morphology, and it is advantageous to form an austenite phase with a small and uniform particle size.

[0019] And in the case of thick materials, in order to ensure a certain reduction ratio, the hot-rolled thickness must be relatively thickly ensured so that there is a problem that the load is large and the operability decreases during subsequent cold rolling. When the reduction ratio is low during the production of thick materials, the non-uniformity of the structure due to unrecrystallized ferrite during annealing becomes large, the yield strength becomes high, and the directionality of cold rolling is maintained in the fine structure so that there is a problem that the workability decreases. Therefore, in the case of thick materials, the characteristics of the dimensions ​​​Due to its properties, material variation in the thickness direction is unavoidable, and therefore processability and usability are affected. To improve performance, technologies are needed to homogenize the materials as much as possible.

[0020] On the other hand, Patent Document 1 describes using Ti, Mo, etc. to form fine precipitates and as a microstructure. Composed of ferrite, bainite, and martensite phases, it offers excellent hole-expanding and extensibility properties. They have disclosed that it is possible to secure a certain percentage of revenue.

[0021] However, this document is intended to introduce carbon and bainite to form micro-precipitation. Excessive silicon addition to the material presents problems with weldability and liquid metal embrittlement (LME). Furthermore, problems still exist due to differences in hardness between the soft and hard phases, and high hole-expanding properties remain. Therefore, by forming a bainite phase in excess, the yield strength is high and it is difficult to process. It has the drawback of having a lower growth rate.

[0022] Judging from the aforementioned conventional technology, the formability of thick, high-strength steel, such as elongation and hole-expanding properties, is... To improve both simultaneously, it is necessary to form a uniform structure within the steel while lowering the yield strength. Furthermore, there is a need to develop methods that can improve processability. [Prior art documents] [Patent Documents]

[0023] [Patent Document 1] Korean Published Patent No. 10-2021-0095156 [Overview of the project] [Problems that the invention aims to solve]

[0024] One aspect of the present invention provides a material suitable for automotive structural members and the like, with a low yield ratio and high strength. A high-strength, thick steel sheet that possesses, and whose formability, such as hole-expanding properties, is improved due to enhanced ductility, and a manufacturing method thereof. This aims to provide a method for doing so.

[0025] The problems of the present invention are not limited to those described above. It can be understood from this, and even a person with ordinary skill in the art to which the present invention belongs Therefore, there is no difficulty in understanding further issues of the present invention. [Means for solving the problem]

[0026] One aspect of the present invention is a composition in which, by weight, carbon (C): 0.05~0.12%, manganese (Mn) :2.0~3.0%, Silicon (Si):0.5% or less (excluding 0%), Chromium (Cr) : 1.0% or less (excluding 0%), Niobium (Nb): 0.1% or less (excluding 0%), Titanium (Ti): 0.1% or less (excluding 0%), Boron (B): 0.003% or less (excluding 0%) ), Aluminum (sol.Al): 0.02~0.05%, Phosphorus (P): 0.05% or less Below (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01 Contains less than % (excluding 0%), including iron (Fe) and other unavoidable impurities. In terms of microstructure, it consists of ferrite with an area fraction of 10-30% and recrystallized ferrite with an area fraction of 10-25%. Bridge, containing 20-30% bainite and the remainder martensite. The invention provides high-strength, thick steel plates with excellent hole-expanding properties and ductility.

[0027] Another aspect of the present invention is a step of preparing a steel slab and a step of 1100-130 The process involves heating the steel slab in a temperature range of 0°C, and then hot-rolling the heated steel slab to produce a hot-rolled steel sheet. The manufacturing stage, the stage of winding the above hot-rolled steel sheet in a temperature range of 400 to 700°C, and the winding After cutting, the hot-rolled steel sheet is cooled to room temperature, and the cooled hot-rolled steel sheet is then reduced to 55-80%. The process involves cold rolling at a specified cold reduction ratio to produce a cold-rolled steel sheet, and then continuously annealing the cold-rolled steel sheet. The process involves a step-by-step cooling rate of 1-10°C / s to a temperature range of 650-700°C after the continuous annealing described above. The process involves a primary cooling stage, followed by a temperature reduction to a range of 450-500°C at a rate of 5-50°C / s after the primary cooling. This includes a step of secondary cooling at the average cooling rate, The above continuous annealing is carried out in a facility equipped with a heating zone, a soaking zone, and a cooling zone, and the cold-rolled steel sheet is processed A key feature is that when the temperature rises to the heating zone, it goes through a recrystallization zone where it is maintained at 600-700°C for 1-3 minutes. The present invention provides a method for manufacturing high-strength, thick steel plates with excellent hole-expanding properties and ductility. [Effects of the Invention]

[0028] According to the present invention, despite having high strength, it exhibits excellent hole-expandability, moldability, and impact resistance. This allows us to provide thick steel plates with improved properties.

[0029] Thus, the steel sheet of the present invention, which has improved formability, is less prone to cracks or wrinkles during press forming. This prevents processing defects, making it suitable for structural and other parts that require processing into complex shapes. It has the effect of being suitably applied to the product. Furthermore, automobiles to which such parts are applied inevitably... In the event of a collision, we manufacture materials with improved impact resistance so that defects such as cracks are less likely to occur. It is also effective in that regard. [Brief explanation of the drawing]

[0030] [Figure 1]This shows the thermal history and phase transformation history during continuous annealing according to one embodiment of the present invention. In Figure 1, the dashed line shows the thermal history during conventional continuous annealing, and the solid line shows the thermal history during continuous annealing according to the present invention. [Figure 2] (a) shows the void formation mechanism within the tissue, and (b) shows the interface strengthening mechanism within the tissue in an example of an invention according to one embodiment of the present invention. [Figure 3] This shows a microstructural photograph of a comparative example according to one embodiment of the present invention. [Figure 4] This shows a microstructural photograph of an example of the present invention according to one embodiment of the present invention (the arrows in Figure 4 represent the recrystallized ferrite bridge structure). [Modes for carrying out the invention]

[0031] The inventors of this invention have found that among automotive materials, parts that require processing into complex shapes are preferable. We conducted intensive research to develop a material with a suitable level of moldability for practical use.

[0032] In particular, the inventors have developed a method for thick steel sheets used in automobiles, where the cold reduction ratio must be relatively low. In this process, we derive a microstructure that can enhance crack resistance between hard phases, and By refining the hard phase and controlling the crystal grain shape, which is advantageous for preventing the generation and propagation of voids, We confirmed that the target conditions could be achieved, and thus completed the present invention.

[0033] In particular, the present invention provides a structure that connects the hard phases to each other so as to eliminate the unidirectional nature of the hard phase. By introducing recrystallized ferrite bridges with a specific structure, this type of microstructure is formed. In doing so, optimizing the alloy composition and manufacturing conditions is of technical significance.

[0034] The present invention will be described in detail below.

[0035] A high-strength thick steel sheet with excellent hole-expanding properties and ductility according to one aspect of the present invention is characterized by a carbon content of ( C): 0.05~0.12%, Manganese (Mn): 2.0~3.0%, Silicon (Si) : 0.5% or less (excluding 0%), Chromium (Cr): 1.0% or less (excluding 0%), Niobium (Nb): 0.1% or less (excluding 0%), Titanium (Ti): 0.1% or less (excluding 0%) Boron (B): 0.003% or less (excluding 0%), Aluminum (sol.Al): 0 0.02-0.05%, Phosphorus (P): 0.05% or less (excluding 0%), Sulfur (S): 0.0 It can contain 1% or less (excluding 0%), and nitrogen (N): 0.01% or less (excluding 0%). ru.

[0036] The following explains the reason for limiting the alloy composition of the high-strength thick steel plate provided in the present invention as described above. I will explain this in detail.

[0037] On the other hand, unless otherwise specified in this invention, the content of each element is based on weight, and the proportion of the tissue is based on area. This will be used as the standard.

[0038] Carbon (C): 0.05~0.12% Carbon (C) is an important element added for solid solution strengthening, and such C is a precipitate source. By bonding with elements and forming fine precipitates, it contributes to improving the strength of steel.

[0039] When the content of C exceeds 0.12%, the hardening ability increases, and during the manufacturing of steel, during cooling, The formation of rutensite leads to an excessive increase in strength, but also to a decrease in elongation. There are problems. Furthermore, it has poor weldability, and there is a risk of welding defects occurring during processing into parts. On the other hand, if the content of C is less than 0.05%, it becomes difficult to achieve the target level of strength.

[0040] Therefore, the above C can include 0.05-0.12%. More favorably, 0.0 It can be included at concentrations of 6% or more, and at concentrations of 0.10% or less.

[0041] Manganese (Mn): 2.0-3.0% Manganese (Mn) causes sulfur (S) in steel to precipitate as MnS, and FeS is produced. It is an element that prevents hot brittleness and is advantageous for solid solution strengthening of steel.

[0042] If the Mn content is less than 2.0%, not only will the above-mentioned effects not be obtained, It is difficult to achieve the target level of strength. On the other hand, if the content exceeds 3.0%, weldability and heat There is a high possibility of problems occurring in terms of inter-rollability, and the increase in hardening ability may lead to... Because martensite is easily formed, there is a risk of reduced ductility. Also, within the tissue Excessive Mn-band (Mn oxide band) formation occurs, resulting in defects such as processing cracks. There is a problem that the risk of this happening increases. And, during annealing, Mn oxide dissolves onto the surface and becomes irritated. There is a problem in that it greatly inhibits the properties of the plant.

[0043] Therefore, the above Mn can be contained in an amount of 2.0-3.0%. More favorably, 2.2 It can be included in amounts between % and 2.8%.

[0044] Silicon (Si): 0.5% or less (excluding 0%) Silicon (Si) is a ferrite-stabilizing element that promotes ferrite transformation and targets It is advantageous for ensuring a certain level of ferrite fraction. Furthermore, it has good solid solution strengthening ability, and ferrite It is effective in increasing the strength of the steel and ensuring strength without reducing the ductility of the steel. It is a useful element.

[0045] When the Si content exceeds 0.5%, the solid solution strengthening effect becomes excessive, and ductility is actually reduced. This reduces the surface scale defects, negatively impacting the surface quality of the plating. There is a problem in that it hinders processing performance.

[0046] Therefore, the above Si can be included in amounts of 0.5% or less, and 0% can be excluded. To be advantageous, it can be included at a rate of 0.1% or higher.

[0047] Chromium (Cr): 1.0% or less (excluding 0%) Chromium (Cr) exhibits a hardening effect during cooling, facilitating the formation of the bainite phase. During blunt heat treatment, the formation of the martensite phase is suppressed, while fine carbides are formed to improve strength. It is an element that contributes to the above.

[0048] Furthermore, in this invention, by including the above Cr at an appropriate level, ferrite is formed during heating. It acts as a stabilizing element, delaying the austenite phase transformation reaction, while the phase changes at higher temperatures. Because the transformation begins, it remains for a long time in the region (Trex~A1) where only recrystallization occurs during heating. As a result, a recrystallized ferrite bridge structure can be secured.

[0049] When the Cr content exceeds 1.0%, the intended recrystallized ferrite bridge is formed. Furthermore, the ductility and hole-expanding properties of the steel decrease, and when carbides are formed at the grain boundaries, the strength and elongation are affected. It may be inferior to other methods. Furthermore, there is the problem of increased manufacturing costs.

[0050] Therefore, the above Cr can be included in amounts of 1.0% or less, and 0% can be excluded. To be advantageous, it can be included at a rate of 0.01% or higher.

[0051] Niobium (Nb): 0.1% or less (excluding 0%) Niobium (Nb) segregates at austenite grain boundaries, and austenite crystals form during annealing heat treatment. This element suppresses grain coarsening and contributes to improved strength by forming fine carbides.

[0052] When the Nb content exceeds 0.1%, coarse carbides precipitate, and carbon in steel A reduction in the amount of material may result in inferior strength and elongation, leading to increased manufacturing costs. There is.

[0053] Therefore, the above Nb can be included in amounts of 0.1% or less, and can be excluded if it is 0%.

[0054] Titanium (Ti): 0.1% or less (excluding 0%) Titanium (Ti) is an element that forms fine carbides, ensuring yield strength and tensile strength. It contributes to this. In addition, Ti causes N in the steel to precipitate as TiN, and A which is inevitably present in the steel. It has the effect of suppressing the formation of AlN by l, reducing the possibility of crack formation during continuous casting. It has the effect of causing it.

[0055] When the Ti content exceeds 0.1%, coarse carbides precipitate, and carbon in steel Reducing the amount of material may decrease strength and elongation. Furthermore, nozzle clogging may occur during continuous casting. There is a risk of causing lumps, which increases manufacturing costs.

[0056] Therefore, the above Ti can be included in amounts of 0.1% or less, and 0% can be excluded.

[0057] Boron (B): 0.003% or less (excluding 0%) Boron (B) undergoes a transformation of austenite into pearlite during the cooling process after annealing. Although it is an element that delays the process, if its content exceeds 0.003%, excess B will form on the surface. This can lead to increased concentration and deterioration of plating adhesion.

[0058] Therefore, the above-mentioned B can be included at a level of 0.003% or less, and is an unavoidable level of addition. By taking this into consideration, we can exclude 0%.

[0059] Aluminum (sol.Al): 0.02~0.05% Aluminum (sol.Al) is an element added to steel for grain refinement and deoxidation. If the content is less than 0.02%, then aluminum-killed steel can be produced in a stable state. It cannot be manufactured. On the other hand, if its content exceeds 0.05%, the crystal grains become finer. Although it has the effect of improving strength, excessive inclusion formation occurs during continuous casting operations in steelmaking, resulting in plated steel. The risk of surface defects occurring on the board increases.

[0060] Therefore, the above sol.Al can be included in an amount of 0.02 to 0.05%.

[0061] Phosphorus (P): 0.05% or less (excluding 0%) Phosphorus (P) is the substitutional element with the greatest solid solution strengthening effect, improving in-plane anisotropy. It is an advantageous element for ensuring strength without significantly reducing moldability. However, this When excessive amounts of P are added, the likelihood of brittle fracture increases significantly during hot rolling. This increases the likelihood of slab plate fracture and impairs the surface properties of the plating, which is a problem. ru.

[0062] Therefore, in the present invention, the content of P can be controlled to 0.05% or less, and it is not possible to Considering the level added to avoidance, 0% can be excluded.

[0063] Sulfur (S): 0.01% or less (excluding 0%) Sulfur (S) is an element that is inevitably added as an impurity element in steel, and it inhibits ductility. Therefore, it is preferable to control its content to be as low as possible. In particular, S causes red-hot brittleness. Because this increases the likelihood of causing the problem, its content should be controlled to 0.01% or less. This is preferable. However, 0% is excluded considering the level that will inevitably be added during the manufacturing process. can.

[0064] Nitrogen (N): 0.01% or less (excluding 0%) Nitrogen (N) is a solid solution strengthening element, but if its content exceeds 0.01%, brittleness occurs. This increases the risk of excessive AlN deposition by bonding with Al in the steel, thus affecting continuous casting quality. It may inhibit something.

[0065] Therefore, the above N can be included in an amount of 0.01% or less, and the level that is inevitably added is We can consider this and exclude 0%.

[0066] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, the raw material or surrounding Because unintended impurities from the environment can inevitably be introduced, eliminating them is necessary. It's not possible. These impurities are something any engineer in a normal manufacturing process would be able to identify. Therefore, this specification will not specifically mention all of its contents.

[0067] The steel sheet of the present invention having the alloy composition described above has a soft phase as its microstructure. Ferrite (se) and bainite and malleolus (hard phase) Recrystallized ferrite bridges (br It can be composed of organizations.

[0068] In the present invention, the greatest change in the microstructure phase due to the formation of recrystallized ferrite bridges. This is because the existing ferrite exhibits a significant loss of rolling directionality and a large degree of bonding around the hard phase. And so on. Furthermore, during heating, the formation of a recrystallized ferrite bridge causes the reverse transformation of austenite. The formation site of austenite is reduced, the formation of austenite at high temperatures is delayed, and the formation of smaller austenites after cooling is reduced. A secondary phase of the smallest size can be generated. The unrecrystallized ferrite region has a rolling directionality. It is an existing stretched structure that remains as an irregular, rough interface, and is recrystallized ferrite. Bridge grain boundaries are characterized by having smooth, polygonal interfaces. One method for determining the itbridge is, for example, EBSD crystal orientation (electron bac Classification by kscatter diffraction orientation Alternatively, a hydrogen peroxide solution (e.g., 140 ml of distilled water, 100 ml of hydrogen peroxide, etc.) can be prepared. Etching with 4g oxalic acid, 2ml sulfuric acid, and 1.5ml hydrofluoric acid can optically separate the material. can.

[0069] Specifically, the steel sheet of the present invention has a ferrite phase of 10-30% area fraction, and recrystallized ferrite It contains bridge phase at an area fraction of 10-25% and hard phase at 20-30%. It may contain bainite and the remainder martensite phase. In addition, trace amounts of residual It may contain an austenite phase.

[0070] In the present invention, the above-mentioned recrystallized ferrite bridge phase is one of the hard phases. Voids are generated along the grain boundaries of the hard phase by eliminating directional bias, and the propagation of these voids... A tissue advantageous for suppressing seeding, and existing granular ferrite (polygonal f It is an organization distinct from errite.

[0071] Furthermore, the above-mentioned recrystallized ferrite bridge is a typical recrystallized ferrite. It is a tissue that is also distinguishable from other tissues, and is relatively coarse, preferably with an equivalent diameter (equi Based on the valent circular diameter, the average size is 1-6 μm. It has the following characteristics. If the size of the recrystallized ferrite bridge phase is less than 1 μm, the hard phase It is difficult to eliminate the directional bias, and the desired effect cannot be obtained. On the other hand, its size is 6μ If the thickness exceeds m, the tissue becomes excessively coarse, which may impair physical properties such as strength. .

[0072] As shown in Figure 2, when the hard phase is formed to have directionality (a), The voids formed along the crack become easier to connect, which is advantageous for crack propagation. On the other hand, the structure intended in the present invention (b), in particular, the recrystallized ferrite bridge phase is one of the hard phases. When the directional properties are removed and the hard phases are formed as a structure that connects them to each other, the voids become crystal grains. Because it becomes more difficult for the materials to fuse along the boundary, the occurrence and propagation of cracks are significantly suppressed. It is effective.

[0073] If the fraction of such recrystallized ferrite bridge phases is excessively high, The fraction of the hard phase decreases, making it impossible to secure the target level of strength. Taking this into consideration, The above recrystallized ferrite bridge phase may be contained in an amount of 25% or less. On the other hand, if that fraction is less than 10%, the above-mentioned effect (elimination of unidirectionality in the hard phase and voids) The suppression of propagation (and other effects) becomes insufficient, resulting in poor hole-expanding ability.

[0074] In other words, the present invention relates to a ferrite phase which is a soft phase and a bainite phase which is a hard phase, and In addition to the crystalline phase, a recrystallized ferrite bridge phase is introduced, and the above recrystallized ferrite bridge The technical significance lies in further improving moldability by controlling the shape and distribution of the diphase. be.

[0075] If the ferrite phase fraction is less than 10%, it is unfavorable for ensuring the ductility of the steel, on the other hand, When the fraction of the hard phase exceeds 30%, the relative fraction of the hard phase decreases, ensuring the target level of strength. There are difficulties in doing so.

[0076] If the bainite phase fraction is less than 20%, it will be difficult to ensure sufficient strength, and the material will be soft. There is a problem in that the hardness difference between the solid phase and the martensite phase becomes large. On the other hand, the fraction When the percentage exceeds 30%, the proportion of the soft phase decreases, making it difficult to ensure ductility.

[0077] The above ferrite phase, recrystallized ferrite bridge phase, and bainite phase are removed. Of the structures, the martensite phase does not have a specific fractional limit, but tensile To ensure ultra-high strength of 980 MPa or more, it can contain an area fraction of 15% or more. However, if the fraction of the martensite phase exceeds 60%, the ductility will decrease and will not reach the target level. This makes it difficult to ensure moldability.

[0078] On the other hand, it is advantageous that the residual austenite phase described above does not exceed 3% in proportion, This demonstrates that even at a percentage level, it is possible to ensure the intended physical properties.

[0079] The high-strength thick steel sheet of the present invention having the above-described microstructure has a tensile strength of 980 MPa or more. It has a tensile strength of 550-700 MPa and an elongation rate (total elongation rate) of 14% or more, and is high in strength as well. It can possess high ductility properties.

[0080] Furthermore, the above steel plate has a hole expansion ratio of 30% or more. By having o, HER), it provides resistance to cracks that may occur during processing and impact resistance. It has the effect of providing excellent resistance to fracture.

[0081] Furthermore, the high-strength thick steel plate of the present invention has a thickness of 1 to 3 mm, more preferably 1 It can have a thickness of 0.5 to 2.5 mm.

[0082] The following describes how to manufacture a high-strength thick steel plate with excellent hole-expanding properties and ductility according to another aspect of the present invention. I will explain the method in detail.

[0083] Simply put, this invention involves [heating a steel slab - hot rolling - coiling - cold rolling - continuous annealing]. The desired steel plate can be manufactured through the following process, and each process will be explained in detail below. ru.

[0084] [Heating steel slabs] First, a steel slab satisfying the alloy composition described above can be prepared and then heated. .

[0085] This process ensures a smooth subsequent hot rolling process and allows for the acquisition of the desired physical properties of the steel sheet. This is done for the purpose of heating. In this invention, there are no particular restrictions on the conditions of such heating steps. Under normal conditions, this is acceptable. For example, a heating process in the temperature range of 1100-1300°C. It is possible to do so.

[0086] [Hot rolling] As described above, the heated steel slab can be hot-rolled to produce a hot-rolled steel sheet. Furthermore, finish hot rolling can be performed at an exit temperature of Ar3 or higher and 1000°C or lower.

[0087] During the above-mentioned finish hot rolling, if the exit temperature is below Ar3, the hot deformation resistance increases rapidly. In addition, the top, tail, and edge portions of the hot-rolled coil are single-phase This region increases in-plane anisotropy and may degrade moldability. On the other hand, if the temperature is 1 Above 000℃, the rolling load decreases relatively, which is advantageous for productivity, but thick acid There is a risk of scaling occurring.

[0088] More specifically, the above finish hot rolling can be performed in a temperature range of 760 to 940°C. .

[0089] [Rewind] The hot-rolled steel sheet manufactured as described above can be wound into a coil.

[0090] The above winding can be performed in a temperature range of 400-700°C. If the winding temperature is 4 Below 0°C, martensite or recrystallized ferrite bridges form. Excessive phase formation leads to an excessive increase in the strength of the hot-rolled steel sheet, and during subsequent cold rolling, the load... Problems such as shape defects may occur. On the other hand, if the winding temperature exceeds 700°C, the surface There is a problem in that scale increases and the pickling properties deteriorate.

[0091] [cooling] The above-mentioned rolled hot-rolled steel sheet is cooled to room temperature at an average rate of 0.1°C / s or less (excluding 0°C / s). It is preferable to cool it at a speed. At this time, the wound hot-rolled steel sheet is transported and stored. Cooling can be performed after any of the processes, but the pre-cooling processes are not limited to these. To clarify that this is not the case.

[0092] In this way, by cooling the rolled hot-rolled steel sheet at a constant rate, austenitic It is possible to obtain a hot-rolled steel sheet in which carbides, which serve as nucleation sites, are finely dispersed. Cut.

[0093] [Cold rolling] As described above, the wound hot-rolled steel sheet can be cold-rolled to produce cold-rolled steel sheet, and this development In the Ming Dynasty, the above cold rolling can be carried out with a cold reduction ratio of 55-80%.

[0094] This invention applies an appropriate level of cold reduction ratio during cold rolling, and then proceeds to the subsequent continuous annealing process. The heating period allows for further promotion of ferrite recrystallization, which in turn creates finer crystals. Ferrite formation is induced, and the austenite formed at the ferrite grain boundaries is also small and uniform. It can be formed into one.

[0095] If the reduction ratio during cold rolling is less than 55%, ferrite recrystallization is delayed and the resulting fineness is reduced. If the recrystallization rate exceeds 80%, it becomes difficult to obtain a uniform austenite phase. This reduces the yield strength too much, making it impossible to achieve the target strength level. A more advantageous approach is 78 It can be done in less than %.

[0096] In the present invention, the above cold reduction ratio is obtained by reducing the thickness of a hot-rolled material having a constant thickness using a high-temperature reduction (ZRM) equipment. Not only can this be achieved with high rolling capacity (for example, at the 5000 KN / mm level), By using a reversing mill, the eye is repeatedly rolled. The process may also include achieving the standard reduction ratio. As a non-limiting example, the above hot-rolled material is 4 It can have a thickness of ~8 mm, and if the thickness of the above hot-rolled material is 6 mm or more, This document demonstrates that a cold rolling process can be performed using a bursting mill.

[0097] A reversing rolling mill is a type of rolling mill generally used for rolling thin materials. This refers to a rolling mill that rolls material by moving it back and forth between a pair of rolls. You can set one leg of a round trip as a single pass.

[0098] The present invention allows hot-rolled steel sheets to be pickled before cold rolling, and the pickling process is To clarify that it can be done using the usual methods.

[0099] [Continuous annealing] It is preferable to subject the cold-rolled steel sheet produced as described above to continuous annealing treatment. This can be done, for example, in a continuous annealing furnace (CAL).

[0100] Typically, a continuous annealing furnace (CAL) consists of [heating zone - homogenization zone - cooling zone (slow cooling zone and rapid cooling zone) - ( It can be configured with an over-aging zone as needed, but cold-rolled steel sheets are charged into such a continuous annealing furnace. Afterward, it is heated to a specific temperature in the heating zone, and once the target temperature is reached, it is maintained for a certain period of time in the uniform zone. It will involve several steps.

[0101] In this invention, the temperatures of the heating zone and the uniform zone during continuous annealing can be controlled to be the same, This means controlling the end temperature of the heating zone and the start temperature of the homogenized zone to be the same (Figure 1).

[0102] Specifically, the temperatures of the heating zone and the uniform zone can be controlled to 790-850°C.

[0103] If the temperature of the above heating zone is below 790°C, sufficient heat input for recrystallization will not be applied. It becomes impossible. On the other hand, if the temperature exceeds 850°C, productivity decreases, and austenite is produced. An excessive phase formation occurs, and after subsequent cooling, the proportion of the hard phase increases significantly. Furthermore, it may result in poor ductility of the steel.

[0104] Furthermore, if the temperature of the sonic zone is below 790°C, excessive cooling is required at the end temperature of the heating zone. This is economically disadvantageous and may result in insufficient heat for recrystallization. On the other hand, if the temperature exceeds 850°C, the austenite fraction becomes excessive, and hardening occurs during cooling. An increase in the phase may reduce moldability.

[0105] Increasing the temperature of the homogeneous zone within the aforementioned temperature range increases the fraction of the hard phase in the final tissue. By adding this, the yield strength is increased, and by introducing bainite, the hardness difference between the phases is reduced, thus creating a hole. Improve the expansion rate.

[0106] On the other hand, the present invention achieves recrystallization by causing sufficient recrystallization during the annealing process. It has the characteristic of inducing the creation of bridges.

[0107] Specifically, the present invention provides a method for raising the temperature of the cold-rolled steel sheet to the temperature range of the heating zone, while maintaining a constant temperature at an intermediate temperature. A recrystallization zone that is maintained over time can be introduced, more preferably at a temperature of 600-700°C. It is preferable to maintain the temperature within the specified range for 1 to 3 minutes (dashed line graph in Figure 2).

[0108] If the temperature of the above recrystallization zone is less than 600°C or the maintenance time is less than 1 minute, Ferrey Because the recrystallization of the ferrite is insufficient, the recrystallized ferrite bridge phase will not form at the target fraction. It is not possible. On the other hand, if the temperature exceeds 700°C or the duration exceeds 3 minutes, Excessive recrystallization may lead to a decrease in strength and a deterioration of physical properties due to grain coarsening.

[0109] The present invention provides a final microstructure with an appropriate ratio of hard phase and soft phase through the above recrystallization zone process. By introducing a recrystallized ferrite bridge phase, the strength is maintained. However, by strengthening crack toughness, or in other words, crack resistance, processability is improved. It can have the effect of [achieving a specific effect].

[0110] [Gradual cooling] As described above, by cooling the cold-rolled steel sheet that has undergone continuous annealing treatment as described above, the target is achieved. This allows for the formation of tissue, and at this time, cooling is performed stepwise. It is preferable.

[0111] In the present invention, the above stepwise cooling can consist of primary cooling - secondary cooling, and specifically Then, after the above continuous annealing, the temperature is cooled to a range of 650-700°C at an average cooling rate of 1-10°C / s. After the initial cooling, secondary cooling is performed at an average cooling rate of 5-50°C / s until the temperature reaches a range of 450-500°C. It is possible to perform a rejection.

[0112] In this case, by performing the primary cooling more slowly than the secondary cooling, the subsequent rapid cooling phase is relatively slower. This method can suppress defects in plate shape caused by a rapid temperature drop during the secondary cooling process.

[0113] If the end temperature during the primary cooling described above is below 650°C, the carbon diffusion activity will be affected by the low temperature. The mobility is low, and the carbon concentration in ferrite is high, while the carbon concentration in austenite is low. As the material hardens, the proportion of the hard phase becomes excessive, increasing the yield ratio, which in turn causes cracking during processing. The tendency for cracks to occur increases. Also, the cooling rate between the homogeneous zone and the cooling zone (slow cooling zone) is high. The problem arises when the temperature becomes too high, resulting in an uneven shape of the board. The above termination temperature is 700°C. If it exceeds this, there is a drawback in that an excessively high cooling rate is required during subsequent cooling (secondary cooling). ru.

[0114] Furthermore, if the average cooling rate during the primary cooling described above exceeds 10°C / s, sufficient carbon diffusion will occur. This will no longer be possible. On the other hand, considering productivity, the above primary cooling is performed at an average cooling rate of 1°C / s or higher. It is possible.

[0115] As mentioned above, after the primary cooling is complete, rapid cooling (secondary cooling) is performed at a cooling rate above a certain level. This can be done. In this case, if the secondary cooling completion temperature is less than 450°C, the width direction of the steel plate Furthermore, cooling variations may occur in the longitudinal direction, potentially leading to deterioration of the plate shape. On the other hand, the temperature If the temperature exceeds 500°C, it may become impossible to secure a sufficient hard phase, potentially leading to a decrease in strength. .

[0116] Furthermore, if the average cooling rate during the secondary cooling described above is less than 5°C / s, the fraction of the soft phase becomes excessive. There is a risk that this may happen. On the other hand, if the temperature exceeds 50°C / s, there is a risk that the hard phase will become insufficient. be.

[0117] Furthermore, if necessary, overaging treatment can be performed after the above-mentioned stepwise cooling is completed.

[0118] The above overaging treatment is a process of maintaining the temperature for a certain period of time after the secondary cooling completion temperature, and the coil Uniform heat treatment in both the width and length directions improves shape quality. For this reason, the above overtime processing can be performed for 200 to 800 seconds.

[0119] Since the above overaging treatment can be performed immediately after the completion of the secondary cooling, the temperature at that time is the same as the secondary cooling. It may be the same as the cooling completion temperature, or within the range of the above secondary cooling completion temperature, It can also be done at lower temperatures. More advantageously, in the temperature range of 300-450°C. To clarify what can be done.

[0120] The high-strength thick steel sheet of the present invention manufactured as described above has a microstructure consisting of a hard phase and a soft phase. Composed of a particularly optimized cold rolling and annealing process that maximizes ferrite recrystallization. As a result, the hard martensite phase is formed in the ferrite matrix that is ultimately recrystallized. It can have a uniformly distributed structure. Furthermore, the hard phase is connected relatively By introducing a coarse recrystallized ferrite bridge phase, the processing is improved. It has the effect of increasing rack resistance.

[0121] Therefore, the thick steel plate of the present invention has high strength, with a tensile strength of 980 MPa or more. Nevertheless, by ensuring a low yield ratio and high ductility, excellent formability such as hole expansion properties can be ensured. It is possible. [Examples]

[0122] The present invention will be described in more detail below with reference to examples. However, such implementation The examples provided are for illustrative purposes only, and such descriptions of examples do not necessarily represent the actual implementation of the present invention. The present invention is not limited thereto. The scope of rights of the present invention is as described in the claims. It is determined by the matters and matters that can be reasonably inferred therefrom.

[0123] (Examples) After fabricating steel slabs having the alloy compositions shown in Table 1 below, each steel slab is subjected to 120 After heating at 0°C for 1 hour, the material is finished hot-rolled at a finishing rolling temperature of 880-920°C to produce hot-rolled steel. The plates were manufactured. After that, each hot-rolled steel plate was wound at 650°C, and then heated at 0.1°C / s. The hot-rolled steel sheet was cooled to room temperature at the cooling rate. After that, the rolled hot-rolled steel sheet was subjected to the following conditions as shown in Table 2 below. After cold rolling and continuous annealing, the material is cooled in stages (primary-secondary) and then annealed at 360°C for 520 seconds. After intermittent aging treatment, a final steel plate with a thickness of 1.8 mm was manufactured.

[0124] In this case, the primary cooling during stepwise cooling has an average cooling rate of 3°C / s, and the secondary cooling has an average cooling rate of 20°C / s. This was done using the average cooling rate.

[0125] The microstructure of each steel sheet manufactured as described above was observed, and its tensile and workability characteristics were determined. After evaluating the usable physical property indices for the processing steps, such as the hole expansion rate, the results are shown in Table 3 below.

[0126] At this time, the tensile test on each test piece is performed perpendicular to the rolling direction, using JIS No. 5 tensile strength. After taking a tensile test specimen from Iz, the strain rate was 0.01 A tensile test was performed at / s.

[0127] On the other hand, the hole expansion rate (HER, %) measurement test was performed according to the ISO 16630 standard. Specifically, when a circular hole is punched into the test piece and then expanded using a cone-shaped punch, This indicates the amount of hole enlargement as a percentage of the initial hole size, until a crack that formed at the edge of the hole penetrates through to the thickness. The dimensions of the test specimen were 120mm x 120mm, and the clearance was (clear). The nce is 12%, the punch hole diameter is 10 mm, and the punching holding load is The test load was set to 20 tons, and the test speed was set to 12 mm / min.

[0128] And the bainite phase, which corresponds to the hard phase among the tissue phases, is picral (p (Icral) etching, the martensite phase after nital etching, 2 Observations were made via SEM at 000x and 5000x magnification.

[0129] Furthermore, regarding the ferrite phase and the recrystallized ferrite bridge (phase) phase, SEM and image analysis program after nital etching. The fractions were measured using the R program.

[0130] [Table 1]

[0131] [Table 2]

[0132] [Table 3]

[0133] As shown in Tables 1-3 above, the alloy composition of steel and manufacturing conditions, particularly cold rolling and continuous annealing processes, are as follows: However, in the examples 1 to 6 of the present invention that satisfy all the conditions proposed in this invention, the annealing process after cold rolling Then, due to sufficient recrystallization of ferrite, the hard phase is recrystallized into ferrite bridges (bridg e) Formed by being connected by a phase. This gives it high strength while being suitable for plate processing. It had a high yield strength and excellent elongation. In addition, the homogeneous distribution of the hard phase gave it good hole-expanding properties. It can be confirmed that it is excellent and that the target level of moldability can be ensured.

[0134] On the other hand, Comparative Examples 1-4 did not go through the recrystallization zone during the heating process in continuous annealing during the steel sheet manufacturing process. In comparative examples 6-9, recrystallization did not occur sufficiently, resulting in a recrystallized ferrite bridge phase. The results were insufficient. Of these, Comparative Examples 1-4, which had relatively low continuous annealing temperatures, showed poor elongation and hole expansion. Comparative Examples 6-9, which are inferior in at least one of the physical properties and have a relatively high continuous annealing temperature, are... The inite phase was excessively formed, resulting in excessively high yield strength and poor elongation.

[0135] Comparative Examples 5 and 10 show an annealing temperature for recrystallization driving and sufficient oats for ensuring strength. Although it has tenite stability, insufficient reduction ratio prevents sufficient recrystallization. As a result of failing to form a uniform structure, the elongation rate was inferior, while the yield strength was relatively high.

[0136] Furthermore, in comparative examples 11-14, where the secondary cooling temperature was very low, the yield strength was excessively high, making processing difficult. There is a high risk of crack formation inside, and because there is no recrystallized ferrite bridge phase, elongation The ratio was inferior.

[0137] Figure 3 shows microstructural images of Comparative Examples 1, 4-5, and 9, and Figure 4 shows microstructural images of Invention Examples 1, 3-4, and 6. This shows a microscopic image of the tissue.

[0138] As shown in Figure 3, Comparative Examples 1, 4, and 9 introduced a recrystallization zone process during the heating process of continuous annealing. Because it did not occur, the hard phase is connected by recrystallized ferrite bridges. It is almost impossible to confirm this. Also, in Comparative Example 5, due to insufficient reduction ratio, the recrystallization zone Due to the low fraction and low driving force, recrystallization is insufficient, and a structure is formed in which the hard phases aggregate together. This resulted in the formation of a tissue with low resistance to crack propagation.

[0139] On the other hand, as shown in Figure 4, the inventive example uses a recrystallization process to produce a relatively coarse recrystallized fella. A bridge phase was observed, and this phase indicates one side of the hard phase. It can be seen that the orientation has been resolved.

Claims

1. In weight percent, carbon (C): 0.05–0.12%, manganese (Mn): 2.0–3.0% Silicon (Si): 0.5% or less (excluding 0%), Chromium (Cr): 1.0% or less (0%) (excluding %), Niobium (Nb): 0.1% or less (excluding 0%), Titanium (Ti): 0.1% The following (excluding 0%), Boron (B): 0.003% or less (excluding 0%), Aluminum ( Sol. Al): 0.02-0.05%, Phosphorus (P): 0.05% or less (excluding 0%) Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%) ), iron (Fe) and other unavoidable impurities, The microstructure consists of ferrite with an area fraction of 10-30% and recrystallized ferrite with an area fraction of 10-25%. Bridge, containing 20-30% bainite and the remainder martensite. High-strength, thick steel plate with excellent hole-expanding properties and ductility.

2. The aforementioned recrystallized ferrite bridge has an average circular equivalent diameter of 1 to 6 μm. The high-strength, thick steel plate with excellent hole-expanding properties and ductility as described in claim 1.

3. The steel plate contains a martensite phase in an area fraction of 15% or more, as described in claim 1. High-strength, thick steel plate with excellent properties and ductility.

4. The steel sheet further contains retained austenite phase in an area fraction of 3% or less (including 0%). A high-strength, thick steel plate with excellent hole-expanding properties and ductility as described in claim 1.

5. The steel plate has a tensile strength of 980 MPa or more, a yield strength of 550 to 700 MPa, and a total elongation of 1 A high-strength thick steel plate with excellent hole-expanding properties and ductility, as described in claim 1, having a density of 4% or more.

6. The steel plate has a hole expansion ratio (HER) of 30% or more, and possesses the hole-expanding properties described in claim 1. High-strength, thick steel plate with excellent ductility.

7. The steel plate has a thickness of 1 to 3 mm and is the same as described in claim 1, having excellent hole-expanding properties and ductility. High strength thick steel plate.

8. In weight percent, carbon (C): 0.05–0.12%, manganese (Mn): 2.0–3.0% Silicon (Si): 0.5% or less (excluding 0%), Chromium (Cr): 1.0% or less (0%) (excluding %), Niobium (Nb): 0.1% or less (excluding 0%), Titanium (Ti): 0.1% The following (excluding 0%), Boron (B): 0.003% or less (excluding 0%), Aluminum ( Sol. Al): 0.02-0.05%, Phosphorus (P): 0.05% or less (excluding 0%) Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%) The steps include preparing a steel slab containing iron (Fe) and other unavoidable impurities, The steps include heating the steel slab to a temperature range of 1100 to 1300°C, The steps include: hot rolling the heated steel slab to produce a hot-rolled steel sheet; The steps include winding the hot-rolled steel sheet at a temperature range of 400 to 700°C, After the aforementioned winding, the hot-rolled steel sheet is cooled to room temperature. The cooled hot-rolled steel sheet is cold-rolled at a cold reduction ratio of 55-80% to produce a cold-rolled steel sheet. The stage, The steps include: continuously annealing the cold-rolled steel sheet, After the aforementioned continuous annealing, the temperature range is reduced to 650-700°C at an average cooling rate of 1-10°C / s. The cooling stage, After the primary cooling, a secondary cooling process is performed at an average cooling rate of 5 to 50°C / s down to a temperature range of 450 to 500°C. This includes a cooling step, The continuous annealing process is carried out in a facility equipped with a heating zone, a soaking zone, and a cooling zone, and the cold-rolled steel sheet is processed as follows: A key feature is that when the temperature rises to the heating zone, it passes through a recrystallization zone where it is maintained at 600-700°C for 1-3 minutes. A method for manufacturing high-strength thick steel plates with excellent hole-expanding properties and ductility.

9. The aforementioned hot rolling is performed as a finish hot rolling at an exit temperature of Ar3 or higher and 1000°C or lower. A method for manufacturing a high-strength thick steel plate with excellent hole-expanding properties and ductility, as described in claim 8.

10. The cooling after winding shall be carried out at a cooling rate of 0.1°C / s or less (excluding 0°C / s). The method for manufacturing a high-strength thick steel plate with excellent hole-expanding properties and ductility, as described in claim 8.

11. The heating zone and the uniform zone are controlled to a temperature range of 790 to 850°C, as described in claim 8. A method for manufacturing high-strength thick steel plates with excellent hole-expanding properties and ductility.

12. The process further includes a step of performing overaging treatment after the aforementioned secondary cooling, The aforementioned over-aging treatment is performed for 200 to 800 seconds, and the hole-expanding properties are superior as described in claim 8. A method for manufacturing high-strength, thick steel plates.

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