Steel plate and method for manufacturing same

Abstract

The present invention relates to a steel plate and a manufacturing method and, more specifically, to an ultra-high strength composite structure hot-rolled steel plate having excellent punching resistance, and a method for manufacturing same.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel plate and a manufacturing method, and more particularly, to an ultra-high strength composite structure hot-rolled steel plate having excellent punching resistance and a manufacturing method thereof.

[0002] High-strength hot-rolled steel sheets used in conventional automobile chassis and frames are undergoing a process of lightweight reduction, while simultaneously requiring superior formability considering the shape of the component. Furthermore, demand for yield strength is increasing to maximize component durability. In particular, as electric vehicle platforms are being developed, thinning of steel materials is becoming increasingly important compared to the past to offset the weight increase caused by battery adoption. However, as steel materials become stronger, local formability (typically, hole expandability) deteriorates. Therefore, microstructure and alloy design that achieve both ultra-high strength and excellent hole expandability are crucial.

[0003] Patent Document 1 relates to a method for manufacturing a composite structure steel having a microstructure of 10 to 55% ferrite and 45 to 90% of bainite and martensite in total, that is, a microstructure of 90% or more of ferrite, bainite, and martensite in total, and a tensile strength of 950 MPa or more.

[0004] However, only the case where the bainite and martensite composite structure is the main phase was considered, and there was no mention of yield strength, so it is judged difficult to guarantee high crashworthiness.

[0005] Patent Document 2 discloses a hot-rolled composite phase steel comprising, in area %, bainitic ferrite: 55% or more, martensite / austenite composite phase (MA): 10% or more, the sum of bainitic ferrite and martensite / austenite composite phase (MA): 95% or more, and a residual total of less than 5% of granular ferrite, retained austenite, and carbides. In addition, the bainitic ferrite has a microstructure characterized by an area fraction of particles having an intra-granular orientation difference of 1.5° or more and 10.5° or less, which is 55% or more of the total area of ​​the bainitic ferrite, an average grain size of the martensite / austenite composite phase (MA) of 2.0 μm or less, an average spacing of 0.3 μm or more, and a method for manufacturing a hot-rolled composite phase steel having mechanical properties of a yield strength of 750 MPa or more, a tensile strength of 950 MPa or more, an elongation of 8% or more, and a hole expandability of 25% or more is described.

[0006] However, the patent does not describe a manufacturing method or development concept for simultaneously securing ultra-high strength of 1180 MPa or more and hole expandability of 30% or more.

[0007] (Patent Document 1) Korean Patent Publication No. 10-2020-0011475

[0008] (Patent Document 2) Korean Patent Publication No. 10-2022-0039946

[0009] According to one embodiment of the present invention, it is an object to provide a steel plate and a manufacturing method.

[0010] According to one embodiment of the present invention, an object is to provide an ultra-high strength composite structure hot-rolled steel sheet having excellent punching resistance and a method for manufacturing the same.

[0011] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0012] According to one embodiment of the present invention, in weight %, C: 0.13 to 0.25%, Si: 0.01 to 1.00%, Mn: 1.8 to 2.5%, Al: 0.010 to 0.100%, Cr: 0.005 to 0.500%, Mo: 0.005 to 0.300%, P: 0.001 to 0.050%, S: 0.001 to 0.010%, N: 0.001 to 0.010%, the remainder being Fe and unavoidable impurities.

[0013] The microstructure of the surface layer contains 30 to 70% ferrite by area%.

[0014] The microstructure of the central portion excluding the surface layer may be a steel plate containing tempered martensite and lower bainite in a total of 95% or more in area%.

[0015] The above steel plate may further contain one or more of V, Nb, Ti, and B in a total amount of 0.05% or less.

[0016] The above central microstructure may include at least one of granular bainite, fresh martensite, MA phase, and pearlite as the residual structure, at 5% or less.

[0017] The above steel plate may have a yield strength of 950 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 5% or more, and a hole expandability (HER) of 40% or more.

[0018] The above steel plate may have an H value defined in the following relational expression 1 of 0.65 or less.

[0019] [Relationship 1]

[0020] H = (HER1-HER2) / HER1

[0021] (In the formula, HER1 is the hole expansion ability value measured by punching a hole through electrical discharge machining when processing a hole for a hole expansion test (HER test), and HER2 is the hole expansion ability value measured by punching a hole for a hole expansion test (HER test).)

[0022] According to one embodiment of the present invention, there is provided a step of reheating a steel slab containing, in wt%, C: 0.13 to 0.25%, Si: 0.01 to 1.00%, Mn: 1.8 to 2.5%, Al: 0.010 to 0.100%, Cr: 0.005 to 0.500%, Mo: 0.005 to 0.300%, P: 0.001 to 0.050%, S: 0.001 to 0.010%, N: 0.001 to 0.010%, the remainder Fe and unavoidable impurities;

[0023] A step of rolling the reheated steel slab;

[0024] A step of descaling the above-mentioned rolled steel sheet 3 to 6 times;

[0025] A step of finishing rolling the above descaled steel sheet at a temperature range of 850°C or higher;

[0026] A step of cooling and coiling the above-mentioned finished rolled steel sheet to a temperature range of Ms-120℃ to Ms-30℃ at an average cooling rate of 50℃ / s or more; and

[0027] It may be a steel plate manufacturing method including a step of cooling the above-mentioned rolled steel plate to a temperature range of 150°C or less at an average cooling rate of 0.1 to 25°C / h.

[0028] The above steel slab may further contain one or more of V, Nb, Ti, and B in a total amount of 0.05% or less.

[0029] The above reheating step is performed at a temperature range of 1150 to 1350°C,

[0030] The above rolling step is performed at a temperature range of 900 to 1150°C.

[0031] The above descaling step can be performed at a water pressure of 170 bar or more.

[0032] The step of pickling and oiling the cooled steel plate may be further included.

[0033] The method may further include a step of heating the cooled steel plate to a temperature range of 450 to 740°C and then performing hot-dip galvanizing.

[0034] According to one embodiment of the present invention, a steel plate and a manufacturing method can be provided.

[0035] According to one embodiment of the present invention, an ultra-high strength composite structure hot-rolled steel sheet having excellent puncture resistance and a method for manufacturing the same can be provided.

[0036] According to one embodiment of the present invention, a composite structure hot-rolled steel sheet having excellent formability and hole-forming properties and excellent strength can be provided, and can be used for components used in automobile chassis parts, including members, lower arms, reinforcing materials, connecting materials, and frames.

[0037] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0038] Figure 1 shows the HER2 value (the hole expansion ability value measured by punching when processing a hole for a hole expansion test (HER test)) according to the H value of relational expression 1 according to one embodiment of the present invention.

[0039] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0040] Hereinafter, the present invention will be described in detail.

[0041] Below, the steel composition of the present invention is described in detail.

[0042] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0043] A steel sheet according to one embodiment of the present invention may include, in wt%, C: 0.13 to 0.25%, Si: 0.01 to 1.00%, Mn: 1.8 to 2.5%, Al: 0.010 to 0.100%, Cr: 0.005 to 0.500%, Mo: 0.005 to 0.300%, P: 0.001 to 0.050%, S: 0.001 to 0.010%, and N: 0.001 to 0.010%.

[0044] Carbon (C): 0.13~0.25%

[0045] The above carbon (C) is the most economical and effective element for strengthening steel, and as the amount added increases, the precipitation strengthening effect or the low-temperature phase fraction increases, thereby increasing the tensile strength. If the content is less than 0.13%, sufficient hardenability is not secured, making it difficult to form a low-temperature phase, making it difficult to secure the target strength. According to one embodiment of the present invention, it may be 0.14% or more. On the other hand, if the content exceeds 0.25%, the strength increases too much, carbides are excessively formed, resulting in poor formability, and there is a disadvantage in that the carbon equivalent is increased, which generally results in poor weldability. In addition, when an excessive carbon (C) content is added, depending on the characteristics of the microstructure formed, there is a possibility that the low-temperature phase may deteriorate and additional excess carbides may be formed during additional heat treatment after hot rolling, which may significantly reduce the tensile strength after heat treatment. According to one embodiment of the present invention, it may be 0.24% or less.

[0046] Silicon (Si): 0.01~1.00%

[0047] The above silicon (Si) deoxidizes molten steel, has a solid solution strengthening effect, and is advantageous in improving formability by delaying the formation of coarse carbides. In addition, it also has an effect of suppressing carbide formation during heat treatment in the range of 300 to 600°C. However, if the content is less than 0.01%, the effect of delaying carbide formation is small, making it difficult to improve formability and may have little effect in improving strength. According to one embodiment of the present invention, it may be 0.30% or more. According to one embodiment of the present invention, it may be 0.40% or more. On the other hand, if the content exceeds 1.00%, when hot rolling, red scale is formed on the surface of the steel sheet due to Si, which not only greatly deteriorates the surface quality of the steel sheet, but also may cause problems in that ductility and weldability are reduced. According to one embodiment of the present invention, it may be 0.90% or less.

[0048] Manganese (Mn): 1.8~2.5%

[0049] The above manganese (Mn), like Si, is an effective element for strengthening steel through solid solution, and can additionally improve the hardenability of steel, delaying ferrite transformation at the same cooling rate and facilitating the formation of low-temperature phases such as bainite and martensite. However, if the content is less than 1.8%, the effects of solid solution strengthening and improving hardenability are small, so there may be a problem that the intended strength increase effect cannot be obtained. According to one embodiment of the present invention, it may be 1.9% or more. On the other hand, if it exceeds 2.5%, the hardenability increases significantly, causing the martensite fraction to exceed the intended purpose, and in the casting process, segregation develops significantly in the center of the thickness during slab casting, resulting in poor formability and deteriorating welding quality. According to one embodiment of the present invention, it may be 2.2% or less.

[0050] Aluminum (Al): 0.010~0.100%

[0051] The above aluminum (Al) is a component mainly added for deoxidation, and if the content is less than 0.01%, the effect of addition may be insufficient. According to one embodiment of the present invention, it may be 0.027% or more. According to one embodiment of the present invention, it may be 0.031% or more. On the other hand, if the content exceeds 0.100%, it combines with nitrogen to form AlN, which easily causes corner cracks to occur in the slab during casting and casting, and easily causes defects due to the formation of inclusions. According to one embodiment of the present invention, it may be 0.090% or less.

[0052] Chromium (Cr): 0.005~0.500%

[0053] The above chromium (Cr) can help strengthen steel and delay ferrite phase transformation during cooling, thereby assisting in the formation of bainite. However, if the content is less than 0.005%, there is a problem in that the above effect due to addition cannot be obtained. According to one embodiment of the present invention, it may be 0.200% or more. On the other hand, if the content exceeds 0.500%, ferrite transformation is excessively delayed, resulting in inferior elongation due to the formation of martensite phase. In addition, similar to manganese, segregation zones develop significantly in the center of thickness, making the thickness direction microstructure non-uniform, resulting in inferior elongation flangeability. In addition, if the chromium (Cr) content is excessively high, it can make the corrosion resistance of the material inferior. Therefore, according to one embodiment of the present invention, it may be 0.400% or less.

[0054] Molybdenum (Mo): 0.005~0.300%

[0055] Molybdenum (Mo) can increase the hardenability of steel and facilitate the formation of bainite structures. However, if the content is less than 0.005%, the above-mentioned effects cannot be achieved. According to one embodiment of the present invention, the content may be 0.100% or more. On the other hand, if the content exceeds 0.300%, excessive hardenability increases, resulting in the formation of a martensite phase, which may drastically reduce formability. Furthermore, it is economically disadvantageous and may also be detrimental to weldability. According to one embodiment of the present invention, the content may be 0.200% or less.

[0056] Phosphorus (P): 0.001~0.050%

[0057] The above phosphorus (P), like silicon (Si), simultaneously has the effects of strengthening and promoting ferrite transformation. However, manufacturing at a content of less than 0.001% is economically disadvantageous due to the high manufacturing costs, and may not be sufficient to achieve sufficient strength. On the other hand, if the content exceeds 0.050%, embrittlement due to grain boundary segregation occurs, microcracks easily form during molding, and ductility and impact resistance can be significantly deteriorated.

[0058] Sulfur (S): 0.001~0.010%

[0059] Sulfur (S) is an impurity present in steel. Manufacturing steel with a sulfur content below 0.001% requires significant time during steelmaking, potentially reducing productivity. Meanwhile, if its content exceeds 0.010%, it combines with manganese and other elements to form non-metallic inclusions, which can easily cause microscopic cracks during steel cutting.

[0060] Nitrogen (N): 0.001~0.010%

[0061] The above nitrogen (N), along with carbon, is a representative solid solution strengthening element and forms coarse precipitates together with titanium, aluminum, etc. In general, the solid solution strengthening effect of nitrogen (N) is superior to that of carbon, but there is a problem that the toughness decreases significantly as the amount of N in the steel increases. Therefore, according to one embodiment of the present invention, the upper limit of the nitrogen (N) content can be limited to 0.010%. On the other hand, in order to manufacture steel with the content less than 0.001%, a lot of time is required during the steelmaking operation, which may lower productivity.

[0062] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0063] A steel plate according to one embodiment of the present invention may further include one or more of V, Nb, Ti, and B in a total amount of 0.05% or less.

[0064] The above vanadium (V), niobium (Nb), titanium (Ti), and boron (B) have the characteristic of segregating at grain boundaries or precipitating carbides or nitrides. If the amount of the carbides or nitrides increases excessively, the fracture resistance desired in the present invention may be deteriorated. Therefore, in the present invention, the sum of these can be controlled to 0.05% or less.

[0065] Below, the steel microstructure of the present invention is described in detail.

[0066] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0067] A steel plate according to one embodiment of the present invention is composed of tempered martensite and lower bainite as the main phases in terms of strength enhancement, and by forming ferrite in a certain area from the surface of the steel plate, it is possible to secure strength and puncture resistance at the same time.

[0068] Accordingly, according to one embodiment of the present invention, the steel plate can be divided into a surface portion having a certain thickness based on the thickness direction and a central portion, which is the remaining area excluding the surface portion, and the microstructure can be controlled for each area.

[0069] According to one embodiment of the present invention, the remaining area excluding the surface portion may be divided into the center portion. According to one embodiment of the present invention, the surface portion of the steel sheet may mean the depth to the point where the ferrite fraction decreases. According to one embodiment of the present invention, the surface portion of the steel sheet may mean the depth to the point where the ferrite fraction decreases rapidly. According to one embodiment of the present invention, it may appear as a depth level that is 7% of the thickness of the steel sheet in the direction of the center of the thickness from the surface of the steel sheet. That is, although the interface between the surface portion and the center portion is not clear, if the fraction of the hard phase (tempered martensite and lower bainite) increases to 95% or more, it can be defined as the center portion. Meanwhile, according to one embodiment of the present invention, the area (thickness) of the surface portion may vary depending on the thickness of the steel sheet being manufactured.

[0070] The microstructure of the surface layer of the steel plate according to one embodiment of the present invention may include 30 to 70% of ferrite in terms of area%.

[0071] If the ferrite fraction of the surface layer is less than 30%, the resistance to fracture due to ferrite may be reduced during a hole expansion test. On the other hand, if the fraction exceeds 70%, the overall strength of the steel may decrease, making it impossible to secure the target tensile properties. There is no particular limitation on the microstructure other than ferrite in the surface layer, but according to one embodiment of the present invention, the microstructure other than ferrite may include upper and lower bainite, martensite, etc.

[0072] According to one embodiment of the present invention, the microstructure of the central portion excluding the surface portion of the steel plate may include tempered martensite and lower bainite in an area % of 95% or more in total, and the remaining structure may include at least one of granular bainite, fresh martensite, MA phase, and pearlite in an area % of 5% or less.

[0073] According to one embodiment of the present invention, the central microstructure can be measured at a point 1 / 4 from the surface of the steel plate in the direction of the center of thickness.

[0074] In the present invention, in order to simultaneously secure excellent hole expandability and ultra-high strength properties, tempered martensite and lower bainite may be included in an amount of 95% or more. If the sum of the fractions is less than 95%, there may be a problem in that both hole expandability and strength are deteriorated. The hole expandability of steel is greatly affected by the composition of the microstructure. This is because when several phases with different strengths exist simultaneously, the hole expandability is deteriorated due to the difference in hardness between the phases. In addition, in ultra-high strength steels of giga grade or higher, securing strength by the fraction of martensite and lower bainite phases is very important, and the strength can easily decrease due to the introduction of upper bainite and ferrite. According to one embodiment of the present invention, tempered martensite and lower bainite may be 100%.

[0075] A steel plate according to one embodiment of the present invention may have a yield strength of 950 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 5% or more, a hole expandability (HER) of 40% or more, and an H value defined in the following relational expression 1 of 0.65 or less.

[0076] In addition, since the steel sheet according to one embodiment of the present invention can maintain the mechanical properties even after heat treatment at 300 to 600°C, it can be easily used when manufacturing a hot-rolled steel sheet plated using molten zinc or the like.

[0077] [Relationship 1]

[0078] H = (HER1-HER2) / HER1

[0079] (In the formula, HER1 is the hole expansion ability value measured by punching a hole through electrical discharge machining when processing a hole for a hole expansion test (HER test), and HER2 is the hole expansion ability value measured by punching a hole for a hole expansion test (HER test).)

[0080] The main factors related to HER can be broadly divided into intrinsic factors related to the inherent properties of the material and extrinsic factors that occur around the hole during punching. The intrinsic factor is defined as HER1, and the extrinsic factor △HER is defined as (HER1-HER2).

[0081] The intrinsic factor is the hole expandability that appears due to the unique microstructural / mechanical properties of the material, and the extrinsic factor is the hole expandability value that takes into account the decline due to external factors such as the hole punching process for performing the hole expansion test, clearance during punching, and wear condition of the punching punch in addition to the unique properties of the material. In the present invention, only the hole punching process by punching was considered as an external factor.

[0082] Therefore, the intrinsic factor can be implemented by creating holes through electrical discharge machining instead of the general punching method while minimizing the damage that occurs to the hole during the punching process. The hole expandability value when external damage is minimized through electrical discharge machining is defined as HER1. The hole expandability value obtained through the punching process is defined as HER2. The hole expandability is reported as the average value of the results of a total of three evaluations at room temperature.

[0083] If the H value defined in relational expression 1 exceeds 0.65, it means that the difference between HER1 and HER2 is large, and that the hole expansion ability that decreases during punching is large. Therefore, according to one embodiment of the present invention, in order to secure a steel material with excellent punching surface quality and in which the hole expansion ability decreases little even due to external factors, the H value can be limited to 0.65 or less.

[0084] Figure 1 shows the HER2 (hole expansion ability value measured by punching when processing a hole for a hole expansion test (HER test)) value according to the H value of relational expression 1 according to one embodiment of the present invention. As shown in Figure 1, when the H value has a value of 0.65 or less, HER2 can have the hole expansion ability at the proposed level.

[0085] Below, the steel manufacturing method of the present invention is described in detail.

[0086] A steel plate according to one embodiment of the present invention can be manufactured by reheating, hot rolling, cooling, coiling, and cooling a steel slab satisfying the above-described alloy composition.

[0087] Reheating

[0088] A steel slab satisfying the alloy composition according to one embodiment of the present invention can be reheated to a temperature range of 1150 to 1350°C.

[0089] If the reheating temperature is below 1150℃, the hot rolling entry temperature will be lowered, potentially resulting in poor surface quality. Conversely, if the temperature exceeds 1350℃, there is a risk of strength loss due to coarsening of austenite grains.

[0090] rough rolling

[0091] According to one embodiment of the present invention, a process in which continuous casting and hot rolling processes are directly connected can be applied.

[0092] The above reheated steel slab can be rolled at a temperature range of 900 to 1150°C.

[0093] During rough rolling, the conditions are not particularly limited and may be conditions applicable in the same technical field. According to one embodiment of the present invention, rough rolling may be performed at a temperature range of 900 to 1150°C. During rough rolling, if rolling is initiated at a temperature exceeding 1150°C, the temperature of the steel sheet increases, resulting in a coarser grain size and increased oxide formation on the surface of the steel sheet, which may result in poor quality.

[0094] Descaling

[0095] The above-mentioned rolled steel sheet can be descaled 3 to 6 times.

[0096] The hot rolling process may include rough rolling and finish rolling, and according to one embodiment of the present invention, a descaling step may be performed after rough rolling and before finish rolling.

[0097] According to one embodiment of the present invention, after the rough rolling step, the steel sheet can be descaled 3 to 6 times across the entire width of the steel sheet at a hydraulic pressure of 170 bar or more.

[0098] The above descaling causes a temperature difference between the surface and other parts of the steel plate by performing surface cooling, which results in the temperature of the surface being instantly lowered below the ferrite phase transformation temperature, thereby introducing a certain amount of ferrite into the surface. The ferrite introduced into the surface increases the resistance of the extreme surface to hole expansion processing during a hole expansion test, thereby significantly improving hole expandability.

[0099] If the above descaling is performed less than three times, surface cooling may be insufficient, resulting in insufficient ferrite formation, which may result in poor hole expandability. If the descaling is performed more than six times, excessive ferrite formation may occur, making it impossible to secure the desired strength.

[0100] Finish rolling

[0101] The above descaled steel sheet can be finished rolled at a temperature range of 850°C or higher.

[0102] When the temperature is lower than 850℃ during final rolling, excessive recrystallization delay may lead to the development of elongated grains, which may result in severe anisotropy and poor formability.

[0103] During final rolling, the upper temperature limit is not particularly limited, but according to one embodiment of the present invention, it may be 1150°C or lower.

[0104] Winding

[0105] The above hot-rolled steel sheet can be cooled and coiled to a temperature range of Ms-120℃ to Ms-30℃ at an average cooling rate of 50℃ / s or more.

[0106] In order to simultaneously secure ultra-high strength and excellent hole expandability proposed in the present invention, the desired microstructure can be secured by controlling the cooling and coiling temperatures.

[0107] When cooling, if the end temperature is lower than Ms-120℃, the final microstructure may be composed mainly of tempered martensite, which may result in excessively high strength compared to the target and poor formability. According to one embodiment of the present invention, it may be higher than Ms-100℃. On the other hand, if the end temperature exceeds Ms-30℃, the fraction of tempered martensite may decrease when forming the final microstructure, and the strength may be poor due to the formation of low-strength granules or upper bainite by reheating during coiling. According to one embodiment of the present invention, it may be lower than Ms-50℃.

[0108] When cooling, if the average cooling rate is less than 50°C / s, ferrite and upper bainite are introduced, which may be detrimental to strength and may result in poor hole expandability. According to one embodiment of the present invention, the average cooling rate may be 58°C / s or higher. The present invention does not specifically limit the upper limit of the average cooling rate, but may be 73°C / s or lower.

[0109] [ceremony]

[0110] Ms = 539-423[C]-30.4[Mn]-12.1[Cr]-7.5[Mo]

[0111] (In the formula, [C], [Mn], [Cr] and [Mo] are the weight percent of each element.)

[0112] cooling

[0113] The above-mentioned rolled steel plate can be cooled to a temperature range of 150°C or less at an average cooling rate of 0.1 to 25°C / h.

[0114] If the average cooling rate exceeds 25°C / h, the tempering of martensite in the steel may be insufficient, which may result in a decrease in the yield strength of the steel and poor elongation flangeability. According to one embodiment of the present invention, it may be 10°C / h or less. On the other hand, in order to control the average cooling rate to less than 0.1°C / h, separate heating equipment, etc. may be required, which may be economically disadvantageous. According to one embodiment of the present invention, it may be 1°C / h or more.

[0115] According to one embodiment of the present invention, a step of acid-treating the cooled steel sheet and then applying oil may be further included, if necessary. Furthermore, according to one embodiment of the present invention, a step of plating the cooled steel sheet to produce a plated steel sheet may be further included, if necessary.

[0116] Plating conditions are not particularly limited, but typical plating conditions can be applied, and according to one embodiment of the present invention, hot-dip galvanizing can be performed. According to one embodiment of the present invention, after heating a cooled steel plate to a temperature range of 450 to 750°C, hot-dip galvanizing can be performed using a plating bath containing 0.01 to 30% Mg, 0.01 to 50% Al, and the remainder Zn.

[0117] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0118] (Example)

[0119] After manufacturing a steel slab having the composition shown in Table 1 below, a hot-rolled steel sheet having a thickness of 2.5 mm was manufactured under the manufacturing conditions shown in Table 2 below. At this time, the reheating temperature was performed within the temperature range of 1150 to 1350°C, and the rough rolling was applied in the same temperature range of 1050°C. In addition, descaling was performed at a hydraulic pressure of 180 bar.

[0120] Steel alloy composition (weight %) CSiMnAlPSNCrMoA0.090.601.60.0310.0070.0010.0030.2000.100 B0.020.502.30.0320.0090.0020.0040.4000.200 C0.170.501.10.0250.0110.0030.0040.2000.200 D0.290.402.20.0270.0110.0010.0030.1000.300 E0.160.703.50.0350.0140.0020. 0030.2000.200F0.140.702.20.0280.0090.0010.0040.2000.100G0.160.901.90.0270.0120.0020.0030.4000.200H0.190.802.20. 0290.0110.0020.0030.3000.200I0.210.402.10.0310.0130.0010.0040.1000.100J0.240.501.90.0280.0080.0020.0030.3000.100

[0121] Specimen numberSteel gradeDescalingFinish rollingMs(℃)Number of coiling cooling timesTemperature(℃)Temperature(℃)Average cooling rate(℃ / s)End temperature(℃)Average cooling rate(℃ / h)1A39124494126552122B3905454402704653C3922430376624884D5885346268563585E68793612877328166F39114103635925227G38984073227029148H4880387309733639I5912384 2675831910J390537529565421611F488941055076442012G586640739515392013H6725387329 65271414I39053843972221515J4907375301434916F191041037262291917F8912410365653214

[0122] [Formula]Ms = 539-423[C]-30.4[Mn]-12.1[Cr]-7.5[Mo]

[0123] (In the formula, [C], [Mn], [Cr] and [Mo] are the weight percent of each element.)

[0124] The microstructure and physical properties of the manufactured steel plate were measured and are shown in Table 3 below.

[0125] The central microstructure fraction was observed at 1 / 4 points from the surface of the manufactured steel sheet toward the center of the thickness, and measured from the results analyzed using SEM at x4000 and x8000 magnifications. To read tempered martensite and fresh martensite, analysis was performed at x1000 magnification using an optical microscope and image analyzer after etching with Nital and Lepera. Here, lower bainite and tempered martensite have very similar microstructural properties, so they can be distinguished only by local analysis in very small areas such as TEM analysis. However, since it is difficult to calculate the representative fraction of steel with these results, the lower bainite and tempered martensite were combined and the fractions were calculated together.

[0126] In addition, the surface microstructure was observed from the surface of the steel plate to a depth of 7% of the steel plate thickness in the direction of the center of thickness, and the average value was measured by observing three random areas using SEM at x1000 magnification. In addition to ferrite, upper and lower bainite and martensite were observed as surface microstructures.

[0127] In addition, the results of yield strength (YS), tensile strength (TS), elongation (El) of the manufactured steel plate and hole expandability related to Equation 1 were presented. The tensile material was tested by taking a JIS standard test piece in a direction perpendicular to the rolling direction and the tensile evaluation was measured at room temperature.

[0128] The hole expandability (HER) test was conducted with a punching clearance (cl) of 12%, and the results are presented based on the JFST 1001-1996 standard. The HER1 and HER2 evaluation values ​​were expressed as the average values ​​after three tests. In addition, the H value of Equation 1 was calculated and presented based on the obtained values.

[0129] Specimen numberSteel gradeMicrostructure (area%)Mechanical propertiesClassificationCenter Surface TM+LBGB+UBFM+MAFYS(MPa)TS(MPa)El(%)HER1HER2△HER(%)Relationship 1(H)1A71236458631092119822760.78Comparative example 12B4241173565482617136321040.76Comparative example 23C52361239921113211106 28780.74 Comparative Example 34D9811511426169848628580.67 Comparative Example 45E9712611322156559625710.74 Comparative Example 56F9622629821221711256560.50 Invention Example 17G97215310021253711053570.52 Invention Example 28H97124911531298610 642640.60 Invention Example 39I96224612351356610950590.54 Invention Example 410J9901521296142779959400.40 Invention Example 511F681136061182918148361120.76 Comparative Example 612G486105565288916142341080.76 Comparative Example 713H76101445 8611275811924950.80Comparative Example 814I122866212691429510118830.82Comparative Example 915J2944598251189611622940.81Comparative Example 1016F1088219941287711020900.82Comparative Example 1117F9154757651122811545700.61Comparative Example 12

[0130] * TM: tempered martensite, LB: lower bainite, GB: granular bainite, UB: upper bainite, FM: fresh martensite, MA: MA phase, F: ferrite [Relationship 1]

[0131] H = (HER1-HER2) / HER1

[0132] (In the formula, HER1 is the hole expansion ability value measured by punching a hole through electrical discharge machining when processing a hole for a hole expansion test (HER test), and HER2 is the hole expansion ability value measured by punching a hole for a hole expansion test (HER test).)

[0133] As shown in Table 3 above, in the case of an invention example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention could also be secured.

[0134] On the other hand, Comparative Examples 1 to 5 are examples that fall outside the alloy composition range proposed by the present invention. In Comparative Examples 1 and 2, the carbon content was insufficient, so the tempered martensite and lower bainite fractions proposed by the present invention were not satisfied, and the strength was also inferior. In Comparative Example 3, the manganese content was insufficient, so the target strength could not be secured. In Comparative Example 4, the carbon content was excessively high, so the elongation and hole expandability were inferior. In Comparative Example 5, the manganese content exceeded the range of the present invention, so the elongation and hole expandability were inferior.

[0135] In the case of Comparative Examples 6 to 10, the composition range of the present invention is satisfied, but the manufacturing conditions are examples that go beyond the scope of the present invention. In the case of Comparative Example 6, it can be confirmed that the coiling temperature is higher than the Ms temperature, so the fractions of upper bainite and granular bainite are excessively high, and accordingly, the strength is greatly reduced. Even if they have the same bainite structure, the upper and lower bainite can have a large difference in the formation temperature and strength, and even if it is not tempered martensite, if a large amount of fresh martensite is contained, the tensile strength can be greatly improved, but the yield strength does not increase as much as the tempered martensite structure, so it can be confirmed that a high yield ratio steel could not be manufactured and the hole expandability is poor. In the case of Comparative Examples 7 and 10, the average cooling rate from hot rolling to the coiling temperature is low, so the fractions of upper bainite and granular bainite become excessively high. Accordingly, it can be confirmed that the strength is far below the level targeted by the present invention, as in the case of Comparative Example 6. In the case of Comparative Example 8, it can be confirmed that the desired microstructure fraction cannot be secured when the hot rolling temperature is lower than the target range. In the case of Comparative Example 9, it is a case where the temperature is lower than the target coiling temperature range, and in this case, the target microstructure fraction is satisfied, but it can be confirmed that the targeted elongation and hole expandability are greatly reduced. In the case of the coiling temperature, fresh martensite is generated at a temperature lower than the Mf (martensite transformation end temperature) of the steel, and then tempered at a low temperature to form a tempered martensite structure, but it can be confirmed that △HER related to Equation 1 is very large.

[0136] Comparative Examples 11 and 12 are examples that did not satisfy the descaling conditions proposed in the present invention. In Comparative Example 11, the number of descaling cycles was insufficient, resulting in insufficient formation of ferrite in the surface layer. As a result, the strength was excessively high and the pore expandability was poor. In Comparative Example 12, the number of descaling cycles was excessive, resulting in excessive formation of ferrite in the surface layer. As a result, the desired strength could not be secured.

[0137] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, by weight%, C: 0.13~0.25%, Si: 0.01~1.00%, Mn: 1.8~2.5%, Al: 0.010~0.100%, Cr: 0.005~0.500%, Mo: 0.005~0.300%, P: 0.001~0.050%, S: 0.001~0.010%, N: 0.001~0.010%, the remainder being Fe and unavoidable impurities. The microstructure of the surface layer contains 30 to 70% ferrite by area%. A steel plate having a microstructure in the central portion excluding the surface layer, in which the combined area % of tempered martensite and lower bainite is 95% or more.

2. In claim 1, The above steel plate is a steel plate that further contains at least one of V, Nb, Ti, and B in a total amount of 0.05% or less.

3. In claim 1, A steel plate in which the above central microstructure contains at least one of granular bainite, fresh martensite, MA phase, and pearlite as the residual structure, at 5% or less.

4. In claim 1, The above steel plate is a steel plate having a yield strength of 950 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 5% or more, and a hole expandability (HER) of 40% or more.

5. In claim 1, The above steel plate is a steel plate having an H value of 0.65 or less as defined in the following relational expression 1. [Relationship 1] H = (HER1-HER2) / HER1 (In the formula, HER1 is the hole expansion ability value measured by punching a hole through electrical discharge machining when processing a hole for the hole expansion test (HER test), and HER2 is the hole expansion ability value measured by punching a hole for the hole expansion test (HER test).) 6. A step of reheating a steel slab containing, by weight%, C: 0.13 to 0.25%, Si: 0.01 to 1.00%, Mn: 1.8 to 2.5%, Al: 0.010 to 0.100%, Cr: 0.005 to 0.500%, Mo: 0.005 to 0.300%, P: 0.001 to 0.050%, S: 0.001 to 0.010%, N: 0.001 to 0.010%, the remainder being Fe and unavoidable impurities; A step of rolling the above reheated steel slab; A step of descaling the above rolled steel plate 3 to 6 times; A step of finish rolling the above descaled steel sheet at a temperature range of 850°C or higher; A step of cooling and coiling the above-mentioned finished rolled steel sheet to a temperature range of Ms-120℃ to Ms-30℃ at an average cooling rate of 50℃ / s or more; and A method for manufacturing a steel plate, comprising: a step of cooling the rolled steel plate to a temperature range of 150°C or less at an average cooling rate of 0.1 to 25°C / h.

7. In claim 6, The above steel slab is a method for manufacturing a steel plate, wherein the steel slab further contains at least one of V, Nb, Ti, and B in a total amount of 0.05% or less.

8. In claim 6, The above reheating step is performed at a temperature range of 1150 to 1350℃. The above rolling step is performed at a temperature range of 900 to 1150℃. A method for manufacturing a steel plate, wherein the above descaling step is performed at a water pressure of 170 bar or more.

9. In paragraph 6, A method for manufacturing a steel plate further comprising the steps of pickling and oiling the cooled steel plate.

10. In paragraph 6, A method for manufacturing a steel plate further comprising the step of heating the cooled steel plate to a temperature range of 450 to 740°C and then performing molten zinc plating.

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