Ultra-high strength steel sheet excellent in elongation and hole expansion property and method for producing the same

Abstract

The present invention relates to an ultra-high strength steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet excellent in elongation and hole expansion property and further excellent in strength, and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to an ultra-high strength steel sheet and a method for manufacturing the same, and more particularly, to an ultra-high strength steel sheet having excellent elongation and hole expansion properties and a method for manufacturing the same.

Background Art

[0002] Recently, in the automotive field, research for reducing the vehicle body weight due to fuel consumption regulations and performance improvement has been actively carried out in developed countries such as Europe. In the case of steel parts, in order to meet the requirements of such vehicle manufacturers for weight reduction, efforts have been made to increase the strength and further reduce the steel sheet thickness to the same level as competing materials (Mg, Al, CFRP, etc.). In addition to weight reduction, due to the strengthened CO 2 emission regulations and the rapid change to the electric vehicle era, the stability and high strength of vehicle body materials are also required to be enhanced due to the strengthening of safety regulations for vehicle passengers and pedestrians. In particular, the demand for high-strength steel materials of 980 to 1180 MPa or higher is increasing. In the case of 980 MPa grade steel materials, in order to be used as a member for absorbing collision energy, not only is it necessary to have a high elongation rate for forming a complex shape, but also it is necessary to be excellent in hole expansion property because fracture should not occur with respect to axial deformation. Such structural members are required to have high yield strength and hole expansion property in order to advantageously absorb impact energy.

[0003] As a typical manufacturing method for increasing the yield strength, there is a method that utilizes water cooling during continuous annealing. In a typical prior art of such a method, Patent Document 1 discloses that after continuously annealing a steel material containing 0.18 to 0.3% carbon, it is water-cooled to room temperature, and then an overaging treatment is performed at a temperature of 120 to 300°C for 1 to 15 minutes to produce a steel material in which the martensite volume fraction is 80 to 97% and the balance is ferrite. After annealing a cold-rolled steel sheet in the two-phase region or the single-phase region and then rapidly cooling it to room temperature level, an ultra-high-strength steel can be produced by a tempering method. However, when manufacturing in this way, although the yield strength and hole expansion property are excellent, there is a problem that the shape quality of the coil deteriorates due to temperature deviation in the width direction and the length direction, and problems such as material defects and reduced workability may occur during roll forming part processing.

[0004] In addition, generally, as the strength of the steel sheet increases, the elongation rate decreases and the formability deteriorates, so its application as a material for cold stamping is limited. In order to form a steel material with a complex shape, basically the elongation rate must be high. As a typical method for increasing the elongation rate, as in Patent Document 2, when a large amount of ferrite is introduced to ensure additional elongation rate in addition to retained austenite, the yield strength and hole expansion property may deteriorate.

[0005] Therefore, in order to solve the above-mentioned problems, the development of an ultra-high-strength cold-rolled steel sheet with a tensile strength of 980 MPa or more, which is excellent in both elongation rate and hole expansion property, is required.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

[0007] According to one aspect of the present invention, it is to provide an ultra-high strength steel sheet excellent in both elongation and hole expansion property, and a method for manufacturing the same.

[0008] The problems of the present invention are not limited to the above-described content. An ordinary technician can easily understand further problems of the present invention from the overall content of this specification. [Means for Solving the Problems]

[0009] One aspect of the present invention provides a steel sheet containing, by weight%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.1%, the balance being iron (Fe) and other inevitable impurities, and having, as a microstructure, ferrite in an amount of 5 to 50 area%, a total of tempered martensite and bainite in an amount of 35 to 80 area%, retained austenite in an amount of 7 to 15 area%, and fresh martensite in an amount of 10 area% or less.

[0010] The above steel sheet may further contain antimony (Sb): 0.03% or less.

[0011] The above steel sheet may have a total content of chromium (Cr) and molybdenum (Mo) of 0.01% or less.

[0012] The above steel sheet may have an A value defined by the following relational expression 1 of 40 or less and a B value of 12 or more. [Relational Expression 1] A = ([U] / [X]) * 100 B = ([Z] - [Y]) * 1000 (In the formula, [U]: the percentage value of the IQ value of 40,000 or less, [X]: the percentage value of the IQ value of 40,000 to 70,000, [Y]: the maximum percentage value of the IQ value of 40,000 or less, [Z]: the maximum percentage value of the IQ value of 40,000 to 70,000, which is based on the Image Quality (IQ) Chart (35 bar graphs) obtained by EBSD measurement.)

[0013] The above steel plate can have a tensile strength of 980 MPa or more and an R value defined by the following relational expression 2 of 30,000 to 60,000 MPa·%. [Relational Expression 2] R = [YS] × ([Total-El] + (2 * [HER])) (In the formula, [YS] is the yield strength (MPa), [Total-El] is the total elongation rate (%), and [HER] is the hole expansion property (%).)

[0014] The above steel plate can have a yield strength of 600 MPa or more, an elongation rate of 21% or more, and a hole expansion property (HER) of 20% or more.

[0015] Another aspect of the present invention is to prepare a cold-rolled steel sheet containing, by weight%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.1%, and the balance iron (Fe) and other inevitable impurities; Annealing heat treatment by heating the above cold-rolled steel sheet in the temperature range of 780 °C to Ac3 and holding for 30 seconds or more; Primary cooling the annealed heat-treated steel sheet to the temperature range of 630 to 750 °C at an average cooling rate of 1 to 10 °C / s; Secondary cooling the primary-cooled steel sheet to the temperature range of 180 °C to Ms at an average cooling rate of 30 to 80 °C / s; and A method for manufacturing a steel sheet including a reheating and overaging step of heating the secondary-cooled steel sheet to the temperature range of Ms - 50 to 450 °C and holding for 1 to 30 minutes can be provided. [Formula] Ac3 = 910 - 203√([C]) - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W] (In the formula, [C], [Ni], [Si], [V], [Mo], and [W] are the weight percentages of the respective elements.) Ms = 539 - 423[C] - 30.4[Mn] - 7.5[Si] + 30[Al] - 12.1[Cr] - 17.7[Ni] - 7.5[Mo] (In the formula, [C], [Mn], [Si], [Al], [Cr], [Ni], and [Mo] are the weight percentages of the respective elements.)

[0016] The cold-rolled steel sheet can further contain antimony (Sb): 0.03% or less.

[0017] The cold-rolled steel sheet can have a total content of chromium (Cr) and molybdenum (Mo) of 0.01% or less.

[0018] The step of preparing the cold-rolled steel sheet includes: Reheating the steel slab in the temperature range of 1100 to 1300°C; Hot-rolling the reheated slab at a finish hot-rolling temperature of Ar3 or higher to obtain a hot-rolled steel sheet; Cooling and coiling the hot-rolled steel sheet at a temperature of 700°C or lower; and Cold-rolling the coiled hot-rolled steel sheet at a cold reduction rate of 30 to 80% can be included.

[0019] During the first cooling, the average cooling rate is 2 to 5°C / s, During the second cooling, the average cooling rate can be 30 to 60°C / s.

Advantages of the Invention

[0020] According to one aspect of the present invention, it is possible to provide a steel sheet excellent in elongation and hole expansion property, and further excellent in strength, and a method for manufacturing the same.

Brief Description of the Drawings

[0021]

Figure 1

Mode for Carrying Out the Invention

[0022] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. This embodiment is provided to explain the present invention in more detail to an ordinary technician in the technical field to which the invention belongs.

[0023] In the present invention, by controlling the content ranges of component elements such as C, Mn, Si, P, and S in the alloy composition, and particularly by controlling the conditions of heat treatment, primary cooling, secondary cooling, reheating, and overaging stages among the manufacturing process conditions, it was confirmed that both the elongation rate, hole expansion property, and strength can be ensured. As a result, the present invention was completed.

[0024] Hereinafter, the present invention will be described in detail.

[0025] Hereinafter, the steel composition of the present invention will be described in detail.

[0026] Unless otherwise specified in the present invention, % representing the content of each element is based on weight.

[0027] The steel according to one aspect of the present invention contains, by weight %, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.1%, and the balance iron (Fe) and other inevitable impurities.

[0028] Carbon (C): 0.15 to 0.25% Carbon (C) is the most effective and important element for improving the strength of steel as an interstitial solid solution element, and is an element that must be added indispensably to ensure the strength of martensitic steel. In order to obtain ultra-high strength steel that satisfies the yield ratio and tensile strength targeted in the present invention, it is preferable that carbon (C) is added in an amount of 0.15% or more, more preferably 0.18% or more, and even more preferably 0.2% or more. However, when its content exceeds 0.25%, as martensite is excessively formed during cooling due to an increase in hardenability, the strength increases rapidly, and there is a possibility that the elongation rate may be inferior. In addition, since an increase in the carbon (C) content has a problem of inhibiting weldability, it is preferable to limit its upper limit to 0.25%. A more preferable upper limit can be 0.24%.

[0029] Manganese (Mn): 1.5 - 2.5% Manganese (Mn) is an element added to ensure strength. When the content of the above manganese (Mn) is less than 1.5%, it may be difficult to ensure the strength level required in the present invention. A more preferable lower limit of the manganese (Mn) content can be 1.8%, and even more preferably 2.0%. On the other hand, when its content exceeds 2.5%, the Ms temperature becomes low during cooling after annealing, so it may be difficult to ensure a smooth initial martensite structure. This is because it is difficult to ensure both the strength, elongation rate, and hole expansion property targeted in the present invention due to a decrease in the fraction of tempered martensite in the Q&P (Quenching&Partitioning) process. In addition, there is a problem that manganese segregates in the thickness direction and it is easy to form a manganese band (Mn band) in the slab, and the occurrence of defects during the rolling process increases together with continuous casting cracks. More preferably, the upper limit of the manganese (Mn) content can be 2.4%.

[0030] Silicon (Si): 1.0 - 2.0% Silicon (Si) is a core element of TRIP (Transformation Induced Plasticity) steel, which increases the fraction of retained austenite and elongation by inhibiting the precipitation of cementite. When the content of the above-mentioned silicon (Si) is less than 1.0%, the control of cementite precipitation is not smooth during the reheating and overaging stages, and the fraction of the finally formed retained austenite may be small or the stability may be low, so the final elongation may be inferior. The lower limit of the more preferable silicon (Si) content can be 1.2%. On the other hand, when the content of the above-mentioned silicon (Si) exceeds 2.0%, the physical properties of the welded part deteriorate due to the formation of LME (Liquid Metal Embrittlement) cracks, and the surface characteristics and plating properties of the steel material deteriorate. The upper limit of the more preferable silicon (Si) content can be 1.8%.

[0031] Phosphorus (P): 0.1% or less Phosphorus (P) is an impurity element contained in steel. Considering the inevitable inclusion during the manufacturing process, 0% is excluded. On the other hand, when the content of phosphorus (P) exceeds 0.1%, the weldability deteriorates and the brittleness of the steel may occur, so the upper limit can be restricted to 0.1%. The more preferable upper limit can be 0.03%.

[0032] Sulfur (S): 0.03% or less Sulfur (S) is an impurity inevitably contained in steel, similar to P, and is an element that inhibits the ductility and weldability of the steel plate. Therefore, it is preferable to control the content as low as possible. Accordingly, in the present invention, the content of sulfur (S) can be restricted to 0.03% or less. More preferably, it can be restricted to 0.005% or less. Note that 0% is excluded considering the inevitable inclusion during the manufacturing process.

[0033] Aluminum (Al): 0.01 - 0.1% Aluminum (Al) can be added for oxygen removal in molten steel and, like Si, is effective in suppressing cementite precipitation and stabilizing retained austenite during reheating and overaging stages. If the content of the above-mentioned aluminum (Al) is less than 0.01%, deoxidation of the steel may not be sufficiently carried out, which may impair the cleanliness of the steel. On the other hand, when the content of the above-mentioned aluminum (Al) exceeds 0.1%, not only does the castability of the slab deteriorate, but the temperature required for single-phase region heating during annealing also increases, resulting in production and equipment problems. More preferably, it can be limited to 0.05% or less.

[0034] In addition to the above-described composition, the steel according to the present invention can contain the remaining iron (Fe) and unavoidable impurities. Since unavoidable impurities may be unintentionally mixed in during normal manufacturing processes, it is impossible to eliminate them. Such impurities are known to any person skilled in the ordinary steel manufacturing field, and thus all of their details are not particularly mentioned in this specification.

[0035] The steel according to one aspect of the present invention can further contain antimony (Sb): 0.03% or less.

[0036] Antimony (Sb): 0.03% or less Antimony (Sb) is distributed at the grain boundaries and suppresses surface enrichment of oxides by delaying the diffusion of oxidizing elements such as Mn, Si, and Al through the grain boundaries. It also has an excellent effect of suppressing coarsening of the surface concentrate due to temperature rise and changes in the hot rolling process. However, when the content of antimony (Sb) exceeds 0.03%, not only does the above-mentioned effect saturate, but it also leads to an increase in manufacturing costs and a problem of deteriorated workability. Therefore, the above-mentioned antimony (Sb) can be added at 0.03% or less and may not be added in some cases. Since antimony (Sb) may segregate at the grain boundaries and adversely affect the strength of the welded part, it may not be added when it is necessary to apply it to members that require particularly high welding characteristics compared to other materials.

[0037] The steel according to one aspect of the present invention can have a total content of chromium (Cr) and molybdenum (Mo) of 0.01% or less.

[0038] Chromium (Cr) and molybdenum (Mo): 0.01% or less Chromium (Cr) and molybdenum (Mo) are typical elements that can improve hardenability. However, in the present invention, the balance of strength, elongation, and hole expansion property is important, and since it is a steel material of 980 MPa grade in tensile strength, it is not necessary to add them to improve hardenability and facilitate the formation of martensite. When adding these elements, there is a problem of increasing the cost of alloy iron. Therefore, in the present invention, it is preferable not to add chromium (Cr) and molybdenum (Mo) for the above-mentioned special purposes. Thus, in the present invention, the total of these contents can be limited to 0.01% or less. On the other hand, considering the case where it is inevitably contained during the manufacturing process, the lower limit can be 0.003%.

[0039] Hereinafter, the steel microstructure of the present invention will be described in detail.

[0040] Unless otherwise specified in the present invention, % representing the fraction of the microstructure is based on the area.

[0041] The steel according to one aspect of the present invention can contain 5 to 50 area% of ferrite, a total of 35 to 80 area% of tempered martensite and bainite, 7 to 15 area% of retained austenite, and 10 area% or less of fresh martensite as the microstructure.

[0042] In the present invention, ferrite can be contained in an amount of 5% or more for improving the elongation rate, and tempered martensite and bainite can be contained in a total amount of 35% or more in order to ensure both a strength of 600 MPa or more in yield strength and an expandability of 20% or more. On the other hand, when the ferrite exceeds 50%, it becomes difficult to ensure the strength and expandability targeted in the present invention, and when the total fraction of tempered martensite and bainite exceeds 80%, the elongation rate may be insufficient. In the present invention, a more preferable fraction of ferrite can be 10% or more. A more preferable total fraction of tempered martensite and bainite can be 75% or less, and more preferably 70% or less.

[0043] By ensuring the fractions of the above-mentioned tempered martensite and bainite, it is possible to finally contain 7 to 15% of stable retained austenite at room temperature. At this time, when the fraction of retained austenite is less than 7%, it may be difficult to ensure the elongation rate at the level targeted in the present invention. On the other hand, when the fraction exceeds 15%, the stability of the retained austenite is insufficient, and this may also make it difficult to ensure the targeted elongation rate.

[0044] Also, in the present invention, fresh martensite can be contained in an amount of 10% or less for a steel material having excellent properties in terms of strength, elongation rate, and expandability.

[0045] The steel according to one aspect of the present invention can have an A value defined by the following relational expression 1 of 40 or less and a B value of 12 or more.

[0046] In the present invention, in addition to limiting the fraction of each microstructure, in order to more precisely control the relationship based on the fraction of the microstructure, the following relational expression 1 is proposed based on the EBSD Image Quality (IQ) value. By limiting the phase fraction ratio according to the IQ value, it may be more advantageous to ensure the target microstructure and physical properties in the present invention. In the present invention, as shown in the following relational expression 1, in an IQ chart using 35 bar graphs, the ratio of the phase fraction value of 40,000 or less and 40,000 to 70,000 and the maximum phase fraction value of 40,000 or less and 40,000 to 70,000 can represent the relative ratio of the fraction of the microstructure.

[0047] When the A value defined by the following relational expression 1 exceeds 40, it means that the fraction of the final fresh martensite is large with respect to the fraction of bainite and tempered martensite which are the core microstructures of the present invention. In this case, there may be problems such as a decrease in the final yield strength, an increase in the tensile strength, and deterioration of the hole expansion property. More preferably, it can be 38 or less, and still more preferably, it can be 35 or less. Further, a more preferable lower limit can be 1. Also, when the B value is less than 12, even if the A value is good, the structure of the final fresh martensite is strongly formed, and in this case, there may also be the problems mentioned above. More preferably, it can be 13 or more, and still more preferably, it can be 15 or more. Also, a more preferable upper limit can be 50. [Relational Expression 1] A = ([U] / [X]) * 100 B = ([Z] - [Y]) * 1000 (In the formula, [U]: phase fraction value of IQ value 40,000 or less, [X]: phase fraction value of IQ value 40,000 to 70,000, [Y]: maximum phase fraction value of IQ value 40,000 or less, [Z]: maximum phase fraction value of IQ value 40,000 to 70,000, and it is based on the Image Quality (IQ) Chart (35 bar graphs) obtained by EBSD measurement.)

[0048] Hereinafter, the steel manufacturing method of the present invention will be described in detail.

[0049] The steel according to one aspect of the present invention can be manufactured by heat-treating, primary cooling, secondary cooling, reheating, and overaging a cold-rolled steel sheet that satisfies the above-described alloy composition. The cold-rolled steel sheet can be manufactured by reheating a steel slab, hot rolling, coiling, and cold rolling.

[0050] Reheating The slab that satisfies the alloy composition of the present invention can be reheated in the temperature range of 1100 to 1300°C.

[0051] The reheating process can be performed to smoothly carry out the subsequent hot rolling process and sufficiently obtain the physical properties of the target steel sheet. If the reheating temperature is less than 1100°C, there may be a problem that the hot rolling load rapidly increases. On the other hand, if the temperature exceeds 1300°C, the surface scale amount may excessively increase, and the material yield may decrease.

[0052] Hot rolling The hot-rolled steel sheet can be obtained by hot rolling the reheated slab to a finish hot rolling temperature of Ar3 or higher.

[0053] When the finish hot rolling temperature is less than Ar3 (the temperature at which austenite starts to transform into ferrite during cooling), rolling in the two-phase region of ferrite + austenite or the ferrite region may be performed, and a mixed grain structure may be formed, and there may be a risk of malfunction due to fluctuations in the hot rolling load.

[0054] Cooling and coiling The hot-rolled steel sheet can be cooled and coiled at a temperature of 700°C or lower.

[0055] If the coiling temperature exceeds 700°C, an excessive oxide film may be generated on the steel sheet surface, which may cause defects. Although the lower the coiling temperature, the higher the strength of the hot-rolled steel sheet and the higher the rolling load in the subsequent cold rolling process may be, since it is not a factor that makes actual production impossible, the lower limit of the coiling temperature is not restricted in the present invention. In the present invention, the oxide layer formed on the surface of the steel sheet after coiling can be removed in the pickling process.

[0056] Cold rolling The wound steel sheet can be cold-rolled with a cold rolling reduction rate of 30% to 80%.

[0057] When the cold rolling reduction rate is less than 30%, it is not only difficult to ensure the target steel sheet thickness, but also the remaining grains of the hot rolling may affect the austenite formation and final physical properties during the annealing heat treatment. On the other hand, when the reduction rate exceeds 80%, non-uniformity of the reduction amount rolled in the length and width directions due to work hardening generated during cold rolling may cause material variation in the final steel sheet, and it may be difficult to ensure the target thickness due to the rolling load.

[0058] Annealing heat treatment The cold-rolled steel sheet can be subjected to an annealing heat treatment by heating it in a two-phase region temperature range of 780 °C to Ac3 and holding it for 30 seconds or more.

[0059] During the heat treatment, when heating in the temperature range of 780 °C or higher and Ac3 or lower, the elongation rate targeted in the present invention can be ensured, and a part of annealing ferrite can be formed in addition to retained austenite. A more preferable temperature range can be 800 °C or higher, and more preferably 880 °C or lower. [Formula] Ac3 = 910 - 203√([C]) - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W] (In the formula, [C], [Ni], [Si], [V], [Mo] and [W] are the weight percentages of each element.)

[0060] Primary cooling The heat-treated steel sheet can be primarily cooled at an average cooling rate of 1 to 10 °C / s to a temperature range of 630 to 750 °C.

[0061] During the first cooling, when the average cooling rate is less than 1 °C / s, it may be difficult to ensure the target strength by ferrite formation during cooling. On the other hand, when the cooling rate exceeds 10 °C / s, during the secondary cooling, the average cooling rate may decrease, making it difficult to ensure a sufficient fraction of martensite. This leads to a decrease in the fraction of tempered martensite, and it may become difficult to ensure both the strength and hole expansion properties targeted in the present invention. The lower limit of the more preferable average cooling rate can be 2 °C / s, and the upper limit of the more preferable average cooling rate can be 5 °C / s.

[0062] During the first cooling, when the cooling temperature is less than 630 °C, phases such as ferrite and bainite may be formed, potentially reducing the strength. On the other hand, when the temperature exceeds 750 °C, there may be problems in the actual production line.

[0063] Secondary cooling The above-mentioned steel plate cooled in the first stage can be secondarily cooled at an average cooling rate of 30 - 80 °C / s to a temperature range of 180 °C to Ms.

[0064] In order to ensure the physical properties targeted in the present invention, a structure of tempered martensite must be ensured. To ensure a structure of tempered martensite at the targeted level, it is necessary to cool in the temperature range between the martensite transformation start temperature (Ms) and the martensite transformation end temperature (Mf) during the secondary cooling after the first cooling. In the present invention, it is preferably cooled in the temperature range of 180 °C to Ms.

[0065] When the secondary cooling temperature is less than 180 °C, the fraction of tempered martensite may become excessively large, ultimately reducing the fraction of retained austenite and deteriorating the elongation rate. On the other hand, when the temperature exceeds Ms, it is difficult to form a structure of tempered martensite, and it may be difficult to ensure the targeted strength and hole expansion properties.

[0066] During the secondary cooling, if the average cooling rate is less than 30 °C / s, some bainite structure can be formed from the primary cooling section to the secondary cooling. On the other hand, if it exceeds 80 °C / s, the surface shape of the steel plate may deteriorate due to the rapid martensite transformation rate at the secondary cooling time, and there may be a problem of material variation in the width direction. A more preferable upper limit can be 60 °C / s. [Formula] Ms = 539 - 423[C] - 30.4[Mn] - 7.5[Si] + 30[Al] - 12.1[Cr] - 17.7[Ni] - 7.5[Mo] (In the formula, [C], [Mn], [Si], [Al], [Cr], [Ni] and [Mo] are the weight percentages of each element.)

[0067] Reheating and overaging Reheating and overaging can be performed by heating the above secondary-cooled steel plate to a temperature range of Ms - 50 to 450 °C and holding it for 1 to 30 minutes.

[0068] In the present invention, it is intended to improve the toughness by changing the hard martensite with a high potential density formed during the secondary cooling into tempered martensite by reheating and overaging. Furthermore, by ensuring a sufficient amount of tempered martensite and bainite transformation, C concentration (Partitioning) occurs in the retained austenite remaining from the annealing. The martensite transformation start temperature (Ms) of the austenite in which C is concentrated in this process becomes lower than room temperature, and finally a large amount of retained austenite is formed, so that the physical properties aimed at in the present invention can be ensured. In the present invention, in order to sufficiently ensure such a Partitioning effect, the reheating temperature can be limited to Ms - 50 to 450 °C. If the above temperature range is not satisfied, it may be difficult to ensure the fractions of the intended tempered martensite, bainite, and retained austenite, and it may be difficult to ensure the intended physical properties.

[0069] If the heating and holding time is less than 1 minute, sufficient transformation cannot proceed, and it is difficult to obtain the above-mentioned desired Partitioning effect. If the time exceeds 30 minutes, it is necessary to make the reheating and the overaging temperature range very long, which takes a lot of time and reduces productivity, so it is difficult to apply to an actual production line.

[0070] The steel of the present invention manufactured in this way has a tensile strength of 980 MPa or more, and the R value defined by the following relational expression 2 can be 30,000 to 60,000 MPa·%.

[0071] The following relational expression 2 is an expression showing the relationship among the yield strength, the total elongation rate, and the hole expansion property. When the R value defined by relational expression 2 is 30,000 to 60,000 MPa·%, both the elongation rate and the hole expansion property can be ensured for the ultra-high strength steel with a tensile strength of 980 MPa or more, which is the object of the present invention. When the R value deviates from the target range, the strength level targeted in the present invention is different, or one of the elongation rate or the hole expansion property deteriorates, making it difficult to use for the member for absorbing collision energy targeted in the present invention. More preferably, the yield strength can be 600 MPa or more, the elongation rate can be 21% or more, and the hole expansion property (HER) can be 20% or more. [Relational Expression 2] R = [YS] × ([Total-El] + (2 * [HER])) (In the formula, [YS] is the yield strength (MPa), [Total-El] is the total elongation rate (%), and [HER] is the hole expansion property (%)).)

Example

[0072] Hereinafter, the present invention will be described more specifically by way of examples. However, it should be noted that the following examples are for illustrating and explaining the present invention in more detail, and are not for limiting the scope of the rights of the present invention.

[0073] (Example) Test pieces Nos. 1 to 13 in Table 1 below were manufactured using steel slabs having a composition of 0.2 to 0.24 [C] - 1.2 to 1.8 [Si] - 2.0 to 2.4 [Mn] - 0.01 to 0.05 [Al] (each element is in wt%, and the balance is Fe and other inevitable impurities). On the other hand, test piece 14 has a composition of 0.2 to 0.24 [C] - 1.2 to 1.8 [Si] - 2.0 to 2.4 [Mn] - 0.01 to 0.05 [Al] - 0.1 [Mo], which is a case where the Mo content is outside the scope of the present invention, and test piece 15 has a composition of 0.2 to 0.24 [C] - 1.2 to 1.8 [Si] - 1.0 to 1.4 [Mn] - 0.01 to 0.05 [Al], which is a case where the Mn content is outside the scope of the present invention. Test pieces were manufactured using steel slabs deviating from the composition within the range proposed in the present invention.

[0074] As the manufacturing method, steel plates were manufactured by reheating, hot rolling, coiling, cold rolling, heat treatment, primary cooling, secondary cooling, reheating, and overaging under the conditions shown in Table 1 below. At this time, reheating was carried out at 1100 to 1300 °C, and finish hot rolling was carried out in the temperature range of 850 to 950 °C. Also, coiling was carried out in the temperature range of 200 to 700 °C, and cold rolling was carried out with a cold reduction rate of 45 to 65%. During annealing, heat treatment was carried out for 100 to 300 seconds, and an overaging time of 1 to 30 minutes was applied. The Ac3 temperature of test pieces 1 to 13 in Table 1 below is 882 °C, the Ms temperature is 367 °C, the Ac3 temperature of test piece 14 is 885 °C, the Ms temperature is 366 °C, the Ac3 temperature of test piece 15 is 882 °C, and the Ms temperature is 399 °C.

[0075]

Table 1

[0076] Table 2 below shows the microstructure and physical properties of the produced steel sheets. The microstructure was measured at the 1 / 4 thickness point of the steel sheet using XRD (X-ray Diffraction) and EBSD (Electron Backscattered Diffraction) to show the phase fraction. The phase fractions of ferrite (F), bainite (B), and the total of tempered martensite (TM), fresh martensite (FM), and austenite (γ) were calculated using an IQ chart via EBSD. Furthermore, the yield strength (YS), tensile strength (TS), total elongation rate (Total-EL), and uniform elongation rate (uEL) were measured by performing a tensile test on the steel sheet processed according to the JIS standard (gauge length width × length: 25 × 50 mm, total length of the test piece: 200 - 260 mm) under the condition of a test speed of 28 mm / min. The hole expansion property (HER) was measured according to the ISO 16330 standard, and the hole was sheared using a 10 mm diameter punch with a 12% clearance. Also, the A value and B value of relational expression 1 were calculated and shown from the measured physical property values.

[0077]

Table 2

[0078]

Table 3

[0079] As shown in Table 2, in the case of Invention Examples 1 to 3 that satisfy the alloy composition and production conditions of the present invention, the characteristics of the microstructure proposed by the present invention were satisfied, and the physical properties intended by the present invention were ensured.

[0080] FIG. 1 shows the (a) EBSD IQ chart and (b) EBSD IQ + Phase map of Invention Example 2 of the present invention. (a) shows the fraction values according to the IQ value in a chart, and the boundary values of each phase for defining Relational Expression 1 can be confirmed. In the case of (b), it shows the fraction of retained austenite, and the fraction level of retained austenite in the steel of the present invention can be confirmed.

[0081] On the other hand, Comparative Example 1 is a case where the annealing conditions of the present invention were not satisfied, and the fraction of the microstructure and Relational Expressions 2 and 3 were not satisfied. In particular, the fraction of ferrite was excessive and the strength deteriorated.

[0082] Comparative Examples 2 and 6 are cases where the primary cooling temperature proposed by the present invention was not satisfied. Due to the formation of a soft phase during the primary cooling, the transformation of tempered martensite and bainite did not proceed smoothly during the secondary cooling and overaging processes, and the fraction of the final fresh martensite increased, resulting in deterioration of the yield strength and hole expansion property.

[0083] Comparative Example 3 did not satisfy not only the primary cooling conditions but also the secondary cooling conditions. Only the bainite transformation was promoted without the tempered martensite transformation. Due to the decrease in the overall transformation fraction, the fraction of the final fresh martensite increased and the fraction of retained austenite decreased. As a result, the intended level of physical properties could not be ensured.

[0084] Comparative Examples 4 and 5 are cases where the secondary cooling conditions of the present invention were not satisfied. In the case of Comparative Example 4, the cooling end temperature was excessively high and the average cooling rate was insufficient. As a result, only bainite transformation occurred without tempered martensite, and the final fresh martensite increased. Consequently, the desired physical properties could not be ensured. In the case of Comparative Example 5, the cooling end temperature was excessively low, the fraction of tempered martensite was excessive, almost no final retained austenite was ensured, and the elongation rate deteriorated significantly.

[0085] Comparative Examples 7 and 8 are cases where the reheat and overaging conditions were not satisfied. During overaging, bainite transformation did not proceed smoothly, and the fraction of fresh martensite increased. As a result, the fraction of retained austenite decreased, and the final desired physical properties could not be ensured.

[0086] Comparative Example 9 is a case where the cooling conditions during primary cooling do not satisfy the conditions of the present invention, and equipment problems such as equipment life may occur. Furthermore, during cooling, there was almost no transformation of ferrite and bainite, and the fraction of final retained austenite was insufficient due to tempered martensite and bainite transformation during subsequent reheat and overaging, and the desired physical properties could not be ensured.

[0087] Comparative Example 10 is a case where the secondary average cooling rate is insufficient. During cooling, sufficient transformation of martensite and bainite did not proceed, and the fraction of fresh martensite in the final microstructure increased. As a result, the desired level of physical properties could not be ensured.

[0088] Comparative Example 11 is a case where the total content of Cr and Mo exceeded the content proposed in the present invention. By adding hardening elements as described above, the soft phases (ferrite, bainite, etc.) during annealing transformed slowly, and tempered martensite was formed as the main constituent phase. As a result, although the strength was excellent overall due to the lack of soft phase, the elongation rate was insufficient.

[0089] Comparative Example 12 is a case where the Mn content is insufficient, and the strength deteriorates, and the physical properties at the target level cannot be ensured.

[0090] As described above, the present invention has been described in detail through the examples, but examples in different forms are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. by weight, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.1%, the balance being iron (Fe) and other inevitable impurities, a steel sheet containing, as a microstructure, 5 to 50 area% of ferrite, a total of 35 to 80 area% of tempered martensite and bainite, 7 to 15 area% of retained austenite, and 10 area% or less of fresh martensite.

2. The steel sheet according to claim 1, further containing antimony (Sb): 0.03% or less.

3. The steel sheet according to claim 1, wherein the total content of chromium (Cr) and molybdenum (Mo) is 0.01% or less.

4. The steel sheet according to claim 1, wherein the A value defined by the following relational expression 1 is 40 or less and the B value is 12 or more. [Relational Expression 1] A = ([U] / [X]) * 100 B = ([Z] - [Y]) * 1000 (In the formula, [U]: the phase fraction value with an IQ value of 40,000 or less, [X]: the phase fraction value with an IQ value of 40,000 to 70,000, [Y]: the maximum phase fraction value with an IQ value of 40,000 or less, [Z]: the maximum phase fraction value with an IQ value of 40,000 to 70,000, which is based on the Image Quality (IQ) Chart (35 bar graphs) obtained by EBSD measurement.)

5. The steel sheet according to claim 1, having a tensile strength of 980 MPa or more and an R value defined by the following relational expression 2 of 30,000 to 60,000 MPa·%. [Relational Expression 2] R = [YS] × ([Total-El] + (2 * [HER])) (In the formula, [YS] is the yield strength (MPa), [Total-El] is the total elongation rate (%), and [HER] is the hole expansion property (%).)

6. The steel sheet according to claim 5, having a yield strength of 600 MPa or more, an elongation rate of 21% or more, and a hole expansion property (HER) of 20% or more.

7. preparing a cold-rolled steel sheet containing, by weight, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less, sulfur (S): 0.03% or less, aluminum (Al): 0.01 to 0.1%, the balance being iron (Fe) and other inevitable impurities; Annealing heat treatment of heating the cold-rolled steel sheet in a temperature range of 780 °C to Ac3 and holding for 30 seconds or more; Primary cooling the annealed heat-treated steel sheet at an average cooling rate of 1 to 10 °C / s to a temperature range of 630 to 750 °C; Secondary cooling the primary-cooled steel sheet at an average cooling rate of 30 to 80 °C / s to a temperature range of 180 °C to Ms; and A method for manufacturing a steel sheet, including a reheating and overaging step of heating the secondary-cooled steel sheet to a temperature range of Ms - 50 to 450 °C and holding for 1 to 30 minutes. [Formula] Ac3 = 910 - 203√([C]) - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W] (In the formula, [C], [Ni], [Si], [V], [Mo], and [W] are the weight percentages of each element.) Ms = 539 - 423[C] - 30.4[Mn] - 7.5[Si] + 30[Al] - 12.1[Cr] - 17.7[Ni] - 7.5[Mo] (In the formula, [C], [Mn], [Si], [Al], [Cr], [Ni], and [Mo] are the weight percentages of each element.)

8. The method for manufacturing a steel sheet according to claim 7, wherein the cold-rolled steel sheet further contains antimony (Sb): 0.03% or less.

9. The method for manufacturing a steel sheet according to claim 7, wherein the total content of chromium (Cr) and molybdenum (Mo) in the cold-rolled steel sheet is 0.01% or less.

10. The step of preparing the cold-rolled steel sheet includes: Reheating the steel slab in a temperature range of 1100 to 1300 °C; Obtaining a hot-rolled steel sheet by hot-rolling the reheated slab at a finish hot-rolling temperature of Ar3 or higher; Cooling and coiling the hot-rolled steel sheet at a temperature of 700 °C or lower; and The method for manufacturing a steel sheet according to claim 7, including a step of cold-rolling the coiled hot-rolled steel sheet at a cold rolling reduction rate of 30 to 80%.

11. During the primary cooling, the average cooling rate is 2 to 5 °C / s, During the secondary cooling, the average cooling rate is 30 to 60 °C / s, the method for manufacturing a steel sheet according to claim 7.

Images