Ultrahigh-strength steel sheet having excellent elongation and hole expandability, and method for producing 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 having excellent strength while having excellent elongation and hole expandability, and a method for manufacturing the same.

Description

Technical Field

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

[0002] In recent years, in the automotive field, developed countries led by Europe have been actively conducting research to reduce the weight of the vehicle body in order to meet the lightweight requirements of automobile companies. In the case of steel parts, efforts are being made to achieve higher strength and further reduce the thickness of steel plates at the same level compared to competing materials (Mg, Al, CFRP, etc.). In addition to lightweighting, due to CO 2 The strengthening of emission control and the rapid transition to the era of electric vehicles have led to the strengthening of safety control for car passengers and pedestrians, which has led to a trend of requiring stability and high strength of body materials. In particular, the demand for high-strength steels above 980-1180MPa is increasing. In the case of 980MPa grade steels, in order to be used as collision energy absorbing parts, not only high elongation is required for forming into complex shapes, but also excellent hole expansion is required so that they should not break due to axial deformation. Such structural parts need to have high yield strength and high hole expansion to facilitate the absorption of impact energy.

[0003] A representative manufacturing method for improving yield strength is a method using water cooling during continuous annealing. As a representative prior art of this method, Patent Document 1 relates to a technology for manufacturing a steel material having a volume fraction of martensite of 80-97% and a balance of ferrite by continuously annealing a steel material having a carbon content of 0.18-0.3%, then water cooling it to room temperature, and then performing an aging treatment at a temperature of 120-300°C for 1-15 minutes. Ultra-high strength steel can be manufactured by rapidly cooling a cold-rolled steel sheet to room temperature after dual-phase annealing or single-phase annealing, and then using a tempering method. However, in the case of this manufacturing, although the yield strength and hole expansion are excellent, the shape quality of the coil deteriorates due to temperature deviations in the width direction and the length direction, and problems such as poor material quality and reduced operability may occur depending on the part when processing roll-formed parts.

[0004] In addition, generally, as the strength of the steel sheet increases, the elongation decreases, so the problem of reduced formability occurs, and therefore its application as a material for cold stamping is limited. In order to form a steel material into a complex shape, it is basically necessary to have a high elongation, but as a representative method for improving the elongation, as in Patent Document 2, in addition to the retained austenite, when a large amount of ferrite is introduced to ensure additional elongation, the yield strength and hole expansion may be poor.

[0005] Therefore, in order to solve the above problems, it is necessary to develop an ultra-high strength cold-rolled steel sheet having excellent elongation and hole expandability and a tensile strength of 980 MPa or more.

[0006] [Prior art literature]

[0007] (Patent Document 1) Japanese Patent Publication No. 1992-289120 (published on October 14, 1992)

[0008] (Patent Document 2) Japanese Patent Publication No. 2004-211157 (published on July 29, 2004) Summary of the invention

[0009] 1. Technical issues to be resolved

[0010] An object according to one aspect of the present invention is to provide an ultra-high strength steel sheet having excellent elongation and hole expandability and a method for manufacturing the same.

[0011] The technical problems of the present invention are not limited to the above contents. A person skilled in the art can easily understand the additional technical problems of the present invention from the overall content of this specification.

[0012] (II) Technical solution

[0013] One aspect of the present invention can provide a steel plate, which contains, by weight%, carbon (C): 0.15-0.25%, manganese (Mn): 1.5-2.5%, silicon (Si): 1.0-2.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.1%, the remainder of iron (Fe) and other inevitable impurities, and the steel plate contains as a fine structure: 5-50 area% of ferrite, 35-80 area% of tempered martensite and bainite in total, 7-15 area% of retained austenite, and less than 10 area% of fresh martensite.

[0014] The steel sheet may further include antimony (Sb): 0.03% or less.

[0015] The total content of chromium (Cr) and molybdenum (Mo) in the steel sheet may be 0.01% or less.

[0016] The steel plate may have an A value of 40 or less and a B value of 12 or more, as defined in the following Relationship 1.

[0017] [Equation 1]

[0018] A=([U] / [X]) 100

[0019] B=([Z]-[Y]) 1000

[0020] (Wherein, [U]: phase fraction value with an IQ value of 40,000 or less, [X]: phase fraction value with an IQ value of 40,000 to 70,000, [Y]: maximum phase fraction value with an IQ value of 40,000 or less, [Z]: maximum phase fraction value with an IQ value of 40,000 to 70,000, and based on the Image Quality (IQ) Chart (35 bars) obtained by EBSD measurement.)

[0021] The tensile strength of the steel plate may be greater than 980 MPa, and the R value defined in the following equation 2 may be 30,000-60,000 MPa·%.

[0022] [Equation 2]

[0023] R = [YS] × ( [Total - El] + (2 [HER]))

[0024] (In the formula, [YS] is the yield strength (MPa), [Total-El] is the total elongation (%), and [HER] is the hole expandability (%).)

[0025] The steel plate may have a yield strength of 600 MPa or more, an elongation of 21% or more, and a hole expandability (HER) of 20% or more.

[0026] Another aspect of the present invention can provide a method for manufacturing a steel plate, the manufacturing method comprising the following steps: preparing a cold-rolled steel plate, the cold-rolled steel plate comprising, by weight%, carbon (C): 0.15-0.25%, manganese (Mn): 1.5-2.5%, silicon (Si): 1.0-2.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.1%, the remainder of iron (Fe) and other inevitable impurities; performing annealing heat treatment to heat the cold-rolled steel plate. The steel plate is heated to a temperature range of 780°C to Ac3 and maintained for more than 30 seconds; the steel plate subjected to the annealing heat treatment is cooled to a temperature range of 630-750°C at an average cooling rate of 1-10°C / second; the steel plate subjected to the primary cooling is cooled to a temperature range of 180°C to Ms at an average cooling rate of 30-80°C / second; and the steel plate subjected to the secondary cooling is heated to a temperature range of Ms-50°C to 450°C and maintained for 1-30 minutes by reheating and overaging.

[0027] [Mode]

[0028] Ac3=910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0029] (In the formula, [C], [Ni], [Si], [V], [Mo], and [W] are the weight % of each element.)

[0030] Ms=539-423[C]-30.4[Mn]-7.5[Si]+30[Al]-12.1[Cr]-17.7[Ni]-7.5[Mo]

[0031] (In the formula, [C], [Mn], [Si], [Al], [Cr], [Ni], and [Mo] are the weight % of each element.)

[0032] The cold-rolled steel sheet may further include antimony (Sb): 0.03% or less.

[0033] The total content of chromium (Cr) and molybdenum (Mo) in the cold-rolled steel sheet may be 0.01% or less.

[0034] The step of preparing the cold-rolled steel sheet may include the following steps: reheating the steel slab in a temperature range of 1100-1300°C; hot rolling the reheated slab at a hot finishing rolling temperature above Ar3 to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to a temperature below 700°C and coiling; and cold rolling the coiled hot-rolled steel sheet at a cold rolling reduction rate of 30-80%.

[0035] During the primary cooling, the average cooling rate may be 2-5°C / second, and during the secondary cooling, the average cooling rate may be 30-60°C / second.

[0036] (III) Beneficial effects

[0037] According to one aspect of the present invention, a steel sheet having excellent elongation and hole expandability and excellent strength, and a method for manufacturing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 (a) EBSD IQ map and (b) EBSD IQ+ phase map (Phase map) of Inventive Example 2 are shown. Best Mode for Carrying Out the Invention

[0039] The preferred specific embodiments of the present invention are described below. The specific embodiments of the present invention can be modified into various forms, and it should not be interpreted that the scope of the present invention is limited to the specific embodiments described below. This specific embodiment is provided to explain the present invention in more detail to those skilled in the art.

[0040] In the present invention, it has been confirmed that elongation, hole expandability and strength can be simultaneously ensured by controlling the content range of the component elements such as C, Mn, Si, P, S, etc. as the alloy composition and controlling the conditions of the manufacturing process, especially the conditions of the heat treatment, primary cooling, secondary cooling, reheating and overaging steps. As a result, the present invention has been completed.

[0041] Hereinafter, the present invention will be described in detail.

[0042] Hereinafter, the composition of the steel of the present invention will be described in detail.

[0043] Unless otherwise specified, % representing the content of each element in the present invention is based on weight.

[0044] The steel according to one aspect of the present invention may contain, by weight%, carbon (C): 0.15-0.25%, manganese (Mn): 1.5-2.5%, silicon (Si): 1.0-2.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.1%, and the balance of iron (Fe) and other inevitable impurities.

[0045] Carbon (C): 0.15-0.25%

[0046] Carbon (C) is an interstitial solid solution element, the most effective and important element for improving the strength of steel, and an element that must be added to ensure the strength of martensitic steel. In order to obtain an ultra-high strength steel that satisfies the yield strength ratio and tensile strength desired by the present invention, it is preferred that more than 0.15% carbon (C) be added, more preferably more than 0.18% carbon (C) be added, and more preferably more than 0.2% carbon (C) be added. However, when the content of the carbon (C) exceeds 0.25%, due to the increase in hardenability, too much martensite is formed during the cooling process, so the strength increases sharply, thereby the elongation may deteriorate. In addition, the increase in carbon (C) content has the problem of damaging weldability, so it is preferred that the upper limit of the carbon (C) content be limited to 0.25%. A more preferred upper limit can be 0.24%.

[0047] Manganese (Mn): 1.5-2.5%

[0048] Manganese (Mn) is an element added to ensure strength. When the content of manganese (Mn) is less than 1.5%, it may be difficult to ensure the strength of the level required by the present invention. The lower limit of the more preferred manganese (Mn) content may be 1.8%, and the lower limit of the more preferred manganese (Mn) content may be 2.0%. On the other hand, when the content of manganese (Mn) exceeds 2.5%, the Ms temperature decreases during cooling after annealing, so it may be difficult to smoothly ensure the initial martensitic structure. This means that due to the reduction of the tempered martensite fraction in the quenching and distribution (Quenching & Partitioning, Q & P) process, it may be difficult to simultaneously ensure the strength, elongation and hole expansion desired by the present invention. In addition, due to the segregation of manganese in the thickness direction, a manganese band (Mnband) is easily formed in the slab, so there may be a problem of increasing the occurrence of defects during the rolling process while generating continuous casting cracks. The upper limit of the more preferred manganese (Mn) content may be 2.4%.

[0049] Silicon (Si): 1.0-2.0%

[0050] Silicon (Si) is a core element of the transformation induced plasticity (TRIP) steel that plays a role in improving the fraction and elongation of retained austenite by hindering the precipitation of cementite. When the content of the silicon (Si) is less than 1.0%, the control of cementite precipitation in the reheating and overaging steps is not smooth, and the fraction of retained austenite finally formed is small, or the stability may be reduced, so the final elongation may deteriorate. The lower limit of the more preferred silicon (Si) content can be 1.2%. On the other hand, when the content of the silicon (Si) exceeds 2.0%, due to the formation of liquid metal embrittlement (Liquid Metal Embrittlement, LME) cracks, the physical properties of the weld deteriorate, so the surface characteristics and the coating properties of the steel deteriorate. The upper limit of the more preferred silicon (Si) content can be 1.8%.

[0051] Phosphorus (P): less than 0.1%

[0052] Phosphorus (P) is an impurity element contained in steel, and 0% is excluded in consideration of the fact that it is inevitably contained in the manufacturing process. In addition, when the content of phosphorus (P) exceeds 0.1%, weldability may deteriorate and brittleness of steel may be caused, so the upper limit of the phosphorus (P) content may be limited to 0.1%. More preferably, the upper limit of the phosphorus (P) content may be 0.03%.

[0053] Sulfur (S): 0.03% or less

[0054] Like P, sulfur (S) is an impurity inevitably contained in steel and is an element that impairs the ductility and weldability of the steel plate, so it is preferable to control the content of sulfur (S) as low as possible. Therefore, in the present invention, the content of sulfur (S) can be limited to 0.03% or less. More preferably, the content of sulfur (S) can be limited to 0.005% or less. In addition, considering the case where it is inevitably contained in the manufacturing process, 0% is excluded.

[0055] Aluminum (Al): 0.01-0.1%

[0056] Aluminum (Al) may be added to remove oxygen from molten steel, and like Si, aluminum (Al) is an effective element that inhibits the precipitation of cementite during the reheating and overaging steps and stabilizes the retained austenite. When the content of the aluminum (Al) is less than 0.01%, the deoxidation of the steel cannot be fully achieved, and the cleanliness of the steel may be impaired. On the other hand, when the content of the aluminum (Al) exceeds 0.1%, the castability of the slab may deteriorate, and the temperature required for single-phase region heating during annealing increases, so there may be production and equipment problems. More preferably, the content of the aluminum (Al) may be limited to 0.05% or less.

[0057] In the steel of the present invention, in addition to the above composition, the remaining amount of iron (Fe) and inevitable impurities may be contained. In the conventional manufacturing process, the inevitable impurities may be accidentally mixed, and therefore, these impurities cannot be eliminated. These impurities are well known to those skilled in the art of conventional steel manufacturing, and therefore, all of them are not particularly described in this specification.

[0058] The steel according to one aspect of the present invention may further contain antimony (Sb): 0.03% or less.

[0059] Antimony (Sb): 0.03% or less

[0060] Antimony (Sb) is distributed at the grain boundaries and suppresses the enrichment of oxides on the surface by delaying the grain boundary diffusion of oxidizing elements such as Mn, Si, and Al. In addition, antimony (Sb) has a significant effect on suppressing the coarsening of surface enrichments that change according to temperature rise and hot rolling process. However, when the content of antimony (Sb) exceeds 0.03%, the above effect is saturated, and leads to an increase in manufacturing costs, and there may be a problem of poor processability. Therefore, the amount of antimony (Sb) added can be less than 0.03%, and antimony (Sb) may not be added depending on the situation. Antimony (Sb) segregates at the grain boundaries and may have an adverse effect on the strength of the weld, so when it is necessary to apply it to parts that require welding properties more importantly than other materials, antimony (Sb) may not be added.

[0061] The steel according to one aspect of the present invention may have a total content of chromium (Cr) and molybdenum (Mo) of 0.01% or less.

[0062] Chromium (Cr) and molybdenum (Mo): less than 0.01%

[0063] Chromium (Cr) and molybdenum (Mo) are representative elements that can improve hardenability, but in the present invention, the balance of strength, elongation, and hole expansion is very important, and because the tensile strength is 980MPa grade steel, it is not necessary to add chromium (Cr) and molybdenum (Mo) for the purpose of improving hardenability so as to form martensite. When adding these elements, there is the problem of increased alloy iron cost, therefore, in the present invention, for the above-mentioned special purpose, preferably do not add chromium (Cr) and molybdenum (Mo). Therefore, in the present invention, the sum of the content of chromium (Cr) and molybdenum (Mo) can be limited to less than 0.01%. In addition, considering the situation that inevitably comprises chromium (Cr) and molybdenum (Mo) in the manufacturing process, the lower limit of chromium (Cr) and molybdenum (Mo) content can be 0.003%.

[0064] Hereinafter, the microstructure of the steel of the present invention will be described in detail.

[0065] Unless otherwise specified, the percentage representing the fraction of fine structures in the present invention is based on the area.

[0066] The microstructure of the steel according to one aspect of the present invention may include 5-50 area % of ferrite, 35-80 area % of tempered martensite and bainite in total, 7-15 area % of retained austenite, and 10 area % or less of fresh martensite.

[0067] In the present invention, in order to improve the elongation, ferrite of 5% or more may be included, and in order to simultaneously ensure a yield strength of 600 MPa or more and a hole expansion of 20% or more, the sum of tempered martensite and bainite may be 35% or more. On the other hand, when ferrite exceeds 50%, it may be difficult to ensure the strength and hole expansion desired by the present invention, and when the sum of the fractions of tempered martensite and bainite exceeds 80%, the elongation may be insufficient. In the present invention, a more preferred fraction of ferrite may be 10% or more. A more preferred sum of the fractions of tempered martensite and bainite may be 75% or less, and more preferably may be 70% or less.

[0068] By ensuring the fractions of tempered martensite and bainite, 7-15% of stable retained austenite can be finally contained at room temperature. At this time, when the fraction of retained austenite is less than 7%, it may be difficult to ensure the elongation at the desired level of the present invention. On the other hand, when the fraction of retained austenite exceeds 15%, the stability of the retained austenite is insufficient, and thus it may be difficult to ensure the desired elongation.

[0069] Furthermore, in the present invention, in order to obtain a steel material having excellent strength, elongation, and hole expandability characteristics at the same time, 10% or less of fresh martensite may be contained.

[0070] The steel according to one aspect of the present invention may have an A value of 40 or less and a B value of 12 or more, as defined in the following Relationship 1.

[0071] In the present invention, in addition to defining the fraction of each microstructure, the following relational formula 1 is proposed based on the EBSD image quality (IQ) value to more strictly control the relationship according to the fraction of the microstructure. By defining the phase fraction ratio according to the IQ value, it is more conducive to ensuring the microstructure and physical properties desired by the present invention. In the present invention, as shown in the following relational formula 1, in an IQ diagram using 35 bars, the ratio of the phase fraction values ​​below 40,000 and 40,000 to 70,000 and the maximum phase fraction values ​​below 40,000 and 40,000 to 70,000 can represent the relative proportion of the microstructure fraction.

[0072] When the A value defined in the following relational formula 1 exceeds 40, it means that the fraction of the final newly formed martensite is greater than the fraction of bainite and tempered martensite as the core structure of the present invention, which may cause the following problems, such as reduced final yield strength, increased tensile strength, poor hole expansion, etc. The A value may more preferably be 38 or less, and more preferably be 35 or less. In addition, the more preferred lower limit of the A value may be 1. In addition, when the B value is less than 12, even if the A value is good, the final newly formed martensite structure is strongly formed, which may lead to the above-mentioned problems. The B value may more preferably be 13 or more, and more preferably be 15 or more. In addition, the more preferred upper limit of the B value may be 50.

[0073] [Equation 1]

[0074] A=([U] / [X]) 100

[0075] B=([Z]-[Y]) 1000

[0076] (wherein, [U]: phase fraction value with an IQ value of 40,000 or less, [X]: phase fraction value with an IQ value of 40,000 to 70,000, [Y]: maximum phase fraction value with an IQ value of 40,000 or less, [Z]: maximum phase fraction value with an IQ value of 40,000 to 70,000, and based on an image quality (IQ) diagram (35 bars) obtained by EBSD measurement).

[0077] Hereinafter, the method for producing the steel of the present invention will be described in detail.

[0078] 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 satisfying the above alloy composition, and the cold-rolled steel sheet can be manufactured by reheating, hot rolling, coiling and cold rolling a steel billet.

[0079] Reheating

[0080] The slab satisfying the alloy composition of the present invention may be reheated within a temperature range of 1100-1300°C.

[0081] The reheating process may be performed to smoothly perform the subsequent hot rolling process and fully obtain the desired physical properties of the steel sheet. When the reheating temperature is lower than 1100°C, a problem of a sharp increase in hot rolling load may occur. On the other hand, when the reheating temperature exceeds 1300°C, the amount of surface scale increases excessively, and thus the yield of the material may decrease.

[0082] Hot Rolling

[0083] The reheated slab may be hot rolled at a hot finishing rolling temperature of Ar3 or higher to obtain a hot rolled steel sheet.

[0084] When the hot finishing rolling temperature is lower than Ar3 (the temperature at which austenite begins to transform into ferrite during cooling), a mixed crystal structure may be formed by rolling in a two-phase region of ferrite + austenite or a ferrite region, and there is a possibility of failure caused by changes in hot rolling load.

[0085] Cooling and winding

[0086] The hot rolled steel sheet may be cooled to a temperature of 700° C. or less and coiled.

[0087] When the coiling temperature exceeds 700°C, too much oxide film is generated on the surface of the steel sheet, which may cause defects. The lower the coiling temperature, the higher the strength of the hot-rolled steel sheet, so there may be a disadvantage that the rolling load of the cold rolling as a post-process becomes higher, but it is not a factor that makes it impossible to implement actual production, so the lower limit of the coiling temperature is not limited in the present invention. In the present invention, the oxide layer formed on the surface of the steel sheet after coiling can be removed by a pickling process.

[0088] Cold Rolling

[0089] The coiled steel sheet may be cold rolled at a cold rolling reduction rate of 30-80%.

[0090] When the cold rolling reduction ratio is less than 30%, it is difficult to ensure the desired steel plate thickness, and due to the residual hot rolled grains, the formation of austenite and the final physical properties may be affected during annealing heat treatment. In addition, when the reduction ratio exceeds 80%, the rolling reduction in the length and width directions is uneven due to the work hardening that occurs during cold rolling, and the material deviation of the final steel plate may occur, and it may be difficult to ensure the target thickness due to the rolling load.

[0091] Annealing heat treatment

[0092] The cold-rolled steel sheet may be heated to a two-phase region temperature range of 780° C. to Ac3 and maintained for more than 30 seconds to perform an annealing heat treatment.

[0093] During heat treatment, when heated to a temperature range of 780°C or higher and Ac3 or lower, the desired elongation of the present invention can be ensured, and in addition to the retained austenite, partial annealed ferrite can be formed. A more preferred temperature range may be 800°C or higher, and more preferably 880°C or lower.

[0094] [Mode]

[0095] Ac3=910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0096] (In the formula, [C], [Ni], [Si], [V], [Mo], and [W] are the weight % of each element.)

[0097] One cooling

[0098] The heat-treated steel plate may be cooled once at an average cooling rate of 1-10° C. / sec to a temperature range of 630-750° C.

[0099] During primary cooling, when the average cooling rate is less than 1°C / sec, ferrite is generated during cooling, so it may be difficult to ensure the desired strength. On the other hand, when the average cooling rate exceeds 10°C / sec, the average cooling rate during secondary cooling decreases, so it may be difficult to ensure a sufficient fraction of martensite. As a result, the fraction of tempered martensite decreases, and it may be difficult to simultaneously ensure the strength and hole expandability desired by the present invention. A more preferred lower limit of the average cooling rate may be 2°C / sec, and a more preferred upper limit of the average cooling rate may be 5°C / sec.

[0100] During primary cooling, when the cooling temperature is lower than 630° C., ferrite, bainite, and the like may be formed, thereby possibly reducing strength. On the other hand, when the cooling temperature exceeds 750° C., there may be problems in actual production lines.

[0101] Secondary Cooling

[0102] The steel plate that has been subjected to the primary cooling may be subjected to secondary cooling at an average cooling rate of 30-80° C. / sec to a temperature range of 180° C. to Ms.

[0103] In order to ensure the desired physical properties of the present invention, it is necessary to ensure the tempered martensite structure. In order to ensure the desired level of tempered martensite structure, it is necessary to cool to a temperature range between the martensite transformation start temperature (Ms) and the martensite transformation end temperature (Mf) during the secondary cooling after the primary cooling. In the present invention, it is preferably cooled to a temperature range of 180°C to Ms.

[0104] When the secondary cooling temperature is lower than 180°C, the fraction of tempered martensite increases excessively, and the fraction of retained austenite decreases, resulting in a possible deterioration in elongation. In addition, when the secondary cooling temperature exceeds Ms, it is difficult to form a tempered martensite structure, and thus it may be difficult to ensure the desired strength and hole expandability.

[0105] During secondary cooling, when the average cooling rate is less than 30°C / sec, a partial bainite structure can be formed during secondary cooling from the primary cooling interval. On the other hand, when the average cooling rate exceeds 80°C / sec, the surface shape of the steel plate may deteriorate due to the rapid martensitic transformation rate at the starting point of secondary cooling, and the problem of material deviation in the width direction may occur. A more preferred upper limit of the average cooling rate may be 60°C / sec.

[0106] [Mode]

[0107] Ms=539-423[C]-30.4[Mn]-7.5[Si]+30[Al]-12.1[Cr]-17.7[Ni]-7.5[Mo]

[0108] (In the formula, [C], [Mn], [Si], [Al], [Cr], [Ni], and [Mo] are the weight % of each element.)

[0109] Reheating and over-aging

[0110] The secondary cooled steel plate may be heated to a temperature range of Ms-50° C. to 450° C. and maintained for 1 to 30 minutes for reheating and overaging.

[0111] The present invention aims to improve toughness by reheating and overaging the hard martensite with high dislocation density formed during secondary cooling to transform it into tempered martensite. In addition, by ensuring a sufficient amount of tempered martensite and bainite transformation, C is enriched (partitioned) to the residual austenite remaining at the beginning of annealing. In this process, the martensitic transformation starting temperature (Ms) of the C-enriched austenite is reduced to below room temperature, and a large amount of residual austenite is finally formed, so that the desired physical properties of the present invention can be ensured. In the present invention, in order to fully ensure this distribution effect, the reheating temperature can be limited to Ms-50°C to 450°C. When the above temperature range cannot be met, it is difficult to ensure the desired tempered martensite, bainite, and residual austenite fractions, and it may be difficult to ensure the desired physical properties.

[0112] When the heating holding time is less than 1 minute, sufficient phase change cannot be carried out, so it is difficult to obtain the above-mentioned desired distribution effect. When the heating holding time exceeds 30 minutes, the reheating and over-aging temperature zones need to be very long and require a lot of time, thereby reducing productivity, so it may be difficult to apply to actual production lines.

[0113] The tensile strength of the steel of the present invention prepared as described above may be 980 MPa or more, and the R value defined in the following relational expression 2 may be 30000-60000 MPa·%.

[0114] The following equation 2 is an equation representing the relationship between yield strength, total elongation and hole expansion. When the R value defined in equation 2 is 30000-60000 MPa·%, the ultra-high strength steel with a tensile strength of 980 MPa or more desired by the present invention can simultaneously ensure elongation and hole expansion. When the R value is not within the desired range, the strength level desired by the present invention is different, or any one of the elongation or hole expansion properties deteriorates, making it difficult to use as the collision energy absorbing component desired by the present invention. More preferably, the yield strength can be 600 MPa or more, the elongation can be 21% or more, and the hole expansion (HER) can be 20% or more.

[0115] [Equation 2]

[0116] R = [YS] × ( [Total - El] + (2 [HER]))

[0117] (In the formula, [YS] is the yield strength (MPa), [Total-El] is the total elongation (%), and [HER] is the hole expandability (%).) DETAILED DESCRIPTION

[0118] The present invention is described in more detail below by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention in more detail and do not limit the scope of the present invention.

[0119] (Example)

[0120] For the test pieces No. 1 to 13 in Table 1 below, the test pieces were manufactured using a steel billet having a composition of 0.2-0.24[C]-1.2-1.8[Si]-2.0-2.4[Mn]-0.01-0.05[Al] (each element is weight %, and the balance is Fe and other inevitable impurities). In addition, the test piece 14 has a composition of 0.2-0.24[C]-1.2-1.8[Si]-2.0-2.4[Mn]-0.01-0.05[Al]-0.1[Mo], which is a case where the content of Mo is not within the range of the present invention, and the test piece 15 has a composition of 0.2-0.24[C]-1.2-1.8[Si]-1.0-1.4[Mn]-0.01-0.05[Al], which is a case where the content of Mn is not within the range of the present invention. The test pieces were manufactured using a steel billet having a composition outside the range proposed in the present invention.

[0121] As for the manufacturing method, the steel sheet is manufactured by reheating, hot rolling, coiling, cold rolling, heat treatment, primary cooling, secondary cooling, reheating and overaging under the conditions of Table 1 below. At this time, reheating is performed at 1100-1300°C, and hot finishing rolling is performed at a temperature range of 850-950°C. In addition, coiling is performed at a temperature range of 200-700°C, and cold rolling is performed at a cold rolling reduction rate of 45-65%. During annealing, heat treatment is performed for 100-300 seconds, and overaging time is applied for 1-30 minutes. 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, and the Ac3 temperature of test piece 15 is 882°C, and the Ms temperature is 399°C.

[0122] [Table 1]

[0123]

[0124] The microstructure and physical properties of the prepared steel plate are shown in Table 2 below. The microstructure is measured at the 1 / 4 thickness position of the steel plate using X-ray diffraction (XRD) and electron backscattered diffraction (EBSD) and the phase fraction is shown. The phase fractions of ferrite (F), the sum of bainite (B) and tempered martensite (TM), fresh martensite (FM) and austenite (γ) are measured using an IQ diagram by EBSD. In addition, the yield strength (YS), tensile strength (TS), total elongation (total-EL) and uniform elongation (uEL) are measured by processing the steel plate according to the JIS standard (gauge length width × length: 25 × 50 mm, total length of the test piece: 200-260 mm) and conducting a tensile test at a test speed of 28 mm / min. Hole expansion (HER) is measured according to ISO 16330, and the hole is sheared using a punch having a diameter of 10 mm with a clearance of 12%. In addition, the A value and the B value of relational expression 1 are calculated from the measured physical property values ​​and are shown.

[0125] [Table 2]

[0126]

[0127] [Equation 1]

[0128] A=([U] / [X]) 100

[0129] B=([Z]-[Y]) 1000

[0130] (wherein, [U]: phase fraction value with an IQ value of 40,000 or less, [X]: phase fraction value with an IQ value of 40,000 to 70,000, [Y]: maximum phase fraction value with an IQ value of 40,000 or less, [Z]: maximum phase fraction value with an IQ value of 40,000 to 70,000, and based on an image quality (IQ) diagram (35 bars) obtained by EBSD measurement).

[0131] [Table 3]

[0132]

[0133] [Equation 2]

[0134] R = [YS] × ( [Total - El] + (2 [HER]))

[0135] (In the formula, [YS] is the yield strength (MPa), [Total-El] is the total elongation (%), and [HER] is the hole expandability (%).)

[0136] As shown in Table 2, in the case of Invention Examples 1 to 3 that satisfy the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed by the present invention are satisfied, and the physical properties expected by the present invention are ensured.

[0137] Figure 1 (a) EBSD IQ diagram and (b) EBSD IQ+ phase diagram of Inventive Example 2 are shown. (a) is a diagram showing the fractional value according to the IQ value, and the boundary value of each phase used to define Relational Formula 1 can be confirmed. In the case of (b), the fraction of retained austenite is shown, and the level of retained austenite fraction of the steel of the present invention can be confirmed.

[0138] On the other hand, Comparative Example 1 is a case where the annealing conditions of the present invention are not satisfied, and the fine structure fraction and Relational Expressions 2 and 3 cannot be satisfied. In particular, the ferrite fraction is too high, so the strength is poor.

[0139] Comparative Examples 2 and 6 are cases where the primary cooling temperature proposed by the present invention is not met. Since a soft phase is formed during the primary cooling process, the tempered martensite and bainite phase transformation cannot be smoothly carried out during the secondary cooling and overaging process. Ultimately, the fraction of newly formed martensite increases, resulting in poor yield strength and hole expandability.

[0140] Comparative Example 3 not only fails to meet the primary cooling conditions, but also fails to meet the secondary cooling conditions, so that only bainite transformation is promoted without tempering martensite transformation. Due to the decrease in the overall transformation fraction, the fraction of newly formed martensite increases and the fraction of retained austenite decreases. As a result, the desired level of physical properties cannot be ensured.

[0141] Comparative Examples 4 and 5 are cases where the secondary cooling conditions of the present invention are not satisfied. In the case of Comparative Example 4, the cooling end temperature is too high and the average cooling rate is insufficient, so only bainite transformation occurs without tempered martensite, and the final newly formed martensite increases, resulting in failure to ensure the desired physical properties. In addition, in the case of Comparative Example 5, the cooling end temperature is too low, the fraction of tempered martensite is too high, and the final retained austenite is almost impossible to ensure, so the elongation is very poor.

[0142] Comparative Examples 7 and 8 do not meet the reheating and overaging conditions, and the bainite transformation cannot be smoothly carried out during the overaging process, and the fraction of new martensite increases. As a result, the fraction of retained austenite decreases, and the desired physical properties cannot be ensured.

[0143] Comparative Example 9 is a case where the cooling conditions during the primary cooling do not meet the conditions of the present invention, and equipment problems such as equipment life may occur. In addition, there is almost no ferrite phase transformation and bainite phase transformation during the cooling process, and due to the tempering martensite and the subsequent reheating and overaging process bainite phase transformation, the final residual austenite fraction is insufficient, and the desired physical properties cannot be ensured.

[0144] Comparative Example 10 is a case where the secondary average cooling rate is insufficient, and sufficient martensitic transformation and bainitic transformation cannot be performed during cooling, and the fraction of newly formed martensite in the final microstructure increases. As a result, the desired level of physical properties cannot be ensured.

[0145] Comparative Example 11 is a case where the sum of the contents of Cr and Mo exceeds the content proposed in the present invention. Due to the addition of the hardenability element as described above, the soft phase (ferrite, bainite, etc.) slowly transforms during annealing, thereby forming tempered martensite as the main constituent phase. As a result, due to the lack of soft phase, the overall strength is excellent but the elongation is insufficient.

[0146] Comparative Example 12 is a case where the Mn content is insufficient, and the strength is poor, so that the desired level of physical properties cannot be ensured.

[0147] The present invention has been described in detail above by using the embodiments, but it can also be implemented in other forms of embodiments. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

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

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

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

4. The steel plate according to claim 1, wherein: The steel plate has an A value of 40 or less and a B value of 12 or more, as defined in the following relational formula 1. [Equation 1] A=([U] / [X]) 100 B=([Z]-[Y]) 1000 Wherein, [U]: phase fraction value with an IQ value of 40,000 or less, [X]: phase fraction value with an IQ value of 40,000 to 70,000, [Y]: maximum phase fraction value with an IQ value of 40,000 or less, [Z]: maximum phase fraction value with an IQ value of 40,000 to 70,000, and based on an image quality (IQ) diagram obtained by EBSD measurement, the image quality (IQ) diagram being a 35-bar diagram.

5. The steel plate according to claim 1, wherein: The tensile strength of the steel plate is above 980 MPa, and the R value defined in the following equation 2 is 30000-60000 MPa·%. [Equation 2] R = [YS] × ( [Total - El] + (2 [HER])) Wherein, [YS] is the yield strength in MPa, [Total-El] is the total elongation in %, and [HER] is the hole expandability in %.

6. The steel plate according to claim 5, wherein: The steel plate has a yield strength of 600 MPa or more, an elongation of 21% or more, and a hole expandability (HER) of 20% or more.

7. A method for manufacturing a steel plate, comprising the following steps: A cold-rolled steel sheet is prepared, wherein the cold-rolled steel sheet contains, by weight%, carbon (C): 0.15-0.25%, manganese (Mn): 1.5-2.5%, silicon (Si): 1.0-2.0%, phosphorus (P): less than 0.1%, sulfur (S): less than 0.03%, aluminum (Al): 0.01-0.1%, and the balance of iron (Fe) and other inevitable impurities; Performing annealing heat treatment, heating the cold-rolled steel sheet to a temperature range of 780° C. to Ac3 and maintaining the temperature for more than 30 seconds; Cooling the annealed steel plate at an average cooling rate of 1-10°C / s to a temperature range of 630-750°C; The steel plate subjected to the primary cooling is subjected to secondary cooling at an average cooling rate of 30-80°C / second to a temperature range of 180°C to Ms; and Reheating and overaging, heating the steel plate after the secondary cooling to a temperature range of Ms-50°C to 450°C and maintaining it for 1-30 minutes, [Mode] 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 % 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 % of each element.

8. The method for manufacturing a steel plate 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 plate according to claim 7, wherein: The sum of the contents of chromium (Cr) and molybdenum (Mo) in the cold-rolled steel sheet is less than 0.01%.

10. The method for manufacturing a steel plate according to claim 7, wherein: The step of preparing the cold rolled steel sheet comprises the following steps: Reheating the steel billet at a temperature in the range of 1100-1300°C; Hot rolling the reheated slab at a hot finishing rolling temperature of Ar3 or higher to obtain a hot rolled steel sheet; Cooling the hot rolled steel sheet to a temperature below 700° C. and coiling the hot rolled steel sheet; and The coiled hot-rolled steel sheet is cold rolled at a cold rolling reduction rate of 30-80%.

11. The method for manufacturing a steel plate according to claim 7, wherein: During the primary cooling, the average cooling rate is 2-5°C / second, and during the secondary cooling, the average cooling rate is 30-60°C / second.

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