Cold-rolled steel sheet and method for manufacturing same

Abstract

The present invention relates to a cold-rolled steel sheet and a method for manufacturing same and, more specifically, to a high-strength cold-rolled steel sheet having excellent moldability and a method for manufacturing same.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having excellent formability and a method for manufacturing the same.

[0002] The automotive industry is currently focusing on lightweighting and ensuring crashworthiness to improve vehicle fuel efficiency and safety, along with regulations on greenhouse gas emissions due to global warming. Consequently, demand is growing for manufacturing technology for high-strength steel plates.

[0003] Among these, high-strength steel sheets for cold forming, which combine formability with high strength, offer superior economic efficiency by increasing productivity and are also advantageous in terms of final component safety. Therefore, automotive parts utilizing high-strength steel sheets require not only strength but also excellent usable properties, such as excellent elongation and light-mechanical elongation (LME) resistance, for forming and weldability.

[0004] In order to improve the formability of steel plates, a method of introducing retained austenite and utilizing the TRIP (TRansformation Induced Plasticity) phenomenon to increase elongation is widely used.

[0005] However, in the case of these TRIP steel plates, it is common to add large amounts of elements such as C, Si, Mn, and Al to the steel to introduce residual austenite. However, if the content of the added elements is excessive or insufficient compared to the target properties of the steel type, the material may be excessive or insufficient. In addition, steel plates containing these elements may form oxides on the surface of the steel plate during the annealing heat treatment process, which may cause the plating properties to be inferior when the steel plate is immersed in a hot-dip galvanizing bath.

[0006] Therefore, appropriate control of additive elements such as C, Si, Mn, and Al is essential to satisfy the target material.

[0007] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same are provided.

[0008] According to one embodiment of the present invention, it is an object to provide a high-strength cold-rolled steel sheet having excellent formability and a method for manufacturing the same.

[0009] 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.

[0010] According to one embodiment of the present invention, it contains, in wt%, C: 0.050 to 0.250%, Si: 0.10 to 3.00%, Al: 0.005 to 3.000%, Mn: 1.00 to 3.00%, P: 0.0400% or less, S: 0.0100% or less, N: 0.0100% or less, the remainder being Fe and other unavoidable impurities.

[0011] The R1 value defined in the following relational expression 1 is 0.35 or greater,

[0012] The R2 value defined in the following relational expression 2 is 0.59 or less,

[0013] The microstructure may be a cold rolled steel sheet containing, in area %, 50.0 to 85.0% ferrite, 0.5 to 10.0% retained austenite, 3.0 to 30.0% bainite, and 30.0% or less fresh martensite.

[0014] [Relationship 1]

[0015] R1 = [C] + 0.15×[Mn]

[0016] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0017] [Relationship 2]

[0018] R2 = [C] + 0.2333×[Mn]

[0019] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0020] The above cold rolled steel sheet may further include, in weight %, one or more of Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.300% or less, and Cr: 0.200% or less.

[0021] The above cold rolled steel sheet may further contain, in weight %, one or more of Nb, Ti and V in a total amount of 0.100% or less.

[0022] The above cold rolled steel sheet may have a tensile strength of 690 MPa or more and an elongation of 25% or more.

[0023] The above cold rolled steel sheet may further include a zinc plating layer on at least one side.

[0024] According to one embodiment of the present invention, there is provided a step of heating a steel slab containing, in wt%, C: 0.050 to 0.250%, Si: 0.10 to 3.00%, Al: 0.005 to 3.000%, Mn: 1.00 to 3.00%, P: 0.0400% or less, S: 0.0100% or less, N: 0.0100% or less, the remainder Fe and other unavoidable impurities, and having an R1 value defined by the following relational expression 1 of 0.35 or more and an R2 value defined by the following relational expression 2 of 0.59 or less;

[0025] A step of finishing hot rolling the above heated steel slab;

[0026] A step of winding the above-mentioned hot-rolled steel sheet;

[0027] A step of cold rolling the above-mentioned rolled steel plate;

[0028] A step of continuously annealing the above cold-rolled steel sheet at a temperature range of 800 to 900°C;

[0029] A step of first cooling the continuously annealed steel plate to a temperature range of 550 to 720°C at an average cooling rate of 1.0°C / s or more;

[0030] A step of secondary cooling the above-mentioned primary cooled steel plate to a temperature range of 150 to 480°C at an average cooling rate of 10.0°C / s or more; and

[0031] It may be a method for manufacturing a cold-rolled steel sheet, including a step of reheating the secondarily cooled steel sheet to a temperature range of 350 to 480°C.

[0032] [Relationship 1]

[0033] R1 = [C] + 0.15×[Mn]

[0034] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0035] [Relationship 2]

[0036] R2 = [C] + 0.2333×[Mn]

[0037] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0038] The above steel slab may further include, in weight %, one or more of Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.300% or less, and Cr: 0.200% or less.

[0039] The above steel slab may further contain, in weight %, one or more of Nb, Ti, and V in a total amount of 0.100% or less.

[0040] The above heating step is performed at a temperature range of 1150 to 1250°C,

[0041] The above finishing hot rolling step is performed at a temperature range of 830 to 980°C.

[0042] The above winding step is performed in a temperature range of 450 to 700°C by cooling at an average cooling rate of 10 to 100°C / s.

[0043] The above cold rolling step can be performed at a cold rolling reduction ratio of 30 to 60%.

[0044] A step of plating the above reheated steel plate in a zinc plating bath at 450 to 470°C; and

[0045] The method may further include a step of alloying the plated steel sheet at a temperature range of 470 to 550°C.

[0046] The above continuous annealing step can be performed in an atmosphere with a dew point temperature of -15.0 to 30.0°C.

[0047] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0048] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet having excellent formability and a method for manufacturing the same can be provided.

[0049] According to one embodiment of the present invention, a cold-rolled steel sheet having excellent strength and ductility and applicable to galvanized steel sheets used in the automobile industry and a method for manufacturing the same can be provided.

[0050] According to one embodiment of the present invention, a cold-rolled steel sheet having excellent strength and ductility and excellent plating adhesion after plating and a method for manufacturing the same can be provided.

[0051] 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.

[0052] Although not essential, it should be noted that the technical solutions according to each aspect of the present invention can also be usefully applied to other aspects of the present invention. Furthermore, the compositions and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to achieve beneficial effects.

[0053] 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.

[0054] Hereinafter, the present invention will be described in detail.

[0055] Below, the steel composition of the present invention is described in detail.

[0056] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0057] A cold-rolled steel sheet according to one embodiment of the present invention may include, in weight %, C: 0.050 to 0.250%, Si: 0.10 to 3.00%, Al: 0.005 to 3.000%, Mn: 1.00 to 3.00%, P: 0.0400% or less, S: 0.0100% or less, and N: 0.0100% or less.

[0058] Carbon (C): 0.050~0.250%

[0059] Carbon (C) is an element that secures the strength of steel through solid solution strengthening and precipitation strengthening, and is an effective element for stabilizing retained austenite to secure high elongation. If the carbon (C) content is less than 0.050%, the desired tensile strength may not be achieved. According to one embodiment of the present invention, it may be 0.060% or more. According to one embodiment of the present invention, it may be 0.070% or more. On the other hand, if the content exceeds 0.250%, the desired tensile strength range may be exceeded, or weldability, etc. may deteriorate due to the increase in carbon (C) content. According to one embodiment of the present invention, it may be 0.230% or less. According to one embodiment of the present invention, it may be 0.220% or less.

[0060] Silicon (Si): 0.10~3.00%

[0061] Silicon (Si) is a useful element for increasing the strength of steel sheets through solid solution strengthening and precipitation hardening. Since it suppresses the formation of cementite, it has the effect of promoting the enrichment of C in austenite, and it is an essential element for increasing the strength and elongation of steel by forming retained austenite after annealing. If the silicon (Si) content is less than 0.10%, retained austenite may not be formed, making it difficult to obtain a uniform elongation. 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 0.50% or more. On the other hand, if the silicon (Si) content exceeds 3.00%, the weld properties deteriorate due to LME cracking, and the surface properties and plating properties of the steel may deteriorate. According to one embodiment of the present invention, it may be 2.50% or less. According to one embodiment of the present invention, it may be 2.00% or less.

[0062] Aluminum (Al): 0.005~3.000%

[0063] Aluminum (Al) is an element that has a deoxidizing effect on molten steel, and similarly to Si, it improves the stability of austenite and is effective in increasing elongation. If the content of aluminum (Al) is less than 0.005%, deoxidation of the steel is not sufficiently performed, and the cleanliness of the steel may be impaired. According to one embodiment of the present invention, it may be 0.015% or more. According to one embodiment of the present invention, it may be 0.020% or more. On the other hand, if the content of aluminum (Al) is excessive, the transformation temperature rises significantly and the fraction of ferrite increases, which may cause a problem in that ultra-high strength cannot be achieved. Therefore, in the present invention, the upper limit of the content of aluminum (Al) may be limited to 3.000%. According to one embodiment of the present invention, it may be 2.500% or less. According to one embodiment of the present invention, it may be 2.000% or less.

[0064] Manganese (Mn): 1.00~3.00%

[0065] Manganese (Mn) is an element added to secure strength. If the manganese (Mn) content is less than 1.00%, it may be difficult to secure strength. According to one embodiment of the present invention, it may be 1.10% or more. According to one embodiment of the present invention, it may be 1.20% or more. On the other hand, if the content exceeds 3.00%, the bainite transformation rate may be slowed down, excessive fresh martensite may be formed, and it may be difficult to obtain high elongation. In addition, a band structure may be formed due to the segregation of manganese (Mn), which may deteriorate the material uniformity and formability of the material. According to one embodiment of the present invention, it may be 2.70% or less. According to one embodiment of the present invention, it may be 2.50% or less.

[0066] Phosphorus (P): 0.0400% or less

[0067] Phosphorus (P) is contained as an impurity and segregates at grain boundaries, thereby lowering the toughness. Therefore, it is advantageous to control its content as low as possible. If the above phosphorus (P) is added excessively, the toughness of the steel deteriorates, so the upper limit may be limited to 0.0400%. According to one embodiment of the present invention, it may be 0.0300% or less. Meanwhile, the lower limit of the above phosphorus (P) content is not particularly limited, but may be 0.0020%.

[0068]

[0069] Sulfur (S): 0.0100% or less

[0070] Sulfur (S) is contained as an impurity in steel, similar to the above P. Sulfur (S) combines with manganese (Mn) to form inclusions, which reduces hole expandability and may also reduce weldability and hot-rollability, so it is advantageous to control it as low as possible. According to one embodiment of the present invention, the upper limit of the sulfur (S) content may be limited to 0.0100% in consideration of cases where it is unavoidably included. According to one embodiment of the present invention, it may be 0.0021% or less. Meanwhile, the lower limit of the sulfur (S) content is not particularly limited, but may be 0.0009%.

[0071] Nitrogen (N): 0.0100% or less

[0072] Nitrogen (N) is included in steel as an impurity, and it is advantageous to control its content as low as possible. According to one embodiment of the present invention, the upper limit of the nitrogen (N) content may be limited to 0.0100%, taking into account cases where it is unavoidably included.

[0073] 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.

[0074] A cold-rolled steel sheet according to one embodiment of the present invention may further include, in weight %, at least one of Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.300% or less, and Cr: 0.200% or less.

[0075] At least one of the following: copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.300% or less, and chromium (Cr): 0.200% or less

[0076] Copper (Cu), nickel (Ni), molybdenum (Mo), and chromium (Cr) are elements that increase the strength of steel. These elements are elements that increase the strength and hardenability of steel. However, if added in excessive amounts, the target strength grade may be exceeded, and since they are expensive elements, their upper limits may be limited to 0.1%, 0.1%, 0.300%, and 0.200%, respectively, from an economical perspective. Meanwhile, since copper (Cu), nickel (Ni), molybdenum (Mo), and chromium (Cr) act as solid solution strengthening elements, when adding one or more of copper (Cu), nickel (Ni), molybdenum (Mo), and chromium (Cr), if added in amounts less than 0.03%, the solid solution strengthening effect may be minimal, and therefore, when added, they may be added in amounts of 0.03% or more, respectively.

[0077] A cold-rolled steel sheet according to one embodiment of the present invention may further include, in weight percent, one or more of Nb, Ti, and V in a total amount of 0.100% or less.

[0078] The total amount of one or more of niobium (Nb), titanium (Ti), and vanadium (V): 0.100% or less

[0079] Niobium (Nb), titanium (Ti), and vanadium (V) are elements that increase the strength of steel. However, if the content of the above elements is excessive, there is a concern that recrystallization may be delayed due to local grain fixation, thereby damaging the uniformity of the structure, so the total amount of the above elements may be limited to 0.100% or less. When adding one or more of niobium (Nb), titanium (Ti), and vanadium (V), if added in an amount less than 0.030%, the strength-enhancing effect may be minimal, so when added, the total amount of these may be added in an amount of 0.030% or more.

[0080] A cold rolled steel sheet according to one embodiment of the present invention may have an R1 value defined in the following relational expression 1 of 0.35 or more.

[0081] [Relationship 1]

[0082] R1 = [C] + 0.15×[Mn]

[0083] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0084] According to one embodiment of the present invention, strength and ductility can be secured simultaneously by controlling the content relationship of C and Mn through relational expression 1.

[0085] If the R1 value defined in the above relational expression 1 is less than 0.35, there may be a problem in that the target tensile strength (TS) cannot be sufficiently secured. According to one embodiment of the present invention, it may be 0.36 or more. Meanwhile, the upper limit of the R1 value is not particularly limited, but according to one embodiment of the present invention, it may be 0.45.

[0086] A cold rolled steel sheet according to one embodiment of the present invention may have an R2 value defined in the following relational expression 2 of 0.59 or less.

[0087] [Relationship 2]

[0088] R2 = [C] + 0.2333×[Mn]

[0089] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0090] In the present invention, strength and ductility can be secured simultaneously by additionally controlling the content relationship of C and Mn through relational expression 2.

[0091] If the R2 value defined in relational expression 2 exceeds 0.59, the target tensile strength range may be exceeded, and it may be difficult to secure the target elongation. According to one embodiment of the present invention, it may be 0.57 or less. Meanwhile, the lower limit of the R2 value is not particularly limited, but according to one embodiment of the present invention, it may be 0.40. According to one embodiment of the present invention, it may be 0.45 or more.

[0092] Below, the steel microstructure of the present invention is described in detail.

[0093] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0094] The microstructure of a cold-rolled steel sheet according to one embodiment of the present invention may include, in terms of area %, 50.0 to 85.0% ferrite, 0.5 to 10.0% retained austenite, 3.0 to 30.0% bainite, and 30.0% or less fresh martensite.

[0095] When the ferrite fraction is less than 50.0%, it is difficult to secure elongation due to the insufficient soft ferrite fraction, and the fractions of bainite, retained austenite, and fresh martensite increase, which may result in excessively high strength. On the other hand, when the fraction exceeds 85.0%, it may be difficult to sufficiently secure the desired strength.

[0096] If the retained austenite fraction is less than 0.5%, it may be difficult to secure elongation. According to one embodiment of the present invention, the retained austenite fraction is 1.0% or more. On the other hand, if the fraction exceeds 10.0%, there is a problem in that elongation cannot be achieved due to insufficient ferrite fraction.

[0097] If the bainite fraction is less than 3.0%, there is a risk that the desired strength and elongation will not be adequately achieved. On the other hand, if the fraction exceeds 30%.0, the initial austenite to bainite transformation will be excessive, resulting in a lack of fresh martensite transformation, which may result in a decrease in tensile strength.

[0098] If the fresh martensite fraction exceeds 30.0%, the hard martensite may become excessive, exceeding the desired level of tensile strength. According to one embodiment of the present invention, the lower limit of the fresh martensite fraction may be 4.5%.

[0099] A cold-rolled steel sheet according to one embodiment of the present invention may have a tensile strength of 690 MPa or more and an elongation of 25% or more.

[0100] According to one embodiment of the present invention, the tensile strength may be 780 MPa or less.

[0101] According to one embodiment of the present invention, the cold-rolled steel sheet may include a plating layer on at least one surface. The type of plating layer is not particularly limited in the present invention, but may be a zinc plating layer. According to one embodiment of the present invention, the zinc plating layer may be a hot-dip galvanized layer. Furthermore, according to one embodiment of the present invention, it may be an alloyed hot-dip galvanized layer.

[0102] Below, the steel manufacturing method of the present invention is described in detail.

[0103] A cold-rolled steel sheet according to one embodiment of the present invention can be manufactured by heating, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, and reheating a steel slab satisfying the above-described alloy composition.

[0104] heating

[0105] A steel slab satisfying the alloy composition according to one embodiment of the present invention can be heated to a temperature range of 1150 to 1250°C.

[0106] The method for manufacturing the above steel slab is not particularly limited, and a continuous casting slab may be used, or a slab manufactured using a thin slab caster, etc. In addition, a hot rolling process, which is a post-process, may be performed immediately after continuous casting.

[0107] When heating steel slabs, if the temperature falls below 1150℃, the final hot rolling temperature can easily drop excessively, potentially increasing the rolling load. Meanwhile, from a manufacturing cost perspective, the upper limit of the heating temperature can be limited to 1250℃ or lower.

[0108] Finishing hot rolling

[0109] The above heated steel slab can be finished by hot rolling at a temperature range of 830 to 980°C.

[0110] During the above finishing hot rolling, if the temperature is below 830°C, the rolling load may be large and the shape may be poor, which may lead to poor productivity. According to one embodiment of the present invention, it may be 880°C or higher. On the other hand, if the finishing hot rolling temperature exceeds 980°C, there is a concern that the surface quality may deteriorate due to an increase in oxides caused by excessively high temperature work. According to one embodiment of the present invention, it may be 950°C or lower. According to one embodiment of the present invention, it may be 930°C or lower.

[0111] Winding

[0112] The above hot-rolled steel sheet can be cooled at an average cooling rate of 10 to 100°C / s and coiled in a temperature range of 450 to 700°C.

[0113] When the temperature exceeds 700°C during coiling, thick internal hot-rolled oxidation of the steel sheet surface may occur, and the pickling properties may deteriorate. According to one embodiment of the present invention, the temperature may be 670°C or lower. According to one embodiment of the present invention, the temperature may be 640°C or lower. Meanwhile, in order to improve toughness by refining the effective grain size, the lower limit of the coiling temperature may be limited to 450°C. According to one embodiment of the present invention, the temperature may be 480°C or higher. According to one embodiment of the present invention, the temperature may be 500°C or higher.

[0114] When cooling to coiling temperature after hot rolling, if the average cooling rate is less than 10℃ / s, the productivity of hot rolling decreases, and in actual production, there may be a problem in which a cooling medium with low cooling capacity must be deliberately adopted. On the other hand, if the average cooling rate exceeds 100℃ / s, the temperature deviation within the steel sheet becomes uneven, which can lead to poor shape and excessively high strength.

[0115] cold rolling

[0116] The above-mentioned rolled steel plate can be cold rolled at a cold reduction ratio of 30 to 60%.

[0117] During cold rolling, if the cold reduction ratio is less than 30%, it will be difficult to achieve the desired thickness accuracy and shape correction of the steel sheet may become difficult. Conversely, if the reduction ratio exceeds 60%, the risk of cracks occurring at the steel sheet edges increases, and the load during cold rolling may be excessively high.

[0118] continuous annealing

[0119] The above cold-rolled steel sheet can be continuously annealed in a temperature range of 800 to 900°C.

[0120] During continuous annealing, if the temperature is below 800℃, sufficient recrystallization and austenite transformation may not occur, making it difficult to secure the desired martensite and bainite fractions after annealing. On the other hand, if the continuous annealing temperature exceeds 900℃, productivity decreases, coarse austenite may form, deteriorating the material, and degrading surface quality, such as plating peeling.

[0121] According to one embodiment of the present invention, continuous annealing can be performed in an atmosphere having a dew point temperature of -15.0 to 30.0°C.

[0122] During continuous annealing, internal oxidation of the steel sheet can be achieved by controlling the dew point temperature. This internal oxidation can secure plating properties and light-mechanical oxidation (LME) resistance.

[0123] If the dew point temperature is lower than -15.0℃, the amount of oxygen flowing into the steel decreases, which only aggravates surface oxidation and does not cause internal oxidation, so a large amount of oxides exist on the surface, and there is a concern that the above-mentioned beneficial effects cannot be obtained. According to one embodiment of the present invention, the dew point temperature may be higher than -10.0℃. On the other hand, if the dew point temperature exceeds 30.0℃, internal oxidation increases, which suppresses the diffusion of alloying elements and increases the effect of suppressing surface oxidation, but there is a problem in that the capacity of the humidifying equipment must be unnecessarily increased because the amount of water vapor supplied increases rapidly. In addition, the cooled water vapor may condense and be applied for a long period of time in continuous annealing, which may cause equipment problems. According to one embodiment of the present invention, it may be lower than 20.0℃.

[0124] According to one embodiment of the present invention, when the dew point temperature is outside the suggested range, when manufacturing a cold-rolled steel sheet, the target material may be satisfied, but there may be a problem in which plating peels off after plating.

[0125] In addition, according to one embodiment of the present invention, in order to obtain a significant effect during internal oxidation, the hydrogen concentration in the atmosphere gas may be included at 1% or more in volume% during annealing.

[0126] If the hydrogen concentration is less than 1%, the trace amount of oxygen that is inevitably included in the H2 and N2 gases cannot be effectively oxidized and removed, which increases the oxygen partial pressure and may cause surface oxidation of the steel plate. On the other hand, if the hydrogen concentration exceeds 70%, there is an explosion risk in the event of a gas leak and the cost of high-hydrogen work increases, so the hydrogen concentration may be limited to 70% or less. Excluding impurity gases that are inevitably included other than the hydrogen, the hydrogen may be substantially nitrogen.

[0127] According to one embodiment of the present invention, the continuous annealing can be performed in a continuous alloying molten plating continuous furnace.

[0128] Primary cooling

[0129] The above continuously annealed steel plate can be first cooled to a temperature range of 550 to 720°C at an average cooling rate of 1.0°C / s or more.

[0130] In the case of primary cooling, the cooling end temperature can be defined as the point at which secondary cooling begins by additionally applying the rapid cooling equipment that was not applied in the primary cooling.

[0131] According to one embodiment of the present invention, when the cooling process is divided into primary and secondary cooling and performed in stages, the temperature distribution of the steel plate can be made uniform in the slow cooling stage, thereby reducing the final temperature and material deviation, and securing the desired organizational structure.

[0132] In the above first cooling, if the end temperature is less than 550℃, the bainite fraction may become excessively high, and it may be difficult to cool to a temperature range below 550℃ at the target cooling rate under actual equipment conditions. On the other hand, if the first cooling end temperature exceeds 720℃, the cooling amount to the second cooling end temperature may increase, resulting in a poor steel plate shape, and there is a concern that the bainite fraction may be lower than the target level.

[0133] When the average cooling rate is less than 1.0℃ / s during the first cooling, the ferrite fraction transformed during the first cooling increases, and it may be difficult to secure the desired strength and work hardening ability due to the insufficient martensite and retained austenite fractions that affect the strength and work hardening ability. Meanwhile, the upper limit of the average cooling rate is not particularly limited, but may be limited to 100.0℃ / s or less for the sake of operability. According to one embodiment of the present invention, it may be limited to 10.0℃ / s or less.

[0134] Secondary cooling

[0135] The above-mentioned primary cooled steel plate can be secondary cooled to a temperature range of 150 to 480°C at an average cooling rate of 10.0°C / s or more.

[0136] As mentioned above, according to one embodiment of the present invention, secondary cooling may additionally apply a quenching facility that was not applied in the first cooling. The present invention is not particularly limited to the type of the quenching facility, but according to one embodiment, a hydrogen quenching facility may be used. According to one embodiment of the present invention, the hydrogen quenching facility may use a gas composed of 50 to 80% hydrogen by volume and the remainder nitrogen. If the hydrogen volume fraction exceeds 80%, there may be a disadvantage in that management, such as explosion control of the facility, becomes difficult. On the other hand, if the volume fraction is less than 50%, there may be a disadvantage in that it becomes difficult to utilize the efficient heat transfer characteristics of hydrogen, which is a light element.

[0137] When the secondary cooling is performed at a final temperature lower than 150°C, the initial martensite transformation amount may be excessive, resulting in excessively high yield strength and tensile strength and poor formability. According to an embodiment of the present invention, when the secondary cooling is performed, the final temperature may be set to be higher than the Ms temperature, thereby limiting martensite transformation from occurring during cooling. According to an embodiment of the present invention, when the secondary cooling is performed, the final temperature may be higher than 200°C. According to an embodiment of the present invention, it may be higher than 250°C. On the other hand, when the secondary cooling final temperature exceeds 450°C, when cooling is performed, it may be higher than the starting point of bainite transformation, and thus sufficient bainite may not be generated. Ultimately, there is a high probability that a large amount of fresh martensite will be generated, which may exceed the appropriate tensile strength range.

[0138] When the average cooling rate is less than 10.0°C / s during secondary cooling, there may be a problem of reduced operability. According to one embodiment of the present invention, it may be 20.0°C / s or higher. Meanwhile, the upper limit of the average cooling rate during secondary cooling is not particularly limited, but according to one embodiment of the present invention, it may be 60.0°C / s.

[0139] Reheating

[0140] The above secondarily cooled steel plate can be reheated to a temperature range of 350 to 480°C.

[0141] According to one embodiment of the present invention, a bainite phase transformation can be achieved through the reheating process. According to one embodiment of the present invention, during the reheating, the end point temperature may be conveniently referred to as the reheating temperature.

[0142] When reheating, if the temperature is below 350℃, the strength may become excessively high and the elongation may be poor. On the other hand, when the temperature exceeds 450℃, the austenite may not be transformed and may remain, and then turn into fresh martensite during the final cooling, which may have a negative effect on the elongation. Meanwhile, the so-called nose temperature at which bainite transformation is most active is approximately 400-450℃. According to one embodiment of the present invention, the lower limit of the temperature during reheating may be 400℃. According to one embodiment of the present invention, the upper limit of the reheating temperature may be 450℃.

[0143] plating

[0144] The above reheated steel plate can be plated in a zinc plating bath at 450 to 470°C.

[0145] According to one embodiment of the present invention, the reheated steel sheet may be galvanized as needed. Additionally, temper rolling may be performed.

[0146] According to one embodiment of the present invention, the temperature of the zinc plating bath is not particularly limited and may be a typical condition applicable in the same technical field.

[0147] alloying heat treatment

[0148] The above-mentioned plated steel plate can be subjected to alloying heat treatment at a temperature range of 470 to 550°C.

[0149] According to one embodiment of the present invention, the galvanized steel sheet may be subjected to an alloying heat treatment as needed. The alloying heat treatment may be performed to obtain an appropriate alloying level, and the temperature thereof may be determined according to the surface condition of the steel sheet. By controlling the surface condition of the steel, the upper limit of the alloying heat treatment temperature may be limited to 550°C, thereby preventing softening of the steel sheet and loss of residual austenite due to excessive tempering. Meanwhile, in order to rapidly progress the alloying, the alloying heat treatment temperature may be higher than the molten zinc plating temperature, and the lower limit thereof may be limited to 470°C.

[0150] According to one embodiment of the present invention, after the alloying heat treatment, in order to correct the shape of the steel plate and adjust the yield strength, the alloying heat treated steel plate may be cooled to room temperature and then additionally subjected to temper rolling at a reduction ratio of 1% or less.

[0151] 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.

[0152] (Example)

[0153] After preparing a steel slab having the alloy composition described in Table 1 below, it was heated in the temperature range of 1180 to 1220°C, and a steel plate was manufactured under the conditions described in Table 2 below. At this time, hot rolling was performed in the temperature range of 880 to 930°C, and coiling was performed in the temperature range of 480 to 640°C after cooling at an average cooling rate of 10°C / s or more after hot rolling.

[0154] Steel alloy composition (weight %) Relationship 1 Relationship 2 CSiMnPSAlTiNbNCrMoR1R2 A0.0801.501.300.00700.00200.030--0.00400.0190.0050.280.38 B0.1101.501.300.00700.00200.030--0.00400.0200.0030.310.41 C0.0801.501.450.00700.00200.005--0.00900.0150.0030.300.42 D0.0801.501.600.00700.00200.005--0.00 900.0170.0050.320.45E0.0801.501.700.00700.00200.080--0.00400.0 130.0070.340.48F0.1201.501.600.01000.00200.030--0.00600.0050.00 90.360.49G0.1101.401.600.00700.00200.0800.0150.020.00400.0120. 0050.350.48H0.2001.501.600.01000.00200.030--0.00600.0180.0060.4 40.57I0.2001.471.51--0.028--0.00500.0220.0020.430.55J0.2001.47 1.51--0.028--0.00700.0110.0040.430.55K0.1001.502.000.01000.0020 0.030--0.00600.0300.0050.400.57L0.2171.511.53--0.048--0.00500. 0260.0060.450.57M0.2201.501.53--0.048--0.00600.0110.0050.450.58 N0.1001.502.000.01000.00200.030--0.00600.0400.0080.400.57O0.11 01.401.900.00700.00200.0800.0150.020.00400.0090.0020.400.55P0.1 001.492.100.00620.00240.084--0.00170.0150.0090.420.59Q0.2301.0 52.00--0.990--0.00600.0210.0020.530.70R0.2191.501.72--0.047--0.00600.0250.0050.480.62S0.2391.531.71--0.030---0.0210.0050.50 0.64T0.0901.452.100.00700.00200.0800.00600.0150.0020.410.58.

[0155] [Relationship 1] R1 = [C] + 0.15×[Mn]

[0156] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0157] [Relationship 2]

[0158] R2 = [C] + 0.2333×[Mn]

[0159] (In the formula, [C] and [Mn] are the weight percent of each element.)

[0160] Specimen numberSteel gradeContinuous annealing1st cooling2nd coolant heatingTemperature (℃)Dew point temperature (℃)Temperature (℃)Average cooling rate (℃ / s)Temperature (℃)Average cooling rate (℃ / s)Temperature (℃)1A78015.66404.440017.54402B80014.46405.040017.54403C78014.86404.440017.54404D80014.66405.040017.54405E80015.96405.0400 17.54406F800-40.36505.040018.24307G82015.16405.740017.54408H80015. 86504.740018.23709I830-45.26505.740018.240010J83015.67505.740018.2 40011K84014.86506.040018.243012L80015.56504.740018.240013M80015.76 504.740018.240014N78014.96504.448518.248515O82015.26405.740017.544 016P81015.06405.444014.644017Q89015.77006.040019.740018R83015.1650 5.730025.540019S80014.96504.730025.540020T850-45.96405.330010.5300

[0161] For the manufactured steel plate, the microstructure and physical properties were measured and are shown in Table 3 below.

[0162] The microstructure was observed at 1 / 4 points from the surface of the steel plate toward the center of the thickness using electron backscatter diffraction (EBSD) and the area fraction was calculated through a phase map, and the retained austenite fraction was measured using X-ray diffraction (XRD).

[0163] In addition, yield strength (YS), tensile strength (TS), and elongation (T-El) were measured and presented through a tensile test in the direction perpendicular to the rolling. At this time, a gauge length of 50 mm and a tensile specimen width of 25 mm were used as the test specimen specifications.

[0164] In addition, after plating the manufactured steel sheet, the plating properties were visually evaluated and shown in Table 3 below. To evaluate the plating properties, after hot-dip galvanizing, an automotive structural sealer was applied to a thickness of approximately 5 mm and cured at a temperature of 150 to 170°C. After cooling the plated steel sheet to room temperature, the sealer was peeled off by bending it at a 90° angle. If the plating layer adhered to the sealer and the entire interface between the zinc plating and the base steel sheet was peeled off, the plating adhesion was judged to be poor. If no peeling occurred after the plating properties evaluation, it was indicated by '○', and if peeling occurred and the plating adhesion was poor, it was indicated by '×'.

[0165] Specimen number Steel grade Microstructure (area %) Property classification FRABFMYS (MPa) TS (MPa) T-El (%) Plating 1A 90.3 1.9 2.6 5.2 4 5 0 5 8 9 32.5 ○ Comparative example 12B 87.13 23.4 6.3 4 0 6 1 5 31.5 ○ Comparative example 23C 89.31 7 5.7 3.3 4 0 4 6 1 8 36.0 ○ Comparative example 34D 85.62 44.87.2 4 1 4 6 2 8 34.4 ○ Comparative example 45E86.52.93.57.140464132.7 ○Comparative Example 56F756.43.714.940569529.8 ×Invention Example 17G73.25.5615.347469831.2 ○Invention Example 28H74.14.75.116.143572525.6 ○Invention Example 39I71.96.2813.948672933.1 ×Invention Example 410J47.61.719.431.34 9076322.5 ○Comparative Example 611K68.22.412.217.241073430.4 ○Invention Example 512L75.32.56.515.740274026.4 ○Invention Example 613M78.73.56.311.540274026.4 ○Invention Example 714N64.11.79.831.439079621.8 ○Comparative Example 715O77.35.4314.348274531.3 ○Invention Example 816P78.76.210.34.832674825.2 ○Invention Example 917Q43.94.217.334.643886323.4 ○Comparative Example 818R71.40.44.823.449581023.8 ○Comparative Example 919S68.911.55.713.947690121.2 ○Comparative Example 1020T69.12.32.725.942577523.1 ×Comparative Example 11

[0166] * F: Ferrite, RA: Retained Austenite, B: Bainite, FM: Fresh Martensite As shown in Table 3 above, in the case of the invention examples that satisfy 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 were also secured.

[0167] Meanwhile, Invention Examples 1 and 4 are examples in which the properties of the cold-rolled steel sheet proposed in the present invention were achieved, but the dew point temperature during continuous annealing was outside the range of the present invention, resulting in peeling after plating. It can be confirmed that when the dew point temperature during continuous annealing is outside the range proposed in the present invention, the plating adhesion of the plated steel sheet is degraded after plating.

[0168] On the other hand, Comparative Examples 1 to 5 are examples that do not satisfy the relational expression 1 proposed in the present invention. As a result, ferrite was formed excessively, and the desired level of strength was not secured.

[0169] In Comparative Example 6, the end temperature exceeded the suggested range during the first cooling. As a result, the ferrite fraction was insufficient and the elongation was poor.

[0170] In Comparative Example 7, the end temperature exceeded the suggested range during secondary cooling, and the temperature also exceeded the recommended range during reheating. As a result, fresh martensite was excessively formed in the final microstructure, resulting in poor elongation.

[0171] Comparative Examples 8 to 10 are examples that do not satisfy the relationship 2 proposed in the present invention. As a result, the proposed microstructure fraction was not properly secured, and the desired level of elongation was not secured.

[0172] In Comparative Example 11, the temperature fell below the suggested range during reheating, preventing sufficient bainite formation. As a result, the elongation was reduced.

[0173] 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, in wt%, C: 0.050 to 0.250%, Si: 0.10 to 3.00%, Al: 0.005 to 3.000%, Mn: 1.00 to 3.00%, P: 0.0400% or less, S: 0.0100% or less, N: 0.0100% or less, the remainder being Fe and other unavoidable impurities. The R1 value defined in the following relational expression 1 is 0.35 or greater, The R2 value defined in the following relational expression 2 is less than or equal to 0.59, Cold rolled steel sheet containing, in area %, 50.0 to 85.0% ferrite, 0.5 to 10.0% retained austenite, 3.0 to 30.0% bainite, and 30.0% or less fresh martensite. [Relationship 1] R1 = [C] + 0.15×[Mn] (In the formula, [C] and [Mn] are the weight percent of each element.) [Relationship 2] R2 = [C] + 0.2333×[Mn] (In the formula, [C] and [Mn] are the weight percent of each element.) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further containing, in weight %, at least one of Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.300% or less, and Cr: 0.200% or less.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further containing, in weight %, at least one of Nb, Ti, and V in a total amount of 0.100% or less.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength of 690 MPa or more and an elongation of 25% or more.

5. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further including a zinc plating layer on at least one surface.

6. A step of heating a steel slab containing, by weight%, C: 0.050 to 0.250%, Si: 0.10 to 3.00%, Al: 0.005 to 3.000%, Mn: 1.00 to 3.00%, P: 0.0400% or less, S: 0.0100% or less, N: 0.0100% or less, the remainder being Fe and other unavoidable impurities, and having an R1 value defined by the following relational expression 1 of 0.35 or more and an R2 value defined by the following relational expression 2 of 0.59 or less; A step of finishing hot rolling the above heated steel slab; A step of coiling the above-mentioned hot-rolled steel sheet; A step of cold rolling the above-mentioned rolled steel plate; A step of continuously annealing the above cold-rolled steel plate at a temperature range of 800 to 900°C; A step of first cooling the continuously annealed steel plate to a temperature range of 550 to 720°C at an average cooling rate of 1.0°C / s or more; A step of secondarily cooling the above-mentioned first-cooled steel plate to a temperature range of 150 to 480°C at an average cooling rate of 10.0°C / s or more; and A method for manufacturing a cold rolled steel sheet, comprising: a step of reheating the secondarily cooled steel sheet to a temperature range of 350 to 480°C. [Relationship 1] R1 = [C] + 0.15×[Mn] (In the formula, [C] and [Mn] are the weight percent of each element.) [Relationship 2] R2 = [C] + 0.2333×[Mn] (In the formula, [C] and [Mn] are the weight percent of each element.) 7. In claim 6, A method for manufacturing a cold rolled steel sheet, wherein the steel slab further contains, in weight %, at least one of Cu: 0.1% or less, Ni: 0.1% or less, Mo: 0.300% or less, and Cr: 0.200% or less.

8. In claim 6, A method for manufacturing a cold rolled steel sheet, wherein the above steel slab further contains, in weight %, one or more of Nb, Ti, and V in a total amount of 0.100% or less.

9. In claim 6, The above heating step is performed at a temperature range of 1150 to 1250℃. The above finishing hot rolling step is performed at a temperature range of 830 to 980°C. The above-mentioned winding step is performed at a temperature range of 450 to 700°C by cooling at an average cooling rate of 10 to 100°C / s. A method for manufacturing cold rolled steel sheets, wherein the above cold rolling step is performed at a cold rolling reduction ratio of 30 to 60%.

10. In claim 6, A step of plating the above reheated steel plate in a zinc plating bath at 450 to 470°C; and A method for manufacturing a cold rolled steel sheet, further comprising the step of alloying the plated steel sheet at a temperature range of 470 to 550°C.

11. In claim 6, A method for manufacturing cold rolled steel sheets, wherein the above continuous annealing step is performed in an atmosphere having a dew point temperature of -15.0 to 30.0°C.