HIGH-STRENGTH COLD-ROLLED STEEL SHEET WITH HIGH POINT EXPANSION RATIO, HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET AND MANUFACTURING METHOD FOR IT

Description

[Technical Field]

[0001] The present invention relates to a high-strength cold rolled steel sheet and high-strength hot-dip galvanized steel sheet having a high hole expansion ratio, and a manufacturing method thereof.[Background Art]

[0002] In recent years, development of a technology of manufacturing a steel sheet having high strength has been promoted to reduce the weight of automobiles. A steel sheet having both high strength and formability may increase productivity, so it is excellent in terms of economy and is more advantageous in terms of safety of final parts. In particular, demand for steel having high tensile strength (TS) of 1180 MPa or higher has increased because a steel sheet having high tensile strength (TS) has a high bearing load until fracture occurs. In the related art, many attempts have been made to improve strength of the existing steel, but it was found that simple improvement of the strength degrades ductility and hole expansion ratio (HER). Meanwhile, transformation induced plasticity (TRIP) steel sheet in which a large amount of Si or Al is added may be a related art which overcomes the aforementioned shortcomings. However, in the case of TRIP steel sheet, it is possible to obtain an elongation of 14% or more at TS 1180 MPa class but liquid metal embrittlement (LME) resistance is deteriorated due to the addition of a large amount of Si and Al, which leads to poor weldability, and thus, commercialization of TRIP steel sheet as a material for automobile structures is limited.

[0003] In addition, various yield ratios are pursued in the same tensile strength class according to usages and purposes, and it is not easy to produce a steel having high hole expansion ratio with a steel sheet having a low yield ratio. The reason is because it is usually necessary to introduce a martensite or ferrite phase as a second phase to lower a yield ratio but such a structural characteristics is a factor that impairs the hole expansion ratio.

[0004] Patent document 1 discloses a high-strength cold rolled steel sheet having yield ratio, strength, hole expansion ratio, delayed fracture resistance characteristics and having a high elongation of 17.5% or more. However, Patent document 1 has a disadvantage in that weldability is poor due to an occurrence of LME due to a high Si addition.

[0005] Therefore, the present disclosure proposes a 1180 MPa-class steel material exhibiting high strength and excellent hole expansion ratio of 25% or more, elongation of 5% to 13%, and excellent weldability even at a low yield ratio, and a manufacturing method thereof.

[0006] Patent document 2 relates to a method for producing a high strength coated steel sheet having improved ductility and formability and to a coated sheet obtained with this method.[Related art document]

[0007] (Patent document 1) Korean Patent Laid-open Publication No. 2017-7015003 (Patent document 2) WO 2017 / 109541 A1 [Disclosure][Technical Problem]

[0008] An aspect of the present disclosure may provide a high-strength cold rolled steel sheet having elongation suitable for machining, a high hole expansion ratio, and good weldability, while having a high strength and a low yield ratio, a high-strength hot-dip galvanized steel sheet manufactured using the same, and a manufacturing method thereof.[Technical Solution]

[0009] A high-strength cold rolled steel according to the invention is defined in the independent claim claim 1. A method of manufacturing the high-strength cold-rolled sheet according to the invention is defined in the independent claim 4. A high strength hot-dip galvanized sheet is defined in claim 2 and a method of manufacturing the high strength hot-dip galvanized steel sheet is defined in claim 6. A high strength hot-dip galvanized sheet comprising an alloyed hot-dip zinc layer is defined in claim 3. A method of manufacturing the high strength hot-dip galvanized steel sheet of claim 3 is defined in claims 7. Preferred embodiments are defined in the dependent claims.[Advantageous Effects]

[0010] According to exemplary embodiments in the present invention, a high-strength cold rolled steel sheet and hot-dip galvanized steel sheet having a high hole expansion ratio of 25% or more and an elongation of 5% to 13%, while having high tensile strength of 1180 MPa or more and a low yield ratio of 0.65 to 0.85, may be provided.

[0011] In addition, the high-strength hot-dip galvanized steel sheet of the present invention has characteristics that exhibit excellent weldability due to excellent LME resistance after galvanizing.[Best Mode]

[0012] Singular forms as used herein also include plural forms unless obviously indicate otherwise.

[0013] As used in the disclosure, the meaning of "including" specifies a specific characteristics, regions, integers, steps, operations, elements and / or components, and do not exclude presence or addition of other specific characteristics, regions, integers, steps, operations, elements, components and / or groups.

[0014] Unless indicated otherwise, it is to be understood that all the terms used in the specification, including technical and scientific terms have the same meaning as those that are understood by those skilled in the art to which the present invention pertains.

[0015] Hereinafter, a high-strength cold rolled steel sheet and a high strength hot-dip galvanized steel sheet according to an aspect of the present invention will be described in detail.

[0016] First, an alloy composition of a high-strength cold rolled steel sheet provided in the present invention will be described in detail. In this case, the content of each component refers to weight% unless otherwise specified.Carbon (C): 0.17 to 0.21%

[0017] Carbon is a basic element that supports strength of steel through solid solution strengthening and precipitation strengthening. If the amount of carbon is less than 0.17%, it is difficult to obtain strength equivalent to tensile strength (TS) of 1180 MPa, while satisfying other materials. Meanwhile, if the amount of carbon exceeds 0.21%, weldability deteriorates and a target hole expansion ratio value cannot be obtained. Therefore, in the present invention, the content of carbon is limited to 0.17 to 0.21%. A lower limit of C is preferably 0.18%, and an upper limit of C is preferably 0.20%.Silicon (Si): 0.3 to 0.8%

[0018] Silicon is a key element of transformation induced plasticity (TRIP) steel that acts to increase a retained austenite fraction and elongation by inhibiting precipitation of cementite in a bainite region. If silicon is less than 0.3%, the elongation is too low as retained austenite rarely remains. Meanwhile, if silicon exceeds 0.8%, deterioration of physical properties of a weld portion due to formation of LME cracks cannot be prevented and surface characteristics and plating properties of the steel deteriorate. Therefore, in the present invention, the content of silicon is limited to 0.3 to 0.8%. A lower limit of Si is preferably 0.4% and an upper limit of Si is preferably 0.6%.Manganese (Mn): 2.7 to 3.3%

[0019] In the present invention, the amount of manganese may be 2.7 to 3.3%. If the manganese content is less than 2.7%, it is difficult to secure strength, and if the manganese content exceeds 3.3%, a bainite transformation rate is slowed to form too much fresh martensite, making it difficult to obtain high hole expansion ratio. In addition, if the content of manganese is high, a start temperature of martensite formation is lowered and a cooling end temperature required to obtain an initial martensite phase in an annealing water cooling step is too low. Therefore, in the presentinvention, the content of manganese is limited to 2.7 to 3.3%. A lower limit of Mn is preferably 2.8% and an upper limit of Mn is preferably 3.1%.Chromium (Cr): 0.3 to 0.7%

[0020] In the present invention, the amount of chromium may be 0.3 to 0.7%. If the amount of chromium is less than 0.3%, it is difficult to obtain target tensile strength, and the amount of chromium exceeds an upper limit of 0.7%, a transformation speed of bainite is slow, making it difficult to obtain a high hole expansion ratio. Therefore, in the presentinvention, the content of chromium is limited to 0.3 to 0.7%. A lower limit of Cr is preferably 0.4% and an upper limit of Cr is preferably 0.6%.Aluminum (Al): 0.01 to 0.3%

[0021] In the present invention, the amount of aluminum may be 0.01 to 0.3%. If the amount of aluminum is less than 0.01%, the steel may not be sufficiently deoxidized and cleanliness is impaired. Meanwhile, the amount of aluminum exceeds 0.3%, castability of the steel is deteriorated. Therefore, in the present invention, the content of aluminum is limited to 0.01 to 0.3%. A lower limit of Al is preferably 0.03% and an upper limit of Al is preferably 0.2%.Titanium (Ti): 0.01 to 0.03%, boron (B): 0.001 to 0.003%

[0022] In the present invention, 0.01 to 0.03% of titanium and 0.001 to 0.003% of boron are added to increase hardenability of the steel. If the content of titanium is less than 0.01%, boron may be bonded to nitrogen, so that the effect of strengthening hardenability of boron is lost, and the content of titanium exceeds 0.03%, castability of the steel is deteriorated. Meanwhile, if the content of boron is less than 0.001%, an effective hardenability strengthening effect cannot be obtained, and if boron is contained in excess of 0.003%, a boron carbide may be formed, significantly impairing hardenability. Therefore, in the presentinvention, the content of titanium is limited to 0.01 to 0.03%, and the content of boron is prefrably limited to 0.001 to 0.003%. A lower limit of Ti is preferably 0.015% and an upper limit of Ti is preferably 0.025%. A lower limit of B is preferably 0.015% and an upper limit of B is preferably 0.0025%.Phosphorus (P): 0.04% or less

[0023] Phosphorus exists as an impurity in the steel and it is advantageous to control its content as low as possible, but phosphorus is also intentionally added to increase strength of the steel. However, if the phosphorus is excessively added, toughness of the steel may be deteriorated. Therefore, in order to prevent this, in the present invention, an upper limit is limited to 0.04%. Preferably, the content of P is 0.01% or less.Sulfur (S): 0.02% or less

[0024] Like phosphorus, sulfur exists as an impurity in the steel, and it is advantageous to control its content as low as possible. In addition, since sulfur deteriorates ductility and impact properties of the steel, an upper limit is limited to 0.02% or less. The content of S is preferably 0.003% or less.Nitrogen (N): 0.01% or less

[0025] In the present invention, nitrogen is included in the steel as an impurity, and it is advantageous to control the content of nitrogen as low as possible. If a large amount of nitrogen is added, an excessive amount of nitride may be formed to degrade rollability due to excessive structure refinement, to make it impossible to control a target structure, and to impair final quality such as impact characteristics, etc. Therefore, an upper limit thereof is limited to 0.01% or less. The content of N is more preferably 0.0060% or less.

[0026] In addition to the aforementioned alloy composition, the steel sheet of the present invention may additionally include 0.1% or less of copper (Cu), 0.1% or less of nickel (Ni), and 0.1% or less of molybdenum (Mo).Copper (Cu): 0.1% or less, nickel (Ni): 0.1% or less, molybdenum (Mo): 0.1% or less

[0027] Copper, nickel, and molybdenum are elements that increase strength of steel and are included as optional components in the present invention, and an upper limit of addition of each element is limited to 0.1%. These elements increase strength and hardenability of steel, but addition of an excessive amount thereof may exceed a target strength class, and since they are expensive elements, an upper limit of their addition is preferably limited to 0.1% in terms of economical efficiency. Meanwhile, since copper, nickel and molybdenum act as solid solution strengthening, an addition thereof less than 0.03% may be too insignificant to achieve solid solution strengthening effect, and therefore, when copper, nickel and molybdenum are added, a lower limit thereof may be limited to 0.03% or more. An upper limit of each of Cu, Ni, and Mo is preferably 0.06%.

[0028] In addition to the alloy composition described above, the steel sheet of the present disclosure may additionally include 0.03% or less of niobium (Nb) and 0.01% or less of vanadium (V).Niobium (Nb): 0.03% or less, vanadium (V): 0.01% or less

[0029] Niobium and vanadium are elements that increase yield strength of steel through precipitation hardening and may be optionally added to increase yield strength in the present invention. However, excessive content thereof may significantly lower elongation and cause brittleness of the steel, and thus, in the present disclosure, upper limits of niobium and vanadium are limited to 0.03% and 0.01% or less, respectively. Meanwhile, since niobium and vanadium cause precipitation hardening, even a small amount of addition thereof may be effective, but if niobium and vanadium is added less than 0.005%, the effect may be insignificant. Therefore, when niobium and vanadium is added, a lower limit thereof may be limited to 0.005% or more. Upper limits of Nb and V are preferably 0.02% and 0.008%, respectively. 0.20 % ≤ C + Si + Al / 5 ≤ 0.35 % wherein [C], [Si], [Al] refer to weight percents of C, Si, and Al, respectively.

[0030] In addition to the aforementioned contents of C, Si and Al, C, Si, and Al satisfy Equation (1) above. Liquid metal embrittlement (LME) of plated steel occurs as liquid zinc penetrates an austenite grain boundary when tensile stress is formed at an austenite grain interface of the steel sheet in a state in which plated zinc becomes liquid during spot welding. Since the LME phenomenon is particularly severe in the steel sheet to which Si and Al are added, an addition amount of Si and Al is limited through Equation (1) above in the present disclosure. In addition, if the C content is high, an A3 temperature of the steel is lowered to cause an austenite region vulnerable to LME to be expanded and weaken toughness of the material, and thus, the addition amount of C is limited through Equation (1) above.

[0031] If the value of Equation (1) exceeds 0.35%, LME resistance is deteriorated during spot welding as described above, and thus, there are LME cracks after the spot welding, which impairs fatigue characteristics and structural safety. Meanwhile, as the value of Equation (1) is smaller, spot weldability and LME resistance are improved, so a lower limit thereof may not be separately set. However, if the value is less than 0.20%, it may be difficult to obtain high tensile strength of 1180 MPa class together with an excellent hole expansion ratio although spot weldability and LME resistance are improved. Therefore, the lower limit is set to 0.25%.

[0032] The remaining component of the present invention is iron (Fe). In other ordinary steel manufacturing processes, unintended impurities may inevitably be mixed from raw materials or a surrounding environment. Since these impurities are known to anyone of ordinary skill in the steel manufacturing process, all the contents thereof are not specifically mentioned in the present invention.

[0033] Meanwhile, the high-strength cold rolled steel sheet of the presentinvention that satisfies the aforementioned steel composition has a microstructure including, by area fraction, 3 to 7% of retained austenite, 5 to 15% of fresh martensite, 5% or less (including 0%) of ferrite, and the balance of bainite or tempered martensite, and a cementite phase, as a second phase, is precipitated and distributed in a bainite lath boundary or in a lath or grain boundary of the tempered martensite, and a volume fraction thereof is 1 to 3%.

[0034] In the high-strength cold rolled steel sheet according to the present invention, part of cementite precipitates and grows in the microstructure by limiting the content of Si and Al that stabilizes austenite by inhibiting the growth of cementite, by the condition of Equation (1). This cementite is precipitated in a martensite lath or grain boundary when martensite formed by secondary cooling is reheated, or is formed in a portion in which carbon is concentrated between bainite ferrite laths when bainite transformation occurs during reheating after secondary cooling. In the cold rolled steel sheet according to the presentinvention, cementite having a volume fraction of 1% or more is precipitated by limiting the upper limits of Si and Al by Equation (1), but, nevertheless, austenite remains due to the presence of partial Si and Al and carbon is distributed inside the retained austenite, and thus, the amount of precipitated cementite is less than 3%. In addition, since Si and Al are added to some extent, austenite remains present in the steel of the present disclosure at a level of 3 to 7%, but a high fraction of retained austenite as in typical TRIP steels with very high Si and Al content is not distributed in the steel of the present disclosure.

[0035] In addition, in the present invention, fresh martensite structure is introduced at a level of 5 to 15% to obtain a low yield ratio. If an austenite phase fraction is high after the secondary cooling and reheating, the carbon content in the austenite is low and stability is insufficient, and part of the austenite is transformed into fresh martensite in a subsequent cooling process, resulting in a lower yield ratio.

[0036] In addition, in the present invention, the ferrite structure is not desirable for the hole expansion ratio, but may exist at a level of 5% or less (including 0%) during the manufacturing process. In addition, the balance in the microstructure of the present invention is of bainite or tempered martensite structure.

[0037] By having the alloy components and microstructure as described above, the high-strength cold rolled steel sheet of the present invention exhibits a high hole expansion ratio of 25% or more even at a tensile strength of 1180 MPa or more and a low yield ratio of 0.65 to 0.85. As described above, the low yield ratio of the high-strength cold rolled steel sheet according to the present invention is due to the introduction of fresh martensite. The inventors of the present application found that more than 25% or more hole expansion ratio even with the fresh martensite under the alloy component and the structure control condition according to the present invention. In addition, since the high-strength cold rolled steel sheet according to the present disclosure limits the content of Si and Al, the TRIP effect is weak and an elongation of 5 to 13% is shown.

[0038] The present invention also provides a hot-dip galvanized steel sheet obtained by performing a hot-dip galvanizing on a surface of the high-strength cold rolled steel sheet and an alloyed hot-dip galvanized steel sheet obtained by performing annealing for alloying on the hot-dip galvanized steel sheet.

[0039] Next, a method of manufacturing a high-strength cold rolled steel sheet and a high strength hot-dip galvanized steel sheet according to another aspect of the present invention will be described in detail.

[0040] The high-strength cold rolled steel sheet according to the present invention is manufactured by undergoing heating a steel slab satisfying the steel component composition described above - hot rolling - cooling - coiling - cold rolling - continuous annealing - primary and secondary cooling - reheating, and details thereof are as follows.Steel slab preparation and heating process

[0041] First, a slab having the aforementioned alloy composition and satisfying Equation (1) is prepared and heated to a temperature of 1150°C to 1250°C. Here, if a slab temperature is less than 1150°C, it may be impossible to perform a next step, hot rolling. Meanwhile, if the slab temperature exceeds 1250°C, a lot of energy is unnecessarily required to increase the slab temperature. Therefore, a heating temperature is limited to a temperature of 1150°C to 1250°C. A lower limit of the heating temperature is preferably 1190°C and an upper limit of the heating temperature is preferably 1230°C.Hot rolling process

[0042] The heated slab is hot-rolled to a thickness suitable for an intended purpose under the condition that a finish delivery temperature (FDT) is 900°C to 980°C. If the FDT is lower than 900°C, a rolling load is large and shape defects increase, resulting in poor productivity. Meanwhile, if the FDT exceeds 980°C, surface quality deteriorates due to an increase in oxides due to an excessive high-temperature operation. Therefore, hot rolling is performed under the condition that the FDT is 900°C to 980°C. A lower limit of the FDT is preferably 910°C and an upper limit of the FDT is preferably 950°C.Coiling process and cold rolling process

[0043] The hot-rolled steel sheet is cooled to a coiling temperature at an average cooling rate of 10°C / s to 100°C / s, and coiling is performed in a typical temperature in a range of 500°C to 700°C. After coiling, the hot-rolled steel sheet is rolled at a cold-rolling reduction ratio of 30% to 60% to obtain a cold rolled steel sheet. If the average cooling rate is less than 10°C / s, hot rolling productivity may be excessively deteriorated, and if it exceeds 100°C / s, strength of an edge portion increases, thereby increasing a material deviation in a width direction. A lower limit of the average cooling rate is preferably 20°C / s and an upper limit of the average cooling rate is preferably 80°C / s. A lower limit of a temperature for coiling is preferably 550°C and an upper limit of the temperature for coiling is preferably 650°C. If the cold rolling reduction ratio is less than 30%, it may be difficult to secure target thickness accuracy and it may be difficult to correct a shape of the steel sheet. Meanwhile, if the cold-rolling reduction rate exceeds 60%, a possibility of cracking at the edge of the steel sheet may increase and the cold-rolling load is excessively large. Therefore, in the present invention, the cold rolling reduction rate at the cold rolling step is preferably limited to 30 to 60%. A lower limit of the cold-rolling reduction ratio is more preferably 35% and an upper limit of the cold-rolling reduction ratio is more preferably 50%.Continuous annealing process

[0044] In the present invention, the cold rolled steel sheet is subjected to continuous annealing in a temperature in a range of (Ae3 + 30°C to Ae3 + 80°C). More preferably, continuous annealing may be performed in a temperature in a range of 830°C to 880°C. In addition, the continuous annealing may be carried out in a continuous alloying hot-dip plating furnace. The continuous annealing step is to form austenite close to 100% by heating up to a single phase of austenite and use the austenite for subsequent phase transformation. If the continuous annealing temperature is lower than Ae3+30°C or less than 830°C, sufficient austenite transformation is not performed, so that desired martensite and bainite fractions cannot be secured after annealing. Meanwhile, if the continuous annealing temperature exceeds Ae3+80°C or 880°C, productivity may decrease and coarse austenite may be formed, resulting in material deterioration. In addition, oxides may grow during annealing, making it difficult to secure surface quality of a plated material. Ae3 may be calculated using thermodynamic software utilizing a computer coupling of phase diagrams and thermochemistry (CALPHAD) method commonly used in the art.Primary and secondary cooling process

[0045] The continuously annealed steel sheet is primarily cooled at an average cooling rate of 10°C / s or less up to a temperature in a range of 560 to 700°C, and secondarily cooled at an average cooling rate of 10°C / s or more up to a temperature in a range of 270 to 330°C to introduce martensite. Here, a temperature for terminating the primary cooling is defined as a time point at which rapid cooling is started by additionally applying a quenching facility that has not been applied in the primary cooling. When a cooling process is divided into primary and secondary cooling and is carried out by stages, a temperature distribution of the steel sheet may become uniform in a slow cooling step to reduce a final temperature and material deviation and it is also advantageous to obtain a required phase composition.

[0046] The primary cooling is slow cooling at an average cooling rate of 10°C / s or less, and a cooling end temperature thereof may be in a temperature in a range of 560 to 700°C. If the primary cooling end temperature is lower than 560°C, a ferrite phase is excessively precipitated to deteriorate a final hole expansion ratio. Meanwhile, if it exceeds 700°C, the secondary cooling is excessively loaded and a plate speed of the continuous annealing line has to be slowed, resulting in lower productivity. A lower limit of the primary cooling end temperature is more preferably 580°C and an upper limit of the primary cooling end temperature is more preferably 670°C.

[0047] For the secondary cooling, a quenching facility not applied in the primary cooling is additionally applied, and a hydrogen quenching facility using H 2 gas may be used. Here, it is important to control a secondary cooling end temperature to 270 to 330°C at which an appropriate initial martensite fraction may be obtained. If the secondary cooling end temperature is lower than 270°C, the initial martensite fraction transformed during the secondary cooling is too high, so that there is no space for obtaining various phase transformations required in a subsequent process and a shape and workability of the steel sheet deteriorate. Meanwhile, if the secondary cooling end temperature exceeds 330°C, the initial martensite fraction is low and high hole expansion ratio cannot be obtained. A lower limit of the secondary cooling end temperature is more preferably 290°C and an upper limit of the secondary cooling end temperature is more preferably 320°C. If the average cooling rate during the secondary cooling is less than 10°C / s, a ferrite / bainite phase or the like may be formed during cooling, resulting in a decrease in strength and making it difficult to finally secure a desired microstructure.Reheating process and hot-dip galvanizing process

[0048] The cooled steel sheet is reheated at a temperature in a range of 380°C to 460°C at a temperature increase rate of 5°C / s or less to temper the martensite obtained in the previous step, induce bainite transformation, and concentrate carbon in untransformed austenite adjacent to bainite. Here, it is important to control a reheating temperature to 380 to 460°C, and if reheating temperature is lower than 380°C or exceeds 460°C, the amount of phase transformation of bainite is small, so too much fresh martensite is formed in a final cooling process, significantly hurting elongation and hole expansion ratio. A lower limit of the reheating temperature is more preferably 440°C and an upper limit of the reheating temperature is more preferably 440°C. When the temperature increase rate during reheating exceeds 5°C / s, tempering of the martensite phase formed during the secondary cooling may be insufficient and there may be a possibility of not sufficiently obtaining bainite phase transformation during the temperature increase.

[0049] After reheating, hot-dip galvanizing is performed at a temperature in a range of 430°C to 490°C to obtain the high strength hot-dip galvanized steel sheet comprising a hot-dip zinc plating layer on a surface of the high-strength cold rolled sheet.

[0050] Annealing for alloying is performed to obtain the high strength hot-dip galvanized steel sheet comprising an alloyed hot-dip zinc plating layer on a surface of the high-strength cold rolled sheet, and then cooling is performed to room temperature.

[0051] After cooling to the room temperature, a process of performing temper rolling less than 1% may be performed to correct the shape of the steel sheet and adjust yield strength.[Mode for Invention](Example)

[0052] Hereinafter, the present invention will be described in more detail through examples.

[0053] After preparing the slab having the alloy composition of Table 1, a cold rolled steel sheet was manufactured through heating the steel slab under the conditions described in Tables 2 and 3 - hot rolling - cooling - coiling - cold rolling - continuous annealing - primary and secondary cooling - reheating. Meanwhile, the FDT shown in Tables 2 and 3 below refer to a finish delivery temperature, CT refers to a hot-rolled coiling temperature, SS refers to a continuous annealing temperature, SCS refers to a primary cooling end temperature, RCS refers to secondary cooling end temperature, and RHS refers to a reheating temperature.

[0054] After measuring a microstructure, mechanical properties, and maximum LME crack size for the prepared cold rolled steel sheet, the results are shown in Table 3 below.

[0055] As for the maximum LME crack size is, a sample was spot-welded under severe conditions of dome radius 6mm, pressing force 3.54kN, welding time 234ms, H / T 100ms, tilting 5 degrees, and gap 1.0mm, a certain cross section across a nugget was taken, and a maximum length of an existing LME crack was then measured.

[0056] A type and fraction of the microstructure were measured through XRD peak analysis in the case of retained austenite, and the fractions of the remaining fresh martensite, ferrite, cementite, bainite and tempered martensite phase were measured through a scanning electron microscope EBSD analysis. [Table 1]Ste el typeAlloy composition (wt%)CSiMnCrAlTiBPSCuNiMoNbVNC+(Si+Al ) / 5A0.1 70.7 262.5 80.4 990.0 530.0 190.0 0180.0 090.0 060.0 20.0 00.0 580.0 030.0 030.0 0450.33B0.1 730.5 442.7 60.0 180.0 520.0 190.0 0190.0 100.0 030.0 10.0 20.0 620.0 040.0 020.0 0650.29C0.1 70.5 252.60.50.2 050.0 190.0 0190.0 060.0 020.0 30.0 10.0 630.0 010.0 050.0 0340.32D0.1 620.5 012.50.4 70.4 500.0 180.0 0180.0 070.0 040.0 20.0 10.0 620.0 010.0 030.0 0770.35E0.1 550.7 42.6 60.5 20.0 450.0 20.0 0190.0 070.0 040.0 10.0 20.0 610.0 040.0 050.0 0900.31F0.2 370.6 962.40.4 80.0 430.0 180.0 0180.0 090.0 030.0 10.0 10.0 570.0 010.0 040.0 0880.38G0.1 820.7 23.5 80.5 140.0 480.0 220.0 0210.0 120.0 040.0 20.0 10.0 080.0 030.0 030.0 0720.34H0.1 80.7 31.6 72.5 600.0 530.0 20.0 020.0 060.0 030.0 20.0 10.0 150.0 010.0 010.0 0470.34I0.1 840.7 42.8 70.5 020.0 530.0 20.0 0190.0 060.0 060.0 30.0 10.0 050.0 040.0 040.0 0420.34J0.1 810.7 23.1 70.4 920.0 500.0 20.0 0210.0 040.0 060.0 30.0 10.0 060.0 020.0 020.0 0670.34K0.20.5 122.9 50.5 060.2 000.0 230.0 0220.0 080.0 040.0 40.0 10.0 080.0 020.0 040.0 0640.34L0.1 840.5 23.1 40.4 940.2 020.0 190.0 0200.0 070.0 030.0 10.0 00.0 090.0 020.0 040.0 0540.33M0.1 771.5 42.6 30.5 10.0 550.0 220.0 0220.0 090.0 030.0 10.0 00.0 570.0 030.0 020.0 0460.50 [Table 2] Classif icationStee l typeSlab heating temperat ure (°C)Hot rolled thickne ss (mm)FDT (°C)Average cooling rate after hot rolling (°C / s)CT (°C)Cold rolled thickness (mm)Cold rollin g reduct ion ratio( %)Compara tive Example 1A11962.4955585561.442Compara tive Example 2B12102.59356615451.636Compara tive Example 3C12031.8932486070.950Compara tive Example 4D12212.0942356331.050Compara tive Example 5E12442.4938555221.442Compara tive Example 6F11892.1966565251.243Compara tive Example 7G12022.3952475651.439Compara tive Example 8H12342.5922555451.540Compara tive Example 9I12312.5925615671.636Compara tive Example 10J11982.7945665521. 833Inventi ve Example 1K12122.2949585551.245Inventi ve Example 2L12482.1930575651.243Compara tive Example 11M12452.5947495521.636 [Table 3] Classif icationSte el typ eSS (°C)Average cooling rate for primary cooling (°C / s)SCS (°C)Average cooling rate for secondary cooling (°C / s)RCS (°C)RHS (°C)Reheating rate (°C / s)Compara tive Example 1A8592.668519.23064210.7Compara tive Example 2B8613.963718.63154181.4Compara tive Example 3C8623.962016.13234170.7Compara tive Example 4D8574.257712.93224450.9Compara tive Example 5E8463.960515.53194311. 8Compara tive Example 6F8474.459517.03014220.9Compara tive Example 7G8605.058717.82974242.6Compara tive Example 8H8473.762217.43024130.8Compara tive Example 9I8472.467515.63434170.6Compara tive Example 10J8573.463314.53474221.2Inventi ve Example 1K8404.060717.63024161.5Inventi ve Example 2L8483.661216.12954151. 8Compara tive Example 11M8424.160617.63014453.1 [Table 4] Classif icationStee l typeYS (MPa)TS (MPa )EL (%)YRHER (%)Fractio n of retaine d austeni te (area%)Fracti on of fresh marten site (area% )Fracti on of ferrit e (area% )Fractio n of cementi te (volume %)Maximum LME crack (µm)Compara tive Example 1A940112610. 80.8332.75%5%1%180Compara tive Example 2B962110210. 40.8733.74%3%0%269Compara tive Example 3C89811399.70.7922.14%15%3%260Compara tive Example 4D959108611. 30.8851.15%0%2%287Compara tive Example 5E978115211. 30.8528.45%5%0%177Compara tive Example 6F96312329.60.7822.45%10%1%1107Compara tive Example 7G744130410. 00.5710.94%20%4%176Compara tive Example 8H1092125312. 20.8725.66%8%0%165Compara tive Example 9I779123910. 60.6317.27%19%2%171Compara tive Example 10J91711939.80.7716. 65%17%1%177Inventi ve Example 1K919118111. 00.7836.54%5%0%266Inventi ve Example 2L839123511. 30.6837.95%10%1%269Compara tive Example 11M1042119515. 20.8741.19%3%0%0149

[0057] First, Comparative Examples 1 to 5 are cases in which steel types A to E were applied, respectively. Steel types A to E have the contents of C, Mn, or Cr lower than that of the range of the present invention, in which strength of TS 1180 MPa class could not be obtained. Even for steels, like steel types A to E, in which the alloy component addition amount is outside of the range of the components of the present invention, tensile strength higher than 1180 MPa may be obtained if annealing heat treatment conditions are significantly changed, but in this case, it is necessity to introduce an excessively large amount of fresh martensite and a high hole expansion ratio cannot be obtained. Comparative Example 6 is a case to which steel type F having a C content exceeding the range of the presentinvention was applied, in which a high hole expansion ratio could not be obtained even if the process conditions suggested in the present invention were satisfied.

[0058] Steel type G of Comparative Example 7 is a case in which the Mn content exceeds the range of the present invention, whereby a ratio of fresh martensite reaches 20%, so that a hole expansion ratio is significantly deteriorated and a yield ratio is also too low. In addition, steel type H of Comparative Example 8 was a steel type in which Cr was increased instead of Mn, and it was difficult to obtain a low yield ratio.

[0059] To Comparative Examples 9 and 10, steel types I and J satisfying the alloy composition of the present invention were applied, but as an annealing and quenching temperature exceeded 330°C, the ratio of fresh martensite increased and a hole expansion ratio was significantly deteriorated.

[0060] Inventive Examples 1 and 2 are cases to which steel types K and L satisfying the alloy composition of the invention present are applied and in which all process conditions are satisfied, and here, a hole expansion ratio of 25% or more and elongation suitable for processing of 5% to 13% may be obtained at a low yield ratio of 0.65 to 0.85.

[0061] Steel types F and M applied to Comparative Examples 6 and 11, respectively, have an alloy amount that does not satisfy Equation 1, and due to this, it can be seen that a maximum size of the LME crack in the weld portion exceeded 100 µm, and thus, LME crack resistance was inferior.

[0062] Meanwhile, cracks in an overlapping portion, which are not allowed to exist as severe LME cracks, were not present in all of the test materials.

[0063] While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A high-strength cold rolled steel sheet comprising: by weight percent, 0.17 to 0.21% of carbon (C), 0.3 to 0.8% of silicon (Si), 2.7 to 3.3% of manganese (Mn), 0.3 to 0.7% of chromium (Cr), 0.01 to 0.3% of aluminum (Al), 0.01 to 0.03% of titanium (Ti), 0.001 to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen (N), the balance of iron (Fe), and other inevitable impurities, optionally further comprising 0.1% or less of copper (Cu), 0.1% or less of nickel (Ni), 0.1% or less of molybdenum (Mo), 0.03% or less of niobium (Nb) and 0.01% of less of vanadium (V), wherein the contents of carbon (C), silicon (Si), and aluminum (Al) satisfy Equation 1 below, a microstructure thereof consists of, by area fraction, 3 to 7% of retained austenite, 5 to 15% of fresh martensite, 5% or less, including 0%, of ferrite, 1 to 3% of a cementite, and the balance of bainite or tempered martensite, and, wherein the cementite is precipitated and distributed between bainite laths or in the laths or grain boundary of a tempered martensite phase, wherein, a type and fraction of the microstructure were measured through XRD peak analysis in the case of retained austenite, and the fractions of the remaining fresh martensite, ferrite, cementite, bainite and tempered martensite phase were measured through a scanning electron microscope EBSD analysis, 0.20 % ≤ C + Si + Al / 5 ≤ 0.35 % , wherein [C], [Si], [Al] refer to weight percents of C, Si, and Al, respectively.

2. A high strength hot-dip galvanized steel sheet further comprising a hot-dip zinc plating layer on a surface of the high-strength cold rolled steel sheet of claim 1.

3. A high strength hot-dip galvanized steel sheet further comprising an alloyed hot-dip zinc plating layer on a surface of the high-strength cold rolled steel sheet of claim 1.

4. A method of manufacturing a high-strength cold rolled steel sheet according to claim 1, the method comprising: preparing a slab including, by weight percent, 0.17 to 0.21% of carbon (C), 0.3 to 0.8% of silicon (Si), 2.7 to 3.3% of manganese (Mn), 0.3 to 0.7% of chromium (Cr), 0.01 to 0.3% of aluminum (Al), 0.01 to 0.03% of titanium (Ti), 0.001 to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.01% or less of nitrogen (N), the balance of iron (Fe), and other inevitable impurities, optionally further comprising 0.1% or less of copper (Cu), 0.1% or less of nickel (Ni), 0.1% or less of molybdenum (Mo), 0.03% or less of niobium (Nb) and 0.01% of less of vanadium (V), wherein the contents of carbon (C), silicon (Si), and aluminum (Al) satisfy Equation 1 below; heating the slab to a temperature in a range of 1,150°C to 1,250°C; finish hot rolling the heated slab within a finish delivery temperature (FDT) range of 900°C to 980°C to obtain a hot rolled steel sheet; cooling the hot rolled steel sheet at an average cooling rate of 10°C / sec to 100°C / sec after the finish hot rolling; coiling the cooled hot rolled steel sheet in a temperature in a range of 500°C to 700°C; cold rolling the coiled hot rolled steel sheet at a cold-rolling reduction ratio of 30% to 60% to obtain a cold rolled steel sheet; continuously annealing the cold rolled steel sheet at a temperature in a range of Ae3+30°C to Ae3+80°C; primarily cooling the continuously annealed steel sheet at an average cooling rate of 10°C / s or less to a temperature in a range of 560°C to 700°C and secondarily cooling the steel sheet at an average cooling rate of 10°C / s or more to a temperature in a range of 270°C to 330°C, where a temperature for terminating the primary cooling is defined as a point in time at which rapid cooling is started by additionally applying a quenching facility that has not been applied in the primary cooling; and reheating the cooled steel sheet at a temperature increase rate of 5°C / s or lower to a temperature in a range of 380°C to 460°C, 0.20 % ≤ C + Si + Al / 5 ≤ 0.35 % , wherein [C], [Si], and [Al] refer to weight percent of C, Si, and Al, respectively.

5. The method of claim 4, wherein the continuous annealing is performed at a temperature in a range of 830°C to 880°C.

6. A method of manufacturing a high strength hot-dip galvanized steel sheet according to claim 2, comprising: performing method steps according to claim 4, performing hot-dip zinc plating on the reheated cold rolled steel sheet at a temperature in a range of 430°C to 490°C.

7. A method of manufacturing a high strength hot-dip galvanized steel sheet according to claim 3, comprising: performing method steps according to claim 6, wherein, after the hot-dip zinc plating, annealing for alloying is performed, and cooling is then performed to room temperature.

8. The method of claim 7, wherein, after cooling to the room temperature, temper rolling less than 1% is performed.