Cold rolled steel sheet and method of manufacturing same

Abstract

One aspect of the present invention provides a cold rolled steel sheet and a method of manufacturing same. One preferred aspect of the present invention provides a cold rolled steel sheet having excellent formability, strength, and LME resistance, and a method of 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 specifically, to a cold-rolled steel sheet and a method for manufacturing the same that can be suitably applied to automobile crash members and structural members.

[0002] Recently, automotive steel sheets have been required to have higher strength to improve fuel efficiency and durability due to various environmental and energy consumption regulations. In particular, with the recent expansion of automobile impact safety regulations, high-strength steels with superior yield strength are being adopted for structural components such as members, seat rails, and pillars to enhance the impact resistance of the body. These structural components have the characteristic of having a higher yield strength relative to the tensile strength, i.e., a higher yield ratio (yield strength / tensile strength), which is advantageous for impact energy absorption. However, as the strength of steel sheets generally increases, the elongation decreases, resulting in a problem of reduced formability. Therefore, the development of materials that can compensate for this problem is urgently needed.

[0003] Conventional methods for strengthening steel include solid solution strengthening, precipitation strengthening, grain refinement strengthening, and transformation strengthening. However, among the above methods, solid solution strengthening and grain refinement strengthening have the disadvantage of making it very difficult to produce high-strength steel with a tensile strength of 490 MPa or higher.

[0004] Meanwhile, precipitation-strengthened high-strength steel is a technology that secures strength by precipitating carbon and nitrides by adding carbon and nitride-forming elements such as Cu, Nb, Ti, and V to strengthen the steel sheet, or by refining the grains by suppressing grain growth due to fine precipitates. This technology has the advantage of easily obtaining high strength at a low manufacturing cost, but has the disadvantage of requiring high-temperature annealing to ensure sufficient recrystallization and ductility because the recrystallization temperature rises rapidly due to fine precipitates. In addition, precipitation-strengthened steel, which seeks strengthening by precipitating carbon and nitrides in a ferrite matrix, has the problem that it is difficult to obtain high-strength steel of the 600 MPa class or higher.

[0005] Meanwhile, various types of transformation-strengthened high-strength steels have been developed, including dual-phase ferrite-martensite steels that incorporate hard martensite into a ferrite matrix, TRIP (Transformation Induced Plasticity) steels that utilize the transformation-induced plasticity of retained austenite, and CP (Complexed Phase) steels composed of ferrite and hard bainite or martensite structures. In addition, hot press forming steels that secure final strength through rapid cooling through direct contact with a die that is formed at high temperatures and then cooled by water are attracting attention as structural members to ensure crashworthiness. However, the high investment in facilities and the high costs of heat treatment and processing have hindered their expansion into application.

[0006] In particular, the demand for TRIP steel as a replacement for hot press forming steel has been gradually increasing recently, but in the case of galvanized steel sheets containing Si in weight percent of 1% or more, the expansion of application is hindered due to the liquid metal embrittlement (LME) that occurs when spot welding is applied.

[0007] A representative manufacturing method for securing LME resistance in TRIP steel involves controlling the dew point of the annealing furnace, thereby forming a soft layer by decarburizing the steel surface during annealing or plating heat treatment, thereby suppressing LME generation. However, when decarburizing during annealing or plating heat treatment, the dew point change range of the annealing furnace is essential. Therefore, securing the desired decarburization range requires a sufficient amount of decarburization in the connecting section, which inevitably leads to a decrease in manufacturing yield. Furthermore, the connecting section also requires a minimum of 30 minutes to several hours of time.

[0008] Therefore, a technology is required to secure LME resistance by forming a soft layer on the surface without decarburization during annealing or plating heat treatment.

[0009] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0010] A preferred aspect of the present invention is to provide a cold-rolled steel sheet having excellent formability, strength and LME resistance and a method for manufacturing the same.

[0011] One embodiment of the present invention provides a cold-rolled steel sheet comprising, in wt%, C: more than 0.150% and 0.30% or less, Si: 3.0% or less (excluding 0%), Mn: 0.90 to 5.0%, Al: 3.0% or less (excluding 0%), P: 0.150% or less (excluding 0%), S: 0.030% or less (excluding 0%), N: 0.030% or less (excluding 0%), the remainder being Fe and other unavoidable impurities, and satisfying the following relational expressions 1 to 3.

[0012] [Relationship 1] 0.15 ≤ A1 ≤ 0.8

[0013] [Relationship 2] 0.15 ≤ A2 ≤ 0.8

[0014] [Relationship 3] 0.6 ≤ B ≤ 1.5

[0015] (However, in the above relational expressions 1 to 3, A means (C0-C1) / C0, and the C0 means the average Mn content of the steel plate, the C1 means the average Mn content at a position of 1 ㎛ in the thickness direction from the surface of the steel plate, A1 means the A value at the edge in the width direction of the steel plate, A2 means the A value at the center in the width direction of the steel plate, and B means A1 / A2.)

[0016] The above cold rolled steel sheet may have a microstructure including ferrite, fresh martensite, tempered martensite, bainite, and retained austenite.

[0017] The above cold rolled steel sheet may have a tensile strength of 980 MPa or more.

[0018] The above cold rolled steel sheet may have an elongation of 15% or more.

[0019] The above cold rolled steel sheet may not develop B type LME cracks during spot welding.

[0020] The above cold rolled steel sheet may have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on at least one surface.

[0021] Another embodiment of the present invention comprises the steps of: heating a slab comprising, in wt%, C: more than 0.150% and 0.30% or less, Si: 3.0% or less (excluding 0%), Mn: 0.90 to 5.0%, Al: 3.0% or less (excluding 0%), P: 0.150% or less (excluding 0%), S: 0.030% or less (excluding 0%), N: 0.030% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; finishing hot-rolling the heated slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 500 to 680°C to obtain a coil; heat-treating the coiled coil at 500 to 750°C for 5 to 48 hours; cooling the heat-treated coil at a cooling rate of 0.005°C / s or more; And a step of cold rolling the cooled coil to obtain a cold rolled steel sheet is provided.

[0022] Heating of the above slab can be performed at 1000 to 1350°C.

[0023] Heating of the above slab can be carried out for 60 to 1000 minutes.

[0024] The above finishing hot rolling can be performed at 800 to 1000°C.

[0025] After the above cold rolling, a step of continuous annealing at 730 to 900°C may be additionally included.

[0026] After the above continuous annealing, a step of obtaining a hot-dip galvanized steel sheet by hot-dip galvanizing the continuously annealed cold-rolled steel sheet at 450 to 490°C may be additionally included.

[0027] After the above-mentioned hot-dip galvanizing, a step of temper rolling the hot-dip galvanized steel sheet at a reduction ratio of 0.01 to 0.5% may be additionally included.

[0028] After the above-described hot-dip galvanizing, a step of obtaining an alloyed hot-dip galvanized steel sheet by subjecting the hot-dip galvanized steel sheet to an alloying heat treatment at 450 to 580°C may be additionally included.

[0029] After the above alloying heat treatment, a step of temper rolling the alloyed hot-dip galvanized steel sheet at a reduction ratio of 0.01 to 0.5% may be additionally included.

[0030] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0031] According to a preferred aspect of the present invention, a cold-rolled steel sheet having excellent formability, strength and LME resistance and a method for manufacturing the same can be provided.

[0032] Figure 1 is an example of a graph measured using a glow discharge spectrometer (GDS) analysis method.

[0033] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition will be described. Unless otherwise specified, the alloy composition described below refers to weight percent.

[0034] C: 0.150% or more and 0.30% or less

[0035] C is an element that is advantageous in securing strength, and also plays a role in stabilizing retained austenite, which increases the ductility of the steel sheet. If the C content is less than 0.150%, it may be difficult to secure the desired tensile strength. If the C content exceeds 0.30%, cold rolling may be difficult during the production of the steel sheet. Therefore, the C content is preferably in the range of more than 0.150% and less than 0.30%. The lower limit of the C content is more advantageously 0.160%, and 0.170% is even more advantageous. The upper limit of the C content is more advantageously 0.280%, and 0.270% is even more advantageous, and 0.250% is most advantageous.

[0036] Si: 3.0% or less (excluding 0%)

[0037] Si is an element that improves strength through solid solution strengthening, and plays a role in strengthening ferrite, uniformizing the microstructure, and improving workability. In addition, Si can contribute to the formation of retained austenite by suppressing cementite precipitation. If the Si content exceeds 3.0%, plating defects such as under-plating may occur in the plating process during the manufacture of the steel sheet, or the weldability of the steel sheet may deteriorate. Therefore, the Si content is preferably 3.0% or less (excluding 0%). The Si content is more advantageously 2.70% or less, and even more advantageously 2.30% or less. Meanwhile, the present invention does not specifically limit the lower limit of the Si content, but as an example, the lower limit of the Si content may be 0.50%. The lower limit of the Si content is more advantageously 0.70%, and even more advantageously 1.0%.

[0038] Mn: 0.90~5.0%

[0039] Manganese (Mn) is an element that helps improve both strength and ductility. If the Mn content is less than 0.90%, it may be difficult to sufficiently secure the above-described effect. If the Mn content exceeds 5.0%, the carbon enrichment in austenite may not be sufficient due to the increase in bainite transformation time, making it difficult to sufficiently secure retained austenite. Therefore, the Mn content is preferably in the range of 0.90 to 5.0%. The lower limit of the Mn content is more advantageously 1.0%, and the upper limit is more advantageously 1.50%. The upper limit of the Mn content is more advantageously 4.0%, and the upper limit is more advantageously 3.0%.

[0040] Al: 3.0% or less (excluding 0%)

[0041] Al combines with oxygen in steel to have a deoxidizing effect. In addition, Al helps stabilize residual austenite by suppressing cementite precipitation. If the Al content exceeds 3.0%, the workability of the steel sheet may deteriorate and inclusions may increase. Therefore, the Al content is preferably 3.0% or less (excluding 0%). It is more advantageous when the Al content is 2.0% or less, and even more advantageous when it is 1.0% or less. Meanwhile, the present invention does not specifically limit the lower limit of the Al content, but as an example, the lower limit of the Al content may be 0.0150%. It is more advantageous when the lower limit of the Al content is 0.020%, and even more advantageous when it is 0.050%.

[0042] P: 0.150% or less (excluding 0%)

[0043] P is an impurity that is inevitably included in the manufacturing process, and if its content exceeds 0.150%, it may deteriorate impact toughness. Therefore, it is preferable that the P content be in the range of 0.150% or less (excluding 0%). It is more advantageous that the P content be 0.10% or less, and even more advantageous that it be 0.080% or less. Meanwhile, the present invention does not specifically limit the lower limit of the P content, but as an example, the lower limit of the P content may be 0.00010%. It is more advantageous that the lower limit of the P content be 0.00030%, and even more advantageous that it be 0.00050%.

[0044] S: 0.030% or less (excluding 0%)

[0045] S is an impurity that is inevitably included in the manufacturing process, and when its content exceeds 0.030%, it may form MnS in the steel sheet and deteriorate ductility. Therefore, the S content is preferably in the range of 0.030% or less (excluding 0%). It is more advantageous that the S content is 0.010% or less, and even more advantageous that it is 0.0080% or less. Meanwhile, the present invention does not specifically limit the lower limit of the S content, but as an example, the lower limit of the S content may be 0.0010%. It is more advantageous that the lower limit of the S content is 0.0020%, and even more advantageous that it is 0.0030%.

[0046] N: 0.030% or less (excluding 0%)

[0047] N is an impurity that is inevitably included in the manufacturing process, and if its content exceeds 0.030%, it may form nitrides during continuous casting, causing cracks in the slab. Therefore, the N content is preferably in the range of 0.030% or less (excluding 0%). It is more advantageous that the N content is 0.010% or less, and even more advantageous that it is 0.0080% or less. Meanwhile, the present invention does not specifically limit the lower limit of the N content, but as an example, the lower limit of the N content may be 0.0010%. It is more advantageous that the lower limit of the N content is 0.0020%, and even more advantageous that it is 0.0030%.

[0048] The remaining component is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0049] It is preferable that the cold-rolled steel sheet of the present invention satisfy the following equations 1 to 3. Fig. 1 is an example of a graph measured using a glow discharge spectrometer (GDS) analysis method. Referring to Fig. 1, in the following equations 1 to 3, A means (C0-C1) / C0, C0 below means the average Mn content of the steel sheet, C1 below means the average Mn content at a position of 1 ㎛ in the thickness direction from the surface of the steel sheet, A1 means the A value at the edge in the width direction of the steel sheet, A2 means the A value at the center in the width direction of the steel sheet, and B means A1 / A2. Meanwhile, the edge may be, for example, a position 10 mm away from the end in the width direction of the steel sheet.

[0050] [Relationship 1] 0.15 ≤ A1 ≤ 0.8

[0051] [Relationship 2] 0.15 ≤ A2 ≤ 0.8

[0052] The above equations 1 and 2 are intended to suppress LME resistance by controlling the Mn deficiency rate of the surface layer. When the values ​​of A1 and A2 are less than 0.15, LME may occur during welding. When the values ​​of A1 and A2 exceed 0.8, the strength of the surface layer may decrease due to excessive Mn deficiency, which may result in poor durability such as fatigue properties. Therefore, the values ​​of A1 and A2 are preferably in the range of 0.15 to 0.8. The lower limit of the values ​​of A1 and A2 is more advantageously 0.16, more advantageously 0.17, and most advantageously 0.18. The upper limit of the values ​​of A1 and A2 is more advantageously 0.7, more advantageously 0.6, and most advantageously 0.5.

[0053] [Relationship 3] 0.6 ≤ B ≤ 1.5

[0054] If a large Mn deficiency layer is formed on the surface of the steel sheet before plating, the diffusion of Fe into the Zn plating layer is promoted during the alloying heat treatment for manufacturing GA steel sheets, thereby increasing the alloying degree of the plating layer and improving plating adhesion. The above relationship 3 is an index representing the homogeneity of the Mn deficiency rate to prevent color differences on the surface of the plating layer from occurring due to alloying deviations or plating amount deviations during the manufacturing of GA steel sheets. When the value of B is less than 0.6, the plating amount may be excessive or alloying may occur in the widthwise center of the steel sheet compared to the edge. When the value of B exceeds 1.5, the plating amount may be excessive or alloying may occur in the widthwise edge of the steel sheet compared to the center. Therefore, the value of B is preferably in the range of 0.6 to 1.5. The lower limit of the value of B is more advantageously 0.65, more advantageously 0.68, and most advantageously 0.7. The upper limit of the value of B above is more advantageous when it is 1.4, more advantageous when it is 1.35, and most advantageous when it is 1.3.

[0055] The above cold-rolled steel sheet may have a microstructure including ferrite, fresh martensite, tempered martensite, bainite, and retained austenite. By having the above microstructure, desired mechanical properties such as tensile strength and elongation can be easily secured.

[0056] As described above, the cold-rolled steel sheet of the present invention may have a tensile strength of 980 MPa or more. Furthermore, the elongation may be 15% or more. Furthermore, B-type LME cracks may not occur during spot welding.

[0057] Meanwhile, the cold rolled steel sheet of the present invention may have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on at least one surface.

[0058] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0059] First, a slab satisfying the aforementioned alloy composition is heated. The present invention does not specifically limit the slab heating conditions, and conventional conditions used in the relevant technical field can be applied. However, as an example, the slab may be heated at 1000 to 1350°C.

[0060] Thereafter, the heated slab is subjected to final hot rolling to obtain a hot-rolled steel sheet. The present invention does not specifically limit the final hot rolling conditions, and conventional conditions used in the relevant technical field may be applied. However, as an example, the final hot rolling may be performed at 800 to 1000°C.

[0061] In general, since the center of the coil in the width direction remains at a higher temperature for a longer period of time than the edge, a larger Mn-deficient layer is formed in the center than in the edge. On the other hand, during heat treatment, since the heat source is supplied from the outside of the coil, the edge of the coil remains at a higher temperature for a longer period of time than the center, and a larger Mn-deficient layer is formed in the edge than in the center. That is, in the present invention, a Mn-deficient layer is uniformly formed in the surface by appropriately controlling the following winding process and heat treatment process.

[0062] Thereafter, the hot-rolled steel sheet is coiled at 500 to 680°C to obtain a coil. If the coiling temperature is less than 500°C, the coiling process may not be easily performed, and a Mn-deficient layer may not be sufficiently formed on the surface, or the Mn-deficient layer may be formed unevenly. If the coiling temperature exceeds 680°C, scale generated on the surface of the hot-rolled steel sheet may form into the interior of the steel sheet, making the pickling process difficult. Therefore, the coiling temperature is preferably in the range of 500 to 680°C. The lower limit of the coiling temperature is more advantageously 510°C, more advantageously 520°C, and most advantageously 530°C. The upper limit of the coiling temperature is more advantageously 670°C, more advantageously 660°C, and most advantageously 650°C.

[0063] Thereafter, the coil is heat-treated at 500 to 750°C for 5 to 48 hours. The heat treatment is to eliminate the unevenness of the Mn deficiency layer in the width direction of the steel sheet and reduce the cold rolling load. If the heat treatment temperature is less than 500°C, the cold rolling load may not be sufficiently reduced, the Mn deficiency layer may not be sufficiently formed at the width direction edge of the coil, or the Mn deficiency layer may be unevenly formed. If the heat treatment temperature exceeds 750°C, scale may be formed deep into the steel sheet, making the pickling process difficult. Therefore, the heat treatment temperature is preferably in the range of 500 to 750°C. The lower limit of the heat treatment temperature is more advantageously 520°C, more advantageously 540°C, and most advantageously 550°C. The upper limit of the heat treatment temperature is more advantageously 720°C, more advantageously 700°C, and most advantageously 680°C. If the heat treatment time is less than 5 hours, heat transfer to the center of the coil may not be sufficiently performed, so that the cold rolling load may not be sufficiently reduced, or a problem may arise in that a Mn-deficient layer may not be sufficiently formed at the edge of the width direction of the coil. If the heat treatment time exceeds 48 hours, scale may be formed deep into the steel sheet, making the pickling process difficult. Therefore, the heat treatment time is preferably in the range of 5 to 48 hours. The lower limit of the heat treatment time is more advantageously 6 hours, more advantageously 7 hours, and most advantageously 8 hours. The upper limit of the heat treatment time is more advantageously 40 hours, more advantageously 30 hours, and most advantageously 24 hours.

[0064] Thereafter, the heat-treated coil is cooled at a cooling rate of 0.005°C / s or more. The cooling is to prevent excessive formation of a Mn-deficient layer that may occur after the heat treatment of the coil. If the cooling rate is less than 0.005°C / s, the Mn-deficient layer may be formed excessively or unevenly. In addition, coarse carbides may be formed on the steel sheet, which may cause a problem of lowering the toughness. Therefore, the cooling rate is preferably in the range of 0.005°C / s or more. It is more advantageous when the cooling rate is 0.006°C / s or more, more advantageous when it is 0.007°C / s or more, and most advantageous when it is 0.008°C / s or more. In the present invention, there is no particular limitation on the upper limit of the cooling rate, but as an example, the upper limit of the cooling rate may be 10°C / s. The upper limit of the above cooling rate is more advantageously 5°C / s, more advantageously 3°C / s, and most advantageously 1°C / s. The above cooling can be achieved by applying various cooling methods such as fan cooling, mist cooling, and water cooling. Meanwhile, the above cooling rate may be measured at a midpoint between the outermost and innermost portions of the edge portion of the winding coil. In addition, since the present invention performs cooling on the winding coil, in the case of general air cooling, the cooling rate may be less than 0.005°C / s.

[0065] Thereafter, the cooled coil is cold rolled to obtain a cold rolled steel sheet. In the present invention, there are no particular limitations on the cold rolling conditions, and typical conditions used in the relevant technical field can be applied.

[0066] After the above cold rolling, a continuous annealing step may be additionally included. The present invention does not specifically limit the conditions for the continuous annealing, and conventional conditions used in the relevant technical field may be applied. However, as an example, the continuous annealing may be performed at 730 to 900°C. Meanwhile, cooling to 250 to 500°C may be performed after the continuous annealing.

[0067] After the continuous annealing, an additional step of hot-dip galvanizing the continuously annealed cold-rolled steel sheet to obtain a hot-dip galvanized steel sheet may be included. The present invention does not specifically limit the hot-dip galvanizing conditions, and typical conditions used in the relevant technical field may be applied. However, as an example, the hot-dip galvanizing may be performed at 450 to 490°C.

[0068] After the above-described hot-dip galvanizing, an additional step of subjecting the hot-dip galvanized steel sheet to an alloying heat treatment to obtain an alloying hot-dip galvanized steel sheet may be included. The present invention does not specifically limit the alloying heat treatment conditions, and conventional conditions used in the relevant technical field may be applied. However, as an example, the alloying heat treatment may be performed at 450 to 580°C.

[0069] After the above-described hot-dip galvanizing or the above-described alloying heat treatment, a step of temper rolling the hot-dip galvanized steel sheet or the above-described alloyed hot-dip galvanized steel sheet may be additionally included. The present invention does not specifically limit the conditions for the temper rolling, and typical conditions used in the relevant technical field may be applied. However, as an example, the temper rolling may be performed at a reduction ratio of 0.01 to 0.5%.

[0070] Hereinafter, the present invention will be described in more detail through examples. However, the following examples are merely illustrative examples for further explaining the present invention and do not limit the scope of the present invention.

[0071] (Example)

[0072] A slab having the alloy composition shown in Table 1 below was heated at 1200°C for 1 hour and final hot-rolled at 900°C to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was coiled under the conditions shown in Table 2 below to obtain a coiled coil. Thereafter, the coiled coil was heat-treated and cooled under the conditions shown in Table 2 below, and then cold-rolled to manufacture a cold-rolled steel sheet. At this time, during the cooling, the cooling rate was measured by measuring the temperature at half of the inner and outer coils at the side of the coil, and the average cooling rate during cooling to 400°C was measured. For the cold-rolled steel sheet thus manufactured, A1 and A2, tensile strength, elongation, LME resistance, and GA alloying deviation were measured, and the results are shown in Table 3 below. Meanwhile, heat treatment was not performed on the coiled coils for Comparative Examples 1 and 3.

[0073] A1 and A2 were measured using a glow discharge spectrometer (GDS) analysis method. Here, A means (C0-C1) / C0, C0 means the average Mn content of the steel sheet, C1 means the average Mn content at a position of 1 ㎛ in the thickness direction from the surface of the steel sheet, A1 means the A value at the edge (10 mm away from the end) in the width direction of the steel sheet, and A2 means the A value at the center in the width direction of the steel sheet. In addition, B described in Table 3 below means A1 / A2.

[0074] Tensile strength and elongation were measured by taking JIS No. 5 tensile test specimens from the above cold-rolled steel sheet and conducting a tensile test.

[0075] LME resistance was measured for the occurrence of B type LME cracks after spot welding the cold rolled steel plate according to the SEP 1220-2 standard. At this time, the occurrence of B type LME cracks was measured for the edge and center in the width direction of the steel plate, with the edge being 10 mm away from the end of the steel plate and the center being 1 / 2 of the distance.

[0076] GA alloying deviation was measured by continuously annealing and cooling the cold-rolled steel sheet under the conditions described in Table 2 below, hot-dip galvanizing at 460°C, temper rolling at a reduction ratio of 0.2%, and alloying heat treatment at 520°C to manufacture an alloyed hot-dip galvanized steel sheet. The alloying degree (Fe content, wt%) of the plating layer was measured through XRF analysis for this alloyed hot-dip galvanized steel sheet, and the deviation between the center and the edge was calculated. At this time, if the alloying deviation was 3 or less, it was evaluated as good, and if it was more than 3, it was evaluated as bad.

[0077] Steel grade No. Alloy composition (weight %) CSiMnBAlPSN10.201.452.250.00140.0210.0090.0030.0020.171.712.540.00240.0310.0110.0040.00530.251.301.810.00180.3000.0110.0040.00540.211.551.920.00220.0200.0 110.0030.00550.181.502.600.00170.0250.0120.0030.00560.211.551.920.00220.0200.0110.0030.00570.181.502.600.00170.0250.0120.0030.00580.171.712.540.00240.0310.0110.0040.005

[0078] Classification Steel grade No. Coiling temperature (℃) Heat treatment temperature (℃) Heat treatment time (hours) Cooling speed (℃ / s) Continuous annealing temperature (℃) Cooling end temperature (℃) Invention example 1 157 564 09 0.01 28 1142 1 Invention example 2 26 355 30 15 0.01 8 7240 9 Invention example 3 36 00 58 0 11 0.00 8 3544 8 Comparative example 1 452 0--0.005 825410Comparison Example 2 5490450120.011821403Comparison Example 36665--0.005825410Comparison Example 4 745251080.01820401Comparison Example 5 7680420100.003818405Comparison Example 6 8480700120.011835413Comparison Example 7 8635530100.002863411

[0079] Classification A1A2B Tensile strength (MPa) Elongation (%) Edge LME crack occurrence Center LME crack occurrence GA alloying deviation Invention example 10.3640.2451.5102123 Not occurred Not occurred Good Invention example 20.1960.3010.7129016 Not occurred Not occurred Good Invention example 30.2870.2661.199127 Not occurred Not occurred Good Comparative example 10.0350.0560.6100125 Occurred Occurred Good Comparative example 20.0910.0146.511521 8 Occurrence Occurrence Defect Comparison Example 30.0420.2870.1102024 Occurrence Non-occurrence Defect Comparison Example 40.1540.0285.5113119 Non-occurrence Occurrence Defect Comparison Example 50.1750.4480.4110220 Non-occurrence Non-occurrence Defect Comparison Example 60.4060.1682.4126715 Non-occurrence Non-occurrence Defect Comparison Example 70.1870.3510.5128615 Non-occurrence Non-occurrence Defect

[0080] As can be seen from Tables 1 to 3 above, in the case of Invention Examples 1 to 3 that satisfy the conditions proposed by the present invention, not only is the formability and strength good, but also the LME resistance is excellent and the GA alloying deviation is also good.

[0081] On the other hand, in the case of Comparative Examples 1 to 7 that do not satisfy the conditions proposed by the present invention, it can be confirmed that the formability and strength are good, but the LME resistance or GA alloying deviation is poor.

Claims

1. Contains, in weight%, C: more than 0.150% but not more than 0.30%, Si: 3.0% or less (excluding 0%), Mn: 0.90 to 5.0%, Al: 3.0% or less (excluding 0%), P: 0.150% or less (excluding 0%), S: 0.030% or less (excluding 0%), N: 0.030% or less (excluding 0%), and the remainder is composed of Fe and other unavoidable impurities. Cold rolled steel sheet satisfying the following relationships 1 to 3. [Relationship 1] 0.15 ≤ A1 ≤ 0.8 [Relationship 2] 0.15 ≤ A2 ≤ 0.8 [Relationship 3] 0.6 ≤ B ≤ 1.5 (However, in the above relational expressions 1 to 3, A means (C0-C1) / C0, wherein C0 means the average Mn content of the steel plate, C1 means the average Mn content at a position of 1 ㎛ in the thickness direction from the surface of the steel plate, A1 means the A value at the edge in the width direction of the steel plate, A2 means the A value at the center in the width direction of the steel plate, and B means A1 / A2.) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a microstructure including ferrite, fresh martensite, tempered martensite, bainite, and retained austenite.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength of 980 MPa or more.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having an elongation of 15% or more.

5. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet that does not develop B type LME cracks during spot welding.

6. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer formed on at least one surface.

7. A step of heating a slab comprising, by weight%, C: more than 0.150% and 0.30% or less, Si: 3.0% or less (excluding 0%), Mn: 0.90 to 5.0%, Al: 3.0% or less (excluding 0%), P: 0.150% or less (excluding 0%), S: 0.030% or less (excluding 0%), N: 0.030% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; A step of obtaining a hot-rolled steel sheet by final hot-rolling the above heated slab; A step of coiling the hot-rolled steel plate at 500 to 680°C to obtain a coil; A step of heat treating the above-mentioned winding coil at 500 to 750°C for 5 to 48 hours; A step of cooling the heat-treated coil at a cooling rate of 0.005℃ / s or higher; and A method for manufacturing a cold rolled steel sheet, comprising: a step of cold rolling the cooled coil to obtain a cold rolled steel sheet.

8. In claim 7, A method for manufacturing cold rolled steel sheets in which the above slab is heated at 1000 to 1350°C.

9. In claim 7, A method for manufacturing cold rolled steel sheets, wherein heating of the above slab is performed for 60 to 1000 minutes.

10. In claim 7, The above finishing hot rolling is a method for manufacturing cold rolled steel sheets performed at 800 to 1000°C.

11. In claim 7, A method for manufacturing a cold rolled steel sheet, further comprising a step of continuously annealing at 730 to 900°C after the cold rolling.

12. In claim 11, A method for manufacturing a cold rolled steel sheet, further comprising, after the continuous annealing, a step of hot-dip galvanizing the continuously annealed cold rolled steel sheet at 450 to 490°C to obtain a hot-dip galvanized steel sheet.

13. In claim 12, A method for manufacturing a cold rolled steel sheet, further comprising, after the above-mentioned hot-dip galvanizing, a step of temper rolling the hot-dip galvanized steel sheet at a reduction ratio of 0.01 to 0.5%.

14. In claim 12, A method for manufacturing a cold rolled steel sheet, further comprising the step of, after the above-mentioned hot-dip galvanizing, performing an alloying heat treatment on the hot-dip galvanized steel sheet at 450 to 580°C to obtain an alloyed hot-dip galvanized steel sheet.

15. In claim 14, A method for manufacturing a cold rolled steel sheet, further comprising, after the above alloying heat treatment, a step of temper rolling the above alloyed hot-dip galvanized steel sheet at a reduction ratio of 0.01 to 0.5%.

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