Cold-rolled steel sheet and manufacturing method therefor

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method therefor and, more specifically, to a cold-rolled steel sheet applicable to a collision energy absorbing member requiring high formability among body-in-white (BIW) structural members, and a manufacturing method therefor.

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 applicable to a BIW (Body-in-white) structural member requiring high formability, such as a collision energy absorbing member, and a method for manufacturing the same.

[0002] In the automotive industry, advanced countries, led by Europe, are actively conducting research to reduce vehicle weight for fuel efficiency regulations and improved performance. To meet these demands, the steel industry is striving to increase strength and reduce plate thickness within the same grade compared to competing materials (e.g., magnesium, aluminum, CFRP). In addition to this weight reduction, tightening CO2 emission regulations and the rapid shift toward electric vehicles are also driving the need for stronger safety regulations for vehicle passengers and pedestrians, necessitating greater safety and higher strength in body materials. Furthermore, demand for cold-rolled steel sheets is increasing to reduce processing costs, and research is also being conducted on forming multiple components into a single part at once. To form such complex shapes in one go, materials with superior formability are required.

[0003] In addition to formability, these structural members must also have a high yield ratio to advantageously absorb impact energy. A representative manufacturing method for increasing the yield strength is to utilize water cooling during continuous annealing. A representative prior art of this method is Patent Document 1. Patent Document 1 is a technology for manufacturing steel having a martensite volume ratio of 80 to 97% and the remainder ferrite by continuously annealing a steel having a carbon content of 0.18 to 0.3%, cooling it to room temperature with water, and then performing an overaging treatment at a temperature of 120 to 300°C for 1 to 15 minutes. In this way, when a cold-rolled steel sheet is rapidly cooled to room temperature after two-phase or single-phase annealing and then tempered, ultra-high strength steel can be manufactured. However, according to this method, although the yield strength and hole expandability are excellent, the shape quality of the coil may deteriorate due to temperature deviation in the width direction and length direction, and problems such as material defects and reduced workability may occur depending on the area when processing roll-forming parts.

[0004] In addition, as the strength of the steel sheet generally increases, the elongation decreases, which causes a problem of lowering the formability, so its application as a material for cold stamping is limited. In order to form steel into complex shapes, the elongation must be high, and a representative method for increasing the elongation is widely used by utilizing the TRIP phenomenon by introducing retained austenite, as in Patent Document 2. However, in the existing patent for the development of 780 MPa-class steel grade using the TRIP phenomenon, the elongation x TS level does not stably reach the level of 21800 MPa%. Therefore, in order to solve the above-mentioned problems, it is necessary to develop a high-strength, ultra-high elongation cold-rolled steel sheet with excellent elongation and a tensile strength of 780 MPa or more.

[0005] [Prior Art Literature]

[0006] [Patent Document]

[0007] (Patent Document 1) Japanese Patent Publication No. 1992-289120

[0008] (Patent Document 2) Japanese Patent Publication No. 2002-382250

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

[0010] More specifically, one aspect of the present invention is to provide a cold-rolled steel sheet having a tensile strength of 780 MPa, high elongation, and an appropriate balance of strength and elongation, and a method for manufacturing the same.

[0011] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall content of this specification.

[0012] According to one aspect of the present invention, a cold-rolled steel sheet comprises, in wt%, C: 0.20 to 0.30%, Si: 1.45 to 2.0%, Mn: 1.2 to 1.85%, Al: 0.010 to 1.0%, P: 0.100% or less (excluding 0%), S: 0.0300% or less (excluding 0%), N: 0.01000% or less (excluding 0%), and the remainder is made of Fe and other inevitable impurities, wherein the C, Si, and Mn simultaneously satisfy the following equations 1, 2, and 3, and the microstructure comprises, in area%, 8 to 20% of retained austenite, 5 to 60% of bainite, 20% or less of fresh martensite, and the remainder of ferrite, and the product of total elongation and tensile strength (T-El × TS) may be 21800 MPa% or more.

[0013] [Relationship 1]

[0014] 0.315 ≤ C Mn ≤ 0.415

[0015] (In the above relational expression 1, C Mn means the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0016] [Relationship 2]

[0017] Si Parameter = -2 x C Mn + 2.53

[0018] (In the above relational expression 2, C Mn means the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0019] [Relationship 3]

[0020] Si Parameter - 0.175 ≤ Si ≤ Si Parameter + 0.220

[0021] (In the above relational expression 3, Si Parameter means the value derived from the above relational expression 2, and the Si means the Si content included in the cold-rolled steel sheet, and each unit is excluded.)

[0022] The cold-rolled steel sheet described above may further include, in weight percent, one or more of niobium (Nb), titanium (Ti), and vanadium (V), with a total content of 0.100% or less.

[0023] The area fraction of the above-described ferrite can be 35 to 75 area%.

[0024] According to another aspect of the present invention, a galvanized steel sheet may have one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer formed on at least one surface of the cold-rolled steel sheet described above.

[0025] The cold rolled steel sheet described above may have a yield strength (YS) of 400 MPa or more, a tensile strength (TS) of 780 MPa or more, and a total elongation (T-El) of 28% or more.

[0026] According to another aspect of the present invention, there is provided a method for manufacturing a cold-rolled steel sheet, comprising the steps of: heating a slab comprising, in wt%, C: 0.20 to 0.30%, Si: 1.45 to 2.0%, Mn: 1.2 to 1.85%, Al: 0.010 to 1.0%, P: 0.100% or less (excluding 0%), S: 0.0300% or less (excluding 0%), N: 0.01000% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; finishing hot-rolling the slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; pickling the hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet for 30 seconds or more at a temperature of Ae3-15°C or less and a γ phase fraction of 50% or more; A first cooling step of cooling the cold-rolled steel sheet at a cooling stop temperature of 600°C or higher and 750°C or lower and an average cooling rate of 2°C / s or higher; a second cooling step of cooling the cold-rolled steel sheet at an average cooling rate of 10°C / s or higher to a temperature of Ms°C or higher and 500°C or lower; and a second cooling step of cooling the cold-rolled steel sheet at a cooling stop temperature of 600°C or higher and 750°C or lower and an average cooling rate of 2°C / s or higher; and a second cooling step of cooling the cold-rolled steel sheet at an average cooling rate of 10°C / s or higher to a temperature of Ms°C or higher and 500°C or lower ... nose ) may include a step of performing an over-aging heat treatment or a constant temperature heat treatment in which the slab is reheated to a temperature range of ±100℃ and then maintained for 1 minute or more and 30 minutes or less; and C, Si and Mn included in the slab may simultaneously satisfy the following relationships 1, 2 and 3.

[0027] [Relationship 1]

[0028] 0.315 ≤ C Mn ≤ 0.415

[0029] (In the above relational expression 1, C Mn means the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0030] [Relationship 2]

[0031] Si Parameter = -2 x C Mn + 2.53

[0032] (In the above relational expression 2, C Mnmeans the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0033] [Relationship 3]

[0034] Si Parameter - 0.175 ≤ Si ≤ Si Parameter + 0.220

[0035] (In the above relational expression 3, Si Parameter means the value derived from the above relational expression 2, and the Si means the Si content included in the cold-rolled steel sheet, and each unit is excluded.)

[0036] The above-described slab may further contain, in weight %, one or more of niobium (Nb), titanium (Ti), and vanadium (V), with a total content of 0.100% or less.

[0037] The cooling stop temperature RCS in the secondary cooling stage described above can satisfy the following relationship 4.

[0038] [Relationship 4]

[0039] B Nose temperature - 100℃ ≥ RCS > Ms

[0040] The above-described heating step can be performed at 1100°C or higher and 1300°C or lower, the above-described finishing hot rolling can be performed at Ar3 or higher, the above-described coiling step can be performed at 700°C or lower, and the cold reduction ratio during the above-described cold rolling can be 30% or higher and 80% or lower.

[0041] Lastly, the method for manufacturing a galvanized steel sheet according to another aspect of the present invention may additionally include a step of electrogalvanizing after the over-aging heat treatment or constant temperature heat treatment, or may additionally include a step of immersing in a molten zinc plating bath of 440 to 480°C.

[0042] In addition, the method for manufacturing the above-described galvanized steel sheet may include a step of alloying heat treatment at 450 to 520°C after the immersion.

[0043] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0044] More specifically, according to one aspect of the present invention, a cold-rolled steel sheet having a tensile strength of 780 MPa, high elongation, and an appropriate balance of strength and elongation, and a method for manufacturing the same can be provided.

[0045] Figure 1 is a photograph showing an example of a dilatometer experiment simulating the annealing process of steel grade A.

[0046] Fig. 2 is an SEM photograph observing the microstructure of a cold-rolled steel sheet according to an example of the present invention.

[0047] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0048] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0049] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the contents of the alloy composition described below are in weight percent.

[0050] A cold-rolled steel sheet according to one embodiment of the present invention may include, in wt%, C: 0.20 to 0.30%, Si: 1.45 to 2.0%, Mn: 1.2 to 1.85%, Al: 0.010 to 1.0%, P: 0.100% or less (excluding 0%), S: 0.0300% or less (excluding 0%), N: 0.01000% or less (excluding 0%), and the remainder may be made of Fe and other unavoidable impurities.

[0051] C: 0.20~0.30%

[0052] C is an interstitial solid solution element, and is the most effective and important element for improving the strength of steel. It is an element that must be added to secure the strength of bainitic and martensitic steels. In order to obtain an ultra-high strength steel that satisfies the target elongation and tensile strength of the present invention, it is preferable that the C be added in an amount of 0.20% or more. However, if the content exceeds 0.30%, martensite may be excessively formed during cooling due to an increase in hardenability, or the effect of solid solution strengthening may be excessively large, resulting in a rapid increase in strength and a deterioration in elongation. Therefore, the C content is preferably in the range of 0.20 to 0.30%. As another example, the C content may be 0.23 to 0.25%, and as another example, it may be 0.20 to 0.22%. In addition, the C content of the cold-rolled steel sheet according to an example of the present invention tends to depend on the Mn content, and as a result, it is necessary to satisfy the following relationship 1.

[0053] Si: 1.45~2.0%

[0054] Silicon (Si) is a key element of TRIP (Transformation Induced Plasticity) steels, which inhibit the precipitation of cementite and thereby increase the fraction of retained austenite and elongation. If the Si content is less than 1.45%, the control of cementite precipitation in the reheating and overaging heat treatment step or the isothermal heat treatment step may not be sufficient, so that the fraction of retained austenite ultimately formed may be small or its stability may be low, and the final elongation may not satisfy the target material. On the other hand, if the Si content exceeds 2.0%, the surface properties and plating properties of the steel deteriorate, and the elongation may be inhibited due to the increase in strength caused by excessive solid solution strengthening. Therefore, the Si content is preferably in the range of 1.45 to 2.0%. As another example, the Si content may be 1.5% to 1.65%, and as another example, 1.75% to 1.9%. The Si content of the cold-rolled steel sheet according to an example of the present invention tends to depend on the C and Mn contents, and as a result, it is necessary to satisfy the following relational expressions 2 and 3.

[0055] Mn: 1.2~1.85%

[0056] Mn is an element added to secure strength and residual austenite stability. If the Mn content is less than 1.2%, it is difficult to secure the strength level desired in the present invention. On the other hand, if the Mn content exceeds 1.85%, the transformation of ferrite and bainite is excessively delayed, so that the ferrite transformation introduced during cooling after annealing hardly progresses, and the transformation of fresh martensite progresses excessively, making it difficult to secure the high elongation desired in the present invention. In addition, there is a problem that Mn segregates in the thickness direction, easily forming Mn bands within the slab, which increases the occurrence of defects during the rolling process along with continuous casting cracks. Therefore, the Mn content is preferably in the range of 1.2 to 1.85%. As another example, the Mn content may be 1.3 to 1.5%, and as another example, 1.6 to 1.8%. In a cold-rolled steel sheet according to an example of the present invention, control of the Mn content is very important, and since the Mn content depends on the C content, it is necessary to satisfy the following relational expression 1.

[0057] Al: 0.010~1.0%

[0058] Al can be added to remove oxygen in molten steel, and if the Al content is less than 0.010%, deoxidation of the steel is not sufficiently performed, and the cleanliness of the steel is impaired. On the other hand, if the Al content exceeds 1.0%, not only the castability of the slab deteriorates, but also the temperature required for single-phase heating during annealing increases, which may cause production and facility problems. Therefore, the Al content is preferably in the range of 0.010 to 1.0%. The lower limit of the Al content is more preferably 0.020%. The upper limit of the Al content is more preferably 0.90%, and even more preferably 0.80%.

[0059] P: 0.100% or less (excluding 0%)

[0060] P is an impurity element contained in steel, and 0% is excluded in consideration of cases where it is unavoidably included during the manufacturing process. However, if the content of P exceeds 0.100%, weldability deteriorates and there is a risk of steel becoming brittle, so the upper limit may be limited to 0.100%. A more preferable upper limit of the P content may be 0.0300%.

[0061] S: 0.0300% or less (excluding 0%)

[0062] S, like P, is an impurity contained in steel, and 0% is excluded in consideration of cases in which it is unavoidably included during the manufacturing process. Since S is an element that inhibits the ductility and weldability of steel plates, it is desirable to manage the content as low as possible. Therefore, in the present invention, it is desirable to limit the content of S to 0.0300% or less. A more preferable upper limit of the S content may be 0.00500%.

[0063] N: 0.01000% or less (excluding 0%)

[0064] Nitrogen (N) is an impurity that is inevitably contained in steel, and if its content exceeds 0.01000%, there is a concern that it may deteriorate the performance quality by combining with Al in the steel to form AlN.

[0065] Therefore, the above N may be included in an amount of 0.01000% or less, more advantageously in an amount of 0.007000% or less, and even more advantageously in an amount of 0.005000% or less.

[0066] A cold rolled steel sheet according to a non-limiting embodiment of the present invention may further include one or more of Nb, Ti, and V in a total content of 0.100% or less.

[0067] Although the above Nb, Ti and V can improve the yield strength of steel even with a small amount of addition, their effect on improving ductility is minimal, so when added, they can be included in an amount of 0.100% or less based on the total content of the above Nb, Ti and V.

[0068] 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.

[0069] A cold-rolled steel sheet according to an example of the present invention can satisfy the following relational expression 1 regarding the relative contents between C and Mn.

[0070] [Relationship 1]

[0071] 0.315 ≤ C Mn ≤ 0.415

[0072] (In the above relational expression 1, C Mn means the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0073] The above relationship 1 is intended to secure both high strength and high elongation characteristics. Specifically, both C and Mn are austenite stabilizing elements, but in the case of C, partitioning, which occurs when additional diffusion occurs into the retained austenite in the rapid cooling section (RCS) - rapid heating section (RHS) temperature range during the annealing process, can occur actively. As a result, the addition of C can further increase the final retained austenite fraction and stability after the annealing process is completed. On the other hand, since Mn causes the above-mentioned partitioning phenomenon very weakly, the effect of increasing the elongation when adding the Mn content compared to C in TRIP steel is less than the increase in strength. However, if Mn is not added or the amount added is greatly reduced, the pearlite phase may be induced by the high C content, making it difficult to achieve the desired microstructure and material.

[0074] Accordingly, the inventors of the present invention recognized the need to control the relative contents of C and Mn, and derived the above relational expression 1. If the cold-rolled steel sheet does not satisfy the relationship between C and Mn in relational expression 1 within the above-described C and Mn ranges, the cold-rolled steel sheet may not be able to secure high strength and high elongation properties or may not obtain the target steel sheet material. As another example, the CMn may be 0.315 to 0.330, and as another example, it may be 0.364 to 0.380.

[0075] In addition, a cold-rolled steel sheet according to an example of the present invention can satisfy the following relationships 2 and 3.

[0076] [Relationship 2]

[0077] Si Parameter = -2 x C Mn + 2.53

[0078] (In the above relational expression 2, C Mnmeans the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0079] [Relationship 3]

[0080] Si Parameter - 0.175 ≤ Si ≤ Si Parameter + 0.220

[0081] (In the above relational expression 3, Si Parameter means the value derived from the above relational expression 2, and the Si means the Si content included in the cold-rolled steel sheet, and each unit is excluded.)

[0082] The above equations 2 and 3 are intended to secure high strength and high elongation, while also ensuring excellent weldability, particularly by controlling the Si content. That is, the inventors of the present invention experimentally confirmed that even if the above-described Si range is satisfied, if equations 2 and 3 are not satisfied, it may be difficult to secure the strength, elongation, and weldability at the levels targeted by the present invention, and thus derived equations 2 and 3.

[0083] That is, one embodiment of the present invention is Si content through the above relational expression 3. Parameter - 0.175 By doing this, not only can the fraction and stability of retained austenite be increased, but also the elongation and strength can be increased simultaneously in TRIP steel through solid solution strengthening. Therefore, when the above-described relational expression 3 is satisfied, the cold-rolled steel sheet according to an embodiment of the present invention can achieve the target material. In another embodiment, the Si content is Si Parameter - It can be 1.70 or more, and in another example, Si Parameter - It can be 0.100 or more. However, the above Si content is Si Parameter+ If it exceeds 0.220, the elongation may be reduced due to excessive solidification strengthening effect and the weldability may be greatly reduced, so one embodiment of the present invention sets the upper limit of the Si content to Si. Parameter +0.210 or Si Parameter + 0.190 can be done.

[0084] Meanwhile, a cold-rolled steel sheet according to one embodiment of the present invention may include, in terms of area %, 8 to 20% of retained austenite, 5 to 60% of bainite, 20% or less of fresh martensite, and residual ferrite as its microstructure.

[0085] First, the cold-rolled steel sheet according to an example of the present invention may include 8 to 20 area% of retained austenite in its microstructure. The retained austenite, together with ferrite, is an essential structure for securing elongation. If the fraction of the retained austenite is less than 8 area%, it may be difficult to secure the elongation targeted by the present invention. On the other hand, if the fraction of the retained austenite exceeds 20 area%, the stability of the retained austenite may be insufficient, making it difficult to secure the target elongation. As another example, the area fraction of the retained austenite may be 8 to 10 area%, or 12 to 20 area%.

[0086] A cold-rolled steel sheet according to an example of the present invention may include 5 to 60 area% of bainite in its microstructure. Bainite is a structure necessary for securing strength and elongation, and when phase transformation progresses so that the bainite fraction reaches 5 to 60 area%, the residual austenite, which is stable at room temperature, can ultimately be included in the range of 8 to 20 area% through diffusion of C into the untransformed austenite. When the bainite fraction is less than 5 area%, it may be difficult to secure the residual austenite fraction targeted by the present invention due to insufficient total transformation amount, and it may be difficult to secure both high strength and high elongation at the same time due to excessive formation of fresh martensite. On the other hand, when the bainite fraction exceeds 60 area%, it may be difficult to secure high elongation because the fractions of ferrite and residual austenite phases are relatively low. As another example, the area fraction of the bainite may be 10 to 20 area%, or 25 to 35 area%.

[0087] A cold-rolled steel sheet according to one embodiment of the present invention may include fresh martensite in its microstructure at a fraction of 20 area% or less. If the area fraction of fresh martensite exceeds 20 area%, it may be difficult to obtain a cold-rolled steel sheet having both excellent strength and elongation.

[0088] Finally, the cold-rolled steel sheet according to one embodiment of the present invention may include ferrite as a remainder. In this case, the ferrite may include both annealed ferrite and ferrite formed during cooling. In one example of the present invention, by using the ferrite, which is a structure advantageous for securing elongation, as a remainder, a cold-rolled steel sheet having excellent strength and elongation properties can be obtained. The area fraction of the ferrite may be controlled to an appropriate level by limiting the area fraction of other microstructures, but as an example, the area fraction of the ferrite may be 35 to 75 area%.

[0089] As described above, the cold-rolled steel sheet of the present invention may have a yield strength of 400 MPa or more, a tensile strength of 780 MPa or more, and a total elongation of 28% or more. Meanwhile, in the present invention, the higher the yield strength, tensile strength, and elongation, the more advantageous they are, and therefore, there is no particular limitation on their upper limits. However, as an example, they may be 650 MPa or less, 980 MPa or less, and 40% or less, respectively.

[0090] In addition, as described above, the present invention has excellent properties in both strength and elongation, and according to one embodiment of the present invention, the product of the total elongation and the tensile strength (T-El × TS) may be 21800 MPa% or more.

[0091] Meanwhile, the cold-rolled steel sheet of the present invention may have at least one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer formed on one surface. In the present invention, there are no particular limitations on the hot-dip galvanized layer, the alloyed hot-dip galvanized layer, and the electrogalvanized layer, and all types commonly used in the relevant technical field may be used.

[0092] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0093] A method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention comprises the steps of: heating a slab comprising, in wt%, C: 0.20 to 0.30%, Si: 1.45 to 2.0%, Mn: 1.2 to 1.85%, Al: 0.010 to 1.0%, P: 0.100% or less (excluding 0%), S: 0.0300% or less (excluding 0%), N: 0.01000% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; finishing hot-rolling the slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; pickling the hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet for 30 seconds or more at a temperature of Ae3-15°C or less and a γ phase fraction of 50% or more; The method may include a first cooling step of cooling the cold-rolled steel sheet at a cooling stop temperature of 600°C or higher and 750°C or lower and an average cooling rate of 2°C / s or higher; a second cooling step of cooling the cold-rolled steel sheet at an average cooling rate of 10°C / s or higher to a temperature of Ms°C or higher and 500°C or lower; and a step of subjecting the cold-rolled steel sheet to overaging heat treatment or constant-temperature heat treatment. Each step is described in detail below.

[0094] First, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention comprises, in wt%, C: 0.20 to 0.30%, Si: 1.45 to 2.0%, Mn: 1.2 to 1.85%, Al: 0.010 to 1.0%, P: 0.100% or less (excluding 0%), S: 0.0300% or less (excluding 0%), N: 0.01000% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities, and may include a step of heating the slab, and C, Si and Mn included in the slab may simultaneously satisfy the following relationships 1, 2 and 3.

[0095] [Relationship 1]

[0096] 0.315 ≤ C Mn ≤ 0.415

[0097] (In the above relational expression 1, CMn means the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0098] [Relationship 2]

[0099] Si Parameter = -2 x C Mn + 2.53

[0100] (In the above relational expression 2, C Mn means the product of the C content and the Mn content contained in the above cold-rolled steel sheet, and each unit is excluded.)

[0101] [Relationship 3]

[0102] Si Parameter - 0.175 ≤ Si ≤ Si Parameter + 0.22

[0103] (In the above relational expression 3, Si Parameter means the value derived from the above relational expression 2, and the Si means the Si content included in the cold-rolled steel sheet, and each unit is excluded.)

[0104] The alloy composition of the slab and the above-mentioned relationships 1, 2 and 3 have been described above, so they will be omitted. The above-mentioned slab heating can be performed to smoothly perform the subsequent hot rolling process and to obtain the target properties of the steel plate.

[0105] In addition, according to a non-limiting example of the present invention, the temperature range in the heating step may be 1100°C or more and 1300°C or less. If the slab heating temperature is less than 1100°C, a problem of a rapid increase in hot rolling load may occur. On the other hand, if the slab heating temperature exceeds 1300°C, the amount of surface scale may increase, which may lower productivity.

[0106] As described above, after heating the slab, one example of the present invention can obtain a hot-rolled steel sheet by finishing hot rolling. In addition, as a non-limiting example, the temperature during the finishing hot rolling may be Ar3 or higher. If the finishing hot rolling temperature is lower than Ar3, a dual-phase region of ferrite + austenite or a ferrite region rolling occurs, resulting in the formation of a mixed grain structure, and equipment malfunction may occur due to fluctuations in the hot rolling load. Meanwhile, the Ar3 can be obtained through the following equation 1.

[0107] [Formula 1] Ar3(℃) = 910-203√vC+44.7Si+31.5Mo-30Mn-11Cr+700P+400Al+400Ti

[0108] Next, according to one embodiment of the present invention, the hot-rolled steel sheet can be coiled. In addition, as a non-limiting example, the coiling can be performed at 700°C or lower. If the coiling temperature exceeds 700°C, an oxide film may be excessively formed on the surface of the steel sheet, which may cause defects. The coiling temperature is more preferably 650°C or lower. On the other hand, as the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, which has the disadvantage of increasing the rolling load of the cold rolling process, which is a subsequent process. However, since this does not make actual production impossible, the present invention does not specifically limit the lower limit thereof. However, as an example, the lower limit of the coiling temperature may be 300°C.

[0109] Afterwards, the coiled hot-rolled steel sheet may be pickled and then cold-rolled to obtain a cold-rolled steel sheet. The pickling is a process for removing an oxide layer formed on the surface of the coiled hot-rolled steel sheet. In addition, as a non-limiting example, the cold rolling may be performed at a cold reduction ratio of 30 to 80%. If the cold reduction ratio is less than 30%, not only is it difficult to secure the target thickness, but there is also a concern that the formation of austenite and securing of physical properties may be affected during annealing heat treatment due to the residual crystal grains formed during hot rolling. If the cold reduction ratio exceeds 80%, material deviation may occur due to uneven rolling amount in the longitudinal and transverse directions due to work hardening that occurs during cold rolling, and it may be difficult to secure the target thickness due to the rolling load.

[0110] Thereafter, an example of the present invention is that the cold-rolled steel sheet can be continuously annealed for 30 seconds or more at a temperature of Ae3-15℃ or lower and a γ phase fraction of 50% or higher.

[0111] The inventors of the present invention have discovered that the annealing temperature during the continuous annealing process is very important in securing the desired levels of strength and elongation. More specifically, after careful consideration, the inventors of the present invention have discovered that when the alloy composition and the relationships 1, 2, and 3 described above are satisfied while simultaneously controlling the annealing temperature range, it is possible to manufacture a steel having a tensile strength of 780 MPa or more and an elongation of 28% or more.

[0112] If the above annealing temperature exceeds Ae3-15℃, the fraction of annealed ferrite may be insufficient due to heating at the single-phase level, resulting in poor elongation. As another example, the upper limit of the above annealing temperature may be Ae3-25℃.

[0113] On the other hand, when the annealing temperature is such that the high-temperature equilibrium austenite phase fraction is less than 50%, excessive annealing ferrite may be formed, making it difficult to secure excellent strength and elongation. According to another example of the present invention, the annealing temperature may be such that the high-temperature equilibrium austenite phase fraction is 60% or more, and the annealing temperature may be such that the high-temperature equilibrium austenite phase fraction is 75% or more.

[0114] In addition, if the annealing time is less than 30 seconds, there is a disadvantage in that a sufficient annealing effect cannot be obtained. On the other hand, since the annealing time is advantageous the longer it is, the present invention does not specifically limit the upper limit thereof. However, as an example, the upper limit of the annealing time may be 500 seconds. The equilibrium phase fraction temperature including the Ae3 temperature mentioned above follows the calculation values ​​of commercial thermodynamic software, including Thermo calc. and J-mat pro, for each component system.

[0115] Thereafter, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention may include a first cooling step of cooling the cold-rolled steel sheet at a cooling stop temperature of 600°C or higher and 750°C or lower and an average cooling rate of 2°C / s or higher.

[0116] If the cooling end temperature in the first cooling step is less than 600°C, there is a risk that phases such as ferrite or bainite may form, resulting in a decrease in strength. If the cooling end temperature exceeds 750°C, problems may occur in actual production lines. The lower limit of the cooling end temperature is more preferably 610°C, and even more preferably 630°C. The upper limit of the cooling end temperature is more preferably 740°C, and even more preferably 730°C.

[0117] When the average cooling rate is less than 2°C / s, the amount of ferrite formed during cooling may be excessive, which may affect the phase fractions of bainite and retained austenite, making application undesirable. When the average cooling rate is fast, for example, when the average cooling rate exceeds 10°C / s, the cooling ferrite fraction decreases, so if the temperature during annealing is set to a range that can secure a ferrite fraction of 35% or more, the target microstructure can be secured, and therefore, an upper limit on the cooling rate is unnecessary. However, in a typical annealing heat treatment process, a cooling rate of 10°C / s or less is advantageous for target temperature control, and therefore, the upper limit of the average cooling rate may preferably be 10°C / s. In addition, when the ferrite fraction presented in this patent is sufficiently secured in the annealing process, if the first cooling process is applied at a fast cooling rate exceeding 10℃ / s, the second cooling process, which generally has a faster cooling rate, and the first cooling process can be cooled at almost the same cooling rate, so there may be cases where the distinction between the first cooling process and the second cooling process is unnecessary, and even in such cases, the target material can be secured.

[0118] After the first cooling step, the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may include a second cooling step of cooling the cold-rolled steel sheet to a temperature range of bainite transformation, which is Ms℃ or higher and 500℃ or lower, at an average cooling rate of 10℃ / s or higher.

[0119] That is, one embodiment of the present invention is to secure the target strength and elongation by secondary cooling to Ms℃ or higher and 500℃ or lower (B nose ) Bainite transformation can be induced by reheating to ±100℃ and then performing overaging heat treatment or isothermal heat treatment in the same temperature range for 1 to 30 minutes.

[0120] However, more preferably, in the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention, the cooling stop temperature RCS in the second cooling step can satisfy the following relational expression 4.

[0121] [Relationship 4]

[0122] Ms < RCS ≤ (B nose ) - 100℃

[0123] The temperature range suggested in the above relational expression 4 is the lower bainite formation temperature, and although full-scale bainite transformation is limited during the secondary cooling up to the temperature range of relational expression 4, some of the lower bainite transformation nuclei are introduced, so that the bainite transformation can be accelerated during the subsequent over-aging heat treatment or constant temperature heat treatment.

[0124] That is, one example of the present invention can secure an additional bainite fraction through the above relational expression 4, thereby sufficiently securing the area fraction of retained austenite and stabilizing the retained austenite. Accordingly, one embodiment of the present invention can be advantageous in stably securing the target material and elongation when satisfying the above relational expression 4.

[0125] In addition, since it is advantageous to perform the secondary cooling at a critical cooling rate that can minimize the formation of ferrite, pearlite, etc., the lower limit of the average cooling rate during the secondary cooling may be 10°C / s in one embodiment of the present invention. That is, if the average cooling rate is less than 10°C / s, even if the target cooling temperature is reached, high-temperature transformation structures such as ferrite and pearlite may be generated during cooling, making it difficult to secure high-strength properties. The upper limit of the average cooling rate is not limited, but if it exceeds 45°C / s in a general process, it is difficult to control the secondary cooling end temperature, which may result in deterioration of the surface shape of the steel sheet due to rapid martensite transformation, and even if it is not the problem described above, material deviation may occur due to cooling unevenness in the width direction, so in a non-limiting example of the present invention, the upper limit of the average cooling rate may be 45°C / s.

[0126] After the above secondary cooling, one example of the present invention is the cold rolled steel sheet (B nose ) may include a step of overheating or constant temperature heat treatment in which the temperature is reheated to a temperature range of ±100°C and maintained for 1 minute or more and 30 minutes or less. That is, in one embodiment of the present invention, the B is located near 400°C. nose By maximizing the transformation activation, C can be concentrated (partitioned) in the remaining untransformed austenite that is not transformed during the annealing process, thereby stabilizing the retained austenite ultimately obtained at room temperature. At this time, the B nose The temperature can be derived by performing repeated annealing simulation experiments using a dilatometer to change the reheating heat treatment temperature, thereby obtaining the reheating heat treatment temperature at which the greatest amount of bainite transformation occurs. In general, the B nose The temperature can be 400~450℃.

[0127] If the above over-aging heat treatment or constant temperature heat treatment temperature is (B nose) - If it is below 100℃, the fraction of fresh martensite, which has a negative effect on the material, may increase. As another example, the over-aging or constant temperature heat treatment temperature is (B nose )-40℃ or higher, and as another example, (B nose )-25℃ or higher.

[0128] On the other hand, (B nose ) If the over-aging or isothermal heat treatment is performed at a temperature exceeding +100℃, the final retained austenite may be unstable due to the delay in bainite transformation. As another example, the over-aging or isothermal heat treatment temperature is (B nose ) can be below +40℃, and as another example, (B nose ) may be below +25℃.

[0129] In addition, if the over-aging heat treatment or constant temperature heat treatment time is less than 1 minute, sufficient transformation may not occur, making it difficult to obtain the desired partitioning effect. If it exceeds 30 minutes, the over-aging heat treatment or constant temperature heat treatment section must be very long, and productivity may decrease, making it difficult to apply to an actual production line.

[0130] Meanwhile, after the over-aging heat treatment or constant temperature heat treatment, the cold rolled steel sheet may be immersed in a molten zinc plating bath at 440 to 480°C to form a molten zinc plating layer. If the molten zinc plating bath temperature is lower than 440°C, management of the molten zinc plating bath may be difficult, and if the molten zinc plating bath temperature exceeds 480°C, the final elongation may decrease.

[0131] In addition, after the formation of the hot-dip galvanized layer, the cold-rolled steel sheet on which the hot-dip galvanized layer is formed can be subjected to alloying heat treatment at 450 to 520°C. If the alloying heat treatment temperature is less than 450°C, it may be difficult to form a sufficient Fe-Zn alloying plating layer, and if the alloying heat treatment temperature exceeds 520°C, the final elongation may be inferior due to the decomposition of the residual austenite formed in the previous step. Although not particularly limited, the zinc-based plating layer may be a zinc-plated layer mainly containing zinc, or a zinc alloy plating layer containing aluminum and / or magnesium in addition to zinc.

[0132] On the other hand, an electrogalvanized layer can be formed after the above-mentioned over-aging heat treatment or constant temperature heat treatment.

[0133] 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 further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0134] (Example)

[0135] A steel slab having an alloy composition shown in Table 1 below was heated in a temperature range of 1150 to 1200°C, then hot-rolled at a finishing hot-rolling temperature of 930°C or higher, and then subjected to the [coiling - cold rolling - continuous annealing - primary cooling - secondary cooling - over-aging heat treatment or constant temperature heat treatment] process according to the conditions shown in Tables 2 and 3 below to manufacture each cold-rolled steel sheet.

[0136] The microstructure, physical properties, etc. of the cold-rolled steel sheet manufactured as described above were measured, and the results are shown in Table 4 below.

[0137] At this time, the microstructure of each steel plate was observed through a scanning electron microscope (SEM) and then measured using a point counting method. However, the fraction of the retained austenite phase was measured using XRD.

[0138] The tensile strength (TS), yield strength (YS), and elongation (El) of each steel plate were evaluated through a tensile test in the rolling direction, and a test specimen specification of 50 mm in gauge length and 25 mm in width was used.

[0139] At this time, the following B nose As described above, the temperature was derived from the reheating heat treatment temperature at which the most bainite transformation occurs through repeated annealing simulation experiments using a dilatometer to change the reheating heat treatment temperature, and the Ms temperature is the temperature value measured after rapidly cooling to room temperature at -50℃ / s after implementing the conditions in Tables 2 and 3 using a dilatometer up to each continuous annealing temperature and the first cooling temperature.

[0140] Steel CSiMnAlPSNTiNbVC Mn Relationship 1Si Parameter Relationship 3A0.2001.711.810.020.0100.0020.005---0.36O1.81OB0.2231.691.570.030.0100.0020.005---0.35O1.83OC0.2502.001.510.040.0070.0020.003---0.38O1.78OD0.2501.951.500.030.0100.00 20.0050.0150.0250.050.38O1.78OE0.21.601.560.040.0070.0020.003---0.31X1.91XF0.2851.5 01.600.050.0070.0020.003---0.46X1.62OG0.2171.511.530.050.0070.0020.003---0.33O1.87X

[0141] Classification Steel grade Ae3-15 (℃ Thermodynamic γ 50% (℃ Hot rolled CT (℃ Cold rolling reduction ratio (%) Continuous annealing stage temperature (℃ Time (sec) Invention example 1A8357754505280035 Invention example 2A8357754505280065 Invention example 3B8387754504883078 Invention example 4B83877545048830100 Invention example 5C8417755205082045 Invention example 6C8417755205082042 Comparative example 1C8417755205076534 Invention example 7D8527704505082545 Comparative example 2E8437804505084033Comparison Example 3F8127455205080570Comparison Example 4G8367704504580365

[0142] Classification 1st cooling stage 2nd cooling stage Heat treatment stage Ms(℃B nose -100℃(℃Relationship 4Cooling stop temperature(℃Cooling speed(℃Cooling stop temperature(℃Cooling speed(℃Temperature(℃Time(min)Invention example 17004.540013440128128320XInvention example 2700330014300128128320OInvention example 3680444028440135135340XInvention example 46802.430011300135135340OInvention example 5650240010.5400119119300XInvention example 66503.530018400119119300OComparative example 1620540010400108108300XInvention example 76204.540034400111111300X Comparison Example 2640540011.5400142142310X Comparison Example 3620620010.4400109109300O Comparison Example 4650440016400107107310X

[0143] Classification Steel grade TS*ElYSTST-ElU-ElP-El Microstructure (fraction %)FBRAFM Invention example 1A2302645579429.022.18.656.417.014.012.6 Invention example 2A2433944279830.523.57.656.419.517.86.3 Invention example 3B2481242678531.624.37.343.725.416.014.9 Invention example 4B2625644078633.423.310.143.726.718.411.2 Invention example 5C2564646782231.223.25.266.57.413.212.9 Invention Example 6C2744047182933.126.17.066.513.217.03.3 Comparative Example 1C2764842876836.029.46.678.44.214.52.9 Invention Example 7D2758848085632.225.46.852.418.314.814.5 Comparative Example 2E2123438675528.120.87.448.225.014.512.3 Comparative Example 3F2298854096124.320.43.952.324.011.012.7 Comparative example 4G2279641882026.823.04.861.511.47.719.5

[0144] Comparative Example 1 had an alloy composition that satisfied the range proposed in the present invention, but the temperature during continuous annealing was too low, and thus high strength characteristics could not be secured due to the introduction of excessive annealed ferrite.

[0145] Comparative example 2 is C Mn The value was less than 0.315 suggested in relational expression 1, so the target yield strength was not achieved, and on the contrary, comparative example 3 was C Mn The elongation was lowered by the amount of strength increased due to excessive strengthening, exceeding 0.415 suggested in relational equation 1, and thus high elongation characteristics were not secured.

[0146] Comparative Example 4 did not satisfy Equation 3, and thus did not meet the target elongation due to insufficient retained austenite.

[0147] On the other hand, Examples 1 to 7 were able to secure high strength and high elongation characteristics by satisfying the conditions of the alloy components and manufacturing method proposed in the present invention. In particular, looking at Examples 2, 4, and 6, when the cooling stop temperature during secondary cooling is set to satisfy the relationship 4 proposed in the present invention, it can be confirmed that the final bainite fraction and retained austenite fraction are measured higher even though continuous annealing and primary cooling were performed under the same conditions for the same steel type. Accordingly, when the above relationship 4 is satisfied, the cold-rolled steel sheet of the present invention can secure higher TS x T-El.

[0148] The following Fig. 1 is a photograph showing an example of a dilatometer experiment simulating the annealing process of steel grade A. In Fig. 1, the y-axis represents the changed sample length after expansion or contraction according to temperature change, which can infer the transformation amount of steel grade A in Table 1, expressed as a fraction compared to the initial sample length, and the x-axis represents temperature (℃). As shown in Fig. 1 (b), even if the same heat treatment temperature (RHS) of 400℃ is maintained, it can be confirmed that additional bainitic transformation occurs under the condition of the secondary cooling temperature (RCS) of 300℃, which satisfies relational expression 4. Therefore, a higher retained austenite fraction can be expected in the specimen satisfying relational expression 4.

Claims

1. Contains, in wt%, C: 0.20 to 0.30%, Si: 1.45 to 2.0%, Mn: 1.2 to 1.85%, Al: 0.010 to 1.0%, P: 0.100% or less (excluding 0%), S: 0.0300% or less (excluding 0%), N: 0.01000% or less (excluding 0%), and the remainder is composed of Fe and other unavoidable impurities. The above C, Si and Mn simultaneously satisfy the following relationships 1, 2 and 3, The microstructure contains 8 to 20% of retained austenite, 5 to 60% of bainite, 20% or less of fresh martensite, and residual ferrite in area %. Cold rolled steel sheet having a product of total elongation and tensile strength (T-El × TS) of 21800 MPa% or more. [Relationship 1] 0.315 ≤ C Mn ≤ 0.415 (In the above relational expression 1, C Mn means the product of the C content and the Mn content contained in the above cold rolled steel sheet, excluding each unit.) [Relationship 2] Si Parameter = -2 x C Mn + 2.53 (In the above relational expression 2, C Mn means the product of the C content and the Mn content contained in the above cold rolled steel sheet, excluding each unit.) [Relationship 3] Si Parameter - 0.175 ≤ Si ≤ Si Parameter + 0.220 (In the above relational expression 3, Si Parameter means the value derived from the above relational expression 2, and the above Si means the Si content included in the cold rolled steel sheet, and each unit is excluded.) 2. In paragraph 1, Cold rolled steel sheet further containing, in weight%, at least one of niobium (Nb), titanium (Ti), and vanadium (V), with a total content of 0.100% or less.

3. In paragraph 1, Cold rolled steel sheet having an area fraction of the above ferrite of 35 to 75 area%.

4. In at least one of paragraphs 1 to 3, Cold rolled steel sheet with a yield strength (YS) of 400 MPa or more, a tensile strength (TS) of 780 MPa or more, and a total elongation (T-El) of 28% or more.

5. In at least one of paragraphs 1 to 3, A galvanized steel sheet having one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer formed on at least one surface of the cold-rolled steel sheet.

6. A step of heating a slab comprising, by weight%, C: 0.20 to 0.30%, Si: 1.45 to 2.0%, Mn: 1.2 to 1.85%, Al: 0.010 to 1.0%, P: 0.100% or less (excluding 0%), S: 0.0300% or less (excluding 0%), N: 0.01000% 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 slab; A step of coiling the above hot-rolled steel plate; A step of obtaining a cold rolled steel sheet by pickling the hot rolled steel sheet and then cold rolling it; A step of continuously annealing the cold rolled steel sheet for 30 seconds or longer at a temperature of Ae3-15℃ or lower and a γ phase fraction of 50% or higher; A first cooling step in which the cold rolled steel sheet is cooled at a cooling stop temperature of 600℃ or higher and 750℃ or lower and an average cooling speed of 2℃ / s or higher; A secondary cooling step of cooling the cold rolled steel sheet to a temperature of Ms℃ or higher and 500℃ or lower at an average cooling rate of 10℃ / s or higher; and The above cold rolled steel plate (B nose) includes a step of heat treatment or constant temperature heat treatment in which the heat is reheated to a temperature range of ±100℃ and maintained for 1 minute or more and 30 minutes or less; A method for manufacturing a cold rolled steel sheet in which C, Si and Mn contained in the above slab simultaneously satisfy the following relationships 1, 2 and 3. [Relationship 1] 0.315 ≤ C Mn ≤ 0.415 (In the above relational expression 1, C Mn means the product of the C content and the Mn content contained in the above cold rolled steel sheet, excluding each unit.) [Relationship 2] Si Parameter = -2 x C Mn + 2.53 (In the above relational expression 2, C Mn means the product of the C content and the Mn content contained in the above cold rolled steel sheet, excluding each unit.) [Relationship 3] Si Parameter - 0.175 ≤ Si ≤ Si Parameter + 0.220 (In the above relational expression 3, Si Parameter means the value derived from the above relational expression 2, and the above Si means the Si content included in the cold rolled steel sheet, and each unit is excluded.) 7. In paragraph 6, A method for manufacturing a cold rolled steel sheet, wherein the above slab further contains, in weight %, at least one of niobium (Nb), titanium (Ti), and vanadium (V), with a total content of 0.100% or less.

8. In paragraph 6, A method for manufacturing a cold rolled steel sheet in which the cooling stop temperature RCS in the above second cooling step satisfies the following relationship 4. [Relationship 4] B Nose temperature - 100℃ ≥ RCS > Ms 9. In paragraph 6, The above heating step is performed at a temperature of 1100℃ or higher and 1300℃ or lower, The above finishing hot rolling is performed at Ar3 or higher. The above-mentioned winding step is performed at 700℃ or lower, A method for manufacturing a cold rolled steel sheet having a cold reduction ratio of 30% or more and 80% or less during the above cold rolling.

10. In paragraph 6, A method for manufacturing a galvanized steel sheet, further comprising the step of performing electrolytic zinc plating after the above-mentioned overheat treatment or constant temperature heat treatment.

11. In paragraph 6, A method for manufacturing a galvanized steel sheet, further comprising the step of immersing in a molten zinc plating bath of 440 to 480°C after the above-mentioned overheating or constant temperature heat treatment.

12. In paragraph 11, A method for manufacturing a galvanized steel sheet, comprising the step of performing alloying heat treatment at 450 to 520°C after the above immersion.

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