Cold-rolled steel sheet and method for producing same

Abstract

The invention relates to an ultra-high-strength cold-rolled steel sheet which is mainly used for automobile collision and structural parts and has the tensile strength of 1470 MPa and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to an ultra-high strength cold-rolled steel sheet having a tensile strength of 1470 MPa grade, mainly used for automotive collision and structural components, and a manufacturing method thereof. Background Art

[0002] In recent years, due to various environmental regulations and energy laws, steel sheets for automobiles are required to have higher strength to improve fuel efficiency or durability. In particular, in recent years, with the expansion of automotive impact stability regulations, high-strength steels with excellent yield strength are used in structural components such as members, seat rails, and pillars to improve the impact resistance of the vehicle body. The structural components are characterized in that the higher the ratio of the yield strength to the tensile strength, that is, the higher the yield ratio (yield strength / tensile strength), the more favorable it is for the impact energy absorption ability. However, generally, as the strength of the steel sheet increases, the elongation rate decreases, resulting in a problem of reduced formability. Therefore, it is necessary to develop a material that can enhance formability.

[0003] Generally, the strengthening methods of steel include solid solution strengthening, precipitation strengthening, strengthening by grain refinement, phase transformation strengthening, etc. However, among the above methods, the disadvantages of strengthening by solid solution strengthening and grain refinement are that it is very difficult to manufacture high-strength steels with a tensile strength of 490 MPa grade or more.

[0004] In addition, precipitation-strengthened high-strength steels are technologies that strengthen the steel sheet by precipitating carbonitrides by adding carbon and nitride forming elements such as Cu, Nb, Ti, V, etc., or by suppressing grain growth by fine precipitates to refine the grains, thereby ensuring strength. The technology has the advantage of easily obtaining high strength at low manufacturing cost, but the disadvantage is that the recrystallization temperature rises sharply due to the fine precipitates, and in order to ensure ductility by causing sufficient recrystallization, high-temperature annealing must be carried out. In addition, precipitation-strengthened steels that are strengthened by precipitating carbonitrides in the ferrite matrix have the problem of being difficult to obtain high-strength steels with a strength of 600 MPa grade or more.

[0005] In addition, various types of transformation-strengthened high-strength steels have been developed, including ferrite-martensite dual-phase steels containing hard martensite in a ferrite matrix, transformation-induced plasticity (TRIP) steels utilizing the transformation-induced plasticity of retained austenite, or complexed-phase (CP) steels composed of ferrite and hard bainite or martensite structures. However, when the tensile strength achievable in such advanced high-strength steels is 1500 MPa, the elongation is limited to approximately 8%. In addition, hot press forming steels have received much attention for their application in structural components ensuring collision safety. The hot press forming steels ensure the final strength through rapid cooling by direct contact with a water-cooled die after being formed at high temperature. However, due to the high equipment investment cost and high heat treatment and process costs, the expansion of their application is limited.

[0006] In recent years, in order to further improve the safety of passengers during a collision, the high-strength and lightweight of vehicle seat components have been carried out simultaneously. Such components are manufactured by two methods: roll forming and stamping. Seat components are components connecting passengers to the vehicle body and must support passengers with high stress during a collision to prevent passengers from being thrown out. For this purpose, high yield strength and high yield ratio are required. In addition, most of the processed components are components requiring stretch flange performance, and steels with excellent hole expansion properties need to be applied.

[0007] In addition, Patent Document 1 (Japanese Patent Publication No. 3729108) discloses a high-tensile cold-rolled steel sheet having a single-phase martensite structure and a tensile strength of 880 - 1170 MPa by optimizing the composition and heat treatment conditions of the steel sheet. In addition, Patent Document 2 (Japanese Patent Laid-Open No. 2005-272954) discloses a method for manufacturing a high-tensile steel sheet, in which a steel sheet in which the volume ratio of a low-temperature transformation phase composed of martensite and retained austenite accounts for more than 90% of the entire metal structure is heated and held in a two-phase region, controlled to have a microstructure of fine ferrite and austenite of the low-temperature transformation phase-containing laths, and finally, through subsequent cooling, a metal structure in which ferrite and the low-temperature transformation phase are finely dispersed in the lath phase is formed. These patents claim that high yield strength can be obtained without water-cooling treatment, but the ductility is significantly deteriorated, or a large amount of austenite is generated in the steel, so there is a disadvantage of poor stretch flange performance.

[0008] (Patent Document 1) Japanese Patent Publication No. 3729108

[0009] (Patent Document 2) Japanese Patent Publication No. 2005-272954 Summary of the Invention

[0010] (1) Technical Problem to be Solved

[0011] According to one aspect of the present invention, an object is to provide a super high strength cold rolled steel sheet having excellent formability and a method for manufacturing the same.

[0012] The technical problems of the present invention are not limited to the above. Those skilled in the art to which the present invention pertains can easily understand the additional technical problems of the present invention based on the entire text of the specification of the present invention.

[0013] (2) Technical Solution

[0014] One aspect of the present invention provides a cold rolled steel sheet which, by weight %, comprises: C: 0.1 - 0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5 - 3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03 - 0.25%, Al: 0.1% or less (excluding 0%), P: 0.001 - 0.015%, S: 0.001 - 0.01%, N: 0.001 - 0.01%, B: 0.001 - 0.005%, the balance being Fe and other impurities, and which, by area %, comprises as the fine structure: the sum of bainite and tempered martensite: 75 - 90%, retained austenite: 10% or less (excluding 0%) and the balance being fresh martensite, and the cold rolled steel sheet satisfies the following relational expressions 1 and 2.

[0015] [Relational Expression 1]

[0016] 1.0 ≤ [C] + (1.3 × [Si] + [Mn]) / 6 + ([Cr] + 1.2 × [Mo]) / 5 + 100 × [B] ≤ 1.2

[0017] (In the relational expression 1, the [C], [Si], [Mn], [Cr], [Mo] and [B] represent the weight % contents of the respective elements in parentheses)

[0018] [Relational Expression 2]

[0019] 106 ≤ ([Cr] + [Mo]) × [Si] / [Al] ≤ 275

[0020] (In the relational expression 2, the [Cr], [Mo], [Si] and [Al] represent the weight % contents of the respective elements in parentheses)

[0021] By area %, the fine structure may comprise 4.8 - 9.6% of retained austenite.

[0022] In terms of area percentage, the fine structure may include: the sum of bainite and tempered martensite: 81 - 89%.

[0023] In addition, another aspect of the present invention provides a method for manufacturing a cold-rolled steel sheet, the manufacturing method including the following steps: reheating a slab, which, in terms of weight percentage, includes: C: 0.1 - 0.3%, Si: not more than 2.0% (except 0%), Mn: 1.5 - 3.0%, Cr: not more than 1.2% (except 0%), Mo: 0.03 - 0.25%, Al: not more than 0.1% (except 0%), P: 0.001 - 0.015%, S: 0.001 - 0.01%, N: 0.001 - 0.01%, B: 0.001 - 0.005%, the balance being Fe and other impurities, and the slab satisfies the following relational expressions 1 and 2; hot finish rolling the reheated slab at a temperature of Ar3 to Ar3 + 50 °C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 500 - 750 °C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at 800 - 900 °C; performing a first cooling on the continuously annealed cold-rolled steel sheet at an average cooling rate of 1 - 10 °C / second until a first cooling termination temperature of 650 - 700 °C is reached; performing a second cooling on the cold-rolled steel sheet after the first cooling at an average cooling rate exceeding 10 °C / second and not exceeding 20 °C / second until a second cooling termination temperature is reached; and performing overaging heat treatment on the cold-rolled steel sheet after the second cooling at 250 - 350 °C.

[0024] [Relational expression 1]

[0025] 1.0 ≤ [C] + (1.3 × [Si] + [Mn]) / 6 + ([Cr] + 1.2 × [Mo]) / 5 + 100 × [B] ≤ 1.2

[0026] (In the relational expression 1, [C], [Si], [Mn], [Cr], [Mo] and [B] represent the weight percentage contents of the respective elements within the brackets)

[0027] [Relational expression 2]

[0028] 106 ≤ ([Cr] + [Mo]) × [Si] / [Al] ≤ 275

[0029] (In the relational expression 2, [Cr], [Mo], [Si] and [Al] represent the weight percentage contents of the respective elements within the brackets)

[0030] The manufacturing method may satisfy the following relational expression 3.

[0031] [Relational expression 3]

[0032] 8 ≤ 3.1×([SS] - [Ac1]) + 2.2×([RCS] - [Ms]) ≤ 180

[0033] (In the said relation 3, [Ac1] represents the value defined by the following relation 4, [RCS] represents the secondary cooling termination temperature (°C), and [Ms] represents the martensite phase transformation start temperature (°C).)

[0034] [Relation 4]

[0035] [Ac1] = 723 - 10.7×[Mn] - 16.9×[Ni] + 29.1×[Si] + 16.9×[Cr]

[0036] (In the said relation 4, [Mn], [Ni], [Si], and [Cr] represent the weight % contents of the respective elements in the parentheses)

[0037] The above manufacturing method may further include a step of skin pass rolling the cold-rolled steel sheet after the overaging heat treatment within a range of 0.1 - 1.0%.

[0038] (III) Beneficial effects

[0039] According to one aspect of the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet with excellent formability and a manufacturing method thereof.

[0040] Various beneficial advantages and effects of the present invention are not limited to the above, and various beneficial advantages and effects of the present invention will be more easily understood during the process of describing the specific embodiments of the present invention. Description of the drawings

[0041] Figure 1 A photograph showing the microstructure of a test piece obtained from Invention Example 1 taken with a scanning electron microscope (SEM). Best mode for carrying out the invention

[0042] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be deformed into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely describe the present invention to those skilled in the art.

[0043] In addition, the terms used in this specification are intended to describe specific embodiments and are not intended to limit the present invention. For example, unless the relevant definition clearly indicates the contrary meaning, the singular forms used in this specification also include the plural forms. In addition, the meanings of "comprising" or "including" used in the specification are used to specifically describe the constitution and do not exclude the existence or addition of other constitutions.

[0044] As described above, in order to produce a steel material of the present invention with excellent hole expansion ratio (HER) of more than 20% and excellent elongation of more than 10%, it is very important to control the microstructure. As a method for simultaneously increasing the stretch flangeability and elongation, a technique for ensuring a uniform structure is required. Generally, among the low-temperature structures, the structure with the highest strength is martensite. It is well known that the easiest way to produce martensite is to hold for a sufficient time during annealing to fully form austenite, then perform water quenching and tempering treatment. However, the water quenching method may result in poor productivity due to problems such as material deviation and shape defects. Therefore, in the present invention, an attempt is made to ensure martensite by controlling alloying elements. That is, a technique for ensuring martensite even at a low cooling rate by adding hardenability elements such as Mn and Cr in an amount above a certain level. However, this method may cause problems such as poor weldability due to the addition of high alloying elements. Therefore, in the present invention, an attempt is made to minimize the carbon content, which has the greatest impact on weldability. In the steel of the present invention, the carbon content is limited to 0.3% or less.

[0045] In order to ensure a high yield ratio under the cooling conditions such as the steel of the present invention, as many alloying elements as possible must be added. However, this attempt may also easily cause problems such as poor weldability and increased hot rolling strength. Therefore, it is necessary to solve this problem. Therefore, through various studies, the present inventors found that without adding excessive alloying elements, by controlling the size of martensite and nano-precipitates, the stretch flangeability and yield ratio proposed in the steel of the present invention can be satisfied, thus completing the present invention. Hereinafter, the alloy composition and microstructure characteristics having an ultra-high strength required by the present invention, a hole expansion ratio of more than 20%, and an elongation of more than 10% will be described in detail.

[0046] First, the reasons for adding and the reasons for limiting the content of the alloy components of the cold-rolled steel sheet of the present invention will be described in detail. It should be noted that unless otherwise specified, the content of each of the following components is based on weight%.

[0047] C: 0.1 - 0.3%

[0048] In steel, carbon (C) is a very important element added to strengthen the phase transformation structure. Carbon promotes high strength and the formation of martensite in the phase transformation structure steel. When the carbon content is less than 0.1%, it is very difficult to ensure the strength of the martensite proposed in the present invention. Therefore, the carbon content is set to 0.1% or more. In addition, when the carbon content increases, the amount of martensite in the steel increases. However, when the carbon content exceeds 0.3%, the strength of the martensite increases, but the strength difference from ferrite with a low carbon concentration increases. This strength difference reduces the flange stretching performance because when stress is applied, fracture is likely to occur at the phase interface. In addition, the weldability is poor, and welding defects occur during processing of the components. Preferably, the carbon content can be 0.10 - 0.30%.

[0049] Si: 2.0% or less (except 0%)

[0050] In steel, silicon (Si) promotes ferrite phase transformation, increases the carbon content in the untransformed austenite, and forms a composite structure of ferrite and martensite, thereby hindering the increase in the strength of martensite. In addition, since the addition of Si not only causes surface scale defects related to surface characteristics but also reduces the chemical conversion treatment property, it is preferably limited as much as possible. Therefore, in the present invention, the Si content is controlled to be 2.0% or less. However, considering the inevitable inclusion, the lower limit of the Si content excludes 0%. Preferably, the Si content can be 2.00% or less.

[0051] Mn: 1.5 - 3.0%

[0052] In steel, manganese (Mn) is an element that refines grains without impairing ductility, precipitates sulfur in the steel completely in the form of MnS, thereby preventing hot brittleness caused by the formation of FeS, strengthening the steel, and at the same time playing a role in reducing the critical cooling rate for obtaining the martensite phase, so that martensite can be more easily formed. When the Mn content is less than 1.5%, it is difficult to ensure the strength desired in the present invention. On the other hand, when the Mn content exceeds 3.0%, the possibility of problems such as weldability and hot rolling property occurs is high. Therefore, the Mn content is set in the range of 1.5 - 3.0%. Preferably, the Mn content can be 1.50 - 3.00%. In addition, in terms of further improving the above effects, the lower limit of the Mn content can be 2.0%, or the upper limit of the Mn content can be 2.9%.

[0053] P: 0.001 - 0.015%

[0054] In steel, phosphorus (P) is the substitutional alloying element that maximizes the solution strengthening effect, playing a role in improving in-plane anisotropy and increasing strength. Therefore, when the content of P is less than 0.001%, not only can the above effects not be ensured, but also problems with manufacturing costs arise. On the other hand, when the content of P is excessively added and exceeds 0.015%, the stamping formability deteriorates, and brittleness of the steel may occur. Therefore, the content of P is set to 0.001 - 0.015%. Additionally, in terms of further improving the above effects, the lower limit of the content of P can be 0.003%, or the upper limit of the content of P can be 0.014%.

[0055] S: 0.001 - 0.01%

[0056] In steel, sulfur (S) is an impurity element that hinders the ductility and weldability of the steel sheet. Therefore, when the content of S exceeds 0.01%, the possibility of hindering the ductility and weldability of the steel sheet is high. Thus, the content of S is set to 0.01% or less. On the other hand, considering the inevitable inclusion, the lower limit of the content of S is set to 0.001%. Additionally, in terms of further improving the above effects, the lower limit of the content of S can be 0.002%, or the upper limit of the content of S can be 0.009%.

[0057] Al: 0.1% or less (excluding 0%)

[0058] In steel, soluble aluminum (Al) combines with oxygen to have a deoxidizing effect and is an effective component that, together with Si, distributes carbon in ferrite to austenite to improve the hardenability of martensite. Therefore, to ensure the above effects of Al, the lower limit of the content of Al excludes 0%. However, when the content of Al exceeds 0.1%, not only do the above effects saturate, but also the manufacturing cost increases. Thus, the content of Al is set to 0.1% or less. Preferably, the content of Al can be 0.10% or less. Additionally, in terms of further improving the above effects, the lower limit of the content of Al can be 0.001%, or the upper limit of the content of Al can be 0.09%.

[0059] N: 0.001 - 0.01%

[0060] In steel, nitrogen (N) is a component that plays an effective role in stabilizing austenite. When the content of N exceeds 0.01%, the risk of cracking during continuous casting increases significantly due to the formation of AlN, etc. Therefore, it is preferred to limit the upper limit of the content of N to 0.01%. Additionally, considering the inevitable inclusion, the lower limit of the content of N is set to 0.001%. Additionally, in terms of further improving the above effects, the lower limit of the content of N can be 0.002%, or the upper limit of the content of N can be 0.009%.

[0061] Cr: below 1.2% (except 0%)

[0062] In steel, chromium (Cr) is an element added to improve the hardenability of the steel and ensure high strength. In the present invention, it plays a very important role in forming martensite as a low-temperature transformation phase. To ensure the above effects, the lower limit of the content of Cr excludes 0%. However, when the content of Cr exceeds 1.2%, not only its effect saturates, but also problems such as poor cold rolling performance occur due to excessive increase in hot rolling strength. Therefore, the content of Cr is set to 1.2% or less. Preferably, the content of Cr can be 1.20% or less. Additionally, in terms of further improving the above effects, the lower limit of the content of Cr can be 0.01%, or the upper limit of the content of Cr can be 1.19%.

[0063] B: 0.001 - 0.005%

[0064] In steel, B is an element that delays the transformation of austenite to pearlite during the cooling process of annealing. B is added to inhibit the formation of ferrite and promote the formation of martensite. However, when the content of B is less than 0.001%, it is difficult to obtain the above effects. When the content of B exceeds 0.005%, cost deterioration occurs due to excessive alloying iron. Therefore, the content of B is set to 0.001 - 0.005%.

[0065] Mo: 0.03 - 0.25%

[0066] Molybdenum (Mo) is an element added to ensure strength and hardenability. When Mo is added together with Ti, Mo and Ti form carbides together. To obtain the strengthening effect of the organization generated by the formation of such carbides, the added content of Mo must be 0.03% or more. However, Mo is an expensive element. When added in excess, not only the economy deteriorates, but also the phase transformation is excessively delayed, which may trigger the formation of new martensite. Therefore, the content of Mo is set to 0.25% or less. Preferably, the content of Mo can be 0.030 - 0.250%. Additionally, in terms of further improving the above effects, the lower limit of the content of Mo can be 0.04%, or the upper limit of the content of Mo can be 0.24%.

[0067] In addition, although not particularly limited, according to one aspect of the present invention, the cold-rolled steel sheet may selectively further contain one or more elements of Ti and Nb, which are elements that can effectively improve the strength of the steel sheet and refine the grains through nano-precipitates in the steel. In the present invention, when the Ti or Nb is added, the content of Ti may be set to 0.01-0.08%, or the content of Nb may be in the range of 0.01-0.05%. In addition, when a large amount of Ti and Nb are added as in the present invention, very fine nano-precipitates are formed in combination with carbon. Such nano-precipitates serve to strengthen the matrix structure and reduce the hardness difference between the phases.

[0068] Except for the above composition, the rest is Fe. However, in the usual manufacturing process, undesirable impurities are inevitably mixed in from the raw materials or the surrounding environment, so these impurities cannot be excluded. These impurities are well known to those of ordinary skill in the art, so all of their contents are not specifically mentioned in this specification, but the description of representative impurities is as follows.

[0069] Next, in the cold-rolled steel sheet according to the present invention, in terms of area%, the fine structure includes: the sum of bainite and tempered martensite: 75-90%, retained austenite: 10% or less (except 0%), and the balance being fresh martensite.

[0070] In the present invention, the sum of bainite and tempered martensite as the phase transformation structure must be controlled to be 75% or more and 90% or less, and the retained austenite must be controlled to be 10% or less. In order to increase the hole expansion ratio (HER) and the yield ratio (YR), the higher the possible phase transformation structure fraction, the better, but considering the elongation, it is preferably controlled to be 90% or less. As in the present invention, when the carbon content is as low as 0.3% or less, considering the weldability and the hot rolling strength, when alloying elements are added, the increase in the strength of the formed martensite is limited. That is, when the martensite does not contain sufficient carbon, the increase in strength is limited. However, the present inventors can provide a cold-rolled steel sheet with ultra-high strength having the desired level of the present invention even when the carbon content is as low as 0.3% or less.

[0071] In addition, in terms of further improving the above effects, the lower limit of the area ratio of the retained austenite may be 4.8%, or the upper limit of the area ratio of the retained austenite may be 9.6%. Or, the lower limit of the area ratio of the sum of bainite and tempered martensite may be 81%, or the upper limit of the area ratio of the sum of bainite and tempered martensite may be 89%.

[0072] In addition, the cold-rolled steel sheet according to the present invention satisfies the following relational expressions 1 and 2. That is, in the present invention, in order to ensure strength while obtaining elongation, through a large number of experiments on the steel materials existing within the composition range proposed for the steel of the present invention, it has been confirmed that in order to minimize the hole expansion ratio (HER) to 20% or more while ensuring a certain ductility and ensuring an elongation (El) of 10% or more, the composition and manufacturing conditions of the steel produced at this time are very important, and it is important that these factors satisfy the following relational expressions 1 and 2.

[0073] [Relational expression 1]

[0074] 1.0 ≤ [C] + (1.3×[Si] + [Mn]) / 6 + ([Cr] + 1.2×[Mo]) / 5 + 100×[B] ≤ 1.2

[0075] (In the said relational expression 1, the [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight % contents of the respective elements within the brackets)

[0076] [Relational expression 2]

[0077] 106 ≤ ([Cr] + [Mo])×[Si] / [Al] ≤ 275

[0078] (In the said relational expression 2, the [Cr], [Mo], [Si], and [Al] represent the weight % contents of the respective elements within the brackets)

[0079] Next, a specific description will be given of the manufacturing method of the cold-rolled steel sheet according to the present invention.

[0080] The slab having the composition formed according to the above alloy design method is reheated and then hot-rolled. It is preferable to perform rolling so that the temperature on the finishing mill exit side during the hot-rolling process is between Ar3 and Ar3 + 50°C. That is, when the temperature on the finishing mill exit side is lower than Ar3, there is a high possibility that the hot deformation resistance will increase rapidly, and the upper (top), lower (tail), and edge portions of the hot-rolled coil will become single-phase regions, which will increase the in-plane anisotropy and deteriorate the formability. However, when the temperature on the finishing mill exit side exceeds Ar3 + 50°C, not only will excessive scale be generated, but there is also a high possibility that the fine structure of the steel sheet will become coarse. In addition, the Ar3 can be obtained by methods generally known in the art, so there is no particular limitation thereto. Furthermore, the finishing mill can be more specifically carried out within a temperature range of 880 - 920°C.

[0081] After completing the hot finish rolling, coiling is carried out at 500 - 750°C. When the coiling temperature is lower than 500°C, excessive martensite or bainite is generated, resulting in an excessive increase in the strength of the hot-rolled steel sheet, and manufacturing problems such as shape defects may occur due to the load during cold rolling. On the other hand, when the coiling temperature exceeds 750°C, due to the increase in surface scale, the pickling property deteriorates. Therefore, the coiling temperature is preferably limited to 500 - 750°C.

[0082] The hot-rolled steel sheet manufactured in the above manner is pickled and then cold-rolled to obtain a cold-rolled steel sheet.

[0083] The cold-rolled steel sheet thus obtained is continuously annealed at 800 - 900°C as the continuous annealing temperature (SS). When the continuous annealing temperature is low, a large amount of ferrite is generated, and thus YS and TS cannot be ensured. On the other hand, when the continuous annealing temperature is too high, due to the increase in austenite grain size caused by high-temperature annealing, the martensite packet size produced during cooling increases, and it is difficult to ensure the physical properties desired in the present invention.

[0084] The cold-rolled steel sheet after the continuous annealing is cooled at an average cooling rate of 1 - 10°C / second for the first cooling to the first cooling termination temperature of 650 - 700°C. The purpose of the first cooling is to suppress the ferrite phase transformation and transform most of the austenite into martensite.

[0085] Next, the cold-rolled steel sheet after the first cooling is cooled at an average cooling rate of more than 10°C / second and 20°C / second or less for the second cooling to the second cooling termination temperature (RCS). The cold-rolled steel sheet after the second cooling is subjected to overaging treatment, and the overaging treatment is to hold and perform heat treatment at 250 - 350°C. This second cooling termination temperature (RCS) is a very important temperature condition for ensuring the shape in the width direction and length direction of the coiled sheet and ensuring high YR and high HER. When the cooling termination temperature is low, the amount of martensite increases excessively during the overaging treatment, while the yield strength and tensile strength increase, and the ductility deteriorates significantly. In particular, due to rapid cooling, shape deterioration may occur, and thus poor workability is expected during the processing of automotive parts. In addition, when the second termination temperature is too high, the austenite generated during annealing cannot be transformed into martensite, and bainite, granular bainite, etc. as high-temperature transformation phases are generated, resulting in a problem of rapid deterioration of the yield strength. If such a structure is produced, the yield ratio decreases, and the hole expansion property also deteriorates, and it is impossible to manufacture the high yield ratio type high-strength steel with excellent stretch flangeability proposed in the present invention.

[0086] According to one aspect of the present invention, the manufacturing method of the cold-rolled steel sheet can be controlled to satisfy the following relational expression 3. By satisfying the following relational expression 3, a cold-rolled steel sheet having excellent strength and hole expansion property simultaneously can be effectively provided.

[0087] [Relational expression 3]

[0088] 8 ≤ 3.1×([SS] - [Ac1]) + 2.2×([RCS] - [Ms]) ≤ 180

[0089] (In the relational expression 3, [Ac1] represents the value defined by the following relational expression 4, [RCS] represents the secondary cooling termination temperature (°C), and [Ms] represents the martensite transformation start temperature (°C).)

[0090] [Relational expression 4]

[0091] [Ac1] = 723 - 10.7×[Mn] - 16.9×[Ni] + 29.1×[Si] + 16.9×[Cr]

[0092] (In the relational expression 4, [Mn], [Ni], [Si], and [Cr] represent the weight % contents of the respective elements in the parentheses)

[0093] At this time, the [Ms] refers to the value defined by the following relational expression 5.

[0094] [Relational expression 5]

[0095] Ms = 539 - 423×[C] - 30.4×[Mn] - 12.1×[Cr] - 7.5×[Mo]

[0096] (In the relational expression 5, [C], [Mn], [Cr], and [Mo] represent the weight % contents of the respective elements in the parentheses)

[0097] The cold-rolled steel sheet after the aging heat treatment is subjected to temper rolling in the range of 0.1 - 1.0%. When temper rolling is performed on a normal phase transformation structure steel, the yield strength is increased by at least 50 Mpa or more, while the tensile strength hardly increases. When the rolling rate of the temper rolling is less than 0.1%, it is very difficult to control the shape in the ultra-high strength steel such as the steel of the present invention. When the rolling rate of the temper rolling in operation exceeds 1.0%, due to the high elongation operation, the workability becomes very unstable. Therefore, this value is set to 0.1 - 1.0%. Detailed implementation mode

[0098] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and are not used to limit the scope of rights of the present invention. This is because the scope of rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom.

[0099] (Example)

[0100] The steel billet having the composition shown in Table 1 below was subjected to vacuum melting, heated in a heating furnace at a reheating temperature of 1200 °C for 1 hour, hot-rolled, and then coiled. As shown in Table 2 below, the temperature conditions during the hot-rolling operation were based on Ar3 or higher in each case. In order to satisfy the range from Ar3 to Ar3 + 50 °C, hot-rolling was completed in the temperature range of 880 - 920 °C, and the coiling temperature was controlled at 500 - 680 °C. The hot-rolled steel plate was pickled and then cold-rolled. For the cold-rolled steel plate thus obtained, under the conditions shown in Table 2 below, continuous annealing was performed at the continuous annealing temperature (SS), and then primary cooling was performed at an average cooling rate of 5 °C / second until the primary cooling termination temperature of 650 - 700 °C was reached. Subsequently, secondary cooling was performed at an average cooling rate of 15 °C / second until the secondary cooling termination temperature (RCS) was reached. Then, the cold-rolled steel plate after the secondary cooling was subjected to over-aging heat treatment, and the over-aging heat treatment was to hold the cold-rolled steel plate at 250 - 350 °C, and then the final skin pass rolling rate was fixed at 0.2%.

[0101] The sum of bainite and tempered martensite, and the area ratios of retained austenite and fresh martensite of each steel plate manufactured according to the changes in the steel composition and annealing conditions were measured and shown in Table 3 below.

[0102] In addition, JIS No. 5 tensile test pieces were prepared, and the yield strength (YS), tensile strength (TS), elongation (El), and hole expansion property (HER) were measured according to the JIS standard. The results are shown in Table 3 below together with the comparative examples. In the case of the hole expansion property, when D o is set as the initial hole diameter (mm) and D h is set as the hole diameter after fracture (mm), the calculation is performed according to the following formula.

[0103] HER (%) = (D h - D o ) / D o × 100

[0104] [Table 1]

[0105]

[0106] [Table 2]

[0107]

[0108] Ac1 = 723 - 10.7×[Mn] - 16.9×[Ni] + 29.1×[Si] + 16.9×[Cr]

[0109] [Table 3]

[0110]

[0111] r: Austenite, B: Bainite, TM: Tempered martensite [remaining FM (fresh martensite)]

[0112] From the experimental results of Table 3 above, it can be seen that in the case of Invention Examples 1 to 6 that satisfy the alloy composition and manufacturing conditions of the present invention, it is confirmed that the tensile strength is 1470 MPa or more, having ultra-high strength, excellent yield strength, and excellent elongation and hole expansion properties at the same time.

[0113] On the other hand, in the case of Comparative Examples 1 to 7 that do not satisfy one or more of the alloy composition and manufacturing conditions of the present invention, it is confirmed that one or more of the properties of tensile strength, yield strength, elongation, and hole expansion are worse than those of the above invention examples.

Claims

1. A cold-rolled steel sheet, by weight %, the cold-rolled steel sheet contains: C: 0.1 - 0.3%, Si: less than 2.0% and excluding 0%, Mn: 1.5 - 3.0%, Cr: less than 1.2% and excluding 0%, Mo: 0.03 - 0.25%, Al: less than 0.1% and excluding 0%, P: 0.001 - 0.015%, S: 0.001 - 0.01%, N: 0.001 - 0.01%, B: 0.001 - 0.005%, the balance of Fe and other impurities, by area %, as the fine microstructure contains: the sum of bainite and tempered martensite: 75 - 90%, retained austenite: less than 10% and excluding 0% and the balance of fresh martensite, the cold-rolled steel sheet satisfies the following relational expressions 1 and relational expression 2, [Relational expression 1] 1.0 ≤ [C] + (1.3×[Si] + [Mn]) / 6 + ([Cr] + 1.2×[Mo]) / 5 + 100×[B] ≤ 1.2 In the relational expression 1, the [C], [Si], [Mn], [Cr], [Mo] and [B] represent the weight % contents of the respective elements in the brackets, [Relational expression 2] 106 ≤ ([Cr] + [Mo])×[Si] / [Al] ≤ 275 In the relational expression 2, the [Cr], [Mo], [Si] and [Al] represent the weight % contents of the respective elements in the brackets.

2. The cold-rolled steel sheet according to claim 1, wherein, By area %, as the fine microstructure contains 4.8 - 9.6% of retained austenite.

3. The cold-rolled steel sheet according to claim 1, wherein, By area %, as the fine microstructure contains: the sum of bainite and tempered martensite: 81 - 89%.

4. A method for manufacturing a cold-rolled steel sheet, which includes the following steps: Reheat the slab, by weight %, the slab contains: C: 0.1 - 0.3%, Si: less than 2.0% and excluding 0%, Mn: 1.5 - 3.0%, Cr: less than 1.2% and excluding 0%, Mo: 0.03 - 0.25%, Al: less than 0.1% and excluding 0%, P: 0.001 - 0.015%, S: 0.001 - 0.01%, N: 0.001 - 0.01%, B: 0.001 - 0.005%, the balance of Fe and other impurities, and the slab satisfies the following relational expressions 1 and relational expression 2; Hot finish roll the reheated slab at Ar3 to Ar3 + 50 °C to obtain a hot-rolled steel sheet; Coil the hot-rolled steel sheet at 500 - 750 °C; Cold roll the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; Continuously anneal the cold-rolled steel sheet at 800 - 900 °C; Cool the continuously annealed cold-rolled steel sheet at an average cooling rate of 1 - 10 °C / second for the first cooling until the first cooling termination temperature of 650 - 700 °C; Cool the cold-rolled steel sheet after the first cooling at an average cooling rate exceeding 10 °C / second and not exceeding 20 °C / second for the second cooling until the second cooling termination temperature; and Perform over-aging heat treatment on the cold-rolled steel sheet after the second cooling at 250 - 350 °C, [Relationship 1] 1.0 ≤ [C] + (1.3×[Si] + [Mn]) / 6 + ([Cr] + 1.2×[Mo]) / 5 + 100×[B] ≤ 1.2 In the above Relationship 1, [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight % contents of the respective elements within the brackets. [Relationship 2] 106 ≤ ([Cr] + [Mo]) × [Si] / [Al] ≤ 275 In the above Relationship 2, [Cr], [Mo], [Si], and [Al] represent the weight % contents of the respective elements within the brackets.

5. The manufacturing method of the cold-rolled steel sheet according to claim 4, wherein, The following Relationship 3 is satisfied. [Relationship 3] 8 ≤ 3.1×([SS] - [Ac1]) + 2.2×([RCS] - [Ms]) ≤ 180 In the above Relationship 3, [Ac1] represents the value defined by the following Relationship 4, [RCS] represents the secondary cooling termination temperature in °C, and [Ms] represents the martensite transformation start temperature in °C. [Relationship 4] [Ac1] = 723 - 10.7×[Mn] - 16.9×[Ni] + 29.1×[Si] + 16.9×[Cr] In the above Relationship 4, [Mn], [Ni], [Si], and [Cr] represent the weight % contents of the respective elements within the brackets.

6. The manufacturing method of the cold-rolled steel sheet according to claim 4, wherein, The manufacturing method further includes a step of skin pass rolling the cold-rolled steel sheet that has undergone the overaging heat treatment within the range of 0.1 - 1.0%.

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