Cold-rolled steel sheet and its manufacturing method

Abstract

The present invention relates to an ultra-high strength cold rolled steel sheet with a tensile strength of 1470 MPa class, which is mainly used for collision and structural components of automobiles, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to an ultra-high strength cold rolled steel sheet with a tensile strength of 1470 MPa class, which is mainly used for collision and structural components of automobiles, and a method for manufacturing the same. [Background technology]

[0002] In recent years, various environmental and energy use regulations have led to a demand for higher strength steel sheets for automobiles to improve fuel efficiency and durability. In particular, as regulations regarding the impact stability of automobiles have recently become more widespread, high-strength steels with excellent yield strength have been adopted for structural components such as members, seat rails, and pillars to improve the impact resistance of vehicle bodies. The higher the yield strength relative to the tensile strength, i.e., the higher the yield ratio (tensile strength / yield strength), the more advantageous the impact energy absorption capabilities of these structural components. However, generally, as the strength of steel sheets increases, the elongation decreases, resulting in a problem of reduced formability. Therefore, there is a need for the development of materials that can address this issue.

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

[0004] On the other hand, precipitation-hardened high-strength steel is a technology that strengthens steel sheets by precipitating carbonitrides through the addition of carbonitride-forming elements such as Cu, Nb, Ti, and V, or by refining grains through the suppression of grain growth by fine precipitates. While these technologies offer the advantage of easily achieving high strength at low manufacturing costs, they also have the disadvantage of requiring high-temperature annealing to induce sufficient recrystallization and ensure ductility because the fine precipitates rapidly increase the recrystallization temperature. Another problem with precipitation-hardened steel, which is strengthened by the precipitation of carbonitrides in a ferrite matrix, is that it is difficult to obtain high-strength steels of 600 MPa or higher.

[0005] Various transformation-strengthened high-strength steels have been developed, including ferrite-martensite dual-phase steels, which incorporate hard martensite into a ferrite matrix; TRIP (Transformation Induced Plasticity) steels, which utilize the transformation-induced plasticity of retained austenite; and CP (Complexed Phase) steels, which combine ferrite with hard bainite or martensite. However, the elongation limit of these advanced high-strength steels is approximately 8% at a tensile strength of 1500 MPa. Furthermore, for structural applications to ensure crashworthiness, hot press-formed steels, which are formed at high temperatures and then rapidly cooled by direct contact with a water-cooled die, have been gaining attention. However, their application has not expanded significantly due to excessive capital investment costs and the high costs of heat treatment and processing.

[0006] Recently, efforts have been made to simultaneously increase the strength and reduce the weight of vehicle seat parts in order to further improve passenger stability during a collision. These parts are manufactured using two methods: roll forming and press forming. Seat parts connect passengers to the vehicle body and must support them with high stress to prevent them from being thrown out during a collision. This requires high yield strength and yield ratio. In addition, most of the parts that are processed require stretch flangeability, so the use of steel materials with excellent hole expandability is required.

[0007] Meanwhile, Patent Document 1 discloses a high-tensile cold-rolled steel sheet with a martensite single-phase structure and a tensile strength of 880 to 1170 MPa, which is achieved by optimizing the steel sheet composition and heat treatment conditions. Patent Document 2 also discloses a method for manufacturing a high-tensile steel sheet, in which a steel sheet in which a low-temperature transformation phase consisting of martensite and retained austenite accounts for 90% or more of the volume ratio of the entire metallographic structure is heated and held in a two-phase region to control a fine ferrite and austenite structure including laths of the low-temperature transformation phase, and then cooling to finally obtain a metallographic structure in which the ferrite and low-temperature transformation phase are finely dispersed on the laths. While these patents claim that high yield strength can be achieved without water cooling, they have the drawback of significantly degrading ductility or degrading stretch flangeability due to the generation of a large amount of austenite in the steel. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-161336 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-272954 Summary of the Invention [Problem to be solved by the invention]

[0009] According to one aspect of the present invention, there are provided an ultra-high strength cold-rolled steel sheet with excellent formability and a method for producing the same.

[0010] The object of the present invention is not limited to the above content. Anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0011] One aspect of the present invention is In weight percent, C: 0.1 to 0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5 to 3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03 to 0.25%, Al: 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, with the balance being Fe and other impurities. The microstructure includes, by area%, 75 to 90% of bainite and tempered martensite in total, 10% or less of retained austenite (excluding 0%), and the remainder of fresh martensite. A cold-rolled steel sheet is provided that satisfies the following relations 1 and 2. [Equation 1] 1.0≦[C]+(1.3×[Si]+[Mn]) / 6+([Cr]+1.2×[Mo]) / 5+100×[B]≦1.2 (In the above Relational Formula 1, the above [C], [Si], [Mn], [Cr], [Mo], and [B] indicate the weight percent content of each element in parentheses.)

[0012] [Equation 2] 106≦([Cr]+[Mo])×[Si] / [Al]≦275 (In the above Relational Formula 2, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

[0013] The above-mentioned microstructure may contain, in area %, 4.8 to 9.6% retained austenite.

[0014] The above-mentioned microstructure may contain, in area %, 81 to 89% in total of bainite and tempered martensite.

[0015] Furthermore, still another aspect of the present invention is a step of reheating a slab containing, by weight percent, C: 0.1 to 0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5 to 3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03 to 0.25%, Al: 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, the balance being Fe and other impurities, and satisfying the following relations 1 and 2: a step of finish hot rolling the reheated slab at Ar3 to Ar3+50°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 500 to 750°C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at 800 to 900°C; performing primary cooling of the continuously annealed cold-rolled steel sheet to a primary cooling end temperature of 650 to 700°C at an average cooling rate of 1 to 10°C / s; Secondary cooling the primarily cooled cold-rolled steel sheet to a secondary cooling end temperature at an average cooling rate of more than 10°C / s and not more than 20°C / s; and and a step of overaging the second-cooled cold-rolled steel sheet at 250 to 350°C.

[0016] [Equation 1] 1.0≦[C]+(1.3×[Si]+[Mn]) / 6+([Cr]+1.2×[Mo]) / 5+100×[B]≦1.2 (In the above Relational Formula 1, the above [C], [Si], [Mn], [Cr], [Mo], and [B] indicate the weight percent content of each element in parentheses.)

[0017] [Equation 2] 106≦([Cr]+[Mo])×[Si] / [Al]≦275 (In the above Relational Formula 2, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

[0018] The above production method can satisfy the following relational expression 3.

[0019] [Equation 3] 8≦3.1×([SS]-[Ac1])+2.2×([RCS]-[Ms])≦180 (In the above relational expression 3, [Ac1] is a value defined by the following relational expression 4, [RCS] represents the secondary cooling end temperature (°C), and [Ms] represents the martensitic transformation start temperature (°C).)

[0020] [Equation 4] [Ac1]=723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr] (In the above Relational Formula 4, [Mn], [Ni], [Si], and [Cr] represent the weight percent content of each element in parentheses.)

[0021] The over-aging heat-treated cold rolled steel sheet may further include a step of skin-pass rolling in the range of 0.1 to 1.0%. [Effects of the Invention]

[0022] According to one aspect of the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet having excellent formability and a method for manufacturing the same.

[0023] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0024] [Figure 1] 1 shows a photograph of the microstructure of the test piece obtained from Inventive Example 1 taken with a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0026] However, the terms used herein are intended to describe specific embodiments and are not intended to limit the present invention. For example, the singular forms "a," "an," and "the" as used herein include the plural forms unless the relevant definition clearly indicates otherwise. Furthermore, the term "comprises" as used herein embodies a configuration and does not exclude the presence or addition of other configurations.

[0027] As described above, microstructural control is crucial for producing steel materials with excellent hole expansion ratios (HER) of 20% or more and elongation of 10% or more, as presented in the present invention. To simultaneously increase stretch flangeability and elongation, a technique for ensuring a uniform microstructure is required. Generally, martensite has the highest strength among low-temperature microstructures. As is well known, the easiest way to produce martensite is to anneal the steel for a sufficient time for austenite to form, followed by water quenching and tempering. However, water quenching can lead to poor productivity due to material variations and shape defects. Therefore, the present invention aims to ensure martensite by controlling alloying elements. Specifically, this technique involves adding a certain amount of hardening elements, such as Mn and Cr, to ensure martensite even at low cooling rates. However, this method can lead to problems such as poor weldability due to the addition of high-alloying elements. Therefore, the present invention aims to minimize the carbon content, which has the greatest impact on weldability. In the present invention, the carbon content is limited to 0.3% or less.

[0028] To ensure a high yield ratio under cooling conditions such as those of the present invention steel, alloying elements must be added in as large an amount as possible. However, such attempts have led to further problems such as deterioration of weldability and an increase in hot rolling strength, which require solutions. Therefore, through extensive research, the inventors have discovered that the stretch flangeability and yield ratio required for the present invention steel can be achieved by controlling the size of martensite and nanoprecipitates without adding excessive alloying elements, leading to the completion of the present invention. Below, we will explain in detail the alloy composition and microstructural characteristics required for ensuring hole expandability of 20% or more and elongation of 10% or more while maintaining the ultra-high strength required in the present invention.

[0029] First, the reasons for adding alloying elements to the cold-rolled steel sheet of the present invention and the reasons for limiting the content thereof will be described in detail. It should be noted that the content of each element described below is based on weight percent unless otherwise specified.

[0030] C: 0.1 to 0.3% Carbon (C) is a very important element added to steel to strengthen the transformation structure. Carbon increases strength and promotes the formation of martensite in the transformation structure steel. If the carbon content is less than 0.1%, it is very difficult to ensure the martensite strength proposed in the present invention, so the carbon content is set to 0.1% or more. On the other hand, as the carbon content increases, the amount of martensite in the steel increases. However, if the carbon content exceeds 0.3%, although the strength of martensite increases, the strength difference with ferrite, which has a low carbon concentration, increases. Such a strength difference easily causes fracture at the interphase interface when stress is applied, resulting in reduced stretch flangeability. Furthermore, poor weldability leads to weld defects during part processing. Preferably, the carbon content may be 0.10 to 0.30%.

[0031] Si: 2.0% or less (excluding 0%) Silicon (Si) in steel accelerates ferrite transformation, increases the carbon content in untransformed austenite, and forms a composite structure of ferrite and martensite, inhibiting the increase in martensite strength. Furthermore, silicon not only induces surface scale defects related to surface properties but also reduces phosphatability. Therefore, it is preferable to limit its addition as much as possible. Therefore, in the present invention, the Si content is controlled to 2.0% or less. However, considering cases where Si is unavoidably contained, 0% is excluded as the lower limit of the Si content. Preferably, it may be 2.00% or less.

[0032] Mn: 1.5 to 3.0% Manganese (Mn) in steel refines grains without damaging ductility, completely precipitates sulfur in the steel as MnS, prevents hot embrittlement due to the formation of FeS, and strengthens the steel. At the same time, it lowers the critical cooling rate at which martensite is obtained, making martensite easier to form. If the Mn content is less than 1.5%, it becomes difficult to achieve the strength targeted in the present invention. However, if it exceeds 3.0%, problems such as weldability and hot rollability are likely to occur. Therefore, the Mn content is set to a range of 1.5 to 3.0%. Preferably, it may be 1.50 to 3.00%. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the Mn content may be 2.0%, or the upper limit of the Mn content may be 2.9%.

[0033] P: 0.001 to 0.015% In steel, phosphorus (P) is a substitutional alloying element with the greatest solid solution strengthening effect, and it plays a role in improving in-plane anisotropy and increasing strength. Therefore, if the P content is less than 0.001%, not only will the above-mentioned effects not be ensured, but it will also cause problems with production costs. On the other hand, if the P content is added in excess of 0.015%, press formability will deteriorate and the steel may become brittle, so the P content is set to 0.001 to 0.015%. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the P content may be 0.003%, or the upper limit of the P content may be 0.014%.

[0034] S: 0.001 to 0.01% In steel, sulfur (S) is an impurity element that impairs the ductility and weldability of steel sheet. Therefore, if the S content exceeds 0.01%, there is a high possibility that the ductility and weldability of the steel sheet will be impaired, so the S content is set to 0.01% or less. However, taking into consideration cases where S is unavoidably contained, the lower limit of the S content is set to 0.001%. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the S content may be 0.002%, or the upper limit of the S content may be 0.009%.

[0035] Al: 0.1% or less (excluding 0%) In steel, soluble aluminum (Al) is an effective component for combining with oxygen to provide deoxidation and, like Si, for distributing carbon in ferrite to austenite to improve martensite hardening. Therefore, to ensure the above-mentioned effects of Al, the lower limit of the Al content is set to 0%. However, if the Al content exceeds 0.1%, not only will the above-mentioned effects saturate but production costs will increase, so the Al content is set to 0.1% or less. Preferably, it may be 0.10% or less. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the Al content may be 0.001%, or the upper limit of the Al content may be 0.09%.

[0036] N: 0.001 to 0.01% Nitrogen (N) in steel is an element that effectively stabilizes austenite. If its content exceeds 0.01%, the risk of cracks occurring during continuous casting due to the formation of AlN, etc., increases significantly. Therefore, it is preferable to limit its upper limit to 0.01%. Taking into consideration cases where N is unavoidably contained, the lower limit of the N content is set to 0.001%. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the N content may be 0.002%, or the upper limit of the N content may be 0.009%.

[0037] Cr: 1.2% or less (excluding 0%) Chromium (Cr) is an element added to steel to improve its hardenability and ensure its high strength. In the present invention, Cr plays a very important role in forming martensite, a low-temperature transformation phase. To ensure the above-mentioned effects, the lower limit of the Cr content is set to 0%. However, if the Cr content exceeds 1.2%, not only will the effect saturate, but the cold rolling property will deteriorate due to an excessive increase in hot rolling strength. Therefore, the Cr content is set to 1.2% or less. Preferably, it may be 1.20% or less. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the Cr content may be 0.01%, or the upper limit of the Cr content may be 1.19%.

[0038] B: 0.001 to 0.005% In steel, B is a component that delays the transformation of austenite to pearlite during the cooling process during annealing, and is added as an element that suppresses the formation of ferrite and promotes the formation of martensite. However, if the B content is less than 0.001%, it is difficult to obtain the above-mentioned effects, and if it exceeds 0.005%, excessive ferroalloy content causes cost deterioration, so the B content is set to 0.001 to 0.005%.

[0039] Mo: 0.03 to 0.25% Molybdenum (Mo) is an element added to ensure strength and hardenability. When added together with Ti, it forms carbides with Ti. To obtain the effect of strengthening the structure through the formation of such carbides, the Mo content must be 0.03% or more. However, since Mo is an expensive element, adding too much not only reduces economic efficiency but also may excessively delay phase transformation, potentially inducing the formation of fresh martensite. Therefore, the Mo content is set to 0.25% or less. Preferably, it may be 0.030 to 0.250%. On the other hand, from the viewpoint of further improving the above-mentioned effect, the lower limit of the Mo content may be 0.04%, or the upper limit of the Mo content may be 0.24%.

[0040] Furthermore, according to one aspect of the present invention, the cold-rolled steel sheet may further include, but is not limited to, one or more of Ti and Nb, which are effective in increasing the strength of the steel sheet and refining the grain size through nano-precipitates. When Ti or Nb is added in the present invention, the Ti content may be in the range of 0.01 to 0.08%, or the Nb content may be in the range of 0.01 to 0.05%. When Ti and Nb are added in large amounts, as in the present invention, they combine with carbon to form very fine nano-precipitates. These nano-precipitates strengthen the matrix structure and reduce the hardness difference between phases.

[0041] In addition to the above composition, the remainder is Fe. However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone with ordinary skill in the art, not all of them will be particularly mentioned in this specification, but typical impurities will be mentioned as follows.

[0042] Next, the cold rolled steel sheet according to the present invention contains, as a microstructure, in area %, 75 to 90% of bainite and tempered martensite in total, 10% or less (excluding 0%) of retained austenite, and the balance fresh martensite.

[0043] In the present invention, the total of the transformed structures, bainite and tempered martensite, must be controlled to 75% to 90% and the retained austenite must be controlled to 10% or less. To increase the hole expandability (HER) and yield ratio (YR), it is preferable to have as high a transformed structure fraction as possible. However, considering the elongation, it is better to control it to 90% or less. When the carbon content is low, such as 0.3% or less, adding alloy elements in consideration of weldability and hot rolling strength limits the strength increase of the resulting martensite. In other words, if sufficient carbon is not contained in the martensite, there is a limit to the strength increase. However, the present inventors have been able to provide a cold-rolled steel sheet with the ultra-high strength desired by the present invention, even with a carbon content as low as 0.3% or less.

[0044] On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the area fraction of retained austenite may be 4.8%, or the upper limit of the area fraction of retained austenite may be 9.6%, or the lower limit of the total area fraction of bainite and tempered martensite may be 81%, or the upper limit of the total area fraction of bainite and tempered martensite may be 89%.

[0045] The cold-rolled steel sheet according to the present invention satisfies the following relations 1 and 2. That is, in the present invention, in order to obtain elongation while ensuring strength, it has been confirmed through numerous experiments using steel materials having composition ranges presented in the steel of the present invention that in order to ensure a certain level of ductility, in order to ensure a hole expandability (HER) of at least 20% and an elongation (El) of 10% or more, the composition and manufacturing conditions of the steel produced at this time are very important, and that it is important that these factors satisfy the following relations 1 and 2.

[0046] [Equation 1] 1.0≦[C]+(1.3×[Si]+[Mn]) / 6+([Cr]+1.2×[Mo]) / 5+100×[B]≦1.2 (In the above Relational Formula 1, the above [C], [Si], [Mn], [Cr], [Mo], and [B] indicate the weight percent content of each element in parentheses.)

[0047] [Equation 2] 106≦([Cr]+[Mo])×[Si] / [Al]≦275 (In the above Relational Formula 2, [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

[0048] The method for producing a cold-rolled steel sheet according to the present invention will be specifically described below.

[0049] The slab whose components are composed according to the above-described alloy design method is reheated and then hot-rolled. The finish rolling in the hot rolling is preferably performed so that the outlet temperature is between Ar3 and Ar3 + 50°C. That is, if the outlet temperature of the finish rolling is lower than Ar3, there is a high possibility that the hot deformation resistance will increase sharply, and the top, tail, and ends of the hot-rolled coil will become single-phase regions, increasing the in-plane anisotropy and deteriorating the formability. However, if the outlet temperature of the finish rolling exceeds Ar3 + 50°C, there is a high possibility that not only an excessively thick oxide scale will be formed but also the microstructure of the steel sheet will become coarse. Meanwhile, the Ar3 can be determined by methods commonly known in the art and is not particularly limited. More specifically, the finish rolling can be performed at a temperature range of 880 to 920°C.

[0050] After the hot finish rolling is completed, the steel sheet is coiled at 500 to 750°C. If the coiling temperature is less than 500°C, excess martensite or bainite is generated, which leads to an excessive increase in strength of the hot-rolled steel sheet, and this may cause manufacturing problems such as defective shape due to the load during cold rolling. On the other hand, if the coiling temperature exceeds 750°C, the surface scale increases, which deteriorates the pickling properties, so the coiling temperature is preferably limited to 500 to 750°C.

[0051] The hot-rolled steel sheet produced by the above method is pickled and then cold-rolled to obtain a cold-rolled steel sheet.

[0052] The cold-rolled steel sheet thus obtained is subjected to continuous annealing at a continuous annealing temperature (SS) of 800 to 900°C. If the continuous annealing temperature is low, a large amount of ferrite is produced, making it impossible to ensure YS and TS. On the other hand, if the continuous annealing temperature is too high, the austenite grain size increases due to high-temperature annealing, and the martensite packet size produced during cooling increases, which may make it difficult to ensure the physical properties targeted in the present invention.

[0053] The continuously annealed cold-rolled steel sheet is subjected to primary cooling at an average cooling rate of 1 to 10°C / s to a primary cooling end temperature of 650 to 700°C. The primary cooling is intended to suppress ferrite transformation and transform most of the austenite into martensite.

[0054] The primarily cooled cold-rolled steel sheet is then subjected to secondary cooling at an average cooling rate of more than 10°C / s and not more than 20°C / s to a secondary cooling finish temperature (RCS), followed by overaging treatment, in which the secondary-cooled cold-rolled steel sheet is held at 250-350°C. This secondary cooling finish temperature (RCS) is an extremely important temperature condition for ensuring high YR and high HER as well as maintaining the coil's shape in the width and length directions. If the cooling finish temperature is too low, the amount of martensite increases excessively during overaging treatment, resulting in increased yield strength and tensile strength, while significantly deteriorating ductility. In particular, rapid cooling can cause shape deterioration, which can lead to poor workability during automotive part processing. On the other hand, if the secondary finish temperature is too high, the austenite formed during annealing cannot transform to martensite, resulting in the formation of high-temperature transformation phases such as bainite and granular bainite, resulting in a rapid decrease in yield strength. The occurrence of such a structure is accompanied by a decrease in the yield ratio and deterioration of hole expandability, making it impossible to produce the high-yield-ratio high-strength steel with excellent stretch-flange formability proposed in the present invention.

[0055] According to one aspect of the present invention, the method for producing the cold-rolled steel sheet can be controlled so as to satisfy the following relational expression 3. By satisfying the following relational expression 3, it is possible to effectively provide a cold-rolled steel sheet that is simultaneously excellent in strength and hole expandability.

[0056] [Equation 3] 8≦3.1×([SS]-[Ac1])+2.2×([RCS]-[Ms])≦180 (In the above relational expression 3, [Ac1] is a value defined by the following relational expression 4, [RCS] represents the secondary cooling end temperature (°C), and [Ms] represents the martensitic transformation start temperature (°C).)

[0057] [Equation 4] [Ac1]=723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr] (In the above Relational Formula 4, [Mn], [Ni], [Si], and [Cr] represent the weight percent content of each element in parentheses.)

[0058] In this case, the above [Ms] is a value defined by the following relational expression 5.

[0059] [Equation 5] Ms=539-423×[C]-30.4×[Mn]-12.1×[Cr]-7.5×[Mo] (In the above Relational Formula 5, [C], [Mn], [Cr], and [Mo] represent the weight percent content of each element in parentheses.)

[0060] The cold-rolled steel sheet that has been subjected to the above-mentioned overaging heat treatment is subjected to skin-pass rolling at a ratio in the range of 0.1 to 1.0%. Normally, when a transformed structure steel is skin-pass rolled, there is almost no increase in tensile strength, but an increase in yield strength of at least 50 MPa or more occurs. If the reduction ratio in the skin-pass rolling is less than 0.1%, it is very difficult to control the shape of an ultra-high strength steel such as the steel of the present invention. If the reduction ratio in the skin-pass rolling is greater than 1.0%, the operability becomes very unstable due to the high elongation operation, so the value is set to 0.1 to 1.0%. [Example]

[0061] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0062] (Example) Steel slabs having the compositions shown in Table 1 below were vacuum melted, reheated in a heating furnace at a temperature of 1200°C for 1 hour, hot rolled, and then coiled. The temperature conditions during the hot rolling operation were as shown in Table 2 below. For each example, hot rolling was completed in a temperature range of 880 to 920°C, with Ar3 or higher as the reference, satisfying the range of Ar3 to Ar3 + 50°C, and the coiling temperature was controlled to 500 to 680°C. The hot-rolled steel sheets were pickled and then cold-rolled. The cold-rolled steel sheets thus obtained were subjected to continuous annealing at a continuous annealing temperature (SS) under the conditions shown in Table 2 below. Then, primary cooling was performed at an average cooling rate of 5°C / s to a primary cooling end temperature of 650 to 700°C, followed by secondary cooling at an average cooling rate of 15°C / s to a secondary cooling end temperature (RCS). Next, the secondarily cooled cold-rolled steel sheet was subjected to an overaging heat treatment in which it was held at 250 to 350°C, and then the final skin-pass rolling reduction was fixed at 0.2%.

[0063] The total area ratio of bainite and tempered martensite, and the area ratio of retained austenite and fresh martensite were measured for each steel plate manufactured according to the changes in the steel composition and annealing conditions, and the results are shown in Table 3 below.

[0064] In addition, JIS No. 5 tensile test pieces were prepared, and the yield strength (YS), tensile strength (TS), elongation (El), and hole expandability (HER) were measured according to the JIS standard. The results are shown in Table 3 below, along with those of comparative examples. On the other hand, in the case of hole expandability, D o is the initial hole diameter (mm), and D h is the hole diameter (mm) after fracture, it was calculated according to the following formula.

[0065] HER(%)=(D h -D o ) / D o ×100

[0066] [Table 1]

[0067] [Table 2]

[0068] [Table 3]

[0069] As can be seen from the experimental results in Table 3 above, in the case of Invention Examples 1 to 6, which satisfy the alloy composition and manufacturing conditions of the present invention, the tensile strength is 1470 MPa or more, and it has been confirmed that it has ultra-high strength, excellent yield strength, and excellent elongation and hole expandability.

[0070] In contrast, in the case of Comparative Examples 1 to 7, which do not satisfy one or more of the alloy compositions and manufacturing conditions of the present invention, it was confirmed that one or more of the properties of tensile strength, yield strength, elongation, and hole expandability were inferior to the above-mentioned invention examples.

Claims

1. In weight percent, C: 0.1 to 0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5 to 3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03 to 0.25%, Al: 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, with the balance being Fe and other impurities, The microstructure contains, by area percentage, 75 to 90% of bainite and tempered martensite in total, 10% or less (excluding 0%) of retained austenite, and the remainder fresh martensite, A cold-rolled steel sheet that satisfies the following relations 1 and 2. [Relationship 1] 1.0≦[C]+(1.3×[Si]+[Mn]) / 6+([Cr]+1.2×[Mo]) / 5+100×[B]≦1.2 (In the above Relational Formula 1, the [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight percent content of each element in parentheses.) [Relationship 2] 106≦([Cr]+[Mo])×[Si] / [Al]≦275 (In the above Relational Formula 2, the [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

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

3. The cold-rolled steel sheet according to claim 1, wherein the microstructure contains, in area %, 81 to 89% of bainite and tempered martensite in total.

4. a step of reheating a slab containing, by weight, C: 0.1 to 0.3%, Si: 2.0% or less (excluding 0%), Mn: 1.5 to 3.0%, Cr: 1.2% or less (excluding 0%), Mo: 0.03 to 0.25%, Al: 0.1% or less (excluding 0%), P: 0.001 to 0.015%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, B: 0.001 to 0.005%, the balance being Fe and other impurities, and satisfying the following relations 1 and 2: Finish hot rolling the reheated slab at Ar3 to Ar3+50°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 500 to 750°C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at 800 to 900°C; subjecting the continuously annealed cold-rolled steel sheet to a primary cooling end temperature of 650 to 700°C at an average cooling rate of 1 to 10°C / s; Secondarily cooling the primarily cooled cold-rolled steel sheet to a secondary cooling end temperature at an average cooling rate of more than 10°C / s and not more than 20°C / s; and and overaging the second-cooled cold-rolled steel sheet at 250 to 350°C. [Relationship 1] 1.0≦[C]+(1.3×[Si]+[Mn]) / 6+([Cr]+1.2×[Mo]) / 5+100×[B]≦1.2 (In the above Relational Formula 1, the [C], [Si], [Mn], [Cr], [Mo], and [B] represent the weight percent content of each element in parentheses.) [Relationship 2] 106≦([Cr]+[Mo])×[Si] / [Al]≦275 (In the above Relational Formula 2, the [Cr], [Mo], [Si], and [Al] represent the weight percent content of each element in parentheses.)

5. The method for producing a cold-rolled steel sheet according to claim 4, wherein the following relational expression 3 is satisfied: [Relationship 3] 8≦3.1×([SS]-[Ac1])+2.2×([RCS]-[Ms])≦180 (In the above-mentioned relational expression 3, [Ac1] is a value defined by the following relational expression 4, [RCS] represents the secondary cooling end temperature (°C), and [Ms] represents the martensitic transformation start temperature (°C).) [Relationship 4] [Ac1]=723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr] (In the above Relational Formula 4, [Mn], [Ni], [Si], and [Cr] represent the weight percent contents of each element in parentheses.)

6. The method for manufacturing a cold-rolled steel sheet according to claim 4, further comprising the step of skin-pass rolling the over-aging heat-treated cold-rolled steel sheet in a range of 0.1 to 1.0%.

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