Cold-rolled steel sheet and plated steel sheet having excellent low-temperature bake hardenability, and manufacturing methods therefor

Abstract

The purpose of an aspect of the present invention is to provide: a cold-rolled steel sheet and a plated steel sheet for vehicles, each simultaneously having excellent low-temperature bake hardenability and good aging resistance, and excellent workability; and methods for manufacturing same.

Description

Cold-rolled steel sheet with excellent low-temperature bake hardening properties, galvanized steel sheet, and method for manufacturing the same

[0001] The present invention relates to a cold-rolled steel sheet with excellent low-temperature bake hardening properties and room-temperature aging resistance, which is mainly suitable for use as exterior panels of automobiles, and a method for manufacturing the same.

[0002] There is a continuous demand for reducing steel thickness through increased strength to achieve lightweighting for improved fuel efficiency. Bake-hardened steel is known to be the most suitable material for exterior panels to meet these characteristics. Bake-hardening is a phenomenon in which solid solution carbon and nitrogen, activated during paint baking, adhere to dislocations generated during pressing, thereby increasing yield strength. Steels with excellent bake-hardened properties facilitate easy forming before paint baking, and improved dent resistance in the final product allows for superior formability and strength simultaneously. However, due to the solid solution elements within the steel, bake-hardened steel is susceptible to aging degradation, such as yield point elongation, when maintained at room temperature for extended periods. Therefore, it is required to possess a certain level of room-temperature aging resistance to guarantee aging over a specific duration.

[0003] Generally, the manufacturing method for cold-rolled steel sheets with bake-hardenability has mainly involved simply coiling low-carbon, P-added Al-killed steel at a low temperature of 400–500°C and then performing annealing to obtain steel with a bake-hardenability of approximately 40–50 MPa. This is because annealing is a method that facilitates the simultaneous achievement of formability and bake-hardenability. However, in the case of P-added Al-killed steel produced by the continuous annealing method, while it is easy to secure bake-hardenability due to the use of a relatively fast cooling rate, there is a problem where formability deteriorates due to rapid heating and short-time annealing, so its application has been limited only to automotive body panels where workability is not required. Recently, thanks to the rapid advancement of steelmaking technology, it has become possible to control the appropriate amount of dissolved elements in the steel. By using Al-killed steel sheets with added strong carbonitride-forming elements such as Ti or Nb, bake-hardenable cold-rolled steel sheets with excellent formability have been manufactured and are widely used for automotive body panels requiring dent resistance.

[0004] Meanwhile, recently, some automakers have been attempting to lower the baking temperature after painting as a measure to reduce costs and CO2 emissions. In particular, there is a growing trend among automakers to use non-ferrous lightweight materials, such as aluminum or plastic (or CFRP), instead of steel for exterior panels to achieve vehicle weight reduction. However, currently, steel and non-ferrous materials are press-processed separately, then painted and baked, before being assembled into parts at the final stage. Nevertheless, there is a continuing increase in attempts to press-process and assemble steel and non-ferrous materials and bake them at the same temperature for the purposes of production process efficiency, energy cost reduction, and environmental protection. In this context, there is a growing demand to lower the baking temperature to below 120°C, taking into account the curing temperature of non-ferrous materials such as plastic. As a result, steel baked at such low temperatures exhibits a sharp decline in strength compared to conventional bake-hardened steel obtained at 170°C, making it impossible to secure adequate dent resistance.

[0005] Typically, lowering the bake hardening temperature of a steel sheet delays the amount of solid solution carbon and nitrogen adhering and the time required for adhering, thereby reducing bake hardenability. Regarding the bake hardening phenomenon, it is common practice to apply a prestrain of several percent and perform a heat treatment at 170°C for 20 minutes after painting, and the amount of hardening required is at least 30 MPa, considering the dent resistance of automotive parts. Therefore, in order to lower the baking temperature while securing bake hardenability above an appropriate level, bake hardenability must be maximized at high temperatures; however, if the bake hardenability of the steel sheet increases, conversely, the aging resistance of the steel sheet deteriorates, leading to a problem where the likelihood of surface defects such as stretcher strain occurring during part processing increases.

[0006] Therefore, the most desirable outcome is to control the difference between the bake hardening amount obtained from the conventional 170°C heat treatment and the BH value obtained from the target low-temperature bake heat treatment to be low. However, considering that the solid solution elements in steel depend exponentially on temperature, it is very difficult to reduce the difference between the BH value obtained at a high temperature of 170°C and the BH value obtained at a low temperature of 120°C or lower.

[0007] Various methods have been proposed to secure an appropriate bake hardening value at low temperatures and to simultaneously secure superior bake hardening properties and corresponding aging resistance at normal conditions of 170°C.

[0008] For example, Patent Documents 1 to 3 propose a method to suppress room temperature aging deterioration by increasing the elongation of temper rolling.

[0009] However, since cold-rolled steel sheets manufactured using this technology aim to improve aging resistance by introducing mobile dislocations into the sheet, the amount of deformation required to enhance the tendency to bake becomes very large. To achieve such large deformation, it is necessary to increase the elongation rate of temper rolling; however, there is a limit to the temper rolling reduction rate that can be applied to high-strength steel sheets, which poses a problem that makes it practically difficult to apply in a continuous line.

[0010] Meanwhile, Patent Document 4 presents a method for controlling the distribution of iron carbide precipitates as a technology for improving low-temperature bake hardening. This technology is a hardening technology that utilizes precipitation strengthening at low temperatures and improves yield strength by utilizing the phenomenon of solid solution C, N and dislocations being fixed.

[0011] [Prior Art Literature]

[0012] [Patent Literature]

[0013] (Patent Document 1) Japanese Published Patent Application No. JP Hei No. 07-75803

[0014] (Patent Document 2) Japanese Published Patent Application No. 2001-140038

[0015] (Patent Document 3) Japanese Published Patent Application No. 2001-200337

[0016] (Patent Document 4) Japanese Published Patent Application No. JP Hei No. 06-73498

[0017] According to one embodiment of the present invention, cold-rolled steel sheets and galvanized steel sheets for automobiles having excellent low-temperature bake hardening properties, good aging resistance, and excellent processability, and a method for manufacturing the same can be provided.

[0018] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0019] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight%, C: 0.015~0.025%, Si: 0.200% or less, Mn: 1.0~2.0%, Cr: 0.5~1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.002~0.008% and Al: 0.010~0.060%, and the remainder is Fe and unavoidable impurities, satisfying the following equation 1, and may include ferrite, a transformation structure and the remainder is an unavoidable impurity structure as a microstructure, and the transformation structure may include at least one of martensite, acicular ferrite, or bainite.

[0020] [Relation 1] 2.00≤([Cr] / [N]) / ([Mn] / [C])≤7.00

[0021] Here, [C], [Mn], [Cr], and [N] represent the atomic percent content of each element in the steel sheet.

[0022] The ferrite described above is included in an area of ​​95 to 99.5 percent, and the transformation structure described above may be included in an area of ​​0.5 to 2.0 percent.

[0023] The above-mentioned unavoidable impurity structure may be at least one of carbides or oxides, and the said unavoidable impurity structure may be included in an area of ​​4.5% or less.

[0024] A plated steel sheet according to another embodiment of the present invention may include the above-described cold-rolled steel sheet; and a plating layer located on at least one surface of the above-described cold-rolled steel sheet, wherein the above-described plating layer may be a zinc-based plating layer or an alloyed zinc-based plating layer.

[0025] The above-described galvanized steel sheet may have a BH value of 30 MPa or more after baking heat treatment at 120°C, and a yield point elongation (YPel) of 0.2% or less measured after artificial aging at 100°C for 1 hour.

[0026] A method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention comprises the steps of: reheating a slab satisfying the following equation 1, wherein the slab comprises, in weight percent, C: 0.015~0.025%, Si: 0.200% or less, Mn: 1.0~2.0%, Cr: 0.5~1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.002~0.008%, and Al: 0.010~0.060%, and the remainder being Fe and unavoidable impurities; hot-rolling the slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; pickling the surface of the hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet at a temperature range of 760~830℃. and the method includes the step of cooling the cold-rolled steel sheet to 650°C first and then cooling it to 400°C to 500°C second, and can satisfy the following equation 2.

[0027] [Relation 1] 2.00≤([Cr] / [N]) / ([Mn] / [C])≤7.00

[0028] Here, [C], [Mn], [Cr], and [N] represent the atomic percent content of each element in the steel sheet.

[0029] [Relationship 2] 2.40≤{log 10(SS) / [Cr]+log 10 (SS) / RCS} / [Mn]≤4.50

[0030] Here, SS represents the annealing temperature (°C) in the annealing heat treatment step, RCS represents the cooling rate (°C / s) in the secondary cooling step, and [Cr] and [Mn] represent the content (weight%) of each component.

[0031] The reheating temperature in the aforementioned reheating step is 1100~1250℃, the finishing temperature during hot rolling in the aforementioned step of obtaining hot-rolled steel sheet is 880℃ or higher, the coiling temperature in the aforementioned coiling step is 550~700℃, and the cumulative reduction rate during cold rolling in the aforementioned step of obtaining cold-rolled steel sheet may be 60~90%.

[0032] The cooling rate during the first cooling described above may be 3℃ / s or less.

[0033] The cooling rate in the aforementioned secondary cooling step may be 5 to 20℃ / s.

[0034] A method for manufacturing a galvanized steel sheet according to another embodiment of the present invention may include the step of preparing a cold-rolled steel sheet manufactured according to the method for manufacturing a cold-rolled steel sheet described above; the step of manufacturing a zinc-based galvanized steel sheet by immersing the cold-rolled steel sheet in a zinc plating bath at 450 to 500°C; and optionally the step of alloying the zinc-based galvanized steel sheet at 450 to 540°C.

[0035] The above-described method for manufacturing galvanized steel sheets may additionally perform a temper rolling treatment of 0.5 to 1.5% using a skin pass roll having a roughness (Ra) of 0.8 to 1.6 μm.

[0036] The present invention can provide cold-rolled steel sheets and galvanized steel sheets having properties suitable for use as exterior panels for automobiles, such as excellent low-temperature bake-hardenability and excellent room-temperature aging resistance, and a method for manufacturing the same.

[0037] As such, the present invention can reduce manufacturing costs and CO2 emissions by lowering the bake hardening temperature. As a result, it is possible to achieve further weight reduction of the automobile body and efficiency improvement of the body manufacturing process.

[0038] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0039] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0040] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0041] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0042] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0043] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0044] Generally, when transformation structures such as martensite, acicular ferrite, and bainite are present in steel, they contain abundant mobile dislocations in the surroundings, which can improve room temperature aging resistance.

[0045] However, creating such a transformed structure requires the addition of hardenable elements such as Mn, which can lead to an increase in strength and a decrease in ductility.

[0046] In addition, while aging resistance can be improved by increasing the fraction of the transformed structure, an excessive transformed structure inhibits the movement of solid solution elements, leading to a problem where low-temperature bake-hardenability is reduced.

[0047] Accordingly, as a result of conducting various experiments, the inventors found that it is necessary to appropriately secure the fraction of the transformed structure by controlling the conditions of the preparation process along with the relative contents of C, Mn, Cr, and N.

[0048] First, the reasons for the addition and limitation of the basic components of the cold-rolled steel sheet according to one embodiment of the present invention will be explained below.

[0049] C: 0.015~0.025%

[0050] Carbon (C) is an important element for securing the martensite fraction in the steel of the present invention. That is, in order to secure an appropriate level of transformation structure, a certain level of C must be added. To this end, the lower limit of the C addition amount is restricted to 0.015%. However, if C is added excessively, the transformation structure is excessively formed, which may lead to an excessive increase in strength or a decrease in elongation. In this case, there is a problem in that the likelihood of bending defects occurring on the product surface increases during part processing at the customer. Therefore, the present invention limits the upper limit of the C content to 0.025%.

[0051] Si: 0.200% or less

[0052] Si is an element that contributes to the increase in steel strength through solid solution strengthening. Since the desired physical properties can be secured without adding Si, the present invention may not contain any Si. However, as an example, the Si may be included in an amount of 0.001% or more. Meanwhile, since there is a problem of deteriorating plating surface characteristics when the Si content exceeds a certain level, the present invention may limit the upper limit of the Si content to 0.200%. As another example, the Si may be included in an amount of 0.150% or less.

[0053] Mn: 1.0~2.0%

[0054] Manganese (Mn) is one of the hardenable elements, such as C and Cr, in the present invention and contributes to securing the area fraction of the transformation structure desired in the present invention. In order to secure excellent low-temperature bake hardenability and room-temperature aging resistance by securing a minimum transformation fraction in the present invention, the Mn content may be added at a minimum of 1.0% or more. However, if Mn exceeds 2.0%, the fraction of the transformation structure in the steel is excessively formed, which may actually reduce low-temperature bake hardenability. Furthermore, annealing oxides may be formed due to the excessive addition of Mn, causing problems on the surface of plated products, and a decrease in elongation may occur, resulting in inferior processability. Therefore, the present invention may set the Mn content to 1.0~2.0%. As another example, the Mn may be included in an amount of 1.1~1.7%.

[0055] Cr: 0.5~1.0%

[0056] Cr is a solid solution strengthening element. Cr increases the hardenability of steel and effectively contributes to the formation of martensite; it is also an element that effectively contributes to securing the elongation of composite microstructure steel by minimizing the decrease in elongation associated with increased strength. Therefore, to achieve these effects, the present invention may add Cr at a concentration of 0.5% or more. If Cr is added at a concentration lower than 0.5%, the hardenability of the steel decreases, making it impossible to secure a sufficient fraction of the transformation phase desired in the steel of the present invention. This results in a decrease in low-temperature bake hardenability. However, if Cr is added at a concentration exceeding 1.0%, excessive martensite formation may occur, which may lead to a deterioration in low-temperature bake hardenability. Therefore, the present invention may set the above Cr content to 0.5% to 1.0%. As another example, the above Cr may be included at a concentration of 0.6% to 0.9%.

[0057] P: 0.030% or less

[0058] P is the most effective element for securing the strength of steel through solid solution strengthening without significantly impairing drawability. However, since the achievement of the objective of the present invention is not affected even if the above-mentioned P is not added, the present invention may not contain the above-mentioned P at all. On the other hand, if P is added excessively, the possibility of brittle fracture increases, which can not only cause plate breakage of the slab during hot rolling but also significantly degrade the surface characteristics of the galvanized steel sheet. Therefore, the present invention may limit the upper limit of the P content to 0.030%.

[0059] S: 0.010% or less

[0060] S is an impurity present in steel and is inevitably added, but in order to secure excellent welding characteristics, it is desirable to keep its content as low as possible. In particular, since S in steel can cause red-hot brittleness, the upper limit of the S content in the present invention is limited to 0.010%.

[0061] N: 0.002~0.008%

[0062] Nitrogen (N) is also an element that inevitably enters as an impurity present in steel. Therefore, while its content is generally managed to be as low as possible, in the present invention, N is utilized as an important element to secure low-temperature bake hardenability and room-temperature aging resistance. As mentioned earlier, nitrogen has a very fast diffusion rate and can simultaneously induce bake hardenability and aging deterioration. To suppress aging deterioration caused by nitrogen, a certain amount of transformation structure must be secured through an appropriate combination of C, Mn, and Cr; furthermore, to simultaneously improve low-temperature bake hardenability in steel containing such transformation structure, the addition of a certain amount of nitrogen is required. If the nitrogen content is less than 0.002%, the aforementioned effects cannot be obtained, and if it is added at a high level exceeding 0.008%, it becomes difficult to simultaneously secure low-temperature bake hardenability and room-temperature aging resistance. Therefore, the range of nitrogen in the present invention can be set to 0.002~0.008%.

[0063] Al: 0.010~0.060%

[0064] Al is a component added to steel for grain refinement and deoxidation. In order to produce Al-killed steel in a stable state, the lower limit of the Al content can be set to 0.010%. However, if Al is added excessively, while strength increases due to grain refinement, excessive inclusions are formed during continuous steelmaking operations, which not only degrades the surface quality of the steel sheet but also leads to an increase in manufacturing costs. Therefore, the upper limit of the Al content can be set to 0.060%.

[0065] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. Since these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification. As an example, Cu, Ni, Sn, Mo, etc., may be included in the steel as impurities.

[0066] Meanwhile, it is preferable that the steel material according to one embodiment of the present invention satisfies the alloy composition described above and simultaneously satisfies the following relationship 1.

[0067] [Relation 1] 2.00≤([Cr] / [N]) / ([Mn] / [C])≤7.00

[0068] Here, [C], [Mn], [Cr], and [N] represent the atomic percent content of each element in the steel sheet.

[0069] The above Equation 1 represents the correlation between solid solution elements and hardenability elements in steel. Cr and Mn can improve aging resistance by increasing the fraction of the transformation structure; however, if added excessively, they may hinder the movement of solid solution elements during low-temperature baking treatment, thereby reducing bake hardenability. Conversely, if the Cr and Mn content is reduced, a problem arises in which sufficient low-temperature bake hardenability is not obtained. Considering the above phenomena, the value calculated by the above Equation 1 may be 2.00 to 7.00 in the present invention. As another example, the value calculated by the above Equation 1 may be 2.10 to 6.95.

[0070] The microstructure of the cold-rolled steel sheet of the present invention is described below.

[0071] A cold-rolled steel sheet according to one embodiment of the present invention having the alloy composition described above may include ferrite, a transformation structure, and a remainder of unavoidable impurity structure in its microstructure.

[0072] As an example, the area fraction of the ferrite may be 95% or more.

[0073] That is, the steel of the present invention can secure excellent low-temperature bake hardening properties by having a ferrite matrix structure of 95 area% or more in its microstructure. Meanwhile, as described below, the present invention can secure room-temperature aging resistance and low-temperature bake hardening properties by including a transformation structure of 0.5 area% or more; considering this, the ferrite may be included in an amount of 99.5% or less.

[0074] In addition, the cold-rolled steel sheet of the present invention may include a transformation structure in addition to the ferrite matrix structure described above. The transformation structure may include at least one of martensite, acicular ferrite, or bainite.

[0075] At this time, the area fraction of the transformed tissue may be 0.5 to 2.0% in area %.

[0076] If the above-mentioned transformation structure is included in an area of ​​less than 0.5%, sufficient mobile potentials may not be obtained, which may result in reduced low-temperature bake hardenability and deterioration of room-temperature aging resistance. On the other hand, if the above-mentioned transformation structure is included in an area of ​​more than 2.0%, as described above, the excessive transformation structure may improve room-temperature aging resistance but may instead cause a problem in which low-temperature bake hardenability deteriorates.

[0077] In addition, the present invention may include, in addition to the ferrite and the transformed structure, a remainder of unavoidable impurity structure.

[0078] These impurity structures may include inclusions such as oxides and carbides. Here, the carbides refer to inclusion structures containing iron carbides such as Fe3C or alloy carbides such as TiC and NbC, which are inevitably formed when carbon combines with metal elements during the manufacturing process. Additionally, the oxides refer to non-metallic inclusion structures formed due to deoxidation reactions or slag incorporation, such as Al2O3 and SiO2. Although it is difficult to completely remove these carbide and oxide structures during the steel manufacturing process, their size and distribution can be appropriately controlled through the composition and process conditions according to the present invention, allowing them to exist within a range that does not significantly impair the strength and low-temperature impact toughness of the steel. That is, since these impurity structures may hinder the securing of the physical properties intended by the present invention, the present invention may include the impurity structures in an area of ​​4.5% or less.

[0079] A plated steel sheet according to one aspect of the present invention comprises a plating layer formed on one surface of the cold-rolled steel sheet described above. In one embodiment of the present invention, the plating layer may be a zinc-based plating layer or an alloyed zinc-based plating layer comprising zinc as a main component, and the composition of a plating layer conventionally applied in the art may be applied identically to the zinc-based plating layer and the alloyed zinc-based plating layer.

[0080] A plated steel sheet according to one aspect of the present invention may have excellent low-temperature hardening properties and room-temperature aging resistance.

[0081] Specifically, the galvanized steel sheet may have a BH value of 30 MPa or higher measured after baking heat treatment at 120°C, and a yield point elongation (YPel) of 0.2% or lower measured after artificial aging at 100°C for 1 hour.

[0082] The above-described method for manufacturing cold-rolled steel sheets will be explained in detail below. However, this does not mean that the cold-rolled steel sheets of the present invention must necessarily be manufactured by the following manufacturing method.

[0083] A method for manufacturing a cold-rolled steel sheet according to one aspect of the present invention may include the step of reheating a slab satisfying the composition and Equation 1 described above.

[0084] The slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0085] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0086] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in this invention may be included in the molten steel within permissible limits.

[0087] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0088] The heating of the slab described above is performed to facilitate the subsequent hot rolling and to sufficiently obtain the desired physical properties of the steel sheet.

[0089] In addition, the reheating temperature in the above reheating step may be 1100 to 1250℃.

[0090] If the above reheating temperature is less than 1100℃, slab inclusions, etc. are not sufficiently remelted, so material variation or surface defects may occur after hot rolling. On the other hand, if the above reheating temperature exceeds 1250℃, strength may be reduced due to excessive growth of austenite grains and excessive scale may be formed, which may degrade the surface quality of the steel sheet.

[0091] A method for manufacturing a cold-rolled steel sheet according to one aspect of the present invention may include the step of hot-rolling the slab to obtain a hot-rolled steel sheet. When such hot-rolling is performed in the austenite single-phase region, it forms pancake-shaped austenite grains and deformation zones, which is advantageous in terms of refining the final microstructure.

[0092] As a specific example, the finishing temperature during the hot rolling described above may be 880℃ or higher.

[0093] If hot rolling is performed at a temperature below 880°C, it corresponds to rolling in the austenite and ferrite phases, which can cause material non-uniformity and lead to excessive rolling load. Therefore, in order to complete hot rolling in the austenite single phase in the present invention, the finishing temperature during hot rolling can be limited to 880°C or higher. More preferably, it is advantageous to limit the temperature range during hot rolling to 880 to 950°C.

[0094] Then, a method for manufacturing a cold-rolled steel sheet according to one aspect of the present invention may include a step of winding a hot-rolled steel sheet obtained according to the above description.

[0095] In the above winding step, the winding temperature may be 550 to 700°C.

[0096] If the coiling temperature is below 550°C, the shape of the steel sheet becomes poor, and a large amount of low-temperature transformation phases such as martensite or bainite are formed, which can lead to an excessive increase in the strength of the steel sheet. On the other hand, if the coiling temperature exceeds 700°C, coarse carbides and nitrides are likely to form, which can degrade the material properties of the steel. In addition, due to the high coiling temperature, surface oxides such as Mn and Si increase, and even if some oxides remain during the pickling process or are completely removed, concentrated oxides form on the surface layer of the steel sheet, which can cause surface defects during plating.

[0097] Next, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may include the step of pickling the surface of the hot-rolled steel sheet and then cold-rolling it to obtain a cold-rolled steel sheet.

[0098] In the step of obtaining the above cold-rolled steel sheet, the cumulative reduction rate during cold rolling may be 60 to 90%.

[0099] If the above cumulative reduction rate is less than 60%, the recrystallization driving force by cold rolling is insufficient, so the recrystallization of ferrite is not completed and unrecrystallized ferrite structure remains. On the other hand, if it exceeds 90%, the grain size becomes very fine, which may result in a decrease in elongation along with an increase in strength. In addition, in this case, the cold rolling roll load is very severe, which may lead to a deterioration in the shape of the steel sheet and, in particular, cracks may occur at the edges of the steel sheet.

[0100] Subsequently, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may include a step of annealing the cold-rolled steel sheet at a temperature range of 760 to 830°C. If the annealing temperature is managed below 760°C, recrystallization may not be sufficiently completed, and there is a risk of an unrecrystallized structure occurring. Furthermore, below 760°C, it corresponds to ferrite single-phase annealing, making it impossible to obtain the transformation structure intended for the steel of the present invention. On the other hand, if the annealing temperature is exceeded 830°C, the carbon concentration within the austenite decreases due to the formation of an excessive austenite fraction, thereby reducing the stability of the austenite. Consequently, when cooling after annealing, reverse transformation back to ferrite may easily occur, or even if some austenite transforms into martensite, the insufficient carbon concentration results in insufficient hardenability, making it impossible to secure excellent bake hardenability and room temperature aging resistance.

[0101] Next, the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention can first cool the cold-rolled steel sheet to 650°C.

[0102] This is intended to partially transform the austenite generated during annealing into ferrite through slow cooling, and to enrich the carbon within the remaining austenite. This ensures the stability of the austenite. For the cooling rate from the annealing crack to the first cooling, a slow cooling condition of 3°C / s or less is desirable to prevent coil meandering.

[0103] A method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may perform a first cooling as described above, and then secondarily cool the cold-rolled steel sheet to 400 to 500°C.

[0104] Through a second cooling step, the present invention can transform the austenite obtained from the first cooling after annealing cracking into a transformation structure such as martensite.

[0105] Within the scope of achieving the above-mentioned purpose, the cooling rate during the second cooling step is not separately limited, but as an example, the cooling rate during the second cooling step may be 5 to 20℃ / s.

[0106] If the cooling rate is less than 5℃ / s, sufficient cooling capacity may not be secured, and thus the fraction of the transformation structure targeted in the present invention may not be secured. However, if the secondary cooling rate exceeds 20℃ / s, all existing austenite may form into a transformation structure, which may reduce low-temperature bake-hardenability. In addition, rapid cooling exceeding 20℃ / s reduces the temperature hit rate before entering the plating bath, and as a result, if the coil enters a plating bath at 400℃ or lower, various plating defects such as non-alloying or dross may occur on the surface of the steel plate.

[0107] In addition to the aforementioned manufacturing conditions, optionally or in combination with one embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet can satisfy the following relationship 2.

[0108] [Relationship 2] 2.40≤{log 10 (SS) / [Cr]+log 10 (SS) / RCS} / [Mn]≤4.50

[0109] Here, SS represents the annealing temperature (°C) in the annealing heat treatment step, RCS represents the cooling rate (°C / s) in the secondary cooling step, and [Cr] and [Mn] represent the content (weight%) of each component.

[0110] Equation 2 above represents the correlation between Cr and Mn content, annealing temperature, and the cooling rate during secondary cooling. In this case, the present invention can control the ratio of ferrite and austenite formed during annealing in the decomposition zone through the annealing temperature, and can control the fraction of austenite formed during the crack annealing stage that transforms into a transformation structure such as martensite through primary slow cooling and secondary rapid cooling through the cooling rate during secondary cooling after annealing. These annealing temperature and secondary cooling rate after annealing depend on the content of Mn and Cr, which are hardenable elements.

[0111] That is, if Equation 2 is less than 2.40, sufficient transformed tissue may not be obtained, and if it is 4.50 or higher, excessive transformed tissue may be formed.

[0112] Hereinafter, a method for manufacturing a plated steel sheet according to another example of the present invention using the cold-rolled steel sheet described above will be described.

[0113] First, a cold-rolled steel sheet manufactured according to the above description can be prepared.

[0114] In this case, regarding the above-mentioned cold-rolled steel sheet, the contents described above apply equally, so they are not described separately.

[0115] Then, the present invention can produce a zinc-plated steel sheet by immersing the cold-rolled steel sheet in a zinc plating bath at 450 to 500°C.

[0116] This is a typical condition for manufacturing hot-dip galvanized steel sheets, where under-plating may occur below 450°C and over-plating may occur above 500°C.

[0117] Next, the present invention may optionally alloy the zinc-plated steel sheet at 450 to 540°C.

[0118] If the alloying temperature is lower than 450℃, unplated areas may occur across the entire width of the annealed steel sheet, and if the alloying temperature exceeds 540℃, powdering characteristics may be inferior due to the influence of brittle Fe-Zn intermetallic compounds caused by excessive alloying.

[0119] As a non-limiting example, the present invention may additionally perform a temper rolling treatment of 0.5 to 1.5% using a skin pass roll having a roughness (Ra) of 0.8 to 1.6 μm on the aforementioned cold-rolled steel sheet or the aforementioned plated steel sheet. If the temper rolling elongation is less than 0.5%, sufficient dislocations may not be formed, and it is also disadvantageous in terms of plate shape. Furthermore, in this case, there is a risk of plating surface defects occurring, and it is also disadvantageous in terms of aging resistance. On the other hand, if it exceeds 1.5%, adverse effects such as plate breakage may occur due to the limit of equipment capacity, in addition to material deterioration caused by an excessive increase in dislocation density in the surface layer.

[0120] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0121] (Example)

[0122] After preparing a slab having the alloy composition of Table 1 below, it was reheated to 1180~1200℃ and finished hot-rolled at 900~920℃, which is above the Ar3 temperature. Subsequently, the hot-rolled steel sheet obtained according to the above was coiled at a temperature of 650℃, the surface of the coiled hot-rolled steel sheet was pickled, and then cold-rolled with a cumulative reduction rate of 70%. The rolled cold-rolled steel sheet was continuously annealed under the annealing temperature conditions described in Table 2 below, then first cooled to 650℃ by furnace cooling at a cooling rate of 3℃ / s or less, and then cooled to 400~500℃ under the second cooling rate conditions of Table 2 below.

[0123] Next, to manufacture a hot-dip galvanized steel sheet, the above cold-rolled steel sheet was immersed in a hot-dip galvanizing bath maintained at a temperature of approximately 460°C, which is a normal condition, to perform hot-dip galvanizing, and subsequently, the final hot-dip galvanized steel sheet was manufactured by temper rolling with a rolling reduction rate of 0.8% using a skin pass roll having a roughness (Ra) of 0.8 to 1.6 μm.

[0124] For each of the above-mentioned hot-dip galvanized steel sheets, a tensile test was performed in the rolling direction using ASTM standards to measure BH120 and AI values, and the results are shown in Table 2 below.

[0125] BH120 was evaluated by measuring the lower yield stress (L-BH) after baking at 120°C for 20 minutes following 2% pre-strain for the same specifications, and room temperature aging resistance (AI) was expressed by measuring the yield point elongation (YPel) obtained by tensile testing after holding the specimen in a water bath maintained at 100°C for 1 hour.

[0126] And the area fraction of the transformation structure in the microstructure was measured by analyzing the microstructure using the point counter method after taking 10 images at a magnification of 2,000x with a scanning electron microscope at the point from the center of the steel plate thickness 1 / 4t after Nital etching.

[0127] Steel Grade CSiMnPSS.AlCrN[Relationship 1]*10.0150.1001.20.0120.0050.0250.60.0042.3120.0150.0501.30.0150.0060.0350.90.0026.9330.0200.1001.50.0100.0080.0400.70.0052.8840.0200.0111.60.0100.0050.0350.60.0043.0 850.0190.0151.50.0150.0020.0400.90.0062.9360.0220.0101.10.0200.0050.0400.70.0053.1670.0160.0501.20.0150.0060.0350.90.0072.1180.0230.0501.60.0100.0040.0500.80.0072.7090 .0220.1001.40.0120.0050.0440.90.0063.39100.0200.1001.50.0100.0050.0300.90.0072.64110.0170.0152.00.0120.0060.0350.40.00351.99120.0240.1002.50.0120.0070.0400.50.0052.461 30.0160.0500.80.0200.0040.0200.60.0081.23140.0200.0501.70.0150.0030.0350.50.0101.03150.0060.0501.40.0250.0070.0550.30.0050.37160.0450.1001.00.0200.0090.0400.50.0045.78

[0128] * [Relation 1] (Cr / N) / (Mn / C)

[0129] * In Table 1, the remaining components are Fe and unavoidable impurities.

[0130] Classification Steel Type SS(°C) RCS(°C / s) [Relationship 2] * Mechanical Properties Transformation Structure Area Fraction (Area %) BH120(MPa)AI(%) Invention Example 1 18 10 10 4.28 36.20 1.1 Comparative Example 1 17 60 25.20 22.10 2.6 Comparative Example 2 18 60 54.56 27.50 30.4 Invention Example 2 28 00 15.2.63 40.20 1.3 Invention Example 3 37 90 33.40 35.40 1.5 Invention Example 4 48 20 20 3.13 38.10 0.9 Comparative Example 3 47 00 10 3.14 32.10 80 Invention Example 5 58 20 52.55 36.10 0.7 Invention Example 6 68 10 10 4.04 40.90 1.6 Invention Example 7 78 00 10 2.93 42 .301.8 Invention Example 8880052.6343.101.1 Invention Example 99780102.5038.501.4 Comparative Example 49780252.3821.603.3 Invention Example 101079052.5336.401.1 Comparative Example 51181053.9328.80.10.4 Comparative Example 612820102.4520.103.9 Comparative Example 713790104.0325.40.40.3 Comparative Example 814810123.5620.11.10.5 Comparative Example 915800107.1243.60.90 Comparative Example 101682086.1912.105.4

[0131] * [Relationship 2]

[0132] 2.40≤{log10(SS) / [Cr]+log10(SS) / RCS} / [Mn]≤4.50

[0133] As can be seen in Tables 1 and 2 above, the hot-dip galvanized steel sheets of Invention Examples 1 to 10, manufactured using the steel composition and manufacturing process of the present invention, exhibited excellent low-temperature bake-hardenability and room-temperature aging resistance. That is, the hot-dip galvanized steel sheets of Invention Examples 1 to 10 described above had a BH120 value of 30 MPa or higher and an AI value of 0.2% or lower. Meanwhile, the area fraction of the transformation structure, which is the low-temperature transformation phase of Invention Examples 1 to 10, also satisfied the range presented in the present invention.

[0134] On the other hand, Comparative Example 1 satisfied the alloy composition of the present invention, but as the value of Equation 2 exceeded the range of the present invention, the area fraction of the transformation structure was very high at 2.6 area%, and as a result, the BH120 value was low.

[0135] In Comparative Example 2, the annealing temperature was very high at 860°C, resulting in insufficient carbon concentration in the austenite. During cooling, most of the austenite was reverse-transformed into ferrite, leading to a low area fraction of the transformation structure. The resulting transformation phase also lacked stability, resulting in a very low BH120 value and deteriorated aging resistance.

[0136] Comparative Example 3 had a very low annealing temperature of 700°C, so annealing took place in the ferrite single-phase region, and as a result, no transformation phase was obtained. This caused deterioration in room temperature aging resistance.

[0137] Comparative Example 4 did not satisfy Equation 2 because the secondary cooling rate after annealing was 25℃ / s, which is higher than the condition presented in the steel of the present invention. As a result, due to the high cooling rate, the area fraction of the transformation structure increased compared to the condition presented in the steel of the present invention, and consequently, the BH120 value did not meet the standard.

[0138] Comparative Example 5 is a case where the Cr component was added at a lower level than the range of the present invention. Due to the low Cr content, the area fraction of the transformation structure was insufficient, and the value of Equation 1 did not satisfy the conditions of the present invention, so the BH120 value deviated from the standard.

[0139] In addition, Comparative Examples 6 and 7 are cases where the Mn content exceeded or fell short of the conditions of the steel of the present invention. As a result, the area fraction of the transformed structure was insufficient or excessive.

[0140] Comparative Example 8 had a high N content. As a result, the value of Equation 1 was very low, and the BH120 value did not satisfy the conditions of the steel of the present invention.

[0141] Comparative Example 9 is a case that deviates from the conditions of the present invention, with a very low C content of 0.006% and a Cr content of 0.3%. Due to the low C and Cr content, sufficient hardenability could not be secured, and as a result, no transformation phase occurred.

[0142] Meanwhile, Comparative Example 10 is a case where the hardenability was significantly increased due to the addition of excessive C, and the area fraction of the transformation structure was very high at 5.4%. As such, due to the excessive transformation phase, the BH120 value was much lower than the conditions presented in the steel of the present invention.

[0143] Although embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and it will be obvious to those skilled in the art that various modifications and variations are possible within the scope of the technical concept of the present invention as described in the claims.

Claims

1. Contains, in wt%, C: 0.015~0.025%, Si: 0.200% or less, Mn: 1.0~2.0%, Cr: 0.5~1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.002~0.008% and Al: 0.010~0.060%, and the remainder consists of Fe and unavoidable impurities, satisfying the following Equation 1, and The microstructure includes ferrite, transformed structure, and residual unavoidable impurity structure, The above transformation structure comprises at least one of martensite, acicular ferrite, or bainite, a cold-rolled steel sheet. [Relation 1] 2.00≤([Cr] / [N]) / ([Mn] / [C])≤7.00 Here, [C], [Mn], [Cr], and [N] represent the atomic percent content of each element in the steel sheet.

2. In Paragraph 1, A cold-rolled steel sheet in which the ferrite is included in an area of ​​95 to 99.5% and the transformation structure is included in an area of ​​0.5 to 2.0%.

3. In Paragraph 1, A cold-rolled steel sheet in which the above-mentioned unavoidable impurity structure comprises at least one of carbides or oxides, and the area fraction of the above-mentioned unavoidable impurity structure is 4.5 area% or less.

4. Cold-rolled steel sheet according to any one of paragraphs 1 to 3; It includes a plating layer located on at least one surface of the above cold-rolled steel sheet, and A plated steel sheet in which the above plating layer is a zinc-based plating layer or an alloyed zinc-based plating layer.

5. In Paragraph 4, A galvanized steel sheet having a BH value of 30 MPa or more after baking heat treatment at 120℃, and a yield point elongation (YPel) of 0.2% or less measured after artificial aging at 100℃ for 1 hour.

6. A step of reheating a slab comprising, in weight%, C: 0.015~0.025%, Si: 0.200% or less, Mn: 1.0~2.0%, Cr: 0.5~1.0%, P: 0.030% or less, S: 0.010% or less, N: 0.002~0.008% and Al: 0.010~0.060%, with the remainder being Fe and unavoidable impurities, satisfying the following Equation 1; A step of obtaining a hot-rolled steel sheet by hot-rolling the above slab; Step of winding the above hot-rolled steel sheet; A step of obtaining a cold-rolled steel sheet by pickling the surface of the hot-rolled steel sheet and then cold-rolling it; A step of annealing the above cold-rolled steel sheet at a temperature range of 760 to 830°C; and The method includes a step of first cooling the above cold-rolled steel sheet to 650℃ and then second cooling it to 400~500℃, and A method for manufacturing cold-rolled steel sheets satisfying the following relationship 2. [Relation 1] 2.00≤([Cr] / [N]) / ([Mn] / [C])≤7.00 Here, [C], [Mn], [Cr], and [N] represent the atomic percent content of each element in the steel sheet. [관계식 2] 2.40≤{log 10 (SS) / [Cr]+log 10 (SS) / RCS} / [Mn]≤4.50 Here, SS represents the annealing temperature (°C) in the annealing heat treatment step, RCS represents the cooling rate (°C / s) in the secondary cooling step, and [Cr] and [Mn] represent the content (weight%) of each component.

7. In Paragraph 6, The reheating temperature in the above reheating step is 1100~1250℃, and In the step of obtaining the above hot-rolled steel sheet, the finishing temperature during hot rolling is 880℃ or higher, and In the above winding step, the winding temperature is 550~700℃, and A method for manufacturing a cold-rolled steel sheet, wherein the cumulative reduction rate during cold rolling in the step of obtaining the cold-rolled steel sheet is 60 to 90%.

8. In Paragraph 6, A method for manufacturing a cold-rolled steel sheet, wherein the cooling rate during the first cooling is 3℃ / s or less.

9. In Paragraph 6, A method for manufacturing a cold-rolled steel sheet, wherein the cooling rate in the above-mentioned second cooling step is 5 to 20℃ / s.

10. A step of preparing a cold-rolled steel sheet manufactured according to a method for manufacturing a cold-rolled steel sheet according to any one of paragraphs 6 to 9; A step of manufacturing a zinc-plated steel sheet by immersing the above cold-rolled steel sheet in a zinc plating bath at 450~500℃; A method for manufacturing a plated steel sheet, comprising optionally a step of alloying the zinc-based plated steel sheet at 450 to 540°C.

11. In Paragraph 10, A method for manufacturing a plated steel sheet, wherein a 0.5~1.5% temper rolling treatment is additionally performed using a skin pass roll having a roughness (Ra) of 0.8~1.6㎛.