Cold-rolled steel sheet with excellent workability and its manufacturing method

Abstract

A cold rolled steel sheet having excellent workability according to one embodiment of the present invention contains, by weight, C: 0.012 to 0.060%, Si: 0.03% or less (except 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: 0.015% or less (except 0%), S: 0.015% or less (except 0%) and N: 0.006% or less (except 0%), with the balance containing Fe and other unavoidable impurities.

Description

[Technical field]

[0001] An embodiment of the present invention relates to a cold-rolled steel sheet having excellent workability and a manufacturing method thereof. More particularly, the present invention relates to a cold-rolled steel sheet having excellent workability before and after processing, which is easy to manufacture intermediate parts, and a manufacturing method thereof. [Background technology]

[0002] Cold-rolled steel sheets are manufactured and used as final structures through various stages of mechanical or thermal processing after production. In general, mechanical processing reduces ductility and makes the steel difficult to work with, so after the first mechanical processing, thermal processing is performed in an intermediate stage to improve the workability again, and then the final shape is obtained through the second mechanical processing. In this case, not only the mechanical properties immediately after the production of the cold-rolled steel sheet, but also the mechanical properties after mechanical and thermal processing are important. In general, immediately after the production of the cold-rolled steel sheet, ductility is required for the first mechanical processing, and after thermal processing, not only ductility for the second mechanical processing but also strength above a certain level is required to ensure the final strength after the mechanical processing. In addition, ductility may decrease due to aging from thermal processing to final processing, so low aging properties above a certain level are also required to ensure sufficient workability.

[0003] In the production of final processed products, various methods such as solid solution strengthening, precipitation strengthening, work hardening, and hard phase control are used to impart strength to steel sheet materials in order to maintain their shape. However, in order to ensure sufficient ductility after thermal processing, a strength imparting method suitable for the thermal processing conditions must be used. Thermal processing is generally performed by controlling the time and temperature, and the shorter the time and the lower the temperature, the easier the control and the more economical it is. In order to significantly recover ductility after mechanical processing, it is most effective to cause recrystallization by heat, so steel sheets in which recrystallization occurs at low temperatures for a short period of time are easy to thermally process.

[0004] Among the above methods, solid solution strengthening is the simplest method for increasing strength, which involves adding alloy elements that can form solid solutions. Since the effect of solid solution strengthening remains the same during processing and after thermal processing, it can be effectively used to ensure the strength of the final product. However, substitutional elements such as Mn and Si, which are generally used to achieve the desired effect, must be added in large amounts, which reduces the economics, and interstitial elements such as C and N are increasingly prone to causing a decrease in workability due to aging.

[0005] Work hardening is a phenomenon in which strength increases due to mechanical processing at room temperature. However, even if strength is increased by work hardening, recrystallization occurs during thermal processing, and the strength increase effect is largely lost, so it cannot be used to increase the strength of the final formed product. In addition, ductility is significantly reduced during work hardening, making it difficult to actively use for materials that require mechanical workability. In order to overcome the decrease in ductility, a method has been proposed in which low-carbon steel is annealed at a low temperature after work hardening. However, the main purpose of this method is simply to ensure initial strength and elongation, and in order to obtain desired physical properties during steel sheet processing and after thermal processing, it is necessary to closely understand the correlation between the initial state of the steel sheet and the changes in physical properties due to the thermal processing conditions during processing.

[0006] Precipitation strengthening is a method to increase strength through fine precipitates that are stable at high temperatures. However, precipitates that are very stable at high temperatures hinder recrystallization, so very high temperatures or long times are required to cause recrystallization, making it unsuitable. A method has been proposed to manufacture high-strength steel by finely precipitating TiN, NbC, and TiC using Ti and Nb, which have a high effect of improving the recrystallization temperature, and then performing recovery annealing, but the increase in recrystallization temperature places relatively high restrictions on thermal processing equipment, making it less economical.

[0007] In the case of hard phase control, a metastable phase is formed to a desired degree mainly through a fast cooling rate during steel sheet manufacturing. A method has been proposed to secure high strength of 800 MPa or more by utilizing a hard phase. This method is difficult to maintain during recrystallization, and precise control of the cooling rate during thermal processing is required to obtain the recrystallized phase, making it difficult to use in applications that involve thermal processing. In addition, since it is mostly used in applications that require very high strength but relatively low elongation, it has the disadvantage of significantly reducing formability.

[0008] Therefore, in order to manufacture a steel sheet having desired physical properties after mechanical and thermal processing, it is necessary to utilize the characteristics of the above strengthening methods in a well-combined manner. Summary of the Invention [Problem to be solved by the invention]

[0009] An embodiment of the present invention provides a cold-rolled steel sheet having excellent workability and a manufacturing method thereof. More specifically, the present invention provides a cold-rolled steel sheet having excellent workability before and after processing, and which is easy to manufacture intermediate parts in processing, and a manufacturing method thereof. [Means for solving the problem]

[0010] A cold rolled steel sheet having excellent workability according to one embodiment of the present invention contains, by weight, C: 0.012 to 0.060%, Si: 0.03% or less (except 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: 0.015% or less (except 0%), S: 0.015% or less (except 0%) and N: 0.006% or less (except 0%), with the balance containing Fe and other unavoidable impurities. The cold rolled steel sheet having excellent formability according to an embodiment of the present invention may have a strengthening index defined by the following Equation 1 of 1.0 to 3.0. [Formula 1] Reinforcement index=[([C] / 12.011)×6+([Mn] / 54.938)]×100 (In formula 1, [C] and [Mn] represent the weight percentage of each component.) The cold rolled steel sheet having excellent workability according to an embodiment of the present invention may have a grain shape ratio defined by the following Equation 2 of 1.25 to 2.50. [Formula 2] Grain shape ratio = (average grain diameter in rolling direction) / (average grain diameter in thickness direction) The cold-rolled steel sheet having excellent workability according to an embodiment of the present invention may have a recrystallized area ratio of 3% or less. The cold-rolled steel sheet having excellent workability according to an embodiment of the present invention may have an average grain size of 8 to 12 μm. The cold rolled steel sheet having excellent workability according to an embodiment of the present invention may further include one or more of Cu: 0.003% or less, Nb: 0.01% or less by weight, Sb: 0.03% or less by weight, Sn: 0.03% or less by weight, Ni: 0.03% or less by weight, Cr: 0.03% or less by weight, Ti: 0.01% or less by weight, and Mo: 0.03% or less by weight. The cold-rolled steel sheet having excellent workability according to an embodiment of the present invention has a dislocation density of 2.5×10 15 / m 2 It can be the following: A plated steel sheet according to an embodiment of the present invention includes a cold-rolled steel sheet and a plating layer located on one or both sides of the cold-rolled steel sheet. According to an embodiment of the present invention, there is provided a method for producing a cold rolled steel sheet having excellent workability, comprising, by weight percent, C: 0.012 to 0.060%, Si: 0.03% or less (except 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: 0.015% or less (except 0%), S: 0.015% or less (except 0%), and N: 0.006% or less (except 0%), with the balance being Fe and other unavoidable impurities, and comprising a slab having a strengthening index defined by the following formula 1 of 1.0 to 3.0. Made hot rolling the slab to produce a hot rolled steel sheet; coiling the hot rolled steel sheet at 600 to 700°C; cold rolling the coiled hot rolled steel sheet at a rolling reduction of 20 to 60% to produce a cold rolled steel sheet; and annealing the cold rolled steel sheet at a temperature of 400 to 580°C. [Formula 1] Reinforcement index=[([C] / 12.011)×6+([Mn] / 54.938)]×100 (In formula 1, [C] and [Mn] represent the weight percentage of each component.) The method may further include a step of heating the slab to 1150° C. or higher prior to the step of producing the hot rolled steel sheet. In the stage of manufacturing hot-rolled steel sheets, Ar 3 This completes the hot finish rolling. After the annealing step, the method may further include a step of temper rolling the annealed sheet to a rolling reduction of 0.4 to 2.0%. A method for manufacturing a plated steel sheet according to an embodiment of the present invention includes the steps of: preparing a cold-rolled steel sheet; and forming a plating layer on one or both sides of the cold-rolled steel sheet by hot-dip galvanizing or electroplating. Effect of the Invention

[0011] According to the present invention, it is possible to provide a cold-rolled steel sheet for processing, which is easy to mechanically process, has increased ductility after thermal processing, has appropriate strength, and is easy to perform additional mechanical processing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Terms such as first, second and third are used to describe various parts, components, regions, layers and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Thus, a first part, component, region, layer or section described below can be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.

[0013] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms include the plural forms unless the phrase clearly indicates otherwise. As used herein, the meaning of "comprising" embodies certain features, regions, integers, steps, operations, elements and / or components and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements and / or components.

[0014] Moreover, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight.

[0015] In one embodiment of the present invention, the inclusion of an additional element means that the remaining iron (Fe) is replaced by the additional amount of the additional element.

[0016] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in dictionaries used in general are additionally interpreted as having a meaning consistent with the relevant technical literature and the currently disclosed content, and are not interpreted as having an ideal or very formal meaning unless defined.

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to exemplary embodiments thereof so that those skilled in the art can easily practice the present invention. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0018] One embodiment of the present invention relates to a cold-rolled steel sheet used as various structural materials after being formed through mechanical and thermal processing, and the material for this purpose must not only have excellent initial workability, but also be capable of additional workability improvement during thermal processing, while ensuring a certain level of strength as a structural material. For this purpose, it is necessary to simultaneously consider not only the physical properties of the initial material, but also the changes in physical properties due to mechanical and thermal processing, to facilitate step-by-step processing, and to satisfy the final physical properties.

[0019] In order to achieve the above object, the present inventors have found that a cold rolled steel sheet having the above target physical properties can be manufactured by optimizing the types and contents of alloy elements and manufacturing conditions, and have arrived at the present invention.

[0020] A cold rolled steel sheet having excellent workability according to one embodiment of the present invention contains, by weight, C: 0.012 to 0.060%, Si: 0.03% or less (except 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: 0.015% or less (except 0%), S: 0.015% or less (except 0%) and N: 0.006% or less (except 0%), with the balance containing Fe and other unavoidable impurities.

[0021] Hereinafter, the composition of the cold rolled steel sheet provided in one embodiment of the present invention will be described in detail, and the content of each component means weight percent unless otherwise specified.

[0022] Carbon (C): 0.0120 to 0.0600% by weight If the C content is low, the strength is low and it is difficult to use it as a structural material, and if the C content is too low, an additional refining process is required, which reduces productivity, so the C content may be 0.012 wt% or more. Although even a small C content can effectively increase strength, if the C content is excessive, the workability can be significantly reduced, so the upper limit can be limited to 0.060 wt% or less. More specifically, the C content may be 0.0035 wt% or less. More specifically, the C content may be 0.0130 to 0.0550 wt%.

[0023] Silicon (Si): 0.03% by weight or less Since Si is an element that can be used as a decarburizing agent and contributes to improving strength through solid solution strengthening, it is difficult to completely eliminate it. However, if it is excessive, Si-based oxides are generated on the surface during annealing, which can induce defects during plating and reduce plating properties. Therefore, taking this into consideration, the upper limit can be limited to 0.03 wt% or less. More specifically, Si can be contained at 0.015 wt%. More specifically, Si can be contained at 0.005 to 0.015 wt%.

[0024] Manganese (Mn): 0.100 to 0.400% by weight Mn is an element that prevents hot shortness caused by solute S by combining with solute S in steel and precipitating as MnS. To achieve this effect, Mn can be contained in an amount of 0.1 wt% or more. It also has the effect of increasing the strength of steel together with C by being dissolved in steel. However, if it is contained in excess, the workability of the steel decreases, so it can be limited to 0.4 wt% or less. More specifically, Mn can be contained in an amount of 0.150 to 0.390 wt%.

[0025] In one embodiment of the present invention, the correlation between the C and Mn contents is important for ensuring strength and workability. It has been found that the desired strength and workability can be obtained when the strengthening index defined by the following Relational Formula 1, which is an index showing the strengthening effect, is 1.000 to 3.000.

[0026] [Formula 1] Reinforcement index=[([C] / 12.011)×6+([Mn] / 54.938)]×100 (In formula 1, [C] and [Mn] represent the weight percentage of each component.)

[0027] More specifically, the reinforcement index may be between 1.500 and 2.700.

[0028] Aluminum (Al): 0.015 to 0.050% by weight Al is an element with a very large deoxidizing effect, and reacts with N in the steel to precipitate AlN, thereby preventing the deterioration of formability due to solute N. Therefore, Al may be contained in an amount of 0.015% by weight or more. However, when a large amount is added, ductility is rapidly reduced, so the content may be limited to 0.05% by weight or less. More specifically, Al may be contained in an amount of 0.020 to 0.048% by weight.

[0029] Phosphorus (P): 0.0150% by weight or less The addition of P up to a certain amount can increase the strength of steel without significantly decreasing its ductility, but if it is added in excess of 0.015 wt%, it segregates in the grain system, hardening the steel excessively and reducing the elongation rate, so the amount should be limited to 0.015 wt% or less. More specifically, P can be included in an amount of 0.015 wt% or less. More specifically, P can be included in an amount of 0.0010 to 0.0100 wt%.

[0030] Sulfur (S): 0.0150% by weight or less Since S is an element that induces red shortness when in solid solution, the precipitation of MnS must be induced by adding Mn. In addition, excessive precipitation of MnS hardens the steel, which is undesirable in terms of softening the steel in the present invention. Therefore, the upper limit of S can be limited to 0.015 wt.%. More specifically, S can be included in an amount of 0.0010 to 0.0100 wt.%.

[0031] Nitrogen (N): 0.0060% by weight or less, N is an unavoidable element in steel, but N present in a solid solution state causes aging and significantly reduces workability. In order to minimize the decrease in ductility due to aging, it is preferable to limit the upper limit to 0.0060 wt% or less. More specifically, N may be contained at 0.0035 wt% or less. More specifically, N may be contained at 0.0010 to 0.0050 wt%.

[0032] The cold-rolled steel sheet having excellent workability according to an embodiment of the present invention may further include one or more of Cu: 0.003% or less, Nb: 0.01% or less by weight, Sb: 0.03% or less by weight, Sn: 0.03% or less by weight, Ni: 0.03% or less by weight, Cr: 0.03% or less by weight, and Mo: 0.03% or less by weight.

[0033] The balance other than the above composition preferably includes Fe and unavoidable impurities, and the steel material of the present invention does not exclude the addition of other compositions. The above unavoidable impurities are those that can be unintentionally mixed from raw materials or the surrounding environment in a normal steel manufacturing process, and cannot be eliminated. The above unavoidable impurities are understandable to any engineer in the field of normal steel manufacturing.

[0034] Hereinafter, the texture characteristics of the cold rolled steel sheet having excellent workability according to an embodiment of the present invention will be described in detail.

[0035] The cold rolled steel sheet having excellent workability according to an embodiment of the present invention may have a grain shape ratio defined by the following Equation 2 of 1.25 to 2.50.

[0036] [Formula 2] Grain shape ratio = (average grain diameter in rolling direction) / (average grain diameter in thickness direction)

[0037] The rolling direction (RD) average grain diameter can be calculated by dividing the number of grains in any length in the rolling direction. The thickness direction (ND) average grain diameter can also be calculated by dividing the number of grains in the thickness of the steel sheet.

[0038] If the grain shape ratio is too small, it is difficult to secure the desired strength, and the driving force for recrystallization is small, so that recrystallization during heat treatment may be difficult. If the grain shape ratio is too large, the difference in workability between the rolling direction and the direction perpendicular to the rolling direction is too large, so that the formability may be poor. More specifically, the grain shape ratio may be 1.30 to 2.00.

[0039] The cold-rolled steel sheet having excellent workability according to an embodiment of the present invention has a dislocation density of 2.50×10 15 / m 2 The dislocation density can be measured by XRD (X-ray Diffraction). If the dislocation density is too high, there may be a problem of cracks occurring during processing. More specifically, the dislocation density is 1.00 to 2.00X10 15 / m 2 It could be.

[0040] The cold rolled steel sheet having excellent workability according to an embodiment of the present invention may have a recrystallized area ratio of 3% or less. In an embodiment of the present invention, recrystallized and non-recrystallized are distinguished by confirming newly nucleated and grown crystal grains from the elongated structure by rolling through optical observation. The recrystallized area ratio may be measured based on a surface parallel to the rolled surface (ND surface) of the steel sheet. If the area ratio is too high, the strength may be reduced, and the recrystallized portion may not recrystallize due to insufficient driving force for recrystallization after processing. More specifically, the recrystallized area ratio may be 2.5% or less.

[0041] The cold rolled steel sheet having excellent workability according to an embodiment of the present invention may have an average grain size of 8.0 to 12.0 μm. If the average grain size is too small, the strength may be too high. If the average grain size is too large, local material deviation may be large, and defects may occur during processing. The average grain size may be measured based on a plane parallel to the rolling surface (ND plane) and may be determined by measuring the diameter of a circle having the same area as the grain. More specifically, the average grain size may be 8.3 to 11.5 μm.

[0042] As described above, the cold rolled steel sheet according to an embodiment of the present invention has excellent strength and workability at the same time, and also has excellent strength and workability at the same time after working. Specifically, the yield strength before working may be 650.0 MPa or less, and the elongation may be 7.0% or more. More specifically, the yield strength before working may be 450.0 to 650.0 MPa, and the elongation may be 7.5 to 12.0%.

[0043] The post-processing yield strength may be 180.0 MPa or more, and the elongation ratio may be 25.0% or more. More specifically, the post-processing yield strength may be 180.0 to 250.0 MPa, and the elongation ratio may be 25.0 to 35.0%. In this case, the processing may be a process of mechanically processing to elongate by 20%, followed by heating to 740°C at a rapid rate of 50°C / sec, and then gradually cooling to 25°C at -5°C / sec.

[0044] A plated steel sheet according to an embodiment of the present invention includes a cold-rolled steel sheet and a plating layer located on one or both sides of the cold-rolled steel sheet.

[0045] A method for manufacturing a cold-rolled steel sheet having excellent workability according to an embodiment of the present invention includes the steps of: hot rolling a slab to manufacture a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

[0046] Each step will be described in detail below.

[0047] First, a slab is hot rolled to produce a hot rolled steel sheet.

[0048] The alloy composition of the slab is the same as that of the cold-rolled steel sheet described above, so the overlapping description will be omitted. Since the alloy composition does not substantially change during the manufacturing process of the cold-rolled steel sheet, the alloy composition of the slab and the cold-rolled steel sheet is substantially the same.

[0049] The slab may be reheated to a temperature of 1150°C or higher before hot rolling. A temperature of 1150°C or higher may be necessary because most of the precipitates present in the steel must be redissolved. More specifically, the steel may be heated to 1200°C or higher to dissolve the precipitates well.

[0050] The cooled slab was then placed in Ar 3 Hot-rolled steel sheets are manufactured by hot-rolling at the above temperature. 3The reason for the above limitation is that rolling is performed in the austenite single phase region.

[0051] Ar 3 The temperature can be calculated as follows: Ar 3 =910-(310×[C])-(80×[Mn])-(20×[Cu])-(15×[Cr])-(55×[Ni])-(80×[Mo])-(0.35×(25.4-8)) [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are the contents (weight%) of C, Mn, Cu, Cr, Ni and Mo in the steel plate, respectively. If not included, it is calculated as 0.

[0052] More specifically, the finish rolling temperature can be 900° C. or higher.

[0053] The hot-rolled steel sheet is coiled at 600 to 700°C. The size of the grains in the hot-rolled steel sheet changes depending on the coiling temperature. When the coiling temperature is low, the grains are fine, and when the coiling temperature is high, the grains are coarse. The size of the grains in the hot-rolled steel sheet is important in the present invention. Since the present invention is characterized in that complete recrystallization does not occur during the annealing process after cold rolling, the size of the grains in the hot-rolled steel sheet directly affects the physical properties of the final steel sheet. In order to obtain the desired workability and strength of the steel sheet, it is preferable to control the coiling temperature of the hot-rolled steel sheet to 600.0 to 700.0°C. More specifically, the coiling temperature may be 610.0 to 690.0°C.

[0054] Next, the hot-rolled steel sheet is cold-rolled.

[0055] At this time, cold rolling is performed at a reduction rate of 20.0 to 60.0% to manufacture a cold rolled steel sheet. The reduction rate not only determines the final thickness of the cold rolled steel sheet, but also increases the strength of the steel sheet due to work hardening during cold rolling. A cold reduction rate of 20% or more is necessary to obtain the desired strength. However, if it is too high, the strength becomes too high and processing becomes difficult, or recrystallization during annealing is promoted, resulting in a significant decrease in strength. In order to prevent recrystallization, the annealing temperature must be further reduced, but if it is too low compared to the annealing temperature of normal steel sheets, the annealing temperature difference between steel sheets becomes large when various types of steel sheets are produced, reducing productivity. In consideration of this, it is preferable to control the upper limit to 60% or less. More specifically, the reduction rate can be adjusted to 35.0 to 56.0%.

[0056] Next, the cold-rolled steel sheet is annealed at a temperature of 400.0 to 580.0°C. The annealing temperature in one embodiment of the present invention is lower than the normal recrystallization annealing temperature and corresponds to a recovery annealing temperature that removes some of the dislocations accumulated in the steel during cold rolling. The reason for removing a significant amount of the dislocations accumulated during cold rolling through recovery is to improve the elongation rate. If annealing is performed at a temperature below 400°C, the dislocations generated during cold rolling are not sufficiently removed, resulting in a decrease in ductility, and if the temperature exceeds 580°C, recrystallization occurs and the strength is greatly reduced. More specifically, annealing can be performed at a temperature of 430.0 to 575.0°C.

[0057] Thereafter, one or both sides of the cold rolled steel sheet are subjected to hot-dip galvanization or electroplating to form a plating layer, thereby producing a plated steel sheet. EXAMPLES

[0058] The present invention will be described in more detail with reference to the following examples, however, these examples are merely illustrative of the present invention and are not intended to limit the present invention.

[0059] Working Example Steel having the composition shown in Table 1 below was manufactured, and the components are shown as actual values. A steel slab containing the composition shown in Table 1 with the balance being Fe and unavoidable impurities was manufactured.

[0060] [Table 1]

[0061] The slab was reheated to 1230°C and hot rolled at 900°C or higher, and then coiled, cold rolled, and annealed according to the manufacturing conditions in Table 2 below to obtain an annealed steel sheet with a thickness of 1 mm. At this time, annealing was performed by a method of continuous annealing in a coil state, where the temperature was raised to the temperature, maintained for 1 minute, and then cooled to room temperature. In the case of Comparative Steel 3, cracks were generated during hot rolling, so cold rolling and annealing were not performed, and therefore subsequent physical property measurements were omitted.

[0062] [Table 2]

[0063] The strengthening index, grain shape ratio, dislocation density, recrystallized area ratio, and average grain size of the manufactured annealed steel sheet were measured and calculated as shown in Table 3 below. The strengthening index was defined as the following Relation 1, and calculated from the steel components. In addition, the average grain size, the recrystallized grain area, and the recrystallized area ratio were measured by optical microstructural observation, and are also shown. The grain shape ratio defined by the following Relation 2 was statistically calculated from optical microstructure observation, and the dislocation density was measured by XRD (X-ray Diffraction).

[0064] [Formula 1] Reinforcement index=[([C] / 12.011)×6+([Mn] / 54.938)]×100 (In formula 1, [C] and [Mn] represent the weight percentage of each component.)

[0065] [Formula 2] Grain shape ratio = (average grain diameter in rolling direction) / (average grain diameter in thickness direction)

[0066] [Table 3]

[0067] Table 4 shows the yield strength and elongation of the material. In order to confirm the change in physical properties after mechanical and thermal processing, the yield strength and elongation after processing were also measured. The processing was performed by mechanical processing of 20% elongation at room temperature, followed by thermal processing of heating to 740°C at a fast rate of 50°C / sec and gradually cooling to 25°C at -5°C / sec. When the physical properties after mechanical processing did not meet the standards for yield strength and elongation, thermal processing was not performed. The physical properties after thermal processing were measured one month after processing, in order to consider the aging effect caused by the passage of time before use due to the transportation and storage of the parts.

[0068] [Table 4]

[0069] Developed steels 1 to 14 in Table 4 appropriately satisfy the strengthening index, grain shape ratio, dislocation density, recrystallized area ratio, and grain size, and have a yield strength of 650 MPa or less and an elongation of 7% or more, which are suitable for mechanical processing as structural materials. In addition, all of the developed steels have a yield strength of 180 MPa or more and an elongation of 25% or more after the mechanical and thermal processing described above. An elongation of less than 25% is unsuitable for complex forming, and a yield strength of less than 180 MPa is unsuitable for maintaining the shape as a structural material.

[0070] Comparative steel 1 has a C content of less than 0.012 wt% and a strengthening index of less than 1.0. This results in a low yield strength of less than 180 MPa after processing, which is unsuitable. Conversely, comparative steel 2 has a C content of more than 0.060 wt% and a strengthening index of more than 3.0. This results in a yield strength of the annealed steel sheet of more than 650 MPa and an elongation ratio of less than 7%, making it difficult to form as described above.

[0071] In comparative steel 3, the Mn content was less than 0.1 wt%, and embrittlement due to S during hot rolling occurred, causing cracks in the hot-rolled steel sheet. When the S in the steel combines with Mn and is sufficiently precipitated in the form of MnS, brittleness is suppressed, but when there is insufficient Mn, brittleness becomes pronounced. In comparative steel 4, the Mn content was more than 0.4 wt%, the strengthening index was more than 3.0, the yield strength was more than 650 MPa, and the elongation was less than 7%, which is low.

[0072] Comparative steel 5 has an N content of more than 0.006 wt %, which results in good physical properties before processing and excellent yield strength of more than 180 MPa after processing. However, due to aging caused by N, the elongation rate is low at less than 25%.

[0073] Comparative steels 6 and 7 have a strengthening index of less than 1.0, and have a low post-processing yield strength of less than 180 MPa, making it difficult to retain the shape. On the other hand, comparative steels 8 and 9 have a strengthening index of more than 3.0, and the strength of the annealed steel sheet is 650 MPa. Pa It can be confirmed that the elongation rate is low, less than 7%, beyond the limit.

[0074] In Comparative Steel 10, the coiling temperature was set to less than 600° C., and the crystal grain size was formed small at 7.8 μm due to the low coiling temperature. In this case, the crystal grains generated in the recrystallization process during thermal processing after mechanical processing are also formed small, and the strength is high to a sufficient level, but the elongation is low at 25% or less, which is unsuitable.

[0075] On the other hand, in Comparative Steel 11, the grain size is very large at 13.7 μm due to the coiling temperature exceeding 700° C. When the grains are large, recrystallization does not easily occur even when thermal processing is performed. As a result, the stress caused by mechanical processing cannot be fully relieved, and the strength is very high, but the elongation is significantly reduced at less than 20%, making it difficult to use.

[0076] Comparative steel 12 has a low grain shape ratio of less than 1.25, which is a low cold reduction of less than 20%. When the cold reduction is low, the initial strength is low and the elongation is high, which is advantageous for mechanical processing. However, recrystallization does not occur sufficiently during thermal processing, and the elongation after processing is low at 15% or less, making it unsuitable for use.

[0077] Conversely, when the cold rolling reduction is high, exceeding 60%, as in the case of comparative steel 13, the grain shape ratio exceeds 2.5 and the dislocation density is also 2.5×10 15 / m 2 In this way, when the driving force for recrystallization is high, recrystallization occurs partially when the cold-rolled steel sheet is annealed, resulting in a recrystallized area ratio exceeding 3%. In the part of the cold-rolled steel sheet where recrystallized grains are formed after annealing, the stress is almost completely relieved, so the driving force for recrystallization to occur even after mechanical processing and thermal processing is relatively low. For this reason, it can be confirmed that recrystallization does not occur sufficiently after thermal processing, resulting in a low elongation rate of less than 20%.

[0078] In Comparative Steel 14, the annealing temperature of the cold-rolled steel sheet is low, less than 400°C, so recovery does not occur sufficiently and the crystal grain size is small. As a result, the elongation rate after annealing is low, less than 7%, and workability is reduced. In contrast, in Comparative Steel 15, the annealing temperature exceeds 580°C, and some recrystallization occurs actively, resulting in a recrystallized area ratio of more than 3%. As a result, it can be confirmed that the recrystallized area, as described above, does not have a sufficient recrystallization driving force even when mechanical processing is performed in the recrystallized area, so recrystallization does not occur after thermal processing, resulting in a low elongation rate of 20% or less.

[0079] The present invention is not limited to the embodiments, and can be manufactured in various different forms, and a person having ordinary skill in the art to which the present invention pertains should understand that the present invention can be embodied in other specific forms without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the above-described embodiments are illustrative in all respects and are not limiting.

Claims

1. In weight percent, it comprises C: 0.012 to 0.060%, Si: 0.03% or less (except 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: 0.015% or less (except 0%), S: 0.015% or less (except 0%) and N: 0.006% or less (except 0%), with the balance being Fe and other unavoidable impurities; The reinforcement index defined by the following formula 1 is 1.0 to 3.0, The crystal grain shape ratio defined by the following formula 2 is 1.25 to 2.50, The recrystallized area ratio is 3% or less, A cold-rolled steel sheet having an average crystal grain size of 8 to 12 μm and excellent workability. [Formula 1] Strengthening index = [([C] / 12.011)×6+([Mn] / 54.938)]×100 (In formula 1, [C] and [Mn] represent the weight percentage of each component.) [Formula 2] Grain shape ratio = (average grain diameter in rolling direction) / (average grain diameter in thickness direction)

2. 2. The cold rolled steel sheet according to claim 1, further comprising one or more of Cu: 0.003% or less, Nb: 0.01% or less by weight, Sb: 0.03% or less by weight, Sn: 0.03% or less by weight, Ni: 0.03% or less by weight, Cr: 0.03% or less by weight, and Mo: 0.03% or less by weight.

3. Dislocation density is 2.5X10 １５ / m ２ 3. The cold-rolled steel sheet having excellent workability according to claim 1 or 2, wherein:

4. A plated steel sheet comprising the cold-rolled steel sheet according to any one of claims 1 to 3 and a plating layer located on one or both sides of the cold-rolled steel sheet.

5. A method for producing a cold-rolled steel sheet having excellent workability according to any one of claims 1 to 3, comprising: a step of producing a slab containing, in weight percent, C: 0.012-0.060%, Si: 0.03% or less (except 0%), Mn: 0.1-0.4%, Al: 0.015-0.050%, P: 0.015% or less (except 0%), S: 0.015% or less (except 0%) and N: 0.006% or less (except 0%), the balance being Fe and other unavoidable impurities, and having a strengthening index, as defined by the following formula 1, of 1.0-3.0; Slab Ar ３ A step of producing a hot-rolled steel sheet by hot finish rolling the above-mentioned steps; coiling the hot-rolled steel sheet at 600 to 700°C; cold rolling the coiled hot rolled steel sheet at a rolling reduction of 20 to 60% to produce a cold rolled steel sheet; and The method for producing the cold rolled steel sheet having excellent workability includes annealing the cold rolled steel sheet at a temperature of 400 to 580°C. [Formula 1] Strengthening index = [([C] / 12.011)×6+([Mn] / 54.938)]×100 (In formula 1, [C] and [Mn] represent the weight percentage of each component.)

6. The method for producing a cold rolled steel sheet having excellent workability according to claim 5, further comprising the step of heating a slab to 1150°C or more before the step of producing the hot rolled steel sheet.

7. In the step of producing the hot-rolled steel sheet, ３ The method for producing a cold rolled steel sheet having excellent workability according to claim 5 or 6, wherein hot finish rolling is performed at the above temperature.

8. The method for producing a cold rolled steel sheet having excellent workability according to any one of claims 5 to 7, further comprising a step of temper rolling the annealed sheet at a rolling reduction of 0.4 to 2.0% after the annealing step.

9. Producing a cold rolled steel sheet by the method according to any one of claims 5 to 8; and The method of manufacturing a plated steel sheet includes the step of forming a plating layer on one or both sides of the cold rolled steel sheet by hot-dip plating or electroplating.

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