Cold-rolled steel sheet having excellent workability and method for manufacturing the same

Abstract

A cold-rolled steel sheet having excellent workability according to an embodiment of the present application, wherein the steel sheet contains, in terms of mass%, C: 0.012 to 0.060%, Si: 0.03% or less (excluding 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: 0.015% or less (excluding 0%), S: 0.015% or less (excluding 0%), N: 0.006% or less (excluding 0%), and the balance of Fe and inevitable impurities.

Description

Technical Field

[0001] One embodiment of the present invention relates to a cold-rolled steel sheet with excellent processability and a method for manufacturing the same. More specifically, one embodiment of the present invention relates to a cold-rolled steel sheet with excellent processability before and after processing, and a method for manufacturing the same, which facilitates the manufacture of intermediate stage components. Background Technology

[0002] After cold-rolled steel sheets are manufactured, they undergo several stages of mechanical or heat treatment to prepare them for final use. Generally, machining reduces ductility and machinability. Therefore, often, after heat treatment to improve machinability in the intermediate stage after the first machining operation, a second machining operation is performed to obtain the final shape.

[0003] In this context, not only are the mechanical properties of the cold-rolled steel sheet important immediately after manufacturing, but also its mechanical properties after machining and hot working. Immediately after manufacturing, the cold-rolled steel sheet needs to have sufficient ductility for primary machining. After hot working, a certain level of strength is also required to ensure sufficient ductility for secondary machining and the final strength after machining.

[0004] Furthermore, since the aging process from hot working to final processing may reduce ductility, a certain degree of low aging characteristics is required to ensure sufficient machinability.

[0005] To maintain the shape of steel sheets during final product manufacturing, methods for imparting strength include solid solution strengthening, precipitation strengthening, work strengthening, and hard phase control. However, to ensure sufficient ductility after hot working, a strength-imparting method suitable for the hot working conditions should be employed. Hot working is generally carried out by controlling time and temperature; shorter times and lower temperatures are easier to control and more economical. To significantly restore ductility after machining, recrystallization via heat is the most effective method; therefore, steel sheets that recrystallize quickly at low temperatures are easier to heat treat.

[0006] Of the methods described above, solid solution strengthening is the simplest way to improve strength, involving the addition of alloying elements that can be dissolved in solids. Since the effects of solid solution strengthening are maintained even after heat treatment during processing, it can be effectively used to ensure the strength of the final product. However, the substitutional elements such as Mn and Si, which are typically used to achieve the desired effect, require large quantities, reducing economic efficiency. When using interstitial elements such as C and N, there is a greater tendency for machinability to deteriorate due to aging.

[0007] Work hardening is the phenomenon of increased strength through mechanical processing at room temperature. However, even with increased strength through work hardening, recrystallization occurs during heat treatment, almost negating the strength-enhancing effect, making it difficult to use for increasing the strength of the final formed part. To overcome the reduction in ductility, a method of low-temperature annealing of low-carbon steel after work hardening has been proposed. However, the primary objective is simply to ensure initial strength and elongation. During steel sheet processing, to obtain the desired post-heat-work properties, it is necessary to carefully study the correlation between property changes and the heat-work conditions during processing.

[0008] Precipitation strengthening is a method to increase strength by producing fine precipitates that are stable at high temperatures. However, since these highly stable precipitates can interfere with recrystallization, very high temperatures or long periods of time are required to induce recrystallization, making it unsuitable. While methods have been proposed to utilize recrystallization temperatures to effectively precipitate TiN, NbC, and TiC fine precipitates from Ti and Nb, followed by recovery annealing to produce high-strength steels, the increased recrystallization temperature leads to greater constraints on heat treatment equipment and reduced economic viability.

[0009] Hard phase control is a method of forming a metastable phase of a desired degree during steel sheet manufacturing by using a high cooling rate. Methods to ensure high strengths above 800 MPa have been proposed using hard phases. However, this method is difficult to maintain during recrystallization, and obtaining the recrystallized phase requires precise control of the cooling rate during heat treatment, making it difficult to apply in heat treatment processes. Furthermore, since it is mostly used for applications requiring relatively low elongation while demanding very high strength, it suffers from significantly low formability.

[0010] Therefore, in order to manufacture steel plates with the desired physical properties after machining and heat treatment, it is necessary to effectively combine the aforementioned strengthening methods. Summary of the Invention

[0011] (a) Technical problems to be solved

[0012] One embodiment of the present invention aims to provide a cold-rolled steel sheet with excellent processability and a method for manufacturing the same. More specifically, one embodiment of the present invention aims to provide a cold-rolled steel sheet with excellent processability before and after processing, and a method for manufacturing the same, which is easy to manufacture intermediate stage components.

[0013] (II) Technical Solution

[0014] According to an embodiment of the present invention, the cold-rolled steel sheet with excellent workability comprises, by weight %, C: 0.012 to 0.060%, Si: less than 0.03% (excluding 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: less than 0.015% (excluding 0%), S: less than 0.015% (excluding 0%), N: less than 0.006% (excluding 0%), and the balance being Fe and unavoidable impurities.

[0015] According to an embodiment of the present invention, the cold-rolled steel sheet with excellent workability has a strengthening index defined by Formula 1 as being from 1.0 to 3.0.

[0016] [Formula 1]

[0017] Enhancement Index = [([C] / 12.011)×6 + ([Mn] / 54.938)]×100

[0018] (In Equation 1, [C] and [Mn] represent the weight percentage of each component.)

[0019] According to an embodiment of the present invention, the cold-rolled steel sheet with excellent workability has a grain shape ratio defined by Formula 2 as being from 1.25 to 2.50.

[0020] [Equation 2]

[0021] Grain shape ratio = (average grain diameter in the rolling direction) / (average grain diameter in the thickness direction)

[0022] According to an embodiment of the present invention, the recrystallization area ratio of the cold-rolled steel sheet with excellent processability can be less than 3%.

[0023] According to an embodiment of the present invention, the cold-rolled steel sheet with excellent processability has an average crystal grain size of 8 to 12 μm.

[0024] The cold-rolled steel sheet with excellent processability according to an embodiment of the present invention further comprises one or more of the following: Cu: less than 0.003%, Nb: less than 0.01%, Sb: less than 0.03%, Sn: less than 0.03%, Ni: less than 0.03%, Cr: less than 0.03%, and Mo: less than 0.03%.

[0025] According to an embodiment of the present invention, a cold-rolled steel sheet with excellent workability has a dislocation density of 2.5 x 10⁻⁶. 15 / m 2 the following.

[0026] According to an embodiment of the present invention, coated steel may include a cold-rolled steel sheet and a coating located on one or both sides of the cold-rolled steel sheet.

[0027] A method for manufacturing a cold-rolled steel sheet with excellent workability according to an embodiment of the present invention includes: a step of manufacturing a slab, wherein the slab comprises, by weight %,: C: 0.012 to 0.060%, Si: less than 0.03% (excluding 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: less than 0.015% (excluding 0%), S: less than 0.015% (excluding 0%), N: less than 0.006% (excluding 0%), and the balance being Fe and unavoidable impurities, with a strengthening index of 1.0 to 3.0 as defined by Formula 1; a step of hot-rolling the slab at Ar3 or higher to manufacture a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet at 600 to 700°C; a step of cold-rolling the coiled hot-rolled steel sheet at a reduction rate of 20 to 60% to manufacture a cold-rolled steel sheet; and a step of annealing the cold-rolled steel sheet at a temperature of 400 to 580°C.

[0028] [Formula 1]

[0029] Enhancement Index = [([C] / 12.011)×6 + ([Mn] / 54.938)]×100

[0030] (In Equation 1, [C] and [Mn] represent the weight percentage of each component.)

[0031] Prior to the step of manufacturing hot-rolled steel sheet, the process further includes a step of heating the slab to above 1150°C.

[0032] In the process of manufacturing hot-rolled steel sheets, hot final rolling is performed at Ar3 or higher.

[0033] Following the step of annealing the cold-rolled steel sheet, the process further includes a step of temper rolling the annealed sheet at a reduction rate of 0.4 to 2.0%.

[0034] A method for manufacturing coated steel according to an embodiment of the present invention includes: the step of manufacturing a cold-rolled steel sheet; and the step of forming a coating on one or both sides of the cold-rolled steel sheet by hot-dip galvanizing or electroplating.

[0035] (III) Beneficial Effects

[0036] The present invention provides a cold-rolled steel sheet that is easy to machine, has increased ductility after hot working, and has appropriate strength and is easy to further machine, and a method thereof. Detailed Implementation

[0037] In this specification, the terms "first," "second," "third," etc., are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another part, component, region, layer, and / or segment. Therefore, without departing from the scope of the invention, the first part, component, region, layer, and / or segment described below can also be described as a second part, component, region, layer, and / or segment.

[0038] The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, the singular forms used herein are intended to include the plural forms as well. The word "comprising" as used in this specification can specifically refer to a particular feature, field, integer, step, action, element, and / or component, but does not exclude the presence or addition of other features, fields, integers, steps, actions, elements, components, and / or groups.

[0039] Additionally, unless otherwise specified, % indicates weight, and 1 ppm is 0.0001 wt%.

[0040] In one embodiment of the present invention, the inclusion of an additional element means that a portion of the remaining iron (Fe) is replaced by an additional element, the amount of which is equivalent to the amount of the additional element added.

[0041] Although not otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries should be interpreted as having the same meaning as disclosed in relevant technical literature and herein, and should not be interpreted in an idealized or overly formal sense.

[0042] Embodiments of the present invention will be described in detail below to enable those skilled in the art to implement the invention. However, the invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0043] One embodiment of the present invention relates to a cold-rolled steel sheet, which, after being formed by mechanical and thermal processing, can be used as a various structural material. Materials intended for such applications should not only possess excellent initial processing properties but also require additional improvements in processability during thermal processing and a certain degree of strength. Therefore, it is necessary to simultaneously consider the physical properties of the initial material and the changes in physical properties due to mechanical and thermal processing, ensuring ease of processing at each stage and meeting the final physical properties.

[0044] The inventors discovered that, in order to achieve the above-mentioned objectives, cold-rolled steel sheets with the aforementioned target properties can be manufactured by optimizing the types and contents of alloying elements and the manufacturing conditions, leading to this invention.

[0045] According to an embodiment of the present invention, a cold-rolled steel sheet with excellent workability comprises, by weight %, C: 0.012 to 0.060%, Si: less than 0.03% (excluding 0%), Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: less than 0.015% (excluding 0%), S: less than 0.015% (excluding 0%), N: less than 0.006% (excluding 0%), and the balance being Fe and unavoidable impurities.

[0046] The composition of the cold-rolled steel sheet provided in an embodiment of the present invention will be described in detail below. Unless otherwise stated, the content of each component refers to weight percent.

[0047] Carbon (C): 0.0120 to 0.0600% by weight

[0048] When the carbon content is low, it is difficult to use as a structural material due to its low strength, and excessive reduction of the carbon content requires additional refining processes, leading to reduced productivity. Therefore, it is necessary to include at least 0.012% by weight. Even a low carbon content can effectively improve strength, but excessive amounts significantly reduce processability; therefore, the upper limit can be limited to below 0.060% by weight. More specifically, it can be below 0.0035% by weight. More specifically, it can be between 0.0130 and 0.0550% by weight of carbon.

[0049] Silicon (Si): less than 0.03% by weight

[0050] Si is an element that can act as a decarburizing agent, and because it can contribute to increased strength through solid solution strengthening, it is difficult to completely exclude. However, if in excess, Si-based oxides form on the surface during annealing, inducing defects during plating and reducing plating performance. Therefore, taking this into consideration, its upper limit can be limited to below 0.03 wt%. More specifically, it may contain 0.015 wt% Si. More specifically, it may contain 0.005 to 0.015 wt% Si.

[0051] Manganese (Mn): 0.100 to 0.400% by weight

[0052] Mn is an element that prevents hotshortness caused by dissolved sulfur in steel by combining with dissolved sulfur (S) and precipitating as MnS. To achieve this effect, its content can be 0.1% by weight or more. Furthermore, dissolved in steel, along with carbon (C), it has the effect of increasing the strength of the steel. However, if excessive, the workability of the steel will decrease, so it can be limited to below 0.4% by weight. More specifically, it can contain 0.150 to 0.390% by weight of Mn.

[0053] In one embodiment of the invention, the correlation between the contents of C and Mn is important for ensuring strength and processability. The desired strength and processability can be obtained when the reinforcement index, defined by Formula 1 as an indicator of reinforcement effect, is between 1.000 and 3.000.

[0054] [Formula 1]

[0055] Enhancement Index = [([C] / 12.011)×6 + ([Mn] / 54.938)]×100

[0056] (In Equation 1, [C] and [Mn] represent the weight percentage of each component.)

[0057] More specifically, the enhancement index can be between 1.500 and 2.700.

[0058] Aluminum (Al): 0.015 to 0.050% by weight

[0059] Al is an element with a very high deoxidizing effect. It reacts with nitrogen in steel to precipitate AlN, thereby preventing the deterioration of workability caused by solid solution nitrogen. Therefore, it can contain more than 0.015% by weight of Al. However, since ductility decreases rapidly when added in large quantities, the content can be limited to less than 0.05% by weight. More specifically, it can include 0.020 to 0.048% by weight of Al.

[0060] Phosphorus (P): less than 0.0150% by weight

[0061] Adding phosphorus (P) in amounts below a certain threshold does not significantly reduce the ductility of steel and is an element that can improve strength. However, if the addition exceeds 0.015 wt%, it will cause segregation at grain boundaries, leading to over-hardening of the steel and reduced elongation. Therefore, it can be limited to below 0.015 wt%. Specifically, it can include less than 0.015 wt% of P. More specifically, it can include 0.0010 to 0.0100 wt% of P.

[0062] Sulfur (S): less than 0.0150% by weight

[0063] Since sulfur (S) is an element that causes red-hot brittleness in solid solutions, Mn should be added to induce the precipitation of MnS. However, since excessive precipitation of MnS hardens the steel, it is not preferred from the viewpoint of softening the steel in this invention. Therefore, the upper limit of S can be limited to 0.015% by weight. More specifically, it can include 0.0010 to 0.0100% by weight of S.

[0064] Nitrogen (N): less than 0.0060% by weight

[0065] Nitrogen (N) is an unavoidable element in steel, but N existing in a solid solution state undergoes aging, significantly deteriorating workability. It is preferable to limit its upper limit to below 0.0060 wt% to minimize the reduction in ductility due to aging. Specifically, the N content can be below 0.0035 wt%. More specifically, it can include 0.0010 to 0.0050 wt% N.

[0066] The cold-rolled steel sheet with excellent processability according to an embodiment of the present invention further comprises one or more of the following: Cu: less than 0.003%, Nb: less than 0.01%, Sb: less than 0.03%, Sn: less than 0.03%, Ni: less than 0.03%, Cr: less than 0.03%, and Mo: less than 0.03%.

[0067] Furthermore, the remaining component of the present invention is iron (Fe). However, since unforeseen impurities from raw materials or the surrounding environment are inevitably introduced during the general manufacturing process, this cannot be excluded. Since these impurities are known to those skilled in the art, not all impurities are specifically mentioned in this specification.

[0068] The following is a detailed description of the texture characteristics of a cold-rolled steel sheet with excellent processability according to an embodiment of the present invention.

[0069] According to an embodiment of the present invention, the cold-rolled steel sheet with excellent workability has a grain shape ratio defined by Formula 2 as being from 1.25 to 2.50.

[0070] [Equation 2]

[0071] Grain shape ratio = (average grain diameter in the rolling direction) / (average grain diameter in the thickness direction)

[0072] The average grain size in the rolling direction (RD direction) can be obtained by dividing any given length in the rolling direction by the number of grains present along that length. The average grain size in the thickness direction (ND direction) can also be obtained by considering the number of grains present along the thickness of the steel plate.

[0073] If the grain size ratio is too small, it is difficult to ensure the desired strength because the recrystallization driving force is small, and recrystallization may be difficult during heat treatment. If the grain size ratio is too large, the difference in workability between the rolling direction and the direction orthogonal to the rolling direction is too large, resulting in reduced workability. Specifically, the grain size ratio can be from 1.30 to 2.00.

[0074] According to an embodiment of the present invention, a cold-rolled steel sheet with excellent workability can have a diameter of 2.50 × 10⁻⁶ mm. 15 / m 2 The following is a dislocation density. Dislocation density can be measured using XRD (X-ray diffraction). If the dislocation density is too high, cracks may occur during processing. Specifically, the dislocation density can be from 1.00 to 2.00 × 10⁻⁶. 15 / m 2 .

[0075] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent workability can have a recrystallization area ratio of less than 3%. In one embodiment of the invention, recrystallization and non-recrystallization are distinguished by optical observation of the formation and growth of new nuclei in the rolled tensile structure. The recrystallization area ratio can be determined with reference to a plane parallel to the rolled surface (ND surface) of the steel sheet. If the area ratio is too large, the strength is low, the recrystallization driving force after processing is insufficient, and the recrystallization region may not be able to recrystallize. More specifically, the recrystallization area ratio can be less than 2.5%.

[0076] A cold-rolled steel sheet with excellent workability according to an embodiment of the present invention.

[0077] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent workability can have an average grain size of 8.0 to 12.0 μm. If the average grain size is too small, excessive strength may occur.

[0078] An excessively large average grain size leads to significant local material deviations and increases the likelihood of defects during processing. The average grain size can be measured using a plane parallel to the rolling surface (ND surface) as a reference, and can be determined by measuring the diameter of a circle with the same area as the grain. More specifically, the average grain size can range from 8.3 to 11.5 μm.

[0079] As described above, the cold-rolled steel sheet of one embodiment of the present invention possesses both excellent strength and workability, and its strength and workability remain excellent after processing. Specifically, the yield strength before processing can be 650.0 MPa or less, and the elongation can be 7.0% or more. More specifically, the yield strength before processing can be 450.0 to 650.0 MPa, and the elongation can be 7.5 to 12.0%.

[0080] The yield strength after processing can be above 180.0 MPa, and the elongation can be above 25.0%.

[0081] More specifically, the yield strength after processing can be 180.0 to 250.0 MPa, and the elongation can be 25.0 to 35.0%. In this case, the processing can be a machining process with 20% elongation followed by rapid heating to 740°C at 50°C / second and then slow cooling to 25°C at -5°C / second.

[0082] According to one embodiment of the present invention, the coated steel sheet includes a cold-rolled steel sheet and a coating located on one or both sides of the cold-rolled steel sheet.

[0083] A method for manufacturing a cold-rolled steel sheet with excellent workability according to an embodiment of the present invention includes the steps of heating a slab to manufacture a hot-rolled steel sheet, winding 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.

[0084] The following is a detailed explanation of each step.

[0085] First, hot-rolled steel sheets are manufactured by hot rolling slabs.

[0086] Since the alloy composition of the slab is the same as that of the cold-rolled steel sheet, repeated explanations are omitted. Because the alloy composition remains essentially unchanged during the manufacturing process of cold-rolled steel sheet, the alloy composition of the slab and the cold-rolled steel sheet is substantially the same.

[0087] Before hot rolling, the slab can be reheated to a temperature above 1150°C. This may require temperatures above 1150°C because most of the precipitates present in the steel must be re-dissolved. More specifically, for better dissolution of the precipitates, it needs to be heated to above 1200°C.

[0088] Hot-rolled steel sheets are manufactured by hot-rolling a slowly cooled slab at a temperature above Ar3. The reason for limiting the hot-rolling temperature to above Ar3 is to ensure that rolling takes place in the austenitic single-phase region.

[0089] The temperature of Ar3 can be calculated using the following formula.

[0090] Ar3=910-(310×[C])-(80×[Mn])-(20×[Cu])-(15×[Cr])-(55×[Ni])-(80×[Mo])-(0.35×(25.4-8))

[0091] [C], [Mn], [Cu], [Cr], [Ni], and [Mo] represent the contents (by weight%) of C, Mn, Cu, Cr, Ni, and Mo in the steel plate, respectively. If these contents are not included, they are counted as 0.

[0092] More specifically, the final rolling temperature can be above 900℃.

[0093] The hot-rolled steel sheet is coiled at 600 to 700°C. The grain size of the hot-rolled steel sheet changes with the coiling temperature; the grains are finer at lower temperatures and coarser at higher temperatures.

[0094] In this invention, the grain size of the hot-rolled steel sheet is crucial. Because complete recrystallization does not occur during the annealing process after cold rolling, the grain size of the hot-rolled steel sheet directly affects the physical properties of the final steel sheet. To obtain the desired workability and strength of the steel sheet, the coiling temperature of the hot-rolled steel sheet is preferably controlled between 600.0 and 700.0°C. More specifically, the coiling temperature can be between 610.0 and 690.0°C.

[0095] Next, the hot-rolled steel sheet is cold-rolled.

[0096] At this point, cold-rolled steel sheets are manufactured by cold rolling at a reduction rate of 20.0% to 60.0%.

[0097] The reduction rate not only determines the final thickness of the cold-rolled steel sheet but also increases its strength through work hardening during the cold rolling process. A cold rolling reduction rate of over 20% is required to achieve the desired strength. However, if it is too high, the high strength makes it difficult to process or promotes recrystallization during annealing, leading to a significant decrease in strength. While it is necessary to lower the annealing temperature to prevent recrystallization, if it is too low compared to the annealing temperature of typical steel sheets, the differences in annealing temperatures among various steel sheets become larger, resulting in reduced productivity. Considering this, it is preferable to control the upper limit below 60%. More specifically, the reduction rate can be adjusted to between 35.0% and 56.0%.

[0098] Next, the cold-rolled steel sheet is annealed at a temperature of 400.0 to 580.0°C. In one embodiment of the invention, the annealing temperature is lower than the typical recrystallization annealing temperature and corresponds to the recovery annealing temperature for removing some of the dislocations that have accumulated in the steel during cold rolling. This is to improve elongation by recovering and removing the large number of dislocations accumulated during cold rolling. When annealing is performed below 400°C, the dislocations generated during cold rolling cannot be sufficiently eliminated, resulting in poor ductility. When 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.

[0099] Subsequently, coated steel sheets can be manufactured by forming a coating on one or both sides of the cold-rolled steel sheet through hot-dip galvanizing or electroplating.

[0100] The present invention will be further described in detail below by way of examples. However, the following examples are only for illustrating the invention in more detail by way of example and are not intended to limit the scope of the invention.

[0101] Example

[0102] Steel with the composition shown in Table 1 below was manufactured, with the composition being measured values. Steel slabs containing the composition shown in Table 1, plus the balance of Fe and unavoidable impurities, were manufactured.

[0103] Table 1

[0104]

[0105] The slab was reheated to 1230℃ and hot-rolled above 900℃. Following the manufacturing conditions in Table 2, it was coiled, cold-rolled, and annealed to obtain an annealed steel sheet with a thickness of 1 mm. At this point, while in coil condition, the temperature was continuously raised to the appropriate level using a continuous annealing method, held for 1 minute, and then cooled to room temperature. Comparative steel 3 developed cracks during the hot rolling process; therefore, cold rolling and annealing were not performed, and subsequent property measurements were omitted.

[0106] Table 2

[0107] distinguish Winding temperature (°C) Cold rolling reduction rate (%) Annealing temperature (°C) Comparison of steel 1 654.3 40.6 543.5 Development Steel 1 650.9 40.1 542.8 Development Steel 2 652.9 40.3 536.1 Develop steel 3 652.9 40.9 545.0 Develop steel 4 653.1 40.7 533.2 Comparison of steel 2 649.3 40.6 531.0 Comparison of steel 3 651.1 - - Develop steel 5 646.3 40.8 543.1 Develop steel 6 652.7 40.2 546.4 Development Steel 7 648.4 40.5 540.3 Comparison of steel 4 647.8 40.0 543.2 Develop steel 8 650.5 39.3 540.9 Comparison of steel 5 647.8 39.1 549.6 Comparison of steel 6 651.6 40.1 546.5 Comparison of steel 7 654.9 39.9 545.3 Comparison of steel 8 649.9 39.5 536.7 Comparison of steel 9 651.3 38.5 539.6 Comparison steel 10 580.2 40.0 545.0 Development Steel 9 611.5 40.2 546.6 Development Steel 10 689.8 39.9 530.6 Comparison Steel 11 720.4 40.3 537.6 Comparison steel 12 653.6 16.8 535.3 Development Steel 11 650.7 24.4 530.8 Development Steel 12 645.0 55.4 530.8 Comparison steel 13 651.3 60.5 535.3 Comparison steel 14 650.0 40.8 385.1 Development Steel 13 654.4 40.4 431.5 Development Steel 14 650.9 39.6 572.5 Comparison steel 15 652.4 39.4 590.8

[0108] For the prepared annealed steel sheets, as shown in Table 3 below, the strengthening index, grain shape ratio, dislocation density, recrystallization area ratio, and average grain size are displayed through measurement and calculation. The strengthening index is defined by Equation 1 below and calculated from the steel composition. Furthermore, the average grain size, recrystallized grain area, and recrystallization area ratio are measured by optical microscopy. The grain shape ratio, defined by Equation 2 below, is statistically calculated through optical microscopy observation, and the dislocation density is measured by XRD (X-ray diffraction).

[0109] [Formula 1]

[0110] Enhancement Index = [([C] / 12.011)×6 + ([Mn] / 54.938)]×100

[0111] (In Equation 1, [C] and [Mn] represent the weight percentage of each component.)

[0112] According to an embodiment of the present invention, the cold-rolled steel sheet with excellent workability has a grain shape ratio defined by Formula 2 as being from 1.25 to 2.50.

[0113] [Equation 2]

[0114] Grain shape ratio = (average grain diameter in the rolling direction) / (average grain diameter in the thickness direction)

[0115] Table 3

[0116]

[0117] Table 4 shows the measured yield strength and elongation of the materials, and the yield strength and elongation after processing were measured to confirm the changes in physical properties after machining and hot working.

[0118] The processing is performed sequentially as follows: After machining at room temperature with 20% elongation, the material is rapidly heated to 740°C at 50°C / second, and then slowly cooled to 25°C at -5°C / second. If the yield strength and elongation do not meet the standards when measuring physical properties during machining, heat treatment is not performed. The physical properties after heat treatment are measured one month after processing to account for aging effects caused by the passage of time before use due to the transportation and storage of the parts.

[0119] Table 4

[0120]

[0121] The development steels 1 to 14 in Table 4 fully meet the requirements for strengthening indicators, grain shape ratio, dislocation density, recrystallization area ratio, grain size, yield strength below 650 MPa, and elongation above 7%, making them suitable for machining into structural materials.

[0122] Furthermore, all developed steels, after the aforementioned mechanical and hot working, possess a yield strength of 180 MPa or higher and an elongation of 25% or higher. Elongation less than 25% is unsuitable for complex forming, and yield strength less than 180 MPa is unsuitable as a structural material for maintaining shape.

[0123] Comparative steel 1 has a carbon content of less than 0.012 wt% and a strengthening index of less than 1.0. Therefore, its yield strength after processing is less than 180 MPa, which is unsuitable. Conversely, comparative steel 2 has a carbon content of more than 0.060 wt% and a strengthening index of more than 3.0. Therefore, the yield strength of the annealed steel sheet exceeds 650 MPa, and the elongation is less than 7%, making it difficult to form as described above.

[0124] Comparative steel 3 has a Mn content of less than 0.1% by weight, and during hot rolling, brittleness caused by sulfur leads to cracks in the hot-rolled steel sheet. When sulfur in the steel combines with Mn and is fully excreted in the form of MnS, brittleness is suppressed, but when Mn is insufficient, brittleness is stronger. Comparative steel 4 has an Mn content greater than 0.4% by weight, a strengthening index greater than 3.0, a yield strength greater than 650 MPa, and an elongation of less than 7%.

[0125] Compared to steel 5, which has an N content exceeding 0.006% by weight, it has good physical properties before processing and excellent yield strength after processing, with a yield strength of over 180 MPa. However, it has the disadvantage of having an elongation of less than 25% due to aging caused by N.

[0126] Comparative steels 6 and 7 have a strengthening index of less than 1.0, and their yield strength after processing is less than 180 MPa, making it difficult to maintain their shape. In contrast, comparative steels 8 and 9 have a strengthening index of over 3.0, and their annealed steel sheets have a strength exceeding 650 MPa, with an elongation of less than 7%.

[0127] Compared to steel 10, which has a coiling temperature below 600°C, the grain size decreases to as small as 7.8 μm as the coiling temperature decreases. In this case, the grains produced during recrystallization in the heat treatment after machining also become smaller. Although the strength is sufficiently high, the elongation is below 25%, making it unsuitable.

[0128] In contrast, compared to steel 11, the grain size is very large at 13.7 μm when the coiling temperature exceeds 700 °C. With such a large grain size, recrystallization is difficult to occur even after hot working. Therefore, the stress from machining cannot be adequately relieved; while the strength is high, the elongation is significantly reduced to below 20%, making it difficult to use.

[0129] Comparative steel 12 has a low cold-rolling reduction rate of less than 20% and a low grain shape ratio of less than 1.25. A low cold reduction rate results in low initial strength and high elongation, which is beneficial for machining. However, due to insufficient recrystallization during hot working, the elongation after processing is less than 15%, making it unsuitable for use.

[0130] Conversely, for example, in contrast steel 13, where the cold-rolling reduction exceeds 60%, the grain shape ratio exceeds 2.5 and the dislocation density exceeds 2.5 x 10⁻⁶. 15 / m 2 Its characteristics.

[0131] Thus, when the driving force for recrystallization is high, recrystallization occurs partially during the annealing of cold-rolled steel sheets, resulting in a recrystallization area exceeding 3%. After annealing, most of the stress is eliminated at the sites where recrystallized grains form. Therefore, even after machining and hot working, the driving force for recrystallization is relatively low. For these reasons, recrystallization is not fully achieved after hot working, resulting in a reduction in elongation to less than 20%.

[0132] Comparative steel 14, with an annealing temperature below 400°C, suffers from insufficient recovery and small crystal grain size. Consequently, its elongation after annealing is less than 7%, resulting in poor machinability. Conversely, comparative steel 15, due to an annealing temperature exceeding 580°C, undergoes partial recrystallization, thus exhibiting a recrystallized area ratio exceeding 3%. Therefore, as described above, even with machining, the recrystallization driving force is insufficient in the recrystallized portion, preventing recrystallization after hot working and leading to an elongation below 20%.

[0133] This invention can be implemented in various ways and is not limited to the embodiments described above. Those skilled in the art will understand that the invention can be implemented in other specific ways without altering its technical concept or essential features. Therefore, it should be understood that the above embodiments are exemplary in all respects and are not restrictive.

Claims

1. A cold-rolled steel sheet with excellent workability, wherein, The cold-rolled steel sheet, by weight percent, comprises C: 0.012 to 0.060%, Si: less than 0.03% and 0% free, Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: less than 0.015% and 0% free, S: less than 0.015% and 0% free, N: less than 0.006% and 0% free, and the balance being Fe and unavoidable impurities. The enhancement index, as defined in Equation 1, ranges from 1.0 to 3.

0. The grain shape ratio, as defined in Equation 2, is between 1.25 and 2.

50. The recrystallization area ratio is below 3%. The average crystal grain size is 8 to 12 mm. [Formula 1] Enhancement Index = [([C] / 12.011)×6 + ([Mn] / 54.938)]×100 In Formula 1, [C] and [Mn] represent the weight percentage of each component. [Equation 2] Grain shape ratio = (average grain diameter in the rolling direction) / (average grain diameter in the thickness direction).

2. The cold-rolled steel sheet with excellent workability according to claim 1, wherein, It further comprises one or more of Cu: less than 0.003%, Nb: less than 0.01%, Sb: less than 0.03%, Sn: less than 0.03%, Ni: less than 0.03%, Cr: less than 0.03%, and Mo: less than 0.03%.

3. The cold-rolled steel sheet with excellent workability according to claim 1, wherein, The dislocation density is 2.5 x 10⁻⁶. 15 / m 2 the following.

4. A type of plated steel, wherein, It includes the cold-rolled steel sheet as described in claim 1 and the coating located on one or both sides of the cold-rolled steel sheet.

5. A method for manufacturing a cold-rolled steel sheet with excellent workability, comprising: The step of manufacturing a slab, in weight percent, the slab comprising: C: 0.012 to 0.060%, Si: less than 0.03% and 0% free, Mn: 0.1 to 0.4%, Al: 0.015 to 0.050%, P: less than 0.015% and 0% free, S: less than 0.015% and 0% free, N: less than 0.006% and 0% free, and the balance Fe and unavoidable impurities, wherein the strengthening index of the slab as defined in Formula 1 is 1.0 to 3.0; The steps of hot-rolling slabs at Ar3 or higher to produce hot-rolled steel plates; The step of coiling the hot-rolled steel sheet at 600 to 700°C; The steps of cold rolling hot-rolled steel sheets with a reduction rate of 20% to 60% are used to manufacture cold-rolled steel sheets. as well as The step of annealing the cold-rolled steel sheet at a temperature of 400 to 580°C; [Formula 1] Enhancement Index = [([C] / 12.011)×6 + ([Mn] / 54.938)]×100 In Formula 1, [C] and [Mn] represent the weight of each component.

6. The method for manufacturing cold-rolled steel sheet according to claim 5, wherein, Prior to the step of manufacturing hot-rolled steel sheet, the process further includes a step of heating the slab to above 1150°C.

7. The method for manufacturing cold-rolled steel sheet according to claim 5, wherein, After the step of annealing heat treatment of the cold-rolled steel sheet It further includes the step of temper rolling the annealed sheet at a reduction rate of 0.4 to 2.0%.

8. A method for manufacturing coated steel, comprising: The steps for manufacturing cold-rolled steel sheets according to claim 5; as well as The step of forming a coating on one or both sides of the cold-rolled steel sheet by hot-dip galvanizing or electroplating.

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