Hard cold-rolled steel sheet having excellent formability and method of manufacturing same

Abstract

A hard cold-rolled steel sheet having excellent formability according to one embodiment of the present invention comprises, in weight%, 0.001 to 0.065% of C, 0.001 to 0.55% of Mn, 0.01 to 0.045% of Al, 0.01% or less of Si (excluding 0%), and 0.055% or less of P (excluding 0%), with the balance being Fe and inevitable impurities, wherein the surface roughness (Ra) of the steel sheet in the rolling direction is 0.1 to 0.3 μm, and the surface roughness (Ra) of the steel sheet in the direction perpendicular to the rolling direction is 0.2 to 0.45 μm.

Description

Hard cold-rolled steel sheet with excellent formability and method for manufacturing the same

[0001] One embodiment of the present invention relates to a hard cold-rolled steel sheet with excellent formability and a method for manufacturing the same. More specifically, one embodiment of the present invention relates to a hard cold-rolled steel sheet with excellent formability that does not undergo additional heat treatment after cold rolling and a method for manufacturing the same.

[0002] Cold-rolled steel sheets are required to possess both high strength and excellent formability simultaneously. Methods to increase strength include solid solution strengthening, precipitation strengthening, and microstructural strengthening utilizing alloying elements, as well as work hardening and bake hardening utilizing manufacturing processes.

[0003] In the case of work hardening, cold rolling forms a large number of dislocations to increase strength but significantly impairs formability; therefore, formability is secured by reducing dislocations through recovery and recrystallization via an annealing step after cold rolling.

[0004] However, when undergoing the annealing stage, the strength tends to decrease again, and at this point, solid solution, precipitation, and microstructural strengthening become effective.

[0005] Since such work hardening is a good means to increase strength in low-alloy systems, if formability can be improved simultaneously, process elimination is possible, thereby reducing manufacturing costs.

[0006] Meanwhile, slide rails are widely utilized in various fields such as refrigerators, washing machines, and furniture. Since they are manufactured through processes such as stamping and bending into various shapes, formability is important as well as strength, and surface characteristics are also required to ensure sliding performance.

[0007] One embodiment of the present invention aims to provide a hard cold-rolled steel sheet with excellent formability and a method for manufacturing the same. More specifically, one embodiment of the present invention aims to provide a hard cold-rolled steel sheet with excellent formability that does not undergo additional heat treatment after cold rolling, and a method for manufacturing the same.

[0008] A hard cold-rolled steel sheet with excellent formability according to one embodiment of the present invention comprises, in weight%, C: 0.001 to 0.065%, Mn: 0.001 to 0.55%, Al: 0.010 to 0.045%, Si: 0.01% or less (excluding 0%), P: 0.055% or less (excluding 0%), and the remainder being Fe and unavoidable impurities, and the surface roughness (Ra) of the steel sheet in the rolling direction is 0.1 to 0.38 μm, and the surface roughness (Ra) of the steel sheet in the rolling vertical direction is 0.2 to 0.45 μm.

[0009] A cold-rolled steel sheet according to one embodiment of the present invention may have an average equiaxed order of grains (rolling direction / rolling perpendicular direction) of 3 or less.

[0010] A cold-rolled steel sheet according to one embodiment of the present invention does not crack when r=0 bending in the rolling direction and the rolling perpendicular direction and when it is folded 180°, and may have a yield strength of 400 MPa or more and an elongation in the rolling direction of 5% or more.

[0011] A cold-rolled steel sheet according to one embodiment of the present invention may include a coating layer containing cold rolling oil on the surface of the steel sheet.

[0012] A method for manufacturing a hard cold-rolled steel sheet with excellent formability according to one embodiment of the present invention comprises the steps of: manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, in weight%, C: 0.001 to 0.065%, Mn: 0.001 to 0.55%, Al: 0.01 to 0.045%, Si: 0.01% or less (excluding 0%), P: 0.055% or less (excluding 0%), and the remainder being Fe and unavoidable impurities; and manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet.

[0013] In the step of manufacturing cold-rolled steel sheets, the surface roughness (Ra) in the rolling direction of the rolling roll of the last pass is 0.1 to 0.38 μm, and the surface roughness (Ra) in the rolling vertical direction is 0.2 to 0.45 μm.

[0014] The ratio of the hardness of the cold-rolled steel sheet to the hardness of the hot-rolled steel sheet may be 1.3 to 2.

[0015] In the step of manufacturing cold-rolled steel sheets, the reduction rate may be 15 to 42%.

[0016] A hard cold-rolled steel sheet with excellent formability according to one embodiment of the present invention has excellent strength and formability simultaneously.

[0017] A hard cold-rolled steel sheet with excellent formability according to one embodiment of the present invention can reduce manufacturing costs by omitting the heat treatment process after cold rolling.

[0018] A hard cold-rolled steel sheet with excellent formability according to one embodiment of the present invention has excellent surface characteristics and can be usefully utilized for slide rails, etc.

[0019] In this specification, 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 solely to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section without departing from the scope of the invention.

[0020]

[0021] In this specification, when a part is described as "comprising" a certain component, it means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0022] In this specification, technical terms used are intended merely to refer to specific embodiments and are not intended to limit the invention. Singular forms used herein include plural forms unless phrases clearly indicate otherwise. The meaning of "comprising" as used in this specification specifies a particular characteristic, area, integer, step, action, element, and / or component, and does not exclude the presence or addition of other characteristics, areas, integers, steps, actions, elements, and / or components.

[0023] In this specification, the term “combination thereof” included in a Markush-type expression means one or more mixtures or combinations selected from a group consisting of components described in the Markush-type expression, and means including one or more selected from the group consisting of said components.

[0024] In this specification, when a part is referred to as being "on" or "on" another part, it may be immediately on or on the other part, or other parts may be involved between them. In contrast, when a part is referred to as being "immediately on" another part, no other parts are interposed between them.

[0025] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or highly formal sense unless otherwise defined.

[0026] Also, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

[0027] In one embodiment of the present invention, the meaning of including additional elements is that the remainder of iron (Fe) is replaced by an amount of the additional element.

[0028] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0029] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight%, C: 0.001 to 0.065%, Mn: 0.001 to 0.55%, Al: 0.010 to 0.045%, Si: 0.01% or less (excluding 0%), P: 0.055% or less (excluding 0%), and the remainder being Fe and unavoidable impurities.

[0030] First, I will explain the reason for limiting the composition of the steel plate.

[0031] C: 0.001 to 0.065 weight%

[0032] Carbon (C) is a representative solid solution strengthening element and a carbide-forming element. If C is too low, the initial strength is low, making it impossible to obtain the desired combination of strength-elongation and microstructure; if C is too high, carbides precipitate, leading to stress concentration during subsequent work hardening and potentially resulting in inferior elongation. Therefore, C is included in an amount of 0.001 to 0.065 weight%. More specifically, it may be included in an amount of 0.001 to 0.060 weight%.

[0033] Mn: 0.001 to 0.550 wt%

[0034] Manganese (Mn) is a representative solid solution strengthening element and an element that can react with S at high temperatures to cause red-hot brittleness. If the Mn content is too low, it may be difficult to obtain the desired material because sufficient initial strength cannot be secured. If too much Mn is included, Mn bands may form within the steel, which can lead to internal cracks or a decrease in elongation during work hardening. Therefore, Mn is included in an amount of 0.001 to 0.550 weight%. More specifically, it may be included in an amount of 0.001 to 0.500 weight%.

[0035] Al: 0.010 to 0.045 wt%

[0036] Aluminum (Al) is a representative deoxidizer that deoxidizes the large amount of oxygen introduced during the steelmaking process through the formation of Al2O3. If too little Al is introduced, sufficient deoxidation does not occur, which may lead to a decrease in ductility due to a large amount of inclusions in the steel. In addition, if too much Al is included, Al-based oxides or precipitates may develop, which may cause a decrease in elongation. Therefore, the Al content may be 0.01 to 0.045 weight%. More specifically, the Al may be 0.010 to 0.040 weight%.

[0037] Si: 0.01 wt% or less (excluding 0%)

[0038] Silicon (Si) is a major deoxidizer and a solid solution strengthening element. When Si is included in large amounts, it can cause brittleness and weaken the surface due to the formation of Si-based oxides; therefore, in the present invention, it is limited to 0.01 weight% or less. More specifically, it may be included in an amount of 0.005 to 0.008 weight%.

[0039] P: 0.055 wt% or less (excluding 0%)

[0040] Phosphorus (P) is an element that causes room temperature brittleness, but it is also a representative solid solution strengthening element. When P is actively utilized to secure initial strength, it causes work hardening, and since too much work hardening is not required to achieve the desired strength, there is an advantage in that sufficient formability can be obtained. Therefore, P is included in an amount of 0.055 weight% or less. More specifically, it may be included in an amount of 0.001 to 0.050 weight%.

[0041]

[0042] In addition to the aforementioned components, the present invention comprises Fe and unavoidable impurities. The addition of effective components other than those mentioned above is not excluded. If additional components are included, they may be added to replace the remainder of Fe.

[0043]

[0044] A hard cold-rolled steel sheet with excellent formability according to one embodiment of the present invention has a surface roughness (Ra) of the steel sheet in the rolling direction of 100 to 380 nm and a surface roughness (Ra) of the steel sheet in the rolling vertical direction of 200 to 450 nm.

[0045] The surface roughness (Ra) of the steel plate in the rolling direction (RD direction) affects the noise and durability when a part manufactured using the steel of the present invention is utilized as a component of a slide rail that operates by the movement of an intermediate bearing. A lower roughness means lower noise and less friction even during long-term use, which reduces material wear and prevents sagging during long-term testing. Furthermore, in one embodiment of the present invention, since additional heat treatment and temper rolling are not performed after cold rolling, the surface roughness (Ra) of the steel plate may be lower than that of a steel plate that undergoes heat treatment. If the surface roughness (Ra) of the steel plate in the rolling direction (RD direction) is too low, slip may occur during rolling, causing risks to equipment and safety; additionally, the rolling load may be too low, leading to problems in terms of shape control. If the surface roughness (Ra) of the steel plate in the rolling direction (RD direction) is too high, the sliding characteristics may be inferior due to noise and sagging when manufactured as a slide rail. More specifically, the surface roughness (Ra) of the steel sheet in the rolling direction may be 0.110 to 0.280 μm. The surface roughness (Ra) of the steel sheet in the rolling direction (RD direction) can be measured using a three-dimensional optical coherence illuminometer. In this case, the surface roughness (Ra) refers to the value of the arithmetic mean of the height and depth of the peaks and valleys relative to the average height of the steel sheet surface.

[0046] The surface roughness (Ra) of the steel sheet in the rolling vertical direction (TD direction) is related to productivity and die wear during slide rail manufacturing. The rolling vertical direction is the region where the greatest force is applied by the die as the material bends during production. In this context, for some forming methods, a lower surface friction coefficient improves formability by enhancing material penetration, whereas an excessively high coefficient can cause die wear.

[0047] If the surface roughness (Ra) of the steel sheet in the rolling vertical direction (TD direction) is too low, the rolling load during the production process may be too low, causing the sheet to slip out of the rolling rolls. This may result in damage to the work equipment or threaten the safety of the workers. Additionally, the low rolling load may lead to defective sheet shapes. If the surface roughness (Ra) of the steel sheet in the rolling vertical direction (TD direction) is too high, the manufacturing load increases when manufacturing with slide rails, and it may cause wear on the mold. More specifically, the surface roughness (Ra) of the steel sheet in the rolling vertical direction may be 0.200 to 0.440 μm. The surface roughness (Ra) of the steel sheet in the rolling vertical direction (TD direction) can be measured using a 3D optical coherence roughness meter, just as it is in the rolling direction.

[0048] The difference between the surface roughness (Ra) in the rolling direction (RD direction) and the surface roughness (Ra) in the rolling vertical direction (TD direction) (TD-RD) can be 0.05 to 0.30 μm. If the difference between the surface roughness (Ra) in the rolling direction (RD direction) and the surface roughness (Ra) in the rolling vertical direction (TD direction) is too large, the sliding characteristics may be inferior when manufactured into a slide rail. More specifically, the difference between the surface roughness (Ra) in the rolling direction (RD direction) and the surface roughness (Ra) in the rolling vertical direction (TD direction) (TD-RD) can be 0.05 to 0.10 μm.

[0049] A cold-rolled steel sheet according to one embodiment of the present invention may have an average equiaxed order of grains (rolling direction / rolling perpendicular direction) of 3 or less. Immediately after cold rolling and annealing, the grain size within the steel sheet is generally isotropic, with the lengths in the rolling direction and the rolling perpendicular direction being similar. Although there may be differences in mechanical properties depending on the texture formed within the steel sheet, in such cases, the average equiaxed order is close to 1. However, when processing occurs and processing is concentrated in a specific direction, the material difference between the rolling direction and the rolling perpendicular direction becomes greater. In this case, equiaxed order refers to the ratio of the lengths in the rolling direction and the rolling perpendicular direction within a single grain, and the average value of the equiaxed order of grains within the steel sheet observation surface is defined as the average equiaxed order.

[0050] If the average equiaxial alignment is too large, anisotropy with respect to a specific direction may occur. Anisotropy refers to a phenomenon where the properties of a plate—not only regarding material properties such as strength and elongation, but also surface characteristics, aging phenomena, and drawability—vary depending on the direction within the plate. In particular, when performing 90-degree bending, the tendency differs depending on whether the bending line, which is the axis at the center of the bend, is perpendicular to the rolling direction or horizontal to the rolling direction; this tendency becomes particularly pronounced when the equiaxial alignment exceeds 3. That is, the tendency for cracks to occur is more severe when bending perpendicular to the rolling direction (when the bending line is in the rolling direction) compared to bending parallel to the rolling direction (when the bending line is perpendicular to the rolling direction), and if a 180-degree fold test is performed after bending, cracks may connect and fracture may occur. However, more specifically, the average equiaxial alignment can be between 1.50 and 2.90. More specifically, the average equiaxial order can be 1.50 to 2.50. The average equiaxial order can be determined by observing the grains using a scanning electron microscope and calculating the number of grains within a length for any length in the rolling direction and the rolling perpendicular direction.

[0051] As described above, the cold-rolled steel sheet according to one embodiment of the present invention has excellent workability and strength. Specifically, the cold-rolled steel sheet according to one embodiment of the present invention does not crack when r=0 bending and folding in the rolling direction and the rolling perpendicular direction, and may have a yield strength of 400 MPa or more and an elongation in the rolling direction of 5% or more.

[0052] Cracks may not occur in cold-rolled steel sheets when bent in the rolling direction and the direction perpendicular to the rolling. In this case, the bending conditions involve forming the material by bringing it into contact with a die mounted above, which has an inner angle of 90° and an outer angle of 270° with an upper V-shaped die having a curvature of r=0. The die drop speed is fixed at 10 mm / min, and the experiment is conducted by maintaining the material in a fully bent state (where the upper and lower dies are in contact) for 10 seconds before releasing the stress. A crack can be determined if it is formed with a length of 5 mm or more and a depth exceeding 20% ​​of the material thickness. The width varies depending on the degree of processing and location, and a width exceeding at least 1 mm is determined to be a crack.

[0053] In this case, since the length of the outer surface of the bend is determined by the thickness of the material, the length of the bend line perpendicular to the bend indicates the direction in which the crack occurs; if no crack occurs for a length of at least 50 mm, it can be determined that no crack has occurred. In the case of sliding rails, r=0 bending processing in the rolling direction and the direction perpendicular to the rolling is essential, and it is very important that no crack occurs during this process.

[0054] In addition, cracks may not occur even when a specimen with an outer angle of 270 degrees, bent at r=0, is completely folded to 180 degrees using a die. In this case, the folding condition is to align both ends of the bent specimen with the center of positional movement of the die, fix the die drop speed at 10 mm / min, and conduct the experiment by maintaining the material in a completely folded state (where both end surfaces of the material are in contact) for 10 seconds and then releasing the stress. Cracks are determined under the same conditions as the bending.

[0055] Yield strength is measured using a uniaxial tensile test with a JIS No. 5 specimen. The uniaxial tensile test specified in this invention is performed by taking a specimen parallel to the rolling direction, mounting the specimen using INSTRON equipment, and measuring the stress-strain curve obtained with the Cross-head Speed ​​fixed at 10 mm / min. Yield strength generally refers to the strength measured by the 0.2% offset method, and total elongation is calculated by including the reduced area after passing through the elastic region following the removal of stress from the specimen.

[0056] In the case of the present invention, the yield strength is 400 MPa or more, and more specifically, the yield strength may be 410 to 550 MPa.

[0057] The elongation in the rolling direction may be 5% or more. The elongation in the rolling direction is important as a measure of resistance to cracks when manufacturing intermediate stoppers, etc., on slide rails. Specifically, the elongation in the rolling direction may be 5 to 20%. The elongation can be measured in the uniaxial tensile test described above.

[0058] The elongation in the vertical direction of rolling may be 5% or more. The elongation in the vertical direction of rolling is the part where deformation occurs during roll bending in the rolling direction, which is the most important aspect of the slide rail. At this time, the elongation in the vertical direction of rolling is generally lower than the elongation in the rolling direction during work hardening, but in the present invention, by securing 5% or more, it serves to prevent processing cracks during roll bending. More specifically, the elongation in the vertical direction of rolling may be 5 to 15%.

[0059] A cold-rolled steel sheet according to one embodiment of the present invention may include a coating layer containing cold rolling oil on the surface of the steel sheet. In one embodiment of the present invention, the manufacturing process is completed during the cold rolling process, and a separate heat treatment process may not be added. Accordingly, some of the cold rolling oil sprayed onto the steel sheet during the cold rolling process may remain and exist as a coating layer. The cold rolling oil is not particularly limited and may include any generally used cold rolling oil without limitation.

[0060]

[0061] A method for manufacturing a hard cold-rolled steel sheet with excellent formability according to one embodiment of the present invention comprises the steps of: manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, in weight%, C: 0.001 to 0.065%, Mn: 0.001 to 0.55%, Al: 0.01 to 0.045%, Si: 0.01% or less (excluding 0%), P: 0.055% or less (excluding 0%), and the remainder being Fe and unavoidable impurities; and manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet.

[0062] First, a slab satisfying the aforementioned composition is prepared. Molten steel, with its composition adjusted to the aforementioned composition during the steelmaking stage, can be manufactured into a slab through continuous casting. The alloy composition of the slab is substantially the same as that of the aforementioned cold-rolled steel sheet. Since the alloy composition has been described above, a redundant explanation is omitted.

[0063] Before hot rolling the slab, the manufactured slab can be heated. By heating, the subsequent hot rolling process can be performed smoothly, and the slab can be homogenized. More specifically, heating may mean reheating.

[0064] At this time, the slab heating temperature may be 1150 to 1280°C. If the slab heating temperature is too low, the rolling load in the subsequent hot rolling process may increase rapidly, which can worsen workability. On the other hand, if the slab heating temperature is too high, not only will energy costs increase, but the amount of surface scale may increase, leading to material loss. More specifically, it may be 1180 to 1260°C.

[0065] After that, the heated slab is hot-rolled to produce a hot-rolled steel sheet.

[0066] At this time, the finishing hot rolling temperature may be 880 to 950°C. If the finishing hot rolling temperature is too low, the mixing of grains proceeds rapidly as rolling is finished in a low-temperature region, which may lead to a decrease in rollability and workability. On the other hand, if the finishing hot rolling temperature is too high, the peelability of surface scale decreases, and as uniform hot rolling is not achieved across the thickness, a decrease in impact toughness due to grain growth may occur. More specifically, the finishing hot rolling temperature may be 890 to 940°C.

[0067] Afterward, the hot-rolled steel sheet manufactured after hot rolling undergoes a coiling process. More specifically, it may be a hot-rolled coiling process.

[0068] At this time, the coiling temperature can be 500 to 700°C. Hot-rolled steel sheets may be cooled on a run-out table (ROT) before coiling. If the hot-rolled coiling temperature is too low, temperature non-uniformity in the width direction occurs during the cooling and holding process, which leads to variations in the formation of low-temperature precipitates. This can cause material variation and adversely affect workability. In particular, since cold-rolled steel sheets for slide rails are slit in the width direction and used in a skelp form, quality variations may occur depending on the material if the width direction differs.

[0069] On the other hand, if the coiling temperature is too high, corrosion resistance decreases as carbide aggregation progresses and P intergranular segregation is promoted, which not only reduces cold rolling performance but also causes problems such as poor workability due to microstructure coarsening in the final product. More specifically in the present invention, the coiling temperature can be 550 to 680°C. The coiling temperature is an important factor in determining the hot-rolled microstructure and the material range during cold rolling.

[0070] The thickness of the hot-rolled steel sheet can be 1.3 to 5.5 mm, which is the final product.

[0071] The coiled hot-rolled steel sheet may additionally include a step of pickling the steel sheet before cold rolling.

[0072] Afterwards, the coiled hot-rolled steel sheet is manufactured into a cold-rolled steel sheet through cold rolling.

[0073] In one embodiment of the present invention, the surface roughness (Ra) of the steel plate surface is a very important factor, and to appropriately control this, the surface roughness (Ra) in the rolling direction of the last pass rolling roll is 0.1 to 0.38 μm, and the surface roughness (Ra) in the rolling vertical direction is 0.2 to 0.45 μm. A pass refers to the number of times a rolling roll is passed through during cold rolling, and when cold rolling is performed through n passes, the last pass refers to the nth pass. The surface roughness in the rolling direction of the rolling roll refers to the circumferential direction of the cylindrical rolling roll, and the rolling vertical direction refers to the length direction of the cylinder. More specifically, the surface roughness (Ra) in the rolling direction of the last pass rolling roll may be 0.15 to 0.35 μm, and the surface roughness (Ra) in the rolling vertical direction may be 0.21 to 0.44 μm.

[0074] The cold reduction ratio can be 15 to 42%. If the cold reduction ratio is too low, the thickness of the hot-rolled sheet required to obtain the final desired thickness becomes too low, making production difficult, and it is difficult to secure the strength of the cold-rolled steel sheet due to insufficient work hardening. However, if the cold reduction ratio is too high, workability becomes inferior, and in particular, workability in the vertical direction of rolling becomes very poor, which can cause processing cracks during slide rail processing. More specifically, the cold reduction ratio can be 20 to 40%.

[0075] The ratio of the hardness of the cold-rolled steel sheet to the hardness of the hot-rolled steel sheet (hardness of the cold-rolled steel sheet / hardness of the hot-rolled steel sheet) may be 1.3 to 2.0. The ratio of the hardness of the cold-rolled steel sheet to the hardness of the hot-rolled steel sheet (hardness of the cold-rolled steel sheet / hardness of the hot-rolled steel sheet) is a value indicating the degree of work hardening. If this ratio is too large, work hardening has occurred significantly, resulting in poor workability and a greater tendency for cracks to occur during processing; if it is too small, sufficient work hardening cannot occur, and the desired strength cannot be secured. Furthermore, the material variation in the width and length directions of the hot-rolled steel sheet is not improved from that of the cold-rolled steel sheet and is maintained, which may result in the manufacture of a material with large material variation. More specifically, the hardness ratio may be 1.50 to 1.80.

[0076] In one embodiment of the present invention, processing at the customer can be performed after cold rolling without additional heat treatment.

[0077]

[0078] The present invention will be explained in more detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and explain the invention in more detail, and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0079]

[0080] Examples

[0081] A slab was manufactured through a converter, secondary refining, and continuous casting process using an alloy composition containing the composition shown in Table 1 below in weight percent, with the remainder being iron (Fe) and unavoidable impurities. This slab was held in a furnace maintained at 1200°C for 3 hours, after which hot rolling was performed. At this time, the finishing hot rolling temperature was controlled to 900°C and the coiling temperature to 650°C, and the final thickness was processed to 2.86 mm. The hardness of the hot-rolled steel sheet was measured and summarized in Table 1. The hardness of the hot-rolled steel sheet was measured in HRB units using a Rockwell hardness tester.

[0082] The hot-rolled specimens were subjected to cold rolling at the reduction ratios listed in Table 1 after removing the oxide film from the surface through pickling. The cold rolling was performed using a 5-stage rolling process with five rolling rolls arranged side by side, and the surface roughness of the final pass rolling roll was adjusted as shown in Table 1. The surface roughness of the rolling rolls was fabricated by applying patterns in the rolling direction and the rolling perpendicular direction, and was measured using a linear roughness meter.

[0083] The roughness of the cold-rolled steel sheets produced in this way is summarized in Table 2.

[0084] In addition, the hardness of the cold-rolled steel sheets was measured using a Rockwell hardness tester and summarized in Table 2.

[0085] In addition, the equiaxial alignment of cold-rolled steel sheets was measured using an optical microscope and an image analyzer and summarized in Table 2.

[0086] For the manufactured steel plates, the presence or absence of cracks during 90-degree bending, the presence or absence of cracks during 180-degree folding, the yield strength in the rolling direction, the elongation in the rolling direction, the elongation in the vertical rolling direction, the presence or absence of meandering during cold rolling, and the results of the long-term durability evaluation were measured and summarized in Table 3 below.

[0087] Presence or absence of cracks during 90° bending: When bending, cracks are detected by measuring surface defects using a point micrometer to detect defects with a depth of 20% or more of the material thickness, and then photographing them with an optical microscope. If the width and length of the defects determined to be cracks are 1mm and 5mm or more, respectively, it is determined to be a crack, and if there is even one crack on the observation surface, it is determined that a crack has occurred.

[0088] Presence or absence of cracks when folding 180 degrees: Similarly, regarding the presence or absence of cracks when folding, the folding part was cut and a surface defect area with a depth of 20% or more of the thickness was identified using a point micrometer, and after optical imaging, if there was even one crack with a width of 1 mm and a length of 5 mm or more, it was determined that a crack had occurred.

[0089] Yield strength and elongation: Measured using a uniaxial tensile test with a JIS No. 5 specimen.

[0090] Presence or absence of meandering during cold rolling: As a defect where the plate deviates from the rolling rolls due to the plate's surface roughness being too low during cold rolling, the occurrence of meandering is determined by utilizing the position of the plate's center measured by a laser at the measuring instrument (Automatic Shape Controller) located in the middle of the cold rolling mill, and it is determined that meandering has occurred if the scrap deviates by more than 20 mm from the center of the rolling scrap.

[0091] Long-term durability evaluation: With the slide rail fastened, a 30kg weight was placed inside the drawer to apply stress, and the drawer was opened and closed 50,000 times at a speed of once every 5 seconds. If the rail sagged by more than 20mm or broke, it was deemed unsatisfactory.

[0092]

[0093] Classification CMnAlSiP Rolling Roll Rolling Direction Roughness (㎛) Rolling Roll Rolling Vertical Direction Roughness (㎛) Cold Rolling Rate (%) 10.00 10.2000.025 0.005 0.01 0.35 0.4030 20.06 0.2000.025 0.005 0.01 0.35 0.4030 30.0400.00 10.025 0.005 0.01 0.35 0.4030 40.0400.5000.025 0.005 0.01 0.35 0.4030 50.0400.2000.01 0.005 0.01 0.35 0.4030 60.0400.2000.0400.005 0.01 0.35 0.4030 70.0400.2000 .0250.0080.010.350.403080.0400.2000.0250.0050.050.350.403090.0400.2000.0250.0050.010.150.4030100.0400.2000.0250.0050.010.300.4030110.0400.2000.0250.0050.010.350.2130120.0400.2000.0250.0050.010.350.4430130.0400.2000.0250.0050.010.350.40 20140.0400.2000.0250.0050.010.350.4040150.0700.2000.0250.0050.010.350.4030160.0400.6000.0250.0050.010.350.4030170.0400.2000.0050.0050.010.350.4030180.0400.2000.0500.0050.010.350.4030190.0400.2000.0250.0150.010.350.4030200.0400.2000.025 0.0050.060.350.4030210.0400.2000.0250.0050.010.050.4030220.0400.2000.0250.0050.010.400.4030230.0400.2000.0250.0050.010.350.1030240.0400.2000.0250.0050.010.350.5030250.0400.2000.0250.0050.010.350.4010260.0400.2000.0250.0050.010.350.4045

[0094] Classification Cold-rolled sheet rolling direction roughness (㎛) Cold-rolled sheet rolling vertical direction roughness (㎛) Hot-rolled hardness (HRB) Hot-rolled yield strength (MPA) Cold-rolled sheet hardness (HRB) Hardness ratio (Cold-rolled Hardness / Hot Rolled Hardness) Average Equilibrium Maintenance 10.2000.32044.5102.9075.01.691.8820.2000.31052.0181.6085.01.631.8630.2000.31146.0118.6079.51.731.8940.2000.31251.5176.4084.01.631.9150.2000.32049.0150.1081.41.661.7660.2000.30548.5144.9081.81.691.8570.2000.29 549.3153.3081.61.661.8480.2000.32551.2173.2083.91.641.8790.1100.28549.0150.1081.71.671.88100.2800.38949.1151.2081.51.661.89110.2000.20048.9149.1081.31.661.88120.2000.43048.8148.0081.01.661.90130.2800.43049.1151.2073.81.501 .52140.1500.22049.8158.5087.01.752.45150.2000.32052.5186.8788.01.681.89160.2000.32051.5176.3787.51.701.91170.2000.32048.5144.8981.61.681.76180.2000.32049.5155.3883.01.681.85190.2000.32049.7157.4883.51.681.84200.2000.32052. 3184.7787.31.671.87210.0500.32049.0150.1382.01.671.88220.3900.32049.1151.1882.31.681.89230.2000.05049.0150.1382.31.681.88240.2000.46048.4143.8481.91.691.85250.2850.32049.1151.1870.01.431.40260.1400.20049.0150.1392.01.883.00

[0095] Classification Presence or absence of cracks during 90-degree bending Presence or absence of cracks during 180-degree folding Yield strength in rolling direction (MPa) Elongation in rolling direction (%) Elongation in vertical rolling direction (%) Presence or absence of meandering during cold rolling Long-term durability evaluation results Remarks 1XX423.013.19.6X Accepted Invention Example 2XX528.07.84.3X Accepted Invention Example 3XX470.310.97.4X Accepted Invention Example 4XX517.58.65.1X Accepted Invention Example 5XX490.29.96.4X Accepted Invention Example 6XX494.49.76.2X Accepted Invention Example 7XX492.39.8 6.3X Accepted Invention Example 8XX516.48.75.2X Accepted Invention Example 9XX493.49.86.3X Accepted Invention Example 10XX491.39.96.4X Accepted Invention Example 11XX489.210.06.5X Accepted Invention Example 12XX486.010.16.6X Accepted Invention Example 13XX410.413.510.0X Accepted Invention Example 1 4XX549.06.75.5X Passed Invention Example 15OO559.56.63.1X Passed Comparative Example 16OO554.26.93.4X Passed Comparative Example 17XO492.39.86.3X Passed Comparative Example 18XO507.09.15.6X Passed Comparative Example 19XO512.28.95.4X Passed Comparative Example 20OO552.17.03 .5X Pass Comparison Example 21XX496.59.66.1O Pass Comparison Example 22XX499.79.56.0X Fail Comparison Example 23XX499.79.56.0O Pass Comparison Example 24XX495.59.76.2X Fail Comparison Example 25XX370.615.612.1X Pass Comparison Example 26OO601.54.62.3X Pass Comparison Example

[0096] As can be seen in Tables 1 and 2 above, when the roughness of the rolling roll of the last pass was properly adjusted, the roughness of the cold-rolled sheet could be properly adjusted, and as a result, it was confirmed that the productivity issue could be resolved when manufacturing slide rails using this material, and the noise and operational performance of the rail could also be reduced.

[0097]

[0098] The present invention is not limited to the above embodiments and can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without changing the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1. In weight%, comprising C: 0.001 to 0.065%, Mn: 0.001 to 0.55%, Al: 0.01 to 0.045%, Si: 0.01% or less (excluding 0%), P: 0.055% or less (excluding 0%), and the remainder being Fe and unavoidable impurities, The surface roughness (Ra) of the steel sheet in the rolling direction is 0.1 to 0.38 μm, and Hard cold-rolled steel sheet with excellent formability, having a surface roughness (Ra) of 0.2 to 0.45 μm in the direction perpendicular to rolling.

2. In Paragraph 1, Hard cold-rolled steel sheet with excellent formability, having an average grain equiaxed order (rolling direction / rolling perpendicular direction) of 3 or less.

3. In Paragraph 1, A hard cold-rolled steel sheet with excellent formability, having no cracks when r=0 bending in the rolling direction and the direction perpendicular to rolling, and when folded 180°, a yield strength of 400 MPa or more, and an elongation in the rolling direction of 5% or more.

4. In Paragraph 1, A hard cold-rolled steel sheet with excellent formability comprising a coating layer containing cold rolling oil on the surface of the steel sheet.

5. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab comprising, in weight%, C: 0.001 to 0.065%, Mn: 0.001 to 0.55%, Al: 0.01 to 0.045%, Si: 0.01% or less (excluding 0%), P: 0.05% or less (excluding 0%), and the remainder being Fe and unavoidable impurities; The method includes the step of manufacturing a cold-rolled steel sheet by cold-rolling the above hot-rolled steel sheet, and A method for manufacturing a hard cold-rolled steel sheet with excellent formability, wherein in the step of manufacturing the cold-rolled steel sheet, the surface roughness (Ra) in the rolling direction of the rolling roll of the last pass is 0.1 to 0.38 μm and the surface roughness (Ra) in the rolling vertical direction is 0.2 to 0.45 μm.

6. In Paragraph 5, A method for manufacturing a hard cold-rolled steel sheet with excellent formability, wherein the ratio of the hardness of the cold-rolled steel sheet to the hardness of the hot-rolled steel sheet is 1.3 to 2.

7. In Paragraph 5, A method for manufacturing a hard cold-rolled steel sheet with excellent formability, wherein the reduction rate in the step of manufacturing the cold-rolled steel sheet is 15 to 42%.