High-strength cold-rolled steel sheet for construction and manufacturing method therefor

Abstract

A cold-rolled steel sheet according to an embodiment of the present invention comprises, in weight%, 0.0015 to 0.0050% of C, 0.10 to 0.50% of Mn, 0.01 to 0.05% of Al, 0.01 to 0.05% of Nb, 0.02 to 0.10% of Cu, 0.0005 to 0.0030% of B, 0.04 to 0.08% of P, and the balance being Fe and inevitable impurities.

Description

High-strength cold-rolled steel sheet for construction and method of manufacturing the same

[0001] One embodiment of the present invention relates to a high-strength cold-rolled steel sheet for construction and a method for manufacturing the same. More specifically, one embodiment of the present invention relates to a high-strength cold-rolled steel sheet used as a safety platform or scaffolding at a construction site and a method for manufacturing the same.

[0002] Steel plates used for safety platforms or scaffolding at construction sites are a type of steel that is essential and widely used.

[0003] Recently, due to the aging workforce, there is an increasing need for lightweighting through high-strength, thin-walled materials.

[0004] The reason strength is required for steel plates used for safety platforms or scaffolding is that increasing the strength allows for a reduction in thickness. Consequently, reducing the thickness leads to a decrease in weight, which facilitates transportation and handling during construction. This not only reduces transportation costs but also prevents safety accidents through lightweight steel plates and enables a reduction in construction costs.

[0005] As a method to improve strength, precipitation strengthening with elements such as C, Mn, and Ti can be considered; however, this method has the problem of reduced productivity because the material becomes too strong, leading to poor rollability during cold rolling.

[0006] One embodiment of the present invention aims to provide a high-strength cold-rolled steel sheet for construction and a method for manufacturing the same. More specifically, one embodiment of the present invention aims to provide a high-strength cold-rolled steel sheet used as a safety platform or scaffolding at a construction site and a method for manufacturing the same.

[0007] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight%, C: 0.0015 to 0.0050%, Mn: 0.10 to 0.50%, Al: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Cu: 0.02 to 0.10%, B: 0.0005 to 0.0030%, P: 0.04 to 0.08%, and the remainder being Fe and unavoidable impurities.

[0008] A cold-rolled steel sheet according to one embodiment of the present invention may further include one or more of Si: 0.30 wt% or less, S: 0.050 wt% or less, N: 0.005 wt% or less, Ti: 0.010 wt% or less, and V: 0.010 wt% or less.

[0009] The area ratio of grains with a length ratio of the rolling direction / vertical rolling direction of 1.5 to 3 may be 15% or less.

[0010] The precipitate has an area fraction of 0.02 to 0.04%, and the average particle size may be 20 nm or less.

[0011] The yield strength may be 550 to 750 MPa, the tensile strength may be 550 to 750 MPa, and the elongation may be 5 to 15%.

[0012] A method for manufacturing a cold-rolled steel sheet 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: 0.0015 to 0.0050%, Mn: 0.10 to 0.50%, Al: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Cu: 0.02 to 0.10%, B: 0.0005 to 0.0030%, P: 0.04 to 0.08%, and the remainder being Fe and unavoidable impurities; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; and an annealing step for the cold-rolled steel sheet.

[0013] In the step of manufacturing cold-rolled steel sheets, the reduction rate may be 30 to 70%.

[0014] During the annealing stage of the cold-rolled sheet, the cracking temperature may be 600 to 700°C.

[0015] A cold-rolled steel sheet according to one embodiment of the present invention has excellent strength, and in particular, even when thin, it has excellent strength and excellent formability.

[0016] 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.

[0017]

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] Also, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

[0024] 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.

[0025] 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.

[0026] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight%, C: 0.0015 to 0.0050%, Mn: 0.10 to 0.50%, Al: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Cu: 0.02 to 0.10%, B: 0.0005 to 0.0030%, P: 0.04 to 0.08%, and the remainder being Fe and unavoidable impurities.

[0027] First, I will explain the reason for limiting the composition of the steel plate.

[0028] C: 0.0015 to 0.0050 wt%

[0029] Carbon (C) is a representative solid solution strengthening element and an element that forms carbides. In the present invention, C also exists in solid solution or in the form of fine precipitates with Nb, thereby having the effect of delaying recrystallization. If the amount of C is too small, sufficient NbC cannot be formed even if Nb is added, so the recrystallization delay effect is reduced; if the amount of C is too large, the initial strength is increased, making it impossible to obtain the desired combination of strength-elongation and microstructure even after annealing heat treatment. Therefore, C is included in an amount of 0.0015 to 0.0050 weight%. More specifically, it may be included in an amount of 0.0018 to 0.040 weight%.

[0030] Mn: 0.10 to 0.50 wt%

[0031] 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, coarse MnS is formed, making it difficult to induce a recrystallization delay effect; therefore, the Mn content is limited to 0.5 or less. Accordingly, Mn is included in the range of 0.10 to 0.50 weight%. More specifically, it may be included in the range of 0.15 to 0.45 weight%.

[0032] Al: 0.010 to 0.050 wt%

[0033] Aluminum (Al) is a representative deoxidizing agent 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 0.010 to 0.050 weight%. More specifically, the Al may be 0.015 to 0.045 weight%.

[0034] Nb: 0.010 to 0.050 wt%

[0035] Niobium (Nb) is an element that actively forms carbonitrides with C and N. It is also an element that expands the unrecrystallized region of austenite during the hot rolling stage, thereby providing a grain refinement effect. Therefore, Nb may be included in an amount of 0.010 to 0.050 weight%. If too little Nb is added, an appropriate amount of carbonitrides that can affect the delay of recrystallization during annealing cannot be formed; if too much Nb is added, the unrecrystallized region of hot rolling expands, causing the initial strength to become excessively high, making it difficult to obtain the desired strength-elongation combination; and since an excessive amount does not significantly affect carbonitride formation, it is limited. More specifically, Nb may be included in an amount of 0.012 to 0.045 weight%.

[0036] Cu: 0.02 to 0.10 wt%

[0037] Copper (Cu) is an element that forms S and CuS precipitates. Since CuS precipitates, like Nb(C,N) precipitates, play a role in raising the recrystallization temperature during annealing, this is advantageous in the concept of securing strength in the present invention. If too little Cu is added, CuS cannot be sufficiently formed, and if too much is added, it acts as a solid solution element, making it impossible to secure the strength-elongation combination, and it does not significantly affect the formation of S precipitates. Therefore, Cu may be included in an amount of 0.020 to 0.100 weight%. More specifically, Cu may be included in an amount of 0.03 to 0.08 weight%.

[0038] B: 0.0005 to 0.0030 weight%

[0039] Boron (B) is an element that combines with nitrogen in steel to form BN. When boron nitride is formed finely, it plays a role in expanding the unrecrystallized region, which is advantageous for recovery annealed steel. Additionally, B is optionally added as it can help refine the grain size of the weldment. Since the addition of B at 30 ppm or more causes grain boundary precipitation and impairs formability, it is not added in the present invention. Therefore, B may be included in an amount of 0.0005 to 0.003 weight%. More specifically, B may be included in an amount of 0.0005 to 0.0028 weight%.

[0040] P: 0.040 to 0.080 wt%

[0041] Phosphorus (P) is a representative solid solution strengthening element. In the present invention, the initial strength of steel immediately after cold rolling is secured by adding P. However, if the P content is added in small amounts, the desired strength cannot be secured, and if too much P is added, room temperature brittleness occurs and the strength after cold rolling becomes too high, making it impossible to obtain the desired strength-elongation combination; therefore, the present invention limits the upper limit to 0.08 weight%. More specifically, P may be included in an amount of 0.045 to 0.075 weight%.

[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 cold-rolled steel sheet according to one embodiment of the present invention may have an area ratio of grains with a length ratio of 1.5 to 3.0 in the rolling direction (RD) to the rolling direction (TD) of 15% or less. When cold rolling is performed, grains with long lengths in the rolling direction are formed; however, since the formability in the cold-rolled state itself is very low, it cannot be used in members requiring forming. Therefore, ductility is secured through heat treatment, but if recrystallization occurs actively, ductility can be secured, but strength also drops sharply, so it cannot be used in members requiring high strength. Accordingly, by appropriately adjusting the annealing temperature to perform heat treatment in a range where the maximum elongation can be secured without recrystallization, appropriate formability can be achieved even when grains with long lengths in the rolling direction are maintained. More specifically, the area ratio of grains with a length ratio of 1.5 to 3.0 in the rolling direction (RD) to the rolling direction (TD) may be 5% or less. In this case, the rolling direction (RD direction) may be the direction in which the grain length is longest on the ND plane or the direction in which the tensile strength (TS) is lowest during uniaxial tension. The rolling perpendicular direction (TD) may be a direction perpendicular to the rolling direction on the rolling plane.

[0045] However, if the heat treatment temperature is not properly controlled, equiaxed recrystallized grains are formed at some grain boundaries long in the rolling direction, where the ratio of the length in the rolling direction (RD) to the length in the rolling perpendicular direction (TD) is too small. Since the dislocation density within these equiaxed recrystallized grains is very low, the strength of the material may be reduced. The grains can be determined by observing the cross-section of the steel sheet under a microscope and measuring the longest length in the rolling direction and the longest length in the rolling perpendicular direction among the grains, respectively, and the value is 1 or greater.

[0046] A cold-rolled steel sheet according to one embodiment of the present invention may have an area ratio of precipitates of 0.02 to 0.04%. In addition, the average particle size of the precipitates is 20 nm or less. In the present invention, precipitates play a role in weakening the driving force for recrystallization in steel. However, if precipitates that are too coarse with an average of 20 nm or more are formed, their number is small and they cannot sufficiently exert a recrystallization delay effect, so the average particle size is limited to 20 nm or less.

[0047] The area ratio is also limited to an upper limit because if it is 0.02 or less, a sufficient amount of precipitates cannot be formed to exhibit the recrystallization delay effect, and if it is 0.04% or more, it may instead increase strength or cause defects.

[0048] Precipitates can be determined by measuring the cross-section of the steel plate with a transmission electron microscope and performing compositional analysis and image analysis using EDS. At this time, at least 30 images must be taken at random locations from photographs of 1,000 nm x 1,000 nm or larger, and then image analysis must be performed to eliminate statistical errors.

[0049] A cold-rolled steel sheet according to one embodiment of the present invention may have a yield strength of 550 to 750 MPa, a tensile strength of 550 to 750 MPa, and an elongation of 5 to 15%.

[0050] 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.

[0051] Elongation can be measured in the uniaxial tensile test described above.

[0052] More specifically, a cold-rolled steel sheet according to one embodiment of the present invention may have a yield strength of 600 to 730 MPa, a tensile strength of 640 to 730 MPa, and an elongation of 7.0 to 14.0%.

[0053]

[0054] A method for manufacturing a cold-rolled steel sheet 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: 0.0015 to 0.0050%, Mn: 0.10 to 0.50%, Al: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Cu: 0.02 to 0.10%, B: 0.0005 to 0.0030%, P: 0.04 to 0.08%, and the remainder being Fe and unavoidable impurities; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; and an annealing step for the cold-rolled steel sheet.

[0055] 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.

[0056] 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.

[0057] At this time, the slab heating temperature may be 1150 to 1250°C. If the slab heating temperature is too low, the rolling load in the subsequent hot rolling process increases rapidly, which can impair 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 will also increase, leading to material loss. More specifically, it may be 1180 to 1230°C.

[0058] After that, the heated slab is hot-rolled to produce a hot-rolled steel sheet.

[0059] At this time, the finishing hot rolling temperature may be 920 to 980°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 930 to 960°C.

[0060] Afterward, the hot-rolled steel sheet manufactured after hot rolling undergoes a coiling process. More specifically, it may be a hot-rolled coiling process.

[0061] 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.

[0062] 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.

[0063] The thickness of the hot-rolled steel sheet can be 1.0 to 5.5 mm.

[0064] The coiled hot-rolled steel sheet may additionally include a step of pickling the steel sheet before cold rolling.

[0065] Afterwards, the coiled hot-rolled steel sheet is manufactured into a cold-rolled steel sheet through cold rolling.

[0066] The cold reduction ratio can be 30 to 70%. 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, internal stress is formed through cold rolling and acts as energy for recrystallization, which reduces the recrystallization temperature delay effect. More specifically, the cold reduction ratio can be 35 to 65%.

[0067] Next, the cold-rolled steel sheet is annealed during the cold-rolled sheet annealing stage. At this time, the cracking temperature may be 600 to 700°C. If the cracking temperature is too low, too much dummy material is required when connecting with other product groups that utilize recrystallization, and the degree of elongation recovery during the heat treatment process is reduced; however, if the cracking temperature is too high, recrystallization occurs, which can cause the strength of the steel itself to drop sharply. More specifically, the cracking temperature may be 620°C to 700°C.

[0068]

[0069] 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.

[0070]

[0071] Examples

[0072] A slab was manufactured through a converter, secondary refining, and continuous casting process using an alloy composition containing the composition of 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 and then hot-rolled. At this time, the finish hot-rolling temperature was controlled to 930°C and the coiling temperature to 650°C, and the final thickness was 1.1 mm.

[0073] The hot-rolled specimens were cold-rolled at a reduction rate of 60% after removing the oxide film on the surface through pickling. Afterward, they were annealed at 680°C.

[0074] In addition, the grain ratio of cold-rolled steel sheets was measured using an optical microscope and an image analyzer and summarized in Table 2.

[0075] In addition, the precipitate area ratio and average particle size were measured using a transmission electron microscope, and the results were summarized in Table 2.

[0076] Yield strength, tensile strength, and elongation: Measured using a uniaxial tensile test with a JIS No. 5 specimen.

[0077] Steel grade (wt%) CMnAlNbCuBP10.0020.200.0250.0300.070.00100.07020.0040.200.0250.0300.070.00100.07030.0020.150. 0250.0300.070.00100.07040.0020.450.0250.0300.070.00100.07050.0020.200.0250.0120.070.00100.07060.0020 .200.0250.0450.070.00100.07070.0020.200.0250.0300.030.00100.07080.0020.200.0250.0300.060.00100.07090.0020.200.0250.0300.070.00050.070100.0020.200.0250.0300.070.00280.070110.0020.200.0250.0300.070.0010 0.050120.0020.200.0250.0300.070.00100.075130.0080.200.0250.0300.070.00100.070140.0020.080.0250.0300.070.00100.070150.0020.600.0250.0300.070.00100.070160.0020.200.0250.0080.070.00100.070170.0020.200.02 50.0560.070.00100.070180.0020.200.0250.0300.010.00100.070190.0020.200.0250.0300.120.00100.070200.0020.200.0250.0300.070.00030.070210.0020.200.0250.0300.070.00100.010220.0020.200.0250.0300.070.00100.120

[0078] Steel Grade Average Precipitate Area (%) Average Precipitate Grain Size (nm) Aspect Ratio 1.5-3 Area Ratio (%) Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Recrystallization Initiation Temperature (°C) 10.0 30 120 640 686.7 11.0 710 20.0 371 90 65 570 2.8 10.5 715 30.0 30 133 620 665.2 12.5 690 40.0 35 180 680 729.6 8.5 720 50.0 2315 262 5670.6 11.8 69 560.0 391 30 64 56 92.111.073070.030130635681.311.269080.037170650697.410.073090.030134610654.513.5695100.037170630676.012.5725110.030135600643.814.069512 0.037170670718.99.5720130.042220730783.34.8710140.030132530568.716.0680150.056350720772.55.5710160.0151022450482.811.0660170.0423207207 72.55.6730180.0251215480515.016.0670190.065420700751.16.5730200.0301313500536.514.0680210.0251211540579.415.5690220.030130750804.73.5730

[0079] As can be seen in Tables 1 and 2, when the steel composition is properly controlled, it can be confirmed that the yield strength, tensile strength, and elongation are all excellent.

[0080] As can be seen in steel grade 13, when the C content is high, the elongation becomes inferior, and when Mn is low or high, there was a problem in that the strength-elongation combination could not be secured. Looking at the influence of Nb in Comparative Examples 16 and 17, when Nb is added below the lower limit, the fraction of fine precipitates decreases, and consequently, the recrystallization temperature is lowered, causing partial recrystallization to occur; however, when Nb is added above the upper limit, the strength is high, resulting in an inferior strength-elongation combination. In the case of Cu, B, and P, the addition of CuS, B, or P raises the recrystallization temperature, but when added in small amounts, the strength decreases due to the drop in the recrystallization temperature.

[0081] 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. A cold-rolled steel sheet comprising, in weight%, C: 0.0015 to 0.0050%, Mn: 0.10 to 0.50%, Al: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Cu: 0.02 to 0.10%, B: 0.0005 to 0.0030%, P: 0.04 to 0.08%, and the remainder being Fe and unavoidable impurities.

2. In Paragraph 1, A cold-rolled steel sheet further comprising one or more of Si: 0.30 wt% or less, S: 0.050 wt% or less, N: 0.005 wt% or less, Ti: 0.010 wt% or less, and V: 0.010 wt% or less.

3. In Paragraph 1, Cold-rolled steel sheet having a length ratio of the rolling direction to the rolling perpendicular direction of 1.5 to 3 and an area ratio of grains of 15% or less.

4. In Paragraph 1, A cold-rolled steel sheet having an area ratio of precipitates of 0.02 to 0.04% and an average particle size of precipitates of 20 nm or less.

5. In Paragraph 1, Cold-rolled steel sheet having a yield strength of 550 to 750 MPa, a tensile strength of 550 to 750 MPa, and an elongation of 5 to 15%.

6. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab comprising, in weight%, C: 0.0015 to 0.0050%, Mn: 0.10 to 0.50%, Al: 0.01 to 0.05%, Nb: 0.01 to 0.05%, Cu: 0.02 to 0.10%, B: 0.0005 to 0.0030%, P: 0.04 to 0.08%, and the remainder being Fe and unavoidable impurities; A step of manufacturing a cold-rolled steel sheet by cold-rolling the above hot-rolled steel sheet and A method for manufacturing a cold-rolled steel sheet comprising a cold-rolled sheet annealing step for annealing the above cold-rolled steel sheet.

7. In Paragraph 6, A method for manufacturing a cold-rolled steel sheet in which the reduction rate in the step of manufacturing the cold-rolled steel sheet is 30 to 70%.

8. In Paragraph 6, A method for manufacturing a cold-rolled steel sheet in which the cracking temperature during the annealing step of the cold-rolled sheet is 600 to 700℃.