Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet having excellent chemical conversion treatability and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet having excellent chemical conversion treatability and a method for manufacturing the same. Background Art

[0002] Generally, before painting treatment of cold-rolled steel sheets, chemical conversion treatment is performed for the purpose of paint adhesion and temporary rust prevention. At this time, the chemical conversion treatment process is carried out in the order of alkaline degreasing - water washing - surface conditioner - phosphate treatment - water washing process. In order for the growth of a dense phosphate film, it is crucial to uniformly disperse and adsorb the surface conditioner in the material.

[0003] In addition, in order to improve chemical conversion treatability by improving the solutions used in the process and the base iron itself, research on reducing annealing oxides, improving surface roughness, etc. is also being actively carried out.

[0004] According to Patent Document 1, since there are differences in phosphate reactions depending on the thickness of the oxide film of the base iron, it is important to form a uniform oxide film.

[0005] In addition, according to Patent Document 2, an attempt is being made to improve the surface roughness of the base iron by an etching solution.

[0006] However, in the cold rolling process of steel sheets, without introducing additional processes, attempts to increase the adsorption amount of the surface conditioner of the steel sheet by changing conditions such as cold reduction rate and annealing temperature are insufficient.

[0007] [Prior Art Documents]

[0008] [Patent Documents]

[0009] (Patent Document 1) Korean Patent Publication No. 1998-0044917 (published on September 15, 1998)

[0010] (Patent Document 2) Korean Patent Publication No. 2022-0089430 (published on June 28, 2022) Summary of the Invention

[0011] (1) Technical Problems to be Solved

[0012] An object of an embodiment of the present invention is to provide a steel sheet and a method for manufacturing the same.

[0013] An object of an embodiment of the present invention is to provide a steel sheet having excellent chemical conversion treatability and a method for manufacturing the same.

[0014] The technical problems of the present invention are not limited to the above. Those of ordinary skill in the art can easily understand the additional technical problems of the present invention from the overall content of this specification.

[0015] (II) Technical Solution

[0016] According to an embodiment of the present invention, a steel plate is provided. By weight%, the steel plate contains carbon (C): 0.02 - 0.10%, silicon (Si): 0.03% or less, manganese (Mn): 0.1 - 0.4%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% or less, the balance being iron (Fe) and inevitable impurities, and a microstructure that, by area%, contains 89.00% or more ferrite, 1.00 - 5.00% cementite, and 11.00% or less pearlite. The number of the cementite is 30,000 / mm 2 or more.

[0017] The ratio of the major axis to the minor axis (major axis / minor axis) of the cementite may be 2.0 to 9.0.

[0018] The major axis length of the cementite may be 0.35 - 1.80 μm, and the minor axis length may be 0.20 - 0.50 μm.

[0019] The steel plate may have a surface roughness with a ratio of Rpm to Rz (Rpm / Rz) of 0.50 or more.

[0020] (Wherein, Rpm refers to the average of 5 consecutive measurement data from the center line of the highest peak within the reference length to Rp representing the height, and Rz refers to the average roughness of 10 points.)

[0021] The coverage rate after chemical conversion treatment of the steel plate may be 80% or more.

[0022] According to an embodiment of the present invention, a method for manufacturing a steel plate can be provided, which includes the following steps: reheating a steel billet that, by weight%, contains carbon (C): 0.02 - 0.10%, silicon (Si): 0.03% or less, manganese (Mn): 0.1 - 0.4%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% or less, the balance being iron (Fe) and inevitable impurities; hot rolling the reheated steel billet; coiling the hot-rolled steel plate; cold rolling the coiled steel plate with a cumulative reduction rate of 50 - 90%; annealing the cold-rolled steel plate within a temperature range of 700 - 780 °C; and starting to cool the annealed steel plate within a temperature range of 650 °C or more and cooling it at an average cooling rate of 15 - 20 °C / second until it reaches a temperature range of 200 - 400 °C.

[0023] The temperature of the reheating can be 1200°C or higher.

[0024] During the hot rolling, the finish rolling temperature can be 800 - 950°C.

[0025] The temperature of the coiling can be 500 - 650°C.

[0026] The reduction ratio during the cold rolling can be 50 - 70%.

[0027] (III) Advantageous Effects

[0028] According to an embodiment of the present invention, a steel plate and a manufacturing method thereof can be provided.

[0029] According to an embodiment of the present invention, a steel plate having excellent chemical conversion treatability and a manufacturing method thereof can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a microstructural photograph of Invention Example 5 according to an embodiment of the present invention.

[0031] Figure 2 is a microstructural photograph of Comparative Example 1 deviating from an embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

[0032] The following describes preferred specific embodiments of the present invention. The specific embodiments of the present invention can be modified into various forms, and the scope of the present invention should not be construed as being limited to the specific embodiments described below. These specific embodiments are provided to more specifically explain the present invention to those of ordinary skill in the technical field to which the present invention pertains.

[0033] The inventors of the present invention confirmed that by controlling the shape and distribution of fine cementite, during chemical conversion treatment, the chemical conversion treatability can be improved by increasing the adsorption force of the surface conditioner, thereby completing the present invention.

[0034] The following describes the present invention in detail.

[0035] The following describes the steel composition of the present invention in detail.

[0036] In the present invention, unless otherwise specifically stated, the % representing the content of each element is based on weight.

[0037] The steel plate according to an embodiment of the present invention contains, by weight %, carbon (C): 0.02 - 0.10%, silicon (Si): 0.03% or less, manganese (Mn): 0.1 - 0.4%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% or less, the balance being iron (Fe) and inevitable impurities.

[0038] Carbon (C): 0.02 - 0.10%

[0039] When the content of carbon (C) is less than 0.02%, the formation of secondary phases does not occur, and thus there may be a possibility of not generating local electrochemical grading phenomena caused by the desired microstructure differences. As an embodiment of the present invention, the content of carbon (C) can be 0.04% or more. On the other hand, when the content of carbon (C) exceeds 0.10%, excessive carbide may be formed, and thus a phenomenon of exceeding the desired strength may occur. As an embodiment of the present invention, the upper limit of the carbon (C) content can be 0.06%.

[0040] Silicon (Si): 0.03% or less

[0041] When the silicon (Si) content in steel is excessive, SiO2 may be formed on the steel surface, and Fe2SiO4, a composite phase of SiO2 and Fe oxide, may also be formed, thus possibly triggering a large amount of red scale. This red scale is difficult to remove during pickling after cold rolling, and during cold rolling annealing, it forms Si oxide, thus having the possibility of reducing acid reactivity during phosphate treatment. Therefore, in the present invention, the upper limit of the silicon (Si) content can be limited to 0.03%. As an embodiment of the present invention, the silicon (Si) content can be 0.02% or less. Additionally, considering the level inevitably added to the steel, except for 0%.

[0042] Manganese (Mn): 0.1 - 0.4%

[0043] Manganese (Mn) is a typical element that forms oxides on the surface during the annealing heat treatment process of cold-rolled steel sheets. However, manganese (Mn) is also an element that forms Mn-Si-based composite oxides that are difficult to remove during pickling during hot rolling and cold rolling annealing. In the present invention, since the silicon (Si) content is controlled to be 0.03% or less, it is not an environment where a large amount of Si oxide can be formed, but the upper limit of the manganese (Mn) content can be limited to 0.4%. According to an embodiment of the present invention, the manganese (Mn) content can be 0.3% or less. Additionally, when the manganese (Mn) content is too low, the formation of stable Si oxide is induced, thus having the possibility of hindering acid reactivity. Therefore, the lower limit of the manganese (Mn) content can be limited to 0.1%. According to an embodiment of the present invention, the lower limit of the manganese (Mn) content can be 0.2%.

[0044] Phosphorus (P): 0.02% or less

[0045] Phosphorus (P) is a solid solution strengthening element, but when too much phosphorus (P) is added, brittleness of the steel may occur. Therefore, the upper limit of the phosphorus (P) content can be limited to 0.02%. Additionally, excluding 0%, considering the levels that are inevitably added to the steel.

[0046] Sulfur (S): 0.003% or less

[0047] Sulfur (S) is an impurity element in the steel, which may hinder the ductility and weldability of the steel. Therefore, the upper limit of the sulfur (S) content can be limited to 0.003%. Additionally, excluding 0%, considering the levels that are inevitably added to the steel.

[0048] The steel of the present invention, in addition to the above components, may contain the balance of iron (Fe) and inevitable impurities. Inevitable impurities may be inadvertently mixed in during the normal manufacturing process, so these impurities cannot be excluded. These impurities are well known to those skilled in the art of ordinary steel manufacturing, so all of their details are not particularly mentioned in this specification.

[0049] Hereinafter, the fine microstructure of the steel of the present invention will be described in detail.

[0050] In the present invention, unless otherwise specifically stated, the % representing the fraction of the fine microstructure is based on area.

[0051] The fine microstructure of the steel plate according to an embodiment of the present invention, in area %, may contain 89.00% or more of ferrite, 1.00 - 5.00% of cementite, and 11.00% or less of pearlite.

[0052] In the present invention, in order to increase the adsorption amount of the surface conditioner, 1.00% or more of cementite may be contained. Within the carbon range proposed in the present invention, the fraction of cementite that can theoretically be formed can be about 2.00% or so. Therefore, the lower limit of the cementite area fraction can be limited to 2.00%. When manufacturing steel, the fraction of cementite can be increased by controlling the cooling rate, but in the present invention, the upper limit of the cementite can be limited to 5.00%.

[0053] Additionally, in order to contain a certain level or more of cementite, in the present invention, pearlite can be limited to 11.00% or less. According to an embodiment of the present invention, the pearlite can be limited to 4.00% or less.

[0054] According to the report of Parkerizing in Japan (Surface Technology (Japan), 2010), in the chemical conversion treatment step, large differences are shown in terms of phosphate grain size and coverage depending on whether surface conditioning treatment is performed. Therefore, it can be known that surface conditioning treatment is necessary. As a surface conditioner, Na4TiO(PO4)2 hydrate in the form of a disc with a layer-layer spacing of several Å is usually used. However, when this particle adsorbs on the surface of the base iron, micro-cells are formed, increasing the starting points of base iron etching and film precipitation. For this reason, it is necessary to increase the number of active points on the surface of the metal. When the number of active points increases, the number of crystal nuclei increases during chemical conversion treatment, and as a result, a fine and uniform phosphate crystal film can be formed.

[0055] The number of cementite according to an embodiment of the present invention can be 30,000 / mm 2 or more.

[0056] In the present invention, in order to increase the adsorption amount of the surface conditioner, a large amount of cementite can be formed. When the number of cementite is less than 30,000 / mm 2 (per unit area), fine cementite cannot be formed within the grains, resulting in the problem that the above effects cannot be obtained. As an embodiment of the present invention, the number of cementite can be 50,000 / mm 2 or less.

[0057] The major axis length of the cementite of the steel plate according to an embodiment of the present invention can be 0.35 - 1.80 μm, and the minor axis length can be 0.20 - 0.50 μm.

[0058] The cementite according to an embodiment of the present invention can be formed in a rod shape. Such rod-shaped cementite can have the effect of facilitating the adsorption of the surface conditioner onto the surface of the steel plate.

[0059] When the major axis length of the cementite is less than 0.35 μm, there may be a problem that it is difficult for the surface conditioner particles to adsorb. According to an embodiment of the present invention, the major axis length of the cementite can be 0.45 μm or more. On the other hand, when the major axis length of the cementite exceeds 1.80 μm, since the total number of the generated cementite decreases, there is a problem of reduced continuity.

[0060] In addition, when the minor axis length of the cementite is less than 0.20 μm, there may be a problem that it is difficult for the surface conditioner particles to adsorb. According to an embodiment of the present invention, the minor axis length of the cementite can be 0.25 μm or more. On the other hand, when the minor axis length exceeds 0.50 μm, there is a problem of reduced continuity due to the decrease in the total number of the generated cementite. According to an embodiment of the present invention, the minor axis length of the cementite can be 0.45 μm or less.

[0061] According to an embodiment of the present invention, the ratio of the major axis to the minor axis (major axis / minor axis) of the cementite can be 2.0 to 9.0.

[0062] In the present invention, an attempt is made to enhance the effect by controlling both the major axis length and the minor axis length of the cementite, and at the same time, controlling the ratio of the major axis to the minor axis (major axis / minor axis) of the cementite. The ratio of the major axis to the minor axis (major axis / minor axis) of the cementite can represent the shape in which the surface conditioner can be uniformly adsorbed, and in the present invention, the phosphate coverage rate can be increased by controlling the ratio of the major axis to the minor axis.

[0063] When the ratio of the major axis to the minor axis of the cementite is less than 2.0, it tends to be square, thus having a problem that is disadvantageous for the adsorption of the surface conditioner in terms of structure and geometry. On the other hand, when the ratio of the major axis to the minor axis of the cementite exceeds 9.0, the cementite may form a layered structure together with the ferrite rather than existing alone. When the ratio of the major axis to the minor axis of the cementite is in the range of 2.0 to 9.0, the fine cementite can be uniformly dispersed separately at a predetermined interval. According to an embodiment of the present invention, the ratio of the major axis to the minor axis of the cementite can be 2.2 or more. According to an embodiment of the present invention, the ratio of the major axis to the minor axis of the cementite can be 8.0 or less.

[0064] A more preferable steel plate according to an embodiment of the present invention can have a surface roughness with a ratio of Rpm to Rz (Rpm / Rz) of 0.50 or more. In addition, the coverage rate after the chemical conversion treatment can be 80% or more, thereby having excellent chemical conversion treatability.

[0065] In the present invention, there are various parameters representing the fine roughness, but in the present invention, the center line of the highest peak within the reference length to Rp representing the height and Rpm obtained by adding and averaging five consecutive measurement data of Rp are used as the reference. More specifically, a relatively small Rpm means wide peaks and narrow valleys, and a relatively large Rpm means a sparsely spiky surface.

[0066] Therefore, in the present invention, the shape of the material cross-section can be more clearly and quantitatively determined by the Rz representing the 10-point average roughness and the ratio of Rpm to Rz (Rpm / Rz). In the present invention, when the ratio of Rpm / Rz is 0.50 or more as a reference, it can be regarded as the sharp-ridged type, and when the ratio of Rpm / Rz is less than 0.50 as a reference, it can be regarded as the round-ridged type.

[0067] Therefore, it is preferable that Rpm is small and the ratio of Rpm / Rz is 0.5 or more in the present invention. The value of Rpm / Rz has no maximum limit for the desired large characteristic, but considering the current technical and economic characteristics of steel plate manufacturing, Rpm / Rz can be 1.00 or less.

[0068] Hereinafter, the manufacturing method of the steel of the present invention will be described in detail.

[0069] The steel plate according to an embodiment of the present invention can be manufactured by reheating, hot rolling, coiling, cold rolling, annealing, and cooling a steel slab that satisfies the above alloy composition.

[0070] Reheating

[0071] The steel slab that satisfies the alloy composition of the present invention can be reheated to a temperature range of 1200 °C or higher.

[0072] In order to redissolve most of the precipitates present in the steel, it can be reheated to a temperature of 1200 °C or higher. As an embodiment of the present invention, the reheating temperature can be 1250 °C or higher.

[0073] Hot rolling

[0074] The reheated steel slab can be hot rolled at a finish rolling temperature of 800 - 950 °C.

[0075] During hot rolling, when the finish rolling temperature is lower than 800 °C, hot rolling is terminated in a relatively low-temperature region, so there may be problems of reduced workability and rollability. As an embodiment of the present invention, the finish rolling temperature can be 850 °C or higher. On the other hand, when the finish rolling temperature exceeds 950 °C, uniform hot rolling cannot be achieved throughout the thickness, resulting in a problem of insufficient grain refinement. As an embodiment of the present invention, the upper limit of the finish rolling temperature can be 930 °C.

[0076] Coiling

[0077] The hot-rolled steel plate can be coiled in a temperature range of 500 - 650 °C.

[0078] The coiling temperature can affect the fraction of phases such as cementite other than ferrite. The higher the coiling temperature, the higher the fraction of cementite may be. In the present invention, in order to form the desired level of cementite, coiling can be carried out in a temperature range above 500 °C. Additionally, in order to ensure the desired level of physical properties of the present invention, the upper limit of the coiling temperature can be limited to 650 °C.

[0079] In the present invention, the cooling conditions from after hot rolling to the coiling temperature are not particularly limited, and cooling can be carried out under conventional conditions applied in the same technical field. As an embodiment of the present invention, air cooling can be carried out.

[0080] Cold rolling

[0081] The coiled steel sheet can be cold rolled with a cumulative reduction ratio of 50 - 90%.

[0082] In the present invention, the cumulative reduction ratio can be expressed as the ratio of the difference in thickness between the hot-rolled material and the cold-rolled material to the thickness of the hot-rolled material. According to an embodiment of the present invention, from the aspect of micro-roughness, the lower the reduction ratio, the more advantageous. However, when the reduction ratio is less than 50%, due to inaccurate control of the roll and tension, sheet distortion may occur. On the other hand, when the reduction ratio exceeds 90%, due to the load on the roll, it may not be possible to produce the product. According to an embodiment of the present invention, in order to more effectively control the roughness, the reduction ratio can be limited to 80% or less. According to an embodiment of the present invention, the reduction ratio can be limited to 70% or less.

[0083] Annealing

[0084] The cold-rolled steel sheet can be annealed in a temperature range of 700 - 780 °C.

[0085] During annealing, due to the formation of oxides caused by the surface enrichment of oxygenophilic elements such as Mn, Al, and Si, there is a possibility of reducing the phosphate reaction during chemical conversion treatment. Therefore, the annealing temperature can be limited to 780 °C or less. On the other hand, when the annealing temperature is lower than 700 °C, recrystallization is not completed, and thus there is a possibility of not ensuring the desired material properties.

[0086] Cooling

[0087] The annealed steel sheet can be cooled starting from a temperature range above 650 °C and at an average cooling rate of 15 - 20 °C / second, and cooled to a temperature range of 200 - 400 °C.

[0088] Upon cooling, in order to supersaturate the carbide and precipitate fine cementite, the cooling rate can be controlled. When the average cooling rate is less than 15 °C / second, it may not be easy to precipitate fine cementite. On the other hand, when the average cooling rate exceeds 20 °C / second, there is a problem of being difficult to achieve due to equipment load.

[0089] When the cooling start temperature is lower than 650 °C, a large amount of pearlite phase transformation has occurred, and thus there may be a problem that the precipitation of fine cementite is restricted.

[0090] In addition, upon cooling, when the termination temperature is lower than 200 °C, part of the austenite transforms into martensite, resulting in a problem of exceeding the desired material. On the other hand, when the termination temperature exceeds 400 °C, there is a problem that fine cementite cannot be formed.

[0091] In the present invention, in order to homogenize the structure of the steel plate to the temperature at which the desired cooling starts after annealing, slow cooling can be performed. The slow cooling conditions are not particularly limited, and slow cooling can be performed by a conventional method. Detailed implementation mode

[0092] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and are not used to limit the scope of the rights of the present invention.

[0093] (Example)

[0094] Prepare a steel billet which, by weight percentage, has C: 0.049%, Si: 0.005%, Mn: 0.3%, P: 0.0126%, S: 0.006%, and the balance Fe, and manufacture a steel plate under the conditions shown in Table 1 below.

[0095] [Table 1]

[0096]

[0097] As shown in Table 2 below, the microstructure and physical properties of the manufactured steel plate were measured and shown. First, the microstructure fraction, the number of cementite, the major axis length, and the minor axis length of the manufactured steel plate were measured and shown, and the major axis / minor axis ratio was calculated. The microstructure fraction was measured using an optical microscope after being installed along the surface direction of the steel plate. The fraction of cementite was prepared by etching the surface of the steel plate with Picral etchant (2 - 4 g of picric acid, 100 ml of ethanol), and then the microstructure was photographed with a scanning electron microscope at a magnification of ×1000, and then measured using Image Analyzer software. In addition, the shape of fine cementite was specified by color in the Image Analyzer software, and the major axis length and minor axis length of the fine cementite that became colored were measured, and the average value thereof was shown.

[0098] The surface roughness is represented by Rpm, which is obtained by adding and averaging five consecutive measurement data of Rp representing the height from the center line of the highest peak within the reference length. The Rz representing the 10 - point average roughness of Rz was measured and shown.

[0099] In addition, the manufactured test piece was subjected to chemical conversion treatment in the order of degreasing - water washing 1 - surface conditioning - phosphate treatment - water washing 2. The test piece subjected to the chemical conversion treatment was observed with a scanning electron microscope at a magnification of 150, and the phosphate coverage area was calculated using Image Analyzer software and shown in Table 3 below. The specific chemical conversion treatment conditions are as follows, and the phosphate coverage rate is as described below, and is represented by grades 1 to 5 in ascending order from low value to high value.

[0100] ○ Chemical conversion treatment

[0101] - Degreasing: FC - 4460A 20 g / L, FC - 4460B 12 g / L (DAE HAN PARKERIZING CO., LTD), treatment time is 90 seconds, temperature is 60 °C

[0102] - Water washing 1: Treatment time is 10 seconds, normal temperature

[0103] - Surface conditioning: PL - Z 5 g / L (DAE HAN PARKERIZING CO., LTD), concentration is pH 7.5 - 11, treatment time is 10 - 20 seconds, normal temperature

[0104] - Phosphate treatment: PB-3111 28.2 g / L, NT-4055 5.8 g / L (DAE HAN PARKERIZING CO., LTD), free acidity (FA) / total acidity (TA) are 1.1 - 1.5 / 11.1 - 11.8 respectively, treatment time is 40 seconds, and the temperature of the phosphate treatment solution is 40 - 45 °C

[0105] - Water washing 2: Treatment time is 10 seconds, at room temperature

[0106] - Coverage rate standard

[0107] 1: Above 50%, less than 60%

[0108] 2: Above 60%, less than 70%

[0109] 3: Above 70%, less than 80%

[0110] 4: Above 80%, less than 90%

[0111] 5: Above 90%

[0112] [Table 2]

[0113]

[0114] F: Ferrite, P: Pearlite, C: Cementite, M: Martensite

[0115] As shown in Table 2, in the case of the inventive examples that meet the conditions of the present invention, the fine microstructure characteristics proposed by the present invention are satisfied, and the physical properties desired by the present invention can also be ensured.

[0116] Figure 1 is the microstructural photograph of Inventive Example 5 according to an embodiment of the present invention. As Figure 1 shown, it can be confirmed that a large amount of fine cementite is formed in the ferrite matrix structure.

[0117] On the other hand, Comparative Examples 1 to 7 are examples that do not reach the cooling rate range proposed by the present invention. As a result, the cementite density desired by the present invention cannot be satisfied. In particular, the ratio of the major axis to the minor axis of the cementite in Comparative Examples 2 to 4 is outside the scope of the present invention.

[0118] Figure 2 is the microstructural photograph of Comparative Example 1 that deviates from an embodiment of the present invention. Compared with Figure 1 it, it can be confirmed that Figure 2 a small amount of cementite is formed.

[0119] Comparative Example 8 is an example where the cooling rate exceeds the scope of the present invention. Also, the cooling termination temperature is out of the scope of the present invention and is cooled to an excessively low temperature. As a result, martensite is formed as the fine structure, and the amount of cementite is also insufficient, resulting in poor phosphate coverage.

[0120] Comparative Example 9 and Comparative Example 10 are examples where the annealing temperature exceeds the temperature range proposed by the present invention, and the desired level of cementite cannot be formed, and its shape also does not meet the conditions of the present invention. As a result, the chemical conversion treatability is poor.

[0121] The present invention has been described in detail above through examples, but the present invention can also be implemented in other forms of examples. Therefore, the examples do not limit the technical idea and scope of the claims described in the present invention.

Claims

1. A steel plate, by weight %, the steel plate contains carbon (C): 0.02 - 0.10%, silicon (Si): below 0.03%, manganese (Mn): 0.1 - 0.4%, phosphorus (P): below 0.02%, sulfur (S): below 0.003%, the balance being iron (Fe) and inevitable impurities, The microstructure, by area %, contains ferrite of 89.00% or more, cementite of 1.00 - 5.00% and pearlite of 11.00% or less. The number of the cementite is 30,000 / mm 2 or more.

2. The steel plate according to claim 1, wherein, The ratio of the major axis to the minor axis of the cementite, i.e., major axis / minor axis, is 2.0 to 9.

0.

3. The steel plate according to claim 1, wherein, The length of the major axis of the cementite is 0.35 - 1.80 μm, and the length of the minor axis is 0.20 - 0.50 μm.

4. The steel plate according to claim 1, wherein, The steel plate has a surface roughness with a ratio of Rpm to Rz, i.e., Rpm / Rz, of 0.50 or more. Wherein, Rpm refers to the average of 5 consecutive measurement data from the center line of the highest peak within the reference length to Rp representing the height, and Rz refers to the average roughness of 10 points.

5. The steel plate according to claim 1, wherein, The coverage rate after chemical conversion treatment of the steel plate is 80% or more.

6. A method for manufacturing a steel plate, which includes the following steps: Reheat the steel billet. By weight %, the steel billet contains carbon (C): 0.02 - 0.10%, silicon (Si): below 0.03%, manganese (Mn): 0.1 - 0.4%, phosphorus (P): below 0.02%, sulfur (S): below 0.003%, the balance being iron (Fe) and inevitable impurities; Hot roll the reheated steel billet; Coil the hot - rolled steel plate; Cold roll the coiled steel plate with a cumulative reduction ratio of 50 - 90%; Anneal the cold - rolled steel plate within the temperature range of 700 - 780 °C; And Start cooling the annealed steel plate within the temperature range above 650 °C and cool it at an average cooling rate of 15 - 20 °C / second until the temperature range of 200 - 400 °C.

7. The method for manufacturing a steel plate according to claim 6, wherein, The temperature of the reheating is 1200 °C or more, During the hot rolling, the finish rolling temperature is 800 - 950 °C, The temperature of the coiling is 500 - 650 °C.

8. The manufacturing method of the steel plate according to claim 6, wherein, The reduction ratio during the cold rolling is 50 - 70%.

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