STEEL SHEET AND METHOD FOR MANUFACTURING SAME

The present invention pertains to a steel sheet and a method for manufacturing same. More specifically, the present invention pertains to a steel sheet having excellent chemical conversion treatability and a method for manufacturing same.

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Description
TECHNICAL FIELD

The present invention relates to a steel sheet and a method for manufacturing the same.

BACKGROUND ART

Normally, a cold rolled steel sheet is subject to a chemical conversion treatment before painting for the purpose of coating adhesion and temporary rust prevention. In this case, the chemical conversion treatment process is carried out in the order of alkaline degreasing-washing-surface activator-phosphate treatment-washing, and uniform dispersion and adsorption of a surface activator on a material is important for the growth of a dense phosphate film.

In the meantime, research has been actively conducted on reducing annealing oxides and improving surface roughness for the purpose of improving the chemical conversion treatment through the improvement of an original base steel material as well as the solution used in the process.

According to Patent Document 1, since the phosphate reaction differs depending on a thickness of an oxide film of a base steel material, it is important to form a uniform oxide film.

In addition, according to Patent Document 2, efforts have been made to improve the surface roughness of the steel sheet using an etchant.

However, there has been little attempt to increase an adsorption amount of a surface activator on the steel sheet by changing a cold reduction rate and annealing temperature conditions without introducing an additional process during the cold rolling process of the steel sheet.

PRIOR ART DOCUMENT Patent Document

    • (Patent Document 1) Korean Patent Publication No. 1998-0044917 (published on Sep. 15, 1998)
    • (Patent Document 2) Korean Patent Publication No. 2022-0089430 (published on Jun. 28, 2022)

DISCLOSURE OF INVENTION Technical Problem

An embodiment of the present invention is to provide a steel sheet and a method for manufacturing the same.

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

The aspects of the present invention are not limited to the above-described content. Those skilled in the art will have no difficulty in understanding additional aspects of the present invention from the overall content of this specification.

Solution to Problem

According to an embodiment of the present invention, provided is a steel sheet comprising: by wt %, carbon (C): 0.02 to 0.10%, silicon (Si): 0.03% or less, manganese (Mn): 0.1 to 0.4%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% or less, and a balance of iron (Fe) and inevitable impurities,

    • wherein a microstructure comprises, in area %, ferrite of 89.00% or more, cementite of 1.00 to 5.00%, and pearlite of 11.00% or less, and
    • the number of cementite is 30,000 grains/mm2 or more.

A ratio of a major axis to a minor axis of the cementite (major axis/minor axis) may be 2.0 to 9.0.

A major axis length of cementite is 0.35 to 1.80 μm, and a minor axis length thereof may be 0.20 to 0.50 μm.

The steel sheet may have a surface roughness in which a ratio of Rpm to Rz (Rpm/Rz) is 0.50 or more.

(Here, Rpm refers to an average of five consecutive measurement data of Rp, which means a height from a center line of the highest peak within a reference length, and Rz refers to an average roughness at 10 points.)

A coverage rate after a chemical conversion treatment of the steel sheet may be 80% or more.

According to an embodiment of the present invention, provided is a method for manufacturing a steel sheet comprising: reheating a steel slab comprising, by wt %, carbon (C): 0.02 to 0.10%, silicon (Si) 0.03% or less, manganese (Mn): 0.1 to 0.4%, phosphorus (P) 0.02% or less, sulfur (S): 0.003% or less, and a balance of iron (Fe) and inevitable impurities;

    • hot-rolling the reheated steel slab;
    • coiling the hot-rolled steel sheet;
    • cold-rolling the coiled steel sheet at a cumulative reduction ratio of 50 to 90%;
    • annealing the cold-rolled steel sheet at a temperature within a range of 700 to 780° C.; and
    • cooling the annealed steel sheet from a start temperature within a range of 650° C. or higher to a temperature within a range of 200 to 400° C. at an average cooling rate of 15 to 20° C./s.

The reheating temperature may be 1200° C. or higher.

A finishing rolling temperature during the hot rolling may be 800 to 950° C.

A coiling temperature may be 500 to 650° C.

A reduction ratio may be 50 to 70% during the cold rolling.

Advantageous Effects of Invention

According to an embodiment of the present invention a steel sheet and a method for manufacturing the same may be provided.

According to an embodiment of the present invention, a steel sheet having excellent chemical conversion treatability and a method for manufacturing the same may be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a microstructure photograph of Inventive Example 5 according to an embodiment of the present invention.

FIG. 2 is a microstructure photograph of Comparative Example 1, which deviates from an embodiment of the present invention.

 BEST MODE FOR THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention belongs.

The inventors of the present invention have confirmed that chemical conversion treatment properties may be improved by controlling the shape and distribution of fine cementite to increase adsorption power of a surface activator during a chemical conversion treatment, and have completed the present invention.

The present invention will be described in detail below.

A steel composition of the present invention will be described in detail below.

Unless otherwise specifically stated in the present invention, % indicating the content of each element is based on weight.

A steel sheet according to an embodiment of the present invention may include, by wt %, carbon (C): 0.02 to 0.10%, silicon (Si): 0.03% or less, manganese (Mn): 0.1 to 0.4%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% or less, and a balance of iron (Fe) and inevitable impurities.

Carbon (C): 0.02 to 0.10%

When the content of carbon (C) is less than 0.02%, because the formation of a secondary phase does occur, there is a concern that a local electrochemical polarization phenomenon due to a desired microstructural difference may not occur. In an embodiment of the present invention, the content of carbon (C) may be 0.04% or more. On the other hand, when the content thereof exceeds 0.10%, a phenomenon of exceeding the desired strength may occur due to excessive carbide formation. In an embodiment of the present invention, an upper limit of the carbon (C) content may be 0.06%.

Silicon (Si): 0.03% or less

When the content of silicon (Si) in the steel is excessive, SiO2 may be formed on a steel surface, and Fe2SiO4, a composite phase of SiO2 and a Fe oxide, may also be formed, and a large amount of red scale may be induced. The red scale may be difficult to remove during pickling after cold rolling, and during cold rolling annealing, the red scale may be also formed as a Si oxide, which may reduce acid reactivity during phosphate treatment. Accordingly, in the present invention, an upper limit of the silicon (Si) content may be limited to 0.03%. In an embodiment of the present invention, the content thereof may be 0.02% or less. Meanwhile, considering a level that is inevitably added to the steel, 0% is excluded.

Manganese (Mn): 0.1 to 0.4%

Manganese (Mn) is an element that typically forms oxides on a surface during the annealing heat treatment of a cold rolled steel sheet. However, during hot rolling and cold rolling annealing, manganese (Mn) is also an element that forms Mn—Si complex oxides not easily removed during the pickling process. In the present invention, the content of Si may be controlled to be 0.03% or less, and thus, an environment in which a large amount of Si oxides may be formed is not provided, an upper limit of the manganese (Mn) content may be limited to 0.4%. According to an embodiment of the present invention, the content thereof may be 0.3% or less. Meanwhile, when the content of manganese (Mn) is excessively low, this may cause the formation of stable Si oxides, thereby inhibiting acid reactivity. Accordingly, a lower limit of the manganese (Mn) content may be limited to 0.1%. According to an embodiment of the present invention, the lower limit may be 0.2%.

Phosphorus (P): 0.02% or less

Phosphorus (P) is a solid solution strengthening element, but when phosphorus (P) is excessively added, This may cause brittleness of the steel, so that an upper limit of the content may be limited to 0.02%. Meanwhile, considering a level that is inevitably added to the steel, 0% is excluded.

Sulfur (S): 0.003% or less

Sulfur (S) is an impurity element in the steel, and since the ductility and weldability of the steel may be inhibited, an upper limit thereof may be limited to 0.003%. Meanwhile, considering a level that is inevitably added during the steel, 0% is excluded.

The steel of the present invention may include the remaining iron (Fe) and inevitable impurities in addition to the composition described above. Since inevitable impurities may be unintentionally incorporated during the normal manufacturing process, the impurities may not be excluded. Since the impurities are known to those skilled in the art of normal steel manufacturing, not all of the contents are specifically mentioned in this specification.

Hereinafter, the microstructure of the steel of the present invention will be described in detail.

Unless specifically stated otherwise in the present invention, % indicating the fraction of the microstructure is based on the area.

A microstructure of the steel sheet according to an embodiment of the present invention may include, in area %, ferrite of 89.00% or more, cementite of 1.00 to 5.00%, and pearlite of 11.00% or less.

In the present invention, cementite may be included in an amount of 1.00% or more in order to increase the amount of surface activator adsorption. The theoretically producible cementite fraction within the carbon range proposed in the present invention may be approximately 2.00%, so that a lower limit of the cementite area fraction may be limited to 2.00%. During steel manufacturing, the fraction may be increased by controlling a cooling rate, but in the present invention, an upper limit may be limited to 5.00%.

Meanwhile, in order to include cementite at a certain level or more, the pearlite may be limited to 11.00% or less in the present invention. According to an embodiment of the present invention, the content thereof may be limited to 4.00% or less.

Meanwhile, according to a report by Nihon Parkerizing (Surface Technology (Japan), 2010), there is a large difference in a phosphate crystal grain size and a coverage rate depending on whether or not surface adjustment treatment is performed during a chemical conversion treatment operation, so that it may be seen that surface adjustment treatment is essential. As a surface activator, Na4TiO(PO4)2 hydrate having a disc-shaped layer-layer gap of several A may be usually used, and the particles may form micro-cells after being adsorbed on a surface of a base steel material, thereby increasing a starting point of etching and film deposition of the base steel material. For this reason, it may be necessary to increase the number of active sites on a surface of a metal, and when the number of active sites increases, the number of crystal nuclei increases during the chemical conversion treatment, resulting in the deposition of a fine and uniform phosphate crystal film.

The number of cementite according to an embodiment of the present invention may be 30,000 ea/mm2 or more.

In the present invention, cementite may be formed in large quantities to increase the amount of surface activator adsorption. When the cementite is less than 30,000 per mm2 unit area, fine cementite may not be formed in the grains, which may cause a problem in that the above-described effect may not be secured. In an embodiment of the present invention, the cementite may be 50,000 ea/mm2 or less.

According to an embodiment of the present invention, in the steel sheet, a major axis length of the cementite may be 0.35 to 1.80 μm, and a minor axis length thereof may be 0.20 to 0.50 μm.

According to an embodiment of the present invention, the cementite may be formed in a rod shape. Such rod-shaped cementite may have an effect of facilitating surface activator adsorption on a surface of the steel sheet.

When the major axis length of the cementite is less than 0.35 μm, there may be a problem that it may be difficult to absorb surface activator particles. According to an embodiment of the present invention, the major axis length thereof may be 0.45 μm or more. On the other hand, when the major axis length exceeds 1.80 μm, there may be a problem that the total number of cementite grains produced may decrease and the continuity may decrease.

Additionally, when the minor axis length of the cementite is less than 0.20 μm, there may be a problem that it may be difficult to adsorb the surface activator particles. According to an embodiment of the present invention, the minor axis length thereof may be 0.25 μm or more. On the other hand, when the minor axis length exceeds 0.50 μm, there may be a problem that the total number of cementite produced may decrease and the continuity may decrease. According to an embodiment of the present invention, the minor axis length thereof may be 0.45 μm or less.

According to an embodiment of the present invention, a ratio of the major axis to the minor axis (major axis/minor axis) of the cementite may be 2.0 to 9.0.

In the present invention, in addition to controlling the lengths of the major and minor axes of cementite, the effect may be increased by controlling a ratio of the major axis to the minor axis of cementite (major axis/minor axis). The ratio of the major axis to the minor axis of cementite (major axis/minor axis) may mean a shape in which the surface activator may be uniformly adsorbed, and in the present invention, the phosphate coverage rate may be increased by controlling the ratio.

If the ratio of the major axis to the minor axis of cementite is less than 2.0, there may be a problem in that the surface activator adsorption may be structurally and geometrically disadvantageous because this converges to a square. On the other hand, when the ratio exceeds 9.0, there may be a problem in that cementite may be formed in a lamellar shape with ferrite rather than existing alone. In a ratio range of 2.0 to 9.0, fine cementite may be independently and uniformly dispersed at a certain interval from each other. According to an embodiment of the present invention, the ratio may be 2.2 or more. According to an embodiment of the present invention, the ratio may be 8.0 or less.

A more preferable steel sheet according to an embodiment of the present invention may have a surface roughness in which a ratio of Rpm to Rz (Rpm/Rz) is 0.50 or more. In addition, since the coverage rate after the chemical conversion treatment is 80% or more, this may have excellent chemical conversion treatment properties.

In the present invention, there are various parameters indicating microscale surface roughness, but in the present invention, Rp, which means a height from a center line of the highest peak within a reference length, and Rpm, which is the average of the five consecutive measurement data of Rp, are used as the standard. More specifically, a relatively small Rpm may mean wide peaks & narrow valleys, and a relatively large Rpm may mean a sparsely spiky surface.

Accordingly, in the present invention, the shape of a cross-section of a material may be more clearly and quantitatively determined through Rz, which indicates the 10-point average roughness, and a ratio of Rpm to Rz (Rpm/Rz) In the present invention, when the ratio of Rpm/Rz is 0.50 or more, this may be considered a sharp-ridged type, and when the ratio thereof is less than 0.50, this may be considered a round-ridged type.

Accordingly, in the present invention, it is preferable that Rpm is small and a ratio of Rpm/Rz is 0.5 or more for this purpose. A value of Rpm/Rz is not limited to a maximum value due to its lager-the-better characteristics of the steel sheet, but the ratio may be 1.00 or less considering the technical and economic characteristics of the current steel sheet manufacturing.

Hereinafter, a method for manufacturing steel of the present invention will be described in detail.

According to an embodiment of the present invention, a steel sheet may be manufactured by reheating, hot rolling, coiling, cold rolling, annealing, and cooling a steel slab satisfying the alloy composition described above.

Reheating

A steel slab satisfying the alloy composition of the present invention may be reheated in a temperature within a range of 1200° C. or higher.

In order to solid-dissolve most of the precipitates present in the steel again, the steel slab may be reheated at a temperature of 1200° C. or higher. In an embodiment of the present invention, the reheating temperature may be 1250° C. or higher.

Hot Rolling

A reheated steel slab may be hot rolled at a finishing rolling temperature of 800 to 950° C.

During the hot rolling, when a finishing rolling temperature is lower than 800° C., the hot rolling is completed in a relatively low temperature range, so that there may be a problem of reduced workability and rollability. In an embodiment of the present invention, the finishing rolling temperature may be 850° C. or higher. On the other hand, when the finishing rolling temperature exceeds 950° C., there may be a problem that uniform hot rolling is not performed through an entire thickness, resulting in insufficient grain refinement. In an embodiment of the present invention, an upper limit may be 930° C.

Coiling

The hot-rolled steel sheet may be coiled at a temperature within a range of 500 to 650° C.

A coiling temperature may affect the fraction of phases such as cementite other than ferrite, and as the coiling temperature increases, the cementite fraction increases. In the present invention, coiling may be performed at a temperature within a range of 500° C. or higher in order to form a desired level of cementite. Meanwhile, in order to secure the desired level of physical properties in the present invention, an upper limit of the coiling temperature may be limited to 650° C.

In the present invention, the cooling conditions to the coiling temperature after the hot rolling are not particularly limited, and the cooling may be performed under the usual conditions applied in the same technical field. In an embodiment of the present invention, air cooling may be performed.

Cold Rolling

The coiled steel sheet may be cold rolled at an accumulated reduction ratio of 50 to 90%.

In the present invention, an accumulated reduction ratio may be expressed as a ratio of the thickness difference between a hot rolled material and a cold rolled material to the thickness of the hot rolled material. According to an embodiment of the present invention, a lower reduction ratio is advantageous in terms of fine roughness, but when the reduction ratio is less than 50%, the rolling roll and tension control may be inaccurate, causing the plate to twist. On the other hand, when the reduction ratio exceeds 90%, the production of the product may be impossible due to the load of the rolling roll. According to an embodiment of the present invention, in order to control the roughness more effectively, the reduction ratio may be limited to 80% or less. According to an embodiment of the present invention, the reduction ratio may be limited to 70% or less.

Annealing

The cold-rolled steel sheet may be annealed at a temperature within a range of 700 to 780° C.

During annealing, there may be a concern that the phosphate reaction may be reduced during the chemical conversion treatment due to the formation of oxides by surface concentration of oxidizing elements such as Mn, Al, and Si, so that the annealing temperature may be limited to 780° C. or less. On the other hand, when the annealing temperature is less than 700° C., recrystallization is not completed, and thus, there may be a concern that the target material may not be secured.

Cooling

The annealed steel sheet may be cooled to a temperature within a range of 200 to 400° C. at an average cooling rate of 15 to 20° C./s, starting with cooling at a temperature within a range of 650° C. or higher.

During cooling, the cooling rate may be controlled in order to precipitate solid carbide supersaturation and fine cementite. When the average cooling rate is less than 15° C./s, fine cementite precipitation may not be easy. On the other hand, when the cooling rate exceeds 20° C./s, there may be a problem that it may be difficult to implement due to the equipment load.

When a cooling start temperature is less than 650° C., there may be a problem that pearlite transformation has already progressed significantly, resulting in limiting the precipitation of fine cementite.

In addition, when an end temperature is less than 200° C. during cooling, there may be a problem that some austenite is formed into martensite, resulting in exceeding a target material. On the other hand, when the temperature exceeds 400° C., there is a problem that fine cementite is not formed.

In the present invention, slow cooling may be performed to uniformize the structure of the steel sheet up to the temperature at which the desired cooling starts after annealing. Slow cooling conditions are not particularly limited, and the slow cooling may be performed by a conventional method.

MODE FOR INVENTION

Hereinafter, the present invention will be described more specifically through examples. However, it should be noted that the examples below are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.

Example

A steel slab having C: 0.049%, Si: 0.005%, Mn: 0.3%, P: 0.0126%, S: 0.006%, and a balance of Fe in wt % was prepared, and a steel sheet was manufactured under the conditions of Table 1 below.

TABLE 1 Hot Cold Anneal- Cooling Reheating Rolling Coiling Rolling ing Start End Tempera- Tempera- Tempera- Reduction Tempera- Tempera- Tempera- Cooling Specimen ture ture ture Rate ture ture ture Rate Number (° C.) (° C.) (° C.) (%) (° C.) (° C.) (° C.) (° C./s)  1 1201 870 500 90 780 650 400 11  2 1203 872 500 80 766 650 400 10  3 1231 921 500 70 769 650 400 9  4 1248 928 610 60 772 650 400 10  5 1229 900 610 90 766 650 300 11  6 1227 910 610 80 762 650 300 12  7 1204 945 610 70 769 650 300 13  8 1251 920 622 50 772 650 300 15  9 1253 918 617 55 766 650 300 18 10 1259 947 621 55 771 650 300 16 11 1257 942 614 60 767 650 300 20 12 1250 930 609 65 757 650 300 16 13 1229 924 501 90 790 650 100 25 14 1238 932 521 80 784 650 400 8 15 1221 925 531 85 783 650 400 9

As shown in Table 2, a microstructure and physical properties of the manufactured steel sheet were measured and are shown in Table 2 below. First, a microstructure fraction, the number of cementite, and major and minor axis lengths of the manufactured steel sheet were measured and are shown, and the major/minor axis ratio was calculated. The microstructure fraction was measured using an optical microscope after mounting toward a surface of the steel sheet, and a cementite fraction was measured by etching the surface of the steel sheet through Picral etchant (picric acid 2-4 g, ethanol 100 ml) to prepare a sample, and capturing a structure at a magnification of ×1000 using a scanning electron microscope and then using an Image Analyzer program. In addition, a shape of the fine cementite was specified through coloring in the Image Analyzer program, from which the major and minor axis lengths of the colored fine cementite were measured and the average values were shown.

Surface roughness was expressed as Rpm by adding five consecutive measurement data of Rp, which means a height from a center line of the highest peak within a reference length, and calculating an average thereof. Rz, which indicates an average roughness of Rz 10 points, was measured and expressed.

In addition, the manufactured specimens were subject to the chemical conversion treatment in the order of degreasing-washing 1-surface adjustment-phosphate treatment-washing 2. The specimens subjected to the chemical conversion treatment were observed with a scanning electron microscope at 150× magnification, and a phosphate coating area was calculated using Image Analyzer software, and is shown in Table 3 below. The specific chemical treatment conditions were represented as follows, and the phosphate coverage rate was graded from 1 to 5 in order of low to high, as shown below.

Chemical Conversion Treatment

    • Degreasing: FC-4460A 20 g/L, FC-4460B 12 g/L (Daehan Parkarizing Co., Ltd.), Treatment time 90 seconds, Temperature 60° C.
    • Washing 1: Treatment time 10 seconds, Room temperature
    • Surface Adjustment: PL-Z 5 g/L (Daehan Parkarizing Co., Ltd.), Concentration pH 7.5 to 11, Treatment time 10 to 20 seconds, Room temperature
    • Phosphate Treatment: PB-3111 28.2 g/L, NT-4055 5.8 g/L (Daehan Parkarizing Co., Ltd.), FA (free acidity)/TA (total acidity) 1.1 to 1.5/11.1 to 11.8, Treatment time 40 seconds, Phosphate Treatment Solution Temperature 40 to 45° C.
    • Washing 2: Treatment Time 10 seconds, Room temperature

Coverage Rate Standard

    • 1: 50% or more, Less than 60%
    • 2: 60% or more, less than 70%
    • 3: 70% or more, less than 80%
    • 4: 80% or more, less than 90%
    • 5: 90% or more

TABLE 2 Microstructure F P C Physical properties Frac- Frac- Frac- Major Phos- tion tion tion Number Major Minor axis/ Roughness phate Specimen (area (area (area (grains/ axis axis Minor Rpm Rz Rpm/ Coverage Number %) % ) % ) mm2) (μm) (μm) axis (μm) (μm) Rz Rate Division 1 97.62 1.00 1.38 8933 0.61 0.22 2.8 39 85 0.38 2 Comparative Example 1 2 97.57 0.90 1.53 11394 0.62 0.36 1.7 25 66 0.38 2 Comparative Example 2 3 97.29 1.10 1.61 19294 0.49 0.49 1.0 15 39 0.47 3 Comparative Example 3 4 97.46 0.80 1.74 18474 0.55 0.41 1.3 12 30 0.41 4 Comparative Example 4 5 97.45 0.60 1.95 11789 0.57 0.20 2.8 28 55 0.52 2 Comparative Example 5 6 97.83 0.60 1.57 12184 1.64 0.43 3.8 25 36 0.69 2 Comparative Example 6 7 97.48 0.70 1.82 29017 0.64 0.24 2.7 22 30 0.74 4 Comparative Example 7 8 97.49 0.50 2.01 34092 0.60 0.28 2.2 39 45 0.87 5 Inventive Example 1 9 97.47 0.50 2.03 32998 0.97 0.36 2.7 32 38 0.84 5 Inventive Example 2 10 97.45 0.50 2.05 31904 1.03 0.32 3.2 45 53 0.85 5 Inventive Example 3 11 97.41 0.50 2.09 36462 1.44 0.35 4.2 15 20 0.75 5 Inventive Example 4 12 97.33 0.60 2.07 34638 1.79 0.43 4.2 48 78 0.62 5 Inventive Example 5 13 99.02 0.60 0.38 4843 0.46 0.35 1.3 35 73 0.48 4 Comparative (M) Example 8 14 97.52 1.20 1.28 12732 0.80 0.46 1.7 30 56 0.54 3 Comparative Example 9 15 97.48 1.40 1.12 13546 0.83 0.46 1.8 31 64 0.48 3 Comparative Example 10 * F: Ferrite, P: Perlite, C: Sementite, M: Martensite

As shown in Table 2, in the case of the Inventive Examples satisfying the conditions of the present invention, the microstructure characteristics proposed by the present invention were satisfied, and the properties targeted by the present invention were also secured.

FIG. 1 is a microstructure photograph of Inventive Example 5 according to an embodiment of the present invention. As shown in FIG. 1, it may be confirmed that a large amount of fine cementite was formed in the ferrite matrix structure.

On the other hand, Comparative Examples 1 to 7 are examples that fall short of the cooling rate range proposed by the present invention. As a result, the cementite density targeted by the present invention was not satisfied, and specifically, Comparative Examples 2 to 4 had a ratio of the major axis to the minor axis of cementite that deviated from the range of the present invention.

FIG. 2 is a microstructure photograph of Comparative Example 1 deviating from an embodiment of the present invention. It may be confirmed that a small amount of cementite was formed in FIG. 2 as compared to FIG. 1.

Comparative Example 8 is an example in which the cooling rate exceeded the range of the present invention. The cooling end temperature was also excessively low, outside the scope of the present invention. As a result, martensite was formed as a microstructure, and the number of cementite was also insufficient, resulting in a poor phosphate coverage rate.

Comparative Examples 9 and 10 are examples in which an annealing temperature exceeded the temperature range proposed by the present invention, so that cementite was not formed to a desired level, and the shape thereof also did not satisfy the conditions of the present invention. As a result, the chemical conversion treatment properties were poor.

Although the present invention has been described in detail through examples, other forms of examples are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. A steel sheet, comprising: by wt %, carbon (C): 0.02 to 0.10%, silicon (Si): 0.03% or less, manganese (Mn): 0.1 to 0.4%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% or less, and a balance of iron (Fe) and inevitable impurities,

wherein a microstructure comprises, in area %, ferrite of 89.00% or more, cementite of 1.00 to 5.00%, and pearlite of 11.00% or less, and
the number of cementite is 30,000 ea/mm2 or more.

2. The steel sheet of claim 1, wherein a ratio of a major axis to a minor axis of the cementite (major axis/minor axis) is 2.0 to 9.0.

3. The steel sheet of claim 1, wherein a major axis length of cementite is 0.35 to 1.80 μm, and a minor axis length thereof is 0.20 to 0.50 μm.

4. The steel sheet of claim 1, wherein the steel sheet has a surface roughness in which a ratio of Rpm to Rz (Rpm/Rz) is 0.50 or more,

where Rpm refers to an average of five consecutive measurement data of Rp, which means a height from a center line of the highest peak within a reference length, and Rz refers to an average roughness at 10 points.

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

6. A method for manufacturing a steel sheet, comprising:

reheating a steel slab including, by wt %, carbon (C): 0.02 to 0.10%, silicon (Si): 0.03% or less, manganese (Mn): 0.1 to 0.4%, phosphorus (P): 0.02% or less, sulfur (S): 0.003% or less, and a balance of iron (Fe) and inevitable impurities;
hot-rolling the reheated steel slab;
coiling the hot-rolled steel sheet;
cold-rolling the coiled steel sheet at a cumulative reduction ratio of 50 to 90%;
annealing the cold-rolled steel sheet at a temperature within a range of 700 to 780° C.; and
cooling the annealed steel sheet from a start temperature within a range of 650° C. or higher to a temperature within a range of 200 to 400° C. at an average cooling rate of 15 to 20° C./s.

7. The method for manufacturing a steel sheet of claim 6, wherein the reheating temperature is 1200° C. or higher,

a finishing rolling temperature during the hot rolling is 800 to 950° C., and
a coiling temperature is 500 to 650° C.

8. The method for manufacturing a steel sheet of claim 6, wherein a reduction ratio is 50 to 70% during the cold rolling.

Patent History
Publication number: 20260201494
Type: Application
Filed: Nov 23, 2023
Publication Date: Jul 16, 2026
Applicant: POSCO CO., LTD (Pohang-si, Gyeongsangbuk-do)
Inventors: Kang-Min Lee (Gwangyang-si, Jeollanam-do), Jong-Kook Kim (Gwangyang-si, Jeollanam-do), Dong-Yoeul Lee (Gwangyang-si, Jeollanam-do), Jin-Ho Jung (Gwangyang-si, Jeollanam-do), Kwon-Il Kim (Gwangyang-si, Jeollanam-do), Yong-Kyun Cho (Seoul)
Application Number: 19/138,589
Classifications
International Classification: C21D 9/46 (20060101); C21D 1/84 (20060101); C21D 6/00 (20060101); C21D 8/0221 (20260101); C21D 8/0247 (20260101); C22C 38/00 (20060101); C22C 38/02 (20060101); C22C 38/04 (20060101);