Steel plate and its manufacturing method

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 technology]

[0002] Cold-rolled steel sheets are usually treated with chemical conversion coating before painting to improve paint adhesion and temporarily prevent rust. The chemical conversion coating process is carried out in the following order: alkaline degreasing, water rinsing, surface conditioner, phosphate treatment, and water rinsing. It is important that the surface conditioner is uniformly dispersed and adsorbed on the substrate to form a dense phosphate coating.

[0003] On the other hand, research is being actively conducted to improve the chemical conversion treatability not only by improving the solutions used in the process but also by improving the base steel itself, such as reducing annealing oxides and improving surface roughness.

[0004] According to Patent Document 1, the thickness of the iron oxide film on the substrate causes differences in the phosphate reaction, so it is important to form a uniform oxide film.

[0005] Furthermore, according to Patent Document 2, efforts have been made to improve the surface roughness of the base iron using an etching solution.

[0006] However, attempts to increase the amount of adsorption of a surface conditioner on a steel sheet by changing the cold rolling reduction rate and annealing temperature conditions without introducing an additional process during the cold rolling process of the steel sheet are inadequate. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 1998-0044917 (Published on September 15, 1998) [Patent Document 2] Korean Patent Publication No. 2022-0089430 (Published on June 28, 2022) Summary of the Invention [Problem to be solved by the invention]

[0008] According to one embodiment of the present invention, a steel sheet and a method for manufacturing the same are provided.

[0009] According to one embodiment of the present invention, a steel sheet having excellent chemical conversion treatability and a method for manufacturing the same are provided.

[0010] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the general content of this specification. [Means for solving the problem]

[0011] According to one embodiment of the present invention, the composition contains, in weight percent, 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, the balance being iron (Fe) and unavoidable impurities, The microstructure contains, by area%, 89.00% or more ferrite, 1.00 to 5.00% cementite, and 11.00% or less pearlite; The number of cementite particles is 30,000 pieces / mm 2 It is possible to provide a steel sheet having the above properties.

[0012] The ratio of the major axis to the minor axis of the cementite (major axis / minor axis) may be 2.0 to 9.0.

[0013] The cementite may have a major axis length of 0.35 to 1.80 μm and a minor axis length of 0.20 to 0.50 μm.

[0014] The steel plate may have a surface roughness in which the ratio of Rpm to Rz (Rpm / Rz) is 0.50 or more. (Here, Rpm means the average of five consecutive measurement data of Rp, which means the height from the center line of the highest peak within the reference length, and Rz means the average roughness at 10 points.)

[0015] The coverage of the steel sheet after chemical conversion treatment may be 80% or more.

[0016] According to one embodiment of the present invention, a method for producing a steel slab containing, in weight percent, 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, the balance being iron (Fe) and unavoidable impurities, is provided. hot rolling the reheated steel slab; coiling the hot-rolled steel sheet; cold rolling the coiled steel sheet at a cumulative reduction of 50 to 90%; Annealing the cold-rolled steel sheet at a temperature range of 700 to 780 ° C.; and cooling the annealed steel sheet starting from a temperature range of 650°C or higher to a temperature range of 200 to 400°C at an average cooling rate of 15 to 20°C / s.

[0017] The reheating temperature can be 1200°C or higher.

[0018] The finish rolling temperature during the hot rolling may be 800 to 950°C.

[0019] The coiling temperature may be 500 to 650°C.

[0020] The reduction rate during the cold rolling may be 50 to 70%. [Effects of the Invention]

[0021] According to an embodiment of the present invention, a steel sheet and a manufacturing method thereof can be provided.

[0022] According to one embodiment of the present invention, a steel sheet having excellent chemical conversion treatability and a manufacturing method thereof can be provided. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a microstructure photograph of Example 5 according to an embodiment of the present invention. [Figure 2] 1 is a photograph of the microstructure of Comparative Example 1, which is not an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified in various different forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to explain the present invention in more detail to those skilled in the art to which the invention pertains.

[0025] The inventors of the present invention have confirmed that chemical conversion treatability can be improved by controlling the shape and distribution of fine cementite and increasing the adsorption power of a surface conditioner during chemical conversion treatment, and have completed the present invention.

[0026] The present invention will be described in detail below.

[0027] The steel composition of the present invention will be described in detail below.

[0028] Unless otherwise specified in the present invention, the percentage indicating the content of each element is based on weight.

[0029] The steel sheet according to one embodiment of the present invention contains, by weight, 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, the balance being iron (Fe) and unavoidable impurities.

[0030] Carbon (C):0.02~0.10% If the carbon (C) content is less than 0.02%, secondary phases may not be formed, and the local electrochemical polarization phenomenon due to the intended microstructural differences may not occur. In one embodiment of the present invention, the carbon (C) content may be 0.04% or more. On the other hand, if the carbon (C) content exceeds 0.10%, excessive carbide formation may occur, resulting in a phenomenon in which the intended strength is exceeded. In one embodiment of the present invention, the upper limit of the carbon (C) content may be 0.06%.

[0031] Silicon (Si): 0.03% or less If the silicon (Si) content in steel is excessive, SiO2 may form on the steel surface, and Fe2SiO4, a composite phase of SiO2 and Fe oxide, may also form, potentially resulting in the formation of a large amount of red scale. This red scale is difficult to remove during pickling after cold rolling, and may also form as Si oxide during cold rolling annealing, potentially reducing acid reactivity during phosphating. Therefore, in the present invention, the upper limit of the silicon (Si) content may be limited to 0.03%. In one embodiment of the present invention, the upper limit may be 0.02% or less. However, 0% is excluded, taking into account the level of silicon that is inevitably added to steel.

[0032] Manganese (Mn): 0.1 to 0.4% Manganese (Mn) is an element that typically forms oxides on the surface of cold-rolled steel sheets during annealing. However, it also forms Mn-Si composite oxides that cannot be completely removed during the pickling process during hot rolling and cold rolling annealing. In the present invention, the Si content is controlled to 0.03% or less, which is not an environment in which a large amount of Si oxide is formed. However, the upper limit of the manganese (Mn) content can be limited to 0.4%. According to one embodiment of the present invention, it can be 0.3% or less. On the other hand, an excessively low manganese (Mn) content can induce the formation of stable Si oxides, which can inhibit acid reactivity. Therefore, the lower limit of the manganese (Mn) content can be limited to 0.1%. According to one embodiment of the present invention, the lower limit can be 0.2%.

[0033] Phosphorus (P): 0.02% or less Phosphorus (P) is a solid solution strengthening element, but if added in excess, it can cause steel embrittlement, so its upper limit can be limited to 0.02%. However, 0% is excluded in consideration of the level that is inevitably added to steel.

[0034] Sulfur (S): 0.003% or less Sulfur (S) is an impurity element in steel and may impair the ductility and weldability of steel, so its upper limit can be limited to 0.003%. On the other hand, 0% is excluded in consideration of the level of unavoidable addition to steel.

[0035] The steel material of the present invention may contain the remaining iron (Fe) and unavoidable impurities in addition to the above-mentioned composition. The unavoidable impurities cannot be excluded because they may be unintentionally mixed in during the normal manufacturing process. Since such impurities are known to all engineers in the field of normal steel manufacturing, all of the details thereof will not be specifically mentioned in this specification.

[0036] The steel microstructure of the present invention will now be described in detail.

[0037] Unless otherwise specified in the present invention, the percentages indicating the fraction of the microstructure are based on the area.

[0038] The microstructure of the steel sheet according to an embodiment of the present invention may contain, in area %, 89.00% or more of ferrite, 1.00 to 5.00% of cementite, and 11.00% or less of pearlite.

[0039] In the present invention, cementite can be contained in an amount of 1.00% or more to increase the adsorption amount of the surface modifier. Since the cementite fraction that can be theoretically produced within the carbon range proposed in the present invention is approximately 2.00%, the lower limit of the cementite area fraction can be set to 2.00%. While the cementite fraction can be increased by controlling the cooling rate during steel production, the upper limit can be set to 5.00% in the present invention.

[0040] Meanwhile, in order to contain cementite at a certain level or more, the present invention limits pearlite to 11.00% or less, and in one embodiment of the present invention limits pearlite to 4.00% or less.

[0041] Meanwhile, a report by Nihon Parkerizing (Surface Technology (Japan), 2010) found that surface conditioning during the chemical conversion treatment process significantly affects the size and coverage of phosphate crystal grains, making surface conditioning essential. The surface conditioner typically used is Na4TiO(PO4)2hydrate, a disc-shaped material with a layer-to-layer spacing of a few angstroms. These particles adsorb onto the surface of the base steel, forming microcells that increase the number of starting points for etching the base steel and depositing the coating. For this reason, it is necessary to increase the number of active sites on the metal surface. Increasing the number of active sites increases the number of crystal nuclei during the chemical conversion treatment, resulting in the formation of a fine, uniform phosphate crystal coating.

[0042] According to one embodiment of the present invention, the number of cementite particles is 30,000 particles / mm 2 It can be more than that.

[0043] In the present invention, a large amount of cementite can be formed in order to increase the amount of adsorption of the surface conditioner. 2 If the number of particles per unit area is less than 30,000, fine cementite is not formed within the grains, and the above-mentioned effects cannot be ensured. 2 It can be:

[0044] In the steel sheet according to an embodiment of the present invention, the cementite may have a major axis length of 0.35 to 1.80 μm and a minor axis length of 0.20 to 0.50 μm.

[0045] The cementite according to an embodiment of the present invention may be formed in a rod-like shape. Such rod-like cementite may have the effect of facilitating the adsorption of a surface conditioner on the steel sheet surface.

[0046] If the major axis length of the cementite is less than 0.35 μm, there may be a problem that the surface conditioner particles are difficult to adsorb. According to one embodiment of the present invention, the major axis length may be 0.45 μm or more. On the other hand, if the major axis length exceeds 1.80 μm, there may be a problem that the total number of cementite particles generated decreases and the continuity decreases.

[0047] Furthermore, if the minor axis length of the cementite is less than 0.20 μm, there may be a problem that the surface conditioner particles are difficult to adsorb. According to one embodiment of the present invention, it may be 0.25 μm or more. On the other hand, if the minor axis length exceeds 0.50 μm, there may be a problem that the total number of cementite particles generated decreases and the continuity decreases. According to one embodiment of the present invention, it may be 0.45 μm or less.

[0048] The cementite according to one embodiment of the present invention may have a ratio of its major axis to its minor axis (major axis / minor axis) of 2.0 to 9.0.

[0049] In the present invention, in addition to controlling the lengths of the major and minor axes of cementite, the effect is enhanced by controlling the 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) can indicate the form in which the surface conditioner can be uniformly adsorbed, and in the present invention, controlling this ratio can increase the coverage rate of phosphate.

[0050] If the ratio of the major axis to the minor axis of cementite is less than 2.0, the shape approaches a square, which may cause structural and geometric problems in terms of the adsorption of surface modifiers. On the other hand, if the ratio exceeds 9.0, there may be a problem that cementite forms a lamellar shape with ferrite rather than existing alone. Within the range of 2.0 to 9.0, fine cementite particles can be independently and uniformly dispersed at regular intervals. According to one embodiment of the present invention, the ratio may be 2.2 or more. According to one embodiment of the present invention, the ratio may be 8.0 or less.

[0051] A more preferred steel sheet according to an embodiment of the present invention may have a surface roughness of 0.50 or more, where the ratio of Rpm to Rz (Rpm / Rz) is 0.50 or more, and a coverage rate after chemical conversion treatment of 80% or more, thereby exhibiting excellent chemical conversion treatability.

[0052] In the present invention, there are various parameters that represent micro-roughness, but Rp, which means the height from the center line of the highest peak within the reference length, and Rpm, which is the average of five consecutive Rp measurements, are used as the basis. More specifically, a relatively small Rpm can mean wide peaks and narrow valleys, and a relatively large Rpm can mean a sparsely spiky surface.

[0053] Therefore, in the present invention, the cross-sectional shape of a material can be more clearly and quantitatively determined through Rz, which represents the 10-point average roughness, and the ratio of Rpm to Rz (Rpm / Rz). In the present invention, the Rpm / Rz ratio is considered to be sharp-ridged type if it is 0.50 or higher, and round-ridged type if it is lower.

[0054] Therefore, in the present invention, it is preferable that Rpm is small and the ratio of Rpm / Rz is 0.5 or more. The value of Rpm / Rz is a desirable characteristic and there is no maximum limit, but it can be 1.00 or less in consideration of the technical and economic characteristics of current steel sheet manufacturing.

[0055] The steel production method of the present invention will be described in detail below.

[0056] The steel sheet according to an embodiment of the present invention can be manufactured by reheating, hot rolling, coiling, cold rolling, annealing, and cooling a steel slab having the above-mentioned alloy composition.

[0057] reheating A steel slab satisfying the alloy composition of the present invention can be reheated at a temperature range of 1200°C or higher.

[0058] In order to redissolve most of the precipitates present in the steel, the steel can be reheated at a temperature of 1200°C or higher. In one embodiment of the present invention, the reheating temperature can be 1250°C or higher.

[0059] hot rolling The reheated steel slab can be hot rolled at a finish rolling temperature of 800 to 950°C.

[0060] If the finish rolling temperature during hot rolling is less than 800°C, the hot rolling will end in a relatively low temperature range, which may result in reduced workability and rollability. In one embodiment of the present invention, the finish rolling temperature may be 850°C or higher. On the other hand, if the finish rolling temperature exceeds 950°C, uniform hot rolling will not be achieved throughout the thickness, resulting in insufficient grain refinement. In one embodiment of the present invention, the upper limit may be 930°C.

[0061] Winding The hot-rolled steel sheet can be coiled at a temperature in the range of 500 to 650°C.

[0062] The coiling temperature may affect the fraction of phases other than ferrite, such as cementite, and the higher the coiling temperature, the higher the cementite fraction. In the present invention, coiling can be performed at a temperature range of 500°C or higher to form a desired level of cementite. Meanwhile, in the present invention, the upper limit of the coiling temperature can be limited to 650°C to ensure a desired level of physical properties.

[0063] In the present invention, the cooling conditions to the coiling temperature after hot rolling are not particularly limited, and cooling can be performed under normal conditions applied in the same technical field. In one embodiment of the present invention, air cooling can be performed.

[0064] cold rolling The coiled steel sheet can be cold rolled at a cumulative reduction of 50 to 90%.

[0065] In the present invention, the cumulative reduction 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 one embodiment of the present invention, a lower reduction is advantageous in terms of micro-roughness. However, if the reduction is less than 50%, the roll and tension control may be inaccurate, which may result in distortion of the plate. On the other hand, if the reduction is more than 90%, the load on the roll may make it impossible to produce the product. According to one embodiment of the present invention, the reduction can be limited to 80% or less to more effectively control roughness. According to another embodiment of the present invention, the reduction can be limited to 70% or less.

[0066] annealing The cold-rolled steel sheet can be annealed at a temperature in the range of 700 to 780°C.

[0067] During annealing, the phosphate reaction may be reduced during chemical conversion treatment due to the formation of oxides caused by the surface concentration of oxidizing elements such as Mn, Al, and Si, so the annealing temperature can be limited to 780°C or less.On the other hand, if the annealing temperature is less than 700°C, recrystallization may not be completed, and the target material properties may not be achieved.

[0068] cooling The annealed steel sheet can be cooled starting from a temperature range of 650°C or higher, and cooled to a temperature range of 200 to 400°C at an average cooling rate of 15 to 20°C / s.

[0069] During cooling, the cooling rate can be controlled to achieve supersaturation of solute carbides and the precipitation of fine cementite. If the average cooling rate is less than 15°C / s, the precipitation of fine cementite may be difficult. On the other hand, if the cooling rate exceeds 20°C / s, it may be difficult to achieve due to the equipment load.

[0070] If the cooling start temperature is less than 650°C, pearlite transformation has already progressed significantly, which may cause a problem of limiting the precipitation of fine cementite.

[0071] Furthermore, if the cooling end temperature is less than 200°C, austenite is partially transformed into martensite, resulting in a problem of exceeding the target material quality, whereas if the temperature exceeds 400°C, fine cementite is not formed.

[0072] In the present invention, slow cooling can be performed after annealing to homogenize the structure of the steel sheet up to the target cooling start temperature. The slow cooling conditions are not particularly limited, and slow cooling can be performed by a conventional method. [Example]

[0073] The present invention will be described in more detail with reference to the following examples, but it should be noted that the following examples are intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.

[0074] (Example) A steel slab containing, by weight, C: 0.049%, Si: 0.005%, Mn: 0.3%, P: 0.0126%, S: 0.006%, and the balance Fe was prepared, and steel plates were produced under the conditions in Table 1 below.

[0075] [Table 1]

[0076] The microstructure and physical properties of the manufactured steel sheets were measured and reported as shown in Table 2 below. First, the microstructure fraction, cementite count, and major and minor axis lengths of the manufactured steel sheets were measured and reported, and the major / minor axis ratio was calculated. The microstructure fraction was measured using an optical microscope after mounting the steel sheet in the surface direction. The cementite fraction was measured by etching the surface of the steel sheet with picral etchant (2-4 g of picric acid, 100 ml of ethanol) to prepare a sample. The microstructure was then photographed at 1000x magnification using a scanning electron microscope and measured using the Image Analyzer program. The shape of the fine cementite was also identified using coloring within the Image Analyzer program, and the major and minor axis lengths of the colored fine cementite were measured and reported as average values.

[0077] Surface roughness is expressed in Rpm by adding up the Rp, which means the height from the center line of the highest peak within the reference length, and averaging it over five consecutive measurement data. Rz, which indicates the 10-point average roughness, is measured and expressed.

[0078] The prepared specimens were then subjected to chemical conversion treatment in the following order: degreasing, water washing 1, surface preparation, phosphate treatment, and water washing 2. The chemically treated specimens were observed under a scanning electron microscope at 150x magnification, and the phosphate coverage area was calculated using Image Analyzer software, and the results are shown in Table 3 below. The specific chemical conversion treatment conditions were as follows, and the phosphate coverage was ranked from 1 to 5, from lowest to highest, as follows:

[0079] Chemical conversion treatment - Degreasing: FC-4460A 20g / L, FC-4460B 12g / L (Daehan Parkerizing Co., Ltd.), treatment time 90 seconds, temperature 60℃ -Wash 1: 10 seconds, room temperature -Surface conditioning: PL-Z 5g / L (Daihan Parkerizing Co., Ltd.), pH concentration 7.5~11, treatment time 10~20 seconds, room temperature Phosphate treatment: PB-3111 28.2g / L, NT-4055 5.8g / L (Daehan Parkerizing Co., Ltd.), FA (free acidity) / TA (total acidity) 1.1-1.5 / 11.1-11.8 respectively, treatment time 40 seconds, phosphate treatment solution temperature 40-45℃ -Wash 2: 10 seconds, room temperature -Coverage criteria 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: Over 90%

[0080] [Table 2] TIFF2025540843000004.tif83165

[0081] As shown in Table 2, in the case of the invention examples that satisfy the conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied and the physical properties aimed at in the present invention were also secured.

[0082] 1 is a microstructure photograph of Example 5 according to an embodiment of the present invention. As shown in FIG. 1, it can be seen that a large amount of fine cementite was formed in the ferrite matrix structure.

[0083] On the other hand, Comparative Examples 1 to 7 are examples in which the cooling rate does not reach the range proposed by the present invention. As a result, the cementite density targeted by the present invention is not satisfied, and in particular, in Comparative Examples 2 to 4, the ratio of the major axis to the minor axis of cementite is outside the range of the present invention.

[0084] 2 is a microstructure photograph of Comparative Example 1, which is not an example of the present invention. In FIG. 2, it can be seen that a small amount of cementite was formed compared to FIG.

[0085] Comparative Example 8 is an example in which the cooling rate exceeded the range of the present invention. The cooling end temperature was also outside the range of the present invention, being cooled to an excessively low temperature. As a result, martensite was formed as a microstructure, the number of cementite particles was insufficient, and the phosphate coverage was poor.

[0086] Comparative Examples 9 and 10 are examples in which the annealing temperature exceeded the temperature range proposed by the present invention, and cementite was not formed to the desired level, and its shape did not satisfy the conditions of the present invention. As a result, the chemical conversion treatability was poor.

[0087] Although the present invention has been described in detail with reference to the above embodiments, other embodiments are possible, and therefore the spirit and scope of the following claims should not be limited to the embodiments.

Claims

1. In weight percent, 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, the balance being iron (Fe) and unavoidable impurities, The microstructure comprises, by area percent, 89.00% or more ferrite, 1.00 to 5.00% cementite, and 11.00% or less pearlite; The number of cementite particles is 30,000 pieces / mm ２ That's it, steel plate.

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

0.

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

4. The steel sheet according to claim 1, wherein the steel sheet has a surface roughness in which the ratio of Rpm to Rz (Rpm / Rz) is 0.50 or more. (Here, Rpm means the average of five consecutive measurement data of Rp, which means the height from the center line of the highest peak within the reference length, and Rz means the average roughness at 10 points.)

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

6. Reheating a steel slab containing, by weight, 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, the balance being iron (Fe) and unavoidable impurities; hot rolling the reheated steel slab; coiling the hot-rolled steel sheet; cold rolling the coiled steel sheet at a cumulative reduction of 50 to 90%; Annealing the cold-rolled steel sheet at a temperature range of 700 to 780°C; and cooling the annealed steel sheet from a temperature range of 650°C or higher at an average cooling rate of 15-20°C / s to a temperature range of 200-400°C.

7. The reheating temperature is 1200°C or higher, The finish rolling temperature during the hot rolling is 800 to 950°C, The steel sheet manufacturing method according to claim 6, wherein the coiling temperature is 500 to 650°C.

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

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