Steel sheet excellent in phosphate reactivity and method for manufacturing the same

Abstract

A steel sheet excellent in phosphate reactivity according to one embodiment of the present application contains, in mass%, carbon (C): 0.02 to 0.06%, silicon (Si): 0.01% or less (excluding 0%), manganese (Mn): 0.1 to 0.24%, aluminum (Al): 0.02% or less (excluding 0%), phosphorus (P): 0.015 to 0.04%, with the balance containing Fe and inevitable impurities. An oxide layer having a thickness of 10 nm or less is present in the direction from the surface to the interior of the steel sheet, and satisfies the following formula 1. Formula 1 ([Mn] + [Si] + [Al]) / (3 x [P]) ≤ 0.6, in formula 1, [Mn], [Si], [Al] and [P] represent the highest content of each element when element analysis is performed in the thickness direction of the oxide layer.

Description

Technical Field

[0001] According to one embodiment of the present invention, a steel sheet with excellent phosphate reactivity and a method for manufacturing the same are provided. Specifically, according to one embodiment of the present invention, when the surface of a steel sheet used as a raw material for rollers is subjected to phosphate treatment to impart corrosion resistance, the phosphate crystals generated on the surface after phosphate treatment are characterized by being small in size and uniformly distributed on the surface of the steel sheet. According to one embodiment of the present invention, a steel sheet with excellent corrosion resistance and phosphate-treated surface properties, and a method for manufacturing the same are provided. Background Technology

[0002] Phosphate treatment is used to ensure rust prevention on steel surfaces, aiming to improve long-term corrosion resistance and enhance adhesion before coating.

[0003] In this phosphate treatment method, an electrochemical potential difference is generated during the contact between the phosphate solution and the steel plate, causing the steel plate to dissolve. Fe ions generate electrons, increasing the pH value, and stable metallic phosphate crystals form on the steel plate surface. This process of formation and growth results in a surface treatment. Phosphate treatment is a process used to impart paintability and corrosion resistance to raw steel plates such as automotive steel plates, roller steel plates, and electrical steel plates.

[0004] The solution typically used for phosphate treatment is zinc phosphate. Depending on the crystal shape of the phosphate crystals formed on the steel plate surface, phosphate treatment results in a two-phase crystal structure of phosphophyllite and hopeite, or a mixed two-phase crystal structure. Phosphophyllite is a spherical, dense crystal formed when Fe ions are present in the phosphate crystals and a reaction occurs, while hopeite has a granular, narrow structure. Both phases densely cover the steel. In this case, phosphophyllite (P) exhibits superior resistance to acid and alkali corrosion compared to hopeite (H), and phosphate treatment with a higher P content results in better corrosion resistance. Therefore, in vapor deposition methods where iron leached from the steel plate is easily contained in the coating, the proportion of P on the surface increases. However, in the case of spray treatment, although it depends on the treatment solution, the H value is relatively high.

[0005] The performance of phosphate treatment ultimately depends on the density of phosphate crystal coverage on the steel plate surface after phosphate treatment, which is determined by the size and coverage of the phosphate crystals.

[0006] Factors hindering the acid reactivity of steel plates are typically the type and thickness of the oxides covering the steel plate surface. In particular, when the oxides are thicker, the dissolution rate of Fe, which serves as the phosphate nucleus, slows down, and the density of the phosphate nuclei decreases, characterized by coarsening of phosphate crystals and low phosphate nucleus coverage.

[0007] Recent environmental regulations have led to increasingly diluted phosphate solutions, causing problems with phosphate treatment. For phosphate to adhere effectively to the steel surface, a high density of phosphate nuclei must form during the reaction with phosphoric acid. However, wastewater treatment issues have reduced the phosphate solution concentration, hindering the initial acid reaction and impeding nuclei formation. Consequently, there is a problem of coarse phosphate crystals that fail to cover the entire surface of the steel.

[0008] That is, if sufficient reactivity cannot be ensured at low phosphoric acid concentrations, it will continue to have an adverse effect on phosphate treatability. Summary of the Invention

[0009] (a) Technical problems to be solved

[0010] According to one embodiment of the present invention, a steel sheet with excellent phosphate reactivity and a method thereof are provided. Specifically, according to one embodiment of the present invention, when the surface of a steel sheet used as a raw material for rollers is subjected to phosphate treatment to impart corrosion resistance, the phosphate crystals generated on the surface after phosphate treatment are characterized by being small in size and uniformly distributed on the surface of the steel sheet. According to one embodiment of the present invention, a steel sheet with phosphate-treated surface properties exhibiting excellent corrosion resistance and a method thereof are provided.

[0011] (II) Technical Solution

[0012] According to one embodiment of the present invention, the steel sheet with excellent phosphate reactivity comprises, by weight %, carbon (C): 0.02 to 0.06%, silicon (Si): less than 0.01% (excluding 0%), manganese (Mn): 0.1 to 0.24%, aluminum (Al): less than 0.02% (excluding 0%), phosphorus (P): 0.015 to 0.04%, with the balance containing Fe and unavoidable impurities.

[0013] According to an embodiment of the present invention, a steel plate with excellent phosphate reactivity has an oxide layer with a thickness of less than 10 nm present in the direction from the surface to the interior of the steel plate, and satisfies the following formula 1.

[0014] Formula 1

[0015] ([Mn]+[Si]+[Al]) / (3×[P])≤0.6

[0016] In Equation 1, [Mn], [Si], [Al] and [P] represent the highest content of each element when performing elemental analysis in the thickness direction of the oxide layer.

[0017] According to one embodiment of the present invention, a steel sheet with excellent phosphate reactivity contains cementite with an area fraction of 2% or more, and the remainder contains ferrite.

[0018] According to one embodiment of the present invention, the steel plate with excellent phosphate reactivity has a pickling lag time of less than 20 seconds when immersed in a 5% sulfuric acid aqueous solution at 30°C.

[0019] According to one embodiment of the present invention, a steel plate with excellent phosphate reactivity exhibits a corrosion reduction rate of 0.55 mg / cm² when immersed in a 5% sulfuric acid aqueous solution at 30°C. 2 / hr or more.

[0020] According to one embodiment of the present invention, a steel plate with excellent phosphate reactivity has a yield strength of 220 to 270 MPa.

[0021] According to one embodiment of the present invention, the steel plate exhibits excellent phosphate reactivity, and the average major axis length of the phosphate particles formed after phosphate treatment is less than 10 μm.

[0022] According to one embodiment of the present invention, the steel plate exhibits excellent phosphate reactivity, wherein the phosphate particles formed after phosphate treatment account for more than 90% of the surface area of ​​the steel plate.

[0023] A method for manufacturing a steel sheet with excellent phosphate reactivity according to an embodiment of the present invention includes: a step of hot rolling a slab to manufacture a hot-rolled sheet, wherein the slab comprises, by weight %, 0.02 to 0.06% carbon (C), less than 0.01% silicon (Si), 0.1 to 0.24% manganese (Mn), less than 0.02% aluminum (Al), 0.015 to 0.04% phosphorus (P), with the balance being Fe and unavoidable impurities; a step of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled sheet; a step of annealing the cold-rolled steel sheet; and a step of temper rolling the annealed cold-rolled steel sheet.

[0024] In the process of manufacturing hot-rolled steel sheets, the coiling temperature is 650 to 750°C; the soaking temperature in the annealing step is 700 to 780°C. The sheets then undergo a quenching and tempering rolling process.

[0025] In the hot-rolled plate manufacturing process, the final hot rolling temperature (FDT) is 800 to 950°C.

[0026] In the aforementioned cold-rolled sheet manufacturing process, the reduction rate is 70% to 85%.

[0027] After the annealing step and before the temper rolling step, the cold-rolled steel sheet is cooled to a final cooling temperature of 80 to 150°C.

[0028] The annealing step is carried out in an atmosphere containing more than 5% by volume of hydrogen and the remainder of nitrogen, and at a dew point below -30°C.

[0029] (III) Beneficial Effects

[0030] According to one embodiment of the present invention, a steel sheet with excellent phosphate reactivity can be effectively used as a raw material for phosphate-treated steel sheets to impart paintability and rust resistance to the steel sheet.

[0031] According to one embodiment of the present invention, the steel sheet exhibits excellent phosphate reactivity, which can easily ensure phosphate treatment even at low phosphate concentrations, and can be used not only for containers but also for automobiles and household appliances. Attached Figure Description

[0032] Figure 1 This is a schematic cross-sectional view of a steel plate according to an embodiment of the present invention.

[0033] Figure 2 These are photographs taken using a scanning electron microscope after the steel plates manufactured according to Example 1 and Comparative Example 4 have undergone phosphate treatment.

[0034] Figure 3 This is a Glow Dispersion Spectroscopy (GDS) graph of the phosphorus content of the steel plates manufactured in Examples 1, 5, 4, and 5. Detailed Implementation

[0035] The terms "first," "second," "third," etc., are used to describe parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another. Therefore, without departing from the scope of the invention, the first part, component, region, layer, and / or segment described below can also be described as a second part, component, region, layer, and / or segment.

[0036] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, the singular forms used herein are intended to include the plural forms as well. As used in the specification, "comprising" can specifically refer to a feature, field, integer, step, action, element, and / or component, but does not exclude the presence or addition of other features, fields, integers, steps, actions, elements, components, and / or groups.

[0037] Additionally, unless otherwise specified, % indicates weight, and 1 ppm is 0.0001 wt%.

[0038] In one embodiment of the present invention, the inclusion of additional elements refers to the replacement of a portion of the remaining iron (Fe) by additional elements, the replacement amount being equivalent to the amount of additional elements added.

[0039] Although not otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries should be interpreted as having the same meaning as disclosed in relevant technical literature and herein, and should not be interpreted in an idealized or overly formal sense.

[0040] Embodiments of the present invention will be described in detail below to enable those skilled in the art to implement the invention. However, the invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0041] According to one embodiment of the present invention, the steel sheet with excellent phosphate reactivity comprises, by weight %, carbon (C): 0.02 to 0.06%, silicon (Si): less than 0.01% (excluding 0%), manganese (Mn): 0.1 to 0.24%, aluminum (Al): less than 0.02% (excluding 0%), phosphorus (P): 0.015 to 0.04%, with the balance containing Fe and unavoidable impurities.

[0042] Below, we will first provide a detailed explanation of the composition of the steel plate.

[0043] As described below, Al, Mn, Si, and P in the steel plate are concentrated in the oxide layer, exhibiting a concentration gradient from the surface to the interior. In one embodiment of the present invention, the elemental content in the steel plate refers to the average content along the thickness direction of the steel plate.

[0044] Carbon (C): 0.02 to 0.06% by weight

[0045] According to one embodiment of the invention, the carbon (C) content of the steel plate can be from 0.02 to 0.06% by weight. If the carbon (C) content is too low, the formation of a second phase will not occur, and therefore the expected localized corrosion will not occur. If the carbon (C) content is too high, excessive carbon (C) will form, potentially leading to a situation where the required strength is exceeded. Therefore, the carbon (C) content can be from 0.02 to 0.06% by weight. More specifically, it can be from 0.025 to 0.055% by weight.

[0046] Silicon (Si): less than 0.01% by weight

[0047] According to one embodiment of the present invention, the silicon (Si) content can be less than 0.01% by weight. When the silicon (Si) content is too high, SiO2 may form on the surface, or a complex phase of SiO2 and Fe oxide may form, producing a large amount of red oxide scale. These red oxide scales may cause defects that are not eliminated during cold rolling pickling, and may form Si oxides themselves during cold rolling annealing, thereby reducing acid reactivity. Therefore, the maximum Si content can be less than 0.01% by weight. Specifically, it can be from 0.001% to 0.01% by weight. More specifically, it can be from 0.003% to 0.009% by weight.

[0048] Mn: 0.10 to 0.24% by weight

[0049] Manganese (Mn) is an element that typically forms oxides on the surface of cold-rolled steel sheets during annealing heat treatment. According to one embodiment of the invention, the Si content is limited to below 0.01 wt% because Si oxide itself creates an environment that forms surface oxides and inhibits acid reactivity during annealing heat treatment. Mn oxide formation can be effectively suppressed by controlling the Mn content to below 0.24 wt%. However, Mn is a typical solid solution strengthening element, and insufficient Mn content may lead to a decrease in strength. Therefore, Mn can be contained in amounts from 0.10 to 0.24 wt%. More specifically, it can be contained in amounts from 0.11 to 0.24 wt%.

[0050] Aluminum (Al): less than 0.020% by weight

[0051] Aluminum (Al) is a representative element of deoxidizers. However, according to one embodiment of the invention, Al also forms Al oxides on the surface of the steel, and when Al oxides are formed, acid reactivity may be inhibited. Therefore, the Al content can be less than 0.020% by weight. More specifically, Al can be contained in 0.001 to 0.020% by weight. More specifically, it can be contained in 0.010 to 0.019% by weight.

[0052] Phosphorus (P): 0.015 to 0.040% by weight

[0053] In one embodiment of the invention, when the steel is in an acidic environment, P acts as a catalyst for the dissolution of Fe. Therefore, the content of P can be limited to 0.015% by weight or more. However, since P is a representative element causing brittleness at room temperature, and when Fe3P precipitates at grain boundaries, it weakens formability, the upper limit can be limited to 0.040% by weight. Therefore, P can be contained from 0.015 to 0.040% by weight. More specifically, it can be contained from 0.016 to 0.038% by weight.

[0054] In addition to the aforementioned elemental components, the present invention also contains Fe and unavoidable impurities. These impurities are well known in the art and will not be described further. In one embodiment of the invention, the addition of other elements besides the foregoing components is not excluded, and when additional elements are further included, they replace a portion of the balance of Fe.

[0055] like Figure 1 The figure shows a cross-sectional schematic diagram of a steel plate according to an embodiment of the present invention.

[0056] like Figure 1 As shown, in a steel plate (10) according to an embodiment of the present invention, an oxide layer (20) exists from the surface of the steel plate toward the interior. Figure 1 As shown, the oxide layer (20) exists only on one side of the steel plate, but the oxide layer (20) can also exist on both sides.

[0057] The oxide layer (20) refers to the depth from the surface of the steel plate to the point where the oxygen peak in the Fe-O diagram shown in the GDS results becomes “0”.

[0058] The thickness of the oxide layer (20) is less than 10.0 nm. If the oxide layer (20) is too thick, the acid reactivity may be too slow, which is not suitable. More specifically, the thickness of the oxide layer (20) can be from 1 to 10.0 nm.

[0059] In the steel plate manufacturing process described later, the Mn, Si, Al, P and other components contained in the steel plate diffuse from the interior of the steel plate to the surface of the steel plate and are concentrated in the oxide layer (20).

[0060] At this time, the contents of Mn, Si, Al and P present in the oxide layer (20) can satisfy the following equation 1.

[0061] Formula 1

[0062] ([Mn]+[Si]+[Al]) / (3×[P])≤0.6

[0063] In Equation 1, [Mn], [Si], [Al] and [P] represent the highest content of each element when performing elemental analysis along the thickness direction of the oxide layer.

[0064] When Formula 1 exceeds 0.6, it indicates that the content of P in the oxide layer is low or the content of Mn, Si, and Al is high. When the P content in the oxide layer is low, P, as an element that ensures acid reactivity, is reduced, and adequate phosphate reactivity cannot be obtained. Conversely, when the content of Mn, Si, and Al is high, a large amount of Mn, Si, and Al oxides are formed, which also prevents adequate phosphate reactivity. Therefore, as described above, the content of Formula 1 can be 0.60 or less. More specifically, the value of Formula 1 can be from 0.20 to 0.60.

[0065] The maximum content of P in the oxide layer (20) is 1.0 to 3.0 wt%, the maximum content of Mn is 0.80 to 1.5 wt%, the maximum content of Si is 0.50 to 1.50 wt%, and the maximum content of Al is 0.30 to 1.0 wt%.

[0066] According to one embodiment of the present invention, a steel sheet with excellent phosphate reactivity may contain cementite with an area fraction of 2% or more, and the remainder may contain ferrite. It is well known that one phenomenon causing corrosion in acid reactions is the formation of small circuits in the electrolyte. In this case, when only a stable ferrite-based Fe single phase induces a cathodic reaction, the acid reaction will not occur. Furthermore, the reaction of cathodic sites, such as cementite, can be promoted. However, in the case of such a cathode, due to the low dissolution potential in an acidic environment, the acid reactivity may actually worsen when the amount of anode is too large. More specifically, the cementite may contain 2.0 to 5.0 area percent. Other phases of 0.5 area percent or less may also be included.

[0067] According to one embodiment of the invention, the steel plate has excellent phosphate reactivity, excellent corrosion resistance, suitable yield strength, and excellent productivity.

[0068] According to one embodiment of the present invention, phosphate reactivity is measured using the pickle lag (P / L) measurement method. A 75×100 mm specimen surface is degreased with alkali in a 5% (w / w) sulfuric acid aqueous solution, and the water wettability is confirmed to be 100%, thus confirming the degreasing performance. This is an indirect method for determining acid reactivity, measuring the time required for hydrogen to cover the entire area. Therefore, a longer P / L time indicates a greater influence of surface oxides, leading to poorer acid reactivity and consequently, poorer phosphate treatment results. According to one embodiment of the present invention, the pickle lag time when the steel plate is immersed in a 5% (w / w) sulfuric acid aqueous solution at 30°C is 20 seconds or less. More specifically, the pickle lag time may be 5 to 20 seconds.

[0069] In one embodiment of the invention, pickle lag is measured by observing the surface of the steel plate with a camera, but this presents limitations due to the presence of microscopic hydrogen gas invisible to the naked eye. In another embodiment, in addition to the pickle lag time, the steel plate is directly immersed in a 5% sulfuric acid aqueous solution and reacted at 30°C for 5 minutes. Phosphate reactivity is quantified by measuring the corrosion reduction rate, which is obtained by dividing the initial weight and final weight of the sample by the immersion time and immersion area. That is, the corrosion reduction rate is an indicator of acid reactivity, representing the rate at which Fe ions dissolve when the steel plate is exposed to a certain concentration of acid. In other words, a higher corrosion loss rate indicates easier Fe elution, easier formation of phosphate nuclei, and a higher phosphate nuclei density, making phosphate treatment easier.

[0070] According to one embodiment of the present invention, the corrosion reduction rate when immersed in a 5% sulfuric acid aqueous solution at 30°C is 0.550 mg / cm². 2 / hr or more. More specifically, the corrosion reduction rate can be from 0.550 mg to 0.700 mg / cm³. 2 / hr.

[0071] In the case of manufacturing products from steel plates according to one embodiment of the present invention, it is necessary to ensure the integrity of formability during the manufacturing process. That is, it is necessary to ensure strengths such as pressure resistance and dent resistance under the service environment. Therefore, in one embodiment of the present invention, the steel plate may have a yield strength of 220 to 270 MPa. If the yield strength is too high, formability may become a problem; if the yield strength is too low, pressure resistance and dent resistance may be problematic.

[0072] As described above, the steel plate according to one embodiment of the present invention is readily subjected to phosphate treatment, and after phosphate treatment, fine phosphate particles with an average long axis of less than 10 μm can exist on the surface of the steel. This can cover more than 90% of the total area of ​​the observed surface.

[0073] The phosphate particles formed according to one embodiment of the invention are primarily foliated hopeite particles. The major axis length of a hopeite particle is defined as the length of the longest axis when a single phosphate particle is observed from the observation surface. To calculate an average value, the average of the measurements can be calculated after measuring 30 or more randomly selected individual phosphate particles. The observation surface can be a surface parallel to the rolling surface (ND surface).

[0074] At this point, zinc phosphate treatment refers to applying a zinc phosphate solution to the steel plate and then treating it at a temperature of 30 to 40°C for 60 to 120 seconds. More specifically, phosphate treatment involves shaping the steel plate according to its intended use, degreasing it to remove surface oil, surface conditioning it, and then applying a zinc phosphate solution by immersion or spraying and treating it at a temperature of 30 to 40°C for 120 seconds.

[0075] A method for manufacturing a steel sheet with excellent phosphate reactivity according to an embodiment of the present invention includes: hot rolling a slab to manufacture a hot-rolled sheet; cold rolling the hot-rolled sheet to manufacture a cold-rolled sheet; final annealing the cold-rolled sheet; and temper rolling the annealed cold-rolled steel sheet.

[0076] The steps are explained in detail below.

[0077] First, hot rolling is performed on the slab to produce hot-rolled plates.

[0078] As for the alloy composition of the slab, the alloy composition of the steel plate has already been described, so it will not be repeated here. The alloy composition does not change substantially during the manufacturing process of the steel plate; therefore, the alloy composition of the steel plate and the slab is actually the same.

[0079] Before hot rolling, the slab can be heated. The heating temperature of the slab can be above 1200°C, and temperatures of 1200°C or higher may be required because most of the precipitates present in the steel must be redissolved. More specifically, the heating temperature of the slab can be above 1250°C.

[0080] In the process of manufacturing hot-rolled steel sheets, the final rolling temperature (FDT) can be between 800°C and 950°C. Specifically, it can be between 850°C and 930°C.

[0081] In the manufacturing process of hot-rolled steel sheets, the coiling temperature can be 650°C or lower. The coiling temperature affects the fraction of ferrite and other phases such as cementite; the higher the coiling temperature, the higher the cementite fraction. Appropriately adjusted cementite fraction may have a beneficial effect on improving phosphate reactivity.

[0082] After the hot-rolled steel sheet manufacturing step, cold-rolled steel sheet is manufactured by cold-rolling the hot-rolled steel sheet. At this point, the reduction rate can be 70% to 85%. Within this range, the surface fiber texture (γ-fiber texture) is maximized, which is beneficial for phosphate reactivity.

[0083] Next, the cold-rolled sheet undergoes final annealing.

[0084] At this point, the soaking temperature can be between 700 and 780°C. As the annealing temperature decreases, the proportion of oxides formed on the steel surface decreases, which is beneficial to acid reactivity. However, at low annealing temperatures, the diffusion of phosphorus (P) to the surface decreases, and since this also inhibits acid reactivity, an appropriate lower limit temperature is required.

[0085] In the annealing step, annealing can be carried out in an atmosphere containing more than 5% by volume of hydrogen and the remainder of nitrogen, and under conditions where the dew point is below -30°C. By managing the annealing atmosphere in a reducing manner and at a low dew point temperature, oxides formed on the surface can be suppressed as much as possible.

[0086] Next, the annealed cold-rolled steel sheet is subjected to quenching and tempering rolling. The quenching and tempering rolling can be performed with a reduction rate of 1.0% to 3.0%. A more suitable reduction rate varies with the sample thickness, but can be between 1.0% and 2.0%.

[0087] After the annealing step, the cold-rolled steel sheet can be cooled to a final cooling temperature of 80°C to 150°C before the temper rolling step. A lower final cooling temperature is more advantageous, but for operating conditions, it can be cooled to 90°C to 120°C.

[0088] The invention will be described in more detail below by way of examples. However, the following examples are merely illustrative and the invention is not limited to them.

[0089] Example 1

[0090] Cold-rolled steel sheets are manufactured by hot rolling, cold rolling, annealing, and temper rolling of slabs with the composition shown in Table 1 below. After hot rolling, the coiling temperature is fixed at 700°C, the cold rolling reduction rate is 80%, the annealing temperature is 760°C, and the final cooling temperature after annealing is 100°C. The temper rolling reduction rate is adjusted to 1.5%, and the final thickness is 1.0 mm. During annealing heat treatment, the hydrogen concentration is controlled at 4.5%, and the dew point is controlled at -40°C.

[0091] GDS analysis was performed on the final manufactured cold-rolled sheet, and the results are shown in Table 1. Additionally, the indices of the surface elements shown in Equation 1 are also presented.

[0092] GDS analysis, based on the Zn Galv RF measurement method, was performed by applying a 21W potential at a voltage of 700V and a current of 30mA, with measurements taken at a scan rate of 1000 points per second. After measuring along the thickness direction from the surface to a depth of 0.01μm, the content of each element was calculated using a calibration factor of 0.7.

[0093] In addition, the oxide layer thickness of the manufactured steel plate and the pickle lag time (the time required for hydrogen bubbles to cover the entire area of ​​the steel plate after immersion in 5% sulfuric acid at 30°C) were analyzed by GDS. The corrosion loss rate (representing corrosion loss per unit time), yield strength of the steel, and crack formation tendency of the folded part when folded at 180 degrees were also analyzed by immersion in the same solution for 5 minutes. These results are summarized in Table 2.

[0094] Pickle lag (P / L) is measured by depositing a 75×100mm sample in a 5% wt% sulfuric acid aqueous solution. After confirming 100% water wettability and degreasing performance, the degree of H2 formation due to Fe ion elution on the surface is measured, and the time required for hydrogen to cover the entire area is also measured.

[0095] The corrosion reduction rate is calculated by dividing the initial weight and final weight of the sample by the immersion time and immersion area after immersion in a 5% by weight sulfuric acid aqueous solution at 30°C for 5 minutes.

[0096] Additionally, after the sample is manufactured by 180-degree folding, it is determined whether cracks appear in the sample.

[0097] The cementite fraction was measured after polishing the steel sheet surface (i.e., the surface to which phosphate was applied).

[0098] The long axis of the phosphate particles was treated by applying a zinc phosphate solution and then holding it at 30–40°C for 60–120 seconds. The length of the longest axis was measured by observing the individual phosphate particles formed on the steel plate surface. After measuring more than 30 randomly selected individual phosphate particles, the average of the measurements was calculated.

[0099] Table 1

[0100]

[0101] Table 2

[0102]

[0103] In Comparative Examples 2, 3, and 6, excessive amounts of Mn, Al, and Si were added to the steel plates, thus failing to satisfy Equation 1, and resulting in a thick oxide layer. Consequently, the pickle lag time increased, and the corrosion reduction rate decreased. That is, the phosphate reactivity was poor.

[0104] Comparative Example 4 did not satisfy Equation 1 due to insufficient added phosphorus (P). Because it lacked adequate P to promote acid reactivity, the pickle lag time was longer and the corrosion reduction rate was lower. In other words, phosphate reactivity was poor.

[0105] In Comparative Examples 7 and 8, the carbon content was either too high or too low, and cementite did not form properly, resulting in increased pickle lag time and decreased corrosion reduction rate. That is, phosphate reactivity was poor. Furthermore, it was confirmed that cracking occurred when the yield strength was insufficient or too high.

[0106] In the case of Comparative Example 1, by controlling the Mn content, which has a solid solution strengthening effect, to be low, there is a problem of insufficient strength.

[0107] In Comparative Example 5, it can be confirmed that the yield strength increases and cracks are generated due to the excessively high P content.

[0108] Figure 2 These are photographs taken using a scanning electron microscope (SEM) of the outer surface of the steel plates manufactured in Example 1 and Comparative Example 4 after phosphate treatment.

[0109] As can be seen, Example 1, which has a shorter pickle lag time and a higher corrosion reduction rate, has finer phosphate particles compared to Comparative Example 4, and these particles are evenly distributed across the entire surface of the steel plate (approximately 100%).

[0110] Figure 3 The results of glow dispersion spectroscopy (GDS) analysis of the P content of the steel plates manufactured in Examples 1, 5, 4, and 5 are shown.

[0111] like Figure 3 As shown, it can be confirmed that the P content in the oxide layer also increases with the increase of P content.

[0112] Example 2

[0113] The slab having the composition of Example 1 was hot-rolled, cold-rolled, annealed, and temper-rolled with a reduction rate of 1.5% to produce cold-rolled steel sheet. However, the conditions in each process were adjusted as shown in Table 3 below.

[0114] Table 3

[0115]

[0116] As shown in Table 3, it was found that the manufacturing conditions of the steel plate affected the reactivity of phosphate.

[0117] As shown in Comparative Examples 9 and 10, the higher the winding temperature, the higher the fraction of ideal cementite. When the fraction is low, such as in Comparative Example 9, acid reactivity is suppressed, and even when the fraction is too high, such as in Comparative Example 10, the reaction zone of the cementite phase with low acid reactivity widens, and the phenomenon of decreased reactivity can be confirmed.

[0118] Comparative Examples 11 and 12 illustrate the influence of annealing temperature. Lower annealing temperatures result in a lower proportion of oxides forming on the steel surface, which is beneficial for acid reactivity but also has adverse effects. That is, when the annealing temperature is too high or too low, it can be confirmed that the value of Equation 1 is not satisfied and acid reactivity decreases.

[0119] This invention can be implemented in various ways and is not limited to the embodiments described. Those skilled in the art will understand that the invention can be implemented in other specific ways without altering its technical concept or essential features. Therefore, it should be understood that the embodiments described are exemplary in all respects and are not restrictive.

[0120] Symbol Explanation

[0121] 10: Steel plate; 20: Oxide layer

Claims

1. A steel plate with excellent phosphate reactivity, wherein, The steel plate, by weight percent, contains carbon (C): 0.02 to 0.06%, silicon (Si): less than 0.01% and 0%, manganese (Mn): 0.1 to 0.24%, aluminum (Al): less than 0.02% and 0%, phosphorus (P): 0.015 to 0.04%, with the balance being Fe and unavoidable impurities. The steel plate has an oxide layer with a thickness of less than 10 nm extending from its surface to its interior. And it satisfies the following equation 1, It contains cementite with an area fraction of 2% or more, and the remainder contains ferrite. [Formula 1] ([Mn] + [Si] + [Al]) / (3 × [P]) ≤ 0.6 In Equation 1, [Mn], [Si], [Al] and [P] represent the highest content of each element when performing elemental analysis in the thickness direction of the oxide layer.

2. The steel plate with excellent phosphate reactivity according to claim 1, wherein, The pickling lag time when immersing steel plates in a 5% sulfuric acid aqueous solution at 30°C is less than 20 seconds.

3. The steel plate with excellent phosphate reactivity according to claim 1, wherein, The corrosion reduction rate of steel plates immersed in a 5% sulfuric acid aqueous solution at 30°C was 0.55 mg / cm². 2 / hr or more.

4. The steel plate with excellent phosphate reactivity according to claim 1, wherein, The yield strength is 220 to 270 MPa.

5. The steel plate with excellent phosphate reactivity according to claim 1, wherein, The average major axis length of the phosphate particles formed after phosphate treatment is less than 10 μm.

6. The steel plate with excellent phosphate reactivity according to claim 1, wherein, Phosphate particles formed after phosphate treatment account for more than 90% of the surface area of ​​the steel plate.

7. A method for manufacturing a steel plate with excellent phosphate reactivity, comprising: The step of hot rolling a slab to produce a hot-rolled steel sheet, wherein the slab comprises, by weight %, carbon (C): 0.02 to 0.06%, silicon (Si): less than 0.01% and excluding 0%, manganese (Mn): 0.1 to 0.24%, aluminum (Al): less than 0.02% and excluding 0%, phosphorus (P): 0.015 to 0.04%, with the balance being Fe and unavoidable impurities; The step of cold rolling the hot-rolled steel sheet to manufacture cold-rolled steel sheet; The step of annealing the cold-rolled steel sheet; The step of quenching and tempering the annealed cold-rolled steel sheet. In the process of manufacturing the hot-rolled steel sheet, the coiling temperature is 650 to 750°C. In the annealing step, the soaking temperature is 700 to 780°C, and After the annealing step of the cold-rolled steel sheet, and before the temper rolling step of the annealed cold-rolled steel sheet. Cool to a final cooling temperature of 80 to 150°C.

8. The method for manufacturing a steel plate with excellent phosphate reactivity according to claim 7, wherein, In the process of manufacturing the hot-rolled steel sheet The final hot rolling temperature is 800 to 950°C.

9. The method for manufacturing a steel plate with excellent phosphate reactivity according to claim 7, wherein, In the process of manufacturing cold-rolled steel sheets by cold rolling... The reduction rate is 70% to 85%.

10. The method for manufacturing a steel plate with excellent phosphate reactivity according to claim 7, wherein, In the annealing step, the annealing process is carried out in an atmosphere containing more than 5% by volume of hydrogen and the remainder of nitrogen, and at a dew point below -30°C.

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