Steel sheet with excellent phosphate reactivity and manufacturing method thereof

Abstract

A steel sheet having excellent phosphate reactivity according to one embodiment of the present invention contains, by weight, 0.02 to 0.06% carbon (C), 0.01% or less (excluding 0%) silicon (Si), 0.1 to 0.24% manganese (Mn), 0.02% or less (excluding 0%) aluminum (Al), 0.015 to 0.04% phosphorus (P), and the balance being iron (Fe) and unavoidable impurities, and has an oxide layer having a thickness of 10 nm or less from the surface toward the inside of the steel sheet, and satisfies the following formula 1. [Formula 1] ([Mn]+[Si]+[Al]) / (3×[P])≦0.60 (In formula 1, [Mn], [Si], [Al], and [P] represent the maximum content of each element when the oxide layer is subjected to elemental analysis in the thickness direction.)

Description

[Technical Field]

[0001] One embodiment of the present invention relates to a steel sheet having excellent phosphate reactivity and a manufacturing method thereof. Specifically, one embodiment of the present invention relates to a steel sheet having excellent phosphate-treated surface properties with excellent corrosion resistance, characterized in that, in a phosphate treatment performed to impart corrosion resistance to the surface of a steel sheet used as a raw material for drums, the phosphate crystals formed on the surface after the phosphate treatment are fine and uniformly distributed over the entire surface of the steel sheet, and a manufacturing method thereof. [Background technology]

[0002] The surface of steel materials is treated with phosphate to ensure rust prevention and improve long-term corrosion resistance, as well as to improve adhesion before and during painting.

[0003] Phosphating is a process in which an electrochemical potential difference is generated when the phosphate solution comes into contact with the steel sheet, dissolving the steel sheet and ionizing the iron, generating electrons. As the pH rises, stable metallic phosphate crystals are formed and grow on the steel sheet surface. Phosphating is a process used to impart paintability and corrosion resistance to steel sheets for automobiles, drums, and electrical steel sheets.

[0004] The solution typically used for phosphating is zinc phosphate. Depending on the crystalline form of the phosphate formed on the steel sheet surface, the resulting crystalline structure can be either phosphophyllite or hopeite, or a mixture of the two. Phosphophyllite is a spherical, dense crystal that forms when iron ions react with each other within the phosphate crystals. Hopeite has a granular structure with narrow and wide morphologies, with both phases densely covering the steel. Phosphophyllite (P) has better corrosion resistance against acids and alkalis than hopeite (H), and phosphating with a relatively high P content results in even better corrosion resistance. Therefore, when the immersion treatment method is used, which is a condition that makes it easy for iron eluted from the steel sheet to be contained in the coating, the ratio of P increases on the surface, but when the spray treatment is used, the ratio of H is relatively high, although this differs depending on the treatment solution.

[0005] The quality of phosphating ability is ultimately determined by how densely the phosphate crystals cover the steel sheet surface after the phosphating process, which is determined by the size and coverage of the phosphate crystals.

[0006] Factors that inhibit the acid reactivity of steel sheets are generally the type and thickness of oxides covering the steel sheet surface. In particular, when the oxides are thick, the rate of Fe dissolution for the growth of phosphate nuclei that become the nuclei of phosphate slows down, reducing the density of the phosphate nuclei, and the sparsely formed phosphate nuclei lead to coarsening of phosphate crystals and low coverage.

[0007] Recently, environmental regulations have led to a gradual dilution of the concentration of phosphating solutions, resulting in problems with smooth phosphating. For phosphate to be applied effectively to the steel sheet surface, the steel sheet must react with phosphoric acid to rapidly form a high density of phosphate nuclei. However, a decrease in the concentration of phosphating solutions due to wastewater treatment issues hinders the smooth initial acid reaction, inhibiting the formation of phosphate nuclei. This results in coarsening of phosphate crystals, which fail to cover the entire steel surface. In other words, even low phosphoric acid concentrations that do not ensure sufficient reactivity continue to negatively impact phosphating performance. Summary of the Invention [Problem to be solved by the invention]

[0008] One embodiment of the present invention provides a steel sheet with excellent phosphate reactivity and a manufacturing method thereof. Specifically, one embodiment of the present invention provides a steel sheet having excellent phosphate-treated surface properties with excellent corrosion resistance, characterized in that, in a phosphate treatment performed to impart corrosion resistance to the surface of a steel sheet used as a raw material for drums, the phosphate crystals formed on the surface after the phosphate treatment are fine and uniformly distributed over the entire surface of the steel sheet. [Means for solving the problem]

[0009] A steel plate having excellent phosphate reactivity according to one embodiment of the present invention contains, by weight, 0.02 to 0.06% carbon (C), 0.01% or less (excluding 0%) silicon (Si), 0.1 to 0.24% manganese (Mn), 0.02% or less (excluding 0%) aluminum (Al), 0.015 to 0.04% phosphorus (P), and the remainder being iron (Fe) and unavoidable impurities.

[0010] The steel sheet having excellent phosphate reactivity according to one embodiment of the present invention has an oxide layer having a thickness of 10 nm or less from the surface toward the interior of the steel sheet, and satisfies the following formula 1. [Formula 1] ([Mn]+[Si]+[Al]) / (3×[P])≦0.60 (In formula 1, [Mn], [Si], [Al], and [P] represent the maximum content of each element when the oxide layer is subjected to elemental analysis in the thickness direction.)

[0011] The steel sheet having excellent phosphate reactivity according to one embodiment of the present invention may contain cementite in an area fraction of 2% or more, with the remainder being ferrite.

[0012] The steel sheet having excellent phosphate reactivity according to one embodiment of the present invention may have a pickle lag time of 20 seconds or less when immersed in a 5% aqueous sulfuric acid solution at 30°C.

[0013] The steel sheet having excellent phosphate reactivity according to one embodiment of the present invention has a corrosion weight loss ratio of 0.55 mg / cm when immersed in a 5% aqueous sulfuric acid solution at 30°C. 2 / hr or more.

[0014] The steel sheet having excellent phosphate reactivity according to one embodiment of the present invention may have a yield strength of 220 to 270 MPa.

[0015] In the steel sheet having excellent phosphate reactivity according to one embodiment of the present invention, the average major axis length of phosphate particles formed after phosphate treatment may be 10 μm or less.

[0016] In the steel sheet having excellent phosphate reactivity according to one embodiment of the present invention, phosphate particles formed after phosphating can occupy 90% or more of the area of ​​the steel sheet surface.

[0017] A method for producing a steel sheet having excellent phosphate reactivity according to one embodiment of the present invention includes the steps of: hot rolling a slab containing, by weight, 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%, and the remainder being iron (Fe) and unavoidable impurities, to produce a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet; and temper rolling the annealed cold-rolled steel sheet.

[0018] When manufacturing hot-rolled steel sheets, the coiling temperature is 650-700°C. 4 The soaking temperature is 700 to 780°C in the annealing step, and then the steel sheet undergoes a temper rolling step.

[0019] In the stage of producing the hot-rolled steel sheet, the final hot rolling temperature (FDT) may be 800 to 950°C.

[0020] In the step of producing a cold-rolled steel sheet by cold rolling, the reduction may be 70 to 85%.

[0021] After the step of annealing the cold-rolled steel sheet, it can be cooled to a final cooling temperature of 80 to 150°C before the step of temper rolling.

[0022] In the annealing step, the annealing can be performed in an atmosphere containing 5% by volume or more of hydrogen and the remainder nitrogen, with a dew point of -30°C or less. [Effects of the Invention]

[0023] The steel sheet having excellent phosphate reactivity according to one embodiment of the present invention can be effectively used as a raw material for steel sheets to be subjected to phosphate treatment in order to impart paintability and rust resistance to the steel sheets.

[0024] The steel sheet having excellent phosphate reactivity according to one embodiment of the present invention can easily ensure phosphate treatment even at low phosphoric acid concentrations, and can be used not only for containers but also for automobiles and home appliances. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a schematic cross-section of a steel plate according to an embodiment of the present invention. [Figure 2] 1 is a photograph of the outer surface of the steel sheets manufactured in Example 1 and Comparative Example 4 after phosphate treatment, analyzed by a scanning electron microscope (SEM). [Figure 3] 1 is a graph showing the results of a GDS (Glow Dispersion Spectroscopy) analysis of the P content of steel sheets manufactured in Example 1, Example 5, Comparative Example 4, and Comparative Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0026] Terms such as "first," "second," and "third" are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a first part, component, region, layer, or section described below can be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0027] The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. As used in the specification, the term "comprising" refers to the inclusion of specific features, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0028] Unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight.

[0029] In one embodiment of the present invention, the inclusion of an additional element means that the remaining iron (Fe) is replaced by the additional amount of the additional element.

[0030] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted as having an ideal or very formal meaning unless otherwise defined.

[0031] Although the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein, the present invention will be described in detail below so that those skilled in the art can easily practice the present invention.

[0032] A steel plate having excellent phosphate reactivity according to one embodiment of the present invention contains, by weight, 0.02 to 0.06% carbon (C), 0.01% or less (excluding 0%) silicon (Si), 0.1 to 0.24% manganese (Mn), 0.02% or less (excluding 0%) aluminum (Al), 0.015 to 0.04% phosphorus (P), and the remainder being iron (Fe) and unavoidable impurities.

[0033] First, the composition of the steel sheet will be described in detail. As described below, Al, Mn, Si, and P in the steel sheet are concentrated in the oxide layer, and have a concentration gradient from the surface to the interior. In one embodiment of the present invention, the element content in the steel sheet refers to the average content in the thickness direction of the steel sheet.

[0034] Carbon (C): 0.02~0.06% by weight

[0035] The carbon content of the steel sheet in the present invention may be 0.02 to 0.06 wt%. If the carbon content in the steel is too low, secondary phases may not form, preventing the expected localized corrosion phenomenon. If the carbon content is too high, excessive carbide may be formed, resulting in a phenomenon in which the strength exceeds the intended level. Therefore, in one embodiment of the present invention, the carbon content is limited to 0.02 to 0.06 wt%. More specifically, it may be 0.025 to 0.055 wt%.

[0036] Silicon (Si): 0.01% by weight or less

[0037] The silicon content of the steel sheet in the present invention may be 0.01 wt% or less. If the silicon content in the steel is excessively high, SiO2 may form on the surface, and a composite phase of SiO2 and Fe oxide may also form, resulting in a large amount of red scale. This red scale may cause defects that are not removed during cold rolling and pickling, and Si oxide itself may form during cold rolling and annealing, reducing acid reactivity. Therefore, in one embodiment of the present invention, the maximum Si content is limited to 0.01 wt% or less. More specifically, it may be 0.001 to 0.01 wt%. Even more specifically, it may be 0.003 to 0.009 wt%.

[0038] Manganese (Mn): 0.10 to 0.24% by weight

[0039] Mn is an element that typically forms oxides on the surface during the annealing heat treatment of cold-rolled steel sheets. In one embodiment of the present invention, the Si content, which can form surface oxides and inhibit acid reactivity during the annealing heat treatment, is limited to 0.01 wt% or less. Because the Si oxide itself can be formed in large amounts in this environment, the Mn content is controlled to 0.24 wt% or less to actively suppress Mn oxide formation. However, since Mn is a typical solid solution strengthening element, an excessively low Mn content can result in a decrease in strength. Therefore, Mn may be included in an amount of 0.10 to 0.24 wt%. More specifically, Mn may be included in an amount of 0.11 to 0.24 wt%.

[0040] Aluminum (Al): 0.020% by weight or less,

[0041] Al is a typical element used as a deoxidizer. However, in one embodiment of the present invention, Al also forms Al oxides on the surface of the steel material, and when Al oxides are formed, they can inhibit acid reactivity. Therefore, Al may be contained in an amount of 0.020 wt% or less. More specifically, Al may be contained in an amount of 0.001 to 0.020 wt%. Even more specifically, Al may be contained in an amount of 0.010 to 0.019 wt%.

[0042] Phosphorus (P): 0.015 to 0.040% by weight

[0043] In one embodiment of the present invention, P acts to cause a leaching reaction of Fe when the steel is placed in an acidic environment. Therefore, the P content can be limited to 0.015 wt% or more. However, since P is a typical element that causes room temperature embrittlement, and FeP can weaken formability when precipitated at grain boundaries, the upper limit can be limited to 0.040 wt%. Therefore, P can be contained in an amount of 0.015 to 0.040 wt%. More specifically, P can be contained in an amount of 0.016 to 0.038 wt%.

[0044] In addition to the aforementioned elemental components, the present invention also includes Fe and unavoidable impurities. Since unavoidable impurities are widely known in the art, detailed description thereof will be omitted. In one embodiment of the present invention, the addition of other effective components in addition to the aforementioned components is not excluded, and when additional components are further included, the remaining Fe can be replaced with the additional components.

[0045] Fig. 1 shows a schematic cross section in the thickness direction of a steel sheet according to one embodiment of the present invention. As shown in Fig. 1, a steel sheet 10 according to one embodiment of the present invention has an oxide layer 20 extending from the surface of the steel sheet toward the interior of the steel sheet. Although Fig. 1 shows that the oxide layer 20 is present on only one side of the steel sheet, it is also possible for the oxide layer 20 to be present on both sides.

[0046] The oxide layer 20 refers to the depth from the steel sheet surface to the point where the oxygen peak becomes '0' in the Fe-O diagram shown as the GDS result.

[0047] The thickness of the oxide layer 20 may be 10.0 nm or less. If the oxide layer 20 is too thick, the acid reactivity may be slow and inappropriate. More specifically, the thickness of the oxide layer 20 may be 1 to 10.0 nm.

[0048] During the steel sheet manufacturing process described below, elements such as Mn, Si, Al, and P contained in the steel sheet diffuse from the inside of the steel sheet to the surface of the steel sheet and are concentrated in the oxide layer 20 .

[0049] At this time, the contents of Mn, Si, Al, and P present in the oxide layer 20 may satisfy the following formula 1. [Formula 1] ([Mn]+[Si]+[Al]) / (3×[P])≦0.60 (In formula 1, [Mn], [Si], [Al], and [P] represent the maximum content of each element when the oxide layer is subjected to elemental analysis in the thickness direction.)

[0050] When the value of formula 1 exceeds 0.6, the oxide layer contains little P or contains a large amount of Mn, Si, or Al. When the oxide layer contains little P, the amount of P, which is an element that ensures acid reactivity, is reduced, making it impossible to obtain appropriate phosphate reactivity. Furthermore, when the content of Mn, Si, or Al is high, large amounts of oxides of Mn, Si, or Al are formed, making it impossible to obtain appropriate phosphate reactivity. Therefore, as described above, the content of formula 1 may be 0.60 or less. More specifically, the value of formula 1 may be 0.20 to 0.60.

[0051] The maximum P content in the oxide layer 20 may be 1.0 to 3.0 wt %, the maximum Mn content may be 0.80 to 1.5 wt %, the maximum Si content may be 0.50 to 1.50 wt %, and the maximum Al content may be 0.30 to 1.0 wt %.

[0052] According to one embodiment of the present invention, a steel sheet with excellent phosphate reactivity may contain 2.0 to 5.0 area percent cementite, with the remainder being ferrite. It is known that corrosion due to acid reaction occurs due to the formation of small circuits within the electrolyte. In this case, if only a stable ferrite-based Fe single phase, which causes a cathodic reaction, is present, the acid reaction does not occur, and cathodic sites, such as cementite, can promote the reaction. However, since such a cathode has low dissolution potential in an acidic environment, an excessively large amount of cementite may actually worsen the acid reactivity. More specifically, the steel sheet may contain 2.0 to 5.0 area percent cementite. Other phases may also be present at up to 0.5 area percent.

[0053] In one embodiment of the present invention, the steel plate has excellent phosphate reactivity, excellent corrosion resistance, suitable yield strength, and excellent productivity.

[0054] In one embodiment of the present invention, phosphate reactivity is measured using a pickle lag (P / L) test. This test involves immersing a 75 x 100 mm specimen in a 5 wt% sulfuric acid aqueous solution, degreasing the surface with alkali, and then confirming 100% water wettability to confirm degreasing performance. The test then measures the amount of H gas formed by the dissolution of Fe ions on the surface, thereby indirectly measuring acid reactivity. This test measures the time it takes for the entire surface area to be covered with hydrogen gas. Consequently, a longer P / L time indicates a greater influence of surface oxides, resulting in poor acid reactivity and, therefore, poor phosphate treatability. In one embodiment of the present invention, the pickle lag time may be 20 seconds or less when the steel sheet is immersed in a 5% sulfuric acid aqueous solution at 30°C. More specifically, the pickle lag time may be 5 to 20 seconds.

[0055] While pickle lag is measured by observing the steel sheet surface with a camera, it is possible that minute hydrogen gas particles, which are invisible to the naked eye, may not be detected. In one embodiment of the present invention, in addition to the pickle lag time, a steel sheet is directly immersed in a 5 wt% aqueous sulfuric acid solution at 30°C for 5 minutes. The corrosion weight loss ratio is calculated by dividing the initial and final weights of the specimen by the immersion time and immersion area to quantify phosphate reactivity. The corrosion weight loss ratio is an indicator of acid reactivity and indicates how quickly iron ions are released from a steel sheet when it is exposed to an acid environment of a certain concentration. In other words, a specimen with a higher corrosion weight loss ratio exhibits easier iron release, facilitating phosphate nucleation and a higher density of phosphate nuclei, which indicates easier phosphate treatment.

[0056] In one embodiment of the present invention, the corrosion weight loss ratio is 0.550 mg / cm when immersed in a 5% sulfuric acid aqueous solution at 30°C. 2 / hr or more. More specifically, the corrosion weight loss rate may be 0.550 to 0.700 mg / cm 2 / hr may also be used.

[0057] When manufacturing a product using a steel sheet according to one embodiment of the present invention, it is necessary to ensure formability for soundness during the manufacturing process. That is, it is necessary to ensure strength for pressure resistance, dent resistance, and the like in the usage environment. Therefore, in one embodiment of the present invention, the steel sheet may have a yield strength of 220 to 270 MPa. If the yield strength is excessively high, formability may become an issue, and if the yield strength is excessively low, problems may arise in terms of pressure resistance and dent resistance.

[0058] As described above, the steel sheet according to one embodiment of the present invention is easily phosphating-resistant, and after phosphating, fine phosphate particles having an average major axis length of 10 μm or less are present on the surface of the steel sheet, which can cover 90% or more of the entire observation area.

[0059] The phosphate particles formed in the present invention are mainly leaf-shaped hopeite particles. The length of the long axis of a hopeite particle is defined as the length of the longest axis when observing a single phosphate particle on the observation surface. To calculate the average value, 30 or more randomly calculated single phosphate particles can be measured and then the average of the measured values ​​can be calculated. The observation surface may be a surface parallel to the rolling surface (ND surface).

[0060] In this case, phosphating refers to applying a zinc phosphate solution to a steel sheet and then treating it at a temperature of 30 to 40°C for 60 to 120 seconds. More specifically, phosphating refers to forming the steel sheet to suit its intended use, then removing any oil applied to the surface through a degreasing process, conditioning the surface, and then applying a zinc phosphate solution by immersion or spraying it, followed by treating it at a temperature of 30 to 40°C for 60 to 120 seconds.

[0061] A method for manufacturing a steel sheet having excellent phosphate reactivity according to an embodiment of the present invention includes the steps of hot rolling a slab to manufacture a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; annealing the cold-rolled steel sheet; and temper rolling the annealed cold-rolled steel sheet.

[0062] Each step will be explained in detail below.

[0063] First, a slab is hot rolled to produce a hot rolled steel sheet.

[0064] The alloy composition of the slab has been explained above for the steel plate, so a duplicate explanation will be omitted. Since the alloy composition does not substantially change during the manufacturing process of the steel plate, the alloy composition of the steel plate and the alloy composition of the slab are substantially the same.

[0065] The slab can be heated before hot rolling. The slab heating temperature may be 1200°C or higher. A temperature of 1200°C or higher is required because most of the precipitates present in the steel must be redissolved. More specifically, the slab heating temperature may be 1250°C or higher.

[0066] In the stage of producing the hot-rolled steel sheet, the final hot rolling temperature (FDT) may be 800 to 950°C. More specifically, it may be 850 to 930°C.

[0067] In the process of manufacturing hot-rolled steel sheets, the coiling temperature is 650-7 4 The coiling temperature may be 0°C. The coiling temperature affects the fraction of two phases such as cementite in addition to the ferrite single phase, and the higher the coiling temperature, the higher the cementite fraction. A properly adjusted cementite fraction can have an advantageous effect on improving phosphate reactivity.

[0068] After the step of producing the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. At this time, the rolling reduction may be 70 to 85%. Within the above range, the surface γ-fiber texture is maximized, which is advantageous for phosphate reactivity.

[0069] Next, the cold rolled steel sheet is annealed.

[0070] In this case, the soaking temperature may be 700 to 780°C. A lower annealing temperature is advantageous for acid reactivity because it has the effect of reducing the fraction of oxides formed on the steel surface. However, at a low annealing temperature, the diffusion of P to the surface decreases, which also inhibits acid reactivity, so an appropriate lower limit temperature is necessary.

[0071] The annealing step can be performed in an atmosphere containing 5% or more by volume of hydrogen and the remainder of nitrogen, with a dew point of -30°C or less. By controlling the annealing atmosphere to be reducing and with a low dew point, oxides formed on the surface can be minimized.

[0072] Next, the annealed cold-rolled steel sheet is temper rolled. Temper rolling can be performed at a reduction rate of 1.0 to 3.0%. A more suitable reduction rate varies in proportion to the thickness of the specimen, but may be 1.0 to 2.0%.

[0073] After annealing the cold-rolled steel sheet, it can be cooled to a final cooling temperature of 80 to 150°C before temper rolling. The lower the final cooling temperature, the more advantageous it is, but it can be cooled to 90 to 120°C depending on the operating conditions.

[0074] The present invention will be described in more detail below through examples, but these examples are merely for illustrative purposes and are not intended to limit the scope of the present invention. [Example]

[0075] Experimental Example 1

[0076] A slab having the composition shown in Table 1 below was hot rolled, cold rolled, annealed, and temper rolled to produce a cold-rolled steel sheet. After hot rolling, the coiling temperature was fixed at 700°C, the cold reduction was 80%, the annealing temperature was 760°C, and the final cooling temperature after annealing was 100°C. The temper rolling reduction was adjusted to 1.5%, producing a final thickness of 1.0 mm. During the annealing heat treatment, the hydrogen concentration was controlled at 4.5%, and the dew point was controlled at -40°C.

[0077] The final cold-rolled sheet was analyzed by GDS analysis, and the results are shown in Table 1. The surface element indexes shown in Equation 1 are also shown.

[0078] GDS analysis was performed using the Zn Galv RF measurement method, applying a voltage of 700V, a current of 30mA, and a potential of 21W at a scan rate of 1000 points per second. Measurements were taken from the surface to a depth of 0.01μm in the thickness direction, and the content of each element was calculated using a calibration factor of 0.7.

[0079] In addition, the oxide layer thickness analyzed through GDS of the manufactured steel sheets, the pickle lag time (the time it takes for hydrogen bubbles to cover the entire area of ​​the steel sheet after immersion in 5% sulfuric acid at 30°C), the corrosion weight loss ratio (the corrosion weight loss per unit surface area and unit time when immersed in the same solution for 5 minutes), the yield strength of the steel, and the tendency for crack formation in the folding area when folded 180 degrees were measured and summarized in Table 2.

[0080] Pickle lag (P / L) was measured by first degreasing the surface of a 75 x 100 mm specimen with a 5 wt% aqueous sulfuric acid solution, then confirming that the wettability was 100% to confirm the degreasing performance. After that, the specimen was immersed in the solution to measure the amount of H2 gas formed on the surface by the elution of Fe ions, and the time it took for the hydrogen gas to cover the entire surface was measured.

[0081] The corrosion weight loss ratio was calculated by immersing the specimen in a 5 wt% sulfuric acid aqueous solution at 30°C for 5 minutes, and then dividing the initial weight and final weight of the specimen by the immersion time and immersion area.

[0082] In addition, the manufactured specimens were 0 degrees After folding, it was determined whether cracks occurred in the folded portion of the specimen.

[0083] The cementite fraction was measured after polishing the surface of the steel sheet on which the phosphate was applied.

[0084] The longest axis of the phosphate particles was measured by applying a zinc phosphate solution, maintaining it at 30-40°C for 60-120 seconds, and observing the single phosphate particles formed on the steel sheet surface. The average of 30 or more randomly calculated single phosphate particles was calculated.

[0085] [Table 1]

[0086] [Table 2] In Comparative Examples 2, 3, and 6, excessive amounts of Mn, Al, and Si were added to the steel sheets, which did not satisfy Formula 1 and resulted in a thick oxide layer, which resulted in a longer pickle lag time and a smaller corrosion weight loss ratio, i.e., poorer phosphate reactivity.

[0087] In Comparative Example 4, the P content was insufficient, so Equation 1 was not satisfied. The P that promotes acid reactivity was not adequately contained, resulting in a long pickle lag time and a small corrosion weight loss ratio. In other words, the phosphate reactivity was poor.

[0088] In Comparative Examples 7 and 8, the C content was either excessive or insufficient, resulting in inadequate cementite formation, prolonged pickle lag time, and reduced corrosion weight loss. This resulted in poor phosphate reactivity. Additionally, the yield strength was either not achieved or was too high, resulting in cracks.

[0089] In the case of Comparative Example 1, there was a problem that the strength was not achieved due to the content of Mn, which has a solid solution strengthening effect, being controlled to a low level.

[0090] In Comparative Example 5, the P content was too high, which increased the yield strength and caused cracks.

[0091] FIG. 2 is a photograph of the outer surface of the steel sheets manufactured in Example 1 and Comparative Example 4 after phosphate treatment, analyzed by a scanning electron microscope (SEM).

[0092] It can be seen that Example 1, which has a short pickle lag time and a large corrosion weight loss ratio, has finer phosphate particles than Comparative Example 4, and is uniformly distributed over the entire surface (nearly 100%) of the steel sheet.

[0093] FIG. 3 shows the results of GDS (Glow Dispersion Spectroscopy) analysis of the P content of the steel sheets manufactured in Example 1, Example 5, Comparative Example 4, and Comparative Example 5.

[0094] As shown in FIG. 3, it can be seen that the P content in the oxide layer increases as the P content increases.

[0095] Experimental Example 2

[0096] A slab having the composition of Example 1 below was hot-rolled, cold-rolled, annealed, and temper-rolled at a reduction rate of 1.5% to produce a cold-rolled steel sheet, where the conditions for each process were adjusted as shown in Table 3 below.

[0097] [Table 3] As shown in Table 3, it became clear that the manufacturing conditions of the steel sheet affect the phosphate reactivity.

[0098] As shown in Comparative Examples 9 and 10, the higher the coiling temperature, the higher the fraction of two-phase cementite. When the fraction is low, as in Comparative Example 9, acid reactivity is inhibited, and when the fraction is excessively high, as in Comparative Example 10, the reaction area of ​​the cementite phase, which has low acid reactivity, becomes wider, and this phenomenon of reduced reactivity can be confirmed.

[0099] Comparative Examples 11 and 12 demonstrate the influence of annealing temperature. Lower annealing temperatures are beneficial for acid reactivity because they reduce the fraction of oxides formed on the steel surface. However, the P content, which is effective in the present invention, is low at low annealing temperatures, which has the opposite effect of inhibiting acid reactivity. In other words, when the annealing temperature is too high or too low, the value of Equation 1 is not satisfied, and it can be seen that acid reactivity is reduced.

[0100] The present invention is not limited to the examples, and can be manufactured in various different forms, and those skilled in the art will understand that the present invention can be embodied in other specific forms without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the above-described examples are illustrative in all respects and are not limiting. [Explanation of symbols]

[0101] 10: Steel plate 20: Oxide layer

Claims

1. In weight percent, carbon (C): 0.02 to 0.06%, silicon (Si): 0.01% or less (0% excluded), manganese (Mn): 0.1 to 0.24%, aluminum (Al): 0.02% or less (0% excluded), phosphorus (P): 0.015 to 0.04%, and the balance being iron (Fe) and inevitable impurities; an oxide layer having a thickness of 10 nm or less is present from the surface toward the inside of the steel sheet, The following formula 1 is satisfied: A steel plate with excellent phosphate reactivity, containing at least 2% cementite by area fraction, with the remainder being ferrite. [Formula 1] ([Mn]+[Si]+[Al]) / (3×[P])≦0.60 (In formula 1, [Mn], [Si], [Al], and [P] represent the maximum content of each element when the oxide layer is subjected to elemental analysis in the thickness direction.)

2. 2. The steel sheet having excellent phosphate reactivity according to claim 1, wherein the pickle lag time when the steel sheet is immersed in a 5% aqueous sulfuric acid solution at 30°C is 20 seconds or less.

3. 3. A steel sheet having excellent phosphate reactivity according to claim 1, wherein the steel sheet has a corrosion weight loss rate of 0.55 mg / cm<2> / hr or more when immersed in a 5% aqueous sulfuric acid solution at 30[deg.] C.

4. The steel plate having excellent phosphate reactivity according to any one of claims 1 to 3, wherein the steel plate has a yield strength of 220 to 270 MPa.

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

6. 6. A steel sheet having excellent phosphate reactivity according to claim 1, wherein phosphate particles formed after the phosphate treatment occupy 90% or more by area of ​​the steel sheet surface.

7. a step of manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, in weight percent, 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%, and the balance being iron (Fe) and unavoidable impurities; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet; and temper rolling the annealed cold-rolled steel sheet, In the step of manufacturing the hot-rolled steel sheet, the coiling temperature is 650 to 740°C, The method for producing a steel sheet having excellent phosphate reactivity according to any one of claims 1 to 6, wherein the soaking temperature in the step of annealing the cold-rolled steel sheet is 700 to 780°C.

8. In the step of manufacturing the hot-rolled steel sheet, The method for producing a steel sheet having excellent phosphate reactivity according to claim 7, wherein the final hot rolling temperature (FDT) is 800 to 950°C.

9. a step of producing a cold-rolled steel sheet by cold rolling; The method for producing a steel sheet having excellent phosphate reactivity according to claim 7 or 8, wherein the rolling reduction is 70 to 85%.

10. After the step of annealing the cold-rolled steel sheet and before the step of temper rolling The method for producing a steel sheet excellent in phosphate reactivity according to any one of claims 7 to 9, wherein the steel sheet is cooled to a final cooling temperature of 80 to 150°C.

11. The method for producing a steel sheet having excellent phosphate reactivity according to any one of claims 7 to 10, wherein the annealing step comprises annealing in an atmosphere containing 5% by volume or more of hydrogen and the remainder of nitrogen at a dew point of -30°C or less.

Images