Low-cost hot-rolled steel plate for electrostatic dry powder enamel and manufacturing method for steel plate

Abstract

A low-cost hot-rolled steel plate for electrostatic dry powder enamel and a manufacturing method for the steel plate. The steel plate comprises the following chemical components in percentage by weight: C: 0.015%-0.05%; Si≤0.03%; Mn: 0.1%-0.5%; Al: 0.01%-0.05%; Cr: 0.01%-0.04%; Cu: 0.01%-0.06%; N: 0.004%-0.012%; B: 0.001%-0.0035%; and V: 0.002%-0.015%; and the balance being iron and other inevitable impurities, and the following condition is met: 1.27×[B] / ([N]×[V]×103)>1.0. The steel plate has a yield strength greater than or equal to 200 MPa, a tensile strength greater than or equal to 290 MPa, an elongation greater than or equal to 40%, and a reaming rate greater than or equal to 95%, and can meet the scale explosion proof requirement required by the electrostatic enamel process.

Description

A low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel and its manufacturing method Technical Field

[0001] The invention belongs to the technical field of enameled steel plate processing, and in particular relates to a low-cost hot-rolled steel plate suitable for electrostatic dry powder enameling and a manufacturing method thereof. Background Art

[0002] Enameled steel is a composite material made by applying porcelain enamel to a steel substrate through high-temperature enameling. It is widely used in ovens, decorative panels, kitchen and bathroom appliances, enameled steel modular tanks, and electric water heater linings. Enameling processes primarily include wet enameling and electrostatic dry powder enameling. Dry powder enameling uses electrostatic adsorption to adhere the glaze to the steel surface, resulting in a more uniform enamel layer, higher surface quality, and increased production efficiency. In recent years, electrostatic dry powder enameling has been a trend in many enameled steel product production areas to replace wet enameling.

[0003] For specialized steel plates suitable for porcelain enamel, excellent anti-scale performance is a fundamental requirement. According to hydrogen theory, steel absorbs hydrogen generated during the enameling reaction at high temperatures. The solubility of hydrogen in steel increases at high temperatures, but decreases with cooling. During the firing process of enameled products, the steel reaches saturation with hydrogen. However, as the product cools in air until the enamel solidifies, hydrogen has no time to escape, resulting in a supersaturated state within the steel. This supersaturated hydrogen, trapped where it will later precipitate, generates a pressure dependent on the amount of hydrogen released from the steel at high temperatures and the degree of porosity within the porcelain layer. When this pressure builds to a level that can cause the porcelain layer to detach, the "fish scale" flaking defect forms. Because dry-powdered enamel powder has a finer particle size, the resulting enamel layer is denser than wet-process enamel, making it more difficult for hydrogen to escape. Consequently, higher requirements are placed on the steel plate for anti-scale performance.

[0004] Chinese patent application CN101139684A discloses a hot-rolled high-strength steel plate for electrostatic dry powder enamel and its manufacturing method. The steel plate's main components, by weight, include: C: 0.02-0.1%, Si ≤ 0.1%, Mn: 0.2-1.5%, P ≤ 0.035%, S ≤ 0.035%, Al: 0.01-0.1%, N: 0.001-0.01%, Ti: 0.01-0.1%, Nb: 0.001-0.1%, with the balance being iron and unavoidable impurities. The composition design utilizes the addition of Ti and Nb to form corresponding carbides or nitrides, or other secondary phase particles, that act as hydrogen traps, achieving anti-scale properties in the enamel.

[0005] Chinese patent application CN115537653A discloses a hot-rolled enameled steel sheet and its preparation method. The steel sheet uses low carbon steel. Its composition, by weight, includes C: 0.03-0.08%, Si ≤ 0.05%, Mn: 0.40-0.80%, Ti: 0.04-0.10%, Al: 0.02-0.35%, P ≤ 0.001%, S ≤ 0.002%, Mo: 0.05-0.10%, N ≤ 0.004%, with the remainder being Fe and unavoidable impurities. Ti and Mo alloying elements are added to the composition to form precipitates such as (Ti, Mo)C, which serve as hydrogen storage traps.

[0006] Chinese patent application CN101812630A discloses hot-rolled high-strength enameled steel sheet for deep drawing and its manufacturing method; CN102181805A discloses a thin slab continuous casting and rolling line for producing steel sheet for water heater liner enameling and its method; CN103540845A discloses a hot-rolled thin-sheet enameled steel with a yield strength of 330 MPa and its manufacturing method; and CN106148855A discloses a method for producing hot-rolled enameled steel with excellent fishscale resistance. The steel sheet designs described in these patents all incorporate S, Ti, and N into their composition, creating precipitates and inclusions such as TiC, Ti4S2C2, and TiN in the final steel sheet, which act as hydrogen traps and achieve fishscale resistance.

[0007] Existing manufacturing technologies for hot-rolled steel plates suitable for electrostatic dry powder enamel mostly use the addition of alloying elements such as Ti, Nb, and Mo to form a sufficient number of precipitates or inclusions such as carbides or carbonitrides to serve as hydrogen storage traps, thereby obtaining anti-scale explosion properties of the enamel.

[0008] There are also designs that use high-B and high-N element additions, using BN second-phase particles as hydrogen storage traps. For example, Chinese patent application CN116426839A discloses a "cold-rolled enameled steel sheet and its preparation method." The steel is designed to contain 0.03-0.15% Alt, 0.004-0.015% B, and 0.01-0.03% N. Through a series of hot rolling, pickling, cold rolling, annealing and other processes, a cold-rolled steel sheet with excellent enamel anti-scale performance is finally obtained.

[0009] Summary of the Invention

[0010] The object of the present invention is to provide a low-cost hot-rolled steel plate suitable for electrostatic dry powder enameling and a method for manufacturing the same. The obtained steel plate meets the anti-scale requirements required by the electrostatic enameling process; moreover, no high content of precious alloy elements such as Ti, Nb, and Mo need to be added, and the alloy cost is low.

[0011] To achieve the above object, the technical solution of the present invention is:

[0012] A low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel, wherein the chemical composition by weight is as follows: C: 0.015-0.050%, Si≤0.030%, Mn: 0.10-0.50%, Al: 0.010-0.050%, Cr: 0.010-0.040%, Cu: 0.010-0.060%, N: 0.0040-0.0120%, B: 0.0010-0.0035%, V: 0.002-0.015%, the balance including Fe and other unavoidable impurities; and the following conditions must also be met: 1.27×[B] / ([N]×[V]×10 3 )>1.0.

[0013] Furthermore, the balance is Fe and other inevitable impurities.

[0014] In some embodiments, the hot rolled steel sheet of the present invention does not contain Ti, Nb, and Mo.

[0015] The microstructure of the steel plate of the present invention is ferrite + a small amount of pearlite. Preferably, the volume ratio of pearlite is less than 5%. The ferrite grain size is grade 7-10, and (B, V)N composite precipitates with a size (i.e., maximum length or diameter) of 50-600 nm are dispersed in the matrix.

[0016] The steel plate of the present invention has a yield strength of ≥200 MPa, a tensile strength of ≥290 MPa, an elongation of ≥40%, and a hole expansion ratio of ≥95%. In some embodiments, the yield strength of the steel plate of the present invention is ≥220 MPa. In some embodiments, the yield strength of the steel plate of the present invention is 200-250 MPa. In some embodiments, the tensile strength of the steel plate of the present invention is ≥310 MPa. In some embodiments, the tensile strength of the steel plate of the present invention is 290-340 MPa. In some embodiments, the elongation of the steel plate of the present invention is 40-50%, such as 42-47%. In some embodiments, the hole expansion ratio of the steel plate of the present invention is 95-120%.

[0017] In the chemical composition design of the low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel according to the present invention:

[0018] Carbon: In the present invention, only trace amounts of V, a carbide-forming element, are added. The carbon in the steel primarily forms pearlite, strengthening the structure. However, excessive carbon content can impair the formability of the steel sheet. Furthermore, excessively high carbon content can lead to excessive pearlite formation, which can cause large amounts of gases like CO to be generated during the enamel firing process. This can lead to poor pore structure in the enamel layer and defects such as pinholes, seriously compromising the quality of the enamel. Therefore, the carbon content in the present invention is controlled between 0.015% and 0.050%.

[0019] Silicon: Silicon element will deteriorate the plasticity of steel and also affect the adhesion between the steel plate and the enamel. It is controlled as a residual element. Therefore, the present invention controls the Si content to ≤0.030%.

[0020] Manganese: While manganese can improve steel plate strength, excessive strength can cause springback after the water heater inner tank is rolled, compromising welding performance and weld quality. Furthermore, manganese expands the austenite phase and lowers the Ac3 point, which negatively impacts the steel plate's enameling properties. Because austenite dissolves hydrogen more strongly than ferrite, cooling makes scale more likely, so excessive addition is advised. Therefore, the manganese content in this invention is controlled within a range of 0.10% to 0.50%.

[0021] Copper and chromium: Adding appropriate amounts of copper and chromium facilitates surface deposition, thereby improving adhesion to the enamel and anti-scale performance. The chromium in the steel partially replaces iron to form the alloy cementite (Fe, Cr)3C, enhancing its stability. A portion dissolves into ferrite, resulting in solid solution strengthening and increasing the ferrite's strength and hardness. However, excessive copper and chromium content not only increases costs but also enhances the steel's corrosion resistance, negatively impacting its adhesion during the enameling process. Therefore, in the present invention, the copper content is controlled between 0.010 and 0.060%, and the chromium content is controlled between 0.010 and 0.040%.

[0022] Aluminum: Aluminum is a strong deoxidizing element. To keep the oxygen content in the steel as low as possible, aluminum is often used for deoxidation in medium- and low-carbon steels. Dissolved aluminum in the steel also combines with free nitrogen to precipitate AlN, a high precipitation temperature that refines the austenite grains. This fine grain structure not only promotes fine-grain strengthening but also improves the hydrogen storage capacity of the steel plate. Therefore, the aluminum content in the present invention is controlled to 0.010-0.050%.

[0023] Vanadium and Boron: The addition of trace amounts of vanadium (V) to the steel of this invention forms a secondary phase called VN. Because V has a stronger affinity for N than B, VN precipitates before BN. When BN precipitates, it forms a (B, V)N composite precipitate with VN as its core. This composite precipitate avoids the tendency of pure BN to segregate at grain boundaries, leading to microcracks in the ingot and poor formability of the steel sheet. Furthermore, this composite precipitate exhibits greater high-temperature stability than BN, effectively acting as a hydrogen trap and meeting the anti-scale requirements of electrostatic dry powder enamel. To minimize alloy cost, only a trace amount of vanadium (0.0020-0.015%) is added in this invention. Dissolved boron in steel increases the strength of the steel sheet while reducing formability. Therefore, the B content should not be too high. In this invention, the boron content is controlled within a range of 0.0010-0.0035%.

[0024] Nitrogen: Under normal circumstances, the nitrogen content in steel should be as low as possible. In the present invention, the higher nitrogen content is mainly used to form a (B, V)N composite precipitate phase. Excess nitrogen can also form an AlN precipitate phase with Al. In the present invention, the nitrogen content is controlled at 0.004-0.012%.

[0025] The contents of the above-mentioned B, N, and V elements must also satisfy the following relationship: 1.27×[B] / ([N]×[V]×10 3 )>1.0.

[0026] The above design can form a good matching relationship between the B, N, and V elements in the steel, making full use of the difference in the precipitation order of VN and BN, and forming a (B, V)N composite precipitate phase with good high-temperature stability in the steel as a hydrogen storage trap, meeting the anti-scale performance requirements of electrostatic dry powder enamel. In some embodiments, in the steel plate of the present invention, the 1.27×[B] / ([N]×[V]×10 3 ) is in the range of 1.10 to 6.00.

[0027] The present invention also provides a method for manufacturing a low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel, comprising the following steps:

[0028] 1) Smelting, refining, and continuous casting

[0029] Smelting, RH or LF refining and casting into billets according to the above composition;

[0030] 2) Hot rolling

[0031] Heating temperature: 1150-1250°C, rough rolling and finishing rolling are carried out in sequence, the rough rolling temperature is ≥850°C, such as 850-1100°C or 880-1080°C, the finishing rolling start temperature is 900-1050°C, and the finishing rolling final temperature is 860-900°C;

[0032] 3) Coiling

[0033] The steel plate after hot rolling is laminar cooled at a cooling rate of 10-35°C / s, and then coiled to obtain hot rolled coil at a coiling temperature of 650-700°C.

[0034] 4) Pickling

[0035] The hot-rolled coil is pickled, dephosphorized and straightened to obtain a hot-rolled pickled steel plate.

[0036] In the design of the manufacturing method of the steel plate of the present invention:

[0037] Smelting and refining ensure that the basic composition of the molten steel meets the design requirements of the present invention. Casting can be carried out by continuous casting or die casting to ensure that the internal composition of the ingot is uniform and the surface quality is good. If die casting is adopted, the cast ingot needs to be rolled into a billet by a primary rolling mill.

[0038] The ingot is heated at 1150-1250℃, so that the slab is transformed into austenite structure and fully homogenized during the heating process; then it is rough-rolled into an intermediate billet, and the rough-rolling temperature is greater than 850℃.

[0039] The present invention employs a finishing rolling start temperature of 900-1050°C and a finishing rolling temperature of 860-900°C. Laminar cooling is employed, with water cooling to the coiling temperature at a cooling rate of 10-35°C / s, followed by air cooling to room temperature. This process forms a (B, V)N composite precipitate phase with VN as the core and coated with BN, distributed within the grains, preventing BN from aggregating and precipitating at grain boundaries. Furthermore, coarse grain size is avoided, resulting in a ferrite structure with a grain size of 7-10. The precipitate phase size is controlled within a range of 50-600 nm, resulting in a steel plate with a good combination of mechanical properties and formability.

[0040] The coiling temperature is controlled at 650-700°C. Coiling in this temperature range is beneficial to the refinement of ferrite grains and the homogenization of the (B, V)N composite precipitation phase, so as to obtain excellent mechanical properties and anti-scale performance.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] The present invention adopts the design of adding B, N, and a trace amount of V in the composition, and controls the B, N, and V elements to meet the following relationship: 1.27×[B] / ([N]×[V]×10 3 )>1.0. By utilizing the difference in the precipitation order of VN and BN, a (B,V)N composite precipitate with excellent high-temperature stability is formed in the steel, acting as a hydrogen trap and meeting the anti-scale performance requirements of electrostatic dry powder enamel. Unlike conventional enameled steel designs that require high levels of precious alloying elements such as Ti, Nb, and Mo to achieve anti-scale performance, this is a low-cost alloy composition design.

[0043] Based on the composition design, the present invention combines the controlled rolling and controlled cooling process to obtain a fine ferrite + a small amount of pearlite grain structure, and the ferrite grain size level is 7 to 10, which is conducive to the steel plate obtaining higher yield strength and good comprehensive performance.

[0044] The steel plate of the present invention has a yield strength of ≥200 MPa, a tensile strength of ≥290 MPa, an elongation of ≥40%, and a hole expansion rate of ≥95%, achieving a good match between the mechanical properties and enameling properties of the steel plate and meeting the anti-scale explosion performance requirements of electrostatic dry powder enamel. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG1 is a microstructure photograph of the steel plate according to Example 1 of the present invention.

[0046] FIG2 is a physical photograph of the enamel anti-scale test specimens of Comparative Example 1 (left) and Example 1 (right). DETAILED DESCRIPTION

[0047] The present invention is further illustrated below by way of examples and drawings, but this is not intended to limit the present invention. Those skilled in the art may make modifications or improvements based on the basic idea of ​​the invention, but as long as they do not depart from the basic idea of ​​the invention, they are all within the scope of the present invention.

[0048] The compositions of the embodiments of the present invention and the comparative examples are shown in Table 1, wherein the balance of the compositions comprises Fe and unavoidable impurities. The process parameters of the embodiments of the present invention and the comparative examples are shown in Table 2.

[0049] The performance tests were performed on the samples obtained in Examples 1-9 of the present invention, and the test results are listed in Table 3.

[0050] Tensile test: According to GB / T 228.1-2010 “Room temperature tensile test method for metallic materials”, the test was carried out using an SCL233 room temperature tensile testing machine with a tensile speed of 3 mm / min and a JIS5 tensile specimen.

[0051] Hole expansion test: According to GB / T 24524-2009 “Metallic Materials Sheet and Strip Test Method for Hole Expansion”, the test was carried out using an SCL250 cupping tester at a test speed of 6 mm / min.

[0052] Drop hammer test: According to the drop hammer test method described in European standard BS EN 10209-1996, the enamel adhesion performance test is carried out using the corresponding drop hammer test device.

[0053] Ferrite grain: According to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals", the average grain size is evaluated by comparison with the standard series rating chart using a metallographic microscope and the comparison method.

[0054] Enameling Performance: Ferro TR1042 electrostatic powder was used for single-side enameling. The enameling process was 840°C for 5 minutes, and the enamel layer thickness was 120-150μm. After enameling, the steel plate was left to stand for 48 hours and observed to show no scale cracking. A drop hammer test was used to verify the excellent adhesion between the steel plate and the enamel.

[0055] As shown in FIG1 , a microstructure photograph of the steel plate obtained in Example 1 of the present invention is shown. From the photograph, it can be seen that the microstructure consists of ferrite and a small amount of pearlite, and the grain size of the ferrite is level 8.

[0056] As shown in Table 3, the hot-rolled steel sheets obtained by the present invention exhibited yield strengths of ≥200 MPa, tensile strengths of ≥290 MPa, elongation of ≥40%, and hole expansion of ≥95%. Surface observation of the resulting enameled steel sheets after 48 hours revealed no fishscale (Figure 2, right, shows a photograph of the enameled anti-fishscale test specimen from Example 1). Drop hammer tests demonstrated excellent adhesion between the steel sheet and the porcelain layer, fully meeting user requirements.

[0057] In Comparative Example 1, element B was not added, and the relationship between the elements was not limited. Although the mechanical properties of the steel plate were similar to those of the present invention, scale failure occurred ( FIG. 2 , left).

[0058] In Comparative Example 2, no V element was added, and the relationship between the elements was not limited. Although the mechanical properties of the steel plate were similar to those of the present invention, scale failure occurred.

[0059] It should be noted that the embodiments listed above are only specific embodiments of the present invention. Within the scope of protection of the present invention, similar changes or modifications made can be directly derived or easily associated with by those skilled in the art from the contents disclosed in the present invention, and should all fall within the scope of protection of the present invention.

[0060] Table 1 (Unit: weight percentage)

[0061] Table 2

[0062] Table 3

[0063] Note: ○ represents no scale-blasting defect, × represents scale-blasting defect.

Claims

1. A hot-rolled steel sheet suitable for electrostatic dry powder enamel, wherein the chemical composition by weight is: C: 0.015-0.050%, Si≤0.03%, Mn: 0.10-0.50%, Al: 0.010-0.050%, Cr: 0.010-0.040%, Cu: 0.010-0.060%, N: 0.0040-0.0120%, B: 0.0010-0.0035%, V: 0.002-0.015%, the balance includes Fe and other unavoidable impurities; the contents of B, N and V satisfy: 1.27×[B] / ([N]×[V]×10 3 )>1.

0.

2. The hot-rolled steel sheet suitable for electrostatic dry powder enameling according to claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The hot-rolled steel sheet suitable for electrostatic dry powder enameling according to claim 1, characterized in that: 1.27×[B] / ([N]×[V]×10 3 ) is in the range of 1.10 to 6.

00.

4. The hot-rolled steel sheet suitable for electrostatic dry powder enameling according to claim 1 or 2, characterized in that: The microstructure of the steel plate is ferrite+pearlite, wherein the volume ratio of pearlite is less than 5%; the ferrite grain size is 7 to 10, and a (B, V)N composite precipitate phase with a size of 50 to 600 nm is dispersed in the matrix.

5. The hot-rolled steel sheet suitable for electrostatic dry powder enameling according to claim 1, 2 or 3, characterized in that: The steel plate has a yield strength of ≥200 MPa, a tensile strength of ≥290 MPa, an elongation of ≥40%, and a hole expansion rate of ≥95%.

6. The hot-rolled steel sheet suitable for electrostatic dry powder enameling according to claim 5, characterized in that: The steel plate has a yield strength of ≥220 MPa, a tensile strength of ≥310 MPa, an elongation of ≥42%, and a hole expansion rate of ≥100%.

7. The hot-rolled steel sheet suitable for electrostatic dry powder enameling according to claim 1, characterized in that: The steel plate has a yield strength of 200-250 MPa, a tensile strength of 290-340 MPa, an elongation of 40-50%, such as 42-47%, and a hole expansion rate of 95-120%.

8. The method for manufacturing a hot-rolled steel sheet suitable for electrostatic dry powder enameling according to any one of claims 1 to 7, characterized in that: The steps include: 1) Smelting, refining and continuous casting Smelting, RH or LF refining, and casting into billets according to the composition of claim 1, 2, or 3; 2) Hot rolling Heating temperature: 1150~1250℃, rough rolling and finish rolling are carried out in sequence, rough rolling temperature ≥850℃, finish rolling starts The temperature is 900-1050°C, and the final rolling temperature is 860-900°C; 3) Coiling The steel plate after hot rolling is laminar cooled, and the cooling rate is controlled at 10-35°C / s. After cooling, the hot rolled coil is coiled, and the coiling temperature is 650-700°C. 4) Pickling The hot-rolled coil is pickled, dephosphorized and straightened to obtain a hot-rolled pickled steel plate.

9. The manufacturing method according to claim 8, characterized in that: The casting is continuous casting or mold casting.

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