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

Abstract

The invention discloses a low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel and a manufacturing method of the low-cost hot-rolled steel plate. The low-cost hot-rolled steel plate comprises the following chemical components in percentage by weight: 0.015-0.05% of C, less than or equal to 0.03% of Si, 0.1-0.5% of Mn, 0.01-0.05% of Al, 0.01-0.04% of Cr, 0.01-0.06% of Cu, 0.004-0.012% of N, 0.001-0.0035% of B, 0.002-0.015% of V and the balance of iron and other inevitable impurities. And 1.27 * [B] / ([N] * [V] * 103) is more than 1.0. The yield strength of the steel plate is larger than or equal to 200 MPa, the tensile strength is larger than or equal to 290 MPa, the ductility is larger than or equal to 40%, the hole expansion rate is larger than or equal to 95%, and the fish scaling resistance requirement of the electrostatic enamel technology can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of enameled steel plate processing, and particularly 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 melting porcelain enamel on a steel plate substrate through high-temperature enameling. It is widely used in ovens, building decorative panels, kitchen and bathroom appliances, enameled steel assembled tanks, and electric water heater tanks. From the perspective of enameling technology, it mainly includes wet enameling and electrostatic dry powder enameling. Dry powder enamel uses electrostatic adsorption to make the glaze adhere to the surface of the steel plate. The thickness of the enamel layer is more uniform, the surface quality of the enamel is higher, and the production efficiency is high. In recent years, in many enameled steel product production fields, there has been a trend of replacing wet enameling with electrostatic dry powder enamel.

[0003] For special steel plates suitable for enamel, good enamel anti-scale explosion performance is the most basic requirement. According to the hydrogen theory, steel will absorb hydrogen produced during the enameling reaction at high temperature. The solubility of steel in hydrogen increases at high temperature, and its solubility decreases after cooling. During the firing process of enameled products, hydrogen reaches saturation in steel, and when the product is cooled in the air until the enamel solidifies, hydrogen has no time to escape, so that it reaches a supersaturated state in the steel. This supersaturated hydrogen left in the position to be precipitated later generates a certain pressure, the size of which is related to the amount of hydrogen precipitated in the steel at high temperature and the degree of porosity of the porcelain layer. When this pressure increases to the point where the porcelain layer can fall off, a "fish scale" explosion defect is formed. Because the particle size of dry powder enamel powder is finer, the enamel layer it forms is denser than the wet enamel layer, which makes it more difficult for hydrogen to escape, so the steel plate has higher requirements for anti-scale explosion performance.

[0004] Chinese patent CN101139684A discloses a hot-rolled high-strength steel plate for electrostatic dry powder electrostatic enamel and its manufacturing method. The main components of the steel plate include by weight: 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%, and the balance is iron and unavoidable impurities. In terms of component design, Ti and Nb are added to form corresponding second-phase particles such as carbides or nitrides as hydrogen storage traps to achieve anti-scale explosion performance of enamel.

[0005] Chinese patent CN115537653A discloses a hot-rolled enameled steel plate and its preparation method, which uses low carbon, and the steel plate composition 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%, and the balance is Fe and inevitable impurities. Ti and Mo alloy elements are added to the composition to form precipitated phases such as (Ti, Mo) C as hydrogen storage traps.

[0006] Chinese patent CN101812630A discloses a hot-rolled high-strength enameled steel plate for deep drawing and a manufacturing method thereof; CN102181805A discloses a thin slab continuous casting and rolling line to produce steel plates for water heater liner enameling and a method thereof; CN103540845A discloses a hot-rolled thin-plate enameled steel with a yield strength of 330MPa and a manufacturing method thereof; CN106148855A discloses a production method of hot-rolled enameled steel with excellent anti-scale explosion performance. The steel plate compositions designed in the above patents all adopt the design of adding S, Ti, and N elements, forming TiC, Ti4S2C2, TiN and other precipitates and inclusions in the final steel plate as hydrogen storage traps, thereby achieving the anti-scale explosion performance of the enamel.

[0007] The existing manufacturing technology of hot-rolled steel plates suitable for electrostatic dry powder enamel mostly adopts the addition of alloy elements such as Ti, Nb, Mo, etc. to form a sufficient number of precipitates or inclusions such as carbides or carbonitrides to serve as hydrogen storage traps, thereby obtaining the anti-scale explosion performance of the enamel.

[0008] There are also designs that add high B and high N elements, using BN second phase particles as hydrogen storage traps. For example, Chinese patent CN116426839A discloses "A cold-rolled enameled steel plate and its preparation method", in which Alt: 0.03-0.15%, B: 0.004-0.015%, N: 0.01-0.03% are added to the steel, and a series of hot rolling, pickling, cold rolling, annealing and other processes are carried out to finally obtain a cold-rolled steel plate with good enamel anti-scale explosion performance. Summary of the invention

[0009] 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, wherein the obtained steel plate meets the anti-scale explosion requirements of the electrostatic enameling process; moreover, there is no need to add high content of precious alloy elements such as Ti, Nb, Mo, etc., and the alloy cost is relatively low.

[0010] To achieve the above object, the technical solution of the present invention is:

[0011] A low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel, the chemical composition weight percentage of which is: 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%, V: 0.002-0.015%, the balance includes Fe and other unavoidable impurities; and it is also necessary to meet the following requirements at the same time:

[0012] 1.27×[B] / ([N]×[V]×10 3 )>1.0.

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

[0014] The microstructure of the steel plate of the present invention is ferrite+pearlite; the ferrite grain size is 7-10, and the (B, V)N composite precipitate phase with a size of 50-600nm is dispersed in the matrix.

[0015] 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%.

[0016] In the chemical composition design of low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel described in the present invention:

[0017] Carbon: In the present invention, only a trace amount of V carbide forming element is added. The C in the steel mainly forms pearlite structure, which plays a role in strengthening the structure. However, too high a C content will deteriorate the formability of the steel plate. In addition, when the carbon content is too high, the proportion of pearlite formed is too high, which will lead to a large amount of CO and other gases being generated during the enamel firing process, resulting in poor bubble structure of the enamel layer, as well as defects such as pinholes, which seriously affect the quality of the enamel. Therefore, the carbon content in the present invention is controlled at 0.015-0.05%.

[0018] 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 ≤ 0.03%.

[0019] Manganese: Manganese can improve the strength of steel plates. However, too high strength will cause the inner tank of the water heater to rebound after rolling, which is not conducive to welding technology and weld quality. In addition, manganese has the effect of expanding the austenite phase and reducing the Ac3 point temperature, which is not conducive to the enamel performance of the steel plate, because the austenite phase has a stronger ability to dissolve hydrogen than the ferrite phase, and it is more likely to cause scale explosion defects after cooling, so it is not suitable to add excessively. Therefore, the manganese content in the present invention is controlled at 0.1-0.5%.

[0020] Copper and chromium: The addition of appropriate amounts of copper and chromium elements is beneficial to surface deposition, thereby improving the adhesion between the steel and the enamel and improving the anti-scale performance. A portion of the Cr in the steel replaces iron to form alloy cementite (Fe, Cr)3C, improving its stability, and a portion of it dissolves into ferrite to produce solid solution strengthening, improving the strength and hardness of the ferrite; however, excessive copper and chromium content will not only increase the cost, but also enhance the corrosion resistance of the steel plate, which will be detrimental to the adhesion of the steel plate during the enameling process. Therefore, in the present invention, the copper content is controlled at 0.01-0.06%, and the chromium content is controlled at 0.01-0.04%.

[0021] Aluminum: Aluminum is a strong deoxidizing element. In order to ensure that the oxygen content in the steel is as low as possible, aluminum deoxidation is often used in medium and low carbon steel. At the same time, the aluminum dissolved in the steel will combine with free nitrogen to precipitate AlN, and its precipitation temperature is relatively high, which can play a role in refining the austenite grains. The fine grain structure can not only play a fine grain strengthening mechanism, but also help to improve the hydrogen storage capacity of the steel plate. Therefore, the aluminum content in the present invention is controlled at 0.01-0.05%.

[0022] Vanadium and boron: A trace amount of V element is added to the steel of the present invention, and a VN second phase can be formed in the steel. Since the affinity between V and N is stronger than that between B and N, VN is precipitated before BN. When BN precipitates, VN is used as the core to form a (B, V)N composite precipitation phase. The formation of this composite precipitation phase can avoid the problem that simple BN is easily segregated and precipitated at the grain boundary, resulting in micro cracks in the ingot and poor formability of the steel plate. On the other hand, the high temperature stability of this composite precipitation phase is better than that of BN, and it can better play the role of hydrogen storage trap and meet the anti-scale explosion performance requirements of electrostatic dry powder enamel. In order to avoid the increase of alloy cost, only a trace amount of V is added in the present invention: 0.002-0.015%. The solid solution of boron in the steel will increase the strength of the steel plate and reduce the formability, so the content of B element should not be too high. In the present invention, the boron content is controlled at 0.001-0.0035%.

[0023] Nitrogen: In general, 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 precipitation phase. Excess nitrogen can also form an AlN precipitation phase with Al. In the present invention, the nitrogen content is controlled at 0.004-0.012%.

[0024] And the above B, N, V ​​elements must also satisfy the following relationship:

[0025] 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, make full use of the difference in the precipitation order of VN and BN, and generate a (B, V)N composite precipitation phase with good high-temperature stability in the steel as a hydrogen storage trap, meeting the anti-scale explosion performance requirements of electrostatic dry powder enamel.

[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℃, rough rolling and finish rolling are carried out in sequence, the rough rolling temperature is ≥850℃, the finish rolling start temperature is 900-1050℃, and the finish rolling final temperature is 860-900℃;

[0032] 3) Coiling

[0033] 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.

[0034] 4) Pickling

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

[0036] In 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 ingot cast by the die casting 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 roughly rolled into an intermediate slab, and the rough rolling temperature is greater than 850℃.

[0039] The present invention designs the start-rolling temperature of the finishing rolling to be 900-1050°C, the final rolling temperature of the finishing rolling to be 860-900°C, and adopts laminar cooling, and water-cools to the coiling temperature at a cooling rate of 10-35°C / s, and then air-cools to room temperature. On the one hand, a (B, V)N composite precipitate phase with VN as the core and coated by BN is formed and distributed in the grains, avoiding the aggregation and precipitation of BN at the grain boundaries. On the other hand, coarse grain size can be avoided, and a ferrite grain size level of 7-10 is obtained. The precipitate phase size is controlled at 50-600nm, so that the final steel plate obtains a comprehensive match of good mechanical properties and forming properties.

[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 explosion performance.

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

[0042] The present invention adopts a design of adding B, N, and a trace amount of V in the composition, and controls the B, N, and V elements to satisfy the following relationship: 1.27×[B] / ([N]×[V]×10 3 )>1.0, using the difference in the order of VN and BN precipitation, a (B, V)N composite precipitate phase with good high temperature stability is generated in the steel as a hydrogen storage trap, meeting the requirements of anti-scale explosion performance of electrostatic dry powder enamel. Different from the conventional enameled steel composition, which needs to add high content of precious alloy elements such as Ti, Nb, Mo to achieve the anti-scale explosion performance of enamel, it is a low-cost alloy composition design scheme.

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

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

[0045] Figure 1 This is a microstructure photograph of the steel plate of Example 1 of the present invention. DETAILED DESCRIPTION

[0046] The present invention is further described below by way of embodiments 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 deviate from the basic idea of ​​the invention, they are all within the scope of the present invention.

[0047] The compositions of the embodiments of the present invention and the comparative examples are shown in Table 1, wherein the balance of the compositions includes Fe and unavoidable impurities. The process parameters of the embodiments of the present invention and the comparative examples are shown in Table 2.

[0048] 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.

[0049] Tensile test: According to GB / T 228.1-2010 "Room temperature tensile test method for metallic materials", the test was carried out using a SCL233 room temperature tensile testing machine, the tensile speed was 3mm / min, and the tensile specimen was a JIS5 tensile specimen.

[0050] Hole expansion test: According to GB / T 24524-2009 “Metallic Sheet and Strip Hole Expansion Test Method”, the test was carried out using SCL250 cupping tester at a test speed of 6mm / min.

[0051] 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.

[0052] Ferrite grains: 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 a comparison method.

[0053] Enameling performance: Ferro TR1042 electrostatic powder is used for single-sided enameling, the enameling process is 840℃ for 5 minutes, and the enamel layer thickness is 120-150μm; after enameling, the steel plate is left to stand for 48 hours and no scale explosion occurs on the surface. The drop hammer test is used to verify the excellent level of adhesion between the steel plate and the enamel.

[0054] like Figure 1 As shown, the microstructure photo of the steel plate obtained in Example 1 of the present invention, it can be seen from the picture that the microstructure is composed of ferrite and pearlite, and the grain size of ferrite is level 8.

[0055] As can be seen from Table 3, the yield strength of the hot-rolled steel plate obtained by the present invention is ≥200MPa, the tensile strength is ≥290MPa, the elongation is ≥40%, and the hole expansion rate is ≥95%. After observing the surface of the finally obtained enameled steel plate after 48 hours, no scale explosion phenomenon occurs; after the drop hammer test, the adhesion between the steel plate and the porcelain layer is excellent, which fully meets the user's use requirements.

[0056] In Comparative Example 1, element B is not added, and the relationship between the elements is not limited. Although the mechanical properties of the steel plate are not much different from those of the present invention, scale failure occurs.

[0057] In Comparative Example 2, no V element is added, and the relationship between the elements is not limited. Although the mechanical properties of the steel plate are not much different from those of the present invention, scale failure occurs.

[0058] It should be noted that the embodiments listed above are only specific embodiments of the present invention. Within the protection scope of the present invention, similar changes or modifications made can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and should all fall within the protection scope of the present invention.

[0059]

[0060]

[0061]

Claims

1. A low-cost hot-rolled steel sheet suitable for electrostatic dry powder enamel, wherein the chemical composition by weight is: 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%, V: 0.002-0.015%, the balance includes Fe and other unavoidable impurities; and the following chemical compositions are also required to be met: 1.27×[B] / ([N]×[V]×10 3 )>1.0。 2. The low-cost 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 low-cost 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; 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.

4. The low-cost 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%.

5. The method for manufacturing a low-cost hot-rolled steel sheet suitable for electrostatic dry powder enamel according to any one of claims 1 to 4, 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 or 2; 2) Hot rolling Heating temperature: 1150-1250℃, rough rolling and finish rolling are carried out in sequence, the rough rolling temperature is ≥850℃, the finish rolling start temperature is 900-1050℃, and the finish rolling final temperature is 860-900℃; 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.

6. The manufacturing method according to claim 5, characterized in that: The casting is continuous casting or mold casting.

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