A low cost hot rolled steel sheet suitable for electrostatic dry powder porcelain and a method of manufacturing the same

Abstract

A low-cost hot-rolled steel sheet for electrostatic dry powder porcelain and a method of manufacturing the same, the chemical composition of the steel sheet being as follows: 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%, and the balance including iron and other inevitable impurities, and the steel sheet also satisfying 1.27×[B] / [N]×[V]×10 3 >1.0. The steel sheet has a yield strength of ≥200 MPa, a tensile strength of ≥290 MPa, an elongation of ≥40%, a hole expansion ratio of ≥95%, and can also satisfy the scale explosion resistance required by the electrostatic porcelain process.

Description

Technical Field

[0001] This invention belongs to the field of enamel steel plate processing technology, and specifically relates to a low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel and its manufacturing method. Background Technology

[0002] Enameled steel is a composite material made by fusing enamel onto a steel substrate at high temperatures. It is widely used in ovens, building decorative panels, kitchen and bathroom appliances, enamel-lined steel tanks, and electric water heater liners. From an enamel-making process perspective, it mainly includes two types: wet enamel and electrostatic dry powder enamel. Dry powder enamel uses electrostatic adsorption to adhere the enamel to the steel plate surface, resulting in a more uniform enamel layer thickness, higher surface quality, and higher production efficiency. In recent years, many enamel-lined steel product manufacturing sectors have seen a trend towards replacing wet enamel with electrostatic dry powder enamel.

[0003] For specialized steel plates used in enamel enamel production, excellent resistance to enamel spalling is a fundamental requirement. According to the hydrogen theory, steel absorbs hydrogen produced during the enamel firing process at high temperatures. The solubility of hydrogen in steel increases at high temperatures, but decreases upon cooling. During the firing process of enamel products, hydrogen reaches saturation in the steel. However, when the product cools in air until the enamel solidifies, the hydrogen cannot escape in time, resulting in a supersaturated state in the steel. This supersaturated hydrogen, remaining at potential precipitation sites, generates pressure, the magnitude of which depends on the amount of hydrogen precipitated from the steel at high temperatures and the porosity of the enamel layer. When this pressure increases to the point where the enamel layer can detach, "fish scale" spalling defects are formed. Because dry powder enamel has a finer particle size, the resulting enamel layer is denser than that of wet enamel, making it more difficult for hydrogen to escape. Therefore, the requirements for the steel plate's resistance to spalling are even higher.

[0004] Chinese patent CN101139684A discloses a hot-rolled high-strength steel plate for electrostatic dry powder enamel and its manufacturing method. The main components of the steel plate, by weight, are: 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 incorporates the addition of Ti and Nb to form corresponding carbides or nitrides, etc., as second-phase particles that act as hydrogen storage traps, thereby achieving anti-scaling properties in the enamel.

[0005] Chinese patent CN115537653A discloses a hot-rolled enamel steel plate and its preparation method. The plate uses low-carbon steel, and 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 balance being Fe and unavoidable impurities. Ti and Mo alloying elements are added to the composition to form (Ti,Mo)C precipitates that act as hydrogen storage traps.

[0006] Chinese patent CN101812630A discloses a hot-rolled high-strength enamel steel plate for deep drawing and its manufacturing method; CN102181805A discloses a method for producing enamel steel plates for water heater inner tanks using a thin slab continuous casting and rolling line; CN103540845A discloses a hot-rolled thin-plate enamel steel with a yield strength of 330MPa and its manufacturing method; CN106148855A discloses a method for producing hot-rolled enamel steel with excellent anti-scaling properties. All of these patents employ a design that adds S, Ti, and N elements to the steel plate composition, forming precipitates such as TiC, Ti4S2C2, and TiN in the final steel plate as hydrogen storage traps, thereby achieving anti-scaling properties in the enamel.

[0007] Existing manufacturing technologies for hot-rolled steel sheets suitable for electrostatic dry powder enamel mostly involve adding alloying elements such as Ti, Nb, and Mo to form a sufficient number of carbides or carbonitrides as precipitates or inclusions to act as hydrogen storage traps, thereby achieving anti-scaling properties of the enamel.

[0008] Some designs incorporate elements with high levels of B and N, utilizing BN second-phase particles as hydrogen storage traps. For example, Chinese patent CN116426839A discloses "A cold-rolled enamel steel plate and its preparation method," which incorporates Alt: 0.03–0.15%, B: 0.004–0.015%, and N: 0.01–0.03% in steel. Through a series of processes including hot rolling, pickling, cold rolling, and annealing, a cold-rolled steel plate with good enamel anti-scaling properties is finally obtained. Summary of the Invention

[0009] The purpose of this invention is to provide a low-cost hot-rolled steel plate suitable for electrostatic dry powder enamel and its manufacturing method. The resulting steel plate meets the anti-scaling requirements of the electrostatic enamel process. Moreover, it does not require the addition of high contents of expensive alloying elements such as Ti, Nb, and Mo, resulting in lower alloy costs.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] A low-cost hot-rolled steel sheet suitable for electrostatic dry powder enamel has the following chemical composition by weight percentage: 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%, with the balance including Fe and other unavoidable impurities; it also needs to simultaneously meet the following requirements:

[0012] 1.27×[B] / [N]×[V]×10 3 >1.0.

[0013] Furthermore, the balance consists of Fe and other unavoidable impurities.

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

[0015] The steel plate of the present invention has a yield strength ≥200MPa, a tensile strength ≥290MPa, an elongation ≥40%, and a hole expansion rate ≥95%.

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

[0017] Carbon: In this invention, only trace amounts of V, a carbide-forming element, are added. The carbon in the steel primarily forms pearlite, which strengthens the structure. However, excessive carbon content can worsen the formability of the steel sheet. Furthermore, when the carbon content is too high, the proportion of pearlite formed is too high, leading to the generation of large amounts of CO and other gases during enamel firing. This results in poor bubble structure and pinholes in the enamel layer, severely affecting the enamel quality. Therefore, the carbon content in this invention is controlled between 0.015% and 0.05%.

[0018] Silicon: Silicon can reduce the plasticity of steel and affect the adhesion between the steel plate and the enamel. As a residual element, it is controlled. Therefore, the Si content is controlled to be ≤0.03% in this invention.

[0019] Manganese: Manganese can increase the strength of steel plates; however, excessive strength can cause the inner tank of the water heater to spring back after rolling, which is detrimental to welding processes and weld quality. Additionally, manganese expands the austenite phase region and lowers the Ac3 point temperature, which is unfavorable for the enamel properties of the steel plate because the austenite phase has a stronger hydrogen-soluble capacity than the ferrite phase, making it more prone to scaling defects after cooling. Therefore, it should not be added in excess. Thus, the manganese content in this invention is controlled at 0.1–0.5%.

[0020] Copper and Chromium: The addition of appropriate amounts of copper and chromium is beneficial for surface deposition, thereby improving the adhesion between the copper and enamel and enhancing resistance to blistering. In steel, some Cr replaces iron to form alloy cementite (Fe, Cr)3C, improving its stability; some dissolves into ferrite, resulting in solid solution strengthening and increasing the strength and hardness of the ferrite. However, excessively high copper and chromium content not only increases costs but also enhances the corrosion resistance of the steel plate, which is detrimental to the adhesion of the enamel coating during the enamel process. Therefore, in this 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. To ensure the oxygen content in steel is as low as possible, aluminum deoxidation is often used for medium and low carbon steel. Simultaneously, dissolved aluminum in the steel combines with free nitrogen to precipitate AlN, and its precipitation temperature is relatively high, which can refine the austenite grains. The fine grain structure not only exerts a grain-refining strengthening mechanism but also helps improve the hydrogen storage capacity of the steel plate. Therefore, the aluminum content in this invention is controlled at 0.01–0.05%.

[0022] Vanadium and Boron: In this invention, trace amounts of vanadium (V) are added to the steel, forming a VN second phase. Since V has a stronger affinity for N than boron (B), VN precipitates preferentially over BN. When BN precipitates, it forms a (B, V)N composite precipitate phase with VN as the core. This composite precipitate phase avoids the problems of pure BN easily agglomerating at grain boundaries, leading to microcracks in the cast billet and poor formability of the steel plate. Furthermore, this composite precipitate phase exhibits better high-temperature stability than BN, better fulfilling its hydrogen trapping function and meeting the anti-scaling requirements of electrostatic dry powder enamel. To avoid increasing alloy costs, only trace amounts of V (0.002–0.015%) are added in this invention. Solid-solution boron in steel increases the strength of the steel plate but reduces its formability; therefore, the boron content should not be too high. In this invention, the boron content is controlled at 0.001–0.0035%.

[0023] Nitrogen: Under normal circumstances, the nitrogen content in steel should be as low as possible. In this invention, the higher nitrogen content is mainly to form (B, V)N composite precipitates. Excess nitrogen can also form AlN precipitates with Al. In this invention, the nitrogen content is controlled at 0.004 to 0.012%.

[0024] Furthermore, the aforementioned B, N, and 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 B, N, and V elements in steel, and make full use of the difference in the order of VN and BN precipitation to generate a (B, V)N composite precipitation phase with good high-temperature stability in the steel as a hydrogen storage trap, thus meeting the anti-scaling performance requirements of electrostatic dry powder enamel.

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

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

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

[0030] 2) Hot rolling

[0031] Heating temperature: 1150~1250℃, rough rolling and finish rolling are carried out in sequence, rough rolling temperature ≥850℃, finish rolling start temperature is 900~1050℃, finish rolling finish temperature is 860~900℃;

[0032] 3) Winding

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

[0034] 4) Pickling

[0035] Hot-rolled coils are pickled, descaled, and straightened to obtain hot-rolled pickled steel sheets.

[0036] In the design of the manufacturing method for the steel plate described in this invention:

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

[0038] The billet is heated at 1150–1250℃ to transform it into an austenitic structure and make it fully homogenized during the heating process; then it is rough rolled into an intermediate billet at a temperature greater than 850℃.

[0039] This invention designs a finishing rolling start temperature of 900–1050℃ and a finishing rolling finish temperature of 860–900℃, employing laminar flow cooling. The steel is water-cooled to the coiling temperature at a cooling rate of 10–35℃ / s, and then air-cooled to room temperature. On one hand, this results in the formation of a (B, V)N composite precipitate with VN as the core and BN coating, distributed within the grains, preventing BN from agglomerating and precipitating at grain boundaries. On the other hand, it avoids coarse grain size, obtaining a ferrite grain size of grade 7–10, with the precipitate size controlled at 50–600 nm, thus achieving a good balance of mechanical and formability properties in the final steel sheet.

[0040] The winding temperature is controlled at 650-700℃. Winding within this temperature range is beneficial for refining ferrite grains and homogenizing the (B, V)N composite precipitates, thereby obtaining excellent mechanical properties and anti-explosion properties.

[0041] Compared with the prior art, the beneficial effects of the present invention are:

[0042] This invention employs a design that adds B, N, and trace amounts of V, and controls the B, N, and V elements to satisfy the following relationship: 1.27 × [B] / [N] × [V] × 10 3 With a concentration >1.0, the difference in the order of VN and BN precipitation is utilized to generate a (B, V)N composite precipitate phase with good high-temperature stability in the steel as a hydrogen storage trap, meeting the anti-scaling performance requirements of electrostatic dry powder enamel. Unlike conventional enamel steel designs that require the addition of high contents of expensive alloying elements such as Ti, Nb, and Mo to achieve anti-scaling performance, this is a low-cost alloy composition design scheme.

[0043] Based on the composition design, this invention combines controlled rolling and controlled cooling processes to obtain a fine ferrite + pearlite grain structure with a ferrite grain size of 7 to 10, which is beneficial for the steel plate to obtain higher yield strength and good comprehensive performance.

[0044] The steel plate described in this invention has a yield strength ≥200MPa, tensile strength ≥290MPa, elongation ≥40%, and hole expansion rate ≥95%, achieving a good match between the mechanical properties and enameling performance of the steel plate and meeting the anti-scaling performance requirements of electrostatic dry powder enamel. Attached Figure Description

[0045] Figure 1 This is a micrograph of the steel plate from Embodiment 1 of the present invention. Detailed Implementation

[0046] The present invention will be further described below with reference to embodiments and accompanying drawings. However, this is not a limitation of the present invention. Those skilled in the art can 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 present invention, they are all within the scope of the present invention.

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

[0048] The samples obtained in Examples 1-9 of this invention were subjected to performance tests, and the test results are listed in Table 3.

[0049] Tensile test: The test was conducted in accordance with GB / T 228.1-2010 "Metallic materials, tensile test at room temperature", using an SCL233 room temperature tensile testing machine. The tensile speed was 3 mm / min, and the tensile specimen was a JIS5 tensile specimen.

[0050] Hole enlargement test: The test was conducted using an SCL250 cupping tester in accordance with GB / T 24524-2009 "Test method for hole enlargement of thin plates and strips of metallic materials" at a test speed of 6 mm / min.

[0051] Drop weight test: The enamel adhesion performance was tested using the appropriate drop weight test apparatus according to the drop weight test method described in European standard BS EN 10209-1996.

[0052] Ferrite grains: The average grain size was assessed by comparing it with a standard series rating chart using a metallographic microscope and the comparative method, in accordance with GB / T 6394-2017 "Method for determination of average grain size of metals".

[0053] Enameling performance: Single-sided enamel was applied using Ferro TR1042 electrostatic powder, with a firing process of 840℃ for 5 minutes, resulting in an enamel layer thickness of 120–150 μm. After enamel application, the steel plate was left to stand for 48 hours, and no surface cracking was observed. A drop weight test was used to verify the excellent adhesion between the steel plate and the enamel.

[0054] like Figure 1 As shown in the photograph of the steel plate obtained in Example 1 of the present invention, the microstructure is composed of ferrite and pearlite, and the grain size of the ferrite is grade 8.

[0055] As shown in Table 3, the hot-rolled steel sheet obtained by this invention has a yield strength ≥200MPa, tensile strength ≥290MPa, elongation ≥40%, and hole expansion rate ≥95%. After 48 hours, no scaling or bursting phenomena were observed on the surface of the finally obtained enamel-lined steel sheet. Drop hammer tests showed excellent adhesion between the steel sheet and the enamel layer, fully meeting the user's requirements.

[0056] In Comparative Example 1, no element B was added, and the relationship between the elements was not defined. Although the mechanical properties of the steel plate were not much different from those of the present invention, scale burst defects appeared.

[0057] In Comparative Example 2, no V element was added, and the relationship between the elements was not defined. Although the mechanical properties of the steel plate were not much different from those of the present invention, scale burst defects appeared.

[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 that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention 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 percentage 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%, with the balance being Fe and other unavoidable impurities; it also needs to simultaneously satisfy: 1.27×[B] / [N]×[V]×10 3 >1.0; The microstructure of the steel plate is ferrite + pearlite; the ferrite grain size is 7 to 10, and the matrix contains (B, V)N composite precipitates with a size of 50 to 600 nm.

2. The low-cost hot-rolled steel sheet suitable for electrostatic dry powder enamel as described in claim 1, characterized in that, The steel plate has a yield strength ≥200MPa, tensile strength ≥290MPa, elongation ≥40%, and hole expansion rate ≥95%.

3. The method for manufacturing low-cost hot-rolled steel sheets suitable for electrostatic dry powder enamel as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Smelting, refining, and continuous casting Smelting, refining with RH or LF, and casting into billets according to the composition described in claim 1; 2) Hot-rolled Heating temperature: 1150~1250℃, rough rolling and finish rolling are carried out in sequence, rough rolling temperature ≥850℃, finish rolling start temperature is 900~1050℃, finish rolling finish temperature is 860~900℃; 3) Winding The hot-rolled steel plate is cooled by laminar flow at a rate of 10~35℃ / s. After cooling, it is coiled to obtain a hot-rolled coil at a temperature of 650~700℃, and then air-cooled to room temperature. 4) Pickling Hot-rolled coils are pickled, descaled, and straightened to obtain hot-rolled pickled steel sheets.

4. The manufacturing method as described in claim 3, characterized in that, The casting is either continuous casting or die casting.

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