Hot-rolled steel plate suitable for electrostatic dry powder enamel and having enamel firing strengthening property and manufacturing method thereof

Abstract

The invention discloses a hot-rolled steel plate suitable for electrostatic dry powder enamel and having enamel firing strengthening performance and a manufacturing method of the hot-rolled steel plate. The hot-rolled steel plate comprises the following chemical components in percentage by weight: 0.03 to 0.08 percent of C, less than or equal to 0.03 percent of Si, 0.3 to 0.8 percent of Mn, 0.01 to 0.05 percent of Al, 0.01 to 0.04 percent of Cr, 0.02 to 0.08 percent of Cu, 0.004 to 0.012 percent of N, 0.001 to 0.005 percent of B, 0.005 to 0.05 percent of Mo and the balance of Fe and other inevitable impurities. And [B] / [N] * [Mo] * 103 is more than 5. The high-temperature stability of a precipitated phase is ensured by utilizing the strengthening effect of the Mo element on BN and AlN precipitation; meanwhile, the air-cooling hardenability of the steel plate is improved by using the solid-solution-state B and Mo elements, so that the steel plate achieves the purpose that the yield strength of the steel plate is increased after high-temperature enameling on the premise that the steel plate meets the fish scaling resistance and adherence performance requirements of an electrostatic dry powder enameling process.

Description

Technical Field

[0001] The invention belongs to the technical field of enameled steel plate processing, and particularly relates to a hot-rolled steel plate suitable for electrostatic dry powder enameling and having enameling strengthening properties, 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] In addition, because the enamel sintering process usually needs to be completed at a high temperature of more than 800℃ for a period of time, the final strength of the enameled product depends on the strength of the enameled steel after high-temperature enameling. Under normal circumstances, the strength of the steel plate will decrease significantly after such high-temperature heat treatment. This is because the microstructure of the steel plate will experience a decrease in dislocation density, growth of ferrite grains, and coarsening of nano-precipitation phases during the high-temperature heat treatment process. These changes lead to the weakening of dislocation strengthening, fine grain strengthening, and precipitation strengthening. How to improve the strength of steel plates after high-temperature enameling has always been a key research topic in the field of enameled steel development, and it also has important practical application value.

[0005] Chinese patent CN101812630A discloses "hot-rolled high-strength enameled steel plate for deep drawing and its manufacturing method", the chemical composition of the steel plate is C: 0.02-0.10%, Si≤0.10%, Mn: 0.05-1.00%, P≤0.05%, S: 0.005-0.035%, Al: 0.01-0.10%, N≤0.015%, Ti<0.10%, and the balance is iron and unavoidable impurities.

[0006] Chinese patent CN103540845A discloses "hot-rolled thin-plate enameled steel with a yield strength of 330 MPa and a manufacturing method". The composition of the hot-rolled thin-plate enameled steel is C: 0.02-0.07%, Si≤0.05%, Mn: 0.10-0.50%, P≤0.020%, S≤0.010%, Ti: 0.04-0.10%, Al: 0.02-0.08%, N≤0.008%, and the rest is Fe and unavoidable inclusions, and Ti / C=1.0-1.5.

[0007] Chinese patent CN102181805A discloses "a thin slab continuous casting and rolling line to produce steel plates for water heater liner enamel and method", the chemical composition of the steel plate is as follows: carbon 0.03-0.10%, manganese 0.15-0.40%, silicon ≤0.06%, sulfur 0.004-0.040%, phosphorus ≤0.15%, aluminum 0.03-0.05%, nitrogen 0.002-0.008%, titanium 0.02-0.10%, the rest is iron and unavoidable impurities

[0008] The above hot-rolled enameled steels are mostly designed with Ti added in their composition, which forms TiC, Ti(C, N) and other precipitation phases with C and N, and plays the role of hydrogen storage trap and precipitation strengthening. However, it should be noted that the above-mentioned technical solutions are only applicable to wet enameling, and the yield strength of the enameled steels obtained by them after high-temperature enameling is lower than the yield strength of the hot-rolled state. Summary of the invention

[0009] The object of the present invention is to provide a hot-rolled steel plate suitable for electrostatic dry powder enameling and having enameling strengthening properties and a manufacturing method thereof. After high-temperature enameling, the yield strength of the steel plate will not decrease but will increase, thereby effectively improving the strength of the final enameled product; at the same time, the obtained steel plate meets the anti-scale explosion and adhesion performance requirements required by the electrostatic dry powder enameling process, and can be widely used in fields such as water heater tanks.

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

[0011] A hot-rolled steel plate suitable for electrostatic dry powder enameling and having enameling strengthening properties, wherein the chemical composition by weight percentage is: C: 0.03-0.08%, Si≤0.03%, Mn: 0.3-0.8%, Al: 0.01-0.05%, Cr: 0.01-0.04%, Cu: 0.02-0.08%, N: 0.004-0.012%, B: 0.001-0.005%, Mo: 0.005-0.05%, and the balance includes Fe and other unavoidable impurities; and the following chemical compositions are also required to be met at the same time:

[0012] [B] / [N]×[Mo]×10 3 >5.

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

[0014] The microstructure of the steel plate of the present invention is equiaxed ferrite+pearlite; the ferrite grain size is 7-9, BN and AlN precipitate phases are dispersed in the ferrite matrix, and the precipitate phase size is 10-800nm.

[0015] The yield strength of the steel plate of the present invention is 285-345 MPa. After being calcined at 840° C., the yield strength of the steel plate is 340-395 MPa.

[0016] In the chemical composition design of the hot-rolled steel plate with electrostatic dry powder enameling and enameling strengthening properties of the present invention:

[0017] Carbon: C in steel mainly forms pearlite structure, which plays a role in strengthening the structure. However, too high a carbon 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 and defects such as pinholes, which seriously affect the quality of the enamel. Therefore, the carbon content in the present invention is controlled at 0.03-0.08%.

[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 0.3-0.8%.

[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 0.02-0.08%, 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] Boron: Boron has a very low solubility in steel and mainly combines with free nitrogen in the steel to precipitate in the form of BN. In the present invention, the BN precipitate phase serves as the main hydrogen storage trap, playing a role in preventing scale explosion in the enamel. However, solid-solution boron in 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 0.001-0.005%.

[0023] Nitrogen: In general, the nitrogen content in steel should be as low as possible. In the present invention, an appropriate amount of nitrogen is added mainly to form a BN 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] Molybdenum: In the technical solution described in the present invention, on the one hand, molybdenum can be dissolved in ferrite, austenite and carbide to play a role in solid solution strengthening. At the same time, it can also interact with solid solution boron to improve the hardenability of the steel plate, so that the steel plate can produce the effect of improving strength under the cooling rate of air cooling; on the other hand, molybdenum can also improve the stability of BN and AlN precipitation phases, reduce the aggregation and coarsening of precipitation phases caused by high-temperature annealing (usually the annealing temperature reaches 830-850°C), thereby improving the high-temperature stability of steel and avoiding the decrease in yield strength of the steel plate due to the weakening of precipitation strengthening after annealing. However, excessive addition of Mo elements will significantly increase the manufacturing cost. Therefore, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, the mass percentage of Mo elements is limited to 0.005-0.05%.

[0025] The above-mentioned B, N, and Mo elements must also satisfy the following relationship:

[0026] [B] / [N]×[Mo]×10 3 >5.

[0027] Satisfying this relationship can make the steel plate have enameling strengthening performance, and the yield strength can be increased by more than 10% after high-temperature enameling at 840°C. This is because: on the one hand, a sufficient amount of BN precipitation phase can be obtained, and at the same time, sufficient Mo elements can be guaranteed to ensure the high-temperature stability of the precipitation phase; on the other hand, a certain amount of B and Mo elements can be guaranteed to exist in a solid solution state, which can improve the air-cooling hardenability of the steel plate, so that the steel plate can obtain an increase in strength under air-cooling conditions after high-temperature enameling.

[0028] The composition is designed with the addition of B, N, Mo, and Al, forming BN and AlN second phases in the steel as hydrogen storage traps to play the role of anti-scale explosion in enamel. The Mo element is used to improve the stability of BN and AlN precipitated phases, reduce the aggregation and coarsening of precipitated phases caused by high-temperature enamelling, and thus improve the high-temperature stability of the steel. At the same time, the interaction between solid solution boron and molybdenum is used to improve the air-cooling hardenability of the steel plate, so that the high-temperature enamelling of the steel plate can produce the effect of increasing strength under air-cooling conditions, achieving the purpose of enamel strengthening, so that the yield strength of the steel plate will not decrease but increase after high-temperature enamelling.

[0029] The present invention also provides a method for manufacturing a hot-rolled steel plate suitable for electrostatic dry powder enamel and having enameling strengthening properties, comprising the following steps:

[0030] 1) Smelting, refining and continuous casting

[0031] Smelting, RH or LF refining, and continuous casting into billets according to the above composition;

[0032] 2) Hot rolling

[0033] 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℃;

[0034] 3) Coiling

[0035] 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.

[0036] 4) Pickling

[0037] The hot-rolled coil is pickled, dephosphorized and straightened to obtain a hot-rolled pickled steel plate.

[0038] In the method for manufacturing the steel plate of the present invention:

[0039] 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.

[0040] 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℃.

[0041] The present invention designs the finishing rolling start temperature to be 900-1050°C, the finishing rolling final rolling temperature to be 860-900°C, adopts laminar cooling, water-cools to a coiling temperature of 650-700°C at a cooling rate of 10-35°C / s, and then air-cools to room temperature. The present invention optimizes the design of C, B, Al, and N components, combines the controlled rolling and controlled cooling process of high-temperature finishing rolling, high-temperature coiling, and rapid cooling, which is conducive to the full recovery and recrystallization of ferrite grains during hot rolling, thereby obtaining a uniform equiaxed ferrite grain structure, which can make the final steel plate have excellent comprehensive mechanical properties.

[0042] In addition, the final rolling temperature is controlled at 860-900℃, which is conducive to strengthening the combination of element Mo with BN and AlN precipitation phases, and then using Mo element to improve the high temperature stability of BN and AlN precipitation phases. In addition, the final rolling temperature is higher than the enameling temperature (830-850℃) during the enameling process, which can reduce the influence of the enameling temperature on the structure and performance of the steel plate and improve the quality of the final enameled product.

[0043] 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 BN and AlN precipitation phases, so as to obtain excellent mechanical properties and anti-scale explosion performance.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The present invention adopts a high N, B, Al component system in the composition, forms BN and AlN second phases in the steel as hydrogen storage traps, exerts excellent enamel anti-scale explosion effect, and can meet the use requirements of electrostatic dry powder enamel. At the same time, Mo element is used to improve the stability of BN and AlN precipitated phases, reduce the aggregation and coarsening of precipitated phases caused by high-temperature enamelling, thereby improving the high-temperature stability of steel. At the same time, the interaction between the solid solution boron and molybdenum elements is used to improve the air-cooling hardenability of the steel plate, so that the steel plate produces an effect of improving strength under the cooling rate condition of air cooling after high-temperature enamelling, so that the yield strength of the steel plate after high-temperature enamelling will not decrease but increase, so as to achieve the purpose of enamel strengthening, and the yield strength of the steel plate after enamelling can be increased by more than 10%.

[0046] The present invention, based on the composition design, combines the controlled rolling and controlled cooling process to obtain a fine equiaxed ferrite + pearlite grain structure, the ferrite grain size level is 7 to 9, which is conducive to the steel plate to obtain a higher yield strength and good comprehensive performance.

[0047] The yield strength of the steel plate of the present invention is 285-345 MPa. After high-temperature sintering at 840°C, the yield strength of the steel plate is 340-395 MPa, 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

[0048] Figure 1 This is a microstructure photograph of the steel plate of Example 1 of the present invention. DETAILED DESCRIPTION

[0049] 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.

[0050] 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 other unavoidable impurities. The process parameters of the embodiments of the present invention and the comparative examples are shown in Table 2.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] As can be seen from Table 3, the yield strength of the hot-rolled steel plate obtained by the present invention is 285-345 MPa, and after high-temperature sintering at 840°C, the yield strength of the steel plate is 340-395 MPa. After observing the surface of the finally obtained enameled steel plate after 48 hours, no scale explosion phenomenon occurred; 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.

[0059] In Comparative Examples 1 to 3, [B] / [N]×[Mo]×10 3 <5.0, scale defects appear on the steel plate after electrostatic dry powder enameling, and the yield strength after enameling is significantly reduced.

[0060] 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.

[0061]

[0062]

[0063]

Claims

1. A hot-rolled steel plate suitable for electrostatic dry powder enameling and having enameling strengthening properties, wherein the chemical composition by weight is: C: 0.03-0.08%, Si≤0.03%, Mn: 0.3-0.8%, Al: 0.01-0.05%, Cr: 0.01-0.04%, Cu: 0.02-0.08%, N: 0.004-0.012%, B: 0.001-0.005%, Mo: 0.005-0.05%, and the balance includes Fe and other unavoidable impurities; and the following chemical compositions are also required to be met at the same time: [B] / [N]×[Mo]×10 3 5.

2. The hot-rolled steel sheet suitable for electrostatic dry powder enameling and having enameling strengthening properties 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 and having enameling strengthening properties according to claim 1 or 2, characterized in that: The microstructure of the steel plate is equiaxed ferrite+pearlite; the ferrite grain size is 7-9, BN and AlN precipitate phases are dispersed in the ferrite matrix, and the size of the precipitate phases is 10-800nm.

4. The hot-rolled steel sheet suitable for electrostatic dry powder enameling and having enameling strengthening properties according to claim 1, 2 or 3, characterized in that: The yield strength of the steel plate is 285-345 MPa. After being sintered at 840° C., the yield strength of the steel plate is 340-395 MPa.

5. The method for producing a hot-rolled steel sheet suitable for electrostatic dry powder enameling and having enameling strengthening properties 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 continuous casting into billets according to 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 obtained steel plate has a thickness of 1.7 to 3.0 mm.

Images