Enamel cold-rolled steel sheet and method for manufacturing the same

Abstract

A kind of cold-rolled steel plate for enamel and its manufacturing method, its component mass percentage is as follows: C≤0.002%, Si≤0.05%, Mn 0.20~0.70%, P≤0.03%, S 0.020~0.055%, Al≤0.012%, O 0.008~0.022%, B 0.0015~0.0060%, N 0.0010~0.0070%, Nb0.01~0.035%, V≤0.01%, Ti≤0.01%, Ca 0.0005~0.003%, Mg0.0002~0.003%, also contain one or more than one of Cu, Cr, Ni, Mo, Cu≤0.08%, Cr≤0.08%, Ni≤0.08%, Mo≤0.08%, the rest includes Fe and inevitable impurities, and simultaneously satisfy: 0.035%≤Cu+Cr+Ni+Mo≤0.15%, 3%≤(3.44×O 2 +1.72×S 2 +78×N 2 )×10 5 ≤6%, 0.1%≤Mn-(3.44×O+1.72×S)≤0.6%, B / 11+Nb / 93>N / 14, Mn / S≥10, B / N≥0.8.Reduce the total oxygen content of steel plate, make it suitable for multi-furnace continuous casting production, improve production efficiency, also can have excellent forming property and higher yield strength, and meet the requirements of no bubble, scale explosion, poor adhesion and other surface defects after enamel.

Description

Technical Field

[0001] This invention relates to the field of manufacturing technology of cold-rolled steel sheets for enamel, and specifically to a cold-rolled steel sheet for enamel and its manufacturing method. Background Technology

[0002] Enamel products, as an advanced metal-based composite material, not only possess the high strength and easy molding characteristics of metal materials, but also the corrosion resistance, acid and alkali resistance, wear resistance, aesthetic appeal, and easy cleaning properties of enamel layers. They are widely used in light industry, home appliances, building decoration, petrochemicals, environmental protection, pharmaceuticals, and other fields.

[0003] In the environmental protection field, desulfurization and denitrification equipment (SCR), air preheaters (APH), and flue gas heaters (GGH) in thermal power plants mainly realize the desulfurization and denitrification process of flue gas, among which enamel-lined heat exchange elements are key core components. Because enamel-lined elements are in long-term contact with sulfur oxides and nitrogen oxides and are in an acidic environment, the requirements for the corrosion resistance of enamel-lined elements are extremely high. In other words, the requirements for the steel plate's resistance to pinholes, scaling, and adhesion are higher than those for steel used in light industry and household appliances.

[0004] In recent years, the coating technology in the desulfurization and denitrification industry has made significant progress and development, mainly reflected in the following aspects:

[0005] In terms of glaze, its characteristic is that the proportion of SiO2 component, which plays a role in acid resistance, is higher than that of civilian glaze. Due to the high SiO2 content, the viscosity of the glaze increases, which is not conducive to the escape of gas in the enamel layer during firing, resulting in more defects such as bubbles and pinholes in the enamel layer. At the same time, too much SiO2 is also not conducive to the adhesion strength of the enamel.

[0006] In terms of pretreatment, traditional pretreatment processes are inefficient, have harsh working environments, and cause serious environmental pollution. As pretreatment steps are gradually simplified or even eliminated, higher requirements are placed on the anti-pinhole performance and adhesion performance of steel plates.

[0007] In terms of enamel coating process, both domestic and foreign companies currently use a one-time enamel coating process. Electrostatic dry powder spraying has advantages such as high degree of mechanization, recyclable enamel powder, and thin and uniform enamel, and is gradually being adopted by enterprises. However, the thin enamel requires the steel plate to have excellent resistance to pinholes.

[0008] In addition, enamel components are also subject to airflow impact during operation, which can cause enamel cracks and peeling, leading to corrosion and perforation. Therefore, the higher the yield strength of the steel plate, the longer the service life of the enamel components.

[0009] Therefore, in order to meet the requirements of green environmental protection and high-quality development in the desulfurization and denitrification industry, the steel used for enamel heat exchange elements not only needs to have good formability and high yield strength, but also needs to ensure extremely high resistance to pinholes, anti-explosion performance and adhesion performance.

[0010] Chinese patent CN201810663362 discloses "A low-cost high-oxygen enamel steel produced by CSP process and its manufacturing method," Chinese patent CN202010515728 discloses "A cold-rolled steel plate for enamel and its preparation method," and Chinese patent CN200780035777 discloses "An enamel steel plate with significantly excellent anti-scaling properties and its manufacturing method." All of these patents utilize high oxygen content (500-850 ppm). The high oxygen content facilitates decarburization and allows oxygen to form numerous oxide inclusions in the steel, improving the steel plate's anti-scaling and adhesion properties. However, high-oxygen steel severely corrodes the stopper rod during continuous casting. Firstly, the thinner stopper rod cannot be completely shut off at the end of continuous casting, potentially leading to leakage. Secondly, the stopper rod contains a high carbon content, which, after corrosion, enters the molten steel, causing carbon enrichment. Therefore, in actual production, molten steel with an oxygen content exceeding 250 ppm is difficult to meet the requirements for multiple heats (3 heats or more) of continuous casting. In addition, the higher the oxygen content in the steel, the greater the amount of oxide inclusions formed, and the larger their size, which seriously damages the formability and surface quality of the steel plate.

[0011] Based on existing research on enamel steels that not only have good formability and high yield strength, but also need to ensure extremely high resistance to pinholes, scale bursts, and adhesion, it is difficult to meet the requirements of multi-furnace continuous casting production. Summary of the Invention

[0012] The purpose of this invention is to provide a cold-rolled steel sheet for enamel coating and its manufacturing method, which reduces the total oxygen content of the steel sheet, making it suitable for multi-furnace continuous casting production and improving production efficiency. The cold-rolled steel sheet has excellent formability and high yield strength (180–230 MPa), tensile strength (280–330 MPa), elongation at break ≥40%, and hydrogen permeability (TH2) ≥7.0 min / mm². 2 It meets the requirements of no surface defects such as bubbles, cracks, and poor adhesion after enamel coating.

[0013] To achieve the above objectives, the technical solution of the present invention is as follows:

[0014] A cold-rolled steel sheet for enamel coating has the following chemical composition by mass percentage: C ≤ 0.002%, Si ≤ 0.05%, Mn: 0.20–0.70%, P ≤ 0.03%, S: 0.020–0.055%, Al ≤ 0.012%, O: 0.008–0.022%, B: 0.0015–0.0060%, N: 0.0010–0.0070%, Nb: 0.01–0.035%, V ≤ 0.01%, Ti ≤ 0.01%, Ca: 0.0005–0.003%, Mg: 0.0002–0.003%. It also contains one or more of Cu, Cr, Ni, and Mo, with Cu ≤ 0.08%, Cr ≤ 0.08%, Ni ≤ 0.08%, and Mo ≤ 0.08%. The remainder includes Fe and other unavoidable impurities, and must simultaneously satisfy the following:

[0015] 0.035%≤Cu+Cr+Ni+Mo≤0.15%,

[0016] 3% ≤ (3.44 × O 2 +1.72×S 2 +78×N 2 )×10 5 ≤6%,

[0017] 0.1% ≤ Mn - (3.44 × O + 1.72 × S) ≤ 0.6%,

[0018] B / 11+Nb / 93>N / 14,

[0019] Mn / S≥10 and B / N≥0.8.

[0020] Furthermore, the remainder consists of Fe and other unavoidable impurities.

[0021] The microstructure of the steel plate described in this invention is equiaxed ferrite with a grain size of 7 to 10. Inclusions and precipitates are dispersed on the ferrite. The inclusions are composite inclusions formed by the combination of MnO and MnS, and are spindle-shaped along the rolling direction. The precipitates are boron-containing precipitates and Nb-containing precipitates with a diameter ≤50nm.

[0022] The steel plate described in this invention has a yield strength of 180–230 MPa, a tensile strength of 280–330 MPa, an elongation at break ≥40%, and a hydrogen permeability value (TH2) ≥7.0 min / mm. 2 .

[0023] C: In the cold-rolled steel sheet for enamel coating described in this invention, the carbon content affects the mechanical properties and enamel coating performance of the steel sheet. Carbon is an interstitial solid solution element; the lower its content, the better the formability and anti-aging properties of the steel sheet, meaning the better its processing performance. Furthermore, during the enamel coating process, carbon in the steel sheet reacts with the water of crystallization in the enamel to generate CO gas. The higher the carbon content, the more numerous and larger the CO bubbles generated, making them more likely to penetrate the entire enamel layer and form pinhole defects on the surface, thus affecting the surface quality of the enamel. Considering the high requirements for the formability and pinhole rate of the steel sheet under the service conditions of enamel heat exchange elements, and the current high level of carbon content control in smelting and refining (carbon content can be as low as below 15 ppm in the molten state), the C content is controlled to be ≤0.002% in this invention.

[0024] Si: In the cold-rolled steel sheet for enamel as described in this invention, silicon exists in a solid solution form. The higher the content, the worse the formability of the steel sheet. Furthermore, silicon in the steel inevitably combines with oxygen to form silica, which is deposited on the surface of the steel sheet. When the silicon content is too high, a large amount of silica is deposited on the surface of the steel sheet, forming a dense oxide film. This hinders the chemical reaction and diffusion between the enamel ions and the iron ions in the steel substrate during high-temperature enamel firing, greatly reducing the adhesion performance of the enamel. Therefore, the lower the silicon content, the better. Thus, this invention controls the Si content to ≤0.05%.

[0025] Mn: In the cold-rolled steel sheet for enamel as described in this invention, manganese reacts with sulfur and oxygen to form manganese sulfide and manganese oxide inclusions. During cold rolling, these inclusions create tiny cavities between themselves and the matrix to store hydrogen generated during enameling, thus ensuring the steel sheet's resistance to blistering. However, these inclusions also impair the steel sheet's formability. Besides forming inclusions, an excess of manganese in solid solution is also required to ensure the steel sheet's basic strength. However, when the manganese content is too high, it affects the unevenness of the interface between the steel sheet and the enamel layer, resulting in poor enamel adhesion and defects such as bubbles. Therefore, this invention controls the Mn content to be between 0.20% and 0.70%.

[0026] P: In the cold-rolled steel sheet for enamel as described in this invention, phosphorus is an unavoidable impurity element. It is completely soluble in ferrite, increasing the strength and hardness of the steel sheet, but reducing its formability. Furthermore, an appropriate amount of phosphorus can accelerate the pickling process in pretreatment steps involving pickling. However, if the phosphorus content is too high, excessive accumulation of pickling residue that cannot be removed in time will lead to defects such as bubbles and black spots on the enamel surface. Therefore, this invention controls the P content to ≤0.03%.

[0027] S: In the cold-rolled steel sheet for enamel coating described in this invention, an appropriate amount of sulfur combines with manganese to form manganese sulfide inclusions, improving the steel sheet's resistance to blistering and eliminating the hot brittleness caused by sulfur during hot rolling. Manganese sulfide is a ductile inclusion, typically distributed in steel in chain or strip-like patterns. The higher the sulfur content, the greater the number and size of the manganese sulfide inclusions formed, and the greater the damage to the transverse formability of the steel sheet. Therefore, this invention controls the sulfur content to be between 0.020% and 0.055%.

[0028] O: In the cold-rolled steel sheet for enamel as described in this invention, oxygen plays the same role as sulfur, mainly combining with manganese to form manganese oxide inclusions, thereby improving the steel sheet's resistance to enamel spalling. However, high oxygen content has many adverse effects. First, molten steel with high oxygen content severely corrodes the refractory material during continuous casting, causing steel leakage and carbon increase problems, making it impossible to achieve precise control of key elements such as C and O, and continuous casting of 3 heats or more, seriously affecting production rhythm and cost control. Second, high oxygen content leads to coarse and excessive manganese oxide inclusions in the steel, resulting in poor formability and uneven resistance to enamel spalling. Third, high oxygen content inevitably combines with Mn, Ti, etc. in the steel, reducing enamel adhesion and affecting the enamel surface quality. Therefore, this invention controls the O content to be between 0.008% and 0.022%.

[0029] Al: In the cold-rolled steel sheet for enamel as described in this invention, aluminum has strong oxidizing properties and usually acts as a deoxidizer. In this technical solution, a certain amount of oxygen needs to be retained. Therefore, the lower the Al content, the better. This invention controls the Al content to be ≤0.012%.

[0030] B: In the cold-rolled steel sheet for enamel as described in this invention, boron mainly forms boron nitride particles with nitrogen and combines with N to form fine, dispersed BN particles, which are uniformly distributed at ferrite grain boundaries or within grains. This can compensate for the problem of insufficient chain inclusions due to low oxygen content, i.e., insufficient effective hydrogen traps, and effectively improve the anti-scaling performance of the steel sheet. In addition, boron may also exist in a solid solution state or form boron carbide particles with carbon. However, the boron content in this technical solution should not be too high. This technical solution retains a certain oxygen content. Boron is an easily oxidized element, and there is some loss of boron during steelmaking, resulting in a reduced boron yield and failing to fully exert the anti-scaling effect of boron in this technical solution. Therefore, this invention controls the B content to be between 0.0015% and 0.0060%.

[0031] N: In the cold-rolled steel sheet for enamel coating described in this invention, nitrogen, like carbon, is an interstitial solid solution element. The higher the nitrogen content, the worse the plasticity of the steel sheet and the more likely it is to cause aging, which is very detrimental to the processing of the steel sheet. In this technical solution, boron is mainly used to fix the nitrogen element in the steel. The formation of boron nitride particles can improve the anti-scaling performance of the steel sheet and avoid the aging problem. However, high nitrogen content will have negative effects. On the one hand, higher nitrogen content leads to the formation of a large number of boron nitride particles, which precipitate at the austenite grain boundaries, easily causing transverse cracks at the corners of the continuously cast billet. On the other hand, higher nitrogen content results in larger and more numerous boron nitride particles, impairing the formability of the steel sheet. Therefore, this invention controls the N content to be between 0.0010 and 0.0070%.

[0032] Niobium (Nb): In the cold-rolled steel sheet for enamel as described in this invention, niobium is added as a strengthening element. Niobium is a strong carbide and nitride forming element, forming niobium carbide and niobium nitride particles in the steel. These particles can both pin the ferrite grain boundaries, effectively inhibiting grain growth, and act as effective hydrogen traps, improving the hydrogen storage capacity of the steel sheet and supplementing its anti-scaling properties. Niobium can also dissolve in the steel, increasing the austenite recrystallization temperature and effectively maintaining the austenite deformation state, thereby obtaining a refined ferrite structure, which refines the grains and improves the strength of the steel sheet. However, excessive niobium content can cause poor enamel adhesion and surface defects such as pinholes. Therefore, this invention controls the Nb content to be between 0.01% and 0.035%.

[0033] Ti and V: In the cold-rolled steel sheet for enamel coating described in this invention, titanium and vanadium are controlled as impurity elements. Besides combining with carbon and nitrogen, titanium and vanadium readily react with oxygen at high temperatures, consuming some oxygen and generating corresponding oxides. If the steel sheet contains excessive titanium and vanadium, a large amount of Ti... m O n V m O n Deposits on the steel plate surface impair its adhesion, thus affecting the stability of the coating. Additionally, Ti formed at high temperatures... m O n During continuous casting, solidification on the inner wall of the nozzle can easily cause nozzle blockage, disrupting the continuous casting capability of the molten steel. It can also lead to carbon increase in the molten steel, resulting in unstable control of its composition and properties. Therefore, this invention controls V ≤ 0.01% and Ti ≤ 0.01%.

[0034] Ca and Mg: In the cold-rolled steel sheet for enamel coating described in this invention, both calcium and magnesium play a role in improving the morphology of sulfides. Calcium and magnesium initially combine with oxygen in the steel, and only combine with sulfur when present in excess, forming fine, dispersed CaO, CaS, MgO, and MgS particles. These particles can serve as nuclei for heterogeneous MnS formation, significantly reducing the proportion of elongated and chain-like sulfides. It is worth noting that calcium and magnesium are added only in trace amounts. Adding excessive calcium leads to high CaS content, reducing the castability of the steel, while adding excessive magnesium weakens the ability to form fine, dispersed oxide nuclei. Therefore, this invention controls the Ca content to be between 0.0005% and 0.003%, and the Mg content to be between 0.0002% and 0.003%.

[0035] Cu, Cr, Ni, and Mo: In the cold-rolled steel sheet for enamel coating described in this invention, copper, chromium, nickel, and molybdenum are unavoidable residual elements that have a certain impact on the processing and enamel coating performance of the steel sheet. Controlling these elements within appropriate ranges can improve the adhesion of the enamel coating during high-temperature firing by improving the microscopic unevenness of the interface between the steel sheet and the enamel layer. Furthermore, an appropriate amount of chromium can improve resistance to blistering. Excessive addition not only hinders the reaction between the steel sheet and the enamel, leading to a decrease in adhesion strength, but also significantly increases the alloy cost. In addition, excessive copper, chromium, and nickel can reduce the formability and local plasticity of the steel sheet. Therefore, this invention controls the content of Cu, Cr, Ni, and Mo to be below 0.08%, and satisfies 0.035% ≤ Cu + Cr + Ni + Mo ≤ 0.15%.

[0036] Furthermore, in the cold-rolled steel sheet for enamel coating described in this invention, in order to ensure that the steel sheet has sufficient hydrogen storage traps, the following condition should be met: 3% ≤ (3.44 × O 2 +1.72×S 2 +78×N 2 )×10 5 A concentration of ≤6% ensures the formation of large-sized composite inclusions of MnS and MnO within the steel plate, as well as small-particle precipitates such as BN, B(C,N), and NbN, Nb(C,N). These inclusions or precipitates of different shapes and sizes generate numerous tiny cavities around them after cold rolling, which can inhibit hydrogen diffusion and effectively improve the steel plate's resistance to blistering. If O, S, and N do not satisfy the above relationship, when the concentration is below 3, the steel does not have sufficient hydrogen storage traps, and it cannot guarantee that blistering will not occur on one or both sides of the steel plate's enamel. When the concentration is above 6, the inclusions and precipitates in the steel are too large and their content is too high, which has very limited effect on improving the resistance to blistering and is detrimental to the formability of the steel plate.

[0037] Furthermore, in the cold-rolled steel sheet for enamel coating described in this invention, in addition to combining with oxygen and sulfur, manganese should also exist in a suitable amount in solid solution form to ensure the strength and enamel coating performance of the steel sheet. Therefore, the elements Mn, O, and S should also satisfy the following limiting relationship: 0.1% ≤ Mn - (3.44 × O + 1.72 × S) ≤ 0.6%. When it is below 0.1%, it indicates that the content of manganese in solid solution in the steel is too low, and the yield strength and resistance to high-temperature enamel softening of the steel sheet are low, which cannot guarantee the resistance of the enamel heat exchange element to airflow impact. When it is above 0.6%, it indicates that the steel has a high content of manganese in solid solution, which not only damages the formability of the steel sheet, but also easily causes surface defects such as poor adhesion and pinholes after enamel coating.

[0038] Furthermore, in the cold-rolled steel sheet for enamel as described in this invention, nitrogen is one of the main elements that cause aging of the steel sheet. B, Nb and N should satisfy: B / 11 + Nb / 93 > N / 14, to ensure that the content of N in solid solution is as low as possible, thereby improving the processing performance of the steel sheet. If the above relationship is not satisfied, the content of N existing in the form of interstitial solid solution is too high, which leads to an increase in the strength and hardness of the steel sheet and a decrease in plasticity.

[0039] Furthermore, in the cold-rolled steel sheet for enamel as described in this invention, to avoid billet cracks and improve the manufacturability of the steel sheet, Mn / S ≥ 10 and B / N ≥ 0.8 should be satisfied. If Mn / S < 10, MnS tends to distribute along the austenite grain boundaries, severely reducing the ductility of the steel. Under external force, the billet will develop internal cracks, affecting the internal quality of the product. When the cracks are severe, the billet cannot be repaired. If B / N is not properly controlled, the billet is prone to corner transverse cracks, which in turn affects the surface quality and mechanical properties of the product. In order to ensure the quality of subsequent processes, measures such as corner cracking in the steelmaking finishing process or edge trimming in the hot rolling process must be taken, which greatly reduces the hot delivery rate and yield.

[0040] The method for manufacturing cold-rolled steel sheet for enamel coating according to the present invention includes the following steps:

[0041] 1) Pre-desulfurization of molten iron, smelting, refining, and continuous casting into billets;

[0042] 2) Hot rolling

[0043] The slab heating temperature is 1150~1280℃, the heating time is ≥t×1, the unit is min, t is the slab thickness, the unit is mm, and the final rolling temperature is 850~940℃;

[0044] 3) Winding

[0045] Cool to 650–750°C before winding;

[0046] 4) Pickling and cold rolling

[0047] The cold rolling reduction rate is 60-85%;

[0048] 5) Continuous annealing

[0049] The annealing temperature is 750–850℃, the annealing time is 150–300s, the over-aging temperature is 420–230℃, and the over-aging time is 200–300s.

[0050] 6) Flat.

[0051] Preferably, in step 6), the flatness is controlled between 0.6% and 1.3%.

[0052] In the cold-rolled steel sheet manufacturing process described in this invention:

[0053] The purpose of smelting is to achieve metallurgical processes such as decarburization, dephosphorization, and heating, and to ensure that indicators such as carbon, phosphorus, sulfur, and free oxygen meet the process requirements of steel grades. Refining mainly involves processes such as deep decarburization, adjustment of final oxygen, and degassing to obtain molten steel that meets the requirements of purity and internal control composition.

[0054] The slab heating temperature is controlled between 1150 and 1280℃ to ensure the full dissolution and diffusion of chemical elements in the slab, especially precipitates such as NbN and BN segregated at grain boundaries, resulting in a uniform austenitic structure. During high-temperature heating, B-containing and Nb-containing precipitates partially dissolve, and during subsequent hot rolling and coiling, these precipitates precipitate again as finer particles. This not only refines the grains and improves the steel plate's strength, plasticity, and resistance to high-temperature softening during hot rolling, but also creates micro-vacuum around these precipitates after cold rolling, enhancing the steel plate's hydrogen storage capacity. The heating time is related to the slab thickness and should ensure sufficient homogenization of the slab structure. Generally, the heating time should be ≥ t × 1 (in minutes), where t is the slab thickness in mm. Insufficient heating time may lead to uneven temperature and composition in the slab, while excessive heating time can cause defects such as overheating, burning, and oxidation.

[0055] During hot rolling, on the one hand, elements such as niobium and boron dissolved in solid solution will redeprecipitate in the form of nitrides, carbides, or carbonitrides. On the other hand, austenite grains elongate, forming a large number of nucleation particles at grain boundaries, preparing for the refinement of ferrite grains. In this technical solution, the final hot rolling temperature is controlled between 850 and 940°C. If the temperature is too low, the final rolling will fall into the two-phase region, resulting in a mixed-grain structure; if the temperature is too high, a coarse hot-rolled microstructure is easily obtained, which will be inherited by subsequent processes and affect the finished product structure. Both excessively high and low temperatures will weaken the mechanical properties of the product.

[0056] Water cooling or laminar flow cooling is used, followed by coiling. During coiling, the main processes are the transformation and growth of ferrite grains, as well as the precipitation of precipitates. A coiling temperature of 650–750℃ ensures sufficient growth of the recrystallized ferrite grains, improving the formability of the steel sheet. Simultaneously, nitrides, carbides, or carbonitrides are fully precipitated, enhancing the hydrogen storage capacity of the steel sheet.

[0057] The purpose of pickling is to remove the iron oxide scale from the surface of the slab.

[0058] The total cold rolling reduction is controlled between 60% and 85%. The degree of cold rolling is closely related to the mechanical properties and hydrogen storage capacity of the steel sheet. On the one hand, the greater the cold rolling reduction, the more nucleation points there are for ferrite recrystallization, which is more conducive to obtaining a uniform and fine microstructure and helps to improve the strength and plasticity of the steel sheet. On the other hand, the composite inclusions of MnO and MnS formed at high temperatures do not dissolve or precipitate with the increase or decrease of temperature during hot rolling and coiling. After cold rolling deformation, micropores are generated between the inclusions and precipitates and the ferrite matrix to store hydrogen atoms generated during pickling and enamelting. The greater the cold rolling deformation, the more micropores there are, and the higher the hydrogen storage capacity of the steel sheet. In other words, the greater the cold rolling reduction, the better the mechanical properties and anti-scaling properties of the steel sheet. In this technical solution, after cold rolling deformation, the composite inclusions of MnO and MnS, as well as the boron- and niobium-containing precipitates, deform inconsistently with the ferrite matrix, easily forming micro-voids between them. The number of voids directly affects the steel plate's resistance to flaking. However, excessive cold rolling deformation will significantly increase the workload of the cold rolling equipment. Therefore, the cold rolling reduction is controlled between 60% and 85%.

[0059] The soaking temperature is 750–850℃, the soaking time is 150–300 s, the over-aging temperature is 420–230℃, and the over-aging time is 200–300 s. This continuous annealing process of heating, holding, and over-aging serves two purposes: firstly, it ensures complete recrystallization and sufficient growth of ferrite, and the full development of the recrystallized texture, ensuring that the ferrite grain size is controlled at level 7–10; secondly, a small amount of niobium- and boron-containing precipitates may redissolve and precipitate during the continuous annealing process, thus both hindering excessive ferrite grain growth and ensuring the steel plate's resistance to flaking. Therefore, the continuous annealing process of this invention ensures that the steel plate achieves excellent comprehensive performance in terms of strength, formability, and hydrogen storage.

[0060] The flattening elongation rate is controlled at 0.6-1.3%. The purpose of flattening is to eliminate the yield plateau of low alloy steel and avoid slip lines during processing. In addition, flattening can also improve the straightness and surface quality of the steel plate.

[0061] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0062] This invention employs an extremely low carbon (C≤0.0020%) and low oxygen content (O: 0.008~0.022%) composition design, adding Mn, B, and Nb, and controlling the content to 3%≤(3.44×O). 2 +1.72×S 2 +78×N 2 )×10 5 The content of Mn, O, and S should be ≤6% to ensure the formation of large-sized composite inclusions of MnS and MnO within the steel plate, as well as small precipitates such as BN, B(C,N) and NbN, Nb(C,N), with the particle diameter controlled below 50 nm. These inclusions or precipitates of different shapes and sizes generate numerous microcavities around them after cold rolling, which can inhibit hydrogen diffusion and effectively improve the anti-scaling performance of the steel plate. Simultaneously, the content of Mn, O, and S should meet the following requirements: 0.1% ≤ Mn - (3.44 × O + 1.72 × S) ≤ 0.6% to ensure an appropriate amount of manganese exists in solid solution form, guaranteeing the strength and coating performance of the steel plate. Furthermore, the content of dissolved N should be controlled to be as low as possible, ensuring B / 11 + Nb / 93 > N / 14, Mn / S ≥ 10, and B / N ≥ 0.8, thereby improving the processing and forming performance of the steel plate, avoiding billet cracks, and enhancing the manufacturability of the steel plate. Adding elements such as Cu, Cr, Ni, and Mo can improve the adhesion performance of enamel.

[0063] Compared with existing high-oxygen-content (0.0050–0.0080%) enamel steel, the oxygen content of this invention is below 0.0022%, solving the process bottlenecks such as steel leakage and carbon increase in the continuous casting process of high-oxygen steel. This increases the number of continuous casting furnaces, manufacturability, and production efficiency, while keeping alloy and manufacturing costs relatively low. Furthermore, it ensures that the steel plate has no surface defects such as bubbles, blistering, or poor adhesion after enamel coating.

[0064] Based on compositional design, this invention achieves stable control of ferrite grain size (grades 7-10) through high final rolling temperature, coiling temperature, cold rolling reduction rate, and continuous annealing processes, thereby improving the strength and toughness of the steel plate. Simultaneously, the composite inclusions of MnO and MnS formed at high temperatures (during liquid and solidification processes), as well as the boron-containing and nitrogen-containing precipitates generated during hot rolling and coiling, are transformed by cold rolling deformation into micropores between the composite inclusions of MnO and MnS, the boron-containing precipitates, and the nitrogen-containing precipitates and the ferrite matrix. This enhances the hydrogen storage capacity of the steel plate and ensures its resistance to scaling. Finally, the microstructure of the steel plate is equiaxed ferrite with a grain size of 7-10. Inclusions and precipitates are dispersed on the ferrite. The inclusions are composite inclusions formed by the combination of MnO and MnS, which are spindle-shaped along the rolling direction. The precipitates are boron-containing precipitates and Nb-containing precipitates with a diameter ≤50nm. This solves the matching problem between anti-scaling performance and deep drawing performance, and also improves continuous casting performance.

[0065] The cold-rolled steel sheet produced by this invention possesses excellent mechanical properties, combining strength and formability. The yield strength of the cold-rolled steel sheet described in this invention is 180–230 MPa; while the actual yield strength range of existing enameled cold-rolled steel sheets is approximately 140–190 MPa. This demonstrates a significant improvement in the yield strength of the steel sheet, which can significantly enhance the resistance of enameled heat exchange elements to gas impact and purging, thus extending their service life. Furthermore, the elongation at break of the enameled cold-rolled steel sheet described in this invention is ≥40%, meeting the requirements for ultra-deep drawing and enabling the stamping of complex-shaped corrugated plates, positioning plates, etc.

[0066] The cold-rolled steel sheet of this invention has excellent enameling properties, with a hydrogen penetration value TH2 ≥ 7.0 min / mm. 2 After one coating or one direct coating, the surface will not produce enamel defects such as pinholes, scaling, or poor adhesion. It is particularly suitable for desulfurization and denitrification industries with extremely high requirements for corrosion resistance, and especially meets the anti-scaling requirements of double-sided enamel. Attached Figure Description

[0067] Figure 1 This is a micrograph of the steel plate from Embodiment 1 of the present invention.

[0068] Figure 2 This is a microscopic photograph of the inclusions containing spherical particles in the steel plate microstructure of Example 1 of the present invention.

[0069] Figure 3 This is an energy dispersive spectroscopy (EDS) analysis diagram of the spherical particle portion containing spherical particle inclusions in the microstructure of the steel plate in Example 1 of the present invention.

[0070] Figure 4This is an energy dispersive spectroscopy (EDS) analysis diagram of the spherical particle portion containing spherical particle inclusions in the microstructure of the steel plate in Example 1 of the present invention.

[0071] Figure 5 This is an energy dispersive spectroscopy (EDS) analysis diagram of the spindle-shaped portion of spherical particle inclusions in the microstructure of the steel plate in Example 1 of the present invention.

[0072] Figure 6 This is a microscopic photograph of the inclusions in the steel plate of Example 1 of the present invention that do not contain spherical particles.

[0073] Figure 7 This is an energy dispersive spectroscopy (EDS) analysis diagram of the steel plate microstructure in Example 1 of the present invention, without spherical particle inclusions.

[0074] Figure 8 This is an energy dispersive spectroscopy (EDS) analysis diagram of the steel plate microstructure in Example 1 of the present invention, without spherical particle inclusions. Detailed Implementation

[0075] The present invention will be further explained and described below with reference to specific embodiments and accompanying drawings. However, such explanation and description do not constitute an improper limitation on the technical solution of the present invention.

[0076] Tables 1 and 2 show the mass percentage of the chemical composition of the steel plates in the embodiments and comparative examples of the present invention. Table 3 shows the process parameters of the embodiments and comparative examples of the present invention. Table 4 shows the performance of the steel plates in the embodiments and comparative examples of the present invention.

[0077] Figure 1 The metallographic microstructure of the steel plate in Embodiment 1 of the present invention is shown below. Figure 1 It can be seen that the steel plate has an equiaxed ferrite structure with an average grain size of 22.15 μm and a grain size of grade 8. The matrix is ​​dispersed with composite inclusions and precipitates formed by the combination of MnO and MnS. The composite inclusions are spindle-shaped along the rolling direction, and the precipitates have a diameter of ≤50 nm.

[0078] Figure 2 and Figure 6 The images shown are microscopic morphology photographs of the steel plate containing spherical particles and the steel plate without spherical particles, respectively, according to Example 1 of the present invention. Figure 2 It can be seen that the composite inclusions are spindle-shaped along the rolling direction, with two dark gray spherical particles embedded within them. Figure 6 No obvious spherical particles were found in the spindle-shaped composite inclusions.

[0079] Figure 3 and Figure 4 This is an energy dispersive spectroscopy (EDS) analysis diagram of the spherical particle portion containing spherical particle inclusions in the microstructure of the steel plate in Example 1 of the present invention. Figure 3 Energy dispersive spectroscopy (EDS) analysis confirmed that the spherical particles of the composite inclusions had a relatively high MnO content.

[0080] Figure 5 This is an energy dispersive spectroscopy (EDS) analysis diagram of the spindle-shaped portion of spherical particle inclusions in the microstructure of the steel plate in Example 1 of the present invention. Figure 7 and Figure 8 This is an energy dispersive spectroscopy (EDS) analysis diagram of the microstructure of the steel plate in Example 1 of the present invention, excluding spherical particle inclusions. Figure 5 and Figure 7 , Figure 8 It can be seen that the spindle-shaped inclusions are Mn(O,S) composite inclusions, but the relative content of MnO is low. Because the composite inclusions contain a certain amount of MnS, they also have good plastic deformation capacity, meaning they will exhibit an elongated shape along the rolling direction after deformation. Regardless of whether the composite inclusions contain spherical particles, they undergo uncoordinated deformation with the ferrite matrix, generating tiny cavities around the composite inclusions. These cavities are irreversible hydrogen storage traps; the more hydrogen storage traps there are, the better the steel plate's resistance to scale bursting.

[0081] The steel plates obtained in the embodiments and comparative examples of this invention were tested for transverse mechanical properties, and their yield strength, tensile strength and elongation at break were determined.

[0082] According to European Standard 10209-2013, the hydrogen penetration value TH2 of Examples 1-10 and Comparative Examples 1-2 was tested using an electrochemical method to measure the anti-scaling performance of the steel plates.

[0083] The steel plates of Examples 1-10 and Comparative Examples 1-2 were pretreated, electrostatically dry powder sprayed, and sintered at 850°C. After the enamel plates were air-cooled to room temperature, the quality of the enamel on their surface was inspected to see if there were any defects such as bubbles.

[0084] According to European Standard 10209-2013, drop ball impact tests were conducted at different heights based on the original steel plate thickness to verify the adhesion performance of the enamel plate. After the enamel plate was placed for 7 days, the surface was inspected for cracking. Table 3 lists the test results.

[0085] As shown in Table 3, the yield strength of the cold-rolled steel sheet for enamel coating obtained in the embodiments of the present invention is between 180 and 230 MPa, the tensile strength is between 280 and 330 MPa, the elongation at break is ≥40%, and the hydrogen penetration value TH2 is ≥7.0 min / mm. 2 .

[0086] The obtained cold-rolled steel sheet is suitable for (direct) one-time enamel enamel application, meaning that after pretreatment, it can be glazed or even directly glazed. Furthermore, after high-temperature firing and air cooling to room temperature, the enamel sheet showed no surface cracking, bubbles, pinholes, or other defects after standing for 7 days, indicating good enamel surface quality. At the same time, the drop hammer impact test results showed that the bonding strength between the steel sheet and the enamel layer was high.

[0087] In Comparative Example 1, the amounts of Mn and O added both exceeded the control range, and (3.44 × O) 2 +1.72×S 2 +78×N 2 )×10 5 =9.07%, which is outside the 3-6% range specified in this invention; Mn-(3.44×O+1.72×S)=0.73%, which is outside the 0.1-0.6% range specified in this invention. In steel, it combines to form a large number of large MnO and MnS particles, which impairs the plasticity of the steel plate; the high Mn content forms an oxide film on the surface of the steel plate in a weak oxidizing environment, which is detrimental to the adhesion of enamel and causes bubble defects on the enamel surface; the O content is as high as 380ppm, which seriously corrodes the refractory material in actual production, and can only be done in two consecutive furnaces, which greatly increases the manufacturing cost.

[0088] In Comparative Example 2, the contents of Mn and S were below the lower limit of the control range (3.44 × O). 2 +1.72×S 2 +78×N 2 )×10 5 =1.8%, which is outside the 3-6% range specified in this invention; Mn-(3.44×O+1.72×S)=0.08%, which is outside the 0.1-0.6% range specified in this invention. This results in a small number of MnS and MnO particles precipitated in the steel. In other words, the steel lacks a sufficient number of hydrogen storage traps, which is not conducive to the anti-scaling performance of the steel plate and cannot meet the anti-scaling requirements of double-sided enamel for enamel heat exchange elements. The low Mn content also results in low strength of the steel plate. After high-temperature firing, the strength will be further reduced, which seriously damages the resistance of the enamel heat exchange element to airflow impact and purging, and reduces the service life of the element.

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Claims

1. A cold-rolled steel sheet for enamel, having a chemical composition in mass percent of: C ≤ 0.002%, Si ≤ 0.05%, Mn: 0.20-0.70%, P ≤ 0.03%, S: 0.020-0.055%, Al ≤ 0.012%, O: 0.008-0.022%, B: 0.0015-0.0060%, N: 0.0010-0.0070%, Nb: 0.01-0.035%, V ≤ 0.01%, Ti ≤ 0.01%, Ca: 0.0005-0.003%, Mg: 0.0002-0.003%, and further containing one or more of Cu, Cr, Ni, Mo, Cu ≤ 0.08%, Cr ≤ 0.08%, Ni ≤ 0.08%, Mo ≤ 0.08%, the balance being Fe and other unavoidable impurities, and simultaneously satisfying: 0.035% ≤ Cu+Cr+Ni+Mo ≤ 0.15%, 0.1% ≤ Mn-(3.44xO+1.72xS) ≤ 0.6%, B / 11+Nb / 93 > N / 14, Mn / S ≥ 10 and B / N ≥ 0.8; the microstructure of the steel sheet being equiaxed ferrite, the grain size being 7-10, and the ferrite being dispersedly distributed with inclusions and precipitates; the inclusions being complex inclusions of MnO and MnS combined together, and being spindle-shaped along the rolling direction; the precipitates being B-containing precipitates and Nb-containing precipitates, the diameter of the precipitates being ≤ 50 nm.

2. A method for manufacturing the cold-rolled steel sheet for enamel according to claim 1, comprising the following steps: 1) pre-desulphurization of molten iron, smelting, refining, and continuous casting into a slab; 2) hot rolling; the slab heating temperature being 1150-1280°C, and the heating time being ≥ t x 1, unit min, t being the slab thickness, unit mm; the finish rolling temperature being 850-940°C; 3) coiling; cooling to 650-750°C for coiling; 4) pickling and cold rolling; the total cold rolling reduction being 60-85%; 5) continuous annealing; the annealing soaking temperature being 750-850°C, the soaking time being 150-300 s, the overaging temperature being 420-230°C, and the overaging time being 200-300 s; 6) skin pass. 3% < (3.44 x O 2 + 1.72 x S 2 + 78 x N 2 ) x 10 5 ≤ 6%, In step 6), the skin pass rate is controlled to be 0.6-1.3%. ​ ​ ​ The steel sheet has a yield strength of 180-230 MPa, a tensile strength of 280-330 MPa, an elongation at break of ≥ 40%, and a hydrogen permeation value TH2 of ≥ 7.0 min / mm 2 . ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 3. The production method according to claim 2, wherein ​

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