JP7611270B2 - Cold-rolled enameled steel sheet for deep-drawing inner pot and its manufacturing method - Google Patents

Description

The present invention relates to a metallic material and a manufacturing method thereof, and more particularly to an enameled steel sheet and a manufacturing method thereof.

There are two types of structures for enameled kettle, including two-stage and three-stage, which are mainly differentiated by the forming and welding process. The two-stage kettle is made by welding two circular plates by deep drawing, so there is only one annular weld seam on the whole kettle, while the three-stage kettle is made by welding two end caps and one kettle body, so the end caps are stamped and the kettle body is edge-rolled, so there are two annular weld seams and one straight seam on the whole kettle.

In recent years, two-stage enameled inner kettles have been rapidly developed in industrial production from the viewpoint of improving product quality and reducing overall costs. In the forming process of two-stage inner kettles, a high degree of forming is performed by deep drawing and ironing die processing (for example, for a typical 80L inner kettles, when the diameter is 382 mm, the press processing depth is about 430 mm), so the steel plate of the two-stage enameled inner kettles is required to have good formability. In addition, since the finished enameled inner kettles must undergo high-pressure circulating water pressure tests more than 20,000 times, if the strength of the steel plate after sintering is insufficient, the inner kettles will deform during the pressure process and the enamel will peel off, so the steel plate used for the two-stage enameled inner kettles is required to have high strength even after cloisonné firing.

In addition, the nail-pop defect is one of the most important problems in the quality of enamel products, and the occurrence of the nail-pop defect is related to the solubility of hydrogen atoms in steel. There are three sources of hydrogen: hydrogen dissolved in the base steel sheet; hydrogen generated during pickling and firing; and most importantly, hydrogen generated during firing by the reaction of the steel sheet with the water vapor in the furnace and the crystal water of the glaze polishing additive. During the high-temperature sintering process, hydrogen atoms generated by the chemical reaction between water and the iron and carbon atoms of the steel sheet enter the steel sheet and exist near the structures of the steel sheet, such as dislocations, crystal boundaries, voids, impurities and precipitates. As the steel sheet cools, the solubility of hydrogen atoms in the steel decreases and they gather at the interface between the steel sheet and the enamel layer. As the pressure of the hydrogen gas exceeds the tensile strength of the enamel layer, it breaks through the glaze layer and causes the so-called nail-pop defect. Under the strict production conditions of a certain water heater manufacturer, all open ends of vertical units face downward during the cloisonné process, which is unfavorable for the evaporation of water vapor, so the steel plate itself must have sufficient hydrogen absorption areas to meet the high requirements for nail-breaking resistance.

In order to ensure that the two-stage enameled inner kettle fits well in terms of forming, pressure resistance, welding and glazing, the steel plate used in the two-stage enameled inner kettle must have good deep drawing performance, high strength after cloisonné, good welding performance and excellent glazing performance, and in particular, excellent resistance to nail chipping.

The characteristics of conventional cold-rolled deep-drawing enamel steels are based on ultra-low carbon, high sulfur and nitrogen content, and an appropriate amount of alloying elements (most commonly titanium) are added. Titanium can fix free carbon, nitrogen and sulfur atoms, reduce the amount of solid solution of impurity elements, and improve the press processing performance of the steel sheet; on the other hand, titanium can combine with carbon, nitrogen and sulfur to form inclusions or second phases, which can act as effective hydrogen storage traps to enhance the hydrogen storage capacity of the steel sheet. However, it should be noted that although this design can simultaneously improve the deep drawing performance and anti-toe jumping performance of the steel sheet, the yield strength of the substrate is low, and the yield strength after high-temperature sintering is further reduced, so the pressure resistance requirements of the enamel inner kettle cannot be met, and the manufacturing cost is high due to the addition of a large amount of alloying elements in this design.

Currently, there is little specialized development of steel plates specifically for two-stage enameled kettles in China, and the steel for two-stage enameled kettles used by Chinese electric water heater manufacturers is mainly ordinary steel SPCC, which does not contain alloy elements such as titanium, niobium, or boron, and has only finely adjusted elements such as carbon and manganese, which creates some problems with the quality of the glazing and welding.

One object of the present invention is to provide a cold-rolled enameled steel sheet for deep drawing inner pots, which is obtained by adding an appropriate amount of boron, nitrogen, and a trace amount of titanium and niobium to a base low carbon steel , and also adding alloy elements such as copper and chromium to improve the adhesion of the enamel, thereby obtaining a uniform and fine microstructure of ferrite + lamellar and pseudo-pearlite. Pearlite has the effect of strengthening the substrate, and the cementite pellets in the pearlite can improve the nail-flying resistance of the steel as an irreversible hydrogen trap. In designing the chemical composition, by adding an appropriate amount of boron and nitrogen to the steel, the BN pellets formed and dispersed and precipitated by the combination of the two can not only significantly improve the nail-flying resistance of the steel as an effective hydrogen trap, but also effectively increase the growth ability of ferrite, and give the steel good formability. The trace amount of titanium added to the steel can replace a part of the boron and combine with nitrogen to form TiN particles, which reduces the risk of cracking of the continuously cast slab due to BN, and is also advantageous for welding performance.

The cold-rolled enameled steel sheet for deep drawing inner kettles has a ferrite grain size of 7.5-8 grade, a yield strength of 220-280 MPa, an elongation of 38-43%, a hydrogen permeation time of ≥8 min, and a yield strength of ≥200 MPa after high-temperature cloisonné treatment at least 850° C. for at least 12 min. It has good formability, high strength after high-temperature cloisonné treatment, excellent welding performance, and excellent resistance to nail chipping, and can be effectively applied to enameled inner kettles of electric water heaters, which have high forming requirements.

In order to achieve the above-mentioned object, the present invention provides a cold-rolled enamelled steel sheet for deep drawing inner kettles, containing the following chemical elements in the following weight percentages:
C: 0.02-0.06%, 0<Si≦0.08%, Mn: 0.1-0.7%, P≦0.055%, S≦0.03%, Al: 0.01-0.07%, N: 0.002-0.010%, B: 0.002-0.009%, Ti: 0.002-0.015%, Nb: 0.002-0.012%, Cr: 0.01-0.08%, and at least one of Cu, Ni, and Mo; Cu+Ni+Mo≦0.40%, where Cu, Ni, and Mo each represent the mass percent content of the respective element.

Furthermore, in the cold-rolled enameled steel sheet for deep drawing inner kettle according to the present invention, the mass percentages of each chemical element are:
C: 0.02-0.06%, 0<Si≦0.08%, Mn: 0.1-0.7%, P≦0.055%, S≦0.03%, Al: 0.01-0.07%, N: 0.002-0.010%, B: 0.002-0.009%, Ti: 0.002-0.015%, Nb: 0.002-0.012%, Cr: 0.01-0.08%, and at least one of Cu, Ni and Mo; Cu+Ni+Mo≦0.40%; the balance being Fe and other unavoidable impurities.

In the present invention, the design principles of each chemical element in the cold-rolled enameled steel sheet for deep drawing inner pot are as follows:
C: In the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention, C is the most basic strengthening element in steel, and carbon is mainly present in the steel of the present invention in the form of a small amount of lamellar and pseudo-pearlite. The cementite pellets in the pearlite act as suitable hydrogen storage traps to reinforce the nail-flying resistance of the steel sheet. Meanwhile, the small amount of lamellar and pseudo-pearlite generates a very small amount of CO, _CO2 gas during high-temperature cloisonné firing, which greatly reduces the bubbles or pinhole defects in the enamel layer and is advantageous for improving the quality of the glaze layer. However, it should be noted that with an increase in the percentage content of C in the steel, the strength of the steel increases and the plasticity and toughness decrease accordingly. Since the steel sheet of the present invention is used as a steel sheet for deep-drawing inner pot of an enamel inner pot, it is preferable that the mass percentage of C is not too high. Therefore, in the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention, the mass percentage of C is 0.02-0.06%.

In a preferred embodiment, the mass percentage of C may be 0.02-0.04%.
Si: In the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention, Si does not form carbides in the steel, but exists in the form of a solid solution, so that it has the effect of solid-solution strengthening and can reduce the plasticity and toughness of the steel to a certain extent. Si can also affect the adhesion performance of the enameled steel, and the _SiO2 thin film formed from an appropriate amount of Si in the high-temperature cloisonné process is advantageous for the penetration and penetration of the glaze into the steel, but if the Si content is too high, the chemical reaction between the glaze and the metal interface is inhibited. In addition, if the Si content is too high, the product is prone to chipping defects. Therefore, in consideration of the forming performance, enamel adhesion performance and chipping resistance performance, the mass percentage of Si in the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention is 0<Si≦0.08%.

In a preferred embodiment, the mass percent of Si may be 0<Si≦0.05%. In a preferred embodiment, the mass percent of Si may be 0.005-0.08%, preferably 0.005-0.05%.

Mn: In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the addition of Mn to the steel has the effect of solid solution strengthening. Mn has the effect of deoxidizing and desulfurizing during steelmaking, and can remove FeO and FeS from the steel. At the same time, the generated MnO and MnS act as effective "hydrogen traps" and can reduce the nail jump sensitivity to some extent. Furthermore, the mixed precipitation of MnO and MnS can improve the elongated shape of MnS to a spindle or ellipse shape, so that the adverse effect on the lateral formability of the steel sheet due to the precipitation of MnS alone can be avoided. However, it should be noted that if the Mn content in the steel exceeds 0.7%, the plasticity, welding performance, and enamel adhesion performance of the steel may be reduced. Therefore, in the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the mass percent of Mn is 0.1-0.7%.

In a preferred embodiment, the weight percentage of Mn may be 0.2-0.5%.
P: In the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention, P is an inevitable impurity element. Although it can improve the strength and hardness of steel, its severe segregation increases temper brittleness and reduces the plasticity and toughness of steel; it also has a negative effect on welding performance. Its content must be strictly controlled. Therefore, in the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention, the mass percentage of P is P≦0.055%.

In a preferred embodiment, the mass percentage of P may be P≦0.035%.
S: In the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention, S is a harmful element in ordinary steel, and the S element remaining in the steel inevitably reacts with Fe to form FeS, which causes the steel to become hot brittle and reduces the ductility and toughness of the steel. S is also detrimental to the welding performance of the steel and reduces the corrosion resistance of the steel. Therefore, in the cold-rolled enameled steel sheet for deep-drawing inner pot according to the present invention, the mass percentage of S is S≦0.03%.

In a preferred embodiment, the mass percentage of S may be S≦0.015%.
Al: In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, Al is usually added to steel as a deoxidizer in steelmaking. In addition, to prevent solute N atoms from causing aging, Al is usually added to form AlN, which refines the grains and can also be an effective "hydrogen trap". However, in the present invention, since the nitride-forming element B is added, the main effect of Al is deoxidization or adjustment of the oxygen content. In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the mass percent of Al is 0.01-0.07%.

In a preferred embodiment, the mass percentage of Al may be 0.01-0.04%.

B and N: In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, B is a strong nitride-forming element. In the present invention, B preferentially forms BN particles with N. There is a strong affinity between the BN pellets and hydrogen atoms. Meanwhile, the large amount of voids generated around the BN pellets after cold rolling can also effectively store hydrogen atoms, and BN strongly affects the diffusion and capacity of hydrogen atoms in steel, and can significantly improve the anti-claw-flying performance of the steel sheet. At the same time, the preferentially precipitated BN pellets suppress the precipitation of fine AlN, so that the growth ability of crystal grains is enhanced and the size of ferrite crystal grains is increased, and the enamel steel can have good deep drawing performance. It should be noted that B also has the effect of suppressing high-temperature deformation of the steel sheet, so that the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention can effectively prevent high-temperature deformation during welding and glazing processes after forming. Therefore, taking into consideration the overall resistance to snapping, deep drawing performance, and production stability, in the cold-rolled enamel steel sheet for deep drawing inner pots according to the present invention, the mass percentage of B is 0.002-0.009%, and the mass percentage of N is 0.002-0.010%.

In a preferred embodiment, the mass percentage of B may be 0.0025-0.0065% and the mass percentage of N may be 0.003-0.008%.

Ti: In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, Ti is a strong carbon, nitride-forming element. In low carbon steel, Ti has a large effect on the strength of the steel sheet. Since the enameled steel according to the present invention requires good formability, a trace amount of titanium treatment is performed. A trace amount of Ti can combine with N at high temperature to form dispersed TiN pellets, which consumes some N and prevents the formation of too many BN pellets, effectively reducing the risk of transverse cracks occurring at the corners of the continuously cast slab. At the same time, the stable and dispersed TiN pellets can control the size of the austenite grains in the welding heat affected zone and improve the welding performance of the steel. It should also be noted that the dispersed and precipitated TiN pellets can further improve the nail-flip resistance of the steel. Therefore, in the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the mass percent of Ti is 0.002-0.015%.

In a preferred embodiment, the mass percentage of Ti may be 0.002-0.008%.

Nb: In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the addition of a small amount of Nb can effectively improve the performance of the steel sheet in the 45° direction, reduce the anisotropy of the steel sheet, reduce the earring defects of the steel sheet after deep drawing, and easily increase the production efficiency and product rate. In addition, Nb exists in the form of solid solution Nb and NbC, and has the effects of solid solution tension and precipitation pinning in the recrystallization process, so that fine crystal grains can be obtained and the strength of the steel sheet after high-temperature cloisonné can be effectively ensured. Therefore, in the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the mass percent of Nb is 0.002-0.012%.

In a preferred embodiment, the mass percentage of Nb may be 0.002-0.006%.

Cr: In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, an appropriate amount of Cr can adjust the unevenness of the steel sheet surface during the production process, and effectively increase the bonding strength between the steel sheet substrate and the enamel glaze. If the Cr content in the steel is too low, the effect of improving the enamel adhesion cannot be achieved, but if the Cr content in the steel is too high, not only the production cost increases, but also the pickling process progresses slowly, making it difficult to obtain a rough surface. Therefore, in the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the mass percentage of Cr is 0.01-0.08%.

In a preferred embodiment, the mass percentage of Cr may be 0.02-0.08%.

Cu, Ni and Mo: In the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the presence of one or more of Cu, Ni and Mo is unavoidable, and all of these elements have an effect on the adhesion performance of enamel. In the high-temperature cloisonné process, the appropriate content of Cu, Ni and Mo can promote the infiltration of the glaze into the metal, and promote the dissolution of iron and iron oxide in the glaze, so as to form a good transition layer structure in which the metal and the enamel layer are closely bonded. In addition, nickel oxide is favorable for the formation of a relatively uniform bubble structure by the enamel layer, and improves the quality of the enamel layer. However, it should be noted that if the content of Cu, Ni and Mo is too high, not only will the alloy cost increase, but also the bonding strength between the metal and the glaze will decrease. Therefore, in the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, Cu+Ni+Mo≦0.40%.

In a preferred embodiment, the mass percentages of Cu, Ni and Mo may be Cu+Ni+Mo≦0.25%. In a more preferred embodiment, the mass percentages of Cu, Ni and Mo may be Cu+Ni+Mo≦0.10%, more preferably ≦0.05%.

In one embodiment, the weight percent of Cu is 0.005-0.02%. In one embodiment, the weight percent of Ni is ≦0.02%, preferably ≦0.01%. In one embodiment, the weight percent of Mo is ≦0.02%, preferably ≦0.01%. In one embodiment, the cold rolled enameled steel sheet for deep drawing inner pots of the present invention contains Cu 0.005-0.02%; Ni ≦0.02%, preferably ≦0.01%; and Mo ≦0.02%, preferably ≦0.01%. Preferably, Cu+Ni+Mo≦0.05%.

Furthermore, in the cold-rolled enameled steel sheet for deep drawing inner kettle according to the present invention, the mass percent contents of B and N satisfy B/N=0.8-1.5, where B and N each represent the mass percent content of the respective element.

In the above technical proposal, in the cold rolled enameled steel sheet for deep drawing inner pot according to the present invention, the content of each element is controlled, and at the same time, the mass percent content of B and N is controlled to satisfy B/N=0.8-1.5. This is because B is an element that is easily burned during the steelmaking process, and it is difficult to control it stably. The higher the B content, the more difficult it is to control. If the B/N ratio is too high, that is, if too much B is added, fine _Fe23 (CB) ₆ is formed, which inhibits the growth of crystal grains and impairs the deep drawing performance of the steel sheet. And if the B/N ratio is too low, the resistance of the steel to finger-snapping is poor.

Furthermore, in the cold-rolled enameled steel sheet for deep drawing inner kettle according to the present invention, each chemical element satisfies at least one of the following formulae:
0.4≦B×N×10 ⁵ ≦10;
-2≦Ti×(N-14/11B)×10 ⁵ ≦12;
In the above formula, B, Ti and N each represent the mass percent content of the respective element.

In the technical solution of the present invention, the synergistic relationship between B, Ti and N in the cold rolled enameled steel sheet for deep drawing inner pot of the present invention affects the performance of the steel sheet. If B and N do not satisfy 0.4≦B×N×105 ^≦ 10, a small amount of BN cannot improve the anti-claw-flying performance of the steel sheet because BN is an effective irreversible hydrogen trap in steel, whereas an excessive amount of BN pellets precipitates at the grain boundaries of austenite, and these particles are subjected to the action of stress during the continuous casting process. When the stress is concentrated, the formation and expansion of cracks is accelerated, and finally the corner transverse cracks of the continuously cast slab are caused. This increases the edge cracking amount of the steel sheet and reduces the product rate.

If B, Ti and N do not satisfy -2≦Ti×(N-14/11B)×10 ⁵ ≦12, the risk of the corner cracking problem of the continuously cast slab caused by BN pellets increases significantly if the Ti content is too low; the larger the solid solubility product of Ti and N, the easier it is to form coarse TiN pellets in the molten steel. Coarse TiN pellets cannot inhibit the growth of crystal grains, which is disadvantageous to both welding performance and ensuring strength after high-temperature cloisonné. The larger the TiN pellet, the smaller the effective surface area around it, that is, the less space is available for storing hydrogen atoms, which is disadvantageous to improving the nail-flying resistance performance. Therefore, in the cold-rolled enamel steel sheet for deep drawing inner pot according to the present invention, if the B element, Ti element and N element satisfy the above relationship, it is easy to realize an appropriate, fine and dispersed distribution of BN and TiN pellets in the steel. Preferably, -2≦Ti×(N-14/11B)×10 ⁵ ≦0.

Furthermore, in the cold-rolled enameled steel sheet for deep drawing inner kettle according to the present invention, the mass percent content of each chemical element satisfies at least one of the following conditions:
C: 0.02-0.04%, 0<Si≦0.05%, Mn: 0.2-0.5%, P≦0.035%, S≦0.015%, Al: 0.01-0.04%, N: 0.003-0.008%, B: 0.0025-0.0065%, Ti: 0.002-0.008%, Nb: 0.002-0.006%, Cr: 0.02-0.08%, and at least one of Cu, Ni and Mo, Cu+Ni+Mo≦0.25%; preferably Cu 0.005-0.02%; Ni ≦0.02%, preferably ≦0.01%; Mo ≦0.02%, preferably ≦0.01%, and preferably Cu+Ni+Mo≦0.05%.

Furthermore, in the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, its microstructure is uniform and fine ferrite, lamellar pearlite and pseudo-pearlite, the relative proportion (volume ratio) of pearlite is <3%, and a large amount of voids exist between the cementite pellets and ferrite in the pearlite (including lamellar pearlite and pseudo-pearlite). The voids are generated during the cold rolling process and are determined by the number and size of the second phase particles and the cold rolling reduction ratio, and the more and finer the second phase particles are and the larger the cold rolling reduction ratio is, the more voids are generated.

Furthermore, in the cold-rolled enameled steel sheet for deep drawing inner pots according to the present invention, the lamellar pearlite is located at the triple grain boundary line of ferrite, and the cementite pellets in the pseudo-pearlite are located at the ferrite grain boundaries and inside the grains.

Furthermore, in the cold-rolled enameled steel sheet for deep drawing inner pots according to the present invention, the crystal grain size of ferrite is 7.5-8 class.

Furthermore, the cold-rolled enameled steel sheet for deep drawing inner pots according to the present invention has properties that satisfy at least one of the following: yield strength 220-280 MPa, elongation 38-43%, hydrogen permeation time ≥ 8 min, and yield strength ≥ 200 MPa after high-temperature cloisonné annealing at least 850°C for at least 12 min. In one embodiment, the tensile strength of the cold-rolled enameled steel sheet for deep drawing inner pots according to the present invention is 320-390 MPa.

Another object of the present invention is to provide a method for producing a cold-rolled enameled steel sheet for deep drawing inner pot. This method can realize the distribution of boron, titanium and niobium carbonitride in steel by using a suitable rolling control and cooling control process and a high-temperature rapid continuous annealing process, and can obtain a uniform and fine ferrite + small amount of lamellar and pseudo-pearlite structure. The cold-rolled enameled steel sheet for deep drawing inner pot produced by this method has a ferrite grain size of 7.5-8 grade, a yield strength of 220-280 MPa, an elongation of 38-43%, a hydrogen permeation time of ≥8 min, and a yield strength of ≥200 MPa after high-temperature cloisonné at least 850°C for at least 12 min. It has good formability, high strength after high-temperature cloisonné, excellent welding performance and excellent resistance to nail chipping, and is suitable for producing enameled inner pots of electric water heaters and other products that have high requirements for forming.

In order to achieve the above object, the present invention provides a method for producing the above-mentioned cold-rolled enameled steel sheet for deep drawing inner kettle, which includes the following steps:
(1) Smelting, refining and continuous casting;
(2) Heating the slab;
(3) hot rolling and coiling;
(4) Pickling;
(5) Cold rolling: cold rolling reduction ratio is 60-70%;
(6) Continuous annealing: soaking temperature is 800-830°C, soaking time is 100-150s, overaging temperature is 350-450°C, and overaging time is 250-350s;
(7) Leveling is performed.

In the manufacturing method of the cold rolled enameled steel sheet for deep drawing inner pot according to the present invention, the smelting and refining operations in step (1) can remove harmful gases such as nitrogen gas and hydrogen gas in the molten steel, realize decarbonization and deoxidization, and ultimately homogenize the composition and temperature of the molten steel. In the continuous casting operation in step (1), the molten steel is sent to a turntable and passes through equipment such as an intermediate tank, a crystallizer, a secondary cooling stage, and a tension leveler, to obtain a continuously cast slab with good shape, surface quality, and internal structure.

In the manufacturing method according to the present invention, the pickling in step (4) removes the oxide scale on the surface of the hot-rolled steel strip, making subsequent operations easier.

In the cold rolling operation of step (5), the cold rolling reduction affects the formability and anti-seizure performance of the steel. Usually, the larger the reduction, the larger the deformation storage energy of the steel sheet, and the easier it is for recrystallization to occur during annealing, and the easier it is for the recrystallization structure to fully develop. The second phase pellets are crushed during the cold rolling process, and gaps are formed between the surroundings and the ferrite substrate, which makes it easier for hydrogen atoms to diffuse, and the larger the cold rolling reduction, the more gaps there are and the stronger the anti-seizure performance. If the cold rolling reduction is less than 60%, it is difficult to obtain good forming performance, and if the cold rolling reduction exceeds 70%, the deformation resistance of the cold rolled strip steel is too large and exceeds the support capacity of the mechanical system. Therefore, in step (5) of the manufacturing method according to the present invention, the cold rolling reduction is 60-70%. In one embodiment, the thickness of the cold rolled sheet may be within the range of 1.5-2.5 mm, for example, 1.5-2.0 mm.

In contrast, the above step (6) uses a high-temperature rapid continuous annealing process, with a soaking temperature of 800-830°C, soaking time of 100-150s, overaging temperature of 350-450°C, and overaging time of 250-350s. By combining with the cold rolling process, the formation and development of a recrystallized structure can be realized, uniform and fine ferrite grains can be obtained, and the simultaneous improvement of the strength and plasticity of the steel sheet can be promoted. At the same time, the cementite pellets can be re-dissolved and precipitated during the annealing process, making it easier for the steel sheet to exhibit its anti-claw chipping performance.

Furthermore, in the manufacturing method according to the present invention, the heating temperature in step (2) is 1100-1250°C.

In the manufacturing method of the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, when the heating temperature in step (2) is 1100-1250°C, a uniform austenitic structure is obtained. When the heating temperature is lower than 1100°C, the deformation resistance of the steel is large, and hot rolling does not proceed easily. When the heating temperature exceeds 1250°C, problems such as decarbonization of the steel slab and thick oxide scale are likely to occur during the heating process. The heating time in step (2) may be 120-200 minutes, for example 130-180 minutes.

Furthermore, in the manufacturing method according to the present invention, in step (3), the finishing rolling temperature of the hot rolling is 880-920°C, and the coiling temperature is 680-720°C.

In the manufacturing method of the cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention, the finish rolling temperature of the hot rolling is set to 880-920°C in the above step (3). If the finish rolling temperature is less than 880°C, the finish rolling will enter a two-phase region, resulting in a fine ferrite mixed structure formed by transition from massive pro-eutectoid ferrite and deformed austenite, which will have a detrimental effect on the mechanical properties of the steel sheet. If the finish rolling temperature exceeds 920°C, the deformed austenite will easily recover, which is disadvantageous for refining the ferrite grains.

The present invention also uses a high coiling temperature. A coiling temperature of 680-720°C not only improves the formability of the steel sheet, but also facilitates sufficient dispersion and precipitation of second-phase pellets, effectively improving the resistance of the steel to chipping.

Furthermore, in the manufacturing method according to the present invention, the leveling reduction rate in step (7) is 0.8-1.2%.

In the above embodiment, when leveling is performed in step (7) with a leveling reduction rate of 0.8-1.2%, not only can the flatness and surface gloss of the steel sheet be improved, but also a "yield shelf" is not generated when the steel sheet is pressed after a certain storage time.

The cold-rolled enameled steel sheet for deep drawing inner pots and the manufacturing method thereof according to the present invention have the following advantages and beneficial effects compared with the prior art:
The cold-rolled enameled steel sheet for deep drawing inner pot according to the present invention is made by adding an appropriate amount of boron, nitrogen, and trace amounts of titanium and niobium to the basic low carbon steel, and also adding alloy elements such as copper and chromium to improve the adhesion performance of the enamel, thereby obtaining a uniform and fine microstructure of ferrite + lamellar and pseudo-pearlite. Pearlite has the effect of strengthening the substrate, and the cementite pellets in the pearlite can improve the nail-flying resistance of the steel as an irreversible hydrogen trap. In the design of the chemical composition, by adding an appropriate amount of boron and nitrogen to the steel, the BN pellets formed and dispersed and precipitated by the combination of the two can not only significantly improve the nail-flying resistance of the steel as an effective hydrogen trap, but also effectively increase the growth ability of ferrite and give the steel good formability. The trace amount of titanium added to the steel can replace a part of the boron and combine with nitrogen to form TiN particles, which reduces the risk of cracking of the continuously cast slab due to BN, and is also advantageous for welding performance.

The cold-rolled enameled steel sheet for deep drawing inner kettles has a ferrite grain size of 7.5-8 grade, a yield strength of 220-280 MPa, an elongation of 38-43%, a hydrogen permeation time of ≥8 min, and a yield strength of ≥200 MPa after high-temperature cloisonné treatment at least 850° C. for at least 12 min. It has good formability, high strength after high-temperature cloisonné treatment, excellent welding performance, and excellent resistance to nail chipping, and can be effectively applied to enameled inner kettles of electric water heaters, which have high forming requirements.

In addition, the manufacturing method of the present invention uses an appropriate rolling control/cooling control process and a high-temperature rapid continuous annealing process to realize a dispersed distribution of carbon nitrides of boron, titanium and niobium. As a result, the cold-rolled enameled steel sheet for deep drawing inner pots obtained by the manufacturing method of the present invention has a uniform and fine ferrite + small amount of lamellar and pseudo-pearlite structure. The lamellar pearlite is located at the ferrite grain boundary triple line, and the cementite pellets of the pseudo-pearlite are located at the ferrite grain boundaries and inside the grains, so that the cold-rolled enameled steel sheet for deep drawing inner pots obtained can ensure properties such as high strength after high-temperature cloisonné tempering, high formability and excellent resistance to nail chipping.

FIG. 1 is a microstructure diagram of the cold-rolled enameled steel sheet for deep drawing inner kettle of Example 1. FIG. 2 shows a typical pearlite morphology in the microstructure of the cold-rolled enameled steel sheet for deep drawing inner kettle of Example 1. FIG. 3 shows the pseudo-pearlite morphology in the microstructure of the cold-rolled enameled steel sheet for deep drawing inner kettle of Example 1. FIG. 4 shows the location of boron element in the cold-rolled enamel steel sheet for deep drawing inner kettle of Example 1.

The cold-rolled enameled steel sheet for deep drawing inner pot and the manufacturing method thereof according to the present invention will be described in more detail below with reference to specific embodiments and drawings, but the description does not limit the technical solution of the present invention.

Examples 1-6 and Comparative Examples 1-2
Table 1 shows the mass percentages of each chemical element in the cold-rolled enameled steel sheets for deep drawing inner kettles of Examples 1-6 and the enameled steel sheets of Comparative Examples 1-2.

The cold-rolled enameled steel sheets for deep drawing inner pots of Examples 1-6 according to the present invention and the enameled steel sheets of Comparative Examples 1-2 were both produced by the following steps:
(1) Smelting, refining and continuous casting;
(2) Slab heating: the heating temperature is set to 1100-1250°C, the heating time is controlled, and the slab is sufficiently austenitized, and then hot rolling is performed;
(3) Hot rolling and coiling: the finishing rolling temperature of hot rolling is 880-920°C, and water cooling, air cooling or slow cooling is performed after rolling, and then the coiling temperature is 680-720°C, and coiling is performed;
(4) Pickling;
(5) Cold rolling: cold rolling reduction ratio is 60-70%;
(6) Continuous annealing: soaking temperature is 800-830°C, soaking time is 100-150s, overaging temperature is 350-450°C, and overaging time is 250-350s;
(7) Leveling: The leveling reduction ratio is 0.8-1.2%.

Tables 2-1 and 2-2 show specific process parameters for the cold-rolled enameled steel sheets for deep drawing inner kettles of Examples 1-6 and the enameled steel sheets of Comparative Examples 1-2.

It should be noted that in the actual operation process, the overaging temperature changes and is not stable at a fixed value. During the overaging time, the temperature gradually decreases, so the overaging temperature in step (6) of Table 2-2 does not indicate a single value in each example and comparative example, but rather indicates a range of values.

The cold-rolled enameled steel sheets for deep drawing inner pots of Examples 1-6 and the enameled steel sheets of Comparative Examples 1-2 were subjected to various performance tests. The specific test methods were as follows:
A high-temperature simulated cloisonné test was conducted on the cold-rolled enamel steel sheets of Examples 1-6 and Comparative Examples 1-2, with the cloisonné temperature set to 850° C. and the furnace time set to 12 minutes. The mechanical properties were measured, and the results are shown in Table 3.

For the cold-rolled enamelled steels of Examples 1-6 and Comparative Examples 1-2, a wet glazing process was used. First, the steel sheets were pretreated to obtain a surface suitable for glazing. Then, the steel sheets with the glaze applied were placed in a drying oven at 110°C and dried. Finally, they were fired in a high-temperature oven. The cloisonné temperature was 850°C and the time in the oven was 12 minutes. After the enamelled steel sheets were air-cooled to room temperature, the adhesion performance was measured according to European standard EN 10209-2013. A steel ball was dropped from a height of 750 mm and collided with the enamelled steel sheet surface. The adhesion level was judged by observing the degree of bonding between the glaze and the steel sheet, and the enamelled steel sheets were left for 72 hours or more to observe the condition of nail chipping. Furthermore, to evaluate the nail-breaking resistance of the steel plate, the hydrogen permeation value of the steel plate was measured according to the European standard EN 10209-2013 and converted into the hydrogen permeation time at a thickness of 1 mm. Here, the yield strength and tensile strength were measured according to GB/T 228.1-2010, and the elongation was measured according to GB/T 228.1-2010.

Table 3 shows the measurement results of each performance of the cold-rolled enameled steel sheets for deep drawing inner kettle of Examples 1-6 and the enameled steel sheets of Comparative Examples 1-2.

As can be seen from Tables 1 to 3, the cold-rolled enamel steel of Comparative Example 1 has a low B content, so the number of effective hydrogen traps in the steel sheet is insufficient, and the nail-fly resistance cannot be effectively improved, and nail-fly defects are likely to occur during high-temperature cloisonné, especially in special equipment with all openings facing downward. The cold-rolled enamel steel of Comparative Example 2 has a high Ti content, so in addition to the formation of coarse TiN and TiS pellets, finely dispersed TiC also precipitates, resulting in an increase in strength and a decrease in elongation. In addition, Ti is prone to form _TiO2 at high temperatures, so the adhesion level of the steel sheet is significantly reduced.

As can be seen from Table 3, the cold-rolled enameled steel sheets for deep drawing inner pots of Examples 1-6 of the present invention all had a pearlite phase ratio of <3%, all had ferrite grain sizes of 7.5-8 grade, all had longitudinal yield strengths of 220-280 MPa, and elongations of 38.0-43.0%, and the yield strengths after high-temperature cloisonné annealing at at least 850°C for at least 12 minutes were ≧200 MPa. In addition, the steel sheets after wet glazing on one side had a good bond between the enamel layer and the steel sheet, which was grade A1, and did not suffer from the nail-popping phenomenon even after being left for a long time. Measurements showed that the hydrogen permeability of the steel sheet (calculated for a thickness of 1 mm) was ≧8 min, exceeding the threshold value of 6.7 min specified in the European standard EN 10209-2013. It was found that the cold-rolled enamel steel sheets for deep-drawn inner kettles according to the respective embodiments of the present invention did not cause chipping even when glazed on both sides, and had excellent chipping resistance.

FIG. 1 is a microstructure diagram of the cold-rolled enameled steel sheet for deep drawing inner kettle of Example 1.
As shown in FIG. 1, in the cold-rolled enameled steel sheet for deep drawing inner pot of Example 1 according to the present invention, its microstructure is polygonal ferrite and a small amount of lamellar and pseudo-pearlite, the lamellar pearlite is distributed at the triple grain boundary line of ferrite, and the cementite pellets of the pseudo-pearlite are distributed at the ferrite grain boundaries and inside the grains.

FIG. 2 shows a typical pearlite morphology in the microstructure of the cold-rolled enameled steel sheet for deep drawing inner kettle of Example 1.

FIG. 3 shows the pseudo-pearlite morphology in the microstructure of the cold-rolled enameled steel sheet for deep drawing inner kettle of Example 1.

As can be seen from Fig. 2 and Fig. 3 taken together, in the cold-rolled enameled steel sheet for deep drawing inner pot of Example 1 according to the present invention, the pearlite contains not only typical lamellar pearlite but also pseudo-pearlite structure. The cementite is not continuous but tends to be elliptical or rectangular. This may be because the addition of B reduces the content of solute carbon in the steel, which makes it impossible to form a complete lamellar pearlite structure. As can be seen from Fig. 3, pseudo-pearlite cementite pellets exist at the ferrite grain boundaries and inside the grains. Research has shown that BN or boron carbide segregates at the grain boundaries, which inhibits the precipitation of carbides at the grain boundaries, which tend to precipitate on the substrate.

FIG. 4 shows the location of boron element in the cold-rolled enamel steel sheet for deep drawing inner kettle of Example 1.

In the cold rolled enameled steel sheet for deep drawing inner pots of Example 1 of the present invention, the B element in the steel does not exist in the form of a single atom, but combines with the N element or the C element to form complex compound BN or boron carbide pellets, which segregate on the grain boundaries or on the cementite within the crystals.

It should be noted that the above-mentioned embodiments are only specific embodiments of the present invention, and the present invention is not limited to the above-mentioned embodiments, and similar changes and modifications can be directly obtained or easily conceived by those skilled in the art from the disclosure of the present invention, and therefore, they are within the protection scope of the present invention.

Claims (9)

A cold-rolled enamelled steel sheet for deep drawing inner kettles, the mass percentages of chemical elements being:
C: 0.02-0.06%, 0<Si≦0.08%, Mn: 0.1-0.7%, P≦0.055%, S≦0.03%, Al: 0.01-0.07%, N: 0.002-0.010%, B: 0.002-0.009%, Ti: 0.002-0.015%, Nb: 0.002-0.0097%, Cr: 0.01-0.08%, and at least one of Cu, Ni and Mo; Cu+Ni+Mo≦0.40%; the balance being Fe and other unavoidable impurities,
The chemical elements contained in the cold rolled enameled steel sheet for deep drawing inner kettle satisfy the following formulas:
B/N=0.8-1.5;
0.4≦B×N×10 ⁵ ≦10;
-2≦Ti×(N-14/11B)×10 ⁵ ≦12;
The cold-rolled enameled steel sheet for deep drawing inner pots has a microstructure which is uniform and fine ferrite, lamellar pearlite and pseudo-pearlite, the total phase ratio of the lamellar pearlite and the pseudo-pearlite being <3%; voids exist between the cementite pellets in the pearlite and the ferrite, and the crystal grain size of the ferrite is 7.5-8 class. The cold rolled enamel steel sheet for deep drawing inner pots according to claim 1, wherein the mass percent content of each of the chemical elements satisfies at least one of the following items: C: 0.02-0.04%, 0<Si≦0.05%, Mn: 0.2-0.5%, P≦0.035%, S≦0.015%, Al: 0.01-0.04%, N: 0.003-0.008%, B: 0.0025-0.0065%, Ti: 0.002-0.008%, Nb: 0.002-0.006%, Cr: 0.02-0.08%, and at least one of Cu, Ni and Mo; Cu+Ni+Mo≦0.25%. 2. The cold-rolled enameled steel sheet for deep drawing inner pots according to claim 1, wherein the lamellar pearlite is located at the triple grain boundary line of ferrite, and the cementite pellets in the pseudo-pearlite are located at the ferrite grain boundaries or inside the grains. The cold-rolled enameled steel sheet for deep drawing inner kilns according to claim 1, wherein the cold-rolled enameled steel sheet for deep drawing inner kilns satisfies at least one of the following properties: yield strength 220-280 MPa, elongation 38-43%, hydrogen permeation time ≥ 8 min, and yield strength ≥ 200 MPa after high-temperature cloisonné annealing at at least 850°C for at least 12 min. The cold-rolled enamel steel sheet for deep drawing inner pots according to claim 1, wherein Cu: 0.005-0.02%, Ni: ≦0.02%, Mo: ≦0.02%, and Cu + Ni + Mo ≦0.05%. A manufacturing method for manufacturing the cold-rolled enameled steel sheet for a deep drawing inner kettle according to any one of claims 1 to 5, comprising the following steps:
(1) Smelting, refining and continuous casting activities;
(2) Heat the slab;
(3) hot rolling and coiling;
(4) Pickling;
(5) Cold rolling: cold rolling reduction ratio is 60-70%;
(6) Continuous annealing: soaking temperature is 800-830°C, soaking time is 100-150s, overaging temperature is 350-450°C, and overaging time is 250-350s;
(7) Leveling is performed. The manufacturing method according to claim 6, wherein in step (2), the heating temperature is 1100-1250°C. The manufacturing method described in claim 6, in which in step (3), the finishing rolling temperature of the hot rolling is 880-920°C and the coiling temperature is 680-720°C. The manufacturing method according to claim 6, in which the leveling reduction rate in step (7) is 0.8-1.2%.

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