High-strength cold-rolled steel plate for double-sided enameled liner and manufacturing method thereof

Abstract

The invention discloses a high-strength cold-rolled steel plate for a double-sided enameled liner. The steel plate contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.06-0.12%, 0<Si≤0.08%, Mn: 0.5-1.2%, P: 0.01-0.05%, S: 0.005-0.05%, Al: 0.008-0.06%, N≤0.006%, Ti: 0.03-0.1%, B: 0.0002-0.0035%, Cr: 0.01-0.06%, Cu: 0.01-0.06%, and Mg: 0.0005-0.03%. Correspondingly, the present invention also discloses a method for manufacturing the high-strength cold-rolled steel sheet for double-sided enameled liner, which comprises the following steps: (1) smelting, refining and continuous casting; (2) heating the ingot; (3) hot rolling and coiling: controlling the final rolling temperature of the hot rolling to be 810-880°C and the coiling temperature to be 620-680°C; (4) descaling; (5) cold rolling: controlling the cold rolling reduction ratio to be 60-70%; (6) continuous annealing: soaking temperature to be 780-850°C, soaking time to be 120-200s, over-aging temperature to be between 165-450°C, and over-aging time to be 250-350s; and (7) leveling.

Description

Technical Field

[0001] The present invention relates to a metal material and a manufacturing method thereof, and in particular to a high-strength cold-rolled steel and a manufacturing method thereof. Background Art

[0002] In the existing technology, the common enameled liner manufacturing process is: the steel plate is stamped and rolled into the end cover and barrel body, and then welded to form the liner base. Then it is pretreated to remove impurities such as residual oil and iron oxide on the surface to obtain a surface suitable for enameling. Finally, wet enameling and high-temperature sintering are carried out to obtain the finished enameled liner.

[0003] Steel plate is a key material for producing enameled liner products. It determines not only the overall compressive strength of the liner but also, to a large extent, the quality and service life of the enamel. Therefore, the market generally places high demands on the performance of steel used in enameled liner products, requiring high yield strength, excellent formability, superior weldability, and good enameling properties.

[0004] In the prior art, some researchers have studied high-strength steel sheets for producing enameled liners. For example, patents CN103510011A, CN101139684A, CN101255530A, CN202199726A, and CN103643118 A all design high-strength steel sheets for producing enameled liners. These designs utilize low-carbon steel as a base, increasing the manganese and titanium content with the addition of niobium or rare earth elements to enhance the steel's strength and fishscale resistance.

[0005] In the above-mentioned patented technical solutions, the main focus is on the strength of the steel plates. The yield strength of the steel plates in the examples listed is all above 350 MPa, but there are still some shortcomings: these steel plates are mainly suitable for single-sided enameling processes and cannot meet the requirements of double-sided enameling without scaling.

[0006] During the actual service life of the enameled inner tank, as hot water drains down the drainpipe, it also removes a significant amount of heat. Currently, electric heating companies employ a foam layer between the inner tank and the outer shell to achieve this insulation. To improve the efficiency of electric water heaters, prevent heat loss, and reduce energy consumption, many manufacturers have attempted to develop new heat exchange inner tank structures. This requires coating not only the inner wall of the enameled inner tank that contacts the water, but also the outer surface that contacts the residual heat to prevent steam corrosion.

[0007] However, to date, there is no data or application practice in China to show that high-strength cold-rolled steel sheets for double-sided enameled liner have been developed. Based on this, in order to meet market demand, the present invention aims to obtain a new high-strength cold-rolled steel sheet for double-sided enameled liner and a manufacturing method thereof. Summary of the Invention

[0008] One of the purposes of the present invention is to provide a high-strength cold-rolled steel plate for double-sided enameled liner, which not only has high strength, good plasticity and excellent enameling performance, but also can meet the requirements of double-sided enameling.

[0009] The high-strength cold-rolled steel plate for double-sided enameled inner liner has good comprehensive properties in forming, enameling, welding and pressure resistance, especially excellent welding performance, good anti-scale performance, adhesion performance and anti-pinhole and bubble defect performance and high strength after high-temperature enameling. It is particularly suitable for making double-sided enameled inner liner.

[0010] In order to achieve the above object, the present invention provides a high-strength cold-rolled steel sheet for double-sided enameled liner, which contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages:

[0011] C: 0.06-0.12%, 0<Si≤0.08%, Mn: 0.5-1.2%, P: 0.01-0.05%, S: 0.005-0.05%, Al: 0.008-0.06%, N≤ 0.006%, Ti: 0.03-0.1%, B: 0.0002-0.0035%, Cr: 0.01-0.06%, Cu: 0.01-0.06%, Mg: 0.0005-0.03%.

[0012] Furthermore, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, the mass percentages of the chemical elements are as follows:

[0013] C: 0.06-0.12%, 0<Si≤0.08%, Mn: 0.5-1.2%, P: 0.01-0.05%, S: 0.005-0.05%, Al: 0.008-0.06%, N≤0.006%, Ti: 0.03-0.1%, B: 0.0002-0.0035%, Cr: 0.01-0.06%, Cu: 0.01-0.06%, Mg: 0.0005-0.03%, and the balance is Fe and other inevitable impurities.

[0014] The chemical composition design of the present invention primarily utilizes C, Mn, and P for strengthening, supplemented by appropriate amounts of Ti and B, a moderately increased S content, and supplemented by alloying elements such as Cu, Cr, and Mg. By adding an appropriate amount of Ti, increasing the S content, and controlling the N content, Ti, C, S, and N form small, dispersed second-phase particles, which not only improve fishscale resistance but also control ferrite grain growth during enameling, enhancing the steel sheet's resistance to high-temperature softening. Furthermore, an appropriate amount of B can mitigate the secondary brittleness caused by P, further improving fishscale resistance and increasing the steel sheet's high-temperature strength.

[0015] In the present invention, by rationally adjusting the chemical elements and their addition amounts, in particular, by defining the relationship between Ti, C, S and N, which have an impact on anti-scale performance, anti-pinhole defect performance and strength, and by controlling the processing technology, the development of high-strength cold-rolled steel sheets for double-sided enameled liners is achieved, ensuring that the steel sheets have high strength and good plasticity while meeting the requirements of anti-scale in double-sided enameling and good bubble structure in the enamel layer.

[0016] In the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, the design principles of each chemical element are as follows:

[0017] C: In the high-strength cold-rolled steel sheet for double-sided enameled liner described herein, C is the most fundamental strengthening element. As the C content in the steel increases, the strength of the steel sheet also increases, but its plasticity and toughness decrease accordingly. Adding an appropriate amount of C to the steel not only ensures the steel sheet's basic strength but also combines with the Ti element in the steel to form TiC particles, allowing the C to exist in the form of pearlite and TiC particles. Accordingly, process control can ensure that the formed TiC particles are uniformly dispersed throughout the ferrite matrix. These TiC particles not only enhance the steel sheet's strength through precipitation strengthening but also serve as effective hydrogen traps, improving its anti-scale performance. Furthermore, during high-temperature enameling, they can control the coarsening of austenite grains and the growth of ferrite grains, effectively preventing softening of the steel sheet after enameling. However, it is important to note that the C content in the steel should not be too high. Excessive C content in the steel will form a large amount of pearlite, which not only impairs the steel sheet's formability but also generates a large amount of CO gas during enameling, resulting in bubbles and pinholes on the enameled surface. Therefore, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, the mass percentage of the C element is controlled between 0.06-0.12%.

[0018] Si: In the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, the Si element can be dissolved in ferrite and austenite, and improve the hardness and strength of the steel, but too high a content will significantly reduce the plasticity and toughness of the steel. Generally speaking, adding an appropriate amount of Si to the steel will not affect the adhesion performance of the steel sheet, but when the Si content in the steel is too high, a SiO2 film will form on the surface of the steel sheet when heated, hindering the infiltration of the porcelain glaze into the steel sheet and the chemical reaction between the two, thereby reducing the adhesion strength between the steel sheet and the porcelain glaze. In addition, Si will also accelerate the adsorption of hydrogen by the steel sheet during the pickling process, exacerbating the possibility of scale explosion. Therefore, taking into account the beneficial effects and adverse effects of the Si element on the performance of the steel sheet, in the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, the mass percentage of Si is controlled to satisfy: 0<Si≤0.08%.

[0019] Mn: In the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, Mn is a commonly used strengthening element. It can be added in a solid solution state and play a role in strengthening the ferrite matrix. At the same time, Mn can also react with S to form MnS, which not only overcomes the hot brittleness caused by sulfur but also acts as an effective hydrogen trap, playing an important role in anti-scale performance. However, MnS is an elongated inclusion, which can be detrimental to the transverse properties of the steel sheet. Therefore, an appropriate amount of Ti is added to the steel design so that the elongated MnS can be gradually replaced by spherical Ti4C2S2, thereby improving the transverse plasticity and toughness of the steel sheet. It is important to note that the Mn content in the steel should also not be too high. When the Mn content in the steel is too high, it not only reduces the material's plasticity and adhesion properties, but also affects the steel sheet's flexural properties. This is because Mn lowers the temperature at which ferrite transforms to austenite, causing the steel sheet to easily flex and deform during high-temperature enameling. Therefore, in order to ensure the performance of the steel plate, in the high-strength cold-rolled steel plate for double-sided enameled liner of the present invention, the mass percentage of Mn is controlled between 0.5-1.2%.

[0020] Phosphorus (P) in the high-strength cold-rolled steel sheet for double-sided enameled liner described herein effectively improves the sheet's fishscale resistance. Furthermore, P, dissolved in ferrite, increases the steel's strength and hardness. However, severe P segregation not only increases the steel's cold brittleness but also significantly reduces its plasticity and toughness. Therefore, the present invention mitigates the secondary processing brittleness caused by P segregation by adding appropriate amounts of alloying elements such as boron and molybdenum. Therefore, the mass percentage of P in the high-strength cold-rolled steel sheet for double-sided enameled liner described herein is controlled between 0.01% and 0.05%.

[0021] S: In the high-strength cold-rolled steel sheet for double-sided enameled liner applications described herein, appropriately increasing the S content can form complex inclusions with titanium and manganese, beneficially preventing scale breakage during enameling. By properly controlling the S content and process, these complex inclusions can be further prevented from becoming oversized, which would otherwise affect the steel sheet's anti-scale breakage and formability. Therefore, to maximize the beneficial effects of S in steel, the mass percentage of S in the high-strength cold-rolled steel sheet for double-sided enameled liner applications described herein is controlled between 0.005% and 0.05%.

[0022] Al: In the high-strength cold-rolled steel sheet for double-sided enameled liner described herein, Al not only deoxidizes and refines grains, but also fixes nitrogen in the steel, reducing its aging tendency and improving its low-temperature toughness. However, it is important to note that the Al content in the steel should not be too high, as this can create difficulties in smelting and casting. Therefore, the mass percentage of Al in the high-strength cold-rolled steel sheet for double-sided enameled liner described herein is controlled between 0.008% and 0.06%.

[0023] N: In the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, nitrogen, like carbon, has a higher N content in the steel, resulting in poorer formability and aging, and therefore an upper limit on the N content is required. Furthermore, in the present invention, N can combine with Ti to form corresponding nitrides, which can improve the anti-scale burst performance of the steel sheet. It should be noted that the N content in the steel should not be too high. When the N content in the steel is too high, the size of the nitrides formed is too large, which has a very limited effect on improving the anti-scale burst performance and impairs the plasticity of the steel. Therefore, in the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, the mass percentage of N is N≤0.006%.

[0024] Ti: In the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, Ti is an extremely active metal element. It can produce stable compounds with carbon, nitrogen and sulfur. Through process control, these compounds are evenly dispersed in the ferrite matrix, playing the role of preventing scale explosion and strengthening the matrix. In addition, Ti element also has certain benefits for adhesion. Ti element oxidized to form Ti m O n It can be enriched on the surface of the steel plate, making the adhesion layer between the steel plate and the enamel significantly widened. However, if Ti m O n Too much Ti will hinder the physical and chemical reaction between the steel plate and the enamel. Therefore, in order to maximize the beneficial effects of Ti, the mass percentage of Ti in the high-strength cold-rolled steel plate for double-sided enameled liner of the present invention is controlled between 0.03-0.1%.

[0025] B: In the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, the B element can effectively improve the secondary brittleness problem caused by P segregation; in addition, B forms boron carbide in the steel, partially existing in the form of solid solution, which can further improve the material's anti-scale performance; however, too high a B content can also cause transverse cracks in the corners of the continuous casting ingot. In addition, the B element can also segregate at the austenite grain boundaries and prevent element diffusion and grain boundary migration during high-temperature enameling, thereby improving the high-temperature strength of the steel sheet. Therefore, in order to exert the beneficial effects of the B element, in the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, the mass percentage of the B element is controlled between 0.0002-0.0035%.

[0026] Cr: In the high-strength cold-rolled steel sheet for double-sided enameled interior containers described herein, Cr effectively enhances the steel's strength while reducing its toughness. A moderate amount of Cr improves the steel's adhesion properties. However, it's important to note that excessive Cr content in the steel should be avoided, as it can cause scale cracking. Therefore, the mass percentage of Cr in the high-strength cold-rolled steel sheet for double-sided enameled interior containers described herein is controlled between 0.01% and 0.06%.

[0027] Cu: In the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, a trace amount of Cu has certain benefits on the enamel adhesion performance under the pre-treatment process including pickling; Cu element exists in the steel mainly in the form of solid solution, and Cu dissolved in the acid solution during pickling 2+ A replacement reaction will re-form metallic Cu or Cu2S compounds on the steel sheet surface. The Cu or Cu2S residue after pickling forms a porous film that acts as a cathode during high-temperature enameling, causing galvanic corrosion and increasing the surface roughness of the steel sheet, thereby improving the adhesion of the enamel. However, in areas where copper is locally enriched, a large number of bubbles will form within the enamel layer. To maximize the beneficial effects of Cu, the mass percentage of Cu in the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention is controlled between 0.01% and 0.06%.

[0028] Mg: Adding an appropriate amount of Mg to the high-strength cold-rolled steel sheet for double-sided enameled liner described herein can effectively improve the morphology of inclusions and enhance the steel's plasticity and toughness. Therefore, in the present invention, the mass percentage of Mg is controlled between 0.0005% and 0.03%.

[0029] Furthermore, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, among the inevitable impurities, O≤0.008%, Ni≤0.1%, and Mo≤0.1%.

[0030] In the technical solution described in the present invention, O, Ni and Mo are all inevitable impurity elements in steel. Under the premise that technical conditions and production costs permit, it is necessary to control the content of impurity elements in steel to be as low as possible.

[0031] O: In the present invention, O element will affect the processing performance and enameling performance of the steel plate. On the one hand, too high O element content will lead to excessive oxide inclusions in the steel, deteriorating the plasticity and toughness of the steel plate; on the other hand, too high O element content will consume a large amount of Ti, forming excessive Ti m O n , which is detrimental to adhesion and reduces the formation of second-phase particles TiN, Ti4C2S2, and TiC, which are beneficial for anti-scale performance. Therefore, in the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, the content of the impurity element O must be strictly controlled, with the mass percentage of O being controlled to meet the requirement of O ≤ 0.008%.

[0032] Ni and Mo: In the present invention, Ni and Mo elements are beneficial to improving the adhesion performance of enamel. This is because during high-temperature enameling, Ni and Mo elements can promote the infiltration and penetration of porcelain enamel into the steel plate, and promote the mutual dissolution and diffusion of ions in the porcelain enamel and iron ions. In addition, Ni also has the function of preventing the diffusion of hydrogen in steel, which can improve the anti-scale explosion performance of the steel plate. However, Ni and Mo elements are both precious alloys. Excessive content of Ni and Mo elements will not only increase the cost, but also reduce the adhesion performance. In general, the adverse effects brought about by the impurity elements Ni and Mo outweigh their beneficial effects. Therefore, in the high-strength cold-rolled steel plate for double-sided enameled liner described in the present invention, the content of impurity elements Ni and Mo must be strictly controlled, and the mass percentage of Ni and Mo elements must be controlled to meet the following requirements: Ni≤0.1%, Mo≤0.1%.

[0033] Furthermore, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, each chemical element also satisfies at least one of the following formulas:

[0034] 0.05%≤C-(Ti-3.43N-1.5S) / 4≤0.1%;

[0035] 0.08%≤Ti+0.875C≤0.25%;

[0036] 0.65≤2.5Ti / (1.2C+8.57N+3.75S)≤1.35;

[0037] The element symbols in the formula all represent the mass percentage of the corresponding elements.

[0038] In the existing technical literature, there is no reference to the limiting relationship between Ti, C, N, and S. According to the formation free energy and solid solubility of titanium compounds, Ti first fixes N and S in the steel, and only when there is a surplus of Ti will it combine with C to form TiC. If excess titanium = Ti-(4C+3.43N+1.5S)>0, it means that all the carbon is fixed by titanium; if excess titanium ≤0, it means that titanium is only combined with part of the carbon or not combined with carbon, then the carbon exists in the form of pearlite. The control of pearlite composition has a great influence on the enameling performance of the steel plate, especially the resistance to pinhole defects. Pearlite decomposes to produce CO gas during the firing process. Generally, the higher the pearlite content, the higher the firing temperature or the longer the firing time, the more gas is produced, which can easily produce excessive and large bubbles in the enamel layer, causing surface bubbles and pinhole defects, and damaging the quality of the porcelain layer.

[0039] In the above technical solution, in the high-strength cold-rolled steel plate for double-sided enameled liner described in the present invention, while controlling the content of a single element, the mass percentages of C, Ti, N and S are further controlled to satisfy: 0.05%≤C-(Ti-3.43N-1.5S) / 4≤0.1%.

[0040] This is because: in the present invention, carbon mainly exists in the form of TiC particles, a low-temperature precipitate of titanium, and pearlite. This relationship limits the content of free carbon that forms the pearlite structure. On the one hand, it is necessary to ensure that more than 0.05% C remains in the steel to form pearlite to ensure the strength and anti-scale performance of the steel plate. On the other hand, the pearlite content must be limited to not be too high, otherwise a large amount of CO gas will be generated during high-temperature enameling, resulting in poor bubble structure of the enamel layer, surface pinhole defects, and damage to the plasticity of the steel plate.

[0041] In addition, in order to ensure that a sufficient amount of second phase particles are formed in the steel plate, the mass percentages of Ti and C may be preferably controlled to satisfy the following: 0.08%≤Ti+0.875C≤0.25%.

[0042] Furthermore, because the type, size, and morphology of precipitates significantly impact the performance of the steel sheet, the present invention, while controlling the content of individual elements, further controls the mass percentages of Ti, C, N, and S to satisfy the following relationship: 0.65 ≤ 2.5Ti / (1.2C + 8.57N + 3.75S) ≤ 1.35. This ensures that the carbon, nitrogen, and sulfide particles of titanium are fine and evenly dispersed within the ferrite matrix. This not only improves the steel sheet's fishscale resistance, but also provides precipitation strengthening and suppresses ferrite grain growth during high-temperature enameling, thereby increasing the steel sheet's strength and post-enameling strength. If this relationship is not satisfied, the size of the second-phase particles formed in the steel sheet will be too large, reducing the steel sheet's formability and fishscale resistance.

[0043] Furthermore, the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention further contains: Nb: 0.005-0.04%.

[0044] In the above technical solution of the present invention, Nb element can be further added to the steel. Like Ti, Nb is a strong carbon and nitride forming element, and part of Nb exists in a solid solution state.

[0045] In the present invention, Nb increases the recrystallization temperature of steel, inhibits austenite recrystallization, and effectively maintains the deformation properties of austenite, thereby refining ferrite grains. This grain-refining effect of Nb prevents softening of the steel plate after enameling and prevents grain coarsening in the heat-affected zone during welding. Furthermore, Nb carbon and nitride precipitates contribute to the steel plate's hydrogen storage capacity. Therefore, considering the beneficial effects of Nb, an appropriate amount of Nb is preferably added to the high-strength cold-rolled steel plate for double-sided enameled liner described herein, with the mass percentage of Nb controlled to between 0.005% and 0.04%.

[0046] Furthermore, in the high-strength cold-rolled steel plate for double-sided enameled liner described in the present invention, the matrix of its microstructure is uniform and fine ferrite + pearlite, wherein the phase ratio of pearlite is less than 8%; wherein the pearlite is located at the ferrite triple grain boundary.

[0047] Furthermore, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, its microstructure includes second-phase particles, and the second-phase particles include fine, dispersed Ti second-phase particles.

[0048] Furthermore, in the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, the second phase particles of Ti include TiN, Ti4C2S2 and TiC, wherein the diameter of TiN precipitates is 50-300nm, the diameter of Ti4C2S2 precipitates is 30-200nm, and the diameter of TiC precipitates is 1-15nm.

[0049] Furthermore, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, the ferrite grain size is 10-11.

[0050] Furthermore, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, its performance meets at least one of the following items: yield strength ≥360MPa, elongation at break at a gauge length of 80mm ≥28.0%, hydrogen permeability ≥7.5min / mm 2 The yield strength after high temperature calcination at least 850℃ for at least 12 minutes is ≥330MPa.

[0051] Accordingly, another object of the present invention is to provide a method for manufacturing a high-strength cold-rolled steel sheet for double-sided enameled liner, which is simple and feasible. The yield strength of the high-strength cold-rolled steel sheet for double-sided enameled liner is ≥360MPa, the elongation at break at a gauge length of 80mm is ≥28.0%, and the hydrogen permeability value is ≥7.5min / mm 2 After high-temperature enameling at least 850℃ for at least 12 minutes, the yield strength is ≥330MPa, which can meet the requirements of double-sided enameling.

[0052] In order to achieve the above object, the present invention proposes a method for manufacturing the high-strength cold-rolled steel sheet for double-sided enameled liner, which comprises the steps of:

[0053] (1) Smelting, refining and continuous casting;

[0054] (2) Heating of the casting blank;

[0055] (3) Hot rolling and coiling: Control the hot rolling finishing temperature to 810-880℃ and the coiling temperature to 620-680℃;

[0056] (4) Descaling;

[0057] (5) Cold rolling: Control the cold rolling reduction rate to 60-70%;

[0058] (6) Continuous annealing: soaking temperature is 780-850℃, soaking time is 120-200s, over-aging temperature is between 165-450℃, over-aging time is 250-350s;

[0059] (7) Flat.

[0060] In the manufacturing method of the high-strength cold-rolled steel sheet for double-sided enameled liner described in the present invention, in step (1), the molten steel composition that meets the basic requirements can be obtained through pre-desulfurization of molten iron, top and bottom composite blowing of converter, and alloying of steel tapping, and then CAS refining treatment can be used to obtain molten steel with uniform temperature and composition and stability, and finally continuous casting is performed to form a continuous casting billet with the above composition.

[0061] During the hot rolling and coiling process of step (3), the hot rolling process is entirely carried out in the austenite single phase region, and the final rolling temperature is set above and close to the Ar3 phase transformation temperature (the transformation temperature of austenite to ferrite during cooling). The purpose is to accumulate sufficient deformation in the austenite non-recrystallization region, and the austenite grain boundaries and the "deformation bands" inside the grains together serve as ferrite nucleation cores, so that the hot rolled plate obtains a refined ferrite structure. Therefore, the present invention controls the final rolling temperature between 810-880°C; accordingly, during the coiling process, the coiling temperature has a significant effect on the microstructure and properties of the steel plate. In order to ensure that the steel plate has a high strength and at the same time fully precipitate pearlite and TiC particles, the present invention controls the coiling temperature between 620-680°C. When the coiling temperature is too low, the yield strength ratio of the material increases, which is not conducive to the forming performance.

[0062] In the manufacturing method of the present invention, in step (4), descaling can effectively remove the iron oxide scale on the surface of the hot-rolled strip to facilitate subsequent operations.

[0063] During the cold rolling operation in step (5), the cold rolling reduction rate affects both the mechanical properties and fishscale resistance of the steel plate. First, the size of the reduction rate affects the strength of the steel plate. The greater the cold rolling reduction rate, the greater the degree of elongation of the structure along the rolling direction, resulting in an increase in the nucleation rate during the recrystallization process. When recrystallization is completed, the ferrite grains are finer, thereby increasing the yield strength of the steel plate. Secondly, the room temperature structure of the high-strength cold-rolled steel plate for double-sided enameled liner described in the present invention contains cementite and titanium carbon, nitrogen, sulfide and other hard phases. These particles do not deform during cold rolling and are inconsistent with the deformation of the matrix. Therefore, a large number of vacancies will be formed around the particles, increasing the hydrogen storage capacity of the steel plate. The greater the cold rolling reduction rate, the more vacancies there are, and the stronger the fishscale resistance of the steel plate. Taking into account the strength, fishscale resistance and load capacity of the steel plate and the rolling mill, in step (5) of the manufacturing method described in the present invention, the cold rolling reduction rate is controlled to be 60-70%.

[0064] Accordingly, in the above step (6), a high-temperature rapid continuous annealing process is adopted, wherein the soaking temperature is controlled to be 780-850°C, the soaking time is controlled to be 120-200s, the overaging temperature is controlled to be between 165-450°C, and the overaging time is controlled to be 250-350s. Through this technical solution, the recrystallization process can be completed at high temperature in a short time, which can avoid the coarsening of ferrite grains, thereby obtaining a high-strength cold-rolled steel plate with good strength and plasticity; the short-term overaging treatment allows cementite and titanium low-temperature precipitates TiC particles to be fully precipitated, effectively improving the anti-scale explosion performance of the steel plate.

[0065] Furthermore, in the manufacturing method of the present invention, in step (2), the heating temperature is 1100-1230° C., and the time in the furnace is ≥360 min.

[0066] In the method for manufacturing high-strength cold-rolled steel sheets for double-sided enameled liner of the present invention, in step (2), the heating temperature is preferably controlled to be 1100-1230°C, and the furnace time is ≥360min, so that the following two beneficial effects can be obtained: first, the plasticity of the steel can be improved, the deformation resistance can be reduced, and a larger reduction can be obtained during rolling; second, the alloy elements can be fully dissolved to obtain an austenitic structure with uniform composition.

[0067] During the heating process of the billet in step (2), if the heating temperature or time is lower than the set value, the above two purposes cannot be achieved; if the heating temperature is too high or the time is too long, it will cause oxidation and decarburization of the billet, resulting in serious burnout of the billet and affecting the yield rate.

[0068] Furthermore, in the manufacturing method of the present invention, in step (7), the leveling reduction rate is controlled to be 0.6-1.2%.

[0069] In the above scheme, in step (7), the leveling reduction rate is controlled to be 0.6-1.2%, and the annealed cold-rolled strip is subjected to secondary cold rolling with a small reduction rate, which can achieve the following two beneficial effects: first, the "yield platform" on the stress-strain curve can be reduced or eliminated, avoiding the occurrence of "Lüders band" during stamping; second, the flatness of the steel plate and the smoothness of the plate surface can be improved.

[0070] Compared with the prior art, the high-strength cold-rolled steel sheet for double-sided enameled liner and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0071] (1) The present invention is based on the design concept of low alloy cost and low processing cost. In terms of composition design, it is based on carbon, manganese and phosphorus strengthening, and relatively low-cost titanium and boron elements are added, and alloying elements such as copper, chromium and magnesium are added as auxiliary elements. In terms of processing technology, by controlling the continuous casting billet heating temperature, hot rolling final rolling temperature, coiling temperature and cold rolling reduction rate, and adopting a high-efficiency high-temperature rapid continuous annealing process, it is ensured that the steel plate has excellent surface quality, mechanical properties and enameling properties.

[0072] (2) The microstructure of the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention comprises a uniform and fine matrix of ferrite + pearlite, wherein the pearlite phase ratio is less than 8%, and the ferrite grain size is between 10 and 11 levels. The present invention primarily utilizes solid solution strengthening, precipitation strengthening, and grain refinement strengthening to improve the strength of the steel sheet, ensuring that the yield strength of the steel sheet is ≥360 MPa and the elongation at break at a gauge length of 80 mm is ≥28.0%. After high-temperature enameling at 850°C for 12 minutes, the fine, dispersed titanium second-phase particles can effectively control the growth of ferrite grains, and the boron element can improve the high-temperature strength of the steel, ensuring that the yield strength of the steel sheet remains above 330 MPa after enameling, better meeting the pressure resistance requirements of the electric heating liner.

[0073] (3) In the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, the anti-scale burst performance of the steel sheet is mainly improved by pearlite at the ferrite triple grain boundary and the fine, dispersed second phase particles of titanium. In addition, boron can further improve the anti-scale burst performance, and the refined ferrite grains can also improve the anti-scale burst performance, because the grain boundaries play a role in storing hydrogen. The finer the grains, the larger the grain boundary area, and the higher the hydrogen storage capacity. The hydrogen permeability value of the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention is ≥7.5min / mm 2 , meeting the anti-scale explosion requirements of double-sided enameling. In addition, in the high-strength cold-rolled steel sheet for double-sided enameled liner of the present invention, the free carbon content that forms pearlite is limited, which suppresses the occurrence of bubbles and pinhole defects during the enameling process. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 The metallographic microstructure of the high-strength cold-rolled steel sheet for double-sided enameled liner of Example 1 is shown.

[0075] Figure 2 and Figure 3 The distribution state of the second phase particles of the high-strength cold-rolled steel sheet for double-sided enameled liner of Example 1 observed under a transmission electron microscope is shown.

[0076] Figure 4 The metallographic microstructure of the high-strength cold-rolled steel plate for double-sided enameled liner of Example 1 after simulated high-temperature sintering is shown.

[0077] Figure 5 and Figure 6 The figure shows the distribution state of the second phase particles of the high-strength cold-rolled steel sheet for double-sided enameled inner container of Example 1 after simulated high-temperature sintering observed under a transmission electron microscope.

[0078] Figure 7 The bubble structure of the enamel layer of the high-strength cold-rolled steel plate for the double-sided enameled inner container in Example 1 after double-sided glazing and high-temperature sintering is shown.

[0079] Figure 8 The bubble structure of the enamel layer of the comparative steel plate of Comparative Example 1 after double-sided glazing and high-temperature sintering is shown. DETAILED DESCRIPTION

[0080] The high-strength cold-rolled steel plate for double-sided enameled liner and its manufacturing method described in the present invention will be further explained and illustrated below in combination with specific embodiments and drawings of the specification. However, such explanation and illustration do not constitute an improper limitation to the technical solution of the present invention.

[0081] Examples 1-6 and Comparative Examples 1-2

[0082] Table 1-1 and Table 1-2 list the mass percentages of various chemical elements in the high-strength cold-rolled steel sheets for double-sided enameled liner of Examples 1-6 and the comparative steel sheets of Comparative Examples 1-2.

[0083] Table 1-1. (wt%, the balance is Fe and other inevitable impurities except O, Ni, and Mo)

[0084]

[0085]

[0086] Table 1-2

[0087] serial number M* N* Q* Example 1 0.068 0.122 0.87 Example 2 0.086 0.153 0.80 Example 3 0.056 0.099 0.76 Example 4 0.062 0.092 0.69 Example 5 0.099 0.205 1.33 Example 6 0.079 0.162 0.73 Comparative Example 1 <![CDATA[ 0.15 ]]> 0.174 <![CDATA[ 0.33 ]]> Comparative Example 2 <![CDATA[ 0.049 ]]> <![CDATA[ 0.064 ]]> <![CDATA[ 0.40 ]]>

[0088] Note: M*=C-(Ti-3.43N-1.5S) / 4; N*=Ti+0.875C; Q*=2.5Ti / (1.2C+8.57N+3.75S). The element symbols in the above formula represent the mass percentage of the corresponding elements.

[0089] The high-strength cold-rolled steel sheets for double-sided enameled inner containers of Examples 1-6 of the present invention and the comparative steel sheets of Comparative Examples 1-2 were prepared by the following steps:

[0090] (1) Smelting, refining and continuous casting are carried out according to the chemical composition ratios shown in Table 1-1 and Table 1-2: After pre-desulfurization of molten iron, combined blowing at the top and bottom of the converter, and alloying of the steel, the molten steel composition that meets the basic requirements is obtained, and then after CAS refining treatment, the molten steel with uniform temperature and composition and stability is obtained, and finally continuous casting is carried out to form continuous casting billets with the chemical composition shown in Table 1-1 and Table 1-2.

[0091] (2) Heating of slab: Control the heating temperature to 1100-1230℃ and the heating time in the furnace to ≥360min, so that the slab is fully austenitized, a uniform austenitic structure is obtained, and the deformation resistance of the steel is reduced.

[0092] (3) Hot rolling and coiling: The final rolling temperature of hot rolling is controlled at 810-880℃. After rolling, the steel is cooled to the coiling temperature through laminar flow and then coiled. The coiling temperature is controlled at 620-680℃.

[0093] (4) Descaling: Fully remove the iron oxide scale on the surface of the hot-rolled coil.

[0094] (5) Cold rolling: Control the cold rolling reduction rate to 60-70%.

[0095] (6) Continuous annealing: soaking temperature is 780-850℃, soaking time is 120-200s, over-aging temperature is between 165-450℃, over-aging time is 250-350s.

[0096] (7) Leveling: Control the leveling reduction rate to 0.6-1.2%.

[0097] It should be noted that in the present invention, the high-strength cold-rolled steel sheets for double-sided enameled liner in Examples 1-6 were all produced using the above steps, and their chemical compositions and related process parameters all met the control requirements of the design specifications of the present invention. Although the comparative steel sheets in Comparative Examples 1-2 were also produced using the above steps, their chemical compositions all contained parameters that did not meet the control requirements of the design specifications of the present invention.

[0098] Table 2-1 and Table 2-2 list the specific process parameters of the high-strength cold-rolled steel sheets for double-sided enameled liner in Examples 1-6 and the comparative steels in Comparative Example 1-2.

[0099] Table 2-1.

[0100]

[0101] Table 2-2.

[0102]

[0103] It should be noted that the over-aging temperature varies during the actual operation process and is not stable at a fixed value. Instead, the temperature gradually decreases during the over-aging period. Therefore, the over-aging temperature in step (6) of Table 2-2 is presented as a range of values ​​rather than a point value in each embodiment and comparative example.

[0104] The double-sided enameled inner container high-strength cold-rolled steel sheets of Examples 1-6 and the comparative steel materials of Comparative Examples 1-2 prepared by the above manufacturing process were subjected to performance tests, and the test results are listed in Tables 3-1 and 3-2 below. The specific test methods are as follows:

[0105] (1) The special feature of enameled steel is that it must be sintered at high temperature before being put into service. Therefore, in order to verify the change in yield strength of the steel plates of each embodiment and comparative example before and after high-temperature enameling, the present invention directly conducts tensile tests on the steel plates of Examples 1-6 and Comparative Examples 1-2 after continuous annealing. Samples are taken along the rolling direction of the annealed steel plates and processed into tensile specimens with a gauge length of 80 mm according to JIS13A standard. The conventional mechanical properties of the tensile specimens of each embodiment and comparative example are then tested to obtain the yield strength, tensile strength and elongation at break A at a gauge length of 80 mm. 80 .

[0106] Accordingly, the steel plates of Examples 1-6 and Comparative Examples 1-2 were subjected to a simulated high-temperature enameling test, and the enameling temperature was controlled to 850°C and the furnace time was 12 min. Then, samples were taken along the rolling direction of the steel plates after the simulated high-temperature enameling, and tensile tests were performed to obtain the yield strength of the steel materials of each Example and Comparative Example after simulated enameling (850°C×12 min).

[0107] (2) The steel sheets of Examples 1-6 and Comparative Examples 1-2 were wet-glazed on both sides and sintered at 850°C for 12 min. The steel sheets were then left for 72 hours and observed for fishscale. The cross-sections of the enameled sheets were then microscopically examined using an optical microscope to examine the bubble structure of the enamel layer. To further quantify the fishscale resistance of the steel sheets, the TH2 values ​​of the unenamelled steel sheets were measured using the electrochemical hydrogen permeation method specified in European standard EN 10209-2013.

[0108] Table 3-1 and Table 3-2 list the performance test results of the high-strength cold-rolled steel sheets for double-sided enameled liner in Examples 1-6 and the enameled steel in Comparative Example 1-2.

[0109] Table 3-1.

[0110]

[0111]

[0112] Table 3-2.

[0113]

[0114] As shown in Table 3, in the present invention, the yield strength of the high-strength cold-rolled steel sheets for double-sided enameled liner of Examples 1-6 is relatively high, ranging from 363 to 409 MPa, and the tensile strength is between 447 and 502 MPa. The elongation at break A under a gauge length of 80 mm is 80 Between 28.5-32.5%, it can meet the forming requirements of three-section inner liner barrel body rolling or head stamping.

[0115] Correspondingly, in the high-strength cold-rolled steel sheets for double-sided enameled liner of Examples 1-6, the phase ratio of pearlite is between 2.17% and 6.03%, and the ferrite grain size is all 10.5.

[0116] After simulated high-temperature enameling (850°C×12 min), the high-strength cold-rolled steel sheets for double-sided enameled liner in Examples 1-6 still maintain a relatively high yield strength, and their yield strengths are all greater than 330 MPa and between 332-368 MPa. This indicates that the cold-rolled enameled steel in Examples 1-6 described in the present invention has good resistance to high-temperature softening, can effectively improve the pressure resistance of the enameled liner, and extend the service life of the liner.

[0117] The high-strength cold-rolled steel sheets for double-sided enameled liner of Examples 1-6, after double-sided glazing, have a good enamel layer bubble structure, with small and evenly dispersed bubbles; the hydrogen penetration time of the steel sheets exceeds the threshold of 6.7 min / mm specified in the European standard EN10209-2013. 2 , meeting the anti-scale requirements during double-sided enameling.

[0118] However, while the Ti content in Comparative Example 1 is within the control range designed by the present invention, calculations show that Ti-3.43N-1.5S is less than 0, indicating that all Ti has been consumed by combining with N and S, leaving no remaining Ti to combine with C. Furthermore, the C content in Comparative Example 1 exceeds the control range, meaning that all C is used to form pearlite, and the pearlite content exceeds the specified range of the present invention. This results in a large amount of gas generated during enameling, which is not fully released during porcelain fusion. This results in dense and unevenly sized bubbles in the porcelain layer, leading to a poor bubble structure.

[0119] In Comparative Example 2, the C and Ti contents were both below the control ranges of the present invention, preventing the formation of sufficient pearlite and titanium carbon, sulfur, and nitrides, thereby reducing the steel plate's strength and hydrogen absorption capacity. After double-sided glazing and high-temperature sintering, the comparative steel plate in Comparative Example 2 exhibited further strength degradation, failing to meet the requirements for high-pressure water circulation testing and failing to meet the requirements for double-sided enameling without scale cracking.

[0120] Figure 1 The metallographic microstructure of the high-strength cold-rolled steel sheet for double-sided enameled liner of Example 1 is shown.

[0121] Figure 2 and Figure 3 The distribution state of the second phase particles of the high-strength cold-rolled steel sheet for double-sided enameled liner of Example 1 observed under a transmission electron microscope is shown.

[0122] like Figure 1As shown, in this embodiment, the microstructure matrix of the high-strength cold-rolled steel sheet for double-sided enameled liner of Example 1 is uniform and fine ferrite + pearlite, wherein the average grain diameter of the ferrite is 8.14 μm.

[0123] like Figure 2 and Figure 3 As shown, in this embodiment, the double-sided enameled liner of Example 1 uses high-strength cold-rolled steel plates with TiN, Ti4C2S2, and TiC particles dispersed in the ferrite matrix, wherein the diameter of the TiN precipitate phase is 76nm, the diameter of the Ti4C2S2 precipitate phase is 41nm, and the size of the TiC precipitate phase is mainly between 1-7nm.

[0124] Figure 4 The metallographic microstructure of the high-strength cold-rolled steel plate for double-sided enameled liner of Example 1 after simulated high-temperature sintering is shown. Figure 5 and Figure 6 The figure shows the distribution state of the second phase particles of the high-strength cold-rolled steel sheet for double-sided enameled inner container of Example 1 after simulated high-temperature sintering observed under a transmission electron microscope.

[0125] like Figure 4 As shown, in this embodiment, the microstructure of the high-strength cold-rolled steel plate for double-sided enameled liner in Example 1 after simulated high-temperature enameling is uniform and fine ferrite + pearlite, that is, the same as the organizational state after cold rolling annealing, wherein the average grain diameter of the ferrite is 9.52 μm. It can be seen that the ferrite grain size does not change significantly after high-temperature sintering.

[0126] like Figure 5 and Figure 6 As shown, in this embodiment, after the high-strength cold-rolled steel plate for the double-sided enameled liner of Example 1 is subjected to simulated high-temperature enameling, TiN, Ti4C2S2, and TiC particles are dispersed in the ferrite matrix, wherein the diameter of the TiN precipitate phase is 82nm, the diameter of the Ti4C2S2 precipitate phase is 73nm, and the size of the TiC precipitate phase is mainly between 3-25nm, indicating that the precipitate phase has a tendency to aggregate and grow during the high-temperature sintering process.

[0127] Figure 7 The bubble structure of the enamel layer of the high-strength cold-rolled steel plate for the double-sided enameled inner container in Example 1 after double-sided glazing and high-temperature sintering is shown.

[0128] like Figure 7 As shown, in this embodiment, the bubbles in the enamel layer of the double-sided enameled inner liner of Example 1 are small and evenly dispersed after double-sided glazing and high-temperature sintering using high-strength cold-rolled steel plates, which has a good bubble structure.

[0129] Figure 8The bubble structure of the enamel layer of the comparative steel plate of Comparative Example 1 after double-sided glazing and high-temperature sintering is shown.

[0130] like Figure 8 As shown, the pearlite content in the comparative steel plate of Comparative Example 1 is too high, and a large amount of gases such as CO are generated during the enameling process. These gases cannot be fully released before the enamel layer is fused, forming a large number of bubbles of uneven sizes in the porcelain layer. Some of the larger bubbles have a diameter of up to 300 μm and almost penetrate the entire enamel layer, which can easily form pinhole defects and affect the performance of the enameled products.

[0131] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0132] It should also be noted that the above examples are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above examples, and many similar variations are possible. Any variations that can be directly derived or conceived by those skilled in the art from the present disclosure are intended to fall within the scope of protection of the present invention.

Claims

1. A high-strength cold-rolled steel plate for double-sided enameled liner, characterized in that: The mass percentage of each chemical element is: C: 0.06-0.12%, Si: 0.033-0.08%, Mn: 0.5-1.2%, P: 0.019-0.05%, S: 0.005-0.05%, Al: 0.008-0.06%, N≤0.006%, Ti: 0.03-0.1%, B: 0.0002-0.0035%, Cr: 0.01-0.06%, Cu: 0.01-0.06%, Mg: 0.0005-0.03%; the balance is Fe and other inevitable impurities; Each chemical element also satisfies the following formulas: 0.05%≤C-(Ti-3.43N-1.5S) / 4≤0.1%; 0.08%≤Ti+0.875C≤0.25%; 0.65≤2.5Ti / (1.2C+8.57N+3.75S)≤1.35; Its microstructure includes second phase particles, the second phase particles include fine, dispersed Ti second phase particles, the Ti second phase particles include TiN, Ti4C2S2 and TiC, wherein the diameter of the TiN precipitate is 50-300nm, the diameter of the Ti4C2S2 precipitate is 30-200nm, and the diameter of the TiC precipitate is 1-15nm; Its performance meets the requirement of a yield strength of ≥330 MPa after high-temperature calcination at a temperature of at least 850°C for at least 12 minutes.

2. The high-strength cold-rolled steel sheet for double-sided enameled liner according to claim 1, characterized in that: Among the inevitable impurities, O≤0.008%, Ni≤0.1%, and Mo≤0.1%.

3. The high-strength cold-rolled steel sheet for double-sided enameled liner according to claim 1, characterized in that: It also contains: Nb: 0.005-0.04%.

4. The high-strength cold-rolled steel sheet for double-sided enameled liner according to claim 1, characterized in that: The matrix of its microstructure is uniform and fine ferrite + pearlite, wherein the volume phase ratio of pearlite is less than 8%; and the pearlite is located at the ferrite triple grain boundary.

5. The high-strength cold-rolled steel sheet for double-sided enameled liner according to claim 4, characterized in that: The ferrite grain size is 10-11.

6. The high-strength cold-rolled steel sheet for double-sided enameled liner according to claim 1, characterized in that: Its performance meets at least one of the following: yield strength ≥360MPa, elongation at break at a gauge length of 80mm ≥28.0%, hydrogen permeability ≥7.5min / mm 2 .

7. A method for manufacturing a high-strength cold-rolled steel sheet for a double-sided enameled liner according to any one of claims 1 to 6, characterized in that: Including steps: (1) Smelting, refining and continuous casting; (2) Heating of the casting blank; (3) Hot rolling and coiling: Control the hot rolling finishing temperature to 810-880℃ and the coiling temperature to 620-680℃; (4) Descaling; (5) Cold rolling: Control the cold rolling reduction rate to 60-70%; (6) Continuous annealing: soaking temperature is 780-850℃, soaking time is 120-200s, over-aging temperature is between 165-450℃, over-aging time is 250-350s; (7) Flat.

8. The manufacturing method according to claim 7, wherein: In step (2), the heating temperature is 1100-1230°C, and the time in the furnace is ≥360 min.

9. The manufacturing method according to claim 7 or 8, characterized in that: In step (7), the leveling reduction rate is controlled to be 0.6-1.2%.

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