Ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel coating and method for manufacturing the same

Abstract

The present invention provides an ultra-low carbon cold rolled high strength steel suitable for electrostatic powder enamel and a method for producing the same. The ultra-low carbon cold rolled high strength steel of the present invention contains Fe, inevitable impurity elements, and the following chemical elements in the following mass percent contents: C: 0.002-0.010%, Si≦0.05%, Mn: 0.25-1.0%, P: 0.04-0.08%, S: 0.01-0.04%; Al: 0.01-0.05%, Cu: 0.02-0.08%, Ti: 0.05-0.12%, Mo: 0.02-0.10%, N: 0.004-0.012%; wherein each chemical element satisfies at least one of the following formulas: M*>0, where M*=Ti-S×1.5-N×3.4-C×4>0; N*≧0.012, where N*=(Mn+P)×Mo. The ultra-low carbon cold-rolled high strength steel described in the present invention can meet the requirements for anti-skid performance, enamel adhesion performance and enamel surface quality of electrostatic powder double-sided enamel.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and particularly to a low-carbon cold-rolled high-strength steel sheet and a method for manufacturing the same.

Background Art

[0002] It is known that enamel products are obtained by firing a metal and a glassy inorganic material at a high temperature. Two materials with completely different properties of the metal and the glassy inorganic material are combined and integrated, and their respective defects are compensated for each other to better embody their respective advantages.

[0003] For example, when the enamel layer of an enamel product is subjected to an external impact, the metal has high strength and can withstand such an external force to protect the product from damage or significantly reduce the degree of destruction; on the other hand, when the enamel product is chemically eroded, the enamel layer plays a role in protecting the metal material from corrosion so that the product is not damaged or discarded. Therefore, enamel products have the excellent characteristics of a series of metals and glassy inorganic materials.

[0004] Enamel steel products are composite materials formed by melting and covering a glassy inorganic material on the surface of a steel sheet. In recent years, they have been widely applied in the light industry, household appliances, environmental protection and construction industries, and are used in the manufacture of products such as ovens, toasters, water heaters, bathtubs, denitration equipment (SCR) in thermal power plants, air preheaters (APH) and flue gas heaters (GGH), architectural decorative panels, and various enamel plate-embedded tanks.

[0005] Currently, there are two main types of enamel coating methods for manufactured enamel steel: wet methods and dry methods. Of these, the wet method involves applying a glaze slurry to the metal substrate, and the three basic methods of the wet method are immersion enamel coating, spray enamel coating, and pour enamel coating. Dry enamel coating mainly refers to electrostatic powder enamel coating. This method employs the principle of high-pressure generation using an electrostatic spray gun, creating a negatively charged electrostatic field with a voltage of 70-100kV at the nozzle. When the workpiece to be sprayed is grounded, and the workpiece passes through the spray gun in the spray booth, the spray gun begins to spray powder, and the positively charged workpiece and negatively charged powder attract each other, forming a powder coating.

[0006] Among the various enamel coating methods described above, the surface quality of enamel steel produced by electrostatic powder enamel is the best, but this process also places higher demands on the base steel sheet.

[0007] This is because the enamel layer obtained using the electrostatic powder enamel method is denser, and hydrogen generated during the enamel firing process does not easily escape, making it prone to chipping defects. In addition, because the electrostatic powder enamel layer is thin (usually less than 200 μm), defects such as bubbles and pinholes are easily formed on the enamel surface, affecting surface quality. Therefore, during design and development, steel plates suitable for electrostatic powder enamel must have resistance to chipping, pinholes, and bubbles.

[0008] Furthermore, it is important to note that the steel sheets used in the manufacture of enamel products must possess not only excellent resistance to chipping, pinholes, and air bubbles, but also, depending on the specific application, high yield strength and minimal strength reduction after high-temperature enamel firing. This is because, with the advancement of the enamel industry, the quality of enamel products has also improved. For example, in the field of manufacturing enamel steel sheets for architectural decorative panels, it is necessary to manufacture wider enamel steel sheets to reduce joints and improve aesthetics. However, such enamel steel sheets have strict requirements regarding their shape, and the larger the workpiece size, the more likely deformation problems are to occur during the high-temperature enamel firing process. Therefore, the enamel steel sheet used as the substrate needs sufficient high-temperature deformation resistance to prevent significant deformation after high-temperature enamel firing at 840-870°C.

[0009] Therefore, the present invention aims to obtain an ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel coating in order to meet the needs of the current market. This ultra-low carbon cold-rolled high-strength steel does not produce chipping defects after enamel firing, has good adhesion between the enamel layer and the steel plate, has excellent enamel surface quality, and meets the various usage requirements for electrostatic powder double-sided enamel coating. At the same time, this ultra-low carbon cold-rolled high-strength steel has a yield strength of 200 MPa or more, and the decrease in yield strength after high-temperature enamel firing at 840-870°C is within 10%, avoiding deformation problems after high-temperature enamel firing. Ultimately, it significantly improves the strength of enamel products, provides excellent usability, and extends the lifespan of enamel products.

[0010] While several conventional technologies exist for manufacturing cold-rolled enamel steel suitable for electrostatic powder enamel, there are still significant differences in the design of the chemical composition, unlike steel materials that require specific strength and performance as in the present invention.

[0011] For example, Chinese patent document No. CN101356295A, published on January 28, 2009, titled "Continuous Cast Enamel Steel Sheet with Excellent Resistance to Splinting and Method for Manufacturing the Same," discloses a continuously cast enamel steel sheet with excellent resistance to splinting, having a chemical element composition of C: 0.010% or less, Mn: 0.03~1.30%, Si: 0.100% or less, Al: 0.030% or less, N: 0.0055% or less, P: 0.035% or less, S: 0.08% or less, O: 0.005~0.085%, and B: 0.0003~0.0250%, and having non-integrated or integrated oxides with different mass concentrations of b or Mn within the steel sheet, and a method for manufacturing the same.

[0012] For example, Chinese patent document No. CN105518174A, published on April 20, 2016, titled "Cold-rolled steel sheet for enamel coating, method for manufacturing the same, and enamel products," discloses a cold-rolled steel sheet for enamel coating, a method for manufacturing the same, and enamel products, with chemical elemental composition of C: 0.0005~0.0050%, Mn: 0.05~1.50%, Si: 0.001~0.015%, Al: 0.001~0.01%, N: 0.0010~0.0045%, O: 0.0150~0.0550%, P: 0.04~0.10%, S: 0.0050~0.050%, Nb: 0.020~0.080%, Cu: 0.015~0.045%, with the remainder being Fe and impurities.

[0013] For example, a Chinese patent document titled "Cold-rolled steel sheet for enamel coating and method for manufacturing the same," with publication number CN106560523A and publication date April 12, 2017, discloses a cold-rolled steel sheet for enamel coating and a method for manufacturing the same, in which the chemical elemental composition is C: 0.005% or less (excluding 0%), Mn: 0.05% to 0.3%, Al: 0.005% or less (excluding 0%), P: 0.03% or less (excluding 0%), S: 0.02% or less (excluding 0%), Si: 0.01% or less (excluding 0%), Ti: 0.005% to 0.01%, Y: 0.01% to 0.02%, N: 0.003% or less (excluding 0%), with the remainder being Fe and other unavoidable impurities. [Overview of the project] [Problems that the invention aims to solve]

[0014] The object of the present invention is to provide an ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel coating. This ultra-low carbon cold-rolled high-strength steel does not produce chipping defects after enamel firing, has good adhesion between the enamel layer and the steel plate, has excellent enamel surface quality, and satisfies the various usage requirements of electrostatic powder double-sided enamel coating. At the same time, this ultra-low carbon cold-rolled high-strength steel has a yield strength of 200 MPa or more, and the decrease in yield strength after high-temperature enamel firing at 840-870°C is within 10%, avoiding deformation problems after high-temperature enamel firing. Ultimately, it significantly improves the strength of the enamel product, provides excellent usability, and extends the lifespan of the enamel product. [Means for solving the problem]

[0015] The ultra-low carbon cold-rolled high-strength steel of the present invention can be widely applied to products that require large widths of architectural decorative enamel panels, bathtubs, and other electrostatic powder enamel coatings, and that demand high yield strength performance after enamel firing, thus possessing considerable application value.

[0016] To achieve the above objective, the present invention provides an ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel, containing Fe and unavoidable impurity elements, and further containing the following chemical elements in the following mass percentages: C:0.002~0.010%, Si≦0.05%, Mn:0.25~1.0%, P:0.04~0.08%, S:0.01~0.04%;Al:0 .01~0.05%, Cu:0.02~0.08%, Ti:0.05~0.12%, Mo:0.02~0.10%, N:0.004~0.012%; Here, each chemical element satisfies at least one of the following equations: M*>0, where M* = Ti - S × 1.5 - N × 3.4 - C × 4; N* ≥ 0.012, where N* = (Mn + P) × Mo; Substitute the numerical value preceding the mass percentage symbol of each chemical element into the formula for each chemical element symbol.

[0017] Furthermore, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, the mass percentage content of each chemical element is: C: 0.002-0.010%, Si ≤ 0.05%, Mn: 0.25-1.0%, P: 0.04-0.08%, S: 0.01-0.04%; Al: 0.01-0.05%, Cu: 0.02-0.08%, Ti: 0.05-0.12%, Mo: 0.02-0.10%, N: 0.004-0.012%; the remainder is Fe and unavoidable impurity elements; Here, each chemical element satisfies at least one of the following equations: M*>0, where M* = Ti - S × 1.5 - N × 3.4 - C × 4; N* ≥ 0.012, where N* = (Mn + P) × Mo; Substitute the numerical value preceding the mass percentage symbol of each chemical element into the formula for each chemical element symbol.

[0018] In the ultra-low carbon cold-rolled high-strength steel described in the present invention, the design principles for each chemical element are as follows: C: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, all C elements can combine with strong carbide-forming elements such as Ti and Nb to form finely dispersed precipitate phases. These precipitate phases effectively improve the hydrogen storage performance of the steel sheet and provide anti-splitting properties for the enamel, while nano-level TiC and other precipitate phases also exhibit precipitation strengthening effects. However, it is important to note that the C element content should not be too high, and the amount added should not be excessive relative to the alloying elements. This is because, during enamel firing, excess free carbon generates a large amount of gas such as CO, which leads to poor pore structure in the enamel layer, further causing defects such as pinholes and bubbles, and affecting the quality of the enamel surface. This effect is even greater in the electrostatic powder enamel process. Therefore, considering the influence of C elements on the performance of the steel, the mass percentage content of C elements in the ultra-low carbon cold-rolled high-strength steel described in the present invention is controlled to be between 0.002 and 0.010%.

[0019] Si: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, Si is a residual element, and if the Si content in the steel is too high, the plasticity of the steel deteriorates. Also, in the enamel firing process, a high Si content affects the adhesion performance between the steel plate and the glaze. Therefore, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, the mass percentage content of Si is controlled to Si ≤ 0.05%. In some embodiments, the mass percentage content of Si is 0.01~0.05% or 0.01~0.045%.

[0020] Mn, P: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, both Mn and P are major solid-solution strengthening elements. Generally, in steels with high Ti and P content, a TiFe(P) precipitate phase is formed, and this precipitate phase tends to precipitate unevenly at ferrite grain boundaries, degrading the mechanical performance of the material. On the other hand, in the embodiments of the present invention, by controlling the hot-rolling end temperature and the winding temperature, the temperature range for TiFe(P) phase precipitation is avoided, thus avoiding this situation and allowing the P element to exert only its solid-solution strengthening effect. Furthermore, if the P element is too high, it may lead to poor formability and secondary brittleness of the material. Therefore, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, the mass percentage content of Mn is limited to between 0.25 and 1.0%, and the mass percentage content of P is limited to between 0.04 and 0.08%.

[0021] S: Generally, in steels with a large addition of Mn element, the S element combines with the Mn element to form strip-shaped or linear MnS inclusions, and such inclusions deteriorate the mechanical properties of the material. On the other hand, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, since a relatively large amount of Ti element is added, the formation of spherical or polygonal TiMn(s) inclusions is promoted by the action of the Ti element, and instead, the mechanical properties of the material are improved. Also, in this embodiment, the S element mainly combines with Ti and C in the steel to form Ti4C2S2, TiS precipitation phases or inclusions. All of these precipitation phases and inclusions formed in the steel can act as irreversible hydrogen storage traps and play the role of preventing the enamel from peeling. Therefore, in order to exert the beneficial effect of the S element, in the present invention, the mass percentage content of the S element is controlled between 0.01% and 0.04%.

[0022] Mo: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, molybdenum dissolves in ferrite, austenite, and carbides and exhibits a solid solution strengthening effect; also, molybdenum enhances the stability of carbides such as TiC and NbC, reduces the aggregation and coarsening phenomena of carbide precipitation phases during high-temperature enamel firing (usually the enamel firing temperature reaches 840 - 870 °C), enhances the high-temperature stability of the steel, and can avoid a significant decrease in yield strength due to the attenuation of the precipitation strengthening effect in the steel plate after enamel firing. However, it should be noted that the content of Mo element in the steel should not be too high. Adding an excessive amount of Mo element will significantly increase the manufacturing cost. Therefore, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, the mass percentage content of the Mo element is controlled between 0.02% and 0.10%.

[0023] Al: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, Al is a strong deoxidizing element. In medium and low carbon steels, deoxidation by Al is often necessary to keep the O content in the steel low. Also, the dissolved Al element in the steel can combine with free nitrogen to precipitate AlN. Its precipitation temperature is relatively high, which plays a role in refining austenite crystal grains and is beneficial for grain refinement and fine crystal strengthening. Therefore, in order to exert the beneficial effects of the Al element, in the present invention, the mass percentage content of the Al element is controlled between 0.01% and 0.05%.

[0024] Ti: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, Ti is a main element for obtaining good hydrogen storage performance in the steel. The Ti element in the steel can form fine and dispersed TiC and Ti(C, N) precipitation phases through an appropriate thermo-mechanical control process. These precipitation phases improve the hydrogen storage performance of the steel sheet as irreversible hydrogen storage traps, and thus play a role in preventing the steel from peeling. However, if the Ti element in the steel is too high, it will lead to deterioration of the formability of the material and an increase in cost. Therefore, considering the mechanical properties and cost of the steel comprehensively, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, the mass percentage content of the Ti element is limited between 0.05% and 0.12%.

[0025] Cu: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, the Cu element deposits on the bonding interface between the glaze and the steel during the high-temperature enamel firing process, improving the adhesion performance between the steel and the glaze and enhancing the anti-peeling performance of the steel material. However, the content of the Cu element in the steel should not be too high either. If the content of the Cu element in the steel is too high, it will similarly lead to deterioration of the forming performance of the material and an increase in cost. Therefore, considering the mechanical properties and cost of the steel comprehensively, in the present invention, the mass percentage content of the Cu element is limited between 0.02% and 0.08%.

[0026] N: In the ultra-low carbon cold-rolled high-strength steel described in the present invention, the main role of element N is to form inclusions or precipitate phases such as TiN, NbN, Ti(C,N), and Nb(C,N) with alloying elements such as Ti and Nb, and further to exhibit the hydrogen storage trap effect of enamel, which prevents chipping. In addition, during the cold rolling process, nitride inclusions are partially crushed, and minute pores are formed around them, which are also very important and effective hydrogen storage traps. Based on this, in order to exhibit the beneficial effects of element N, the mass percentage content of element N in the ultra-low carbon cold-rolled high-strength steel described in the present invention is limited to between 0.004 and 0.012%.

[0027] In the above-described embodiment of the present invention, the present invention can control the mass percentage content of a single chemical element in steel while simultaneously controlling the elements in the steel to satisfy the limiting relationship "M*>0, where M*=Ti-S×1.5-N×3.4-C×4". According to the inventors' experimental studies, when the element content in steel satisfies the above limiting relationship, the C and N gap atoms in the steel are completely fixed, and the structure of the steel is guaranteed to be a single ferrite. After electrostatic powder enamel firing of a steel sheet having such structural characteristics, the quality of the enamel surface is excellent, and there is no occurrence of defects such as pinholes or bubbles. In some embodiments, 0.001≦M*≦0.02. In some embodiments, 0.001≦M*≦0.013.

[0028] Therefore, the present invention can control the mass percentage content of a single chemical element in steel, while simultaneously controlling the elements in the pressure vessel steel sheet to satisfy the limiting relationship "N*≧0.012, where N*=(Mn+P)×Mo". According to the inventors' experimental research, when this limiting relationship is satisfied, the steel sheet can guarantee a yield strength performance of 200 MPa or more, and moreover, even after high-temperature enamel firing treatment, the steel sheet maintains a high yield strength value, with a decrease in strength from the original strength of less than 10%. In some embodiments, 0.012≦N*≦0.10. In some embodiments, 0.012≦N*≦0.06.

[0029] Furthermore, the ultra-low carbon cold-rolled high-strength steel described in the present invention may contain B ≤ 0.003% and Nb ≤ 0.06%.

[0030] Furthermore, the ultra-low carbon cold-rolled high-strength steel described in the present invention contains at least one of the following chemical elements: B: 0.0006~0.003%; Nb: 0.02~0.06%.

[0031] In this invention, in order to obtain a high-strength, ultra-low-carbon cold-rolled steel with even better performance, in some embodiments, appropriate amounts of B and Nb elements may be added.

[0032] Nb: In the embodiment of the present invention, the Nb element may combine with the C and N elements to form fine precipitate phases such as NbC and NbN. Of course, Nb may also adhere to the previously formed TiC precipitate phase to form a (Nb,Ti)C phase. These precipitate phase particles act as irreversible hydrogen storage traps, exhibiting both anti-splitting properties for the enamel and a precipitation strengthening effect. Furthermore, these precipitate phase particles effectively suppress the deformation and recrystallization of austenite during the hot rolling process, inhibiting grain growth and exhibiting a fine-grain strengthening effect. Based on this, in order to exhibit the beneficial effects of the Nb element, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, it is preferable to add only a small amount of Nb, limiting the mass percentage content of the Nb element to between 0.02 and 0.06%.

[0033] B: In the embodiment of the present invention, adding an appropriate amount of element B forms B(C,N) in the steel sheet, increasing the number of second-phase particles and hydrogen traps, thereby improving the steel sheet's resistance to chipping. Furthermore, B(C,N) also acts as a nucleus for crystals, which is advantageous for the formation of equiaxed crystals, inhibiting grain growth, and improving the enamel performance and mechanical performance of the steel sheet. Therefore, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, B is preferably added in small amounts, and the mass percentage content of element B is limited to between 0.0006 and 0.003%.

[0034] Furthermore, the ultra-low carbon cold-rolled high-strength steel described in the present invention has a single ferrite microstructure.

[0035] In the ultra-low carbon cold-rolled high-strength steel designed according to the present invention, the matrix of its microstructure is a single ferrite, and inclusions and precipitated phases such as TiN, TiC, Ti(C,N), and Nb(C,N) are scattered within the matrix.

[0036] Furthermore, in the ultra-low carbon cold-rolled high-strength steel described in the present invention, the ferrite grain size is of grade 9 to 11.

[0037] Furthermore, the ultra-low carbon cold-rolled high-strength steel described in the present invention has a thickness of 0.7 to 3.5 mm.

[0038] Furthermore, the ultra-low carbon cold-rolled high-strength steel described in the present invention satisfies the requirements of yield strength ≥ 200 MPa, tensile strength ≥ 400 MPa, and elongation ≥ 30%, and the reduction in yield strength is less than 10% after high-temperature enamel firing at 840-870°C.

[0039] Furthermore, the ultra-low carbon cold-rolled high-strength steel described in this invention satisfies the following performance requirements: yield strength ≥ 220 MPa, tensile strength ≥ 410 MPa, and elongation ≥ 34%. The reduction in yield strength is less than 6.5% after 5 minutes of heat-sealed enamel firing at 840-870°C.

[0040] Furthermore, the ultra-low carbon cold-rolled high-strength steel described in this invention satisfies the following performance requirements after heat-sealing enamel firing at 840-870°C for 5 minutes: yield strength ≥ 210 MPa, tensile strength ≥ 370 MPa, and elongation ≥ 34%.

[0041] Therefore, another object of the present invention is to provide a method for manufacturing an ultra-low carbon cold-rolled high-strength steel sheet suitable for electrostatic powder enamel, the manufacturing process of which is simple, the manufacturing method enables efficient production of the ultra-low carbon cold-rolled high-strength steel sheet described in the present invention, and which has good enamel suitability and can be effectively applied to electrostatic powder enamel.

[0042] To achieve the above objective, the present invention proposes a method for manufacturing the above-mentioned ultra-low carbon cold-rolled high-strength steel sheet, comprising the following steps: (1) Smelting and casting; (2) Billet heating; (3) Hot rolling; (4) Laminar flow cooling: Cool the steel sheet to the winding temperature while controlling the cooling rate to 10-30°C / s; (5) Winding: The winding temperature is 600-680°C, and then it is air-cooled to room temperature; (6) Pickling; (7) Cold rolling; (8) Annealing; (9) Flattening.

[0043] In the present invention, the casting process in step (1) above can be specifically carried out by employing a continuous casting process, thereby ensuring uniformity of the internal components of the billet and good surface quality. Of course, in some other embodiments, casting can also be carried out using a die-casting method, and the die-cast ingot needs to be rolled in a bloc rolling mill to produce a steel billet.

[0044] Furthermore, in the manufacturing method of the present invention, in step (2), the heating temperature is controlled to 1180 to 1260°C.

[0045] Furthermore, in the manufacturing method described in the present invention, in step (3), the heated billet is roughly rolled into an intermediate billet, and then the intermediate billet is finish-rolled, with the rough-rolling temperature controlled to 900°C or higher, for example, 900 to 1080°C; the starting temperature for finish-rolling is 900 to 1050°C, and the final rolling temperature for finish-rolling is 860 to 940°C.

[0046] In the manufacturing method designed in this invention, by controlling the thermal processing control process of hot rolling in steps (3) and (4), a fine ferrite grain structure can be secured in the steel material of this invention, and the yield strength of the steel sheet can be improved by the fine grain strengthening effect. It is also advantageous in that it contributes to the acquisition of precipitated phases of alloying elements such as Ti and Nb that are finely dispersed, thereby improving the resistance of the steel sheet to chipping. Furthermore, the formation of the TiFe(P) precipitated phase can be effectively avoided, as this precipitated phase tends to precipitate unevenly at the ferrite grain boundaries, degrading the mechanical properties of the material.

[0047] Furthermore, in the manufacturing method of the present invention, in step (7), the cold rolling reduction ratio is controlled to 70-95%.

[0048] In step (7) above, the cold rolling reduction ratio can be controlled between 70 and 95%. As the cold rolling reduction ratio increases, the recrystallization temperature of the steel is lowered, which is advantageous for forming micro-vacancies around inclusions and precipitated phases in the steel, thereby increasing the hydrogen storage capacity of the steel and further improving the enameling resistance of the steel sheet.

[0049] Furthermore, in the manufacturing method described in the present invention, continuous annealing is employed in step (8), and the annealing temperature is 770 to 830°C.

[0050] Furthermore, in the manufacturing method described in the present invention, bell-type annealing is employed in step (8), and the annealing temperature is 690 to 740°C.

[0051] In this invention, the annealing process can employ either a continuous annealing process or a bell annealing process as needed. When employing a continuous annealing process, the continuous annealing temperature can be controlled to 770°C to 830°C; when employing a bell annealing process, the bell annealing temperature can be controlled to 690°C to 740°C. Furthermore, sufficient recrystallization and microstructure development can be ensured, which is advantageous in obtaining good workability and formability of the steel sheet.

[0052] Therefore, after completing the annealing process in step (8), it is necessary to flatten the steel sheet or strip in order to improve the shape and surface quality of the sheet.

[0053] The ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel coating and its manufacturing method, as described in the present invention, have the following advantages and beneficial effects compared to the prior art: Compared to conventional technologies, the inventors of the present invention have devised a new chemical composition design, using a specific P, Mn, and Mo component blend in combination with a Ti alloy element and C, S, and N component blend, and further combining a specific hot rolling thermal processing control process with cold rolling and annealing processes, thereby enabling the effective production of ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel coating. This ultra-low carbon cold-rolled high-strength steel possesses high yield strength performance and can meet the requirements for anti-splitting performance, enamel adhesion performance, and enamel surface quality for electrostatic powder double-sided enamel coating.

[0054] The ultra-low carbon cold-rolled high-strength steel manufactured in the manner of the present invention has a yield strength of 200 MPa or more, and the reduction in yield strength by high-temperature enamel firing at 840 to 870°C is within 10%, avoiding the deformation problem of the steel sheet after high-temperature enamel firing, ultimately significantly improving the strength of the enamel product and extending its lifespan.

[0055] As described above, the ultra-low carbon cold-rolled high-strength steel of the present invention can be widely applied to products that require electrostatic powder enamel, such as wide architectural decorative enamel boards, and that demand high yield strength performance after enamel firing, thus possessing considerable application value. [Brief explanation of the drawing]

[0056] [Figure 1] Microstructure photograph of the ultra-low carbon cold-rolled high-strength steel of Example 1. [Figure 2] Photograph of the precipitated phase in ultra-low carbon cold-rolled high-strength steel of Example 1. [Modes for carrying out the invention]

[0057] The following interpretation and explanation will be based on specific examples and drawings and will further describe the ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel described in the present invention and its manufacturing method, but this interpretation and explanation will not unduly limit the technical solutions of the present invention.

[0058] Examples 1-8 and Comparative Examples 1-2 The ultra-low carbon cold-rolled high-strength steel sheets suitable for electrostatic powder enamel in Examples 1 to 8 of the present invention, and the comparative steel sheets in Comparative Examples 1 to 2, are all manufactured in the following steps: (1) Smelting and casting were carried out based on the chemical composition shown in Tables 1-1 and 1-2: After vacuum degassing the smelted molten steel, continuous casting was performed to obtain a continuously cast billet.

[0059] (2) Billet heating: The obtained continuous cast billets were heated, and the heating temperature was controlled to 1180-1260°C.

[0060] (3) Hot rolling: The heated billet was roughly rolled into an intermediate billet, and then the intermediate billet was subjected to finish rolling. However, the rough rolling temperature was controlled to be 900°C or higher, the starting temperature for finish rolling was 900 to 1050°C, and the final rolling temperature for finish rolling was controlled to be 860 to 940°C.

[0061] (4) Laminar flow cooling: Laminar flow water cooling was performed, and the steel plate was cooled to the winding temperature while controlling the cooling rate to 10-30°C / s; (5) Winding: The winding temperature was controlled to 600-680°C, and then air-cooled to room temperature.

[0062] (6) Pickling: The scale was removed from the surface of the steel plate. (7) Cold rolling: The cold rolling reduction ratio was controlled to 70-95%.

[0063] (8) Annealing: Annealing was performed using either a continuous annealing process or a bell-type annealing process. When using the continuous annealing process, the annealing temperature was controlled to 770°C to 830°C; when using the bell-type annealing process, the annealing temperature was controlled to 690°C to 740°C.

[0064] Furthermore, in this invention, the chemical composition design and related processes of the ultra-low carbon cold-rolled high-strength steels of Examples 1 to 8 designed by the inventors all satisfy the requirements of the design standards of this invention. On the other hand, the comparative steel sheets of Comparative Examples 1 to 2 were also manufactured using the above steps (1) to (8), and in both the chemical composition design and related processes, there are parameters that satisfy the design requirements of this invention.

[0065] Tables 1-1 and 1-2 show the mass percentage composition of each chemical element in the ultra-low carbon cold-rolled high-strength steel of Examples 1-8 and the comparative steel sheets of Comparative Examples 1-2.

[0066] [Table 1-1]

[0067] [Table 1-2]

[0068] Note: In the table above, the formula for M* is "M* = Ti - S × 1.5 - N × 3.4 - C × 4", and the formula for N* is "N* = (Mn + P) × Mo". In the above relationships, the number before the mass percentage symbol of the chemical element is substituted for the corresponding element symbol in the formula.

[0069] Table 2 shows the specific process parameters in steps (1) to (8) of the above manufacturing method for the ultra-low carbon cold-rolled high-strength steel of Examples 1 to 8 and the comparative steel sheets of Comparative Examples 1 to 2.

[0070] [Table 2]

[0071] To further demonstrate the performance of the ultra-low carbon cold-rolled high-strength steel sheets suitable for electrostatic powder enamel in Examples 1 to 8 of the present invention and the comparative steel sheets in Comparative Examples 1 to 2 after enamel firing, the inventors sampled the ultra-low carbon cold-rolled high-strength steel sheets of Examples 1 to 8, which are the finished products obtained in the above process steps, and the comparative steel sheets in Comparative Examples 1 to 2, and subjected them to enamel treatment.

[0072] The enamel treatment involved performing electrostatic powder enamel treatment on both sides of the sample steel plates of each example and comparative example: using TR1042 glaze from Fukuroku Co., Ltd., the enamel firing temperature was controlled to 840-870°C, and after being kept warm for 5 minutes, the steel plates were air-cooled to obtain the enamel-fired plates. In this invention, after enamel treatment of the ultra-low carbon cold-rolled high-strength steel of Examples 1-8 and the comparative steel plates of Comparative Examples 1-2, corresponding enamel steel plates were obtained.

[0073] After completing the above operations, the enamel-coated steel sheets of Examples 1-8 and Comparative Examples 1-2 were observed and tested. The quality of the enamel surface was observed after the enamel-coated steel sheets were left to stand for 48 hours. The adhesion performance between the steel sheet and the glaze was verified using a drop weight test, and the grade was evaluated. The tensile performance of the steel sheets of each example and comparative example before and after enamel firing was determined by tensile tests at room temperature.

[0074] The relevant performance testing methods and techniques are as follows: (1) Tensile test: Tensile tests were performed in accordance with GB / T 228.1-2010 "Room temperature tensile test method for metallic materials". Using an SCL233 room temperature tensile testing machine, the tensile speed was controlled to 3 mm / min, and the tensile samples were JIS 5 tensile samples. The yield strength, tensile strength, and elongation of each example and comparative example were measured.

[0075] When conducting the above tensile tests, two test groups were designed: one for the enamel-coated steel sheets obtained by enamel firing in Examples 1-8 and Comparative Examples 1-2, and another for the original finished products manufactured according to the present invention: the ultra-low carbon cold-rolled high-strength steel of Examples 1-8 and the comparative steel sheets of Comparative Examples 1-2. These two test groups allowed for the measurement of the room-temperature tensile performance of each example and comparative steel sheet before and after enamel firing.

[0076] (2) Drop weight test and adhesion performance evaluation: For the enamel steel sheets of Examples 1 to 8 and Comparative Examples 1 to 2 obtained after enamel firing, enamel adhesion performance tests and grade evaluations were performed using the corresponding drop weight test apparatus in accordance with the drop weight test method described in European standard BS EN 10209-1996.

[0077] Table 3 shows the mechanical properties of the sheet materials of Examples 1-8 and Comparative Examples 1-2 before enamel treatment, and the detection results of the enamel performance and mechanical properties of the enamel steel sheets obtained after enamel treatment for each example and comparative example.

[0078] [Table 3]

[0079] As shown in Table 3 above, in the present invention, the yield strength of the original plates of the ultra-low carbon cold-rolled high-strength steel of Examples 1 to 8 is between 223 and 246 MPa, its tensile strength is between 418 and 449 MPa, and its elongation A 50 The yield strength is between 34% and 38%. These ultra-low carbon cold-rolled high-strength steels of Examples 1 to 8 still exhibit excellent mechanical properties after high-temperature enamel firing, with a yield strength reduction of between 3.04% and 6.38%, and a yield strength of 200 MPa or higher, between 212 and 230 MPa.

[0080] Therefore, when the enamel surfaces of the enamel steel sheets of Examples 1 to 8 were observed after 48 hours, none of these enamel steel sheets exhibited the toe-skip phenomenon, and no pinholes or air bubble defects were found on the surface. Furthermore, when the enamel steel sheets manufactured in accordance with Examples 1 to 8 were measured using a drop weight test based on the European standard BS EN 10209:2013, the adhesion performance between the steel sheet and the enamel layer was excellent, reaching A1 grade.

[0081] However, the performance of the comparative steel sheets in Comparative Examples 1-2 was clearly inferior to that of the ultra-low carbon cold-rolled high-strength steel sheets in Examples 1-8. The composition of the comparative steel material in Comparative Example 1 did not meet the N* value design requirements of the present invention, and the yield strength before and after enamel firing was low. The yield strength before enamel firing was 183 MPa, and the decrease after high-temperature enamel firing reached 12.57%, dropping to 160 MPa. Furthermore, the composition of the steel in Comparative Example 2 did not meet the M* value design requirements of the present invention, resulting in poor quality of the enamel surface with many pinholes and air bubble defects (>200 per m²). 2 ).

[0082] Figure 1 is a photograph of the microstructure of the ultra-low carbon cold-rolled high-strength steel of Example 1. As shown in Figure 1, in this embodiment, the microstructure of the ultra-low carbon cold-rolled high-strength steel of Example 1 is a ferrite structure having the characteristics of an IF steel structure, and the grain size of the ferrite is evaluated as grade 10 according to the GB / T 6394-2017 standard.

[0083] Figure 2 is a photograph of the precipitated phase in the ultra-low carbon cold-rolled high-strength steel of Example 1. Figure 2 shows the morphology and distribution of precipitated phases in the matrix of the ultra-low carbon cold-rolled high-strength steel of Example 1. As shown in Figure 2, in this embodiment, the ultra-low carbon cold-rolled high-strength steel of Example 1 contains numerous finely scattered precipitated phases such as TiC, Ti(C,N), and Nb(C,N), which are distributed within the steel matrix.

[0084] Furthermore, the combinations of technical features in this application are not limited to the combinations described in the claims or the specific embodiments, and all technical features described in this application can be freely combined or combined in any form, as long as they do not contradict each other.

[0085] Furthermore, it should be noted that the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the above embodiments, and it is clear that any similar changes or modifications that a person skilled in the art can directly derive from the disclosure of the present invention or readily conceive are also covered within the scope of the present invention.

Claims

1. An ultra-low carbon cold-rolled high-strength steel suitable for electrostatic powder enamel coating, The mass percentage content of each chemical element in the aforementioned steel is: C: 0.002–0.010%, Si ≤ 0.05%, Mn: 0.25–1.0%, P: 0.04–0.08%, S: 0.01–0.04%; Al: 0.01–0.05%, Cu: 0.02–0.08%, Ti: 0.05–0.12%, Mo: 0.02–0.10%, N: 0.004–0.012%; the remainder is Fe and unavoidable impurity elements; It is characterized by the following: Here, each chemical element satisfies at least one of the following equations: M* > 0, where M* = Ti - S × 1.5 - N × 3.4 - C × 4 > 0; N* ≥ 0.012, where N* = (Mn + P) × Mo; Substitute the numerical value preceding the mass percentage symbol of each chemical element into the formula for each chemical element symbol.

2. The ultra-low carbon cold-rolled high-strength steel according to Claim 1, wherein the mass percentage content of Si is Si: 0.01% to 0.045%.

3. The ultra-low carbon cold-rolled high-strength steel according to claim 1, further characterized in that it contains at least one of the following chemical elements: B: 0.0006 to 0.003% and Nb: 0.02 to 0.06%.

4. The ultra-low carbon cold-rolled high-strength steel according to claim 1, characterized in that its microstructure is a single ferrite.

5. The ultra-low carbon cold-rolled high-strength steel according to claim 4, characterized in that the ferrite grain size is of grade 9 to 11.

6. The ultra-low carbon cold-rolled high-strength steel according to claim 1, characterized in that its thickness is 0.7 to 3.5 mm.

7. The ultra-low carbon cold-rolled high-strength steel according to claim 1 is characterized in that it satisfies the performance requirements of yield strength ≥ 200 MPa, tensile strength ≥ 400 MPa, and elongation ≥ 30%, and the reduction in yield strength is less than 10% after high-temperature enamel firing at 840 to 870°C for 5 minutes.

8. The ultra-low carbon cold-rolled high-strength steel according to claim 7 is characterized in that it satisfies the performance requirements of yield strength ≥ 220 MPa, tensile strength ≥ 410 MPa, and elongation ≥ 34%, and the reduction in yield strength is less than 6.5% after enamel firing at 840 to 870°C for 5 minutes.

9. The ultra-low carbon cold-rolled high-strength steel according to claim 1, characterized in that, after enamel firing at 840-870°C for 5 minutes, the performance satisfies yield strength ≥ 210 MPa, tensile strength ≥ 370 MPa, and elongation ≥ 34%.

10. A method for producing ultra-low carbon cold-rolled high-strength steel according to any one of 1 to 9, characterized by comprising the following steps: (1) Smelting and casting; (2) Billet heating; (3) Hot rolling; (4) Laminar flow cooling: Cool the steel sheet to the winding temperature so that the cooling rate is controlled to 10-30°C / s; (5) Winding: The winding temperature is 600-680°C, and then it is air-cooled to room temperature; (6) Pickling; (7) Cold rolling; (8) Annealing; (9) Flattening.

11. The manufacturing method according to claim 10, characterized in that in step (2), the heating temperature is controlled to 1180 to 1260°C.

12. The manufacturing method according to claim 10, characterized in that in step (3), the heated billet is roughly rolled into an intermediate billet, and then the intermediate billet is finish-rolled, with a rough-rolling temperature of 900°C or higher; the starting temperature for finish-rolling is 900 to 1050°C, and the final rolling temperature for finish-rolling is 860 to 940°C.

13. The manufacturing method according to claim 10, characterized in that in step (3), the rough rolling temperature is controlled to 900 to 1080°C.

14. The manufacturing method according to claim 10, characterized in that in step (7), the cold rolling reduction ratio is controlled to 70 to 95%.

15. The manufacturing method according to claim 10, characterized in that in step (8), continuous annealing is employed and the annealing temperature is 770 to 830°C.

16. The manufacturing method according to claim 10, characterized in that, in step (8), bell-type annealing is employed and the annealing temperature is 690 to 740°C.

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