Ultra-low carbon cold-rolled high strength steel suitable for electrostatic powder enameling and its manufacturing method

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 manufacturing method thereof, and more particularly to a low carbon cold rolled high strength steel sheet and a manufacturing method thereof. [Background technology]

[0002] Enamel products are known to be made by firing metal and inorganic glass materials at high temperatures. The two completely different materials, metal and inorganic glass materials, are combined and integrated, compensating for each other's shortcomings and better embodying the advantages of each.

[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, protecting the product from injury or greatly reducing the degree of destruction; on the other hand, when the enamel product is chemically eroded, the enamel layer plays a role in protecting the product from being destroyed or discarded due to corrosion of the metal material. Therefore, enamel products combine the excellent properties of a series of metals and glassy inorganic materials.

[0004] Enameled steel products are composite materials formed by dissolving a glassy inorganic material onto the surface of a steel plate and coating it. In recent years, they have been widely used in the light industry, home appliances, environmental protection and construction industries, and are used to manufacture products such as ovens, toasters, water heaters, bathtubs, selective catalytic reduction (SCR) equipment in thermal power plants, air preheaters (APH) and gas flue gas heaters (GGH), architectural decorative panels and various enamel-embedded tanks.

[0005] Among the currently manufactured enamel steel products, the enameling methods are mainly divided into two types: wet method and dry method. The wet method is the method of applying glaze slurry to the metal base, and the three basic wet methods are immersion enameling, spray enameling, and flow enameling. Dry enameling mainly refers to electrostatic powder enameling, which adopts the principle of high voltage generation by electrostatic spray gun, and the nozzle forms a negatively charged electrostatic field with a voltage of 70-100kV. When the workpiece to be sprayed is grounded, the spray gun starts spraying powder when the workpiece passes through the spray booth, and the positively charged workpiece and the negatively charged powder are attracted to each other to form a powder coating.

[0006] Among the above several enameling methods, the surface quality of the enameled steel product produced by electrostatic powder enamel is the best, but such a process means also places higher requirements on the base steel plate.

[0007] This is because the enamel layer obtained by the electrostatic powder enamel method is denser, and hydrogen generated during the enamel firing process is less likely to escape, making it easier for pinch defects to occur. In addition, because the electrostatic powder enamel layer is thin (usually less than 200 μm), defects such as air bubbles and pinholes are easily generated on the enamel surface, affecting the surface quality. Therefore, during design and development, steel sheets suitable for electrostatic powder enamel must be resistant to pinch defects, pinholes, and air bubbles.

[0008] It should also be noted that the steel sheets used in the manufacture of enamel products must have high yield strength and small strength loss after high-temperature enamel firing in addition to excellent resistance to pinholes and air bubbles, in specific applications. This is because the quality of enamel products is also improving with the progress of the enamel industry, and for example, in the field of manufacturing enamel steel sheets for architectural decorative panels, it is necessary to manufacture enamel steel sheets with a larger width to reduce splices and improve aesthetics. However, such enamel steel sheets have strict requirements for the plate shape, and the larger the size of the workpiece, the more likely it is that deformation problems will occur during high-temperature enamel firing. In order to prevent large deformation after high-temperature enamel firing at 840 to 870 °C, the substrate steel sheet for enamel must have sufficient high-temperature deformation resistance.

[0009] Therefore, the present invention aims to obtain an ultra-low carbon cold-rolled high strength steel suitable for electrostatic powder enamel, which does not cause pinch defects after enamel firing, has good adhesion between the enamel layer and the steel sheet, and has excellent enamel surface quality, meeting all the requirements for use of electrostatic powder double-sided enamel. At the same time, this ultra-low carbon cold-rolled high strength steel has a yield strength of more than 200 MPa, and the yield strength reduction after high-temperature enamel firing at 840-870°C is within 10%, avoiding the deformation problem after high-temperature enamel firing, and finally, it can significantly improve the strength of enamel products, achieve excellent usage performance, and extend the life of enamel products.

[0010] In the prior art, there are several manufacturing technologies for cold-rolled enamel steel suitable for electrostatic powder enamel, but unlike the steel materials required for specific strength and performance such as those of the present invention, there are still significant differences in the design of chemical components.

[0011] For example, a Chinese patent document entitled "Continuously cast enameled steel sheet with excellent scuff resistance and manufacturing method thereof" with publication number CN101356295A and publication date Jan. 28, 2009, discloses a continuously cast enameled steel sheet with excellent scuff resistance, the chemical element compositions of which are 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 the steel sheet has non-integral or integral oxides with different mass concentrations of B or Mn within the steel sheet, and a manufacturing method thereof.

[0012] For example, a Chinese patent document entitled “Cold-rolled steel sheet for enamel, its manufacturing method, and enamel product” with publication number CN105518174A and publication date Apr. 20, 2016, discloses a cold-rolled steel sheet for enamel, its manufacturing method, and enamel product, the chemical element compositions of which are 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%, and the balance being Fe and impurities.

[0013] For example, a Chinese patent document titled "Cold rolled steel sheet for enamel and its manufacturing method" with publication number CN106560523A and publication date April 12, 2017, discloses a cold rolled steel sheet for enamel and its manufacturing method having chemical element compositions of C: 0.005% or less (except 0%), Mn: 0.05% to 0.3%, Al: 0.005% or less (except 0%), P: 0.03% or less (except 0%), S: 0.02% or less (except 0%), Si: 0.01% or less (except 0%), Ti: 0.005% to 0.01%, Y: 0.01% to 0.02%, N: 0.003% or less (except 0%), with the balance being Fe and other unavoidable impurities. Summary of the Invention [Problem to be solved by the invention]

[0014] The object of the present invention is to provide an ultra-low carbon cold-rolled high strength steel suitable for electrostatic powder enamel, which does not produce any pinch defects after enamel firing, has good adhesion between the enamel layer and the steel sheet, has excellent enamel surface quality, and meets all the requirements for use of electrostatic powder double-sided enamel. At the same time, this ultra-low carbon cold-rolled high strength steel has a yield strength of more than 200 MPa, and the yield strength is reduced by less than 10% after high-temperature enamel firing at 840-870°C, avoiding the deformation problem after high-temperature enamel firing, and finally, it can significantly improve the strength of enamel products, achieve excellent usage performance, and extend the life of enamel products. [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 electrostatic powder enamel and high yield strength performance after enamel firing, such as large width architectural decorative enamel plates and bathtubs, and has significant application value.

[0016] In order to achieve the above object, the present invention provides an ultra-low carbon cold rolled high strength steel suitable for electrostatic powder enamel, which contains Fe and inevitable impurity elements, and further contains 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; N* ≥ 0.012, where N* = (Mn + P) × Mo; Substitute the numerical value preceding the mass percent symbol for each chemical element in the formula.

[0017] Furthermore, in the ultra-low carbon cold rolled high strength steel according to the present invention, the mass percent 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 balance is Fe and unavoidable impurity elements; 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; N* ≥ 0.012, where N* = (Mn + P) × Mo; Substitute the numerical value preceding the mass percent symbol for each chemical element in the formula.

[0018] In the ultra-low carbon cold rolled high strength steel described in the present invention, the design principles of each chemical element are as follows: C: In the ultra-low carbon cold-rolled high strength steel described in the present invention, all of the C element can combine with strong carbide forming elements such as Ti and Nb to form finely dispersed precipitation phases. These precipitation phases effectively improve the hydrogen storage performance of the steel sheet and exert the anti-pinching effect of the enamel, while the nano-level precipitation phases such as TiC also exert a precipitation strengthening effect. However, it should be noted that the content of the C element should not be too high, and its addition amount should not be excessive with respect to the alloying elements, because during enamel firing, excessive free carbon will generate a large amount of gas such as CO during enamel firing, and these gases will cause the bubble structure of the enamel layer to become poor, and even defects such as pinholes and bubbles will occur, which will affect the quality of the enamel surface. In the electrostatic powder enamel process, this effect is even greater. Therefore, considering the effect of the C element on the performance of the steel material, in the ultra-low carbon cold-rolled high strength steel described in the present invention, the mass percent content of the C element is controlled between 0.002 and 0.010%.

[0019] Si: In the ultra-low carbon cold rolled high strength steel described in the present invention, the Si element is a residual element, and if the content of the Si element in the steel is too high, the plasticity of the steel material will deteriorate. In addition, in the enamel firing process, a high Si element content also affects the adhesion performance between the steel sheet and the glaze. Therefore, in the ultra-low carbon cold rolled high strength steel described in the present invention, the mass percent content of the Si element is controlled to Si≦0.05%. In some embodiments, the mass percent 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, Mn and P are both major solid solution strengthening elements. In general, in steels with high Ti and P content, TiFe(P) precipitation phases are formed, and this precipitation phase tends to be unevenly precipitated at ferrite grain boundaries, which deteriorates the mechanical performance of the material. On the other hand, in the embodiment of the present invention, the hot rolling end temperature and coiling temperature are controlled to avoid the precipitation temperature range of TiFe(P) phase, and such a situation is avoided, and the P element only exerts its solid solution strengthening effect. In addition, if the P element is too high, it is considered that it may cause the formability and secondary embrittlement of the material. Therefore, in the ultra-low carbon cold-rolled high strength steel described in the present invention, the mass percent content of the Mn element is limited to between 0.25 and 1.0%, and the mass percent content of the P element is limited to between 0.04 and 0.08%.

[0021] S: In steels containing a large amount of Mn, the S element usually bonds with the Mn element to form stripe- or line-shaped MnS inclusions, and these inclusions deteriorate the mechanical performance of the material. On the other hand, in the ultra-low carbon cold-rolled high strength steel described in the present invention, a relatively large amount of Ti element is added, and the action of the Ti element promotes the formation of spherical or polygonal TiMn(s) inclusions, which in turn improves the mechanical performance of the material. In addition, in this embodiment, the S element mainly bonds with Ti and C in the steel, and the Ti 4 C 2 S 2, forming TiS precipitates or inclusions. These precipitates and inclusions formed in the steel can act as irreversible hydrogen storage traps and exert the anti-chip action of the enamel. In order to exert the beneficial effects of the S element, the mass percent content of the S element is controlled to be between 0.01 and 0.04% in the present invention.

[0022] Mo: In the ultra-low carbon cold rolled high strength steel described in the present invention, molybdenum dissolves in ferrite, austenite and carbides to exert a solid solution strengthening effect; molybdenum also enhances the stability of carbides such as TiC and NbC, reduces the aggregation and coarsening phenomenon of carbide precipitation phases caused by high temperature enamel firing (the enamel firing temperature usually reaches 840-870 ° C), enhances the high temperature stability of steel, and can avoid a significant decrease in yield strength due to the weakening of the precipitation strengthening effect in the steel sheet after enamel firing. However, the content of Mo element in steel should also not be too high, and it should be noted that excessive addition 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 percent content of Mo element is controlled between 0.02-0.10%.

[0023] Al: In the ultra-low carbon cold-rolled high strength steel described in the present invention, Al is a strong deoxidizing element, and in order to keep the O content in the steel low, deoxidization with Al is often required in medium and low carbon steels. In addition, the Al element dissolved in the steel can combine with free nitrogen to precipitate AlN, the precipitation temperature of which is relatively high and plays a role in refining austenite grains, which is advantageous for refining grains and strengthening fine crystals. Therefore, in order to exert the beneficial effects of the Al element, in the present invention, the mass percent content of the Al element is controlled to be 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 the main element that gives the steel good hydrogen storage performance, and the Ti element in the steel can form fine and dispersed TiC, Ti(C,N) precipitate phases through an appropriate thermomechanical control process, and these precipitate phases improve the hydrogen storage performance of the steel sheet as irreversible hydrogen storage traps, and thus exert the anti-scratch effect of enamel. However, if the Ti element in the steel is too high, it will cause deterioration of the formability of the material and increase in cost, so in consideration of the mechanical performance and cost of the steel, the mass percent content of the Ti element in the ultra-low carbon cold-rolled high strength steel described in the present invention is limited to between 0.05% and 0.12%.

[0025] Cu: In the ultra-low carbon cold-rolled high strength steel described in the present invention, Cu element is deposited at the bonding interface between the glaze and the steel during the high temperature enamel firing process, which improves the adhesion performance between the steel and the glaze and improves the anti-finging performance of the steel material. However, the content of Cu element in the steel should not be too high, which also leads to deterioration of the forming performance of the material and increase in cost. Therefore, taking into consideration the mechanical performance and cost of the steel comprehensively, the present invention limits the mass percent content of Cu element to 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 N element is to form inclusions or precipitate phases such as TiN, NbN, Ti(C,N), Nb(C,N) with alloy elements such as Ti and Nb, and further exert the hydrogen storage trap function for the anti-toe enamel. In addition, in the process of cold rolling, the nitride inclusions are partially crushed, and around them, micro voids are formed, which are also very important and effective hydrogen storage traps. Based on this, in order to exert the beneficial effect of N element, the mass percent content of N element is limited to between 0.004 and 0.012% in the ultra-low carbon cold-rolled high strength steel described in the present invention.

[0027] In the above aspect of the present invention, the present invention can control the mass percent content of a single chemical element in the steel and at the same time control the elements in the steel to satisfy the limiting relationship "M*>0, but M*=Ti-S×1.5-N×3.4-C×4". According to the inventors' experimental research, when the element contents in the steel satisfy the above limiting relationship, the C, N interstitial atoms in the steel are completely fixed, and the structure of the steel is guaranteed to be a single ferrite. After the steel sheet with such structure characteristics undergoes electrostatic powder enamel firing, the quality of the enamel surface is excellent, and there is no occurrence of defects such as pinholes and 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 percent content of a single chemical element in the steel, and at the same time, control the elements in the steel plate for pressure vessels to satisfy the limiting relationship of "N*≧0.012, where N*=(Mn+P)×Mo". According to the inventors' experimental research, when this limiting relationship is satisfied, the steel plate can guarantee a yield strength performance of 200 MPa or more, and even after high-temperature enamel baking treatment, the steel plate maintains a high yield strength value, with a decrease of less than 10% compared to the original strength. 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 according to the present invention may contain B≦0.003% and Nb≦0.06%.

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

[0031] In the present invention, in order to obtain an ultra-low carbon cold rolled high strength steel with better performance, in an embodiment, appropriate amounts of B and Nb elements may be further added.

[0032] Nb: In the embodiment of the present invention, Nb element may combine with C and N elements to form fine precipitate phases such as NbC and NbN, and of course Nb may 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 to prevent pinching of enamel and also to strengthen the precipitation. In addition, these precipitate phase particles effectively suppress the deformation recrystallization of austenite during hot rolling, inhibit the growth of crystal grains, and strengthen the fine grains. Based on this, in order to exert the beneficial effects of Nb element, in the ultra-low carbon cold rolled high strength steel described in the present invention, Nb is preferably added in a small amount, and the mass percent content of Nb element is limited to between 0.02% and 0.06%.

[0033] B: In the embodiment of the present invention, adding an appropriate amount of B element can form B(C,N) in the steel sheet, increase the number of second phase particles, increase the number of hydrogen traps, and improve the pinch resistance of the steel sheet. In addition, B(C,N) also acts as a crystal nucleus, which is advantageous for the formation of equiaxed crystals, inhibits the growth of crystal grains, and is advantageous for improving the enameling 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 a small amount, and the mass percent content of B element 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 microstructure that is single ferrite.

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

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

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

[0038] Furthermore, the performance of 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 by high-temperature enamel firing at 840 to 870°C is less than 10%.

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

[0040] Furthermore, the properties of the ultra-low carbon cold rolled high strength steel described in the present invention satisfy the following after enameling baking at 840 to 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 manufacturing method for ultra-low carbon cold rolled high strength steel sheet suitable for electrostatic powder enamel, the production process of the manufacturing method is simple, the ultra-low carbon cold rolled high strength steel sheet described in the present invention can be efficiently manufactured by the manufacturing method, and the ultra-low carbon cold rolled high strength steel sheet has good enameling suitability and can be effectively applied to electrostatic powder enamel.

[0042] In order to achieve the above object, the present invention proposes a method for producing the above ultra-low carbon cold rolled high strength steel plate, which includes the following steps: (1) Smelting and foundry; (2) billet heating; (3) Hot rolling; (4) Laminar cooling: The steel sheet is cooled to the coiling temperature by controlling the cooling rate at 10-30℃ / s; (5) Winding: The winding temperature is 600-680°C, and then air-cooled to room temperature; (6) Pickling; (7) Cold rolling; (8) Annealing; (9) Flattening.

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

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

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

[0046] In the manufacturing method designed in the present invention, by controlling the thermal processing control process of the hot rolling in step (3) and step (4), the fine ferrite grain structure can be ensured in the steel material of the present invention, and the yield strength of the steel plate can be improved by the fine grain strengthening effect. It also contributes to the acquisition of finely scattered alloy element precipitates such as Ti, Nb, etc., which is advantageous in improving the fingering resistance of the steel plate. It also effectively avoids the formation of TiFe(P) precipitates, which tend to be unevenly precipitated at the ferrite grain boundaries, which deteriorates the mechanical performance of the material.

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

[0048] In the above step (7), the cold rolling reduction rate can be controlled between 70% and 95%. As the cold rolling reduction rate increases, the recrystallization temperature of the steel material decreases, while it is advantageous for forming micropores around inclusions and precipitates in the steel, thereby increasing the hydrogen storage capacity of the steel and further improving the enamel pinch resistance of the steel sheet.

[0049] Furthermore, in the manufacturing method described in the present invention, continuous annealing is adopted 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 adopted in step (8), and the annealing temperature is 690 to 740°C.

[0051] In the present invention, the annealing process can adopt a continuous annealing process or a bell-type annealing process according to need, and when the continuous annealing process is adopted, the continuous annealing temperature can be controlled to 770°C to 830°C; when the bell-type annealing process is adopted, the bell-type annealing temperature can be controlled to 690°C to 740°C. Furthermore, sufficient recrystallization and structure development can be ensured, which is advantageous for obtaining good processing and forming performance of the steel sheet.

[0052] Therefore, after finishing the annealing process in step (8), the steel plate or strip needs to be flattened to improve the plate shape and surface quality of the steel plate or strip.

[0053] The ultra-low carbon cold rolled high strength steel suitable for electrostatic powder enamel and its manufacturing method according to the present invention have the following advantages and beneficial effects compared with the prior art: Compared with the prior art, in the present invention, the inventors have devised a new chemical composition design, combining a specific composition of P, Mn, and Mo with a composition of Ti alloying elements and a composition of C, S, and N, and further combining a specific hot rolling thermal processing control process with a cold rolling and annealing process, thereby effectively producing ultra-low carbon cold rolled high strength steel suitable for electrostatic powder enamel. This ultra-low carbon cold rolled high strength steel has high yield strength performance and can meet the requirements of electrostatic powder double-sided enamel pinch resistance performance, enamel adhesion performance, and enamel surface quality.

[0054] The ultra-low carbon cold rolled high strength steel produced in the embodiment of the present invention has a yield strength of 200 MPa or more, and the reduction in yield strength after high-temperature enamel firing at 840-870°C is within 10%, which avoids the deformation problem of the steel plate after high-temperature enamel firing, and ultimately significantly improves the strength of the enamel products and extends the service life of the enamel products.

[0055] As a result, the ultra-low carbon cold-rolled high strength steel of the present invention can be widely applied to products that require electrostatic powder enamel and high yield strength performance after enamel firing, such as large width architectural decorative enamel plates, and has significant application value. [Brief description of the drawings]

[0056] [Figure 1] Photograph of the metal structure of the ultra-low carbon cold-rolled high strength steel of Example 1. [Diagram 2] Photograph of a precipitate phase in the ultra-low carbon cold-rolled high strength steel of Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] The ultra-low carbon cold rolled high strength steel suitable for electrostatic powder enamel and the manufacturing method thereof according to the present invention will be further explained with reference to the following specific examples and the accompanying drawings, but the explanations are not intended to unduly limit the technical solution of the present invention.

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

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

[0060] (3) Hot rolling: The heated billet is rough rolled into an intermediate billet, and then the intermediate billet is finish rolled, with the rough rolling temperature being controlled to 900°C or higher, the start temperature of the finish rolling being controlled to 900-1050°C, and the final rolling temperature of the finish rolling being controlled to 860-940°C.

[0061] (4) Laminar cooling: The steel sheet was cooled to the coiling temperature by laminar water cooling, with the cooling rate controlled to 10 to 30°C / s; (5) Winding: The winding temperature was controlled to 600 to 680°C, and then the wire was air-cooled to room temperature.

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

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

[0064] In the present invention, the chemical composition designs 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 criteria of the present invention. On the other hand, the comparative steel sheets of Comparative Examples 1 and 2 were also manufactured using the above steps (1) to (8), and both the chemical composition designs and related processes have parameters that satisfy the design requirements of the present invention.

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

[0066] [Table 1-1]

[0067] [Table 1-2]

[0068] Note: In the above table, 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 formula, the numerical value before the mass percent symbol of the chemical element is substituted for the corresponding element symbol in the formula.

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

[0070] [Table 2]

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

[0072] The enameling treatment includes performing double-sided electrostatic powder enameling treatment on the sample steel plates of each Example and Comparative Example: using TR1042 glaze from Fukuroku Company, the enameling firing temperature was controlled at 840-870°C, and the steel plates were kept warm for 5 minutes and then air-cooled to obtain enameled steel plates. In the present invention, the ultra-low carbon cold-rolled high strength steels of Examples 1-8 and the comparative steel plates of Comparative Examples 1-2 were enameled, and then the corresponding enameled steel plates could be obtained.

[0073] After the above operations were completed, the enameled steel sheets of Examples 1 to 8 and Comparative Examples 1 and 2 that had been subjected to the enameling treatment were observed and tested. The enameled steel sheets were left to stand for 48 hours, and the quality of the enamel surface was observed; a drop weight experiment was used to verify the adhesion performance between the steel sheet and the glaze, and the grade was evaluated; the tensile performance at room temperature of the steel sheets of each Example and Comparative Example before and after enamel firing was determined by a tensile test.

[0074] The relevant performance test methods and procedures are as follows: (1) Tensile test: A tensile test was performed according to GB / T 228.1-2010 "Room temperature tensile test method for metallic materials". The tensile speed was controlled at 3 mm / min using an SCL233 room temperature tensile tester. The tensile sample was a JIS5 tensile sample. The yield strength, tensile strength, and elongation of each of the examples and comparative examples were measured.

[0075] When conducting the above tensile tests, two test groups were designed: the enameled steel sheets obtained by enameling in Examples 1 to 8 and Comparative Examples 1 and 2, and the ultra-low carbon cold-rolled high strength steel sheets of Examples 1 to 8, which are the original finished products manufactured according to the present invention, and the comparative steel sheets of Comparative Examples 1 and 2. These two test groups make it possible to measure the room temperature tensile properties of the steel sheets of each of the Examples and Comparative Examples before and after enameling.

[0076] (2) Drop weight test and adhesion performance evaluation: The enamel steel sheets of Examples 1 to 8 and Comparative Examples 1 and 2 obtained after enamel firing were subjected to an enamel adhesion performance test and grade evaluation using a corresponding drop weight test device 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 plate materials of Examples 1 to 8 and Comparative Examples 1 and 2 before the enamel treatment, and the results of detection of the enamel performance and mechanical properties of the enameled steel sheets obtained after the enamel treatment of 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 plate 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 ultra-low carbon cold rolled high strength steels of Examples 1 to 8 still have excellent mechanical properties after high temperature enamel baking treatment, with a yield strength reduction range of 3.04 to 6.38%, and a yield strength of 200 MPa or more, between 212 and 230 MPa.

[0080] Therefore, when the enamel surfaces of the enameled steel sheets of Examples 1 to 8 were observed after 48 hours, none of these enameled steel sheets showed any pinholes or air bubble defects on the surface. Furthermore, when the enameled steel sheets manufactured in accordance with Examples 1 to 8 were measured by 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 Class A1.

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

[0082] FIG. 1 is a photograph of the metal structure of the ultra-low carbon cold-rolled high strength steel of Example 1. As shown in FIG. 1, in this embodiment, the metal structure of the ultra-low carbon cold rolled high strength steel of Example 1 is a ferritic 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] FIG. 2 is a photograph of the precipitate phase in the ultra-low carbon cold rolled high strength steel of Example 1. Figure 2 shows the morphology and distribution of the precipitated phase 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 a large number of precipitated phases such as TiC, Ti(C,N), and Nb(C,N) that are finely scattered and distributed in the steel matrix.

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

[0085] In addition, it should be noted that the above-mentioned embodiments are merely specific embodiments of the present invention. The present invention is not limited to the above-mentioned embodiments, and it is clear that any similar changes or modifications that can be directly derived or easily conceived by those skilled in the art from the disclosure of the present invention are included in the protection scope of the present invention.

Claims

1. An ultra-low carbon cold rolled high strength steel suitable for electrostatic powder enamel, containing Fe and inevitable impurity elements, and further containing 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; Substitute the numerical value preceding the mass percent symbol for each chemical element in the formula.

2. The mass percent 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%; preferably, Si: 0.01%-0.045%; the balance being Fe and inevitable impurity elements; 2. The ultra-low carbon cold rolled high strength steel according to claim 1 . 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; Substitute the numerical value preceding the mass percent symbol for each chemical element in the formula.

3. The ultra-low carbon cold rolled high strength steel according to claim 1 or 2, further comprising at least one of the chemical elements B: 0.0006-0.003% and Nb: 0.02-0.06%.

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

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

6. 3. The ultra-low carbon cold rolled high strength steel according to claim 1 or 2, characterized in that the thickness is 0.7 to 3.5 mm.

7. 3. The ultra-low carbon cold rolled high strength steel according to claim 1 or 2, characterized in that the performance satisfies the requirements of yield strength ≧200 MPa, tensile strength ≧400 MPa, and elongation ≧30%, and the yield strength decreases by less than 10% due to high-temperature enamel firing at 840 to 870°C, and preferably the enamel firing temperature retention time is 5 minutes.

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

9. 3. The ultra-low carbon cold rolled high strength steel according to claim 1 or 2, characterized in that after enameling firing at 840-870°C for 5 minutes, the properties satisfy the following: yield strength ≥ 210 MPa, tensile strength ≥ 370 MPa, and elongation ≥ 34%.

10. A method for producing an ultra-low carbon cold rolled high strength steel according to any one of claims 1 to 9, comprising the steps of: (1) Smelting and foundry; (2) billet heating; (3) hot rolling; (4) Laminar cooling: The steel sheet is cooled to the coiling temperature while controlling the cooling rate to be 10 to 30°C / s; (5) Winding: the winding temperature is 600-680°C, and then air-cooled to room temperature; (6) Pickling; (7) cold rolling; (8) annealing; (9) Flattening.

11. The method according to claim 10, wherein 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 rough rolled into an intermediate billet, and then the intermediate billet is finish rolled, and the rough rolling temperature is controlled to be above 900°C, preferably 900-1080°C; the starting rolling temperature of the finish rolling is 900-1050°C, and the final rolling temperature of the finish rolling is 860-940°C.

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

14. The method according to claim 8, characterized in that in step (8), continuous annealing is adopted, and the annealing temperature is 770-830°C.

15. The method according to claim 10, wherein in step (8), bell-type annealing is adopted, and the annealing temperature is 690-740°C.

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