600MPa Grade Cold-Rolled Steel with Low Yield Ratio Based on Bell Annealing Treatment and Its Preparation Method

Abstract

The present application discloses a cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment and a preparation method thereof. The method includes providing a continuous casting billet containing the following components by weight percentage: C: 0.10% - 0.12%, Si: 0.75% - 0.85%, Mn: 3.40% - 3.60%, Al: 0.02% - 0.05%, Cr: 0.4% - 0.5%, P ≤ 0.015%, S ≤ 0.015%, and the balance being iron and unavoidable impurities; heating the continuous casting billet to 1180°C - 1220°C for hot rolling, the hot rolling including rough rolling and finish rolling carried out in sequence, the rough rolling process including rolling the heated continuous casting billet to obtain an intermediate billet; the finish rolling including rolling the intermediate billet, the final rolling temperature of the hot rolling being 870°C - 940°C to obtain a hot-rolled finished product; removing the surface scale of the hot-rolled finished product for rewinding; cold rolling the rewound hot-rolled finished product to obtain a cold-rolled finished product; subjecting the cold-rolled finished product to bell annealing, the annealing temperature being 640°C - 660°C and the holding time being 8 hours to 10 hours; slowly cooling the annealed cold-rolled finished product, the slow cooling time ≥ 20 hours to obtain a cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment.

Description

Technical Field

[0001] This application belongs to the technology of steel smelting, and particularly relates to a cold-rolled steel with a tensile strength of 600 MPa grade and a low yield ratio based on bell annealing treatment and a preparation method thereof. Background Art

[0002] With the continuous increase in the demand in application fields such as the automotive industry, the development and production research of high-strength cold-rolled steel plates have become a hot topic. High-strength cold-rolled steel is mainly used to manufacture products such as automobile bodies, automobile chassis, and high-strength guardrail structural parts, which can significantly reduce the weight of automobiles and conform to the current lightweight technology development trend of automobiles. The substantial improvement in the control level of the manufacturing process technology of cold-rolled high-strength steel provides a guarantee for replacing thick-specification hot-rolled low-grade products with thin-specification cold-rolled high-strength products. While the product has high strength, it also has excellent cold bending forming performance and impact resistance, and the prepared components are not easily cracked.

[0003] Currently, the general methods for manufacturing high-strength cold-rolled steel plates generally include two types: one is to improve the strength by microalloying, and the other is by phase transformation strengthening methods, such as Trip steel, DP dual-phase steel, etc. The former method requires the addition of more alloying elements. Although the latter method has a low production cost for cold-rolled dual-phase steel and the tensile strength can reach 600 MPa, its yield strength is relatively high, seriously affecting its forming performance.

[0004] It is relatively easy to achieve the mechanical properties of cold-rolled high-strength steel by annealing in a continuous annealing furnace, but there are also the following disadvantages:

[0005] 1. The equipment investment of the continuous annealing furnace is large, and processes such as rapid cooling and segmented cooling are required. The annealing process is complex, especially the requirements for the automation of the production line and operators are high, and the process operation is difficult. It is more suitable for the annealing treatment of steel grades such as dual-phase steel and complex-phase steel;

[0006] 2. The continuous annealing production line is suitable for producing products with large batches and single specifications, and can achieve continuous production. For steel grades without continuous annealing treatment equipment or restricted by the continuous annealing production process, the cost increases significantly and the process adjustment is frequent. Summary of the Invention

[0007] In order to solve the above problems, the embodiment of this application provides a preparation method of a cold-rolled steel with a tensile strength of 600 MPa grade and a low yield ratio based on bell annealing treatment, which can obtain a cold-rolled steel with a tensile strength of 600 MPa grade and a low yield ratio based on bell annealing treatment with a yield strength less than 410 MPa, a tensile strength greater than 600 MPa, a yield ratio less than 70%, and an elongation of 15% - 25%, which is suitable for processing to meet the requirements of automotive steel for low cost, high strength, good plasticity, and forming performance.

[0008] In a first aspect, the present application provides a method for preparing a cold-rolled steel with a yield ratio of 600 MPa and a low yield ratio based on bell annealing treatment. The method includes:

[0009] Providing a continuous casting billet, wherein the chemical composition of the continuous casting billet, by weight percentage, contains the following components in the following contents:

[0010] C: 0.10% to 0.12%, Si: 0.75% to 0.85%, Mn: 3.40% to 3.60%, Al: 0.02% to 0.05%, Cr: 0.4% to 0.5%, P ≤ 0.015%, S ≤ 0.015%, and the balance is iron and unavoidable impurities;

[0011] Heating the continuous casting billet with a thickness of 210 mm to 260 mm to 1180°C to 1220°C;

[0012] Hot-rolling the heated continuous casting billet. The hot-rolling includes rough rolling and finish rolling carried out in sequence. The rough rolling process includes rolling the heated continuous casting billet for 5 to 7 passes to obtain an intermediate billet with a thickness of 25 mm to 40 mm; the finish rolling includes rolling the intermediate billet for 6 to 8 passes. The final rolling temperature of the hot-rolling is 870°C to 940°C, and the cooling rate of the laminar cooling is 5°C / s to 15°C / s to obtain a hot-rolled finished product with a thickness of 3.0 mm to 6.0 mm;

[0013] Removing the surface scale of the hot-rolled finished product and then rewinding it, and the coiling temperature is 590°C to 620°C;

[0014] Cold-rolling the rewound hot-rolled finished product. After completing one cold-rolling pass, the total reduction ratio is 75% to 88% to obtain a cold-rolled finished product with a thickness of 0.7 mm to 1.5 mm;

[0015] Subjecting the cold-rolled finished product to bell annealing, with the annealing temperature being 640°C to 660°C and the holding time being 8 hours to 10 hours;

[0016] Slow-cooling the annealed cold-rolled finished product, with the slow-cooling time ≥ 20 hours to obtain a cold-rolled steel with a yield ratio of 600 MPa and a low yield ratio based on bell annealing treatment.

[0017] In an embodiment of the present application, heating the continuous casting billet includes heating the continuous casting billet to 1180°C to 1220°C in a heating furnace, and the heating time is 110 minutes to 130 minutes.

[0018] In an embodiment of the present application, removing the surface scale of the hot-rolled finished product and then rewinding it includes pickling the surface scale with an acid solution and then rewinding it.

[0019] In some embodiments, the acid solution is a hydrochloric acid solution selected from different concentrations.

[0020] In one embodiment of the present application, cold rolling the hot-rolled finished product after recoiling includes rolling the hot-rolled finished product after recoiling in 5 to 11 passes to obtain a cold-rolled finished product.

[0021] In one embodiment of the present application, slow cooling the cold-rolled finished product after annealing includes slow cooling the cold-rolled finished product after annealing for 20 to 24 hours, preferably 20 to 23 hours.

[0022] In a second aspect, the present application provides a cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment. By mass percentage, it has the following components: C: 0.10% - 0.12%, Si: 0.75% - 0.85%, Mn: 3.40% - 3.60%, Al: 0.02% - 0.05%, Cr: 0.4% - 0.5%, P ≤ 0.015%, S ≤ 0.015%, and the rest are iron and unavoidable impurities.

[0023] In one embodiment of the present application, the product thickness of the cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment is 0.7 mm - 1.5 mm, the tensile strength ≥ 600 MPa, the yield ratio is less than 70%, and the elongation is 15% - 25%.

[0024] In one embodiment of the present application, the product of the cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment is in strip or plate shape, i.e., steel strip or steel plate.

[0025] The preparation method of the cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment in the embodiments of the present application, on the basis of the composition of ordinary carbon structural steel, uses the cooperation of Si and Mn elements and the steel strip after hot rolling controlled rolling and controlled cooling technology as raw materials, conducts cold rolling and annealing treatment, and controls the grain size of the hot-rolled steel strip by using the characteristics of ordinary hot strip mills and the fine grain strengthening mechanism, so as to manufacture cold-rolled steel with a low yield ratio whose tensile strength and elongation meet the requirements at a low cost, and can meet the requirements of automotive steel sheets. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required to be used in the embodiments of the present application. For those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative efforts.

[0027] Figure 1 It is a schematic flow chart of the preparation method of the cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment provided by an embodiment of the present application;

[0028] Figure 2It is the metallographic diagram of the cold-rolled steel with a yield ratio of 600 MPa and a low yield ratio based on bell annealing treatment after annealing and heat preservation at 650 °C for 10 hours in a bell annealing furnace provided by another embodiment of the present application;

[0029] Figure 3 It is the metallographic diagram of the cold-rolled steel with a yield ratio of 600 MPa and a low yield ratio based on bell annealing treatment after annealing and heat preservation at 660 °C for 10 hours in a bell annealing furnace provided by another embodiment of the present application. Detailed implementation manners

[0030] The features and exemplary embodiments of various aspects of the present application will be described in detail below. To make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present application, rather than limiting the present application. For those skilled in the art, the present application can be implemented without some of these specific details. The following description of the embodiments is only intended to provide a better understanding of the present application by showing examples of the present application.

[0031] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, the elements defined by the statement "including..." do not exclude the existence of additional identical elements in the process, method, article or device including the said elements.

[0032] To solve the problems of the prior art, the embodiment of the present application provides a preparation method of cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment. First, the preparation method of cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment provided by the embodiment of the present application will be introduced below.

[0033] Figure 1 The flowchart of the preparation method of cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment provided by an embodiment of the present application is shown. As Figure 1 shown, the preparation method of cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment includes:

[0034] S1. Provide a continuous casting billet, wherein the chemical composition of the continuous casting billet, by weight percentage, contains the following components:

[0035] C: 0.10% - 0.12%, Si: 0.75% - 0.85%, Mn: 3.40% - 3.60%, Al: 0.02% - 0.05%, Cr: 0.4% - 0.5%, P ≤ 0.015%, S ≤ 0.015%, and the balance is iron and unavoidable impurities;

[0036] S2. Heat the continuous casting billet with a thickness of 210 mm - 260 mm to 1180°C - 1220°C;

[0037] S3. Hot-roll the heated continuous casting billet. The hot rolling includes rough rolling and finish rolling carried out in sequence. The rough rolling process includes rolling the heated continuous casting billet for 5 to 7 passes to obtain an intermediate billet with a thickness of 25 mm - 40 mm; the finish rolling includes rolling the intermediate billet for 6 to 8 passes. The final rolling temperature of the hot rolling is 870°C - 940°C, and the cooling rate of the laminar cooling is 5°C / s - 15°C / s to obtain a hot-rolled finished product with a thickness of 3.0 mm - 6.0 mm;

[0038] S4. Remove the surface scale of the hot-rolled finished product and rewind it. The coiling temperature is 590°C - 620°C;

[0039] S5. Cold-roll the rewound hot-rolled finished product. After completing one cold rolling pass, the total reduction ratio is 75% - 88% to obtain a cold-rolled finished product with a thickness of 0.7 mm - 1.5 mm;

[0040] S6. Bell anneal the cold-rolled finished product. The annealing temperature is 620°C - 680°C, and the holding time is 8 hours to 10 hours;

[0041] S7. Slowly cool the annealed cold-rolled finished product. The slow cooling time ≥ 20 hours to obtain a cold-rolled steel with a yield strength ratio of 600 MPa and a low yield ratio based on bell annealing treatment.

[0042] The preparation method of the cold-rolled steel with a yield strength ratio of 600 MPa and a low yield ratio based on bell annealing treatment in the embodiments of the present application, on the basis of the composition of ordinary carbon structural steel, uses the cooperation of Si and Mn elements and the steel strip after adopting the hot rolling controlled rolling and controlled cooling technology as raw materials, conducts cold rolling and annealing treatments, and utilizes the characteristics of ordinary hot continuous rolling mills and the fine grain strengthening mechanism to control the grain size of the hot-rolled state steel strip, and manufactures cold-rolled steel with a low yield ratio that meets the requirements of tensile strength and elongation at a low cost, which can meet the requirements of automotive steel sheets.

[0043] Among them, the composition combination of Si and Mn elements enables the steel after cold rolling to have high strength, with a tensile strength of over 600 MPa, a yield strength of less than 410 MPa, a yield ratio of less than 70%, and an elongation rate reaching 15% - 25%, meeting the usage requirements of automotive steel sheets.

[0044] The preparation method of the cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell-type annealing treatment in the embodiments of the present application can produce thin-gauge steel strips on a conventional hot continuous rolling mill and an ordinary reversible cold rolling mill. While having the advantage of low cost, the prepared steel meets the requirements of automotive steel sheets.

[0045] It should be noted that the continuous casting billets used in the present application are prepared according to the existing technology, that is, the steelmaking processes before hot rolling are all carried out according to the design requirements of the chemical composition of the steel grades in the existing technology for batching, converter smelting, refining, and continuous casting. The continuous casting billets can be obtained by casting molten steel in a mold for shaping.

[0046] In an embodiment of the present application, heating the continuous casting billet includes heating the continuous casting billet in a heating furnace to 1180°C - 1220°C, and the heating time is 110 minutes - 130 minutes.

[0047] In an embodiment of the present application, the cooling rate of laminar flow cooling is 9°C / s - 12°C / s.

[0048] In an embodiment of the present application, removing the surface scale of the hot-rolled finished product and rewinding it includes pickling the surface scale of the hot-rolled finished product with acid solution and then rewinding it. Pickling generally involves feeding the hot-rolled finished product that needs to remove the surface scale into a pickling tank for pickling. Then, the pickled hot-rolled finished product is re-rolled into a steel coil for convenient storage and transportation. The acid solution is an aqueous solution of hydrochloric acid.

[0049] In an embodiment of the present application, cold rolling the rewound hot-rolled finished product includes rolling the rewound hot-rolled finished product for 5 - 11 passes to obtain a cold-rolled finished product.

[0050] In an embodiment of the present application, the annealing temperature is further preferably 640°C - 660°C, the holding time of annealing is 6 hours to 12 hours, and the holding time of annealing is preferably 8 hours to 10 hours.

[0051] Due to equipment limitations, the bell-type annealing furnace cannot achieve high-temperature annealing and can only achieve strength indicators by controlling the size of annealing grain size. In the present application, the annealing time and annealing temperature of bell-type annealing play an important role in the steel strength reaching 600 MPa level. Therefore, the bell-type annealing temperature needs to be strictly guaranteed to be between 620°C - 680°C, preferably between 640°C - 660°C, and the holding time is controlled between 6 hours and 12 hours, preferably 8 hours to 10 hours.

[0052] The production of small-batch, multi-specification, and multi-variety products can be achieved by bell-type annealing. The annealing temperature is low, the process is simple, the operability is strong, and the cost is low.

[0053] In an embodiment of the present application, slow cooling of the cold-rolled finished product after annealing includes slow cooling the cold-rolled finished product after annealing for 20 to 24 hours, preferably 20 to 23 hours.

[0054] In a second aspect, the present application provides a cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment. By mass percentage, it has the following components: C: 0.10% - 0.12%, Si: 0.75% - 0.85%, Mn: 3.40% - 3.60%, Al: 0.02% - 0.05%, Cr: 0.4% - 0.5%, P ≤ 0.015%, S ≤ 0.015%, and the rest is iron and inevitable impurities.

[0055] In an embodiment of the present application, the product thickness of the cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment is 0.7 mm - 1.5 mm, the tensile strength ≥ 600 MPa, the yield ratio is less than 70%, and the elongation is 15% - 25%.

[0056] In an embodiment of the present application, the product of the cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment is in strip or plate shape, that is, steel strip or steel plate.

[0057] Example

[0058] The following specific examples are listed to illustrate the present invention. It should be noted that the examples are only used to further illustrate the present invention and do not limit the protection scope of the present invention. Non-essential modifications and adjustments made by others based on the present invention still fall within the protection scope of the present invention.

[0059] According to the chemical composition ranges set in Table 1, continuous casting billets are prepared according to the following process flow. Using chemical components C, Si, Mn, Cr, Al, and Fe as raw materials, converter smelting, refining, and continuous casting are carried out in sequence to obtain billets, and the billets are directly heated or soaked.

[0060] Table 1 Chemical composition ratio table

[0061]

[0062] Example 1

[0063] A preparation method of a cold-rolled steel with a yield ratio less than 70% and a tensile strength of 600 MPa based on bell-type annealing treatment includes:

[0064] Provide a continuous casting billet containing the components and contents shown in Example 1 of Table 1;

[0065] Heat a continuous casting billet with a thickness of 230 mm to 1200 °C for 120 minutes;

[0066] Hot-roll the heated continuous casting billet. The hot rolling includes rough rolling and finish rolling carried out in sequence. The rough rolling process includes rolling the heated continuous casting billet for 7 passes to obtain an intermediate billet with a thickness of 36 mm; the finish rolling includes rolling the intermediate billet for 7 passes. The final rolling temperature of the hot rolling is 870 °C, and the cooling rate of the laminar cooling is 10 °C / s to obtain a hot-rolled finished product with a thickness of 5.4 mm;

[0067] Remove the surface scale of the hot-rolled finished product by pickling and rewind it. The coiling temperature is 600 °C;

[0068] Cold-roll the rewound hot-rolled finished product on a reversible cold rolling mill. Roll it in 9 passes. After completing one cold rolling process, the total reduction ratio is 78% to obtain a cold-rolled finished product with a thickness of 1.2 mm;

[0069] Anneal the cold-rolled finished product in a bell-type annealing furnace at an annealing temperature of 650 °C for 10 hours;

[0070] Slowly cool the annealed cold-rolled finished product for 20 hours to obtain a cold-rolled steel with a yield strength ratio of 65% and a tensile strength of 600 MPa grade and a low yield ratio based on bell-type annealing treatment. Its tensile strength is 627 MPa, the yield strength is 406 MPa, and the elongation is 20.6%. Figure 2 This is the metallographic diagram after annealing and heat preservation at 650 °C for 10 hours in the bell-type annealing furnace in this example. It can be seen from it that the metallographic structure is mainly ferrite and carbide, which is beneficial to obtaining good strength and plasticity.

[0071] Example 2

[0072] A preparation method of a cold-rolled steel with a yield strength ratio of 65% and a tensile strength of 600 MPa grade based on bell-type annealing treatment includes:

[0073] Provide a continuous casting billet containing the components and contents shown in Example 2 of Table 1;

[0074] Heat a continuous casting billet with a thickness of 230 mm to 1200 °C for 120 minutes;

[0075] Hot-roll the heated continuous casting billet. The hot rolling includes rough rolling and finish rolling carried out in sequence. The rough rolling process includes rolling the heated continuous casting billet for 7 passes to obtain an intermediate billet with a thickness of 36 mm; the finish rolling includes rolling the intermediate billet for 7 passes. The final rolling temperature of the hot rolling is 870 °C, and the cooling rate of the laminar cooling is 12 °C / s to obtain a hot-rolled finished product with a thickness of 5.4 mm;

[0076] The hot-rolled finished product is pickled to remove the surface scale and then recoiled, with the coiling temperature being 600°C;

[0077] The recoiled hot-rolled finished product is cold-rolled on a reversible cold rolling mill, rolled in 9 passes. After completing one cold rolling process, the total reduction ratio is 78%, and a cold-rolled finished product with a thickness of 1.2 mm is obtained;

[0078] The cold-rolled finished product is subjected to box annealing, with the annealing temperature being 660°C and the holding time being 10 hours;

[0079] The annealed cold-rolled finished product is slowly cooled for 20 hours to obtain a cold-rolled steel with a yield strength ratio of less than 600 MPa grade based on box annealing treatment, with a tensile strength of 671 MPa, a yield strength of 419 MPa, a yield strength ratio of 62%, and an elongation of 20%. Figure 3 This is the metallographic diagram after annealing and holding at 660°C for 10 hours in the box annealing furnace for this example. It can be seen that the metallographic structure is mainly ferrite and carbide, and the grain size is Figure 2 further refined compared to [reference], so the yield strength is also further increased compared to Example 1, and the elongation hardly decreases.

[0080] The average grain diameters of the metallographic structures of the cold-rolled steels with a yield strength ratio of less than 600 MPa grade based on box annealing treatment prepared in Examples 1 and 2 after annealing and holding in the box annealing furnace are shown in Table 2 below:

[0081] Table 2 Comparison table of the average grain diameters of the metallographic structures of the cold-rolled steels in Examples 1 and 2 after annealing and holding

[0082]

[0083]

[0084] From the process parameters of the above Examples 1 and 2 and the average grain sizes in Table 2, it can be concluded that the preparation method of the cold-rolled steel with a yield strength ratio of less than 600 MPa grade based on box annealing treatment in the examples of this application, on the basis of the composition of ordinary carbon structural steel, uses the strip steel after the cooperation of Si and Mn elements and the application of hot rolling controlled rolling and controlled cooling technology as the raw material, conducts cold rolling and annealing treatment, utilizes the characteristics of ordinary hot continuous rolling mills and the fine grain strengthening mechanism, and controls the grain size of the hot-rolled strip steel by controlling and increasing the speed of laminar cooling and the annealing temperature, making the average grain size of the hot-rolled strip steel finer and more uniform, and manufacturing a cold-rolled steel with a yield strength ratio that meets the requirements of tensile strength, yield strength, and elongation at a lower cost, which can meet the requirements of automotive steel sheets.

[0085] Comparative Example 1

[0086] A preparation method of a low yield ratio cold-rolled steel includes:

[0087] Providing a continuous casting billet containing the components and contents shown in Comparative Example 1 in Table 1;

[0088] Heating the continuous casting billet of Comparative Example 1 with a thickness of 230 mm to 1200 °C for 120 minutes;

[0089] Performing hot rolling on the heated continuous casting billet. The hot rolling includes rough rolling and finish rolling carried out in sequence. The rough rolling process includes rolling the heated continuous casting billet for 7 passes to obtain an intermediate billet with a thickness of 36 mm; the finish rolling includes rolling the intermediate billet for 7 passes. The final rolling temperature of the hot rolling is 880 °C, and the cooling rate of laminar cooling is 10 °C / s to obtain a hot-rolled finished product with a thickness of 5.4 mm;

[0090] Removing the surface scale of the hot-rolled finished product by pickling and performing rewinding, and the coiling temperature is 650 °C;

[0091] Performing cold rolling on the rewound hot-rolled finished product on a reversible cold rolling mill, rolling in 9 passes. After completing one cold rolling process, the total reduction ratio is 78% to obtain a cold-rolled finished product with a thickness of 1.2 mm;

[0092] Performing bell annealing on the cold-rolled finished product, the annealing temperature is 640 °C, and the holding time is 8 hours;

[0093] Slowly cooling the annealed cold-rolled finished product for 20 hours to obtain a low yield ratio cold-rolled steel with a tensile strength of 500 MPa and an elongation of 6%.

[0094] The mechanical properties of the products of Examples 1-2 and Comparative Example 1 were tested according to the method specified in GB / T 228.1-2021. It can be seen from the test results that the cold-rolled steels with a tensile strength of 600 MPa grade and a low yield ratio based on bell annealing treatment in Examples 1 and 2 can both maintain the tensile strength of 600 MPa grade, and have a higher elongation, and can achieve a tensile elongation rate ≥ 15%. While the tensile strength of the cold-rolled steel obtained by the method of conventional cold-rolled steel production in Comparative Example 1 only reaches 500 MPa, and the elongation is only 6%, which is much smaller than the elongation of the cold-rolled steel in this application. It is proved that the tensile strength, ductility, and yield strength ratio performance of the cold-rolled steel with a tensile strength of 600 MPa grade and a low yield ratio based on bell annealing treatment prepared by the preparation method of the cold-rolled steel with a tensile strength of 600 MPa grade and a low yield ratio based on bell annealing treatment in this application are superior to those of the cold-rolled steel obtained by the method of conventional cold-rolled steel production in Comparative Example 1.

[0095] As described above, this is only the specific implementation manner of the present application. Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working process described above can refer to the corresponding process in the foregoing method embodiment, and will not be elaborated herein. It should be understood that the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application.

Claims

1. A preparation method of cold-rolled steel with a yield ratio less than 1.0 and a tensile strength of 600 MPa based on bell annealing treatment, characterized in that, Including: Providing a continuous casting billet, wherein the chemical composition of the continuous casting billet contains the following components by weight percentage: C: 0.10% - 0.12%, Si: 0.75% - 0.85%, Mn: 3.40% - 3.49%, Al: 0.02% - 0.05%, Cr: 0.4% - 0.5%, P ≤ 0.015%, S ≤ 0.015%, and the balance is iron and unavoidable impurities; Heating the continuous casting billet with a thickness of 210 mm - 260 mm to 1180°C - 1220°C, and the heating time is 110 minutes - 130 minutes; Hot rolling the heated continuous casting billet. The hot rolling includes rough rolling and finish rolling carried out in sequence. The rough rolling process includes rolling the heated continuous casting billet for 5 passes - 7 passes to obtain an intermediate billet with a thickness of 25 mm - 40 mm; the finish rolling includes rolling the intermediate billet for 6 passes - 8 passes. The finish rolling temperature of the hot rolling is 870°C - 940°C, and the cooling rate of the laminar cooling is 9°C / s - 12°C / s to obtain a hot rolled finished product with a thickness of 3.0 mm - 6.0 mm; Removing the surface scale of the hot rolled finished product and then rewinding it, and the coiling temperature is 590°C - 620°C; Cold rolling the rewound hot rolled finished product. After completing one cold rolling pass, the total reduction ratio is 75% - 88% to obtain a cold rolled finished product with a thickness of 0.7 mm - 1.5 mm; Carrying out bell annealing on the cold rolled finished product, the annealing temperature is 620°C - 680°C, and the holding time is 6 hours to 12 hours; Slow cooling the annealed cold rolled finished product, the slow cooling time ≥ 20 hours to obtain a cold rolled steel with a yield strength of 600 MPa grade and a low yield ratio based on bell annealing treatment, and the yield ratio of the cold rolled steel is less than 70%; 2. The preparation method of the cold-rolled steel with a yield ratio of less than 0.7 and a tensile strength of 600 MPa based on bell annealing treatment according to claim 1, wherein, Heating the continuous casting billet includes heating the continuous casting billet to 1180°C - 1220°C in a heating furnace.

3. The preparation method of the cold-rolled steel with a yield ratio of less than 0.8 and a tensile strength of 600 MPa based on bell annealing treatment according to claim 1, wherein Removing the surface scale of the hot rolled finished product and then rewinding it includes pickling the surface scale with acid solution and then rewinding it.

4. The preparation method of the cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment according to claim 1, characterized in that, Cold rolling the rewound hot rolled finished product includes rolling the rewound hot rolled finished product for 5 passes - 11 passes to obtain a cold rolled finished product.

5. The preparation method of the cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment according to claim 1, characterized in that, Slow cooling the annealed cold rolled finished product includes slow cooling the annealed cold rolled finished product for 20 hours - 24 hours, or slow cooling for 20 hours - 23 hours.

6. The preparation method of the cold-rolled steel with a tensile strength of 600 MPa and a low yield ratio based on bell annealing treatment according to claim 1, characterized in that Carrying out bell annealing on the cold rolled finished product, the annealing temperature is 640°C - 660°C, and the holding time is 8 hours to 10 hours.

7. A cold-rolled steel with a yield ratio of less than 0.7 and a tensile strength of 600 MPa, which is subjected to bell annealing treatment, characterized in that, Prepared according to the preparation method described in any one of claims 1 - 6, and by mass percentage, it contains the following components: C: 0.10% - 0.12%, Si: 0.75% - 0.85%, Mn: 3.40% - 3.49%, Al: 0.02% - 0.05%, Cr: 0.4% - 0.5%, P ≤ 0.015%, S ≤ 0.015%, and the balance is iron and unavoidable impurities.

8. The cold-rolled steel with a yield ratio of less than 0.7 and a tensile strength of 600 MPa based on bell annealing treatment according to claim 7, characterized in that The thickness of the cold rolled steel is 0.7 mm - 1.5 mm, the tensile strength ≥ 600 MPa, the yield ratio is less than 70%, and the elongation is 15% - 25%.

9. The cold-rolled steel with a yield ratio of less than 0.8 and a tensile strength of 600 MPa based on bell annealing treatment according to claim 7, wherein, The cold rolled steel is in strip or plate shape.

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