A 450MPa grade cold-rolled steel plate based on hood annealing, preparation method and application thereof

Abstract

The present invention provides a 450MPa cold-rolled steel sheet based on hood annealing, a preparation method and an application thereof. The preparation method of the present invention is used to prepare a low-cost 450MPa cold-rolled steel sheet with good yield strength and tensile strength and low yield strength ratio for manufacturing automobile parts, which solves the problems of high cost, large performance fluctuation and high yield strength ratio of cold-rolled steel sheets for manufacturing automobile parts produced by related processes.

Description

Technical Field

[0001] The present invention relates to the technical field of steel heat treatment, and in particular to a 450MPa-grade cold-rolled steel plate based on hood annealing, a preparation method and an application thereof. Background Art

[0002] With the increasing demand in the automotive industry and other application fields, the development and production of high-strength cold-rolled steel sheets has become a hot topic. Since the deformation resistance of steel sheets during cold rolling is already very large, it is difficult to control the shape and dimensional accuracy of the steel sheets. In addition, the requirements of the automotive industry for the welding performance of steel sheets also require that the chemical composition of high-strength steel sheets be maintained at a low carbon equivalent level. Therefore, the development of modern cold-rolled high-strength steel is mainly focused on the development of annealing / hot-dip galvanizing processes after cold rolling, that is, not only the strength of the steel should be controlled by controlling the chemical element composition, but also the strength grade of the steel sheet should be improved by the annealing process after cold rolling.

[0003] The composition of cold-rolled high-strength steel is mainly C, Mn, and Si. Some also add Nb and Ti and other metal composite micro-alloying composition designs and low-carbon composition designs to achieve high strength of the plate and strip through methods such as fine grain and precipitation strengthening. However, the steel plate obtained by the above-mentioned existing preparation method has high cost, large performance fluctuations and high yield strength ratio. In terms of its annealing process, continuous annealing has become the main annealing process in recent years. Although continuous annealing is easier to obtain the target annealing structure so that the steel plate can achieve the target mechanical properties, due to the high cost of continuous annealing equipment, some manufacturers do not have the ability to implement continuous annealing and need other annealing methods for heat treatment, such as traditional hood annealing. However, the performance of the steel plate obtained by hood annealing in the related technology is lower than that of the steel plate obtained by continuous annealing, which does not meet market demand.

[0004] In summary, there is an urgent need for a 450MPa grade cold-rolled steel plate based on hood annealing, a preparation method and an application to solve the above problems. Summary of the invention

[0005] The main purpose of the present invention is to provide a 450MPa grade cold-rolled steel plate based on hood annealing, a preparation method and an application thereof, so as to solve the technical problems of high cost, large performance fluctuation and high yield strength ratio of the steel plate obtained in the prior art.

[0006] To achieve the above-mentioned purpose, the present invention provides a 450MPa grade cold-rolled steel sheet based on hood annealing, wherein the components thereof include, by mass percentage, C: 0.07-0.09%, Si: 0.75-0.85%, Mn: 1.80-2.00%, P<0.012%, S<0.008, Al: 0.02-0.05%, Cr: 0.4-0.5%, and the remainder is Fe and unavoidable impurities, the 450MPa grade cold-rolled steel sheet.

[0007] The thickness of the cold-rolled steel plate is 0.7-1.5 mm, and the thickness of the cold-rolled steel plate is further optimized to be 1.2-1.3 mm.

[0008] The cold-rolled steel sheet has a yield strength of 251 MPa-286 MPa, a tensile strength of 450 MPa-471 MPa, a yield strength ratio of 0.55-0.61, and an elongation of more than 25%.

[0009] The beneficial effects of the present invention are:

[0010] (1) The composition of the 450MPa grade cold-rolled steel sheet provided by the present invention eliminates the commonly used metal composite microalloying components such as Nb and Ti in the related art, and uses Cr as the microalloying component instead. By using appropriate components and corresponding proportions, more pearlite with good plasticity and ferrite and cementite with high strength are formed inside the cold-rolled steel sheet, thereby maintaining the yield strength and tensile strength of the cold-rolled steel sheet and reducing the yield strength ratio. In addition, Cr is cheaper than Nb and Ti, so the cost of the 450MPa grade cold-rolled steel sheet is also reduced.

[0011] (2) The 450MPa grade cold-rolled steel plate provided by the present invention controls the thickness of the steel plate so that the yield strength of the cold-rolled steel plate can be maintained at 251MPa-286MPa and the tensile strength can be maintained at 450MPa-471MPa, so that the provided cold-rolled steel plate has better performance.

[0012] The present invention also provides a method for preparing the above-mentioned 450MPa grade cold-rolled steel sheet, comprising the following steps:

[0013] The ingots of the corresponding components of the above 450MPa grade cold-rolled steel plate are hot-rolled and cold-rolled in sequence to obtain cold-rolled steel;

[0014] The cold-rolled steel is hood-annealed, furnace-cooled to 250-350° C. after annealing, and then air-cooled to room temperature to obtain the 450MPa grade cold-rolled steel sheet; wherein the annealing temperature is 690° C.-730° C., and the holding time is 4-5 hours.

[0015] Preferably, the hot rolling comprises: heating the ingot and sequentially performing rough rolling, finish rolling, laminar cooling and coiling to obtain hot-rolled steel; the ingot has a thickness of 25-40 mm after rough rolling, and the hot-rolled steel has a thickness of 3.0-6.0 mm.

[0016] Preferably, the target heating temperature in the hot rolling step is 1180-1220°C, the heating time is 110-130 min; the finishing rolling temperature is 870°C-940°C, the cooling rate of laminar cooling is 5-15°C / S; and the coiling temperature is 600°C-650°C.

[0017] Preferably, before the cold rolling, the process also includes removing the oxide layer on the surface of the hot-rolled steel; in the cold rolling step: the reduction rate is 70%-77%, and the thickness of the cold-rolled steel is 0.7-1.5 mm.

[0018] Preferably, pickling is used to remove the oxide layer on the surface of the hot-rolled steel; the acid used for pickling includes: one or more of sulfuric acid, hydrochloric acid and hydrofluoric acid.

[0019] Preferably, before performing the hood annealing, an antioxidant is applied on the surface of the cold-rolled steel; the antioxidant includes an anti-oxidation paint.

[0020] The beneficial effects of the present invention are:

[0021] (1) The cold-rolled steel sheet prepared by the preparation method provided by the present invention has good comprehensive performance, the yield strength can be maintained at 251MPa-286MPa, and the tensile strength is maintained at 450MPa-471MPa. By selecting suitable components and corresponding proportions of the ingot, hot rolling and cold rolling are performed, and then hood annealing is performed to obtain a cold-rolled steel sheet having ferrite, cementite and pearlite. The cold-rolled steel sheet obtained by the method has a yield strength of 251MPa-286MPa, a tensile strength of 450MPa-471MPa, a yield strength ratio of 0.55-0.61, and an elongation of>25%.

[0022] (2) The present invention effectively increases the contents of ferrite, cementite and pearlite in the cold-rolled steel sheet by controlling the parameters of hood annealing. When hood annealing is performed at 690°C, cementite fails to precipitate discretely and is distributed in a band in the steel. The grains are fine, so that the yield strength reaches about 380 MPa. When the temperature is further increased, the grains grow. When the holding time reaches 4 hours, cementite precipitates, and a large amount of ferrite and carbide are generated in the cold-rolled steel sheet, further increasing the yield strength. When the temperature reaches 710°C, pearlite is formed, which improves the plasticity of the cold-rolled steel sheet and further improves its tensile strength.

[0023] (3) The annealing process adopted in the present invention is hood annealing. Compared with the existing continuous annealing process, its equipment cost is low. Moreover, for some manufacturers that do not have the conditions for continuous annealing, the cold-rolled steel sheets prepared by the preparation method provided by the present invention have the same performance as the cold-rolled steel sheets obtained by continuous quenching. While reducing costs and production conditions, the performance of the products is guaranteed, which is conducive to promoting the development of the steel industry.

[0024] The present invention also provides an application of the above-mentioned 450MPa cold-rolled steel sheet or the 450MPa cold-rolled steel sheet prepared by the above-mentioned preparation method in the manufacture of automobile parts.

[0025] The beneficial effects of the present invention are: providing a low-cost cold-rolled steel plate with good yield strength and tensile strength and low yield strength ratio for manufacturing automobile parts, solving the problems of high cost, large performance fluctuation and high yield strength ratio of the existing cold-rolled steel plates used for manufacturing automobile parts. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] It should be noted that all directional indications (such as up, down, etc.) in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0028] In addition, in the present invention, the descriptions such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0029] Furthermore, the technical solutions between the various embodiments of the present invention may be combined with each other, but this must be based on the fact that they can be implemented by ordinary technicians in the field. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0030] The present invention provides a 450MPa cold-rolled steel sheet based on hood annealing, wherein the components include by mass percentage: C: 0.07-0.09%, Si: 0.75-0.85%, Mn: 1.80-2.00%, P<0.012%, S<0.008, Al: 0.02-0.05%, Cr: 0.4-0.5%, and the remainder is Fe and unavoidable impurities, and the crystal forms in the 450MPa cold-rolled steel sheet include ferrite, cementite and pearlite. The commonly used metal composite microalloying components such as Nb and Ti in the related art are eliminated from the composition of the 450MPa cold-rolled steel sheet, and Cr is used as the microalloying component instead. By using appropriate components and corresponding proportions, more pearlite with good plasticity and ferrite and cementite with high strength are formed inside the cold-rolled steel sheet, thereby maintaining the yield strength and tensile strength of the cold-rolled steel sheet and reducing the yield strength ratio. In addition, Cr is cheaper than Nb and Ti, thus reducing the cost of 450MPa grade cold-rolled steel sheets.

[0031] In the present invention, the thickness of the cold-rolled steel plate is 0.7-1.5 mm, and the further preferred thickness of the cold-rolled steel plate is 1.2-1.3 mm. By controlling the thickness of the steel plate, the yield strength of the cold-rolled steel plate can be maintained at 251 MPa-286 MPa, and the tensile strength can be maintained at 450 MPa-471 MPa, so that the provided cold-rolled steel plate has better performance.

[0032] In the present invention, the yield strength of the cold-rolled steel sheet is 251MPa-286MPa, the tensile strength is 450MPa-471MPa, the yield strength ratio is 0.55-0.61, and the elongation is >25%.

[0033] The present invention also provides a method for preparing the above-mentioned 450MPa grade cold-rolled steel sheet, comprising the following steps:

[0034] The ingots of the corresponding components of the above-mentioned 450MPa grade cold-rolled steel plate are hot-rolled and cold-rolled in sequence to obtain cold-rolled steel; the ingots with suitable components and corresponding proportions are selected, hot-rolled and cold-rolled, and then hood-annealed to obtain cold-rolled steel plates having ferrite, cementite and pearlite. The cold-rolled steel plates obtained by this method have a yield strength of 251MPa-286MPa, a tensile strength of 450MPa-471MPa, a yield strength ratio of 0.55-0.61, and an elongation of >25%.

[0035] The cold-rolled steel is hood-annealed, and after annealing, it is furnace-cooled to 250-350°C, and then air-cooled to room temperature to obtain the 450MPa grade cold-rolled steel sheet; wherein, the annealing temperature is 690°C-730°C, and the holding time is 4-5h. By controlling the parameters of hood annealing, the contents of ferrite, cementite and pearlite in the cold-rolled steel sheet are effectively increased. When hood annealing is performed at 690°C, cementite fails to precipitate discretely, and is distributed in a band in the steel, and the grains are fine, so that its yield strength reaches about 380MPa. When the temperature is further increased, its grains grow; when the holding time reaches 4h, cementite precipitates, and a large amount of ferrite and carbide are generated in the cold-rolled steel sheet, further increasing the yield strength. When the temperature reaches 710°C, pearlite is formed, which improves the plasticity of the cold-rolled steel sheet, thereby improving its tensile strength.

[0036] And because the annealing process adopted in the present invention is hood annealing, compared with the existing continuous annealing process, its equipment cost is low. For some manufacturers that do not have the conditions for continuous annealing, the cold-rolled steel sheets prepared by the preparation method provided by the present invention have the same performance as the cold-rolled steel sheets obtained by continuous quenching. While reducing costs and production conditions, the performance of the products is guaranteed, which is conducive to promoting the development of the steel industry.

[0037] The hot rolling includes: heating the ingot and then performing rough rolling, finish rolling, laminar cooling and coiling in sequence to obtain hot-rolled steel; the ingot has a thickness of 25-40 mm after rough rolling, and the hot-rolled steel has a thickness of 3.0-6.0 mm. Hot rolling production within this thickness specification range has a good effect of recrystallization structure refinement, which is conducive to obtaining products with uniform mechanical properties.

[0038] The target heating temperature in the hot rolling step is 1180-1220°C, and the heating time is 110-130min; the finishing rolling temperature is 870°C-940°C, and the cooling rate of laminar cooling is 5-15°C / s. By controlling the hot rolling temperature, austenite rolling is achieved under low energy consumption conditions, and the conditions for dynamic recrystallization are achieved; the coiling temperature is 600°C-650°C, and the coiling temperature range affects the content of ferrite, cementite and pearlite in the finished product.

[0039] The method further includes removing an oxide layer on the surface of the hot-rolled steel before the cold rolling.

[0040] In the cold rolling step, the reduction rate is 70%-77%, the thickness of the cold rolled steel is 0.7-1.5 mm, and the cold rolled steel having a thickness similar to that of the finished cold rolled steel plate is obtained by cold rolling. Subsequently, the finished cold rolled steel plate with excellent performance can be obtained by hood annealing.

[0041] Pickling is used to remove the oxide layer on the surface of the hot-rolled steel; the acid used for pickling includes: one or more of sulfuric acid, hydrochloric acid and hydrofluoric acid.

[0042] The method further includes coating an antioxidant on the surface of the cold-rolled steel before performing the hood annealing; the antioxidant includes an anti-oxidation paint.

[0043] An application of the above 450MPa cold-rolled steel sheet or the 450MPa cold-rolled steel sheet prepared by the above preparation method in the manufacture of automobile parts. A low-cost cold-rolled steel sheet with good yield strength and tensile strength and low yield strength ratio is provided for the manufacture of automobile parts, which solves the problems of high cost, large performance fluctuation and high yield strength ratio of the existing cold-rolled steel sheets used for the manufacture of automobile parts.

[0044] For further understanding of the present invention, examples are given below:

[0045] According to the chemical composition and proportion set in Table 1, different ingots are prepared according to the following process flow, with chemical components C, Si, Mn, Cr, Al, and the remainder Fe and inevitable impurities as raw materials, and converter smelting, refining, continuous casting, direct heating or soaking of the ingots are carried out.

[0046] Table 1 Chemical composition (unit: wt%, the remainder is Fe and unavoidable impurities)

[0047] project C Si Mn P S Al Cr Ratio 1 0.08 0.78 1.90 0.0055 0.0024 0.03 0.45 Ratio 2 0.09 0.85 2.00 0.012 0.008 0.05 0.40 Ratio 3 0.07 0.75 1.80 0.0050 0.0020 0.02 0.50 Ratio 4 0.09 0.85 2.00 0.012 0.008 0.05 0.30 Ratio 5 0.07 0.75 1.70 0.0050 0.0020 0.02 0.60

[0048] Example 1: A method for preparing a 450MPa grade cold-rolled steel sheet based on hood annealing comprises the following steps: taking a cast slab having a chemical composition as described in Mix 1 in Table 1 (the thickness of the cast slab is 210 mm), and hot rolling the cast slab:

[0049] Heat to 1220°C in a heating furnace for 118 minutes; perform 7 passes in the rough rolling process and 7 passes in the finish rolling process, hot rolling temperature is 870°C, then perform laminar cooling at a cooling rate of 5-15°C / s, cool to a coiling temperature of 30°C, coil, and the hot rolled steel thickness is 5.5mm.

[0050] Then cold rolling is performed, specifically: the hot rolled steel is pickled (the acid used for pickling is hydrochloric acid) to remove the oxide scale and then cold rolled on a cold rolling mill with a reduction rate of 76% and a target thickness of 1.3 mm (i.e., the thickness of the cold rolled steel plate).

[0051] Finally, hood annealing is carried out, specifically: anti-oxidation paint (QY600 anti-oxidation paint) is applied on the surface of the cold-rolled steel, and then hood annealing is carried out. Nitrogen protection is used during annealing, the annealing temperature is 730°C, the insulation time is 4h, and after annealing, it is cooled to 250-350°C with the furnace under the acceleration of cooling water, and then air-cooled to about 90°C with the hood, and air-cooled to room temperature after being taken out of the furnace. After annealing, it is cooled to 300°C with the furnace, and then air-cooled to room temperature.

[0052] Example 2: The difference from Example 1 is that the annealing temperature is 710°C and the holding time is 5 hours.

[0053] Example 3: The difference from Example 1 is that the annealing temperature is 690°C and the holding time is 5 hours.

[0054] Example 4: The difference from Example 1 is that a casting billet with the chemical composition described in Mixture 2 in Table 1 is taken.

[0055] Example 5: The difference from Example 1 is that a casting billet with the chemical composition described in Ratio 3 in Table 1 is taken.

[0056] Example 6: The difference from Example 1 is that the thickness of the cold-rolled steel plate is 1.2 mm.

[0057] Example 7: The difference from Example 1 is that the thickness of the cold-rolled steel plate is 0.7 mm.

[0058] Example 8: The difference from Example 1 is that the thickness of the cold-rolled steel plate is 1.5 mm.

[0059] Comparative Example 1: The difference from Example 1 is that the annealing temperature is 800°C and the holding time is 6 hours.

[0060] Comparative Example 2: The difference from Example 1 is that the annealing temperature is 600°C and the holding time is 3 hours.

[0061] Comparative Example 3: The difference from Example 1 is that a casting billet with the chemical composition described in Mixture 4 in Table 1 is taken.

[0062] Comparative Example 4: The difference from Example 1 is that a casting billet with the chemical composition described in Mixture 5 in Table 1 is taken.

[0063] Comparative Example 5: The difference from Example 1 is that the thickness of the cold-rolled steel plate is 0.5 mm.

[0064] Comparative Example 6: The difference from Example 1 is that the thickness of the cold-rolled steel plate is 1.7 mm.

[0065] Comparative Example 7: The difference from Example 1 is that Cr in the ratio 1 in Table 1 is replaced by Nb.

[0066] Comparative Example 8: The difference from Example 1 is that Cr in the ratio 1 in Table 1 is replaced by Ti.

[0067] Comparative Example 9: The difference from Example 1 is that the hood annealing is replaced by the continuous annealing in the related art.

[0068] The cold-rolled steel sheets obtained in Examples 1-8 and Comparative Examples 1-9 were subjected to yield strength tests and tensile strength tests according to the test methods specified in GB / T 228.1-2020. The results are shown in Table 2:

[0069] Table 2 Performance test results of Examples 1-8 and Comparative Examples 1-9

[0070]

[0071]

[0072] It can be seen from Examples 1-8 that the cold-rolled steel sheet prepared by the present invention has a yield strength of 251 MPa-286 MPa, a tensile strength of 450 MPa-471 MPa, a yield strength ratio of 0.55-0.61, and an elongation of >25%.

[0073] It can be seen from Examples 1-3 and Comparative Examples 1-2 that when the hood annealing temperature is controlled to be 690°C-730°C, the yield strength of the cold-rolled steel sheet is 253-261 MPa, the tensile strength is 455-468 MPa, and the yield strength ratio is 0.55-0.56.

[0074] When the annealing temperature is higher than 730°C, due to the high temperature, more pearlite is generated in the cold-rolled steel sheet, while the relative contents of ferrite and cementite decrease, resulting in a decrease in both the yield strength and the tensile strength of the obtained cold-rolled steel sheet. For example, as shown in the performance effect of Comparative Example 1, the yield strength is 232 MPa, the tensile strength is 442 MPa, and the yield strength ratio is 0.52, both of which are much lower than those of Examples 1-3.

[0075] When the annealing temperature is lower than 690°C, such as in Comparative Example 2, although the yield strength and tensile strength are better, the early cold rolling causes the previously formed grains to break, and the later temperature is relatively low, so that the cold-rolled steel sheet cannot be recrystallized, resulting in uneven product performance at this temperature, incomplete annealing, and failure to meet factory requirements. In addition, since the yield strength of the cold-rolled steel sheet in Comparative Example 2 is too high, it is difficult to process and shape it for use in manufacturing automotive parts.

[0076] It can be seen from Examples 1, 4, 5 and Comparative Examples 3-4 that the ratio of the chemical components of the cold-rolled steel sheet in the present invention is crucial. In particular, by controlling the content of Cr in the entire cold-rolled steel sheet, a cold-rolled steel sheet with a yield strength of 253MPa-286MPa, a tensile strength of 455MPa-471MPa, and a yield strength ratio of 0.55-0.61 can be obtained.

[0077] When the Cr content in the cold-rolled steel sheet is too high, for example, the cold-rolled steel sheet obtained in Comparative Example 4 has excessive plasticity due to the excessive Cr content. In the performance test, its yield strength is 245 MPa and its tensile strength is 434 MPa, which are lower than the cold-rolled steel sheets provided in Examples 1, 4, and 5.

[0078] When the Cr content in the cold-rolled steel sheet is too low, such as the cold-rolled steel sheet obtained in Comparative Example 3, due to the low Cr content, although the yield strength and tensile strength are good, the obtained cold-rolled steel sheet is easy to rust, and the yield strength is too high, making it difficult to process and shape and use it in manufacturing automotive parts.

[0079] It can be seen from Example 1, Examples 6-8 and Comparative Examples 5-6 that controlling the appropriate thickness of the cold-rolled steel plate can effectively control the yield strength and tensile strength, so that the cold-rolled steel plate can be better used for manufacturing automotive parts; when the thickness of the cold-rolled steel plate is too thin, its yield strength and tensile strength are low, and it cannot meet the performance requirements of 450MPa-grade cold-rolled steel plates, and cannot be used for manufacturing automotive parts; and when the thickness of the cold-rolled steel plate is too thick, its yield strength and tensile strength increase significantly, resulting in processing difficulties, and it is difficult to process and shape and use it to manufacture automotive parts.

[0080] It can be seen from Example 1 and Comparative Example 7 that when Cr in the cold-rolled steel sheet of the present application is replaced by Nb, although the yield strength is increased, the tensile strength decreases significantly, and the yield strength ratio is larger. The cold-rolled steel sheet obtained by Example 1 has better plasticity than that of Comparative Example 7, and can be better applied to the manufacture of automotive parts, indicating that the present application adopts Cr as the microalloying component of the cold-rolled steel sheet, which can reduce the production cost while ensuring the performance of the cold-rolled steel sheet.

[0081] It can be seen from Example 1 and Comparative Example 8 that replacing Cr in the cold-rolled steel sheet of the present application with Ti can significantly improve both the yield strength and the tensile strength, and using Cr to replace Ti reduces the production cost of the cold-rolled steel sheet.

[0082] It can be seen from Example 1 and Comparative Example 9 that the cold-rolled steel sheet obtained by the hood annealing in the present application has a lower yield strength than the cold-rolled steel sheet obtained by the continuous annealing in the related art, but is relatively close in tensile strength. The yield strength ratio of the cold-rolled steel sheet obtained by the hood annealing in the present application is lower than that of the cold-rolled steel sheet obtained by the continuous annealing in the related art, and the cold-rolled steel sheet has better plasticity, can be better processed and formed, and used to manufacture automotive parts.

[0083] The above technical solutions of the present invention are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. All equivalent structural changes made using the contents of the present invention specification under the technical concept of the present invention, or directly / indirectly applied in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A 450MPa grade cold-rolled steel sheet based on bell annealing, characterized in that: Its components include, by mass percentage: C: 0.07-0.09%, Si: 0.75-0.85%, Mn: 1.80-2.00%, P<0.012%, S<0.008, Al: 0.02-0.05%, Cr: 0.4-0.5%, the remainder is Fe and unavoidable impurities, and the crystal forms in the 450MPa grade cold-rolled steel sheet include ferrite, cementite and pearlite; Wherein, the thickness of the cold-rolled steel plate is 0.7-1.5 mm; The method for preparing the 450MPa grade cold-rolled steel sheet comprises the following steps: The ingots of the corresponding components of the above-mentioned 450MPa grade cold-rolled steel plate are hot-rolled and cold-rolled in sequence to obtain cold-rolled steel; The cold-rolled steel is hood-annealed, furnace-cooled to 250-350° C. after annealing, and then air-cooled to room temperature to obtain the 450MPa grade cold-rolled steel sheet; wherein the annealing temperature is 690° C.-730° C., and the holding time is 4-5 hours.

2. The 450MPa grade cold-rolled steel sheet according to claim 1, characterized in that: The cold-rolled steel sheet has a yield strength of 251 MPa-286 MPa, a tensile strength of 450 MPa-471 MPa, a yield strength ratio of 0.55-0.61, and an elongation of more than 25%.

3. The 450MPa grade cold-rolled steel sheet according to claim 1, characterized in that: The hot rolling comprises: heating the ingot and then sequentially performing rough rolling, finish rolling, laminar cooling and coiling to obtain hot-rolled steel; the ingot has a thickness of 25-40 mm after rough rolling, and the hot-rolled steel has a thickness of 3.0-6.0 mm.

4. The 450MPa grade cold-rolled steel sheet according to claim 3, characterized in that: The target heating temperature in the hot rolling step is 1180-1220°C, and the heating time is 110-130min; the finishing rolling temperature is 870°C-940°C, and the cooling rate of laminar cooling is 5-15°C / S; and the coiling temperature is 600°C-650°C.

5. The 450MPa grade cold-rolled steel sheet according to claim 1, characterized in that: The method further includes removing an oxide layer on the surface of the hot-rolled steel before cold rolling; In the cold rolling step, the reduction ratio is 70%-77%, and the thickness of the cold rolled steel is 0.7-1.5 mm.

6. The 450MPa grade cold-rolled steel sheet according to claim 5, characterized in that: Pickling is used to remove the oxide layer on the surface of the hot-rolled steel; the acid used for pickling includes: one or more of sulfuric acid, hydrochloric acid and hydrofluoric acid.

7. The 450MPa grade cold-rolled steel sheet according to claim 1, characterized in that: In the method for preparing the 450MPa grade cold-rolled steel sheet, the method further comprises coating an antioxidant on the surface of the cold-rolled steel before performing the hood annealing; the antioxidant comprises an anti-oxidation paint.

8. Application of the 450 MPa grade cold-rolled steel sheet according to any one of claims 1 to 7 in the manufacture of automobile parts.

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