A steel for a truck bed and a method of making the same

Abstract

The application relates to the technical field of steel smelting and rolling, in particular to a steel for a carriage plate and a preparation method thereof. The method comprises the following steps: continuously casting molten steel with a set chemical composition, and controlling a pulling speed of the continuous casting to obtain a slab; heating the slab; stage rolling the heated slab, then cooling and coiling after cooling to obtain a hot-rolled coil; pickling the hot-rolled coil, then cold rolling, and controlling a reduction rate of the cold rolling to obtain a cold-rolled coil; semi-annealing the cold-rolled coil, controlling process parameters of the semi-annealing, then flattening to obtain the steel for the carriage plate. The application solves the technical problem that the existing steel for the carriage plate is difficult to simultaneously consider high strength and low cost.

Description

Technical Field

[0001] This application relates to the field of steel smelting and rolling technology, and in particular to a steel for car body panels and its preparation method. Background Technology

[0002] Currently, many domestic manufacturers of light and small trucks primarily use cold-rolled annealed steel sheets such as SPCC and DC01 for their truck bodies. These sheets have a yield strength below 300MPa and a thickness mainly between 0.8-2.5mm. The low strength of these products makes them unsuitable for lightweighting requirements. Some advanced manufacturers, however, are using 550MPa grade products for lightweighting light and small trucks, such as low-alloy high-strength steel 550LA or duplex steel 550 and 780DP.

[0003] Currently, the high alloy content and long production process result in high overall costs. The structure of a light truck body mainly consists of corrugated sheets and transverse and longitudinal beams, processed by methods such as roll forming and stamping, requiring only a 90-degree cold forming capability to meet its forming requirements. The body panels and beams are then connected by arc welding or electric welding, demanding high welding performance. Summary of the Invention

[0004] This application provides a steel for vehicle body panels and a method for preparing the same, in order to solve the technical problem that existing steel for vehicle body panels cannot simultaneously achieve high strength and low cost.

[0005] In a first aspect, this application provides a method for preparing steel for a vehicle body panel, the method comprising:

[0006] molten steel with a set chemical composition is continuously cast, and the casting speed is controlled to obtain a slab.

[0007] The slab is heated;

[0008] The heated slab is rolled in stages, then cooled and coiled to obtain a hot-rolled coil.

[0009] The hot-rolled coil is pickled, then cold-rolled, and the reduction rate of the cold rolling is controlled to obtain a cold-rolled coil.

[0010] The cold-rolled coil is semi-annealed, and the process parameters of the semi-annealing are controlled. Then it is leveled to obtain steel for the car body panel.

[0011] Optionally, the semi-annealing process parameters include: hot spot temperature of 540-580℃, cold spot temperature of 500-540℃, and semi-annealing time of 8-10h.

[0012] Optionally, the cold rolling reduction rate is 40-50%.

[0013] Optionally, the casting speed of the continuous casting is 5.5-6.5 m / min.

[0014] Optionally, the specified chemical composition includes: C, Si, Mn, P, S, Al, Nb, and Fe; wherein,

[0015] The C content is 0.030-0.045 wt%, the Si content is ≤0.05 wt%, the Mn content is 0.30-0.90 wt%, the P content is ≤0.010 wt%, the S content is ≤0.003 wt%, the Al content is 0.02-0.05 wt%, and the Nb content is 0.025-0.050 wt%.

[0016] Optionally, the step of performing staged rolling, followed by cooling and coiling of the heated slab to obtain a hot-rolled coil includes:

[0017] The heated slab is rough rolled, and the exit temperature of the rough rolling mill is controlled to obtain an intermediate slab;

[0018] Under a set temperature condition, the intermediate billet is induction heated and then rolled to obtain a hot-rolled plate;

[0019] The hot-rolled plate is subjected to laminar flow cooling and then coiled after cooling, and the coiling temperature is controlled to obtain a hot-rolled coil.

[0020] Optionally, the exit temperature of the roughing mill is ≥960℃, the set temperature is 1040-1090℃, and the winding temperature is 600-650℃.

[0021] Optionally, the heating temperature is 1100-1140℃.

[0022] Optionally, the elongation of the flattened surface is ≤0.2%.

[0023] Secondly, this application provides a steel for a vehicle body panel, which is prepared by the method described in any embodiment of the first aspect.

[0024] The technical solutions provided in this application have the following advantages compared with the prior art:

[0025] The method for preparing the steel for the carriage body provided in this application adopts a low-C-low-Mn-low-Si+Nb microalloying composition system. Its purpose is to: ensure a certain strength in the base material by controlling the elemental content of C, Mn, and Nb, with minimal strength fluctuations; control the semi-annealing process parameters to retain some of the work hardening effect from the cold rolling stage while eliminating large residual stresses caused by significant cold rolling deformation; control the continuous casting speed to ensure uniformity of the billet structure, eliminate banded structures, reduce segregation, and improve plasticity and toughness; and control the cold... The purpose of cold rolling reduction is to increase the proportion of dislocation strengthening by controlling the cold rolling reduction rate, while controlling the degree of fiberization of the microstructure to ensure the plasticity matching of the finished product; and combined with hot rolling process, to produce 0.8mm~1.6mm strip steel for car body panels with excellent cold forming and welding performance, yield strength of over 650MPa, tensile strength of over 700MPa, elongation A50 of over 12%, meeting the requirement of not cracking when bending 180°, and meeting the requirements of roll forming and stamping to manufacture corrugated sheets, rectangular tubes and other parts, thus solving the technical problem that existing car body panel steel cannot simultaneously achieve high strength and low cost. Attached Figure Description

[0026] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0027] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A schematic flowchart illustrating a method for preparing steel for a vehicle body panel, provided in an embodiment of this application;

[0029] Figure 2 Metallographic diagram of steel for a car body panel provided in an embodiment of this application. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0031] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0032] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0033] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0034] Firstly, this application provides a method for preparing steel for carriage panels; please refer to [link to relevant documentation]. Figure 1 The method includes:

[0035] S1. Continuously cast molten steel with a set chemical composition and control the casting speed to obtain a slab;

[0036] S2. Heat the slab;

[0037] S3. The heated slab is rolled in stages, then cooled and coiled after cooling to obtain a hot-rolled coil.

[0038] S4. Pickle the hot-rolled coil, then cold-roll it, and control the reduction rate of the cold rolling to obtain a cold-rolled coil;

[0039] S5. The cold-rolled coil is semi-annealed, and the process parameters of the semi-annealing are controlled. Then it is leveled to obtain steel for the carriage panel.

[0040] The method for preparing the steel for the carriage body provided in this application adopts a low-C-low-Mn-low-Si+Nb microalloying composition system. Its purpose is to: ensure a certain strength in the base material by controlling the elemental content of C, Mn, and Nb, and to minimize the strength fluctuation of the base material; control the semi-annealing process parameters to retain some of the work hardening effect from the cold rolling stage while eliminating the large residual stress caused by the large cold rolling deformation; control the continuous casting speed to ensure the uniformity of the billet structure, eliminate banded structures, and reduce segregation, which is beneficial for improving plasticity and toughness; and control the cold rolling reduction rate to increase the proportion of dislocation strengthening while controlling the degree of fibrousness in the structure to ensure the plasticity matching of the finished product.

[0041] In some embodiments, the semi-annealing process parameters include: hot spot temperature of 540-580℃, cold spot temperature of 500-540℃, and semi-annealing time of 8-10h.

[0042] "Hot spot temperature" refers to the temperature of the outer ring of the steel coil during uncoiling. Controlling the hot spot temperature to 540-580℃ has the following positive effects: ensuring microstructure recovery, preventing recrystallization, and eliminating residual stress. If the hot spot temperature is too high, recrystallization may occur to some extent, significantly reducing the strength of the finished product; if the hot spot temperature is too low, it may be difficult to guarantee a product with excellent plasticity. Specifically, the hot spot temperature can be 540℃, 550℃, 560℃, 570℃, 580℃, etc.

[0043] "Cold point temperature" refers to the temperature of the inner coil of the steel coil during unwinding. Controlling the cold point temperature to 500-540℃ has the following positive effects: ensuring microstructure recovery, preventing recrystallization, and eliminating residual stress. If this cold point temperature is too high, it may lead to excessively long holding times, increasing costs; if it is too low, it may be difficult to guarantee a product with excellent plasticity. Specifically, this cold point temperature can be 500℃, 510℃, 520℃, 530℃, 540℃, etc.

[0044] The positive effects of controlling the semi-annealing time to 8-10 hours include: ensuring sufficient time for recovery and eliminating some dislocation structures. If the semi-annealing time is too long, it can lead to excessive dislocation elimination, resulting in insufficient strength; if the semi-annealing time is too short, it may be difficult to guarantee a product with excellent plasticity. Specifically, the semi-annealing time can be 8 hours, 9 hours, 10 hours, etc.

[0045] In some embodiments, the cold rolling reduction rate is 40-50%.

[0046] The positive effects of controlling the cold rolling reduction rate to 40-50% include: controlling the microstructure deformation by controlling the cold rolling reduction rate, generating a large number of dislocation structures, and improving strength. If the cold rolling reduction rate is too high, it will lead to severe fibrosis of the microstructure and significant loss of plasticity; if the cold rolling reduction rate is too low, it will be difficult to guarantee high strength. Specifically, the cold rolling reduction rate can be 40%, 44%, 48%, 50%, etc.

[0047] In some embodiments, the casting speed is 5.5-6.5 m / min.

[0048] The positive effects of controlling the casting speed to 5.5-6.5 m / min include: achieving uniform billet quality through high casting speed. If the casting speed is too high, it may exceed the equipment's limits, leading to production instability; if the casting speed is too low, it may increase billet segregation. Specifically, the casting speed can be 5.5 m / min, 6.0 m / min, 6.5 m / min, etc.

[0049] In some embodiments, the specified chemical composition includes: C, Si, Mn, P, S, Al, Nb, and Fe;

[0050] The composition of C is 0.030-0.045 wt%, Si is ≤0.05 wt%, Mn is 0.30-0.90 wt%, P is ≤0.010 wt%, S is ≤0.003 wt%, Al is 0.02-0.05 wt%, and Nb is 0.025-0.050 wt%.

[0051] The positive effects of controlling the carbon content to 0.030-0.045 wt%: Carbon is the most economical solid solution strengthening element in steel. However, if the carbon content is too high, the weldability and formability of the steel will deteriorate, especially for cold rolling + semi-annealing processes, where carbon has a greater impact on cold formability. Since this product requires good formability, low carbon content control is recommended. Furthermore, in this embodiment, the hot-rolled raw material is produced through a continuous casting and rolling process, and the carbon content needs to avoid the peritectic region to prevent defects such as billet cracks; generally, it is required to be below 0.05 wt%. On the other hand, if the carbon content is too low, its contribution to strength is small, especially with the addition of microalloying elements, which makes it difficult to form second-phase precipitates, affecting strength improvement. Specifically, the carbon content can be 0.030 wt%, 0.035 wt%, 0.040 wt%, 0.045 wt%, etc.

[0052] The positive effects of controlling the Si content to ≤0.05 wt%: Si is also a solid solution strengthening element. However, a high Si content easily leads to the formation of strip-shaped iron oxide scale that is pressed in, which is difficult to remove during the pickling stage and affects the surface quality of the sheet. Specifically, the Si content can be 0.05 wt%, 0.045 wt%, 0.04 wt%, etc.

[0053] The positive effects of controlling the Mn content to 0.30-0.90 wt%: Mn is also a solid solution strengthening element. Adding a certain amount of Mn not only strengthens the matrix through solid solution but also stabilizes austenite, refines ferrite, and increases the contribution of fine grain strengthening. Therefore, the Mn content cannot be too low. However, the Mn content should not be too high either, because this invention requires strict control of the cold rolling compression ratio of the raw material, resulting in a thinner finished product. Therefore, the hot-rolled coil is also thinner, requiring higher rolling stability. If the Mn content is high, it will not only increase costs but also increase the deformation resistance during rolling, which is detrimental to rolling stability. Specifically, the Mn content can be 0.30 wt%, 0.40 wt%, 0.50 wt%, 0.60 wt%, 0.70 wt%, 0.80 wt%, 0.90 wt%, etc.

[0054] The positive effects of controlling the P content to ≤0.010 wt% and the S content to ≤0.003 wt% are as follows: P and S are impurity elements in steel. P easily causes center segregation in steel, worsening its weldability and ductility; S readily forms MnS inclusions with Mn, reducing toughness. Specifically, the P content can be 0.010 wt%, 0.009 wt%, 0.0095 wt%, etc., and the S content can be 0.003 wt%, 0.0025 wt%, 0.002 wt%, etc.

[0055] The positive effects of controlling the Al content to 0.02-0.05 wt%: Al is mainly added to steel for deoxidation. Incomplete deoxidation will lead to a decrease in the cold formability of the material. To meet the formability requirements of steel sheets, the Al content should be ≥0.02 wt%. However, excessive Al content will result in too many AlN inclusions in the steel, reducing the elongation of the material. Specifically, the Al content can be 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, etc.

[0056] The positive effects of controlling the Nb content to 0.025-0.050 wt% include: Adding Nb to steel can expand the non-recrystallized austenite region, increase the overall reduction in the finishing rolling section, retain more deformation defects and increase the nucleation rate, resulting in significant grain refinement strengthening. Simultaneously, it combines with C and N to form second-phase precipitates, providing precipitation strengthening and significantly contributing to strength. However, because Nb inhibits recrystallization, it increases deformation resistance during rolling, making it less effective for controlling the rolling stability of thin-gauge products. Specifically, the Nb content can be 0.025 wt%, 0.030 wt%, 0.035 wt%, 0.040 wt%, 0.045 wt%, 0.050 wt%, etc.

[0057] In some embodiments, the step of performing staged rolling on the heated slab, followed by cooling and cold coiling to obtain a hot-rolled coil includes:

[0058] The heated slab is rough rolled, and the exit temperature of the rough rolling mill is controlled to obtain an intermediate slab;

[0059] Under a set temperature condition, the intermediate billet is induction heated and then rolled to obtain a hot-rolled plate;

[0060] The hot-rolled plate is subjected to laminar flow cooling and then coiled after cooling, and the coiling temperature is controlled to obtain a hot-rolled coil.

[0061] In some embodiments, the exit temperature of the roughing mill is ≥960°C, the set temperature is 1040-1090°C, and the winding temperature is 600-650°C.

[0062] The positive effect of controlling the exit temperature of the roughing mill to ≥960℃ is that it ensures sufficient recrystallization during the roughing stage. Specifically, the exit temperature of the roughing mill can be 960℃, 970℃, 980℃, etc.

[0063] "Set temperature" refers to the temperature of the aforementioned induction heating. The positive effects of controlling the induction heating temperature to 1040-1090℃ are: ensuring rolling stability during the finishing rolling stage and ensuring the dissolution of the microalloying element Nb. Specifically, the induction heating temperature can be 1040℃, 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, etc.

[0064] The positive effect of controlling the winding temperature at 600-650℃ is that it ensures the microstructure is ferrite of appropriate size. Specifically, the winding temperature can be 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, etc.

[0065] In some embodiments, the heating temperature is 1100-1140°C.

[0066] "Heating" refers to heating in a furnace. The positive effect of controlling the heating temperature at 1100-1140℃ is to ensure the uniformity of the billet structure. Specifically, the heating temperature can be 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, etc.

[0067] In some embodiments, the elongation of the flattened surface is ≤0.2%.

[0068] The positive effect of controlling the elongation of the flatness to ≤0.2% is to ensure good board shape quality of the finished product. Specifically, the elongation of the flatness can be 0.2%, 0.15%, 0.1%, etc.

[0069] Secondly, this application provides a type of steel for a car body panel; please refer to [link to relevant documentation]. Figure 2 The metallographic diagram shown is of the steel used for the carriage panel, which is prepared by the method described in any embodiment of the first aspect.

[0070] The steel used for the carriage body panel is based on the above-described preparation method for the steel used for the carriage body panel. The specific steps of the preparation method for the steel used for the carriage body panel can be referred to the above embodiments. Since the steel used for the carriage body panel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0071] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed. Specific preparation process:

[0072] The molten iron from the multi-mode thin slab continuous casting and rolling production line is processed through KR desulfurization, converter smelting, LF refining and VD refining to obtain steel for car body panels.

[0073] Continuous casting: The above-mentioned molten steel is continuously cast into slabs, wherein the casting speed is 5.5-6.5 m / min and the slab thickness is 110-125 mm;

[0074] Heating: Heating is carried out in a roller hearth furnace at a temperature of 1100-1140℃;

[0075] Continuous rolling: Rough descaling removes the iron oxide scale from the surface of the heated slab using descaling water at a pressure of ≥30MPa; rough rolling is performed in 3 passes, with an inlet temperature of ≥1080℃ and an outlet temperature of ≥960℃; electromagnetic induction heating is used to heat the intermediate slab produced from rough rolling, with the induction heating temperature controlled at 1040-1090℃; the intermediate slab is rolled to the finished thickness in 5 passes; then laminar flow cooling is used to cool the strip to the coiling temperature, with the cooling rate controlled at 5-10℃ / s and the coiling temperature controlled at 600-650℃; finally, the steel coil is air-cooled to room temperature.

[0076] Pickling and rolling: The iron oxide scale on the surface of the strip is removed by pickling, and then the strip is rolled to 0.8-1.6mm by cold rolling, with the cold rolling reduction rate controlled between 40-50%.

[0077] Semi-annealing: A bell-type annealing furnace is used, with a hot spot temperature of 540-580℃, a cold spot temperature of 500-540℃, and a semi-annealing time of 8-10 hours.

[0078] Leveling: In the leveling process, the leveling elongation rate is ≤0.2%.

[0079] Table 1 lists the chemical composition of the steel used for the truck body panels, Table 2 lists the manufacturing process parameters of the steel used for the truck body panels, and Table 3 lists the performance results of the steel used for the truck body panels.

[0080] Table 1 shows the chemical composition (wt%) of the steel used for the carriage panels, with the remainder being Fe and other unavoidable impurities.

[0081] Serial Number C Si Mn P S Alt Nb Example 1 0.030 0.01 0.90 0.0020 0.0020 0.035 0.040 Example 2 0.035 0.02 0.60 0.0015 0.0025 0.030 0.035 Example 3 0.040 0.03 0.40 0.0015 0.0020 0.035 0.030 Example 4 0.045 0.05 0.70 0.0030 0.0015 0.025 0.025 Comparative Example 1 0.07 0.20 1.5 0.008 0.006 0.035 0.015

[0082] Table 2. Manufacturing process parameters for steel used in carriage panels

[0083]

[0084]

[0085] Table 3 Performance results of steel used for carriage panels

[0086] Serial Number Finished product thickness / mm Rp0.2 / MPa Rm / MPa <![CDATA[A 80 / %]]> 180°C cold (d=1a) Example 1 1.5 658 742 13.8 qualified Example 2 1.2 657 738 14.0 qualified Example 3 1.0 652 745 15.0 qualified Example 4 0.8 668 760 12.5 qualified Comparative Example 1 1.2 660 750 5.5 cracking

[0087] As shown in Tables 1-3, the yield strength and tensile strength of the steel used for the truck body panels in this application embodiment are greater than 700 MPa; the elongation is greater than or equal to 12.0%, and the 180°d=a cold bending test is qualified. This steel strip has excellent plate shape and surface quality, and is suitable for the processing and manufacturing of light truck body panels. This truck body panel has high strength, light weight, and good wear resistance. In contrast, the comparative example did not adopt the scheme of this application embodiment, and the steel used for the truck body panels produced was of poorer performance.

[0088] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing steel for a vehicle body panel, characterized in that, The method includes: molten steel with a set chemical composition is continuously cast, and the casting speed is controlled to obtain a slab; The slab is heated; The heated slab is rolled in stages, then cooled and coiled to obtain a hot-rolled coil. The hot-rolled coil is pickled, then cold-rolled, and the reduction rate of the cold rolling is controlled to obtain a cold-rolled coil. The cold-rolled coil is semi-annealed, and the process parameters of the semi-annealing are controlled. Then it is leveled to obtain steel for the car body panel. The semi-annealing process parameters include: The hot spot temperature is 540-580℃, the cold spot temperature is 500-540℃, and the semi-annealing time is 8-10 hours. The reduction rate of the cold rolling is 40-50%; The casting speed is 5.5-6.5 m / min; The heating temperature is 1100-1140℃; The elongation rate of the flattened surface is ≤0.2%; The process of rolling the heated slab in stages, followed by cooling and coiling to obtain a hot-rolled coil includes: The heated slab is rough rolled, and the exit temperature of the rough rolling mill is controlled to obtain an intermediate slab; Under a set temperature condition, the intermediate billet is induction heated and then rolled to obtain a hot-rolled plate; The hot-rolled sheet is subjected to laminar flow cooling and then coiled after cooling, and the coiling temperature is controlled to obtain a hot-rolled coil. The exit temperature of the roughing mill is ≥960℃, the set temperature is 1040-1090℃, and the winding temperature is 600-650℃; The specified chemical composition is: C, Si, Mn, P, S, Al, Nb, and Fe; among which, The C content is 0.030-0.045 wt%, the Si content is ≤0.05 wt%, and the Mn content is 0.30-0.90%. The content of P is ≤0.010% by weight, the content of S is ≤0.003% by weight, the content of Al is 0.02-0.05% by weight, and the content of Nb is 0.025-0.050% by weight.

2. A type of steel for a vehicle body panel, characterized in that, The steel is prepared by the method of claim 1.

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