750MPa-grade automobile beam steel plate with uniform ferritic structure and preparation method of 750MPa-grade automobile beam steel plate

Abstract

The invention discloses a 750MPa-grade automobile beam steel plate with a uniform ferritic structure and a preparation method, and belongs to the technical field of hot continuous rolling plate strip production, the 750MPa-grade automobile beam steel plate comprises the following chemical components in percentage by weight: 0.045-0.060% of C, 0.05-0.10% of Si, 1.60-1.70% of Mn, less than or equal to 0.010% of P, less than or equal to 0.0040% of S, 0.030-0.040% of Nb, 0.070-0.085% of Ti, 0.020-0.040% of Als, less than or equal to 0.0040% of N, and the balance of Fe and inevitable impurities. According to the 750MPa-grade automobile beam steel plate with the uniform ferritic structure and the preparation method, the problems that in the prior art, novel beam steel meeting the requirements of automobile beam steel is high in cost, poor in structure uniformity and large in grain size difference, and consequently subsequent steel plate shearing and stamping cracking is caused can be effectively solved.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled strip production technology. Specifically, it relates to a 750MPa grade automotive beam steel plate with a uniform ferrite structure and its preparation method. Background Technology

[0002] In recent years, the application of high-strength lightweight automotive steel has become increasingly widespread, greatly promoting the low-carbon, energy-saving, and emission-reduction development process of the automotive manufacturing industry. For gasoline vehicles, every 10% reduction in vehicle weight can save 3% to 7% of crude oil; therefore, thinner steel plates should be used while maintaining or even increasing strength. As a key structural component of automobiles, automotive beam steel is mainly used to manufacture longitudinal and transverse beams, and its quality and performance requirements are extremely stringent. With the continuous advancement and development of high-strength lightweight automotive steel, significant progress has been made in material innovation and strength upgrades for beam steel. However, with the rapid pace of product updates, many steel mills often neglect the stability of steel performance while developing and upgrading new products. Various problems arise in the subsequent processing of automotive beam steel plates of different strength grades, such as side cracking, shear delamination, stamping cracking, and tensile delamination, failing to meet relevant processing requirements. The main reason is the uneven microstructure of the beam steel plate, such as differences in grain size or banded structures along the thickness direction, leading to failure behaviors during processing and application. Therefore, under the requirements of green development, it is necessary to develop a new type of beam steel that is low-cost, has excellent comprehensive performance, and meets the requirements of automotive beam steel.

[0003] Chinese patent application publication number CN110846564A discloses a low-cost, high-strength beam steel 750L and its production method. The chemical composition and mass percentage of the steel are as follows: C: 0.065-0.085%, Si: 0.05-0.15%, Mn: 1.30-1.70%, P≤0.015%, S≤0.004%, Nb: 0.035-0.050%, Ti: 0.080-0.100%, Als: 0.015-0.035%, V: 0.040-0.050%, B: 0-0.0020%, N≤0.005%, with the balance being Fe and within permissible ranges. The invention involves adding a large amount of microalloying elements such as Nb, V, Ti, and B to steel to prepare 750L steel, resulting in high production costs. Chinese patent application CN118028692A discloses a 750MPa tensile strength grade commercial vehicle beam steel and its production method. This patent uses Nb and Ti composite microalloying to design the composition of the beam steel to prepare 10-12mm thick hot-rolled plates. The addition amounts (mass percentages) of Nb and Ti are 0.05%-0.06% and 0.085%-0.115%, respectively. The matrix structure is ferrite with a small amount of pearlite, and the ferrite grain size is 11-12. However, the excessive addition of Nb increases the cost, and the provided metallographic structure shows poor uniformity and large grain size differences in the beam steel, which will inevitably lead to subsequent shearing and stamping cracking of the steel plate. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned shortcomings by providing a 750MPa grade automotive beam steel plate with a uniform ferrite microstructure and its preparation method. This solves the problems of high cost, poor microstructure uniformity, and large grain size differences in existing automotive beam steels that meet the requirements, leading to subsequent shearing and stamping cracking issues. To achieve the above objective, this invention provides the following technical solution: A 750MPa grade automotive beam steel plate with a uniform ferrite structure and its preparation method are disclosed. The chemical composition and weight percentage of the plate are as follows: C: 0.045-0.060%, Si: 0.05-0.10%, Mn: 1.60-1.70%, P≤0.010%, S≤0.0040%, Nb: 0.030-0.040%, Ti: 0.070-0.085%, Als: 0.020-0.040%, N≤0.0040%, with the balance being Fe and unavoidable impurities.

[0005] Furthermore, the thickness of the 750MPa grade automotive beam steel plate is 2-8mm, the matrix structure is a uniform ferrite structure, the grain size grade is 12.0-12.5, and the difference in ferrite grain size grade between the core and the edge is ≤0.5.

[0006] Furthermore, the 750MPa grade automotive beam steel plate has a yield strength ≥740MPa, a tensile strength ≥800MPa, and an elongation ≥20%.

[0007] A method for preparing 750MPa grade automotive beam steel plate with uniform ferrite structure includes converter smelting, LF refining, continuous casting, billet heating, controlled rolling, and cooling processes. In the billet heating process, the billet thickness is 200mm-230mm, the billet heating temperature is 1220-1250℃, and the slab is descaled after being taken out of the furnace.

[0008] Furthermore, the controlled rolling process includes 6 passes of rough rolling and 7 passes of finish rolling, wherein the 6 passes of rough rolling are carried out in the recrystallization zone, and an intermediate billet is subsequently obtained with a thickness of 40±1mm.

[0009] Furthermore, after the intermediate billet undergoes 7 passes of precision rolling and then passes through the aforementioned cooling process, a thin-gauge 750MPa grade automotive beam steel plate is obtained; the cooling process is a two-stage controlled cooling.

[0010] Furthermore, the seven-pass finishing rolling process is carried out in the non-recrystallization zone, the cumulative reduction rate must be ≥70%, and the final rolling temperature is controlled at 900-850℃.

[0011] Furthermore, in the two-stage controlled cooling, the first stage is ultra-fast cooling, where the temperature is rapidly cooled to 700-680℃ immediately after final rolling at a cooling rate of ≥80℃ / s.

[0012] Furthermore, in the two-stage controlled cooling, the second stage is low-speed cooling. After the ultra-fast cooling reaches the target temperature, it is slowly cooled to 600-580℃ at a cooling rate of 1-5℃ / s.

[0013] Furthermore, the steel plate, after being cooled in the two-stage controlled manner, is rolled into a coil and then placed in a slow cooling pit for slow cooling.

[0014] The beneficial effects of this invention are: This invention discloses a 750MPa grade automotive frame steel sheet with a uniform ferrite microstructure and its preparation method. It combines ultra-fast cooling (UFC) technology with extremely low-speed phase transformation cooling to obtain a ferrite microstructure, which is the opposite of the conventional approach used to obtain bainite / martensite, and has significant advantages. This process effectively solves the problems of asynchronous phase transformation in the core and edges of hot-rolled steel sheets and uneven ferrite grain growth by using a "fast then slow" cooling mode after rolling, and also reduces the amount of Nb and Ti elements added. This invention can stably produce thin-gauge 750MPa grade automotive frame steel sheets with a uniform ferrite microstructure, where the ferrite grain size in both the core and edges is above grade 12 and the grain size difference is less than grade 0.5. Its microstructure uniformity is superior to that of automotive frame steel of the same strength grade. This automotive frame steel sheet is less prone to failure behaviors such as side cracking, shear delamination, stamping cracking, and tensile delamination during subsequent processing, and can meet various complex processing requirements. Meanwhile, the method for preparing thin-gauge 750MPa grade automotive beam steel plates with uniform ferrite structure provided by the present invention can be extended to other steel grades with different strength levels. Detailed Implementation

[0015] The following embodiments are only used to illustrate the technical solutions of the present invention more clearly, and are therefore only examples and should not be used to limit the scope of protection of the present invention.

[0016] The specific embodiments listed in this invention are merely examples, and the invention is not limited to the specific embodiments described below. For those skilled in the art, any equivalent modifications and substitutions to the embodiments described below are also within the scope of this invention. Therefore, all equivalent transformations and modifications made without departing from the spirit and scope of this invention should be covered within its scope.

[0017] Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. All reagents or instruments without a specified manufacturer are commercially available, conventional products. Numerous specific details are provided in the following detailed embodiments to better illustrate the invention. Those skilled in the art should understand that the invention can be practiced even without certain specific details. In other embodiments, methods, means, equipment, and steps well-known to those skilled in the art are not described in detail in order to highlight the spirit of the invention.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Unless otherwise specified, all units used in this specification are International Standard Units (SI), and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors unavoidable in industrial production.

[0019] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," and "some examples" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0020] Example The functions and mechanisms of each element and main process in this invention are as follows: Carbon (C) is an effective strengthening element in steel. It can dissolve into the matrix to provide solid solution strengthening. Simultaneously, C can form precipitates with Ti and Nb, contributing to grain refinement and precipitation strengthening. However, C is also a crucial element in the formation of bainite, pearlite, and cementite. Appropriately reducing the C content helps lower the phase transformation driving force, inhibiting the formation of bainite and pearlite, promoting ferrite formation, and greatly satisfying the requirement for uniform microstructure. Therefore, this invention controls the C content within the range of 0.045% to 0.060%.

[0021] Si and Mn: Both silicon and manganese have strong solid solution strengthening effects. While improving strength, Mn does not excessively inhibit ferrite phase transformation and can improve the hardenability of steel; while Si can strengthen ferrite, inhibit cementite precipitation, and promote the formation of carbide-free ferrite. Therefore, in this invention, the Mn content is controlled at 1.60-1.70%, and the Si content is controlled at 0.05-0.10%.

[0022] S and N: Sulfur and nitrogen are impurity elements in steel and easily combine with Ti to form Ti4C2S2 and TiN inclusions, respectively. This not only reduces the toughness and plasticity of steel, but also reduces the content of TiC precipitates, thus causing fluctuations in precipitation strengthening effect. Therefore, the present invention limits the content of S and N to ≤0.004% and ≤0.0040%, respectively.

[0023] Nb and Ti: The core role of Nb in steel is to inhibit austenite recrystallization, refine the original austenite grains, and obtain a fine austenite structure containing a large number of deformation bands, providing conditions for subsequent ferrite nucleation and growth. The main role of Ti in steel is to increase the coarsening temperature of austenite, refine the original austenite grain size, and combine with C to form TiC, playing a role in precipitation strengthening. This invention uses Nb-Ti composite microalloying technology to reduce the sensitivity of single Ti microalloyed steel to controlled cooling temperature; and because the interaction between Nb and Ti elements increases the precipitation rate of the second phase, the amount of Nb and Ti elements added can be reduced; secondly, combined with controlled rolling and controlled cooling processes and composition optimization, the Ti-containing composite precipitate phase can be stably precipitated throughout the entire process, improving the performance stability of beam steel; finally, reducing the compositional fluctuation range of Nb and Ti content is beneficial to narrowing the fluctuation range of precipitation strengthening effect. Therefore, this invention limits the Nb and Ti contents to 0.030-0.040% and 0.070-0.085%, respectively.

[0024] According to the composition of the 750MPa grade automotive beam steel plate mentioned above, the steel plate is smelted into a slab, and then the slab is successively heated, rough rolled, fine rolled, cooled and coiled to obtain the 750MPa grade automotive beam steel plate.

[0025] Specifically, in the heating step, heating the slab is to homogenize the as-cast structure and compositional segregation, and at the same time to dissolve the alloying elements. The 750MPa grade automotive beam steel plate of this invention has a high content of alloying elements, and the solid solution temperature of the Nb and Ti precipitates is high. Therefore, the heating process requires a sufficiently high temperature, 1220-1250℃.

[0026] Specifically, regarding the thickness of the roughing and intermediate slabs: the roughing is completed in 6 passes in the recrystallization zone with 6 passes of large reduction to ensure that the original austenite structure is uniform and fine; the intermediate slab directly corresponds to the thickness of the final finished steel, which is 2-8 mm, and meets the requirement that the cumulative reduction rate in the finishing rolling process must be ≥70%. This invention limits the thickness of the intermediate slab to 40±1 mm.

[0027] Specifically, in the finishing rolling step, the massive deformation causes the austenite grains to be severely flattened, elongated, and refined, increasing the grain boundary area of ​​austenite per unit volume. Simultaneously, numerous deformation bands and high-density dislocations are generated within the grains, thereby increasing the ferrite nucleation rate and resulting in a fine microstructure after phase transformation. Therefore, this invention specifies that finishing rolling is completed in 7 passes using an intermediate billet without recrystallization, with a cumulative reduction rate ≥70%, and the final rolling temperature controlled at 900-850℃.

[0028] Specifically, in the cooling process, a two-stage controlled cooling method is used to achieve uniform ferrite grains in the core and edges of the steel plate. The first stage is ultra-fast cooling, which is performed immediately after final rolling at a cooling rate of ≥80℃ / s to 700-680℃. The goal of ultra-fast cooling is to allow the edges and core of the steel plate to rapidly pass through the austenite region at an extremely fast and synchronous cooling rate (≥80℃ / s), preventing grain growth and uniformly cooling to the upper limit temperature of the ferrite phase transformation region (700-680℃). Its function is to suppress the recovery and recrystallization process of high-temperature austenite with extremely fast cooling rate, retaining all deformation defects as nucleation sites for subsequent ferrite. Although the steel plate is very thin at this time, ultra-fast cooling allows the core and edges of the steel plate to cool down almost synchronously, greatly reducing the temperature gradient before phase transformation. The second stage is slow cooling, which is to slowly cool to 600-580℃ at a cooling rate of 1-5℃ / s after ultra-fast cooling reaches the target temperature. Within the ferrite phase transformation temperature range, a very slow and controllable cooling rate allows ferrite sufficient time to nucleate and slowly grow at defects such as deformation bands and high-density dislocations, ultimately resulting in a uniform, fine ferrite microstructure. Therefore, this invention employs a two-stage controlled cooling process. The first stage is ultra-fast cooling, where, immediately after final rolling, the temperature is rapidly cooled to 700-680℃ at a rate of ≥80℃ / s. The second stage is low-speed cooling, where, after ultra-fast cooling reaches the target temperature, the temperature is slowly cooled to 600-580℃ at a rate of 1-5℃ / s.

[0029] This embodiment discloses a 750MPa grade automotive beam steel plate with a uniform ferrite structure, including four embodiments and four comparative examples. Its chemical composition is shown in Table 1, its preparation process parameters are shown in Table 2, and its mechanical properties and microstructure characteristics are shown in Table 3.

[0030] Table 1 Chemical composition (wt%) of the examples and comparative examples

[0031] Table 2 Process parameters of embodiments of the present invention

[0032] Table 3 Mechanical properties and microstructure characteristics of the embodiments and comparative examples of the present invention

[0033] The automotive beam steel of Examples 1 to 4 of this invention is obtained as finished steel plates through a hot continuous rolling process. The mechanical properties and microstructure characteristics meet the requirements, namely, the yield strength of the beam steel plate is ≥740MPa, the tensile strength is ≥800MPa, the elongation is ≥20%, the matrix microstructure is a uniform ferrite microstructure, the grain size grade is 12.0-12.5, and the difference in ferrite grain size grade between the core and the edge is ≤0.5.

[0034] The chemical compositions of the steels in Comparative Examples 1 and 2 are shown in Table 1, and the production processes are consistent with those in Examples 1 and 2. In Comparative Examples 1 and 2, the N and S content of the steels exceeds the requirements of ≤0.0040% and ≤0.0040%, respectively, reaching 0.0080% and 0.0080%. Since N and S combine with Ti to form TiN and Ti4C2S2 inclusions, respectively, the increase in Ti inclusions leads to a decrease in the Ti content responsible for precipitation strengthening, resulting in yield strength and tensile strength both falling below the required values.

[0035] The chemical compositions of the steels in Comparative Examples 3 and 4 are shown in Table 1. Unlike Examples 3 and 4, the post-rolling cooling methods in Comparative Examples 3 and 4 were laminar flow cooling with concentrated front-end cooling, and after finishing rolling, the steels were cooled to 600-580℃ at cooling rates of 28℃ / s and 30℃ / s, respectively. First, the rapid cooling rate led to the formation of a large amount of bainite, resulting in poor uniformity of the steel plate structure. Second, the nucleation and growth of ferrite were hindered, and the large temperature difference between the core and the edge of the steel plate caused asynchronous ferrite phase transformation processes, with a grain size difference of 1~1.5 between the core and the edge. Ultimately, the mechanical properties of the steel plate did not meet the requirements, namely, yield strength ≥740MPa, tensile strength ≥800MPa, elongation ≥20%, uniform ferrite matrix structure, grain size grade 12.0-12.5, and a grain size difference of ≤0.5 between the core and the edge.

[0036] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications and substitutions should be covered within the scope of the claims of the present invention. Technical aspects, shapes, and structures not described in detail in this invention are all well-known technologies.

Claims

1. A 750MPa grade automotive beam steel plate with a uniform ferrite microstructure, characterized in that: Its chemical composition and weight percentage are as follows: C: 0.045~0.060%, Si: 0.05~0.10%, Mn: 1.60~1.70%, P≤0.010%, S≤0.0040%, Nb: 0.030~0.040%, Ti: 0.070~0.085%, Als: 0.020~0.040%, N≤0.0040%, with the balance being Fe and unavoidable impurities.

2. The 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 1, characterized in that: The 750MPa grade automotive beam steel plate has a thickness of 2-8mm, a uniform ferrite matrix, a grain size grade of 12.0-12.5, and a grain size grade difference of ≤0.5 between the core and the edge.

3. The 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 1, characterized in that: The 750MPa grade automotive beam steel plate has a yield strength ≥740MPa, a tensile strength ≥800MPa, and an elongation ≥20%.

4. A method for preparing 750MPa grade automotive beam steel plates with a uniform ferrite structure, comprising converter smelting, LF refining, continuous casting, billet heating, controlled rolling, and cooling processes, characterized in that: In the billet heating process, the billet thickness is 200mm-230mm, the billet heating temperature is 1220-1250℃, and the slab is descaled after being taken out of the furnace.

5. The method for preparing a 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 4, characterized in that: The controlled rolling process includes 6 passes of rough rolling and 7 passes of finish rolling. The 6 passes of rough rolling are carried out in the recrystallization zone, and an intermediate billet is subsequently obtained with a thickness of 40±1mm.

6. The method for preparing a 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 5, characterized in that: After the intermediate billet undergoes 7 passes of precision rolling, it passes through the cooling process to obtain a thin-gauge 750MPa grade automotive beam steel plate; the cooling process is a two-stage controlled cooling.

7. The method for preparing a 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 6, characterized in that: The seven-pass finishing rolling process is carried out in the non-recrystallization zone, the cumulative reduction rate must be ≥70%, and the final rolling temperature is controlled at 900-850℃.

8. The method for preparing a 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 7, characterized in that: In the two-stage controlled cooling, the first stage is ultra-fast cooling, where the temperature is rapidly cooled to 700-680℃ immediately after final rolling at a cooling rate of ≥80℃ / s.

9. The method for preparing a 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 8, characterized in that: In the two-stage controlled cooling, the second stage is low-speed cooling. After the ultra-fast cooling reaches the target temperature, it is slowly cooled to 600-580℃ at a cooling rate of 1-5℃ / s.

10. The method for preparing a 750MPa grade automotive beam steel plate with a uniform ferrite structure according to claim 9, characterized in that: After being cooled in two stages, the steel plate is rolled into a coil and then placed in a slow cooling pit for slow cooling.

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