700MPa-grade automotive beam steel with low cost, high yield ratio and low-temperature impact resistance as well as preparation method and application of 700MPa-grade automotive beam steel

Abstract

The embodiment of the invention discloses low-cost, high-yield-ratio and low-temperature-impact-resistant 700MPa-grade automotive beam steel as well as a preparation method and application thereof. The automobile beam steel comprises the following components in percentage by mass: 0.05 to 0.12 percent of C; si: less than or equal to 0.10%; mn: 1.2 to 2.0%; n: < = 0.005%; 0.015% to 0.03% of Nb; 0.03 to 0.08% of Ti; less than or equal to 0.3% of Cr; 0.001 to 0.003 percent of B and the balance of Fe; according to the automobile beam steel, the tensile strength Rm is not smaller than 750 MPa, the yield strength R0.2 or ReL is not smaller than 700 MPa, the total elongation A after fracture is not smaller than 20%, and the yield ratio is 0.90-0.95. The Charpy impact absorbing energy KV2 at-20 DEG C is not less than 140J; the Charpy impact absorbing energy KV2 at-40 DEG C is not less than 120J; the microscopic structure of the automobile beam steel is composed of acicular ferrite, pearlite and granular bainite, the effective grain size of the acicular ferrite is 3-5 microns, and the volume fraction of the pearlite is smaller than or equal to 10%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material processing, and particularly relates to a low-cost, high yield ratio and low-temperature impact-resistant 700MPa-grade automobile beam steel, a preparation method and application thereof. BACKGROUND

[0002] The weight proportion of the body of a new energy automobile is close to 70%, and the use of high-strength automobile steel to achieve the lightweight design goal of the body becomes a feasible technical means. The lightweight steel and iron materials for automobiles include high-strength steel, advanced high-strength steel and ultrahigh-strength steel. The high-strength steel is usually directly used after hot rolling, and has better strength, plasticity, weldability and formability compared with traditional materials. The proportion of high-strength steel in international mainstream vehicles has exceeded 70%, and the proportion of high-strength steel used in the body of domestic vehicles has also approached 60%. The high-performance hot-rolled automobile steel mainly used for high-strength steel is mainly applied to the frame assembly of commercial vehicles, and is prepared through forming processes such as cutting, stamping and rolling. As a core load-bearing component, it needs to bear the weight of each assembly and the effective load of the automobile, and also needs to bear various dynamic and static loads generated during driving. At the same time, it needs to adapt to harsh service environments such as low temperature. Therefore, the high-performance hot-rolled automobile steel must have high strength and plasticity, and at the same time, good low-temperature impact toughness and deformation resistance should be considered to meet the lightweight design requirements of the body and ensure the safety and service life of the component.

[0003] Chinese patent document CN117512437A discloses a high-toughness high-strength beam steel and a preparation method thereof. The steel contains ≤0.002% B element, and after hot rolling and coiling, ferrite + bainite + nanoscale Ti precipitates are obtained. The maximum Charpy impact energy at -20℃ can reach 206J, but the total elongation is only 18%.

[0004] Chinese patent document CN117947337A discloses a high-fracture-toughness tensile strength 700MPa beam steel and a preparation method thereof. The B content in the steel is 0.0005% to 0.0007%, and 0.014% to 0.016% Ce element is additionally added. After hot rolling and coiling, bainite structure is obtained. However, the highest yield strength is only 675MPa, the highest tensile strength is 750MPa, and the yield strength ratio is 0.9.

[0005] Chinese patent document CN115747643A discloses a preparation method of a 1.2-2.0mm thin-gauge 700MPa-grade hot-rolled beam steel. By adding ≤0.01% rare earth element to the molten steel, a plate with polygonal ferrite microstructure is obtained after hot rolling and coiling. The tensile strength is ≥680MPa, and the yield strength is ≥610MPa. Although the strength level does not reach the level of the present application, the addition of rare earth elements increases the viscosity of the molten steel, thereby increasing the smelting difficulty.

[0006] Chinese patent document CN 113308646A discloses a high fatigue performance 700 MPa grade hot-rolled automobile beam steel strip and a preparation method thereof. The steel is added with ≤0.35% Ti element, and ferrite + bainite microstructure is obtained after hot rolling and coiling, accompanied by nanoscale Ti precipitation. Although the Charpy impact energy of the material at -20°C can reach 120 J, the Ti content is relatively high, and the nitrogen content in the molten steel needs to be strictly controlled during steelmaking to prevent the formation of large-size TiN particles, which will affect the impact and fatigue performance of the material.

[0007] Chinese patent document CN 109161806A discloses a preparation method of a 700 MPa grade automobile beam steel strip, wherein the Nb content in the steel is 0.045%-0.055%, and ferrite + pearlite microstructure is obtained after hot rolling and coiling, and the Charpy impact energy thereof at -20°C is ≥150 J. However, the yield strength of the material is lower than 700 MPa, and the high Nb content leads to high production cost. SUMMARY

[0008] Therefore, in one aspect, some embodiments disclose a low-cost, high yield ratio, low-temperature impact resistant 700 MPa grade automobile beam steel. The automobile beam steel comprises, by mass ratio, C: 0.05-0.12%; Si: ≤0.10%; Mn: 1.2-2.0%; N: ≤0.005%; Nb: 0.015-0.03%; Ti: 0.03-0.08%; Cr: ≤0.3%; B: 0.001-0.003%, and the balance being Fe.

[0009] The tensile strength R m of the automobile beam steel is not less than 750 MPa, the yield strength R 0.2 of the automobile beam steel is not less than 700 MPa, the total elongation A eL after fracture of the automobile beam steel is not less than 20%, the yield ratio of the automobile beam steel is 0.90-0.95, the Charpy impact energy KV2 at -20°C of the automobile beam steel is not less than 140 J, and the Charpy impact energy KV2 at -40°C of the automobile beam steel is not less than 120 J.

[0010] The microstructure of the automobile beam steel is composed of acicular ferrite, pearlite and granular bainite, wherein the effective grain size of the acicular ferrite is 3-5 μm, and the volume fraction of the pearlite is ≤10%.

[0011] Further, some embodiments disclose a low-cost, high yield ratio, low-temperature impact resistant 700 MPa grade automobile beam steel. The content of C is 0.08-0.10%.

[0012] Some embodiments disclose a low-cost, high yield ratio, low-temperature impact resistant 700 MPa grade automobile beam steel. The content of Si is 0.08-0.09%.

[0013] Some embodiments disclose a low-cost, high yield strength ratio, low-temperature impact resistant 700MPa grade automobile beam steel, the content of Mn is 1.45-1.75%.

[0014] Some embodiments disclose a low-cost, high yield strength ratio, low-temperature impact resistant 700MPa grade automobile beam steel, the content of Nb is not more than 0.02%.

[0015] Some embodiments disclose a low-cost, high yield strength ratio, low-temperature impact resistant 700MPa grade automobile beam steel, the content of Ti is 0.05-0.07%.

[0016] Some embodiments disclose a low-cost, high yield strength ratio, low-temperature impact resistant 700MPa grade automobile beam steel, the content of B is 0.015-0.020%.

[0017] In another aspect, some embodiments disclose a method for preparing a low-cost, high yield strength ratio, low-temperature impact resistant 700MPa grade automobile beam steel, comprising the steps of:

[0018] S1, smelting and casting: smelting, refining and continuous casting into a casting blank or ingot according to the components and proportions of the automobile beam steel to obtain a casting blank;

[0019] S2, homogenization of the casting blank: heating the casting blank to 1220-1250℃ for 2-3h to homogenize the composition of the casting blank;

[0020] S3, controlled rolling: adopting two-stage controlled rolling, the first stage is a rough rolling stage, the opening rolling temperature is 1180-1220℃, the cumulative deformation of rough rolling is ≥50% to obtain an intermediate blank; the second stage is a finish rolling stage, the intermediate blank is heated to 950-1000℃, then 4-7 passes of finish rolling are performed with a cumulative deformation of not less than 80%, and the final rolling temperature is 830-850℃ to obtain an automobile beam steel plate;

[0021] S4, controlled cooling and coiling: cooling the automobile beam steel plate to 570-500℃ by adopting continuous laminar flow cooling at a cooling rate of 20-40℃ / s; then coiling the automobile beam steel plate, and slowly cooling to room temperature after coiling at a cooling rate of not more than 20℃ / h; obtaining a low-cost, high yield strength ratio, low-temperature impact resistant 700MPa grade automobile beam steel.

[0022] Further, some embodiments disclose a method for preparing a low-cost, high yield strength ratio, low-temperature impact resistant 700MPa grade automobile beam steel, wherein in step S3, the single pass reduction rate is not less than 15%, and the total hot rolling reduction rate is not less than 90%; the rolling in the second stage is performed in the unrecrystallization zone with a total reduction rate of not less than 40%; and the thickness of the automobile beam steel plate obtained by hot rolling is 5-12mm.

[0023] In another aspect, some embodiments disclose applications of low-cost, high yield strength ratio, low-temperature impact resistant 700 MPa grade automobile beam steels, and the automobile beam steels are applied to prepare a new energy vehicle beam load-bearing component.

[0024] The disclosed preparation method of the low-cost, high yield strength ratio, low-temperature impact resistant 700 MPa grade automobile beam steel in the embodiments of the present application adopts a low-cost alloying process route of low C, low Ti, low Nb and adding trace B elements, and meanwhile, the strength and toughness of the material are ensured; the addition of trace B elements effectively inhibits pearlite phase transformation, promotes bainite phase transformation, and promotes the phase transformation of intragranular acicular ferrite by inhibiting the nucleation of proeutectoid ferrite at the austenite grain boundary; the “interlocking” structure of the acicular ferrite makes the proportion of high-angle grain boundaries increase, thereby improving the low-temperature impact toughness of the material; meanwhile, the B element increases the bainite transformation temperature, so that granular bainite and more precipitated phases can be obtained when coiling at a higher temperature, thereby ensuring that the material has a higher yield strength; compared with the existing same strength grade beam steels, the Nb and Ti contents of the alloy are significantly reduced, and the manufacturing cost is greatly reduced.

[0025] The automobile beam steel obtained by the disclosed preparation method of the low-cost, high yield strength ratio, low-temperature impact resistant 700 MPa grade automobile beam steel in the embodiments of the present application realizes a higher yield strength ratio and low-temperature impact toughness under the same strength grade, the tensile strength of the material at room temperature is not less than 750 MPa, the yield strength is not less than 700 MPa, the yield strength ratio is 0.90-0.95, and the elongation after fracture is not less than 20%; the Charpy impact absorbed energy KV2 at-20 ℃ is not less than 140 J; and the Charpy impact absorbed energy KV2 at-40 ℃ is not less than 120 J. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 , OM image of the beam steel in Example 1;

[0027] Figure 2 , SEM image of the-40 ℃ impact fracture of the beam steel in Example 1. DETAILED DESCRIPTION

[0028] Here, the special term “embodiment” as “exemplary” of any embodiment described does not necessarily mean that it is superior or better than other embodiments. In the performance index test in the embodiments of the present application, unless otherwise specified, the conventional test method in the art is adopted. It should be understood that the terms described in the embodiments of the present application are only for describing the particular embodiments, and are not used to limit the disclosure of the embodiments of the present application.

[0029] 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 to which this application belongs; methods and techniques similar to but not otherwise specifically described herein are well known and available to those in the art.

[0030] The terms "substantially" and "approximately" as used herein are used to describe small fluctuations. For example, they can mean less than or equal to ± 5%, such as less than or equal to ± 2%, such as less than or equal to ± 1%, such as less than or equal to ± 0.5%, such as less than or equal to ± 0.2%, such as less than or equal to ± 0.1%, such as less than or equal to ± 0.05%. Numerical data may, in the text herein, be presented in a range format. It is to be understood that such a range format is used only for convenience and brevity and should be construed as having been followed only to the extent that such a range format permits, and thus should not be construed to limit the values to the explicit endpoints. For example, a numerical range of "1 to 5" should be interpreted to include not only the explicit endpoints 1 and 5, but also to include individual values and sub ranges within the indicated range. For example, included in this range are individual values such as 2, 3.5, and 4, and sub ranges such as 1 to 3, 2 to 4, and 3 to 5, etc. This principle should also apply to ranges reciting only one numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described.

[0031] In this text, including the claims, conjunctions such as "comprise", "include", "have", "with", "contain", "relate to", "accommodate" and the like are understood to be open, i.e. to mean "including but not limited to". Only the conjunctions "consist of" and "consist essentially of" are closed conjunctions.

[0032] For a better understanding of the present application, numerous specific details are given in the following detailed description. It will be understood by those skilled in the art that the present application can be practiced without some of the specific details, and that the present application can be practiced with some variations. In the examples, some methods, means, instruments, devices, etc. well known to those skilled in the art are not described in detail in order to highlight the principles of the present application.

[0033] The technical features disclosed in the embodiments of the present application can be combined in any way, provided that there is no conflict, and the resulting technical solutions belong to the disclosure of the present application.

[0034] In some embodiments, the low-cost, high-ductility-to-strength ratio, low-temperature impact-resistant 700MPa-grade automobile beam steel comprises the following components and contents: C: 0.05-0.12%; Si: ≤0.10%; Mn: 1.2-2.0%; N: ≤0.005%; Nb: 0.015-0.03%; Ti: 0.03-0.08%; Cr: ≤0.3%; B: 0.001-0.003%, and the balance being Fe; wherein the contents of the components are in mass ratio, and the balance usually further comprises inevitable impurity elements in the steel material.

[0035] The tensile strength R of the automobile beam steel m Not less than 750MPa, the yield strength R 0.2 Or R eL Not less than 700MPa, the total elongation A after fracture is not less than 20%, the yield strength ratio is 0.90-0.95; the Charpy impact energy KV2 at-20°C is not less than 140J; the Charpy impact energy KV2 at-40°C is not less than 120J.

[0036] The microstructure of the automobile beam steel is composed of acicular ferrite, pearlite and granular bainite, wherein the effective grain size of the acicular ferrite is 3-5μm, and the volume fraction of the pearlite is ≤10%.

[0037] The principle of each component element of the beam steel alloy in the application is as follows:

[0038] C (carbon); C is one of the most common alloying elements in steel, usually in the form of interstitial atoms, is solid-solved in the lattice interstice of the matrix to form a solid solution, causing lattice distortion, thereby improving the strength and hardness of the steel material; at the same time, C is also an important forming element of micro-alloying element precipitates. However, excessive C content will not only increase the proportion of pearlite or brittle cementite, resulting in a decrease in the plasticity and toughness of the material, but also affect the welding performance of the material. In order to have good yield strength ratio and low-temperature impact toughness, the application adopts low-carbon alloying design.

[0039] In some embodiments, the more preferred C content is 0.08-0.10%.

[0040] Si (silicon); Si is usually solid-solved in the matrix in the form of substitution atoms in steel, which plays a role in solid solution strengthening; in addition, Si can inhibit the diffusion of C in steel, hinder the precipitation of carbides, and suppress the formation of pearlite, but a large amount of Si will increase the adhesion of iron oxide scale on the surface of hot-rolled plate, leading to an increase in the difficulty of hot-rolled dephosphorization, affecting the surface quality of the finished plate. Therefore, in order to obtain good surface quality, the application adopts low-silicon alloying design.

[0041] In some embodiments, the more preferred Si content is 0.08-0.09%.

[0042] Mn (manganese) ; Mn as an element to expand the austenite phase region, can stabilize the austenite phase, prolong the incubation period of phase transformation, and move the "C curve" to the right. In addition, Mn as a weak carbide forming element can inhibit the decomposition of carbon-rich austenite phase, suppress pearlite transformation, and promote granular bainite transformation. Meanwhile, higher Mn content can also refine the M / A islands in granular bainite and improve the toughness of granular bainite. However, higher Mn content can lead to serious segregation and deteriorate the welding performance of the material.

[0043] In some embodiments, in order to have good strength and toughness, the more preferred Mn content is 1.45-1.75%.

[0044] Nb (niobium) ; Nb mainly plays the role of fine-grain strengthening and precipitation strengthening. Nb can significantly increase the recrystallization temperature of austenite at high temperature, expand the unrecrystallized temperature range, expand the hot rolling process window, significantly refine the grain, and improve the toughness of the material. However, the cost of Nb alloy is relatively high.

[0045] In some embodiments, as a method for producing 700MPa grade girder steel at low cost, the more preferred Nb content is not more than 0.02%.

[0046] Ti (titanium) ; Ti is one of the main alloying elements in the present application, which mainly plays the roles of "nitrogen fixation and boron preservation" and precipitation strengthening. Since Ti is a strong carbon and nitride forming element in steel, nitrogen atoms in the steel preferentially combine with Ti to form liquid-precipitated TiN, thereby improving the utilization rate of B in the steel. In addition, Ti can form a large number of nano precipitates during the coiling process, thereby improving the yield strength of the material. However, higher Ti content can lead to the formation of large-size TiN inclusions in the steel, which seriously affects the impact and fatigue performance of the material.

[0047] In some embodiments, the more preferred Ti content is 0.05-0.07%.

[0048] Cr (chromium) ; Cr is a strong ferrite forming element, which can refine ferrite grains, expand the cold speed range of acicular ferrite formation, and refine the M / A islands in granular bainite, thereby improving the toughness and strength of the material.

[0049] B (boron) ; B is one of the important alloying elements in the present application, which mainly plays the role of changing the phase transformation path and designing the microstructure. B element is one of the necessary elements for obtaining bainite phase in low carbon steel under continuous cooling, which can significantly inhibit the nucleation of ferrite at the austenite phase interface, so that the undercooled austenite obtains bainite structure under continuous cooling conditions. Meanwhile, B element can increase the bainite transformation temperature, so that the steel containing B is more likely to obtain bainite. However, B atoms tend to segregate at the austenite grain boundaries to form B-rich regions, and excessive B can cause grain boundary embrittlement, which significantly reduces the impact toughness of the material.

[0050] In some embodiments, the B content is preferably 0.015-0.020%.

[0051] Some embodiments disclose a method for preparing a low-cost, high yield ratio, low-temperature impact-resistant 700 MPa grade automobile beam steel, comprising the steps of:

[0052] S1, smelting and casting: smelting, refining, and continuous casting into a casting blank or ingot according to the components and proportions of the automobile beam steel to obtain a casting blank;

[0053] S2, homogenization of the casting blank: heating the casting blank to 1220-1250°C for 2-3 h to homogenize the composition of the casting blank;

[0054] S3, controlled rolling: two-stage controlled rolling is adopted, the first stage is a rough rolling stage, the opening rolling temperature is 1180-1220°C, the cumulative deformation of rough rolling is ≥50%, and an intermediate blank is obtained; the second stage is a finish rolling stage, the intermediate blank is heated to 950-1000°C, then 4-7 passes of finish rolling are performed and the cumulative deformation is ≥80%, and the final rolling temperature is 830-850°C, to obtain an automobile beam steel plate;

[0055] S4, controlled cooling and coiling: the automobile beam steel plate is cooled to 570-500°C using a continuous laminar flow cooling method, and the cooling rate is 20-40°C / s; then the automobile beam steel plate is coiled, and after coiling, it is slowly cooled to room temperature at a cooling rate of not greater than 20°C / h; a low-cost, high yield ratio, low-temperature impact-resistant 700 MPa grade automobile beam steel is obtained.

[0056] Some embodiments disclose a method for preparing a low-cost, high yield ratio, low-temperature impact-resistant 700 MPa grade automobile beam steel, wherein in step S3, the controlled rolling stage, the single pass reduction rate is not less than 15%, and the total hot rolling reduction rate is not less than 90%; the second stage rolling is performed in the unrecrystallization zone, and the total reduction rate is not less than 40%; the automobile beam steel plate obtained by hot rolling has a thickness of 5-12 mm.

[0057] Generally, the microstructure is an important factor affecting the toughness of steel materials. With the increase of external stress, micro cracks gradually appear in the material and slowly expand. When the crack expands to a large angle grain boundary, the expansion will be hindered. At this time, a large number of dislocations will gather at the grain boundary near the crack tip, causing the internal stress of the material to increase. Under the joint action of external stress and internal stress, the expansion direction of the crack will be deflected. The grain orientation difference on both sides of the large angle grain boundary is large, which makes the expansion direction of the crack change, thereby reducing the driving force of crack expansion, and finally leading to the difficulty of crack expansion and improving the toughness of the material. The acicular ferrite matrix is interwoven by interlaced ferrite laths, and this structure has a high proportion of large angle grain boundaries, so its toughness is relatively high.

[0058] The recrystallization temperature of austenite can be increased and the hot rolling process window in the unrecrystallized region can be expanded by adding a certain amount of niobium (Nb) in the steel. A total reduction of not less than 40% in the second stage of the air cooling process can ensure a large amount of deformation in the unrecrystallized region of austenite, so that a large number of deformation bands are generated in the austenite, the ferrite grains are significantly refined, and the ferrite is nucleated in the austenite grains, promoting the formation of acicular ferrite. The large-size Nb(C, N) precipitated by strain-induced precipitation during hot rolling can serve as nucleation sites for ferrite, significantly refining the ferrite grains and providing certain precipitation strengthening effect.

[0059] Ti is a strong carbon and nitride forming element in steel. Nitrogen atoms in the steel preferentially combine with Ti to form liquid-precipitated TiN, mainly playing the roles of "solid nitrogen and boron preservation" and precipitation strengthening, thereby improving the utilization rate of B in the steel. In addition, Ti can form a large number of nano precipitates during coiling, thereby improving the yield strength of the material. However, a higher Ti content can lead to the formation of large-size TiN inclusions in the steel, seriously affecting the impact and fatigue performance of the material. A certain amount of boron (B) added in the steel preferentially segregates at the original austenite grain boundaries, thereby improving the stability of austenite, reducing the austenite grain boundary energy, inhibiting the nucleation of ferrite at the grain boundaries, and promoting the nucleation of acicular ferrite in the austenite grains. In addition, boron can promote the bainite transformation and refine the bainite structure, thereby improving the strength and toughness of the material.

[0060] Some embodiments disclose the application of low-cost, high yield strength ratio, and low-temperature impact-resistant 700MPa-grade automobile beam steel. The automobile beam steel is applied to prepare a new energy vehicle beam load-bearing component. For example, the transverse and longitudinal beams of special new energy vehicles such as trucks and fire trucks can be manufactured using the low-cost, high yield strength ratio, and low-temperature impact-resistant 700MPa-grade automobile beam steel disclosed in the embodiments.

[0061] The technical details are further exemplarily described below in conjunction with the embodiments.

[0062] Embodiment 1

[0063] In embodiment 1, the preparation method of the low-cost, high yield strength ratio, and low-temperature impact-resistant 700MPa-grade automobile beam steel comprises the following steps:

[0064] S1, smelting and casting: the raw materials are weighed according to the components and proportions of the automobile beam steel, and then smelting, refining, and continuous casting are performed to obtain a casting blank or ingot, thereby obtaining a casting blank; the components and contents are as follows: C: 0.09%; Si: 0.09%; Mn: 1.67%; N: 0.005%; Nb: 0.02%; Ti: 0.07%; Cr: 0; B: 0.002%, and the balance is Fe;

[0065] S2, casting blank homogenization: the casting blank is heated to 1250℃ and kept for 2h to homogenize the composition of the casting blank;

[0066] S3, controlled rolling: two-stage controlled rolling is adopted, the first stage is rough rolling at a temperature of 1220℃, and the cumulative deformation is 50%, to obtain an intermediate blank with a thickness of 60mm; the second stage is that the intermediate blank is heated to 1000℃, and then 7 passes of finish rolling are performed with a cumulative deformation of 80%, and the finish rolling temperature is 830℃, to obtain a 6mm-thick automobile beam steel plate;

[0067] S4, controlled cooling and coiling: the automobile beam steel plate is cooled to 550℃ by using continuous laminar cooling, and the cooling rate is 40℃ / s; then the automobile beam steel plate is coiled, and after coiling, slow cooling is performed to room temperature, and the cooling rate is 15℃ / h; a low-cost, high yield ratio, low-temperature impact resistant 700MPa grade automobile beam steel is obtained.

[0068] Examples 2-5

[0069] In Examples 2-5, the preparation method of the low-cost, high yield ratio, low-temperature impact resistant 700MPa grade automobile beam steel refers to Example 1, the composition and content of the beam steel are set according to Table 1, and the process parameters are set according to Table 2.

[0070] Table 1 lists the composition and content of each beam steel sample in Examples 1-5, and Table 2 lists the process parameters of the preparation method in Examples 1-5.

[0071] Table 1, composition and content list of beam steel samples in Examples 1-5

[0072]

[0073] Table 2, process parameter list of Examples 1-5

[0074]

[0075] The automobile beam steel obtained in Examples 1-5 is tested for performance, and the beam steel samples obtained in each example are tested for conventional mechanical properties on a UTM5386 type universal material tensile testing machine according to the national standard GB / T288-2010 "Metal material warm room tensile test method", the tensile specimen adopts a standard unidirectional tensile specimen with an original gauge length (L0) of 25mm and an extensometer gauge length (L e ) of 25mm. The tensile strength (R m / MPa), yield strength (R 0.2tensile strength (MPa) and total elongation after fracture (A / %) were measured. In order to ensure the accuracy of the experimental data, at least two tensile samples were taken for each sample with different phase content, and the test results were averaged. Charpy V-notch samples with a size of 10x5x55 mm were prepared. The V-notch was perpendicular to the rolling direction. The JB-30B impact testing machine was used to perform Charpy impact tests at -20℃ and -40℃, respectively, and the experimental results were converted into full-sample low-temperature impact toughness values. The results are listed in Table 3.

[0076] Table 3, performance parameter list of automobile beam steel of examples 1-5

[0077]

[0078] As can be seen from Table 3, the high yield ratio, low-temperature impact-resistant 700MPa-grade automobile beam steel obtained by the examples of the present application has excellent comprehensive mechanical properties, with high tensile strength, yield strength and low-temperature impact toughness. Specifically, the tensile strength of the material at room temperature is not less than 750MPa, the yield strength is not less than 700MPa, the yield ratio is between 0.90 and 0.95, and the elongation after fracture is not less than 20%; at -20℃, the Charpy impact energy KV2 is not less than 140J; at -40℃, the Charpy impact energy KV2 is not less than 120J.

[0079] Figure 1 In the figure, (a) and (b) are OM images of the cross-section 1 / 4 and 1 / 2 from the rolling direction of the steel plate, respectively, and it is observed that the microstructure is acicular ferrite, granular bainite and a small amount of pearlite; Figure 2 In the figure, (a) is the macroscopic fracture morphology after the -40℃ Charpy V-notch impact test, (b), (c) and (d) are SEM images of the fiber zone, the radiation zone and the shear lip during low-temperature fracture of the beam steel, respectively, and a large number of equiaxed dimples are found, the fracture mode is ductile fracture, which shows that the automobile beam steel disclosed in the examples of the present application presents a multi-phase microstructure of acicular ferrite, granular bainite and pearlite. With the help of the "interlocking" structure of acicular ferrite and the high strength of granular bainite, the material realizes a good match between strength and low-temperature impact toughness, and can meet the application requirements of high-strength impact-resistant beam steel for special new energy vehicles.

[0080] The disclosed low-cost, high yield strength ratio and low-temperature impact resistant 700MPa grade automobile beam steel preparation method adopts a low-C, low-Ti, low-Nb and trace B element alloying process route, while ensuring the strength and toughness of the material; the addition of trace B element effectively inhibits pearlite phase change, promotes bainite phase change, and promotes the phase change of intragranular acicular ferrite by inhibiting the nucleation of proeutectoid ferrite at the austenite grain boundary; the "interlocking" structure of acicular ferrite increases the proportion of high-angle grain boundaries, thereby improving the low-temperature impact toughness of the material; at the same time, the B element increases the bainite phase change temperature, so that granular bainite and more precipitated phases can be obtained when coiling at a higher temperature, thereby ensuring that the material has a higher yield strength; compared with the existing same strength level automobile beam steel, the Nb and Ti content of the alloy is significantly reduced, and the manufacturing cost is greatly reduced.

[0081] The automobile beam steel prepared by the disclosed low-cost, high yield strength ratio and low-temperature impact resistant 700MPa grade automobile beam steel preparation method has a higher yield strength ratio and low-temperature impact toughness at the same strength level, the tensile strength of the material at room temperature is not less than 750MPa, the yield strength is not less than 700MPa, the yield strength ratio is 0.90-0.95, and the elongation after fracture is not less than 20%; the Charpy impact absorbed energy KV2 at-20 DEG C is not less than 140J; the Charpy impact absorbed energy KV2 at-40 DEG C is not less than 120J.

[0082] The technical solutions disclosed in the embodiments of the present application and the technical details disclosed in the embodiments are only exemplary to illustrate the inventive concept of the present application, and do not constitute a limitation on the technical solutions of the embodiments of the present application, and any conventional changes, substitutions or combinations of the technical details disclosed in the embodiments of the present application have the same inventive concept as the present application, and are within the protection scope of the claims of the present application.

Claims

1. A low-cost, high yield ratio, low-temperature impact resistant 700 MPa grade automobile beam steel, characterized in that: the automobile beam steel comprises the following components and contents in mass ratio: C: 0.05-0.12%; Si: ≤0.10%; Mn: 1.2-2.0%; N: ≤0.005%; Nb: 0.015-0.03%; Ti: 0.03-0.08%; Cr: ≤0.3%; B: 0.001-0.003%, and the balance being Fe; the microstructure of the automobile beam steel is composed of acicular ferrite, pearlite and granular bainite, wherein the effective grain size of the acicular ferrite is 3-5 μm, and the volume fraction of the pearlite is ≤10%. The content of C is 0.08-0.10%. The content of Si is 0.08-0.09%. The content of Mn is 1.45-1.75%. The content of Nb is not more than 0.02%. The automobile beam steel has a tensile strength R m not less than 750 MPa, a yield strength R 0.2 or R eL not less than 700 MPa, a total elongation A after fracture not less than 20%, a yield strength / tensile strength ratio of 0.90-0.95, a Charpy impact absorbed energy KV2 at -20 DEG C not less than 140 J, and a Charpy impact absorbed energy KV2 at -40 DEG C not less than 120 J. The content of Ti is 0.05-0.07%.

2. The low cost, high ratio of ultimate tensile strength to yield strength, low temperature impact resistant 700 MPa grade automotive beam steel of claim 1, wherein, The content of B is 0.015-0.020%.

3. The low cost, high ratio of ultimate tensile strength to yield strength, low temperature impact resistant 700 MPa grade automotive beam steel of claim 1 wherein, The method comprises the following steps:

4. The low cost, high ratio of ultimate tensile strength to yield strength, low temperature impact resistant 700 MPa grade automotive beam steel of claim 1 wherein, S1, smelting and casting 5. The low cost, high ratio of ultimate tensile strength to yield strength, low temperature impact resistant 700 MPa grade automotive beam steel of claim 1 wherein, The raw materials are weighed according to the components and proportions of the automobile beam steel, and smelting, refining and continuous casting are performed to obtain a casting blank; 6. The low cost, high ratio of ultimate tensile strength to yield strength, low temperature impact resistant 700 MPa grade automotive beam steel of claim 1 wherein, S2, homogenization of the casting blank 7. The low cost, high ratio of ultimate tensile strength to yield strength, low temperature impact resistant 700 MPa grade automotive beam steel of claim 1 wherein, The casting blank is heated to 1220-1250 °C for 2-3 h to homogenize the composition of the casting blank; 8. A method of producing a low cost, high yield ratio, low temperature impact resistant 700 MPa grade automotive beam steel according to any one of claims 1 to 7, characterized in that, S3, controlled rolling Two-stage controlled rolling is adopted, the first stage is a rough rolling stage, the opening rolling temperature is 1180-1220 °C, the cumulative deformation of rough rolling is not less than 50%, and an intermediate blank is obtained; the second stage is a finish rolling stage, the intermediate blank is warmed to 950-1000 °C, then 4-7 passes of finish rolling are performed and the cumulative deformation is not less than 80%, and the final rolling temperature is 830-850 °C, to obtain an automobile beam steel plate; S4, controlled cooling and coiling The automobile beam steel plate is cooled to 570-500 °C at a cooling rate of 20-40 °C / s by using continuous laminar cooling, and then coiled, and slowly cooled to room temperature after coiling at a cooling rate of not more than 20 °C / h, to obtain a low-cost, high yield ratio, low-temperature impact resistant 700 MPa grade automobile beam steel. In step S3, the single pass reduction rate is not less than 15%, and the total hot rolling reduction rate is not less than 90%; the rolling in the second stage is carried out in the unrecrystallization zone, and the total reduction rate is not less than 40%; the thickness of the automobile beam steel plate obtained by hot rolling is 5-12 mm. The automobile beam steel is applied to prepare a new energy vehicle beam load-bearing component. ​ ​ ​ 9. The method of producing a low-cost, high yield ratio, low-temperature impact resistant 700 MPa grade automotive beam steel according to claim 8, characterized by, ​ 10. Use of the low-cost, high yield ratio, low-temperature impact resistant 700 MPa grade automotive beam steel according to any one of claims 1 to 7, characterized in that, ​

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