FB dual-phase steel with high broaching performance, controlled rolling and controlled cooling process, application and performance verification method

Abstract

The invention discloses FB dual-phase steel with high reaming performance, which comprises the following chemical components in percentage by weight: 0.07 to 0.09 percent of C, 0.30 to 0.40 percent of Si, 1.50 to 1.60 percent of Mn, 0.03 to 0.04 percent of Al, 0.03 to 0.05 percent of Nb, 0.02 to 0.03 percent of Ti and the balance of Fe. And the balance of Fe and inevitable impurities. The invention also discloses a controlled rolling and controlled cooling process of the FB dual-phase steel, which comprises the following steps of: S1, heating a blank to 1450-1550K on a rolling mill provided with a cooling water system, preserving heat for 1-2 hours, performing air cooling until the initial rolling temperature is 1300-1400 K, performing four-pass rolling in an austenite recrystallization region, performing three-pass rolling in a non-recrystallization region, and controlling the final rolling temperature to be 1080-1100K; s2, carrying out controlled cooling: S21, carrying out first-section water cooling to 920-980K; s22, air cooling is conducted for 8 s to 18 s; s23, carrying out second-stage water cooling to 650 to 750 K; and S24, carrying out isothermal treatment for 1.5 to 2 hours. By means of the process, the FB dual-phase steel with polygonal ferrite and acicular ferrite can be obtained, and the dual-phase steel has the good reaming performance.

Description

Technical Field

[0001] The present invention relates to the technical field of FB duplex steel materials and their heat treatment, and particularly relates to an FB duplex steel with high hole expansion performance, a controlled rolling and controlled cooling process, and an application and performance verification method thereof. Background Art

[0002] Duplex steel belongs to a kind of high-strength steel and advanced high-strength steel, including ferrite / martensite (FM) duplex steel and ferrite / bainite (FB) duplex steel. Research shows that compared with FM duplex steel, FB duplex steel exhibits excellent ductility, fracture toughness, and formability. Therefore, they are very suitable for use in automotive stampings, such as body reinforcements, chassis components, and wheels.

[0003] Chemical composition and process parameters determine the grain size, microstructure morphology, and volume fraction of FB duplex steel, and these parameters jointly affect the mechanical properties and formability of FB duplex steel. Research shows that by adjusting the controlled rolling and controlled cooling process parameters, acicular ferrite and polygonal ferrite can be obtained. Duplex steel with an acicular ferrite microstructure exhibits excellent strength and toughness, but has a higher yield ratio and a lower uniform elongation rate.

[0004] Sheet metal stretch flanging is an important forming problem in the automotive industry. During the stretch flanging process, edge cracks often occur at the deformed part, resulting in defective products. In order to more objectively evaluate the formability of sheet metal, the hole expansion rate has been widely studied and has gradually become an important evaluation parameter for this performance. Due to the excellent formability and good application prospects of FB duplex steel, the hole expansion rate of FB duplex steel is an important research content. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a new chemical composition of FB duplex steel and a controlled rolling and controlled cooling process, obtaining an FB duplex steel with polygonal ferrite and acicular ferrite, which has good hole expansion performance. At the same time, the present invention also provides a performance verification method for the FB duplex steel. Through SEM in-situ tensile experiments, the deformation evolution process and failure mechanism of ferrite and bainite phases under tensile stress are observed, thereby confirming the significant influence of ferrite morphology on the formability of FB duplex steel.

[0006] An FB duplex steel with high hole expansion performance according to the present invention, its chemical composition by weight percentage includes: C: 0.07 - 0.09%, Si: 0.30 - 0.40%, Mn: 1.50 - 1.60%, Al: 0.03 - 0.04%, Nb: 0.03 - 0.05%, Ti: 0.02 - 0.03%; the balance is Fe and unavoidable impurities;

[0007] Further, the chemical composition of the dual-phase steel by weight percentage includes: C: 0.076%, Si: 0.34%, Mn: 1.565%, Al: 0.031%, Nb: 0.041%, Ti: 0.022%, and the balance is Fe and inevitable impurities. Among the inevitable impurities, P ≤ 0.015% and S ≤ 0.005%.

[0008] The present invention also discloses a controlled rolling and controlled cooling process for an FB dual-phase steel with high hole-expanding performance, including the following steps:

[0009] S1. Heat the billet to 1450 - 1550K on a rolling mill equipped with a cooling water system, hold for 1 - 2h, air-cool to the rolling start temperature of 1300 - 1400K, perform four passes of rolling in the austenite recrystallization zone, perform three passes of rolling in the non-recrystallization zone, and the finish rolling temperature is 1080 - 1100K;

[0010] S2. Implement controlled cooling, including the following steps:

[0011] S21. Cool to 920 - 980K in the first stage of water cooling;

[0012] S22. Air-cool for 8 - 18s;

[0013] S23. Cool to 650 - 750K in the second stage of water cooling;

[0014] S24. Isothermal treatment for 1.5 - 2h;

[0015] Further, in step S1, the billet is heated to 1503K and held for 1.5h;

[0016] Further, in step S1, the rolling start temperature is 1373K and the finish rolling temperature is 1093K;

[0017] Further, in step S2, it is cooled to 923K in the first stage of water cooling, air-cooled for 10s, cooled to 723K in the second stage of water cooling, and the isothermal treatment time is 2h.

[0018] The present invention also discloses an application of the FB dual-phase steel in automotive stampings;

[0019] Further, the stampings include body reinforcement parts, wheels or chassis components.

[0020] The present invention also discloses a method for verifying the performance of the FB dual-phase steel with high hole-expanding performance, including the following steps:

[0021] Step 1: Conduct a hole-expanding experiment on a sheet metal forming testing machine to measure the hole-expanding rate of the FB dual-phase steel sample;

[0022] Step 2: Conduct in-situ tensile experiments on the FB dual-phase steel samples, and use a variety of microscopes to observe the microstructure and the mechanisms of tissue deformation and failure.

[0023] Advantages of the present invention:

[0024] (1) The thermo-mechanical control process (TMCP) adopted in the present invention is conducive to the formation of fine acicular ferrite, and these acicular ferrites are dispersed, effectively cutting the undercooled austenite, resulting in the subsequent formation of bainite being dispersed and showing a characteristic of interlaced distribution with the acicular ferrite.

[0025] (2) The reaming experiment shows that the FB dual-phase steel produced by the thermo-mechanical control process adopted in the present invention has a higher reaming rate, the formed microvoids are finer and more dispersed, and it has lower defect sensitivity.

[0026] (3) It can be seen from the SEM in-situ tensile experiment that by adopting the thermo-mechanical control process of the present invention, the acicular ferrite structure can better inhibit the growth of microvoids and the propagation of microcracks. This is beneficial to dispersing the external stress, reducing the stress concentration, and thus effectively delaying the failure process. Therefore, it has higher strength and elongation rate, and at the same time has more excellent formability. Description of the drawings

[0027] The present invention will be further described below in conjunction with the drawings and embodiments:

[0028] Figure 1 It is a schematic diagram of the control cooling process of the present invention.

[0029] Figure 2 It is a schematic diagram of the structure of the SEM in-situ tensile specimen in the present invention.

[0030] Figure 3 It is a photograph of the original microstructure of the FB dual-phase steel of the present invention; among them, Figure 3 (a) is the metallographic photograph of No. 1 steel; Figure 3 (b) is the metallographic photograph of No. 2 steel; Figure 3 (c) is the SEM photograph of No. 1 steel; Figure 3 (d) is the SEM photograph of No. 2 steel; Figure 3 (e) is the TEM photograph of No. 1 steel; Figure 3 (f) is the TEM photograph of No. 2 steel.

[0031] Figure 4 It is a radial microstructure photograph at the center hole position after reaming, among which Figure 4 (a) is No. 1 steel; Figure 4 (b) is No. 2 steel.

[0032] Figure 5 It is the displacement-load curve of the steel SEM in-situ tensile.

[0033] Figure 6 Microstructure characteristic diagrams of No. 1 steel at different displacements during the in-situ tensile test in SEM; where (a) is at a displacement of 0 mm; (b) is at a displacement of 0.3 mm; (c) is at a displacement of 1.0 mm; (d) is at a displacement of 1.5 mm.

[0034] Figure 7 Microstructure characteristic diagrams of No. 2 steel at different displacements during the in-situ tensile test in SEM; where (a) is at a displacement of 0 mm; (b) is at a displacement of 0.3 mm; (c) is at a displacement of 1.0 mm; (d) is at a displacement of 1.5 mm.

[0035] Figure 8 (a) is the average elongation of ferrite and bainite; (b) is the difference in the average elongation of ferrite and bainite.

[0036] Figure 9 Is the formation process of microvoids during in-situ tensile, where (a) is No. 1 steel; (b) is No. 2 steel. Specific implementation mode

[0037] Example 1 A kind of FB dual-phase steel with high hole-expanding performance

[0038] The FB dual-phase steel with high hole-expanding performance in this example, its chemical composition by weight percentage includes: C: 0.076%, Si: 0.34%, Mn: 1.565%, Al: 0.031%, Nb: 0.041%, Ti: 0.022%, the balance is Fe and unavoidable impurities, and in the unavoidable impurities, P≤0.015%, S≤0.005%.

[0039] Example 2 The controlled rolling and controlled cooling process of a kind of FB dual-phase steel with high hole-expanding performance

[0040] The controlled rolling and controlled cooling process in this example includes the following steps:

[0041] S1. Conducted on a φ550mm two-high rolling mill, and the rolling mill is equipped with a cooling water system. Heat the billet (50mm×100mm×120mm) with the composition recorded in Example 1 to 1503K, hold for 1.5h, air-cool to the rolling start temperature of 1373K, conduct four passes of rolling in the austenite recrystallization zone, reduce the thickness from 50mm to 10mm, the total reduction ratio is 80%, conduct three passes of rolling in the non-recrystallization zone, reduce the thickness from 10mm to 4mm, the total reduction ratio is 60%, and the finishing rolling temperature is about 1093K.

[0042] S2. Conduct controlled cooling on No. 1 steel and No. 2 steel respectively, including four processes of "the first-stage water cooling, air cooling, the second-stage water cooling, isothermal", and the controlled cooling process and parameters are as Figure 1As shown in Table 1, where the finish rolling temperature is denoted by T1, the air cooling temperature and air cooling time after the first stage of water cooling are denoted by T2 and t1 respectively, and the isothermal temperature and isothermal time after the second stage of water cooling are denoted by T3 and t2 respectively.

[0043] Table 1 Steel controlled cooling process parameters

[0044]

[0045] The finish rolling process is used to control the initial morphology of austenite and the stored energy of deformation, affecting the ferrite phase transformation rate and morphology; water cooling is used to avoid the formation of non-ideal phases; the air cooling temperature and air cooling time are used to control the ferrite formation temperature and growth time, thereby controlling the ferrite morphology and ferrite content; the isothermal temperature is the key to realizing bainite transformation and also affects the bainite morphology. The finish rolling temperatures of Steel No. 1 and Steel No. 2 are the same, but they have different air cooling temperatures, air cooling times, and bainite isothermal temperatures, so that the two steels can have different microstructures and mechanical properties.

[0046] Performance verification method for FB dual-phase steel with high hole expansion performance in Example 3

[0047] The performance verification method for FB dual-phase steel with high hole expansion performance in this example includes the following steps:

[0048] Step 1: Conduct a hole expansion experiment on a sheet metal forming testing machine to measure the hole expansion rate of the FB dual-phase steel sample.

[0049] The hole expansion experiment is carried out on a BCS-50A sheet metal forming testing machine. The specimen size is 4mm×100mm×100mm. A prefabricated hole is machined at the center of the hole expansion specimen by punching, and then the hole edge is polished to a certain extent. The diameter of the center hole is 10mm. In the hole expansion experiment, a conical punch is used to expand and flanging the steel. The angle of the conical punch is 60°. The loading force is maintained at 20kN, and the punching speed is 20mm / min. Once a crack appears in the thickness direction of the steel, the loading is stopped. The hole expansion rate λ value is defined by the following formula:

[0050]

[0051] In the formula: d i is the diameter of the center hole before hole expansion; d f is the diameter of the center hole when the loading stops.

[0052] Step 2: Conduct an in-situ tensile experiment on the FB dual-phase steel sample, and use a variety of microscopes to observe the microstructure to observe the tissue deformation and failure mechanism.

[0053] An in-situ tensile experiment was carried out using a CS3400 scanning electron microscope with a specification of 5 kN. Samples with a thickness of 1 mm were cut from the core of the steel and then processed into a specific shape. In order to ensure that the observation area corresponds to the maximum deformation area and to be able to simulate the punching defect at the edge of the central hole of the expanded-hole sample, two V-shaped notches of about 60° were machined in the middle of the tensile specimen, as Figure 2 shown. Subsequently, the surface of the sample was subjected to rough grinding, fine grinding, polishing and etching. The etching solution was a 4% nitric acid alcohol solution. Then it was placed in a tensile testing machine and subjected to tensile deformation at a displacement rate of 2 mm / min. By periodically interrupting, the observation of the microstructure images of the preselected area was realized, so as to realize the analysis of the morphological changes of ferrite and bainite tissues, the formation of pores and the crack propagation behavior.

[0054] The microstructure was observed using a Leica DMR type optical microscope and a ZEISS EVO18 type scanning electron microscope. Using Image-tool software, the average ferrite grain size (d f ) and volume fraction (V f ) were measured on at least 10 metallographic pictures at 500 times magnification. The microstructure of each sample was observed using a TECNAI F-20 transmission electron microscope. The samples for optical microscope and scanning electron microscope observation need to be mechanically ground, polished and etched. The etching solution is a 4% nitric acid alcohol solution.

[0055] Microstructure analysis: The metallographic photos, SEM photos and TEM photos of Steel No. 1 and Steel No. 2 are as Figure 3 shown. Figure 3 (a) shows that the microstructure of Steel No. 1 consists of polygonal ferrite and massive bainite. The average ferrite grain size is 6.4 μm, and the ferrite volume fraction is about 81%. Different from Steel No. 1, in the microstructure of Steel No. 2 ( Figure 3 (b)), in addition to polygonal ferrite and massive bainite, there are also a large number of acicular ferrite, and acicular bainite tissues are accompanied around it. The average ferrite grain size of Steel No. 2 is 5.6 μm, and the ferrite volume fraction is about 76%. The ferrite and bainite tissues of Steel No. 2 show fine and diverse tissue morphologies, and the bainite volume fraction is 5% higher.

[0056] From Figure 3As can be seen from (c), the ferrite of Steel No. 1 looks purer and "flatter" because the ferrite formation temperature of Steel No. 1 is higher, the transformation time is longer, the diffusion of carbon element is more sufficient, the lattice distortion inside the ferrite is smaller, so it looks more "flat". This will also lead to a higher carbon content in the austenite that has not undergone phase transformation, and ultimately a higher carbon content in the bainite. On the contrary, due to the low ferrite transformation temperature and short time of Steel No. 2, the ferrite grains are finer, and a lot of acicular ferrite is produced. The supersaturated carbon in the ferrite either precipitates carbides, and the lattice distortion inside the ferrite is larger, as shown in Figure 3 (d).

[0057] It can be clearly seen from the TEM photos that both the polygonal ferrite and the acicular ferrite grains contain mobile dislocations, but they show different internal structural characteristics. The polygonal ferrite in Steel No. 1 has an equiaxed morphology, and the dislocations inside it are entangled, but relatively sparse ( Figure 3 (e)). The acicular ferrite in Steel No. 2 presents a lath shape, and the acicular ferrite is interlaced with each other, and it has a higher density of tangled dislocations inside. Compared with the dislocations in the polygonal ferrite, the dislocations in the acicular ferrite are more dense ( Figure 3 (f)).

[0058] The total deformation amount and the finish rolling temperature of Steel No. 1 and Steel No. 2 are the same. Therefore, the deformation energy storage of austenite before controlled cooling is the same. The air cooling start temperature of Steel No. 1 is 973K, which is 50K higher than that of Steel No. 2. Therefore, the ferrite phase transformation temperature is higher, resulting in a lower ferrite phase transformation driving force and a lower ferrite nucleation rate. Therefore, the number of ferrite grains will be less. However, compared with Steel No. 2, the air cooling time of Steel No. 1 is 15s, and the ferrite has more sufficient growth time. Therefore, larger polygonal ferrite is formed, which also leads to a higher ferrite volume fraction. In contrast, the air cooling temperature in Steel No. 2 is lower, which provides a more sufficient driving force and more nucleation points for the ferrite phase transformation. In addition, due to the lower air cooling temperature and shorter time of Steel No. 2, finer ferrite grains and a large amount of acicular ferrite tissue are produced. Research shows that acicular ferrite forms in the range between polygonal ferrite and bainite, and the air cooling process of Steel No. 2 just goes through this interval ( Figure 1 ). The isothermal temperature of Steel No. 1 is 673K, and the supercooled austenite mainly forms lath bainite tissue. Due to the few ferrite nucleation points, the supercooled austenite is relatively coarse, so the formed bainite clusters are relatively coarse. The isothermal temperature of Steel No. 2 is 723K. Since the acicular ferrite has already divided the supercooled austenite very severely, the size and morphology of the bainite are restricted, so finer and more dispersed acicular bainite is formed.

[0059] Hole-expanding performance: The hole-expanding ratios of Steel No. 1 and Steel No. 2 are 52% and 83% respectively. A main crack and several micro-cracks are generated on the deformed lips of the specimens. These cracks are all approximately 45° to the plane of the deformed lips and the radial direction of the sheet, and all originate from the upper surface of the central hole (i.e., the outer side of the deformed lip). This is mainly because the upper surface of the central hole is a free surface and the amount of bending deformation is larger than that of the lower surface, so the tensile stress is greater; while during the bending deformation of the lower surface, it is subjected to both the compressive stress of the external punch and the compressive stress of the internal sheet, so it is not easy to crack. Since the hole-expanding ratio of Steel No. 2 is larger, the thickness of its deformed lip is thinner.

[0060] The microstructure at the crack tip near the deformed central hole is as Figure 4 shown. During the experiment, the steel underwent tensile deformation along the circumferential direction and bending deformation perpendicular to the plane, and the tensile deformation mainly affects the final hole-expanding ratio of the sheet. For these two types of steel, many micro-voids are at a certain angle to the radial direction, which is consistent with the macroscopic crack direction. With the increase of deformation, the micro-voids expand and merge, and finally evolve into macroscopic cracks.

[0061] Figure 4 It shows that the size of the micro-voids in Steel No. 1 is larger and sparser, while the micro-voids in Steel No. 2 are finer and denser. Due to the acicular ferrite structure, Steel No. 2 has a higher hole-expanding ratio. When the micro-cracks expand, the crack tip is strongly blocked by the acicular ferrite bundles. The acicular ferrite bundles are interlocked and cross-distributed, which will consume more energy and result in a longer and more tortuous crack propagation path. In addition, the dislocation density of the acicular ferrite near the crack tip increases rapidly, which will form many dislocation cells, and finally form a high-density dislocation on the entire stress surface, further inhibiting the crack propagation. Since the central hole in this hole-expanding experiment is prepared by stamping process, there are inevitably many initial micro-cracks at the edge of the central hole, and Steel No. 2 shows a better hole-expanding ratio, which indicates that the acicular ferrite structure has lower defect sensitivity.

[0062] SEM in-situ tensile performance: The SEM in-situ tensile experiment can observe the evolution process of the microstructure during deformation, so it can reveal the influence law of the microstructure of different morphologies of FB dual-phase steel on the mechanical properties and the deformation mechanism. In order to better observe the deformation and evolution process of the microstructure, the observation point is selected at a position about 0.7 mm away from the edge notch. The schematic diagram of the observation point and the displacement-load curve are as Figure 5As shown, where the displacement is represented by d and the load value is represented by f. The slightly decreasing region of the load in the curve corresponds to the position where the tensile stop occurs. When reloaded, the load resumes to its original value and continues to deform. According to the displacement-load curve, Steel No. 1 enters the plastic deformation stage earlier and has a more significant work-hardening effect. This is because the mobile dislocations at the phase interface of ferrite and bainite with larger grain sizes are more likely to achieve multiple slip. Steel No. 2 has a higher yield strength and tensile strength, and at the same time has a better elongation rate. On the one hand, it is because the finer ferrite grains play a role in grain refinement strengthening, and on the other hand, it is because it has a certain amount of acicular ferrite structure. Acicular ferrite is a medium-temperature product when the steel undergoes medium-temperature isothermal transformation or continuous cooling transformation at a medium cooling rate. The formation temperature is above the bainite transformation temperature. Since the air-cooling temperature of Steel No. 2 is relatively low, slightly higher than the bainite transformation region, acicular ferrite is formed. The acicular ferrite contains many tiny carbides, which can effectively improve the strength and toughness of the material.

[0063] The microstructural morphologies corresponding to different displacement deformation stages of Steel No. 1 and Steel No. 2 are respectively as Figure 6 and Figure 7 shown. The four selected positions respectively correspond to the morphologies in the initial stage, elastic deformation stage, uniform plastic deformation stage, and non-uniform plastic deformation stage.

[0064] In the elastic deformation stage (d = 0.3 mm), the tensile stress is released through the elastic deformation of the ferrite matrix. At this time, both Steel No. 1 and Steel No. 2 are in the overall deformation stage, and the strain is relatively small. Therefore, the microstructure has not changed much from the initial state. In the initial stage of plastic deformation, since ferrite is a softer phase, ferrite first undergoes plastic deformation, generating dislocations and starting to slip. When the displacement reaches 1.0 mm, Steel No. 1 is in the latter stage of uniform plastic deformation, and many dislocations have slipped. Relative to the small-sized ferrite, the large ferrite grains are more likely to appear local deformation bands ( Figure 6 yellow square area). When the displacement reaches 1.5 mm, Steel No. 1 has entered the non-uniform plastic deformation stage. It can be seen that a large number of slip bands have appeared in almost every ferrite grain. During the deformation process, bainite undergoes coordinated deformation. The yield strength of bainite with relatively dispersed carbide distribution is closer to that of ferrite, and the coordinated deformation ability is better ( Figure 6 blue circle area). This is mainly because the dispersed carbides do not overly restrict the ferrite matrix, and the ferrite matrix can still undergo relatively sufficient plastic deformation, even forming slip bands ( Figure 6 red triangle area). However, when there are more carbides in the bainite cluster ( Figure 6In the green diamond-shaped area), the plastic deformation ability of bainite will be greatly reduced, and the ability to coordinate deformation will be very poor. Even in the stage of non-uniform plastic deformation (d = 1.5 mm), almost no plastic deformation occurs, which increases the plastic incompatibility between ferrite and bainite, thus reducing the plastic deformation ability of the FB dual-phase steel, and it is easier to generate microvoids at the phase interface.

[0065] The plastic deformation ability of Steel No. 2 is better than that of Steel No. 1. From Figure 5 It can be seen that when the displacement is 0.6 mm, Steel No. 1 has entered the plastic deformation stage, while Steel No. 2 is still in the elastic deformation stage, indicating that Steel No. 2 has a stronger ability to resist deformation. When the displacement of Steel No. 2 reaches 1.5 mm, it is at the end of the uniform plastic deformation stage. At this time, the deformation state of the microstructure of Steel No. 2 is very similar to that of Steel No. 1 when the displacement is 0.8 mm, and obvious slip bands ([ Figure 7 blue circle area and yellow square area) appear only in some polygonal ferrite grains. When the displacement reaches 1.9 mm, Steel No. 2 is in the non-uniform plastic deformation stage. At this time, Steel No. 2 has experienced more severe plastic deformation than Steel No. 1, with more slip bands in ferrite, and slip band intersections ([ Figure 7 orange rectangle area) also appear in some ferrite grains. Due to the intersection of different slip planes, the strengthening effect of double slip is more significant than that of single slip. Although there is also some bainite in Steel No. 2 with a very small deformation amount ([ Figure 7 green diamond-shaped area), but because most of the bainite is finer and more dispersed, the plastic compatibility between most of the bainite and ferrite is better. The bainite can deform better following the deformation of ferrite, and at the same time hinder the movement of the slip bands in ferrite, thus ensuring that Steel No. 2 has good toughness.

[0066] Using the Image-tool software, the elongation (A) of a large number of ferrite grains and bainite grains at different observation points of the steel ([ Figure 5 ) was statistically analyzed and the average value was calculated, and the difference between the ferrite elongation (A f ) and the bainite elongation (A b ) was calculated, as Figure 8 shown. As can be seen from Figure 8 (a), the elongation of ferrite is significantly greater than that of bainite, because ferrite is the matrix and has better plastic deformation ability. Under the same displacement, the ferrite elongation of Steel No. 1 is higher than that of Steel No. 2, while the bainite elongation of Steel No. 1 is lower than that of Steel No. 2. The differences between the ferrite elongation and the bainite elongation in the two steels were statistically analyzed respectively, as Figure 8(as shown in (b)). It can be seen that the difference in elongation between ferrite and bainite in Steel No. 1 is significantly greater than that in Steel No. 2, indicating that the deformation rates of ferrite and bainite in Steel No. 2 are closer, and the plastic compatibility is better.

[0067] Figure 9 are microvoids formed during the in-situ tensile test of the steel under SEM. When the displacement is 1.5 mm, obvious microvoids have appeared in Steel No. 1 ( Figure 9 (a)). Most of the microvoids are formed on the ferrite phase, and the voids are single and expand relatively rapidly. For Steel No. 2, when the displacement is 1.9 mm, more microvoids appear ( Figure 9 (b)). Although most of the voids also appear on the ferrite, the bainite around the voids can better restrict the deformation of the ferrite, making the voids expand more slowly. As a result, the stress will transfer to the periphery of the voids to form new voids, thereby reducing the stress concentration of a single void and inhibiting the growth and expansion of cracks.

[0068] It can be known from the performance verification method of the FB dual-phase steel in this embodiment that:

[0069] (1) For the dual-phase steel of the C-Si-Mn-Nb-Ti system, a higher ferrite formation temperature (973 K) and a longer air-cooling time (15 s) are beneficial to the formation of larger-sized polygonal ferrite; while a lower ferrite formation temperature (923 K) and a shorter air-cooling time (10 s) are beneficial to the formation of fine acicular ferrite. These acicular ferrites are dispersed, effectively cutting the undercooled austenite, resulting in the subsequent formation of bainite being dispersed and showing a characteristic of interlacing with the acicular ferrite.

[0070] (2) The reaming experiment shows that the macroscopic crack propagation direction and the microscopic void distribution direction of both steels are approximately 45° to the radial direction. The reaming rate of Steel No. 2 (83%) is significantly higher than that of Steel No. 1 (52%). The microvoids formed in Steel No. 1 are large and sparse, while the microvoids formed in Steel No. 2 are finer and more dispersed, making Steel No. 2 have lower defect sensitivity.

[0071] (3) It can be known from the SEM in-situ tensile experiment that the acicular ferrite structure can better inhibit the growth of microvoids and the propagation of microcracks. This is beneficial to dispersing the external stress, reducing the stress concentration, and thus effectively delaying the failure process. Therefore, the FB dual-phase steel with an acicular ferrite structure has higher strength and elongation, and at the same time has more excellent forming performance.

[0072] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are all included within the protection scope of the present invention.

Claims

1. A FB dual-phase steel with high hole expansion performance, characterized in that: The chemical composition of the dual-phase steel includes, by weight percentage, C: 0.07-0.09%, Si: 0.30-0.40%, Mn: 1.50-1.60%, Al: 0.03-0.04%, Nb: 0.03-0.05%, Ti: 0.02-0.03%; the balance is Fe and unavoidable impurities.

2. The FB dual-phase steel with high hole expansion performance according to claim 1, characterized in that: The chemical composition of the dual-phase steel includes, by weight percentage, C: 0.076%, Si: 0.34%, Mn: 1.565%, Al: 0.031%, Nb: 0.041%, Ti: 0.022%, and the remainder is Fe and inevitable impurities, among which P≤0.015% and S≤0.005%.

3. A controlled rolling and controlled cooling process for FB dual-phase steel with high hole expansion performance, characterized in that: The following steps are involved: S1. The billet is heated to 1450-1550K on a rolling mill equipped with a cooling water system, kept at this temperature for 1-2h, and air-cooled to the starting rolling temperature of 1300-1400K. Four passes of rolling are performed in the austenite recrystallization zone, and three passes of rolling are performed in the non-recrystallization zone. The final rolling temperature is 1080-1100K. S2. Implementing controlled cooling, comprising the following steps: S21. The first stage is water-cooled to 920-980K; S22. Air cooling 8-18s; S23. The second stage is water-cooled to 650-750K; S24. Isothermal treatment for 1.5-2h.

4. The controlled rolling and controlled cooling process of FB dual-phase steel according to claim 3, characterized in that: In step s1, the billet is heated to 1503 K on a rolling mill equipped with a cooling water system and kept at this temperature for 1.5 h.

5. The controlled rolling and controlled cooling process of FB dual-phase steel according to claim 3, characterized in that: In step s1, the starting rolling temperature is 1373K and the final rolling temperature is 1093K.

6. The controlled rolling and controlled cooling process of FB dual-phase steel according to claim 3, characterized in that: In step s2, the first stage is water-cooled to 923 K, air-cooled for 10 s, and the second stage is water-cooled to 723 K. The isothermal treatment time is 2 h.

7. Use of the FB dual-phase steel according to any one of claims 1 to 6 in automobile stamping parts.

8. The application of FB dual-phase steel in automobile stamping parts according to claim 7 is characterized in that: The stamped parts include body reinforcements, wheels or chassis components.

9. The performance verification method of FB dual-phase steel with high hole expansion performance according to claim 1, characterized in that: The following steps are involved: Step 1: Conduct a hole expansion test on a plate forming test machine to determine the hole expansion rate of the FB dual-phase steel sample; Step 2: In situ tensile tests were performed on the FB dual-phase steel samples, and the microstructure was observed using a variety of microscopes to observe the deformation and failure mechanism of the organization.

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