780MPa-grade low-cost hot-rolled high-strength high-hole-expansion steel and application thereof

Abstract

The invention discloses 780MPa-grade low-cost hot-rolled high-strength high-hole-expansion steel and an application thereof. The microstructure of the 780MPa-grade low-cost hot-rolled high-strength high-hole-expansion steel is a full-ferrite-based microstructure composed of polygonal ferrite and nano carbide; the grain size of the polygonal ferrite is less than or equal to 5 microns; the size of the nano carbide is less than or equal to 10nm; the yield strength of the 780MPa-grade low-cost hot-rolled high-strength high-hole-expansion steel is larger than or equal to 680MPa, the tensile strength is larger than or equal to 780MPa, the ductility is larger than or equal to 18%, and the hole expansion rate is larger than or equal to 70%; the 780MPa-grade low-cost hot-rolled high-strength high-hole-expansion steel comprises the following components in percentage by mass: 0.02-0.1% of C, 0.02-0.1% of Si, 0.02-0.1% of Cr, si: less than or equal to 0.2%; 1.0 to 1.8 percent of Mn; p: less than or equal to 0.02%; s: < = 0.005%; 0.02 to 0.05 percent of Al; n: < = 0.005%; 0.04% to 0.15% of Ti; nb: less than or equal to 0.05%; 0.3 to 0.6 percent of Cr; o: < = 0.003%; and the balance of Fe. The preparation method comprises the following steps: S1, smelting and casting; s2, heating a casting blank or a cast ingot; s3, hot rolling; and S4, cooling and coiling are conducted, and the 780 MPa-grade low-cost hot-rolled high-strength high-hole-expansion steel is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-strength steel, and in particular relates to 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel and applications thereof. Background Art

[0002] With the increasingly stringent environmental laws and regulations and the implementation of restrictions on automobile emissions, high-strength thinning or lightweight automobile structures have become an important development direction in the automotive field, especially in the field of passenger cars. From the perspective of energy conservation and emission reduction, there are two main ways to improve automobile fuel efficiency: one is to develop engines with higher fuel efficiency; the other is to use high-strength steel or lightweight steel to reduce the weight of the car body, thereby reducing emissions.

[0003] Due to the structural complexity of components such as chassis and suspension, passenger cars usually need to use steel plates with higher hole expansion rates to ensure that cracks do not occur during stamping. However, there is a contradictory relationship between the hole expansion and strength of steel plates, and high strength usually leads to lower hole expansion performance. Therefore, in order to manufacture components such as automobile chassis and suspension, automobile manufacturers usually choose high hole expansion steels with lower strength levels such as 440MPa and 590MPa; the microstructure of high hole expansion steels of 440MPa and 590MPa levels is mainly composed of ferrite and bainite, and sometimes contains a small amount of martensite. Ferrite and bainite structures have higher hole expansion performance, but the relative strength is lower, so it is difficult to reach a strength level of 780MPa and above. Although ferrite and martensite structures have lower yield strength ratios and higher strength, due to the large difference in performance between the two phases, their hole expansion performance is the worst among high-strength steels at the same strength level.

[0004] In order to solve the contradiction between the hole expansion and strength of steel plates, it is urgent to develop a steel plate with higher strength, such as 780MPa or even 980MPa, while having hole expansion and flanging performance comparable to low-strength steel plates. Summary of the invention

[0005] In view of this, some embodiments disclose a 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel, wherein the microstructure of the 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel is a full ferrite-based microstructure composed of polygonal ferrite and nanocarbide; the grain size of the polygonal ferrite is ≤5μm; the size of the nanocarbide is ≤10nm; the yield strength of the 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel is ≥680MPa, the tensile strength is ≥780MPa, the elongation is ≥18%, and the hole expansion rate is ≥70%;

[0006] In terms of mass percentage, the components of 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel include:

[0007] C: 0.02~0.1%; Si: ≤0.2%; Mn: 1.0~1.8%; P: ≤0.02%; S: ≤0.005%;

[0008] Al: 0.02~0.05%; N: ≤0.005%; Ti: 0.04~0.15%; Nb: ≤0.05%; Cr: 0.3~0.6%; O: ≤0.003%; the balance is Fe;

[0009] The preparation method of 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel comprises:

[0010] S1. Smelting and casting

[0011] According to the composition of the above-mentioned 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, smelting, refining and continuous casting are carried out into a casting billet or an ingot;

[0012] S2. Heating of billet or ingot

[0013] Heat the billet or ingot at a holding temperature of 1200-1260°C for 1-3 hours;

[0014] S3, hot rolling

[0015] Two-stage controlled rolling is adopted, the starting rolling temperature is 1080-1150℃, the cumulative deformation of rough rolling is ≥50%, then the intermediate billet is heated to 950-1000℃, and then 4-7 passes of finishing rolling are carried out with a cumulative deformation of ≥70%, and the final rolling temperature is 860-900℃;

[0016] S4, cooling and coiling

[0017] After final rolling, the steel plate is cooled to 600-650°C by continuous laminar cooling at a cooling rate of 20-40°C / s. The steel plate is then coiled and slowly cooled to room temperature at a cooling rate of ≤20°C / h to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel.

[0018] Some embodiments disclose 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, and the grain size of polygonal ferrite is 3-5 μm.

[0019] Some embodiments disclose 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, and the thickness of the 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel is 2.0 to 6.0 mm.

[0020] Some embodiments disclose 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, the C content of which is 0.03-0.07%.

[0021] Some embodiments disclose 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, with Mn / Si of 7-10.

[0022] Some embodiments disclose 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, the Ti content of which is 0.05-0.1%.

[0023] Some embodiments disclose 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, with Nb content ≤ 0.03%.

[0024] Some embodiments disclose 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, wherein the nitrogen content is 0.004-0.005.

[0025] Some embodiments disclose an application of 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, where the 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel is used to produce automobile body structural parts.

[0026] The 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention has a high Ti content, so that a large amount of dispersed fine nano-carbides are precipitated in the ferrite during the hot rolling coiling stage, which has a strong dispersion precipitation strengthening effect; the Si content is low, which can avoid the formation of red iron scale on the surface of the strip steel, which is conducive to the stable control of key process parameters in the actual production process; the Cr content is 0.3-0.6%, which can improve the strength of the steel by solid solution strengthening or refinement of the structure; there is no precious metal Mo, and the production cost is low. The 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention has a yield strength of ≥680MPa, a tensile strength of ≥780MPa, an elongation of ≥18%, a hole expansion rate of ≥70%, a thickness of 2.0-6.0mm and a high surface quality. It is suitable for manufacturing automotive body structural parts that require high-strength thinning and hole expansion and flanging, and has broad application prospects. The method for preparing 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention is to quickly cool the strip steel to a suitable coiling temperature at a high cooling rate after the final rolling, so that the strip steel obtains a full ferrite-based microstructure composed of fine polygonal ferrite and nano-carbide, thereby obtaining 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel with excellent strength, plasticity and ultra-high hole expansion rate. The method for preparing 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention can be applied to existing hot rolling production lines to achieve mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Example 1 Typical metallographic photograph of 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel;

[0028] Figure 2Example 1 Typical transmission photograph of 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel;

[0029] Figure 3 Example 3 Typical metallographic photograph of 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel;

[0030] Figure 4 Example 3 Typical transmission photograph of 780MPa grade low cost hot rolled high strength high hole expansion steel. DETAILED DESCRIPTION

[0031] The word "embodiment" used here as an "exemplary" does not necessarily mean that any embodiment described is superior to or better than other embodiments. Unless otherwise specified, the performance index tests in the embodiments of this application are performed using conventional test methods in the art. It should be understood that the terms described in this application are only used to describe specific implementation methods and are not used to limit the content disclosed in this application.

[0032] Unless otherwise specified, the technical and scientific terms used in this document have the same meanings as commonly understood by ordinary technicians in the technical field to which this application belongs; other experimental methods and technical means not specifically specified in this application refer to experimental methods and technical means commonly used by ordinary technicians in this field.

[0033] The terms "substantially" and "approximately" used herein are used to describe small fluctuations. For example, they can refer to 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%. The numerical data represented or presented in the range format herein are used only for convenience and brevity, and should therefore be flexibly interpreted as including not only the values ​​clearly listed as the limits of the range, but also all independent values ​​or sub-ranges contained in the range. For example, the numerical range of "1-5%" should be interpreted as including not only the clearly listed values ​​of 1% to 5%, but also the independent values ​​and sub-ranges within the range shown. Therefore, independent values ​​such as 2%, 3.5% and 4% and sub-ranges such as 1%-3%, 2%-4% and 3%-5% are included in this numerical range. This principle also applies to the range of only one numerical value. In addition, such an interpretation applies regardless of the width of the range or the characteristics described.

[0034] In this document, including in the claims, transitional words such as "comprises," "includes," "with," "having," "containing," "involving," "accommodating," etc. are understood to be open-ended, i.e., meaning "including but not limited to." Only the transitional words "consisting of" and "composed of" are closed transitional words.

[0035] In order to better illustrate the content of the present application, numerous specific details are provided in the specific examples below. It should be understood by those skilled in the art that the present application can also be implemented without certain specific details. In the embodiments, some methods, means, instruments, equipment, etc. well known to those skilled in the art are not described in detail in order to highlight the main purpose of the present application.

[0036] Under the premise of no conflict, the technical features disclosed in the embodiments of the present application can be arbitrarily combined, and the obtained technical solutions belong to the contents disclosed in the embodiments of the present application. It should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like mentioned in the present application indicate the orientation or position relationship based on the orientation or position relationship shown in the accompanying drawings, which is only for the convenience of describing the technical features and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention, unless it conflicts with the context. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance, unless it conflicts with the context.

[0037] In some embodiments, the 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, in terms of mass percentage, comprises the following components:

[0038] C: 0.02-0.1%; Carbon is used to expand the austenite phase region and stabilize austenite. Carbon, as an interstitial atom in steel, can improve the solid solution strengthening ability of steel and form sufficient carbide strengthening phase to ensure the strength of steel; Carbon, titanium and niobium will form carbides and precipitate. Low carbon content will make the solid solution carbon content of carbon in steel too low or zero, causing the thermoplasticity of the slab to deteriorate and cause the slab to break, which will affect the production stability and manufacturability of the product. High carbon content will make the precipitated carbide particles coarse, which is not conducive to the hole expansion performance of the steel plate; the present invention requires that the solid solution carbon content of the steel plate is greater than 50ppm, preferably, the C content is 0.03-0.07%;

[0039] Si: ≤0.2%; Silicon plays a role in solid solution strengthening, and the addition of silicon helps to improve the elongation of the steel plate; the presence of silicon in steel can easily cause surface defects such as red iron scale on the surface of the steel plate, making the surface roughness of the pickled plate uneven, which has an adverse effect on the apparent quality. In addition, too high a silicon content will lead to silicon enrichment on the surface of the steel plate, which is not conducive to the phosphating coating performance of the steel plate; in the present invention, in order to ensure good surface quality, the content of Si should be strictly controlled to be ≤0.2% during the composition design;

[0040] Mn: 1.0-1.8%; Manganese is used to expand the austenite phase, stabilize austenite, refine grains and promote the transformation of austenite to pearlite; manganese is also a solid solution strengthening element. When the manganese content is increased, the plasticity of the steel decreases, causing the segregation of the components of the continuous casting slab, affecting the uniformity of the organization of the hot-rolled plate, and being unfavorable to the hole expansion rate of the steel plate; the strength of the present invention is mainly strengthened by titanium and niobium alloy elements, and the manganese content can be appropriately reduced. The strength, plasticity and hole expansion rate of the steel plate can be balanced by controlling the appropriate ratio of Mn / Si; in some embodiments, Mn / Si is 7-10;

[0041] P: ≤0.02%; Phosphorus is an impurity element in steel. Too high a phosphorus content will affect the weldability of the steel plate. The phosphorus content should be as low as possible, but pursuing too low a phosphorus content will increase the smelting cost. Controlling the phosphorus content below 0.02% can meet the requirements of steel plate production cost control and user use;

[0042] S: ≤0.005%; Sulfur is an impurity element in steel, and the lower the content, the better. Sulfur easily forms MnS with manganese in steel. MnS itself has a certain plasticity. In the subsequent rolling process, MnS is easy to deform along the rolling direction, reducing the transverse plasticity of the steel and increasing the anisotropy of the organization, which is not conducive to the hole expansion performance;

[0043] Al: 0.02-0.05%; Aluminum is a deoxidizing element in steel, which can reduce oxide inclusions in steel, purify steel, and improve the formability of steel plates;

[0044] N: ≤0.005%; Nitrogen is an impurity element in steel, and the lower its content, the better; the present invention adopts a high-titanium design, and nitrogen is easily combined with the strong carbide-forming element Ti to form TiN particles. The TiN particles are square, and there is a large stress concentration between the top corners and the matrix. During the hole expansion deformation process, the stress concentration between the TiN particles and the matrix is ​​easy to form a cracking source, thereby reducing the hole expansion performance of the material; in order to minimize the adverse effects of TiN on hole expansion, the nitrogen content should be controlled below 0.005%; generally, the nitrogen content is 0.004-0.005%;

[0045] Ti: 0.04-0.15%; Titanium is one of the main strengthening alloy elements of the present invention. Titanium plays two main roles in the present invention: first, it combines with the impurity element N in the steel to form TiN, which plays a part of the "nitrogen fixation" role; second, in the coiling phase transformation process, it forms dispersed, uniform and fine nano-scale carbides from the ferrite to improve the strength, plasticity and hole expansion rate of the steel; and the price of titanium alloy is lower than that of niobium alloy, which can reduce costs; generally, the Ti content can be 0.04-0.1%, 0.05-0.1%; preferably, the Ti content is 0.05-0.15%;

[0046] Nb: ≤0.05%; Niobium is one of the main strengthening alloy elements of the present invention. Niobium is finely precipitated in the form of carbides during cooling or coiling after rolling, thereby improving the strength of the steel; when the niobium content is high, the strengthening effect of niobium is weakened. Considering the price of niobium alloy, the niobium content is controlled below 0.05%; preferably, the Nb content is ≤0.03%;

[0047] Cr: 0.3-0.6%; Chromium mainly improves the strength of steel by solid solution strengthening or refining the structure; adding a small amount of chromium can appropriately reduce the yield strength of steel, thereby reducing the yield strength ratio. In addition, the addition of a small amount of chromium can also improve the corrosion resistance;

[0048] O: ≤0.003%; oxygen is an inevitable element in the steelmaking process. For the present invention, the oxygen content in the steel can generally be reduced to less than 30 ppm after deoxidation, which will not cause significant adverse effects on the performance of the steel plate; therefore, the oxygen content in the steel can be controlled within 30 ppm;

[0049] The balance is Fe and unavoidable impurity elements;

[0050] The microstructure of 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel is a full ferrite-based microstructure composed of polygonal ferrite and nano-carbide; the fine grain strengthening played by the tiny ferrite grains and the precipitation strengthening played by the precipitation of nano-scale carbides give the steel plate excellent comprehensive properties;

[0051] The yield strength of 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel is ≥680MPa, the tensile strength is ≥780MPa, the elongation is ≥18%, and the hole expansion ratio is ≥70%.

[0052] In some embodiments, the pore expansion ratio (λ, in %) is calculated by the following method:

[0053] Take a 100×100mm hot-rolled steel plate and punch a hole with a diameter of 10mm on it as the original hole diameter (d1);

[0054] Use a conical punch with a vertex angle of 60° to expand the punched hole, and measure the hole diameter (d2) when the crack generated in the punched part penetrates the plate thickness;

[0055] The hole expansion ratio is calculated from hole expansion ratio (λ) = [(d2-d1) / d1] × 100. Usually, the experiment is performed at least three times, and the average value of the hole expansion ratio is taken. In the present invention, a value of λ of 55% or more is considered acceptable.

[0056] In some embodiments, the grain size of the polygonal ferrite is ≤ 5 μm.

[0057] In some embodiments, the grain size of the polygonal ferrite is 3-5 μm.

[0058] In some embodiments, the size of the nanocarbide is ≤ 10 nm.

[0059] In some embodiments, the thickness of the 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel is 2.0 to 6.0 mm.

[0060] In some embodiments, the method for preparing 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel comprises:

[0061] S1. Smelting and casting

[0062] According to the composition of the above-mentioned 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, smelting, refining and continuous casting are carried out into a casting billet or an ingot;

[0063] S2. Heating of billet or ingot

[0064] Heat the billet or ingot at a temperature ≥ 1200°C for 1 to 2 hours;

[0065] In order to match the high Ti composition design, the heating temperature of the billet or ingot must be ≥1200℃ to ensure that as many Ti atoms as possible are dissolved in the slab; since the solid solution temperature of Ti carbonitride is ≥1300℃, it will precipitate at different stages of steelmaking, continuous casting and rolling, which makes the final Ti content that can be used for precipitation strengthening very low. Therefore, a high heating temperature must be ensured to obtain more nano-carbides in the final coiling process; the upper limit of the heating temperature is based on the actual achievable or tolerable temperature of the on-site heating furnace, as long as it does not exceed the liquidus temperature of the billet;

[0066] The heating time of the billet or ingot is mainly to ensure that the slab can be evenly "burned through". The heating time should not be too long, otherwise the Ti carbonitrides that are not dissolved at high temperature are likely to coarsen and grow. These coarse Ti carbonitrides will precipitate at the austenite grain boundaries, reducing the bonding strength of the grain boundaries and easily causing the slab to "break" during the heating process. Therefore, depending on the thickness of the slab, the heating temperature is usually controlled at 1 to 2 hours.

[0067] S3, hot rolling

[0068] Two-stage controlled rolling is adopted, the starting rolling temperature is 1080-1150℃, the cumulative deformation of rough rolling is ≥50%, then the intermediate billet is heated to 950-1000℃, and then 4-7 passes of finishing rolling are carried out with a cumulative deformation of ≥70%, and the final rolling temperature is 860-900℃; in the rough rolling stage, it is sufficient to roll the intermediate billet to the required thickness, which is usually 40-60mm; and in the finishing rolling stage, 4-7 passes are used with a cumulative deformation of ≥70%, mainly to obtain "hardened" austenite containing more dislocations, so as to facilitate the acquisition of fine ferrite in the subsequent phase transformation process;

[0069] When the slab is rolled out of the furnace, a relatively fast rolling rhythm should be maintained to minimize the precipitation of Ti in the rough rolling and finishing rolling stages. The temperatures of rough rolling and finishing rolling are in the austenite region. The size of Ti carbides or carbonitrides precipitated in this temperature range is mostly tens of microns, which has little effect on the final precipitation strengthening; therefore, the hot rolling stage should be completed as soon as possible to retain more Ti atoms;

[0070] S4, cooling and coiling

[0071] After final rolling, it is generally air-cooled for 2 to 7 seconds, and then the steel plate is cooled to 600 to 650°C by continuous laminar cooling, with a cooling rate of 20 to 40°C / s; the steel plate is then coiled and slowly cooled to room temperature after coiling, with a cooling rate of ≤20°C / h, to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel.

[0072] If the cooling speed is slow after the steel plate is rolled, the deformed austenite inside the steel plate will recrystallize and the grains will grow; when the relatively coarse austenite undergoes ferrite phase transformation, coarse ferrite grains with a size of 5 to 20 μm will be formed, which is not conducive to improving the strength of the steel plate;

[0073] The coiling temperature is controlled at 600-650°C in order to obtain a nearly isothermal transformation with a grain size of ≤5μm, fine and uniform ferrite, and a size of ≤10nm, fine and dispersed nano-precipitates;

[0074] The cooling rate after coiling is ≤20℃ / h to ensure that carbides have sufficient time to precipitate in the temperature range of 600-650℃, thereby obtaining the maximum precipitation strengthening effect.

[0075] The strength contribution of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention mainly comes from the precipitation strengthening mechanism and the fine grain strengthening mechanism;

[0076] Among them, the formula for precipitation strengthening contribution is:

[0077]

[0078] Where G is the shear elastic modulus of the matrix, b is the absolute value of the dislocation Burgers vector, K is the coefficient, 1 / K=1 / 2(1+1 / 1-v), where v is the Poisson's ratio, f is the volume fraction of the precipitated phase, and d is the size of the precipitated phase; in the 780MPa grade low-cost hot-rolled high-strength high-expansion steel disclosed in the embodiment of the present invention, the precipitation strengthening contribution of nanocarbides is approximately between 150 and 300MPa;

[0079] The formula for the contribution of fine grain strengthening is:

[0080] σ g =K y d -1 / 2 .

[0081] Wherein d is the effective grain size, Ky is the proportionality coefficient; in the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention, the fine grain strengthening contribution of the fine and uniform polygonal ferrite grain size is approximately between 200 and 300MPa.

[0082] In some embodiments, 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel is used to produce automotive body structural parts.

[0083] The technical details are further illustrated below in conjunction with embodiments.

[0084] Example 1

[0085] Figure 1 A typical metallographic photograph of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in Example 1; Figure 2 This is a typical transmission photograph of the 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in Example 1.

[0086] Example 1 discloses a method for preparing 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, comprising:

[0087] S1. Smelting and casting

[0088] In percentage by mass, a mixture of 0.058% C, 0.19% Si, 1.59% Mn, 0.003% P, 0.004% S, 0.005% Al, 0.005% N, 0.083% Ti, 0.029% Nb, 0.50% Cr and 0.003% O, with the remainder being Fe and unavoidable impurity elements, is cast into a casting billet or an ingot after smelting in a converter or an electric furnace and secondary refining in a vacuum furnace;

[0089] S2. Heating of billet or ingot

[0090] The billet or ingot is heated at a temperature of 1280°C for 1.3 hours;

[0091] S3, hot rolling

[0092] Two-stage controlled rolling is adopted, the starting rolling temperature is 1140℃, the cumulative deformation of rough rolling is ≥83.1%, then the intermediate billet is heated to 990℃, and then 4 to 7 passes of finishing rolling are carried out with the cumulative deformation ≥85%, and the final rolling temperature is 900℃; the steel plate thickness is 2.0mm;

[0093] S4, cooling and coiling

[0094] After final rolling, the steel plate is air-cooled for 6 seconds, and then cooled to 630°C by continuous laminar cooling at a cooling rate of 20°C / s; the steel plate is then coiled and slowly cooled to room temperature after coiling at a cooling rate of ≤20°C / h to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, such as Figure 1 and Figure 2 As shown in the figure, the microstructure of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel is uniform and fine full ferrite and dispersed nano-carbide precipitation phase.

[0095] The 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel prepared in Example 1 has a yield strength of 718 MPa, a tensile strength of 824 MPa, an elongation of 18.4%, and a hole expansion rate of 76.0%.

[0096] Example 2

[0097] Example 2 discloses a method for preparing a 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, comprising:

[0098] S1. Smelting and casting

[0099] In percentage by mass, a mixture of 0.056% C, 0.20% Si, 1.63% Mn, 0.003% P, 0.005% S, 0.005% Al, 0.005% N, 0.062% Ti, 0.03% Nb, 0.49% Cr and 0.003% O, with the remainder being Fe and unavoidable impurity elements, is cast into a casting blank or an ingot after smelting in a converter or an electric furnace and secondary refining in a vacuum furnace;

[0100] S2. Heating of billet or ingot

[0101] The billet or ingot is heated at a temperature of 1270°C for 1.7 hours;

[0102] S3, hot rolling

[0103] Two-stage controlled rolling is adopted, the starting rolling temperature is 1130℃, the cumulative deformation of rough rolling is ≥85.5%, then the intermediate billet is heated to 980℃, and then 4 to 7 passes of finishing rolling are carried out with the cumulative deformation ≥86.7%, and the final rolling temperature is 900℃; the steel plate thickness is 2.3mm;

[0104] S4, cooling and coiling

[0105] After final rolling, the steel plate is air-cooled for 4 seconds, and then cooled to 650°C by continuous laminar cooling at a cooling rate of 30°C / s; the steel plate is then coiled and slowly cooled to room temperature after coiling at a cooling rate of ≤20°C / h to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel.

[0106] The 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel prepared in Example 2 has a yield strength of 681 MPa, a tensile strength of 784 MPa, an elongation of 20.1%, and a hole expansion rate of 81.1%.

[0107] Example 3

[0108] Figure 3 A typical metallographic photograph of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in Example 3; Figure 4 This is a typical transmission photograph of the 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in Example 3.

[0109] Example 3 discloses a method for preparing 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, comprising:

[0110] S1. Smelting and casting

[0111] In percentage by mass, a mixture of 0.052% C, 0.20% Si, 1.65% Mn, 0.003% P, 0.003% S, 0.004% Al, 0.004% N, 0.071% Ti, 0.028% Nb, 0.45% Cr and 0.003% O, with the remainder being Fe and unavoidable impurity elements, is cast into a casting blank or an ingot after smelting in a converter or an electric furnace and secondary refining in a vacuum furnace;

[0112] S2. Heating of billet or ingot

[0113] The billet or ingot is heated at a temperature of 1240°C for 1.8 hours;

[0114] S3, hot rolling

[0115] Two-stage controlled rolling is adopted, the starting rolling temperature is 1060℃, the cumulative deformation of rough rolling is ≥82.9%, then the intermediate billet is heated to 975℃, and then 4 to 7 passes of finishing rolling are carried out with a cumulative deformation of ≥88%, and the final rolling temperature is 880℃; the steel plate thickness is 2.8mm;

[0116] S4, cooling and coiling

[0117] After final rolling, the steel plate is air-cooled for 2 seconds, and then cooled to 640°C by continuous laminar cooling at a cooling rate of 35°C / s; the steel plate is then coiled and slowly cooled to room temperature after coiling at a cooling rate of ≤20°C / h to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, such as Figure 3 and Figure 4 As shown in the figure, the microstructure of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel is uniform and fine full ferrite and dispersed nano-carbide precipitation phase.

[0118] The 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel prepared in Example 3 has a yield strength of 700 MPa, a tensile strength of 799 MPa, an elongation of 19.0%, and a hole expansion rate of 80.2%.

[0119] Example 4

[0120] Example 4 discloses a method for preparing a 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, comprising:

[0121] S1. Smelting and casting

[0122] In percentage by mass, a mixture of 0.060% C, 0.18% Si, 1.58% Mn, 0.003% P, 0.003% S, 0.004% Al, 0.004% N, 0.079% Ti, 0.03% Nb, 0.42% Cr and 0.003% O, with the remainder being Fe and unavoidable impurity elements, is cast into a casting blank or an ingot after smelting in a converter or an electric furnace and secondary refining in a vacuum furnace;

[0123] S2. Heating of billet or ingot

[0124] The billet or ingot is heated at a temperature of 1230°C for 1.5 hours;

[0125] S3, hot rolling

[0126] Two-stage controlled rolling is adopted, the starting rolling temperature is 1080℃, the cumulative deformation of rough rolling is ≥83.4%, then the intermediate billet is heated to 985℃, and then 4 to 7 passes of finishing rolling are carried out with the cumulative deformation ≥87.5%, and the final rolling temperature is 890℃; the thickness of the steel plate is 3.0mm;

[0127] S4, cooling and coiling

[0128] After final rolling, the steel plate is air-cooled for 5 seconds, and then cooled to 620°C by continuous laminar cooling at a cooling rate of 40°C / s. The steel plate is then coiled and slowly cooled to room temperature after coiling at a cooling rate of ≤20°C / h to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel.

[0129] The 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel prepared in Example 4 has a yield strength of 709 MPa, a tensile strength of 808 MPa, an elongation of 18.7%, and a hole expansion rate of 78.2%.

[0130] Example 5

[0131] Example 5 discloses a method for preparing 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel, comprising:

[0132] S1. Smelting and casting

[0133] In percentage by mass, a mixture of 0.062% C, 0.19% Si, 1.62% Mn, 0.003% P, 0.003% S, 0.004% Al, 0.004% N, 0.069% Ti, 0.029% Nb, 0.52% Cr and 0.003% O, with the remainder being Fe and unavoidable impurity elements, is cast into a casting blank or an ingot after smelting in a converter or an electric furnace and secondary refining in a vacuum furnace;

[0134] S2. Heating of billet or ingot

[0135] The billet or ingot is heated at a temperature of 1250°C for 1.4 hours;

[0136] S3, hot rolling

[0137] Two-stage controlled rolling is adopted, the starting rolling temperature is 1120℃, the cumulative deformation of rough rolling is ≥84.4%, then the intermediate billet is heated to 995℃, and then 4 to 7 passes of finishing rolling are carried out with the cumulative deformation ≥84.1%, and the final rolling temperature is 875℃; the thickness of the steel plate is 3.3mm;

[0138] S4, cooling and coiling

[0139] After final rolling, the steel plate is air-cooled for 7 seconds, and then cooled to 600°C by continuous laminar cooling at a cooling rate of 25°C / s; the steel plate is then coiled and slowly cooled to room temperature after coiling at a cooling rate of ≤20°C / h to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel.

[0140] The yield strength of the 780 MPa grade low-cost hot-rolled high-strength high-hole expansion steel prepared in Example 5 is 690 MPa, the tensile strength is 793 MPa, the elongation is 19.6%, and the hole expansion rate is 80.6%.

[0141] From Examples 1 to 5, it can be seen that the 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiments of the present invention has a yield strength ≥680MPa, a tensile strength ≥780MPa, an elongation A50 ≥18%, and a hole expansion rate ≥70%. It has a good match of high strength, high plasticity and ultra-high hole expansion rate, and is particularly suitable for preparing automobile chassis structural parts, complex parts that require wheel drilling, etc.

[0142] The 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention has a high Ti content, so that a large amount of dispersed fine nano-carbides are precipitated in the ferrite during the hot rolling coiling stage, which has a strong dispersion precipitation strengthening effect; the Si content is low, which can avoid the formation of red iron scale on the surface of the strip steel, which is conducive to the stable control of key process parameters in the actual production process; the Cr content is 0.3-0.6%, which can improve the strength of the steel by solid solution strengthening or refinement of the structure; there is no precious metal Mo, and the production cost is low. The 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention has a yield strength of ≥680MPa, a tensile strength of ≥780MPa, an elongation of ≥18%, a hole expansion rate of ≥70%, a thickness of 2.0-6.0mm and a high surface quality. It is suitable for manufacturing automotive body structural parts that require high-strength thinning and hole expansion and flanging, and has broad application prospects. The method for preparing 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention is to quickly cool the strip steel to a suitable coiling temperature at a high cooling rate after the final rolling, so that the strip steel obtains a full ferrite-based microstructure composed of fine polygonal ferrite and nano-carbide, thereby obtaining 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel with excellent strength, plasticity and ultra-high hole expansion rate. The method for preparing 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel disclosed in the embodiment of the present invention can be applied to existing hot rolling production lines to achieve mass production.

[0143] The technical solutions disclosed in the present invention and the technical details disclosed in the embodiments are merely illustrative of the inventive concept of the present invention and do not constitute a limitation on the technical solutions of the present invention. Any conventional changes, substitutions or combinations of the technical details disclosed in the embodiments of the present invention have the same inventive concept as the present invention and are within the protection scope of the claims of the present invention.

Claims

1. 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, characterized by: The microstructure of the 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel is a full ferrite-based microstructure composed of polygonal ferrite and nanocarbide; the grain size of the polygonal ferrite is ≤5μm; the size of the nanocarbide is ≤10nm; the yield strength of the 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel is ≥680MPa, the tensile strength is ≥780MPa, the elongation is ≥18%, and the hole expansion rate is ≥70%; In terms of mass percentage, the components of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel include: C: 0.02~0.1%; Si: ≤0.2%; Mn: 1.0~1.8%; P: ≤0.02%; S: ≤0.005%; Al: 0.02~0.05%; N: ≤0.005%; Ti: 0.04~0.15%; Nb: ≤0.05%; Cr: 0.3~0.6%; O: ≤0.003%; the balance is Fe; The method for preparing the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel comprises: S1. Smelting and casting According to the composition of the above-mentioned 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel, smelting, refining and continuous casting are carried out into a casting billet or an ingot; S2. Heating of billet or ingot Heat the billet or ingot at a holding temperature of 1200-1260°C for 1-3 hours; S3, hot rolling Two-stage controlled rolling is adopted, the starting rolling temperature is 1080-1150℃, the cumulative deformation of rough rolling is ≥50%, then the intermediate billet is heated to 950-1000℃, and then 4-7 passes of finishing rolling are carried out with a cumulative deformation of ≥70%, and the final rolling temperature is 860-900℃; S4, cooling and coiling After final rolling, the steel plate is cooled to 600-650°C by continuous laminar cooling at a cooling rate of 20-40°C / s. The steel plate is then coiled and slowly cooled to room temperature at a cooling rate of ≤20°C / h to obtain 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel.

2. The 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to claim 1, characterized in that: The grain size of the polygonal ferrite is 3-5 μm.

3. The 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to claim 1, characterized in that: The thickness of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel is 2.0 to 6.0 mm.

4. The 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to claim 1, characterized in that: The C content is 0.03-0.07%.

5. The 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to claim 1, characterized in that: Mn / Si is 7~10.

6. The 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to claim 1, characterized in that: The Ti content is 0.05-0.1%.

7. The 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to claim 1, characterized in that: The Nb content is ≤0.03%.

8. The 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to claim 1, characterized in that: The N content is 0.004-0.

005.

9. Application of the 780MPa grade low-cost hot-rolled high-strength high-hole expansion steel according to any one of claims 1 to 8, characterized in that: The 780MPa-grade low-cost hot-rolled high-strength high-hole expansion steel is used for producing automobile body structural parts.

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