750MPa-grade high-strength steel with high comprehensive formability and preparation method of 750MPa-grade high-strength steel

Abstract

The invention relates to 750MPa-grade high-strength steel with high comprehensive formability and a preparation method, and belongs to the technical field of steel preparation. The high-strength steel comprises the following chemical components in percentage by mass: 0.04%-0.08% of C, less than or equal to 0.2% of Si, 1.4%-2.0% of Mn, less than or equal to 0.01% of P, less than or equal to 0.005% of S, 0.02%-0.05% of Al, 0.02%-0.06% of Nb, 0.08%-0.15% of Ti, 0.1%-0.3% of Mo, less than or equal to 0.004% of N and a matrix element Fe, the microstructure of the high-strength steel comprises, by volume fraction, 60%-80% of ferrite and 20%-40% of bainite. Through reasonable component design, a metallographic structure of a target proportion is obtained, so that the obtained high-strength steel has relatively high elongation and hole expansion rate, and has excellent comprehensive forming performance.

Description

Technical Field

[0001] This application relates to the technical field of steel preparation, and particularly relates to a 750MPa grade high-strength steel with high comprehensive formability and a preparation method thereof. Background Art

[0002] The automotive chassis is an important part of a vehicle. It bears most of the vehicle's weight and provides basic support for functions such as vehicle driving, steering, and braking. With the continuous improvement of the automotive industry's requirements for lightweight, high performance, and safety, high-strength steel is increasingly widely used in automotive manufacturing. Especially in the field of automotive chassis, due to its crucial load-bearing capacity and safety performance, higher requirements are put forward for the comprehensive properties of materials such as strength, elongation, and hole expansion rate.

[0003] Currently, the automotive industry generally uses 600 - 800MPa grade high-strength steel as the chassis material, and among them, the application of 750MPa grade high-strength steel is becoming more and more widespread. Traditional 750MPa grade high-strength steel generally adopts two design ideas. The first is the high-elongation design. The steel coil is hot-rolled and coiled at a high temperature to obtain a ferrite + pearlite structure. The material has a relatively high elongation (≥20%), while the hole expansion rate is only about 30%. The second is the high-hole expansion rate design. The steel coil is hot-rolled and coiled at a medium-low temperature to obtain a ferrite + bainite structure. The material has a relatively high hole expansion rate (≥60%), while the elongation is only about 14%. High-elongation steel can meet the drawing forming of parts, but cannot meet the local flange forming. On the contrary, high-hole expansion steel cannot meet the drawing forming of parts. As a result, when complex chassis parts require both drawing forming and flange forming, the existing high-strength steel materials cannot meet the forming requirements of the parts, and large quantities of stamping cracking problems are likely to occur. Summary of the Invention

[0004] This application provides a 750MPa grade high-strength steel with high comprehensive formability and a preparation method thereof to solve the following technical problems: how to solve the problem that it is difficult to balance the elongation and hole expansion rate of existing high-strength steel.

[0005] In a first aspect, an embodiment of this application provides a 750MPa grade high-strength steel with high comprehensive formability. In terms of mass fraction, the chemical composition of the high-strength steel includes: C: 0.04% - 0.08%, Si ≤ 0.2%, Mn: 1.4% - 2.0%, P ≤ 0.01%, S ≤ 0.005%, Al: 0.02% - 0.05%, Nb: 0.02% - 0.06%, Ti: 0.08% - 0.15%, Mo: 0.1% - 0.3%, N ≤ 0.004%, and the matrix element Fe;

[0006] In terms of volume fraction, the microstructure of the high-strength steel includes: ferrite: 60% - 80%, bainite: 20% - 40%.

[0007] Optionally, the high-strength steel satisfies at least one of the following properties: thickness is 2.0 mm to 6.0 mm, yield strength

[0008] ≥650 MPa, tensile strength ≥750 MPa, elongation ≥23%, and hole expansion rate ≥70%.

[0009] In a second aspect, the present application provides a method for preparing the high-strength steel described in the first aspect, and the method includes:

[0010] Obtaining a continuous casting billet with the chemical composition;

[0011] Successively heating, rough rolling, finish rolling, multi-stage cooling, coiling, and pickling the continuous casting billet to obtain finished high-strength steel.

[0012] Optionally, the temperature of the heating is ≥1240 °C, and the holding time of the heating is 2.5 h to 3.5 h.

[0013] Optionally, the starting rolling temperature of the rough rolling is 1080 °C to 1180 °C, the outlet temperature of the rough rolling is 980 °C to 1060 °C, and the cumulative deformation amount of the rough rolling is ≥80%.

[0014] Optionally, the cumulative deformation amount of the finish rolling is ≥85%, and the outlet temperature of the finish rolling is 850 °C to 920 °C.

[0015] Optionally, the multi-stage cooling includes a rapid cooling stage, an air cooling stage, and a water cooling stage.

[0016] Optionally, the cooling rate of the rapid cooling stage is ≥50 °C / s, and the end temperature of the rapid cooling stage is 680 °C to 750 °C.

[0017] Optionally, the cooling rate of the air cooling stage is ≤5 °C / s, and the time of the air cooling stage is 5 s to 9 s.

[0018] Optionally, the cooling rate of the water cooling stage is 20 °C / s to 40 °C / s.

[0019] Optionally, the coiling temperature is 500 °C to 580 °C.

[0020] Optionally, the descaling and straightening elongation of the pickling is 0.3% to 0.7%, and the pickling speed is 100 m / min to 200 m / min.

[0021] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0022] An embodiment of the present application provides a 750 MPa grade high-strength steel with high comprehensive formability. In terms of mass fraction, the chemical composition of the high-strength steel includes: C: 0.04% - 0.08%, Si ≤ 0.2%, Mn: 1.4% - 2.0%, P ≤ 0.01%, S ≤ 0.005%, Al: 0.02% - 0.05%, Nb: 0.02% - 0.06%, Ti: 0.08% - 0.15%, Mo: 0.1% - 0.3%, N ≤ 0.004%, and the matrix element Fe; in terms of volume fraction, the microstructure of the high-strength steel includes: ferrite: 60% - 80%, bainite: 20% - 40%. Through reasonable composition design, a metallographic structure with a target ratio is obtained, so that the obtained high-strength steel has a high elongation and hole expansion rate, and has excellent comprehensive forming performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings herein are incorporated into the specification and constitute a part of this specification, showing embodiments in line with the present application, and are used together with the specification to explain the principles of the present application.

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the accompanying drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, other accompanying drawings can also be obtained based on these drawings without creative efforts.

[0025] Figure 1 It is a schematic flow chart of a preparation method of a 750 MPa grade high-strength steel with high comprehensive formability provided by an embodiment of the present application;

[0026] Figure 2 It is a metallographic structure diagram provided by Embodiment 1 of the present application;

[0027] Figure 3 It is a hole expansion rate test result diagram provided by Embodiment 1 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0029] The various embodiments of the present application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be construed as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and individual values within that range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and individual numbers within that range, such as 1, 2, 3, 4, 5, and 6, and this applies regardless of the range; in addition, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0030] In this document, terms including "comprising" etc. mean "including but not limited to". Relative terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. "And / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, or B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "a plurality" means two or more; "at least one kind", "at least one of the following items (pieces)" or similar expressions refer to any combination of these items, including any combination of single items (pieces) or plural items (pieces); for example, "at least one of a, b, or c", or, "at least one of a, b, and c" can both mean: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively. "Parts representation method" such as parts by weight, parts by mass, etc. represents the proportional relationship between each component. In the proportional relationships involved in this document, the parameters that need to be described by proportion should be understood as the antecedents of the proportion formula in the order of description, and the proportional numbers should be understood as the consequents of the proportion formula. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one by one with the proportional numbers in the proportion formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0031] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in this document can be obtained through market purchase or can be prepared by existing methods.

[0032] In a first aspect, an embodiment of the present application provides a 750 MPa grade high-strength steel with high comprehensive formability. In terms of mass fraction, the chemical composition of the high-strength steel includes: C: 0.04% - 0.08%, Si ≤ 0.2%, Mn: 1.4% - 2.0%, P ≤ 0.01%, S ≤ 0.005%, Al: 0.02% - 0.05%, Nb: 0.02% - 0.06%, Ti: 0.08% - 0.15%, Mo: 0.1% - 0.3%, N ≤ 0.004%, and the matrix element Fe;

[0033] Positive effect of limiting the mass fraction of C to 0.04% - 0.08%: C is a basic element in steel and one of the important elements of the present application. As an interstitial atom in steel, C can improve the strength of the steel plate through interstitial solid solution, especially the tensile strength is more obvious. In order to obtain a high-formability steel with a tensile strength reaching 750 MPa grade in the present application, it is necessary to ensure that the mass fraction of C is above 0.04%. At the same time, when the mass fraction of C is higher than 0.08%, it is easy to form a small amount of pearlite during the cooling process of the steel plate, which is not conducive to the steel plate obtaining a high hole expansion rate. Therefore, the present application controls the mass fraction of C to 0.04% - 0.08%. Exemplarily, the mass fraction of C can be 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, etc.

[0034] Positive effect of limiting Si ≤ 0.2%: Si is a basic element in steel, which acts as a reducing agent and deoxidizer during the steelmaking process, and at the same time has a strong solid solution strengthening effect. Si can promote the diffusion of C atoms in ferrite into austenite, expanding the formation range of ferrite. However, when the mass fraction of Si is higher than 0.2%, it is easy to form scale defects on the surface of the hot-rolled steel plate, which is difficult to completely remove in the subsequent pickling process, affecting the surface quality of the strip steel. Therefore, the present application controls Si ≤ 0.2%. Exemplarily, the mass fraction of Si can be 0.1%, 0.12%, 0.14%, 0.16%, 0.18%, 0.2%, etc.

[0035] Positive effect of limiting the mass fraction of Mn to 1.4% - 2.0%: Mn is a basic element in steel, which plays a role in solid solution strengthening in steel, and at the same time can improve the hardenability of the steel plate, reduce the critical quenching speed of the steel, and is an important element for expanding the austenite phase region, which can stabilize austenite and delay the transformation of austenite to pearlite. In order to ensure the strength of the steel plate and avoid serious segregation problems caused by too high Mn content in the present application, the mass fraction of Mn is controlled to 1.4% - 2.0%. Exemplarily, the mass fraction of Mn can be 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, etc.

[0036] Positive effects of limiting P ≤ 0.01%: P is an impurity element in steel and is extremely prone to segregation at grain boundaries, forming Fe2P, which reduces the plasticity and toughness of the steel. Therefore, the lower the mass fraction of P, the better. Considering the steelmaking cost, this application controls P ≤ 0.01%. Exemplarily, the mass fraction of P can be 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, 0.01%, etc.

[0037] Positive effects of limiting S ≤ 0.005%: S is an impurity element in steel and easily forms MnS inclusions with Mn in the steel, directly affecting the hole expansion rate and elongation rate of the steel plate, resulting in a reduction in the formability of the steel plate. Since this application limits the mass fraction of Mn to 1.4% - 2.0% to ensure the strength of the steel plate, which is at a relatively high level, the mass fraction of S needs to be controlled as low as possible. This application controls S ≤ 0.005%. Exemplarily, the mass fraction of S can be 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc.

[0038] Positive effects of limiting the mass fraction of Al to 0.02% - 0.05%: Al is an essential deoxidizing element in steel, which can reduce oxide inclusions in the steel and purify the steel quality, being beneficial to improving the formability of the steel plate. However, when the mass fraction of Al is higher than 0.05%, it will affect continuous casting production and have an adverse effect on grain refinement. Therefore, this application controls the mass fraction of Al to 0.02% - 0.05%. Exemplarily, the mass fraction of Al can be 0.02%, 0.025%, 0.03%, 0.035%, 0.04%, 0.045%, 0.05%, etc.

[0039] Positive effects of limiting the mass fraction of Nb to 0.02% - 0.06%: Nb is one of the important strengthening elements in steel. In the austenite region, it combines with C or N to form nano-precipitates NbC / NbN, which can pin grain boundaries and inhibit the growth of austenite grains, thereby refining the grain size of the final structure. However, when the mass fraction of Nb is higher than 0.06%, it will lead to an increase in the finishing rolling load, being unfavorable to the stability of production, and at the same time, it will cause the coarsening of precipitates and an increase in cost. Therefore, this application controls the mass fraction of Nb to 0.02% - 0.06%. Exemplarily, the mass fraction of Nb can be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, etc.

[0040] Positive effects of limiting the mass fraction of Ti to 0.08% - 0.15%: Ti is one of the important elements in this application. Ti can refine grains and combine with C to form nanoscale carbides TiC, which precipitate during the transformation from austenite to ferrite and in the ferrite matrix, having a strong precipitation strengthening effect. Therefore, in this application, the mass fraction of Ti is controlled to be 0.08% - 0.15%. Exemplarily, the mass fraction of Ti can be 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, etc.

[0041] Positive effects of limiting the mass fraction of Mo to 0.1% - 0.3%: Mo is also one of the important elements in this application. Mo can improve the hardenability of steel and refine the grain size of the microstructure, and its effect is stronger than that of Cr and slightly inferior to that of Mn. Mo narrows the austenite phase region and delays the pearlite transformation, making it easier to obtain bainite structure during the cooling process of steel, thereby improving the hole expansion rate of the steel plate. At the same time, the precipitates of Mo and C have high-temperature thermal stability, and the (Ti,Mo)C particles precipitated in combination with Ti are smaller in size and have a stronger precipitation strengthening effect. Therefore, in this application, the mass fraction of Mo is controlled to be 0.1% - 0.3%. Exemplarily, the mass fraction of Mo can be 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, etc.

[0042] Positive effects of limiting N ≤ 0.004%: N is an impurity element in this application. It is easy to form large-sized square TiN with Ti at high temperatures, while the purpose of adding Ti in this application is to obtain fine and dispersed nanoscale carbides rather than nitrides. TiN cannot play a precipitation strengthening role and will cause fluctuations in the performance of the steel plate. Therefore, in this application, N is controlled to be ≤ 0.004%. Exemplarily, the mass fraction of N can be 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, 0.0035%, 0.004%, etc.

[0043] Fe is the matrix element, and the specific content / content range of Fe can be obtained through the upper and lower limit formulas of the components, that is:

[0044] The sum of the percentage contents of each component in a composition should be equal to 100%, and the content ranges of several components should meet the following conditions: the upper limit value of a certain component + the lower limit values of other components ≤ 100; the lower limit value of a certain component + the upper limit values of other components ≥ 100.

[0045] By volume fraction, the microstructure of the high-strength steel includes: ferrite: 60% - 80%, bainite: 20% - 40%.

[0046] In some embodiments, the high-strength steel satisfies at least one of the following properties: the thickness is 2.0 mm to 6.0 mm, the yield strength ≥ 650 MPa, the tensile strength ≥ 750 MPa, the elongation ≥ 23%, and the hole expansion rate ≥ 70%.

[0047] Figure 1 Schematic flow chart of a preparation method of a 750 MPa grade high-strength steel with high comprehensive formability provided by an embodiment of the present application.

[0048] Please refer to Figure 1 , second, the present application provides a preparation method of the high-strength steel described in the first aspect, and the method includes:

[0049] S1. Obtain a continuous casting billet with the chemical composition;

[0050] S2. Heat, rough roll, finish roll, multi-stage cooling, coiling, and pickling the continuous casting billet in sequence to obtain the finished high-strength steel.

[0051] In some embodiments, the temperature of the heating is ≥ 1240 °C, and the holding time of the heating is 2.5 h to 3.5 h.

[0052] In terms of the heating process, since a relatively large amount of Ti is added in the composition design, in order to make full use of the precipitation strengthening effect of Ti, it is necessary to make the Ti atoms in the continuous casting billet fully dissolve in the heating stage, so as to obtain more nano-scale carbides during the coiling process and play a stronger precipitation strengthening role. For low-carbon steel, the solution temperature of Ti carbonitride (Ti(C,N)) is ≥ 1240 °C. Exemplarily, the temperature of the heating can be 1240 °C, 1244 °C, 1248 °C, 1252 °C, 1256 °C, 1260 °C, etc. In order to ensure that the continuous casting billet can be evenly "burned through", the holding time needs to be ≥ 2.5 hours. At the same time, the holding time cannot be too long, otherwise the structure of the continuous casting billet will be too coarse, and the undissolved TiN is likely to coarsen, affecting the hole expansion rate and elongation of the steel plate. Therefore, the holding time is controlled between 2.5 h and 3.5 h. Exemplarily, the holding time of the heating can be 2.5 h, 2.7 h, 2.9 h, 3.1 h, 3.3 h, 3.5 h, etc.

[0053] In some embodiments, the starting rolling temperature of the rough rolling is 1080 °C to 1180 °C, the outlet temperature of the rough rolling is 980 °C to 1060 °C, and the cumulative reduction of the rough rolling is ≥ 80%.

[0054] In some embodiments, the cumulative reduction of the finish rolling is ≥ 85%, and the outlet temperature of the finish rolling is 850 °C to 920 °C.

[0055] In the rough rolling and finish rolling stages, it is necessary to ensure that the strip steel has sufficient deformation. The cumulative deformation in rough rolling is ≥80%, and the cumulative deformation in finish rolling is ≥85%. This is mainly to promote ferrite nucleation during the phase transformation after rolling, refine the grain size of the microstructure, and improve the strength of the steel plate. Exemplarily, the cumulative deformation in rough rolling can be 80%, 81%, 82%, 83%, 84%, 85%, etc.; the cumulative deformation in finish rolling can be 85%, 86%, 87%, 88%, 89%, 90%, etc. The finish rolling exit temperature is controlled between 850°C and 920°C to ensure that the strip steel at the last stand of finish rolling is rolled in the non-recrystallization zone, the austenite does not recrystallize, the deformation can be accumulated and superimposed, increasing the dislocation density in the austenite, promoting deformation-induced precipitation and phase transformation, thereby refining the grains and improving the properties of the steel plate. Exemplarily, the finish rolling exit temperature can be 850°C, 860°C, 870°C, 880°C, 890°C, 900°C, 910°C, 920°C, etc.

[0056] In some embodiments, the multi-stage cooling includes a rapid cooling stage, an air cooling stage, and a water cooling stage.

[0057] In some embodiments, the cooling rate in the rapid cooling stage is ≥50°C / s, and the end temperature of the rapid cooling stage is 680°C - 750°C.

[0058] The multi-stage cooling process aims to regulate the type and proportion of the microstructure in the steel, and it is the core process of this application.

[0059] In the embodiments of this application, the first-stage cooling process of the multi-stage cooling is rapid cooling, and the cooling rate is ≥50°C / s. Rapid cooling can inhibit the growth of austenite grains and promote more ferrite phase transformation, thereby obtaining a finer ferrite microstructure. At the same time, a cooling rate of ≥50°C / s can inhibit the precipitation of second-phase particles in the high-temperature stage, which is beneficial for more Ti to precipitate in the ferrite at the low-temperature stage, obtaining finer carbides and achieving a stronger precipitation strengthening effect. Exemplarily, the cooling rate in the rapid cooling stage can be 50°C / s, 52°C / s, 54°C / s, 56°C / s, 58°C / s, 60°C / s, etc.

[0060] The end temperature of the rapid cooling stage is 680°C - 750°C, which is the nose temperature of the ferrite phase transformation, ensuring that the strip steel immediately undergoes a phase transformation after the ultra-rapid cooling, which can shorten the phase transformation incubation time and improve production efficiency. If the end temperature of the rapid cooling stage is higher than 750°C, it is likely to result in insufficient supercooling of the steel plate, coarsening of the grain size of the microstructure, and an increase in the phase transformation incubation time, and it is impossible to obtain the target proportion of ferrite microstructure; if the end temperature of the rapid cooling stage is lower than 680°C, the steel plate is likely to enter the pearlite phase transformation zone, generating pearlite microstructure, which is not conducive to improving the hole expansion rate of the steel plate. Exemplarily, the end temperature of the rapid cooling stage can be 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, etc.

[0061] In some embodiments, the cooling rate of the air cooling stage is ≤5°C / s, and the duration of the air cooling stage is 5s to 9s.

[0062] In the embodiment of the present application, the second cooling process of the multi-stage cooling is air cooling, the air cooling time is 5s to 9s, and the cooling rate is ≤5℃ / s. The purpose is to obtain the target proportion of ferrite structure and improve the elongation of the steel plate. If the air cooling time is shorter than 5s, sufficient ferrite structure cannot be obtained. If the air cooling time is longer than 9s or the cooling rate is greater than 5℃ / s, pearlite structure is easily obtained, which is not conducive to improving the hole expansion rate of the steel plate. The most ideal process for the second stage cooling is to carry out heat preservation treatment on the strip after the first stage cooling is completed, which is conducive to the strip to fully undergo ferrite phase transformation at this temperature, while not entering the pearlite phase transformation zone to produce pearlite structure, but heat preservation treatment cannot be carried out in the actual industrial production process, so it is necessary to reduce the cooling rate as much as possible, and control the air cooling cooling rate ≤5℃ / s. Exemplarily, the cooling rate of the air cooling section can be 1℃ / s, 2℃ / s, 3℃ / s, 4℃ / s, 5℃ / s, etc.; the time of the air cooling section can be 5s, 6s, 7s, 8s, 9s, etc.

[0063] In some embodiments, the cooling rate of the water cooling section is 20°C / s to 40°C / s.

[0064] In the embodiment of the present application, the third cooling process of the multi-stage cooling is water cooling, and the cooling rate is 20-40°C / s. The purpose is to make the remaining austenite structure of the strip undergo bainite transformation after the ferrite phase transformation. Therefore, it is necessary to avoid the pearlite phase transformation zone and quickly enter the bainite phase transformation zone to obtain the target proportion of bainite structure. Exemplarily, the cooling rate of the water cooling section can be 20°C / s, 24°C / s, 28°C / s, 32°C / s, 36°C / s, 40°C / s, etc.

[0065] After water cooling, coiling is performed.

[0066] In some embodiments, the coiling temperature is 500°C to 580°C.

[0067] A coiling temperature lower than 500°C will result in a decrease in the elongation of the steel plate, and a coiling temperature higher than 580°C will result in pearlite structure in the steel plate, resulting in a decrease in the hole expansion rate, so the coiling temperature is controlled between 500°C and 580°C. For example, the coiling temperature may be 500°C, 520°C, 540°C, 560°C, 580°C, etc.

[0068] In some embodiments, the scale breaking and straightening elongation of the pickling is 0.3% to 0.7%, and the pickling speed is 100 m / min to 200 m / min.

[0069] In the pickling stage, in order to fully remove the scale on the surface of the steel plate and correct the shape of the steel plate, it is necessary to carry out tension leveling on the strip steel with a scale-breaking tension leveling machine at the pickling inlet, so that the scale on the surface of the steel plate is broken and peeled off, thereby improving the pickling production efficiency. The scale-breaking tension leveling elongation needs to be controlled between 0.3% and 0.7%. If the tension leveling elongation is lower than 0.3%, the scale on the surface of the steel plate cannot be fully removed, resulting in surface defects. If the tension leveling elongation is higher than 0.7%, it will affect the mechanical properties and formability of the steel plate. Exemplarily, the scale-breaking tension leveling elongation can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, etc. At the same time, the pickling speed is controlled between 100 m / min and 200 m / min, which is beneficial to obtaining excellent surface quality of the steel plate. If the pickling speed is lower than 100 m / min, it is easy to cause over-pickling and the surface of the steel plate turns yellow. If the pickling speed is higher than 200 m / min, it is easy to cause under-pickling and scale remains on the surface of the steel plate. Exemplarily, the pickling speed can be 100 m / min, 120 m / min, 140 m / min, 160 m / min, 180 m / min, 200 m / min, etc.

[0070] Through reasonable composition and process design, the present invention prepares a high-strength steel with high comprehensive formability that takes into account strength, elongation and hole expansion rate. The microstructure of this high-strength steel consists of ferrite and bainite. The proportion of ferrite is 60% - 80%, and the proportion of bainite is 20% - 40%. While the tensile strength of this steel plate reaches 750 MPa, it still has a high elongation of ≥23% and a high hole expansion rate of ≥70%, and has excellent comprehensive formability.

[0071] The product prepared by the preparation method of the 750 MPa grade high-strength steel with high comprehensive formability is the above-mentioned 750 MPa grade high-strength steel with high comprehensive formability. Since the preparation method of the 750 MPa grade high-strength steel with high comprehensive formability adopts some or all of the technical solutions of the embodiments of the 750 MPa grade high-strength steel with high comprehensive formability, it has at least all the beneficial effects brought by the technical solutions of the embodiments of the 750 MPa grade high-strength steel with high comprehensive formability, which will not be elaborated one by one here.

[0072] The following further elaborates the present application with specific embodiments. For the experimental methods without specific conditions noted in the following embodiments, they are usually determined according to national standards / industry standards; if there are no corresponding national standards / industry standards, they are carried out according to general international standards, conventional conditions or according to the conditions recommended by the manufacturer.

[0073] The chemical composition of the continuous casting billet is shown in Table 1.

[0074] Table 1 Chemical composition (wt%) of the continuous casting billets in the examples and comparative examples, the balance is Fe and unavoidable impurities

[0075]

[0076]

[0077] Based on the chemical compositions of the continuous casting billets in the examples and comparative examples, this example also provides a preparation method of a 750 MPa grade high-strength steel with high comprehensive formability, including the following steps:

[0078] Obtain a continuous casting billet with the said chemical composition;

[0079] Subject the said continuous casting billet to heating, rough rolling, finish rolling, multi-stage cooling, coiling and pickling in sequence to obtain the finished high-strength steel. Please refer to Table 2 for the preparation process parameters.

[0080] Table 2 Preparation process parameters of the examples and comparative examples

[0081]

[0082] Perform performance tests on the products obtained in the above examples and comparative examples, and the obtained data are shown in Table 3.

[0083] Table 3 Summary of performance test results

[0084] Group Yield strength / MPa Tensile strength / MPa Elongation / % Hole expansion rate / % Example 1 670 785 23 76 Example 2 685 793 24 82 Example 3 662 801 23.5 72 Comparative Example 1 640 740 27 72 Comparative Example 2 621 737 27.5 78 Comparative Example 3 702 813 23 55

[0085] As can be seen from Tables 1 to 3, the yield strength of the high-strength steels prepared in Examples 1 to 3 is 670 - 693 MPa, the tensile strength is 776 - 801 MPa, the elongation is 23% - 25%, and the hole expansion rate is 72% - 85%, all within the target range of this application. This high-strength steel has excellent mechanical properties and comprehensive forming properties, meeting the usage requirements of users.

[0086] The content of Mo element in the high-strength steel prepared in Comparative Example 1 is 0.05%, not reaching the target range of this application (0.1% - 0.3%), and the yield strength is less than 650 MPa, and the tensile strength is less than 750 MPa. The rapid cooling rate of the high-strength steel prepared in Comparative Example 2 is 40 °C / s, not reaching the target range of this application (≥50 °C / s). Therefore, its ferrite grains are slightly coarser, resulting in a yield strength of less than 650 MPa and a tensile strength of less than 750 MPa. The end temperature of the rapid cooling section of the high-strength steel prepared in Comparative Example 3 is 625 °C, lower than the target range of this application (680 °C - 750 °C). Therefore, a certain amount of pearlite structure is generated in the steel, resulting in a hole expansion rate of only 55%, not reaching the target value of this invention (≥70%).

[0087] Appendix Figure 2-3 Detailed description:

[0088] Figure 2 Is the metallographic structure diagram provided for Example 1 of this application; as Figure 2As shown, the metallographic structure consists of ferrite + bainite. The average grain size of ferrite is about 4 - 5 μm, the proportion of ferrite is about 70%, and the proportion of bainite is about 30%.

[0089] Figure 3 It is a graph showing the reaming rate test results provided in Embodiment 1 of the present application; as Figure 3 shown, the reaming rate is as high as 76%.

[0090] One or more technical solutions in the embodiments of the present invention at least further have the following technical effects or advantages:

[0091] The 750MPa grade high-strength steel with high comprehensive formability obtained in the embodiments of the present invention can meet the forming requirements of complex automotive chassis parts. Compared with high-strength steels of the same strength level, the stamping cracking rate of parts is reduced from 20% to less than 0.1%, and it has broad application prospects.

[0092] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but rather will conform to the broadest scope consistent with the principles and novel features claimed in the present application.

Claims

1. A 750MPa grade high-strength steel with high comprehensive formability. By mass fraction, the chemical composition of the high-strength steel includes: C： 0.04% to 0.08%, Si ≤ 0.2%, Mn: 1.4% to 2.0%, P ≤ 0.01%, S ≤ 0.005%, Al: 0.02% to 0.05%, Nb: 0.02% to 0.06%, Ti: 0.08% to 0.15%, Mo: 0.1% to 0.3%, N ≤ 0.004%, and matrix element Fe; By volume fraction, the microstructure of the high-strength steel comprises: ferrite: 60% to 80%, bainite: 20% to 40%.

2. The high-strength steel according to claim 1, characterized in that, The high-strength steel satisfies at least one of the following properties: thickness of 2.0 mm to 6.0 mm, yield strength ≥ 650 MPa, tensile strength ≥ 750 MPa, elongation ≥ 23%, hole expansion rate ≥ 70%.

3. A method for preparing the high-strength steel according to any one of claims 1 to 2, characterized in that, The method comprises: obtaining a continuous casting billet having the chemical composition; heating, rough rolling, finish rolling, multi-stage cooling, coiling and pickling the continuous casting billet in sequence to obtain the finished high-strength steel.

4. The method according to claim 3, characterized in that, The temperature of the heating is ≥ 1240°C, and the holding time of the heating is 2.5 h to 3.5 h.

5. The method according to claim 3, characterized in that, The starting rolling temperature of the rough rolling is 1080°C to 1180°C, the outlet temperature of the rough rolling is 980°C to 1060°C, and the cumulative reduction of the rough rolling is ≥ 80%.

6. The method according to claim 3, wherein The cumulative reduction of the finish rolling is ≥ 85%, and the outlet temperature of the finish rolling is 850°C to 920°C.

7. The method according to claim 3, characterized in that, The multi-stage cooling includes a rapid cooling stage, an air cooling stage and a water cooling stage.

8. The method according to claim 7, wherein The cooling rate of the rapid cooling stage is ≥ 50°C / s, and the end temperature of the rapid cooling stage is 680°C to 750°C; and / or, The cooling rate of the air cooling stage is ≤ 5°C / s, and the time of the air cooling stage is 5 s to 9 s; and / or, The cooling rate of the water cooling stage is 20°C / s to 40°C / s.

9. The method according to claim 3, characterized in that, The coiling temperature is 500°C to 580°C.

10. The method according to claim 3, characterized in that, The descaling and straightening elongation of the pickling is 0.3% to 0.7%, and the pickling speed is 100 m / min to 200 m / min.

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