A method for improving the strength-ductility product and performance stability of ultrahigh-strength multiphase steel

Abstract

The application relates to the technical field of high-strength steel material preparation, in particular to a method for improving the strength-ductility product and performance stability of ultrahigh-strength multiphase steel. The specific steps comprise the following: taking C-Si-Mn as basic components to cast a steel billet; controlling the initial structure of hot rolling to be ferrite and pearlite; then performing pickling and cold rolling on the hot-rolled steel coil; performing secondary cold rolling after cover annealing; (Ac3+30) DEG C <= austenitizing annealing heating temperature <= 10 DEG C; cooling to overaging temperature 380 310 DEG C at a cooling speed >= 35 DEG C >= >= after the end of the austenitizing soaking section; final cooling out of the furnace temperature < 110 DEG C; and performing skin pass treatment on the annealed steel coil. The prepared ultrahigh-strength multiphase steel has a strength-ductility product > 23.0 GPa*%, a tensile strength of 1250 MPa 325 MPa, a yield strength of <= 00325 MPa, a yield strength ratio of 0.70-0.75, a simultaneously elongation > 18.0%, and an expansion ratio lambda >= 55%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-strength steel material preparation, and in particular to a method for improving the strength-plasticity product and performance stability of ultrahigh-strength multiphase steel. BACKGROUND

[0002] At present, with the development of the automobile industry, the development direction of automobiles is focused on energy saving, emission reduction, weight reduction and safety improvement. The main method is to use high-strength steel plates, which not only meet the strength requirements, but also meet the demand for reducing the thickness and number of stamping parts, and are easier to meet the safety performance indicators. The effect of "weight reduction" to achieve "lightweight" is remarkable, so the application proportion of high-strength steel plates in the manufacture of body-in-white and chassis structural parts continues to increase. The multiphase steel has the following characteristics: 1. The multiphase microstructure characteristics of multiphase steel can effectively improve the bending and flaring performance of the material; 2. The multiphase steel based on bainite matrix has high density of dislocations and small strength difference between the phases, so it has better flaring and flanging capacity; 3. The multiphase steel has a high yield ratio, and compared with other high-strength materials of the same level, the deformation resistance is enhanced, and it is particularly suitable for the manufacture of automobile safety parts; 4. The multiphase steel has high energy absorption capacity, which can absorb more energy during the collision process, further improving the safety of the automobile. Due to the excellent properties of multiphase steel, it is widely used in automobile chassis suspension parts, B-pillar, bumper, seat slide rail and other automobile structural parts and reinforcing parts, and has broad market prospects. However, the industrialization of ultrahigh-strength multiphase steel still has the following problems: 1. High-cost alloy design: in order to obtain a high tensile strength of more than 1000 MPa and ensure the plasticity of ultrahigh-strength multiphase steel, the composition design idea is to add Mn (≥1.80%) and Cr, Mo, Nb, Ti and other micro-alloy elements, and the cost of steel billet manufacturing is high; 2. Low strength-plasticity product: the strength-plasticity product is an important indicator for evaluating automobile body steel. The higher the strength-plasticity product, the better the forming ability of the material at the same strength level, and the more suitable it is for manufacturing complex or relatively complex automobile body structural parts and reinforcing parts; in the process of collision, the material has stronger energy absorption capacity, and the safety of the automobile structural parts is better. However, the strength-plasticity product of the current ultrahigh-strength multiphase steel is less than 14 GPa%; 3. Poor performance stability: for ultrahigh-strength multiphase steel with a tensile strength of more than 1000 MPa, automobile manufacturers require that the tensile strength be more than 1000 MPa, the yield strength be 780-950 MPa, and the yield strength standard deviation be ±85 MPa on the premise of meeting the plasticity index. However, with the continuous innovation of the production technology and concept of automobile manufacturers, the performance requirements of ultrahigh-strength multiphase steel are also continuously improved, and the performance standard deviation is required to be less than or equal to 40 MPa.

[0003] CN 109338229A provides a low-carbon Si-Mn series hot-rolled multi-phase steel based on the concept of quenching-carbon distribution and a manufacturing method. Although the product has a product strength product > 20 GPa%, it has the following problems: (1) the multi-phase steel prepared by CN 109338229A has a low strength level, the tensile strength > 800 MPa, and the tensile strength > 1000 MPa; (2) the preparation method is relatively complex, the cooling rate after finish rolling is 5~10℃ / s to 820~840℃, then the cooling rate is 100~150℃ / s to a temperature between 230~280℃, and finally the simulation coiling, which requires high cooling equipment and is difficult to industrialize; (3) the hole expansion performance is one of the important indicators for measuring the forming capacity of multi-phase steel, but the multi-phase steel prepared by CN 109338229A has no requirement on the hole expansion performance.

[0004] CN 108624820A provides a high strength and toughness steel for automobiles with a product strength product greater than 45 GPa% and a preparation method. The main problems are as follows: (1) alloy design: the composition design is (0.4~0.6)%C, (7.8~8.0)%Mn, (2.5~3.5)%Al, which greatly increases the smelting difficulty in the actual industrialization process, causing problems such as pipe blockage and ingot corner cracking, in addition, in the rolling process, the deformation resistance is large, and the rolling capacity of the hot rolling and cold rolling equipment is extremely high; (2) preparation method: the primary annealing and secondary annealing adopted are cover annealing, and the annealing temperature is 700~750℃ in the unrecrystallized zone; (3) the multi-phase steel prepared by CN 109338229A has a thickness of 2.2~2.25mm, (4) the hole expansion performance is one of the important indicators for measuring the forming capacity of multi-phase steel, and the high strength and toughness steel prepared by CN 109338229A has no requirement on the hole expansion performance.

[0005] At present, high-strength multi-phase steel mainly has the problems of high cost of alloy composition design, low product strength product, poor stability, no requirement on hole expansion performance, and unsuitable for large-scale production. SUMMARY

[0006] In view of the technical problems of high cost of alloy design, low product strength product, and poor stability of existing ultra-high strength multi-phase steel, the present application provides a method for regulating the product strength product and performance stability of ultra-high strength multi-phase steel.

[0007] The technical solution of the present application is as follows:

[0008] A method for regulating the product strength product and performance stability of ultra-high strength multi-phase steel, comprising the following steps:

[0009] (1) The composition of the billet is designed as C-Si-Mn as the basic component, and the mass percentage of the chemical composition is C: 0.22%~0.25%, Si: 1.6%~1.8%, Mn: 2.1%~2.4%, V: 0.20%~0.24%, N: 0.020%~0.025%, Alt: 0.050%~0.080%, P, S, O three elements are not more than 0.005%, and the balance is Fe and other unavoidable impurities;

[0010] (2) The initial organization regulation process of hot rolling is that the finish rolling temperature is 890±15℃, and then the natural cooling is performed to 635±15℃ for coiling; the regulated initial organization is 50%~60% ferrite and 40%~50% pearlite organization;

[0011] (3) The first cold rolling regulation process is that the hot-rolled steel coil obtained in step (2) is pickled, and then the first cold rolling is performed with a reduction of 40%~50%;

[0012] (4) The cover annealing regulation process is that the cold-rolled steel coil of step (3) is subjected to cover annealing, and the annealing temperature is 670℃, and the holding time is 10~12h;

[0013] (5) The second cold rolling regulation process is that the cover annealing steel coil of step (4) is then subjected to the second cold rolling with a reduction of 33%~45%;

[0014] (6) The annealing microstructure regulation process is that (Ac3+30)℃≤austenitizing annealing heating temperature≤910℃, after the end of the austenitizing soaking section, the rapid cooling is performed to the overaging temperature 380±10℃ at a cooling rate of ≥35℃ / s; the final cooling out of the furnace temperature is <160℃;

[0015] Further, in step (2), the shape of the ferrite is polygonal or equiaxed, and the ferrite grain size is 3~5μm; the pearlite is lamellar pearlite and granular pearlite.

[0016] Further, in step (3), the oxide on the surface of the hot-rolled steel coil is removed by pickling, and the reflectivity of the hot-rolled steel coil after pickling is ≥70%.

[0017] Further, the total reduction of the two cold rollings of step (3) and step (5) is ≥60.0%.

[0018] Further, the quality of the cover annealing process is better than that of the continuous annealing, the mechanical properties are uniform, and the plastic strain ratio and the work hardening index value are higher than those of the products of the continuous annealing.

[0019] Further, the running speed of the strip steel with different thickness specifications in step (6) is as follows: when the thickness specification is 0.6<=delta<=1.0mm, the strip speed of the strip steel is >=100m / min; when the thickness specification is 1.0<delta<1.8mm, the strip speed of the strip steel is 85~100m / min; when the thickness specification is 1.8<=delta<=2.5mm, the strip speed of the strip steel is 80±5m / min.

[0020] Further, the final microstructure of the ultra-high-strength complex-phase steel is a bainite matrix structure and >15.0% of second-phase residual austenite.

[0021] Further, the V(C,N) dispersed precipitates with a particle diameter of <10nm and high-density dislocation pile-ups exist in the bainite matrix.

[0022] Further, the matrix structure morphology in the microstructure is controlled to be bainite lath bundles with different orientations, and the bainite lath bundles have obvious original austenite grain boundaries, and the bainite lath interfaces in different orientation reference units are relatively blurred; the second-phase residual austenite exists in the form of interfaces between the bainite laths.

[0023] Further, the ultra-high-strength complex-phase steel is obtained after the annealing microstructure regulation process and the flattening regulation process, and the flattening process has an elongation of 0.7%±0.1% and a rolling force of 7200~7600KN.

[0024] Further, the obtained ultra-high-strength complex-phase steel has a strength-ductility product of >23.0GPa·%, a tensile strength of 1250MPa±25MPa, a yield strength of 900±25MPa, a stable control of the yield ratio to 0.70~0.75, a elongation of >18.0%, and an expansion ratio lambda of >=55%.

[0025] The beneficial effects of the present application are as follows:

[0026] (1) The ultra-high-strength complex-phase steel prepared by the technical scheme provided by the present application has high strength-ductility product and performance stability, and has the advantages of low cost, good expansion performance, etc., and the strength-ductility product is >23.0GPa·%, the tensile strength is stably controlled to 1250MPa±25MPa, the yield strength is stably controlled to 900±25MPa, the yield ratio is stably controlled to 0.70~0.75, the elongation is >18.0%, and the expansion ratio lambda is >=55%.

[0027] (2) The present application regulates the microstructure of the ultra-high-strength complex-phase steel by the alloy composition and the annealing temperature, so that the final microstructure is regulated to be a bainite matrix structure and >15.0% of second-phase residual austenite, and the V(C,N) dispersed precipitates with a particle diameter of <10nm and high-density dislocation pile-ups exist in the matrix; and further, the excellent effects of high strength and high toughness are achieved.

[0028] (3) The prepared super-high-strength multiphase steel has a thickness of 0.6-2.5 mm, and the thickness of the stamping part is further reduced while the strength requirement is met, so that the effect of "lightweight" is more remarkable through "weight reduction". BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0030] Figure 1 is a typical morphology feature of the hot-rolled initial microstructure of the super-high-strength multiphase steel prepared in Example 1-II.

[0031] Figure 2 is the hot-rolled initial microstructure of the super-high-strength multiphase steel prepared in Example 1-II observed under a transmission electron microscope.

[0032] Figure 3 is a typical microstructure scanning diagram of the super-high-strength multiphase steel product prepared in Example 1-II.

[0033] Figure 4 is the morphology feature of residual austenite of the super-high-strength multiphase steel product prepared in Example 1-II.

[0034] Figure 5 is Figure 4 a dark field image.

[0035] Figure 6 is Figure 4 a diffraction pattern diagram marked by a circle.

[0036] Figure 7 is the V(C, N) dispersion precipitation feature diagram of the super-high-strength multiphase steel product prepared in Example 1-II under a transmission electron microscope.

[0037] Figure 8 is the TEM morphology diagram of the high-density "dislocation pile-up" of the matrix of the super-high-strength multiphase steel product prepared in Example 1-II under a transmission electron microscope. DETAILED DESCRIPTION

[0038] In order to make the person skilled in the art better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the protection scope of the present application.

[0039] The embodiment of the application adopts a kind of super high-strength multiphase steel strength plastic product and performance stability regulation method to prepare super high-strength multiphase steel, specifically comprising the following steps:

[0040] (1) Steel billet preparation: low-cost, economical component design is used to prepare steel billet;

[0041] (2) Hot rolling initial organization regulation process: the finishing temperature is 890±15℃, and then naturally cooled to 635±15℃ for coiling;

[0042] (3) First cold rolling regulation process: the above hot-rolled steel coil is pickled to remove the oxides on the surface of the strip, to ensure that the surface reflectivity of the strip is ≥70%, and then cold-rolled with a reduction of 40%~50%;

[0043] (4) Cover annealing regulation process: the above first cold-rolled strip is annealed in a cover, the annealing temperature is 670℃, and the holding time is 10~12h;

[0044] (5) Second cold rolling regulation process: the above cover annealed steel coil is then cold-rolled with a reduction of 33%~45%; the total reduction of cold rolling is ≥60.0%;

[0045] (6) Annealing microstructure regulation process: (Ac3+30)℃≤austenitizing annealing heating temperature≤910℃, after the end of the austenitizing soaking section, rapidly cool to overaging temperature 380±10℃ at a cooling rate of ≥35℃ / s; control the running speed according to the thickness specification of the strip, wherein when the thickness specification is 0.6≤δ≤1.0mm, the strip speed is ≥100m / min, when the thickness specification is 1.0<δ<1.8mm, the strip speed is 85~100m / min, and when the thickness specification is 1.8≤δ≤2.5mm, the strip speed is 80±5m / min; the final cooling out-of-furnace temperature is <160℃;

[0046] (7) Leveling regulation process: the elongation during the leveling process is set to 0.7±0.1%, and the actual rolling force is 7200~7600KN; to obtain super high-strength multiphase steel.

[0047] Example 1-5

[0048] Example 1-5 is composed of C-Si-Mn as the basic component, and the mass percentage of the chemical composition is C: 0.22~0.25%, Si: 1.6~1.8%, Mn: 2.1~2.4%, V: 0.20~0.24%, N: 0.020~0.025%, Alt: 0.050~0.080%. And limit the control of P, S, O three elements to be not more than 0.005%, the balance is Fe and other unavoidable impurities; see Table 1 for specific steel billet composition

[0049] Table 1 Steel blank composition of the example (mass percentage, %)

[0050]

[0051] The actual production hot rolling initial microstructure regulation process and cold rolling regulation process are shown in Table 2, and the annealing microstructure regulation process parameters and the smoothing regulation process parameters are shown in Table 3.

[0052] Table 2 Hot rolling initial microstructure regulation and cold rolling regulation process parameters of the example

[0053]

[0054] Table 3 Annealing microstructure regulation process and smoothing regulation parameters of the example

[0055]

[0056] In the hot rolling process, the hot rolling initial microstructure of the test steel sample is analyzed, and the hot rolling initial microstructure is regulated to 50% to 60% polygonal or equiaxed ferrite and 40% to 50% pearlite structure by using the technical scheme implemented by the present application. The ferrite grain size is regulated to 3 to 5 μm; the pearlite is mainly in the form of lamellar pearlite, and the continuous cementite lamella with a thickness of 8 to 40 nm is distributed in parallel and divides the pro-eutectoid ferrite; and a small amount of granular pearlite also exists. Figure 1 The typical morphological characteristics of the hot rolling initial microstructure of the super-high-strength complex-phase steel prepared in Example 1-II can be clearly observed, and the hot rolling initial microstructure is mainly composed of F+P (see Figure 1 ), in which the volume fraction of ferrite is about 58%, and a certain amount of equiaxed ferrite also exists; the pearlite mainly exists in the form of lamellar in the matrix, and the continuous cementite lamella with a thickness of 9 to 35 nm is observed in the transmission electron microscope, which is distributed in parallel and divides the pro-eutectoid ferrite, as indicated by Figure 2 “lamellar pearlite”; in addition, a small amount of cementite in the hot rolling initial microstructure under the experimental conditions exists in the form of particles in the matrix, as indicated by Figure 2 “granular pearlite” in the transmission electron microscope.

[0057] The prepared super-high-strength complex-phase steel product is sampled for microstructure analysis and mechanical property test, and the test and analysis results are shown in Table 4.

[0058] Table 4 Mechanical properties and microstructure volume fraction of the example

[0059]

[0060] The technical scheme implemented by the application can regulate the final microstructure of the ultra-high strength complex phase steel to be bainite matrix structure and >15.0% residual gamma phase. The matrix structure in the microstructure is regulated to be bainite lath bundles distributed in different directions, and the bainite lath bundles in different direction "reference units" have relatively blurred bainite lath interfaces; the residual gamma phase exists in the form of interfaces between the bainite lath bundles. Figure 3 Typical microstructure scanning of the ultra-high strength complex phase steel product prepared in Example 1-II; Figure 4 Morphological characteristics of residual austenite of the ultra-high strength complex phase steel product prepared in Example 1-II; Figure 5 Dark field image of Figure 4 Dark field image of Figure 6 Diffraction pattern marked by the circle in Figure 4 Diffraction pattern marked by the circle in Figure 7 TEM morphological diagram of V(C, N) dispersion precipitation in the matrix of the ultra-high strength complex phase steel product prepared in Example 1-II under a transmission electron microscope, Figure 8 TEM morphological diagram of high-density "dislocation pile-up" in the matrix of the ultra-high strength complex phase steel product prepared in Example 1-II under a transmission electron microscope.

[0061] Although the application has been described in detail with reference to the preferred embodiments, the application is not limited to the preferred embodiments. Any modification or replacement made by those skilled in the art to the embodiments of the application without departing from the spirit and essence of the application should be included in the scope of the application. Any modification or replacement made by those skilled in the art to the technical range disclosed in the application should be included in the protection scope of the application.

Claims

1. A method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel, characterized in that, Includes the following steps: (1) Steel billet composition design: It is based on C-Si-Mn as the basic composition. The mass percentage of its chemical composition is C: 0.22%~0.25%, Si: 1.6%~1.8%, Mn: 2.1%~2.4%, V: 0.20%~0.24%, N: 0.020%~0.025%, Alt: 0.050%~0.080%, P, S and O are not greater than 0.005%, and the balance is Fe and other unavoidable impurities; (2) Initial microstructure control process of hot rolling: The finishing rolling temperature is 890±15℃, and then it is naturally cooled to 635±15℃ for coiling; the initial microstructure is controlled to be 50%~60% ferrite and 40%~50% pearlite. (3) Cold rolling control process: The hot-rolled steel coil obtained in step (2) is pickled and then cold rolled in one step with a reduction of 40%~50%; (4) Bell-type annealing control process: The cold-rolled strip steel coil from step (3) is annealed in a bell-type manner. The annealing temperature is 670℃ and the holding time is 10~12h. (5) Secondary cold rolling control process: The annealed steel coil from step (4) is then subjected to secondary cold rolling with a reduction of 33%~45%; (6) Annealing microstructure control process: (Ac3+30)℃≤austenitic annealing heating temperature≤910℃, after the austenitic homogenization heating section, rapidly cool to the over-aging temperature of 380±10℃ at a cooling rate of ≥35℃ / s; final cooling temperature <160℃. The ultra-high strength multiphase steel has a strength-ductility product >23.0 GPa·%, a tensile strength of 1250 MPa ±25 MPa, a yield strength of 900 ±25 MPa, a yield strength ratio stably controlled between 0.70 and 0.75, an elongation >18.0%, and a hole expansion rate λ ≥55%.

2. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 1, characterized in that, In step (2), the ferrite is polygonal or equiaxed and the ferrite grain size is 3~5μm; the pearlite is lamellar pearlite and granular pearlite.

3. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 1, characterized in that, In step (3), the oxides on the surface of the hot-rolled steel coil are removed by pickling, and the reflectivity of the surface of the hot-rolled steel coil after pickling is ≥70%.

4. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 1, characterized in that, The total reduction rate of the two cold rolling processes in steps (3) and (5) is ≥60.0%.

5. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 1, characterized in that, The annealing speeds for strips of different thicknesses in step (6) are as follows: when the thickness is 0.6≤δ≤1.0mm, the strip speed is ≥100m / min; when the thickness is 1.0<δ<1.8mm, the strip speed is 85~100m / min; and when the thickness is 1.8≤δ≤2.5mm, the strip speed is 80±5m / min.

6. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 1, characterized in that, The final microstructure of ultra-high strength multiphase steel is a bainitic matrix and >15.0% second-phase retained austenite.

7. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 6, characterized in that, The bainitic matrix contains dispersed precipitation of V(C,N) particles with a diameter of <10nm and high-density "dislocation pile-up".

8. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 6, characterized in that, The microstructure is morphologically controlled as bainite lath bundles distributed in different orientations, with obvious original austenite grain boundaries, and the bainite lath interfaces in different orientation "reference units" are relatively blurred; the second-phase residual austenite exists in the bainite laths in the form of interfaces.

9. The method for controlling the strength-ductility product and performance stability of ultra-high strength multiphase steel as described in claim 1, characterized in that, After the annealing microstructure control process, a leveling control process is carried out to obtain ultra-high strength multiphase steel. The elongation rate during the leveling process is 0.7%±0.1%, and the rolling force is 7200~7600KN.

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