CN113201690B

CN113201690B - Low-carbon nano bainite complex phase steel and preparation method thereof

Info

Publication number: CN113201690B
Application number: CN202110466341.8A
Authority: CN
Inventors: 郭辉; 陈彦伟; 李强; 范亚萍
Original assignee: Weifang University of Science and Technology
Current assignee: Weifang University of Science and Technology
Priority date: 2021-04-28
Filing date: 2021-04-28
Publication date: 2022-01-28
Anticipated expiration: 2041-04-28
Also published as: CN113201690A

Abstract

The low-carbon nano bainite complex phase steel comprises the following elements in percentage by weight: c: 0.28 to 0.35%, Si: 1.6-1.8%, Mn: 1.6-2.2%, Cr: 2.0-3.0%, Ni: 1.1-1.5% and Nb/V: 0.02-0.025 wt%, and the balance Fe and inevitable impurities, wherein the complex phase steel is low-carbon alloy steel, and Ms is 200-295 ℃. The preparation method comprises the following steps: s1, smelting in a vacuum induction furnace and casting into steel ingots; s2, homogenizing the steel ingot at 1200 ℃ for 24h, and carrying out hot rolling treatment to obtain a hot rolled plate blank; s3, heating the hot rolled plate blank to 900-; s4, rapidly heating the quenched plate blank to a critical zone for annealing, and preserving heat for 30-80min to generate partial pre-ferrite; s5, immediately placing the annealed plate blank into a salt bath furnace at the temperature of 250-350 ℃ for heat preservation for 2min, taking out the annealed plate blank and then carrying out warm rolling deformation at the temperature of 250-350 ℃; s6, immediately placing the warm-rolled plate blank into a 200-350 ℃ salt bath furnace for heat preservation for 0.5-5 h to finish isothermal phase change of the nano bainite, and air-cooling to room temperature.

Description

Low-carbon nano bainite complex phase steel and preparation method thereof

Technical Field

The invention belongs to the field of metal materials, and relates to low-carbon nano bainite complex phase steel and a preparation method thereof.

Background

With the increasing of automobile production and sales and maintenance, the energy and environment problems become more severe, and more severe requirements on energy conservation, environmental protection, safety and the like are put forward in the automobile industry. The development and application of high-strength high-toughness advanced high-strength steel become effective ways and important methods for realizing the lightening and collision safety of automobiles. Nanometer bainite steel (Nanostructured bainitic steel) is one of the third-generation advanced high-strength steels with the greatest development prospect, has extremely high tensile strength (greater than or equal to 2000MPa) and good ductility (20%), can greatly reduce the quality of automobile maintenance while ensuring impact resistance safety, structural rigidity and comfort, and promotes the continuous improvement of fuel economy and energy-saving emission reduction level of passenger cars. At present, the nano bainite steel is widely popularized in the national defense and wear-resistant fields of tank armors, mining machinery and the like, the application of the nano bainite steel in automobile design is further promoted, the strength grade of the existing automobile steel can be improved, and important guidance is provided for structure optimization design and material reasonable selection in the automobile lightweight development process.

The nano bainite steel is prepared into a complex phase structure consisting of nano bainite ferrite laths and carbon-rich film-shaped residual austenite through a low-temperature isothermal quenching process, the preparation process is simple, and the complex phase structure has good strong plasticity matching, so that the nano bainite steel draws wide attention of numerous scholars and industries at home and abroad. However, the process flow is long and tedious due to the characteristic of low-temperature transformation, and the time for completing the phase transformation is dozens of hours, so that the large-scale production and application of the method are restricted. Meanwhile, the high carbon content of the nano bainite steel causes low toughness and poor weldability, and the reduction of the carbon content inevitably causes the temperature increase of bainite and the like, thereby causing the coarsening of bainite laths and the loss of strength. At present, researchers develop a great deal of research, try to explore ways for rapidly preparing a nano-structure bainite structure and improving toughness, and solve the problems of overlong bainite phase change completion time and low impact toughness.

The literature retrieval in the prior art shows that the nano bainite steel containing proeutectoid ferrite can be prepared by adopting a preparation process of cooling by stages and deformation quenching, but the process complexity is higher, the large-scale production is difficult to realize, the carbon content is higher, and the toughness of steel is not obviously improved; the low-carbon high-hardness nano bainite steel is prepared by adopting a rolling deformation process, the experimental steel is austenitized and then rapidly cooled to a temperature higher than a martensite transformation temperature (abbreviated as Ms) point to deform by 50%, and then cooled to a temperature lower than the Ms point to perform isothermal treatment, so that high microhardness is obtained, martensite is easy to exist in a structure, and the toughness of the experimental steel is poor. The superfine crystal ferrite/low-temperature bainite dual-phase low-carbon steel is prepared by adopting a cold rolling troostite critical region heating isothermal quenching process, but the superfine crystal ferrite content is high, the tensile strength of steel is less than 1000MPa, and the requirement of high strength required under special working conditions cannot be met.

Disclosure of Invention

Object of the Invention

The invention provides low-carbon nano bainite complex phase steel for automobiles and a preparation method thereof, and mainly aims to prepare nano bainite complex phase steel which has ultrahigh strength and short preparation period and consists of deformed pre-ferrite and nano bainite.

Technical scheme

The low-carbon nano bainite complex phase steel comprises the following elements in percentage by weight:

c: 0.28 to 0.35%, Si: 1.6-1.8%, Mn: 1.6-2.2%, Cr: 2.0-3.0%, Ni: 1.1-1.5% and Nb/V: 0.02-0.025 percent, and the balance of Fe and inevitable impurities, wherein the complex phase steel is low-carbon alloy steel, and the martensite transformation temperature of the low-carbon alloy steel is 200-295 ℃.

The preparation method of the low-carbon nano bainite complex phase steel comprises the following steps:

s1, smelting in a vacuum induction furnace and casting into a steel ingot according to the component design requirements of the steel part;

s2, homogenizing the ingot at 1200 ℃ for 24h, subsequently hot rolling,

the hot rolling conditions are as follows: rolling the homogenized sample at 1100-1150 ℃ for 5-7 times, wherein the final rolling temperature is not lower than 850 ℃, the total hot rolling reduction is 50-70%, and the thickness of the final hot rolled plate blank is 15mm to obtain a hot rolled plate blank;

s3, heating the hot rolled plate blank in a muffle furnace to 900-950 ℃, preserving heat for 0.5-1h to ensure complete austenitization, then quickly taking out of the furnace, putting into oil, quenching and cooling to room temperature, wherein the cooling speed in the quenched oil is more than or equal to 10 ℃/s to obtain a full martensite structure;

s4, rapidly heating the quenched plate blank obtained in s3 at a heating speed of more than or equal to 20 ℃/s to a critical zone, carrying out 640-700 ℃ annealing treatment, and carrying out heat preservation for 30-80min to generate partial pre-ferrite, wherein the ferrite content is 8-15%;

s5, immediately placing the annealed blank obtained in s4 into a container of 250-Keeping the temperature in a 350 ℃ salt bath furnace for 2min, taking out and carrying out warm rolling deformation at the temperature of 250-350 ℃, wherein the total deformation amount of the warm rolling is controlled to be 20-40%, and the deformation rate is 3s^-1；

s6, immediately putting the warm rolled plate blank obtained in s5 into a salt bath furnace at the temperature of 200-350 ℃, preserving heat for 0.5-5 hours, completing isothermal phase change of the nano bainite, and then air-cooling to room temperature.

The physical metallurgy principle of the technical scheme is as follows: through Mn, Cr and Ni alloying component design, the thermal stability of the super-cooled austenite is improved, the temperature rise of a phase transformation area caused by the reduction of carbon content is compensated, and the low-temperature isothermal phase transformation of bainite is ensured; Nb/V microalloying is adopted to realize grain refinement, fine original austenite grains are obtained, and the bainite nucleation rate is promoted to be improved; the distribution of alloy elements between ferrite and austenite is realized through a critical region annealing process, and the thermal stability and the mechanical stability of super-cooled austenite are improved by combining warm rolling treatment before bainite transformation begins, so that the phase transformation temperature required by the formation of a nano-structure bainite structure is ensured; the bainite nucleation growth condition is effectively improved through the deformation strengthening of the pre-ferrite and the super-cooled austenite in the critical region, the phase change driving force of the super-cooled austenite is improved, and the low-temperature isothermal transformation of the bainite is accelerated; the defect density in the supercooled austenite is increased by warm rolling deformation and fast cooling treatment, and the integral strength of the material is improved by heredity to a bainite structure.

Advantages and effects

The invention takes the industrial field large-scale application as the prospect, designs the process route for preparing the high-strength low-carbon nano bainite complex phase steel for the automobile based on the critical region annealing and warm rolling synergistic effect, obtains the complex phase structure consisting of deformed pre-ferrite and nano bainite, and realizes the rapid preparation of the nano structure complex phase structure. The invention shortens the phase transition time of preparing the nanometer bainite and improves the plasticity and toughness of the nanometer bainite under the condition of ensuring no loss of high strength. The invention further widens the application field of the nano bainite steel and improves the strength grade of the existing automobile steel.

Drawings

FIG. 1 is an electron microscope image of the microstructure of low carbon nano bainite complex phase steel obtained in example 1 of the present invention;

FIG. 2 is the isothermal transformation curve of the low carbon nano bainite complex phase steel obtained in example 1 of the present invention;

FIG. 3 is the engineering stress-strain curve of the low carbon nano bainite complex phase steel obtained in example 1 of the present invention.

Detailed Description

The present invention is further illustrated by, but is not limited to, the following examples.

Example 1

Smelting a low-carbon alloy by using a vacuum induction smelting furnace, wherein the low-carbon alloy comprises the following chemical components in percentage by weight: 0.28% of C, 1.6% of Si, 1.6% of Mn, 2.0% of Cr, 1.1% of Ni and 0.02% of Nb, and the balance of Fe and inevitable impurities. By adding a certain amount of Mn, Cr and Ni elements, the Ms point temperature of the low-carbon alloy steel is 295 ℃, and bainite phase change can be ensured to occur at 300 ℃. The addition of Nb element forms NbC precipitate, pins dislocation, refines original austenite grain, and the grain size is below 20 μm. The specific processing technology is as follows: and heating the steel ingot to 1200 ℃, preserving heat for 24 hours, and carrying out homogenization treatment. And then, starting rolling the homogenized sample at 1100 ℃, rolling for 5 times, wherein the final rolling temperature is 890 ℃, the total rolling reduction of hot rolling is 50%, and the thickness of the final hot-rolled plate blank is 15 mm. And then heating the hot rolled plate blank to 900 ℃ in a muffle furnace, preserving heat for 1h, quickly taking the hot rolled plate blank out of the furnace, putting the hot rolled plate blank into oil, quenching and cooling the hot rolled plate blank to room temperature. And rapidly heating the quenched plate blank to 680 ℃ for annealing treatment, keeping the temperature for 40min to generate 15% of pre-ferrite, increasing the carbon content in the residual austenite, and improving the thermal stability of the steel, wherein the Ms point of the alloy steel is reduced to 280 ℃, and the lowest temperature at which bainite transformation can occur is further reduced. Meanwhile, the generation of the pre-phase transformation ferrite introduces a ferrite/austenite interface, the nucleation growth condition of bainite is improved, more nucleation positions are provided for bainite phase transformation, and the bainite phase transformation rate is improved. Then immediately putting the annealed plate blank into a salt bath furnace at 300 ℃ for heat preservation for 2min, taking out the annealed plate blank, and performing warm rolling deformation at 300 ℃, wherein the total deformation is 20 percent, and the deformation rate is 3s^-1. A large amount of dislocation defects are introduced into the supercooled austenite through warm rolling deformation, a bainite preferential nucleation position is provided, and simultaneously deformation storage is carried outThe energy storage improves the phase transformation driving force and accelerates the low-temperature bainite phase transformation. And immediately putting the warm-rolled plate blank into a salt bath furnace at 300 ℃ for heat preservation for 3h, and then air-cooling to room temperature to finally prepare the low-carbon nano bainite complex phase steel. Compared with the isothermal time of 6h required by a direct isothermal process, the critical zone annealing and warm rolling deformation process can shorten the phase change time by nearly half and obviously shorten the preparation period. The test shows that the yield strength of the steel is 1300MPa, the tensile strength is 1800MPa, the total elongation is 25 percent, and the impact toughness is 60J/cm²。

Example 2

Smelting a low-carbon alloy by using a vacuum induction smelting furnace, wherein the low-carbon alloy comprises the following chemical components in percentage by weight: 0.28 percent of C, 1.6 percent of Si, 1.8 percent of Mn, 2.2 percent of Cr, 1.1 percent of Ni and 0.025 percent of Nb, and the balance of Fe and inevitable impurities. By adding a certain amount of Mn, Cr and Ni elements, the Ms point temperature of the low-carbon alloy steel is 281 ℃, and bainite phase change can be ensured to occur at 300 ℃. The addition of Nb element forms NbC precipitate, pins dislocation, refines original austenite grain, and the grain size is below 20 μm. The specific processing technology is as follows: and heating the steel ingot to 1200 ℃, preserving heat for 24 hours, and carrying out homogenization treatment. Then, the homogenized sample is subjected to initial rolling at 1100 ℃, rolling is carried out for 6 times, the final rolling temperature is 860 ℃, the total rolling reduction of hot rolling is 50%, and the thickness of the final hot-rolled plate blank is 15 mm. And then heating the hot rolled plate blank to 920 ℃ in a muffle furnace, preserving heat for 0.8h, quickly taking the hot rolled plate blank out of the furnace, putting the hot rolled plate blank into oil, quenching and cooling the hot rolled plate blank to room temperature. And rapidly heating the quenched plate blank to 700 ℃ for annealing treatment, and preserving heat for 30min to generate 11% of pre-ferrite. The carbon content in the residual austenite is increased, the thermal stability of the residual austenite is improved, the Ms point of the alloy steel is reduced to 269 ℃, and the lowest temperature at which bainite transformation can occur is further reduced. Meanwhile, the generation of the pre-phase transformation ferrite introduces a ferrite/austenite interface, the nucleation growth condition of bainite is improved, more nucleation positions are provided for bainite phase transformation, and the bainite phase transformation rate is improved. Then immediately putting the annealed plate blank into a 350 ℃ salt bath furnace for heat preservation for 2min, taking out the annealed plate blank, and performing warm rolling deformation at 350 ℃, wherein the total deformation is 40%, and the deformation rate is 3s^-1. Introducing a large amount of dislocation defects in the undercooled austenite through warm rolling deformation to provide bainiteThe position of the nucleus is preferentially formed by the solid, and meanwhile, the phase change driving force can be improved by deformation storage, and the low-temperature bainite phase change is accelerated. And (3) immediately placing the warm-rolled plate blank into a 350 ℃ salt bath furnace, preserving the heat for 0.5h, then air-cooling to room temperature, and finally preparing the low-carbon nano bainite complex phase steel, wherein the phase change time can be shortened by nearly one fourth by the critical zone annealing and warm-rolling deformation process compared with the 2h isothermal time required by the direct isothermal process, and the preparation period is obviously shortened. The test shows that the yield strength of the steel is 1240MPa, the tensile strength is 1650MPa, the total elongation is 33 percent, and the impact toughness is 72J/cm²。

Example 3

Smelting a low-carbon alloy by using a vacuum induction smelting furnace, wherein the low-carbon alloy comprises the following chemical components in percentage by weight: 0.3 percent of C, 1.7 percent of Si, 1.8 percent of Mn, 2.5 percent of Cr, 1.2 percent of Ni and 0.025 percent of Nb, and the balance of Fe and inevitable impurities. By adding a certain amount of Mn, Cr and Ni elements, the Ms point temperature of the low-carbon alloy steel is 263 ℃, and bainite phase transformation can be ensured to occur at 300 ℃.

The addition of Nb element forms NbC precipitate, pins dislocation, refines original austenite grain, and the grain size is below 20 μm. The specific processing technology is as follows: and heating the steel ingot to 1200 ℃, preserving heat for 24 hours, and carrying out homogenization treatment. Then, the homogenized sample is subjected to initial rolling at 1100 ℃, rolling is carried out for 5 times, the final rolling temperature is 860 ℃, the total rolling reduction of hot rolling is 50%, and the thickness of the final hot-rolled plate blank is 15 mm. And then heating the hot rolled plate blank to 930 ℃ in a muffle furnace, preserving heat for 0.6h, quickly taking the hot rolled plate blank out of the furnace, putting the hot rolled plate blank into oil, quenching and cooling the hot rolled plate blank to room temperature. And rapidly heating the quenched plate blank to 690 ℃ for annealing treatment, preserving heat for 40min to generate 10% of pre-ferrite, increasing the carbon content in the residual austenite, and improving the thermal stability of the steel, wherein the Ms point of the alloy steel is reduced to 245 ℃, and the lowest temperature at which bainite transformation can occur is further reduced. Meanwhile, the generation of the pre-phase transformation ferrite introduces a ferrite/austenite interface, the nucleation growth condition of bainite is improved, more nucleation positions are provided for bainite phase transformation, and the bainite phase transformation rate is improved. Then immediately putting the annealed plate blank into a salt bath furnace at 250 ℃ for heat preservation for 2min, taking out the annealed plate blank, and performing warm rolling deformation at 250 ℃, wherein the total deformation is 30 percent, and the deformation rate is 3s^-1. A large number of dislocation defects are introduced into the supercooled austenite through warm rolling deformation, a bainite preferential nucleation position is provided, and meanwhile, the transformation storage can improve the phase transformation driving force and accelerate the phase transformation of the low-temperature bainite. And immediately putting the warm-rolled plate blank into a salt bath furnace at 300 ℃ for heat preservation for 3h, and then air-cooling to room temperature to finally prepare the low-carbon nano bainite complex phase steel. Compared with the isothermal time of 6h required by a direct isothermal process, the critical zone annealing and warm rolling deformation process can shorten the phase change time by nearly half and obviously shorten the preparation period. The test shows that the yield strength of the steel is 1490MPa, the tensile strength is 1900MPa, the total elongation is 21 percent, and the impact toughness is 48J/cm²。

Example 4

Smelting a low-carbon alloy by using a vacuum induction smelting furnace, wherein the low-carbon alloy comprises the following chemical components in percentage by weight: 0.32% of C, 1.7% of Si, 2.0% of Mn, 2.7% of Cr, 1.3% of Ni, 0.025% of Nb and the balance of Fe and inevitable impurities, and casting the molten steel into steel ingots. By adding a certain amount of Mn, Cr and Ni elements, the Ms point temperature of the low-carbon alloy steel is 241 ℃, and bainite phase change can be ensured to occur at 250 ℃. The addition of Nb element forms NbC precipitate, pins dislocation, refines original austenite grain, and the grain size is below 20 μm. The specific processing technology is as follows: the Ms point temperature of the low-carbon alloy steel is 241 ℃. And heating the steel ingot to 1200 ℃, preserving heat for 24 hours, and carrying out homogenization treatment. Then, the homogenized sample is rolled at 1100 ℃ for 5 passes, the final rolling temperature is 870 ℃, the total rolling reduction is 50%, and the thickness of the final hot-rolled plate blank is 15 mm. And then heating the hot rolled plate blank to 950 ℃ in a muffle furnace, preserving heat for 0.5h, quickly taking the hot rolled plate blank out of the furnace, putting the hot rolled plate blank into oil, quenching and cooling the hot rolled plate blank to room temperature. And rapidly heating the quenched plate blank to 680 ℃ for annealing treatment, keeping the temperature for 40min to generate 12% of pre-ferrite, increasing the carbon content in the residual austenite, and improving the thermal stability of the steel, wherein the Ms point of the alloy steel is reduced to 230 ℃, and the lowest temperature at which bainite transformation can occur is further reduced. Meanwhile, the generation of the pre-phase transformation ferrite introduces a ferrite/austenite interface, the nucleation growth condition of bainite is improved, more nucleation positions are provided for bainite phase transformation, and the bainite phase transformation rate is improved. Then the annealed plate blank is immediately put into a salt bath furnace with the temperature of 250 ℃ for heat preservation for 2min, and takenAfter the extrusion, the hot rolling deformation is carried out at 250 ℃, the total deformation is 30 percent, and the deformation rate is 3s^-1. A large number of dislocation defects are introduced into the supercooled austenite through warm rolling deformation, a bainite preferential nucleation position is provided, and meanwhile, the transformation storage can improve the phase transformation driving force and accelerate the phase transformation of the low-temperature bainite. And immediately putting the warm-rolled plate blank into a salt bath furnace at 250 ℃ for heat preservation for 4 hours, and then air-cooling to room temperature to finally prepare the low-carbon nano bainite complex phase steel. For the 12h isothermal time required by the direct isothermal process, the critical zone annealing and warm rolling deformation process can shorten the phase change time by nearly two thirds, and the preparation period is obviously shortened. Through tests, the yield strength of the steel is 1550MPa, the tensile strength is 1920MPa, the total elongation is 18 percent, and the impact toughness is 36J/cm²。

Example 5

Smelting a low-carbon alloy by using a vacuum induction smelting furnace, wherein the low-carbon alloy comprises the following chemical components in percentage by weight: 0.33 percent of C, 1.8 percent of Si, 2.0 percent of Mn, 2.8 percent of Cr, 1.3 percent of Ni and 0.025 percent of Nb, and the balance of Fe and inevitable impurities, and casting the molten steel into steel ingots. By adding a certain amount of Mn, Cr and Ni elements, the Ms point temperature of the low-carbon alloy steel is 230 ℃, and bainite phase change can be ensured to occur at 250 ℃. The addition of Nb element forms NbC precipitate, pins dislocation, refines original austenite grain, and the grain size is below 20 μm. The specific processing technology is as follows: and heating the steel ingot to 1200 ℃, preserving heat for 24 hours, and carrying out homogenization treatment. Then, the homogenized sample is rolled at 1100 ℃ for 5 passes, the final rolling temperature is 870 ℃, the total rolling reduction is 60%, and the thickness of the final hot-rolled plate blank is 15 mm. And then heating the hot rolled plate blank to 950 ℃ in a muffle furnace, preserving heat for 0.5h, quickly taking the hot rolled plate blank out of the furnace, putting the hot rolled plate blank into oil, quenching and cooling the hot rolled plate blank to room temperature. And rapidly heating the quenched plate blank to 660 ℃ for annealing treatment, keeping the temperature for 80min to generate 10% of pre-ferrite, increasing the carbon content in the residual austenite, and improving the thermal stability of the steel, wherein the Ms point of the alloy steel is reduced to 218 ℃, and the lowest temperature at which bainite transformation can occur is further reduced. Meanwhile, the generation of the pre-phase transformation ferrite introduces a ferrite/austenite interface, the nucleation growth condition of bainite is improved, more nucleation positions are provided for bainite phase transformation, and the bainite phase transformation rate is improved. And then. Annealing the plate blankImmediately placing into a 350 deg.C salt bath furnace, holding for 2min, taking out, and performing warm rolling deformation at 350 deg.C with total deformation of 40% and deformation rate of 3s^-1. A large number of dislocation defects are introduced into the supercooled austenite through warm rolling deformation, a bainite preferential nucleation position is provided, and meanwhile, the transformation storage can improve the phase transformation driving force and accelerate the phase transformation of the low-temperature bainite. And immediately putting the warm-rolled plate blank into a salt bath furnace at 300 ℃ for heat preservation for 2 hours, and then air-cooling to room temperature to finally prepare the low-carbon nano bainite complex phase steel. Compared with the isothermal time of 6h required by a direct isothermal process, the critical zone annealing and warm rolling deformation process can shorten the phase change time by nearly half and obviously shorten the preparation period. The test shows that the yield strength of the steel is 1400MPa, the tensile strength is 1860MPa, the total elongation is 23 percent, and the impact toughness is 44J/cm²。

Example 6

Smelting a low-carbon alloy by using a vacuum induction smelting furnace, wherein the low-carbon alloy comprises the following chemical components in percentage by weight: 0.35% of C, 1.8% of Si, 2.2% of Mn, 3.0% of Cr, 1.5% of Ni and 0.025% of Nb, and the balance of Fe and inevitable impurities. By adding a certain amount of Mn, Cr and Ni elements, the Ms point temperature of the low-carbon alloy steel is 200 ℃, and bainite phase change can be ensured to occur at 210 ℃. The addition of Nb element forms NbC precipitate, pins dislocation, refines original austenite grain, and the grain size is below 20 μm. The specific processing technology is as follows: and heating the steel ingot to 1200 ℃, preserving heat for 24 hours, and carrying out homogenization treatment. Subsequently, the homogenized sample was subjected to initial rolling at 1150 ℃ for 7 passes, and the final rolling temperature was 870 ℃, the total rolling reduction was 70%, and the thickness of the final hot-rolled slab was 15 mm. And then heating the hot rolled plate blank to 950 ℃ in a muffle furnace, preserving heat for 0.5h, quickly taking the hot rolled plate blank out of the furnace, putting the hot rolled plate blank into oil, quenching and cooling the hot rolled plate blank to room temperature. And rapidly heating the quenched plate blank to 640 ℃ for annealing treatment, keeping the temperature for 60min to generate 8% of pre-ferrite, increasing the carbon content in the residual austenite, and improving the thermal stability of the steel, wherein the Ms point of the alloy steel is reduced to 191 ℃, and the lowest temperature at which bainite transformation can occur is further reduced. Meanwhile, the generation of pre-phase transformation ferrite is introduced into a ferrite/austenite interface, the nucleation growth condition of bainite is improved, more nucleation positions are provided for bainite phase transformation, and the improvement is realizedThe bainite transformation rate. And then. Immediately putting the annealed plate blank into a salt bath furnace at 250 ℃ for heat preservation for 2min, taking out the annealed plate blank, and performing warm rolling deformation at 250 ℃, wherein the total deformation is 30 percent, and the deformation rate is 3s^-1. A large number of dislocation defects are introduced into the supercooled austenite through warm rolling deformation, a bainite preferential nucleation position is provided, and meanwhile, the transformation storage can improve the phase transformation driving force and accelerate the phase transformation of the low-temperature bainite. And immediately putting the warm-rolled plate blank into a 200 ℃ salt bath furnace for heat preservation for 5 hours, and then air-cooling to room temperature to finally prepare the low-carbon nano bainite complex phase steel. Compared with the isothermal time of 25h required by a direct isothermal process, the critical zone annealing and warm rolling deformation process can shorten the phase change time by nearly four fifths, and obviously shorten the preparation period. The test shows that the yield strength of the steel is 1700MPa, the tensile strength is 2180MPa, the total elongation is 16 percent, and the impact toughness is 33J/cm²。

The low-carbon nano bainite complex phase steel of the technical scheme can be obtained for the electron microscope microstructure, the isothermal phase transition curve, the engineering stress-strain curve and the like obtained in the embodiments 2 to 6.

Claims

1. A preparation method of low-carbon nano bainite complex phase steel is characterized by comprising the following steps:

c: 0.28 to 0.35%, Si: 1.6-1.8%, Mn: 1.6-2.2%, Cr: 2.0-3.0%, Ni: 1.1-1.5% and Nb: 0.02-0.025 percent, and the balance of Fe and inevitable impurities, wherein the complex phase steel is low-carbon alloy steel, and the martensite transformation temperature of the low-carbon alloy steel is 200-295 ℃;

s2, homogenizing the steel ingot at 1200 ℃ for 24h, and then carrying out hot rolling treatment to obtain a hot rolled slab;

s3, heating the hot rolled plate blank in a muffle furnace to 900-950 ℃, preserving heat for 0.5-1h to ensure complete austenitization, then quickly discharging the plate blank out of the furnace, putting the plate blank into oil, quenching and cooling the plate blank to room temperature;

s4, rapidly heating the quenched plate blank obtained in the step s3 to a critical zone for annealing treatment, and preserving heat for 30-80min to generate part of pre-ferrite;

s5, immediately putting the annealed plate blank obtained in s4 into a salt bath furnace at the temperature of 250-350 ℃ for heat preservation for 2min, taking out, and carrying out warm rolling deformation at the temperature of 250-350 ℃;

2. The method for preparing the low-carbon nano bainite complex phase steel according to the claim 1, characterized in that: s2 the hot rolling conditions are: and (3) rolling the homogenized sample at 1100-1150 ℃ for 5-7 times, wherein the final rolling temperature is not lower than 850 ℃, the total rolling reduction of hot rolling is 50-70%, and the thickness of the final hot rolled plate blank is 15 mm.

3. The method for preparing the low-carbon nano bainite complex phase steel according to the claim 1, characterized in that: s3 the cooling speed in the quenched oil is more than or equal to 10 ℃/s, so as to obtain the full martensite structure.

4. The method for preparing the low-carbon nano bainite complex phase steel according to the claim 1, characterized in that: s4 quickly heating the quenched plate blank at a heating speed of more than or equal to 20 ℃/s.

5. The method for preparing the low-carbon nano bainite complex phase steel according to the claim 1, characterized in that: s4, performing critical zone annealing treatment, wherein the annealing temperature is controlled to be 640-700 ℃, and the content of the generated pre-ferrite is 8-15%.

6. The method for preparing the low-carbon nano bainite complex phase steel according to the claim 1, characterized in that: s5 the warm rolling control total deformation amount is 20-40%, and the deformation rate is 3s^-1。

7. A low carbon nano bainite complex phase steel prepared by the method of claim 1, wherein: the weight percentages of the elements are as follows:

c: 0.28 to 0.35%, Si: 1.6-1.8%, Mn: 1.6-2.2%, Cr: 2.0-3.0%, Ni: 1.1-1.5% and Nb: 0.02-0.025 percent, and the balance of Fe and inevitable impurities, wherein the complex phase steel is low-carbon alloy steel, and the martensite transformation temperature of the low-carbon alloy steel is 200-295 ℃.