CN116855829B - Low-carbon nano bainite steel and preparation method thereof - Google Patents
Low-carbon nano bainite steel and preparation method thereof Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 144
- 239000010959 steel Substances 0.000 title claims abstract description 144
- 229910052799 carbon Inorganic materials 0.000 title claims abstract description 62
- 229910001563 bainite Inorganic materials 0.000 title claims abstract description 38
- 238000002360 preparation method Methods 0.000 title abstract description 17
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 38
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 20
- 239000000126 substance Substances 0.000 claims abstract description 19
- 239000012535 impurity Substances 0.000 claims abstract description 5
- 238000005096 rolling process Methods 0.000 claims description 137
- 238000001816 cooling Methods 0.000 claims description 79
- 238000001953 recrystallisation Methods 0.000 claims description 48
- 238000000034 method Methods 0.000 claims description 26
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 22
- 238000005266 casting Methods 0.000 claims description 20
- 230000009467 reduction Effects 0.000 claims description 12
- 230000007704 transition Effects 0.000 claims description 9
- 229910000734 martensite Inorganic materials 0.000 claims description 8
- 238000010438 heat treatment Methods 0.000 claims description 7
- 239000002994 raw material Substances 0.000 claims description 7
- 238000003723 Smelting Methods 0.000 claims description 6
- 238000005303 weighing Methods 0.000 claims description 6
- 238000010583 slow cooling Methods 0.000 claims description 5
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 239000000463 material Substances 0.000 abstract description 7
- 238000004519 manufacturing process Methods 0.000 abstract description 5
- 239000007769 metal material Substances 0.000 abstract description 2
- 230000000052 comparative effect Effects 0.000 description 39
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- 238000001556 precipitation Methods 0.000 description 17
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- 230000006911 nucleation Effects 0.000 description 8
- 238000013461 design Methods 0.000 description 6
- 238000007670 refining Methods 0.000 description 5
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- 238000004904 shortening Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000013459 approach Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
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- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/04—Making ferrous alloys by melting
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
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Abstract
The invention relates to the technical field of metal materials, in particular to low-carbon nano bainitic steel and a preparation method thereof, wherein the low-carbon nano bainitic steel comprises the following chemical components: 0.15-0.30% of C, 1.6-1.9% of Si, 1.8-2.3% of Mn, 0.7-0.9% of Cr, 0.2-0.3% of Mo, less than or equal to 0.010% of P, less than or equal to 0.010% of S, 0.03-0.08% of Nb, and the balance of Fe and unavoidable impurities. The structure of the low-carbon nano bainitic steel mainly comprises bainite, and contains a small amount of residual austenite, wherein the size of bainitic ferrite laths is 80-100nm, and the small amount of residual austenite is fine and uniformly distributed. The low-carbon nano bainite steel material has ultrahigh strength and good plasticity and toughness, and meets the requirements of technical equipment and steel for engineering structure manufacturing.
Description
Technical Field
The invention relates to low-carbon nano bainitic steel and a preparation method thereof, belonging to the technical field of metal materials.
Background
Since the advent of bainitic steels, the steel has been widely used in structural materials because of its excellent comprehensive mechanical properties, and therefore, the development of bainitic steels has become an important approach for improving the strength, plasticity and toughness of steel materials. Research on bainite steel in various countries in the world has achieved great results, such as low-temperature isothermal treatment of medium-high-carbon bainite steel, application of relaxation-precipitation technology in ultralow-carbon and low-carbon bainite steel, and the like.
In recent years, nano-scale bainitic steels have become one of the hot spots of research, and the inventors have called super bainitic steels because of the small laths and high strength due to the transformation of bainite at low temperatures. The common characteristics of the bainite structure are finer grains and nano-scale. The existing preparation process of the medium-low carbon nano bainite steel is relatively complicated, and isothermal operation is required to be carried out in a low temperature area for several days, such as CN103451549B and the like. The invention achieves the purposes of shortening the production period and optimizing the preparation process by utilizing methods of reasonably designing chemical element components, controlling rolling and cooling, utilizing a toughening mechanism of microalloy elements and the like.
Disclosure of Invention
Aiming at the defects existing in the prior art, the invention provides a low-carbon nano bainitic steel and a preparation method thereof, wherein the structure is mainly composed of bainite and contains a small amount of residual austenite, wherein the size of bainitic ferrite laths is 80-100nm, and the small amount of residual austenite is fine and uniformly distributed. The material has ultrahigh strength and good plasticity and toughness, and meets the requirements of technical equipment and steel for engineering structure manufacture.
The technical scheme for solving the technical problems is as follows: a low carbon nano bainitic steel comprising the chemical components of: 0.15-0.30% of C, 1.6-1.9% of Si, 1.8-2.3% of Mn, 0.7-0.9% of Cr, 0.2-0.3% of Mo, less than or equal to 0.010% of P, less than or equal to 0.010% of S, 0.03-0.08% of Nb, and the balance of Fe and unavoidable impurities.
Further, the chemical components of the low-carbon nano bainite steel are as follows: 0.20-0.25% of C, 1.5-1.8% of Si, 1.8-2.2% of Mn, 0.7-0.8% of Cr, 0.2-0.25% of Mo, less than or equal to 0.010% of P, less than or equal to 0.010% of S, 0.03-0.06% of Nb, and the balance of Fe and unavoidable impurities.
Further, the low-carbon nano bainite steel also comprises one or two of V and Ti, and the total content of V and Ti is not higher than 0.3%.
Further, the tensile strength of the low-carbon nano bainite steel is more than or equal to 2000MPa, and the yield is strongThe degree is more than or equal to 1700MPa, the elongation after breaking is more than or equal to 10 percent, and the normal-temperature impact power KV is realized 2 More than or equal to 30J, and the hardness is more than or equal to 600HV10.
Further, the microstructure of the low-carbon nano bainitic steel consists of 80-100nm thick bainitic ferrite and residual austenite.
The invention also discloses a preparation method of the low-carbon nano bainitic steel, which comprises the following steps:
s1, weighing raw materials according to chemical components, smelting and casting, heating a casting blank to austenitizing temperature and carrying out isothermal treatment;
s2, rolling the casting blank after being discharged from the furnace for the first time, rolling in a recrystallization zone, then rolling in a non-recrystallization zone for the second time, and performing in-furnace isothermal treatment at a non-recrystallization finish rolling temperature after the second time of rolling; then oil cooling, wherein the final cooling temperature is more than Ar < 3+ > 20 ℃, and then third rolling is carried out, and the final rolling temperature of the third rolling is higher than Ar < 3 >;
s3, cooling the steel plate prepared in the step S2 again by oil, wherein the final cooling temperature is between the bainite starting transition temperature Bs and the martensite starting transition temperature Ms, and the final cooling temperature is as follows: bs to (ms+50);
s4, slowly cooling the steel plate obtained in the step S3, and performing isothermal treatment, wherein the isothermal temperature is (Ms+5) to (Ms+10);
and S5, cooling to room temperature along with the furnace after the isothermal is finished, and obtaining the low-carbon nano bainitic steel.
Further, in the step S1, a casting blank with the thickness of 120-150mm is heated to austenitizing temperature and isothermally treated for 1.5-2 hours.
In the step S2, the rolling reduction of the first rolling is more than or equal to 50%, and the temperature of the first rolling is 1000-1150 ℃;
the rolling reduction of the second rolling is more than or equal to 60 percent, the temperature range of the second rolling is 880-950 ℃, the steel plate with the thickness of 20-30mm is obtained after the second rolling, and the isothermal treatment time after the second rolling is 5-8 minutes.
In step S2, the temperature range of the third rolling is 820-860 ℃, and the steel plate with the thickness of 12-16mm is obtained after the third rolling.
Further, in the steps S2 and S3, the cooling rate of the oil cooling is 5-15 ℃/S;
the final cooling temperature in the step S3 is 320-350 ℃;
the isothermal temperature of the step S4 is 318-323 ℃, the isothermal treatment time of the step S4 is 8-10 hours, and the slow cooling rate in the step S4 is 0.5-1 ℃/S.
The beneficial effects of the invention are as follows:
(1) In the chemical composition of the low-carbon nano bainite steel, a proper amount of C element is added, so that the strength of the material can be improved, the initial transformation temperature of bainite and the initial transformation temperature of martensite can be reduced, and the welding performance of the material can not be influenced;
the addition of a proper amount of Si element is beneficial to inhibiting the precipitation of carbide in austenite, avoiding cementite from forming among bainitic ferrite laths, and meanwhile, the high Si content carbon-rich austenite has strong stability, and is distributed among laths in the form of film-shaped residual austenite in the bainitic transformation process, so that the toughness of the steel is improved;
the addition of a proper amount of Cr element increases the hardenability of the steel, can reduce the critical cooling speed, provides a wider cooling speed range for bainite transformation, and promotes the bainite transformation;
the addition of a proper amount of Mo element can improve the hardenability, and meanwhile, the elements such as Mo, nb, C and the like can be promoted to form complex and fine second-phase particles, because carbon and other elements in a matrix around the second-phase particles are difficult to enter due to the addition of Mo, the growth of the second-phase particles is hindered, the recrystallization of austenite is effectively inhibited, and the purpose of grain refinement is achieved;
the purpose of improving the toughening effect of the steel can be achieved by adding a proper amount of Nb, the upper limit amount of the Nb can prevent complex phases of undissolved Nb from excessively growing and being large in an austenitizing stage, and the performance of the steel is endangered; the Nb dosage is matched with the two-stage rolling-isothermal-re-rolling process scheme to promote the precipitation of the second phase, thereby achieving the purposes of refining grains and improving the toughness.
(2) According to the invention, the elements are reasonably proportioned and added with Nb, the Nb has the effects of refining grains and improving toughness, and the Nb plays an important role in controlled rolling and cooling:
when austenitizing heating or heat preservation is carried out, the dragging effect of solute atoms of the solid solution Nb and pinning of Nb precipitation phases jointly act;
nb element can significantly delay austenite recrystallization and enable T And then The range of the non-recrystallized region is widened, nb element plays a role in inhibiting the recrystallization process, the recrystallization temperature is increased, the Ar3 temperature is reduced, the growth of austenite grains is inhibited, and the purpose of grain refinement is achieved;
during cooling, nb and C form complex and fine precipitated phases, and the precipitation strengthening effect is exerted.
The strengthening and toughening effects of Nb in the medium-low carbon nanoscale bainitic steel are utilized to make the Nb exert the solid solution strengthening and precipitation strengthening effects, thereby achieving the purposes of refining grains, improving the strength and considering the toughness, and playing important practical significance and practical value in the production and development of the steel for the ultra-high strength structure.
(3) The temperature of the first rolling is controlled in a recrystallization temperature range, and rolling deformation is carried out above the complete recrystallization temperature of the deformed austenite, so that each rolling pass can be recrystallized and refined, and a very fine austenite grain structure is obtained.
The temperature of the second rolling is controlled in a non-recrystallization temperature range, and the non-recrystallization rolling can obtain finer grain structure and greatly improve the toughness of the steel. Since there is a limit to grain refinement after the steel passes through the recrystallization zone. That is, rolling in the recrystallized region can refine the grain size of recrystallized austenite only to a certain extent regardless of the amount of deformation, and deformation in the unrecrystallized region can increase nucleation sites and nucleation rates, so that a fine grain structure can be obtained after transformation.
To achieve the above two-stage rolling, it is necessary to have a wide non-recrystallized region, i.e. a high recrystallization temperature (T And then ) And a lower Ar3 temperature, nb element can significantly delay austenite recrystallization and T can be caused to be And then Increase and make it un-arrangedThe range of the recrystallized region becomes wider.
(4) And the isothermal treatment in the furnace is carried out after the second rolling, which is favorable for the precipitation of complex carbides such as Nb, mo and the like as second phase particles. The rolling in the non-recrystallized region can generate deformation-induced precipitation. The precipitation temperature of the precipitate increases due to the driving of the deformation, and the precipitation rate increases. Fine second phase particles are precipitated in the non-recrystallized region to play a role in pinning grain boundaries and subgrain boundaries, so that the growth of grains is inhibited, the effect of refining the grains is achieved, the toughness is improved, and the strengthening effect is indirectly achieved.
The isothermal treatment time after the second rolling is too short, and the precipitation of second phase particles is not obvious; the isothermal treatment time is 5-8 minutes, which is favorable for obtaining products with high toughness and strength.
(5) The temperature of the third rolling is controlled above Ar3, based on the previous two-time rolling and isothermal precipitation, the third rolling continuously gives a certain degree of refined austenite to rolling deformation, and the grain boundary area of austenite in more unit volume is increased, so that a large number of deformed bands and high-density dislocation are generated in the crystal. These deformation bands act similarly to grain boundaries and can serve as sites for nucleation during phase transformation. The dislocation density of austenite before phase transformation is high, the nucleation rate is increased during phase transformation, and finer grain structure can be obtained after phase transformation.
(6) The method comprises the steps of oil cooling to Bs to (Ms+50) DEG C, slow cooling to isothermal temperature and heat preservation, and a large amount of tissues are prevented from being transformed into martensite at an excessively high cooling speed.
The invention achieves the purposes of shortening the production period and optimizing the preparation process by utilizing methods of reasonably designing chemical element components, controlling rolling and cooling, utilizing a toughening mechanism of microalloy elements and the like.
Drawings
FIG. 1 is a schematic process diagram of the present invention;
FIG. 2 is a scanning electron microscope image of the bainitic steel of example 1;
FIG. 3 is a transmission electron microscope image of the bainitic steel of example 1;
FIG. 4 is a diagram showing precipitation of second phase particles in example 1;
FIG. 5 is a view showing precipitation of second phase particles in comparative example 3;
FIG. 6 is a view showing precipitation of second phase particles in comparative example 4;
FIG. 7 is a TTT curve of experimental steel;
fig. 8 is a CCT curve of experimental steel.
Detailed Description
The following describes the present invention in detail. The present invention may be embodied in many other forms than described herein and similarly modified by those skilled in the art without departing from the spirit of the invention, so that the invention is not limited to the specific embodiments disclosed.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.
As shown in fig. 1, the preparation process method of the low-carbon nano bainitic steel comprises the following steps:
s1, weighing raw materials according to chemical components, smelting and casting, heating a casting blank to austenitizing temperature and carrying out isothermal treatment;
in this example, a cast slab of 120-150mm thickness was heated to austenitizing temperature and isothermally treated for 1.5-2 hours.
S2, rolling the casting blank after being discharged from the furnace for the first time, rolling in a recrystallization zone, then rolling in a non-recrystallization zone for the second time, and performing in-furnace isothermal treatment at a non-recrystallization finish rolling temperature after the second time of rolling; then oil cooling, wherein the final cooling temperature is more than Ar < 3+ > 20 ℃, and then third rolling is carried out, and the final rolling temperature of the third rolling is higher than Ar < 3 >;
in the embodiment, the rolling reduction of the first rolling is more than or equal to 50%, and the temperature range of the first rolling is 1000-1150 ℃; the rolling reduction of the second rolling is more than or equal to 60 percent, the temperature range of the second rolling is 880-950 ℃, the steel plate with the thickness of 20-30mm is obtained after the second rolling, and the isothermal treatment time after the second rolling is 5-8 minutes; the temperature range of the third rolling is 820-860 ℃, and the steel plate with the thickness of 12-16mm is obtained after the third rolling, and the cooling rate of oil cooling is 5-15 ℃/s.
S3, cooling the steel plate prepared in the step S2 again by adopting oil, wherein the final cooling temperature is between the bainite starting transition temperature Bs and the martensite starting transition temperature Ms, and is the final cooling temperature; bs to (ms+50);
in the embodiment, the cooling rate of the oil cooling is 5-15 ℃/s; the final cooling temperature is 320-350 ℃;
s4, slowly cooling the steel plate obtained in the step S3 (cooling rate: 0.5-1 ℃/S), and performing isothermal treatment, wherein the isothermal temperature is (Ms+5) - (Ms+10);
in the embodiment, the isothermal temperature is 318-323 ℃; the isothermal treatment time is 8-10 hours.
And S5, cooling to room temperature along with the furnace after the isothermal is finished, and obtaining the low-carbon nano bainitic steel.
And then measuring the mechanical properties of the low-carbon nano bainite steel.
Example 1
Preparation of low-carbon nano bainitic steel:
s1, weighing raw materials according to chemical components in the table 1, smelting and casting, and heating a casting blank with the thickness of 150mm to the austenitizing temperature of 1150 ℃ and isothermal for 2 hours;
table 1 composition (wt.%) of the cast slab of example 1
S2, rolling the casting blank after discharging from the furnace for the first time, and rolling in a recrystallization zone, wherein the initial rolling temperature in the recrystallization stage is
The final rolling temperature of 1050 ℃ and the rolling reduction of 1000 ℃ in the recrystallization stage;
then carrying out secondary rolling, rolling in a non-recrystallization zone, wherein the initial rolling temperature of a non-recrystallization stage is 940 ℃, the final rolling temperature of a non-recrystallization stage is 880 ℃, the rolling reduction is more than or equal to 65%, carrying out in-furnace isothermal treatment for 8 minutes at the final rolling temperature of the non-recrystallization stage, and rolling into a 20mm thick steel plate by adopting two-stage rolling of recrystallization and non-recrystallization under the condition that second phase particles are separated out as shown in figure 4;
then oil cooling is carried out, the cooling speed is 10 ℃/s, the final temperature of the oil cooling is 860 ℃, then third rolling is carried out, the final rolling temperature of the third rolling is 820 ℃, and the steel plate is rolled into a steel plate with the thickness of 12 mm;
s3, cooling the steel plate prepared in the step S2 again by oil, and cooling the hot rolled steel plate to 350 ℃ at a cooling rate of 10 ℃/S;
s4, slowly cooling to 320 ℃ after the temperature is stable, and carrying out isothermal treatment for 8 hours;
and S5, cooling to room temperature along with the furnace after the isothermal is finished, and obtaining the low-carbon nano bainitic steel.
The mechanical properties of the low-carbon nano bainitic steel prepared in the embodiment are shown in table 2.
TABLE 2 mechanical Properties of the nano bainitic Steel of example 1
The scanning electron microscope structure of the nano bainitic steel prepared in the embodiment is shown in fig. 2, the main structure is a bainitic structure, and the rest of the structure is mainly residual austenite.
The transmission electron microscope image of the nano bainitic steel prepared in the embodiment is shown in fig. 3, and the dimension of bainitic ferrite in the thickness direction is 80-100nm.
Example 1 an alloy of the chemical composition according to the design is subjected to controlled rolling and controlled cooling and isothermal for 8 hours at 320 ℃ in the bainite transformation zone. It can be seen that the mechanical properties all reach the required performance indexes: the tensile strength is 2152MPa, the yield strength is 1806MPa, the elongation after breaking is 10.5%, the normal-temperature impact energy is 31J, and the hardness is 620HV10. The structure is nano-scale lath ferrite and fine residual austenite, wherein the dimension of the lath ferrite in the thickness direction is 80-100nm.
The TTT curve and CCT curve of the steel material of this example 1 are shown in FIGS. 7 and 8. As can be seen from fig. 7, the steel has a wide bainite transformation temperature range: 312-484 ℃, wherein 422 ℃ is the temperature of nose tip, and the phase transition temperature of the steel is as follows:
as can be seen from fig. 8: the phase transition incubation period below the temperature of the nose tip shows a tendency to be shortened, and the phase transition incubation period above the temperature of the nose tip shows an opposite tendency, which shows that the low-temperature region is beneficial to the bainite transformation of experimental steel, increases the bainite transformation degree and promotes the progress of the bainite transformation.
Example 2
Preparation of low-carbon nano bainitic steel:
s1, weighing raw materials according to chemical components in the table 3, smelting and casting, and heating a casting blank with the thickness of 150mm to the austenitizing temperature of 1200 ℃ and isothermal for 2 hours;
table 3 composition (wt.%) of the cast slab of example 2
S2, rolling the casting blank after discharging from the furnace for the first time, and rolling in a recrystallization zone, wherein the initial rolling temperature in the recrystallization stage is
The final rolling temperature of 1100 ℃ and the rolling reduction of 50% in the recrystallization stage is 1000 ℃;
then rolling for the second time at the initial temperature of 950 ℃ in the non-recrystallization stage and the final temperature of 885 ℃ in the non-recrystallization stage, wherein the rolling reduction is more than or equal to 60%, carrying out isothermal treatment in a furnace for 8 minutes at the final temperature of 885 ℃ in the non-recrystallization stage, and rolling into a 30mm thick steel plate by adopting two stages of recrystallization and non-recrystallization;
then oil cooling is carried out, the cooling speed is 10 ℃/s, the final temperature of the oil cooling is 860 ℃, then third rolling is carried out, the final rolling temperature of the third rolling is 830 ℃, and the steel plate is rolled into a 12mm thick steel plate;
s3, cooling the steel plate prepared in the step S2 again by oil, and cooling the hot rolled steel plate to 350 ℃ at a cooling rate of 10 ℃/S;
s4, slowly cooling to 320 ℃ after the temperature is stable, and carrying out isothermal treatment for 8 hours;
and S5, cooling to room temperature along with the furnace after the isothermal is finished, and obtaining the low-carbon nano bainitic steel.
The mechanical properties of the nano bainitic steel prepared in this example are shown in table 4.
TABLE 4 mechanical Properties of the nano bainitic Steel of example 2
The nano bainitic steel prepared according to the process scheme shown in Table 3 and described above has a main structure of bainitic structure, and the rest of the structure is mainly residual austenite. The dimension of bainitic ferrite in the thickness direction is 80-100nm.
Example 2 an alloy of the chemical composition according to the design is subjected to controlled rolling and controlled cooling and isothermal for 8 hours at 320 ℃ in the bainite transformation zone. It can be seen that the mechanical properties all reach the required performance indexes: the tensile strength is 2015MPa, the yield strength is 1742MPa, the elongation after breaking is 11.0%, the normal-temperature impact energy is 32J, and the hardness is 611HV10. The structure is nano-scale lath ferrite and fine residual austenite, wherein the dimension of the lath ferrite in the thickness direction is 80-100nm.
Example 3
Preparation of low-carbon nano bainitic steel:
the same procedure as in example 1 was used to prepare a low carbon nano bainitic steel, except that in step S2, an in-furnace isothermal treatment was performed at a non-recrystallized finish temperature of 880 ℃ for 5 minutes.
The mechanical properties of the nano bainitic steel prepared in this example are shown in table 5.
TABLE 5 mechanical Properties of the nano bainitic Steel of example 3
Example 3 an alloy of the chemical composition according to the design is subjected to controlled rolling and controlled cooling and isothermal for 8 hours at 320 ℃ in the bainite transformation zone. It can be seen that the mechanical properties all reach the required performance indexes: the tensile strength is 2105MPa, the yield strength is 1755MPa, the elongation after breaking is 11.0%, the normal temperature impact energy is 34J, and the hardness is 615HV10. The structure is nano-scale lath ferrite and fine residual austenite, wherein the dimension of the lath ferrite in the thickness direction is 80-100nm.
Example 4
Preparation of low-carbon nano bainitic steel:
the same method as in example 1 was used to prepare a low carbon nano bainitic steel, except that in steps S2 and S3, the cooling rate of the oil cooling was 5 ℃/S.
The mechanical properties of the nano bainitic steel prepared in this example are shown in table 6.
TABLE 6 mechanical Properties of the nano bainitic Steel of example 4
Example 4 an alloy of the chemical composition according to the design is subjected to controlled rolling and controlled cooling and isothermal for 8 hours at 320 ℃ in the bainite transformation zone. It can be seen that the mechanical properties all reach the required performance indexes: the tensile strength is 2135MPa, the yield strength is 1768MPa, the elongation after breaking is 10.5%, the normal-temperature impact energy is 32J, and the hardness is 617HV10. The structure is nano-scale lath ferrite and fine residual austenite, wherein the dimension of the lath ferrite in the thickness direction is 80-100nm.
Example 5
Preparation of low-carbon nano bainitic steel:
the same method as in example 1 was used to prepare a low carbon nano bainitic steel, except that in steps S2 and S3, the oil cooling rate was 15 ℃/S.
The mechanical properties of the nano bainitic steel prepared in this example are shown in table 7.
TABLE 7 mechanical Properties of the nano bainitic Steel of example 5
Example 5 an alloy of the chemical composition according to the design is subjected to controlled rolling and controlled cooling and isothermal for 8 hours at 320 ℃ in the bainite transformation zone. It can be seen that the mechanical properties all reach the required performance indexes: the tensile strength is 2160MPa, the yield strength is 1810MPa, the elongation after breaking is 10.0%, the normal-temperature impact energy is 30J, and the hardness is 621HV10. The structure is nano-scale lath ferrite and fine residual austenite, wherein the dimension of the lath ferrite in the thickness direction is 80-100nm.
Example 6
Preparation of low-carbon nano bainitic steel:
the same method as in example 1 was used to prepare the low carbon nano bainitic steel, except that in step S4, the isothermal treatment time was 10 hours.
The mechanical properties of the nano bainitic steel prepared in this example are shown in table 8.
TABLE 8 mechanical Properties of the nano bainitic Steel of EXAMPLE 6
Example 5 an alloy of the chemical composition according to the design is subjected to controlled rolling and controlled cooling and isothermal for 10 hours at 320 ℃ in the bainite transformation zone. It can be seen that the mechanical properties all reach the required performance indexes: the tensile strength is 2110MPa, the yield strength is 1798MPa, the elongation after breaking is 11.0%, the normal temperature impact energy is 33J, and the hardness is 615HV10. The structure is nano-scale lath ferrite and fine residual austenite, wherein the dimension of the lath ferrite in the thickness direction is 80-100nm.
Comparative example 1
Preparation of low-carbon bainite steel:
s1, weighing raw materials according to chemical components in table 9, smelting and casting, and heating a casting blank with the thickness of 150mm to the austenitizing temperature of 1200 ℃ and isothermal for 2 hours;
table 9 composition (wt.%) of the cast slab of example 1
S2, rolling the casting blank after discharging from the furnace for the first time, and rolling in a recrystallization zone, wherein the initial rolling temperature in the recrystallization stage is
The final rolling temperature in the recrystallization stage is 1000 ℃ and the rolling reduction is 54%;
then rolling for the second time at the initial temperature of 950 ℃ in the non-recrystallization stage and the final temperature of 885 ℃ in the non-recrystallization stage, wherein the rolling reduction is more than or equal to 65%, carrying out isothermal treatment in a furnace for 8 minutes at the final temperature of 885 ℃ in the non-recrystallization stage, and rolling into a 20mm thick steel plate by adopting two stages of recrystallization and non-recrystallization;
then oil cooling is carried out, the cooling speed is 10 ℃/s, the final temperature of the oil cooling is 860 ℃, then third rolling is carried out, the final rolling temperature of the third rolling is 830 ℃, and the steel plate is rolled into a 12mm thick steel plate;
s3, cooling the steel plate prepared in the step S2 again by oil, and cooling the hot rolled steel plate to 350 ℃ at a cooling rate of 10 ℃/S;
s4, slowly cooling to 320 ℃ after the temperature is stable, and carrying out isothermal treatment for 8 hours;
s5, cooling to room temperature along with the furnace after isothermal ending, and obtaining the low-carbon bainitic steel.
The mechanical properties of the test steel obtained in this comparative example 1 are shown in Table 10.
Table 10 mechanical properties of comparative example 1 test steel
The bainitic steel prepared in this comparative example has a main structure of coarse bainitic structure, the rest of the structure is mainly composed of retained austenite, and a small amount of maolybdenum structure is accompanied.
Comparative example 1 the alloy composition of comparative example 1 was isothermal for 8 hours at 320 ℃ in the bainitic transformation zone using the same controlled rolling and cooling process without Nb addition. It can be seen that the mechanical properties cannot reach the required performance index: the tensile strength is 1720MPa, the yield strength is 1544MPa, the elongation after breaking is 13.5%, the normal-temperature impact energy is 40J, and the hardness is 508HV10. The structure is composed of bainite, residual austenite and a small amount of maolympic islands, the bainite is coarse and no longer fine, the residual austenite is in a block shape, and the small amount of maolympic islands are unevenly distributed.
Comparative example 2
A bainitic steel was prepared in the same manner as in example 2, except that Nb was not added to the chemical composition of the raw material.
The mechanical properties of the test steel obtained in this comparative example 2 are shown in Table 11.
Table 11 mechanical properties of comparative example 2 test steels
Comparative example 2 the alloy composition of comparative example 2 was isothermal for 8 hours at 320 ℃ in the bainitic transformation zone using the same controlled rolling and cooling process without Nb addition. It can be seen that the mechanical properties cannot reach the required performance index: the tensile strength is 1700MPa, the yield strength is 1450MPa, the elongation after breaking is 13.0%, the normal-temperature impact energy is 35J, and the hardness is 502HV10. The structure is composed of bainite, residual austenite and a small amount of maolympic islands, the bainite is coarse and no longer fine, the residual austenite is in a block shape, and the small amount of maolympic islands are unevenly distributed.
From the mechanical property data of comparative examples 1 and 2, it can be seen that: since no carbide precipitation of Nb in the steel consumes carbon caused by supercooled austenite, the supercooled austenite has slightly high carbon content and large size, has certain thermal stability at 320 ℃ when isothermal, and has a part of supercooled austenite with large size which is subjected to martensitic transformation due to insufficient stability after tapping. The addition of Nb can refine bainitic ferrite lath and improve the stability of the lath.
Comparative example 3
Test steels were prepared in the same manner as in example 1 except that in step S2, the in-furnace isothermal treatment time was 3 minutes at a final non-recrystallization rolling temperature of 880 ℃. The second phase particles precipitated after the second rolling are shown in fig. 5.
The mechanical properties of the test steel obtained in this comparative example 3 are shown in Table 12.
Table 12 mechanical properties of comparative example 3 test steels
In the comparison between the present comparative example 3 and example 1, the isothermal treatment time after the second rolling was shortened. It can be seen that the mechanical properties do not meet the required performance index: the tensile strength is 1902MPa, the yield strength is 1610MPa, the elongation after breaking is 11.5%, the normal-temperature impact energy is 40J, and the hardness is 552HV10.
Further, as can be seen from a comparison of fig. 4 and 5, when the isothermal treatment time is too short after the second rolling, the precipitation of the second phase particles is not remarkable, and the contribution of the second phase particles of Nb to the improvement of the toughness of the bainitic steel is not remarkable.
Comparative example 4
The same procedure as in example 1 was used to prepare a low carbon nano bainitic steel, except that in step S2, an in-furnace isothermal treatment was performed at a non-recrystallized finish temperature of 880 ℃ for 10 minutes. The second phase particles precipitated after the second rolling are shown in fig. 6.
The mechanical properties of the test steel obtained in this comparative example 4 are shown in Table 12.
Table 12 mechanical properties of comparative example 4 test steel
In the comparison between the present comparative example 4 and example 1, the isothermal treatment time after the second rolling was prolonged. It can be seen that the mechanical properties do not meet the required performance index: the tensile strength is 1854MPa, the yield strength is 1542MPa, the elongation after breaking is 12.0%, the normal-temperature impact energy is 36J, and the hardness is 539HV10.
In addition, as can be seen from the comparison of fig. 4 and fig. 6, the isothermal treatment time is prolonged after the second rolling, and the coarse second phase particles are unfavorable for refining grains, and have damage to mechanical properties and are unfavorable for the mechanical properties of bainitic steel.
Comparative example 5
The same method as in example 1 was used to prepare a low carbon nano bainitic steel, except that the third rolling was not performed in step S2, and other operating conditions were the same.
The mechanical properties of the test steel obtained in this comparative example 5 are shown in Table 13.
Table 13 mechanical properties of comparative example 5 test steel
This comparative example 5 and example 1 are compared, absent a third pass. It can be seen that the mechanical properties do not meet the required performance index: the tensile strength is 1715MPa, the yield strength is 1403MPa, the elongation after breaking is 13.5%, the normal temperature impact energy is 36J, and the hardness is 508HV10.
As can be seen from the comparison of comparative example 5 and example 1, the arrangement of the third rolling process is favorable for obtaining the nano bainitic steel with excellent mechanical properties. The austenite refined to a certain degree is continuously subjected to rolling deformation by the third rolling, and the grain boundary area of the austenite in more unit volume is increased, so that a large number of deformation bands and high-density dislocation are generated in the crystal, the deformation bands and the grain boundary have similar functions, the deformation bands can be used as nucleation sites during phase transformation, the dislocation density of the austenite before the phase transformation is high, the nucleation rate is increased during the phase transformation, and finer grain structures can be obtained after the phase transformation.
Comparative example 6
The same method as in example 1 was used to prepare the low carbon nano bainitic steel, except that the second rolling was not performed in step S2, and the third rolling was performed by directly oil-cooling to 860 ℃ after the first rolling was completed, and other process conditions were the same.
The mechanical properties of the test steel obtained in this comparative example 6 are shown in Table 14.
Table 14 mechanical properties of comparative example 6 test steel
This comparative example 6 and example 1 are compared, absent a second pass. It can be seen that the mechanical properties do not meet the required performance index: the tensile strength is 1815MPa, the yield strength is 1505MPa, the elongation after breaking is 12.0%, the normal-temperature impact energy is 30J, and the hardness is 529HV10.
As can be seen from the comparison of comparative example 6 and example 1, the provision of the second rolling process is advantageous in obtaining nano bainitic steel excellent in mechanical properties. The temperature of the second rolling is controlled in a non-recrystallization temperature range, and finer grain structure and higher toughness of the steel can be obtained in the non-recrystallization rolling. Deformation in the non-recrystallized region can increase nucleation sites and nucleation rates, so that a fine grain structure can be obtained after phase transformation.
Comparative example 7
The same method as in example 1 was used to prepare a low carbon nano bainitic steel, except that the bainitic steel was obtained by rapid cooling after the third rolling, and the other process conditions were the same.
The mechanical properties of the test steel obtained in this comparative example 7 are shown in Table 15.
Table 15 mechanical properties of comparative example 7 test steel
As can be seen from the comparison of this comparative example 7 and example 1: the strength of this comparative example 7 is higher than the required performance index: the tensile strength is 2450MPa, the yield strength is 1956MPa, the elongation after break is 9.5%, the normal-temperature impact energy is 18J, and the hardness is 662HV10.
The reason for this is that the strength of comparative example 7 is high compared with that of example because part of the structure is directly transformed into martensite without the slow cooling process, and the strength is increased while the plasticity and toughness are lost. As can be seen from a comparison of comparative example 7 and example 1, oil cooling followed by slow cooling after the third rolling is advantageous for transformation of bainite, avoiding formation of martensite.
The technical features of the above-described embodiments may be arbitrarily combined, and in order to simplify the description, all possible combinations of the technical features in the above-described embodiments are not exhaustive, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.
It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the appended claims.
Claims (9)
1. The low-carbon nano bainitic steel is characterized by comprising the following chemical components: 0.15-0.30% of C, 1.6-1.9% of Si, 1.8-2.3% of Mn, 0.7-0.9% of Cr, 0.2-0.3% of Mo, less than or equal to 0.010% of P, less than or equal to 0.010% of S, 0.03-0.08% of Nb, and the balance of Fe and unavoidable impurities;
the tensile strength of the low-carbon nano bainitic steel is more than or equal to 2000MPa, the yield strength is more than or equal to 1700MPa, the elongation after breaking is more than or equal to 10%, and the normal-temperature impact energy KV is realized 2 More than or equal to 30J, and the hardness is more than or equal to 600HV10.
2. The low carbon nano bainitic steel according to claim 1, wherein the chemical components of the low carbon nano bainitic steel are: 0.20-0.25% of C, 1.5-1.8% of Si, 1.8-2.2% of Mn, 0.7-0.8% of Cr, 0.2-0.25% of Mo, less than or equal to 0.010% of P, less than or equal to 0.010% of S, 0.03-0.06% of Nb, and the balance of Fe and unavoidable impurities.
3. A low carbon nano bainitic steel according to claim 1 or 2, further comprising one or two of V and Ti, wherein the total content of V and Ti is not higher than 0.3%.
4. The low carbon nano bainitic steel according to claim 1, wherein the microstructure of the low carbon nano bainitic steel is composed of bainitic ferrite and residual austenite having a thickness of 80-100nm.
5. A method for preparing a low carbon nano bainitic steel according to any one of claims 1 to 4, comprising the steps of:
s1, weighing raw materials according to chemical components, smelting and casting, heating a casting blank to austenitizing temperature and carrying out isothermal treatment;
s2, rolling the casting blank after being discharged from the furnace for the first time, rolling in a recrystallization zone, then rolling in a non-recrystallization zone for the second time, and performing in-furnace isothermal treatment at a non-recrystallization finish rolling temperature after the second time of rolling; then oil cooling, wherein the final cooling temperature is more than Ar < 3+ > 20 ℃, and then third rolling is carried out, and the final rolling temperature of the third rolling is higher than Ar < 3 >;
s3, cooling the steel plate prepared in the step S2 again by adopting oil, wherein the final cooling temperature is between the bainite starting transition temperature Bs and the martensite starting transition temperature Ms, and is the final cooling temperature; bs to (ms+50);
s4, slowly cooling the steel plate obtained in the step S3, and performing isothermal treatment, wherein the isothermal temperature is (Ms+5) to (Ms+10);
and S5, cooling to room temperature along with the furnace after the isothermal is finished, and obtaining the low-carbon nano bainitic steel.
6. The method for preparing low carbon nano bainitic steel according to claim 5, wherein in the step S1, a casting blank with a thickness of 120-150mm is heated to austenitizing temperature and isothermally treated for 1.5-2 hours.
7. The method for preparing the low-carbon nano bainitic steel according to claim 5, wherein in the step S2, the rolling reduction of the first rolling is more than or equal to 50%, and the temperature of the first rolling is 1000-1150 ℃;
the rolling reduction of the second rolling is more than or equal to 60 percent, the temperature range of the second rolling is 880-950 ℃, the steel plate with the thickness of 20-30mm is obtained after the second rolling, and the isothermal treatment time after the second rolling is 5-8 minutes.
8. The method for preparing low-carbon nano bainitic steel according to claim 5, wherein in the step S2, the temperature range of the third rolling is 820-860 ℃, and the steel plate with the thickness of 12-16mm is obtained after the third rolling.
9. The method for preparing low-carbon nano bainitic steel according to claim 5, wherein in the steps S2 and S3, the cooling rate of oil cooling is 5-15 ℃/S;
the final cooling temperature in the step S3 is 320-350 ℃;
the isothermal temperature of the step S4 is 318-323 ℃, the isothermal treatment time of the step S4 is 8-10 hours, and the slow cooling rate in the step S4 is 0.5-1 ℃/S.
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