CN112760463B - Continuous annealing method of 780 MPa-grade microalloyed dual-phase steel - Google Patents
Continuous annealing method of 780 MPa-grade microalloyed dual-phase steel Download PDFInfo
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- 229910000885 Dual-phase steel Inorganic materials 0.000 title claims abstract description 37
- 238000000137 annealing Methods 0.000 title claims abstract description 30
- 238000000034 method Methods 0.000 title claims abstract description 24
- 238000001816 cooling Methods 0.000 claims abstract description 85
- 238000010438 heat treatment Methods 0.000 claims abstract description 25
- 238000010583 slow cooling Methods 0.000 claims abstract description 24
- 238000002791 soaking Methods 0.000 claims abstract description 18
- 238000011217 control strategy Methods 0.000 claims abstract description 10
- 229910000831 Steel Inorganic materials 0.000 claims description 77
- 239000010959 steel Substances 0.000 claims description 77
- 229910001563 bainite Inorganic materials 0.000 claims description 9
- 229910000734 martensite Inorganic materials 0.000 claims description 6
- 238000005096 rolling process Methods 0.000 claims description 5
- 229910000859 α-Fe Inorganic materials 0.000 claims description 4
- 239000000126 substance Substances 0.000 claims description 3
- 238000002347 injection Methods 0.000 claims description 2
- 239000007924 injection Substances 0.000 claims description 2
- 238000004519 manufacturing process Methods 0.000 abstract description 7
- 239000000047 product Substances 0.000 description 6
- 238000005098 hot rolling Methods 0.000 description 5
- 239000000203 mixture Substances 0.000 description 4
- 238000004140 cleaning Methods 0.000 description 3
- 238000010791 quenching Methods 0.000 description 3
- 230000000171 quenching effect Effects 0.000 description 3
- 239000002994 raw material Substances 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 229910000742 Microalloyed steel Inorganic materials 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 238000005275 alloying Methods 0.000 description 2
- 238000005452 bending Methods 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000004321 preservation Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- 229910000599 Cr alloy Inorganic materials 0.000 description 1
- 229910001275 Niobium-titanium Inorganic materials 0.000 description 1
- 238000003723 Smelting Methods 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000005097 cold rolling Methods 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000009977 dual effect Effects 0.000 description 1
- 238000004146 energy storage Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 239000000835 fiber Substances 0.000 description 1
- 238000009776 industrial production Methods 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- RJSRQTFBFAJJIL-UHFFFAOYSA-N niobium titanium Chemical compound [Ti].[Nb] RJSRQTFBFAJJIL-UHFFFAOYSA-N 0.000 description 1
- 238000005192 partition Methods 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- 238000006116 polymerization reaction Methods 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000004886 process control Methods 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 239000010936 titanium Substances 0.000 description 1
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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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
-
- 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
- C21D11/00—Process control or regulation for heat treatments
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
-
- 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
-
- 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
-
- 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
- C21D2211/008—Martensite
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- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
- Heat Treatment Of Strip Materials And Filament Materials (AREA)
Abstract
The invention discloses a continuous annealing method of 780 MPa-grade microalloyed dual-phase steel, which reduces the strength fluctuation of a product and improves the local forming performance of the product by adopting a U-shaped control strategy for heating temperature and soaking temperature, an inverted U-shaped control strategy for slow cooling temperature and a segmented cooling strategy of fast cooling I and fast cooling II for a fast cooling section. The strength difference of the inner end of the obtained product roll is not more than 40MPa, the elongation after fracture is 16-23%, and the hole expansion rate is not less than 20%. The method is simple and effective, and accords with the production line reality of a continuous annealing unit.
Description
Technical Field
The invention relates to the technical field of metallurgical plate production, in particular to a continuous annealing method of micro-alloyed dual-phase steel with 780 MPa-grade tensile strength.
Background
Dual phase steels are very important in automotive steel systems due to their excellent strong plastic match. With the continuous development of light weight of automobiles, the structure of parts made of dual-phase steel is increasingly complex, the forming mode is increasingly diversified, and the parts are subjected to flanging and bending in addition to traditional deep drawing and bulging, so that higher requirements are provided for the forming performance of the dual-phase steel. Meanwhile, under the application scene of industrial mass production, the market more favors products with high homogeneity, which puts higher requirements on the performance stability of the dual-phase steel.
In actual production, cold-rolled dual-phase steel is generally subjected to heat treatment in a continuous annealing mode, and a conventional continuous annealing furnace generally comprises a preheating section, a heating section, a soaking section, a slow cooling section, a fast cooling section and an overaging section. The contents and kinds of the alloying elements of the cold-rolled dual-phase steel of the same strength grade are different according to the cooling technology adopted by the annealing furnace. For an annealing unit adopting high-speed gas injection rapid cooling, the alloy content of the cold-rolled dual-phase steel with the tensile strength of 780MPa and above is often higher. With conventional C, Si, Mn, Cr alloy composition designs, it is difficult to obtain outstanding formability properties, especially local deformability (flanging, bending). Steel enterprises mostly carry out microalloying design on the basis of conventional components, however, the microalloyed steel has large inherent strength fluctuation, and the position strength difference in the coil inner head of the product can reach 80 MPa.
In order to improve the problems of large strength fluctuation and poor performance stability of microalloyed steel, CN 111575592A (hereinafter referred to as patent document 1) has found that the ratio fluctuation of large-size carbonitride can be reduced by low-O, low-N and low-Ti control, the structural uniformity of the inner and outer rings of a hot coil can be improved by using high reduction, low-temperature coiling and slow cooling of a hot rolling heat-preserving cover through a finish rolling starting and finishing stand, the deformation energy storage difference between different thicknesses can be balanced through the annealing temperature of a heating section and a soaking section and the thickness difference of the speed of strip steel, and sufficient recrystallization is promoted, so that low-alloy high-strength steel with the yield strength of 460MPa grade with small strength fluctuation is obtained. However, the present inventors have found that the microalloyed dual phase steel does not have the fiber structure in the cold rolled state as described in patent document 1, and the microalloyed dual phase steel is significantly deteriorated by reducing the strength fluctuation by dividing the annealing temperature and the strip speed by the thickness as described in patent document 1. Next, the measures of the high reduction of the finish rolling start and end stand and the slow cooling of the hot rolling heat retaining cover, etc. adopted in patent document 1 can reduce the fluctuation in strength of the microalloyed dual phase steel to some extent, but cannot solve the problem at all. Further, the measure described in patent document 1 requires high process control, and slow cooling of the hot-rolled heat-retaining cover also lowers the production efficiency.
Disclosure of Invention
The invention aims to provide a continuous annealing method of 780 MPa-level microalloying dual-phase steel, which reduces the strength fluctuation of 780 MPa-level microalloying dual-phase steel by controlling the heating temperature in a U shape and the slow cooling temperature in an U shape, more accurately adjusts the hard phase composition by controlling the fast cooling temperature in a segmented manner, reduces the strength fluctuation and improves the local deformation capacity.
In order to achieve the above purpose, the specific technical scheme of the invention is as follows:
a continuous annealing method of 780 MPa-grade microalloyed dual-phase steel comprises the steps of heating, soaking, slow cooling, fast cooling I, fast cooling II, overaging and leveling.
Specifically, in the heating and soaking step, the average heating rate is 1.8-3.5 ℃/s, and the soaking and heat preservation are 85-168 s; the expected target temperature T of the middle part of the strip steel is 780-820 ℃, and a U-shaped control strategy is adopted to ensure that the expected target temperature of the head and the tail of the strip steel is 5-15 ℃ higher than the expected target temperature of the middle part;
further, the initial temperature of the strip steel is set to be T + delta T, the length of the strip steel is set to be T at 1/8-1/6, the length of the strip steel is set to be T + delta T at 4/6-6/8, wherein delta T is annealing temperature compensation, and delta T is larger than or equal to 5 ℃ and smaller than or equal to 15 ℃.
Specifically, in the slow cooling step, the expected target temperature H of the middle part of the strip steel is 660-690 ℃, and a reverse U-shaped control strategy is adopted to ensure that the expected target temperature of the head and the tail of the strip steel is 5-20 ℃ lower than the expected target temperature of the middle part;
further, the initial temperature of the strip steel is set to be H-delta H, the length of the strip steel is set to be H at 1/8-1/6, the length of the strip steel is set to be H-delta H at 4/6-6/8, wherein delta H is slow cooling temperature compensation, and delta H is larger than or equal to 5 ℃ and smaller than or equal to 20 ℃.
Specifically, in the step I of rapid cooling, the expected target temperature K of the temperature I of the rapid cooling is 470 ℃ plus delta K, and the cooling speed I of the rapid cooling is 65-140 ℃/s; wherein delta K is temperature compensation of the quick cooling I, and delta K is more than or equal to minus 30 ℃ and less than or equal to 30 ℃.
Specifically, in the step II of fast cooling, the temperature of the fast cooling II is 280-315 ℃, and the cooling speed of the fast cooling II is 15.0-40.0 ℃/s;
furthermore, the temperature of the quick cooling II can be set according to the thickness of the strip steel, and when the thickness of a hard rolling coil is not less than 0.6mm and not more than 1.2mm, the temperature of the quick cooling II is 300 +/-10 ℃; when the thickness of the rolled hard coil is 1.2mm and the thickness of the rolled hard coil is less than 1.6mm, the temperature of the fast cooling II is 295 +/-10 ℃; when the thickness of the rolled hard coil is less than or equal to 1.6mm and less than or equal to 2.5mm, the temperature of the fast cooling II is 290 +/-10 ℃.
Specifically, in the overaging step, the overaging temperature is 275-310 ℃, and the time is controlled to be 6.0-11.6 min.
Specifically, in the leveling step, the leveling elongation is 0.2-1.0%.
Specifically, the 780 MPa-grade microalloyed dual-phase steel has the thickness of 0.6-2.5 mm and the length of 950-3000 m.
Further, as a more preferable technical scheme, the effective strip steel lengths of the heating section, the soaking section, the slow cooling section, the fast cooling section I and the fast cooling section II in the continuous annealing furnace are respectively 520-550 m, 220-230 m, 20-30 m, 3.5-5.0 m and 11-13 m, and the fast cooling section I and the fast cooling section II adopt a high-speed gas jet cooling process.
Further, as a more preferable technical scheme, the 780 MPa-grade microalloyed dual-phase steel has the following chemical components by mass: ti and Nb are more than or equal to 0.03 percent and less than or equal to 0.10 percent, Mn and Cr are more than or equal to 2.0 percent and less than or equal to 2.8 percent, and Si is less than or equal to 0.8 percent.
The 780 MPa-grade microalloyed dual-phase steel has a microstructure of ferrite, martensite and bainite, wherein the volume fraction of the bainite is 10-20%.
The strength difference of the 780 MPa-grade microalloyed dual-phase steel in the coil inner head is not more than 40MPa, the elongation after fracture is 16-23%, and the hole expansion rate is not less than 20%.
Compared with the prior art, the invention has the beneficial effects that:
(1) the problems of large difference of strength in coils and low middle strength of 780 MPa-level microalloyed dual-phase steel commonly existing in industrial production are obviously improved through simple and easy-to-operate U-shaped control of heating temperature and U-shaped control of slow cooling temperature;
(2) through the adoption of the sectional control of the rapid cooling temperature, the hard phase composition is adjusted more accurately, 10-20% of bainite in volume fraction is obtained, the strength fluctuation is reduced, the elongation after fracture and the hole expansion rate are excellent, and the forming requirement of complex parts can be met.
Drawings
FIG. 1 is a schematic view of a heating target temperature-strip length curve in example 2;
FIG. 2 is a schematic diagram of a slow cooling target temperature-strip length curve in example 2;
FIG. 3 is a graph of the structure of example 2, where M is martensite and B is bainite;
Detailed Description
The invention will be further described with reference to the accompanying figures 1-3 and specific examples,
the invention provides a 780 MPa-grade microalloying dual-phase steel continuous annealing method.
The steel plate strength fluctuation is influenced by many factors, and besides the detection deviation, the strength result is influenced to different degrees by component fluctuation, process fluctuation and equipment state fluctuation. However, these factors are subject to relevant technical regulations in the production control. Therefore, it is necessary to analyze and investigate other factors that affect the intensity fluctuations.
In the traditional continuous annealing process, the strip steel temperature of a heating section, a soaking section and a slow cooling section only has a fixed expected target, and the fast cooling section only controls the inlet and outlet temperature and does not perform sectional or partition control. The inventor of the application finds that the strength of the head and tail parts in the coil of the microalloyed dual-phase steel is often higher than that of the middle part, mainly because the head and tail parts often have less soft phase ferrite and the head and tail parts have finer niobium-titanium precipitates, and in addition, the ratio change of martensite and bainite in a hard phase has certain influence on strength fluctuation, particularly on hole expansion ratio. Under the existing process conditions, it is very difficult to obviously improve the phenomenon. Therefore, the inventors have recognized a breakthrough in solving the problems of strength and hole expansion rate fluctuation of the prior art microalloyed dual phase steel, which is how to reduce the microstructure difference between and within the coils by using a suitable process.
The inventor finds out through a large number of theoretical analyses and practices that: 1. microalloy elements are not completely precipitated in the hot rolling stage, a considerable part of microalloy elements exist in a solid solution state in strip steel, the solid solution state microalloy elements at the head and the tail are relatively higher due to the difference of cooling conditions at the head, the middle and the tail parts, and are re-precipitated during subsequent continuous annealing, and the precipitation size at the head and the tail parts is smaller due to the fact that the time of polymerization growth is shortened relative to that of the precipitation at the hot rolling stage; 2. under the condition that other processes are not changed, the proportion relation between the soft phase and the hard phase cannot be fundamentally changed due to the moderate change of the heating temperature and the soaking temperature; 3. by adopting the sectional or zone control in the quick cooling section of the continuous annealing furnace, the ratio change of martensite and bainite in the hard phase becomes more predictable.
Based on the discovery, the invention provides a 780 MPa-grade microalloying dual-phase steel continuous annealing method, wherein the thickness of a strip steel raw material is 0.6-2.5 mm, and the length is 950-3000 m. When the strip steel is heated and soaked, the average heating rate is 1.8-3.5 ℃/s, the expected target temperature T of the middle part of the strip steel is 780-820 ℃ after soaking and heat preservation are carried out for 85-168 s, and a U-shaped control strategy is adopted to ensure that the expected target temperature of the head and the tail of the strip steel is 5-15 ℃ higher than the expected target temperature of the middle part; preferably, the initial temperature of the strip steel is set to be T + delta T, the length of the strip steel is set to be T at 1/8-1/6, the length of the strip steel is set to be T + delta T at 4/6-6/8, wherein delta T is annealing temperature compensation, and delta T is more than or equal to 5 ℃ and less than or equal to 15 ℃. When the strip steel is slowly cooled, the expected target temperature H of the middle part of the strip steel is 660-690 ℃, and an inverted U-shaped control strategy is adopted to ensure that the expected target temperature of the head and the tail of the strip steel is 5-20 ℃ lower than the expected target temperature of the middle part; preferably, the initial temperature of the strip steel is set to be H-delta H, the length of the strip steel is 1/8-1/6 is set to be H, the length of the strip steel is 4/6-6/8 is set to be H-delta H, wherein delta H is slow cooling temperature compensation, and delta H is more than or equal to 5 ℃ and less than or equal to 20 ℃. When the strip steel is rapidly cooled to I, the expected target temperature K of the rapid cooling temperature I is 470 ℃ plus delta K, and the cooling speed of the rapid cooling temperature I is 65-140 ℃/s; wherein delta K is temperature compensation of the quick cooling I, and delta K is more than or equal to minus 30 ℃ and less than or equal to 30 ℃. When the strip steel is rapidly cooled by II, the temperature of the rapid cooling II is 280-315 ℃, and the cooling speed of the rapid cooling II is 15.0-40.0 ℃/s; preferably, the temperature of the quick cooling II can be set according to the thickness of the strip steel, and when the thickness of a hard rolling coil is more than or equal to 0.6mm and less than or equal to 1.2mm, the temperature of the quick cooling II is 300 +/-10 ℃; when the thickness of the rolled hard coil is 1.2mm and the thickness of the rolled hard coil is less than 1.6mm, the temperature of the fast cooling II is 295 +/-10 ℃; when the thickness of the rolled hard coil is less than or equal to 1.6mm and less than or equal to 2.5mm, the temperature of the fast cooling II is 290 +/-10 ℃. When the strip steel is overaged, the overaging temperature is 275-310 ℃, and the time is controlled to be 6.0-11.6 min. When the strip steel is leveled, the leveling elongation rate is 0.2-1.0%.
It is worth noting that differences in strip composition and production line, among others, can affect product strength. The annealing temperature compensation delta T, the slow cooling temperature compensation delta H and the fast cooling I temperature compensation delta K are empirical values, and specifically comprise the following steps: the micro-alloying dual-phase steel strip steel raw materials are subjected to batch combination tests of delta T, delta H and delta K, the number of steel coils in each batch test is not less than 20, and when the strip steel 100% meets the conditions that the strength difference in the coil inner heads is not more than 40MPa, the elongation after fracture is 16-23% and the hole expansion rate is not less than 20%, the temperature compensation value combination is optimal.
Further preferably, the strip steel is prepared according to the process route of smelting → continuous casting → hot rolling → acid pickling and cold rolling, and the mass fraction of the chemical components meets the following conditions: ti and Nb are more than or equal to 0.03 percent and less than or equal to 0.10 percent, Mn and Cr are more than or equal to 2.0 percent and less than or equal to 2.8 percent, and Si is less than or equal to 0.8 percent.
Further preferably, the effective lengths of the strip steel of the heating section, the soaking section, the slow cooling section, the fast cooling section I and the fast cooling section II in the continuous annealing furnace are 520-550 m, 220-230 m, 20-30 m, 3.5-5.0 m and 11-13 m respectively, and the fast cooling section I and the fast cooling section II adopt a high-speed gas jet cooling process.
Example 1
1) Uncoiling and cleaning, wherein the thickness of the strip steel raw material is 1.2mm, and the length is 1739 m;
2) preheating, and controlling the temperature at 170 ℃;
3) heating at the heating rate of 2.72-2.83 ℃/s, setting the initial temperature to be 820 ℃, setting the strip steel length 1/7 to be 810 ℃, and setting the strip steel length 5/7 to be 820 ℃;
4) soaking for 89.2S, wherein the initial temperature is set to 820 ℃, the length 1/7 of the strip steel is set to 810 ℃, and the length 5/7 of the strip steel is set to 820 ℃;
5) slowly cooling, wherein the slow cooling rate is 12.0-16.5 ℃/s, the initial temperature is set to be 680 ℃, the length 1/7 of the strip steel is set to be 690 ℃, and the length 5/7 of the strip steel is set to be 680 ℃;
6) carrying out rapid cooling I, controlling the temperature to be 485 ℃, wherein the rapid cooling rate is 115-128 ℃/s;
7) carrying out quick cooling II, controlling the temperature at 300 ℃, and controlling the quick cooling rate at 36-39 ℃/s;
8) overaging is carried out, the temperature is controlled at 295 ℃, and the time is 6.2 min;
9) final cooling is carried out, and the temperature is controlled at 150 ℃;
10) performing water quenching, and controlling the temperature below 43 ℃;
11) leveling, wherein the leveling elongation is 0.6%;
12) and coiling to obtain a finished product of the dual-phase steel.
Example 2
1) Uncoiling and cleaning are carried out, and the thickness of the hard coil is 1.6mm, and the length is 1364 m.
2) Preheating, and controlling the temperature at 170 ℃;
3) heating at a heating rate of 2.31-2.42 ℃/s, setting the initial temperature to be 810 ℃, the strip steel length 1/6 to be 800 ℃, and the strip steel length 4/6 to be 810 ℃;
4) soaking for 111.5S, setting the initial temperature to be 810 ℃, the length 1/6 of the strip steel to be 800 ℃, and the length 4/6 of the strip steel to be 810 ℃;
5) slowly cooling, wherein the slow cooling rate is 9.0-13.5 ℃/s, the initial temperature is set to 670 ℃, the length 1/6 of the strip steel is set to 680 ℃, and the length 4/6 of the strip steel is set to 670 ℃;
6) carrying out rapid cooling I, controlling the temperature at 455 ℃, and controlling the rapid cooling rate at 107-115 ℃/s;
7) carrying out quick cooling II, controlling the temperature at 290 ℃, and controlling the quick cooling rate at 25-28 ℃/s;
8) overaging is carried out, the temperature is controlled to be 285 ℃, and the time is 7.7 min;
9) final cooling is carried out, and the temperature is controlled at 150 ℃;
10) performing water quenching, and controlling the temperature below 43 ℃;
11) leveling, wherein the leveling elongation is 0.6%;
12) and coiling to obtain a finished product of the dual-phase steel.
Example 3
1) Uncoiling and cleaning, wherein the thickness of a hard coil is 2.0mm, and the length is 980 m;
2) preheating, and controlling the temperature at 170 ℃;
3) heating at a heating rate of 1.93-2.01 ℃/s, setting the initial temperature to be 800 ℃, setting the strip steel length 1/6 to be 790 ℃, and setting the strip steel length 4/6 to be 800 ℃;
4) soaking for 121.6S, setting the initial temperature to 800 ℃, setting the length 1/6 of the strip steel to 790 ℃, and setting the length 4/6 of the strip steel to 800 ℃;
5) slowly cooling, wherein the slow cooling rate is 9.0-11.5 ℃/s, the initial temperature is set to be 660 ℃, the length 1/6 part of the strip steel is set to be 670 ℃, and the length 4/6 part of the strip steel is set to be 660 ℃;
6) carrying out rapid cooling I, controlling the temperature at 470 ℃, and controlling the rapid cooling rate to be 82.5-92.0 ℃/s;
7) carrying out quick cooling II, controlling the temperature at 290 ℃, and controlling the quick cooling rate at 24.0-28.0 ℃/s;
8) overaging is carried out, the temperature is controlled to be 285 ℃, and the time is 8.4 min;
9) final cooling is carried out, and the temperature is controlled at 150 ℃;
10) performing water quenching, and controlling the temperature below 43 ℃;
11) leveling, wherein the leveling elongation is 0.4%;
12) and coiling to obtain a finished product of the dual-phase steel.
TABLE 1 results of mechanical Properties measurements
As can be seen from FIG. 3 and Table 1, the 780MPa microalloyed dual phase steel prepared by the invention is composed of ferrite, martensite and bainite, the strength difference in head is not more than 40MPa, the elongation after fracture is not less than 16%, and the hole expansion rate is not less than 20%.
Claims (6)
1. A continuous annealing method of 780MPa grade microalloyed dual phase steel, which is characterized by comprising the following steps: heating, soaking, slow cooling, fast cooling I, fast cooling II, overaging and leveling, wherein a U-shaped control strategy is adopted for the heating temperature and the soaking temperature, and an inverted U-shaped control strategy is adopted for the slow cooling temperature;
in the heating and soaking step, the expected target temperature T of the middle part of the strip steel is 780-820 ℃, and a U-shaped control strategy is adopted to ensure that the expected target temperature of the head and the tail of the strip steel is 5-15 ℃ higher than the expected target temperature of the middle part; specifically, the initial temperature of the strip steel is set to be T + delta T, the length of the strip steel is set to be T at 1/8-1/6, and the length of the strip steel is set to be T + delta T at 4/6-6/8;
in the slow cooling step, the expected target temperature H of the middle part of the strip steel is 660-690 ℃, and an inverted U-shaped control strategy is adopted to ensure that the expected target temperature of the head and the tail of the strip steel is 5-20 ℃ lower than the expected target temperature of the middle part; specifically, the initial temperature of the strip steel is set to be H-delta H, the length of the strip steel is set to be H at 1/8-1/6, and the length of the strip steel is set to be H-delta H at 4/6-6/8;
in the step I of fast cooling, the expected target temperature K of the fast cooling temperature I is 470 ℃ plus delta K, and the cooling speed of the fast cooling temperature I is 65-140 ℃/s;
in the step II of fast cooling, the temperature of the fast cooling II is 280-315 ℃, and the cooling speed of the fast cooling II is 15.0-40.0 ℃/s;
wherein,
delta T is annealing temperature compensation, and delta T is more than or equal to 5 ℃ and less than or equal to 15 ℃;
delta H is slow cooling temperature compensation, and delta H is more than or equal to 5 ℃ and less than or equal to 20 ℃;
delta K is temperature compensation of the quick cooling I, and delta K is more than or equal to minus 30 ℃ and less than or equal to 30 ℃.
2. The continuous annealing method of 780MPa grade microalloyed dual phase steel according to claim 1, characterized in that: in the overaging step, the overaging temperature is 275-310 ℃, and the time is controlled to be 6.0-11.6 min; in the leveling step, the leveling elongation is 0.2-1.0%; the thickness of the strip steel is 0.6-2.5 mm, and the length of the strip steel is 950-3000 m.
3. The continuous annealing method of 780MPa grade microalloyed dual phase steel according to claim 1, characterized in that: setting the temperature of the quick cooling II according to the thickness of the strip steel, and when the thickness of a hard rolling coil is not less than 0.6mm and not more than 1.2mm, setting the temperature of the quick cooling II to be 300 +/-10 ℃; when the thickness of the rolled hard coil is 1.2mm and the thickness of the rolled hard coil is less than 1.6mm, the temperature of the fast cooling II is 295 +/-10 ℃; when the thickness of the rolled hard coil is less than or equal to 1.6mm and less than or equal to 2.5mm, the temperature of the fast cooling II is 290 +/-10 ℃.
4. Continuous annealing process of 780MPa grade microalloyed dual phase steel according to any one of claims 1 to 3, characterized in that: the effective strip steel lengths of the heating section, the soaking section, the slow cooling section, the fast cooling section I and the fast cooling section II are respectively 520-550 m, 220-230 m, 20-30 m, 3.5-5.0 m and 11-13 m, and a high-speed gas injection fast cooling process is adopted.
5. The continuous annealing method of 780MPa grade microalloyed dual phase steel according to claim 1, characterized in that: the mass fraction of the chemical components of the strip steel is more than or equal to 0.03 percent and less than or equal to 0.10 percent of Ti + Nb, more than or equal to 2.0 percent and less than or equal to 2.8 percent of Mn + Cr, and less than or equal to 0.8 percent of Si.
6. The continuous annealing method of 780MPa grade microalloyed dual phase steel according to claim 1, characterized in that: the strength difference of the 780 MPa-grade microalloyed dual-phase steel in the coil inner head is not more than 40MPa, the elongation after fracture is 16-23%, and the hole expansion rate is not less than 20%; and has a microstructure of ferrite, martensite and bainite, wherein the volume fraction of bainite is 10 to 20%.
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