CN111893379B - 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel and manufacturing method thereof - Google Patents
780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel and manufacturing method thereof Download PDFInfo
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Abstract
The invention provides 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, belonging to the field of manufacturing of high-strength steel for automobiles, wherein the dual-phase steel comprises the following chemical components in percentage by mass: c: 0.16% -0.2%, Si: 0.35-0.65%, Mn: 1.8% -2.3%, Al: 0.7 to 1.0 percent of the total weight of the alloy, less than or equal to 0.01 percent of P, less than or equal to 0.003 percent of S, and the balance of Fe and inevitable impurities. Compared with the traditional dual-phase steel of the same grade, the dual-phase steel has better forming performance, and solves the problem that the traditional dual-phase steel is difficult to form on a complex stamping structural part. The invention also provides a manufacturing method of the 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel.
Description
Technical Field
The invention belongs to the field of manufacturing of high-strength steel for automobiles, and relates to 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel and a manufacturing method thereof.
Background
From the viewpoints of energy conservation, emission reduction and environmental protection, the light weight of the automobile body is a great trend. Meanwhile, in order to protect the safety of the driver and passengers, it is necessary to improve the collision safety of the vehicle body material. In view of the above requirements, the application of advanced high-strength steel to automobile bodies is gradually expanding.
However, the strength of the steel sheet is increased, which generally results in a reduction in formability, so that some automobile parts having complicated shapes cannot be press-formed from the high-strength steel sheet. Even if the dual-phase steel has excellent strength, plasticity and processing hardening performance, when the strength reaches 780MPa or above, the problem of high risk of stamping forming cracking also exists, which not only increases the waste rate of materials, but also greatly limits the application of the high-strength dual-phase steel. Therefore, the improvement of the forming performance of the steel plate while improving or ensuring the strength of the steel plate is a difficult problem which needs to be overcome in the development and application of advanced high-strength steel.
Disclosure of Invention
In order to solve the technical problem that the forming performance of the existing high-strength steel is poor, the invention provides 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, compared with the traditional dual-phase steel at the same grade, the dual-phase steel has better forming performance, and the problem that the traditional dual-phase steel is difficult to form on a complex stamping structural part is solved.
The invention also provides a manufacturing method of the 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel.
The invention is realized by the following technical scheme:
a 780MPa grade enhanced forming hot-dip galvanized dual-phase steel, the chemical composition of which comprises, in mass fraction:
c: 0.16% -0.2%, Si: 0.35-0.65%, Mn: 1.8% -2.3%, Al: 0.7 to 1.0 percent of the total weight of the alloy, less than or equal to 0.01 percent of P, less than or equal to 0.003 percent of S, and the balance of Fe and inevitable impurities;
in the microstructure of the dual-phase steel, the area ratio of ferrite is 45-60%, the sum of the area ratios of bainite and martensite is 35-45%, the area ratio of retained austenite is 4-9%, and the retained austenite with the equivalent circle diameter of more than 500nm accounts for more than 90% of the total amount of the retained austenite.
Further, the chemical composition of the dual-phase steel further comprises any one of the following elements in mass fraction:
Cr：0.16％～0.25％，Mo：0～0.3％。
further, the contents of Si and Al in the chemical components of the dual-phase steel are calculated by mass fraction:
Si+Al≥1.2％。
further, the surface of the dual-phase steel is provided with a hot dip galvanizing layer.
A method for manufacturing 780MPa grade reinforced forming hot-dip galvanized dual-phase steel, comprising the following steps:
the molten steel is cast into a plate blank in a continuous casting mode, and the chemical components of the molten steel are the same as those of the dual-phase steel;
heating, rough rolling and finish rolling are carried out on the plate blank to obtain a hot rolled plate;
carrying out acid pickling on the hot rolled plate, and then carrying out cold rolling to obtain cold and hard strip copper;
and continuously annealing the cold-hard strip steel, and performing surface galvanization after annealing.
Further, the heating temperature of the plate blank is 1250 +/-20 ℃, soaking is carried out for 30-100 min, and the thickness of the hot rolled plate obtained by finish rolling is 3.0-5.0 mm; wherein the inlet temperature of the finish rolling is 1040-1080 ℃, the finishing temperature is 860-910 ℃, and the coiling temperature is 660 +/-20 ℃.
Furthermore, in the cold rolling process, the total cold rolling reduction rate is 60-70%, and the thickness of the cold-hard strip steel obtained by cold rolling is 1.0-2.0 mm.
Further, in the continuous annealing process, the furnace region dew point is controlled to be-30 ℃ to-10 ℃, the preheating temperature is 210 ℃ to 230 ℃, the heating and soaking temperature is 780 ℃ to 830 ℃, the slow cooling temperature is 680 ℃ to 720 ℃, and the fast cooling outlet temperature is 450 ℃ to 470 ℃.
Furthermore, in the surface galvanizing process, the temperature of a zinc pot is 450-470 ℃, and the finishing elongation of the galvanized strip steel is 0.3-0.6%.
Furthermore, in the continuous annealing process, the heating speed of the copper in the cold and hard zone is 2 ℃/s-4 ℃/s, the soaking time is 60 s-90 s, and the average cooling rate of the fast cooling section is more than 10 ℃/s.
One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:
1. compared with the same-grade traditional hot-dip galvanized dual-phase steel, the 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel greatly reduces or avoids the addition of precious alloy elements Mo, Ti and Nb by increasing the C content, greatly reduces the material cost, and improves the surface quality of the hot-dip galvanized dual-phase steel and the additional value of a steel plate while inhibiting the austenite decomposition and carbide generation in the galvanizing process by accurately controlling the component proportion of Si and Al.
2. The invention relates to a manufacturing method of 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, which improves finish rolling, annealing and hot galvanizing processes, controls the area ratio of ferrite, bainite, martensite and residual austenite in a structure relative to the whole steel plate structure, obviously improves the forming performance of the dual-phase steel and the elongation (A) after fracture80) Can reach 23 percent or more, so that the dual-phase steel is more suitable for producing automobile parts with complex shapes.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
FIG. 1 is a surface quality chart of 780MPa grade reinforced forming hot-dip galvanized dual-phase steel obtained in example 1 of the present invention;
FIG. 2 is a graph comparing the Forming Limit Curve (FLC) of 780MPa grade reinforced Forming hot-dip galvanized dual-phase steel prepared in the invention example 1 with the traditional DP 780.
Detailed Description
The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that these specific embodiments and examples are for the purpose of illustrating the invention and are not to be construed as limiting the invention.
Throughout the specification, unless otherwise specifically noted, terms used herein should be understood as having meanings as commonly used in the art. Accordingly, 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. If there is a conflict, the present specification will control.
Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.
In order to solve the technical problems, the embodiment of the invention provides the following general ideas:
aiming at the technical bottleneck that the forming performance of the existing high-strength steel is poor, the inventor carries out deep research and repeated tests, and the result shows that: a certain amount of austenite is retained in the room-temperature structure of the dual-phase steel, and the area ratio of ferrite, bainite and martensite in the structure relative to the whole steel plate structure is strictly controlled, which contributes to the improvement of the formability of the dual-phase steel. Furthermore, the heating temperature, soaking temperature, slow cooling temperature and fast cooling temperature, including the heating and cooling rates of each stage, in the annealing process need to be precisely controlled. It has also been found that it is critical to improve the formability to reasonably control the size of retained austenite and ensure that most of the retained austenite can generate Transformation-induced plasticity (TRIP) effect during the deformation process.
Based on the above, the invention providesProvides 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel and a manufacturing method thereof, and has better forming performance and post-fracture elongation (A) compared with the same-grade traditional dual-phase steel80) The forming safety margin is higher, and the problem that the traditional dual-phase steel is difficult to form on a complex stamping structural part is solved.
The 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel comprises the following chemical components in percentage by mass:
c: 0.16% -0.2%, Si: 0.35-0.65%, Mn: 1.8% -2.3%, Al: 0.7 to 1.0 percent of the total weight of the alloy, less than or equal to 0.01 percent of P, less than or equal to 0.003 percent of S, and the balance of Fe and inevitable impurities.
Preferably, the chemical composition further contains any one of the following elements in percentage by mass:
Cr：0.16％～0.25％，Mo：0～0.3％。
preferably, the content of Si and Al in the chemical composition satisfies the following requirements in percentage by mass:
Si+Al≥1.2％。
the 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel has the advantages that the area ratio of ferrite in a microstructure is 45-60%, the sum of the area ratios of bainite and martensite is 35-45%, and the area ratio of retained austenite is 4-9%. Wherein the retained austenite with the equivalent circle diameter of more than 500nm accounts for more than 90% of the total amount of the retained austenite.
The invention relates to 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, which is provided with a hot-dip galvanized layer on the surface.
The invention relates to a method for manufacturing 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, which comprises the following steps:
(1): according to the chemical component proportion of 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, steelmaking is carried out by using a converter, and a plate blank is cast by adopting a continuous casting mode;
(2): heating the plate blank to 1250 +/-20 ℃, soaking for 30-100 min, and carrying out rough rolling and finish rolling to obtain a hot rolled plate with the thickness of 3.0-5.0 mm; wherein the inlet temperature of finish rolling is 1040-1080 ℃, the finishing temperature is 860-910 ℃, and the coiling temperature is 660 +/-20 ℃;
(3): pickling a hot rolled plate to remove oxide scales, and cold rolling to obtain cold-hard strip steel with the thickness of 1.0-2.0 mm, wherein the total cold rolling reduction rate is between 60 and 70 percent;
(4): annealing the cold-hard strip steel in a continuous annealing furnace, controlling the dew point of a furnace area between minus 30 ℃ and minus 10 ℃, setting the preheating temperature to be 210 ℃ to 230 ℃, the heating and soaking temperature to be 780 ℃ to 830 ℃, the slow cooling temperature to be 680 ℃ to 720 ℃ and the fast cooling outlet temperature to be 450 ℃ to 470 ℃; then, surface galvanization is carried out, the temperature of a zinc pot is 450-470 ℃, and the finishing elongation of the galvanized strip steel is 0.3-0.6%.
Wherein in the step (4), the heating speed of the strip steel is 2 ℃/s-4 ℃/s, the soaking time is 60 s-90 s, and the average cooling rate of the fast cooling section is more than 10 ℃/s.
1. First, the limitation range and reason of the chemical composition of the 780MPa grade hot-dip galvanized dual-phase steel for reinforcement forming of the present invention will be explained.
[C：0.16％～0.2％]
C is an austenite stabilizing element, can be enriched to austenite in the continuous annealing and galvanizing processes and is vital to the retention of austenite at room temperature; as a solid solution strengthening element, C can also improve the hardness of martensite, thereby ensuring the tensile strength of the dual-phase steel. In order to obtain the above-described effects, the C content needs to be 0.16%, and when the C content exceeds 0.2%, the strength margin increases and the weldability deteriorates. The content of C is preferably 0.16-0.19%.
[Si：0.35％～0.65％]
Si mainly plays a role in solid solution strengthening in the dual-phase steel, and the strength of the dual-phase steel is improved. Meanwhile, Si is insoluble in cementite, so that the formation of cementite in the galvanizing stage can be effectively inhibited, the enrichment of C in austenite is further promoted, and the austenite stabilization is facilitated. However, for a galvanization line without a pre-oxidation function, too high Si content causes severe deterioration in the quality of the galvanized surface. Therefore, the Si is limited to the range of 0.35% to 0.65%, preferably 0.4% to 0.6%.
[Mn：1.8％～2.3％]
Mn is also a solid solution strengthening element, and is beneficial to improving the strength of the dual-phase steel; meanwhile, Mn can be enriched to austenite to stabilize the austenite, which contributes to the improvement of the plasticity of the dual-phase steel. In order to obtain such effects, the Mn content should be controlled to 1.8% or more. However, when the Mn content exceeds 2.3%, hardenability of austenite is too high, martensite is excessively generated, and even more serious segregation occurs. The Mn content is preferably 1.9 to 2.2%.
[Al：0.7％～1.0％]
Similar to Si, Al can effectively inhibit the decomposition of residual austenite and the precipitation of carbide, and Al is adopted to replace partial Si, so that the surface quality of the galvanized steel sheet can be effectively improved. Considering the higher C content and the lower Si content in the dual-phase steel of the present invention, the amount of Al added should not be less than 0.7%. However, since the Al content is too high to ensure the strength of the steel sheet and to easily cause clogging of the nozzle in the continuous casting process, the upper limit of the Al content is set to 1.0%. The Al content is preferably 0.7 to 0.9%.
[P≤0.01％]
P is likely to be segregated at grain boundaries to deteriorate the plasticity of the steel sheet, so that the P content is not more than 0.01%, but excessive P removal causes an increase in production cost, and therefore the P content is preferably 0.005% to 0.01%.
[S≤0.003％]
S is easily bonded to Mn to form coarse MnS inclusions, which deteriorate formability of a steel sheet such as punching, and therefore, the S content is controlled to 0.003% or less, and is preferably 0.0003% to 0.003% in order to avoid an increase in cost due to excessive S removal.
[ Cr: 0.16% -0.25% and Mo: 0 to 0.3% ]
Both Cr and Mo improve hardenability, promote martensite formation, and improve the strength of the steel sheet. In order to avoid segregation due to too high Mn content, a small amount of Cr may be used instead of Mn. Mo functions similarly to Cr, but Mo is an expensive alloying element, so that the content thereof should be limited from the viewpoint of material cost.
[Si+Al≥1.2％]
Both Si and Al can suppress carbide precipitation and austenite decomposition. Different from common cold-rolled annealed dual-phase steel, the hot-galvanized dual-phase steel has long retention time at about 460 ℃ (galvanizing process), carbide is easy to precipitate at the time, and austenite is easy to decompose and difficult to retain. Therefore, in the case of ensuring that the contents of Si and Al are within the respective constraints, the total amount of Si and Al must be 1.2% or more, otherwise the above effects cannot be fully achieved.
2. Next, the scope and reason for defining the microstructure of the 780MPa class reinforced formable hot-dip galvanized dual-phase steel of the present invention will be described.
The 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel has a microstructure which comprises the following components in percentage by area:
ferrite: 45 to 60 percent of
Bainite + martensite: 35 to 45 percent of
Retained austenite: 4 to 9 percent.
Wherein: the retained austenite with the equivalent circle diameter of more than 500nm accounts for more than 90% of the total amount of the retained austenite.
[ ferrite: 45% -60% ]
Ferrite belongs to a soft phase structure in the dual-phase steel and is the most important composition phase for ensuring the plasticity and the formability of the dual-phase steel. In addition to taking up most of the strain during deformation, ferrite may also act as a synergistic deformation when the retained austenite phase changes to hard martensite. When the area ratio of ferrite in the structure is less than 45%, it is difficult to obtain excellent plasticity under the premise that the tensile strength is greater than 780MPa, and when the area ratio of ferrite exceeds 60%, the expected strength cannot be ensured. Therefore, the area ratio of ferrite is set to be 45% to 60%.
[ bainite + martensite: 35% -45% ]
In the case of continuous hot-dip galvanized dual-phase steel, a part of bainite and martensite, both of which belong to hard phases, particularly martensite, are important constituent phases for securing the required strength, are generated in the galvanizing stage and in the subsequent cooling process. When the total area ratio of both is less than 35%, the tensile strength of 780MPa cannot be secured, and when the total area ratio of both exceeds 45%, the strength margin increases and desired plasticity and formability cannot be obtained. Therefore, the total area ratio of bainite and martensite is set to be 35% to 45%.
[ retained austenite: 4% -9% ]
In the deformation process, the residual austenite is transformed into martensite, so that the stress concentration can be relieved, and the plasticity and the formability of the material are improved. In the present invention, the content of the retained austenite is at least 4% or more in order to ensure that sufficient retained austenite is generated to generate TRIP effect during the deformation process. However, when the content of retained austenite exceeds 9%, the average C enrichment in the interior decreases, resulting in a decrease in stability, and phase transformation tends to occur at an early stage of deformation, and too high a content of retained austenite also increases edge crack sensitivity in flange forming. Therefore, the residual austenite content is limited to 4% to 9%.
[ the retained austenite having an equivalent circle diameter of more than 500nm accounts for 90% or more of the total amount of the retained austenite ]
The mechanical stability of the retained austenite determines whether the TRIP effect can occur. The retained austenite in the present invention is distributed mostly at ferrite grain boundaries, but has a non-uniform size. When the size of the retained austenite is too small, the enrichment degree of the interior C, Mn is high, so that the stability is too high; in addition, strain is not easily transmitted to austenite of a small size during deformation, and this causes the retained austenite to be hardly transformed during deformation, and thus the effect of improving plasticity and formability is not exerted. Therefore, in order to obtain good plasticity and formability, it is necessary to ensure that the retained austenite having an equivalent circle diameter of more than 500nm accounts for 90% or more of the total amount of the retained austenite so that most of the retained austenite can generate the TRIP effect.
3. Finally, a method for producing 780MPa grade reinforced formable hot dip galvanized dual phase steel of the present invention will be explained.
The method for manufacturing 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel comprises the following steps:
(1) according to the chemical component proportion of 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, steelmaking is carried out by using a converter, and a plate blank is cast by adopting a continuous casting mode;
the process is not particularly limited as long as a desired slab is obtained.
(2) Heating the plate blank to 1250 +/-20 ℃, soaking for 30-100 min, and carrying out rough rolling and finish rolling to obtain a hot rolled plate with the thickness of 3.0-5.0 mm; wherein the inlet temperature of finish rolling is 1040-1080 ℃, the finishing temperature is 860-910 ℃, and the coiling temperature is 660 +/-20 ℃;
[ finishing temperature: 860 ℃ to 910 DEG C
The finishing temperature is more than 910 ℃, which easily causes coarse grains and increases the difficulty of process control; the finishing temperature is lower than 860 ℃, the deformation resistance of the hot rolled plate is too large, and mixed crystals are easy to appear.
[ coiling temperature: 660 +/-20℃)
The coil collapse is easy to occur when the coiling temperature is too high; bainite and martensite are excessively generated due to too low coiling temperature, and the cold rolling difficulty is increased.
(3) Pickling a hot rolled plate to remove oxide scales, and cold rolling to obtain cold-hard strip steel with the thickness of 1.0-2.0 mm, wherein the total cold rolling reduction rate is between 60 and 70 percent;
[ Cold Rolling Total reduction: 60% -70% ]
The insufficient cold rolling deformation can lead to insufficient deformation energy storage, is not beneficial to recrystallization in the annealing process and further influences the product performance; too large cold rolling deformation can increase the load of the rolling mill and easily cause edge cracking.
(4) Annealing the cold-hard strip steel in a continuous annealing furnace, controlling the dew point of a furnace area between minus 30 ℃ and minus 10 ℃, setting the preheating temperature to be 210 ℃ to 230 ℃, the heating and soaking temperature to be 780 ℃ to 830 ℃, the slow cooling temperature to be 680 ℃ to 720 ℃ and the fast cooling outlet temperature to be 450 ℃ to 470 ℃; then, surface galvanization is carried out, the temperature of a zinc pot is 450-470 ℃, and the finishing elongation of the galvanized strip steel is 0.3-0.6%.
Wherein the heating speed of the strip steel is 2-4 ℃/s, the soaking time is 60-90 s, and the average cooling rate of the fast cooling section is more than 10 ℃/s.
[ furnace zone dew point: minus 30 ℃ to minus 10 DEG C
Inhibit the external oxidation of the strip steel and ensure the platability of the product.
Soaking temperature: 780 ℃ -830℃)
When the soaking temperature is higher than 830 ℃, the proportion of ferrite in the two-phase region is greatly reduced, and the plasticity of the dual-phase steel is influenced; when the soaking temperature is lower than 780 ℃, the proportion of austenite is insufficient, and bainite and martensite in expected proportions cannot be obtained in the subsequent cooling process, so that the strength is insufficient.
[ slow cooling temperature: 680 ℃ -720℃)
The slow cooling temperature higher than 720 ℃ causes the ferrite proportion to be reduced, the average C, Mn content in austenite is reduced, the stability is reduced, and finally, stable residual austenite cannot be obtained; the slow cooling temperature lower than 680 ℃ causes the ferrite proportion to be too high, and the tensile strength of more than 780MPa cannot be ensured.
Soaking time: 60s to 90s ]
The soaking time is regulated and controlled by controlling the running speed of the strip steel in the furnace zone. The soaking time is too short, the recrystallization is insufficient, and a banded structure is reserved; the soaking time is too long, and the crystal grains grow excessively.
[ average cooling rate of fast cooling stage: greater than 10 ℃/s
When the cooling speed of the rapid cooling section is lower than 10 ℃/s, ferrite is generated continuously after slow cooling, and the final structure proportion and the mechanical property of the dual-phase steel are influenced.
In the following, 8 embodiments of the present invention are listed in a table format. Meanwhile, some comparative examples are listed to highlight the technical effects of the present invention and to emphasize the ingredients and process design ideas that must be followed to obtain such technical effects. The chemical compositions of the examples and comparative examples are shown in Table 1, the hot rolling and cold rolling process parameters are shown in Table 2, the continuous hot galvanizing process is shown in Table 3, and the microstructure and mechanical property analysis results are shown in Table 4.
Examples
The invention relates to 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, which comprises the following chemical components in percentage by mass:
c: 0.16% -0.2%, Si: 0.35-0.65%, Mn: 1.8% -2.3%, Al: 0.7 to 1.0 percent of the total weight of the alloy, less than or equal to 0.01 percent of P, less than or equal to 0.003 percent of S, more than or equal to 1.2 percent of Si and Al, and the balance of Fe and inevitable impurities;
further, the chemical composition of the dual-phase steel further comprises any one of the following elements in mass fraction:
Cr：0.16％～0.25％，Mo：0～0.3％。
the chemical compositions of the examples of the invention and the comparative examples are shown in table 1:
table 1 chemical composition (wt.%) of each example and comparative example
The invention relates to a method for manufacturing 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, which comprises the following steps:
(1): according to the chemical component proportion of 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel, steelmaking is carried out by using a converter, and a plate blank is cast by adopting a continuous casting mode;
(2): heating the plate blank to 1250 +/-20 ℃, soaking for 30-100 min, and carrying out rough rolling and finish rolling to obtain a hot rolled plate with the thickness of 3.0-5.0 mm; wherein the inlet temperature of finish rolling is 1040-1080 ℃, the finishing temperature is 860-910 ℃, and the coiling temperature is 660 +/-20 ℃;
(3): pickling a hot rolled plate to remove oxide scales, and cold rolling to obtain cold-hard strip steel with the thickness of 1.0-2.0 mm, wherein the total cold rolling reduction rate is between 60 and 70 percent;
(4): annealing the cold-hard strip steel in a continuous annealing furnace, controlling the dew point of a furnace area between minus 30 ℃ and minus 10 ℃, setting the preheating temperature to be 210 ℃ to 230 ℃, the heating and soaking temperature to be 780 ℃ to 830 ℃, the slow cooling temperature to be 680 ℃ to 720 ℃ and the fast cooling outlet temperature to be 450 ℃ to 470 ℃; then, surface galvanization is carried out, the temperature of a zinc pot is 450-470 ℃, and the finishing elongation of the galvanized strip steel is 0.3-0.6%.
Wherein in the step (4), the heating speed of the strip steel is 2 ℃/s-4 ℃/s, the soaking time is 60 s-90 s, and the average cooling rate of the fast cooling section is more than 10 ℃/s.
Specifically, the rolling process parameters of the examples and comparative examples of the present invention are shown in table 2, and the continuous annealing and hot dip galvanizing process parameters are shown in table 3:
TABLE 2 Rolling Process parameters for the examples and comparative examples
TABLE 3 continuous hot-dip galvanizing process parameters of each example and comparative example
And (3) detecting the performance of the dual-phase steel: a sample for microstructure observation is corroded by a 4% nital solution by volume, and is observed and an image is obtained under a metallographic microscope. Wherein, the ferrite is bright white, and the martensite, the bainite and the residual austenite are gray black, so as to calculate the area ratio of the ferrite; in addition, the area ratio of the retained austenite is measured by Electron Back Scattering Diffraction (EBSD), and the retained austenite proportion with the equivalent circle diameter of more than 500nm is counted; finally, the area ratio of ferrite and retained austenite is subtracted from the total 100% area to obtain the sum of the area ratios of bainite and martensite.
And (3) detecting yield, tensile strength and elongation after fracture by using a ZWICK/Roe | Z100 tensile testing machine according to the GB/T228.1-2010 standard.
The microstructure and mechanical property statistics of the duplex copper prepared in the examples and comparative examples of the present invention are shown in table 4:
TABLE 4 statistics of microstructure and mechanical Properties of the examples and comparative examples
From examples 1 to 8, it can be seen that by improving the components and the preparation process of the dual-phase steel as described above, yield strengths of 452MPa to 485MPa, tensile strengths of 803MPa to 853MPa, and elongations after fracture (A)80) Compared with the traditional galvanized DP780, the novel dual-phase steel with the elongation of more than 23 percent has the advantages that the elongation of the dual-phase steel is greatly improved; the steel sheets of comparative examples 1 to 5, which are different from the present invention in chemical composition or preparation process, could not satisfy both tensile strength and elongation after fracture.
As can be seen from FIG. 1, the dual phase steel obtained in example 1 of the present invention has good galvanized surface quality. The horizontal and vertical coordinates of figure 2 respectively represent the strain of the elliptical grid in the short axis and long axis directions when the plate is expanded and broken, and the larger the ratio of the main strain to the secondary strain is, the better the formability is. FIG. 2 shows that the forming safety margin of the dual-phase steel is obviously improved compared with that of the traditional DP780, and the dual-phase steel is more suitable for forming high-drawing parts with complex shapes. In addition, the 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel has simple alloy components and a manufacturing method, can be produced on a common continuous hot-dip galvanizing production line, and has wide popularization and application.
Finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (9)
1. A 780MPa grade reinforced forming hot-dip galvanized dual-phase steel, characterized in that the chemical composition of the dual-phase steel comprises the following components in percentage by mass:
c: 0.16% -0.2%, Si: 0.35-0.65%, Mn: 1.8% -2.3%, Al: 0.7-1.0 percent of Si, less than or equal to 0.01 percent of P, less than or equal to 0.003 percent of S, and the contents of Si and Al in mass fraction satisfy:
more than or equal to 1.2 percent of Si and Al, and the balance of Fe and inevitable impurities;
in the microstructure of the dual-phase steel, the area ratio of ferrite is 45-60%, the sum of the area ratios of bainite and martensite is 35-45%, the area ratio of retained austenite is 4-9%, wherein the retained austenite with the equivalent circle diameter of more than 500nm accounts for more than 90% of the total amount of the retained austenite, the tensile strength of the dual-phase steel is 803 MPa-853 MPa, and the elongation percentage A after fracture is80Greater than 23%.
2. A 780MPa grade enhanced forming hot galvanized dual phase steel according to claim 1, characterized in that the chemical composition of said dual phase steel further comprises any one of the following elements in mass fraction:
Cr：0.16％～0.25％，Mo：0～0.3％。
3. a 780MPa grade enhanced forming hot galvanized dual phase steel according to claim 1, characterized in that the surface of said dual phase steel has a hot dip galvanising layer.
4. A method of manufacturing 780MPa grade enhanced forming hot galvanized dual phase steel according to any of claims 1-3, characterized in that it comprises:
continuously casting molten steel into a slab, wherein the chemical composition of the molten steel is the same as that of the dual-phase steel of any one of claims 1 to 3;
heating, rough rolling and finish rolling are carried out on the plate blank to obtain a hot rolled plate;
carrying out acid pickling on the hot rolled plate, and then carrying out cold rolling to obtain cold-hard strip steel;
and continuously annealing the cold-hard strip steel, and performing surface galvanization after annealing.
5. The manufacturing method of 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel according to claim 4, characterized in that the slab heating temperature is 1250 +/-20 ℃, soaking is carried out for 30-100 min, and the thickness of the hot rolled plate obtained by finish rolling is 3.0-5.0 mm; wherein the inlet temperature of the finish rolling is 1040-1080 ℃, the finishing temperature is 860-910 ℃, and the coiling temperature is 660 +/-20 ℃.
6. The manufacturing method of 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel according to claim 4, characterized in that in the cold rolling process, the total reduction rate of cold rolling is 60-70%, and the thickness of cold-hard strip steel obtained by cold rolling is 1.0-2.0 mm.
7. The manufacturing method of 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel according to claim 4, characterized in that, in the continuous annealing process, the furnace zone dew point is controlled to be-30 ℃ to-10 ℃, the preheating temperature is 210 ℃ to 230 ℃, the heating and soaking temperature is 780 ℃ to 830 ℃, the slow cooling temperature is 680 ℃ to 720 ℃, and the fast cooling outlet temperature is 450 ℃ to 470 ℃.
8. The method for manufacturing 780 MPa-grade reinforced forming hot-dip galvanized dual-phase steel according to claim 4, wherein in the surface galvanizing process, the temperature of a zinc pot is 450-470 ℃, and the finished elongation of the galvanized steel strip is 0.3-0.6%.
9. The manufacturing method of 780MPa grade enhanced forming hot galvanizing dual-phase steel according to claim 4, characterized in that in the continuous annealing process, the heating speed of the cold-hard strip steel is 2 ℃/s-4 ℃/s, the soaking time is 60 s-90 s, and the average cooling rate of the fast cooling section is more than 10 ℃/s.
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