CN114606446B - High-strength toughness steel, preparation method thereof and preparation method of hot-dip galvanized steel - Google Patents
High-strength toughness steel, preparation method thereof and preparation method of hot-dip galvanized steel Download PDFInfo
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- 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
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- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
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- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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- 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
- C21D2211/001—Austenite
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- 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
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- C21D2211/009—Pearlite
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Abstract
The invention relates to high-strength toughness steel, a preparation method thereof and a preparation method of hot-dip galvanized steel. The high-strength toughness steel comprises the following raw materials in percentage by mass: c:0.18 to 0.25 percent; mn:1.8 to 2.5 percent; si:1.2 to 1.5 percent; the balance of Fe and other unavoidable impurities; the high-strength toughness steel is obtained by taking cold-rolled pearlite and cold-rolled ferrite as initial structures and performing heat treatment on the initial structures, wherein the mass percent of Mn in cementite in the initial structures is more than 5%, the volume fraction of the pearlite in the initial structures is more than 25%, the volume fraction of austenite in the high-strength toughness steel is 70% -90%, and the volume fraction of residual austenite in the high-strength toughness steel is 18% -30%. The high-strength toughness steel of the present invention has excellent tensile strength and high plasticity.
Description
Technical Field
The invention belongs to the field of steel products, particularly relates to the field of heat treatment of steel products, and particularly relates to high-strength and high-toughness steel and a preparation method thereof as well as a preparation method of hot-dip galvanized steel.
Background
With the increasing energy crisis and the worsening environment, the environmental protection consciousness and the safety consciousness of people are continuously improved, and the energy conservation, the environmental protection and the safety are the first problems to be considered on the premise that the materials can meet the use requirements. Based on the requirements of light weight, energy conservation, emission reduction and High safety of future automobiles, various iron and Steel manufacturers in the world develop research and development of Advanced High Strength Steel (AHSS) with High product of Strength and elongation (namely the product of tensile Strength (Rm) and plasticity (A)).
The retained austenite can improve the elongation of the material through deformation-induced plasticity during the deformation process, thereby playing an important role in the design of advanced high-strength steel. Currently, a variety of steel grades having high strength plasticity have been developed, including transformation induced plasticity (TRIP) steel, quench-partitioning (Q & P) steel, medium manganese steel, and the like.
Currently, the approaches to obtain stable retained austenite at room temperature are mainly: austenite stability is enhanced by enrichment of substitutional elements in austenite, such as medium manganese steels; and austenite stability by enrichment of carbon elements from bainite or martensite into austenite, such as transformation induced plasticity (TRIP) steels, quench-partition (Q & P) steels. The medium manganese steel has high volume fraction of retained austenite, but the alloy elements of the medium manganese steel have large requirements and high manufacturing cost; in conventional low-carbon low-alloy TRIP steels and Q & P steels, part of the carbon elements are pinned in bainite or martensite defects, or form carbides, so that the volume fraction of stable austenite obtained at room temperature is limited, typically 8-14%.
Moreover, the heat treatment process is limited by the production facilities of the enterprises in the past, most of the related researches are based on the heating rate (5-20 ℃/s) of the existing conventional heating equipment to austenitize the strip steel, such as that described in document 1, and the austenitizing process needs to be completed under the near-equilibrium condition, so that the tissue regulation means is limited.
In recent years, the development of rapid heating technologies such as transverse magnetic flux induction heating and novel direct fire heating has led to the industrial application of rapid thermal processing. The cold-rolled strip steel can possibly complete the austenitizing process within dozens of seconds from room temperature, thereby greatly shortening the time length of the heating section and improving the speed and the production efficiency of a unit.
The austenitizing process completed in a very short time also provides more methods for controlling the phase transformation, and further allows for more flexible tissue design. Under the condition of short-time austenitizing, the composition inheritance of the replacement element is realized under the condition that the tissue structure is changed by utilizing the extremely large diffusion coefficient difference of the replacement element and the interstitial element. Document 2 discloses a method for achieving a significant improvement in the strength and plasticity of medium manganese steel through rapid heating and compositional inheritance, but the method is limited to medium manganese steel with martensite as the initial structure (Mn >4 wt.%). Document 3 discloses a rapid heat treatment method for cold-rolled low-carbon high-strength steel, which is characterized in that a structure before rapid heating is a bainite/ferrite bidirectional structure, and although the method improves the overall heat treatment efficiency of cold-rolled strip steel, the austenite transformation time is too long, the diffusion of replacement elements is obvious, the genetic effect of components is poor, and more residual austenite cannot be obtained.
The cited documents are:
document 1: CN 104988391A;
document 2: R.Ding, Y.Yao, B.Sun, G.Liu, J.He, T.Li, X.Wan, Z.Dai, D.Ponge, D.Raabe, C.Zhang, A.Godfrey, G.Miyamoto, T.Furuhara, Z.Yang, S.van der Zwaag, H.Chen, chemical boundary engineering: A new route towardlean, ultrastrong year tier, 6 (13) (2020) eaay 1430.
Document 3: CN 105543674A.
Disclosure of Invention
Problems to be solved by the invention
The invention aims to provide high-strength toughness steel, a preparation method thereof and a preparation method of hot-dip galvanized steel. The high-strength toughness steel disclosed by the invention is excellent in tensile strength and high in plasticity, and the volume fraction of a residual austenite structure can be remarkably improved by the preparation method disclosed by the invention, so that the steel plasticity is remarkably improved.
Means for solving the problems
The invention provides high-strength toughness steel, which comprises the following raw materials in percentage by mass:
C：0.18-0.25％；
Mn：1.8-2.5％；
Si：1.2-1.5％；
the balance of Fe and other unavoidable impurities;
the high-strength toughness steel is obtained by heat treatment with cold-rolled pearlite and cold-rolled ferrite as initial structures,
the mass percentage of Mn in cementite in the initial structure is more than 5 percent,
the volume fraction of pearlite in the initial tissue is above 25%,
the volume fraction of austenite in the high-strength toughness steel is 70-90%,
the volume fraction of the retained austenite in the high-strength toughness steel is 18-30%.
According to the high-strength toughness steel, when the content of C in the high-strength toughness steel is 0.18-0.22%, the volume fraction of residual austenite in the high-strength toughness steel is 18-22%; when the content of C in the high-strength toughness steel is 0.22-0.25%, the volume fraction of residual austenite in the high-strength toughness steel is 22-30%.
According to the high-strength toughness steel, the tensile strength of the high-strength toughness steel is 900-1200MPa, and the total elongation is 20-40%.
The high-strength toughness steel further contains at least one of the following elements in percentage by mass:
Nb：0.01-1％；
V：0.01-0.5％；
Ti：0.01-0.5％；
Mo：0.01-0.5％。
the invention also provides a preparation method of the high-strength toughness steel, which comprises the following steps:
(1) Rapidly heating the strip steel from room temperature to an austenite ferrite two-phase region of 800-830 ℃;
(2) Staying for a short time within a two-phase region heating target temperature range, wherein the staying time is 5-30s;
(3) Cooling to the secondary isothermal interval of 400-450 deg.C, and maintaining at the temperature interval for 80-120s;
(4) And rapidly cooling to room temperature.
According to the preparation method provided by the invention, the heating rate of the strip steel in the step (1) is more than 10 ℃/s when the temperature of the strip steel is less than 400 ℃, and the heating rate of the strip steel is more than 100 ℃/s when the temperature of the strip steel is more than 400 ℃, and the heating rate is preferably 200 ℃/s-300 ℃/s.
According to the preparation method, the cooling speed of the cooling in the step (3) is below 80 ℃/s, and the cooling time is 5s-15s.
According to the preparation method, the cooling comprises multi-stage cooling or nonlinear speed cooling.
According to the preparation method, when the temperature interval in the secondary isothermal zone is isothermal, the temperature is raised or lowered without exceeding the secondary isothermal zone.
According to the preparation method, the strip steel is prepared by the following steps:
smelting and continuously casting steel consisting of the raw materials of the high-strength toughness steel to form a casting blank;
and performing hot rolling and laminar cooling, performing hot coiling, performing slow cooling, uncoiling and cold rolling on a hot-rolled coil, and thus obtaining the strip steel.
According to the preparation method, the heat preservation time of the thermal curling at 600-500 ℃ is more than 4 hours.
According to the preparation method of the invention, the reduction of the cold rolling is more than 70%.
The invention also provides a preparation method of the hot-dip galvanized steel, which is characterized by comprising the following steps of:
(1) Smelting and continuously casting steel consisting of the raw materials of the high-strength toughness steel to form a casting blank;
(2) Hot rolling and laminar cooling are carried out on the strip steel, and uncoiling and cold rolling are carried out on a hot rolled coil after slow cooling to obtain the strip steel;
(3) Heating the strip steel from room temperature to a 800-830 ℃ austenite ferrite two-phase region, wherein the heating rate of the strip steel is more than 10 ℃/s when the temperature of the strip steel is less than 400 ℃, and the heating rate of the strip steel is more than 100 ℃/s when the temperature of the strip steel is more than 400 ℃;
(4) Staying for a short time within a two-phase region heating target temperature range, wherein the staying time is 5-30s;
(5) Cooling to the secondary isothermal interval of 400-450 deg.C, and maintaining at the temperature interval for 80-120s;
(6) Tempering to 460-470 ℃, preserving heat and then soaking zinc to finish hot-dip galvanizing;
(7) And rapidly cooling to room temperature to obtain the hot-dip galvanized steel.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the invention, through rapid heating and cooling control, the volume fraction of the retained austenite of the high-strength toughness steel is greatly improved, and the plasticity of the steel is obviously improved; meanwhile, the preparation method shortens the time required by the heat treatment of the cold-rolled strip steel and improves the production efficiency; the microstructure of the high-strength ductile steel of the present invention can suppress the recrystallization of ferrite, and the grains of each phase are nearly equiaxial, and the influence of anisotropy is weak, so that the steel has good deformation compatibility.
Drawings
FIG. 1 is a flow chart of the preparation process of example 1 of the present invention.
FIG. 2 initial structures of pearlite and ferrite in a steel sheet having a thickness of 1.2mm in example 1 of the present invention, in which manganese in cementite is present in a concentration of 5 to 7wt.%, and a magnification of 4000 times.
FIG. 3 is a scanning electron microscope image of the microstructure of the high strength and toughness steel obtained in example 1 of the present invention, at a magnification of 4000 times.
FIG. 4 is an Electron Back Scattering Diffraction (EBSD) photograph of the microstructure of the high strength ductile steel obtained in example 1 of the present invention, in which the dark color is residual austenite and the light color is ferrite.
Testing equipment: a nano Auger PHI-710 probe.
FIG. 5 shows the fluctuation of the manganese element content measured inside the retained austenite grains of the high strength toughness steels of example 1 of the present invention and comparative example 1, according to the test method: and scanning the obtained tissue and the corresponding manganese element distribution under the EBSD tissue in situ by adopting a nano Auger PHI-710 probe.
FIG. 6 is a graph comparing the pull curve of example 1 of the present invention (solid line) with that of comparative example 1 (dotted line), wherein the gauge length of the specimen is 20mm and the pulling rate is 0.2mm/min.
FIG. 7 is a scanning electron micrograph of the microstructure of the steel strip obtained in comparative example 1 of the present invention, in which B is bainite, M/A is martensite/austenite islands, and F is ferrite.
FIG. 8 is an Electron Back Scattering Diffraction (EBSD) photograph of the microstructure of the obtained steel strip in comparative example 1 of the present invention, wherein dark gray is retained austenite and the bulk with internal contrast change is martensite.
Detailed Description
The present invention will be described in detail below. The technical features described below are explained based on typical embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples.
Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a better understanding of the present invention. It will be understood by those skilled in the art that the present invention may be practiced without some of these specific details. In other instances, methods, means, devices and steps which are well known to those skilled in the art have not been described in detail so as not to obscure the invention.
It should be noted that:
in the present specification, the numerical range represented by "numerical value a to numerical value B" means a range including the end points of numerical values a and B.
All units used in the present invention are international standard units unless otherwise stated, and numerical values and numerical ranges appearing in the present invention should be understood to include errors allowed in industrial production.
In the present specification, reference to "some particular/preferred embodiments," "other particular/preferred embodiments," "embodiments," and the like, means that a particular element (e.g., feature, property, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments.
In the present specification, "room temperature" and "room temperature" mean "10 to 40 ℃.
The invention firstly provides high-strength toughness steel, which comprises the following raw materials in percentage by mass:
C：0.18-0.25％；
Mn：1.8-2.5％；
Si：1.2-1.5％；
the balance of Fe and other unavoidable impurities;
the high-strength toughness steel is obtained by heat treatment with cold-rolled pearlite and cold-rolled ferrite as initial structures,
the mass percentage of Mn in cementite in the initial structure is more than 5 percent,
the volume fraction of pearlite in the initial tissue is 25% or more,
the volume fraction of austenite in the high-strength toughness steel is 70-90%,
the volume fraction of the retained austenite in the high-strength toughness steel is 18-30%.
The residual austenite in the steel can induce plasticity through deformation during the deformation process, so that the elongation of the material is improved, and the volume fraction of the residual austenite in the high-strength toughness steel is 18-30%.
Compared with the traditional high-strength steel with the same components, the high-strength toughness steel obtained by the invention has a high volume fraction of retained austenite, generally, the volume fraction of the retained austenite is positively correlated with the volume fraction of pearlite in an initial structure, and specifically, when the content of C in the high-strength toughness steel is 0.18-0.22%, the volume fraction of the retained austenite in the high-strength toughness steel is 18-22%; when the content of C in the high-strength toughness steel is 0.22-0.25%, the volume fraction of residual austenite in the high-strength toughness steel is 22-30%.
According to the high-strength toughness steel, the tensile strength of the high-strength toughness steel is 900-1200MPa, and the total elongation is 20-40%.
Compared with the traditional high-strength steel with the same components, the plasticity of the high-strength toughness steel obtained by the invention is obviously improved, and the elongation of the high-strength toughness steel obtained by the invention is improved by 8-12% compared with the high-strength steel obtained under the traditional condition within the range of 900-1000MPa of tensile strength; in the range of the tensile strength of 1000-1100MPa, the elongation of the high-strength toughness steel obtained by the method is improved by 5-9% compared with the high-strength steel obtained under the traditional condition; the elongation of the high-strength toughness steel obtained by the method is improved by 4-7% compared with the elongation of the high-strength steel obtained under the traditional condition within the range of 1100-1200MPa of tensile strength.
The high-strength toughness steel further contains at least one of the following elements in percentage by mass:
Nb：0.01-1％；
V：0.01-0.5％；
Ti：0.01-0.5％；
Mo：0.01-0.5％。
the invention also provides a preparation method of the high-strength toughness steel, which comprises the following steps:
(1) Rapidly heating the strip steel from room temperature to an austenite ferrite two-phase region of 800-830 ℃;
(2) Staying for a short time within a two-phase region heating target temperature range, wherein the staying time is 5-30s;
(3) Cooling to the secondary isothermal interval of 400-450 deg.C, and maintaining at the temperature interval for 80-120s;
(4) And rapidly cooling to room temperature.
According to the invention, the cold-rolled pearlite and the cold-rolled ferrite with fully distributed elements are used as initial structures, the strip steel is rapidly heated to an austenite phase transformation two-phase region, and the release of deformation energy storage in the austenite phase transformation process is utilized to improve the nucleation rate of austenite and accelerate the austenite phase transformation mechanics. Meanwhile, the dissolution of cementite is realized and the diffusion of replacement elements is limited by using shorter heat preservation time. In the secondary isothermal process, the enrichment of carbon at an austenite interface limits the nucleation of bainite so that ferrite further grows in the isothermal process; after the secondary isothermal treatment, the carbon element is sufficiently enriched in the austenite, and the component fluctuation of the replacement element exists in the austenite, so that the martensite nucleation in the final cooling process is limited; finally, at room temperature, the fine-grained austenite is retained and the volume fraction is far higher than that of the retained austenite obtained by the traditional process, and the plasticity of the high-strength toughness steel is greatly improved under the condition of maintaining the material strength.
According to the preparation method provided by the invention, the heating rate of the strip steel in the step (1) is more than 10 ℃/s when the temperature of the strip steel is less than 400 ℃, and the heating rate of the strip steel is more than 100 ℃/s when the temperature of the strip steel is more than 400 ℃, and the heating rate is preferably 200 ℃/s-300 ℃/s.
The rapid heating in the step (1) can be realized by contact heating, resistance heating or transverse magnetic induction heating.
According to the preparation method, the ferrite phase is changed into austenite to enrich carbon elements, the ferrite phase change is sufficient through controlled cooling, and the carbon elements are efficiently enriched in the austenite, so that according to the preparation method, the cooling speed in the step (3) is preferably below 80 ℃/s, and the cooling time is preferably 5-15 s.
In the step (2), incomplete austenitizing needs to be ensured in the heat preservation process of the two-phase region, and the incomplete austenitizing can ensure that ferrite phase transformation does not need nucleation in the cooling process and is realized only through interface migration, so that the ferrite phase transformation efficiency is improved. Wherein the volume fraction of austenite accounts for 70-90%, the heat preservation temperature is 800-830 ℃, and the austenitizing time is ensured to be 5-30s so as to limit the diffusion of manganese element.
More preferably, the austenitizing heat preservation temperature for a low-carbon system (C: 0.18-0.22 wt.%) is 810-830 ℃, and the austenitizing heat preservation temperature for a slightly high-carbon system (C: 0.22-0.25 wt.%) is 800-820 ℃, and incomplete austenitizing can enable ferrite phase transformation in the cooling process to be realized without nucleation and only through interface migration, so that ferrite phase transformation efficiency is improved.
According to the production method of the present invention, the cooling preferably includes multi-stage cooling or non-linear velocity cooling.
According to the preparation method of the invention, the temperature rise or the temperature reduction which does not exceed the secondary isothermal zone is preferably carried out when the temperature in the secondary isothermal zone is isothermal.
In the step 4), the microstructure of the high-strength ductile steel is composed of the following structures: residual austenite in a nearly equiaxial state, the average grain size is 1-2 mu m, and the volume fraction is 15-30%; nearly equiaxial ferrite, the average grain size is 1-2 μm, and the volume fraction is 60% -80%; bainite or newly formed martensite, and the volume fraction is below 15%. In most of the retained austenite, the manganese element was detected to be distributed at intervals in a thin band form, and it was derived from the uneven distribution of the manganese element in the original pearlite between the cementite and ferrite.
Preferably, the toughness of the high-strength toughness steel can be adjusted by adjusting the residence time of the secondary isothermal zone, the ferrite + new martensite (or bainite) + residual austenite structure can be realized by keeping the temperature for a short time for 80-100s, and the ferrite + residual austenite dual-phase structure can be realized by keeping the temperature for a long time for 100-120s, wherein the ferrite and the residual austenite are both nearly equiaxed fine-grained structures, and the grain size is about 500nm-2 μm.
According to the preparation method, the strip steel is prepared by the following steps:
smelting and continuously casting steel consisting of the raw materials of the high-strength toughness steel to form a casting blank;
and performing hot rolling and laminar cooling, performing hot coiling, performing slow cooling, uncoiling and cold rolling on a hot-rolled coil, and thus obtaining the strip steel.
According to the preparation method, the heat preservation time of the hot coiling at 600-500 ℃ is preferably more than 4 hours, and cementite and ferrite in cold rolled pearlite need to have more sufficient distribution of replacement elements, so that the heat preservation time of the hot strip coiling process needs to be kept at 600-500 ℃ for more than 4 hours, the mass percent of manganese elements in the cementite in the structure of the obtained strip steel is more than 5%, and the volume fraction of the obtained pearlite is more than 25%.
According to the preparation method of the invention, austenite grain refinement can be realized by combining distortion energy in the material with rapid heating, and the reduction of the cold rolling is preferably more than 70% in order to obtain sufficient distortion energy.
The invention also provides a preparation method of the hot-dip galvanized steel, which comprises the following steps:
(1) Smelting and continuously casting steel consisting of the raw materials of the high-strength toughness steel to form a casting blank;
(2) Hot rolling and laminar cooling are carried out on the strip steel, and uncoiling and cold rolling are carried out on a hot rolled coil after slow cooling to obtain the strip steel;
(3) Heating the strip steel from room temperature to a 800-830 ℃ austenite ferrite two-phase region, wherein the heating rate of the strip steel is more than 10 ℃/s when the temperature of the strip steel is less than 400 ℃, and the heating rate of the strip steel is more than 100 ℃/s when the temperature of the strip steel is more than 400 ℃;
(4) Staying for a short time within a two-phase region heating target temperature range, wherein the staying time is 5-30s;
(5) Cooling to the secondary isothermal interval of 400-450 deg.C, and maintaining at the temperature interval for 80-120s;
(6) Tempering to 460-470 ℃, preserving heat and then soaking zinc to finish hot-dip galvanizing;
(7) And rapidly cooling to room temperature to obtain the hot-dip galvanized steel.
Examples
Embodiments of the present invention will be described in detail below with reference to examples, but those skilled in the art will appreciate that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products commercially available.
Example 1:
after steel with the component of 0.2C-1.8Mn-1.4Si (wt.%) is smelted and continuously cast by an electric furnace, a casting blank with the thickness of 250mm is formed. After hot rolling and laminar cooling, the curling is ensured to be 620-660 ℃, the heat preservation time of the hot curling at 600-500 ℃ is 10 hours, the thickness of the strip steel is 4mm, and then a ferrite + pearlite double-phase structure is formed on a matrix through slow cooling. The hot rolled coil was uncoiled and cold rolled to reduce the strip thickness to 1.2mm, and the microstructure of the resulting strip was as shown in FIG. 2. Continuously annealing the cold-rolled strip steel, wherein the heat treatment process comprises the following steps:
1) Heating the strip steel to 810 ℃ at the speed of 300 ℃/s, and preserving the heat for 28s;
2) Cooling the strip steel at the temperature of 70 ℃/s to 430 ℃ and preserving the heat for 120s;
3) Rapidly cooling the strip steel to room temperature;
the microstructure of the high strength and toughness steel obtained after the heat treatment of the steel strip is shown in FIG. 2. Obtaining a two-phase structure of the residual austenite ferrite which is uniformly distributed.
As shown in the EBSD characterization of FIG. 4, the volume fraction of retained austenite in the resulting microstructure was 22.9%, and the austenite ferrite grains were uniformly distributed and had an average grain size of 1.9 μm.
As shown by the solid line in fig. 5, the obtained retained austenite has compositional heterogeneity of the alloying elements, which is characterized by nano auger scanning, and the compositional fluctuation is derived from the compositional inheritance of the austenite composition to the initial cementite/ferrite caused by the short-time austenitizing, and the compositional inheritance is favorable for the austenite to provide continuous deformation induction austenite to martensite phase transformation in the deformation stage, and can improve the coordinated deformability of the structure.
The microstructure obtained had a tensile strength of 980MPa, an elongation of 37.4% and a product of strength and elongation (tensile strength. Times. Elongation) of 36652 MPa. Therefore, compared with the traditional process for producing the strip steel, the high-strength and high-toughness steel has the advantages that the elongation rate is greatly improved under the same strength condition, and the heat treatment efficiency is obviously improved.
Example 2:
after steel with the component of 0.2C-1.8Mn-1.4Si (wt.%) is smelted and continuously cast by an electric furnace, a casting blank with the thickness of 250mm is formed. After hot rolling and laminar cooling, the curling is ensured to be 620-660 ℃, the heat preservation time of the hot curling at 600-500 ℃ is 12 hours, the thickness of the strip steel is 4mm, and then a ferrite + pearlite double-phase structure is formed on a matrix through slow cooling. The hot rolled coil was uncoiled and cold rolled to reduce the thickness of the strip to 1.2mm, and the resulting microstructure was similar to that of example 1.
And continuously annealing the cold-rolled strip steel. The annealing process comprises the following steps:
1) Heating the strip steel to 810 ℃ at the speed of 300 ℃/s, and preserving the heat for 28s;
2) Cooling the strip steel to 430 ℃ at a speed of 70 ℃/s and preserving heat for 60s;
3) Rapidly cooling the strip steel to room temperature;
after the strip steel is treated by the heat treatment process, a uniformly distributed residual austenite/ferrite/new martensite complex phase structure is obtained, and the volume fraction of austenite is 14.9%. The tensile strength of the obtained mesoscopic structure is 1080MPa, and the elongation is 25%. Therefore, compared with the prior art which produces the strip steel under the condition of the same elongation, the high-strength and high-toughness steel applied by the invention has the advantages that the material strength is greatly improved, and the heat treatment efficiency is obviously improved.
Comparative example 1
After steel with the component of 0.2C-1.8Mn-1.4Si (wt.%) is smelted and continuously cast by an electric furnace, a casting blank with the thickness of 250mm is formed. After hot rolling and laminar cooling, the curling is ensured to be 620-660 ℃, the heat preservation time of the hot curling at 600-500 ℃ is 12 hours, the thickness of the strip steel is 4mm, and then a ferrite + pearlite double-phase structure is formed on a matrix through slow cooling. The hot rolled coil was uncoiled and cold rolled to reduce the strip thickness to 1.2mm, and the microstructure of the resulting strip was as shown in FIG. 2. Continuously annealing the cold-rolled strip steel, wherein the heat treatment process comprises the following steps:
1) Heating the strip steel to 810 ℃ at the speed of 5 ℃/s, and preserving the heat for 120s;
2) Cooling the strip steel at the temperature of 70 ℃/s to 430 ℃ and preserving the heat for 120s;
3) Rapidly cooling the strip steel to room temperature;
after the heat treatment of the steel strip, no fluctuation in the manganese content was detected in the retained austenite grains of the obtained steel strip, and the microstructure of the obtained steel strip was as shown in FIG. 7, in which the volume fraction of retained austenite obtained in the microstructure was 11%, as shown in FIG. 8, and the elongation was 29.5%, which was considerably lower than the volume fraction of retained austenite and the corresponding elongation in example 1.
The above examples of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.
Claims (12)
1. A preparation method for preparing high-strength toughness steel comprises the following steps:
(1) Rapidly heating the strip steel from room temperature to an austenite ferrite two-phase region of 800-830 ℃, wherein the heating rate of the strip steel in the step (1) is more than 10 ℃/s when the temperature of the strip steel is less than 400 ℃, and the heating rate of the strip steel is more than 100 ℃/s when the temperature of the strip steel is more than 400 ℃;
(2) Staying for a short time within a two-phase region heating target temperature range, wherein the staying time is 5-28s;
(3) Cooling to the secondary isothermal interval of 400-450 deg.C, and maintaining at the temperature interval for 80-120s;
(4) Rapidly cooling to room temperature;
the high-strength toughness steel comprises the following raw materials in percentage by mass:
C：0.18-0.25％；
Mn：1.8-2.5％；
Si：1.2-1.5％；
the balance of Fe and other unavoidable impurities;
the high-strength toughness steel is obtained by heat treatment with cold-rolled pearlite and cold-rolled ferrite as initial structures,
the mass percentage of Mn in cementite in the initial structure is more than 5 percent,
the volume fraction of pearlite in the initial tissue is above 25%,
the microstructure of the high-strength toughness steel is composed of the following structures: residual austenite in a nearly equiaxial state, the average grain size is 1-2 mu m, and the volume fraction is 18-30%; nearly equiaxial ferrite, the average grain size is 1-2 μm, and the volume fraction is 60% -80%; bainite or newly formed martensite, the volume fraction of which is 15% or less.
2. The method according to claim 1, wherein when the content of C in the high strength toughness steel is 0.18 to 0.22%, the volume fraction of retained austenite in the high strength toughness steel is 18 to 22%; when the content of C in the high-strength toughness steel is 0.22-0.25%, the volume fraction of the retained austenite in the high-strength toughness steel is 22-30%.
3. The production method according to claim 1 or 2, wherein the high strength toughness steel has a tensile strength of 900 to 1200MPa and a total elongation of 20 to 40%.
4. The production method according to claim 1 or 2, characterized in that the high-strength, tough steel further contains at least one of the following elements in mass percent:
Nb：0.01-1％；
V：0.01-0.5％；
Ti：0.01-0.5％；
Mo：0.01-0.5％。
5. the production method according to claim 1, wherein the heating rate at which the temperature of the steel strip in the step (1) is 400 ℃ or higher is 200 ℃/s to 300 ℃/s.
6. The production method according to claim 1 or 5, wherein the cooling in step (3) is performed at a cooling rate of 80 ℃/s or less and for a cooling time of 5s to 15s.
7. The method of claim 6, wherein the cooling comprises multi-stage cooling or non-linear velocity cooling.
8. The production method according to claim 1 or 5, wherein the temperature rise or decrease that does not exceed the secondary isothermal zone is performed while the secondary isothermal zone is isothermal in the temperature zone.
9. The production method according to claim 1 or 5, wherein the steel strip is produced by:
smelting and continuously casting steel consisting of the raw materials of the high-strength toughness steel to form a casting blank;
and performing hot coiling after hot rolling and laminar cooling, and performing uncoiling and cold rolling on the hot rolled coil after slow cooling to obtain the strip steel.
10. The production method according to claim 9, wherein the heat retention time of the hot-rolled sheet at 500 ℃ to 600 ℃ is 4 hours or more.
11. The production method according to claim 9, wherein a reduction amount of the cold rolling is 70% or more.
12. A preparation method of hot-dip galvanized steel is characterized by comprising the following steps:
(1) Smelting and continuously casting steel consisting of raw materials of high-strength toughness steel to form a casting blank;
(2) Hot rolling and laminar cooling are carried out, then hot coiling is carried out, and after slow cooling, uncoiling and cold rolling are carried out on the hot rolled coil, thus obtaining the strip steel;
(3) Heating the strip steel from room temperature to a 800-830 ℃ austenite ferrite two-phase region, wherein the heating rate of the strip steel is more than 10 ℃/s when the temperature of the strip steel is less than 400 ℃, and the heating rate of the strip steel is more than 100 ℃/s when the temperature of the strip steel is more than 400 ℃;
(4) Staying for a short time within a two-phase region heating target temperature range, wherein the staying time is 5-28s;
(5) Cooling to the secondary isothermal zone of 400-450 deg.C, and maintaining at the temperature zone for 80-120s;
(6) Tempering to 460-470 ℃, preserving heat and then soaking zinc to finish hot-dip galvanizing;
(7) Rapidly cooling to room temperature to obtain the hot-dip galvanized steel,
the high-strength toughness steel comprises the following raw materials in percentage by mass:
C：0.18-0.25％；
Mn：1.8-2.5％；
Si：1.2-1.5％；
the balance of Fe and other unavoidable impurities;
the high-strength toughness steel is obtained by heat treatment with cold-rolled pearlite and cold-rolled ferrite as initial structures,
the mass percentage of Mn in cementite in the initial structure is more than 5 percent,
the volume fraction of pearlite in the initial tissue is above 25%,
the microstructure of the high-strength toughness steel is composed of the following structures: residual austenite in a nearly equiaxial state, the average grain size is 1-2 mu m, and the volume fraction is 15-30%; nearly equiaxial ferrite, the average grain size is 1-2 μm, and the volume fraction is 60% -80%; bainite or newly formed martensite, the volume fraction of which is 15% or less.
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| CN109943770A (en) * | 2017-12-20 | 2019-06-28 | 宝山钢铁股份有限公司 | 780MPa rank low-carbon and low-alloy hot galvanizing TRIP steel and its quick heat treatment method |
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