CN117265415B - Acid-washing-free low-density steel and preparation method thereof - Google Patents

Acid-washing-free low-density steel and preparation method thereof Download PDF

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CN117265415B
CN117265415B CN202311532353.1A CN202311532353A CN117265415B CN 117265415 B CN117265415 B CN 117265415B CN 202311532353 A CN202311532353 A CN 202311532353A CN 117265415 B CN117265415 B CN 117265415B
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CN117265415A (en
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余灿生
蒋家乐
李云杰
常智渊
康健
袁国
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Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C21D2211/001—Austenite
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

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Abstract

The invention discloses acid-washing-free low-density steel and a preparation method thereof, wherein the acid-washing-free low-density steel comprises the following chemical components in percentage by weight: c:0.08% -0.25%, si: 0.05-0.30%, mn:3.5% -5.0%, P is less than or equal to 0.020%, S is less than or equal to 0.010%, al:3.5% -4.5%, V: 0.020-0.060%, ti: 0.005-0.050%, wherein N is less than or equal to 0.005%, and the balance is Fe and unavoidable impurities. The preparation method comprises the following steps: smelting, hot rolling and hood annealing. According to the pickling-free low-density steel provided by the invention, the density of steel iron is reduced by adding light element Al, the adverse effect of austenitic phase region shrinkage caused by adding Al element is reduced by properly increasing Mn element content, and the strong plasticity of the product is improved by refining grains and precipitating and separating out micro-alloy element V, ti. The preparation method obtains a thinner and denser oxide layer by regulating and controlling the hot rolling and annealing processes, avoids environmental pollution caused by treatments such as acid washing, shot blasting and the like, and provides technical support for the development of new generation environment-friendly high-strength automobile steel.

Description

Acid-washing-free low-density steel and preparation method thereof
Technical Field
The invention relates to the technical field of low-density steel, in particular to acid-washing-free low-density steel and a preparation method thereof.
Background
The automobile manufacturing industry is the pillar industry of national economy, and along with the increasing prominence of energy shortage and environmental pollution problems, under the national 'double carbon' target constraint, light weight is the focus of carbon emission reduction attention in the automobile industry. At present, three approaches are mainly used for realizing the weight reduction of the automobile: firstly, lightweight raw materials such as aluminum alloy, magnesium alloy, carbon fiber composite material and the like are adopted. Although the use of the lightweight raw materials can reduce the overall weight of the automobile, the defects of complex forming process, poor welding performance, low impact absorption energy, high price, low yield and the like exist, and the market application of the lightweight raw materials is limited. Secondly, the ultra-high strength steel is used for replacing the traditional steel for automobiles, the thickness of the steel plate is reduced to lighten the weight of automobiles, the forming capability is reduced along with the improvement of the strength of the steel plate, the problems of cracking, wrinkling and overlarge rebound occur, in addition, the rigidity of certain motor vehicle parts is excessively reduced along with the reduction of the thickness of the steel plate, and the acoustic problem of uncomfortable conditions for passengers occurs, so that popularization is limited. Thirdly, develop a steel grade which integrates high strength, high elongation and low density, and the low density steel has good mechanical and physical properties and obvious weight reduction effect, so that the low density steel becomes a hot spot for the research of automobiles and suppliers thereof.
The surface of a hot rolled product by the conventional process is generally provided with a severe red iron scale, so that the processing utilization rate of a component is reduced; in the forming process, the oxide layer is easy to fall off unevenly, and the surface quality of the member after the electrophoretic paint spraying is not up to the requirement. Therefore, the raw materials are required to be subjected to treatments such as acid washing and shot blasting, and the environmental protection problems such as environmental pollution and waste acid treatment caused by subsequent treatments are inevitably caused, the processing cost of users is increased, and development of low-density steel with good performance, low density and environmental protection for realizing the development requirements of 'greenization and light weight' of steel for automobiles is urgently needed. The patent which is similar to the environment-friendly acid-free low-density steel is as follows through the inquiry of related patents:
CN 115216704B discloses a short-flow production method of low-density steel based on continuous thin strip casting, which comprises the following chemical components in percentage by weight: c:0.60 to 1.50 percent, mn: 16.00-25.00%, al: 6.00-12.00%, V:0.01 to 0.20 percent, nb:0.01 to 0.2 percent, and the balance of Fe and unavoidable impurities. Transferring the molten steel that is eligible for smelting to a pair of casting rolls which rotate relatively, the molten steel is cooled and solidified on the casting surfaces of the casting rolls and passes downwards through a nip between the casting rolls to form a thin cast strip; the thin cast strip is hot rolled into a thin steel strip through 1-pass hot rolling, the temperature after rolling is 850-1050 ℃, the hot rolled thin steel strip is cooled to 600-800 ℃, and coiled into a coil of the thin steel strip; the coiled thin strip steel is subjected to solution treatment at 900-1150 ℃ for about 1-3 hours; the thin strip steel subjected to solution treatment is subjected to cold rolling with a rolling reduction of 20-50%, then subjected to aging treatment at 650-850 ℃ for about 10-12 hours, and then subjected to air cooling. The addition of higher C, mn content is not favorable for obtaining better welding performance, but also increases the alloy cost; the double-roller thin strip continuous casting process equipment is special, and is not beneficial to popularization of the technology. In addition, the application does not consider the control method of the scale, and does not play a role in avoiding pickling and reducing damage to the environment.
CN 115491614A discloses an austenitic high manganese steel with a strength-plastic product of more than 60 GPa% and a production method, the chemical components in percentage by weight are: c:0.7 to 1.2 percent, mn:13.0 to 21.0 percent, cr:3.0 to 4.0 percent, al:1.0 to 1.5 percent, si:0.05 to 0.3 percent, cu:0.1 to 0.5 percent, S is less than or equal to 0.015 percent, P is less than or equal to 0.005 percent, and the balance is Fe and unavoidable impurity elements. The thickness of the plate blank after smelting and casting into a blank is controlled to be 50-80 mm, the plate blank is heated, the heating temperature is controlled to be 1120-1200 ℃, and the temperature is kept for 120-180 min; controlling the total rolling reduction to be not lower than 90 percent during hot rolling, the initial rolling temperature to be not lower than 1100 ℃, and the final rolling temperature to be 900-1000 ℃; naturally cooling to room temperature, annealing, controlling the annealing temperature to 650-700 ℃, and preserving heat for 20-60 min at the annealing temperature; and controlling the average grain size to be less than 5 mu m; naturally cooling to room temperature again, cold-rolling to the thickness of the product at room temperature, and controlling the total reduction ratio to be 60-90%; performing reverse phase transformation annealing, heating the cold-rolled sheet to 650-850 ℃, and preserving heat at the temperature for 1.5-5 min; the annealed metallographic structure comprises the following components in percentage by volume: 90-100% of austenite and 10-0% of martensite. Cooling, water cooling to 120-150 deg.c at the cooling speed of 50-80 deg.c/s, and natural cooling to room temperature. The high contents of C, mn and Cr are not favorable for obtaining good welding performance, and meanwhile, the cost is greatly increased; the addition amount of Al element is low (1.0-1.5%) and is difficult to achieve the effect of reducing density, copper brittleness is easily caused by the high Cu content (0.1-0.5%), and in addition, the application does not consider a control method of iron scale, and the effects of avoiding acid washing and reducing environmental damage cannot be achieved.
CN 113025794B discloses a method for improving strength of low-density steel of Fe-Mn-Al-C system, which comprises the following chemical components in percentage by weight: c:0.8% -1.6%, mn:15% -30%, al:5% -10%, cr is less than or equal to 5%, mo is less than or equal to 3%, nb is less than or equal to 0.2%, ti is less than or equal to 0.5%, si is less than or equal to 2%, B is less than or equal to 0.6%, and the balance is Fe and unavoidable impurities. The smelted casting blank is rolled (the rolling reduction is 40% -60%) or is subjected to solution treatment after being forged, the temperature of the solution treatment is 950% -1100 ℃, and the heat preservation time is 1-3 hours. The patent adds more C, mn, cr, mo, si, nb, ti, B elements, which is not beneficial to obtaining good welding performance and greatly increases the cost; the product produced by the patent guidance has extremely high strength, but has low plasticity (A value is 8-27%), which is not beneficial to part forming; furthermore, the application does not consider the control method of the scale, which is disadvantageous in reducing the pickling process and thus reducing the environmental impact.
Disclosure of Invention
In order to solve the problems, according to a first aspect of the invention, there is provided a pickling-free low-density steel comprising the following chemical components in percentage by weight: c:0.08% -0.25%, si: 0.05-0.30%, mn:3.5% -5.0%, P is less than or equal to 0.020%, S is less than or equal to 0.010%, al:3.5% -4.5%, V: 0.020-0.060%, ti: 0.005-0.050%, wherein N is less than or equal to 0.005%, and the balance is Fe and unavoidable impurities.
Further, the chemical components in percentage by weight are: c: 0.10-0.18%, si: 0.10-0.20%, mn: 4.2-4.8%, al: 3.8-4.3%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, V:0.030 to 0.055%, ti: 0.015-0.030%, N is less than or equal to 0.004%, and the balance is Fe and unavoidable impurities.
Further, the yield strength is 440-610 MPa, the tensile strength is 780-865 MPa, and the elongation A 50 The value is 33.0-39.0%; the microstructure consists of 15% -20% of high-temperature ferrite, 25% -30% of critical ferrite, 30% -45% of martensite of the lath and 15% -20% of residual austenite; the thickness of the iron scale is less than or equal to 10 mu m, and the iron scale is formed by 80% -85% of Fe 3 O 4 12-17% of Fe and a small amount of dispersed FeO.
According to a second aspect of the invention, there is provided a method for producing the pickling-free low-density steel according to the first aspect of the invention, comprising:
smelting: the chemical components are as follows by weight percent: c:0.08% -0.25%, si: 0.05-0.30%, mn:3.5% -5.0%, P is less than or equal to 0.020%, S is less than or equal to 0.010%, al:3.5% -4.5%, V: 0.020-0.060%, ti: 0.005-0.050%, wherein N is less than or equal to 0.005%, and the rest elements are Fe and unavoidable impurities, so that smelting is performed; and (3) hot rolling: the method comprises rough rolling and finish rolling, wherein rough rolling is carried out above the recrystallization temperature to obtain an intermediate blank with iron oxide scale removed, finish rolling is carried out below the recrystallization temperature, and the thickness and the components of the iron oxide scale are controlled to obtain a hot rolled coil with the target thickness dimension; the hood annealing is carried out in a reducing atmosphere, so that the reverse transformation from martensite to austenite is realized, and the forming performance of the oxide layer is improved.
Further, the rough rolling heats the plate blank obtained by smelting; pre-cooling the plate blank after the plate blank is discharged from the furnace, adopting a five-pass rolling mode, performing full-length and full-scale removal on the plate blank, wherein the scale removal water pressure is more than or equal to 22MPa, the scale removal temperature is more than or equal to 1175 ℃, the initial pass deformation of rough rolling is more than or equal to 18%, and the deformation of each pass is more than or equal to 15%, and the thickness of the obtained intermediate blank is 34+/-3 mm; the finish rolling adopts a seven-pass speed-increasing rolling mode, the initial rolling temperature is 960-1010 ℃, the first pass speed is more than or equal to 1.6m/s, the thickness of the oxide scale is controlled within 1.5 mu m, the rolling speed of each subsequent pass is gradually increased, the last pass rolling speed is more than or equal to 7.0m/s, and the thickness of the oxide scale is as follows: less than or equal to 2.4 mu m, less than or equal to 3.2 mu m, less than or equal to 4.0 mu m, less than or equal to 4.7 mu m, less than or equal to 5.3 mu m and less than or equal to 5.8 mu m, and the finishing temperature is 850-890 ℃.
Further, the slab obtained by smelting is heated, specifically: and slowly heating the slab to 1250+/-20 ℃, keeping the temperature for 2 hours, discharging from the furnace, and keeping the furnace time for 4-6 hours.
Further, the finish rolling is performed with a final pass reduction of 15% or less and lubrication rolling is performed.
Further, the hot rolling further comprises laminar cooling, after finish rolling, the laminar cooling firstly carries out full-open rapid cooling of the header, then carries out sparse cooling of the header, the vertical purging water is started at the tail end of the laminar cooling, and the thickness of the oxidized iron sheet after the laminar cooling is controlled within 7.5 mu m.
Further, the hot rolling further comprises coiling, wherein the coiling temperature is 500-560 ℃ to obtain Fe 3 O 4 Mainly comprises iron oxide scale, and has a thickness less than or equal to 9 μm.
Further, the cap annealing is: placing the hot rolled coil in a hood-type annealing furnace, and adopting H 2 Reducing, slowly heating to 630-670 ℃ at the speed of 2-5 ℃/min, preserving heat for 8-12 hours, and then cooling to room temperature along with a furnace.
According to the pickling-free low-density steel, the density of steel iron is reduced by adding the light element Al, the content of Mn element is properly increased, the adverse effect of the reduction of an austenite phase region caused by adding the Al element is reduced, and the strong plasticity of a product is improved by refining grains and precipitating and separating out micro alloy elements (V, ti). The preparation method has the advantages that a thinner and denser oxide layer is obtained by regulating and controlling the hot rolling and annealing processes, so that environmental pollution caused by treatments such as acid washing, shot blasting and the like is avoided; reverse transformation from martensite to austenite is performed by using a hood-type annealing furnace to improve the stamping effect. The pickling-free low-density steel has low equipment investment and strong operability, and provides technical support for development of new-generation environment-friendly high-strength automobile steel.
Drawings
Fig. 1 is a metallographic photograph of a pickling-free low-density steel provided in an embodiment of the present application.
Fig. 2 is a scanning electron microscope photograph of a pickling-free low-density steel provided in an embodiment of the present application.
Fig. 3 is an XRD detection result of a pickling-free low-density steel provided in the example of the present application.
Detailed Description
According to a first aspect of the embodiment of the invention, the acid-washing-free low-density steel comprises the following chemical components in percentage by weight: c:0.08% -0.25%, si: 0.05-0.30%, mn:3.5% -5.0%, P is less than or equal to 0.020%, S is less than or equal to 0.010%, al:3.5% -4.5%, V: 0.020-0.060%, ti: 0.005-0.050%, wherein N is less than or equal to 0.005%, and the balance is Fe and unavoidable impurities.
According to the pickling-free low-density steel provided by the embodiment of the invention, the density of steel iron is reduced by adding light element Al, the content of Mn element is properly increased, the adverse effect of austenite phase region shrinkage caused by adding Al element is reduced, and the strong plasticity of the product is improved by refining crystal grains and precipitating and separating out micro alloy element (V, ti).
Further, the chemical components in percentage by weight are: c: 0.10-0.18%, si: 0.10-0.20%, mn: 4.2-4.8%, al: 3.8-4.3%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, V:0.030 to 0.055%, ti: 0.015-0.030%, N is less than or equal to 0.004%, and the balance is Fe and unavoidable impurities.
Specifically, it is preferable that the contents of C, mn, and Al satisfy the above conditions in order to ensure the content controlling strength of austenite, martensite, and the like, because Al is a ferrite forming element that greatly reduces the phase region of austenite, and C, mn is an austenite forming element that enlarges the austenite phase region; the V, ti content satisfies the above conditions, and serves to ensure the content of the second phase, thereby achieving the effects of precipitation and grain refinement.
The acid-washing-free low-density steel has the yield strength of 440-610 MPa, the tensile strength of 780-865 MPa and the elongation A 50 The value is 33.0-39.0%; the microstructure consists of 15% -20% of high-temperature ferrite, 25% -30% of critical ferrite, 30% -45% of martensite of the lath and 15% -20% of residual austenite; the thickness of the iron scale is less than or equal to 10 mu m, and the iron scale is formed by 80% -85% of Fe 3 O 4 12-17% of Fe and a small amount of dispersed FeO.
Specifically, the effect of alloy elements in pickling-free low-density steel:
carbon: c is an important austenite element in steel, can stabilize an austenite structure, improve stacking fault energy and induce dynamic strain aging, and plays a role in balancing volume fractions of ferrite and austenite phases in a high-Al-content low-density steel (Fe-Mn-Al-C) system alloy. Through heat treatment process control, the austenite phase of C in steel is enriched to form metastable austenite phase, so that the formability and elongation of the steel plate are improved, and when the content of C is too low, the austenite structure in the steel is unstable, and the strength and toughness of the low-density steel are reduced. Meanwhile, C can react with micro-alloy elements in the steel to generate nano-scale carbide, and can react with Mn and Al elements to generate kappa-carbide ((Fe, mn) 3 AlC), and the two are combined to produce precipitation strengthening, so that the strength of the steel is improved. When the C content is too high, the formation of coarse kappa-carbide in austenite grain boundaries is promoted, and the elongation of the low-density steel is deteriorated, so that the C content of the invention is 0.08% -0.25%, preferably 0.10% -0.18%.
Silicon: si is often used as a deoxidizer in steel, and Si reduces the specific gravity of steel and increases the strength and stacking fault energy of steel similar to Al, but reduces the dynamic strain aging of steel and deteriorates weldability. Si can be dissolved in ferrite and austenite in a solid way to improve the strength of the steel, and the effect of the Si is inferior to C, P and is stronger than elements such as Mn, cr, ti, ni and the like; si can also inhibit the precipitation of carbide in ferrite and fully enrich solid solution C atoms in austenite, so that the residual austenite is difficult to obtain at room temperature due to the excessively low Si content of the solid solution C atoms. However, when the Si content is too high, the surface scale formed by Si in the heating furnace is difficult to remove, the difficulty of descaling is increased, and the surface is poor. Therefore, the Si content of the present invention is 0.05 to 0.30%, preferably 0.10 to 0.20%.
Manganese: the Mn element plays a role in solid solution strengthening and stabilizing austenite in steel. On one hand, mn element improves the strength of the steel plate through the solid solution strengthening effect, and on the other hand, the Mn element promotes the steel plate to form a residual austenite phase through the effect of stabilizing austenite, so that the effect of improving the plasticity of the steel plate is achieved. In addition, the Mn element can improve the stacking fault energy of the steel, inhibit the martensitic transformation, enable the steel to generate dense twin crystals in the deformation process, effectively improve the elongation of the steel, easily cause low austenite content due to low Mn content and have poor stability, and are not beneficial to obtaining good plasticity; when the Mn content is too high, a banded structure is generated in the steel, and the toughness of the steel plate is reduced; meanwhile, austenite is too stable and is difficult to be transformed into martensite in the deformation process, so that the formability of the steel sheet is lowered. Therefore, in the present invention, the Mn content is 3.5% to 5.0%, preferably 4.2 to 4.8%.
Aluminum: al density of 2.7g/cm 3 Far lower than the density of Fe (7.85 g/cm) 3 ) The material density can be significantly reduced. The certain Al content can also obviously improve the heat deformation resistance of the steel, improve the corrosion resistance of the steel and delay dynamic cracking, and the Al can also obviously improve the fault energy of the steel, change the deformation mechanism and have certain buffer effect when the medium manganese steel containing the Al is subjected to violent collision. Al is a strong ferrite element, and an excessively high Al content tends to promote ferrite phase formation and reduce austenite phase content, and in addition, al and C, mn element act together to form kappa-carbide ((Fe, mn) 3 AlC) to produce precipitation strengthening to increase the strength of the steel, but to significantly reduce the plasticity of the density steel. The too low Al content cannot reduce the density of the material, but the too high Al content is unfavorable for obtaining good plasticity, so that the Al content is 3.5% -4.5%, preferably 3.8% -4.3%.
Vanadium: v acts to refine grains and precipitation strengthening by forming carbonitrides in the steel. The high V (C, N) solubility in austenite allows for the use of lower reheat temperatures, which means lower production costs. The deformation mechanism of the steel is mainly dislocation slip, and the addition of V can separate out fine precipitated phases in the steel, so that on one hand, the nucleation rate can be improved, and the growth of crystal grains is prevented to refine the crystal grains; on the other hand, dislocation movement can be blocked to improve the strength, so that the good comprehensive mechanical property is finally obtained. Therefore, the V content is set in a range of 0.020 to 0.060% by mass, preferably 0.030 to 0.055% by mass.
Titanium: the Ti element is very active and is easy to combine with O, and a part of Ti can play a role of a deoxidizer in the smelting process; ti forms TiN at high temperature to inhibit the formation of AlN inclusion in steel, and TiC precipitation at medium temperature effectively inhibits k carbide precipitation, so that the ductility and yield strength of steel are improved. However, if a large amount of Ti is added, the continuous casting nozzle may be plugged or inclusion defects may occur due to excessive TiN crystallization. Meanwhile, the too high Ti content can obviously reduce the diffusion speed of C in austenite, reduce the C content in the austenite, reduce the stability of a matrix and reduce the plasticity; therefore, the Ti content is set to be in the range of 0.005 to 0.050% by mass, preferably 0.015 to 0.030% by mass.
Phosphorus: p in steel is generally solid-dissolved in ferrite and has a strong solid-solution strengthening effect, but during the solidification of a slab, phosphorus (P) segregates along columnar grain boundaries or equiaxed grain boundaries to make the slab brittle at high temperature and room temperature and may cause cracks in the slab. In addition, after working, P increases the ductility-brittle transition temperature of the steel and makes the steel susceptible to hydrogen embrittlement. Therefore, the P content is set in a range of 0.020% by mass or less, preferably 0.015% by mass or less.
Sulfur: s is an impurity element in steel, is easy to generate segregation at a grain boundary, forms FeS with low melting point with Fe in the steel, and reduces the toughness of the steel; in addition, S also forms inclusions such as MnS, thereby causing cracking when the steel is hot-rolled or cold-rolled. Therefore, the S content is set in the range of 0.010% by mass or less, preferably 0.005% by mass or less.
Nitrogen: n is an impurity element in steel, N easily forms TiN, alN and NbN with Ti, al, nb in steel during a solidification stage, during a hot rolling stage, and during an annealing stage after cold rolling, tiN and AlN existing in molten steel cause a risk of inclusion defects, and a large amount of TiN and AlN can promote crack formation in cast slabs. The liquid-out TiN may cause significant deterioration in the ductility and toughness of steel. Therefore, the S content is set in the range of 0.005% by mass or less, preferably 0.004% by mass or less.
According to a second aspect of the embodiment of the invention, the preparation method of the pickling-free low-density steel comprises the following steps:
smelting: smelting according to the chemical components; and (3) hot rolling: the method comprises rough rolling and finish rolling, wherein rough rolling is carried out above the recrystallization temperature to obtain an intermediate blank with iron oxide scale removed, finish rolling is carried out below the recrystallization temperature, and the thickness and the components of the iron oxide scale are controlled to obtain a hot rolled coil with the target thickness dimension; the hood annealing is carried out in a reducing atmosphere, so that the reverse transformation from martensite to austenite is realized, and the forming performance of the oxide layer is improved.
Specifically, after smelting, a slab with the chemical component content is obtained through casting; rough rolling is carried out on the slab above the recrystallization temperature, the thickness of the slab is reduced, oxide scales are removed to obtain an intermediate slab, then finish rolling is carried out below the recrystallization temperature, the thickness and the components of the oxide scales are controlled, and a hot rolled slab coil with the target thickness size is obtained; and finally, performing reverse phase transformation from martensite to austenite by using a hood-type annealing furnace to improve the stamping effect. In a word, through regulating and controlling the hot rolling and the covering and annealing processes, a thinner and denser oxide layer is obtained, environmental pollution caused by treatments such as acid washing, shot blasting and the like is avoided, good strong plasticity and surface quality are obtained, equipment investment is low, and operability is strong.
The rough rolling is carried out, and a plate blank obtained through smelting is heated; pre-cooling the plate blank after the plate blank is discharged from the furnace, adopting a five-pass rolling mode, performing full-length and full-scale removal on the plate blank, wherein the scale removal water pressure is more than or equal to 22MPa, the scale removal temperature is more than or equal to 1175 ℃, the initial pass deformation of rough rolling is more than or equal to 18%, and the deformation of each pass is more than or equal to 15%, and the thickness of the obtained intermediate blank is 34+/-3 mm; the finish rolling adopts a seven-pass speed-increasing rolling mode, the initial rolling temperature is 960-1010 ℃, the first pass speed is more than or equal to 1.6m/s, the thickness of the oxide scale is controlled within 1.5 mu m, the rolling speed of each subsequent pass is gradually increased, the last pass rolling speed is more than or equal to 7.0m/s, and the thickness of the oxide scale is as follows: less than or equal to 2.4 mu m, less than or equal to 3.2 mu m, less than or equal to 4.0 mu m, less than or equal to 4.7 mu m, less than or equal to 5.3 mu m and less than or equal to 5.8 mu m, and the finishing temperature is 850-890 ℃.
Specifically, the thickness of the slab is favorably and quickly compressed through large deformation of rough rolling, meanwhile, the large descaling water can ensure that oxide scales in the heating process are completely removed, the full length of the rolling line is completely descaled, the descaling water pressure is more than or equal to 22MPa, the descaling temperature of the steel billet is more than or equal to 1175 ℃, and the hard and brittle low-melting-point FeO and Fe formed in the heating process are completely removed 2 SiO 4 Finally obtaining an intermediate blank with the thickness of 34+/-3 mm; finish rolling is to process the slab to a desired thickness specification and accumulate large deformationsThe amount refines the grains during cooling.
The iron scale formed during rough rolling is mainly FeO and has a small amount of Fe 3 O 4 And trace Fe 2 O 3 A thinner scale thickness is obtained. And the finish rolling adopts a seven-pass speed-increasing mode to roll, so that the high-temperature zone time of the strip steel is reduced, and the thickness of an oxide layer is reduced.
The slab obtained by smelting is heated, and the slab is specifically: and slowly heating the slab to 1250+/-20 ℃, keeping the temperature for 2 hours, discharging from the furnace, and keeping the furnace time for 4-6 hours.
Specifically, when the furnace time exceeds 6 hours, the furnace needs to be returned, and the furnace needs to be returned for reheating and judging to be of other steel types, so that the defect of incomplete descaling due to excessive thick rough rolling of the iron scale is avoided.
And the finish rolling and final pass reduction rate is less than or equal to 15%, and lubrication rolling is adopted.
Specifically, the reduction rate of the last pass is less than or equal to 15 percent, and lubrication rolling is adopted, so that III times of oxide scales formed in the finish rolling process are prevented from being broken in the last pass.
The hot rolling further comprises laminar cooling, wherein after finish rolling, the laminar cooling firstly carries out full-open rapid cooling of the header, then carries out sparse cooling of the header, the tail end of the laminar cooling is opened with vertical purging water, and the thickness of the oxidized iron sheet after laminar cooling is controlled within 7.5 mu m.
Specifically, after finish rolling of the steel strip, the steel strip is firstly subjected to quick cooling by fully opening a laminar flow header, so that the temperature is quickly reduced, the thickness of the iron oxide scale is prevented from being thickened due to overlong high temperature, then the header adopts a sparse cooling mode (the effect of cooling uniformity is achieved by opening intervals) which is used for enabling the thickness of the iron oxide scale to be uniform, the tail end of the layer cooling is opened to vertically blow water, poor plate shape and the thickness of the iron oxide scale caused by accumulated water are avoided, the thickness of the iron oxide scale after the layer cooling is controlled within 7.5 mu m, and the iron oxide scale is formed to be about 35% FeO+about 65% Fe 3 O 4 +small amount of Fe 2 O 3 。
The hot rolling further comprises coiling, wherein the coiling temperature is 500-560 ℃ to obtain Fe 3 O 4 Mainly comprises iron oxide scale, and has a thickness less than or equal to 9 μm.
Specifically, the coiling temperature is 500-560 ℃ to obtainBy Fe 3 O 4 Mainly comprises iron scales, has the thickness less than or equal to 9 mu m, and avoids the adverse effect on plasticity caused by the generation of coarse kappa-carbide.
The hood-type annealing is as follows: placing the hot rolled coil in a hood-type annealing furnace, and adopting H 2 Reducing, slowly heating to 630-670 ℃ at the speed of 2-5 ℃/min, preserving heat for 8-12 hours, and then cooling to room temperature along with a furnace.
Specifically, by hood annealing, on the one hand, the reverse transformation of martensite into austenite is achieved, and on the other hand, fe in an anoxic environment 2+ Ion continuously diffuses from the steel matrix into the oxide layer, fe 2 O 3 Conversion to Fe 3 O 4 Finally, fe with the thickness less than or equal to 10 mu m and the microstructure of 80% -85% is formed 3 O 4 +12-17% Fe (including alpha-Fe and delta-Fe) +a small amount of iron scale with FeO dispersed therein, and improving the forming performance of the oxide layer.
Example 1 acid-washing-free low-density steel and preparation method thereof
Acid-washing-free low-density steel: the example provides three groups of environment-friendly acid-free low-density high-strength steel, and the chemical composition of the three groups of environment-friendly acid-free low-density high-strength steel is shown in table 1.
Table 1 environmental protection type pickling free low density high strength steel chemical composition (wt.%)
Numbering device C Si Mn P S V Ti Al
1 0.155 0.182 4.52 0.010 0.008 0.040 0.025 4.10
2 0.163 0.154 4.35 0.009 0.006 0.035 0.028 3.98
3 0.145 0.165 4.65 0.008 0.005 0.050 0.015 4.25
The preparation method of the environment-friendly pickling-free low-density high-strength steel comprises the following specific processes:
A. smelting: through a smelting process, an environment-friendly pickling-free low-density high-strength steel slab shown in table 1 is prepared.
B. Hot rolling: slowly heating the slab to 1250+/-20 ℃ and preserving heat for 2 hours, discharging the slab, and pouring soft blowing water for precooling iron sheets in the furnace for 5 hours (a group of water nozzles are used for providing water with the pressure of 10MPa before rough rolling to cool oxidized iron sheets, so that the scale removal effect of scale removal water is improved by shrinkage of the iron sheets), wherein the whole length of a rolling line is fully subjected to scale removal, and the scale removal water pressure is more than or equal to 22MPa; the descaling temperature of the billet is more than or equal to 1175 ℃, the deformation of the first pass of rough rolling is more than or equal to 18%, and the deformation of each subsequent pass is more than or equal to 15%; the initial rolling temperature of finish rolling is 960-1010 ℃, the finish rolling is performed in a speed-up mode, the finish rolling is performed in a seven-pass rolling mode, the first pass speed is more than or equal to 1.6m/s, the thickness of oxide scales is controlled within 1.5 mu m, the rolling speed of each pass is gradually increased to the last pass rolling mill outlet speed to be more than or equal to 7.0m/s, and the thickness of the oxide scales is as follows: iron scale formed during rough rolling is mainly FeO and has a small amount of Fe, wherein the iron scale is less than or equal to 2.4 mu m, less than or equal to 3.2 mu m, less than or equal to 4.0 mu m, less than or equal to 4.7 mu m, less than or equal to 5.3 mu m and less than or equal to 5.8 mu m 3 O 4 And trace Fe 2 O 3 Obtaining a thinner oxide scale thickness, wherein the finishing rolling temperature is 850-890 ℃; the reduction rate of the last pass is less than or equal to 15 percent, and lubrication rolling is adopted, so that the III-time oxide scale formed in the finish rolling process is prevented from being broken in the last pass, the defect that excessive cracks are formed in the oxide layer due to excessive stress in the oxide layer during cooling is avoided, and the forming performance of the oxide layer is influenced. The rapid cooling header of the laminar flow after the steel strip rolling is totally rapid, so that the temperature is rapidly reduced, the thickness thickening of the iron oxide scale caused by overlong high temperature is avoided, the thickness of the iron oxide scale is uniform by adopting a sparse cooling mode of separating the headers one by one, the vertical purging water is started at the tail end of the layer cooling, the poor plate shape and the thickening of the iron oxide scale caused by water accumulation are avoided, the thickness of the iron oxide scale after the layer cooling is controlled within 7.5 mu m, and the iron oxide scale is formed to 35% FeO+65% Fe 3 O 4 +small amount of Fe 2 O 3 The method comprises the steps of carrying out a first treatment on the surface of the The coiling temperature is 500-560 ℃ to obtain Fe 3 O 4 Mainly comprises iron oxide scale with the thickness less than or equal to 9 μm, the same asThe generation of coarse kappa-carbides is avoided from adversely affecting the plasticity. The specific hot rolling process parameters are shown in table 2.
TABLE 2 major process parameters for hot rolling of environmentally friendly pickling-free low-density high-strength steel
C. Hood annealing: hot rolled coils in a coil-tightening manner are placed in a hood-type annealing furnace (H is adopted 2 Reduction), slowly heating the strip steel to 630-670 ℃ at a speed of 2-5 ℃/min, preserving heat for 8-12 hours, and cooling to room temperature along with a furnace, so that on one hand, the reverse transformation from martensite to austenite is realized, and on the other hand, fe is in an anoxic environment 2+ Ion continuously diffuses from the steel matrix into the oxide layer, fe 2 O 3 Conversion to Fe 3 O 4 Finally, fe with the thickness less than or equal to 10 mu m and the microstructure of 80% -85% is formed 3 O 4 The parameters of +12-17%Fe (including alpha-Fe and delta-Fe) +a small amount of iron scale of FeO dispersion, and the forming performance of the improved oxide layer are shown in Table 3.
Table 3 main technological parameters of environment-friendly pickling-free low-density high-strength steel
The microstructure of the environment-friendly pickling-free low-density high-strength steel of case 1 prepared by the process is shown in figure 1, and the performance of the environment-friendly pickling-free low-density high-strength steel is tested according to GB/T228-2010 Metal material room temperature tensile test method, and the mechanical properties are shown in the following table 4.
Table 4 mechanical properties of environmentally friendly pickling-free low-density high-strength steel
The characterization of the pickling-free low-density steel of example No. 1 is shown in fig. 1-3, and the metallographic photograph of fig. 1 can grasp the structure ratio as a whole although the magnification is low. Fig. 2 is an enlarged view of a portion of fig. 1, showing morphology, size and size of each phase. From the XRD pattern of FIG. 3, the content of retained austenite was measured.
It will be readily appreciated by those skilled in the art that the above advantageous ways can be freely combined and superimposed without conflict. The foregoing description of the preferred embodiment of the present invention is not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. The foregoing is merely a preferred embodiment of the present application and it should be noted that it will be apparent to those skilled in the art that several modifications and variations can be made without departing from the technical principles of the present application, and these modifications and variations should also be regarded as the scope of the present application.

Claims (8)

1. The pickling-free low-density steel is characterized by comprising the following chemical components in percentage by weight: c:0.08% -0.25%, si: 0.05-0.30%, mn:3.5% -5.0%, P is less than or equal to 0.020%, S is less than or equal to 0.010%, al:3.5% -4.5%, V: 0.020-0.060%, ti: 0.005-0.050%, wherein N is less than or equal to 0.005%, and the rest elements are Fe and unavoidable impurities; the thickness of the iron scale is less than or equal to 10 mu m, and the iron scale is formed by 80% -85% of Fe 3 O 4 12-17% of Fe and a small amount of dispersed FeO.
2. The acid wash free low density steel of claim 1, wherein the chemical composition comprises, by weight: c: 0.10-0.18%, si: 0.10-0.20%, mn: 4.2-4.8%, al: 3.8-4.3%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, V:0.030 to 0.055%, ti: 0.015-0.030%, N is less than or equal to 0.004%, and the balance is Fe and unavoidable impurities.
3. The acid pickling-free low-density steel according to claim 1, wherein the yield strength is 440-610 mpa, the tensile strength is 780-865 mpa, and the elongation a is 50 The value is 33.0-39.0%; the microstructure is composed of 15% -20% of high-temperature ferrite, 25% -30% of critical ferrite, 30% -45% of martensite of the lath and 15% -20% of residual austenite.
4. A method for producing the pickling-free low-density steel as claimed in any one of claims 1 to 3, comprising:
smelting: the chemical components are as follows by weight percent: c:0.08% -0.25%, si: 0.05-0.30%, mn:3.5% -5.0%, P is less than or equal to 0.020%, S is less than or equal to 0.010%, al:3.5% -4.5%, V: 0.020-0.060%, ti: 0.005-0.050%, wherein N is less than or equal to 0.005%, and the rest elements are Fe and unavoidable impurities, so that smelting is performed;
and (3) hot rolling: the method comprises rough rolling and finish rolling, wherein rough rolling is carried out above the recrystallization temperature to obtain an intermediate blank with iron oxide scale removed, finish rolling is carried out below the recrystallization temperature, and the thickness and the components of the iron oxide scale are controlled to obtain a hot rolled coil with the target thickness dimension; the finish rolling adopts a seven-pass speed-increasing rolling mode, the initial rolling temperature is 960-1010 ℃, the first pass speed is more than or equal to 1.6m/s, the thickness of the oxide scale is controlled within 1.5 mu m, the rolling speed of each subsequent pass is gradually increased, the last pass rolling speed is more than or equal to 7.0m/s, and the thickness of the oxide scale is as follows: less than or equal to 2.4 mu m, less than or equal to 3.2 mu m, less than or equal to 4.0 mu m, less than or equal to 4.7 mu m, less than or equal to 5.3 mu m and less than or equal to 5.8 mu m, and the finishing temperature is 850-890 ℃;
the hot rolling further comprises laminar cooling, wherein after finish rolling, the laminar cooling firstly carries out full-open rapid cooling of the header, then carries out sparse cooling of the header, the tail end of the laminar cooling is opened with vertical purging water, and the thickness of the oxidized iron sheet after the laminar cooling is controlled within 7.5 mu m;
the hot rolling further comprises coiling, wherein the coiling temperature is 500-560 ℃ to obtain Fe 3 O 4 Mainly comprises iron oxide scale with the thickness less than or equal to 9 mu m;
hood annealing: under a reducing atmosphere.
5. The method for producing a pickling-free low-density steel as claimed in claim 4, wherein,
the rough rolling is carried out, and a plate blank obtained through smelting is heated; pre-cooling the plate blank after the plate blank is discharged from the furnace, adopting a five-pass rolling mode, performing full-length and full-scale removal on the plate blank, wherein the scale removal water pressure is more than or equal to 22MPa, the scale removal temperature is more than or equal to 1175 ℃, the initial pass deformation of rough rolling is more than or equal to 18%, and the deformation of each pass is more than or equal to 15%, and the thickness of the obtained intermediate blank is 34+/-3 mm.
6. The method for producing acid-free low-density steel according to claim 5, wherein the slab obtained by smelting is heated, specifically: and slowly heating the slab to 1250+/-20 ℃, keeping the temperature for 2 hours, discharging from the furnace, and keeping the furnace time for 4-6 hours.
7. The method for producing a pickling-free low-density steel according to claim 4, wherein the finish rolling is performed with a final pass reduction of 15% or less and lubrication rolling is performed.
8. The method of producing a pickling-free low-density steel according to claim 4, wherein the cap annealing is: placing the hot rolled coil in a hood-type annealing furnace, and adopting H 2 Reducing, slowly heating to 630-670 ℃ at the speed of 2-5 ℃/min, preserving heat for 8-12 hours, and then cooling to room temperature along with a furnace.
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