CN112522596B - High-strength high-hole-expansion steel based on scrap steel and production method thereof - Google Patents

High-strength high-hole-expansion steel based on scrap steel and production method thereof Download PDF

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CN112522596B
CN112522596B CN201910889397.7A CN201910889397A CN112522596B CN 112522596 B CN112522596 B CN 112522596B CN 201910889397 A CN201910889397 A CN 201910889397A CN 112522596 B CN112522596 B CN 112522596B
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steel
strip
strength
scrap
casting
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CN112522596A (en
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吴建春
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Baoshan Iron and Steel Co Ltd
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Priority to DE112020004399.1T priority patent/DE112020004399T5/en
Priority to PCT/CN2020/115964 priority patent/WO2021052434A1/en
Priority to US17/761,719 priority patent/US20220389534A1/en
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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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0622—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two casting wheels
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0637—Accessories therefor
    • B22D11/064—Accessories therefor for supplying molten metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0637—Accessories therefor
    • B22D11/0648—Casting surfaces
    • B22D11/0651—Casting wheels
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/12—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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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

Abstract

A high-strength and high-hole-expansion steel based on scrap steel and a production method thereof are disclosed, wherein the steel comprises the following chemical components in percentage by weight: 0.01 to 0.05 percent of C, 0.2 to 0.6 percent of Si, 0.8 to 1.5 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.005 percent of S, less than or equal to 0.008 percent of N, als<0.001%, less than or equal to 0.0050% of Ca, 0.01-0.08% of Nb, and total oxygen [ O ]] T : 0.007-0.02% of Cu, 0.1-0.6% of Sn, 0.005-0.04% of one or two elements, and the balance of Fe and inevitable impurities. The method utilizes residual Sn and Cu elements in the scrap steel to carry out smelting, and selectively adds microalloy elements such as Nb and the like; controlling the alkalinity of slag, the type and melting point of inclusions in steel, the free oxygen content in molten steel and the content of acid-soluble aluminum Als; the casting strip is cast by twin-roll thin strip continuous casting, the casting strip directly enters a lower closed chamber with non-oxidizing atmosphere and enters an on-line rolling mill for hot rolling under the closed condition, the cooling after rolling adopts a gas atomization cooling mode, and finally the casting strip is coiled.

Description

High-strength high-hole-expansion steel based on scrap steel and production method thereof
Technical Field
The invention relates to a manufacturing technology of high-strength and high-hole-expansion steel, in particular to high-strength and high-hole-expansion steel based on scrap steel and a production method thereof.
Background
In the traditional steel production flow, tin (Sn) and copper (Cu) are typical residual elements or harmful elements in steel, the Sn and the Cu are difficult and expensive to remove fully in the steel making process, once the steel contains the Sn and the Cu, the Sn and the Cu cannot be completely eliminated basically, and the contents of the Sn and the Cu can be reduced only by diluting molten steel, which causes the increase of the smelting cost of steel products.
In recent years, as scrap steel is continuously recycled, scrap steel resources are more and more, electricity prices are continuously reduced, domestic scrap-based short-flow electric furnace steel making is increasingly started, so that the content of residual elements such as Sn and Cu in steel is gradually increased, sn and Cu in steel are easy to segregate and are easy to concentrate in grain boundaries to cause defects such as cracks, and the like, so that the content of Sn and Cu elements in the traditional process is strictly controlled, and in common structural steel, the content of Sn and Cu has clear requirements: sn (wt%) is less than or equal to 0.005%; cu (wt%) is less than or equal to 0.2%.
Therefore, if the residual elements such as Sn, cu and the like in steel (particularly scrap steel) can be reasonably utilized, the 'harm into benefit' can generate positive influence on the whole metallurgical industry; the method can realize the effective utilization of the existing steel scrap or low-quality inferior ore resources (high tin ore and high copper ore), promote the recycling of steel, reduce the production cost and realize the sustainable development of the steel industry.
The traditional thin strip steel is produced by casting blanks with the thickness of 70-200mm through multi-pass continuous rolling, and the traditional hot rolling process flow is as follows: continuous casting, casting blank reheating and heat preservation, rough rolling, finish rolling, cooling and coiling, namely firstly, a casting blank with the thickness of about 200mm is obtained through continuous casting, the casting blank is reheated and heat preserved, then rough rolling and finish rolling are carried out to obtain a steel strip with the thickness generally larger than 2mm, and finally laminar cooling and coiling are carried out to the steel strip to complete the whole hot rolling production process. The difficulty is relatively great if steel strips with a thickness of less than 1.5mm inclusive are to be produced, which is usually done by subsequent cold rolling and annealing of the hot rolled strip. And the process flow is long, the energy consumption is high, the number of unit equipment is large, the capital construction cost is high, and the production cost is high.
The process flow of the thin slab continuous casting and rolling comprises the following steps: continuous casting, heat preservation and soaking of a casting blank, hot continuous rolling, cooling and coiling. The main differences between the process and the traditional process are as follows: the thickness of a casting blank in the thin slab process is greatly reduced to 50-90mm, and the casting blank is thin, so that the casting blank can be reduced to the required specification before finish rolling only by carrying out 1-2 times of rough rolling (when the thickness of the casting blank is 70-90 mm) or not carrying out the rough rolling (when the thickness of the casting blank is 50 mm) on the casting blank in the traditional process through repeated multi-pass rolling; and the casting blank of the thin slab process directly enters a soaking pit furnace for soaking and heat preservation without cooling or supplementing a small amount of heat, so that the thin slab process greatly shortens the process flow, reduces the energy consumption and the investment, thereby reducing the production cost. However, the strength of steel is improved and the yield ratio is improved due to the faster cooling speed of the thin slab continuous casting and rolling, so that the rolling load is increased, and the thickness specification of a hot rolled product which can be economically produced is unlikely to be too thin, generally more than or equal to 1.5mm, see Chinese patents CN200610123458.1, CN200610035800.2 and CN200710031548.2, and the patents do not relate to Sn and Cu.
The ESP realizes the continuous casting of the plate blank, eliminates a heating furnace with the functions of flame cutting, heat preservation, soaking and plate blank transition, greatly shortens the length of the whole production line to about 190 meters, ensures that the plate blank continuously cast by a continuous casting machine has the thickness of 90-110mm and the width of 1100-1600mm, ensures that the continuously cast plate blank plays the role of heat preservation, soaking and heating through a section of induction heating roller table, and then sequentially enters the procedures of rough rolling, finish rolling, layer cooling and coiling to obtain the hot rolled plate. At present, the process is rapidly developed and popularized, and a plurality of ESP production lines are operated and produced in the world at present.
The process flow shorter than the thin slab continuous casting and rolling is a thin strip continuous casting and rolling process, the thin strip continuous casting technology is a leading-edge technology in the fields of metallurgy and material research, the emergence of the technology brings a revolution to the steel industry, the technology changes the production process of steel strips in the traditional metallurgy industry, integrates continuous casting, rolling, even heat treatment and the like into a whole, and the produced thin slab is subjected to online hot rolling once to form a thin steel strip, so that the production process is greatly simplified, the production period is shortened, and the length of the process line is only about 50 m; the equipment investment is correspondingly reduced, the product cost is obviously reduced, and the method is a low-carbon and environment-friendly hot-rolled thin strip production process. The twin-roll thin strip continuous casting process is a main form of the thin strip continuous casting process and is the only thin strip continuous casting process for realizing industrialization in the world.
The typical process flow of twin roll strip casting is shown in fig. 1, molten steel in a ladle 1 is directly poured into a molten pool 7 surrounded by two oppositely rotating and rapidly cooled crystallizing rolls 8a and 8b and side sealing devices 6a and 6b through a ladle long nozzle 2, a tundish 3, a submerged nozzle 4 and a distributor 5, the molten steel is solidified to form a solidified shell on the circumferential surfaces of the rotating crystallizing rolls 8a and 8b and gradually grows, a cast strip 11 with the thickness of 1-5mm is formed at the minimum gap (nip point) between the two crystallizing rolls, the steel strip is guided to a pinch roll 12 through a guide plate 9 and is fed into a rolling mill 13 to be rolled into a thin strip with the thickness of 0.7-2.5mm, and then the thin strip is cooled through a cooling device 14 and is cut by a flying shear device 16 and is finally fed into a coiler 19 to be coiled.
The high hole expanding steel is an important steel type of advanced high-strength steel (AHSS), has higher strength and elongation, excellent formability and flanging property, can meet the requirements of automobile parts with complex shapes and high forming property requirements, such as automobile chassis rear axle suspension swing arms, and can also be used for other parts needing flange flanging. The flanging capability is expressed by the hole expansion rate, the hole expansion performance is taken as a forming performance index of steel, and the capacity of resisting local cracking of the material in the direction vertical to the hole edge due to overlarge local elongation deformation of the hole edge in the hole expansion process is reflected.
Along with the increasing requirements of automobile design on chassis structures, the forming of parts is more complex, the requirements on flanging and hole expanding performance of steel plates are further improved, the strength and rigidity of the parts can be improved through the flanging and local hole expanding shape design of the automobile parts, the aims of thinning and lightening the automobile steel plates can be achieved, the forming requirements of automobile chassis and cantilever parts can be easily met by the traditional carbon-manganese solid solution strengthened steel and low alloy precipitation strengthened steel structure steel plates, for example, the hole expanding rate of the traditional 440MPa steel plate carbon-manganese solid solution strengthened steel and low alloy precipitation strengthened steel is only 50-70%, and therefore high hole expanding steel is produced. In the 90 s of the 20 th century, hot rolled steel plates with high hole expansion performance of 440-780 MPa level are successively developed in America, japan and the like, the hole expansion rate is 70-131%, the hot rolled steel plates are mainly applied to parts such as automobile chassis, wheels and the like with good formability, particularly flanging performance requirements, the hole expansion performance of the steel plates is related to components, strength and structural uniformity of the steel plates, and ferrite/bainite dual-phase structures can be obtained under the condition of low cooling speed due to the fact that the hot rolled steel plates contain more expensive alloy elements such as Cr, nb, ti, V, mo and the like, but the cost is high.
In the past, two options are generally available for meeting the use conditions of the steel plate on the car chassis, one is to use the steel plate with reduced strength (less than or equal to 300 MPa) to obtain higher hole expansion performance; and the other method is to reduce the flanging amount in the part design so as to reduce the requirement on the steel plate hole expanding performance. With the continuous improvement of the strength of the automobile steel, the hole expansion rate of the traditional automobile steel is reduced, and the requirement of the automobile chassis on the hole expansion rate of the steel plate is difficult to meet. With the increasing requirements of automobile design on chassis structures, the shapes of parts are increasingly complex, the strength requirements are continuously improved, the hole expanding rate of steel plates is increased, and high hole expanding steel becomes an important automobile steel variety.
The strength grades of the high-hole-expansion steel which are most commonly used at present are mainly concentrated on the grades of 440 and 590MPa, and the microstructure of the high-hole-expansion steel is mainly ferrite and bainite and sometimes contains a small amount of martensite. The hole expansion performance of steel sheets is related to a number of factors, which mainly include: inclusion level, performance difference of each phase in the structure, structure uniformity, yield ratio, structure type and the like. From the structure type, the ferrite type and bainite type structures have relatively high hole expansion performance, but the strength is relatively low, and the level of 780MPa and above is difficult to reach, which is also the main reason that the high hole expansion steel is mainly concentrated on two strength levels of 440MPa and 590MPa at present. High-bore-expansion steel has become one of the important varieties of automobile steel plates.
Compared with the traditional hot rolling process, the thin strip continuous casting easily generates bainite type microstructures in the cooling process after rolling due to the natural technological process advantages of the thin strip continuous casting, and the produced product has excellent reaming performance. Therefore, the thin-strip continuous casting method for producing the high-hole-expansion steel has natural advantages.
The high-reaming steel is produced by adopting thin-strip continuous casting, and is mainly aimed at the hot-rolled thin-specification automobile steel market with the thickness of less than 1.8mm (inclusive). Due to the thin thickness, the strip continuous casting process has strong manufacturing and cost advantages. The product specification characteristic thicknesses of the high-reaming-hole steel strip directly supplied in a hot rolling/pickling state are 1.2, 1.25, 1.4, 1.5, 1.6, 1.7mm, 1.8mm and the like, and due to the fact that the product thickness is thin, the traditional thin-gauge high-reaming-hole steel cannot be supplied in full specifications due to the capacity limitation of a traditional hot continuous rolling line of many manufacturers; or the production process of firstly adopting the hot continuous rolling process and then carrying out cold rolling increases the production cost of the thin gauge high reaming steel.
When the hot-rolled strip steel is used as a thin hot-rolled product, the surface quality requirement of the strip steel is not the highest. It is generally required that the thinner the scale on the surface of the strip is, the better the scale is, and it is required to control the scale formation in the subsequent stages of the strip casting, for example, in the typical twin roll strip casting process, a closed chamber device is used in the crystallization roll up to the mill entrance to prevent the oxidation of the strip, and the scale thickness on the surface of the strip can be controlled by adding hydrogen gas in the lower closed chamber, for example, in US6920912 and controlling the oxygen content to be less than 5% in US 20060182989. However, the control of the thickness of the oxide scale during the conveying process from the rolling mill to the coiling is seldom related to patents, and particularly in the process of cooling the strip steel by adopting laminar cooling or spray cooling, the high-temperature strip steel is contacted with cooling water, and the thickness of the oxide scale on the surface of the cast strip is increased rapidly; meanwhile, the contact between the high-temperature strip steel and the cooling water also brings about a plurality of problems:
firstly, water spots (rusty spots) are formed on the surface of strip steel, and the surface quality is influenced;
secondly, cooling water for laminar cooling or spray cooling easily causes uneven local cooling on the surface of the strip steel and uneven microstructure inside the strip steel, thereby causing uneven performance of the strip steel and influencing the product quality;
thirdly, the local cooling of the surface of the strip steel is uneven, which causes the deterioration of the plate shape and influences the quality of the plate shape.
However, the continuous strip casting has the fast solidification process characteristics, so that the produced steel generally has the problems of uneven structure, low elongation, high yield ratio and poor formability; meanwhile, the austenite grains of the cast strip have obvious unevenness, which can cause uneven texture of a final product obtained after austenite phase transformation, thereby causing unstable performance of the product. Therefore, the production of products with high hole expanding performance required by the automobile industry and the petrochemical industry by adopting the thin strip continuous casting production line has certain difficulty and certain challenge, so that the traditional composition process cannot be produced when the thin strip continuous casting is adopted to produce the high-hole expanding steel, and breakthroughs in composition and process are required.
Chinese patent CN103602890 discloses a high-hole-expansion steel plate with 540MPa tensile strength and a manufacturing method thereof. The patent adopts the traditional continuous casting and the traditional hot rolling process to produce, and adopts a one-section type conventional laminar cooling mode.
Chinese patent CN103602890 discloses a high hole expansion steel plate with 440MPa tensile strength and a manufacturing method thereof. The patent adopts the traditional continuous casting and the traditional hot rolling process to produce, and adopts a one-section type conventional laminar cooling mode.
Chinese patents CN105154769 and CN106119702 respectively disclose 780MPa and 980MPa grade hot-rolled high-strength high-hole-expansion steel and manufacturing methods thereof, which are both high-strength steel categories, and the strengthening of steel types is realized by adding Ti, mo and more microalloy elements such as Ti, nb, cr, V and the like, so that the alloy cost is high; meanwhile, the production mode adopts the traditional continuous casting and traditional hot rolling process to produce.
International patent WO200928515 adopts C, si, mn to add a small amount of Nb, ti alloy element, can produce tensile strength at the reaming steel more than 490MPa, the hot rolling must adopt two segmentation laminar cooling modes, can more accurately simulate two segmentation cooling control in the laboratory, obtain fine test result, but in hot rolling production, the steel speed change in the hot rolling is big, can't measure the steel sheet temperature of air cooling section again, adopt two segmentation cooling model control laminar cooling, the actual temperature fluctuation of steel sheet is big, easily lead to the coil of strip head in the tail performance fluctuation big.
Disclosure of Invention
The invention aims to provide high-strength high-hole-expansion steel based on scrap steel and a production method thereof, wherein the scrap steel is fully utilized as a raw material to reduce the cost of molten steel, and the production process cost can be further reduced and the product performance can be improved through strip continuous casting.
In order to achieve the purpose, the technical scheme of the invention is as follows:
the method utilizes residual elements such as Sn, cu and the like in the scrap steel to smelt the molten steel, and selectively adds microalloy elements such as Nb and the like in the steel; in the smelting process, the alkalinity of slag, the type and melting point of inclusions in steel, the free oxygen content in molten steel and the content of acid-soluble aluminum Als are controlled; then, performing double-roller thin-strip continuous casting to cast strip steel with the thickness of 1.5-3mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the strip steel exits from a crystallization roller, and entering an online rolling mill for hot rolling under the closed condition; the rolled strip steel is cooled by adopting an air atomization cooling mode, and the air atomization cooling mode can effectively reduce the thickness of oxide skins on the surface of the strip steel, improve the temperature uniformity of the strip steel and improve the surface quality of the strip steel. The finally produced steel coil can be directly used as a hot rolled plate and can also be used after acid cleaning and flattening.
Specifically, the high-strength and high-hole-expansion steel based on the scrap steel comprises the following chemical components in percentage by weight: c:0.01-0.05%, si:0.2-0.6%, mn:0.8-1.5%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, N is less than or equal to 0.008%, als:<0.001%, ca is less than or equal to 0.0050%, nb:0.01-0.08%, total oxygen [ O ]] T :0.007 to 0.02 percent, the balance being Fe and unavoidable impurities, and simultaneously satisfying:
further contains Cu:0.1-0.6%, sn:0.005-0.04% of one or two elements;
Mn/S>250。
the microstructure of the high reaming steel is ferrite and bainite, wherein the proportion of a bainite phase is more than or equal to 15%.
The yield strength of the high-strength high-hole-expansion steel based on the scrap steel is more than or equal to 440MPa, the tensile strength is more than or equal to 590MPa, the elongation is more than or equal to 19%, and the hole expansion rate is more than or equal to 100%.
In the chemical composition design of the high hole expansion steel, the following steps are carried out:
the reasons for the chemical composition are as follows:
c: c is the most economical and basic strengthening element in steel, and enhances the strength of steel by solid solution strengthening and precipitation strengthening. C is an essential element for precipitating cementite during austenite transformation, so the strength level of the steel is determined to a great extent by the content of C, namely, higher C content corresponds to higher strength level. However, since interstitial solid solution and precipitation of C greatly impair the plasticity and toughness of steel, and an excessively high C content is disadvantageous in weldability, the C content cannot be excessively high, and the strength of steel is compensated by appropriate addition of alloy elements. Meanwhile, for conventional slab continuous casting, casting in a peritectic reaction region is easy to generate surface cracks of a casting blank, and steel leakage accidents can happen in severe cases. The same is true for thin strip casting, where casting a cast strip in the peritectic reaction zone is prone to surface cracking and, in severe cases, strip breakage. Therefore, the thin strip casting of Fe-C alloy also needs to avoid the peritectic reaction zone. Therefore, the content range of C adopted by the invention is 0.01-0.05%.
Si: si plays a role in solid solution strengthening in steel, and the Si added in the invention also plays a role in deoxidation, so that the purity of steel can be improved; meanwhile, the formation range of ferrite can be enlarged by Si, and the appearance of pearlite phase is avoided. However, the steel plate surface after rolling is easy to form red iron sheet defect due to the excessively high Si content. Therefore, the Si content range adopted by the invention is 0.2-0.6%.
Mn: mn is one of the cheapest alloy elements, can improve the hardenability of steel, has considerable solid solubility in the steel, improves the strength of the steel through solid solution strengthening, basically has no damage to the plasticity and the toughness of the steel, is the most main strengthening element for improving the strength of the steel, and can play a role in deoxidizing in the steel. However, too high Mn content results in deterioration of weldability and toughness of weld heat-affected zone. Therefore, the Mn content range adopted by the invention is 0.8-1.5%.
P: high content of P is easy to be segregated in grain boundary, increases cold brittleness of steel, deteriorates welding performance, reduces plasticity and deteriorates cold bending performance. In the thin strip continuous casting process, the solidification and cooling rates of the cast strip are extremely high, and the segregation of P can be effectively inhibited, so that the disadvantages of P can be effectively avoided, and the advantages of P can be fully exerted. Therefore, in the invention, the P content is higher than that in the traditional process, the content of the P element is properly relaxed, the dephosphorization procedure is eliminated in the steelmaking procedure, in the actual operation, the dephosphorization procedure is not required to be carried out intentionally, and no extra phosphorus is required to be added, and the range of the P content is less than or equal to 0.02 percent.
S: in general, S is a harmful element in steel, causes hot brittleness of steel, reduces ductility and toughness of steel, and causes cracks during rolling. S is easy to form MnS in steel, the amount and the form of sulfide in the steel directly influence the hole expansion rate of the steel plate, and the S must be lower than 0.005 percent. The amount and the form of the inclusion elements have great influence on the hole expanding performance of the steel plate, and particularly, the inclusion of strip-shaped sulfides in deformation easily causes cracks. Therefore, in the present invention, S is controlled as an impurity element, and the content range thereof is 0.005% or less, and Mn/S is more than 250.
And (3) Als: in order to control inclusions in steel, the invention requires that Al cannot be used for deoxidation, and in the use of refractory materials, the additional introduction of Al is avoided as much as possible, and the content of acid-soluble aluminum Als is strictly controlled: <0.001%.
N: similar to the element C, the element N can improve the strength of the steel through interstitial solid solution, however, the interstitial solid solution of N has great harm to the plasticity and the toughness of the steel, and the yield ratio of the steel can be improved due to the existence of free N, so that the content of N cannot be too high. The content range of N adopted by the invention is less than or equal to 0.008 percent.
Nb: in the strip casting process, the added alloying element Nb can be caused to exist mainly in a solid solution state in the steel strip due to its unique rapid solidification and rapid cooling characteristics, and precipitation of Nb is hardly observed even if the steel strip is cooled to room temperature. The Nb element which is dissolved in the steel can play a role in solid solution strengthening. The content range of Nb designed by the invention is 0.01-0.08%.
Ca: the form of sulfide in steel can be changed, the long-strip MnS inclusion is converted into spherical CaS inclusion, the plasticity and the toughness of the steel plate are improved, and the hole expansion rate of the steel plate is improved. The invention controls Ca to be less than 0.0050 percent.
Cu: the Cu is an easily segregated element, so that the Cu content is generally controlled strictly in the traditional process flow. The invention improves the upper limit of Cu to 0.60% by applying the rapid solidification effect of thin strip continuous casting. Fig. 2 shows the effect of copper on the interface heat flow, and the copper elements with different compositions are added into the steel, and the experimental result shows that the peak heat flow of the interface heat transfer of the steel is reduced and the average heat flow is also reduced along with the increase of the copper content. When the content of Cu reaches 0.80%, the peak heat flow and the average heat flow are still higher, and when the content of Cu is more than 2.5%, the peak heat flow and the average heat flow are both obviously reduced. The invention controls the content of Cu between 0.1 and 0.6 percent, and the peak heat flow and the average heat flow caused by the Cu element have little influence. The improvement of the Cu content can realize the effective utilization of copper in scrap steel or inferior ore resources (high copper ores) in a certain sense, promote the recycling of steel, reduce the production cost and realize the purpose of sustainable development. It is worth noting that in the invention, the Cu element in the scrap steel is fully utilized, and the additional addition of metal Cu is not needed, which increases the steel-making cost.
Sn: sn is also one of main participating elements in scrap steel and is generally recognized as a harmful element in steel, because Sn is an easily segregated element, a small amount of Sn is enriched in grain boundaries, and defects such as cracks are caused, so that the content of Sn is strictly controlled in the conventional process. Due to the characteristic of rapid solidification, segregation of elements among dendrites is greatly reduced, and the solid solution amount of the elements can be greatly improved, so that the range of Sn elements can be expanded under the condition of a strip continuous casting process, and the steelmaking cost can be greatly reduced. Fig. 3 is a relationship between Sn element and average heat flux density. As can be seen from fig. 3, when the Sn addition is less than 0.04%, the influence on the heat flux density is not great, i.e., the ribbon solidification process is not affected. FIG. 4 is a relationship between Sn content and surface roughness. Since cracks on the surface of the cast strip usually occur at the uneven folds of the surface of the cast strip, the surface roughness is used to characterize the occurrence of surface cracks. If the roughness is large, the probability of occurrence of cracks is high. As is clear from FIG. 4, the increase in Sn content does not adversely affect the surface quality of the cast strip under the rapid solidification conditions. From the results of fig. 3 and 4, it is clear that Sn does not adversely affect the solidification and surface quality of the cast strip. Therefore, in the present invention, the requirement for Sn content can be further relaxed, and the Sn content is designed to be in the range of 0.005-0.04%. It is worth noting that in the invention, sn element in scrap steel is fully utilized, and additional metal Sn is not needed, which increases steel-making cost.
The invention relates to a production method of high-strength high-hole-expansion steel based on scrap steel, which comprises the following steps:
1) Smelting
Smelting according to the chemical components, wherein in the smelting process, the slagging alkalinity a = CaO/SiO 2 Is controlled at a<1.5, preferably a<1.2, or a =0.7 to 1.0; need toTo obtain a low melting point MnO-SiO 2 -Al 2 O 3 Ternary inclusions, mnO-SiO 2 -Al 2 O 3 MnO/SiO in ternary inclusions 2 Controlled at 0.5-2, preferably at 1-1.8; free oxygen [ O ] in molten steel] Free The range is as follows: 0.0005-0.005% of Mn/S in molten steel>250；
2) Continuous casting
The continuous casting adopts double-roller thin strip continuous casting, and a casting strip with the thickness of 1.5-3mm is formed at the position with the minimum gap between two crystallizing rollers; the diameter of the crystallization roller is 500-1500mm, and the preferred diameter is 800mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150m/min; the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;
3) Lower enclosed chamber protection
After the cast strip is taken out of the crystallization roller, the temperature of the cast strip is 1420-1480 ℃, the cast strip directly enters a lower closed chamber, non-oxidizing gas is introduced into the lower closed chamber, the oxygen concentration in the lower closed chamber is controlled to be less than 5%, and the temperature of the cast strip at the outlet of the lower closed chamber is 1150-1300 ℃;
4) In-line hot rolling
The cast strip is sent to a rolling mill in a lower closed chamber through pinch rolls to be rolled into strip steel, and the rolling temperature is 1100-1250 ℃. Controlling hot rolling reduction rate to be 10-50%, preferably, hot rolling reduction rate to be 30-50%; the thickness of the hot-rolled strip steel is 0.8-2.5mm, and the preferable thickness is 1.0-1.8mm;
5) Cooling after rolling
Cooling the strip steel after the online hot rolling, wherein the cooling adopts an air atomization cooling mode, and the cooling rate is more than 50 ℃/s;
6) Strip steel coiling
And cutting the head of the cooled hot-rolled strip steel to remove the head with poor quality, and directly coiling the hot-rolled strip steel into coils at the coiling temperature of 470-570 ℃.
Preferably, in the step 1), 100% of all-waste steel can be selected as smelting raw materials, pre-screening is not needed, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.
Preferably, in step 3), the non-oxidizing gas is N 2 Ar, or CO obtained by sublimation on dry ice 2 A gas.
Preferably, in the step 5), the gas-water ratio of the gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.
Preferably, in step 6), the coiling is performed in a double coiler format or a carrousel coiling format.
In the method for producing the high-strength and high-hole-expansion steel based on the steel scrap, disclosed by the invention, the method comprises the following steps:
smelting is carried out through electric furnace steelmaking, and 100% of smelting raw materials can come from scrap steel; the smelting is carried out by a converter, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed, so that the scrap steel ratio of the converter is improved to the maximum extent, and the production cost is greatly reduced; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.
In order to improve the castability of the strip-cast molten steel, the basicity of the slagging in the steel-making process a = CaO/SiO 2 Is controlled at a<1.5, preferably a<1.2, or a = 0.7-1.0.
To improve the castability of thin strip continuous casting molten steel, it is necessary to obtain MnO-SiO of low melting point 2 -Al 2 O 3 Ternary inclusions, e.g. shaded area of FIG. 5, mnO-SiO 2 -Al 2 O 3 MnO/SiO in ternary inclusions 2 The concentration is controlled to be 0.5 to 2, preferably 1 to 1.8.
In order to improve the castability of thin strip continuous casting molten steel in which O is an essential element for forming oxide inclusions, the present invention requires formation of MnO-SiO with a low melting point 2 -Al 2 O 3 The ternary inclusions of (2) require free oxygen [ O ] in molten steel] Free The range is as follows: 0.0005 to 0.005 percent.
The strip steel after on-line hot rolling is cooled after rolling, and the cooling adopts an air atomization cooling mode, so that the thickness of the oxide scale on the surface of the strip steel can be effectively reduced, the temperature uniformity of the strip steel is improved, and the surface quality of the strip steel is improved. The gas-water ratio of gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa. High-pressure water mist is formed after gas atomization and sprayed on the surface of the steel strip, so that on one hand, the temperature of the steel strip is reduced, on the other hand, the water mist can form a compact air film to be coated on the surface of the steel strip, the anti-oxidation effect of the steel strip is achieved, and the growth of oxide skin on the surface of the hot-rolled steel strip is effectively controlled. The cooling mode can avoid the problems caused by the traditional spraying or laminar cooling, so that the surface temperature of the strip steel is uniformly reduced, the temperature uniformity of the strip steel is improved, and the effect of homogenizing the internal microstructure is achieved; meanwhile, the cooling is uniform, so that the shape quality and the performance stability of the strip steel can be improved; effectively reducing the thickness of the oxide scale on the surface of the strip steel. The cooling rate range of the gas atomization cooling is >50 ℃/s, so that the rolled high-temperature austenite is transformed into a mixed microstructure of ferrite and a small amount of bainite, as shown in fig. 6.
And cutting the head of the cooled hot-rolled strip steel by using a cutting head to remove the head with poor quality, and directly coiling the hot-rolled strip steel into coils at the coiling temperature of 470-570 ℃. The recoiling machine adopts a double-coiling mode and can also adopt a carrousel coiling mode to ensure the continuous production of the strip steel. Preferably, a carrousel take-up form is used.
Description of the possible selection of 100% total scrap without pre-screening as raw material:
in order to save investment cost and production cost, modern iron and steel production enterprises actively carry out technical innovation on the prior production process flow. Aiming at the problems of long process flow, more equipment and complexity of the existing hot strip steel production process, a plurality of manufacturers tightly combine the continuous casting and rolling technology with the traditional process so as to meet the requirements of the continuous casting and rolling process.
The converter steelmaking is adopted to provide molten steel, so that furniture of a production plant needs to be provided with conditions for providing molten iron, and generally, blast furnace ironmaking equipment or non-blast furnace ironmaking equipment is needed, and the converter steelmaking method belongs to the current long-flow steel production mode. However, nowadays, the steel scrap resource is increasingly abundant, the country advocates improving the steel scrap ratio of the converter so as to achieve the purposes of energy saving, consumption reduction and cost reduction, the average level of the steel scrap ratio of the converter is about 8 percent in the past, and the steel scrap ratio of the converter is 15-25 percent in the present and future.
When molten steel is provided by electric furnace steelmaking, scrap steel is used as a main raw material, and the traditional processSuch as die casting or continuous slab casting, with a solidification cooling rate of only 10 -1 10 ℃/s, the residual elements in the steel scrap can generate grain boundary segregation during the solidification process, deteriorate the performance and quality of the steel, and directly generate cracking and breaking phenomena in severe cases, so that in the traditional process, the harmful elements need to be strictly controlled, in the selection of the steel scrap raw materials, some pre-screening needs to be carried out, and in the steel-making process, some special treatment needs to be carried out, such as adding some fine materials for dilution, and the like, which undoubtedly increases the production operation cost. Because the components of steel need to be controlled, certain quality requirements are required for the adopted steel scrap raw materials, and generally, the steel scrap needs to be pre-screened and classified. In order to improve the production efficiency of some domestic electric furnace steel mills, concentrated materials such as outsourced sponge iron, iron carbide and the like are selectively added in the raw material composition to dilute harmful elements which are difficult to remove in the waste steel, so that the quality of the molten steel is improved. Some domestic steel mills with blast furnaces and electric furnaces simultaneously use self-produced molten iron to be added into the electric furnace as raw materials of the electric furnace to improve the production efficiency of the electric furnace, thereby greatly shortening the tapping time of the electric furnace, and the molten iron blending ratio in the electric furnace can reach 30-50%.
The adoption of twin-roll thin-strip casting technology is a typical sub-rapid solidification process, and the solidification cooling speed is as high as 10 2 -10 4 The method has the advantages that (1) DEG C/s, some harmful residual elements in the scrap steel, such as Cu, sn, P and the like, can be dissolved into a steel matrix to the maximum extent without grain boundary segregation, so that 100% of all scrap steel smelting can be realized during electric furnace steelmaking without pre-screening, and the raw material cost is greatly reduced; the converter is adopted for smelting, the scrap steel is added into the converter according to the proportion of more than 20 percent of smelting raw materials, pre-screening is not needed, the scrap steel ratio of the converter is improved to the maximum extent, and the production cost is greatly reduced. These residual elements also act as solid solution strengthening to produce ultra-thin hot rolled strip with excellent properties. Realizes the comprehensive utilization of the production of inferior waste steel resources, and has the effects of turning harmful into beneficial and utilizing waste materials on the harmful residual elements in the waste steel.
The reason why the present invention relates to the castability of the thin strip continuous casting is explained as follows:
the castability is not defined exactly at present, and traditionally it is a frequently used concept closely related to the fluidity of the steel, the chilling tendency, the shrinkage characteristics and the quality of the product, with respect to the metal species and the process factors thereof. The definition "Strip Casting castability (CASC)" refers to the feasibility of twin roll Casting of a steel grade. The castability is good, which means that the restrictive problem that the casting process can not be carried out or the quality of the cast product can not meet the requirement can not occur in the casting process; poor castability means that problems such as poor molten steel fluidity, molten pool agglomeration and bridging, severe belt breakage, surface cracks, surface slag inclusion and the like often occur in the casting process, so that production cannot be carried out normally and stably or the product quality cannot meet requirements.
The strip continuous casting castability of a steel grade is judged through research and analysis on the strip continuous casting castability, and briefly summarized, the following aspects can be considered: firstly, whether uneven solidification shrinkage can be avoided or not; secondly, the uniformity of interface heat transfer can be improved, so that the solidification uniformity is improved; and thirdly, whether the hot brittleness in the solidification process can be improved or controlled. When the strip casting castability of a steel grade is poor, the production process stability is poor, the quality stability of the produced product is also poor, the productivity cannot be exerted, the qualification rate of the product is low, and the product is not suitable for the strip casting process.
The steel grade related to the invention is controlled from the carbon content (avoids peritectic zone to solve uneven solidification shrinkage); alkalinity control, control of Als, free oxygen total oxygen control, and low melting MnO-SiO 2 -Al2O 3 Ternary inclusion control (improving interface heat transfer uniformity to solve solidification uniformity); mn/S control (avoidance of hot shortness) and the like, the castability of strip casting is strictly satisfied.
The reason that the spray cooling is preferably adopted after the thin strip continuous casting hot rolled steel is rolled is as follows:
the traditional continuous casting also adopts spray cooling, but the action area and the temperature are different, the traditional continuous casting carries out spray cooling on the casting blank in the outlet fan-shaped section area of the casting blank outlet crystallizer, the temperature of the casting blank is higher at the moment, and the casting blank is in a high-temperature austenite single-phase area seen on a phase diagram. The main purposes of spray cooling in the area are to control the position of the solidification tail end, accelerate the surface cooling of the casting blank, refine the surface austenite grain structure, improve the surface strength of the casting blank, improve the surface quality of the casting blank and avoid the occurrence of cracks. The invention carries out spray cooling on the ultrathin strip steel after the cast strip is subjected to online hot rolling, the temperature is lower, the ultrathin strip steel is in a solid phase transformation area where high-temperature austenite is transformed into ferrite on a phase diagram, the strip steel is subjected to spray cooling in the area, and the microstructure type after the solid phase transformation can be effectively controlled by adjusting the spray cooling strength, thereby realizing the performance requirement of a final product.
The reason that the strip continuous casting hot rolled steel coil is preferably wound by a carrousel homotopic winder is as follows:
at present, most production lines of ultrathin hot rolled steel coils adopt an underground double-coiling mode or an underground triple-coiling mode, and the main reason is that the production lines also consider the production of thick hot rolled plates, for example, the coiling of an ESP production line of Avedi (Avedi) company adopts the underground triple-coiling mode, and the coiling of an FTSC production line of Danielli (Danieli) introduced by Tang steel adopts the underground double-coiling mode. The Castrip strip casting production line of the Umbelliferae, nippon kok (Nucor) adopts the traditional method and also adopts an underground double-coiling mode. The distance interval between the underground recoiling machine and the recoiling machine is generally 8-10 m (typical value is 9.4 m), when the ultrathin hot rolled strip steel is produced by strip continuous casting, the cooling speed of the strip steel in the air is very high, the interval is enough to influence the difference of the recoiling temperature, the temperature deviation between the two recoiling machines can reach 49 ℃, and the performance deviation of the steel coil can be seriously influenced.
The invention preferably adopts a carrousel coiling mode, can realize the co-position coiling of the hot rolled steel coil, ensures the same coiling temperature and further greatly improves the stability of the performance of the steel coil product. At present, the carrousel coiling machine is widely used in the field of cold rolling, and has the main advantages of realizing thinner strip steel coiling, occupying small area and greatly shortening the length of a production line, but the carrousel coiling machine is easier to realize in the field of cold rolling due to lower strip steel temperature. The invention provides a carrousel coiling method in the field of coiling ultrathin hot rolled strip steel, which considers the high temperature resistance of equipment and realizes the coiling of the ultrathin hot rolled strip steel. The coiling mode is more advanced than that of a Castrip strip continuous casting production line of Nucor (Nucor) in the United states.
The invention is distinguished and improved from the prior art:
there are many patents on the prior thin strip continuous casting for producing thin strip products and processes, but the production of the high-reaming-hole steel related to the invention by adopting the thin strip continuous casting is not directly reported.
The most obvious characteristic of the invention which is different from the prior thin strip continuous casting technology is the roller diameter of the crystallization roller and the corresponding distribution mode. The EUROSTRIP technology is characterized in that the crystallization roller with a large roller diameter of 1500mm phi is large, the molten steel capacity of a molten pool is large, the distribution is easy, and the manufacturing cost and the operation cost of the crystallization roller are high. CASTIP technology is characterized in that the diameter of a crystallization roller is 500mm, the crystallization roller is small, the molten steel capacity of a molten pool is small, the distribution is very difficult, but the manufacturing, operation and maintenance costs of casting machine equipment are low. CASTRIP adopts a three-stage molten steel distribution system (tundish, transition ladle and distributor) to solve the problem of uniform distribution of small molten pools. Because a three-level flow distribution system is adopted, the cost of the refractory material is directly increased; more importantly, the three-stage flow distribution system enables the flowing path of the molten steel to be lengthened, the temperature drop of the molten steel is large, and in order to meet the temperature of molten steel in a molten pool, the tapping temperature needs to be greatly increased. The increase of the tapping temperature causes problems such as increase of steel-making cost, increase of energy consumption, and shortening of the life of refractory.
The diameter of the crystallization roller is 500-1500mm, preferably the diameter of the crystallization roller is 800mm, and a two-stage steel water distribution system (a tundish and a flow distributor) is adopted. Molten steel flowing out of the flow distributor forms different flow distribution modes along the roller surface and the two end surfaces, and flows in two paths without mutual interference. Because a two-stage flow distribution system is adopted, compared with a three-stage flow distribution system, the cost of the refractory material is greatly reduced; the reduction of the flow path of the molten steel reduces the temperature drop of the molten steel, and can reduce the tapping temperature by 30-50 ℃ compared with a three-level flow distribution system. The steel tapping temperature is reduced, so that the steel making cost can be effectively reduced, the energy consumption is saved, and the service life of refractory materials is prolonged. The invention is matched with the crystallization roller with the optimal roller diameter of phi 800mm, and adopts a two-stage steel water distribution system, thereby not only realizing the requirement of stable distribution of molten steel, but also realizing the aims of simple structure, convenient operation and low processing cost.
Chinese patent CN101353757 uses low carbon micro-alloy components to produce reaming steel with tensile strength of 440MPa, and trace amounts of Nb:0-0.25% and Ti:0-0.03%, and because the coiling temperature of 600 ℃ is adopted, the steel is produced by adopting the traditional continuous casting and traditional hot rolling process, and for a carbon manganese steel hot rolled plate, a banded structure often exists, so that the hole expanding rate of the steel plate is reduced, and meanwhile, a plurality of micro alloys are added, so that the steel making cost is increased. The invention is obviously different from the patent in production process, and the invention adopts the thin strip continuous casting process for production, thereby greatly shortening the production flow, avoiding the banded structure, saving the dosage of microalloy and achieving the same or more excellent performance by only adding a small amount of microalloy.
Chinese patent CN101928881 discloses a hot-rolled high-hole-expansion steel plate with tensile strength of 590MPa level and a manufacturing process thereof, wherein trace amount of Nb is added into the components: 0-0.10% and Ti:0-0.04 percent by adopting the traditional continuous casting and the traditional hot rolling process, cooling the finally rolled steel plate to 600-750 ℃ at the cooling speed of 50-100 ℃/s, then cooling the steel plate in the air for 3-10 seconds at the cooling speed of 5-15 ℃/s, then cooling the steel plate to 350-500 ℃ at the cooling speed of 70-150 ℃/s, coiling the steel plate, and then cooling the steel plate to room temperature by air. The follow-up cooling adopts complicated three-section type cooling, the coiling temperature fluctuation is large, the performance fluctuation of the head, the middle and the tail of the steel coil is large, and the hole expansion rate fluctuation is also large. The invention adopts the thin strip continuous casting process to produce, greatly simplifies the production process flow, does not need to adopt complex three-section cooling, and has obvious advancement.
Japanese patent JP2006063394 discloses a hot-rolled high-hole-expansion steel, which contains 0.20-0.48% of carbon, has a tensile strength of 440MPa or more, contains Cr alloy element, has a hole expansion ratio of 70% or more, and is subjected to annealing treatment at 640 ℃ after hot rolling. The carbon content design of the invention reaches the range of medium-high carbon steel, and is obviously higher than the low-carbon design of the invention. The hot-rolled high-strength steel plate disclosed in Japanese patent JP2006305700 adopts a component design of C-Si-Mn + Ti, the tensile strength is obtained to be more than 780MPa, and the hole expanding rate is only more than 68%. The hot-rolled high-hole-expansion steel disclosed in JP2003/016614 has a carbon content of 0.02-0.10%, si content of 0.5% or less, and tensile strength of 590MPa or more, but contains many alloying elements such as Nb, ti, V, cr, RE, etc., and thus has a high steel-making cost and a main objective of good surface coatability. Compared with the patent, the invention adopts a simple alloy component system, adopts a thin strip continuous casting process to realize the performance of the high-hole-expansion steel, and has the characteristics of simplicity and high efficiency.
The hot rolled steel sheet disclosed in U.S. Pat. No. 2006096678 has a strength of 780MPa or more, an elongation of 22% or more, and an expansion ratio of 60% or more. U.S. Pat. No. 4,430,181,181,76 discloses a hot rolled steel sheet having a yield strength of 80ksi (550 MPa) or more and a hole expansion ratio of 58% or more, and reinforced with Nb and V. The production processes adopted by the patents are the production of the traditional continuous casting and the traditional hot rolling process, are different from the production process of the invention, and have lower hole expansion rate of the product.
The invention has the main advantages that:
the high-strength high-reaming steel containing tin (Sn), copper (Cu)/tin (Sn) and copper (Cu) is produced by a thin-strip continuous casting technology, and has no report until now, and the advantages are summarized as follows:
1. the invention omits the complex processes of slab heating, multi-pass repeated hot rolling and the like, and has the advantages of shorter production flow, higher efficiency and greatly reduced production line investment cost and production cost by adopting the working procedures of double-roller thin-strip continuous casting and one-pass online hot rolling.
2. The invention omits a plurality of complex intermediate steps in the traditional high hole expansion steel production, and compared with the traditional high hole expansion steel, the energy consumption and CO of the production 2 Greatly reduces the emission, and is a green and environment-friendly product.
3. Compared with the traditional hot rolling process, the thin strip continuous casting easily generates bainite type microstructures in the cooling process after rolling due to the natural technological process advantages of the thin strip continuous casting, and the produced product has excellent reaming performance.
4. The invention adopts the thin strip continuous casting process to produce the high-reaming-hole steel, the thickness of the cast strip is thinner, the thin-specification product is produced to the desired product thickness through online hot rolling, the thin-specification product is directly supplied to the market for use without cold rolling, the purpose of supplying the thin-specification hot rolled plate is achieved, and the cost performance of the plate and the strip can be obviously improved.
5. The smelting is carried out by an electric furnace, the smelting raw materials can realize 100 percent of full scrap steel smelting in the true sense, pre-screening is not needed, and the raw material cost is greatly reduced; the steel is smelted by a converter, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and the pre-screening is not needed, so that the scrap steel ratio of the converter is improved to the maximum extent, and the smelting cost and the energy consumption are greatly reduced.
6. The invention utilizes the scrap steel containing Cu and Sn to turn the Cu and Sn in the steel into harmful and beneficial, realizes the full utilization of the existing scrap steel or low-quality inferior ore resources (high-tin ore and high-copper ore), promotes the recycling of the scrap steel, reduces the production cost and realizes the sustainable development of the steel industry.
7. The invention adopts the air atomization cooling mode of the rolled strip steel, can avoid the problems brought by the traditional spray or laminar cooling, uniformly reduces the surface temperature of the strip steel, improves the temperature uniformity of the strip steel, and achieves the effect of homogenizing the internal microstructure; meanwhile, the cooling is uniform, so that the shape quality and the performance stability of the strip steel can be improved; effectively reducing the thickness of the oxide scale on the surface of the strip steel.
8. In the traditional process, alloy elements are separated out in the cooling process of the plate blank, and the utilization rate of the alloy elements is reduced because the re-dissolution of the alloy elements is insufficient when the plate blank is reheated. In the thin strip continuous casting process, the high-temperature cast strip is directly hot-rolled, and the added alloy elements mainly exist in a solid solution state, so that the alloy utilization rate can be improved.
9. According to the invention, the hot rolled steel strip carrousel coiling machine is selected, so that the length of a production line is effectively shortened; meanwhile, the control precision of the coiling temperature can be greatly improved by the co-position coiling, and the stability of the product performance is improved.
Drawings
FIG. 1 is a schematic diagram of a process arrangement for a twin roll strip casting process;
FIG. 2 is a schematic illustration of the effect of Cu on interface heat flow;
FIG. 3 is a graph showing the relationship between Sn content and average heat flux density;
FIG. 4 is a schematic representation of the relationship between Sn content and surface roughness of a cast strip;
FIG. 5 shows MnO-SiO 2 -Al 2 O 3 Ternary phase diagram (shaded area: low melting point region);
FIG. 6 is a photograph showing the microstructure of steel according to an example of the present invention.
Detailed Description
The invention is further illustrated below by means of examples and figures, which are not intended to limit the invention in any way. Any variations in the practice of the invention which may occur to those skilled in the art and which fall within the scope of the appended claims will be deemed to fall within the scope of the invention.
Referring to fig. 1, molten steel designed according to the chemical composition of the present invention is directly poured into a molten pool 7 surrounded by two relatively rotating and rapidly-cooled crystallizing rollers 8a and 8b and side sealing plate devices 6a and 6b through a ladle 1, a tundish 2, a tundish 3, a submerged nozzle 4 and a distributor 5, the molten steel is solidified on the circumferential surfaces of the rotating crystallizing rollers 8a and 8b, and a solidified shell is formed and gradually grows to form a casting strip 11 with a thickness of 1.5-3mm at the minimum gap (nip point) between the two crystallizing rollers; the diameter of the crystallization roller in the embodiment is 500-1500mm, and water is introduced into the crystallization roller for cooling; the casting speed of the casting machine ranges from 60 to 150m/min according to the thickness of the cast strip.
After the cast strip 11 comes out of the crystallization rollers 8a and 8b, the cast strip is directly fed into the lower closed chamber 10 at 1420-1480 ℃, the non-oxidizing gas is introduced into the lower closed chamber 10 to protect the strip steel, so as to realize the anti-oxidation protection of the strip steel, and the atmosphere of the anti-oxidation protection can be N 2 Ar, or other non-oxidizing gas, such as CO obtained by sublimation of dry ice 2 Gas, etc., the oxygen concentration in the lower sealed chamber 10 is controlled to be<5 percent; the lower closed chamber 10 protects the cast strip 11 from oxidation to the inlet of a rolling mill 13; the temperature of the casting belt at the outlet of the lower closed chamber 10 is 1150-1300 ℃; then the cast strip is sent to a rolling mill 13 through a swing guide plate 9 and a pinch roll 12, hot rolling is carried out to form a hot rolled strip with the thickness of 0.8-2.5mm, and the hot rolled strip is rolledThe post-cooling adopts an air atomization cooling mode to improve the temperature uniformity of the strip steel; after the head of the hot-rolled strip is cut by the flying shear device 16, the cut head falls into a flying shear pit 18 along a flying shear guide plate 17, and the hot-rolled strip after the head is cut enters a coiler 19 for coiling. And taking the steel coil off the coiling machine, and naturally cooling to room temperature. The finally produced steel coil can be directly used as a hot rolled plate and can also be used after acid cleaning and flattening.
The chemical compositions of the examples of the invention are shown in table 1, and the balance of the compositions is Fe and other unavoidable impurities. The manufacturing method of the invention has the process parameters shown in the table 2, and the properties of the finally obtained product are shown in the table 3.
In conclusion, the high-hole-expansion steel manufactured by the thin-strip continuous casting process technology according to the design range of the steel grade components provided by the invention has the yield strength of more than or equal to 440MPa, the tensile strength of more than or equal to 590MPa, the elongation of more than or equal to 19 percent and the hole expansion rate of more than or equal to 100 percent.
Figure BDA0002208256600000191
Figure BDA0002208256600000201
Figure BDA0002208256600000211

Claims (9)

1. A production method of high-strength and high-hole-expansion steel based on scrap steel comprises the following chemical components in percentage by weight: c:0.01-0.04%, si:0.2-0.6%, mn:0.8-1.5%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, N is less than or equal to 0.008%, als:<0.001%, ca is less than or equal to 0.0050%, nb:0.01-0.08%, total oxygen [ O ]] T :0.007 to 0.02 percent, the balance being Fe and unavoidable impurities, and simultaneously satisfying:
further contains Cu:0.27-0.6%, sn:0.022-0.04 wt%;
Mn/S>250；
the microstructure of the high-strength and high-reaming steel is ferrite and bainite, wherein the proportion of a bainite phase is more than or equal to 15 percent;
the yield strength of the high-strength high-hole-expansion steel is more than or equal to 440MPa, the tensile strength is more than or equal to 590MPa, the elongation is more than or equal to 19 percent, and the hole expansion rate is more than or equal to 100 percent;
the method specifically comprises the following steps:
1) Smelting
Smelting according to the chemical components, wherein in the smelting process, the slagging alkalinity a = CaO/SiO 2 Controlling a = 0.7-1.0; it is necessary to obtain MnO-SiO having a low melting point 2 -Al 2 O 3 Ternary inclusions, mnO-SiO 2 -Al 2 O 3 MnO/SiO in ternary inclusions 2 Controlling the temperature to be 0.5-2; free oxygen [ O ] in molten steel] Free The range is as follows: 0.0005-0.005% of Mn/S in molten steel>250；
2) Continuous casting
The continuous casting adopts double-roller thin strip continuous casting, and a casting strip with the thickness of 1.5-3mm is formed at the minimum position of a gap between two crystallizing rollers; the diameter of the crystallization roller is 500-1500 mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150m/min; a two-stage steel water distribution system is adopted for continuous casting flow distribution, namely a tundish and a flow distributor;
3) Lower sealed chamber protection
After the casting strip is taken out of the crystallization roller, the casting strip is directly fed into a lower closed chamber at the temperature of 1420-1480 ℃, non-oxidizing gas is introduced into the lower closed chamber, the oxygen concentration in the lower closed chamber is controlled to be less than 5 percent, and the temperature of the casting strip at the outlet of the lower closed chamber is 1150-1300 ℃;
4) In-line hot rolling
Conveying the cast strip to a rolling mill in a lower closed chamber through a pinch roll, and rolling the cast strip into strip steel, wherein the rolling temperature is 1100-1250 ℃, and the hot rolling reduction rate is controlled to be 10-50%; the thickness of the hot rolled strip steel is 0.8-2.5mm;
5) Cooling after rolling
Cooling the strip steel after the online hot rolling, wherein the cooling adopts an air atomization cooling mode, and the cooling rate is more than 50 ℃/s;
6) Strip steel coiling
And cutting the head of the cooled hot-rolled strip steel by using a cutting head to remove the head with poor quality, and directly coiling the hot-rolled strip steel into coils at the coiling temperature of 470-570 ℃.
2. The method for producing high-strength and high-bore-expansion steel based on scrap steel according to claim 1, wherein in the step 1), 100% of all scrap steel is selected as a smelting raw material, pre-screening is not required, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.
3. The method for producing a steel scrap-based high-strength and high-bore-expansion steel according to claim 1, wherein in the step 3), the non-oxidizing gas is N 2 CO obtained by sublimation of Ar or dry ice 2 A gas.
4. The method for producing high-strength and high-bore-expansion steel based on steel scrap according to claim 1, wherein in the step 5), the gas-water ratio of the gas atomization cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.
5. The method for producing a high-strength and high-bore-expansion steel based on scrap steel according to claim 1, wherein the coiling in the step 6) is in a form of a double coiler or a carrousel coiler.
6. The method for producing a high-strength and high-bore-expansion steel scrap according to claim 1, wherein in the step 1), the MnO-SiO is 2 -Al 2 O 3 MnO/SiO in ternary inclusions 2 The temperature is controlled to be 1-1.8.
7. The method for producing a high-strength and high-bore-expansion steel based on scrap steel according to claim 1, wherein the diameter of the crystallization roll in the step 2) is 800mm.
8. The method for producing a high-strength and high-bore-expansion steel based on scrap steel according to claim 1, wherein the hot rolling reduction in the step 4) is 30 to 50%.
9. The method for producing a high-strength and high-bore-expansion steel based on scrap steel according to claim 1 or 8, wherein the thickness of the hot-rolled strip steel in the step 4) is 1.0 to 1.8mm.
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