A method for realizing IF steel ferrite rolling in an ultra-short process near net shape

Abstract

A method for realizing near-net-shape IF steel ferritic rolling with an ultra-short process, which adopts thin strip continuous casting + austenite rolling + ferritic rolling. During the rolling process, two rolling operations are carried out, and between the two rolling operations, aerosol cooling is used to effectively and precisely control the temperature of the strip to uniformly decrease to 5 - 50 °C below the Ar3 temperature, ensuring that the strip can undergo ferritic rolling in the second rolling stage, thereby obtaining an ultra-thin IF steel with a uniform ferritic structure. Its thickness can be as thin as 0.5 mm. Compared with the IF steel produced by the existing hot rolling process, it has lower yield strength, tensile strength, yield ratio, and better deep drawing performance. The performance and thickness accuracy reach the level of cold-rolled products of the same specification, realizing "replacing cold rolling with hot rolling". It has the advantages of a simple and efficient overall production process, high production efficiency, energy conservation and environmental protection, etc. The production line occupies a small area and has a low investment cost.

Description

Technical Field

[0001] The invention relates to the field of IF steel manufacturing, and in particular to a method for implementing IF steel ferrite rolling in an ultra-short process near net shape. Background Art

[0002] IF steel is a kind of steel that is made by adding strong carbon and nitrogen compound forming elements such as Ti or Nb to ultra-low carbon steel. It reacts with C and N atoms in the steel to form carbon and nitrogen compounds, making the matrix free of interstitial atoms. It is also called interstitial-free steel. Since IF steel does not contain interstitial atoms, it has the characteristics of high plastic anisotropy (r) value, high work hardening index (n) and no aging. In particular, its excellent deep drawing performance makes IF steel widely used as the third generation of deep drawing steel sheets in the automotive, home appliance and other industries.

[0003] Ferrite rolling was first successfully developed by Professor Appell and his team at the Belgian Steel Research Center in 1994. At that time, the main purpose of developing this technology was to replace cold-rolled products with a thickness range of 1.0-2.0 mm with thin-gauge hot rolling, so as to achieve "replacing cold with heat" and reduce production costs. Under the traditional process flow, the process flow of this technology is: continuous casting ingot + heating and insulation in a heating furnace + rough rolling (austenite rolling) + rapid cooling + finishing rolling (ferrite rolling) + cooling + coiling, that is, firstly, a billet with a thickness of about 200 mm is obtained through continuous casting, and the billet is reheated and kept warm, and then rough rolling and finishing rolling are carried out, and finally the steel strip is laminar cooled and coiled to complete the entire hot rolling production process. What is special is that the rough rolling process is completed in the full austenite state, and a rapid cooling device is set between the rough rolling mill and the finishing mill to reduce the strip temperature to below Ar3 before entering the finishing mill. Before entering the finishing mill, the strip has completed the γ→α phase transformation, so that the entire finishing process is carried out in the ferrite region or in the two-phase region of ferrite and austenite. In the conventional austenite rolling process, both the rough rolling and the finishing rolling processes are completed in the austenite region. The above-mentioned traditional ferrite rolling process has a long process flow, high energy consumption, many units and equipment, high infrastructure costs, and high production costs.

[0004] In the comparative test of ferrite rolling and austenite rolling of IF steel, ferrite rolling can obtain coarse-grained ferrite structure with lower yield strength and tensile strength, lower yield strength ratio, larger r value and better forming performance. Since the temperature of ferrite rolling is significantly lower than that of austenite rolling, which is more than 200℃ lower, the lower finishing temperature will lead to a significant increase in rolling deformation resistance, increased load on the rolling equipment, and increased roll consumption, which is not conducive to long-term rolling; at the same time, the rolling force increases, deformation is difficult, and the plate shape is difficult to control, which cannot ensure the smooth progress of the rolling process. Therefore, not all steel types are suitable for ferrite rolling technology. It is necessary to select some steel types with a large ferrite zone and a low deformation resistance valley within the temperature range of the ferrite zone. The high-temperature deformation resistance characteristics of IF steel meet such characteristics.

[0005] The process flow of thin slab continuous casting and rolling is: continuous casting + heat preservation and soaking of the ingot + hot rolling + cooling + coiling. The main difference between this process and the traditional process is that the thickness of the ingot in the thin slab process is greatly reduced to 50-90mm. Due to the thinness of the ingot, the ingot only needs 1-2 rough rolling (when the thickness of the ingot is 70-90mm) or no rough rolling (when the thickness of the ingot is 50mm), while the continuous casting ingot of the traditional process needs to be repeatedly rolled for multiple times before it can be thinned to the required specifications before finishing rolling; and the ingot in the thin slab process is not cooled, but directly enters the soaking furnace for soaking and soaking, or a small amount of heating, so the thin slab process greatly shortens the process flow, reduces energy consumption, reduces investment, and thus reduces production costs. However, the faster cooling rate of thin slab continuous casting and rolling will lead to increased steel strength and yield strength ratio, thereby increasing the rolling load, so that the thickness specification of economically produced hot-rolled products cannot be too thin, generally ≥1.5mm, see Chinese Patent CN200610123458.1, Chinese Patent CN200610035800.2 and Chinese Patent CN200710031548.2.

[0006] In recent years, a completely headless thin slab continuous casting and rolling process (ESP) has emerged. It is an improved process developed on the basis of the above-mentioned semi-headless thin slab continuous casting and rolling process. ESP realizes headless rolling of slab continuous casting, eliminates slab flame cutting and heating furnaces for heat preservation and transition of slabs, and greatly shortens the length of the entire production line to about 190 meters. The slabs cast by the continuous casting machine are 90-110mm thick and 1100-1600mm wide. The slabs cast by the continuous casting machine are kept warm and heated by a section of induction heating rollers, and then enter the rough rolling, finishing rolling, layer cooling, and coiling processes in sequence to obtain hot-rolled plates. This process can obtain hot-rolled plates with a thickness of 0.8mm thinnest due to headless rolling, which expands the specification range of hot-rolled plates. In addition, the output of a single production line can reach 2.2 million tons / year. At present, this process has been rapidly developed and promoted, and there are many ESP production lines in operation in the world.

[0007] The process flow is shorter than that of thin slab continuous casting and rolling. Thin strip continuous casting is a cutting-edge technology in the field of metallurgy and material research. Since Henry Bessemer proposed this idea in 1865 (US Patent: 49053), it has been developed for more than 150 years. However, due to the immaturity of related technologies such as manufacturing technology and control technology at that time, this technology was basically stagnant. It was not until the middle of the 20th century that it was realized in the continuous casting and rolling process of Al, which once again attracted people's attention in the field of steel manufacturing. Its emergence has brought a revolution to the steel industry, changing the production process of steel strips in the traditional metallurgical industry, integrating continuous casting, rolling, and even heat treatment processes, so that the thin strip blanks produced can be formed into thin steel strips in one go after a single online hot rolling, greatly simplifying the production process from molten steel to steel strips, shortening the production cycle, and its process line length is only about 50m, making the steel production process more compact, continuous, efficient, and environmentally friendly; at the same time, equipment investment is also reduced accordingly, and production costs are significantly reduced. It is a low-carbon and environmentally friendly hot-rolled thin strip production process. Therefore, thin strip continuous casting technology has become a hot spot for countries around the world to compete for development in recent years.

[0008] The typical process flow of the existing twin-roll thin strip continuous casting and rolling technology is as follows: the molten steel in the ladle is directly poured into a molten pool surrounded by two relatively rotating and rapidly cooling crystallizing rollers and a side sealing device through the ladle long nozzle, the tundish, the submerged nozzle and the distributor. The molten steel solidifies on the rotating circumferential surface of the crystallizing roller to form a solidified shell and gradually grows, and then forms a 1-5mm thick steel strip at the smallest gap between the two casting rollers (nip point). The steel strip is fed into the rolling mill through the guide plate to guide the pinch rollers for one rolling, and then cooled by the cooling device. After the head is cut by the flying shear device, the cut head falls into the flying shear pit along the flying shear guide plate, and the hot rolled strip after the head is cut enters the coiler to be coiled into a coil.

[0009] Chinese patent CN200810122770 discloses "a method for achieving ferrite rolling of interstitial-free steel on a conventional hot rolling mill", which mainly solves the technical problem that the finishing rolling of conventional IF steel in the austenite region is prone to fall into the two-phase region. By controlling the process parameters, after the rough rolling in the austenite region is completed, the finishing rolling is carried out in the ferrite region by cooling, thereby achieving ferrite rolling of IF steel. This invention adopts the traditional long-process production process, which has complex procedures, high energy consumption and high cost.

[0010] Chinese patent CN201710960186 discloses "a headless continuous casting and rolling low-carbon steel for deep drawing", and Chinese patent CN201710960187 discloses "ferrite rolling method and device for ultra-low carbon steel for ultra-deep drawing". These two patents adopt a new layout, make local improvements, use a lower finishing rolling start temperature, and require a higher final rolling temperature and coiling temperature to meet the process requirements for recrystallization and recovery of the internal structure of ferrite rolled products. This invention adopts the production process of thin slab continuous casting and rolling combined with local equipment improvements to achieve the conditions for finishing ferrite rolling, but the control is difficult and mixed crystal problems are prone to occur.

[0011] Chinese patent CN201910753893 discloses "a fully continuous production device and method for hot-rolled strip steel for ferrite rolling based on thin slab continuous casting and rolling", which uses a multifunctional cooling control device to integrate high-pressure water descaling and intermediate billet cooling functions, and adopts a 4R+(3-4)F mill layout, four temperature detectors and a close-range down coiler to achieve precise control of process temperature. The method is: continuous casting into billets → high-pressure water rotary descaling → rough rolling of a 4-frame high-pressure roughing mill → drum shearing → high-pressure water dephosphorization and post-cooling in a multifunctional cooling control device → finishing rolling of a 3 or 4-frame finishing mill → air cooling → high-speed flying shear coiling → down coiler coiling, wherein temperature monitoring is performed after rough rolling, before and after finishing rolling, and before down coiler coiling, which can easily achieve ferrite rolling of IF steel and low-carbon steel. This invention adopts the thin slab continuous casting and rolling production process, adopts the 4R+(3-4)F rolling mill layout, and adds a multifunctional cooling control device after rough rolling to perform high-pressure water dephosphorization and rapid cooling on the cast ingot to achieve ferrite rolling in the finishing rolling process. The number of rolling mill arches is large, the rolling roll consumption cost is high, and the control is complex.

[0012] Chinese patent CN20181065733 and Chinese patent CN201610768866 both proposed "a method for producing low-carbon steel by ferrite rolling in an ESP production line". The key control point is to cool the strip steel between the first and second racks and between the second and third racks with cooling water, so that the strip steel completes the transformation from austenite to ferrite before entering the third rack. When the strip steel is rolled between the third and fifth racks, the strip steel is in the ferrite zone, thereby achieving ferrite rolling. This invention is to produce low-carbon steel by ferrite rolling through the ESP production process. Since water cooling is required to control the strip steel temperature between the racks, its production is difficult and the accuracy is difficult to guarantee.

[0013] Chinese patent CN201610759108 discloses "a method for producing low-carbon steel using ferrite rolling process in CSP production line". The key process point is to use F1, F2 and F4, F5, F6, and F7 in the 7-stand finishing mill for rolling, the F3 stand is dummy, and the cooling water between the F1-F3 stands is 60-90%. Through the cooling between the stands, pure ferrite rolling is achieved in F4. After adopting the ferrite rolling process, the strength of the material is significantly reduced and the forming performance is improved. This invention uses the CSP ferrite rolling process to produce low-carbon steel. Similarly, water cooling is required between the F1-F3 stands to control the cooling of the strip. Its production is difficult and the accuracy is difficult to guarantee.

[0014] Chinese patent CN201721755853 discloses "a ferrite rolling control system, in which the slab enters a tunnel-type heating furnace after rough rolling by a rough rolling unit", and the upper and lower surfaces of the slab are heated simultaneously by the upper heating double heat storage gas burner above the water-cooled furnace roller and the lower heating double heat storage gas burner below, and the heating temperature is 900-950℃; the heated slab is descaled by the finishing rolling descaling machine and then enters the finishing rolling unit for ferrite rolling. The key point of the patented process is to use a tunnel-type soaking furnace between rough rolling and finishing rolling to achieve uniform temperature control. The tunnel furnace has high heating energy consumption, high cost per ton of steel, and it is difficult to control the head and tail temperature of the slab uniformly.

[0015] Chinese patent CN201810657331 discloses "a method for producing low-carbon steel by ferrite rolling in an ESP production line". The method includes finishing rolling the strip steel after rough rolling. During finishing rolling, the strip steel enters the first frame, the second frame, the third frame, the fourth frame, and the fifth frame in sequence. The strip steel entering between the first frame and the second frame, and between the second frame and the third frame is cooled by cooling water, so that the strip steel finishing rolling between the third frame and the fifth frame is carried out in the ferrite zone. This invention combines the technical advantages of the ESP production line with the ferrite rolling process. When the ESP production line produces low-carbon steel, it can effectively solve the problem of high yield strength ratio of the ESP production line products. Since water cooling is required to control the temperature of the strip steel between the frames, its production difficulty is relatively large and the accuracy is difficult to guarantee.

[0016] After the emergence of thin slab continuous casting and rolling technology, due to its process characteristics, it can easily produce ultra-thin hot-rolled strip steel, which has created conditions for the application of ferrite rolling technology. At present, most thin slab continuous casting and rolling production lines have reserved ferrite rolling functions. There are many patents for the application of existing ferrite rolling technology in thin slab continuous casting and rolling technology, but in the thin strip continuous casting and rolling technology that also has the characteristics of producing ultra-thin hot-rolled strip steel, there is no proposal for the concept of ferrite rolling process or related technical reports.

[0017] Due to the sub-rapid solidification process characteristics of thin strip continuous casting, the cooling rate of molten steel solidification reaches 10 2 -10 4 ℃ / s, which will lead to the phenomenon that the steel produced generally has a high yield strength ratio and poor formability. For most of the "hot instead of cold" products on the market, the product's yield strength is generally required to be low and easy to bend and stamp. Therefore, when producing such "hot instead of cold" steels, thin strip continuous casting needs to solve the problems of uneven structure and high yield strength ratio, so as to meet the requirements of the steel with a low yield strength ratio and good formability when the strip is used as a "hot instead of cold" product. Summary of the invention

[0018] The purpose of the present invention is to provide a method for realizing ferrite rolling of IF steel in an ultra-short process near the net shape, and to realize ferrite rolling of IF steel by thin strip continuous casting and rolling technology, so as to obtain ultra-thin hot-rolled IF steel, the thickness of which can reach 0.5mm at the thinnest, and the yield strength is 120-160MPa, the tensile strength is 270-310MPa, the elongation is ≥45%, and the yield strength ratio is ≤0.55. Compared with the IF steel produced by the existing hot rolling process, it has lower yield strength, tensile strength, yield strength ratio and better deep drawing performance, and the performance and thickness accuracy reach the level of cold-rolled products of the same specification, realizing "replacing cold with heat". At the same time, the production process has the advantages of simplicity and high efficiency, high production efficiency, energy saving and environmental protection, and the production line occupies a small area and has low investment cost.

[0019] To achieve the above object, the technical solution of the present invention is:

[0020] A method for realizing IF steel ferrite rolling in an ultra-short process near net shape comprises the following steps:

[0021] 1) Smelting

[0022] Smelted according to the following chemical composition, the chemical composition weight percentage is: C: 0.0001-0.005%, Si: 0.001-0.02%, Mn: 0.01-0.20%, P≤0.02%, S≤0.005%, N≤0.005%, Als<0.001%, Ca≤0.0050%, Nb: 0.02~0.08%, total oxygen [O]T: 0.007~0.020%, and the rest is Fe and other inevitable impurities;

[0023] 2) Continuous casting

[0024] The double-roller thin strip continuous casting is adopted to form a 1.5-3mm thick casting strip at the smallest gap between the two crystallization rolls; the diameter of the crystallization roll is between 500 and 1500mm, the crystallization roll is cooled by water inside, and the casting speed is 50-150m / min; the continuous casting flow distribution adopts a two-stage molten steel distribution flow distribution system, i.e., tundish + flow distributor;

[0025] 3) Lower closed protection

[0026] After the casting strip leaves the crystallization roller, the casting strip temperature is 1400-1480°C, and it directly enters the lower closed chamber, in which non-oxidizing gas is passed, and the oxygen concentration in the lower closed chamber is controlled to be less than 5%, and the casting strip temperature at the outlet of the lower closed chamber is 1100-1250°C;

[0027] 4) Austenitic rolling

[0028] The cast strip is sent to the rolling mill in a closed state through the pinch rollers in the lower closed chamber and rolled into a steel strip with a thickness of 1.0 to 2.5 mm. The austenite rolling temperature is 1000 to 1200°C and the austenite rolling reduction rate is 15 to 45%.

[0029] 5) Aerosol cooling

[0030] The steel strip after austenite rolling is cooled to below the Ar3 temperature, and the cooling adopts a mist cooling method, and the mist cooling rate is ≤200°C / s, and the Ar3 temperature of the steel strip is 890-910°C;

[0031] 6) Ferrite rolling

[0032] The cooled steel strip is sent to a rolling mill for ferrite rolling to be rolled into a steel strip with a thickness of 0.5 to 1.5 mm. The ferrite rolling temperature is 800 to 900°C and the ferrite rolling reduction rate is 20 to 50%.

[0033] 7) Coiling

[0034] The steel strip after ferrite rolling is directly coiled into a coil, and the coiling temperature is 700-760℃;

[0035] 8) Slow cooling

[0036] After the steel strip is coiled, it is slowly cooled to below 550°C, then stored and naturally air-cooled.

[0037] Preferably, in step 5), the air-water ratio of the aerosol cooling is 15-10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

[0038] Preferably, in step 6), the ferrite rolling temperature is 830-880°C.

[0039] Preferably, in step 8), the steel strip is slowly cooled in a heat-insulating hood or a slow cooling pit after being coiled.

[0040] Preferably, in step 3), the non-oxidizing gas is nitrogen, argon or CO2 obtained by sublimation of dry ice.

[0041] Preferably, in step 2), the diameter of the crystallization roller is 780-820 mm.

[0042] Preferably, in step 7), the winding is in a double winding form, or in a carrousel winding form, preferably in a carrousel winding form.

[0043] The microstructure of the IF steel of the present invention is uniform polygonal ferrite.

[0044] The grain size of the microstructure of the IF steel of the present invention is 30-100 um.

[0045] The IF steel of the present invention has a yield strength of 120-160 MPa, a tensile strength of 270-310 MPa, an elongation of ≥45%, and a yield strength ratio of ≤0.55.

[0046] In the composition design of the IF steel of the present invention:

[0047] C: The IF steel described in the present invention is an ultra-low carbon steel, such as Figure 2 The deformation resistance curves of the IF steel of the present invention at different deformation temperatures show that there is an obvious deformation resistance valley in the ferrite phase transformation temperature range, which is suitable for the ferrite rolling process. The lower finishing temperature will not increase the rolling deformation resistance, resulting in an increase in the load and roll consumption of the rolling equipment. Therefore, the C content in the present invention is controlled to be 0.0001-0.005%.

[0048] Si: In order to avoid introducing interstitial atoms in IF steel, it is generally not necessary to add Si, but the steel will inevitably contain Si. A high Si content is likely to increase the strength and decrease the elongation. The present invention pursues lower strength and higher elongation, so the Si content is controlled to be 0.001-0.02% in the present invention.

[0049] Mn: Mn is one of the cheapest alloying elements. It can improve the hardenability of steel and has a considerable solid solubility in steel. It can improve the strength of steel through solid solution strengthening, and has little damage to the plasticity and toughness of steel. It is the most important strengthening element for improving the strength of steel and can also play a role in deoxidation in steel. However, too high a Mn content will lead to deterioration of weldability and toughness of the welding heat affected zone. The present invention pursues lower strength and yield strength ratio, and the Mn content does not need to be too high. Therefore, the present invention controls the Mn content to 0.001-0.20%.

[0050] P: High P content is easy to segregate at the grain boundary, increase the cold brittleness of steel, deteriorate welding performance, reduce plasticity, and deteriorate cold bending performance. The traditional process of producing IF steel has strict requirements on P. A dephosphorization process must be included in the steelmaking process, and the P content is generally required to be controlled below 0.01%; in the thin strip continuous casting process, the solidification and cooling rates are extremely fast, which can effectively inhibit the segregation of P, thereby effectively avoiding the disadvantages of P and giving full play to the advantages of P. Therefore, in the present invention, the content of P element can be appropriately relaxed, the dephosphorization process can be cancelled in the steelmaking process, and in actual operation, there is no need to add additional phosphorus. In this case, the P content is generally ≤0.02%.

[0051] S: S is usually a harmful element in steel, which makes steel hot brittle and reduces its ductility and toughness. S is easy to form MnS in steel. The amount and form of sulfides in steel directly affect the formability of steel plates. S must be less than 0.005%. The amount and form of inclusion elements have a great influence on the deep drawing performance of steel plates, especially strip-shaped sulfide inclusions that easily cause cracks during deformation. Therefore, the S content is controlled to be ≤0.005% in the present invention.

[0052] Als: To control inclusions in steel, the present invention requires that Al cannot be used for deoxidation. In the use of refractory materials, additional introduction of Al should also be avoided as much as possible. However, steel will inevitably contain Al, and the content of acid-soluble aluminum Als is strictly controlled to be <0.001%.

[0053] N: The present invention utilizes N in steel to react with C and Nb in steel to generate carbon and nitrogen compounds, so that the matrix is ​​in a state of interstitial atoms, which requires a certain N content in the steel. However, N has a great harm to the plasticity and toughness of steel, and the presence of free N will increase the yield strength ratio of steel, so the N content cannot be too high. The present invention controls the N content to ≤0.005%.

[0054] Ca: can change the form of sulfides in steel, transforming long MnS inclusions into spherical CaS inclusions, improving the plasticity and toughness of the steel plate, and helping to improve the formability of the steel plate. The present invention controls the Ca content to less than 0.0050%.

[0055] Nb: Nb is a strong carbon and nitrogen compound forming element, which can react with C and N atoms in steel to form carbon and nitrogen compounds, making the matrix present a state of no interstitial atoms. Since it does not contain interstitial atoms, IF steel can have the characteristics of high plastic anisotropy (r) value, high work hardening index (n) and no aging, and has excellent deep drawing performance. The present invention controls the Nb content to 0.02-0.08%.

[0056] The present invention utilizes the thin strip continuous casting and rolling technology to perform two rolling processes during the rolling process, firstly performing austenite rolling at 1000-1200°C, and then using mist cooling to effectively and accurately control the temperature of the strip steel to uniformly drop to 5-50°C below the Ar3 temperature, ensuring that the strip steel can be ferrite rolled during the second rolling, thereby realizing the ferrite rolling of IF steel by the thin strip continuous casting and rolling technology, obtaining a product with lower yield strength and tensile strength, lower yield strength ratio, better deep drawing performance, and performance and thickness accuracy reaching the level of cold-rolled products of the same specification, thus realizing "replacing cold with heat". When the obtained product is used as a cold-rolled base material, it effectively solves the problem of high strength and difficult cold-rolling deformation of similar IF steels in thin slab continuous casting or thin strip continuous casting under austenite rolling process.

[0057] The complex intermediate steps in the traditional production process such as flame cutting of ingots, ingot heating furnace, rough rolling, multi-stand finishing rolling, etc. are eliminated. The overall production line has the advantages of small footprint, simplicity and efficiency, high production efficiency, energy saving and environmental protection.

[0058] The molten steel of the present invention can be smelted by electric furnace steelmaking or converter steelmaking, and then enter necessary refining steps, such as LF furnace, VD / VOD furnace, RH furnace, etc.

[0059] The continuous casting adopts double-roller thin strip continuous casting, and a 1.5-3mm thick casting strip is formed at the smallest gap between the two crystallization rollers. After the casting strip comes out of the crystallization roller, the casting strip temperature is 1400-1480℃, and it directly enters the lower closed chamber. The lower closed chamber is filled with non-oxidizing gas, which can realize the anti-oxidation protection of the strip steel on the one hand, and the cooling of the strip steel on the other hand. The non-oxidizing gas can be N2, Ar, or other non-oxidizing gases, such as CO2 gas obtained by sublimation of dry ice.

[0060] The cast strip is sent to the hot rolling mill for austenite rolling in the lower closed chamber through the pinch rollers in a closed state. The thickness of the steel strip after austenite rolling is 1.0-2.5 mm, the austenite rolling temperature is 1000-1200°C, and the austenite rolling reduction rate is 15-45% to ensure static recrystallization of austenite.

[0061] The strip steel after austenite rolling is cooled to 5-50°C below the Ar3 temperature, and the cooling rate is ≤200°C / s. The Ar3 temperature is mainly related to the chemical composition of the strip steel, the austenite grain size, the rolling temperature, the deformation amount and the cooling rate. The Ar3 temperature during the rolling process can be calculated by the following formula: Ar3=910-310×w(C)-80×w(Mn)-20×w(Cu)-15×w(Cr)-55×w(Ni)-80×w(Mo). After calculation, the Ar3 temperature range of the IF steel involved in the present invention is 890-910°C.

[0062] The cooling method adopts aerosol cooling, and the air-water ratio of aerosol cooling is 15-10:1, the air pressure is 0.5-0.8MPa, and the water pressure is 1.0-1.5MPa. The aerosol cooling method can effectively reduce the thickness of the oxide scale on the surface of the strip, improve the temperature uniformity of the strip, and improve the surface quality and forming performance of the strip. After the aerosol, high-pressure water mist is sprayed on the surface of the steel strip. On the one hand, it plays a role in reducing the temperature of the steel strip. On the other hand, the water mist will form a dense air film covering the surface of the strip, which plays a role in preventing the strip from oxidation, thereby effectively controlling the growth of oxide scale on the surface of the hot-rolled strip. This cooling method can avoid the problems caused by traditional spraying or laminar cooling, make the surface temperature of the strip drop evenly, improve the temperature uniformity of the strip, and thus achieve the effect of homogenizing the internal microstructure.

[0063] In traditional continuous casting, mist cooling is also used, but the effect area and temperature are different. In traditional continuous casting, mist cooling is performed on the billet in the outlet sector area of ​​the billet crystallizer. At this time, the temperature of the billet is relatively high, and from the phase diagram, it should be in the high-temperature austenite single-phase area. The main purpose of mist cooling in this area is to control the end position of solidification, accelerate the surface cooling of the billet, refine the surface austenite grain structure, increase the surface strength of the billet, improve the surface quality of the billet, and avoid the occurrence of cracks. The present invention performs mist cooling on the ultra-thin strip after austenite rolling. By adjusting the mist cooling intensity, the strip temperature can be effectively and accurately controlled to be 5 to 50°C below the Ar3 temperature to ensure that the strip can be ferrite rolled during the second rolling process.

[0064] The cooled strip is subjected to ferrite rolling, and the ferrite rolling temperature is 800-900°C, preferably 830-880°C. For the steel type involved in the present invention, this temperature region is in the area where the high-temperature deformation resistance is low, and this area is suitable for ferrite rolling; the reduction rate of ferrite rolling is controlled to be 20-50%, and after rolling, an ultra-thin hot-rolled strip is obtained, and the thickness of the strip is 0.5-1.5mm; ferrite rolling adopts lubrication rolling, which can improve the surface quality of the strip and reduce the rolling loss of the roller.

[0065] After ferrite rolling, the strip steel does not need to be cooled, and directly enters the coiler for high-temperature coiling, with a coiling temperature of 700-760°C; the steel coil is cut by a high-speed flying shear, and the high-temperature steel coil after cutting is unloaded from the coiler and then slowly cooled by an online heat preservation cover, or quickly enters the slow cooling pit for slow cooling, so that the strip steel undergoes a more sufficient recovery and recrystallization, avoiding unqualified performance caused by faster cooling of the head and tail of the steel coil during air cooling, which is conducive to obtaining good product quality and yield, making it easy to stamp and bend, and better meet the performance requirements of "replacing cold with heat" or subsequent cold rolling deformation. When the temperature of the steel coil slowly drops to below 550°C, it can be put into storage for natural air cooling.

[0066] Through the above manufacturing process, the microstructure of the ferrite rolled IF steel finally obtained is a polygonal ferrite structure, with a yield strength of 120-160MPa, a tensile strength of 270-310MPa, an elongation of ≥45%, and a yield strength ratio of ≤0.55. The product has lower strength, lower yield strength ratio, higher elongation, and better formability, which can better meet the performance requirements of "heat instead of cold" products; at the same time, when used as a cold rolling base material, it can also effectively solve the problem of high yield strength ratio and difficult cold rolling deformation of similar IF steel produced by thin slab continuous casting or thin strip continuous casting under austenitic rolling process.

[0067] The reason why the thin strip continuous casting hot-rolled steel coil of the present invention preferably adopts the Carrousel iso-coiler is as follows:

[0068] At present, most of the production lines for ultra-thin hot-rolled steel coils adopt underground double coiling or underground triple coiling. The main reason is that these production lines also take into account the production of thick hot-rolled plates. For example, the coiling of Avedi's ESP production line adopts underground triple coiling, and the coiling of Danieli's FTSC production line introduced by Tangshan Iron and Steel adopts underground double coiling. The Castrip thin strip continuous casting production line of Nucor in the United States adopts the traditional approach and also adopts underground double coiling. The distance between the underground coiler and the coiler is generally 8-10m (typical value 9.4m). When the thin strip continuous casting produces ultra-thin hot-rolled strip steel, the cooling speed of the strip steel in the air is also very fast. This interval is enough to affect the difference in coiling temperature. The temperature deviation between the two coilers can reach 49°C, which will seriously affect the performance deviation of the steel coil.

[0069] The present invention preferably adopts the Carrousel coiling method, which can realize the co-situ coiling of hot-rolled steel coils, ensure the uniformity of the coiling temperature, and thus greatly improve the stability of the performance of the steel coil products. Currently on the market, Carrousel coilers are widely used in the field of cold rolling. Their main advantages are that they can realize the coiling of thinner strips, and they occupy a small area, which can greatly shorten the length of the production line. However, in the field of cold rolling, due to the lower temperature of the strip, it is easier to implement. The present invention proposes to adopt the Carrousel coiling method in the field of ultra-thin hot-rolled strip coiling, taking into account the high temperature resistance of the equipment, and realizing the coiling of ultra-thin hot-rolled strip. This coiling method is more advanced than the coiling method of the Castrip thin strip continuous casting production line of Nucor (Nucor) in the United States.

[0070] Beneficial effects of the present invention:

[0071] 1. The present invention adopts an ultra-short process near-net shape process to produce IF steel, namely, thin strip continuous casting + austenite rolling + ferrite rolling. Two rolling processes are performed during the rolling process, and mist cooling is used between the two rolling processes to effectively and accurately control the temperature of the strip to be uniformly reduced to 5-50°C below the Ar3 temperature, so as to ensure that the strip can be ferrite rolled in the second rolling stage, thereby obtaining an ultra-thin IF steel with uniform ferrite structure, the thinnest of which can reach 0.5mm, while the IF steel obtained by the traditional hot rolling process is the thinnest at 1.5mm, and the IF steel obtained by the thin slab continuous casting and rolling process is the thinnest at 0.8mm, and the production difficulty is extremely great; the IF steel obtained by the present invention has a yield strength of 120-160MPa, a tensile strength of 270-310MPa, an elongation of ≥45%, and a yield strength ratio of ≤0.55. Compared with the IF steel produced by the existing hot rolling process, the IF steel has lower yield strength, tensile strength, yield strength ratio and better deep drawing performance, and the performance and thickness accuracy reach the level of cold-rolled products of the same specification, realizing "replacing cold with heat".

[0072] 2. The present invention adopts an ultra-short process near-net-shape process to produce IF steel with lower yield strength and tensile strength, lower yield strength ratio, and better deep drawing performance, eliminating the complex intermediate steps of flame cutting of ingots, ingot heating furnace, rough rolling, multi-stand finishing rolling, etc. in the traditional production process of IF steel. The production process is simple and efficient, with high production efficiency, energy saving and environmental protection, and the production line occupies a small area and has low investment cost.

[0073] 3. In the IF production process of the present invention, the strip is cooled by mist cooling after austenite rolling, while the conventional process usually uses spraying or laminar cooling. The mist cooling can make the surface temperature of the IF steel drop evenly, improve the temperature uniformity of the strip, and make the IF steel obtain a uniform ferrite structure after ferrite rolling; at the same time, the uniform cooling can improve the plate quality and formability of the IF steel and reduce the oxide scale thickness on the surface of the IF steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 The figure is a schematic diagram of the process flow of an embodiment of the present invention.

[0075] Figure 2 This is the deformation resistance curve of IF steel at different deformation temperatures according to the embodiment of the present invention.

[0076] Figure 3 This is a microstructure photograph of the IF steel according to the embodiment of the present invention. DETAILED DESCRIPTION

[0077] The present invention is further described below by way of embodiments and drawings, but this is not intended to limit the present invention. Those skilled in the art may make modifications or improvements based on the basic idea of ​​the invention, but as long as they do not deviate from the basic idea of ​​the invention, they are all within the scope of the present invention.

[0078] See also Figure 1 The method for achieving IF steel ferrite rolling in an ultra-short process near net shape according to the present invention.

[0079] The smelted molten steel that meets the chemical composition design of the present invention is directly poured from the ladle 1 through the ladle long nozzle 2, the middle ladle 3, the submerged nozzle 4 and the distributor 5 into a molten pool 7 surrounded by two relatively rotating and rapidly cooling double-roller thin strip continuous casting crystallization rollers 8a, 8b and side sealing plates 6a, 6b. The molten steel solidifies on the rotating circumferential surface of the crystallization rollers 8a, 8b, and then forms a 1.5-3mm thick cast strip 11 at the smallest gap between the two crystallization rollers 8a, 8b (nip point). The diameter of the crystallization rollers 8a, 8b described in the present invention is between 500-1500mm, and water is passed inside for cooling. According to the thickness of the cast strip 11, the casting speed of the casting machine ranges from 60-150m / min.

[0080] The casting strip 11 comes out of the crystallization rollers 8a and 8b, and the casting strip temperature is 1400-1480°C. It directly enters the closed chamber 10. The lower closed chamber 10 is passed through a non-oxidizing gas to protect the strip steel. On the one hand, it realizes the anti-oxidation protection of the casting strip 11, and on the other hand, it cools the casting strip 11. The anti-oxidation protection atmosphere can be N2, Ar, or other non-oxidizing gases, such as CO2 gas obtained by sublimation of dry ice. The oxygen concentration in the closed chamber 10 is controlled at <5%. 10 provides anti-oxidation protection for the cast strip 11 to the entrance of the rolling mill 13, and the temperature of the cast strip 11 at the exit of the lower closed chamber 10 is between 1100 and 1250°C; then the cast strip 11 passes through the swing guide plate 9 and the pinch roller 12 and enters the four-roller 1# hot rolling mill 13 for austenite rolling. The austenite rolling temperature is 1000-1200°C, and the hot rolling reduction rate is controlled to be 15-45% to ensure static recrystallization of austenite. The thickness of the steel strip after austenite rolling is 1.0-2.5mm.

[0081] The strip steel after austenite rolling is cooled to below 910°C by gas mist cooling, and the cooling rate is ≤200°C / s; the cooled strip steel enters the online four-roller hot rolling mill 20 (2# rolling mill) for ferrite rolling, the ferrite rolling temperature is 800-900°C, preferably 830-880°C, the hot rolling reduction rate is controlled to be 20-50%, and ultra-thin hot-rolled strip steel is obtained after ferrite rolling, and the thickness of the steel strip is 0.5-1.5mm. The ferrite rolling adopts lubrication rolling, which can improve the surface quality of the strip steel and reduce the rolling loss of the rolling roller; the strip steel after ferrite rolling does not need to be cooled, and after the high-speed flying shear 16 is cut, the cut head is cut along the The flying shear guide plate 17 falls into the flying shear pit 18, and the subsequent strip steel directly enters the coiler 19 for high-temperature coiling, and the coiling temperature is 700-760℃; the steel coil is cut by the high-speed flying shear 16 according to the weight of the strip steel coil, and the high-temperature steel coil after cutting is unloaded from the coiler 19 and then slowly cooled by an online heat preservation cover, or quickly enters the slow cooling pit for slow cooling, so that the strip steel undergoes a more sufficient recovery and recrystallization, avoiding the performance problems caused by the rapid cooling of the head and tail of the steel coil during air cooling, which is conducive to obtaining good product quality and yield, and a lower yield strength ratio, which is easy to stamp and bend, and better meets the performance requirements of "replacing cold with heat" or subsequent cold rolling deformation. When the temperature of the steel coil slowly drops to below 550℃, it can be put into storage for natural air cooling.

[0082] The coiler 19 adopts a double coiling form, and can also adopt a Carrousel coiling form to ensure the continuous production of aluminum alloy thin strips.

[0083] The chemical composition of the IF steel of the embodiment of the present invention is shown in Table 1, and the remainder of the composition is Fe and other inevitable impurities. The process parameters of the preparation method of the embodiment of the present invention are shown in Table 2, and the properties of the IF steel finally obtained are shown in Table 3. Comparative Examples 1 and 2 in the table are the relevant chemical composition, process parameters and product properties of the IF steel produced by the existing typical twin-roll thin strip continuous casting and rolling process technology.

[0084] It can be seen from Table 3 that the IF steel obtained by the process of the present invention has a yield strength of 120-160 MPa, a tensile strength of 270-310 MPa, an elongation of ≥45%, and a yield strength ratio of ≤0.55. Compared with the IF steel obtained in Comparative Examples 1 and 2, it has lower strength, lower yield strength ratio, and better formability.

[0085] After the above manufacturing process, the microstructure of the ferrite rolled IF steel is shown in Figure 3 .from Figure 3 It can be seen from the figure that the microstructure of the IF steel is polygonal ferrite with uniform structure and grain size of 30 to 100 um.

[0086] Compared with the IF steel produced by the existing hot rolling process, the IF steel obtained by the present invention has lower strength, lower yield strength ratio and better forming performance, and can better meet the performance requirements of "replacing cold with heat" products; at the same time, when used as a cold rolling base material, it can also effectively solve the problem of high strength and difficult cold rolling deformation of similar IF steels produced by thin slab continuous casting or thin strip continuous casting under the austenitic rolling process.

[0087]

[0088]

[0089]

Claims

1. A method for realizing IF steel ferrite rolling in an ultra-short process near net shape, characterized in that: The steps include: 1) Smelting Smelted according to the following chemical composition, the chemical composition weight percentage is: C: 0.0001-0.005%, Si: 0.001-0.02%, Mn: 0.01-0.20%, P≤0.02%, S≤0.005%, N≤0.005%, Als<0.001%, Ca≤0.0050%, Nb: 0.02~0.08%, total oxygen [O]T: 0.007~0.020%, and the rest is Fe and other inevitable impurities; 2) Continuous casting The double-roller thin strip continuous casting is adopted to form a 1.5-3mm thick casting strip at the smallest gap between the two crystallization rolls; the diameter of the crystallization roll is 500-1500mm, the crystallization roll is cooled by water inside, and the casting speed is 50-150m / min; the continuous casting flow distribution adopts a two-stage molten steel distribution flow distribution system, i.e., tundish + flow distributor; 3) Lower closed protection After the casting strip leaves the crystallization roller, the casting strip temperature is 1400-1480°C, and it directly enters the lower closed chamber, in which non-oxidizing gas is passed, and the oxygen concentration in the lower closed chamber is controlled to be less than 5%, and the casting strip temperature at the outlet of the lower closed chamber is 1100-1250°C; 4) Austenitic rolling The cast strip is sent to the rolling mill in a closed state through the pinch rollers in the lower closed chamber and rolled into a steel strip with a thickness of 1.0 to 2.5 mm. The austenite rolling temperature is 1000 to 1200°C and the austenite rolling reduction rate is 15 to 45%. 5) Aerosol cooling The steel strip after austenite rolling is cooled to 5-50°C below the Ar3 temperature, and the cooling is carried out by aerosol cooling, and the aerosol cooling rate is ≤200°C / s, and the Ar3 temperature of the steel strip is 890-910°C; 6) Ferrite rolling The cooled steel strip is sent to a rolling mill for ferrite rolling to be rolled into a steel strip with a thickness of 0.5 to 1.5 mm. The ferrite rolling temperature is 800 to 900°C and the ferrite rolling reduction rate is 20 to 50%. 7) Coiling The steel strip after ferrite rolling is directly coiled into a coil, and the coiling temperature is 700-760°C; 8) Slow cooling After the steel strip is coiled, it is slowly cooled to below 550°C, then stored and naturally air-cooled.

2. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape as claimed in claim 1, characterized in that: In step 2), the diameter of the crystallization roller is 780-820 mm.

3. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape as claimed in claim 1, characterized in that: In step 3), the non-oxidizing gas is nitrogen, argon or CO2 obtained by sublimation of dry ice.

4. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape as claimed in claim 1, characterized in that: In step 5), the air-water ratio of the mist cooling is 15-10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

5. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape as claimed in claim 1, characterized in that: In step 6), the ferrite rolling temperature is 830-880°C.

6. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape as claimed in claim 1, characterized in that: In step 7), the winding is carried out in a double winding form or a carrousel winding form.

7. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape as claimed in claim 1, characterized in that: In step 8), the steel strip is slowly cooled in a heat preservation cover or a slow cooling pit after being coiled.

8. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape according to any one of claims 1 to 7, characterized in that: The microstructure of the IF steel is uniform polygonal ferrite.

9. The method for realizing IF steel ferrite rolling in an ultra-short process near net shape according to any one of claims 1 to 7, characterized in that: The grain size in the microstructure of the IF steel is 30-100 um.

10. The method for achieving near-net-shape IF steel ferrite rolling in an ultra-short process as claimed in claim 8, characterized in that: The grain size in the microstructure of the IF steel is 30-100 um.

11. The method for achieving near-net-shape IF steel ferrite rolling in an ultra-short process as claimed in any one of claims 1 to 7 and 10, characterized in that: The IF steel has a yield strength of 120-160 MPa, a tensile strength of 270-310 MPa, an elongation of ≥45%, and a yield strength ratio of ≤0.

55.

12. The method for achieving near-net-shape IF steel ferrite rolling in an ultra-short process as claimed in claim 8, characterized in that: The IF steel has a yield strength of 120-160 MPa, a tensile strength of 270-310 MPa, an elongation of ≥45%, and a yield strength ratio of ≤0.

55.

13. The method for achieving near-net-shape IF steel ferrite rolling in an ultra-short process as claimed in claim 9, characterized in that: The IF steel has a yield strength of 120-160 MPa, a tensile strength of 270-310 MPa, an elongation of ≥45%, and a yield strength ratio of ≤0.55.

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