CN115478143A - A method for realizing IF ferrite rolling of near-net shape in ultra-short process - Google Patents

Abstract

A method for realizing ferritic rolling of IF steel with near-net shape in an ultra-short process, using thin strip continuous casting + austenitic rolling + ferritic rolling, rolling twice during the rolling process, and The gas mist cooling is used between the two rollings to effectively and precisely control the temperature of the strip to be uniformly lowered to 5-50°C below the Ar ₃ temperature, so as to ensure that the strip can be rolled ferritely in the second rolling stage, so as to obtain a uniform Ultra-thin IF steel with ferrite structure, the thinnest thickness can reach 0.5mm, 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, performance and thickness accuracy reach the level of cold-rolled products of the same specification, realizing "replacing cold with heat". The overall production process is simple and efficient, high production efficiency, energy saving and environmental protection and other advantages, the production line occupies a small area, and the investment cost is low.

Description

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

technical field

The invention relates to the field of IF steel manufacturing, in particular to a method for realizing IF steel ferrite rolling in an ultra-short process near net shape.

Background technique

IF steel is a kind of ultra-low carbon steel that adds strong carbon and nitrogen compounds such as Ti or Nb to form elements, and reacts with C and N atoms in the steel to form carbon and nitrogen compounds, making the matrix in a state without interstitial atoms, also known as Interstitial free steel. Because 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, especially its excellent deep drawing performance, so that IF steel is used as The third generation of deep-drawn steel plates has been widely used in industries such as automobiles and home appliances.

Ferritic rolling was first successfully developed by Professor Appell and his team from the Belgian Iron and Steel Research Center in 1994. At that time, the main purpose of developing this technology was to replace cold-rolled products in the thickness range of 1.0-2.0mm with thin-gauge hot-rolled products to achieve "Replacing cold with heat" reduces production costs. Under the traditional process flow, the technical process is: continuous casting billet + heating furnace heating and heat preservation + rough rolling (austenitic rolling) + rapid cooling + finishing rolling (ferritic rolling) + cooling + coiling, That is to say, firstly, the cast slab with a thickness of about 200mm is obtained through continuous casting, after the slab is reheated and kept warm, rough rolling and finish rolling are carried out, and finally the steel strip is laminarly cooled and coiled to complete the whole hot rolling production process . What is special is that the rough rolling process is completed in a fully austenitic state, and a rapid cooling device is installed between the rough rolling mill and the finishing mill to reduce the temperature of the strip steel to below Ar ₃ before entering the finishing mill. , the phase transformation of γ→α has been completed, so that the entire finishing rolling process is carried out in the ferrite region or in the two-phase region of ferrite and austenite. In the conventional austenite rolling process, the rough rolling and finishing rolling processes are all completed in the austenite zone. The above-mentioned traditional ferrite rolling process has long process flow, high energy consumption, many unit equipment, high infrastructure cost and high production cost.

In the comparative test of ferritic rolling and austenitic rolling of IF steel, ferritic rolling can obtain ferrite structure with coarse grains, which has lower yield strength and tensile strength, yield strength Relatively low, larger r value, better formability. Since the temperature of ferritic rolling is significantly lower than that of austenitic rolling, which is lower than that of austenitic rolling by more than 200°C, the lower temperature of finishing rolling will lead to a significant increase in rolling deformation resistance, increase the load of roll equipment, and increase roll consumption , which is not conducive to long-term rolling; at the same time, the rolling force increases, the deformation is difficult, the shape of the plate is difficult to control, and the rolling process cannot be guaranteed, so not all steel types are suitable for the ferritic rolling process. For some steel types with a large range of ferrite zone and low deformation resistance in the temperature range of ferrite zone, the high temperature deformation resistance characteristics of IF steel conform to such characteristics.

Thin slab continuous casting and rolling process is: continuous casting + slab heat preservation and soaking + hot continuous rolling + cooling + coiling. The main difference between this process and the traditional process is that the thickness of the slab in the thin slab process is greatly reduced, which is 50-90mm. time) or does not need to undergo rough rolling (when the thickness of the casting slab is 50mm), while the continuous casting slab of the traditional process has to go through repeated multi-pass rolling before it can be thinned to the required specifications before the finish rolling; and the casting slab of the thin slab process The slab is not cooled, and directly enters the soaking furnace for soaking heat preservation, or a small amount of temperature supplementation, so the thin slab process greatly shortens the process flow, reduces energy consumption, reduces investment, and thus reduces production costs. However, due to the faster cooling rate of thin slab continuous casting and rolling, the steel strength and yield ratio will increase, 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.

An all-endless thin slab continuous casting and rolling process (abbreviation: ESP), which has emerged in recent years, is an improved process developed on the basis of the above-mentioned semi-endless thin slab continuous casting and rolling process. The endless rolling of slab continuous casting cancels the slab flame cutting and the heating furnace that plays the role of heat preservation and soaking and slab transition. The length of the whole production line is greatly shortened to about 190 meters. The slab cast by the continuous casting machine The thickness is 90-110mm, and the width is 1100-1600mm. The continuous casting slab passes through a section of induction heating roller table to keep the slab warm and warm, and then enters rough rolling, finish rolling, layer cooling and coiling in sequence. The hot-rolled sheet is obtained in the process. Due to the realization of endless rolling, this process can obtain the hot-rolled sheet with the thinnest thickness of 0.8mm, which expands the specification range of the hot-rolled sheet. In addition, the output of a single production line can reach 2.2 million t/year scale. At present, the process has been rapidly developed and promoted, and there are already many ESP production lines in operation in the world.

A shorter process than thin slab casting and rolling is thin strip casting and rolling. Thin strip continuous casting technology 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. The development of related technologies such as manufacturing technology and control technology is not mature enough, which makes this technology basically stagnant. It was not realized in the continuous casting and rolling process of Al until the middle of the 20th century, which once again attracted people's attention in the field of steel manufacturing. Its appearance brought a revolution to the iron and steel industry, changed the production process of steel strips in the traditional metallurgical industry, integrated continuous casting, rolling, and even heat treatment, and made the produced thin strips go through on-line processes one by one. Hot rolling forms a thin steel strip at one time, which greatly simplifies the production process from molten steel to steel strip and shortens the production cycle. The length of the process line is only about 50m, making the steel production process more compact, more continuous, more efficient and more environmentally friendly ; At the same time, the equipment investment is also reduced correspondingly, and the production cost is significantly reduced. It is a low-carbon and environmentally friendly hot-rolled strip production process. Therefore, thin strip continuous casting technology has become a hot spot in the development of countries all over the world in recent years.

The typical process flow of the existing twin-roll thin strip continuous casting and rolling technology: molten steel in the ladle is directly poured into a two-roller-rotating and capable In the melting pool surrounded by the rapidly cooled crystallization roll and the side sealing device, the molten steel solidifies on the rotating peripheral surface of the crystallization roll to form a solidification shell and gradually grows, and then forms a 1-5mm shell at the smallest gap (nip point) between the two casting rolls. Thick steel strip, the steel strip is sent into the rolling mill through the guide plate to guide the pinch roller for one rolling, and then cooled by the cooling device, after being cut by the flying shear device, the cut head falls into the flying shear pit along the flying shear guide plate, The cut hot strip enters the coiler to be coiled into coils.

Chinese patent CN200810122770 discloses "a ferritic rolling method for realizing interstitial atom-free steel on a traditional hot rolling mill", which mainly solves the problem that the traditional IF steel rolled in the austenitic zone tends to fall into the two-phase zone in the finish rolling According to the technical problem, through the control of process parameters, after the rough rolling in the austenite area, cooling makes the finishing rolling in the ferrite area, so as to realize the ferritic rolling of IF steel. The invention adopts a traditional long-flow production process, which has complex procedures, large energy consumption and high cost.

Chinese patent CN201710960186 discloses "a low-micro carbon steel for endless continuous casting and rolling for deep drawing", and Chinese patent CN201710960187 discloses "ferrite rolling method and device for ultra-low carbon steel for ultra-deep drawing". The patent adopts a new arrangement form and makes local improvements. It adopts a lower finish rolling start temperature, and requires a higher finish rolling temperature and coiling temperature to meet the needs of recrystallization and recrystallization of the internal structure of ferritic rolled products. Reply with the desired process request. The invention adopts the thin slab continuous casting and rolling production process combined with local equipment improvement to realize the conditions of finishing ferrite rolling, but the control is difficult and the problem of mixed crystals is prone to occur.

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". The billet cooling function is integrated, and the 4R+(3-4)F rolling mill layout, four temperature detectors and close-distance downcoilers are used to achieve precise control of the process temperature. The method is: continuous casting into billets → high-pressure water rotary descaling → rough rolling in 4-stand high-pressure roughing mill → rotary drum shearing → cooling after dephosphorization by high-pressure water in multi-functional cooling device → 3 or 4-stand finishing Rolling mill finishing rolling → air cooling → high-speed flying shear coiling → downcoiler coiling, in which temperature monitoring is carried out separately after rough rolling, before and after finish rolling, and before downcoiler coiling, which can easily realize IF steel and ferritic rolling of low carbon steel. The invention adopts thin slab continuous casting and rolling production technology, adopts 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 of the cast slab to achieve finish rolling In the process of ferrite rolling, the number of rolling mill arches is large, the cost of rolling roll consumption is high, and the control is complicated.

Chinese patent CN20181065733 and Chinese patent CN201610768866 both proposed "a method for producing low-carbon steel by ferrite rolling in the ESP production line". The strip between the second stand and the third stand is cooled by cooling water, so that the strip steel completes the transformation from austenite to ferrite before entering the third stand, and the rolling between the third stand and the fifth stand When making strip steel, the strip steel is in the ferrite zone, thus achieving ferrite rolling. This invention is to produce low-carbon steel by ferrite rolling through ESP production process. Since the temperature of the strip steel needs to be controlled by water cooling between the racks, the production is difficult and the precision is difficult to guarantee.

Chinese patent CN201610759108 discloses "a method for producing low-carbon steel using a ferrite rolling process in a CSP production line". Rolling, the F3 frame is virtual, the cooling water between the F1-F3 frames is 60-90%, through the cooling between the frames, the pure ferrite rolling is realized in F4, and the strength of the material is improved after the ferrite rolling process is adopted. Significantly decreased, the formability improved. This invention adopts the CSP ferrite rolling process to produce low-carbon steel. Also, water cooling is required to control the strip steel between the F1-F3 stands, which is difficult to produce and difficult to guarantee the precision.

Chinese patent CN201721755853 discloses "a ferrite rolling control system. The slab is rough-rolled by the roughing mill and then enters the tunnel-type heating furnace". Heating Double regenerative gas burners heat the upper and lower sides of the slab at the same time, the heating temperature is 900-950°C; 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 heating energy consumption of the tunnel furnace is large, the cost per ton of steel is high, and the temperature at the head and tail of the slab is difficult to control uniformly.

Chinese patent CN201810657331 discloses "a method for producing low-carbon steel by ferrite rolling in an ESP production line". frame, the second frame, the third frame, the fourth frame, the fifth frame, for the strip steel entering between the first frame and the second frame, between the second frame and the third frame Cooling with cooling water is carried out, so that the rolling between the third stand and the fifth stand of the strip finish rolling is carried out in the ferrite area. By combining the technical advantages of the ESP production line with the ferrite rolling process, the invention can effectively solve the problem of high yield ratio of the ESP production line when the ESP production line produces low-carbon steel. Water cooling controls the temperature of the strip steel, which is difficult to produce and the accuracy is difficult to guarantee.

After the emergence of thin slab continuous casting and rolling technology, due to its technological characteristics, it can easily produce ultra-thin hot-rolled strip steel, which creates 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 on the application of the existing ferrite rolling technology in the 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 ferrite rolling technology. Proposal of the rolling process concept or related technical reports.

Due to its own 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 common phenomenon of high yield ratio and poor formability of the steel produced. For most of the products that "replace heat with cold" on the market, it is generally required that the yield strength of the product is relatively low, and it is easy to bend and stamp. Therefore, when thin strip continuous casting produces this kind of "replacing heat with cold" steel, it is necessary to solve the problems of uneven structure and high yield ratio, so as to meet the needs of strip steel when it is used as a "replace heat with cold" product. The steel grade has a low yield ratio and good formability.

Contents of the invention

The purpose of the present invention is to provide a method for realizing ferritic rolling of IF steel in a near-net shape in an ultra-short flow process, and utilize thin strip continuous casting and rolling technology to realize ferritic rolling of IF steel to obtain ultra-thin specification hot-rolled IF steel , the thinnest thickness can reach 0.5mm, its yield strength is 120-160MPa, tensile strength is 270-310MPa, elongation ≥ 45%, yield ratio ≤ 0.55, compared with 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 its performance and thickness accuracy have reached 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 simple and efficient, high production efficiency, energy saving and environmental protection, the production line occupies a small area, and the investment cost is low.

For achieving the above object, technical scheme of the present invention is:

A method for realizing IF ferritic rolling of IF steel in a near-net shape in an ultra-short process, comprising the following steps:

1) Smelting

Smelting according to the following chemical composition, the weight percentage of its chemical composition 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%, the rest is Fe and other unavoidable impurities;

2) Continuous casting

Double-roll thin strip continuous casting is adopted to form a 1.5-3mm thick casting belt at the smallest gap between the two crystallization rolls; the diameter of the crystallization roll is between 500-1500mm, the inside of the crystallization roll is cooled by water, and the casting speed is 50-150m/min ;Continuous casting flow distribution adopts two-stage molten steel distribution distribution system, that is, tundish + flow distributor;

3) Lower closed protection

After the casting strip comes out of the crystallization roller, the temperature of the casting strip is between 1400 and 1480°C, and it directly enters the lower airtight chamber. The non-oxidizing gas is passed through the lower airtight chamber, and the oxygen concentration in the lower airtight chamber is controlled to be less than 5%. At 1100～1250℃;

4) Austenitic rolling

The cast strip is sent to the rolling mill in a closed state through the pinch rollers in the lower airtight chamber, and rolled into a steel strip with a thickness of 1.0-2.5mm. The austenitic rolling temperature is 1000-1200℃, and the austenitic rolling reduction rate : 15~45%;

5) Air mist cooling

The steel strip after austenitic rolling is cooled to below the Ar ₃ temperature, and the cooling adopts the air mist cooling method, the air mist cooling rate is ≤200°C/s, and the Ar ₃ temperature of the steel strip is 890-910°C;

6) Ferrite rolling

The cooled steel strip is sent to the rolling mill for ferrite rolling, and rolled into a steel strip with a thickness of 0.5-1.5mm. The ferrite rolling temperature is 800-900°C, and the ferrite rolling reduction rate: 20 ~50%;

7) coiling

The steel strip after ferrite rolling is directly coiled into coils, 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, and then stored in storage for natural air cooling.

Preferably, in step 5), the gas-water ratio of the gas 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.

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

Preferably, in step 8), the steel strip is slowly cooled in a heat preservation cover or a slow cooling pit after coiling.

Preferably, in step 3), the non-oxidizing gas is nitrogen, argon or CO ₂ obtained after sublimation of dry ice.

Preferably, in step 2), the diameter of the crystallization roll is 780-820 mm.

Preferably, in step 7), the coiling adopts a double coiling form, or adopts a Carrousel coiling form, preferably adopts a Carrousel coiling form.

The microstructure of the IF steel in the present invention is uniform polygonal ferrite.

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

The yield strength of the IF steel in the invention is 120-160 MPa, the tensile strength is 270-310 MPa, the elongation rate is ≥ 45%, and the yield ratio is ≤ 0.55.

In the composition design of the IF steel of the present invention:

C: The IF steel of the present invention is an ultra-low carbon steel. As shown in Figure 2, the deformation resistance curves of the IF steel of the present invention at different deformation temperatures have obvious deformation resistance troughs in the ferrite phase transition temperature range, and are suitable for use The ferritic rolling process will not increase the rolling deformation resistance due to the lower finishing rolling temperature, resulting in an increase in the load on the roll equipment and roll consumption. Therefore, in the present invention, the C content is controlled to be 0.0001-0.005%.

Si: In order to avoid the introduction of interstitial atoms in IF steel, it is generally not necessary to add Si, but the steel will inevitably contain Si, but high Si content will easily lead to increased strength and decreased elongation, and the present invention pursues a lower Strength and higher elongation, so the content of Si is controlled at 0.001-0.02% in the present invention.

Mn: Mn is one of the cheapest alloying elements, it can improve the hardenability of steel, has a considerable solid solubility in steel, improves the strength of steel through solid solution strengthening, and basically affects the plasticity and toughness of steel Without damage, it is the most important strengthening element to improve the strength of steel, and it can also play a role in deoxidation in steel. However, too high Mn content will lead to deterioration of weldability and weld heat-affected zone toughness. 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 be 0.001-0.20%.

P: A high content of P is easy to segregate at the grain boundary, increases the cold brittleness of the steel, deteriorates the welding performance, reduces the plasticity, and deteriorates the cold bending performance. The production of IF steel by the traditional process has strict requirements on P. Dephosphorization process must be carried out 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 , can effectively suppress 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, and the dephosphorization process can be omitted in the steelmaking process. In actual operation, additional phosphorus does not need to be added. In this case, the P content is generally ≤0.02%.

S: In general, S is a harmful element in steel, which causes hot embrittlement of steel and reduces the ductility and toughness of steel. S is easy to form MnS in steel, the amount and form of sulfide in steel directly affect the formability of steel plate, S must be lower than 0.005%. The amount and form of inclusion elements have a great influence on the deep drawing performance of steel plate, especially the strip-shaped sulfide inclusions in the deformation are easy to cause cracks. Therefore, in the present invention, the S content is controlled to be ≤0.005%.

Als: In order to control the inclusions in the steel, the present invention requires that Al should not be used for deoxidation. In the use of refractory materials, the additional introduction of Al should also be avoided as much as possible, but the steel will inevitably contain Al, and the acid-soluble aluminum Als should be strictly controlled. Content <0.001%.

N: The present invention utilizes the reaction of N in the steel with C and Nb in the steel to form carbon and nitrogen compounds, so that the matrix is in a state without interstitial atoms, and a certain amount of N in the steel is required. However, N has great harm to the plasticity and toughness of steel, and the existence of free N will increase the yield ratio of steel, so the N content should not be too high. The present invention controls the N content to be ≤0.005%.

Ca: It can change the form of sulfide in the steel, transform the elongated MnS inclusions into spherical CaS inclusions, improve the plasticity and toughness of the steel plate, and help to improve the formability of the steel plate. The present invention controls the Ca content to <0.0050%.

Nb: Nb is a strong carbon and nitrogen compound forming element, which can react with C and N atoms in the steel to form carbon and nitrogen compounds, making the matrix in a state without interstitial atoms. Because it does not contain interstitial atoms, IF steel can have high plasticity Anisotropy (r) value, high work hardening index (n) and no aging, etc., have excellent deep drawing performance, the invention controls Nb content at 0.02-0.08%.

The invention utilizes thin strip continuous casting and rolling technology to carry out two rollings in the rolling process, first to carry out austenitic rolling at 1000-1200°C, and then to use air mist cooling to effectively and accurately control the temperature of the strip steel to uniformly reduce to Ar ₃ The temperature is 5-50°C below the temperature to ensure that the strip steel can be ferritic rolled during the second rolling, so as to realize the ferritic rolling of IF steel by the thin strip continuous casting and rolling technology, and obtain a lower Excellent yield strength and tensile strength, low yield ratio, good deep drawing performance, performance and thickness accuracy reach the level of cold-rolled products of the same specification, and realize "replacing cold with heat". When the obtained product is used as a cold-rolled base material, it can effectively solve the problems of high strength and difficult cold-rolled deformation of similar IF steels under the austenitic rolling process of thin slab continuous casting or thin strip continuous casting.

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

The molten steel of the present invention can be smelted by an electric furnace or a converter, and then enter necessary refining procedures, such as LF furnace, VD/VOD furnace, RH furnace and the like.

Continuous casting adopts twin-roll thin strip continuous casting, forming a 1.5-3mm thick casting strip at the smallest gap between the two crystallization rolls. After the cast strip comes out of the crystallization roll, the temperature of the cast strip is 1400-1480°C, and it directly enters the lower airtight chamber. The non-oxidizing gas is passed through the lower airtight chamber, on the one hand, it can realize the anti-oxidation protection of the strip steel, and on the other hand, it can realize the cooling of the strip steel. The non-oxidizing gas may be N ₂ , Ar, or other non-oxidizing gases, such as CO ₂ gas obtained by sublimation of dry ice.

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

The strip steel after austenitic rolling is cooled to 5-50°C below the Ar ₃ temperature, and the cooling rate is ≤200°C/s. The Ar ₃ temperature is mainly related to the chemical composition of the strip steel, austenite grain size, rolling temperature, deformation and cooling rate and other factors. The Ar ₃ temperature in the rolling process can be calculated by the following formula: Ar ₃ =910-310× w(C)-80×w(Mn)-20×w(Cu)-15×w(Cr)-55×w(Ni)-80×w(Mo), after calculation, the IF steel Ar involved in the present invention ₃ The temperature range is 890-910°C.

The cooling adopts the air mist cooling method, the air-water ratio of the air mist cooling is 15-10:1, the air pressure is 0.5-0.8MPa, and the water pressure is 1.0-1.5MPa. The air-mist cooling method can effectively reduce the thickness of the oxide skin on the surface of the strip steel. , Improve strip temperature uniformity, improve strip surface quality and formability. After the aerosol forms high-pressure water mist and sprays it on the surface of the steel strip, on the one hand, it can reduce the temperature of the steel strip; , thus effectively controlling the growth of scale on the surface of hot-rolled strip steel. This cooling method can avoid the problems caused by traditional spray or laminar flow cooling, make the surface temperature of the strip drop evenly, improve the temperature uniformity of the strip, and achieve the effect of homogenizing the internal microstructure.

Gas mist cooling is also used in traditional continuous casting, but the area and temperature of the action are different. Traditional continuous casting performs gas mist cooling on the casting slab in the area of the exit sector of the casting slab exiting the crystallizer. At this time, the temperature of the slab is relatively high, from According to the phase diagram, it should be in the high temperature austenite single phase region. The main purpose of gas mist cooling in this area is to control the position of the solidification end, accelerate the cooling of the surface of the slab, refine the surface austenite grain structure, improve the surface strength of the slab, improve the surface quality of the slab, and avoid the occurrence of cracks. After the austenitic rolling of the present invention, the ultra-thin strip steel is subjected to gas mist cooling, and by adjusting the gas mist cooling intensity, the temperature of the strip steel can be effectively and precisely controlled at 5-50°C below the _Ar3 temperature, so as to ensure that the strip steel is in the second Ferrite rolling can be performed during the rolling process.

The cooled steel strip is subjected to ferrite rolling, and the ferrite rolling temperature is 800-900°C, preferably 830-880°C. For the steel types involved in the present invention, this temperature range is in the low temperature deformation resistance area , this area is suitable for ferrite rolling; control the reduction rate of ferrite rolling to 20-50%, and obtain ultra-thin hot-rolled strip steel after rolling, and the thickness of the steel strip is 0.5-1.5mm; Rolling adopts lubricated rolling, which can improve the surface quality of the strip and reduce the rolling loss of the rolls.

The ferrite-rolled steel strip does not need to be cooled, and directly enters the coiler for high-temperature coiling. The coiling temperature is 700-760°C; the steel coil is cut by high-speed flying shears, and the high-temperature steel coil after cutting After the coil is unloaded from the coiler, the online insulation cover is used for slow cooling, or it is quickly entered into the slow cooling pit for slow cooling, so that the strip steel can fully recover and recrystallize, and avoid the performance loss caused by the rapid cooling of the coil head and tail during air cooling. Unqualified 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 below 550°C, it can be put into storage for natural air cooling.

Through the above manufacturing process, the microstructure of the ferritic 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 ratio of ≤0.55. The product has lower strength, lower yield ratio, higher elongation, and better formability, which can better meet the performance requirements of "replacing cold with heat" products; at the same time, as a cold-rolled base material When used, it can also effectively solve the problems of high yield ratio 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.

The reason why the strip continuous casting hot-rolled steel coil of the present invention preferably adopts the Carrousel co-coiler:

At present, the vast majority of ultra-thin hot-rolled steel coil production lines adopt underground double-coil or underground three-coil methods, mainly because these production lines also take into account the production of thick-gauge hot-rolled plates, such as Avedi (Avedi) The coiling of the ESP production line adopts the underground three-coil method, and the coiling of the FTSC production line introduced by Tangsteel Danieli (Danieli) adopts the underground double-coil method. The Castrip thin strip continuous casting production line of Nucor in the United States adopts the traditional method and also adopts the underground double coiling method. The distance between the down coiler and the coiler is generally 8-10m (typical value 9.4m). When 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, and the temperature deviation between the two coilers can reach 49°C, which will seriously affect the performance deviation of the steel coil.

However, the present invention preferably adopts the Carousel coiling method, which can realize co-coiling of hot-rolled steel coils, ensure the uniformity of coiling temperature, and further greatly improve the stability of steel coil product performance. Currently on the market, Carousel coilers are widely used in the field of cold rolling. Due to the lower temperature of the strip steel, it is easier to realize. The invention proposes to adopt the Carousel coiling method in the coiling field of the ultra-thin hot-rolled steel strip, and realizes the coiling of the ultra-thin hot-rolled steel strip in consideration of the high temperature resistance of the equipment. This coiling method is more advanced than that of the Castrip thin strip continuous casting production line of Nucor in the United States.

Beneficial effects of the present invention:

1. The present invention adopts ultra-short flow process to produce IF steel near net shape, that is, thin strip continuous casting+austenite rolling+ferrite rolling. During the rolling process, two rollings are carried out, and in the two rollings Air mist cooling is used to effectively and accurately control the temperature of the strip steel to be uniformly lowered to 5-50°C below the _Ar3 temperature, so as to ensure that the strip steel can be rolled ferritely in the second rolling stage, so as to obtain uniform ferrite The thinnest ultra-thin IF steel can reach 0.5mm, while the thinnest IF steel obtained by the traditional hot rolling process is 1.5mm, and the thinnest IF steel obtained by the thin slab continuous casting and rolling process is 0.8mm, and the production is extremely difficult Large; the IF steel obtained by the present invention has a yield strength of 120-160MPa, a tensile strength of 270-310MPa, an elongation ≥ 45%, and a yield ratio ≤ 0.55, which is lower than that of the IF steel produced by the existing hot rolling process. The yield strength, tensile strength, yield strength ratio and good deep drawing performance, performance and thickness accuracy have reached the level of cold-rolled products of the same specification, realizing "replacing cold with heat".

2. The present invention produces IF steel with lower yield strength and tensile strength, lower yield strength ratio, and better deep drawing performance by adopting an ultra-short flow process near net shape, eliminating the traditional production process of IF steel Medium flame cutting slab, slab heating furnace, rough rolling, multi-stand finish rolling and other complex intermediate steps, the production process is simple and efficient, high production efficiency, energy saving and environmental protection and other advantages, the production line occupies a small area and low investment cost.

3. In the IF process of the present invention, the strip steel is cooled by air mist cooling after austenitic rolling, while spraying or laminar flow cooling is commonly used in the traditional process. Air mist cooling can make the surface temperature of the IF steel uniformly decrease, improving The temperature uniformity of the strip makes the IF steel obtain a uniform ferrite structure after ferrite rolling; at the same time, uniform cooling can improve the shape quality and formability of the IF steel and reduce the thickness of the oxide scale on the surface of the IF steel.

Description of drawings

Fig. 1 is a schematic diagram of the process flow of the embodiment of the present invention.

Fig. 2 is the deformation resistance curve of the IF steel of the embodiment of the present invention at different deformation temperatures.

Fig. 3 is a photo of the microstructure of IF steel according to the embodiment of the present invention.

detailed description

The present invention will be further described below by embodiment and accompanying drawing, but this is not limitation of the present invention, those skilled in the art can make modification or improvement according to the basic idea of the invention, but as long as not departing from the basic idea of the present invention, all in within the scope of the present invention.

Referring to Fig. 1 , the ultra-short process near net shape of the present invention realizes the method of IF ferrite rolling.

The smelted molten steel conforming to the design of the chemical composition of the present invention is directly poured from the ladle 1 through the ladle shroud 2, the tundish 3, the submerged nozzle 4 and the distributor 5 in a two relatively rotating and capable of rapid cooling. In the molten pool 7 surrounded by twin-roll thin strip continuous casting crystallization rolls 8a, 8b and side sealing plates 6a, 6b, molten steel solidifies on the circumferential surface of the crystallization rolls 8a, 8b rotation, and then in the gap between the two crystallization rolls 8a, 8b The smallest point (nip point) forms a 1.5-3 mm thick cast strip 11 . The crystallization rollers 8a and 8b according to the present invention have a diameter between 500-1500mm, and are cooled by passing water inside. According to the thickness of the casting belt 11, the casting speed range of the casting machine is between 60-150m/min.

The cast strip 11 comes out from the crystallization rollers 8a, 8b, the temperature of the cast strip is 1400-1480°C, and it directly enters the airtight chamber 10, and the lower airtight chamber 10 passes through the non-oxidizing gas to protect the strip steel, on the one hand to realize the protection of the cast strip 11. Oxidation protection, cooling cast belt 11 on the other hand, the atmosphere of anti-oxidation protection can be N ₂ , Ar, also can be other non-oxidizing gases, such as CO ₂ gas obtained by sublimation of dry ice, etc., the oxygen concentration in the airtight chamber 10 is controlled At <5%, the lower airtight chamber 10 protects the casting belt 11 against oxidation to the entrance of the rolling mill 13, and the temperature of the casting belt 11 at the outlet of the lower airtight chamber 10 is 1100-1250°C; then the casting belt 11 passes through the swing guide plate 9 and pinch rollers 12. Enter the four-roller 1# hot rolling mill 13 for austenitic rolling. The austenitic rolling temperature is 1000-1200°C, and the hot rolling reduction rate is controlled to be 15-45% to ensure that austenite static After recrystallization, the thickness of the steel strip after austenitic rolling is 1.0-2.5mm.

After austenitic rolling, the strip steel is cooled to below 910°C by means of gas mist cooling, and the cooling rate is ≤200°C/s; the cooled strip steel enters the online four-high hot rolling mill 20 (2# rolling mill ) in ferrite rolling, the ferrite rolling temperature is 800-900°C, preferably 830-880°C, the hot rolling reduction rate is controlled to 20-50%, and the ultra-thin hot rolling is obtained after ferrite rolling Strip steel, the thickness of the steel strip is 0.5-1.5mm, the ferrite rolling adopts lubricated rolling, which can improve the surface quality of the strip steel and reduce the rolling loss of the roll; the strip steel after ferrite rolling does not need to be cooled, After being cut by the high-speed flying shear 16, the cutting head falls into the flying shear pit 18 along the flying shear guide plate 17, and the subsequent strip directly enters the coiler 19 for high-temperature coiling, and the coiling temperature is 700-760°C; The weight of the strip steel coil is cut by high-speed flying shears 16. After the high-temperature steel coil is unloaded from the coiler 19, it is slowly cooled by an online heat preservation cover, or quickly enters the slow cooling pit for slow cooling, so that the strip The steel undergoes relatively sufficient recovery and recrystallization, avoiding the problem of inconsistency in performance 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, low yield ratio, and easy stamping and folding Bending forming, to better meet the performance requirements of "replacing cold with heat" or subsequent cold rolling deformation. When the temperature of the steel coil slowly drops below 550°C, it can be put into storage for natural air cooling.

The coiler 19 adopts the form of double coiling, and can also adopt the form of carousel coiling, so as to ensure the continuous production of aluminum alloy strips.

The chemical composition of IF steel in the embodiment of the present invention is shown in Table 1, and the balance of the composition is Fe and other unavoidable 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 finally obtained IF steel are shown in Table 3. Comparative examples 1 and 2 in the table show the relevant chemical composition, process parameters and product performance of IF steel produced by the existing typical twin-roll strip continuous casting and rolling process technology.

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 ≥ 45%, and a yield ratio ≤ 0.55. Compared with the IF steel obtained in Comparative Examples 1 and 2, it has lower strength, lower yield ratio and better formability.

After the above manufacturing process, the microstructure of the finally obtained ferritic rolled IF steel is shown in Fig. 3 . It can be seen from Figure 3 that the microstructure of the obtained IF steel is polygonal ferrite, the structure is uniform, and the grain size is 30-100um.

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 formability, and can better meet the requirements of "replacing cold with heat" products Performance requirements; at the same time, when used as a cold-rolled base material, it can also effectively solve the problems of high strength and difficult cold-rolled deformation of similar IF steels under the austenitic rolling process of thin slab continuous casting or thin strip continuous casting.

Claims (10)

1. A method for realizing IF iron and steel ferrite rolling near net shape in an ultra-short process, is characterized in that, comprising the steps: 1) Smelting Smelting according to the following chemical composition, the weight percentage of its chemical composition 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%, the rest is Fe and other unavoidable impurities; 2) Continuous casting Double-roll strip continuous casting is used to form a 1.5-3mm thick casting belt at the smallest gap between the two crystallization rolls; the diameter of the crystallization roll is 500-1500mm, the inside of the crystallization roll is cooled by water, and the casting speed is 50-150m/min; The casting flow adopts a two-stage molten steel distribution distribution system, that is, tundish + flow distributor; 3) Lower closed protection After the casting strip comes out of the crystallization roller, the temperature of the casting strip is between 1400 and 1480°C, and it directly enters the lower airtight chamber. The non-oxidizing gas is passed through the lower airtight chamber, and the oxygen concentration in the lower airtight chamber is controlled to be less than 5%. At 1100～1250℃; 4) Austenitic rolling The cast strip is sent to the rolling mill in a closed state through the pinch rollers in the lower airtight chamber, and rolled into a steel strip with a thickness of 1.0-2.5mm. The austenitic rolling temperature is 1000-1200℃, and the austenitic rolling reduction rate : 15~45%; 5) Air mist cooling The steel strip after austenitic rolling is cooled to 5-50°C below the _Ar3 temperature, and the cooling adopts the air mist cooling method, the air mist 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 the rolling mill for ferrite rolling, and rolled into a steel strip with a thickness of 0.5-1.5mm. The ferrite rolling temperature is 800-900°C, and the ferrite rolling reduction rate: 20 ~50%; 7) coiling The steel strip after ferrite rolling is directly coiled into coils, 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, and then stored in storage for natural air cooling. 2. The method for realizing IF ferrite rolling of IF ferrite with near-net shape in ultra-short process according to claim 1, characterized in that, in step 2), the diameter of the crystallization roll is 780-820mm. 3. the ultra-short process flow near net shape as claimed in claim 1 realizes the method for IF steel ferrite rolling, it is characterized in that, in step 3), described non-oxidizing gas obtains after sublimation of nitrogen, argon or dry ice of CO ₂ . 4. The method for realizing IF steel ferrite rolling with ultra-short process near-net shape as claimed in claim 1, characterized in that, in step 5), the gas-water ratio of the gas-mist cooling is 15-10:1, The air pressure is 0.5-0.8MPa, and the water pressure is 1.0-1.5MPa. 5. The method for realizing ferrite rolling of IF ferrite in near-net shape in ultra-short process according to claim 1, characterized in that, in step 6), the ferrite rolling temperature is 830-880°C. 6. The method for realizing IF ferrite rolling of IF ferrite in ultra-short process near-net shape as claimed in claim 1, characterized in that, in step 7), the coiling adopts a double coiling form, or adopts Carrousel The coiled form is preferably a Carrousel coiled form. 7. the ultra-short flow process as claimed in claim 1 realizes the method for IF steel ferrite rolling near net shape, it is characterized in that, in step 8), after described steel strip coils, carry out in insulation cover or slow cooling pit slow cooling. 8. The method for realizing ferrite rolling of IF steel in a near-net shape in an ultra-short process 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 ferrite rolling of IF steel in a near-net shape in an ultra-short process according to any one of claims 1-8, characterized in that the grain size in the microstructure of the IF steel is 30-100 um. 10. The method for realizing the ferrite rolling of IF steel with near-net shape in ultra-short process according to any one of claims 1 to 9, characterized in that, the yield strength of the IF steel is 120 to 160 MPa, and the tensile strength is 270 ~ 310MPa, elongation ≥ 45%, yield ratio ≤ 0.55.

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