CN115475834A

CN115475834A - Method for realizing low-carbon steel ferrite rolling by ultra-short process near net shape

Info

Publication number: CN115475834A
Application number: CN202110599113.8A
Authority: CN
Inventors: 吴建春; 支卫军; 方园
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2021-05-31
Filing date: 2021-05-31
Publication date: 2022-12-16

Abstract

A method for realizing low-carbon steel ferrite rolling with ultra-short process and near net shape adopts thin-strip continuous casting, austenite rolling and ferrite rolling; from carbon content, basicity, als, free oxygen total oxygen and low melting point MnO-SiO ₂ ‑Al ₂ O ₃ Controlling the ternary inclusions and the like in multiple aspects, so that the prepared molten steel can be cast by adopting thin-strip continuous casting; the rolling adopts twice rolling, and the gas mist cooling is adopted between the two times of rolling to effectively and accurately control the temperature of the strip steel to be evenly reduced to Ar ₃ The temperature is 5-50 ℃ below, the strip steel can be subjected to ferrite rolling in the second rolling stage, the ultrathin low-carbon steel with the uniform polygonal ferrite structure is obtained, the thickness of the ultrathin low-carbon steel can be as thin as 0.5mm, the obtained low-carbon steel has lower yield strength, tensile strength, yield ratio and better deep drawing performance compared with the low-carbon steel produced by the existing hot rolling process, the performance and thickness precision reach the level of cold-rolled products of the same specification, and the purpose of replacing cold with heat is realized.

Description

Method for realizing low-carbon steel ferrite rolling by ultra-short process near net shape

Technical Field

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

Background

Ferrite rolling was first successfully developed in 1994 by professor Appell and the team of the Steel research center Appell, belgium, and the main objective of developing the technology at that time was to replace cold-rolled products with the thickness range of 1.0-2.0mm with thin-gauge hot rolling, so as to realize the purposes of replacing cold with hot and reduce the production cost. Under the traditional process flow, the technical process flow is as follows: the production method comprises the steps of continuously casting a casting blank, heating and preserving heat by a heating furnace, roughly rolling (austenite rolling), rapidly cooling, finely rolling (ferrite rolling), cooling and coiling, namely, firstly, the casting blank with the thickness of about 200mm is obtained through continuous casting, the casting blank is reheated and preserved heat, then roughly rolling and finely rolling are carried out, finally, laminar cooling and coiling are carried out on a steel strip, and the whole hot rolling production process is completed. Specially, the rough rolling process is completed in all austenite state, and a quick cooling device is set between the rough rolling mill and the finishing mill to make the strip steel temperature be reduced to Ar before entering the finishing mill ₃ The gamma → alpha transformation is completed before the strip enters the finishing mill, so that the whole 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, both the rough rolling and the finish rolling are performed in the austenite region. The traditional ferrite rolling process has the advantages of long process, high energy consumption, more unit equipment, high capital construction cost and higher production cost.

In a comparative test of carrying out austenite rolling and ferrite rolling on low-carbon steel, ferrite rolling can obtain a ferrite structure with coarse grains, and the ferrite structure has the advantages of lower yield strength and tensile strength, lower yield ratio, larger r value and better forming performance. Because the temperature of ferrite rolling is obviously reduced compared with that of austenite rolling and is lower than 200 ℃, the lower finish rolling temperature can cause the rolling deformation resistance to be obviously improved, the load of roller equipment is increased, the roller consumption is increased, and the long-time rolling is not facilitated; meanwhile, the rolling force is increased, the deformation is difficult, the plate shape is difficult to control, and the smooth rolling process cannot be ensured, so that all steel grades are not suitable for a ferrite rolling process, and some steel grades with large ferrite area range and low deformation resistance in the ferrite area temperature range need to be selected.

The process flow of the thin slab continuous casting and rolling comprises the following steps: continuous casting, heat preservation and soaking of a casting blank, hot continuous rolling, cooling and coiling. The main differences between the process and the traditional process are as follows: the thickness of a casting blank in the thin slab process is greatly reduced to 50-90mm, and the casting blank is thin, so that the casting blank can be reduced to the required specification before finish rolling only by carrying out 1-2 times of rough rolling (when the thickness of the casting blank is 70-90 mm) or not carrying out the rough rolling (when the thickness of the casting blank is 50 mm) on the casting blank in the traditional process through repeated multi-pass rolling; and the casting blank of the thin slab process directly enters a soaking pit furnace for soaking and heat preservation without cooling or supplementing a small amount of heat, so that the thin slab process greatly shortens the process flow, reduces the energy consumption and the investment, thereby reducing the production cost. However, the strength of steel and the yield ratio of the sheet billet can be improved due to the faster cooling speed in the continuous casting and rolling of the sheet billet, so that the rolling load is increased, and the thickness specification of a hot rolled product which can be economically produced cannot be too thin, generally more than or equal to 1.5mm, see Chinese patent CN200610123458.1, chinese patent CN200610035800.2 and Chinese patent CN200710031548.2.

The ESP realizes the continuous casting of the plate blank, eliminates a heating furnace with the functions of flame cutting, heat preservation, soaking and plate blank transition, greatly shortens the length of the whole production line to about 190 meters, ensures that the plate blank continuously cast by a continuous casting machine has the thickness of 90-110mm and the width of 1100-1600mm, ensures that the continuously cast plate blank plays the role of heat preservation, soaking and heating through a section of induction heating roller table, and then sequentially enters the procedures of rough rolling, finish rolling, layer cooling and coiling to obtain the hot rolled plate. At present, the process is rapidly developed and popularized, and a plurality of ESP production lines are operated and produced in the world.

The process flow shorter than the thin slab continuous casting and rolling is the thin strip continuous casting and rolling process. The thin strip continuous casting technology is a leading-edge technology in the metallurgical and material research fields, the development has been over 150 years since the idea was proposed by Henry Bessemer in 1865 (U.S. Patent: 49053), but the technology is basically in a state of stagnation at that time due to the fact that the development of related technologies such as manufacturing technology and control technology is not mature enough. It was not achieved until the middle of the 20 th century in the continuous casting and rolling process of Al, which once again has attracted attention in the field of steel making. The production process of steel strips in the traditional metallurgy industry is changed, the continuous casting, rolling, even heat treatment and other processes are integrated, and the produced thin strip blank is subjected to one-time online hot rolling to form a thin steel strip, so that the production process from molten steel to the steel strip is greatly simplified, the production period is shortened, the length of a process line is only about 50m, and the steel production process is more compact, more continuous, more efficient and more environment-friendly; meanwhile, the equipment investment is correspondingly reduced, the production cost is obviously reduced, and the method is a low-carbon and environment-friendly hot-rolled thin strip production process. Therefore, the strip casting technology has recently become a hot spot of competitive development in countries around the world.

The typical technological process of the existing twin-roll thin strip continuous casting and rolling technology is as follows: molten steel in a large ladle is directly poured into a molten pool enclosed by two relatively rotating crystallizing rollers capable of being rapidly cooled and a side sealing device through a large ladle long water gap, a tundish, an immersion water gap and a flow distributor, the molten steel is solidified on the circumferential surface of the rotating crystallizing rollers to form a solidified shell and gradually grow, a steel strip with the thickness of 1-5mm is further formed at the minimum gap (nip point) of the two casting rollers, the steel strip is fed into a rolling mill through a guide plate guide pinch roller to be rolled for one time, then is cooled through a cooling device, is cut by a flying shear device, is dropped into a flying shear pit along a flying shear guide plate, and the hot rolled strip after being cut enters a coiler to be coiled.

Chinese patent CN20181065733 and Chinese patent CN201610768866 both propose "a method for producing low-carbon steel by ferrite rolling in an ESP production line", and the key control key point is that cooling water is cooled on strip steel entering between a first rack and a second rack and between the second rack and a third rack, so that the transformation from austenite to ferrite is completed before the strip steel enters into the third rack, and when the strip steel is rolled between the third rack and a fifth rack, the strip steel is in a ferrite zone, thereby realizing the ferrite rolling. The invention produces low-carbon steel by ferrite rolling through an ESP production process, and because water cooling is needed to control the temperature of strip steel between frames, the production difficulty is higher and the precision is difficult to guarantee.

Chinese patent CN201610759108 discloses a method for producing low-carbon steel by ferrite rolling process on CSP production line, which has the key process points that F1, F2, F4, F5, F6 and F7 are rolled in a 7-rack finishing mill, an F3 rack is nominal, cooling water between the F1 and F3 racks is 60-90%, pure ferrite rolling is realized on F4 by cooling between the racks, the strength of the material is obviously reduced and the forming performance is improved after the ferrite rolling process is adopted. The invention adopts CSP ferrite rolling technology to produce low-carbon steel, and also needs water cooling to control the cooling of the strip steel between the F1-F3 frames, thus the production difficulty is higher and the precision is difficult to guarantee.

Chinese patent CN201721755853 discloses a ferrite rolling control system, wherein a slab enters a tunnel type heating furnace after being roughly rolled by a rough rolling mill set, and the upper surface and the lower surface of the slab are simultaneously heated by an upper heating double heat storage gas burner above a water cooling furnace roller and a lower heating double heat storage gas burner below the water cooling furnace roller, wherein the heating temperature is 900-950 ℃; and the heated plate blank is descaled by a finish rolling descaler and then enters a finish rolling unit for ferrite rolling. The key point of the patent process is that a tunnel type soaking pit furnace is adopted between rough rolling and finish rolling, the uniform control of the temperature is realized, the heating energy consumption of the tunnel furnace is large, the cost of each ton of steel is high, and the head and tail temperature of a plate blank is difficult to control uniformly.

Chinese patent CN201710960186 discloses "a low micro-carbon steel for endless continuous casting and rolling deep drawing", and Chinese patent CN201710960187 discloses "a ferrite rolling method and device for ultra-low carbon steel for ultra-deep drawing", wherein the two patents adopt a new arrangement form, local improvement is carried out, a lower finish rolling start temperature is adopted, and a higher finish rolling temperature and a higher coiling temperature are required at the same time so as to meet the process requirements for recrystallization and recovery of the internal structure of a ferrite rolled product. The invention combines the production process of sheet billet continuous casting and rolling with local equipment improvement, realizes the rolling condition of finish rolling ferrite, but has larger control difficulty and is easy to have the problem of mixed crystals.

Chinese patent CN201810657331 discloses a method for producing low-carbon steel by ferrite rolling in an ESP production line, which comprises the steps of finish rolling strip steel after rough rolling, wherein the strip steel sequentially enters a first rack, a second rack, a third rack, a fourth rack and a fifth rack during finish rolling, and cooling the strip steel entering between the first rack and the second rack and between the second rack and the third rack by cooling water, so that the strip steel is rolled in a ferrite area between the third rack and the fifth rack during finish rolling. The invention combines the technical advantages of the ESP production line with the ferrite rolling process, can effectively solve the problem of higher yield ratio of products of the ESP production line when the ESP production line produces low-carbon steel, and has larger production difficulty and difficult guarantee of precision because water cooling is needed to control the temperature of strip steel between racks.

Chinese patent CN201910753893 discloses a full-continuous production device and a method for hot-rolled strip steel for ferrite rolling based on thin slab continuous casting and rolling, which adopts a multifunctional cooling control device to integrate high-pressure water descaling and intermediate slab cooling functions, and adopts a 4R + (3-4) F rolling mill arrangement, four temperature detectors and a close-distance underground coiler to realize accurate control of process temperature. The method comprises the following steps: continuous casting blank forming → high-pressure water rotary descaling → 4 frame high-pressure roughing mill group roughing → rotary drum shearing → cooling after high-pressure water dephosphorization in a multifunctional controlled cooling device → 3 or 4 frame finishing mill group finishing mill → air cooling → high-speed flying shear coil splitting → underground coiler coiling, wherein the temperature is monitored after roughing rolling, before and after finishing mill rolling and before coiling by the underground coiler respectively, and the ferrite rolling of low-carbon steel and low-carbon steel can be realized easily. The invention adopts a continuous casting and rolling production process of thin slabs, adopts a 4R + (3-4) F rolling mill arrangement form, is additionally provided with a multifunctional cooling control device after rough rolling to carry out high-pressure water dephosphorization and rapid cooling on casting blanks, realizes ferrite rolling in the finish rolling process, and has more rolling mills, higher rolling roll consumption cost and complex control.

After the continuous casting and rolling technology of thin slabs emerges, because the technological characteristics are that the hot rolled strip steel with ultra-thin specification can be produced easily, which creates conditions for the application of ferrite rolling technology, most of the current continuous casting and rolling production lines of thin slabs have the function of reserved ferrite rolling. The prior ferrite rolling technology is applied to the thin slab continuous casting and rolling technology, and the proposal of the ferrite rolling process concept or the related technical report does not exist in the thin strip continuous casting and rolling technology which has the characteristics of producing ultrathin hot rolled strip steel.

The cooling speed of molten steel solidification reaches 10 due to the characteristic of the thin strip continuous casting process of sub-rapid solidification ² -10 ⁴ The steel grade produced by the method generally has the phenomena of high yield ratio and poor forming performance, and most products which are 'hot instead of cold' in the market generally require low yield ratio and are easy to bend and stamp. Therefore, when the steel grade 'replacing cold with heat' is produced by thin-strip continuous casting, the problems of uneven structure and higher yield ratio need to be solved, so that when the strip steel is used as the product 'replacing cold with heat', the steel grade has lower yield ratio and good forming performance.

Disclosure of Invention

The invention aims to provide a method for realizing ferrite rolling of low-carbon steel by using an ultra-short process near net shape, which realizes ferrite rolling of low-carbon steel by using a thin-strip continuous casting and rolling technology to obtain ultra-thin hot-rolled low-carbon steel, wherein the thickness of the ultra-thin hot-rolled low-carbon steel can be as thin as 0.5mm, the yield strength of the ultra-thin hot-rolled low-carbon steel is 170-210MPa, the tensile strength of the ultra-thin hot-rolled low-carbon steel is 280-330MPa, the elongation of the ultra-thin hot-rolled low-carbon steel is more than or equal to 40%, the yield ratio of the ultra-thin hot-rolled low-carbon steel is less than or equal to 0.65, and compared with the low-carbon steel produced by the existing hot rolling process, the ultra-short process near net shape low-carbon steel has lower yield strength, lower yield ratio and better deep drawing performance, the performance and the thickness precision of the same specification cold rolling product level, and realizes ' replacing cold ' by heat '; meanwhile, the production process has the advantages of simplicity, high efficiency, high production efficiency, energy conservation, environmental protection and the like, and the production line has small floor area and low investment cost.

In order to achieve the purpose, the technical scheme of the invention is as follows:

the invention relates to a method for realizing low-carbon steel ferrite rolling by using ultra-short process near net shape, which comprises the following steps:

1) Smelting of

Smelting according to the following chemical components in percentage by weight: c:0.01 to 0.05%, si:0.01-0.1%, mn:0.15-0.25%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, N is less than or equal to 0.005%, als is less than 0.001%, ca is less than or equal to 0.0050%, B:0.001 to 0.006%, total oxygen [ O ] T:0.007 to 0.020 percent, and the balance of Fe and other inevitable impurities;

in the smelting process, the slagging alkalinity a = CaO/SiO ₂ Control in a<1.5; it is desirable to obtain low melting point MnO-SiO ₂ -Al ₂ O ₃ Ternary inclusions, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ Controlling the temperature to be 0.5-2; free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005-0.005%;

2) Continuous casting

Adopting double-roller thin strip continuous casting to form a casting strip with the thickness of 1.5-3mm at the minimum position of a gap between two crystallizing rollers; the diameter of the crystallization roller is 500-1500mm, the interior of the crystallization roller is cooled by water, and the casting speed is 50-150 m/min; the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;

3) Lower closed protection

After the casting strip is taken out of the crystallization roller, the temperature of the casting strip is 1400-1480 ℃, the casting strip directly enters a lower closed chamber, non-oxidizing gas is introduced into the lower closed chamber, the oxygen concentration in the lower closed chamber is controlled to be less than 5%, and the temperature of the casting strip at the outlet of the lower closed chamber is 1100-1250 ℃;

4) Rolling of austenite

And (2) conveying the cast strip to a rolling mill in a lower closed chamber in a closed state through a pinch roll, rolling the cast strip into a steel strip with the thickness of 1.0-2.5mm, wherein the austenite rolling temperature is 1000-1200 ℃, and the austenite rolling reduction ratio is as follows: 15 to 45 percent;

5) Aerosol cooling

Cooling the austenitic rolled strip to Ar ₃ The temperature is 5-50 ℃, the cooling adopts an aerial fog cooling mode, the aerial fog cooling rate is less than or equal to 300 ℃/s, and the Ar of the steel strip is ₃ The temperature is 875 to 900 ℃;

6) Ferrite rolling

And (3) conveying the cooled steel strip to a rolling mill for ferrite rolling to obtain a steel strip with the thickness of 0.5-1.5mm, wherein the ferrite rolling temperature is 750-850 ℃, and the ferrite rolling reduction rate is as follows: 20 to 50 percent;

7) Coiling

Directly coiling the steel strip rolled by ferrite into a coil at the coiling temperature of 700-760 ℃;

8) Slow cooling

And coiling the steel strip, slowly cooling to below 550 ℃, then warehousing and naturally cooling in air.

Preferably, in step 1), the slagging alkalinity a is less than 1.2, preferably a = 0.7-1.0.

Preferably, in step 1), the MnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The temperature is controlled to be 1-1.8.

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

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

Preferably, in the 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 the step 6), the ferrite rolling temperature is 780 to 830 ℃.

Preferably, in step 7), the coiling is in a double coiling form, or in a carrousel coiling form, preferably in a carrousel coiling form.

The microstructure of the low-carbon steel is uniform polygonal ferrite.

The grain size of the microstructure of the low-carbon steel is 20-50um.

The yield strength of the low-carbon steel is 170-210MPa, the tensile strength is 280-330MPa, the elongation is more than or equal to 40%, and the yield ratio is less than or equal to 0.65.

In the composition design of the low-carbon steel of the invention:

c: c is the most economical and basic strengthening element in steel, and enhances the strength of steel by solid solution strengthening and precipitation strengthening. For conventional slab continuous casting, casting in a peritectic reaction zone is easy to generate surface cracks of a casting blank, and a steel leakage accident can happen in severe cases. Therefore, strip casting of Fe — C alloys also requires avoidance of the peritectic reaction zone.

In the invention, because the temperature of ferrite rolling is obviously reduced compared with that of austenite rolling and is lower than 200 ℃ above, the lower finish rolling temperature can lead to the obvious improvement of rolling deformation resistance, the load on roller equipment is increased, the roller consumption is increased, and the long-time rolling is not facilitated; at the same time, the rolling force is increased, the deformation is difficult, the plate shape is difficult to control, and the stable operation of the rolling process can not be ensured, so that not all steel grades are suitable for adopting the ferrite rolling process, some steel grades with larger ferrite area range and deformation resistance valley in the ferrite area temperature range need to be selected, according to the Fe-C phase diagram, when the C content is less than or equal to 0.0218 percent at 727 ℃, a single-phase ferrite area exists, when the C content reaches 0.05 percent, the ferrite proportion is up to more than 95 percent, meanwhile, according to the deformation resistance curves of low-carbon steel with different carbon contents at different temperatures, as shown in figure 3, the low-carbon steel with the C content of 0.01-0.05 percent has obvious deformation resistance valley in the ferrite area temperature range, and is suitable for adopting the ferrite rolling process, so that the C content is controlled to be 0.01-0.05 percent in the invention.

Si: si is added in the invention to play a role in deoxidation, so that the purity of steel can be improved; meanwhile, si can enlarge the ferrite forming range and is beneficial to ferrite rolling. However, if the Si content is too high, a red iron sheet defect is easily formed on the surface of the rolled steel plate, which can result in the increase of the strength and the reduction of the elongation, the invention pursues lower strength and higher elongation, and the Si content does not need to be too high. Therefore, the content of Si is controlled to be 0.01-0.1 percent in the invention.

Mn: mn is one of the cheapest alloy elements, can improve the hardenability of steel, has considerable solid solubility in the steel, improves the strength of the steel through solid solution strengthening, basically has no damage to the plasticity and the toughness of the steel, is the most main strengthening element for improving the strength of the steel, and can play a role in deoxidizing in the steel. However, too high a Mn content leads to deterioration of weldability and toughness of weld heat-affected zone, lower strength and yield ratio are sought in the present invention, and the Mn content does not need to be too high. Therefore, the Mn content is controlled to be 0.15-0.25 percent in the invention.

P: high content of P is easy to be segregated in grain boundary, increases cold brittleness of steel, deteriorates welding performance, reduces plasticity and deteriorates cold bending performance. The traditional process is adopted to produce the low-carbon steel with thin specification, the requirement on P is strict, a dephosphorization procedure is required in a steelmaking procedure, and the content of P is generally required to be controlled below 0.01 percent; in the thin strip continuous casting process, the solidification and cooling rates are extremely high, and the segregation of P can be effectively inhibited, so that the disadvantages of P can be effectively avoided, and the advantages of P can be fully exerted. Therefore, in the invention, the content of the P element can be properly relaxed, the dephosphorization process can be cancelled in the steelmaking process, and in the actual operation, no extra phosphorus is required to be added, and in this case, the content of the P is generally less than or equal to 0.02 percent.

S: in general, S is a harmful element in steel, causing hot brittleness of steel, and reducing ductility and toughness of steel. S is easy to form MnS in steel, the quantity and the form of sulfide in the steel directly influence the formability of the steel sheet, and S must be less than 0.005%. The quantity and the form of the inclusion elements have great influence on the deep drawing performance of the steel plate, and particularly, the strip-shaped sulfide inclusion is easy to cause cracks in deformation. Therefore, in the present invention, S is controlled as an impurity element, and the content thereof is controlled to be 0.005% or less.

And Als: in order to control inclusions in steel, al cannot be used for deoxidation, and the extra introduction of Al is avoided as much as possible in the use of refractory materials, but Al is inevitably contained in the steel, and the content of acid-soluble aluminum Als is strictly controlled to be less than 0.001%.

N: the invention utilizes the reaction of N in the steel and C and Nb in the steel to generate carbon and nitrogen compounds, so that the matrix is in a gapless atom state and needs a certain content of N in the steel. However, N is harmful to the plasticity and toughness of the steel, and the yield ratio of the steel is improved by the existence of free N, so that the content of N cannot be too high. The invention controls the content of N to be less than or equal to 0.005 percent.

Ca: the form of sulfide in steel can be changed, long-strip MnS inclusion is converted into spherical CaS inclusion, the plasticity and the toughness of the steel plate are improved, and the forming performance of the steel plate is improved. The Ca content is controlled to be less than 0.0050 percent.

B: the hardenability of the steel can be multiplied by trace boron, B can preferentially precipitate BN particles in high-temperature austenite so as to inhibit the precipitation of AlN, weaken the pinning effect of AlN on grain boundaries, improve the growth capability of grains, further homogenize austenite grains and effectively reduce the yield ratio of the steel; in addition, the combination of B and N can effectively prevent the grain boundary low-melting-point phase B ₂ O ₃ Is present. B is an active segregation-prone element and is easy to segregate in a grain boundary, and the content of B is generally controlled to be very strict and is generally about 0.001-0.003% when B-containing steel is produced by a traditional process; in the thin strip continuous casting process, the solidification and cooling rates are high, the segregation of B can be effectively inhibited, and the content of B is more in solid solution, so that the content of B can be properly widened; BN particles can be generated through reasonable process control, and the function of fixing nitrogen is achieved. It has also been shown that B reduces the tendency of C atoms to segregate and avoids grain boundary Fe ₂₃ (C,B) ₆ So that more B can be added. Therefore, in the invention, the content of B is controlled to be in the range of 0.001-0.006% by adopting the content of B higher than that of the traditional process.

The invention uses the thin strip continuous casting and rolling technology, adopts two times of rolling in the rolling process, firstly performs austenite rolling at 1000-1200 ℃, and then effectively and accurately controls the temperature of the strip steel to be uniformly reduced to Ar by adopting aerial fog cooling ₃ The temperature is 5-50 ℃ below, the strip steel can be ensured to be ferrite-rolled when being rolled for the second time, thereby realizing the ferrite-rolling of low-carbon steel by the thin strip continuous casting and rolling technology and obtaining the strip steel with lower yield strength and tensile strengthThe strength, the yield ratio and the deep drawing performance are relatively low, the performance and the thickness precision reach the level of cold rolling products with the same specification, and the purpose of replacing cold with hot is realized. When the obtained product is used as a cold rolling base material, the problems of higher strength and difficult cold rolling deformation of similar low-carbon steel in the thin slab continuous casting or thin strip continuous casting process under the austenite rolling process are effectively solved.

Complicated intermediate steps such as flame cutting of a casting blank, a casting blank heating furnace, rough rolling, multi-stand finish rolling and the like in the traditional process production are omitted, and the production line has the advantages of small occupied area, simplicity, high efficiency, high production efficiency, energy conservation, environmental friendliness and the like.

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

In order to improve the castability of thin-strip continuous casting molten steel, the alkalinity of slagging in the steel-making process is a = CaO/SiO ₂ Control at a<1.5, preferably a<1.2, more preferably a =0.7 to 1.0.

To improve the castability of thin strip continuous casting molten steel, it is necessary to obtain MnO-SiO of low melting point ₂ -Al ₂ O ₃ Ternary inclusions, based on MnO-SiO as shown in FIG. 1 ₂ -Al ₂ O ₃ The ternary phase diagram shows that MnO-SiO is obtained by the method ₂ -Al ₂ O ₃ Ternary inclusions, requiring control of MnO/SiO ₂ From 0.5 to 2, preferably from 1 to 1.8.

In order to improve the castability of thin strip continuous casting molten steel in which O is an essential element for forming oxide inclusions, the present invention requires formation of MnO-SiO with a low melting point ₂ -Al ₂ O ₃ The ternary inclusions of (2) require free oxygen [ O ] in molten steel] _Free The range is 0.0005-0.005%.

The castability is not defined exactly at present, and traditionally it is a frequently used concept closely related to the fluidity of the steel, the chilling tendency, the shrinkage characteristics and the quality of the product, with respect to the metal species and the process factors thereof. The definition "Strip Casting castability (CASC)" refers to the feasibility of twin roll Casting of a steel grade. The castability is good, which means that the restrictive problem that the casting process can not be carried out or the quality of the cast product can not meet the requirement can not occur in the casting process; poor castability means that problems such as poor molten steel fluidity, molten pool agglomeration and bridging, severe belt breakage, surface cracks, surface slag inclusion and the like often occur in the casting process, so that production cannot be carried out normally and stably or the product quality cannot meet requirements.

The strip continuous casting castability of a steel grade is judged through research and analysis on the strip continuous casting castability, and briefly summarized, the following aspects can be considered:

1. whether uneven solidification shrinkage can be avoided;

2. whether the uniformity of the interface heat transfer can be improved, thereby improving the uniformity of solidification;

3. whether the hot shortness during solidification can be improved or controlled. When the castability of the strip continuous casting of a steel grade is poor, the stability of the production process is poor, the quality stability of the produced product is also poor, and finally, the productivity cannot be exerted, the qualification rate of the product is low, and the product is not suitable for the production by adopting the strip continuous casting process.

The steel grade of the invention controls the carbon content (avoids peritectic regions to solve uneven solidification shrinkage); alkalinity control, control of Als, free oxygen total oxygen control, and low melting MnO-SiO ₂ -Al ₂ O ₃ The control of ternary inclusions (improving the uniformity of interface heat transfer and solving the uniformity of solidification) and the like strictly meet the castability of the thin strip continuous casting.

The continuous casting adopts double-roller thin strip continuous casting, a casting strip with the thickness of 1.5-3mm is formed at the minimum position of a gap between two crystallizing rollers, the temperature of the casting strip is 1400-1480 ℃ after the casting strip is discharged from the crystallizing rollers, the casting strip directly enters a lower closed chamber, and non-oxidizing gas is introduced into the lower closed chamber, so that the anti-oxidation protection of strip steel can be realized on one hand, and the cooling of the strip steel is realized on the other hand. The non-oxidizing gas may be N ₂ Ar, but also other non-oxidising gases, e.g. CO obtained by sublimation of dry ice ₂ Gases, and the like. The oxygen concentration in the lower closed chamber is controlled<5 percent. Lower closed chamber pair casting beltThe temperature of the cast strip at the outlet of the lower closed chamber is 1100-1250 ℃.

The theoretical basis of BN precipitated phase involved in the lower sealing process of the cast strip is as follows:

the thermodynamic equation of boron with nitrogen, aluminum and nitrogen in gamma-Fe in steel is as follows:

BN＝B+N；Log[B][N]＝-13970/T+5.24 (1)

AlN＝Al+N；Log[Al][N]＝-6770/T+1.03 (2)

as shown in the thermodynamic curves of BN and AlN precipitation shown in FIG. 2, the precipitation start temperature of BN in the steel is about 1280 ℃, and precipitation of BN at 980 ℃ is in equilibrium, but precipitation of AlN just starts (the precipitation temperature of AlN is about 980 ℃), and precipitation of BN is thermodynamically preferred to precipitation of AlN. Therefore, the invention can complete the combination of B and N in the lower closed chamber, thereby preferentially precipitating BN particles in high-temperature austenite, inhibiting the precipitation of AlN, weakening the pinning effect of AlN on grain boundary, improving the growth capability of grains and further leading the austenite grains to be more uniform; in addition, the combination of B and N can effectively prevent the grain boundary low melting point phase B ₂ O ₃ Is present.

And (3) conveying the cast strip to a hot rolling mill for austenite rolling in a lower closed chamber in a closed state by a pinch roll, wherein the thickness of the steel strip after the austenite rolling is 1.0-2.5mm, the austenite rolling temperature is 1000-1200 ℃, and the hot rolling reduction rate is controlled to be 15-45% so as to ensure that austenite static recrystallization occurs.

Cooling the austenitic rolled strip to Ar ₃ The temperature is 5-50 ℃ below, and the cooling rate is less than or equal to 300 ℃/s. Ar (Ar) ₃ The temperature is mainly related to factors such as the chemical composition of the strip steel, the austenite grain size, the rolling temperature, the deformation, the cooling speed and the like, and Ar is generated in the rolling process ₃ The temperature can be calculated using the following equation: ar (Ar) ₃ =910-310 xw (C) -80 xw (Mn) -20 xw (Cu) -15 xw (Cr) -55 xw (Ni) -80 xw (Mo), and the low carbon steel Ar related to the invention is calculated ₃ The temperature range is 875-900 ℃.

The cooling adopts an aerial fog cooling mode, and the air-water ratio of the aerial fog cooling is 15:1 to 10:1, the air pressure is 0.5-0.8 MPa, the water pressure is 1.0-1.5 MPa, and the thickness of oxide skin on the surface of the strip steel can be effectively reduced by an aerial fog cooling mode, the temperature uniformity of the strip steel is improved, and the surface quality and the forming performance of the strip steel are improved. High-pressure water mist formed after mist spraying is sprayed on the surface of the steel strip, so that the temperature of the steel strip is reduced, and a compact air film is formed on the water mist to coat the surface of the steel strip to prevent the oxidation of the steel strip, so that the growth of oxide scales on the surface of the hot-rolled steel strip is effectively controlled. The cooling mode can avoid the problems caused by traditional spraying or laminar cooling, so that the surface temperature of the strip steel is uniformly reduced, the temperature uniformity of the strip steel is improved, and the effect of homogenizing the internal microstructure is achieved.

The reason that the thin strip continuous casting strip steel is preferably cooled by gas spray after being rolled by austenite is as follows:

the traditional continuous casting also adopts gas mist cooling, but the acting area and the temperature are different, the traditional continuous casting carries out the gas mist cooling on the casting blank in the outlet fan-shaped section area of the casting blank outlet crystallizer, the temperature of the casting blank is higher at the moment, and the casting blank is in a high-temperature austenite single-phase area in a phase diagram. The main purposes of carrying out gas mist cooling in the area are to control the position of the solidification tail end, accelerate the surface cooling of the casting blank, refine the surface austenite grain structure, improve the surface strength of the casting blank, improve the surface quality of the casting blank and avoid the occurrence of cracks. The invention carries out gas fog cooling on the ultrathin strip steel after austenite rolling, and can effectively and accurately control the temperature of the strip steel in Ar by adjusting the gas fog cooling strength ₃ The temperature is 5-50 ℃ below, so as to ensure that the strip steel can be subjected to ferrite rolling in the second hot rolling process.

The cooled strip steel is subjected to ferrite rolling, the ferrite rolling temperature is 750-850 ℃, preferably 780-830 ℃, for the steel grade, the temperature region is just in the region of low high-temperature deformation resistance, and the region is suitable for ferrite rolling; controlling the reduction rate of ferrite rolling to be 20-50%, and obtaining ultrathin hot rolled strip steel after rolling, wherein the thickness of the strip steel 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 roller.

The strip steel rolled by ferrite does not need to be cooled and directly enters a coiling machine for high-temperature coiling, and the coiling temperature is 700-760 ℃; the steel coil is cut by the high-speed flying shear, the cut high-temperature steel coil is slowly cooled by the online heat-insulating cover after being discharged from the coiling machine or rapidly enters the slow cooling pit for slow cooling, so that the strip steel is sufficiently restored and recrystallized, the unqualified performance caused by the rapid cooling of the head and the tail of the steel coil in the air cooling process is avoided, the good product quality, the yield and the low yield ratio are favorably obtained, the stamping and bending forming are easily realized, and the performance requirement of 'replacing cold with heat' or subsequent cold rolling deformation is better met. And when the temperature of the steel coil is slowly reduced to below 550 ℃, the steel coil can be put in a warehouse for natural air cooling.

Through the manufacturing process, the microstructure of the low-carbon steel is a uniform polygonal ferrite structure, the yield strength is 170-210MPa, the tensile strength is 280-330MPa, the elongation is more than or equal to 40%, and the yield ratio is less than or equal to 0.65. The product has lower strength, lower yield ratio and better forming performance, and the performance and thickness precision of the product reach the level of cold-rolled products with the same specification, thereby better meeting the performance requirement of replacing cold products with hot products; meanwhile, when the alloy is used as a cold rolling base material, the alloy can also effectively solve the problems of higher yield ratio and difficult cold rolling deformation of similar low-carbon steel produced by thin slab continuous casting or thin strip continuous casting in an austenite rolling process.

The reason that the strip continuous casting hot rolled steel coil of the invention preferably adopts a carrousel homotopic coiler is as follows:

at present, most production lines of ultrathin hot rolled steel coils adopt an underground double-coiling mode or an underground triple-coiling mode, and the production lines also consider the production of thick hot rolled plates, for example, an ESP production line of Avedi (Avedi) company adopts the underground triple-coiling mode, and an FTSC production line of Danielli (Danieli) introduced by Tang steel adopts the underground double-coiling mode. The Castrip strip casting production line of the Umbelliferae, nippon kok (Nucor) adopts the traditional method and also adopts an underground double-coiling mode. The distance interval between the underground recoiling machine and the recoiling machine is generally 8-10m (typical value is 9.4 m), when the ultrathin hot rolled strip steel is produced by strip continuous casting, the cooling speed of the strip steel in the air is very high, the interval is enough to influence the difference of the coiling temperature, the temperature deviation between the two recoiling machines can reach 49 ℃, and the performance deviation of the steel coil can be seriously influenced.

The invention preferably adopts a carrousel coiling mode, can realize the co-position coiling of the hot rolled steel coil, ensures the same coiling temperature and further greatly improves the stability of the performance of the steel coil product. At present, the carrousel coiler is widely used in the field of cold rolling, and has the main advantages of realizing thinner strip steel coiling, occupying small area and greatly shortening the length of a production line. The invention provides a carrousel coiling method in the field of coiling ultrathin hot rolled strip steel, which considers the high temperature resistance of equipment and realizes the coiling of the ultrathin hot rolled strip steel. The coiling mode is more advanced than that of a Castrip strip continuous casting production line of Nucor (Nucor) in the United states.

The invention has the beneficial effects that:

1. the invention adopts the process of ultra-short flow near-net-shape to produce low-carbon steel, namely thin-strip continuous casting, austenite rolling and ferrite rolling, the rolling process is carried out for two times, and the gas mist cooling is adopted between the two times of rolling to effectively and accurately control the temperature of the strip steel to be uniformly reduced to Ar ₃ The temperature is 5-50 ℃ below, the strip steel can be subjected to ferrite rolling in the second rolling stage, the thinnest of the ultra-thin low-carbon steel with uniform ferrite tissues can reach 0.5mm, the thinnest of the low-carbon steel obtained by the traditional hot rolling process is 1.5mm, the thinnest of the low-carbon steel obtained by the thin slab continuous casting and rolling process is 0.8mm, and the production difficulty is extremely high; the yield strength of the low-carbon steel obtained by the invention is 170-210MPa, the tensile strength is 280-330MPa, the elongation is more than or equal to 40%, the yield ratio is less than or equal to 0.65, and compared with the low-carbon steel produced by the existing hot rolling process, the low-carbon steel has lower yield strength and tensile strength, lower yield ratio and better deep drawing performance, the performance and thickness precision reach the level of cold rolling products of the same specification, and the purpose of replacing cold with hot is realized.

2. The invention controls the carbon content, the alkalinity, the Als, the free oxygen total oxygen and the low melting point MnO-SiO in the smelting process ₂ -Al ₂ O ₃ Ternary inclusions and the like, so that the castability of the obtained molten steel meets the requirement of strip continuous casting,and then, the dual-roll thin strip continuous casting can be adopted for casting, so that the low-carbon steel can realize ferrite rolling by adopting a thin strip continuous casting technology.

3. The invention adopts the ultra-short process near-net shape process to produce the low-carbon steel with lower yield strength and tensile strength, lower yield ratio and better deep drawing performance, omits complex intermediate steps of flame cutting casting blanks, casting blank heating furnaces, rough rolling, multi-frame finish rolling and the like in the traditional low-carbon steel production process, has the advantages of simple and efficient production process, high production efficiency, energy conservation, environmental protection and the like, and has small production line occupied area and low investment cost.

4. According to the invention, trace B element is selectively added in the composition design of low-carbon steel, BN particles are preferentially precipitated in the lower closed protection process so as to inhibit the precipitation of AlN, the pinning effect of fine AlN on grain boundaries is weakened, the growth capacity of austenite grains is improved, the austenite grains are homogenized, the yield strength and the yield ratio of strip steel are favorably reduced, and the forming performance of products is improved.

5. The low-carbon steel produced by the invention adopts an aerial fog cooling mode after austenite rolling, and the traditional process adopts a spraying or laminar cooling mode, the aerial fog cooling can uniformly reduce the surface temperature of the low-carbon steel, improve the temperature uniformity of strip steel and ensure that the low-carbon steel obtains uniform ferrite tissues after ferrite rolling; meanwhile, the uniform cooling can improve the plate shape quality and the forming performance of the low-carbon steel and reduce the thickness of the oxide scale on the surface of the low-carbon steel.

Drawings

FIG. 1 shows MnO-SiO ₂ -Al ₂ O ₃ Ternary phase diagram.

FIG. 2 shows the thermodynamic curves of BN and AlN precipitation.

FIG. 3 is a deformation resistance curve of the low carbon steel of the embodiment of the invention at different deformation temperatures.

FIG. 4 is a schematic process flow diagram according to an embodiment of the present invention.

FIG. 5 is a photograph showing the microstructure of low carbon steel according to an example of the present invention.

Detailed Description

The present invention is further illustrated by the following examples and the accompanying drawings, which are not intended to limit the present invention, and those skilled in the art can make modifications or improvements based on the basic idea of the invention, but within the scope of the present invention, without departing from the basic idea of the invention.

Referring to fig. 4, the method for realizing low-carbon steel ferrite rolling by using ultra-short process near net shape is disclosed by the invention.

Molten steel which is well smelted and designed according to the chemical composition of the invention is directly poured from a ladle 1 through a ladle long nozzle 2, a tundish 3, a submerged nozzle 4 and a flow distributor 5 into a molten pool 7 which is surrounded by two oppositely rotating and rapidly cooled twin-roll thin strip casting crystallizing rollers 8a and 8b and side closing plates 6a and 6b, the molten steel is solidified on the rotating circumferential surfaces of the crystallizing rollers 8a and 8b, and then a casting strip 11 with the thickness of 1.5-3mm is formed at the minimum gap (nip point) of the two crystallizing rollers 8a and 8 b. The diameter of the crystallization rollers 8a and 8b is 500-1500mm, and water is introduced into the crystallization rollers for cooling. The casting speed of the casting machine ranges from 60 to 150m/min depending on the thickness of the cast strip 11.

The casting belt 11 comes out from the crystallizing rollers 8a and 8b, the casting belt temperature is 1400-1480 ℃, the casting belt directly enters the closed chamber 10, the non-oxidizing gas is introduced into the lower closed chamber 10 to protect the strip steel, on one hand, the anti-oxidation protection of the casting belt 11 is realized, on the other hand, the casting belt 11 is cooled, and the anti-oxidation protection atmosphere can be N ₂ Ar, but also other non-oxidising gases, e.g. CO obtained by sublimation of dry ice ₂ Gas, etc., the oxygen concentration in the closed chamber 10 is controlled to be<5 percent, the lower closed chamber 10 protects the casting strip 11 from oxidation to the inlet of a rolling mill 13, and the temperature of the casting strip 11 at the outlet of the lower closed chamber 10 is 1100-1250 ℃; and then, the cast strip 11 enters a four-roll No. 1 hot rolling mill 13 for austenite rolling through a swing guide plate 9 and a pinch roll 12, the austenite rolling temperature is 1000-1200 ℃, the hot rolling reduction rate is controlled to be 15-45% so as to ensure that austenite static recrystallization occurs, and the thickness of the steel strip after austenite rolling is 1.0-2.5mm.

Cooling the band steel after the austenite rolling to below 900 ℃ in an aerosol cooling mode, wherein the cooling rate is less than or equal to 300 ℃/s; the cooled strip steel enters an online four-roll hot rolling mill 20 (2 # rolling mill) to be subjected to ferrite rolling, the ferrite rolling temperature is 750-850 ℃, preferably 780-830 ℃, the hot rolling reduction is controlled to be 20-50%, the ultrathin hot rolled strip steel is obtained after the ferrite rolling, the thickness of the strip steel is 0.5-1.5mm, and the lubrication rolling is adopted for the ferrite rolling, so that the surface quality of the strip steel can be improved, and the rolling loss of the rolls can be reduced; the strip steel rolled by ferrite does not need to be cooled, after the head of the strip steel is cut by a high-speed flying shear 16, the cut head falls into a flying shear pit 18 along a flying shear guide plate 17, and the subsequent strip steel directly enters a coiling machine 19 for high-temperature coiling at the coiling temperature of 700-760 ℃; the steel coil is cut through the high-speed flying shear 16 according to the coil weight of the strip steel, the cut high-temperature steel coil is slowly cooled by adopting an online heat-insulating cover after being discharged from the coiling machine 19, or quickly enters a slow cooling pit to slowly cool, so that the strip steel is relatively fully recovered and recrystallized, the problem of performance incompatibility caused by the fact that the head and the tail of the steel coil are rapidly cooled in air cooling is solved, good product quality and yield are favorably obtained, the yield ratio is low, stamping and bending forming are easily carried out, and the performance requirement of 'replacing cold with heat' or subsequent cold rolling deformation is better met. And when the temperature of the steel coil is slowly reduced to below 550 ℃, the steel coil can be put in a warehouse for natural air cooling.

The coiling machine 19 adopts a double-coiling mode and can also adopt a carrousel coiling mode to ensure the continuous production of the aluminum alloy thin strip.

The chemical components of the low carbon steel of the examples of the present invention are shown in table 1, and the balance of the components is Fe and other unavoidable impurities. The preparation method of the embodiment of the invention has the process parameters shown in the table 2, and the properties of the finally obtained low-carbon steel are shown in the table 3. In the table, comparative examples 1 and 2 show the relevant chemical components, process parameters and product performance of the low-carbon steel manufactured by adopting the existing typical twin-roll thin-strip continuous casting and rolling process flow technology.

As can be seen from Table 3, the yield strength of the low-carbon steel obtained by the process is 170-210MPa, the tensile strength is 280-330MPa, the elongation is more than or equal to 40 percent, and the yield ratio is less than or equal to 0.65. Compared with the low-carbon steel obtained by the comparative examples 1 and 2, the low-carbon steel has lower strength, lower yield ratio and better forming performance.

Through the manufacturing process, the microstructure of the ferrite-rolled low-carbon steel finally obtained is shown in figure 5, and as can be seen from figure 5, the microstructure of the obtained dual-phase steel is uniform polygonal ferrite, the structure is uniform, and the grain size is 20-50um. The low-carbon steel obtained by the method has lower strength, lower yield ratio and better forming performance, and can better meet the performance requirement of replacing cold with hot products; meanwhile, when the alloy is used as a cold rolling base material, the alloy can also effectively solve the problems of higher strength and difficult cold rolling deformation of similar low-carbon steel in the thin slab continuous casting or thin strip continuous casting process under the austenite rolling process.

Claims

1. A method for realizing low-carbon steel ferrite rolling by ultra-short process near net shape is characterized by comprising the following steps:

1) Smelting

in the smelting process, the slagging alkalinity a = CaO/SiO ₂ Control of a<1.5; it is necessary to obtain MnO-SiO having a low melting point ₂ -Al ₂ O ₃ Ternary inclusions, mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ Controlling the temperature to be 0.5-2; free oxygen [ O ] in molten steel] _Free The range is as follows: 0.0005-0.005%;

2) Continuous casting

Adopting twin-roll thin strip continuous casting to form a casting strip with the thickness of 1.5-3mm at the minimum position of the gap between two crystallizing rolls; the diameter of the crystallization roller is 500-1500mm, the inner part of the crystallization roller is cooled by water, and the casting speed is 50-150 m/min; the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;

3) Lower closed protection

4) Rolling of austenite

5) Aerosol cooling

6) Ferritic rolling

7) Coiling

8) Slow cooling

And coiling the steel strip, slowly cooling to below 550 ℃, and then warehousing and naturally cooling in air.

2. The method for ultra-short process near net shape low carbon steel ferrite rolling of claim 1, wherein in step 1), the slagging alkalinity a <1.2, preferably a = 0.7-1.0.

3. Ultrashort as claimed in claim 1 or 2The method for realizing the low-carbon steel ferrite rolling by the near net shape of the process is characterized in that in the step 1), mnO-SiO ₂ -Al ₂ O ₃ MnO/SiO in ternary inclusions ₂ The temperature is controlled to be 1-1.8.

4. The method for ultra-short process near net shape low carbon steel ferrite rolling of claim 1, wherein in step 2), the diameter of the crystallization roll is 780-820 mm.

5. The method of ultra short process near net shape low carbon steel ferrite rolling of claim 1, wherein in step 3), the non-oxidizing gas is nitrogen, argon or CO obtained after sublimation of dry ice, or argon ₂ 。

6. The method for achieving low-carbon steel ferrite rolling with ultra-short process and near net shape as claimed in claim 1, wherein in step 5), the gas-water ratio of the gas-fog cooling is 15-10: 1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

7. The method for ultra-short process near net shape low carbon steel ferrite rolling of claim 1, wherein in step 6), the ferrite rolling temperature is 780-830 ℃.

8. The method of ultra short flow near net shape to achieve low carbon steel ferrite rolling of claim 1 wherein in step 7) the coiling is in a double coiling configuration or in a carossel coiling configuration, preferably carossel coiling configuration.

9. The method of ultra-short pass near net shape for achieving low carbon steel ferrite rolling as claimed in any one of claims 1 to 8, wherein the microstructure of said low carbon steel is uniform polygonal ferrite.

10. The method of ultra-short pass near net shape for achieving low carbon steel ferrite rolling as claimed in any one of claims 1 to 9, wherein the microstructure of said low carbon steel has a grain size of 20 to 50um.

11. The method for ultra-short process near net shape low-carbon steel ferrite rolling as claimed in any one of claims 1-10, wherein the yield strength of said low-carbon steel is 170-210MPa, the tensile strength is 280-330MPa, the elongation is not less than 40%, and the yield ratio is not more than 0.65.