Production and finishing method of a martensitic thin steel strip

Abstract

The present invention provides a production and finishing method for a martensitic thin steel strip, wherein the thickness of the thin steel strip is less than 2 mm, and the thin steel strip is produced by a twin-roll casting process. In this process, molten metal flows into a molten pool formed above the roll gap of a pair of relatively rotating casting rolls after being subjected to electric furnace steelmaking, VD furnace vacuum degassing, and LF refining. The molten metal cools and solidifies on the casting surfaces of the pair of casting rolls and then passes downward through the roll gap between the pair of casting rolls to form a thin cast strip, which is then subjected to hot rolling, rapid cooling, and coiling to form an intermediate product of a thin steel strip with a martensitic structure. Subsequently, the intermediate product of the thin steel strip is subjected to on-line finishing at a temperature below 80 °C, and this finishing process includes the technological steps of uncoiling, trimming, tension leveling, and coiling.

Description

Technical Field

[0001] The present invention relates to the technical field of steel rolling, and particularly to a production and finishing method for martensitic thin steel strips. Specifically, the martensitic steel produced by twin-roll casting is subsequently subjected to a stretch-bending straightening process for trimming and shape optimization to obtain a final martensitic steel product with good shape, thereby improving the product qualification rate. Background Art

[0002] As the steel grade with the highest strength level in commercial applications, hot-rolled martensitic steel can be used for components such as crane booms, mixer truck blades, dump truck bodies, and trailer beams that require high strength, high wear resistance, and simple forming processes, highly meeting the current development trend of using steel for lightweight and energy-saving commercial vehicles and construction machinery.

[0003] Ultra-high strength martensitic steel mainly obtains excellent mechanical properties through martensitic transformation. Specifically, during the production process, in order to achieve high strength, rapid cooling is required. Most hot-rolled martensitic steels are produced by online quenching + tempering or offline quenching + tempering (quenching and tempering) processes, and full martensite structure is obtained through ultra-low cooling temperature and relatively fast cooling rate. This traditional production process not only has high requirements for the equipment capacity of the production line and a long process flow, but also most laminar cooling equipment cannot meet the production requirements of rapid cooling of hot-rolled martensitic steel.

[0004] In addition, during the martensitic transformation process, strong internal stresses will be generated, which may lead to poor shape of the steel plate and reduced flatness; at the same time, due to hydrogen penetration, microcracks are generated, and it is difficult to ensure product quality during the use by downstream customers, and its application in the automotive sheet field is also limited.

[0005] In order to generate martensite structure in an easy-to-operate manner and eliminate the defects caused by internal stresses during the martensitic transformation process, a production and finishing method for martensitic steel is needed to obtain a steel strip product that meets the product quality requirements and qualification rate requirements. Summary of the Invention

[0006] The present invention provides a production method for martensitic steel, in which a twin-roll casting process is used to produce thin steel strips. After one-pass hot rolling, laminar cooling is carried out for rapid cooling (for example, cooling at a rate of 60 - 120 °C / s), and an ultra-rapid cooling rate that is difficult to achieve by a conventional hot-rolling production line can be realized, thereby obtaining a full martensite structure.

[0007] However, due to the strong internal stresses generated during the martensitic transformation process, different wavinesses will appear during the ultra-rapid cooling process, affecting the use by downstream customers. Specifically, the existence of residual stresses may lead to poor shape of the steel plate and reduced flatness; at the same time, due to hydrogen penetration, microcracks are generated, and it is difficult to ensure product quality during the use by downstream customers, and its application in the automotive sheet field is also limited.

[0008] In order to eliminate the defects caused by internal stress during the martensitic transformation process, the present invention further provides a method for finishing martensitic steel to obtain a thin steel strip product that meets the product quality requirements and the qualified rate requirements. Specifically, the martensitic thin steel strip produced by twin-roll casting is subsequently subjected to a stretch-bending straightening process for trimming and shape optimization to obtain a final martensitic thin steel strip product with good shape.

[0009] Specifically, the present invention provides a method for producing and finishing a martensitic thin steel strip, characterized in that the thickness of the martensitic thin steel strip is less than 2 mm, and the martensitic thin steel strip is produced by a twin-roll casting process, the process including: the molten metal flows into a molten pool formed above the roll gap of a pair of relatively rotating casting rolls after being subjected to electric furnace steelmaking, VD furnace vacuum degassing, and LF refining. The molten metal cools and solidifies on the casting surfaces of the pair of casting rolls and passes downward through the roll gap between the pair of casting rolls to form a thin cast strip. The thin cast strip is subjected to hot rolling, rapid cooling, and coiling to form an intermediate product of a thin steel strip with a martensitic structure. Among them, during the LF refining process, only the molten metal is heated up so that its tapping temperature is within the range of 1650 °C to 1700 °C. The molten pool is protected by an atmosphere, and the slightly positive pressure of the molten pool protection atmosphere is selected from within the range of 5 - 30 Pa. And the thin cast strip enters a hot box with a protective atmosphere before hot rolling, and the slightly positive pressure of the hot box protective atmosphere is selected from within the range of 15 - 50 Pa.

[0010] The intermediate product of the thin steel strip is subjected to online finishing at a temperature below 80 °C. The finishing process includes the following steps: uncoiling, where the coiled intermediate product of the thin steel strip produced by twin-roll casting is uncoiled by an uncoiler; trimming, where the two width sides of the thin steel strip are sheared by a shearing machine; stretch leveling: the thin steel strip passes through a bending roll unit and a straightening roll unit in sequence. Among them, the bending roll unit is configured such that the bending roll insertion amount is selected from within the range of 5 - 25 mm, the straightening roll unit is configured such that the straightening roll insertion amount is selected from within the range of 5 - 25 mm, and the stretch leveling tension during the stretch leveling process is selected from within the range of 100 - 250 kN, and the elongation rate is selected from within the range of 0.5% - 1.5%; coiling, where the thin steel strip is coiled into a coil of the thin steel strip by a coiler.

[0011] In a preferred embodiment, the tapping temperature of the LF refining is within the range of 1670 °C to 1690 °C.

[0012] In a preferred embodiment, the protective atmosphere of the molten pool is nitrogen, argon, or other non-oxidizing inert gases, and the slightly positive pressure of the atmosphere protection of the molten pool is selected from within the range of 10 - 20 Pa.

[0013] In a preferred embodiment, the protective atmosphere of the hot box is nitrogen, argon, or other non-oxidizing inert gases, and the slightly positive pressure of the hot box protective atmosphere is selected from within the range of 20 - 30 Pa.

[0014] In a preferred embodiment, the finishing process is performed at a temperature below 50°C.

[0015] In a preferred embodiment, the insertion amount of the bending roller is selected from the range of 10 to 15 mm, and the insertion amount of the straightening roller is selected from the range of 10 to 15 mm.

[0016] In a preferred embodiment, the tension in the straightening process is selected from the range of 120 to 220 kN, and the elongation is selected from the range of 0.8% to 1.0%.

[0017] In a preferred embodiment, during the tension leveling process, the thin steel strip passes through two bending roller units and one straightening roller unit in sequence.

[0018] In a preferred embodiment, in the process of hot rolling, rapid cooling and coiling of the thin cast strip to form a thin steel strip intermediate product with a martensitic structure, the temperature of the thin cast strip is reduced to a range of 800-850°C after a hot rolling, and then laminar cooling is performed at a cooling rate of 60-120°C / s, and then coiling is performed at a coiling temperature below 300°C.

[0019] The present invention also relates to a thin steel strip produced and finished by the above method, wherein the thin steel strip has a martensitic metallographic structure.

[0020] Beneficial technical effects

[0021] Compared with the prior art, the present invention is beneficial in that:

[0022] The martensitic steel produced by rapid cooling in the twin-roll casting process is effectively improved in terms of martensitic plate shape and surface quality by the trimming and straightening action of the stretch bending straightening machine. The invention is easy to operate, has low process improvement cost, and can significantly improve the product qualification rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments are briefly introduced below. Obviously, the drawings in the following description only relate to some embodiments of the present invention, but are not intended to limit the present invention.

[0024] Figure 1 is a schematic diagram of the metallographic structure of the martensitic thin steel strip of the present invention;

[0025] Figure 2 is a schematic side view of a twin-roll continuous casting machine system of the present invention;

[0026] Figure 3 is through Figure 2 A partial cross-sectional view of a casting roll installed in a roll box in a casting position of a twin-roll continuous casting machine;

[0027] Figure 4 is Figure 3 An enlarged schematic view of a part, specifically showing molten metal solidifying on the surface of a casting roll to form a thin steel strip that is conveyed downward;

[0028] Figure 5A A schematic view showing the process of finishing the thin steel strip produced by twin-roll casting again on the production line; Figure 5B An enlarged view showing the tension leveling process of the thin steel strip during finishing on the production line. Detailed implementation manners

[0029] In order to make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0030] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those of ordinary skill in the art to which the present invention pertains.

[0031] In the technical solution of the present invention, a twin-roll casting process is adopted to produce a thin steel strip, for example, a thin steel strip with a thickness of less than 2 mm. After one-pass hot rolling, the temperature of the steel strip drops to, for example, 800 - 850 °C, and then after rapid cooling through laminar flow cooling (for example, air mist cooling, with a cooling rate of 60 - 120 °C / s), a fully martensitic structure is obtained. For example, as Figure 1 shown, it shows the martensite metallographic structure produced by the twin-roll casting process and then formed through ultra-rapid cooling.

[0032] Due to the strong internal stress generated during the martensite phase transformation process, different wavinesses will occur during the ultra-rapid cooling process. Specifically, the existence of residual stress may lead to poor plate shape and reduced flatness of the steel plate; at the same time, due to hydrogen penetration, microcracks are generated, and it is difficult to ensure product quality during the use by downstream customers, and its application in the field of automotive sheets is also limited.

[0033] Through the production and finishing method of the martensitic thin steel strip provided by the present invention, the generation of microcracks can be reduced during the twin-roll casting process, the flatness of the thin steel strip can be improved, surface defects can be reduced, and the product quality and qualification rate can be increased.

[0034] Now referring to Figures 2 - 4 , a twin-roll continuous caster is shown, which includes a main machine frame 10 that stands upright from the floor and supports a pair of casting rolls 12, and this pair of casting rolls 12 are installed in a roll cassette 11 for easy operation and movement.

[0035] A twin-roll continuous casting machine for continuously casting thin steel strips includes a pair of oppositely rotating casting rolls 12, each having a casting surface 12A, with a roll gap 18 formed therebetween. The molten metal (i.e., molten steel) after being refined by an electric furnace steelmaking process, degassed in a VD furnace under vacuum, and then heated up in an LF refining process is transferred from a ladle 13 through an intermediate ladle 14, and then through a refractory slide gate 15 to a tundish 16, and then to a distributor 17 positioned above the roll gap 18 and between the casting rolls 12. The molten metal thus transferred forms a molten pool 19 supported above the roll gap and on the casting surfaces 12A of the casting rolls 12.

[0036] The casting rolls 12 are internally cooled (e.g., internally water-cooled), so that as the casting rolls 12 rotate relative to each other, the molten metal in the molten pool 19 comes into rotational contact with the casting surface 12A and cools and solidifies on the casting surface 12A. During the casting process, the metal shell formed by cooling and solidifying on the casting surface 12A of the casting rolls 12 is brought together at the roll gap 18 between the casting rolls to form a thin cast strip that is conveyed downward from the roll gap.

[0037] Specifically, during the LF refining process, the LF refining only performs a heating operation on the molten steel, so that its tapping temperature is in the range of 1650°C to 1700°C, more preferably in the range of 1670°C to 1690°C, to contribute to smooth casting, avoid the accident of clogging the nozzle due to too low temperature, causing potential safety hazards or reducing product quality.

[0038] During the process where the molten steel tapped from the LF refining flows out of the ladle 13, passes through the intermediate ladle 14, the tundish 16, and forms and flows into the molten pool 19 via the distributor 17, in order to prevent oxygen from entering the molten pool 19 and reacting with the molten steel therein or the distributor 17, and even forming slag inclusions that are rolled into the surface of the steel strip, it is necessary to protect the molten pool with an atmosphere. For example, the protective atmosphere adopted for the molten pool 19 can be selected as non-oxidizing inert gases such as nitrogen and argon. Preferably, the slightly positive pressure (i.e., the value higher than the atmospheric pressure) of the atmosphere protection for the molten pool 19 is selected from the range of 5 - 30 Pa; more preferably, the slightly positive pressure of the atmosphere protection for the molten pool is selected from the range of 10 - 20 Pa.

[0039] The thin cast strip (thin steel strip) formed from the molten steel then enters a hot box with a protective atmosphere to prevent the formation of iron oxide on the surface of the thin steel strip (the formed iron oxide may be pressed into the surface of the thin steel strip in subsequent rolling processes, affecting the surface quality of the final product). In the hot box, the temperature of the thin steel strip is maintained at a relatively high temperature through sealing to facilitate the subsequent hot rolling process. For example, the protective atmosphere of the hot box can be selected as non-oxidizing inert gases such as nitrogen and argon. Preferably, the slightly positive pressure of the protective atmosphere of the hot box is selected from the range of 15 - 50 Pa; more preferably, the slightly positive pressure of the protective atmosphere of the hot box is selected from the range of 20 - 30 Pa.

[0040] After leaving the hot box, the thin steel strip is hot-rolled through at least one pass and then cooled, for example, by air mist cooling. To form a fully martensitic metallographic structure, the steel strip needs to be rapidly cooled, for example, by laminar flow cooling at a cooling rate of 60 - 120 °C / s. Subsequently, the cooled thin steel strip is coiled to form an intermediate product of the thin steel strip, and the coiling temperature is preferably below 300 °C, more preferably below 200 °C.

[0041] Reference Figure 5A , the intermediate product of the thin steel strip produced by the twin-roll casting process needs to be re-fed to the finishing line to eliminate the internal stress formed during the generation of martensite and achieve good surface quality, for example, meeting the application requirements of components such as crane booms, mixer truck blades, dump truck bodies, and trailer beams that require high strength, high wear resistance, and simple forming processes. In a preferred embodiment, the feeding temperature of the finishing line is maintained below 80 °C, more preferably below 50 °C. If the feeding temperature of the finishing line is too high, it may damage the processing rolls (such as bending rolls and straightening rolls), thereby reducing their service durability, and even damaging the surface of the thin steel strip straightened by them. In addition, the thin steel strip straightened at too high a temperature may undergo recovery or springback after cooling, and its elongation and surface quality may deteriorate or decrease due to this, affecting the straightening effect.

[0042] In an exemplary embodiment, the process of feeding the intermediate product of the thin steel strip to the finishing line, for example, includes the following steps:

[0043] 1) Uncoiling

[0044] The steel coil of the intermediate product of the thin steel strip formed by the twin-roll casting process is uncoiled by an uncoiler 100 to open the steel coil into a flat thin steel strip form. Generally, the thickness of this thin steel strip is below 2 mm.

[0045] 2) Edge trimming

[0046] Considering the poor-quality parts such as the burrs on both sides in the width direction of the thin steel strip formed by twin-roll casting or the possibly existing unrolled parts, and according to the dimensional requirements of the final application scenario, edge trimming operation needs to be performed on the thin steel strip.

[0047] In the illustrated embodiment, the two sides in the width of the thin steel strip are sheared by a shearing machine 200 (such as a circular shear) to remove the defective parts of the thin steel strip and make its dimensions meet the specific application scenario or customer requirements.

[0048] 3) Straightening

[0049] After the thin steel strip is trimmed to a suitable width dimension, it will pass through the tension leveling mechanism 300 for shape optimization, such as eliminating internal stress during the formation of martensite, improving or eliminating surface defects such as ribbing, cracks, and waves that may occur on the martensite plate, and optimizing its surface flatness and surface quality.

[0050] In the illustrated embodiment, and with reference to Figure 5A , the thin steel strip is conveyed to the tension leveling mechanism 300, for example, via a set of tension rollers 500 (or other similar steel strip driving devices). The tension leveling mechanism 300 includes two bending roller units 310, 320, and a straightening roller unit 330. Among them, when the thin steel strip passes through the bending roller units 310, 320, the surface of the thin steel strip is corrected by the repeated bending of the two bending roller units 310, 320, and the effect of improving the strip shape and eliminating surface defects can be achieved with a smaller tension. Subsequently, the thin steel strip passes through the straightening roller unit 330 to release the internal stress generated by the repeated bending of the thin steel strip through the bending roller units 310, 320.

[0051] The bending roller unit 310 includes a pair of bending rollers 310A and 310B, which are respectively located on the upper and lower sides of the thin steel strip, and the insertion amount C1 of the bending rollers is selected within the range of 5 - 25 mm, preferably within the range of 10 - 15 mm. In the illustrated embodiment, the bending roller 310A is a fixed roller, and the bending roller 310B is a movable roller. By adjusting the position of the movable roller 310B in the vertical direction, the insertion amount of the bending rollers can be flexibly adjusted. In the context of the present invention, the insertion amount C1 of the bending rollers is defined as the linear distance in the vertical direction that the movable roller 310B on one side of the thin steel strip presses into the fixed roller 310A on the other side of the thin steel strip, as shown in the enlarged view in the appendix Figure 5B as shown.

[0052] Another bending roller unit 320 has a substantially similar structure to the bending roller unit 310, and the insertion amount of its bending rollers can also be selected within the range of 5 - 25 mm, preferably within the range of 10 - 15 mm.

[0053] The straightening roller unit 330 includes two upper straightening rollers 330A, 330B located on the upper side of the thin steel strip, and one lower straightening roller 330C located on the lower side of the thin steel strip. Among them, the two upper straightening rollers 330A, 330B are fixed rollers and are substantially at the same horizontal height, and the lower straightening roller 330C is a movable roller. Among them, the insertion amount C2 of the straightening rollers is selected within the range of 5 - 25 mm, preferably within the range of 10 - 15 mm. The insertion amount C2 of the straightening rollers is defined as the linear distance in the vertical direction that the movable roller 330C presses into the fixed rollers 330A, 330B, as shown in the appendix Figure 5BAs shown in the enlarged view in [Figure number]. In the illustrated embodiment, the fixed rollers 330A and 330B are located on the upper side of the thin steel strip, and the movable roller 330C is located on the lower side of the thin steel strip; however, in an alternative embodiment, the fixed rollers can be arranged on the lower side of the thin steel strip, and the movable rollers can be arranged on the other side.

[0054] In addition, in order to better eliminate the internal stress generated during the martensite formation process and improve the shape and surface quality of the martensitic steel, during the tension leveling process, the tension leveling tension is selected to be relatively high, for example, in the range of 100 - 250 kN, preferably selected from the range of 120 - 220 kN; the elongation rate is selected from the range of 0.5% - 1.5%, preferably selected from the range of 0.8% - 1.0%.

[0055] In an alternative embodiment, according to the actual requirements of the strip shape, a tension leveling mechanism including two bending roller units and two straightening roller units, or a tension leveling mechanism with more bending rollers and / or straightening rollers, can also be used to meet the higher flatness requirements for the steel strip; or a tension leveling mechanism (or straightening mechanism) including a single bending roller unit and / or a single straightening roller unit can also be used, so as to meet the requirements of a certain surface flatness and surface quality at a lower cost and with a shorter production line.

[0056] 4) Coiling

[0057] The tension-leveled thin steel strip forms a good shape and surface quality, and then enters the coiler 400 via, for example, the tension roller group 600 (or other similar steel strip driving devices) and is coiled by the coiler 400 to form a final steel coil product of the thin steel strip that meets the requirements.

[0058] The following further illustrates the embodiments and technical effects of the present invention through multiple comparative examples and examples.

[0059]

Comparative Example 1

[0060] The thin steel strip is produced by a twin-roll casting process. During the twin-roll casting process, the tapping temperature of LF refining is 1650 °C, the slightly positive pressure of the molten pool protective atmosphere is 35 Pa, the slightly positive pressure of the hot box protective atmosphere is 10 Pa, and after one-pass hot rolling, it is cooled and coiled at a coiling temperature of 350 °C to form an intermediate product of the thin steel strip.

[0061] The intermediate product of the thin steel strip is put on the production line for finishing. When finishing, the on-line temperature is 90 °C. After the intermediate product of the thin steel strip is uncoiled and trimmed, it is tension leveled by passing through two bending roller units and one straightening roller unit. During the tension leveling process, the insertion amount of the bending rollers is selected to be 30 mm, the insertion amount of the straightening rollers is selected to be 30 mm, the tension leveling tension is adjusted to 100 kN, and the tension leveling elongation rate is 2.0%.

[0062] Under the process parameters selected in Comparative Example 1, the probability of rejecting the martensitic steel strip product due to surface quality was 1.7%, and the product qualification rate could not meet the requirements.

[0063]

Example 1

[0064] A thin steel strip was produced by a twin-roll casting and rolling process. During the twin-roll casting and rolling process, the tapping temperature of LF refining was 1670 °C, the slightly positive pressure of the molten pool protective atmosphere was 10 Pa, the slightly positive pressure of the hot box protective atmosphere was 20 Pa. After one-pass hot rolling, it was cooled and coiled at a coiling temperature of 200 °C to form an intermediate product of the thin steel strip.

[0065] The intermediate product of the thin steel strip was put on the production line for finishing. The temperature when putting it on the production line for finishing was 40 °C. After uncoiling and trimming the intermediate product of the thin steel strip, the thin steel strip was tension levelled by passing through two bending roll units and one straightening roll unit. During the tension levelling process, the insertion amount of the bending roll was selected as 10 mm, the insertion amount of the straightening roll was selected as 10 mm, the tension levelling tension was adjusted to 200 kN, and the tension levelling elongation was 0.8%.

[0066] Under the process parameters selected in Example 1, the probability of rejecting the martensitic steel strip product due to surface quality was 0.35%, and the product qualification rate met the requirements.

[0067]

Example 2

[0068] A thin steel strip was produced by a twin-roll casting and rolling process. During the twin-roll casting and rolling process, the tapping temperature of LF refining was 1690 °C, the slightly positive pressure of the molten pool protective atmosphere was 20 Pa, the slightly positive pressure of the hot box protective atmosphere was 30 Pa. After one-pass hot rolling, it was cooled and coiled at a coiling temperature of 150 °C to form an intermediate product of the thin steel strip.

[0069] The intermediate product of the thin steel strip was put on the production line for finishing. The temperature when putting it on the production line for finishing was 50 °C. After uncoiling and trimming the intermediate product of the thin steel strip, the thin steel strip was tension levelled by passing through two bending roll units and one straightening roll unit. During the tension levelling process, the insertion amount of the bending roll was selected as 15 mm, the insertion amount of the straightening roll was selected as 15 mm, the tension levelling tension was adjusted to 180 kN, and the tension levelling elongation was 1.0%.

[0070] Under the process parameters selected in Example 2, the probability of rejecting the martensitic steel strip product due to surface quality was 0.5%, and the product qualification rate met the requirements.

[0071] The main technical parameters of the above-mentioned multiple comparative examples and examples are summarized in Table 1 below, and the implementation effects of the above-mentioned multiple comparative examples and examples are summarized in Table 2.

[0072] Table 1 Technical parameters of Comparative Example 1, Example 1, and Example 2

[0073]

[0074] Table 2 Surface quality judgment rate of Comparative Example 1, Example 1, and Example 2

[0075]

[0076] As can be seen from Table 2, the surface quality judgment rate of the martensitic thin steel strip in Comparative Example 1 is 1.7%; while in Example 1 and Example 2, the product rejection rate is reduced, and the product qualification rate and profit level are improved.

[0077] The above detailed description of the embodiments of the present disclosure provided in conjunction with the drawings is not intended to limit the scope of the present disclosure claimed, but merely represents selected embodiments of the present disclosure. Moreover, the drawings are only used to schematically show the various components involved in the present invention, and their dimensions, ratios, etc. are not used to actually define the true proportional relationship of the various components.

[0078] The above are only specific embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the scope of protection of the present invention.

Claims

1. A production and finishing method for a martensitic thin steel strip, characterized in that, The thickness of the martensitic thin steel strip is less than 2 mm. The martensitic thin steel strip is produced by a twin-roll casting process. The process of producing the thin steel strip by twin-roll casting includes: the molten metal flows into a molten pool (19) formed above the roll gap (18) of a pair of relatively rotating casting rolls (12) after being subjected to electric furnace steelmaking, VD furnace vacuum degassing, and LF refining. The molten metal cools and solidifies on the casting surface (12A) of the pair of casting rolls (12) and then passes downward through the roll gap (18) between the pair of casting rolls to form a thin cast strip. The thin cast strip is subjected to hot rolling, rapid cooling, and coiling to form an intermediate product of a thin steel strip with a martensitic structure. Among them, during the LF refining process, only the molten metal is heated up so that its tapping temperature is in the range of 1650 °C to 1700 °C. The molten pool (19) is protected by an atmosphere. The slightly positive pressure of the protective atmosphere of the molten pool is selected from the range of 5 - 30 Pa. And the thin cast strip enters a hot box with a protective atmosphere before hot rolling. The slightly positive pressure of the protective atmosphere of the hot box is selected from the range of 15 - 50 Pa. The intermediate product of the thin steel strip is subjected to on-line finishing at a temperature below 80 °C. The finishing process includes the following steps: Uncoiling: The steel coil of the intermediate product of the thin steel strip produced by twin-roll casting is uncoiled by an uncoiler (100). Edge trimming: The two width sides of the thin steel strip are sheared by a shearing machine (200). Tension leveling: The thin steel strip passes through a bending roll unit (310, 320) and a straightening roll unit (330) in sequence. Among them, the bending roll unit is configured such that the bending roll insertion amount is selected from the range of 5 - 25 mm, the straightening roll unit is configured such that the straightening roll insertion amount is selected from the range of 5 - 25 mm, and the tension leveling tension during the tension leveling process is selected from the range of 100 - 250 kN, and the elongation is selected from the range of 0.5% - 1.5%. Coiling: The thin steel strip is coiled into a coil of the thin steel strip by a coiler (400).

2. The method according to claim 1, characterized in that, The tapping temperature of LF refining is in the range of 1670 °C to 1690 °C.

3. The method according to any one of claims 1-2, characterized in that, The protective atmosphere of the molten pool is nitrogen, argon, or other non-oxidizing inert gases. The slightly positive pressure of the atmosphere protection of the molten pool is selected from the range of 10 - 20 Pa.

4. The method according to any one of claims 1-2, characterized in that, The protective atmosphere of the hot box is nitrogen, argon, or other non-oxidizing inert gases. The slightly positive pressure of the protective atmosphere of the hot box is selected from the range of 20 - 30 Pa.

5. The method according to any one of claims 1-2, characterized in that, The finishing process is carried out at a temperature below 50 °C.

6. The method according to any one of claims 1-2, characterized in that, The bending roll insertion amount is selected from the range of 10 - 15 mm, and the straightening roll insertion amount is selected from the range of 10 - 15 mm.

7. The method according to any one of claims 1-2, characterized in that, The tension leveling tension during the tension leveling process is selected from the range of 120 - 220 kN, and the elongation is selected from the range of 0.8% - 1.0%.

8. The method according to any one of claims 1-2, characterized in that, During the tension leveling process, the thin steel strip passes through two bending roll units (310, 320) and one straightening roll unit (330) in sequence.

9. The method according to any one of claims 1-2, characterized in that, In the process of forming an intermediate product of a thin steel strip with a martensitic structure by subjecting the thin cast strip to hot rolling, rapid cooling, and coiling, after the thin cast strip is hot rolled in one pass and the temperature drops to the range of 800 - 850 °C, it is then subjected to laminar flow cooling at a cooling rate of 60 - 120 °C / s and coiled at a coiling temperature below 300 °C.

10. A thin steel strip produced and finished by the method according to any one of claims 1-9, wherein the thin steel strip has a martensitic metallographic structure.

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