A method for manufacturing a ferritic-martensitic dual-phase steel

Abstract

The application relates to a preparation method of a ferrite-martensite dual-phase steel, which comprises the steps of converter smelting, LF furnace refining, RH refining, tundish treatment and continuous casting, and through controlling specific parameters in each existing step, cooperating with the setting of a continuous casting protective slag and the specific smelting steel grade component content, the ferrite-martensite dual-phase steel can reduce the steel slag reactivity and has good lubricity in the continuous casting process, so that the continuous casting is ensured to be smooth.

Description

Technical Field

[0001] This invention relates to the field of steel preparation technology, specifically to a method for preparing ferritic martensitic dual-phase steel. Background Technology

[0002] Ferritic-martensitic dual-phase steel possesses both high strength and high toughness. Its microstructure is mainly composed of a hard martensite phase and a soft ferrite matrix phase. Ferritic-martensitic dual-phase steel exhibits excellent mechanical properties, including good formability, low yield strength ratio, high work hardening rate, high uniform total elongation, high fatigue resistance, and weldability. It is widely used in many industrial fields, especially in the automotive industry.

[0003] In the continuous casting process of ferritic martensitic duplex steel, the steel contains a certain amount of strong reducing element Al, which easily reacts with silicon dioxide in the continuous casting protective slag. Therefore, many technical solutions currently use low-reactivity continuous casting protective slag that does not contain silicon dioxide or other substances. However, this type of low-reactivity continuous casting protective slag has high basicity and poor lubrication performance, which makes it easy to stick together during the continuous casting process and affect the smooth operation of continuous casting.

[0004] Chinese invention patent publication CN115945658A discloses a continuous casting protective slag for casting high-alumina steel molten steel, its preparation method and application. By adjusting the components and contents of the continuous casting protective slag, the smooth progress of continuous casting is improved. However, this technical solution does not pay attention to the coordination between the continuous casting protective slag and the parameters of each step, so the improvement effect is not ideal.

[0005] Converter smelting, LF furnace refining, RH refining, tundish treatment, and continuous casting are routine steps in steelmaking. However, the specific parameter settings for each step are crucial, and these parameter settings are closely coordinated. Setting certain parameters in an earlier step can affect the smooth operation of subsequent steps or the quality of the steel product. Therefore, how to reduce the reactivity of steel slag by controlling various parameters during smelting and continuous casting has become an urgent technical problem to be solved. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing ferritic-martensitic dual-phase steel.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] A method for preparing ferritic-martensitic dual-phase steel includes the following steps:

[0009] (1) Converter smelting: The raw materials are prepared according to the composition of ferritic martensitic duplex steel, including molten iron, scrap steel, auxiliary materials, and alloy materials. The temperature of the molten iron entering the furnace is 1220-1500℃, the S content of the molten iron entering the furnace is less than or equal to 0.015wt%, and the proportion of scrap steel added is 0.1-20wt%. The converter adopts top and bottom blowing, with oxygen blown from the top and inert gas blown from the bottom. The free oxygen content at the end of the converter smelting is 100-700ppm, C≤0.04wt%, P≤0.01wt%, S≤0.03wt%, and Mo: 0.18-0.23wt%. The final slag basicity R is 2.5–4.0, the converter smelting endpoint temperature is 1635–1685℃, the converter tapping temperature is 1580–1630℃, the settling time before tapping is 30–90 s, the tapping time is 4–7 min, and the molten steel after tapping contains C: 0.05–0.07 wt%, Si ≤ 0.08 wt%, Mn: 1.85–1.95 wt%, and Cr: 0.18–0.25 wt%. The auxiliary materials are quicklime, limestone, fluorite, and / or steel shot aluminum, and the alloy material is medium carbon ferromanganese, low carbon ferrochrome, and / or ferroniobium.

[0010] (2) LF furnace refining: The molten steel after tapping from the converter in step (1) is refined in the LF furnace. The temperature of the molten steel entering the LF furnace is 1560~1610℃, and the temperature of the molten steel exiting the LF furnace is 1603~1628℃. The molten steel exiting the LF furnace contains C: 0.07~0.09wt%, Si≤0.1wt%, Mn: 1.95~2.05wt%, Cr: 0.2~0.3wt%. The LF furnace refining cycle is 30~150min.

[0011] (3) RH refining: The molten steel obtained after refining in the LF furnace in step (2) is refined in the RH furnace. The temperature of the molten steel entering the RH furnace is 1593~1618℃, the temperature of the molten steel leaving the RH furnace is 1593~1618℃, the B content in the molten steel leaving the RH furnace is 0.001~0.0022wt%, the vacuum treatment time is 15~35min, and the pure degassing time is 6~15min.

[0012] (4) Tundish treatment: The molten steel obtained after RH refining in step (3) is injected into the tundish. A low-carbon covering agent is used to cover the molten steel in the tundish. The temperature of the molten steel in the tundish is 1533~1558℃. The argon flow rates for argon blowing through the stopper rod, the top nozzle, and between the plates are all 2~4L / min. The molten steel in the tundish contains C: 0.07~0.09wt%, Si≤0.1wt%, and Mn: 1.95~2.05wt%. , P≤0.015wt%, S≤0.003wt%, Als: 0.55~0.70wt%, Alt: 0.55~0.72wt%, N≤0.005wt%, Cr: 0.2~0 .3wt%, Nb: 0.013~0.028wt%, Ti: 0.010~0.025wt%, Mo: 0.18~0.23wt%, B: 0.001~0.0022wt%.

[0013] (5) Continuous casting: Molten steel from the tundish is continuously poured into the crystallizer through a submerged entry nozzle. The submersion depth of the nozzle is 120–180 mm. Then, continuous casting protective slag is continuously added to the crystallizer. The chemical composition of the continuous casting protective slag is as follows: CaO: 28.80–33.12 parts by weight, MgO: 1.01–2.0 parts by weight, SiO2: 33.12–38.95 parts by weight, Al2O3: 2.01–2.62 parts by weight, Fe2O3: 0.5–0.6 parts by weight, MnO: 0.10–0.16 parts by weight, C: 4.06 The continuous casting protective slag has the following composition: approximately 5.12 parts by weight, Na2O: 9.62–10.58 parts by weight, F: 9.55–11.2 parts by weight, and K2O: 0.1–0.15 parts by weight. The basicity R of the continuous casting protective slag is 0.8–0.88, the melting point is 1030–1039℃, the bulk density is 0.7–0.76 g / ml, and the viscosity is 0.130–0.135 Pa·S. The amount of continuous casting protective slag added is 0.35–0.6 kg / t steel, and the thickness of the continuous casting protective slag is 12–18 mm. Then, the ferritic martensitic duplex steel billet is obtained by straightening and cooling the billet.

[0014] Preferably, the chemical composition of the ferritic martensitic duplex steel product is as follows: C: 0.07–0.095 wt%, Si ≤ 0.12 wt%, Mn: 1.7–2.1 wt%, P ≤ 0.02 wt%, S ≤ 0.012 wt%, Als: 0.50–0.75 wt%, Alt: 0.50–0.76 wt%, N ≤ 0.007 wt%, Cr: 0.2–0.35 wt%, Nb: 0.013–0.030 wt%. %, Ti: 0.010~0.025wt%, Mo: 0.15~0.25wt%, B: 0.001~0.0025wt%, Cu≤0.1wt%, Ni≤0.1wt%, V≤0.010wt%, Nb+Ti≤0.15wt%, Cr+Mo≤1.4wt%, Ni+Cr+Mo≤1.5wt%, Nb+Ti+Cr+Mo+B≤1.5wt%, balance being Fe and unavoidable impurities.

[0015] Preferably, the chemical composition of the ferritic martensitic duplex steel product is as follows: C: 0.07–0.09 wt%, Si ≤ 0.1 wt%, Mn: 1.95–2.05 wt%, P ≤ 0.015 wt%, S ≤ 0.003 wt%, Als: 0.55–0.70 wt%, Alt: 0.55–0.72 wt%, N ≤ 0.005 wt%, Cr: 0.2–0.3 wt%, Nb: 0.013–0.028 wt%. %, Ti: 0.010~0.025wt%, Mo: 0.18~0.23wt%, B: 0.001~0.0022wt%, Cu≤0.1wt%, Ni≤0.1wt%, V≤0.010wt%, Nb+Ti≤0.15wt%, Cr+Mo≤1.4wt%, Ni+Cr+Mo≤1.5wt%, Nb+Ti+Cr+Mo+B≤1.5wt%, balance being Fe and unavoidable impurities.

[0016] As a preferred option, in step (1), the slag blocking device in front of the converter tapping slag blocking device is a slag blocking plug, and the slag blocking device behind the converter tapping slag blocking device is a sliding plate.

[0017] As a preferred option, argon is blown into the bottom throughout the steel tapping process in step (1).

[0018] As a preferred option, quicklime, steel sand aluminum, medium carbon ferromanganese, low carbon ferrochrome and ferroniobium are added sequentially when the converter taps steel in step (1).

[0019] Preferably, the amount of quicklime added is 3-5 kg / t steel.

[0020] Preferably, in step (5), the water flow rate of the wide side of the crystallizer is 3980-4020 L / min, the water inlet pressure of the wide side of the crystallizer is 0.85-1.05 MPa, the water flow rate of the narrow side of the crystallizer is 480-520 L / min, the water inlet pressure of the narrow side of the crystallizer is 0.85-1.05 MPa, the vibration mode of the crystallizer is sinusoidal, and the liquid surface fluctuation of the crystallizer is -3-3 mm.

[0021] Preferably, the converter ladle is continuously baked before step (1) at a temperature of 600-1000℃.

[0022] Preferably, the chemical composition of the continuous casting protective slag in step (5) is as follows: CaO: 30.51-30.88 parts by weight, MgO: 1.38-1.50 parts by weight, SiO2: 33.12-38.95 parts by weight, Al2O3: 2.11-2.21 parts by weight, Fe2O3: 0.51-0.58 parts by weight, MnO: 0.11-0.15 parts by weight, C: 4.62-4.92 parts by weight, Na2O: 10.06-10.28 parts by weight, F: 10.3-10.92 parts by weight, K2O: 0.1-0.13 parts by weight; the basicity R of the continuous casting protective slag is 0.81-0.86, the melting point is 1031-1038℃, the bulk density is 0.7-0.75 g / ml, and the viscosity is 0.130-0.133 Pa·S.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) This invention controls the parameters in the converter smelting, LF furnace refining, RH refining, tundish treatment and continuous casting process, and sets specific continuous casting protective slags. When using these protective slags for continuous casting, the low reactivity of the steel slag is ensured, as well as the lubricity of the protective slag film between the copper wall of the crystallizer and the billet shell. This ensures the smooth operation of continuous casting. In other words, the smelting parameters, steel grades and continuous casting protective slags of this invention are a coordinated whole with a harmonious relationship. Replacing any one of the three will not produce the ferritic martensitic duplex steel that allows for smooth continuous casting of this invention.

[0025] (2) This invention controls the oxygen content at the end of the converter smelting process, the basicity of the final slag, the tapping temperature, and the deoxidation and alloying during the tapping process, so that the content of elements such as carbon, silicon, manganese, sulfur, and phosphorus in the molten steel can be adjusted at this stage, thereby obtaining molten steel that meets the requirements of subsequent processes.

[0026] (3) This invention achieves a reduction in oxygen and sulfur content and a higher purity in molten steel by rationally controlling the temperature and chemical composition of molten steel during the LF refining process; it also achieves a reduction in gas content, fewer inclusions, and higher purity in molten steel by rationally controlling the vacuum degree, temperature, and B content during the RH refining process, thus providing the continuous casting machine with molten steel that has good fluidity, high purity, and meets the casting temperature requirements, which is beneficial for multi-furnace continuous casting production; and it produces qualified ferritic martensitic duplex steel continuous casting billets by rationally controlling the liquid level height in the tundish, casting speed, protective slag consumption, liquid slag layer thickness, and secondary cooling water volume during the continuous casting process. Detailed Implementation

[0027] The process technology solution of the present invention will be further described below with reference to embodiments. Unless otherwise specified, each feature is merely one example of a series of equivalent or similar features. These embodiments are merely for the purpose of aiding understanding the present invention and should not be considered as specific limitations thereof.

[0028] Example 1

[0029] A method for preparing ferritic-martensitic dual-phase steel includes the following steps:

[0030] The converter ladle is first continuously baked at a temperature of 800℃.

[0031] (1) Converter smelting: The raw materials are prepared according to the composition of ferritic-martensitic duplex steel, including molten iron, scrap steel, auxiliary materials, and alloy materials. The temperature of the molten iron entering the furnace is 1220–1500℃, the S content of the molten iron entering the furnace is less than or equal to 0.015wt%, and the proportion of scrap steel added is 12wt%. The converter adopts top and bottom blowing, with oxygen blown from the top and inert gas blown from the bottom. The free oxygen content at the end of the converter smelting is 300–500ppm, C: 0.03wt%, P≤0.01wt%, S≤0.0 The slag composition is 3 wt%, Mo: 0.21 wt%, final slag basicity R: 3.5, and the final temperature of the converter smelting is 1665℃; the converter tapping temperature is 1610℃, the settling time before tapping is 60s, the tapping time is 6min, and the composition of the molten steel after tapping is C: 0.06 wt%, Si≤0.08 wt%, Mn: 1.9 wt%, Cr: 0.18~0.25 wt%. The auxiliary materials are quicklime, fluorite, and steel shot aluminum, and the alloying materials are medium-carbon ferromanganese, low-carbon ferrochrome, and ferroniobium. In this embodiment, the slag blocking front baffle of the converter tapping is a slag blocking plug, and the slag blocking rear baffle is a sliding plate. Bottom blowing oxygen is used throughout the converter tapping process. In this embodiment, quicklime, steel shot aluminum, medium-carbon ferromanganese, low-carbon ferrochrome, and ferroniobium are added sequentially during converter tapping. The amount of quicklime added is 3.5 kg / t steel.

[0032] (2) LF furnace refining: The molten steel after tapping from the converter in step (1) is refined in the LF furnace. The temperature of the molten steel entering the LF furnace is 1590℃, and the temperature of the molten steel exiting the LF furnace is 1618℃. The molten steel exiting the LF furnace contains C: 0.08wt%, Si≤0.1wt%, Mn: 2.0wt%, Cr: 0.25wt%, and the LF furnace refining cycle is 100min.

[0033] (3) RH refining: The molten steel obtained after refining in the LF furnace in step (2) is refined in the RH furnace. The temperature of the molten steel entering the RH furnace is 1608℃, the temperature of the molten steel leaving the RH furnace is 1573℃, the B content in the molten steel leaving the RH furnace is 0.0016wt%, the vacuum treatment time is 25min, and the pure degassing time is 9min.

[0034] (4) Tundish treatment: The molten steel obtained after RH refining in step (3) is injected into the tundish. The molten steel in the tundish is covered with a low-carbon covering agent. The temperature of the molten steel in the tundish is 1553℃. The argon flow rates of the stopper rod blowing, the top nozzle blowing, and the plate blowing are all 2-4 L / min. The molten steel in the tundish contains C: 0.08wt%, Si≤0.1wt%, Mn: 2.0wt%, P≤0.015wt%, S≤0.003wt%, Als: 0.62wt%, Alt: 0.55-0.72wt%, N≤0.005wt%, Cr: 0.25wt%, Nb: 0.021wt%, Ti: 0.018wt%, Mo: 0.21wt%, and B: 0.0016wt%.

[0035] (5) Continuous casting: Molten steel from the tundish is continuously poured into the crystallizer through a submerged entry nozzle (DIV) with a submersion depth of 160 mm. Then, continuous casting protective slag is continuously added to the crystallizer. The chemical composition of the continuous casting protective slag is: CaO: 30.6 parts by weight, MgO: 1.42 parts by weight, SiO2: 36.29 parts by weight, Al2O3: 2.21 parts by weight, Fe2O3: 0.55 parts by weight, MnO: 0.12 parts by weight. The composition of the continuous casting protective slag is as follows: C: 4.66 parts by weight, Na₂O: 10.18 parts by weight, F: 10.6 parts by weight, K₂O: 0.13 parts by weight. The basicity R of the slag is 0.85, the melting point is 1036℃, the bulk density is 0.73 g / ml, and the viscosity is 0.133 Pa·S. The amount of slag added is 0.51 kg / t steel, and the thickness of the slag layer is 15 mm. Ferritic-martensitic duplex steel billets are then obtained through billet straightening and cooling. In this embodiment, the water flow rate on the wide side of the crystallizer is 3980–4020 L / min, the water inlet pressure on the wide side of the crystallizer is 0.85–1.05 MPa, the water flow rate on the narrow side of the crystallizer is 480–520 L / min, the water inlet pressure on the narrow side of the crystallizer is 0.85–1.05 MPa, the vibration mode of the crystallizer is sinusoidal, and the liquid level fluctuation is -3–3 mm.

[0036] The chemical composition of the ferritic martensitic duplex steel product described in this embodiment is as follows: C: 0.08wt%, Si: 0.10wt%, Mn: 2wt%, P≤0.02wt%, S≤0.012wt%, Als: 0.63wt%, Alt: 0.65wt%, N: 0.002wt%, Cr: 0.25wt%, Nb: 0.021wt%, Ti: 0.018wt%, Mo: 0.21wt%, B: 0.0016wt%, Cu: 0.05wt%, Ni: 0.02wt%, V: 0.006wt%, Nb+Ti≤0.15wt%, Cr+Mo≤1.4wt%, Ni+Cr+Mo≤1.5wt%, Nb+Ti+Cr+Mo+B≤1.5wt%, with the balance being Fe and unavoidable impurities.

[0037] Comparative Example 1

[0038] The other settings in this comparative example are the same as in Example 1, except that the continuous casting protective slag used in the continuous casting process is CaO: 39.32 parts by weight, MgO: 0.92 parts by weight, SiO2: 32.26 parts by weight, Al2O3: 1.4 parts by weight, Fe2O3: 0.55 parts by weight, C: 4.22 parts by weight, F: 9.2 parts by weight, and Li2O: 3.7 parts by weight; the basicity R of the continuous casting protective slag is 1.24, the melting point is 1122℃, the bulk density is 0.85 g / ml, and the viscosity is 0.08 Pa·S. Using the same steps and parameters as in Example 1 to prepare the same steel grade as in Example 1, it was found that during the continuous casting process, an increase in the basicity of the protective slag led to an increase in the crystallization temperature, an increase in the critical cooling rate, and a shortening of the crystal incubation time, resulting in the development of slag rings during casting and affecting the smooth operation of continuous casting. It was also found that the produced billets exhibited surface quality defects such as transverse cracks, while Example 1 did not have any of these problems. This also verifies that the selection of the protective slag in this invention is closely coordinated with the various parameters of continuous casting and the steel grade.

[0039] Comparative Example 2

[0040] The other settings in this comparative example are the same as in Example 1, except that the steel grade is different. Specifically, the steel composition is: C: 0.1%–0.35%; Mo: 0.01%–0.5%; Si: 1.2%–3.5%; Mn: 1.5%–3.0%; Cr: 0.8%–3.5%; V: 0.05%–0.25%; Ni: 0.1%–1.5%; with the balance being Fe and unavoidable impurities in a ferritic-martensitic dual-phase steel. Using the same step parameters and continuous casting molded slag as in Example 1, it was found that during continuous casting, the lubrication and heat transfer conditions of the billet shell in the crystallizer were poor, resulting in surface quality defects such as longitudinal cracks and transverse depressions in the produced billets. Example 1, however, did not exhibit these problems at all. This further verifies that the combination of the steel grade, specific smelting parameters, and molded slag in this invention is the optimal configuration, and the coordination between the parameters is close.

[0041] The techniques known in the art involved in this invention have not been described in detail. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing ferritic-martensitic dual-phase steel, characterized in that, Includes the following steps: (1) Converter smelting: The raw materials are prepared according to the composition of ferritic martensitic duplex steel. The raw materials are molten iron, scrap steel, auxiliary materials and alloy materials. The temperature of the molten iron entering the furnace is 1220~1500℃. The S content of the molten iron entering the furnace is less than or equal to 0.015wt%. The proportion of scrap steel added is 0.1~20wt%. The converter adopts top and bottom blowing, with oxygen blown from the top and inert gas blown from the bottom. The free oxygen at the end of the converter smelting is 100~700ppm, P≤0.01wt%, S≤0.03wt%, Mo: 0.18~0.23wt%, and the final slag basicity is... R is 2.5~4.0, the final temperature of converter smelting is 1635~1685℃; the tapping temperature of converter is 1580~1630℃, the settling time before tapping is 30~90s, the tapping time is 4~7min, and the composition of molten steel after tapping is C: 0.05~0.07wt%, Si≤0.08wt%, Mn: 1.85~1.95wt%, Cr: 0.18~0.25wt%; the auxiliary materials are quicklime, limestone, fluorite and / or steel shot aluminum, and the alloy material is medium carbon ferromanganese, low carbon ferrochrome and / or ferroniobium; The chemical composition of ferritic martensitic duplex steel products is as follows: C: 0.07~0.095wt%, Si≤0.12wt%, Mn: 1.7~2.1wt%, P≤0.02wt%, S≤0.012wt%, Als: 0.50~0.75wt%, Alt: 0.50~0.76wt%, N≤0.007wt%, Cr: 0.2~0.35wt%, Nb: 0.013~0.030wt%, Ti: 0.010~0.025wt%, Mo: 0.15~0.25wt%, B: 0.001~0.0025wt%, Cu≤0.1wt%, Ni≤0.1wt%, V≤0.010wt%, Nb+Ti≤0.15wt%, Cr+Mo≤1.4wt%, Ni+Cr+Mo≤1.5wt%, Nb+Ti +Cr+Mo+B≤1.5wt%, balance is Fe and unavoidable impurities; (2) LF furnace refining: The molten steel after tapping from the converter in step (1) is refined in the LF furnace. The temperature of the molten steel entering the LF furnace is 1560~1610℃, and the temperature of the molten steel exiting the LF furnace is 1603~1628℃. In the molten steel exiting the LF furnace, C: 0.07~0.09wt%, Si≤0.1wt%, Mn: 1.95~2.05wt%, Cr: 0.2~0.3wt%. The LF furnace refining cycle is 30~150min. (3) RH refining: The molten steel obtained after refining in the LF furnace in step (2) is refined in the RH furnace. The temperature of the molten steel entering the RH furnace is 1593~1618℃, the temperature of the molten steel exiting the RH furnace is 1593~1618℃, the B content in the molten steel exiting the RH furnace is 0.001~0.0022wt%, the vacuum treatment time is 15~35min, and the pure degassing time is 6~15min. (4) Tundish treatment: The molten steel obtained after RH refining in step (3) is injected into the tundish. A low-carbon covering agent is used to cover the molten steel in the tundish. The temperature of the molten steel in the tundish is 1533~1558℃. The argon flow rates for argon blowing through the stopper rod, the top nozzle, and between the plates are all 2~4L / min. The C content in the molten steel in the tundish is 0.07~0.09wt%, Si ≤0.1wt%, and Mn 1.95~2.05wt%. , P≤0.015wt%, S≤0.003wt%, Als: 0.55~0.70wt%, Alt: 0.55~0.72wt%, N≤0.005wt%, Cr: 0.2~0 .3wt%, Nb: 0.013~0.028wt%, Ti: 0.010~0.025wt%, Mo: 0.18~0.23wt%, B: 0.001~0.0022wt%; (5) Continuous casting: Molten steel from the tundish is continuously injected into the crystallizer through a submerged entry nozzle. The submersion depth of the nozzle is 120~180mm. Then, continuous casting protective slag is continuously added to the crystallizer. The chemical composition of the continuous casting protective slag is: CaO: 28.80~33.12 parts by weight, MgO: 1.01~2.0 parts by weight, SiO2: 33.12~38.95 parts by weight, Al2O3: 2.01~2.62 parts by weight, Fe2O3: 0.5~0.6 parts by weight, MnO: The continuous casting protective slag has the following components: 0.10~0.16 parts by weight, C: 4.06~5.12 parts by weight, Na2O: 9.62~10.58 parts by weight, F: 9.55~11.2 parts by weight, K2O: 0.1~0.15 parts by weight; the basicity R of the continuous casting protective slag is 0.8~0.88, the melting point is 1030~1039℃, the bulk density is 0.7~0.76g / ml, and the viscosity is 0.130~0.135Pa•S. The amount of continuous casting protective slag added is 0.35~0.6kg / t steel, and the thickness of the continuous casting protective slag is 12~18mm. Then, the ferritic martensitic duplex steel billet is obtained by straightening and cooling the billet.

2. The method for preparing ferritic-martensitic dual-phase steel according to claim 1, characterized in that, The chemical composition of the ferritic martensitic duplex steel product is as follows: C: 0.07~0.09wt%, Si≤0.1wt%, Mn: 1.95~2.05wt%, P≤0.015wt%, S≤0.003wt%, Als: 0.55~0.70wt%, Alt: 0.55~0.72wt%, N≤0.005wt%, Cr: 0.2~0.3wt%, Nb: 0.013~0.028wt%, Ti: 0.010~0.025wt%, Mo: 0.18~0.23wt%, B: 0.001~0.0022wt%, Cu≤0.1wt%, Ni≤0.1wt%, V≤0.010wt%, Nb+Ti≤0.15wt%, Cr+Mo≤1.4wt%, Ni+Cr+Mo≤1.5wt%, Nb+Ti +Cr+Mo+B≤1.5wt%, with the balance being Fe and unavoidable impurities.

3. The method for preparing ferritic-martensitic dual-phase steel according to claim 1, characterized in that, In step (1), the slag blocking device in front of the converter tapping slag block is a slag blocking plug, and the slag blocking device behind it is a sliding plate.

4. The method for preparing ferritic-martensitic dual-phase steel according to claim 1, characterized in that, In step (1), argon is blown into the bottom throughout the steel tapping process in the converter.

5. The method for preparing ferritic martensitic dual-phase steel according to claim 1, characterized in that, In step (1), quicklime, steel shot aluminum, medium carbon ferromanganese, low carbon ferrochrome and ferroniobium are added sequentially when the converter taps steel.

6. The method for preparing ferritic martensitic dual-phase steel according to claim 5, characterized in that, The amount of quicklime added is 3~5 kg / t steel.

7. The method for preparing ferritic martensitic dual-phase steel according to claim 1, characterized in that, In step (5), the water flow rate of the wide side of the crystallizer is 3980~4020L / min, the water inlet pressure of the wide side of the crystallizer is 0.85~1.05MPa, the water flow rate of the narrow side of the crystallizer is 480~520L / min, the water inlet pressure of the narrow side of the crystallizer is 0.85~1.05MPa, the vibration mode of the crystallizer is sinusoidal, and the liquid surface fluctuation of the crystallizer is -3~3mm.

8. The method for preparing ferritic-martensitic dual-phase steel according to claim 1, characterized in that, Before step (1), the converter ladle is continuously baked at a temperature of 600~1000℃.

9. The method for preparing ferritic-martensitic dual-phase steel according to claim 1, characterized in that, The chemical composition of the continuous casting protective slag mentioned in step (5) is as follows: CaO: 30.51~30.88 parts by weight, MgO: 1.38~1.50 parts by weight, SiO2: 35.12~38.90 parts by weight, Al2O3: 2.11~2.21 parts by weight, Fe2O3: 0.51~0.58 parts by weight, MnO: 0.11~0.15 parts by weight, C: 4.62~4.92 parts by weight, Na2O: 10.06~10.28 parts by weight, F: 10.3~10.92 parts by weight, K2O: 0.1~0.13 parts by weight; the basicity R of the continuous casting protective slag is 0.81~0.86, the melting point is 1031~1038℃, the bulk density is 0.7~0.75g / ml, and the viscosity is 0.130~0.133Pa•S.