Method for producing hot-rolled ferrite bainite thin strip steel through double-roller cast rolling process

Abstract

The invention belongs to the field of short-process steel and iron manufacturing, and relates to a method for producing hot-rolled ferrite bainite thin strip steel through a double-roller cast rolling process. According to the method, the bainite and ferritic structure can be obtained only by adopting a C-Si-Mn system component system by combining a double-roller casting and rolling process with ultra-fast aerial fog cooling. The temperature of an aerial fog cooling outlet is 500 + / -20 DEG C, and the aerial fog cooling rate reaches 60 DEG C / s. Before coiling, part of austenite is subjected to phase change to separate out ferrite, after coiling, the remaining austenite is subjected to phase change to generate bainite, and finally a ferrite and bainite double-phase structure is formed. The yield strength of the thin strip steel obtained through the method is larger than or equal to 500 MPa. According to the method, the alloy cost is low, and meanwhile, the problem of flat rolling generally existing in thin-gauge hot-rolled high-strength steel is solved.

Description

Technical Field

[0001] This invention belongs to the field of short-process steel manufacturing and relates to a method for producing hot-rolled ferritic bainitic thin strip steel using a twin-roll casting process. More specifically, this invention relates to a twin-roll casting process combined with ultra-fast air-jet cooling, and the method of this invention can obtain hot-rolled thin strip steel with a ferritic-bainitic dual-phase structure using only C-Si-Mn composition. Background Technology

[0002] The traditional hot-rolling process for producing ferritic-bainitic dual-phase steel is lengthy, requiring reheating of the continuously cast slab and undergoing 1-2 stands of roughing, 6-7 stands of finishing, laminar flow cooling, and coiling. Producing thin-gauge steel of 1.5mm and below using traditional processes demands significant mill reductions, presenting a high degree of difficulty and approaching the limits of the production line's design. Furthermore, the addition of microalloying elements such as Cr, Mo, Nb, Ti, V, and B not only increases alloying costs but also improves hardenability, causing the bainite nose temperature to shift towards the higher temperature range. This further exacerbates the volume expansion and flattening problem during coiling, severely impacting yield and downstream customer performance. Additionally, offline annealing or isothermal treatment in the two-phase region is required after coiling to obtain the ferritic-bainitic dual-phase structure, necessitating large-area heat treatment equipment.

[0003] Patent document 202410351360.X discloses a ferritic bainitic steel and its preparation method. Its chemical composition includes: C: 0.18%–0.22%, Si: 1.3%–1.8%, Mn: 2.0%–2.5%, P≤0.01%, S≤0.01%, N≤0.004%, with the balance being Fe and unavoidable impurities. This patent document solves the technical problem of poor porosity and elongation in existing ferritic bainitic steels by reducing coarse MA islands and unstable blocky retained austenite, thus avoiding crack initiation sources. The chemical composition used in this patent document contains high levels of Si and Mn, with a yield strength of 700 MPa–850 MPa. The metallographic structure includes multiple components, and the reduction of coarse MA islands improves porosity and elongation.

[0004] Twin-roll casting involves directly solidifying molten steel into a strip of approximately 2mm thickness by passing it through two opposing rotating copper rolls equipped with cooling mechanisms. This eliminates the need for roughing and finishing rolling, reducing energy consumption by over 70%. Twin-roll casting can produce strips as thin as 0.7mm. Due to the ultra-rapid cooling advantage of twin-roll casting, it is possible to produce hot-rolled thin strip coils with a yield strength ≥500MPa and a ferritic and bainitic microstructure using low-alloy components.

[0005] Therefore, a method is still needed to produce hot-rolled ferritic + bainitic dual-phase thin strip steel using twin-roll casting and rolling processes, which can solve the flat coil problem and also have cost advantages. Summary of the Invention

[0006] Based on the aforementioned objectives, this invention aims to provide a method for producing hot-rolled ferritic bainitic thin strip steel using a twin-roll casting process. The method of this invention leverages the advantages of ultra-rapid cooling in the twin-roll casting process, achieving a dual-phase microstructure of ferrite and bainite using only ordinary C-Si-Mn, with a yield strength ≥500 MPa.

[0007] To achieve the above objectives, the present invention adopts the following technical solution.

[0008] According to a first aspect of the present invention, a method for producing hot-rolled ferritic bainitic thin strip steel using a twin-roll casting process is provided, the method comprising the following steps:

[0009] (1) Steel is obtained by smelting. The molten metal is processed by converter steelmaking, VD vacuum degassing, and LF refining. The molten steel is prepared according to the following chemical composition and mass fraction:

[0010] C: 0.20~0.30%; Si: 0.1~0.5%; Mn: 0.1~0.7%;

[0011] P: ≤0.02%; Al: ≤0.003%; S: ≤0.004%; N: ≤0.005%;

[0012] The balance is Fe and unavoidable impurities; no other alloying components are added to the molten steel.

[0013] (2) The molten steel obtained in step (1) is continuously cast into thin strips, and the molten steel flows into the tundish, transition ladle and molten pool in sequence to form a casting strip with a thickness of 1.8~2.5mm;

[0014] (3) The strip obtained in step (2) is conveyed through a hot box with a protective atmosphere and clamped by pinch rolls to the rolling mill for one hot rolling pass. The reduction rate is controlled at 30%~65%, the pre-rolling temperature is controlled at 1000±10℃, the thickness of the obtained strip is 0.7~1.2mm, and the post-rolling temperature is controlled at 800±10℃.

[0015] (4) The strip obtained in step (3) is subjected to aerosol cooling. The outlet temperature of the aerosol cooling is controlled at 500±20℃, the cooling rate is controlled at 60±10℃ / s, the aerosol cooling distance is controlled at 6~10m, and the strip speed is controlled at 1.5~1.8m / s.

[0016] (5) The thin strip steel cooled by aerosol is coiled by a coiler to form a thin strip steel coil with a ferrite + bainite dual-phase structure, wherein the coiling tension is controlled at 28~32MPa.

[0017] The method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting process of the present invention preferably includes, in step (1), the manganese content in the molten steel is controlled to be Mn: 0.3-0.5%.

[0018] In the method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting and rolling process of the present invention, preferably, in step (3), the pre-rolling temperature is controlled at 1000~1010℃.

[0019] In the method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting and rolling process of the present invention, preferably, in step (3), the post-rolling temperature is controlled at 800~810℃.

[0020] In the method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting process of the present invention, preferably, in step (4), the outlet temperature of the air mist cooling is controlled at 500~520°C.

[0021] In the method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting process of the present invention, preferably, in step (4), the cooling rate is controlled at 60±5℃ / s.

[0022] In the method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting and rolling process of the present invention, preferably, in step (4), the air mist cooling distance is controlled to be 8~10m.

[0023] In the method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting process of the present invention, preferably, in step (4), the strip speed is controlled to be 1.6~1.8m / s.

[0024] The method for producing hot-rolled ferritic bainitic thin strip steel according to the twin-roll casting process of the present invention preferably includes, in step (5), the thin strip steel coil with ferritic + bainitic dual-phase structure does not have the problem of flattening.

[0025] According to a second aspect of the invention, a hot-rolled ferritic bainitic thin strip steel is provided, which is produced using the aforementioned method.

[0026] According to the present invention, the hot-rolled ferritic bainitic thin strip steel preferably has a microstructure of bainite and ferrite, wherein the volume fraction of bainite is greater than the volume fraction of ferrite.

[0027] The hot-rolled ferritic bainitic thin strip steel according to the present invention preferably has a yield strength ≥ 500 MPa, a tensile strength ≥ 650 MPa, and an elongation ≥ 10%.

[0028] Beneficial technical effects:

[0029] Compared with the prior art, the technical concept and corresponding technical solution of the present invention can achieve at least the following beneficial technical effects:

[0030] The method of the present invention adopts a twin-roll casting process, combined with its rapid cooling characteristics, to achieve the effect of obtaining bainitic and ferrite structures using only a C-Si-Mn system composition.

[0031] The ferrite + bainitic thin strip steel obtained by the method of the present invention has a yield strength ≥500MPa and belongs to high-strength steel.

[0032] Compared with traditional hot rolling processes, the method of the present invention significantly reduces the alloy cost of thin-gauge high-strength steel and also solves the problem of flat coils of thin-gauge high-strength steel.

[0033] In summary, the method of this invention enables the production of ferritic + bainitic thin strip steel using a twin-roll casting process. This thin strip steel possesses excellent microstructure and mechanical properties, while also offering the advantage of lower alloy cost. Therefore, the method of this invention further expands the application market for high-strength steel and enhances the profitability and competitiveness of related products. Attached Figure Description

[0034] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below. Obviously, the drawings described below only relate to some embodiments of the present invention and are not intended to limit the present invention.

[0035] Figure 1 The CCT (Continuous Cooling Transformation) curve of ferrite + bainite thin strip steel according to an embodiment of the present invention.

[0036] Figure 2 The microstructure of ferrite + bainite thin strip steel according to an embodiment of the present invention.

[0037] Figure 3 This refers to the hot-rolled coil of ferritic + bainitic thin strip steel according to an embodiment of the present invention. Detailed Implementation

[0038] 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 clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0039] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0040] The main chemical composition of twin-roll cast thin strip steel is as follows: C: 0.25%, Si: 0.13%, Mn: 0.4%, with the balance being Fe and unavoidable impurity elements. No other alloying components are added to the steel.

[0041] After being smelted in a converter, degassed in a vacuum (VD) furnace, and heated in an LF furnace, the molten steel flows through an intermediate ladle and a transition ladle into a twin-roll casting pool with a protective non-oxidizing atmosphere, forming a casting strip with a thickness of 2.0 mm.

[0042] Afterward, the cast strip passes through a hot box with a nitrogen protective atmosphere, and then undergoes one hot rolling pass to a strip thickness of 1.0 mm, with a reduction rate of 50%. The temperature before rolling is 1000℃, and the temperature after rolling is 800℃.

[0043] The thin strip is cooled to 500℃ by aerosol cooling over a distance of 8m. The strip speed is 1.6m / s, the cooling rate is 60℃ / s, and the strip is wound into a thin steel strip with ferrite and bainite structure. The winding tension is 30MPa.

[0044] Figure 1 The CCT (Continuous Cooling Transformation) curve of ferrite + bainite thin strip steel according to an embodiment of the present invention is shown below. Figure 2 This is the microstructure of a ferrite + bainite thin strip steel according to an embodiment of the present invention. For example... Figure 1 As shown, before coiling, some austenite undergoes a phase transformation to precipitate ferrite. After coiling, all the remaining austenite transforms to bainite, ultimately forming a ferrite + bainite dual-phase microstructure, as shown. Figure 2 As shown.

[0045] Figure 3 This refers to the hot-rolled coil of ferritic + bainitic thin strip steel according to an embodiment of the present invention. See also... Figure 3 The hot-rolled coils obtained in this embodiment of the invention do not exhibit the problem of flattening. The corresponding technical principle is explained below. In the prior art, the bainitic phase transformation occurs after coiling, resulting in a very large volume change; under other high-alloy chemical compositions, the final composition is bainite, leading to severe flattening of the strip. In contrast, in the solution of this invention, on the one hand, the thin strip undergoes a partial phase transformation before coiling, thus reducing the volume change caused by the bainitic phase transformation after coiling, thereby avoiding the flattening problem; on the other hand, the solution of this invention increases the coiling tension, further preventing the flattening problem.

[0046] It is evident that the present invention optimizes the steel composition and cooling process. Before coiling, a portion of the austenite in the thin strip undergoes a phase transformation to generate proeutectoid ferrite, increasing the austenite phase transformation ratio of the strip on the laminar flow roller table. This reduces the expansion and plastic deformation caused by the bainite phase transformation after coiling, thus avoiding the problem of flattened coils.

[0047] The above description is only a specific embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing hot-rolled ferritic bainitic thin strip steel using a twin-roll casting and rolling process, characterized in that, The method includes the following steps: (1) Steel is obtained by smelting. The molten metal is processed by converter steelmaking, VD vacuum degassing, and LF refining. The molten steel is prepared according to the following chemical composition and mass fraction: C: 0.20~0.30%; Si: 0.1~0.5%; Mn: 0.3~0.6%; P: ≤0.02%; Al: ≤0.003%; S: ≤0.004%; N: ≤0.005%; The balance is Fe and unavoidable impurities; no other alloying components are added to the molten steel. (2) The molten steel obtained in step (1) is continuously cast into thin strips, and the molten steel flows into the tundish, transition ladle and molten pool in sequence to form a casting strip with a thickness of 1.8~2.5mm; (3) The strip obtained in step (2) is conveyed through a hot box with a protective atmosphere and clamped by pinch rolls to the rolling mill for one hot rolling pass. The reduction rate is controlled at 30%~65%, the pre-rolling temperature is controlled at 1000±10℃, the thickness of the obtained strip is 0.7~1.2mm, and the post-rolling temperature is controlled at 800±10℃. (4) The strip obtained in step (3) is subjected to aerosol cooling. The outlet temperature of the aerosol cooling is controlled at 500±20℃, the cooling rate is controlled at 60±10℃ / s, the aerosol cooling distance is controlled at 6~10m, and the strip speed is controlled at 1.5~1.8m / s. (5) The thin strip steel cooled by aerosol is coiled by a coiler to form a thin strip steel coil with a ferrite + bainite dual-phase structure, wherein the coiling tension is controlled at 28~32MPa.

2. The method for producing hot-rolled ferritic bainitic thin strip steel using the twin-roll casting and rolling process according to claim 1, characterized in that: In step (1), the manganese content in the molten steel is controlled to be Mn: 0.3-0.5%.

3. The method for producing hot-rolled ferritic bainitic thin strip steel using the twin-roll casting and rolling process according to claim 1, characterized in that: In step (3), the pre-rolling temperature is controlled at 1000~1010℃.

4. The method for producing hot-rolled ferritic bainitic thin strip steel using the twin-roll casting and rolling process according to claim 1, characterized in that: In step (3), the temperature after rolling is controlled at 800~810℃.

5. The method for producing hot-rolled ferritic bainitic thin strip steel using the twin-roll casting and rolling process according to claim 1, characterized in that: In step (4), the outlet temperature of the aerosol cooling is controlled at 500~520℃.

6. The method for producing hot-rolled ferritic bainitic thin strip steel using the twin-roll casting and rolling process according to claim 1, characterized in that: In step (4), the cooling rate is controlled at 60±5℃ / s.

7. The method for producing hot-rolled ferritic bainitic thin strip steel using the twin-roll casting and rolling process according to claim 1, characterized in that: In step (4), the aerosol cooling distance is controlled to be 8~10m.

8. The method for producing hot-rolled ferritic bainitic thin strip steel using the twin-roll casting and rolling process according to claim 1, characterized in that: In step (4), the belt speed of the rolling strip is controlled to be 1.6~1.8m / s.

9. The method for producing hot-rolled ferritic bainitic thin strip steel using a twin-roll casting and rolling process according to any one of claims 1 to 8, characterized in that: In step (5), the thin strip steel coil with ferrite + bainite dual-phase structure does not have the problem of flattening.

10. A hot-rolled ferritic bainitic thin strip steel, characterized in that, The hot-rolled ferritic bainitic thin strip steel is produced using the method described in any one of claims 1 to 9.

11. The hot-rolled ferritic bainitic thin strip steel according to claim 10, characterized in that: The microstructure of the hot-rolled ferritic bainitic strip steel consists of bainite and ferrite, with the volume fraction of bainite being greater than the volume fraction of ferrite.

12. The hot-rolled ferritic bainitic thin strip steel according to claim 10 or 11, characterized in that: The hot-rolled ferritic bainitic thin strip steel has a yield strength ≥500MPa, a tensile strength ≥650MPa, and an elongation ≥10%.

Images