A continuous casting process for improving the surface quality of a medium carbon steel casting blank

Abstract

This invention discloses a continuous casting process for improving the surface quality of medium carbon steel billets. Addressing the issue of coarse grains in medium carbon steel, the process first adjusts the steel composition to keep the carbon equivalent away from the region with the highest austenite temperature, thus shortening the austenite transformation time. Then, a higher cooling rate is used to promote austenite grain growth. Finally, ferrite and second-phase particles are used as austenite nucleation sites to facilitate intermittent austenite transformation, further refining the austenite grains in the billet. Through these three measures, the austenite grains on the surface of the medium carbon steel billet are refined, uniform grain boundary precipitation is achieved, and the surface plasticity of the billet is improved, thereby reducing the occurrence of surface cracks and ultimately improving the surface quality of the product.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking technology, specifically relating to a continuous casting process for improving the surface quality of medium carbon steel billets. Background Technology

[0002] In the continuous casting process of metallurgical production, the internal and external quality of the billet is a major concern for metallurgists. The internal quality of the billet is mainly reflected in its uniform composition, absence of porosity, shrinkage cavities, and cracks, while the external quality is characterized by the absence of slag inclusions and cracks. These defects can be inherited by subsequent rolled products, so it is necessary to produce products with excellent internal and external structures.

[0003] Crack formation in billets is the result of the combined effects of metallurgical and mechanical factors during continuous casting. Whether a billet develops cracks depends on the steel's microstructure, solidification metallurgical behavior, casting machine process parameters, and equipment operating conditions. Controlling the billet microstructure is one of the effective ways to improve the overall material properties. In continuous casting, methods such as controlling the supercooling of the molten steel, electromagnetic stirring, and adding nucleating agents such as aluminum, titanium, or rare earth elements are commonly used to control the as-cast microstructure. The surface microstructure of the billet can also be controlled by adjusting the composition, cooling rate, and the billet's heating process. Since the billet is still in the austenitic single-phase region when it exits the crystallizer, the cooling intensity has a crucial impact on the surface microstructure. Surface cracks commonly occur in the straightening section of the casting machine, mainly because the billet is straightened, generating tensile stress on its upper surface. The low-plasticity microstructure of the billet cracks under this tensile stress. Therefore, controlling the cooling rate of the billet in the casting machine before the straightening section can effectively control the billet microstructure and improve its mechanical properties.

[0004] Publication No. CN 110653352 A proposes adding a row of small-angle cooling water nozzles before the straightening section to eliminate surface cracks caused by ferrite in the billet. This provides a narrow area of ​​strong cooling to eliminate proeutectoid ferrite and control surface cracks. Publication No. CN107695313 A similarly employs rapid cooling quenching to eliminate ferrite and carbonitride precipitation on the billet surface, preventing embrittlement and improving the billet's plasticity to reduce surface cracks. Chinese Patent CN 105478704 B establishes a lifting device for the secondary cooling water spray frame, enabling dynamic control of the secondary cooling water area and automatic adjustment of the water volume. This method avoids corner cracks caused by excessively low temperatures or temperature changes due to direct spraying of secondary cooling water onto the corners of the billet. Chinese patents CN107414049B and CN105057626B, based on the corner temperature of the billet and the cooling and grain refinement temperature requirements of microalloyed carbonitride precipitation in the corner microstructure of continuously cast billets of different microalloyed steels, dynamically control the water volume on the wide / narrow face of the continuous casting machine foot rolls to meet the temperature evolution requirements of the corner microstructure of different types of microalloyed steel billets.

[0005] The patent analysis above reveals two main technical approaches to solving surface cracks in cast billets. One approach involves high-temperature operation to increase the surface temperature of the billet, preventing phase transformation and the precipitation of ferrite films and carbonitridium precipitates at grain boundaries. However, this method keeps the continuous casting machine at a constant high temperature, significantly impacting its accuracy and lifespan. The other approach uses a low-temperature method to avoid the third brittle zone of the steel. However, this method requires a large volume of cooling water, significantly impacting energy and the environment. Therefore, it is necessary to find a microstructure that improves the plasticity of the steel itself. Summary of the Invention

[0006] To address the aforementioned deficiencies in existing technologies, the present invention aims to provide a continuous casting process for improving the surface quality of medium carbon steel billets. Specifically addressing the issue of coarse grains in medium carbon steel, the process first adjusts the steel composition to ensure the carbon equivalent is far from the region of highest austenite temperature, thus shortening the austenite transformation time. Then, a higher cooling rate is employed to promote austenite grain growth. Finally, ferrite and second-phase particles are used as austenite nucleation sites for a warm-up austenite transformation, further refining the austenite grains in the billet. Through these three measures, the austenite grains on the surface of the medium carbon steel billet are refined, uniform grain boundary precipitation is achieved, and the surface plasticity of the billet is improved, thereby reducing the occurrence of surface cracks and ultimately improving the surface quality of the product.

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

[0008] This invention provides a continuous casting process for improving the surface quality of medium carbon steel billets, comprising the following steps:

[0009] S1, the carbon equivalent Cp is adjusted by adjusting the composition of medium carbon steel, so that the carbon equivalent Cp < 0.15% or Cp > 0.17%;

[0010] S2, during the secondary cooling process of the billet, when the surface temperature of the billet is lower than the austenite transformation temperature T γ At the same time, the cooling rate should be controlled at ≥5℃ / s to inhibit austenite grain growth;

[0011] S3. When the surface temperature of the billet after step S2 decreases to the ferrite phase region and the ferrite ratio is ≥50%, the cooling rate of the billet surface is reduced to perform warm-up austenitization.

[0012] Preferably, in step S1:

[0013] The formula for calculating the carbon equivalent Cp is:

[0014] Cp=[C]-0.123[Si]+0.04[Mn];

[0015] In the formula, Cp is the carbon equivalent, expressed in % %.

[0016] [C], [Si], and [Mn] represent the contents of C, Si, and Mn in medium carbon steel, respectively, in percentages (%).

[0017] The formula for calculating the carbon equivalent Cp is:

[0018] Cp=[C]-0.1[Si]+0.04[Mn]-0.04[Cr]+0.04[Ni]-0.1[Mo]-0.7[S];

[0019] In the formula, Cp is the carbon equivalent, expressed in % (%).

[0020] [C], [Si], [Mn], [Cr], [Ni], [Mo], and [S] represent the contents of C, Si, Mn, Cr, Ni, Mo, and S in medium carbon steel, respectively, in units of %.

[0021] Preferably, in step S2, during the secondary cooling process of the billet, the cooling rate is 5-20℃ / s.

[0022] Preferably, in step S3, the temperature of the ferrite phase region is 400–800°C.

[0023] Preferably, after step S3, the diameter of the austenite grains in the medium carbon steel billet is 30–1000 μm.

[0024] The continuous casting process for improving the surface quality of medium carbon steel billets provided by this invention has the following beneficial effects:

[0025] The continuous casting process of this invention for improving the surface quality of medium carbon steel billets addresses the issue of coarse grains in medium carbon steel by first adjusting the steel composition to keep the carbon equivalent away from the region with the highest austenite temperature, thus shortening the austenite transformation time. Then, a higher cooling rate is used to promote austenite grain growth. Finally, ferrite and second-phase particles are used as austenite nucleation sites for a warm-up austenite transformation, further refining the austenite grains in the billet. Through these three measures, the austenite grains on the surface of the medium carbon steel billet are refined, uniform grain boundary precipitation is achieved, and the surface plasticity of the billet is improved, thereby reducing the occurrence of surface cracks and improving the surface quality of the product. Detailed Implementation

[0026] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with embodiments.

[0027] The continuous casting process for improving the surface quality of medium carbon steel billets of the present invention includes the following steps:

[0028] S1, adjust the composition of the medium carbon steel according to the carbon equivalent Cp, so that the carbon equivalent Cp is far from 0.15-0.17%;

[0029] The specific process is as follows: The plasticity of the solidification structure of the surface layer of medium carbon steel billets is directly related to the grain size and grain boundary precipitation, especially for medium carbon steel billets with a carbon equivalent (Cp) of 0.08–0.25%. In the initial solidification stage, medium carbon steel billets are single-phase austenite, and grains easily grow. After exiting the crystallizer, the surface temperature of the billet is in the high-temperature region, at which point the surface structure is in the austenite region, and single austenite phase grains grow very easily. The austenitizing temperature T of medium carbon steel is directly related to its carbon equivalent (Cp). γ The austenite phase is relatively high, and it enters the austenite phase very early. Without the inhibition of the liquid and ferrite phases, the austenite grows rapidly, resulting in coarse austenite grains and a billet structure prone to cracking. Therefore, the composition of medium carbon steel is first adjusted according to the carbon equivalent Cp to keep the carbon equivalent C away from the 0.15-0.17% austenite transformation high-temperature region, thereby lowering the austenite transformation temperature T of the steel grade. γ This shortens the single-phase austenite transformation time and inhibits austenite grain growth. Specifically, it involves adjusting the steel composition to achieve a carbon equivalent Cp < 0.15% or Cp > 0.17%.

[0030] In the above process, the composition of medium carbon steel is adjusted by carbon equivalent Cp, which can be calculated by formula (1) or (2) below, or by other carbon equivalent Cp.

[0031] Cp=[C]-0.123[Si]+0.04[Mn]; (1)

[0032] Cp=[C]-0.1[Si]+0.04[Mn]-0.04[Cr]+0.04[Ni]-0.1[Mo]-0.7[S] (2)

[0033] In the formula, Cp is the carbon equivalent, expressed in % (%).

[0034] [C], [Si], [Mn], [Cr], [Ni], [Mo], and [S] represent the contents of C, Si, Mn, Cr, Ni, Mo, and S in medium carbon steel, respectively, in percentages (%).

[0035] According to the calculation formula of carbon equivalent Cp in (1) or (2), the carbon equivalent can be adjusted by adjusting the content of elements such as C, Si, and Mn in medium carbon steel.

[0036] S2, during the secondary cooling process of the billet, when the surface temperature of the billet is lower than the austenite transformation temperature T γ At the same time, the cooling rate should be controlled at ≥5℃ / s to inhibit austenite grain growth;

[0037] The specific process is as follows: During the secondary cooling process of the billet, the billet is cooled by spray water inside the casting machine, and the surface temperature of the billet continuously decreases. When the surface temperature of the billet reaches the austenite transformation temperature T... γ During the secondary cooling process, austenite transforms into ferrite in the billet. At a relatively low cooling rate, proeutectoid ferrite first nucleates at the original austenite grain boundaries and grows along them. With continued cooling, second-phase particles are formed at the grain boundaries. At this point, the original austenite grains are coarse, and a proeutectoid ferrite film and second-phase precipitation occur at the grain boundaries. This microstructure, subjected to tensile stress in the straightening zone, is highly susceptible to cracking under stress, which then gradually propagates. Therefore, during the secondary cooling process of the billet, when the surface temperature reaches the austenite transformation temperature T... γ When this process is carried out, the cooling rate of the billet surface needs to be increased because the production of austenite grains is related not only to the steel composition but also to the cooling rate of the billet surface. Therefore, the cooling rate of the billet surface should be increased and controlled at ≥5℃ / s. In a further preferred scheme, the cooling rate is controlled at 5~20℃ / s, thereby further inhibiting the growth of austenite grains. At this cooling rate, during the austenite phase transformation process of the billet structure, the precipitation of second-phase particles occurs inside the austenite grains, avoiding the precipitation of austenite grain boundaries, and also preventing the precipitation of proeutectoid ferrite at the grain boundaries.

[0038] Among them, the austenite phase transformation temperature T γ It can be calculated using thermodynamic software, such as the commonly used Thermo-Calc and JMatPro.

[0039] S3. When the surface temperature of the billet after step S2 decreases to the ferrite phase region and the ferrite ratio is ≥50%, the cooling rate of the billet surface is reduced to perform warm-up austenitization.

[0040] The specific process is as follows: After step S2, as the billet cools, when the billet surface temperature drops to the ferrite phase region and the proportion of ferrite is ≥50% (i.e., ferrite reaches 50-100%), the cooling rate of the billet surface is reduced, thereby causing the billet surface temperature to rise again. Ferrite phase and second phase particles are used as austenite nucleation points for reheat austenitization, and the austenite grains after reheating are further refined. During the above process, the temperature of the ferrite phase region is 400-800℃, and the cooling rate of the billet surface is <5℃ / s.

[0041] Through the above three steps, the austenitic grains of medium carbon steel can be significantly refined, the surface plasticity of the billet can be improved, and the occurrence of surface cracks in the billet can be reduced. The fine austenite without grain boundaries has high plasticity, and the billet will not develop surface cracks due to stress concentration at the grain boundaries when the billet is over-straightened.

[0042] After processing with this invention, the length of austenite grains in medium carbon steel billets can be controlled between 30 and 1000 μm.

[0043] The continuous casting process for improving the surface quality of medium carbon steel billets according to the present invention will be further described below with specific examples.

[0044] Example 1

[0045] This embodiment uses medium carbon steel with C: 0.1383%, Si: 0.228%, and Mn: 1.519% as an example.

[0046] (1) The carbon equivalent Cp is calculated to be 0.171% based on Cp = [C] - 0.123[Si] + 0.04[Mn]. The austenitic transformation temperature T of this steel grade is calculated using Thermal-calc software. γ At a temperature of 1485℃, the austenite grain size of the cast billet obtained using conventional methods was 1693μm.

[0047] (2) If the C content is reduced to 0.1%, the carbon equivalent Cp is 0.133%, and its austenitic phase transformation T γ At a temperature of 1470℃, the austenite grain size was 1557µm, a reduction of 136µm.

[0048] (3) In order to further suppress the growth of austenite grains, a high cooling rate of 7℃ / S is used for cooling during the secondary cooling of the billet. The austenite grains will be further reduced to 1184um, the grain size will be reduced by 509um, and the ferrite film and the second phase particles will not precipitate at the austenite grain boundaries when the temperature is greater than 5℃ / S.

[0049] (4) In order to further reduce the grain size of the billet surface, when the billet surface temperature is reduced to 690°C in the ferrite phase region, a ferrite phase with a proportion greater than 70% will be obtained. At this time, the cooling rate of the billet surface is reduced to 0.8°C / s, and the ferrite phase and the second phase particles in the phase are used as nucleation particles to carry out the austenitizing phase transformation at room temperature, and austenite grains with a size of 650 μm are obtained. This process is completed before the straightening point.

[0050] This process yielded a fine-grained austenitic structure without grain boundary precipitation, which is very beneficial for reducing surface cracks in medium carbon steel billets. Furthermore, in this embodiment, no cracks appeared in the billet during the subsequent straightening process.

[0051] Example 2

[0052] This embodiment uses medium carbon steel with the following composition: C: 0.18%, Si: 0.85%, Mn: 2.2%, S: 0.0004%, Ni: 0.012%, Cr: 0.034%, Mo: 0.018 as an example.

[0053] (1) The carbon equivalent Cp is calculated to be 0.18% based on Cp=[C]-0.1[Si]+0.04[Mn]-0.04[Cr]+0.04[Ni]-0.1[Mo]-0.7[S]. The austenitic transformation temperature T of this steel grade is obtained by calculation using JMatPro. γ At a temperature of 1450℃, the austenite grain size of the cast billet obtained using conventional methods was 1320μm.

[0054] (2) If the Mn content is increased to 2.7%, the carbon equivalent Cp is 0.2%, and its austenitic phase transformation T γ At a temperature of 1420℃, the austenite grain size was 1150µm, a reduction of 170µm.

[0055] (3) In order to further suppress the growth of austenite grains, a high cooling rate of 5.5℃ / S is used for cooling during the secondary cooling of the billet. The austenite grains will be further reduced to 990um, the grain size is reduced by 330um, and the ferrite film and the second phase particles will not precipitate at the austenite grain boundaries when the temperature is greater than 5℃ / S.

[0056] (4) In order to further reduce the grain size of the surface layer of the billet, when the surface temperature of the billet is reduced to 674°C in the ferrite phase region, a ferrite phase with a proportion greater than 65% will be obtained. At this time, the cooling rate of the billet surface is reduced to 0.78°C / s, and the ferrite phase and the second phase particles in the phase are used as nucleation particles to carry out the austenitizing phase transformation at room temperature, and austenite grains with a size of 729 μm are obtained. This process is completed before the straightening point.

[0057] This process yielded a fine-grained austenitic structure without grain boundary precipitation, which is very beneficial for reducing surface cracks in medium carbon steel billets. Furthermore, in this embodiment, no cracks appeared in the billet during the subsequent straightening process.

[0058] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A continuous casting process for improving the surface quality of a medium carbon steel strand, characterized in that, The method comprises the following steps: S1, adjusting carbon equivalent Cp by adjusting the composition of medium carbon steel, so that the carbon equivalent Cp < 0.15% or Cp > 0.17%; S2, in the process of secondary cooling of the casting blank, when the surface layer temperature of the casting blank is lower than the austenite phase transition temperature T γ , the cooling speed is controlled to be ≥5℃ / s to inhibit the growth of austenite grains; S3, when the surface temperature of the casting blank after step S2 is reduced to the ferrite phase region, and the ferrite proportion is ≥ 50%, the cooling speed of the surface of the casting blank is reduced for reheating austenitization.

2. The continuous casting process for improving the surface quality of a medium carbon steel cast slab according to claim 1, characterized in that, In the step S1: The calculation formula of the carbon equivalent Cp is: Cp = [C] - 0.123 [Si] + 0.04 [Mn]; In the formula, Cp is the carbon equivalent, unit: %; [C], [Si], [Mn] are respectively the content of C, Si, Mn in the medium carbon steel, unit: %; or The calculation formula of the carbon equivalent Cp is: Cp = [C] - 0.1 [Si] + 0.04 [Mn] - 0.04 [Cr] + 0.04 [Ni] - 0.1 [Mo] - 0.7 [S]; In the formula, Cp is the carbon equivalent, unit: %; [C], [Si], [Mn], [Cr], [Ni], [Mo], [S] are respectively the content of C, Si, Mn, Cr, Ni, Mo, S in the medium carbon steel, unit: %.

3. The continuous casting process for improving the surface quality of a medium carbon steel cast slab as claimed in claim 1, wherein, In the step S2, the cooling speed during the secondary cooling of the casting blank is 5-20 ℃ / s.

4. The continuous casting process for improving the surface quality of a medium carbon steel cast slab as claimed in claim 1, wherein, In the step S3, the ferrite phase region temperature is 400-800 ℃.

5. The continuous casting process for improving the surface quality of a medium carbon steel cast slab according to any one of claims 1 to 4, characterized in that, After step S3, the diameter of the austenite grain of the medium carbon steel casting blank is 30-1000 μm.