A nano-precipitated high-strength steel with excellent forming properties for CSP production line and its production method

Abstract

This invention discloses a method for producing high-strength nano-precipitated steel with excellent formability on a CSP production line. The method utilizes a thin slab continuous casting and rolling process, with the following specific steps: 1) After converter smelting, the steel enters an LF furnace for single refining, during which alloying is performed; 2) The molten steel is continuously cast and rolled into thin slabs, with the slab heated in a soaking furnace; then rolled in a finishing mill; 3) Laminar flow cooling; 4) The steel coils are coiled, and for coils with a thickness ≤2.5mm, a head-and-tail U-cooling process is used; 5) After the steel coils come off the production line, they are stacked in a coil warehouse for slow cooling. The physical properties of the nano-precipitated high-strength steel produced by this invention are as follows: yield strength ≥700MPa, tensile strength ≥785MPa, elongation ≥20%, expansion rate ≥60%, and thickness 1.2~4.5mm. It not only possesses high strength but also excellent formability.

Description

Technical Field

[0001] This invention relates to the field of microalloyed steel manufacturing, specifically to a method for producing nano-precipitated high-strength steel with a thickness of 1.2–4.5 mm and possessing high strength, excellent formability, and weldability using a CSP production line. Background Technology

[0002] Achieving structural lightweighting by increasing steel plate strength, thereby reducing energy consumption, alleviating labor intensity, and other benefits, has become an inevitable trend in the development of modern steel materials. For general structural steel, as material strength increases, its formability tends to decrease. Formability characterizes a material's deformation capacity and directly affects the shaping of user-defined parts. Insufficient formability can easily lead to cracking and other problems, rendering the material unusable. Resolving this contradiction between strength and formability has become a key challenge in the research and development of steel materials.

[0003] Research and manufacturing of high-strength steel with a yield strength of 700MPa have been conducted by domestic and international institutions and enterprises. CN1639371A discloses a weather-resistant high-strength steel plate with excellent bending workability and its manufacturing method; CN1757783A discloses a manufacturing method for 700MPa grade F / B high-strength strip steel; and CN101153371A discloses a high-strength cold-formed hot-rolled steel plate and its production method. All of these materials achieve a yield strength of 700MPa, but their compositions contain a significant amount of expensive alloys such as Cu, Ni, V, Mo, and Nb, resulting in high overall production costs. Developing a low-alloy high-strength steel material that simultaneously possesses high strength, high formability, and is also economical and practical presents considerable challenges. Summary of the Invention

[0004] The main technical problem solved by this invention is to develop a thin-gauge nano-precipitated hot-rolled high-strength steel based on the equipment and technical characteristics of CSP production lines. On the one hand, it ensures high strength (yield strength ≥700MPa, tensile strength ≥785MPa) and on the other hand, it has high formability (elongation after fracture ≥20%, hole expansion rate ≥60%). On this basis, it achieves low-cost design and production.

[0005] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0006] A method for producing high-strength nano-precipitated steel with excellent forming properties on a CSP production line utilizes a thin slab continuous casting and rolling process. The process flow includes: converter smelting, LF ladle furnace single refining, thin slab continuous casting and rolling, laminar flow cooling, coiling, and post-rolling stacking slow cooling. The specific steps are as follows:

[0007] 1) After being smelted in a converter, the steel enters the LF furnace for single refining. The chemical composition of the molten steel after alloying treatment during the refining process is as follows (weight percentage): C: 0.03~0.06%, Si: ≤0.15%, Mn: 1.3~1.5%, P: ≤0.012%, S: ≤0.003%, Ti: 0.11~0.15%, Als: 0.02~0.06%, Mo: 0.05~0.12%, As≤0.012%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0008] 2) Thin slab continuous casting and rolling production of molten steel: The thickness of the cast steel billet is 65-80mm; the billet is heated in a soaking furnace, and the heating temperature is controlled at 1200-1230℃; it is rolled in a 7-stand finishing mill, with a finishing rolling start temperature ≥1080℃, a reduction rate of ≥50% for the first three passes, a reduction rate of ≥38% for the F4 pass, a reduction rate of ≥33% for the F5 pass, and a final rolling temperature controlled at 890-930℃;

[0009] 3) Laminar flow cooling: The water cooling rate after exiting the finishing mill is 70-120℃ / s;

[0010] 4) Coiling: The coiling temperature is controlled at 610~650℃ (the target coiling temperature is preferably around 630℃). At the same time, for steel coils with a thickness of ≤2.5mm, the head and tail U-cooling process is adopted.

[0011] 5) After the steel coils come off the production line, they are stacked together in the steel coil warehouse for slow cooling.

[0012] According to the above scheme, in step 2), the preferred chemical composition by weight percentage of the molten steel is: C: 0.04–0.06%, Si: ≤0.12%, Mn: 1.45–1.5%, P: ≤0.010%, S: ≤0.003%, Ti: 0.11–0.15%, Als: 0.03–0.035%, Mo: 0.075–0.09%, As ≤0.005%, N ≤0.005%, with the balance being Fe and unavoidable impurities.

[0013] According to the above scheme, in step 2), the carbon equivalent of the chemical composition of the molten steel is 0.3-0.35%.

[0014] According to the above scheme, in step 2), the rolling thickness is 1.2 to 4.5 mm; and the rolling speed is controlled at 3.0 to 12.0 m / s according to the finished product thickness in order to ensure the stability of the through-coiling process and the overall performance stability.

[0015] According to the above scheme, in step 4), the U-cooling process specifically involves increasing the coiling temperature setting within a 10-meter distance between the head and tail of the strip by 20°C based on the target coiling temperature.

[0016] The physical properties of the nano-precipitated high-strength steel produced by the above method are as follows: yield strength ≥700MPa, tensile strength ≥785MPa, elongation ≥20%, porosity ≥60%, and thickness 1.2~4.5mm. It not only has high strength but also excellent formability.

[0017] The improvements and mechanisms of the main components and processes in this invention are as follows:

[0018] Carbon (C): Carbon is the most economical strengthening element. Excessive carbon content increases the driving force for carbide precipitation, which is detrimental to the material's formability. Furthermore, when the carbon content approaches 0.07%, it can cause peritectic reactions in molten steel during casting, increasing the risk of leaks during continuous casting. Conversely, insufficient carbon content reduces the strengthening effect, potentially leading to unsatisfactory performance. Considering factors such as strength and formability, the carbon content is limited to the range of 0.04%–0.06%.

[0019] Silicon (Si): Silicon can purify ferrite and prevent the formation of coarse carbides during cooling. However, silicon also easily forms Fe2SiO4 and forms eutectoid products with FeO on the surface of the billet, which solidify into an anchor-like structure, making FeO difficult to remove and affecting the final surface quality. Therefore, the silicon content is limited to ≤0.12%.

[0020] Manganese (Mn): Manganese is the most effective element for improving strength and toughness, effectively delaying the austenite transformation and refining the grain size. If its content is less than 1.30%, the material strength requirements cannot be met; however, adding excessive manganese can easily lead to peritectic reaction in molten steel, causing quality problems such as continuous casting cracks. At the same time, the manganese content has a significant impact on welding performance. Therefore, the manganese content is limited to the range of 1.30% to 1.50%.

[0021] Molybdenum (Mo): Molybdenum improves the hardenability of materials, and combined with the effects of manganese, it significantly delays the transformation of pearlite and bainite, refining the microstructure and enhancing the strength and toughness of the material. Simultaneously, molybdenum is a carbide-forming element with a strong affinity for carbon, acting as a solute dragging element to reduce the diffusion rate of carbon atoms and prevent the further growth of nanoscale carbide precipitates, significantly improving the nanoprecipitation strengthening effect. However, molybdenum alloys are expensive, and excessive addition can lead to a significant increase in production costs. Considering both strength and cost, the molybdenum content is controlled at 0.05–0.12% in this invention.

[0022] Titanium (Ti): Titanium has a prominent precipitation strengthening effect and is an economical and effective element to ensure the strength of steel. If the amount of Ti alloying element added is too high, it will easily produce too many TiN hard particles, which will affect the forming performance of the material. If the Ti content is too low, it will not meet the strength requirements of the material. Under the premise of certain carbon and manganese content, the present invention designs the titanium content range to be 0.11-0.15%.

[0023] Phosphorus (P): Phosphorus is prone to center segregation, which affects molding performance. In this invention, the phosphorus content is controlled at ≤0.010%.

[0024] Aluminum (Als): As an effective deoxidizer in the smelting process, it also has a certain grain-refining effect and improves the strength of steel, but it is also prone to forming Al2O3 inclusions. In this invention, the aluminum content is controlled at 0.02-0.06%.

[0025] Arsenic (As): It can significantly reduce the toughness and plasticity of materials. In this invention, the arsenic content is strictly controlled to ≤0.012%.

[0026] Sulfur (S) and nitrogen (N): They readily combine with Ti in steel, affecting the strengthening effect of Ti and greatly impacting the plasticity of the steel. In this invention, sulfur is controlled at ≤0.003% and nitrogen at ≤0.006%.

[0027] The control of the main processes of this invention is described below:

[0028] The billet thickness in this invention is set at 65-80mm, mainly taking into account the mill load of the production line and different finished product thicknesses;

[0029] Heating temperature: On the one hand, the higher the heating temperature, the better the remelting effect of carbides, etc. However, the characteristics of the heating furnace equipment in the CSP production line determine that the heating temperature has an upper limit of 1230℃. At the same time, excessive heating temperature will lead to the growth of austenite grain size. Therefore, this invention controls the heating temperature at 1200~1230℃.

[0030] The reduction rate of each pass is allocated as follows: the reduction rate of the first three passes is ≥50%, the reduction rate of the F4 pass is ≥38%, and the reduction rate of the F5 pass is ≥33%. This is mainly based on the composition characteristics and the mill load, and adopts the largest possible cumulative deformation to break the as-cast dendritic structure, thereby achieving the effect of homogenization and refinement of austenite grains.

[0031] The laminar flow cooling rate is set at 70-120℃ / s, mainly because the product of this invention can undergo as much ferrite transformation as possible within this cooling rate range, while refining the grain size.

[0032] The coiling temperature is set between 610 and 650℃, and the U-shaped cooling process is adopted mainly to ensure uniformity of performance within the coil and to maintain the optimal precipitation temperature range. Within this temperature range, the precipitation strengthening effect of Ti can be maximized. At the same time, the U-shaped cooling process at the beginning and end of the strip can compensate for the difference in performance between the beginning and end of the strip and the middle section caused by insufficient precipitation due to excessively rapid cooling rates of the inner and outer rings, which affects the processing stability of the material at the user end.

[0033] Depending on the thickness of the finished product, the rolling speed is controlled at a constant speed of 3.0 to 12.0 m / s to ensure the stability of the continuous coiling process and the overall performance.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] Compared with existing technologies, the product of this invention not only possesses high strength (yield strength ≥700MPa, tensile strength ≥785MPa) but also high formability (elongation after fracture ≥20%, porosity ≥60%). This invention controls the C content to 0.04–0.06%, reducing carbide precipitation and ensuring good elongation and porosity. Simultaneously, by adding an appropriate amount of Mo to the C+Mn+Ti composition, the size of the precipitated phase is controlled within 10nm, significantly improving the precipitation strengthening effect with minimal impact on the material's plasticity and toughness. Furthermore, the addition of Mo alloying element effectively reduces softening in the weld heat-affected zone, enhancing the material's application performance. Therefore, the nano-precipitated high-strength steel product of this invention can meet various forming processes such as stamping, rolling, and bending, and also possesses excellent welding performance. It can be used in fields with high formability and service requirements, such as automotive parts, and also in the field of high-strength structural components. Attached Figure Description

[0036] Figure 1 and Figure 2 The images show the metallographic structure and nanoprecipitated phase morphology of the present invention, respectively; wherein the metallographic structure type is ferrite or ferrite + a small amount of bainite. Detailed Implementation

[0037] To better understand the present invention, the following embodiments further illustrate the content of the present invention. However, it should be understood that the specific embodiments described herein are merely for explaining the present invention and do not limit the present invention.

[0038] Examples 1-5

[0039] A method for producing high-strength nano-precipitated steel with excellent forming properties on a CSP production line utilizes a thin slab continuous casting and rolling process. The process flow includes: converter smelting, LF ladle furnace single refining, thin slab continuous casting and rolling, laminar flow cooling, coiling, and post-rolling stacking slow cooling. The specific steps are as follows:

[0040] 1) After being smelted in a converter, the steel enters the LF furnace for single refining. The chemical composition of the molten steel after alloying treatment during the refining process is as follows (weight percentage): C: 0.03~0.06%, Si: ≤0.15%, Mn: 1.3~1.5%, P: ≤0.012%, S: ≤0.003%, Ti: 0.11~0.15%, Als: 0.02~0.06%, Mo: 0.05~0.12%, As≤0.012%, N≤0.006%, with the balance being Fe and unavoidable impurities.

[0041] 2) Thin slab continuous casting and rolling production of molten steel: The thickness of the cast steel billet is 65-80mm; the billet is heated in a soaking furnace at a temperature controlled at 1200-1230℃; it is rolled in a 7-stand finishing mill with a finishing rolling temperature ≥1080℃, a reduction rate of ≥50% for the first three passes, a reduction rate of ≥38% for the F4 pass, a reduction rate of ≥33% for the F5 pass, a final rolling temperature controlled at 890-930℃, a rolling speed controlled at 3.0-12.0m / s, and a rolling thickness of 1.2-4.5mm;

[0042] 3) Laminar flow cooling: The water cooling rate after exiting the finishing mill is 70-120℃ / s;

[0043] 4) Coiling: The coiling temperature is controlled at 610~650℃ (the target coiling temperature is preferably 630℃). At the same time, for steel coils with a thickness of ≤2.5mm, the head and tail U-cooling process is adopted. The U-cooling process is that the coiling temperature set within 10 meters of the head and tail of the strip is increased by 20℃ based on the target coiling temperature.

[0044] 5) After the steel coils come off the production line, they are stacked together in the steel coil warehouse for slow cooling.

[0045] Table 1 is a list of chemical components for each embodiment of the present invention; Table 2 is a list of main process parameters and corresponding performance for each embodiment of the present invention.

[0046] Table 1. List of chemical components in various embodiments of the present invention.

[0047]

[0048] Table 2. List of main process parameters and corresponding performance of various embodiments of the present invention.

[0049]

[0050] As shown in Table 2, the nano-precipitated high-strength steel products obtained in Examples 1-5 meet the requirements of yield strength ≥700MPa, tensile strength ≥785MPa, elongation ≥20%, and porosity ≥60%. The metallographic structure of the products of this invention is as follows: Figure 1As shown, the microstructure is ferrite or ferrite with a small amount of bainite. This microstructure has good plasticity, allowing the product to meet the requirements of subsequent processing. The precipitates of the product are as follows: Figure 2 As shown, the precipitated phase is uniformly distributed and its size is within 10 nm, which can greatly improve the precipitation strengthening effect without significantly affecting plasticity and toughness.

[0051] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for producing high-strength nano-precipitated steel with excellent forming performance on a CSP production line, characterized in that, Includes the following steps: 1) After being smelted in a converter, the steel enters the LF furnace for single refining. The chemical composition of the molten steel after alloying treatment during the refining process is as follows (weight percentage): C: 0.03~0.06%, Si: ≤0.15%, Mn: 1.3~1.5%, P: ≤0.012%, S: ≤0.003%, Ti: 0.11~0.15%, Als: 0.02~0.06%, Mo: 0.05~0.09%, As≤0.012%, N≤0.006%, with the balance being Fe and unavoidable impurities. 2) Thin slab continuous casting and rolling production of molten steel: The thickness of the cast steel billet is 65-80mm; the billet is heated in a soaking furnace, and the heating temperature is controlled at 1200-1230℃; it is rolled in a 7-stand finishing mill, with a finishing rolling start temperature ≥1080℃, a reduction rate of ≥50% for the first three passes, a reduction rate of ≥38% for the F4 pass, a reduction rate of ≥33% for the F5 pass, and a final rolling temperature controlled at 890-930℃; 3) Laminar flow cooling: The water cooling rate after exiting the finishing mill is 70-120℃ / s; 4) Coiling: The coiling temperature is controlled at 610~650℃. For steel coils with a thickness of ≤2.5mm, the head and tail U-cooling process is adopted. 5) After the steel coils come off the production line, they are stacked in the steel coil warehouse for slow cooling. The properties of the resulting nano-precipitated high-strength steel are as follows: yield strength ≥700MPa, tensile strength ≥785MPa, elongation ≥20%, hole expansion rate ≥60%, and thickness 1.2~4.5mm.

2. The method for producing high-strength nano-precipitated steel with excellent forming performance on a CSP production line according to claim 1, characterized in that, In step 2), the chemical composition of the molten steel by weight percentage is as follows: C: 0.04-0.06%, Si: ≤0.12%, Mn: 1.45-1.5%, P: ≤0.010%, S: ≤0.003%, Ti: 0.11-0.15%, Als: 0.03-0.035%, Mo: 0.075-0.09%, As ≤0.005%, N ≤0.005%, with the balance being Fe and unavoidable impurities.

3. The method for producing high-strength nano-precipitated steel with excellent forming performance on a CSP production line according to claim 1, characterized in that, In step 2), the carbon equivalent of the molten steel is 0.3-0.35%.

4. The method for producing high-strength nano-precipitated steel with excellent forming performance on a CSP production line according to claim 1, characterized in that, In step 2), the rolling thickness is 1.2 to 4.5 mm; and the rolling speed is controlled at 3.0 to 12.0 m / s according to the thickness of the finished product.

5. The method for producing high-strength nano-precipitated steel with excellent forming performance on a CSP production line according to claim 1, characterized in that, In step 4), the U-cooling process specifically involves increasing the coiling temperature setting within a 10-meter distance between the head and tail of the strip by 20°C based on the target coiling temperature.

6. The method for producing high-strength nano-precipitated steel with excellent forming performance on a CSP production line according to claim 1, characterized in that, Chemical composition by weight percentage: C: 0.04~0.06%, Si: ≤0.12%, Mn: 1.45~1.5%, P: ≤0.010%, S: ≤0.003%, Ti: 0.11~0.15%, Als: 0.03~0.035%, Mo: 0.075~0.09%, As≤0.005%, N≤0.005%, balance being Fe and unavoidable impurities.

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