CSP production line 900MPa-grade hot-rolled high-strength steel and manufacturing method thereof

Abstract

The invention discloses 900MPa-grade hot-rolled high-strength structural steel for a CSP production line. The 900MPa-grade hot-rolled high-strength structural steel comprises the following chemical components in percentage by weight: 0.050-0.069% of C, less than or equal to 0.20% of Si, 1.50-1.70% of Mn, less than or equal to 0.012% of P, less than or equal to 0.003% of S, 0.15-0.20% of Ti, 0.18-0.55% of Cr, 0.02-0.08% of V, 0.03-0.06% of Alt, 0.001-0.004% of Ca, less than or equal to 0.004% of N and the balance of Fe. And 18 < = (Ti + V) * 104 < = 25. The low-cost alloy component design is adopted, the optimized steelmaking and hot rolling procedures are combined, the strength and plasticity of the hot-rolled structural steel can be effectively improved, and low-cost production of a thin steel plate strip with the tensile strength larger than or equal to 900 MPa is achieved; and after-treatment processes such as cold rolling or heat treatment for regulating performance are not needed, so that the production process can be greatly shortened, procedures are saved, and the method is suitable for popularization and application.

Description

Technical Field

[0001] This invention belongs to the field of microalloyed steel manufacturing technology, specifically relating to a 900MPa grade hot-rolled high-strength steel for CSP production lines and its manufacturing method. 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 steel materials. For general structural steel, as material strength increases, its plasticity tends to decrease. Plasticity characterizes a material's deformation capacity; excessively low plasticity directly affects the forming of user-defined parts, easily leading to cracking and other problems, thus impacting the practical application of steel. How to resolve this contradiction between strength and plasticity through cost-effective design has become a challenge in the research and development of steel materials.

[0003] Existing technology discloses a hot-rolled TRIP steel with a tensile strength of 900 MPa and its preparation method. This method involves adding a significant amount of alloying elements (chemical composition by mass percentage: C: 0.10–0.30%, Si ≤ 0.02–0.06%, Mn: 4–8%, P ≤ 0.01%, S ≤ 0.008%, Al: 1.0–2.0%, Nb ≤ 0.04%, V: 0.06–0.10%, with the remainder being Fe and unavoidable impurities). This method requires hot rolling followed by annealing for production, resulting in a long and costly process. Furthermore, the high content of C and Mn alloys drastically reduces the weldability of the material and does not address improvements in low-temperature impact toughness or other related properties. Existing technology also discloses an 850 MPa grade hot-rolled steel sheet for automotive bodies and its production method. This method significantly increases alloy costs by adding a large amount of expensive alloys such as V, Cu, and Ni, and the high C content greatly affects the weldability of the material. Summary of the Invention

[0004] The main objective of this invention is to address the problems and shortcomings of existing technologies by providing a CSP production line for 900MPa grade hot-rolled high-strength steel and its manufacturing method. By combining the CSP production line and low-alloy technology, the preparation of hot-rolled high-strength steel can be achieved through economical alloy composition design and an improved steelmaking + hot rolling process. This eliminates the need for post-processing such as cold rolling or heat treatment for performance control, greatly shortening the production process and reducing production costs.

[0005] This invention uses precipitation strengthening and grain refinement as the main strengthening methods. At the same time, by controlling the microstructure of the finished product to be ferrite + a small amount of bainite, it meets the requirements of high plasticity of the material and realizes the production of hot-rolled high-strength steel with a tensile strength of over 900 MPa. While ensuring high strength (yield strength ≥ 850 MPa, tensile strength ≥ 900 MPa), it also achieves high plasticity (elongation after fracture ≥ 18%), with a strength-plasticity product > 15.3 GPa∙.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A 900MPa grade hot-rolled high-strength structural steel produced on a CSP production line has the following chemical composition and weight percentages: C 0.050-0.069%, Si ≤0.20%, Mn 1.50-1.70%, P ≤0.012%, S ≤0.003%, Ti 0.15-0.20%, Cr 0.18-0.55%, V 0.02-0.08%, Alt 0.03-0.06%, Ca 0.001-0.004%, N ≤0.004%, with the balance being Fe and unavoidable impurities; 18 ≤ (Ti+V)×10⁻⁶ is also controlled. 4 ≤25.

[0007] Furthermore, the Si content is 0.10-0.20%.

[0008] Furthermore, the Cr content is 0.30-0.55%.

[0009] In the above scheme, the metallographic structure of the 900MPa grade hot-rolled high-strength structural steel includes ferrite or ferrite + a small amount of bainite.

[0010] In the above scheme, the thickness of the 900MPa grade hot-rolled high-strength structural steel is 1.2-4.5mm.

[0011] In the design of the chemical composition of this invention: Carbon: Carbon is the most economical strengthening element. However, if the carbon content exceeds 0.07%, it is detrimental to the material's formability and can cause peritectic reactions in molten steel during casting, increasing the risk of leaks during continuous casting. Furthermore, excessively high carbon content affects the weldability of the steel. Conversely, excessively low carbon content affects subsequent precipitation strengthening. Therefore, the carbon content is limited to the range of 0.050-0.069%.

[0012] Silicon: Silicon can purify ferrite and prevent the formation of coarse carbides during cooling. Using silicon-manganese alloys for deoxidation can also reduce alloy costs to some extent. However, silicon also easily forms Fe2SiO4 and forms eutectoid products with FeO on the billet surface, 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 ≤0.20%.

[0013] Manganese: Manganese is the most effective element for improving strength and toughness, and can effectively delay the pearlite transformation. Its content is less than 1.50%, which cannot meet the strength requirements of the material; however, adding too much manganese will lead to peritectic reaction in molten steel, causing quality problems such as continuous casting cracks, and will also be detrimental to welding in subsequent applications of the material. Therefore, the manganese content is limited to the range of 1.50-1.70%.

[0014] Phosphorus: Phosphorus is prone to center segregation, which affects molding performance. In this invention, the phosphorus content is controlled at ≤0.012%.

[0015] Sulfur and nitrogen: They easily combine with Ti in steel, affecting the strengthening effect of Ti and greatly affecting the plasticity of steel. In this invention, sulfur is controlled at ≤0.003% and nitrogen is controlled at ≤0.004%.

[0016] Titanium: Titanium has a prominent precipitation strengthening effect and is an economical and effective element to ensure the strength of steel. The titanium content of this invention is designed to be in the range of 0.15-0.20%.

[0017] Chromium: Chromium is a carbide-forming element with a strong affinity for carbon, which can hinder the diffusion of carbon atoms, slow down the growth of precipitates to a certain extent, and improve the precipitation strengthening effect. It can also play a strengthening role. Adding too little chromium will not provide sufficient strengthening, while adding too much will increase costs and affect welding performance. In this invention, the chromium content is controlled at 0.18-0.55%.

[0018] Vanadium: Vanadium has a prominent precipitation strengthening effect and is a relatively economical alloying element for improving the strength of steel. In this invention, the vanadium content is controlled at 0.02-0.08%.

[0019] Aluminum: As an effective deoxidizer in the smelting process, it also has a certain effect on refining grains and improving the strength of steel, but it is also prone to forming Al2O3 inclusions. In this invention, the aluminum content is controlled at 0.03-0.06%.

[0020] Calcium: Calcium is mainly used as an additive to modify inclusions. It can effectively refine and change the morphology of inclusions and improve the toughness and plasticity of materials. The content should not be too high. In this invention, the Ca content is controlled at 0.001-0.004%.

[0021] Furthermore, the mass percentage content of chemical elements satisfies: (Ti+V)×10 4 Between 18 and 25, (Ti+V)×10 4 If the strength is below 18, the requirements cannot be met; if it is too high, the toughness and plasticity of the material will be reduced.

[0022] In addition to limiting the range of the above chemical components, from the point of view of improving the formability and economy of materials, this invention does not add expensive alloying elements such as Nb, Cu, Ni, and Mo.

[0023] This invention also provides a method for preparing 900MPa grade hot-rolled high-strength structural steel from a CSP production line, comprising the following steps: 1) After converter smelting, the metal is refined in an LF furnace; 2) The obtained molten steel is then used for thin slab continuous casting and rolling production. The key control requirements include the following: Casting and controlling the thickness of the billet; The billet is heated in a soaking furnace; During the rolling process, the reduction rate of the first three passes is ≥45%, and the total reduction rate is ≥85%. Constant speed rolling is adopted according to the thickness specifications of the finished steel. 3) Laminar flow cooling: First, water cooling is used to rapidly cool the temperature to 640-680℃ (temperature after the first stage of cooling), and then air cooling is used to cool the temperature to 600-640℃ (temperature after the second stage of cooling) before winding.

[0024] In the above scheme, during the converter smelting, argon gas is supplied throughout the bottom blowing process, with the argon gas flow rate controlled at 3600-11000 NL / min, and the tapping temperature ≥1630℃. During the LF furnace refining process, after adding Ti wire, the refining time must be ensured to be 6-8 minutes to allow the Ti alloy to fully and uniformly dissolve into the molten steel. The molten steel is then subjected to calcium treatment, which introduces finely dispersed calcium oxides as nucleation sites for liquid-precipitated TiN, refining the TiN size and improving the austenite grain refinement and material plasticity.

[0025] In the above scheme, the thickness of the cast billet is controlled between 58-85mm.

[0026] In the above scheme, the billet is heated in a heating furnace in two stages. The first stage is the heating stage, in which the billet temperature is controlled at 900-1100℃ and the heating time is controlled at 5-8 minutes. At the end of the heating stage, the billet temperature rises to 1240-1270℃. The holding stage temperature is controlled at 1230-1260℃ and the holding time is 30-50 minutes.

[0027] In the above scheme, the rolling step adopts a 7-stand finishing mill, wherein the reduction rate of the first three passes is ≥45%, the reduction rate of the first two passes is >55%, and the total reduction rate is ≥85%, which fully refines the austenite grains.

[0028] In the above scheme, the initial rolling temperature is ≥1080℃; the rolling speed is controlled at 4.0-13.0m / s; and the final rolling temperature is controlled at 870-920℃.

[0029] Furthermore, based on the thickness of the finished steel, the F7 plate speed (the rolling speed of the last mill in the 7-stand finishing mill) is controlled at 4.0-13.0 m / s for constant speed rolling, which can effectively ensure the stability of the through-coil process and ensure the overall performance stability.

[0030] Furthermore, the thickness of the finished steel strip is 1.2-4.5mm, and the relationship between the F7 plate-passing speed and the thickness is v=16.273-2.727×t, where v is the F7 plate-passing speed (m / s) and t is the thickness of the finished steel strip (mm).

[0031] In the above scheme, the water cooling rate is 30-50℃ / s (first stage cooling rate). By delaying the opening of the first stage cooling water valve, the cooling intensity is increased, the driving force for the transformation of austenite to ferrite is increased, and the microstructure of the finished product is mainly ferrite.

[0032] The 900MPa grade hot-rolled high-strength structural steel prepared according to the above scheme has a yield strength ≥850MPa, tensile strength ≥900MPa, elongation after fracture ≥18%, and strength-ductility product >18.3GPa∙; taking into account both high strength and high ductility.

[0033] Compared with the prior art, the beneficial effects of the present invention include: 1) This invention adopts a more economical alloy composition design and combines optimized steelmaking and hot rolling processes to achieve low-cost production of thin steel strips with tensile strength ≥900MPa. 2) The hot-rolled high-strength structural steel obtained by this invention combines high strength and high plasticity, and has an extremely high strength-plasticity product. It can be applied to the field of high-strength structural parts with higher forming requirements, and can meet the user's various forming processes such as rolling and bending, as well as various application requirements and application environments such as welding. 3) This invention eliminates the need for post-processing such as cold rolling or heat treatment for performance control, which can greatly shorten the production process, save steps, and further significantly reduce production costs. Attached Figure Description

[0034] Figure 1 The image shows the metallographic structure of hot-rolled high-strength structural steel obtained in one embodiment. Detailed Implementation

[0035] The technical solutions adopted in this invention are described in detail below through specific implementation examples. These descriptions are merely a part of the invention and do not represent all embodiments. Unless otherwise specified, the experimental methods used in this invention are conventional methods, and the instruments and equipment used are commercial products in this technical field.

[0036] In the following embodiments, the hot-rolled high-strength structural steel is produced according to the following steps: 1) After being smelted in a converter according to the steel composition described in Table 1, the steel is then refined in an LF furnace; In the converter smelting process, argon gas is supplied throughout the bottom blowing process, with the argon gas flow rate controlled at 6000-11000 NL / min and the tapping temperature ≥1630℃. During the LF furnace refining process, after adding Ti wire, it is necessary to ensure that the refining time is 6-8 minutes to allow the Ti alloy to be fully and uniformly dissolved into the molten steel, and then the molten steel is subjected to calcium treatment (by adding silicon-calcium wire), with the amount of silicon-calcium wire added being 2-3 kg / t. 2) The obtained molten steel is then subjected to continuous casting and rolling of thin slabs: The thickness of the cast steel billet is 58-85mm; The billet is heated in a soaking furnace, with the target heating temperature controlled at 1230-1260℃. Specifically, a two-stage heating process is adopted. The first stage is the heating stage, where the billet temperature is controlled at 900-1100℃ and the heating time is controlled at 5-8 minutes. At the end of the heating stage, the billet temperature rises to 1240-1270℃. The second stage is the holding stage, where the temperature is controlled at 1230-1260℃ and the holding time is 30-50 minutes.

[0037] The rolling process is carried out on a 7-stand finishing mill with an initial rolling temperature ≥1080℃, a reduction rate of ≥45% for the first three passes, a reduction rate of >55% for the first two passes, and a total reduction rate of ≥85%. Constant speed rolling is adopted according to the thickness specification, with the rolling speed controlled between 4.0-13.0 m / s, the final rolling temperature controlled between 870-920℃, and the rolling thickness between 1.3-4.5 mm; Among them, the F7 plate speed is controlled at 4.0-13.0m / s for constant speed rolling, and the F7 plate speed v is controlled according to the thickness of the finished steel as v=16.273-2.727×t, where v is the F7 plate speed (m / s) and t is the thickness of the finished steel strip (mm). 3) Laminar flow cooling: First, the water is rapidly cooled to 640-680℃ at a rate of 30-50℃ / s, then air-cooled for 6-10 seconds to 600-640℃, and then coiled to obtain the 900MPa grade hot-rolled high-strength structural steel of the CSP production line.

[0038] The specific main process parameters are shown in Table 2.

[0039] Table 1. List of chemical components (wt%) of various embodiments of the present invention

[0040] Table 2. List of main process parameters and performance data of various embodiments of the present invention

[0041] Figure 1The image shows the metallographic structure of hot-rolled high-strength structural steel obtained in one embodiment of the present invention. It is mainly ferrite with a small amount of bainite, wherein the proportion of bainite is <10%.

[0042] This invention is not limited to the embodiments described above. Those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A 900MPa grade hot-rolled high-strength structural steel for CSP production lines, characterized in that, Its chemical composition and weight percentage include: C 0.050-0.069%, Si ≤0.20%, Mn 1.50-1.70%, P≤0.012%, S≤0.003%, Ti 0.15-0.20%, Cr 0.18-0.55%, V 0.02-0.08%, Alt 0.03-0.06%, Ca 0.001-0.004%, N≤0.004%, with the balance being Fe and unavoidable impurities; 18 ≤ (Ti+V)×10⁻⁶. 4 ≤25.

2. The 900MPa grade hot-rolled high-strength structural steel according to claim 1, characterized in that, Its metallographic structure is dominated by ferrite.

3. The method for preparing 900MPa grade hot-rolled high-strength structural steel from the CSP production line as described in claim 1 or 2, characterized in that, Includes the following steps: 1) After converter smelting, the metal is refined in an LF furnace; 2) The obtained molten steel is then used for thin slab continuous casting and rolling production. The key control requirements include the following: Casting and controlling the thickness of the billet; The billet is heated in a soaking furnace; During the rolling process, the reduction rate of the first three passes is ≥45%, and the total reduction rate is ≥85%. Constant speed rolling is adopted according to the thickness specifications of the finished steel. 3) Laminar flow cooling: After finishing, the strip is rapidly cooled to 640-680℃ by water cooling, and then air cooled to 600-640℃ before being coiled to obtain the finished steel.

4. The preparation method according to claim 3, characterized in that, During the converter smelting process, argon gas is supplied throughout the bottom blowing process, with the argon gas flow rate controlled at 3600-11000 NL / min and the tapping temperature ≥1630℃. During the LF furnace refining process, Ti wire is added and the refining time is 6-8 min, and the molten steel is treated with calcium.

5. The preparation method according to claim 3, characterized in that, The thickness of the cast billet is 58-85 mm.

6. The preparation method according to claim 3, characterized in that, The billet is heated in a heating furnace in two stages. The first stage is the heating stage, where the billet temperature is controlled at 900-1100℃ and the heating time is controlled at 5-8 minutes. At the end of the heating stage, the billet temperature rises to 1240-1270℃. The second stage is the holding stage, where the temperature is controlled at 1230-1260℃ and the holding time is 30-50 minutes.

7. The preparation method according to claim 3, characterized in that, The rolling process employs a 7-stand finishing mill, wherein the reduction rate of the first three passes is ≥45%, the reduction rate of the first two passes is >55%, and the total reduction rate is ≥85%, which fully refines the austenite grains.

8. The preparation method according to claim 7, characterized in that, The initial rolling temperature is ≥1080℃; depending on the thickness of the finished product, the F7 plate speed during the rolling process is controlled at 4.0-13.0m / s, and the final rolling temperature is controlled at 870-920℃.

9. The preparation method according to claim 8, characterized in that, The thickness of the finished steel is 1.2-4.5mm; the relationship between the F7 plate speed v and the thickness t is v=16.273-2.727×t, where v is in m / s and t is in mm.

10. The preparation method according to claim 3, characterized in that, The water cooling rate is 30-50℃ / s.

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