Microalloyed high-strength steel and its corner-free hot-charging rolling method

Abstract

The present invention discloses a microalloyed high-strength steel and a method for hot-rolling without corner cleaning. The microalloyed high-strength steel comprises the following chemical composition, by mass percentage: C: 0.055-0.075%, Si: 0.40-1.00%, Mn: 1.90-2.40%, Cr: 0.20-0.60%, Nb: 0.010-0.025%, Ti: 0.020-0.040%, P ≤ 0.040%, Als: 0.01-0.15%, B ≤ 0.0005%, S ≤ 0.006%, N ≤ 0.0055%, Ca ≤ 0.006%, with the remainder being Fe and unavoidable impurity elements. The method comprises smelting, continuous casting, and hot rolling with a carbon equivalent (CE) of less than 0.07%. The present invention addresses the industry's difficulty in achieving corner-clearing-free hot-rolling of micro-alloyed high-strength steel. By deeply exploring the restrictive factors from the source of composition design, continuous casting process, and hot rolling process, the present invention realizes the industrial practice of corner-clearing-free hot-rolling of Nb-Ti micro-alloyed high-strength steel through integrated design. The key indicators including hot-charging rate, charging temperature, yield rate, and yield rate are very impressive.

Description

Technical Field

[0001] This invention relates to the field of metallurgical sheet metal technology, specifically to a microalloyed high-strength steel and its corner-free hot-charging rolling method. Background Technology

[0002] Hot charging is a revolutionary technology in the steel industry that emerged in the 1980s. Compared to cold charging, hot charging offers significant advantages, including reduced gas consumption, increased machine output, faster billet turnover, reduced corner iron removal losses, and reduced oxidation losses. Driven by the pursuit of ultimate efficiency, cost-effectiveness, and profitability, leading steel companies like Baosteel have placed great emphasis on improving the technical specifications of hot charging.

[0003] With the continuous development of lightweighting in automobiles, high-strength steel has become the preferred material for automotive body structures. As strength levels increase, solid solution strengthening alone is insufficient to achieve the target strength; fine-grain strengthening, precipitation strengthening, phase transformation strengthening, and even dislocation strengthening are also necessary. Whether traditional or advanced high-strength steel, microalloying elements such as Nb, Ti, V, and B play crucial roles. While microalloying composition design can improve formability while achieving high strength, the accompanying strength fluctuations, increased difficulty in acid rolling, and corner cracking issues in cast billets have long plagued industry and academia. Currently, air cooling is the most common cooling technology used in continuous annealing and galvanizing mills, with very few employing roll cooling and water quenching. For high-strength steel continuous annealing bare plates or hot-dip galvanized plates with tensile strengths of 780 MPa and above, the types and contents of alloying elements are numerous and high. Achieving corner-free hot-charging rolling places extremely high demands on casting equipment, casting processes, hot rolling equipment, hot rolling processes, and surface inspection levels. In existing technologies, industry and academia have also conducted a great deal of exploration and research.

[0004] Chinese invention patent application CN200720185022.5 proposes an online corner-cleaning device for slab continuous casting, aiming to clean corner cracks in the slab online and improve production efficiency, but it does not mention the ability to control edge defects. Chinese patent CN105414502B discloses a continuous casting sector-shaped positioning and locking device and its assembly method. By adding the sector-shaped positioning and locking device, it avoids the sector segment using its own weight for positioning, prevents the sector segment from shifting during production, greatly reduces the mechanical stress on the slab, thereby stabilizing the arc connection accuracy and improving the slab quality. Chinese patent CN110653352A adds a row of small-angle cooling water nozzles before the straightening section to eliminate ferrite-induced surface cracks in the slab, providing a narrow area of ​​strong cooling to eliminate surface proeutectoid ferrite and control surface cracks in the slab. Chinese patent CN107695313A also employs rapid cooling quenching to eliminate ferrite and carbonitride precipitation on the billet surface, preventing embrittlement and improving billet plasticity to reduce surface cracks. Chinese patent CN105478704B establishes a secondary cooling water spray rack lifting device, enabling dynamic control of the secondary cooling water area and automatic adjustment of water volume. This avoids corner cracks caused by excessively low temperatures or temperature changes due to direct spraying of secondary cooling water onto the billet corners. Chinese patent CN113843403B improves billet plasticity and reduces surface cracks by controlling the ferrite content on the billet surface above 35% before the straightening point of the casting machine. The Iron and Steel Research Institute proposed and developed patented technologies for chamfered crystallizers and chamfered foot rollers, improving corner crack defects in continuously cast billets. Whether adding positioning and locking devices or controlling the secondary cooling process to adjust the billet surface temperature and microstructure, all these methods heavily rely on equipment upgrades, making industrial implementation still very difficult. Specifically, for certain microalloyed high-strength steels, there is no readily available solution for corner-free hot-charging rolling to reduce corner cracks in cast billets and edge cracks in hot-rolled steel. Summary of the Invention

[0005] To overcome the shortcomings of the existing technology, the purpose of this invention is to provide a micro-alloyed high-strength steel and its corner-free hot-charging rolling method, which solves problems such as corner cracks, edge peeling, and edge cracks in cast billets.

[0006] To achieve the above objectives, the technical concept of the present invention is as follows:

[0007] Typically, high-temperature cast billets exhibit three distinct brittle zones with significant thermoplasticity changes: the first brittle zone within the solidification temperature range, the second brittle zone between 1200 and 950°C, and the third brittle zone between 900 and 600°C. It is generally believed that surface cracks rarely occur in the second brittle zone. For high-strength steel continuously annealed bare plates or hot-dip galvanized plates with tensile strengths of 780 MPa and above, major steel mills have long adopted the composition design of peritectic steel, with carbon content generally falling within the range of 0.08% to 0.12% in industrial practice. During solidification, the transformation from the δ phase to the γ phase in peritectic steel is accompanied by significant volume shrinkage, thus significantly reducing high-temperature plasticity. Achieving corner-free hot-charging rolling places extremely high demands on casting equipment, casting processes, hot rolling equipment, hot rolling processes, and surface inspection levels. However, due to technical secrecy, many key parameters and control points have not been disclosed. Specifically, for certain microalloyed high-strength steels, there is no readily available, corner-free hot-charging rolling solution for reducing corner cracks and edge defects in cast billets. Given the limited technical capabilities, avoiding peritectic / sub-peritetic regions through compositional design is crucial.

[0008] Carbon equivalent calculation model: CE = 0.1C / (0.1 + 0.08Si - 0.02Mn - 0.04P - 0.25S + 0.03Cr + 0.03Ti); where C, Si, Mn, P, S, Cr, and Ti are the mass percentages of the corresponding elements. According to the carbon equivalent calculation model, controlling the carbon equivalent CE within 0.07% can avoid the peritectic and sub-peritectic regions.

[0009] The heat transfer, stress, and phase transformation processes during continuous casting solidification are highly complex, and the elements themselves also affect corner cracks. Carbon (C) is a sensitive element for surface cracks in the billet. Excessive C content not only easily leads to peritectic reactions but may also increase the austenite grain size and even widen the banded structure and the hardness difference between the soft and hard phases in the finished product, severely affecting the formability of microalloyed high-strength steel. Insufficient C content necessitates the addition of more alloying elements, increasing both cost and the difficulty of corner crack control. In this invention, the C content is 0.055~0.075%. Secondly, the precipitation of niobium carbonitrides significantly deteriorates the high-temperature plasticity of the billet in the second brittle zone. Practice shows that when Nb ≤ 0.025%, the difficulty of corner crack control can be reduced. Although Ti also precipitates carbonitrides, its precipitation temperature is higher, which can weaken the influence of Nb and N. An appropriate Ti content is essential, but the significant change in the yield strength ratio of the product by Ti must also be considered. In this invention, the Nb content is 0.010~0.025%, and the Ti content is 0.020~0.040%. Therefore, the chemical composition and mass percentage of the high-strength automotive steel sheet of this invention meet the following conditions: C: 0.055~0.075%, Si: 0.40~1.00%, Mn: 1.90~2.40%, Cr: 0.20~0.60%, Nb: 0.010~0.025%, Ti: 0.020~0.040%, P≤0.040%, Als: 0.01~0.15%, B≤0.0005%, S≤0.006%, N≤0.0055%, Ca≤0.006%, with the balance being Fe and unavoidable impurity elements.

[0010] Compared to high-strength steel without nitrogen (Nb), controlling corner cracks in Nb-containing high-strength steel billets is more challenging, placing more stringent requirements on casting equipment, casting processes, hot rolling equipment, and hot rolling processes. As the strength grade increases, the types and contents of alloying elements further increase, consequently increasing the difficulty of controlling corner cracks in Nb-containing high-strength steel billets. Any unreasonable design or control of arc precision, liquid level fluctuations, mold flux parameters, vibration processes, temperature control, and billet casting speed can lead to corner cracks. Therefore, in this invention, the continuous casting process employs a short mold vibration negative slip time, a high-basicity and low-viscosity mold flux, and a process to increase the target temperature of the billet surface in the straightening zone.

[0011] During hot rolling, corner cracks in the cast billet can evolve into edge peeling defects. Furthermore, improper hot rolling processes can cause edge metal to rise and flatten under alternating side and flat pressure, leading to edge cracks. Edge peeling and edge cracks typically occur within a 35mm range of the strip edge. Reducing and mitigating hot-rolled edge cracks is crucial by increasing edge temperature and minimizing edge metal rise. This can be achieved through techniques such as… Figure 1 The concave hammerhead shown and the control of the width reduction are important means.

[0012] Based on the above ideas, and to achieve the objectives of this invention, the technical solution adopted is as follows:

[0013] A microalloyed high-strength steel has the following chemical composition and mass percentage content: C: 0.055~0.075%, Si: 0.40~1.00%, Mn: 1.90~2.40%, Cr: 0.20~0.60%, Nb: 0.010~0.025%, Ti: 0.020~0.040%, P≤0.040%, Als: 0.01~0.15%, B≤0.0005%, S≤0.006%, N≤0.0055%, Ca≤0.006%, with the balance being Fe and unavoidable impurity elements; the carbon equivalent CE of the microalloyed high-strength steel is <0.07%, and the calculation model for carbon equivalent CE is as follows:

[0014] CE = 0.1C / (0.1 + 0.08Si - 0.02Mn - 0.04P - 0.25S + 0.03Cr + 0.03Ti), in %, where C, Si, Mn, P, S, Cr, and Ti are the mass percentages of the corresponding elements.

[0015] Furthermore, in the microalloyed high-strength steel, N≤0.0050%, Als:0.010~0.040%, P≤0.015%.

[0016] Furthermore, the tensile strength of the microalloyed high-strength steel is ≥780MPa.

[0017] The above-mentioned method for hot-charging microalloyed high-strength steel without corner clearing includes the following steps:

[0018] 1) Smelting: The chemical composition of microalloyed high-strength steel is controlled, and the refining adopts the LF+RH dual process;

[0019] 2) Continuous casting: A straight-crystallized arc-shaped continuous casting machine is used, with the arc accuracy controlled within ±0.5mm, the liquid level fluctuation in the crystallizer controlled within ±5mm, the basicity of the crystallizer protective slag being 1.25±0.25, the viscosity of the crystallizer protective slag being 0.15±0.06Pa·s, and the negative slip time of the crystallizer vibration device being 0.12~0.16s; the target temperature of the billet surface in the straightening zone is ≥920℃, and the billet casting speed is 1.1~1.6m / min;

[0020] 3) Hot rolling, casting billet without clear corners, furnace charging temperature 400~900℃, total width reduction ≤200mm, heating temperature 1230±40℃, side press adopts concave hammer head, concave depth 20~40mm; use heat insulation cover.

[0021] Furthermore, the corner-free hot-rolling method also includes: 4) pickling and cold rolling; 5) continuous annealing or hot-dip galvanizing.

[0022] Furthermore, in step 1), the chemical composition of the microalloyed high-strength steel is controlled, with the converter endpoint C ≤ 0.050%, and the refining process adopts the LF+RH dual-process.

[0023] Furthermore, in step 2), the crystallizer outlet temperature is ≥1050℃, the tundish superheat is 15~35℃, and electromagnetic stirring and dynamic light pressure are employed.

[0024] Furthermore, in step 2), the target thickness of the billet is 230 mm, and the corner shape of the billet is right angle.

[0025] Furthermore, in step 2), the billet casting speed is 1.2~1.4 m / min.

[0026] Furthermore, in step 3), a U-shaped cooling process is used for winding, with a temperature compensation of 30~80℃ within a range of at least 50m from the beginning and end, and the target winding temperature for the remaining middle part is set at 500~600℃.

[0027] Furthermore, in step 3), the final rolling temperature is 900±30℃, and an edge heater is used.

[0028] Furthermore, in step 3), the total width reduction is ≤100mm.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] 1) The microalloyed high-strength steel and its corner-free hot-charging rolling method provided by the present invention effectively avoids the crack-sensitive subperitectic region by designing the composition with carbon equivalent CE<0.07%, 0.055%≤C≤0.075%, and 0.010%≤Nb≤0.025%, thus eliminating the tendency of the billet to crack at the source of composition design.

[0031] 2) This invention reduces and mitigates the defects of corner cracks in the billet from the source of the continuous casting process by rationally designing the arc accuracy, liquid level fluctuation, protective slag parameters, vibration process, temperature control, and billet casting speed.

[0032] 3) By rationally designing the hammer head shape, side pressure reduction amount, heating temperature, furnace loading temperature, and edge insulation of the side press, this invention reduces, mitigates, or even eliminates edge defects (edge ​​peeling + edge cracks) from the source of the hot rolling process. The edge defects can be controlled within 10mm of the edge, which is convenient for subsequent processes to remove them.

[0033] 4) This invention improves the through-coil stability of the microstructure and properties of hot-rolled microalloyed high-strength steel by precisely designing the B content and layer cooling method, and effectively reduces thickness fluctuation defects during subsequent pickling and rolling production.

[0034] In summary, this invention adopts an integrated design of chemical composition, continuous casting process, and hot rolling process, realizing the industrial-scale hot-rolling of Nb-Ti microalloyed high-strength dual-phase steel without corner cleaning. The yield rate of edge defects (edge ​​peeling + edge cracks) within 10mm of the hot-rolled edge can reach more than 90%, the hot charging rate can reach more than 90%, the furnace time can be reduced by more than 20 minutes compared with cold charging rolling, and the hot-rolled yield is increased by more than 0.3%. Attached Figure Description

[0035] Figure 1 This is a physical image of a concave hammerhead used in the corner-free hot-charging rolling method of the present invention;

[0036] Figure 2 This is a pickling diagram of the corner of the cast billet in Comparative Example 1;

[0037] Figure 3 This is a diagram showing the thickness fluctuation process during pickling in Comparative Example 4. Detailed Implementation

[0038] To better explain the present invention, the main contents of the present invention are further illustrated below with reference to the accompanying drawings and specific embodiments, but the contents of the present invention are not limited to the following embodiments.

[0039] The chemical compositions of the microalloyed high-strength steels in Examples 1-3 (S1-S3) and Comparative Examples 1-4 (D1-D4) are shown in Table 1. The target thickness of the billet was 230 mm, and the corners of the billet were right angles.

[0040] In Examples 1-3 and Comparative Example 3, the microalloyed high-strength steel was prepared using the corner-free hot-charging rolling method of the present invention; in Comparative Examples 1, 2, and 4, the microalloyed high-strength steel was not prepared using the method of the present invention. The relevant process parameters are shown in Tables 2 and 3.

[0041] Table 1: Main chemical composition (wt%, balance is Fe and unavoidable impurity elements)

[0042]

[0043] Table 2: Main Smelting and Continuous Casting Process Parameters

[0044]

[0045] Table 3: Main Hot Rolling Process Parameters

[0046]

[0047] Table 4: Comparison of the effects of the proposed solutions

[0048]

[0049] Note: Hot-rolled coil yield refers to the percentage of steel coils with edge defects located within 10mm of the edge.

[0050] The slabs produced using the method of this invention in Examples 1-3 showed good corner quality, and no corner cracks were observed after low-magnification pickling. The effects of Examples 1-3 and Comparative Examples 1-4 are shown in Table 4. Comparative Examples 1-2 had higher C content (CE > 0.07%) and did not avoid the subperitectic region; despite using the continuous casting process of this invention, the slabs still exhibited severe corner cracks, such as... Figure 2 As shown in Table 4, when the corner cracks of the billet are severe, regardless of the hot rolling process used, the yield of products with edge defects within 10 mm of the hot-rolled edge is very low. Comparative Example 3 and Example 1 have the same chemical composition, but the continuous casting process of Comparative Example 3 differs from that of Example 1. The difference in composition between Comparative Example 4 and Example 1 is the addition of 0.0015% B; the continuous casting process is the same, but the hot rolling process differs. The yield of products with edge defects within 10 mm of the hot-rolled edge in Comparative Examples 3-4 is also very low, and edge defects affecting usability will still exist after trimming in subsequent production processes. See Table 4 and... Figure 3 As shown, the thickness of the pickled and rolled steel in Comparative Example 4 fluctuated significantly and could not meet production requirements. The hot-rolled coil yields in all embodiments were very high, with a hot-rolled yield of no less than 90%. Compared to Comparative Example 2, which was cold-rolled, the hot-rolled yield increased by more than 0.4%, and the furnace time was reduced by more than 20 minutes. The tensile strength of all embodiments after heat treatment reached 780 MPa.

[0051] In summary, the method of this invention adopts an integrated design of chemical composition, continuous casting process, and hot rolling process, which can realize the industrial-scale hot rolling of microalloyed high-strength steel without corner cleaning. It has the advantages of low cost, good product surface quality, and simple process flow, and reduces corner cleaning iron loss and oxidation burn loss.

Claims

1. A microalloyed high-strength steel, characterized in that: The chemical composition includes the following components by mass percentage: C: 0.055~0.075%, Si: 0.40~1.00%, Mn: 1.90~2.40%, Cr: 0.20~0.60%, Nb: 0.010~0.025%, Ti: 0.020~0.040%, P≤0.040%, Als: 0.01~0.15%, B≤0.0005%, S≤0.006%, N≤0.0055%, Ca≤0.006%, with the balance being Fe and unavoidable impurity elements; the carbon equivalent CE of the microalloyed high-strength steel is <0.07%, and the calculation model for the carbon equivalent CE is as follows: CE = 0.1C / (0.1 + 0.08Si - 0.02Mn - 0.04P - 0.25S + 0.03Cr + 0.03Ti), in %, where C, Si, Mn, P, S, Cr, and Ti are the mass percentages of the corresponding elements.

2. The microalloyed high-strength steel according to claim 1, characterized in that: In the microalloyed high-strength steel, N≤0.0050%, Als:0.010~0.040%, P≤0.015%.

3. The microalloyed high-strength steel according to claim 1, characterized in that: The tensile strength of the microalloyed high-strength steel is ≥780MPa.

4. The method for hot-rolling microalloyed high-strength steel without corner clearing as described in claim 1, characterized in that: Includes the following steps: 1) Smelting is controlled according to the chemical composition of microalloyed high-strength steel, and refining adopts the LF+RH dual process; 2) Continuous casting: A straight-crystallized arc-shaped continuous casting machine is used, with the arc accuracy controlled within ±0.5mm, the liquid level fluctuation in the crystallizer controlled within ±5mm, the basicity of the crystallizer protective slag being 1.25±0.25, the viscosity of the crystallizer protective slag being 0.15±0.06Pa·s, and the negative slip time of the crystallizer vibration device being 0.12~0.16s; the target temperature of the billet surface in the straightening zone is ≥920℃, and the billet casting speed is 1.1~1.6m / min; 3) Hot rolling, casting billet without corner removal, furnace charging temperature of 400~900℃, total width reduction ≤200mm, heating temperature of 1230±40℃, side press adopts concave hammer head, concave depth of 20~40mm; use heat insulation cover.

5. The corner-free hot-charging rolling method according to claim 4, characterized in that: In step 2), the crystallizer outlet temperature is ≥1050℃, and the superheat of the tundish is 15~35℃; electromagnetic stirring and dynamic light pressure are used.

6. The corner-free hot-charging rolling method according to claim 4, characterized in that: In step 2), the billet casting speed is 1.2~1.4m / min.

7. The corner-free hot-charging rolling method according to claim 4, characterized in that: In step 2), the target thickness of the billet is 230mm, and the corner shape of the billet is right angle.

8. The corner-free hot-charging rolling method according to claim 4, characterized in that: In step 3), a U-shaped cooling process is used for winding, with the temperature compensation within a range of at least 50m from the beginning and end being 30~80℃, and the target winding temperature for the remaining middle part being set at 500~600℃.

9. The corner-free hot-charging rolling method according to claim 4, characterized in that: In step 3), the final rolling temperature is 900±30℃, and the edge heater is put into operation.

10. The method for hot-charging without corner cleaning according to any one of claims 4 to 9, characterized in that: In step 3), the total width reduction is ≤100mm.

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