A method of producing a low alloy high strength structural steel with reduced banded structure

Abstract

A production method for reducing low alloy high strength structural steel strip structure, the method comprises the following process steps: step one: after hot rolling of continuous casting billet, on-line quenching is carried out, the quenching starting temperature is 780-880 DEG C, the quenching medium is high pressure water, the quenching cooling rate of the surface and the core of the hot rolled plate is all greater than or equal to 30 DEG C / s, and the quenching end temperature is water temperature; step two: the quenched hot rolled plate is normalized, the heating temperature is 800-900 DEG C, the heating time is determined according to the plate thickness, is 1.5-2.0 min / mm, and after heating, air cooling to room temperature on the cooling bed is carried out. The heat treatment process of "hot rolling on-line quenching + normalizing" can reduce the final hot rolling strip structure to below 2.0 under the premise of guaranteeing qualified mechanical properties, effectively simplifies the heat treatment process, and is beneficial to realize large-scale industrialized continuous production.

Description

Technical Field

[0001] This invention belongs to the field of steel heat treatment technology and relates to a production method for reducing the banded structure of low-alloy high-strength structural steel. Background Technology

[0002] Low-alloy high-strength structural steel possesses excellent mechanical and weldability properties and is commonly used in engineering machinery, offshore platforms, shipbuilding, and special vehicles. In actual production, the continuously cast slabs of low-alloy high-strength structural steel exhibit a certain degree of dendritic element segregation (mainly C and Mn segregation), which forms elongated element segregation bands along the rolling direction during subsequent hot rolling. During post-rolling air cooling, regions with lower C and Mn content preferentially undergo ferrite phase transformation, while C- and Mn-rich regions primarily transform into pearlite at lower temperatures, ultimately exhibiting a banded microstructure characterized by alternating ferrite and pearlite distribution. The presence of this banded microstructure results in significant anisotropy in the mechanical properties of the hot-rolled plate; its transverse plasticity and toughness are significantly lower than its longitudinal properties, and cracks easily form along the ferrite-pearlite interface during deformation, ultimately greatly reducing the workpiece's lifespan and safety.

[0003] Currently, low-alloy high-strength structural steel mainly reduces or eliminates banded structures through post-rolling offline heat treatment. Traditional post-rolling heat treatment processes (such as normalizing or quenching and tempering) are simple, but their reduction of banded structures is limited. Therefore, some novel heat treatment schemes that can significantly reduce banded structures have been proposed. Chinese patent CN110819773A discloses a heat treatment process for controlling banded structures in low-alloy high-strength weathering steel. This process, based on traditional post-rolling normalizing, incorporates a multi-stage normalizing process. First, the hot-rolled plate undergoes austenitizing annealing and then furnace cooling to the pearlite transformation zone, followed by isothermal treatment and finally air cooling to room temperature. Chinese patent CN110438298A discloses a heat treatment process for eliminating banded structures in acid-resistant container steel plates. This process employs austenitizing annealing followed by rapid cooling after hot rolling, followed by sub-temperature normalizing and tempering. These novel heat treatment processes show significant effects in eliminating banded structures, but the overall process flow is long and energy-intensive, which is not conducive to large-scale industrial production. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a production method for low-alloy high-strength structural steel that has a short process flow and can reduce or eliminate banded structures.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A production method for reducing banded microstructure in low-alloy high-strength structural steel, wherein the thickness of the hot-rolled steel plate is ≤25mm, is characterized by comprising the following process steps: Step 1: After hot rolling the continuously cast billet, online quenching is performed. The quenching start temperature is 780~880℃, the quenching medium is high-pressure water, the quenching cooling rate of the surface and core of the hot-rolled plate is ≥30℃ / s, and the quenching end temperature is the water temperature. Step 2: Normalize the quenched hot-rolled plate at a temperature of 800~900℃ and a heating time of 1.5~2.0 min / mm, depending on the plate thickness. After heating, air cool the plate to room temperature on a cooling bed.

[0006] Furthermore, the chemical composition of the low-alloy high-strength structural steel is as follows (mass percentage): C = 0.15%~0.22%, Mn = 1.00%~1.50%, Si = 0.1%~0.5%, Cu ≤ 0.15%, Nb ≤ 0.05%, V ≤ 0.08%, Ti ≤ 0.03%, P ≤ 0.015%, S ≤ 0.005%, with the balance being Fe and unavoidable inclusions.

[0007] Furthermore, in step one, the thickness of the hot-rolled steel plate is ≤25mm.

[0008] Furthermore, in step one, the microstructure before online quenching of the hot-rolled material is entirely austenitic.

[0009] Furthermore, in step one, the microstructure of the hot-rolled plate after online quenching is entirely martensitic.

[0010] Furthermore, the final mechanical properties of the Q345 grade hot-rolled plate meet the following requirements: yield strength ≥370MPa, tensile strength ≥520MPa, elongation ≥25%, and average impact energy ≥150J at -30℃.

[0011] Furthermore, the final mechanical properties of the Q460 grade hot-rolled plate meet the following requirements: yield strength ≥490MPa, tensile strength ≥640MPa, elongation ≥20%, and average impact energy ≥120J at -30℃.

[0012] The technical principle of this invention is as follows: This invention utilizes a "hot-rolled online quenching + normalizing" process to eliminate banded structures in hot-rolled billets. After hot rolling, the billet is in a fully austenitic state, followed by ultra-fast cold quenching to generate a fully martensitic structure. Because martensitic structures have multi-level substructures (laths, lath bundles, lath groups), they provide numerous nucleation sites for the subsequent austenitization process, thus forming fine austenite grains at high temperatures. During the subsequent air cooling process, these fine austenite grains facilitate the refinement and dispersion of ferrite and pearlite grains, disrupting the continuity of ferrite and pearlite, thereby hindering the formation of banded structures.

[0013] The present invention has the following beneficial effects: It effectively reduces the strip structure in hot-rolled steel through a short-process technology of "online quenching + normalizing". While ensuring the mechanical properties meet requirements, this process can reduce the strip structure grade of hot-rolled steel to below 2.0, and also significantly simplifies the heat treatment process, which is conducive to large-scale industrial continuous production. Attached Figure Description

[0014] Figure 1 Metallographic structure diagram of the steel plate produced in Example 1.

[0015] Figure 2 Metallographic structure diagram of the steel plate produced in Example 2. Detailed Implementation

[0016] The present invention will be further described below with reference to the embodiments. Example 1

[0017] The steel composition corresponding to this embodiment is shown in Table 1. The process steps are as follows: Step 1: After roughing and finishing rolling, the continuously cast billet is made into a medium-thick plate with a thickness of 16.4 mm. Then, it is rapidly cooled from 800℃ to the bottom using an ultra-fast cooling device (the cooling rate of the surface and the core is ≥30℃ / s). Step Two: The quenched hot-rolled plate is reheated to 880℃ and held for 30 minutes, then air-cooled to room temperature on a cooling bed. Ultimately, the mechanical properties in this embodiment are: yield strength 394 MPa, tensile strength 521 MPa, elongation 33%, and average impact energy at -30℃ 200 J, meeting the mechanical property requirements. The metallographic structure corresponding to this embodiment is as follows: Figure 1 As shown, the banded tissue grade is 2.0.

[0018] Examples 2-3: The chemical compositions used in Examples 2 and 3 are shown in Table 2. The process steps are as follows: Step 1: After roughing and finishing rolling, the continuously cast billet is made into a medium-thick plate with a thickness of 15.7 mm. Then, it is rapidly cooled from 860℃ to the bottom using an ultra-fast cooling device (the cooling rate of the surface and the core is ≥30℃ / s). Step 2: The quenched hot-rolled plates were reheated to 800℃ (Example 2) and 880℃ (Example 3) respectively, held for 30 minutes, and then air-cooled to room temperature on a cooling bed. Finally, the mechanical properties of Examples 2 and 3 are shown in Table 3, with the corresponding banded microstructure grade being 1.0. The metallographic structure corresponding to Example 2 is as follows: Figure 2 As shown.

[0019] Table 1. Chemical composition (wt%) of the steel plate rolled in Example 1 .

[0020] Table 2 Chemical composition (wt%) of the steel plates rolled in Examples 2-3 .

[0021] Table 3 Final mechanical properties of the rolled steel plates from Examples 2-3 .

Claims

1. A production method for reducing banded microstructure in low-alloy high-strength structural steel, characterized in that: The low-alloy high-strength structural steel has the following chemical composition by mass percentage: C = 0.15%~0.22%, Mn = 1.00%~1.50%, Si = 0.1%~0.5%, Cu ≤ 0.15%, Nb ≤ 0.05%, V ≤ 0.08%, Ti ≤ 0.03%, P ≤ 0.015%, S ≤ 0.005%, with the balance being Fe and unavoidable inclusions; the process includes the following steps: Step 1: After hot rolling, the continuously cast billet is quenched online. The quenching start temperature is 780~880℃, the quenching medium is high-pressure water, the quenching cooling rate of the surface and core of the hot-rolled plate is ≥30℃ / s, and the quenching end temperature is the water temperature. Before online quenching, the microstructure of the hot-rolled plate is entirely austenitic, and after online quenching, the microstructure of the hot-rolled plate is entirely martensitic. The thickness of the hot-rolled steel plate is ≤25mm. Step 2: Normalize the quenched hot-rolled plate at a temperature of 800~900℃ and a heating time of 1.5~2.0 min / mm, depending on the plate thickness. After heating, air cool the plate to room temperature on a cooling bed.

2. The production method for reducing banded microstructure in low-alloy high-strength structural steel according to claim 1, characterized in that: The final mechanical properties of Q345 grade hot-rolled plate meet the following requirements: yield strength ≥370MPa, tensile strength ≥520MPa, elongation ≥25%, and average impact energy ≥150J at -30℃.

3. The production method for reducing banded microstructure in low-alloy high-strength structural steel according to claim 1, characterized in that: The final mechanical properties of Q460 grade hot-rolled plate meet the following requirements: yield strength ≥490MPa, tensile strength ≥640MPa, elongation ≥20%, and average impact energy ≥120J at -30℃.

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