Production method for reducing banded structures of low-alloy high-strength structural steel

Abstract

A production method for reducing banded structures of low-alloy high-strength structural steel comprises the following process steps that firstly, a continuous casting blank is subjected to hot rolling and then subjected to online quenching, the quenching starting temperature ranges from 780 DEG C to 880 DEG C, high-pressure water serves as a quenching medium, the quenching cooling speed of the surface and the core of a hot-rolled plate is larger than or equal to 30 DEG C / s, and the quenching ending temperature is water temperature; and secondly, the quenched hot rolled plate is subjected to normalizing treatment, the heating temperature ranges from 800 DEG C to 900 DEG C, the heating time is determined according to the plate thickness and ranges from 1.5 min / mm to 2.0 min / mm, and after heating, the hot rolled plate is air-cooled to the room temperature on a cooling bed. According to the method, the heat treatment process of hot rolling on-line quenching and normalizing is adopted, the final hot rolling banded structure can be reduced to be lower than 2.0 level on the premise that the mechanical property is qualified, meanwhile, the heat treatment process is effectively simplified, and large-scale industrial continuous production is achieved.

Description

Technical Field

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

[0002] Low-alloy high-strength structural steel has excellent mechanical properties and welding properties, and is often used in engineering machinery, offshore platforms, shipbuilding, special vehicles and other fields. In the actual production process, there is a certain degree of dendritic element segregation (mainly C and Mn segregation) inside the continuous casting billet of low-alloy high-strength structural steel, and an element segregation band elongated along the rolling direction is formed during the subsequent hot rolling process. During the air cooling process after rolling, the areas with low C and Mn content preferentially undergo ferrite phase transformation, while the C-rich and Mn-rich areas mainly transform into pearlite at a lower temperature, and finally present the characteristics of a banded structure with alternating ferrite / pearlite distribution. The existence of banded structure makes the hot-rolled plate have obvious anisotropy of mechanical properties, and its transverse plasticity and toughness are significantly lower than those in the longitudinal direction. In addition, cracks are easily formed along the interface between ferrite and pearlite during deformation, which ultimately greatly reduces the life and safety of the workpiece.

[0003] At present, low-alloy high-strength structural steel is mainly reduced or eliminated by post-rolling offline heat treatment. The traditional post-rolling heat treatment (such as normalizing or quenching and tempering) process is simple, but the degree of reduction of banded structure is limited. Therefore, some new heat treatment schemes that can significantly reduce banded structure have been proposed. Chinese patent CN110819773A discloses a heat treatment process for controlling the banded structure of low-alloy high-strength weathering steel. The process designs a post-rolling multi-stage normalizing process based on the traditional post-rolling normalizing treatment, that is, the hot-rolled plate is first austenitized annealed and then furnace cooled to the pearlite phase transformation zone, followed by isothermal treatment and finally air cooled to room temperature. Chinese patent CN110438298A discloses a heat treatment process for eliminating the banded structure of acid-resistant container steel plate. The process adopts austenitization annealing + rapid cooling process after hot rolling, followed by sub-temperature normalizing + tempering treatment. These new heat treatment processes have obvious effects on eliminating banded structure, but the overall process flow is long and energy consumption is high, which is not conducive to large-scale industrial production. Summary of the invention

[0004] The object of the present invention is to overcome the above-mentioned deficiencies of the prior art and to provide a production method for the preparation of low-alloy high-strength structural steel with a short process flow and the ability to reduce or eliminate banded structures.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions: A production method for reducing the banded structure of low-alloy high-strength structural steel, wherein the thickness of the steel plate after hot rolling is ≤25 mm, characterized in that it comprises the following process steps: Step 1: After hot rolling, the continuous casting billet is quenched online. The quenching start temperature is 780~880℃, the quenching medium is high-pressure water, the quenching cooling rate of the hot-rolled plate surface and core is ≥30℃ / s, and the quenching end temperature is the water temperature; Step 2: Normalize the hot-rolled plate after quenching at a heating temperature of 800-900°C. The heating time is determined according to the plate thickness, which is 1.5-2.0 min / mm. After heating, cool it to room temperature on a cooling bed.

[0006] Furthermore, the chemical composition mass percentage of the low-alloy high-strength structural steel is 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%, and the balance is Fe and unavoidable inclusions.

[0007] Furthermore, in step 1, the thickness of the steel plate after hot rolling is ≤25 mm.

[0008] Furthermore, in step 1, the microstructure before hot rolling and online quenching is all austenite.

[0009] Furthermore, in step 1, the microstructure of the hot-rolled plate after online quenching is entirely martensite.

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

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

[0012] Technical principle of the present invention: The principle of the present invention to eliminate the hot-rolled banded structure by using the "hot-rolled online quenching + normalizing" process is that the continuous casting billet is in a fully austenitized state after hot rolling, and then ultra-fast cold quenching generates a fully martensitic structure. Since the martensitic structure has a multi-level substructure (laths, lath bundles, lath groups), it can provide a large number of nucleation points for the subsequent austenitization process, so fine austenite grains are formed in the high temperature section. In the subsequent air cooling process, the fine austenite grains are conducive to the refinement and dispersion distribution of ferrite and pearlite grains, destroying the continuity of ferrite and pearlite, thereby hindering the formation of banded structure.

[0013] The present invention has the following beneficial effects: the present invention realizes effective reduction of hot-rolled strip structure through the short process of "hot-rolled online quenching + normalizing". Under the condition of ensuring qualified mechanical properties, the application of this process can reduce the grade of hot-rolled strip to below 2.0, and at the same time greatly simplifies the heat treatment process, which is conducive to large-scale industrial continuous production. BRIEF DESCRIPTION OF THE DRAWINGS

[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 DESCRIPTION

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

[0017] The corresponding steel composition of this embodiment is shown in Table 1. The process steps are as follows: Step 1: The continuous casting billet is prepared into a medium and thick plate with a thickness of 16.4 mm after rough rolling and finish rolling, and then rapidly cooled from 800°C to the bottom using ultra-fast cooling equipment (the cooling rate of the surface and the core is ≥30°C / s); Step 2: After quenching, the hot-rolled plate is reheated to 880°C for 30 minutes, and then air-cooled to room temperature on a cooling bed. Finally, the mechanical properties of this embodiment are: yield strength 394MPa, tensile strength 521MPa, elongation 33%, and average impact energy 200J at -30°C, which meet 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] Embodiment 2~3: The chemical compositions used in Examples 2-3 are shown in Table 2. The process steps are as follows: Step 1: The continuous casting billet is prepared into a medium and thick plate with a thickness of 15.7 mm after rough rolling and finish rolling, and then rapidly cooled from 860°C to the bottom using ultra-fast cooling equipment (the cooling rate of the surface and the core is ≥30°C / s); Step 2: The hot-rolled plates after quenching were reheated to 800°C (Example 2) and 880°C (Example 3) for 30 minutes, and then cooled to room temperature on a cooling bed. Finally, the mechanical properties of Example 2 and Example 3 are shown in Table 3, and the corresponding banded structure grades are both 1.0, and the metallographic structure corresponding to Example 2 is as follows: Figure 2 shown.

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

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

[0021] Table 3 Final mechanical properties of steel plates rolled in Examples 2 to 3 .

Claims

1. A production method for reducing the banded structure of low-alloy high-strength structural steel, characterized in that The process steps include: Step 1: After hot rolling, the continuous casting billet is quenched online. The quenching start temperature is 780~880℃, the quenching medium is high-pressure water, the quenching cooling rate of the hot-rolled plate surface and core is ≥30℃ / s, and the quenching end temperature is the water temperature; Step 2: Normalize the hot-rolled plate after quenching at a heating temperature of 800-900°C. The heating time is determined according to the plate thickness, which is 1.5-2.0 min / mm. After heating, cool it to room temperature on a cooling bed.

2. A method for reducing banded structure of low alloy high strength structural steel according to claim 1, characterized in that: The chemical composition mass percentage of the low-alloy high-strength structural steel is 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%, and the balance is Fe and unavoidable inclusions.

3. The method for reducing the banded structure of low alloy high strength structural steel according to claim 1, characterized in that: The thickness of the steel plate after hot rolling in step 1 is ≤25mm.

4. The method for reducing the banded structure of low alloy high strength structural steel according to claim 1, characterized in that: In step 1, the microstructure before hot rolling and online quenching is all austenite.

5. The method for reducing the banded structure of low alloy high strength structural steel according to claim 1, characterized in that: In step 1, the hot-rolled plate is completely martensite after online quenching.

6. The method for reducing the banded structure of 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 requirements of yield strength ≥370MPa, tensile strength ≥520MPa, elongation ≥25%, and average impact energy ≥150J at -30℃.

7. The method for reducing the banded structure of 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 requirements of yield strength ≥490MPa, tensile strength ≥640MPa, elongation ≥20%, and average impact energy ≥120J at -30℃.

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