Multi-stage controlled rolling and controlled cooling process for improving impact toughness of DH36 steel thick plate

Abstract

The application relates to a multistage controlled rolling and controlled cooling process for improving the impact toughness of DH36 steel thick plates and belongs to the technical field of metal material processing. In view of the problem of insufficient impact toughness caused by coarse grains, element segregation and mixed grains in the center of the thick plate under the traditional process, a multistage controlled rolling and controlled cooling synergic process is provided. The process comprises the following steps: (1) low-temperature homogenization pretreatment, 1250 DEG C annealing for 2-3 hours, adding 0.002 wt.% boron, and reducing the Mn segregation index to below 1.5; (2) two-stage controlled rolling, coarse rolling at 1050-1150 DEG C, cumulative pass reduction rate >=55%, refining austenite grains to <=50 mu m, and fine rolling at 900-1000 DEG C, cumulative pass reduction rate >=60%; (3) gradient controlled cooling, ultrafast cooling >=50 DEG C / s to inhibit grain growth, and slow cooling <=10 DEG C / s to promote residual austenite decomposition. Finally, a multi-scale structure of 'bainite + ferrite + nanometer carbide' is obtained, and the impact energy value at-40 DEG C reaches above 120 J.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, and relates to a rolling process that significantly improves the impact toughness of DH36 marine steel thick plates by optimizing rolling temperature, deformation amount and cooling path to synergistically control microstructure. Background Technology

[0002] DH36 steel, used in ship hull structures, is widely applied in shipbuilding due to its excellent weldability and corrosion resistance. However, under traditional rolling processes, especially for steel plates thicker than 50mm, the problem of uneven microstructure in the core is often encountered, specifically manifested as follows:

[0003] Grain coarsening and mixed grain phenomenon: Although traditional high-temperature rolling (≥1200℃) can reduce deformation resistance, it leads to abnormal coarsening of austenite grains, with an average size >100μm. Furthermore, during cooling, due to temperature gradient differences, there is a significant difference in grain size between the surface and core layers: the fine-grained region on the surface is ≤20μm, while the coarse-grained region in the core is >150μm, forming a mixed grain structure. Mixed grains cause stress concentration, significantly reducing the material's low-temperature impact toughness; the impact energy at -40℃ is <50 J.

[0004] Element segregation and inclusion aggregation: During the solidification process, continuously cast billets are prone to dendrite segregation index Cmax / Cmin > 2.0 of elements such as Mn and C. After rolling, the segregation band remains in the core of the plate and becomes the source of crack initiation. At the same time, coarse sulfides such as MnS and oxide inclusions of Al2O3 are difficult to break during rolling, further deteriorating the mechanical properties.

[0005] Cooling process defects: Conventional cooling processes such as air cooling or water cooling alone fail to control the microstructure due to a mismatch between the cooling rate and phase transformation kinetics. For example, with an air cooling rate ≤5℃ / s, austenite remains in the high-temperature region for too long, resulting in continuous grain coarsening, and the phase transformation products are mainly coarse ferrite >30μm and pearlite, making it difficult to achieve both strength and toughness; with a water cooling rate ≥30℃ / s, although grains can be refined, it easily induces hard and brittle bainite / martensite phases, and the uneven cooling generates residual stress >200 MPa, increasing the risk of plate deformation and cracking.

[0006] Existing improvement processes mostly focus on optimizing single parameters, but these have limitations. These issues severely restrict the application of DH36 steel thick plates in harsh environments such as polar vessels and deep-sea platforms. Therefore, it is necessary to develop a process that can synergistically control rolling temperature, deformation amount, and cooling path to achieve precise matching of grain refinement, elemental homogenization, and phase transformation structure, thereby comprehensively improving the overall mechanical properties of thick plates. Summary of the Invention

[0007] The present invention aims to provide a multi-stage controlled rolling and cooling process to improve the impact toughness of DH36 steel thick plates, achieving precise matching of grain refinement, element homogenization and phase transformation structure, thereby significantly improving the impact toughness of thick plates.

[0008] The technical solution of the invention:

[0009] A multi-stage controlled rolling and controlled cooling process for improving the impact toughness of DH36 steel thick plates includes the following process steps:

[0010] (1) Low-temperature homogenization pretreatment: The continuous casting billet is homogenized and annealed at 1250℃ for 2-3 hours, and 0.002wt.% of B is added to reduce the Mn segregation index Cmax / Cmin to below 1.5;

[0011] (2) Two-stage controlled rolling: In the roughing stage, multiple passes are rolled in the temperature range of 1050-1150℃, with a cumulative pass reduction rate ≥55%; in the finishing stage, rolling is carried out in the temperature range of 900-1000℃, with a cumulative pass reduction rate ≥60% and a cumulative strain energy >0.6, forming flat austenite grains.

[0012] (3) Gradient cooling: The first stage is ultra-fast cooling, which is cooled at a rate of ≥50℃ / s within 3 seconds after final rolling; the second stage is slow cooling, which is cooled to room temperature at a rate of ≤10℃ / s.

[0013] Further, step (1) low-temperature homogenization pretreatment: the homogenization annealing is carried out under an argon protective atmosphere to prevent oxidation of the surface of the continuous casting billet.

[0014] Further, step (1) low-temperature homogenization pretreatment: the added B series is uniformly added to the molten steel through vacuum induction furnace melting.

[0015] Preferably, step (3) gradient cooling: the first stage cooling rate of the gradient cooling is 50~70℃ / s, and the second stage cooling rate is 5~10℃ / s.

[0016] Further, step (3) gradient cooling: the microstructure after gradient cooling is a composite phase structure of 50%~70% ferrite, 20%~45% bainite and 3%~8% retained austenite, with grain size ≤10μm, carbide precipitation size ≤50nm, and high angle grain boundaries (HAGBs) accounting for ≥60%.

[0017] Furthermore, the process is applicable to the production of DH36 steel plates with a thickness ≥50mm, and the final mechanical properties are impact energy ≥100J at -40℃.

[0018] The technical principle of this invention is as follows: The continuously cast billet is placed in a furnace at 1250℃ for homogenization annealing for 2-3 hours. At this temperature, dendritic segregation of elements such as Mn and C within the billet is significantly reduced through diffusion mechanisms, and the segregation index Cmax / Cmin decreases from above 2.5 initially to below 1.5, effectively eliminating the risk of banded structures during subsequent rolling. Adding 0.002 wt.% B during annealing causes boron atoms to preferentially segregate at austenite grain boundaries, suppressing the grain boundary segregation of solute elements such as P and S by occupying grain boundary vacancies, thus reducing grain boundary brittleness. Simultaneously, boron can delay the austenite-to-ferrite phase transformation, providing a wider phase transformation control window for subsequent controlled cooling processes. Through multi-pass large deformation-induced dynamic recrystallization, the initial coarse austenite grains >200 μm are refined to ≤50 μm. Dynamic recrystallization forms equiaxed fine grains through dislocation multiplication and grain boundary migration, while a high strain rate ≥5 s⁻¹ inhibits grain growth, ensuring microstructure uniformity. Through high dislocation density >10¹ 4 The flat austenite grains with an area of ​​m⁻² store deformation energy, providing a driving force for subsequent phase transformation. Rolling in the non-recrystallized region increases the number of phase deformation nucleation sites through dislocation entanglement and subgrain boundary formation, promoting the uniform precipitation of fine ferrite and bainite during cooling. Rapid cooling transforms the undercooled austenite into fine lath bainite, while retaining some residual austenite, with a volume fraction of approximately 5%–8%, to improve toughness. During slow cooling, carbides precipitate uniformly with a size ≤50 nm, avoiding the aggregation of hard and brittle phases, while the ferrite matrix further recovers, improving plasticity and low-temperature toughness.

[0019] This invention provides a reliable technical solution for the application of DH36 steel thick plates in polar ships, deep-sea platforms, and other fields through multi-level synergistic regulation. Compared with the prior art, this invention has the following beneficial effects: 1) Low-temperature homogenization eliminates segregation, providing a uniform initial microstructure for controlled rolling; 2) Rough rolling refines grains, and finish rolling introduces dislocation energy storage, achieving a composite strengthening of "fine grains + high energy storage"; 3) Flat austenite with high dislocation density preferentially nucleates bainite during ultra-fast cooling, and the carbide distribution is optimized through carbon diffusion during the slow cooling stage, ultimately obtaining a multi-scale microstructure of "bainite + ferrite + nano carbides", enabling an impact energy value of over 120J at -40℃. Attached Figure Description

[0020] Figure 1 This is a partial plot of Mn elements.

[0021] Figure 2 This is the EBSD map of high-angle grain boundaries.

[0022] Figure 3 Image of the dimple morphology of the impact fracture surface. Detailed Implementation

[0023] The present invention will be further described below with reference to the embodiments. Example 1

[0024] In this embodiment, the DH36 continuously cast billet has a thickness of 450mm and a rolled thickness of 60mm. The specific process is as follows:

[0025] (1) Low-temperature homogenization pretreatment: 0.002 wt.% boron (B) was added, and the composition was ensured by vacuum induction furnace melting. The continuously cast billet was homogenized and annealed at 1250℃ for 2 hours (argon protective atmosphere to prevent oxidation). After annealing, the Mn segregation index C max / C min It decreased from 2.4 to 1.2, such as Figure 1 As shown.

[0026] (2) Two-stage controlled rolling: Two-stage controlled rolling is adopted, namely roughing and finishing. In the roughing stage, rolling starts at 1150℃, with a cumulative reduction rate of 58% (5 passes, single pass reduction rate of 10%~12%), a pass interval of 25 seconds, and a final rolling temperature of 1050℃. In the finishing stage, rolling starts at 945℃, with a cumulative reduction rate of 75% and a cumulative strain energy of 0.8, forming flat austenite grains.

[0027] (3) Gradient cooling: Cool at an ultra-fast cooling rate of 55℃ / s within 3 seconds after final rolling; then cool to room temperature at a cooling rate of 8℃ / s.

[0028] Final performance: In this embodiment, the metallographic microstructure consisted of ferrite (60%) + bainite (35%) + retained austenite (5%), with a grain size of 8.2 μm, uniform carbide precipitation with a size of 30~50 nm, and HAGBs accounting for 68%. The final impact energy values ​​at -40℃ were 207 J, 198 J, and 212 J. Example 2

[0029] In this embodiment, the DH36 continuously cast billet has a thickness of 450mm and a rolled thickness of 60mm. The specific process is as follows:

[0030] (1) Low-temperature homogenization pretreatment: 0.002 wt.% boron (B) was added, and the composition was ensured by vacuum induction furnace melting. The continuously cast billet was homogenized and annealed at 1250℃ for 2 hours under an argon protective atmosphere to prevent oxidation. After annealing, the Mn segregation index C max / C min It decreased from 2.4 to 1.2.

[0031] (2) Two-stage controlled rolling: Two-stage controlled rolling is adopted, namely roughing and finishing. In the roughing stage, rolling starts at 1148℃, with a cumulative reduction rate of 58% (5 passes, single pass reduction rate of 10%~12%), a pass interval of 25 seconds, and a final rolling temperature of 1042℃. In the finishing stage, rolling starts at 940℃, with a cumulative reduction rate of 83% and a cumulative strain energy of 1.0, forming flat austenite grains.

[0032] (3) Gradient cooling: Cool at an ultra-fast cooling rate of 55℃ / s within 3 seconds after final rolling; then cool to room temperature at a cooling rate of 8℃ / s.

[0033] Final performance: In this embodiment, the metallographic microstructure consisted of ferrite (55%) + bainite (40%) + retained austenite (5%), with a grain size of 7.5 μm, uniform carbide precipitation with a size of 25-40 nm, and HAGBs accounting for 72% (e.g. Figure 2 (As shown). The final impact energy values ​​at -40℃ are 252J, 223J, and 232J. Example 3

[0034] In this embodiment, the DH36 continuously cast billet has a thickness of 450mm and a rolled thickness of 80mm. The specific process is as follows:

[0035] (1) Low-temperature homogenization pretreatment: 0.002 wt.% B was added, and the composition was ensured by vacuum induction furnace melting. The continuously cast billet was homogenized and annealed at 1250℃ for 2 hours under an argon protective atmosphere to prevent oxidation. After annealing, the Mn segregation index C max / C min It decreased from 2.4 to 1.2.

[0036] (2) Two-stage controlled rolling: Two-stage controlled rolling is adopted, namely roughing and finishing. In the roughing stage, rolling starts at 1146℃, with a cumulative pass reduction rate of 55%, i.e., a total of 5 passes, a single pass reduction rate of 10%~12%, a pass interval of 25 seconds, and a final rolling temperature of 1045℃. In the finishing stage, rolling starts at 945℃, with a cumulative pass reduction rate of 62%, a cumulative strain energy of 0.7, and flat austenite grains are formed.

[0037] (3) Gradient cooling: Cooling at an ultra-fast rate of 70℃ / s within 3 seconds after final rolling; then cooling to room temperature at a rate of 5℃ / s.

[0038] Final performance: In this embodiment, the metallographic microstructure consists of 70% ferrite, 25% bainite, and 5% retained austenite, with a grain size of 9.8 μm. The carbide precipitates are uniformly distributed with a size of 35–50 nm, and HAGBs account for 65%. The final impact energy values ​​at -40°C are 143 J, 162 J, and 165 J. Example 4

[0039] In this embodiment, the DH36 continuously cast billet has a thickness of 450mm and a rolled thickness of 80mm. The specific process is as follows:

[0040] (1) Low-temperature homogenization pretreatment: 0.002 wt.% B was added, and the composition was ensured by vacuum induction furnace melting. The continuously cast billet was homogenized and annealed at 1200℃ for 3 hours (argon protective atmosphere to prevent oxidation). After annealing, the Mn segregation index Cmax / C min It decreased from 2.4 to 1.3.

[0041] (2) Two-stage controlled rolling: Two-stage controlled rolling is adopted, namely roughing and finishing. In the roughing stage, rolling starts at 1149℃, with a cumulative reduction rate of 55% (5 passes, single pass reduction rate of 10%~12%, pass interval of 24 seconds, and final rolling temperature of 1052℃); in the finishing stage, rolling starts at 941℃, with a cumulative reduction rate of 61% and a cumulative strain energy of 0.7, forming flat austenite grains.

[0042] (3) Gradient cooling: Cooling at an ultra-fast rate of 70℃ / s within 3 seconds after final rolling; then cooling to room temperature at a rate of 5℃ / s.

[0043] Final performance: In this embodiment, the metallographic microstructure consists of 68% ferrite, 27% bainite, and 5% retained austenite, with a grain size of 10.3 μm. The carbide precipitates are uniformly distributed with a size of 30–50 nm, and HAGBs account for 63%. The final impact energy values ​​at -40°C are 136 J, 142 J, and 156 J.

Claims

1. A multi-stage controlled rolling and controlled cooling process for improving the impact toughness of a DH36 steel thick plate, characterized in that Includes the following steps: (1) Low-temperature homogenization pretreatment: The continuous casting billet with 0.002wt.% B added after vacuum induction melting is homogenized and annealed at 1250℃ for 2~3 hours to reduce the Mn segregation index Cmax / Cmin to below 1.5; (2) Two-stage controlled rolling: In the roughing stage, multiple passes are rolled in the temperature range of 1050-1150℃, with a cumulative pass reduction rate ≥55%; in the finishing stage, rolling is carried out in the temperature range of 900-1000℃, with a cumulative pass reduction rate ≥60% and a cumulative strain energy >0.6, forming flat austenite grains. (3) Gradient cooling: The first stage is ultra-fast cooling, which is cooled at a rate of ≥50℃ / s within 3 seconds after final rolling; the second stage is slow cooling, which is cooled to room temperature at a rate of ≤10℃ / s.

2. The multi-stage controlled rolling and controlled cooling process for improving impact toughness of DH36 steel thick plate according to claim 1, characterized in that Step (1) Low-temperature homogenization pretreatment: The homogenization annealing is carried out under an argon protective atmosphere to prevent oxidation of the surface of the continuous casting billet.

3. The multi-stage controlled rolling and controlled cooling process for improving impact toughness of DH36 steel thick plate according to claim 1, characterized in that Step (1) Low-temperature homogenization pretreatment: The added B series is uniformly added to the molten steel through vacuum induction furnace melting.

4. The multi-stage controlled rolling and controlled cooling process for improving impact toughness of DH36 steel thick plate according to claim 1, characterized in that (3) Gradient cooling: The first stage of the gradient cooling has a cooling rate of 50~70℃ / s, and the second stage has a cooling rate of 5~10℃ / s.

5. The multi-stage controlled rolling and controlled cooling process for improving impact toughness of DH36 steel thick plate according to claim 1, characterized in that (3) Gradient cooling: The microstructure after gradient cooling is a composite phase structure of 50%~70% ferrite, 20%~45% bainite and 3%~8% retained austenite, with grain size ≤10μm, carbide precipitation size ≤50nm, and high angle grain boundaries (HAGBs) accounting for ≥60%.

6. The multi-stage controlled rolling and controlled cooling process for improving impact toughness of DH36 steel thick plate according to claim 1, characterized in that: The process described is applicable to the production of DH36 steel plates with a thickness of ≥50mm, and the final mechanical properties are impact energy ≥100J at -40℃.

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