Process method for improving strength and plasticity of EH51 steel plate

Abstract

The invention belongs to the technical field of steel and iron material production, and relates to a process method for improving the strength and plasticity of an EH51 steel plate. The method specifically comprises the following steps that an EH51 finished product steel billet is heated to 720-740 DEG C, heat preservation is conducted for 2-4 hours, then multi-pass warm rolling is conducted, the finish rolling temperature of warm rolling is not lower than 700 DEG C, the total reduction rate is not lower than 80%, then laminar cooling is conducted on the EH51 finished product steel billet till the surface temperature is 380-420 DEG C, then air cooling is conducted to the room temperature, and the ultra-fine grain ferrite steel plate is obtained. And polishing the upper and lower surfaces of the ultra-fine grain ferrite steel plate, rapidly heating the ultra-fine grain ferrite steel plate by using a laser beam, and carrying out self-cooling quenching on the surface layer to obtain the finished product steel plate. The finished steel plate is of a gradient microstructure with the surface martensite cooperating with the core ultra-fine grain ferrite, and the strength and plasticity of the finished steel plate are improved at the same time through the coupling effect of the gradient microstructure.

Description

Technical Field

[0001] This invention belongs to the field of steel material production technology and relates to an optimization method for the high plasticity of EH51 based on the coupling of deformation-induced fine grains and local phase transformation. Background Technology

[0002] High strength and ductility refer to the property of a material that maintains high strength while possessing good plasticity; that is, it can resist fracture (high strength) under stress, and undergo significant permanent deformation before fracture (high plasticity). This property is usually achieved by optimizing the material's microstructure (such as martensitic substructure, metastable phase modulation, etc.). How to improve the strength of steel materials while maintaining their plasticity has always been a key research direction in steel materials. Fine-grain strengthening is a means to achieve both strength improvement and improved plasticity and toughness. However, the generation of a large number of defects is inevitable during the ultra-fine grain process. With the significant increase in strength level, the plasticity, toughness, and work hardening ability of the material usually decrease. This "inverted" relationship between strength and plasticity / toughness limits further improvement in its performance.

[0003] Recent research indicates that introducing gradient structures into metallic materials can achieve a good balance between strength and toughness. Most current research on gradient structures focuses primarily on grain size gradients. By fabricating ultrafine / nanocrystalline structures on the surface of coarse-grained materials, a grain size gradient distribution that gradually increases from the surface to the core is formed. This gradient material retains some of the high strength and hardness of ultrafine / nanocrystalline materials while also preserving the plasticity and toughness of coarse-grained materials, achieving a good balance between strength and toughness.

[0004] Currently, the main methods for preparing materials with gradient grain size are gradient plastic deformation and gradient physical or chemical deposition.

[0005] Gradient plastic deformation involves subjecting the surface of a coarse-grained material to intense plastic deformation, resulting in a refined surface microstructure down to ultrafine or nanocrystalline layers. The ultrafine-grained surface layer lacks a clear interface with the matrix, preventing separation and detachment between the surface layer and the matrix. However, this method has limitations. Because it requires intense plastic deformation on the surface, it is only suitable for materials with good plasticity, such as Cu, IF steel, and stainless steel; brittle materials may break during processing. Furthermore, the thickness of the prepared surface nanostructure layer is only 100–300 μm, limiting its industrial application prospects.

[0006] While gradient physical or chemical deposition methods can control sample thickness and grain size gradients by manipulating the physical or chemical deposition kinetics, they involve complex equipment, are costly, and are not suitable for mass production.

[0007] Chinese patent CN105821180A discloses a method for constructing a coarse-grained to fine-grained gradient structure on a metal surface. Although it achieves a gradient structure with uniformly reduced grain size from the surface inward through laser surface heat treatment, it is limited to materials that do not undergo phase transformation during heat treatment. Obtaining fine grains in such materials requires significant plastic deformation, resulting in generally thin finished sheets; for example, the finished sheets produced by this patent are only 1-3 mm thick. This limits its application range.

[0008] Chinese patent CN118441139A discloses a high-strength, high-toughness, and wear-resistant heterogeneous steel plate and its preparation method. Although it utilizes electron beam surface heat treatment to transform the surface layer of the steel plate into a ferrite-martensite structure while the core remains a ferrite-pearlite structure, its approach involves first performing surface heat treatment. To avoid changes in the surface and core microstructure, the subsequent warm rolling deformation temperature must be relatively low, actually controlled within the range of 100~400℃, which significantly increases the mill load. Simultaneously, when the core and surface deform simultaneously, the grain refinement efficiency of the core is low, resulting in a relatively large grain size. Summary of the Invention

[0009] In order to overcome the shortcomings of the prior art, the present invention aims to provide an EH51 high plasticity optimization method based on the coupling of deformation-induced fine grains and local phase transformation. By reasonably constructing a gradient microstructure, the problem of low plasticity while improving the strength of conventional ultrafine grain steel plates can be solved.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A process for improving the strength and ductility of EH51 steel plates, wherein the chemical composition of the EH51 continuously cast billet is as follows (weight percentage): C = 0.05%~0.07%, Si = 0.13%~0.17%, Mn = 1.7%~2.2%, Ni = 0.8%~1.2%, Cr = 0.28%~0.32%, Mo = 0.01%~0.02%, Ti = 0.01%~0.02%, S ≤ 0.008%, P ≤ 0.02%, with the balance being iron and unavoidable impurities; the thickness of the finished steel plate is 5~20mm; the process includes the following steps: S1 The EH51 finished steel billet is heated to 720~740℃, held for 2~4 hours and then subjected to multiple warm rolling. The final rolling temperature of the warm rolling is not lower than 700℃ and the total reduction rate is not lower than 80%. Then it is laminar cooled to a surface temperature of 380~420℃ and then air cooled to room temperature to obtain ultrafine crystalline ferritic steel plate. S2 After polishing the upper and lower surfaces of the ultrafine crystalline ferritic steel plate, it is rapidly heated by a laser beam and then self-cooled and quenched on the surface to obtain the finished steel plate. The finished steel plate has a gradient microstructure with surface martensite and core ultrafine ferrite. The depth of the surface martensite is 280~600μm, and the average grain size of the core ultrafine ferrite is less than 1.2μm.

[0011] Furthermore, the multi-pass rolling in step S1 includes 8 to 10 passes, with the first 2 to 3 passes having a pass reduction rate of 8% to 12%, the last 2 to 3 passes having a pass reduction rate of 14% to 18%, and the intermediate passes having a pass reduction rate of 14% to 34%.

[0012] Furthermore, the average cooling rate of the laminar flow cooling described in step S1 is greater than 15°C / s.

[0013] Furthermore, the surface quenching temperature of rapid heating in step S2 is controlled to be 800~1000℃.

[0014] Furthermore, the parameters of the laser beam in step S2 are: power 2~3kW, scanning speed 15mm / s, spot size 20mm×2mm, and defocusing amount 400mm.

[0015] Furthermore, during the rapid heating described in step S2, the interval between each heating cycle is greater than 2 minutes.

[0016] Furthermore, the yield strength of the finished steel plate shall not be less than 620 MPa and the elongation shall not be less than 22%.

[0017] Compared with existing technologies, the technological advancements achieved by this invention, based on the EH51 strong plasticity optimization method coupled with deformation-induced grain refinement and local phase transformation, are as follows: 1) This invention achieves a significant improvement in both the strength and ductility of low-carbon microalloyed steel without altering the material composition. In the embodiments of this application, the mechanical properties of the sheet metal with uniform fine-grained ferrite are: yield strength 559 MPa, tensile strength 810 MPa, and elongation 18.3%. The mechanical properties of the sheet metal with a gradient structure of surface martensite and core ferrite obtained after laser surface hardening are: yield strength 656 MPa, tensile strength 869 MPa, and elongation 24.3%. The yield strength is increased by an average of approximately 17%, and the elongation is increased by an average of approximately 32%, achieving a significant improvement in both strength and ductility.

[0018] 2) The process of this invention is simple, and fine-grained ferrite plates can be obtained in one rolling process. The production cycle is short, energy is saved, and costs are reduced.

[0019] 3) Laser surface hardening is used when the final sheet material is obtained. Laser hardening technology has advantages such as low cost, high efficiency, and no pollution. Its heating layer depth and heating trajectory are easy to control and automate. Compared with other technologies, it is easier to achieve industrial application.

[0020] 4) The thickness of the plate obtained by this invention can reach 5~20mm, which belongs to the category of medium and thick plates, allowing it to have a wider range of applications. Attached Figure Description

[0021] Figure 1 This is a gradient structure diagram of the ultrafine crystalline ferritic steel plate formed after single-sided laser heat treatment in Example 1.

[0022] Figure 2 This is a diagram of the surface martensite microstructure of the gradient structure formed after the ultrafine crystalline ferritic steel plate in Example 1 was subjected to single-sided laser heat treatment.

[0023] Figure 3 This is a microstructure diagram of the core of the ultrafine ferritic steel plate and the finished steel plate in Example 1.

[0024] Figure 4 This is a gradient structure diagram of the ultrafine crystalline ferritic steel plate formed after single-sided laser heat treatment in Example 2.

[0025] Figure 5 This is a diagram of the surface martensite microstructure of the gradient structure formed after the ultrafine crystalline ferritic steel plate in Example 2 was subjected to single-sided laser heat treatment. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments. Example 1

[0027] A process for improving the strength and ductility of EH51 steel plate includes the following steps: EH51 low-carbon microalloyed steel billets were selected as raw materials. In this embodiment, the billet dimensions were 130×110×85mm, with the following chemical composition by mass percentage: C=0.07%, Si=0.14%, Mn=1.9%, Ni=1.1%, Cr=0.3%, Mo=0.013%, Ti=0.012%, S=0.0073%, P=0.02%, with the balance being iron and unavoidable impurities. Before implementing the technical solution of this application, the EH51 billet underwent conventional rough rolling, which initially improved the internal structure of the billet, eliminated some internal defects, and prepared the microstructure for subsequent warm rolling to obtain ultrafine grains. The process steps included: S1 heats the EH51 finished steel billet to 730℃. Excessive temperature will result in insufficient grain refinement, failing to produce fine-grained ferrite; excessive temperature will increase the load on the rolls. After holding at this temperature for four hours, it is placed on a φ450 two-roll reversible hot rolling mill for 10 passes of warm rolling. The specific rolling parameters (mm) are 85→78→70→60→50→40→30→20→17→14→12. To ensure good shape of the final finished steel plate, the reduction rates in the first 2-3 passes and the last 2-3 passes should not be too high, and should be controlled within the ranges of 8%~12% and 14%~18%, respectively, while maintaining the total reduction rate. After rolling, a laminar flow cooling system with an average cooling rate greater than 15℃ / s is used to cool the surface to 400℃, followed by air cooling to room temperature to obtain an ultrafine-grained ferrite steel plate. Using ultrafine-grained steel plates as the matrix can, firstly, improve the strength of steel as much as possible while maintaining its ductility and toughness through fine-grain strengthening; secondly, it can obtain the smallest possible amount of martensite after subsequent laser surface hardening. S2 involves cutting the ultrafine-grained ferritic steel plate from step S1 into templates measuring 80mm × 100mm × 10mm, and polishing the top and bottom surfaces of the templates. Laser surface hardening is then performed using a GW6000 laser with the following parameters: laser power 3kW, scanning speed 15mm / s, spot size 20×2mm, and defocusing distance 400mm. The surface temperature of the steel plate during laser scanning is approximately 950℃. Excessive laser power results in poor surface quality and is unsuitable for this application; insufficient laser power prevents the steel plate surface temperature from reaching the phase transformation point, thus failing to obtain martensite after hardening. The interval between each processing step is greater than 2 minutes to allow the steel plate to cool, preventing heat accumulation between processing passes and excessively high temperatures that could lead to recrystallization in the core. Laser surface hardening yields a low-carbon microalloyed steel plate with a gradient microstructure of surface martensite and a core of ultrafine-grained ferrite.

[0028] The martensitic-ferritic gradient low-carbon microalloyed steel plate prepared in this embodiment has a yield strength of 656 MPa, a tensile strength of 869 MPa, and an elongation of 24.3%.

[0029] The scanning microstructure of the martensitic-ferrite gradient low-carbon microalloyed steel plate prepared in this embodiment is shown in the figure. Figure 1 .from Figure 1 As can be seen from this, the surface of the finished plate in this embodiment is Figure 2 The martensitic structure shown has a thickness of approximately 450 μm, while the core remains the original ultrafine-grained ferrite structure with an average grain size of 1.2 mm. This gradient microstructure of surface martensite and core ferrite simultaneously enhances the strength and ductility of the low-carbon microalloyed steel sheet of this embodiment. Example 2

[0030] Compared with Example 1, in Example 2 the laser power was adjusted to 2 kW, while other process steps and parameters remained unchanged, and the surface temperature of the steel plate was about 800°C when the laser swept across it.

[0031] The martensitic-ferritic gradient low-carbon microalloyed steel plate prepared in this embodiment has a yield strength of 640 MPa, a tensile strength of 859 MPa, and an elongation of 23%.

[0032] The scanning microstructure of the martensitic-ferrite gradient low-carbon microalloyed steel plate prepared in this embodiment is shown in the figure. Figure 4 ,from Figure 4 As can be seen from the image, the finished plate in this embodiment also has a gradient structure of surface martensite and core ferrite. Figure 5 The surface martensite morphology is shown. The difference from Example 1 is that the surface martensite layer is slightly thinner, approximately 340 μm. However, this still simultaneously improves the strength and plasticity of the low-carbon microalloyed steel plate in this embodiment. In fact, for steel plates with a thickness of 5-20 mm, during plastic deformation, the surface martensite structure in the gradient structure of this application can synergistically work with the core ultrafine-grained ferrite structure to further optimize plasticity while improving strength. If the surface martensite is too thin, it cannot play a role in improving strength; if it is too thick, it will worsen plasticity. Therefore, in the technical solution of this application, the depth of the surface martensite is controlled within the range of 280 μm to 600 μm.

[0033] Comparative Example 1 Compared with Example 1, Comparative Example 1 did not undergo surface laser treatment, but other process parameters remained unchanged. The microstructure results showed that its core and surface were composed of relatively uniform ultrafine ferrite, with a yield strength of 559 MPa, a tensile strength of 810 MPa, and an elongation of 18.3%.

[0034] Compared with Comparative Example 1, Examples 1 and 2 show significant improvements in both strength and plasticity. Furthermore, the laser surface hardening process is flexible, easy to control, and suitable for industrial production.

Claims

1. A process for improving the strength and plasticity of EH51 steel plate, characterized in that: The chemical composition of the EH51 continuously cast billet by weight percentage is C=0.05%~0.07%, Si=0.13%~0.17%, Mn=1.7%~2.2%, Ni=0.8%~1.2%, Cr=0.28%~0.32%, Mo=0.01%~0.02%, Ti=0.01%~0.02%, S≤0.008%, P≤0.02%, with the balance being iron and unavoidable impurities; The thickness of the finished steel plate is 5~20mm; The process includes the following steps: S1 The EH51 finished steel billet is heated to 720~740℃, held for 2~4 hours and then subjected to multiple warm rolling. The final rolling temperature of the warm rolling is not lower than 700℃ and the total reduction rate is not lower than 80%. Then it is laminar cooled to a surface temperature of 380~420℃ and then air cooled to room temperature to obtain ultrafine crystalline ferritic steel plate. S2 After polishing the upper and lower surfaces of the ultrafine crystalline ferritic steel plate, it is rapidly heated by a laser beam and then self-cooled and quenched on the surface to obtain the finished steel plate. The finished steel plate has a gradient microstructure with surface martensite and core ultrafine ferrite. The depth of the surface martensite is 280~600μm, and the average grain size of the core ultrafine ferrite is less than 1.2μm.

2. The process for improving the strength and plasticity of EH51 steel plate according to claim 1, characterized in that: The multi-pass rolling in step S1 includes 8 to 10 passes, with the first 2 to 3 passes having a pass reduction rate of 8% to 12%, the last 2 to 3 passes having a pass reduction rate of 14% to 18%, and the intermediate passes having a pass reduction rate of 14% to 34%.

3. The process for improving the strength and plasticity of EH51 steel plate according to claim 1, characterized in that: The average cooling rate of the laminar flow cooling described in step S1 is greater than 15°C / s.

4. The process for improving the strength and plasticity of EH51 steel plate according to claim 1, characterized in that: The surface quenching temperature for rapid heating in step S2 is controlled to be 800~1000℃.

5. The process for improving the strength and plasticity of EH51 steel plate according to claim 1, characterized in that: The parameters of the laser beam in step S2 are: power 2~3kW, scanning speed 15mm / s, spot size 20mm×2mm, and defocusing amount 400mm.

6. The process for improving the strength and plasticity of EH51 steel plate according to claim 1, characterized in that: During the rapid heating described in step S2, the interval between each heating cycle is greater than 2 minutes.

7. The process for improving the strength and plasticity of EH51 steel plate according to claim 1, characterized in that: The yield strength of the finished steel plate shall not be less than 620 MPa and the elongation shall not be less than 22%.

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