Ultra-fine-grained high-hole-expansion hot-rolled pickled 420mpa grade low-alloy high-strength steel and preparation method thereof

Abstract

The present application relates to the technical field of steel material preparation, in particular to a kind of ultra-fine grain high hole expansion rate hot rolled pickling 420MPa grade low alloy high strength steel and its preparation method.Chemical composition contains C, Si, Mn, Nb, Ti, Cu, Ni and other elements, and meet Ti / N=3.0~5.0, (Nb+Ti) / C=0.40~0.70.The method includes hot metal pretreatment, converter smelting, LF+RH refining, continuous casting, slab heating (1140~1160℃), hot rolling, controlled cooling, coiling (580~620℃), flattening and pickling.The steel plate microstructure is ultra-fine grain ferrite and a small amount of pearlite, ferrite grain size 3.0~4.5μm, yield strength 443~512MPa, tensile strength 508~578MPa, elongation 25.5%~35.0%, hole expansion rate 81%~115%, with high strength and excellent flanging formability.

Description

Technical Field

[0001] This invention relates to the field of steel material preparation technology, specifically to a hot-rolled pickled 420MPa grade low-alloy high-strength steel with ultrafine grains and high porosity, and its preparation method. Background Technology

[0002] With increasingly stringent global energy conservation and emission reduction regulations and the growing demand for lightweighting in manufacturing, low-alloy high-strength steel has gained widespread application in automotive structural components, construction machinery, and logistics transportation due to its excellent strength, good formability, and relatively low cost. Among these, hot-rolled and pickled products, which eliminate the need for cold rolling and annealing processes, offer significant advantages such as low production costs and short delivery cycles, making them a key area for the development of high-strength steel products. For components such as commercial vehicle beams, chassis suspension parts, and wheels, 420MPa grade hot-rolled and pickled low-alloy high-strength steel, which combines high strength with excellent hole-expanding and flanging formability, has extremely broad application prospects.

[0003] To achieve a strength of 420 MPa while maintaining formability, existing technologies primarily employ low-carbon compositions with microalloying elements such as niobium and titanium, combined with controlled rolling and cooling processes. However, in practical production and applications, existing technologies still have the following shortcomings: First, existing products struggle to simultaneously meet the demands for both high strength and high porosity. Porosity expansion is a key indicator for evaluating a material's flanging and forming capabilities, crucial for preventing cracking during punching and flanging. For example, Chinese patent application number 202111571699.3 discloses a "manufacturing method for low-alloy cost QStE420TM hot-rolled pickled automotive steel sheet," which improves strength by adding C, Si, Mn, Nb, and Ti elements. However, this method involves high superheat in continuous casting, hindering the acquisition of a uniform billet microstructure. Furthermore, the high billet heating temperature results in poor microstructure uniformity in the final product, making it difficult to consistently obtain ideal fine grains, and its porosity expansion performance is typically limited. Chinese patent application number 202410343350.1 discloses a "low-cost, high-formability, 450MPa grade pickled high-strength steel and its manufacturing method," which uses Si and P composite reinforcement of ferrite and employs a low-temperature coiling process of 350–480℃ to improve flanging and porosity expansion performance. However, excessively low winding temperatures can easily lead to unstable winding performance, while high Si and P content can severely degrade the surface quality and weldability of steel plates, limiting their application in high-end structural components.

[0004] Secondly, existing technologies often sacrifice production efficiency and cost control in pursuit of performance. Chinese patent application No. 202010497258.2 discloses "a hot-rolled pickled low-alloy high-strength steel with enhanced hole-expanding performance and its production method," which adds the costly element Mo and adopts a high Al composition design (0.08%–0.2%). This not only significantly increases alloy costs but also adversely affects the smooth operation of the continuous casting process. More importantly, this method requires the steel coil to be slowly cooled in a heat-insulating pit and held at that temperature for up to 48 hours after coiling. While this can improve the hole-expanding rate to some extent, it severely sacrifices production rhythm and efficiency, failing to meet the demands of large-scale, high-efficiency industrial production.

[0005] Finally, existing technologies lack sufficient control over the purity and microstructure refinement of steel, resulting in limitations on the overall performance of the product. Most existing solutions fail to fully utilize the advantages of multiple microalloying elements (such as Ti, Nb and Cu, Ni), and also fail to achieve extreme control over impurities such as oxygen, nitrogen, and sulfur in the steel. This makes it difficult to obtain an ultrafine ferrite microstructure with a grain size of less than 5 μm, thus failing to fully exploit the material's potential in terms of strength-ductility matching and high porosity. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention provides a hot-rolled pickled 420MPa grade low alloy high-strength steel with ultrafine grain and high porosity, and its preparation method. The steel has an ultrafine grain ferrite structure and high porosity, which achieves a combination of high strength, excellent formability and high surface quality while significantly reducing energy consumption and cost.

[0007] To achieve the above objectives, the present invention employs the following technical solution: A 420MPa grade hot-rolled pickled low-alloy high-strength steel with ultrafine grains and high porosity is composed of the following chemical composition by weight percentage: C: 0.04%–0.08%, Si: 0.10%–0.30%, Mn: 1.00%–1.30%, P: ≤0.020%, S: ≤0.015%, Als: 0.020%–0.060%, Nb: 0.010%–0.030%, Ti: 0.010%–0.030%, Cu: 0.15%–0.25%, Ni: 0.05%–0.10%, RE (rare earth): ≤0.0020%, O: ≤0.0060%, N: ≤0.0060%, with the remainder being Fe and unavoidable impurities. It also satisfies the following conditions: Ti / N = 3.0~5.0, (Nb+Ti) / C = 0.40~0.70, Cu / Ni = 2.5~3.5; its microstructure consists of ferrite and pearlite, the grain size of the ferrite is 3.0~4.5μm, and the ferrite area percentage content is 90%~95%.

[0008] The effect of selecting the above alloying elements and their contents: C: As the main interstitial solid solution strengthening element, its content is controlled within the low carbon range of 0.04% to 0.08%. While providing the necessary basic strength, it maximizes the excellent weldability, low-temperature toughness, and formability of the steel, avoiding the increase of brittle phases and deterioration of formability due to excessive carbon content.

[0009] Silicon (Si): As an inexpensive solid solution strengthening element, it can effectively improve the strength of ferrite. Simultaneously, silicon is an important deoxidizer, preferentially combining with oxygen during steelmaking, helping to reduce iron oxide inclusions in steel and purify the molten steel. Its content is controlled at a low level of 0.10%–0.30%, mainly to prevent adverse effects on surface quality.

[0010] Mn: As a core alloying element, its content ranges from 1.00% to 1.30%. Manganese significantly improves the strength and hardness of steel through solid solution strengthening, while also lowering the γ→α phase transformation temperature and refining the ferrite grains after the phase transformation. In addition, manganese can combine with sulfur to form MnS, fixing the harmful element sulfur, thereby improving the hot working properties and toughness of steel.

[0011] P: Phosphorus is a harmful element that is very prone to segregation in steel, which significantly worsens the cold brittleness, weldability and secondary processing performance of steel. Strictly controlling its content at a low level of ≤0.020% is to prevent its enrichment at grain boundaries and ensure that the product has uniform and stable mechanical properties.

[0012] S: Sulfur is also a major harmful element, easily forming low-melting-point FeS, which leads to hot brittleness. Strictly limiting it to ≤0.015% is to reduce the quantity and size of sulfide inclusions, which is crucial for improving the transverse toughness, fatigue performance, and anisotropy of steel.

[0013] Als: Aluminum is a strong deoxidizer, with a content of 0.020% to 0.060%. It is mainly used for deep deoxidation during the refining process to form fine Al2O3 inclusions, promoting the purity of molten steel. In addition, acid-soluble aluminum (Als) can refine grains and fix free nitrogen in steel, which helps to improve the cold forming properties of steel.

[0014] Nb: Niobium is a grain-refining element with a content of 0.010% to 0.030%. Some Nb dissolves in austenite under low-temperature heating of slab. During rolling, these dissolved Nb will form nanoscale Nb(C,N) precipitates at defects such as dislocations through the "strain-induced precipitation" mechanism, producing a strong precipitation strengthening effect and further preventing recrystallization and refining the grains.

[0015] Ti: Titanium is a grain-refining element, with a content of 0.010% to 0.030%. At high temperatures, titanium forms extremely stable TiN particles with N. During continuous casting and low-temperature heating of slabs, these fine TiN particles can effectively pin the austenite grain boundaries and strongly inhibit the abnormal growth of austenite grains, laying the foundation for obtaining fine phase transformation structures in the future.

[0016] Ti / Nb synergy: By precisely controlling the content of Ti and N, it is ensured that after the formation of sufficient and fine TiN, a certain amount of solid-dissolved N still binds with Nb. This design avoids the excessive binding of Nb with N to form coarse compounds, ensuring the precipitation enhancement potential of Nb.

[0017] Cu: Copper is an effective substitutional solid solution strengthening element, and its content is controlled between 0.15% and 0.25%. On the one hand, its solid solution strengthening in ferrite and precipitation hardening during aging can stably improve the strength of steel; on the other hand, copper can significantly improve the atmospheric corrosion resistance of steel. Its upper limit needs to be strictly controlled to avoid excessive copper accumulation at grain boundaries during hot working, which could lead to "copper embrittlement" defects and ensure thermoplasticity and surface quality.

[0018] Ni: Nickel is an important element for toughness and process compatibility, with its content controlled between 0.05% and 0.10%. Its main role is to form a synergistic effect with copper: nickel can increase the solubility of copper in austenite, effectively suppressing the tendency for high-temperature brittleness caused by copper addition, and ensuring the smooth operation of continuous casting and hot rolling processes. At the same time, nickel itself also has the beneficial effects of mild solid solution strengthening and improving low-temperature toughness.

[0019] Rare Earth Elements (RE): Rare earth elements are extremely effective deoxidizers, desulfurizers, and elements controlling inclusion morphology. Their residual content is strictly limited to ≤0.0020%. By utilizing trace amounts of rare earth elements to combine with impurities such as oxygen and sulfur in steel, fine, dispersed, and spherical high-melting-point rare earth compounds are generated. This achieves deep purification of molten steel and complete modification of harmful inclusions, which greatly improves the steel's transverse toughness, fatigue resistance, and isotropy. Controlling its extremely low content is to ensure its beneficial effects while completely avoiding production problems such as continuous casting nozzle blockage caused by excessive addition.

[0020] Oxygen (O): Oxygen is the main impurity gaseous element in steel. Limiting its content to ≤0.0060% has two main advantages. First, extremely low oxygen content significantly reduces the number and size of brittle inclusions such as Al2O3, fundamentally improving the fatigue life, toughness, and isotropy of steel. Second, it provides a clean steelmaking environment for subsequent control of trace rare earth elements, allowing them to be more effectively used for modified residual sulfides rather than being consumed by large amounts of primary oxides. Ultra-low oxygen and trace rare earth elements are key chemical characteristics for achieving high purity steel and high toughness in products.

[0021] Nitrogen (N) is an interstitial element, and its content is limited to ≤0.0060%. This is to prevent age embrittlement caused by free nitrogen atoms and to ensure the ductility and toughness of the steel. On the other hand, it is to ensure that it combines with sufficient amounts of microalloying elements such as titanium and niobium and is completely transformed into beneficial carbonitride precipitates, thus avoiding the waste of alloying elements.

[0022] The yield strength of the ultrafine-grained, high-porosity 420MPa grade hot-rolled pickled low-alloy high-strength steel is 443–512MPa, the tensile strength is 508–578MPa, the elongation is 25.5%–35.0%, and the porosity is 81%–115%.

[0023] A method for preparing ultrafine-grained, high-porosity, 420MPa grade hot-rolled pickled low-alloy high-strength steel, the method specifically includes the following steps: 1) Hot metal pretreatment; 2) Converter smelting; 3) LF+RH refining; 4) Continuous casting; 5) Slab heating: The heating temperature is 1140~1160℃, and the holding time is 1~3h; 6) Hot rolling: including roughing and finishing rolling, wherein the initial rolling temperature of the roughing rolling is ≥1100℃ and the total reduction rate of the roughing rolling is ≥75%; the initial rolling temperature of the finishing rolling is 1000~1050℃, the final rolling temperature is 840~880℃, and the total reduction rate of the finishing rolling is ≥70%; 7) Controlled cooling: After final rolling, cool to 620-650℃ at a cooling rate of ≥25℃ / s, then air cool for 2-7s; 8) Winding: Winding temperature is 580~620℃; 9) Leveling: The leveling elongation is 0.8-1.2%, and the surface roughness of the work roll is Ra1.2-1.6μm; 10) Pickling.

[0024] Furthermore, in step 1), the KR mechanical stirring method is used to control the mass ratio of magnesium powder to lime powder to be 1:3.5 to 4.5, the final temperature of the pretreatment is ≥1300℃, and the final S content of the pretreatment is ≤0.003%.

[0025] Furthermore, in step 2), the final carbon content is 0.06% to 0.10%, the tapping temperature is 1620 to 1650°C, and the tapping process is protected by argon gas sealing throughout.

[0026] Furthermore, in step 3), the LF furnace produces high-alkalinity white slag with an alkalinity R≥8, a white slag holding time≥25min, and a soft argon blowing time≥15min; the RH furnace has a vacuum degree≤100Pa, a deep degassing time≥18min, an endpoint H content≤2ppm, an endpoint O content≤0.0040%, and an endpoint N content≤0.0040%.

[0027] Furthermore, in step 4), low superheat casting is used, with a superheat of 10-20°C.

[0028] Furthermore, in step 10), the acid concentration is 12% to 18%, the pickling temperature is 75 to 85°C, and the pickling speed is 80 to 180 m / min.

[0029] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention precisely controls the Ti / N ratio to form fine TiN particles that pin austenite grain boundaries, inhibiting abnormal grain growth. Simultaneously, it controls the (Nb+Ti) / C ratio to ensure that dissolved Nb forms nanoscale Nb(C,N) precipitates during rolling through strain-induced precipitation, further preventing recrystallization. Combined with a low-temperature slab heating process (ensuring stable TiN while partially dissolving Nb(C,N)) and high-reduction rolling, high-density dislocations and deformation bands are introduced into the austenite. After final rolling, a cooling path of "rapid cooling → short-time air cooling → medium-temperature coiling" is adopted, ultimately obtaining an ultrafine-grained ferrite microstructure with an average grain size of 3.0~4.5μm. As a result, the finished steel exhibits a yield strength of 443~512MPa, a tensile strength of 508~578MPa, an elongation of 25.5%~35.0%, and a porosity as high as 81%~115%, achieving an excellent balance between high strength and high flanging formability.

[0030] 2. This invention significantly reduces heating energy consumption and oxidation loss through a Ti-Nb-Cu-Ni composite microalloying design combined with a low-temperature slab heating process. Simultaneously, it employs a low-carbon, low-manganese composition design, avoiding the addition of expensive elements such as Mo and V, and eliminates the need for post-coiling heat preservation and slow cooling, enabling stable production on conventional hot continuous rolling lines. This greatly improves production efficiency and reduces production costs.

[0031] 3. This invention controls harmful elements P, S, O, and N to extremely low levels through KR deep desulfurization, LF high-alkalinity white slag refining, and RH vacuum deep degassing. Based on this, trace amounts of rare earth elements are added to generate fine, dispersed, and spherical high-melting-point rare earth compounds, which modify brittle Al2O3 and elongated MnS inclusions. The synergistic effect of ultra-low oxygen and trace rare earth elements significantly improves the steel's transverse toughness, fatigue resistance, and weldability, meeting the stringent requirements of high-safety structural components.

[0032] 4. The cooling path adopted in this invention has a relatively wide process window, low requirements for equipment response, no need for extreme control, and no need for subsequent slow cooling. It can stably produce high-quality products with uniform performance and consistent structure throughout the roll, making it very suitable for large-scale industrial promotion. Attached Figure Description

[0033] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention. Detailed Implementation

[0034] This invention discloses an ultrafine-grained, high-porosity hot-rolled, pickled 420MPa grade low-alloy high-strength steel and its preparation method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the same result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments. Those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0035] A method for preparing ultrafine-grained, high-porosity, hot-rolled, pickled 420MPa grade low-alloy high-strength steel includes: hot metal pretreatment → converter smelting → LF+RH refining → continuous casting → slab heating → hot rolling → controlled cooling → coiling → leveling → pickling, wherein: 1. Hot metal pretreatment: Deep desulfurization is carried out using the KR mechanical stirring method, controlling the ratio of magnesium powder to lime powder to be 1:3.5-4.5, the final temperature of the pretreatment is ≥1300℃, and the final sulfur content of the pretreatment is ≤0.003% to achieve extremely low sulfur content in the subsequent process and ensure efficient modification of rare earth elements into inclusions.

[0036] 2. Converter smelting: The final carbon content is 0.06% to 0.10%, the tapping temperature is 1620 to 1650℃, and the tapping process is protected by argon gas sealing to prevent nitrogen and oxygen from increasing.

[0037] 3. LF+RH Refining: The LF furnace produces high-basicity white slag with R≥8 to ensure deep desulfurization and strong adsorption of Al2O3 inclusions. The white slag holding time is ≥25min, and the soft argon blowing time is ≥15min to promote the full flotation and removal of inclusions. The RH furnace reduces the vacuum degree to ≤100Pa, and the deep degassing time is ≥18min. The final H content is ≤2ppm, the final O content is ≤0.0040%, and the final N content is ≤0.0040%. During this stage, the final fine-tuning of alloying elements such as Ti, Nb, Cu, and Ni is completed, ensuring that the element ratios are within the target range.

[0038] 4. Continuous casting: Low superheat casting is adopted, with a superheat of 10-20℃.

[0039] 5. Slab heating: The heating temperature is 1140~1160℃, and the holding time is 1~3h.

[0040] 6. Hot rolling: The roughing rolling start temperature is ≥1100℃, and the total reduction rate of roughing rolling is ≥75%; the finishing rolling start temperature is 1000~1050℃, the finishing rolling temperature is 840~880℃, and the total reduction rate of finishing rolling is ≥70%. High-density dislocations and deformation bands are introduced into the austenite to provide nucleation sites for strain-induced precipitation of Nb(C,N).

[0041] 7. Controlled cooling: After final rolling, the strip is rapidly cooled to 620-650℃ at a cooling rate of ≥25℃ / s, and then air-cooled to the coiling temperature for 2-7 seconds.

[0042] 8. Coiling: Coil the hot-rolled strip steel that has been air-cooled to 580-620℃, and then air-cool it to room temperature.

[0043] 9. Leveling: Unwind the cooled hot-rolled steel coil and level it. The leveling elongation is 0.8% to 1.2%, and the surface roughness of the work roll is Ra1.2 to 1.6 μm.

[0044] 10. Pickling: The leveled hot-rolled steel strip is pickled with an acid concentration of 12% to 18%, a pickling temperature of 75 to 85°C, and a pickling speed of 80 to 180 m / min.

[0045] The chemical composition of the steel in this embodiment is shown in Table 1. The weight ratios of Ti / N, (Nb+Ti) / C, and Cu / Ni of the steel in this embodiment are shown in Table 2. The hot metal pretreatment process in this embodiment is shown in Table 3. The converter smelting and LF refining process in this embodiment is shown in Table 4. The RH refining and continuous casting process in this embodiment is shown in Table 5. The slab heating and hot rolling process in this embodiment is shown in Table 6. The controlled cooling and coiling process in this embodiment is shown in Table 7. The leveling and pickling process in this embodiment is shown in Table 8. The mechanical properties of the finished steel plate in this embodiment are shown in Table 9.

[0046] Table 1. Chemical composition (wt%) of steel in the embodiments of the present invention: Table 2. Weight ratios of Ti / N, (Nb+Ti) / C, and Cu / Ni in the steel chemical composition of the embodiments of the present invention: Table 3. Iron pretreatment parameters in the embodiments of the present invention: Table 4. Converter smelting parameters and LF refining parameters of the embodiments of the present invention: Table 5. RH refining parameters and continuous casting parameters of the embodiments of the present invention: Table 6. Slab heating parameters and hot rolling parameters in the embodiments of the present invention: Table 7. Cooling and winding parameters in embodiments of the present invention: Table 8. Leveling parameters and pickling parameters of the embodiments of the present invention: Table 9. Test results of mechanical properties of finished steel plates according to embodiments of the present invention: The microstructure of the finished product in Example 1 is shown below. Figure 1 The microstructure consists of ferrite and a small amount of pearlite, with a ferrite area percentage of 93% and a ferrite grain size of 3.4 μm.

[0047] Based on the mechanical performance test results shown in Table 9 and Figure 1 As shown in the typical microstructure photographs, the 420MPa grade finished steel plates prepared in Examples 1-5 all exhibit a microstructure consisting of a fine equiaxed ferrite matrix with an average grain size of 3.0-4.5μm, and uniformly distributed pearlite with an area fraction of 5%-10%. This microstructure was obtained through a Ti-Nb-Cu-Ni composite microalloying composition design, combined with a synergistic design of low-temperature heating and hot rolling processes for the slab. The fine ferrite grains primarily originate from the suppression of austenite recrystallization and the refinement of the phase transformation process by strain-induced precipitated nano-sized Nb(C,N) particles; while the appropriate amount of pearlite is derived from the precisely controlled carbon content and cooling path. Ultimately, the synergistic effect of multiple mechanisms—fine grain strengthening, second-phase (pearlite) strengthening, and nanoprecipitation strengthening—provides high strength. Simultaneously, the highly uniform and clean ultrafine-grained ferrite matrix provides excellent plasticity and flanging formability, achieving a good balance between high strength and high formability.

[0048] Comparative Example 1: This comparative example provides a hot-rolled, pickled, low-alloy high-strength steel with the following chemical composition by mass percentage: C: 0.065%, Si: 0.25%, Mn: 1.20%, P: 0.018%, S: 0.009%, Als: 0.042%, Nb: 0.018%, Ti: 0.035%, Cu: 0% (not added), Ni: 0% (not added), RE: 0%, O: 0.0055%, N: 0.0075%, with the remainder being Fe and unavoidable impurities. Calculations show that Ti / N = 4.67 (within the scope of this invention), (Nb+Ti) / C = 0.82 (above the upper limit of 0.70 of this invention), and Cu / Ni is not applicable (no Cu or Ni added).

[0049] The production method of this comparative example is basically the same as that of Example 1, except that: the slab heating temperature is 1180℃ (higher than the 1140~1160℃ limit of this invention), the finishing rolling temperature is 830℃ (slightly lower than the lower limit of 840℃ of this invention), and the coiling temperature is 550℃ (lower than the lower limit of 580℃ of this invention).

[0050] The mechanical properties of Comparative Example 1 were tested as follows: yield strength 485 MPa, tensile strength 560 MPa, elongation 22.0%, and porosity 52%.

[0051] A comparison of Comparative Example 1 and Example 1 shows that: Comparative Example 1 did not add Cu or Ni elements, and the (Nb+Ti) / C ratio was relatively high. Simultaneously, the slab heating temperature was higher, and the coiling temperature was lower. Although the yield strength and tensile strength met the 420 MPa requirement, the lack of Cu solid solution strengthening and precipitation hardening effects, as well as the synergistic effect of Ni in suppressing hot brittleness, resulted in a significant decrease in the porosity (only 52%, far lower than the 81%–115% of the examples of this invention). Furthermore, the lower coiling temperature (550°C) caused the appearance of some bainite in the microstructure. Although the strength was slightly improved, it worsened the plasticity (elongation only 22.0%) and flanging performance. At the same time, the higher slab heating temperature increased energy consumption and oxidation loss, and the surface quality was also affected.

[0052] Comparative Example 2: This comparative example provides a hot-rolled, pickled, low-alloy high-strength steel with the following chemical composition by mass percentage: C: 0.055%, Si: 0.20%, Mn: 1.25%, P: 0.015%, S: 0.008%, Als: 0.045%, Nb: 0.025%, Ti: 0.028%, Cu: 0.21%, Ni: 0.07%, RE: 0.0012%, O: 0.0045%, N: 0.0065%, with the remainder being Fe and unavoidable impurities. Calculations show that Ti / N = 4.31, (Nb+Ti) / C = 0.96 (significantly higher than the upper limit of 0.70 in this invention), and Cu / Ni = 3.00 (within the scope of this invention).

[0053] The production method of this comparative example is basically the same as that of Example 1, except that: the slab heating temperature is 1220℃ (far higher than the 1140~1160℃ limit of this invention), the finishing rolling temperature is 890℃ (higher than the upper limit of 880℃ of this invention), and after coiling, it is slowly cooled in a heat preservation pit and kept at the heat for 48 hours.

[0054] The mechanical properties of Comparative Example 2 were tested as follows: yield strength 460 MPa, tensile strength 535 MPa, elongation 28.0%, and porosity 78%.

[0055] A comparison of Comparative Example 2 and Example 1 reveals that although Cu and Ni were added to the chemical composition of Comparative Example 2, the (Nb+Ti) / C ratio was excessively high (0.96), indicating an overabundance of microalloying elements relative to carbon content. The excessively high slab heating temperature (1220℃) resulted in almost complete solid solution of Nb(C,N), leading to coarse precipitation during subsequent rolling and weakening the fine-grain strengthening effect. Simultaneously, the excessively high final rolling temperature (890℃) caused austenite grain growth, ultimately resulting in ferrite grains coarsening to approximately 6.5 μm, significantly larger than the 3.0–4.5 μm of the embodiment of this invention. Although a slow cooling process in an insulation pit for 48 hours was used to improve the porosity, it only reached 78%, still lower than the minimum value of 81% in the embodiment of this invention. More importantly, this process severely sacrifices production efficiency (slow cooling for 48 hours leads to a longer coil turnover cycle), and the high-temperature heating increases energy consumption and cost, making it uneconomical.

[0056] As shown in Comparative Examples 1 and 2, simply adjusting a single factor in the composition or process, while deviating from the synergistic design scope of this invention (especially the ratios of Ti / N, (Nb+Ti) / C, and Cu / Ni, as well as the matching of low-temperature heating and controlled rolling / cooling processes for the slab), cannot yield ultrafine-grained hot-rolled pickled low-alloy high-strength steel with ultra-high porosity (≥81%), excellent plasticity (elongation ≥25.5%), and stable strength. This invention, through synergistic control of the entire process, achieves significantly superior overall performance compared to existing technologies while realizing low-cost manufacturing.

[0057] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing ultrafine-grained, high-porosity, 420MPa grade hot-rolled pickled low-alloy high-strength steel, characterized in that, The ultrafine-grained, high-porosity, 420MPa grade hot-rolled pickled low-alloy high-strength steel has the following chemical composition by weight percentage. composition: C: 0.04%~0.08%, Si: 0.10%~0.30%, Mn: 1.00%~1.30%, P: ≤0.020%, S: ≤0.015%, Als: 0.020%~0.060%, Nb: 0.010%~0.030%, Ti: 0.010%~0.030%, Cu: 0.15%~0.25%, Ni: 0.05%~0.10%, RE: ≤0.0020%, O: ≤0.0060%, N: ≤0.0060%, with the remainder being Fe and unavoidable impurities; And it satisfies Ti / N = 3.0~5.0, (Nb+Ti) / C = 0.40~0.70, Cu / Ni = 2.5~3.5; Its microstructure consists of ferrite and pearlite, with the ferrite grain size being 3.0–4.5 μm and the ferrite area percentage content being 90%–95%. Its yield strength is 443–512 MPa, tensile strength is 508–578 MPa, elongation is 25.5%–35.0%, and porosity is 81%–115%. The preparation method specifically includes the following steps: 1) Hot metal pretreatment; 2) Converter smelting; 3) LF+RH refining; LF furnace produces high-basicity white slag, basicity R≥8, white slag holding time≥25min, soft argon blowing time≥15min; RH furnace vacuum degree≤100Pa, deep degassing time≥18min, endpoint H content≤2ppm, endpoint O content≤0.0040%, endpoint N content≤0.0040%; 4) Continuous casting; low superheat casting is used, with a superheat of 10-20℃; 5) Slab heating: The heating temperature is 1140~1160℃, and the holding time is 1~3h; 6) Hot rolling: including roughing and finishing rolling, wherein the initial rolling temperature of the roughing rolling is ≥1100℃ and the total reduction rate of the roughing rolling is ≥75%; the initial rolling temperature of the finishing rolling is 1000~1050℃, the final rolling temperature is 840~880℃, and the total reduction rate of the finishing rolling is ≥70%; 7) Controlled cooling: After final rolling, cool to 620-650℃ at a cooling rate of ≥25℃ / s, then air cool for 2-7s; 8) Winding: Winding temperature is 580~620℃; 9) Smoothing: Smoothing elongation is 0.8% to 1.2%, and the surface roughness of the work roll is Ra1.2 to 1.6 μm; 10) Pickling: The acid concentration is 12% to 18%, the pickling temperature is 75 to 85℃, and the pickling speed is 80 to 180 m / min.

2. The method for preparing ultrafine-grained, high-porosity, 420MPa grade hot-rolled pickled low-alloy high-strength steel according to claim 1, characterized in that, In step 1), the KR mechanical stirring method is used to control the mass ratio of magnesium powder to lime powder to be 1:3.5 to 4.5, the final temperature of the pretreatment is ≥1300℃, and the final S content of the pretreatment is ≤0.003%.

3. The method for preparing ultrafine-grained, high-porosity, 420MPa grade hot-rolled pickled low-alloy high-strength steel according to claim 1, characterized in that, In step 2), the final carbon content is 0.06% to 0.10%, the tapping temperature is 1620 to 1650℃, and the tapping process is protected by argon gas sealing throughout.

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