Method for improving strength and plasticity stability of hot-dip galvanizing giga-pascal steel by microstructure design

Abstract

The present application relates to the technical field of advanced steel material, and particularly discloses a method for improving the strength and plasticity stability of hot-base galvanizing giga-pascal steel by microstructure design, smelting a steel billet; a hot rolling and cooling process: the steel billet is heated, rolled and cooled to form a steel coil; a tempering process: the steel coil is placed in a resistance furnace for heat preservation, and then cooled to room temperature with the furnace, to obtain a hot-rolled base plate; an acid pickling process: the hot-rolled base plate is acid-pickled to remove the iron oxide scale on the surface of the hot-rolled base plate; a galvanizing process: the acid-pickled plate is heated and cooled, and then placed in a zinc pot for galvanizing, and then finished to obtain the hot-base galvanizing giga-pascal steel; the present application can improve plasticity, effectively relax local stress concentration in the deformation process, delay the occurrence of necking, and continuously improve the plasticity; and the present application can also coordinate the deformation, effectively absorb the stress at the tip of micro-crack in the plastic deformation or hole-expanding deformation process, greatly hinder the continuous expansion of cracks, and further improve the plasticity and hole-expanding performance.

Description

Technical Field

[0001] This invention relates to the field of advanced steel materials technology, specifically to a method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel through microstructure design. Background Technology

[0002] Compared to cold-dip galvanized steel sheets, hot-dip galvanized high-strength steel eliminates the cold rolling process, shortening the production flow and offering a significant cost advantage, thus demonstrating strong market competitiveness. In recent years, hot-dip galvanizing technology has been continuously optimized and upgraded, resulting in significant improvements in surface quality and sheet shape. With increasing market demand and continuous technological advancements, hot-dip galvanizing, leveraging its cost advantage, will see wider application and development in the future.

[0003] For a long time, research on hot-dip galvanized high-strength steel in this technical field has mainly focused on improving surface quality. However, with the stabilization of its production process and the continuous improvement of surface quality, hot-dip galvanized steel has gradually gained favor among automakers and is widely used in key parts such as automotive structural components, chassis, and wheels. The strength level has also expanded from the typical CQ level to the current 1000MPa grade high-strength steel.

[0004] In the automotive manufacturing industry, to achieve lightweight vehicles, the strength of parts is often pushed to the limit, leaving little safety margin. Therefore, the performance stability of ultra-high-strength steel at the gigapascal level and above is the bridge for automakers to transform the "performance advantages" of materials into "macroeconomic benefits and safety guarantees" in production. It determines whether automakers and component suppliers can achieve large-scale, low-cost, and highly safe production of lightweight structural components, crash barriers, and safety components.

[0005] However, currently available technologies have not addressed the issue of improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel. Therefore, a method is needed to design, through microstructural design, to improve the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel, thus solving the technical problem of improving the stability of these properties in existing technologies. Summary of the Invention

[0006] To address the problems existing in the prior art, the purpose of this invention is to provide a method for improving the stability of the strength and plasticity of hot-dip galvanized gigapasal steel through microstructure design.

[0007] The technical solution adopted by this invention to solve its technical problem is: a method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel through microstructure design, comprising the following steps:

[0008] S1. Smelting steel billets: The chemical composition of the steel billets by mass percentage is as follows: C: 0.15%-0.17%, Si: 0.45%-0.55%, Mn: 2.4%-2.6%, Alt: 0.65%-0.85%, Cr: 0.35%-0.45%, Mo: 0.25%-0.35%, Nb: 0.045%-0.055%, and satisfies 1.20%≤Si+Alt≤1.35%, 1.25<Cr / Mo<1.60, while controlling impurity elements P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.002%, H≤0.0003%, with the balance being Fe and unavoidable impurities;

[0009] S2. Hot rolling and cooling process: The steel billet is heated, rolled and cooled to form a steel coil;

[0010] S3, Tempering process: The steel coil is placed in a resistance furnace for heat preservation, and then cooled to room temperature with the furnace to obtain a hot-rolled substrate;

[0011] S4. Pickling process: Pickling the hot-rolled substrate to remove the iron oxide scale on the surface of the hot-rolled substrate.

[0012] S5. Galvanizing process: After the pickled plate is heated and cooled, it is placed in a zinc pot for galvanizing, and then finished to obtain hot-dip galvanized GMP steel.

[0013] Specifically, in step S1, the billet thickness is 230mm and the billet length is 7~9m.

[0014] Specifically, the billet heating process in step S2 involves heating the billet to a uniform heating temperature of 1230-1260°C, and after the core temperature of the billet reaches the uniform heating temperature, holding it at that temperature for 30-45 minutes, with the total furnace time controlled at 220-300 minutes.

[0015] Specifically, in step S2, the rolling process is as follows: the roughing exit temperature is 1070~1120℃, the intermediate billet thickness is 28~34mm, and the total reduction rate of roughing is 85% < 88%; the finishing temperature is controlled at 1030~1090℃, the finishing temperature is 900~930℃, the total reduction rate of finishing is 91% < 95%, and the thickness of the hot-rolled steel coil is 1.5~3.0mm.

[0016] Specifically, the cooling process in step S2 is a cooling mode of "stage I pre-ultra-fast cooling + stage II sparse cooling" after finishing rolling. The cooling rate of stage I pre-ultra-fast cooling is >80℃ / s, and the final cooling temperature is 480~520℃. In stage II sparse cooling, one group of manifolds is opened in every 3-4 groups of manifolds, and the cooling rate is controlled at 15~20℃ / s. The strip is cooled to 380~420℃ before being coiled.

[0017] Specifically, in step S3, the steel coil is kept at a temperature of 390~410℃ in the resistance furnace for a holding time of 60~90 minutes.

[0018] Specifically, in step S4, the elongation of the scale-breaking machine during the pickling process is set to 1.3±0.1%, and the leveling rolling force is 2600~2800KN.

[0019] Specifically, in step S5, the pickled plate is heated at a heating rate of 15±5℃ / s to a preheating temperature of 240~270℃; then heated at a heating rate of 5~7℃ / s to a pre-oxidation temperature of 640~660℃, and held for 18~24s for pre-oxidation; then heated at a heating rate of 1.5~2.5℃ / s to a homogenization temperature of 670~700℃ and held for 80~110s; then rapidly cooled at a cooling rate of 30±5℃ / s to a zinc pot temperature of 450~465℃ for galvanizing; after exiting the zinc pot, it is cooled to room temperature; then it is finished, with a finishing elongation rate set to 0.8~1.1%, thus obtaining hot-dip galvanized gigapasal steel.

[0020] Specifically, the hot-dip galvanized gigapasal steel is composed of 70%~80% granular bainite and 20%~30% grain boundary ferrite, wherein the average grain size of the grain boundary ferrite is 0.3~1.5μm and the average grain size of the granular bainite is 3~5μm; the proportion of large-angle grain boundaries is >90%.

[0021] Specifically, the microstructure design of the hot-dip galvanized gigapasal steel results in extremely high performance stability, with a yield strength of 1030±35MPa, tensile strength of 1265±40MPa, yield ratio of 0.81±0.03, elongation after fracture >19%, strength-ductility product >25GPa·%, and expansion rate >45%.

[0022] The present invention has the following beneficial effects:

[0023] This invention relates to a method for improving the strength, ductility, and stability of hot-dip galvanized gigapasal steel through microstructural design. The resulting hot-dip galvanized gigapasal steel exhibits a uniform distribution of grain boundary ferrite at the grain boundaries of granular bainite grains, with a high-angle grain boundary ratio exceeding 90%. The volume fraction of grain boundary ferrite is 20%–30%, with an average grain size of 0.3–1.5 μm; the volume fraction of granular bainite is 70%–80%, with an average grain size of 3–5 μm. The significant effects of this microstructural design on improving the strength, ductility, and performance stability of hot-dip galvanized gigapasal steel include:

[0024] (1) The granular bainitic structure achieves a good balance between strength and plasticity. Its typical microstructure is: ferrite matrix + island-like / short rod-like second phase M / A. The bainitic ferrite matrix not only has high strength but also provides good plastic deformation capacity; while the second phase M / A structure is a C-rich hard phase structure, which significantly improves the strength of the material. This microstructure design ensures the stability of the strength and plasticity of hot-dip galvanized GPa steel, with a yield strength of 1030±35MPa, tensile strength of 1265±40MPa, yield ratio of 0.81±0.03, elongation after fracture >19%, and strength-plasticity product >25GPa·.

[0025] (2) Granular bainite structure significantly improves hole expansion performance. During hole expansion deformation, when the edge of the hole is subjected to complex tensile stress, the second phase M / A structure effectively pins dislocations and inhibits microcrack propagation, thereby avoiding local stress concentration and changing the microcrack propagation path, giving it excellent hole expansion performance with a hole expansion rate of >45%.

[0026] (3) A high proportion of large-angle grain boundaries (>90%) is key to excellent performance matching. Large-angle grain boundaries are defined as interfaces with an orientation difference of >15° between adjacent grains. When a microcrack encounters a large-angle grain boundary during propagation, the crack propagation direction needs to change (i.e., "stress reorientation") due to the significant difference in crystallographic orientation between the grains on both sides. This greatly increases the crack propagation path and the energy required. Simultaneously, large-angle grain boundaries can blunt the sharp crack tip, thereby significantly improving the material's plasticity and resistance to edge cracking, resulting in excellent performance matching and stability.

[0027] (4) The second-phase M / A microstructure in granular bainite simultaneously improves the stability of strength, plasticity, and porosity. Retained austenite in the M / A microstructure plays a key role in improving performance stability. Under certain strain, these metastable retained austenite can undergo the TRIP effect and transform into martensite, which not only further improves the material strength but also relaxes local stress and delays necking, thereby improving the stability of strength, plasticity, and porosity.

[0028] (5) Beneficial effects of grain boundary ferrite. First, it enhances plasticity. During deformation, it can effectively relax local stress concentration, delay the occurrence of necking, and achieve continuous improvement in plasticity. Second, it coordinates deformation. During plastic deformation or pore-expanding deformation, it effectively absorbs the stress at the tip of microcracks, thereby greatly hindering the continued propagation of cracks and achieving further improvement in plasticity and pore-expanding performance. Attached Figure Description

[0029] Figure 1 This is a typical microstructure image of the hot-dip galvanized gigapasal steel prepared in Example 1.

[0030] Figure 2This is a SEM image of the hot-dip galvanized gigap steel prepared in Example 1 under a scanning electron microscope. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0032] Examples 1-10 of this invention use the following method to prepare hot-dip galvanized gigapasal steel:

[0033] Step 1: Smelting steel billets.

[0034] Smelt steel billets according to the following requirements: billet thickness is 230mm, and billet length is 7~9m.

[0035] The chemical composition of the steel billet used in Examples 1-10 of this invention is as follows: C: 0.15%-0.17%, Si: 0.45%-0.55%, Mn: 2.4%-2.6%, Alt: 0.65%-0.85%, Cr: 0.35%-0.45%, Mo: 0.25%-0.35%, Nb: 0.045%-0.055%, and satisfies 1.20%≤Si+Alt≤1.35%, 1.25<Cr / Mo<1.60; at the same time, the impurity elements are controlled to P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.002%, H≤0.0003%, with the balance being Fe and unavoidable impurities.

[0036] The chemical composition of the steel billets in specific embodiments 1 to 10 is shown in Table 1.

[0037] Table 1. Chemical composition and corresponding mass percentage of steel billets in Examples 1-10

[0038]

[0039] Step 2: Hot rolling and cooling process.

[0040] (1) Heating: Heat the above steel billet to a uniform heating temperature of 1230-1260℃. After the core temperature of the steel billet rises to the uniform heating temperature, keep it at the temperature for 30-45 minutes, and control the total time in the furnace to 220-300 minutes.

[0041] (2) Rolling: The exit temperature of the roughing mill is 1070~1120℃, the thickness of the intermediate billet is 28~34mm, and the total reduction rate of the roughing mill is 85% < 88%; the temperature of the finishing mill is controlled at 1030~1090℃, the final rolling temperature of the finishing mill is 900~930℃, and the total reduction rate of the finishing mill is 91% < 95%; the thickness of the hot-rolled steel coil is 1.5~3.0mm.

[0042] (3) Cooling: After finishing rolling, a cooling mode of "stage I pre-ultra-fast cooling + stage II sparse cooling" is adopted. The cooling rate of stage I pre-ultra-fast cooling is >80℃ / s, and the final cooling temperature is 480~520℃; for stage II sparse cooling, one group of manifolds is opened for every 3-4 groups of manifolds, and the cooling rate is controlled at about 15~20℃ / s. The strip is cooled to 380~420℃ before being coiled.

[0043] Step 3: Tempering process.

[0044] The steel coils were placed in an electric resistance furnace and held at 390-410°C for 60-90 minutes. They were then cooled to room temperature in the furnace to obtain the hot-rolled substrate. Specific process parameters for hot rolling, cooling, and tempering are shown in Table 2.

[0045] Table 2 Specific process parameters for hot rolling, cooling, and tempering control in Examples 1-10

[0046]

[0047] Step 4: Pickling process.

[0048] The hot-rolled substrate was pickled to remove the iron oxide scale from its surface. During the pickling process, the elongation of the scale remover was set to 1.3 ± 0.1%, and the leveling rolling force was 2600~2800 KN.

[0049] Step 5: Galvanizing process.

[0050] The pickled plate is heated to a preheating temperature of 240-270℃ at a heating rate of 15±5℃ / s; then heated to a pre-oxidation temperature of 640-660℃ at a heating rate of 5-7℃ / s, and held for 18-24s for pre-oxidation; then heated to a homogenization temperature of 670-700℃ at a heating rate of 1.5-2.5℃ / s and held for 80-110s; subsequently cooled rapidly to a zinc bath temperature of 450-465℃ at a cooling rate of 30±5℃ / s for galvanizing; after removing from the zinc bath, cooled to room temperature; then finished, with a finishing elongation rate set to 0.8-1.1%, thus obtaining the hot-dip galvanized gigapasal steel of the present invention. Specific process parameters for galvanizing process control are shown in Table 3.

[0051] Table 3 Specific process parameters for galvanizing process control in Examples 1-10

[0052]

[0053] The sample preparation and mechanical property testing methods at room temperature were carried out in accordance with the national standard GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test method at room temperature". The mechanical property test results of Examples 1-10 are shown in Table 4.

[0054] Analysis of the mechanical property test results shows that the hot-dip galvanized GPa steel prepared in Examples 1 to 10 of this invention has a yield strength of 1030±35MPa, a tensile strength of 1265±40MPa, a yield ratio of 0.81±0.03, an elongation after fracture of >19%, a strength-ductility product of >25GPa·%, and a hole expansion rate of >45%.

[0055] The microstructure and morphology of hot-dip galvanized gigapasal steel were observed using an Axio Scope A1 optical microscope (OM) and a Gemini SEM 300 scanning electron microscope (SEM), such as... Figures 1-2 As shown; simultaneously, five scanned images were selected, and the average grain size was calculated using the intercept method. According to the national standard GB / T15749-2008 "Quantitative Metallographic Measurement Methods", the bainite content (volume fraction) in the microstructure was statistically analyzed using the built-in analysis software of an Axio Scope A1 optical microscope (OM); and the distribution of large and small angle grain boundaries was statistically analyzed using electron backscatter diffraction (EBSD) experiments. The microscopic analysis results show that the volume fraction of grain boundary ferrite is 20%~30%, with an average grain size of 0.3~1.5 μm; the volume fraction of granular bainite is 70%~80%, with an average grain size of 3~5 μm; and the proportion of large angle grain boundaries is >90%. The microstructure analysis results of Examples 1-10 are shown in Table 4.

[0056] Table 4. Results of mechanical property testing and microstructure analysis of Examples 1-10

[0057]

[0058] This invention is not limited to the above-described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

[0059] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel through microstructure design, characterized in that, Includes the following steps: S1. Smelting steel billets: The chemical composition of the steel billets by mass percentage is as follows: C: 0.15%-0.17%, Si: 0.45%-0.55%, Mn: 2.4%-2.6%, Alt: 0.65%-0.85%, Cr: 0.35%-0.45%, Mo: 0.25%-0.35%, Nb: 0.045%-0.055%, and satisfies 1.20%≤Si+Alt≤1.35%, 1.25<Cr / Mo<1.60, while controlling impurity elements P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.002%, H≤0.0003%, with the balance being Fe and unavoidable impurities; S2. Hot rolling and cooling process: The steel billet is heated, rolled and cooled to form a steel coil; S3, Tempering process: The steel coil is placed in a resistance furnace for heat preservation, and then cooled to room temperature with the furnace to obtain a hot-rolled substrate; S4. Pickling process: Pickling the hot-rolled substrate to remove the iron oxide scale on the surface of the hot-rolled substrate. S5. Galvanizing process: After the pickled plate is heated and cooled, it is placed in a zinc pot for galvanizing, and then finished to obtain hot-dip galvanized GMP steel.

2. The method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel according to claim 1, characterized in that, In step S1, the billet thickness is 230mm and the billet length is 7~9m.

3. The method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel according to claim 1, characterized in that, The billet heating process in step S2 involves heating the billet to a uniform heating temperature of 1230-1260°C, and holding it at the core temperature for 30-45 minutes after it reaches the uniform heating temperature. The total time spent in the furnace is controlled between 220 and 300 minutes.

4. The method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel according to claim 1, characterized in that, The rolling process in step S2 is as follows: the roughing exit temperature is 1070~1120℃, the intermediate billet thickness is 28~34mm, and the total reduction rate of roughing is 85% < 88%; the temperature of entering the finishing mill is controlled at 1030~1090℃, the finishing mill final rolling temperature is 900~930℃, and the total reduction rate of finishing is 91% < 95%; the thickness of the hot-rolled steel coil is 1.5~3.0mm.

5. The method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel according to claim 1, characterized in that, The cooling process in step S2 is a "stage I pre-ultra-fast cooling + stage II sparse cooling" cooling mode after finishing rolling. The cooling rate of stage I pre-ultra-fast cooling is >80℃ / s, and the final cooling temperature is 480~520℃. In stage II sparse cooling, one group of manifolds is opened in every 3-4 groups of manifolds, and the cooling rate is controlled at 15~20℃ / s. The strip is cooled to 380~420℃ before being coiled.

6. The method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel according to claim 1, characterized in that, In step S3, the steel coil is held at a temperature of 390~410℃ in the resistance furnace for a holding time of 60~90min.

7. The method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel according to claim 1, characterized in that, In step S4, the elongation of the scale-breaking machine during the pickling process is set to 1.3±0.1%, and the leveling rolling force is 2600~2800KN.

8. The method for improving the stability of the strength and ductility properties of hot-dip galvanized gigapasal steel according to claim 1, characterized in that, In step S5, the pickled plate is heated at a rate of 15±5℃ / s to a preheating temperature of 240~270℃; then heated at a rate of 5~7℃ / s to a pre-oxidation temperature of 640~660℃, and held for 18~24s for pre-oxidation; then heated at a rate of 1.5~2.5℃ / s to a homogenization temperature of 670~700℃ and held for 80~110s; then rapidly cooled at a rate of 30±5℃ / s to a zinc pot temperature of 450~465℃ for galvanizing; after removing from the zinc pot, cooled to room temperature; then finished, with a finishing elongation rate set at 0.8~1.1%, thus obtaining hot-dip galvanized gigapasal steel.

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