High-yield-ratio ferrite bainite steel with high strength and plasticity and preparation method of high-yield-ratio ferrite bainite steel

Abstract

The invention relates to the technical field of metal material processing, in particular to high-yield-ratio ferrite bainite steel with high strength and plasticity and a preparation method thereof.The high-yield-ratio ferrite bainite steel is prepared from, by mass, 0.18%-0.22% of C, 1.2%-1.5% of Si, 2.4%-2.8% of Mn, 0.04%-0.07% of Nb, 0.3%-0.6% of Cr, 0.8%-1.2% of Alt, smaller than or equal to 0.005% of P, smaller than or equal to 0.003% of S, smaller than or equal to 0.0035% of N, smaller than or equal to 0.005% of O and the balance Fe and inevitable impurities. By regulating and controlling the chemical components and proportion of the steel, the yield ratio of the steel is increased, and the strength and plasticity matching is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metal material processing, and in particular to a high-yield ratio ferrite bainite steel with high strength and plasticity and a preparation method thereof. Background Art

[0002] Compared with traditional fuel vehicles, new energy electric vehicles pay more attention to cruising range, safety and service life. Existing technologies usually extend the cruising range of new energy electric vehicles by increasing the battery capacity of the battery pack. Due to the limited energy density of the battery, while increasing the battery capacity, the weight of the battery pack will also increase, resulting in the weight of new energy electric vehicles being greater than that of fuel vehicles of the same level. Taking a compact SUV as an example, the weight of a fuel vehicle is about 1.4 tons, while the weight of a new energy electric vehicle exceeds 1.7 tons. Among them, the weight of the best-selling new energy electric vehicles on the market is mainly concentrated in 2.2-2.4 tons, which is 300-500kg heavier than the fuel vehicles of the same level. An overweight body will not only shorten the service life of new energy electric vehicles, but also increase fuel consumption, affect handling performance and induce safety hazards. New energy electric vehicles are more urgently in need of lightweighting than fuel vehicles.

[0003] The main ways to achieve lightweight automobiles include lightweight structure, lightweight materials and lightweight manufacturing. Lightweight materials are the most direct way to achieve automobile weight reduction. Compared with aluminum alloys, carbon fiber and other materials, high-strength steel has the characteristics of high strength, simple manufacturing process, high cost performance and good machinability. It also has the characteristics of high safety and low carbon and environmental protection. It is an important material for manufacturing new energy electric vehicles. Dual-phase steel (DP steel for short) refers to steel composed of two-phase structures of martensite, austenite or bainite and ferrite matrix. Among them, ferrite-bainite dual-phase steel not only has good stamping formability, but also has excellent ductility and hole expansion and bending performance. At present, most high-strength steels with tensile strength above 1280MPa are low yield ratio steels, and the effective utilization rate of steel is low, which is not conducive to achieving lightweight automobiles. Summary of the invention

[0004] In view of the technical problems that high-strength steels above 1280MPa have a low yield ratio and a low effective utilization rate of steel, which is not conducive to achieving lightweight automobiles, the present invention provides a high yield ratio ferritic bainite steel with high strength and plasticity and a preparation method thereof. By regulating the chemical composition and proportion of the steel and a reasonable process, the yield ratio of the steel is improved and the strength-plasticity matching is improved.

[0005] In a first aspect, the present invention provides a high yield ratio ferrite bainite steel with high strength and plasticity. The high yield ratio ferrite bainite steel comprises the following chemical components by mass percentage: C 0.18%-0.22%, Si 1.2%-1.5%, Mn 2.4%-2.8%, Nb 0.04%-0.07%, Cr 0.3%-0.6%, Alt 0.8%-1.2%, P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities; The high yield ratio ferritic bainitic steel has a yield ratio of 0.8-0.9, a tensile strength of >1280MPa, an elongation after fracture of >20%, a hole expansion rate of >30%, and a strength-ductility product of >26.5GPa·%.

[0006] Furthermore, in terms of mass percentage, the chemical composition of high yield ratio ferritic bainite steel satisfies the following relationship: 2.2%≤Alt+Si≤2.5%. Aluminum can refine the grains and improve the ability of steel to absorb energy during plastic deformation. Silicon can improve the strength of steel. When the silicon content is low, it can improve the strength of steel without significantly affecting the plasticity and toughness of steel. When the silicon content exceeds 0.8wt%-1.0wt%, while improving the strength of steel, the plasticity of steel will decrease, especially the impact toughness will be significantly reduced. The present invention limits the content of silicon to a lower level by limiting the content of aluminum to 2.2%≤Alt+Si≤2.5%, thereby improving the strength of steel while improving the plasticity of steel.

[0007] Furthermore, in terms of mass percentage, the chemical composition of high yield ratio ferritic bainitic steel satisfies the following relationship: 2.9%≤Mn+Cr≤3.2%. Manganese and iron can form a solid solution to improve the hardness and strength of ferrite in steel. Chromium can significantly improve the strength, hardness and wear resistance of steel, but at the same time it will reduce the plasticity and toughness of steel. The present invention limits the sum of the mass percentages of manganese and chromium elements to 2.9%≤Mn+Cr≤3.2%, thereby achieving control over the bainite reaction and improving the strength of the steel.

[0008] Furthermore, the microstructure of the high yield ratio ferrite bainite steel includes grain boundary ferrite and bainite, the volume percentage of grain boundary ferrite is 15%-25%, the volume percentage of bainite is 75%-85%, the grain size of grain boundary ferrite is less than 2μm, and the average reference unit size of bainite is less than 5.5μm. The volume percentage of grain boundary ferrite and the volume percentage of bainite are regulated in the present invention, which is conducive to promoting the steel to achieve the best strength-ductility balance; by controlling the grain size of grain boundary ferrite to be less than 2μm and the average reference unit size of bainite to be less than 5.5μm, the yield ratio of the steel is improved.

[0009] In a second aspect, the present invention further provides a method for preparing a high yield ratio ferrite bainite steel, comprising the following steps: Step 1: Forging, heating the steel billet for the first time and keeping it warm, and then forging it to obtain a forged billet, wherein the start forging temperature is ≥1160°C and the final forging temperature is ≥950°C; Step 2: Heat treatment, heating the forging blank to the austenitizing heat treatment temperature and keeping it warm; Step 3: rolling, the starting rolling temperature is 1160-1190°C, and the final rolling temperature is 900-930°C; Step 4: Cooling and coiling, using ultra-fast cooling mode, cooling to 320-360°C at a cooling rate of 75-85°C / s, and then coiling, the coiling temperature is ≥320°C, and the hot-rolled steel coil is obtained after coiling; Step 5: leveling and pickling. After the hot-rolled steel coil is cooled to room temperature, it is leveled, and the leveling elongation is 1.6%-2.0%. After leveling, the hot-rolled steel coil is pickled with tension to obtain a pickled steel coil. Step six: hood annealing. The hood annealing temperature is 340°C-380°C and the hood annealing time is 6-10 hours.

[0010] Furthermore, in step 1, the target temperature of the initial heating is 1210°C ± 20°C, and the insulation time is 60-90 minutes.

[0011] Furthermore, in step 2, the austenitizing soaking temperature is 1220°C ± 20°C, and the holding time is 30-40 minutes. Heating the forging blank to the austenitizing soaking temperature and holding the temperature is beneficial to promoting the homogenization of austenite in the microstructure.

[0012] Furthermore, in step 4, the thickness of the hot-rolled steel coil is 2.0-6.0 mm. The hot-rolled steel coil prepared by the present invention is a thinned hot-rolled steel coil, which is beneficial to reducing energy consumption and lightweight production.

[0013] Furthermore, in step five, the cooling method is natural slow cooling with air, and the pickling liquid used for tension pickling is hydrochloric acid.

[0014] Furthermore, in step six, the hood annealing is single stack annealing.

[0015] The beneficial effects of the present invention are: The present invention provides a high-yield ratio ferrite bainite steel with high strength and plasticity and a preparation method thereof. The microstructure obtained by adjusting the ratio of elements such as C, Si, Mn, Nb, Cr, Al, P, S, N and O in the steel and preparing the microstructure through a reasonable process includes 15%-25% grain boundary ferrite and 75%-85% bainite, wherein the grain size of the grain boundary ferrite is less than 2μm, and the average reference unit size of the bainite is less than 5.5μm. Different from the micropore growth aggregation fracture of the ferrite-martensite dual-phase steel, the high-yield ratio ferrite bainite steel with high strength and plasticity prepared by the present invention, during the stretching and hole expansion deformation process, due to the disordered distribution of island phase structures of different sizes in the bainite crystal, the subgrain boundaries of these island phase structures can effectively inhibit the expansion of microcracks. If the crack is to continue to expand, it needs to deflect in direction, which significantly increases the crack expansion path. 15%-25% grain boundary ferrite can effectively release and relax local stress concentration and weaken or passivate the local stress concentration borne by the microcrack tip. Macroscopically, it presents excellent matching strength and plasticity, while improving the tensile strength of the dual-phase steel, and improving the elongation after fracture, hole expansion rate and strength-plasticity product of the dual-phase steel. The dual-phase steel provided by the present invention has a high yield strength ratio, high effective utilization rate of steel, and is conducive to achieving lightweight automobiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 It is a typical engineering stress-engineering strain curve of the yield ratio ferrite bainite steel in Example 1.

[0018] Figure 2 3 is the microstructure diagram of the high yield ratio ferrite bainite steel in Example 1. DETAILED DESCRIPTION

[0019] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0020] Embodiment 1-6 provides a method for preparing a high yield ratio ferrite bainite steel, comprising the following steps: Step 1: Forging, heat the steel billet to 1210℃±20℃ and keep it warm for 60-90min, then forge, control the start forging temperature ≥1160℃, the final forging temperature ≥950℃, and obtain the forging billet.

[0021] Step 2: Heat treatment, heat the forging blank to the austenitizing heat treatment temperature and keep it warm. The austenitizing heat treatment temperature is 1220℃±20℃ and the holding time is 30-40min.

[0022] Step 3: rolling, the starting rolling temperature is 1160-1190°C, and the final rolling temperature is 900-930°C; Step 4: Cooling and coiling, using ultra-fast cooling mode, cooling to 320-360℃ at a cooling rate of 75-85℃ / s and then coiling, the coiling temperature is ≥320℃, and after coiling, a hot-rolled steel coil with a thickness of 2.0-6.0mm is obtained.

[0023] Step 5: leveling and pickling. The hot-rolled steel coil is leveled after being naturally cooled to room temperature by air, and the leveling elongation is 1.6%-2.0%. After leveling, the hot-rolled steel coil is tension pickled, and the pickling liquid used for tension pickling is hydrochloric acid to obtain the pickled steel coil.

[0024] Step six: hood annealing, placing the pickled steel coil in a hood annealing furnace for hood annealing, the hood annealing temperature is 340°C-380°C, the hood annealing time is 6-10h, the hood annealing is single stack annealing, after the hood annealing is completed, a high yield ratio ferrite bainite steel plate is obtained.

[0025] In Examples 1-6, the chemical composition and mass percentage of the steel billet are shown in Table 1, and the specific process parameters of steps 1 to 6 are shown in Table 2.

[0026] Table 1 Chemical composition and mass percentage of the steel billets in Examples 1-6

[0027] Table 2 Specific process parameters of Examples 1-6

[0028] Samples were taken from the steel plate after bell annealing, and A was prepared according to GB / T 228.1-2021 "Tensile test of metallic materials - Part 1: Test method at room temperature" 50 The tensile test specimens were tested on a Zwick / Roell Z100 tensile testing machine for yield strength, tensile strength and elongation after fracture (A 50) detection, and calculate the yield strength ratio and strength-ductility product; according to the standard GB / T 15825.4-2008 "Metallic Sheet Formability and Test Method Part 4: Hole Expansion Test", a (150 × 150) mm sample was prepared, and a hole was made by drilling, and then a hole expansion test was carried out to detect the hole expansion rate. The microstructure of the high yield ratio ferrite bainite steel plate prepared in Example 1-6 was observed under a scanning electron microscope (Gemini SEM 300, Scanning electron microscope, SEM), and the image was statistically analyzed and analyzed using Image J software to obtain the volume fraction of the ferrite structure and bainite structure. The results of the mechanical properties test and the microstructure test of the high yield ratio ferrite bainite steel plate prepared in Example 1-6 are shown in Table 3.

[0029] Table 3 Mechanical properties test results and volume percentage of each phase structure of high yield ratio ferrite bainite steel of Examples 1-6

[0030] As shown in Table 3, the high yield ratio ferrite bainite steel prepared in Example 1-6 has a yield ratio of 0.8-0.9, which has the characteristics of high yield ratio; the high yield ratio ferrite bainite steel prepared in Example 1-6 has a tensile strength of >1280MPa, an elongation after fracture of >20%, a hole expansion rate of >30%, and a strength-plasticity product of >26.5GPa·%, and has excellent strength-plasticity matching, and is particularly suitable for the manufacture of ultra-high strength automotive parts with high formability, hole expansion and anti-collision energy absorption requirements. The microstructure of the high yield ratio ferrite bainite steel prepared in Example 1-6 is grain boundary ferrite and bainite, wherein the volume percentage of grain boundary ferrite is 15%-25%, the volume percentage of bainite is 75%-85%, the grain size of grain boundary ferrite is <2μm, and the average reference unit size of bainite is <5.5μm.

[0031] The typical engineering stress-engineering strain curve of the high yield ratio ferrite bainite steel prepared in Example 1 is as follows: Figure 1 As shown. Figure 1 It can be seen that the tensile process shows the characteristics of continuous yielding, without obvious yield point extension. Its tensile strength reaches 1307MPa, the yield strength ratio is 0.832, and the elongation after fracture reaches 25.6%. It has excellent strength-plasticity matching, and the strength-plasticity product reaches 28.23GPa·%.

[0032] Although the present invention has been described in detail with reference to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art may easily think of changes or substitutions within the technical scope disclosed by the present invention, and these shall be within the scope of protection of the present invention.

Claims

1. A high yield ratio ferrite bainite steel with high strength and plasticity, characterized in that: In terms of mass percentage, the high yield ratio ferritic bainite steel includes the following chemical composition: C 0.18%-0.22%, Si 1.2%-1.5%, Mn 2.4%-2.8%, Nb 0.04%-0.07%, Cr 0.3%-0.6%, Alt 0.8%-1.2%, P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities; The high yield ratio ferritic bainitic steel has a yield ratio of 0.8-0.9, a tensile strength of >1280MPa, an elongation after fracture of >20%, a hole expansion rate of >30%, and a strength-ductility product of >26.5GPa·%.

2. The high-yield ratio ferrite bainite steel with high strength and plasticity as claimed in claim 1, characterized in that: Measured in mass percentage, the chemical composition of high yield ratio ferritic bainite steel satisfies the following relationship: 2.2%≤Alt+Si≤2.5%.

3. The high-yield ratio ferrite bainite steel with high strength and plasticity as claimed in claim 1, characterized in that: Measured in mass percentage, the chemical composition of high yield ratio ferritic bainite steel satisfies the following relationship: 2.9%≤Mn+Cr≤3.2%.

4. The high-yield ratio ferrite bainite steel with high strength and plasticity as claimed in claim 1, characterized in that: The microstructure of high yield ratio ferritic bainitic steel includes grain boundary ferrite and bainite. The volume percentage of grain boundary ferrite is 15%-25%, the volume percentage of bainite is 75%-85%, the grain size of grain boundary ferrite is less than 2μm, and the average reference unit size of bainite is less than 5.5μm.

5. A method for preparing a high yield ratio ferrite bainite steel as claimed in any one of claims 1 to 4, characterized in that: The steps include: Step 1: Forging, heating the steel billet for the first time and keeping it warm, and then forging it to obtain a forged billet, wherein the start forging temperature is ≥1160°C and the final forging temperature is ≥950°C; Step 2: Heat treatment, heating the forging blank to the austenitizing heat treatment temperature and keeping it warm; Step 3: rolling, the starting rolling temperature is 1160-1190°C, and the final rolling temperature is 900-930°C; Step 4: Cooling and coiling, using ultra-fast cooling mode, cooling to 320-360°C at a cooling rate of 75-85°C / s, and then coiling, the coiling temperature is ≥320°C, and the hot-rolled steel coil is obtained after coiling; Step 5: leveling and pickling. After the hot-rolled steel coil is cooled, it is leveled, and the leveling elongation is 1.6%-2.0%. After leveling, the hot-rolled steel coil is pickled with tension to obtain a pickled steel coil. Step six: hood annealing. The hood annealing temperature is 340°C-380°C and the hood annealing time is 6-10 hours.

6. The preparation method according to claim 5, characterized in that: In step 1, the target temperature of the initial heating is 1210°C ± 20°C, and the insulation time is 60-90 minutes.

7. The preparation method according to claim 5, characterized in that: In step 2, the austenitizing uniform heating temperature is 1220°C ± 20°C, and the holding time is 30-40 minutes.

8. The preparation method according to claim 5, characterized in that: In step 4, the thickness of the hot-rolled steel coil is 2.0-6.0 mm.

9. The preparation method according to claim 5, characterized in that: In step 5, the cooling method is natural slow cooling with air, and the pickling liquid used for tension pickling is hydrochloric acid.

10. The preparation method according to claim 5, characterized in that: In step six, the hood annealing is single stack annealing.

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