High-yield-ratio galvanized ferrite bainite dual-phase steel and preparation method thereof

Abstract

The invention relates to the technical field of high-strength steel materials, in particular to a high-yield-ratio galvanized ferrite and bainite dual-phase steel and a preparation method thereof.The adopted steel billet comprises the following chemical components: 0.16-0.20% of C, 0.6-0.8% of Si, 2.5-2.9% of Mn, 0.05-0.08% of Nb, 0.5-0.7% of Cr, 0.8-1.2% of Alt, 1.5% < = Alt + Si < = 1.8%, 3.0% < = Mn + Cr < = 3.6%, P < = 0.005%, S < = 0.003%, N < = 0.0035%, O < = 0.005% and the balance Fe and inevitable impurities. The zinc-plated ferrite and bainite dual-phase steel prepared through the method is simple in preparation process and has excellent comprehensive performance and surface corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of high-strength steel materials, and in particular to a high-yield ratio galvanized ferrite-bainite dual-phase steel and a preparation method thereof. Background Art

[0002] Compared with aluminum alloy, carbon fiber and other materials, high-strength steel materials have the characteristics of high strength, light weight, manufacturing process, high cost performance and good machinability, as well as safety and low carbon environmental protection. They are important materials used in the manufacture of new energy vehicles. At the same time, as the demand for lightweight in the new energy vehicle manufacturing industry continues to escalate, the proportion of high-strength and high-performance steel in automobile manufacturing materials will further increase. Among them, ferrite-bainite dual-phase steel can not only meet the stamping forming performance, but also has excellent ductility and hole expansion and bending performance, so its application prospects are extremely broad.

[0003] For ferrite-bainite dual-phase steel, the prior art has the following technical difficulties: 1. The manufacturing process in the existing technology is relatively complicated, and includes low strength, poor formability such as hole expansion, and poor strength-plasticity matching; the heat treatment method of the cold-hardened coil is relatively complicated, including three consecutive stages of heating section treatment, three stages of cooling section treatment, and partitioning treatment. The shear edge affected area and hardening degree of the existing cold-rolled dual-phase steel are significantly higher than those of some strength grade steels, which affects the local formability and thus affects the hole expansion performance.

[0004] 2. The prior art adds many types of microalloying elements, including Cu: 0-0.01%, Ni: 0-0.01%, and Ti: 0-0.006% in addition to Nb and Cr.

[0005] 3. The surface corrosion resistance strength level of ferrite-bainite dual-phase steel products is low.

[0006] 4. Development of thin-gauge high-strength ferrite-bainite dual-phase steel products to meet the lightweight demands of new energy electric vehicles. This is mainly for hot-rolled products of different strength levels, and the specifications of hot-rolled products are generally >1.8mm. For GPa-level ferrite-bainite dual-phase steel, it is generally >2.0mm, which limits the further thinning of this steel. Summary of the invention

[0007] In view of the technical problems in the prior art that ferrite-bainite dual-phase steel has complex preparation process, low strength, poor forming properties such as hole expansion, and low surface corrosion resistance, the present invention provides a high yield ratio galvanized ferrite-bainite dual-phase steel and a preparation method thereof, which solves the technical problems in the prior art that the process is complex, low strength, poor forming properties such as hole expansion, poor strength-plasticity matching, and low surface corrosion resistance.

[0008] In a first aspect, a high yield ratio galvanized ferrite-bainite dual-phase steel is provided, wherein the steel billet comprises the following chemical compositions in weight percentage: C 0.16%-0.20%, Si 0.6%-0.8%, Mn 2.5%-2.9%, Nb 0.05%-0.08%, Cr 0.5%-0.7%, Alt 0.8%-1.2%, while satisfying 1.5%≤Alt+Si≤1.8%, 3.0%≤Mn+Cr≤3.6%, and limiting P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.005%, and the balance being Fe and unavoidable impurities.

[0009] Furthermore, the galvanized ferrite-bainite dual-phase steel has a yield strength ratio of 0.80-0.85, a tensile strength of >1180 MPa, an elongation after fracture of >20%, a hole expansion rate of >30%, a strength-ductility product of >25 GPa·%, and the dual-phase steel has the following microstructure: an average ferrite grain size of ≤2.5 μm, an average bainite benchmark unit grain size of ≤4.0 μm, and ≥14% of grain boundary retained austenite is retained at the ferrite / ferrite and ferrite / bainite phase interfaces.

[0010] In a second aspect, the present invention provides a method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel, which specifically comprises the following steps: (1) Steelmaking: smelting steel according to chemical composition to obtain steel billets, and heating and forging the steel billets to obtain forging billets; (2) Hot rolling: hot rolling the forged billet, and then rapidly cooling it to obtain a hot-rolled steel coil; (3) Annealing, placing the hot-rolled steel coil in a bell-type annealing furnace for annealing to obtain a bell-type annealed coil; (4) cold rolling, pickling the hood annealed coil and cold rolling it to obtain a cold-hardened strip; (5) Galvanized steelmaking includes subjecting the cold hardened strip to continuous annealing hot-dip galvanizing treatment, heating it to the annealing temperature and then cooling it to the flash cooling temperature, placing it in a zinc pot for galvanizing and then cooling it out of the zinc pot to obtain a ferrite-bainite duplex steel.

[0011] Furthermore, in step (1), the steel billet is heated to 1220°C ± 20°C and kept at this temperature for 80 ± 20 min before forging, with the start forging temperature being ≥ 1150°C and the final forging temperature being ≥ 960°C.

[0012] Furthermore, in step (2), the forging billet is heated to an austenitizing temperature of 1240°C ± 20°C and kept in the soaking section for 35 to 45 minutes; the starting rolling temperature is 1150°C to 1190°C and the final rolling temperature is 865°C to 895°C; after rolling, the billet is rapidly cooled to 580°C to 620°C at a cooling rate of 30°C / s in a laminar cooling mode for coiling.

[0013] Furthermore, in step (3), the temperature of the bell-type annealing furnace is set to 660° C. to 680° C., and the annealing time is 8 to 10 hours.

[0014] Furthermore, in step (4), the cumulative cold rolling reduction ratio is ≥ 60%, and a 0.8-2.5 mm cold-hardened steel strip is obtained.

[0015] Furthermore, in step (5), annealing includes heating the hot rolled coil to an annealing temperature of 840°C to 860°C for 30 to 60 seconds; rapidly cooling to a flash cooling temperature of 330°C to 360°C at a cooling rate of >55°C / s by high hydrogen cooling, rapidly heating to a zinc pot inlet temperature of 465°C ± 5°C by induction heating, and cooling the galvanized steel strip to ≤160°C by a mobile bellows after exiting the zinc pot.

[0016] Furthermore, in step (5), a pre-oxidation-reduction device is used to manufacture continuous annealing hot-dip galvanizing.

[0017] The beneficial effects of the present invention are: In the matrix structure of the high yield ratio galvanized ferrite bainite dual-phase steel prepared by the present invention, the average ferrite grain size is ≤2.5μm, and the average grain size of the bainite reference unit is ≤4.0μm. In addition, ≥14% of the grain boundary residual austenite is retained at the ferrite / ferrite and ferrite / bainite phase interfaces, which can improve the plasticity and toughness of the steel and improve the forming performance of the steel. The high yield ratio galvanized ferrite bainite dual-phase steel prepared by the present invention has high strength in thin specifications, good formability such as hole expansion, excellent strength-plasticity matching, and good surface corrosion resistance. Its thickness specification is 0.8~2.5mm, and its performance characteristics are high yield ratio type, yield ratio is 0.80~0.85, tensile strength>1180MPa, elongation after fracture>20%; at the same time, the hole expansion rate is>30%, and the strength-plasticity product is>25GPa·%. BRIEF DESCRIPTION OF THE DRAWINGS 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.

[0018] Figure 1 It is a typical engineering stress-strain curve of high yield ratio galvanized ferrite-bainite dual-phase steel.

[0019] Figure 2 It is a typical microstructure diagram of high yield ratio galvanized ferrite-bainite dual-phase steel.

[0020] Figure 3This is a typical TEM morphology characteristic diagram of retained austenite in high yield ratio galvanized ferrite-bainite dual-phase steel. DETAILED DESCRIPTION

[0021] 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.

[0022] Example 1 The chemical composition of the steel billet used in the present invention is C: 0.16-0.20%, Si: 0.6-0.8%, Mn: 2.5-2.9%, Nb: 0.05-0.08%, Cr: 0.5-0.7%, Alt: 0.8-1.2%, and at the same time satisfies 1.5%≤Alt+Si≤1.8%, 3.0%≤Mn+Cr≤3.6%, and is limited to P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities, as shown in Table 1.

[0023] Table 1 Chemical composition of the steel billets in Examples 1-4

[0024] The specific preparation method of high yield ratio galvanized ferrite bainite dual-phase steel comprises the following steps: (1) Steelmaking: smelting the chemical components to obtain a steel billet, heating the steel billet to 1220°C ± 20°C and keeping it at that temperature for 80 ± 20 minutes, and then forging it, with the starting forging temperature being ≥ 1150°C and the final forging temperature being ≥ 960°C, to obtain a forged billet; (2) Hot rolling, including the manufacture of hot-rolled coils, wherein the forging billet is hot rolled, the forging billet is heated to an austenitizing temperature of 1240°C ± 20°C, and is kept in the soaking section for 35 to 45 min; the starting rolling temperature is 1150°C to 1190°C, and the final rolling temperature is 865°C to 895°C; after rolling, a laminar cooling mode is adopted, and the billet is rapidly cooled to 580°C to 620°C at a cooling rate of about 30°C / s for coiling, so that a hot-rolled steel coil with a thickness of 2.5 to 6.5 mm can be obtained. The specific manufacturing process parameters of the hot-rolled coil and the bell-annealed coil are shown in Table 2; (3) Annealing, including the manufacture of bell-annealed coils, in which the hot-rolled steel coils are placed in a bell-annealing furnace for annealing, with the bell-annealing temperature set at 660°C to 680°C and the annealing time being 8 to 10 hours; (4) Cold rolling: pickling the hood annealed coil and cold rolling the hood annealed coil, with a cold rolling cumulative reduction ratio of ≥60% to obtain a 0.8-2.5 mm cold-hardened strip. The specific manufacturing process parameters of cold hardening, annealing and galvanizing are shown in Table 3. (5) Galvanized steel stripping: the cold hardened strip is subjected to continuous annealing hot-dip galvanizing treatment, heated to an annealing temperature of 840°C~860°C, and the annealing time is 30~60s; then, high hydrogen cooling is adopted to rapidly cool the strip to a flash cooling temperature of 330°C~360°C at a cooling rate of >55°C / s; then, induction heating is adopted to rapidly heat the strip to a zinc pot temperature of 465°C±5°C, and after exiting the zinc pot, a mobile bellows is used to cool the galvanized steel strip to ≤160°C, thereby obtaining the galvanized high yield ratio galvanized ferrite bainite dual-phase steel.

[0025] Table 2 Specific manufacturing process parameters of hot rolled coils and bell annealed coils

[0026] Table 3 Specific manufacturing process parameters for cold hardening, annealing and galvanizing

[0027] The prepared high yield ratio galvanized ferrite bainite dual-phase steel was sampled for mechanical property testing and microstructure analysis. The test and analysis results are specifically shown in Table 4. From the analysis of the mechanical property test results, it can be seen that the high yield ratio galvanized ferrite bainite dual-phase steel prepared in the present invention has excellent comprehensive performance and surface corrosion resistance.

[0028] Table 4 Mechanical properties and phase volume fraction of high yield ratio galvanized ferrite-bainite dual-phase steel prepared in Example

[0029] like Figure 1 As shown, Figure 1 The typical engineering stress-strain curve of the prepared high yield ratio galvanized ferrite-bainite dual-phase steel is shown in Figure 2. Its performance characteristics are high yield ratio type, with a yield ratio of 0.80~0.85, tensile strength>1180MPa, and elongation after fracture>20%; at the same time, the hole expansion rate is>30%, and the strength-ductility product is>25GPa·%.

[0030] Figure 2 The typical microstructure of the prepared high yield ratio galvanized ferrite-bainite dual-phase steel; Figure 3 The typical TEM morphology of retained austenite in the prepared high yield ratio galvanized ferrite-bainite dual-phase steel. Figure 3 a is the bright field image, b is the dark field image, and c is Figure 3 Diffraction pattern of retained austenite marked by the circle in (a).

[0031] In the matrix structure of the high yield ratio galvanized ferrite-bainite dual-phase steel, the average ferrite grain size is ≤2.5μm, the average grain size of the bainite reference unit is ≤4.0μm, and ≥14% of grain boundary retained austenite is retained at the ferrite / ferrite and ferrite / bainite phase interfaces.

[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 galvanized ferrite-bainite dual-phase steel, characterized in that: The steel billet includes the following chemical compositions in weight percentage: C 0.16%~0.20%, Si 0.6~0.8%, Mn 2.5%~2.9%, Nb 0.05%~0.08%, Cr 0.5%~0.7%, Alt 0.8~1.2%, while satisfying 1.5%≤Alt+Si≤1.8%, 3.0%≤Mn+Cr≤3.6%, and limiting P≤0.005%, S≤0.003%, N≤0.0035%, O≤0.005%, and the balance is Fe and unavoidable impurities.

2. The high yield ratio galvanized ferrite-bainite dual-phase steel according to claim 1, characterized in that: The yield strength ratio of galvanized ferrite-bainite dual-phase steel is 0.80~0.85, the tensile strength is greater than 1180MPa, the elongation after fracture is greater than 20%, the hole expansion rate is greater than 30%, and the strength-ductility product is greater than 25GPa·%. The dual-phase steel has the following microstructure: the average ferrite grain size is ≤2.5μm, the average grain size of the bainite benchmark unit is ≤4.0μm, and ≥14% of the grain boundary retained austenite is retained at the ferrite / ferrite and ferrite / bainite phase interfaces.

3. A method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel as claimed in claim 1, characterized in that: The specific steps include: (1) Steelmaking: smelting steel according to chemical composition to obtain steel billets, and heating and forging the steel billets to obtain forging billets; (2) Hot rolling: hot rolling the forged billet, and then rapidly cooling it to obtain a hot-rolled steel coil; (3) Annealing, placing the hot-rolled steel coil in a bell-type annealing furnace for annealing to obtain a bell-type annealed coil; (4) cold rolling, pickling the hood annealed coil and cold rolling it to obtain a cold-hardened strip; (5) Galvanized steelmaking includes subjecting the cold hardened strip to continuous annealing hot-dip galvanizing treatment, heating it to the annealing temperature and then cooling it to the flash cooling temperature, placing it in a zinc pot for galvanizing and then cooling it out of the zinc pot to obtain a ferrite-bainite duplex steel.

4. The method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel according to claim 3, characterized in that: In step (1), the steel billet is heated to 1220°C ± 20°C and kept at this temperature for 80 ± 20 min, and then forged. The start forging temperature is ≥ 1150°C and the final forging temperature is ≥ 960°C.

5. The method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel according to claim 3, characterized in that: In step (2), the forging billet is heated to an austenitizing temperature of 1240°C ± 20°C and kept in the soaking section for 35 to 45 minutes; the starting rolling temperature is 1150°C to 1190°C and the final rolling temperature is 865°C to 895°C; after rolling, a laminar cooling mode is adopted and the billet is rapidly cooled to 580°C to 620°C at a cooling rate of 30°C / s for coiling.

6. The method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel according to claim 3, characterized in that: In step (3), the temperature of the bell-type annealing furnace is set to 660°C to 680°C, and the annealing time is 8 to 10 hours.

7. The method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel according to claim 3, characterized in that: In step (4), the cumulative cold rolling reduction ratio is ≥ 60%, and a 0.8-2.5 mm cold-hardened strip is obtained.

8. The method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel according to claim 6, characterized in that: In step (5), annealing includes heating the hot rolled coil to an annealing temperature of 840°C to 860°C for 30 to 60 seconds; rapidly cooling to a flash cooling temperature of 330°C to 360°C at a cooling rate of >55°C / s by high hydrogen cooling, rapidly heating to a zinc pot inlet temperature of 465°C±5°C by induction heating, and cooling the galvanized steel strip to ≤160°C by a mobile bellows after exiting the zinc pot.

9. The method for preparing a high yield ratio galvanized ferrite-bainite dual-phase steel according to claim 3, characterized in that: In step (5), a pre-oxidation-reduction device is used to manufacture continuous annealing hot-dip galvanizing.

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