980mpa grade low spot weld carbon equivalent alloyed hot-dip galvanized dual phase steel and method of making

Abstract

The application discloses a 980MPa-grade low-spot welding carbon equivalent alloying hot-dip galvanized dual-phase steel and a preparation method thereof, and belongs to the field of cold-rolled plate strip production. The preparation method comprises the following steps: a smelting process: smelting according to the chemical composition of the dual-phase steel and casting into a slab; a hot rolling process: after the slab is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling, a hot-rolled coil is obtained; an acid pickling process: after the hot-rolled coil is subjected to pickling, cold rolling is performed to obtain a cold-rolled strip steel; a hot-dip galvanizing process: after the cold-rolled strip steel is subjected to soaking and galvanizing treatment, alloying treatment is performed, and after cooling, the 980MPa-grade low-spot welding carbon equivalent alloying hot-dip galvanized dual-phase steel is obtained. Through the process flow of the alloying element and the content thereof in combination with the smelting process, the hot rolling process, the acid pickling process and the hot-dip galvanizing process, the 980MPa-grade low-spot welding carbon equivalent alloying hot-dip galvanizeddual-phase steel is prepared, the yield strength of the steel reaches 650-745MPa, the tensile strength reaches 1015-1083MPa, the elongation A 80 of the steel reaches 11.0-14.5%.

Description

Technical Field

[0001] This invention belongs to the field of cold-rolled sheet and strip production, specifically relating to a 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel and its preparation method. Background Technology

[0002] With increasing awareness of energy conservation and material safety, many automakers are choosing high-strength steel for their vehicles. The automotive industry's use of hot-dip galvanized high-strength steel sheets reduces sheet thickness while improving corrosion resistance, dent resistance, durability, impact strength, and safety. Therefore, automotive steel sheets will inevitably develop towards higher strength, higher toughness, corrosion resistance, and easier forming and processing. As the requirements for corrosion resistance in car bodies increase, galvanized automotive sheets are increasingly used in automobile production. Electro-galvanized automotive sheets have the best surface quality, but the coating thickness is relatively thin, failing to meet the demands of corrosion resistance development. Hot-dip galvanized automotive sheets have a thicker coating and excellent corrosion resistance, but the weldability, paintability, and stone chip resistance of pure zinc (GI) layers are relatively poor. Alloying annealing involves heating the hot-dip galvanized sheet to a certain temperature after hot-dip galvanizing, causing a zinc-iron alloy layer to form through the interdiffusion phase transformation between the iron in the substrate and the zinc in the zinc layer. This coating exhibits good weldability, paintability, and stone chip resistance. 980MPa grade duplex steel is the most widely used ultra-high strength advanced high-strength steel, which combines good strength, plasticity and corrosion resistance, and is widely used in parts such as A-pillars, B-pillars, anti-collision beams and longitudinal beams.

[0003] In the prior art, the literature similar to the preparation method of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel includes:

[0004] CN 108486501 A discloses a 1000MPa grade cold-rolled galvanized duplex steel with enhanced plasticity and its preparation method. The chemical composition by mass percentage is: C: 0.15-0.23%, Si: 0.1-0.5%, Mn: 1.8-2.3%, P≤0.01%, S≤0.01%, Al: 0.5-1.0%, Cr: 0.3-0.6%, Ti: 0.01-0.04%, with the balance being Fe and unavoidable impurities. The continuously cast billet is heated to 1150-1250℃ and held at that temperature. The final rolling temperature is 890±20℃, the coiling temperature is 650±20℃, and the deformation during cold rolling is 45-60%. The cold-rolled thin strip is heated to the two-phase region (annealing temperature) and held at 760-820℃ for a certain period of time. Then, the strip is slowly cooled to 660-720℃. After that, the slowly cooled strip is rapidly cooled to the galvanizing temperature of 450-460℃ under 20% high hydrogen conditions for galvanizing. After galvanizing, the strip is cooled to 410-430℃ and then air-cooled. The temperature of the top roller of the cooling tower is controlled at 250-290℃. The patented steel contains a high amount of carbon (C), which reduces weldability. Simultaneously, the high Al content (0.5-1.0%) increases the viscosity of the molten steel, causing nozzle blockage and affecting casting. It also generates more Al₂O₃ inclusions, leading to increased surface defects. While the high-C, Si+Al design of this patent yields a small amount of retained austenite (approximately 4.0%), achieving good elongation during plastic forming through the TRIP effect, cracks easily form during the transformation of retained austenite into martensite during punching, thus deteriorating hole-expanding performance. Furthermore, the products manufactured under the guidance of this patent are unalloyed, with a surface of pure zinc (GI) rather than alloyed (GA).

[0005] CN 109402525 A discloses a processing method for duplex steel with a yield strength of 780MPa and a tensile strength of 1000MPa. The chemical composition by mass percentage is as follows: C: 0.085~0.115%, Mn: 2.3~2.5%, Si: 0.35~0.50%, P: 0~0.020%, S: 0~0.007%, Cr: 0.2~0.4%, Mo: 0.15~0.30%, Nb: 0.035~0.055%, Ti: 0.035~0.055%, with the balance being Fe and unavoidable impurities. In the pickling and rolling process, the welding power of the hot-rolled raw material is 10000-12000W; the welding speed is 2-6m / min, the wire feeding speed is 2-7m / min, the annealing current is 100-180A, and the heat treatment temperature is 500-900℃. The welded coil is then stretched, bent, and straightened. During this process, the elongation is 0.5-1.1%, the insertion depth of bending unit #1 is 10-20mm, bending unit #2 is 8-18mm, and straightening unit #3 is 4-10mm. The stretched, bent, and straightened coil is then pickled. The pickling process parameters are: pickling temperature 75-90℃. 0℃; pickling speed: 40~220m / min; the pickled coil is rolled, and the rolling process parameters are: the reduction distribution rate of F1~F5 stands are 25~35%, 25~35%, 25~33%, 20~27%, and 0.3~1% respectively; the produced cold-rolled thin strip steel is heated to 780~810℃ and then slowly cooled to 680~700℃ and rapidly cooled to 450~460℃ respectively. After a period of equalization, it is put into a zinc pot for galvanizing treatment. The strip speed of hot-dip galvanized steel strip is controlled at 50~70m / min, and a flattening elongation of 0.7%~0.9% is applied to adjust the strip shape and yield strength. This patent adds precious alloys such as Mo, which increases costs. It does not clearly state that the hot rolling process is not conducive to promotion in similar units. Moreover, it is a continuous annealing product, which is significantly different from hot-dip galvanized products. In addition, it does not consider the hole expansion rate.

[0006] CN 109097676 A discloses an alloyed hot-dip galvanized duplex steel and its preparation method. The chemical composition by weight percentage is: C: 0.10%~0.15%, Si: 0.2%~0.5%, Mn: 2.0%~2.4%, Alt: 0.02%~0.05%, Ti: 0.015%~0.03%, Cr: 0.4%~0.6%, Mo: 0.1%~0.3%, P≤0.01%, S≤0.01%, N≤0.005%, with the balance being Fe and trace elements. The slab heating temperature is 1220–1280℃; the final rolling temperature is 870–920℃; and the coiling temperature is 650–690℃. The hot-rolled strip is pickled and cold-rolled before hot-dip galvanizing, with a cold rolling reduction rate of 50%–60%. The strip is first preheated to 220℃ at a rate of 8℃ / s–12℃ / s. The preheated strip is then further heated to 780℃–820℃ at a rate of 1.5℃ / s–4℃ / s, held at this temperature for 60s–100s, and then subjected to sequential processing. The strip is slowly cooled to a temperature of 720℃~760℃ at a rate of 8℃ / s~12℃ / s. It is then rapidly cooled by air blowing to a galvanizing temperature of 450℃~460℃. After galvanizing, it is cooled by air knife blowing to 420~430℃. After galvanizing, it is heated to an alloying temperature of 480℃~530℃ for 5~30s for alloying treatment. Finally, it is cooled to 250~300℃ by a combination of front-end air cooling between the air knife and the top roller, and rear-end air cooling, at a rate of approximately 6℃ / s~9℃ / s. This patent contains a high amount of carbon (0.10%~0.15%), which is detrimental to obtaining good weldability and hole-expanding performance. The high manganese content (2.0%~2.4%) easily leads to segregation, which is unfavorable for uniform control of microstructure and properties.

[0007] Therefore, existing technologies need to be improved. Summary of the Invention

[0008] To address at least one of the problems in the prior art, the present invention aims to provide a 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel and its preparation method.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] According to a first aspect of the present invention, a method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel is provided, comprising the following steps:

[0011] (a) Smelting process: Smelting is carried out according to the chemical composition of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, and then cast into slabs. The chemical composition of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, by mass percentage, includes: C: 0.05%~0.12%, Si: 0.20%~0.60%, Mn: 1.80%~2.30%, P≤0.018%, S≤0.009%, Als: 0.010%~0.065%, N≤0.0065%, Cr: 0.40%~0.80%, Nb: 0.01%~0.04%, Ti: 0.025%~0.055%, B: 0.0010%~0.0050%, with the balance being Fe and unavoidable impurities;

[0012] (b) Hot rolling process: The slab obtained in step (a) is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled coil;

[0013] (c) Pickling and rolling process: The hot-rolled coil obtained in step (b) is pickled and then cold-rolled into cold-rolled strip steel;

[0014] (d) Hot-dip galvanizing process: After the cold-rolled strip steel obtained in step (c) undergoes homogenization and galvanizing treatment, it is then alloyed and cooled to obtain 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel. The homogenization temperature is 820-860℃, the homogenization holding time is 70-150s, the alloying temperature is 510-560℃, and the alloying holding time is 6-36s.

[0015] Preferably, the chemical composition of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, by mass percentage, includes: C: 0.08%–0.10%, Si: 0.35%–0.50%, Mn: 1.85%–2.15%, P≤0.015%, S≤0.008%, Als: 0.025%–0.055%, N≤0.0060%, Cr: 0.45%–0.65%, Nb: 0.015–0.035%, Ti: 0.030–0.045%, B: 0.0020%–0.0030%, with the balance being Fe and unavoidable impurities.

[0016] According to some embodiments of the present invention, in step (b), the heating temperature is 1215-1250°C, the finishing rolling start temperature is 1035-1130°C, the finishing rolling finish temperature is 850-920°C, the laminar flow cooling adopts the front-stage cooling method, the upper and lower surface cooling rates are 25% and 50% respectively, the coiling temperature is 520-580°C, and the coiling method adopts "U-shaped coiling".

[0017] According to some embodiments of the present invention, in step (c), the cold rolling reduction rate is 44%-60%.

[0018] According to some embodiments of the present invention, in step (d), the cold-rolled strip steel is heated to a uniform heating temperature by a segmented heating method, and after reaching the uniform heating holding time, it is cooled to 440-470°C by a segmented cooling method. After a period of uniform holding, it enters a zinc bath for galvanizing treatment. The composition of the galvanizing bath contains 0.15%-0.30% Al, with the remainder being Zn and unavoidable impurities. The unit speed is 80-120 m / min, and the flattening elongation range is 0.30-0.50%. After galvanizing, it is first cooled to 390-420°C by an air knife, and then heated to the alloying temperature for alloying. After reaching the alloying holding time, it is cooled to room temperature by a fan to obtain 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel.

[0019] According to some embodiments of the present invention, in step (c), as the thickness of the cold-rolled strip increases, the cold rolling reduction rate gradually decreases, and the cold rolling reduction rate decreases by 3 to 5% for every 0.4 mm increase in the thickness of the cold-rolled strip.

[0020] According to some embodiments of the present invention, in step (d), the segmented heating method is as follows: first, heating to 300°C at a heating rate of 8 to 20°C / s, then heating to 700°C at a heating rate of 3 to 10°C / s, and then heating to the homogenization temperature at a heating rate of 0.5 to 3.2°C / s.

[0021] According to some embodiments of the present invention, in step (d), the segmented cooling method is as follows: first, cooling to 710-750°C at a cooling rate of 1.2-7.8°C / s, and then cooling to 440-470°C at a cooling rate of 15-33°C / s.

[0022] According to some embodiments of the present invention, in step (d), as the thickness of the cold-rolled strip increases, the speed of the unit gradually decreases, and the speed of the unit decreases by 10 m / min for every 0.3 mm increase in the thickness of the cold-rolled strip.

[0023] According to some embodiments of the present invention, in step (d), as the thickness of the cold-rolled strip increases, the flattening elongation gradually decreases, and the flattening elongation decreases by 0.05% for every 0.4 mm increase in the thickness of the cold-rolled strip.

[0024] According to some embodiments of the present invention, the thickness of the hot-rolled plate obtained in step (b) is 2.25 to 4.50 mm, and the thickness of the cold-rolled strip obtained in step (c) is 0.9 to 2.5 mm.

[0025] According to a second aspect of the present invention, a 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel is provided, which is prepared by means of the method described in the first aspect of the present invention.

[0026] According to some embodiments of the present invention, the yield strength of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel is 650-745MPa, the tensile strength is 1015-1083MPa, and the elongation A is... 80 The value is 11.0%–14.5%, and the porosity is 44%–54%.

[0027] According to some embodiments of the present invention, the microstructure of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel consists of 30-40% ferrite matrix and 60-65% blocky martensite.

[0028] By adopting the above technical solution, the present invention has the following beneficial effects:

[0029] This invention, through compositional design of alloying elements and their contents, combined with a process flow of smelting + hot rolling + pickling + hot-dip galvanizing, produces 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel. Its yield strength reaches 650–745MPa, tensile strength reaches 1015–1083MPa, and elongation A… 80 The value reaches 11.0% to 14.5%, and the porosity reaches 44% to 54%.

[0030] This invention strictly controls the content of elements such as C, P, and S, which helps reduce the carbon equivalent of spot welding, improve welding performance, and optimize stamping and hole-expanding performance. It adopts a C-Si-Mn route with Ti+Nb+B composite microalloying, utilizing Ti to fix N and fully leveraging the significant hardenability-enhancing effect of B, thus reducing the amount of alloying required. Simultaneously, the formed TiN inhibits austenite coarsening during hot rolling, while the addition of Nb acts as an atomic dragging agent during hot rolling, also inhibiting austenite coarsening and preventing coarsening at grain boundaries during finish rolling. During hot-dip galvanizing, Nb and Ti elements inhibit recrystallization, thereby refining the grains and resulting in a finer finished product with coordinated deformation. Appropriate amounts of Mn and Cr elements improve hardenability and ensure strength. Through alloying treatment of the zinc layer, not only are the coating properties altered, but hole-expanding performance is also improved, better meeting the requirements of high-expanding-hole flanged parts. Attached Figure Description

[0031] Figure 1 A flowchart illustrating the preparation method of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel provided by the present invention.

[0032] Figure 2Metallographic micrograph of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel prepared by the method of the present invention.

[0033] Figure 3 The image shows a scanning electron microscope (SEM) image of a 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel prepared by the method of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0035] Specific embodiments of the invention are disclosed herein as needed; however, it should be understood that the embodiments disclosed herein are merely examples of the invention that may be implemented in various alternative forms. In the following description, various operating parameters and components are described in several contemplated embodiments. These specific parameters and components are provided as examples only and are not intended to be limiting.

[0036] According to a first aspect of the present invention, a method for preparing 980MPa grade low-spot-welding carbon equivalent alloyed hot-dip galvanized duplex steel is provided. For example... Figure 1 As shown, the method includes the following steps:

[0037] (a) Smelting process: Smelting is carried out according to the chemical composition of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, and then cast into slabs. The chemical composition of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, by mass percentage, includes: C: 0.05%~0.12%, Si: 0.20%~0.60%, Mn: 1.80%~2.30%, P≤0.018%, S≤0.009%, Als: 0.010%~0.065%, N≤0.0065%, Cr: 0.40%~0.80%, Nb: 0.01%~0.04%, Ti: 0.025%~0.055%, B: 0.0010%~0.0050%, with the balance being Fe and unavoidable impurities;

[0038] (b) Hot rolling process: The slab obtained in step (a) is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled coil;

[0039] (c) Pickling and rolling process: The hot-rolled coil obtained in step (b) is pickled and then cold-rolled into cold-rolled strip steel;

[0040] (d) Hot-dip galvanizing process: After the cold-rolled strip steel obtained in step (c) undergoes homogenization and galvanizing treatment, it is then alloyed and cooled to obtain 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel. The homogenization temperature is 820-860℃, the homogenization holding time is 70-150s, the alloying temperature is 510-560℃, and the alloying holding time is 6-36s.

[0041] Preferably, the chemical composition of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, by mass percentage, includes: C: 0.08%–0.10%, Si: 0.35%–0.50%, Mn: 1.85%–2.15%, P≤0.015%, S≤0.008%, Als: 0.025%–0.055%, N≤0.0060%, Cr: 0.45%–0.65%, Nb: 0.015–0.035%, Ti: 0.030–0.045%, B: 0.0020%–0.0030%, with the balance being Fe and unavoidable impurities.

[0042] The roles of alloying elements in this 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel are as follows:

[0043] As one of the most important components of duplex steel, carbon primarily affects the volume fraction of austenite formed during annealing. During austenite formation, the diffusion of carbon within austenite or ferrite effectively controls austenite grain growth. With increasing carbon content or critical zone heating temperature, the austenite volume fraction increases, leading to an increase in the martensitic phase formed after cooling, thus increasing the material's strength. If the carbon content is too low, the stability of austenite and the hardenability of martensite decrease, resulting in lower strength; in duplex steel, the carbon content is generally not less than 0.02%. If the carbon content is too high, the plasticity and weldability of duplex steel decrease; in duplex steel, the carbon content is generally not higher than 0.15%. Therefore, the carbon content of this invention is 0.05%–0.12%, preferably 0.08%–0.10%.

[0044] Si plays a significant role in solid solution strengthening in steel and effectively inhibits carbide precipitation and delays pearlite transformation during phase transformation. However, excessive Si content significantly increases deformation resistance during thin-gauge rolling and promotes carbon segregation in manganese-rich regions. During heat treatment in the two-phase region, Si accelerates carbon diffusion into austenite, significantly purifying ferrite, improving ferrite purity in dual-phase steel, promoting ferrite formation, and expanding the ferrite formation process window, thus resulting in a lower yield strength ratio. On the other hand, excessive silicon content increases martensite brittleness, leading to decreased toughness, and the formation of high-melting-point oxides on the steel plate surface affects surface quality. Therefore, it is necessary to minimize the silicon content in the steel. Thus, the Si content in this invention is 0.20–0.60%, preferably 0.35%–0.50%.

[0045] Manganese (Mn) is a good deoxidizer and desulfurizer, and also a commonly used solid solution strengthening element in steel, generally not less than 1.20% in duplex steel. Mn can combine with carbon (C) to form various carbides, playing a precipitation strengthening role, and can also dissolve in the matrix to enhance the solid solution strengthening effect. Mn readily combines with sulfur (S) to form the high-melting-point compound MnS, thereby eliminating or weakening the hot brittleness caused by FeS and improving the hot working properties of steel. Mn can improve the stability of austenite, shifting the C-curve to the right, thus significantly reducing the critical cooling rate of martensite. However, when the Mn content is too high, it easily accumulates on the surface during annealing, forming a large amount of manganese compounds, leading to a decrease in the quality of surface galvanization. Therefore, in this invention, the Mn content is 1.80%–2.30%, preferably 1.85%–2.15%.

[0046] Cr can replace Mn, improving the strength of steel, reducing segregation, and inhibiting pearlite transformation. Furthermore, the addition of a certain amount of Cr in this invention can also improve surface quality. After Cr is added, it can react with oxygen and aggregate at the interface between the iron oxide scale and the iron matrix, forming a dense (Fe,Cr)₂O₃ or (Fe,Cr)₃O₄ spinel film. The presence of this Fe-Cr spinel film hinders oxygen diffusion, reducing the formation of iron oxide scale. Therefore, adding Cr can effectively reduce the thickness of the iron oxide scale and the formation of AlN, and improve the adhesion of the iron oxide scale, thereby effectively reducing pitting defects caused by the indentation of iron oxide scale. Therefore, in this invention, the Cr content is 0.40% to 0.80%, preferably 0.45% to 0.65%.

[0047] Al is a common deoxidizer in steel and can also form AlN studded grain boundaries, thereby refining the grains. Furthermore, Al, similar to Si, can inhibit carbide precipitation, thus ensuring sufficient carbon enrichment in austenite. Therefore, the Al content in this invention is 0.010%–0.065%, preferably 0.025%–0.055%.

[0048] In dual-phase steel, nitrogen (Nb) mainly exists in the form of NbC, which has a significant effect on grain refinement and dispersion precipitation strengthening. During the hot-dip galvanizing annealing process, undissolved NbC particles can pin the ferrite grain boundaries, thereby refining the grains. When the annealing temperature increases to the two-phase region, the NbC dissolution temperature is relatively low, so it dissolves fully in the matrix, while dissolved carbon atoms accumulate in austenite to improve its stability. During cooling, NbC in the ferrite will reprecipitate, resulting in significant precipitation strengthening. Therefore, the Nb content is 0.01–0.04%, preferably 0.015–0.035%.

[0049] Ti readily combines with nitrogen (N) to form TiN, thus fixing the N element. Simultaneously, TiN suppresses austenite coarsening during hot rolling, effectively refining the grains and reducing aging and cold brittleness. Furthermore, Ti combines with carbon (C) to provide significant precipitation strengthening. If the Ti content is too low, N cannot be completely fixed, thus affecting the significant hardenability-enhancing effect of boron (B). If the Ti content is too high, TiN precipitation can easily occur, worsening the material's plasticity and potentially leading to cracking of stamped parts. Therefore, the Ti content is selected as 0.025–0.055%, preferably 0.030–0.045%.

[0050] Boron (B) significantly improves the hardenability of austenite and effectively promotes the formation of martensite in duplex steel. At the same time, it readily combines with nitrogen (N) in steel to form BN, which plays a role in precipitation strengthening. However, excessive B content will reduce the toughness of the steel and increase the manufacturing cost. Therefore, the B content is selected as 0.0010–0.0050%, preferably 0.0020–0.0030%.

[0051] Phosphorus (P) is an impurity element in steel that tends to segregate at grain boundaries, weakening intergranular bonding. During rapid solidification, a high P content can easily lead to cracking in the cast billet. Therefore, in this invention, the P content is ≤0.018%, preferably ≤0.015%.

[0052] Sulfur (S) is an impurity element in steel. It tends to segregate at grain boundaries and forms low-melting-point FeS with Fe in the steel, reducing the toughness of the steel and easily causing fine microcracks on the surface of the cast billet during rapid solidification. Therefore, the S content in this invention is ≤0.009%, preferably ≤0.008%.

[0053] Nitrogen (N) is an impurity element in steel. Due to its small atomic size, it easily enters the interstitial spaces of iron, significantly increasing lattice distortion and thus greatly enhancing strength, but it also noticeably worsens the steel's ductility and toughness. Simultaneously, N readily combines with Al, B, and Ti in steel to form second phases such as AlN, BN, and TiN, which, while strengthening the steel, also deteriorate its ductility and toughness. Especially when the N content is too high, the combined NiN with Ti can precipitate to the micrometer scale, not only failing to provide strengthening but also causing stress concentration and crack initiation. Therefore, the N content must be strictly controlled. Thus, in this invention, the N content is ≤0.0065%, preferably 0.0060%.

[0054] In this invention, by designing the alloying elements and their contents in conjunction with corresponding process steps, a 980MPa grade low spot weld carbon equivalent alloyed hot-dip galvanized duplex steel was obtained. Specifically, by strictly controlling the contents of elements such as C, P, and S, it is beneficial to reduce the spot weld carbon equivalent (Ceq(spot)=C+Si / 30+Mn / 20+2P+4S), improving welding performance and optimizing stamping and hole expansion performance. The C-Si-Mn route using Ti+Nb+B composite microalloying utilizes Ti to fix the N element and fully leverages the significant hardenability-enhancing effect of the B element, thus reducing... The amount of alloy added; the TiN formed simultaneously inhibits austenite coarsening during hot rolling, and the addition of Nb plays an atomic dragging role during hot rolling, inhibiting austenite coarsening. During finishing rolling, grain boundaries are pinned to avoid coarsening; during hot-dip galvanizing, Nb and Ti elements inhibit recrystallization, thereby refining the grains and obtaining a finer finished product structure with coordinated deformation; appropriate amounts of Mn and Cr elements improve hardenability and ensure strength; alloying the zinc layer in an alloying furnace not only changes the properties of the coating but also improves the hole-expanding performance, better meeting the needs of high-expanding-hole-flanging parts.

[0055] In step (b), the heating temperature is 1215–1250°C, the initial finishing rolling temperature is 1035–1130°C, the final finishing rolling temperature is 850–920°C, laminar cooling is achieved using a front-stage cooling method, with upper and lower surface cooling rates of 25% and 50%, respectively, and the coiling temperature is 520–580°C. To maintain the stability of the microstructure, a "U-shaped coiling" method is adopted (i.e., the strip head and tail are higher than the middle of the strip, the coiling temperature is the temperature of the middle part of the strip, the coiling temperature for the first 80m of the strip head is 570–630°C, and the coiling temperature for the last 100m of the strip tail is 580–640°C). Typical but non-limiting heating temperatures are, for example, 1215°C, 1220°C, 1225°C, 1230°C, 1235°C, 1240°C, 1245°C, or 1250°C. The initial finishing rolling temperature is typically, but not limited to, for example, 1035℃, 1040℃, 1045℃, 1050℃, 1055℃, 1060℃, 1065℃, 1070℃, 1075℃, 1080℃, 1085℃, 1090℃, 1095℃, 1100℃, 1105℃, 1110℃, 1115℃, 1120℃, 1125℃, or 1130℃. The final finishing rolling temperature is typically, but not limited to, 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, 910℃, or 920℃. The coiling temperature is typically, but not limited to, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, or 580℃.

[0056] In some embodiments, the thickness of the hot-rolled sheet obtained in step (b) is 2.25 to 4.50 mm. Typical, but not limited, thicknesses of the hot-rolled sheet include, for example, 2.25 mm, 2.30 mm, 2.35 mm, 2.40 mm, 2.45 mm, 2.50 mm, 2.55 mm, 2.60 mm, 2.65 mm, 2.70 mm, 2.75 mm, 2.80 mm, 2.90 mm, 3.00 mm, 3.10 mm, 3.20 mm, 3.30 mm, 3.40 mm, 3.50 mm, 3.60 mm, 3.70 mm, 3.80 mm, 3.90 mm, 4.00 mm, 4.10 mm, 4.20 mm, 4.30 mm, 4.40 mm, or 4.50 mm.

[0057] In step (c), the cold rolling reduction rate is 44%-60%. Typical, but not limited, cold rolling reduction rates are, for example, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, or 60%.

[0058] In some embodiments, the thickness of the cold-rolled strip obtained in step (c) is 0.9-2.5 mm. Typical, but not limited, thicknesses of the cold-rolled strip include, for example, 0.9 mm, 1 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.7 mm, 1.8 mm, 1.9 mm, 2.0 mm, 2.1 mm, 2.2 mm, 2.3 mm, 2.4 mm, or 2.5 mm. The thickness of the cold-rolled strip obtained in step (c) is less than the thickness of the hot-rolled sheet obtained in step (b).

[0059] In some embodiments, the cold rolling reduction rate gradually decreases as the thickness of the cold-rolled strip increases. Preferably, the cold rolling reduction rate decreases by 3-5% for every 0.4 mm increase in the thickness of the cold-rolled strip. This facilitates the adjustment of the cold rolling reduction rate according to the product thickness specifications, resulting in better stability of the product's microstructure and properties through flexible control. More preferably, when the thickness of the cold-rolled strip is 0.9 mm, the cold rolling reduction rate is 60%. Based on this, the cold rolling reduction rate decreases by 3-5% for every 0.4 mm increase in the thickness of the cold-rolled strip.

[0060] In step (d), the soaking temperature is typically, but not limited to, for example, 820°C, 825°C, 830°C, 835°C, 840°C, 845°C, 850°C, 855°C, or 860°C; the soaking time is typically, but not limited to, for example, 70s, 80s, 90s, 100s, 110s, 120s, 130s, 140s, or 150s; the alloying temperature is typically, but not limited to, for example, 510°C, 515°C, 520°C, 525°C, 530°C, 535°C, 540°C, 545°C, 550°C, 555°C, or 560°C; and the alloying time is typically, but not limited to, for example, 6s, 9s, 12s, 15s, 18s, 21s, 24s, 27s, 30s, 33s, or 36s.

[0061] In step (d), the cold-rolled strip steel is heated to a uniform temperature using a segmented heating method. After reaching the uniform heating and holding time, it is cooled to 440–470°C using a segmented cooling method. After a period of uniform holding, it enters the zinc bath for galvanizing. The galvanizing bath contains 0.15%–0.30% Al, with the remainder being Zn and unavoidable impurities. The unit speed (i.e., the belt speed of the hot-dip galvanized steel strip) is 80–120 m / min. To adjust the strip shape and yield strength, the elongation range is 0.30–0.50%. After galvanizing, the strip is first cooled to 390–420°C by an air knife, and then heated to the alloying temperature for alloying. After reaching the alloying holding time, it is cooled to room temperature using a fan to obtain 980 MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel. Cooling temperatures (i.e., galvanizing temperatures) are typically, but not limited to, 440°C, 445°C, 450°C, 455°C, 460°C, 465°C, or 470°C. Unit speeds are typically, but not limited to, 80 m / min, 90 m / min, 100 m / min, 110 m / min, or 120 m / min. Leveling elongation is typically, but not limited to, 0.30%, 0.35%, 0.40%, 0.45%, or 0.50%. Air knife cooling temperatures after galvanizing are typically, but not limited to, 390°C, 395°C, 400°C, 405°C, 410°C, 415°C, or 420°C.

[0062] In step (d), the advantage of using a segmented heating method is that it facilitates recrystallization before austenitization and reduces the influence of banded martensite. The advantage of using a segmented cooling method is that different microstructures are generated at different stages, which is beneficial for precise control of the microstructure and obtaining an ideal microstructure, thus precisely controlling the formation of the product microstructure. In some embodiments, the segmented heating method is as follows: first heating to 300°C at a heating rate of 8–20°C / s, then heating to 700°C at a heating rate of 3–10°C / s, and then heating to the homogenization temperature at a heating rate of 0.5–3.2°C / s; the segmented cooling method is as follows: first slowly cooling to 710–750°C at a cooling rate of 1.2–7.8°C / s, and then cooling to 440–470°C at a cooling rate of 15–33°C / s. The heating rate of the first heating stage is typically, but not limited to, for example, 8°C / s, 10°C / s, 12°C / s, 14°C / s, 16°C / s, 18°C / s, or 20°C / s; the heating rate of the second heating stage is typically, but not limited to, for example, 0.5°C / s, 0.8°C / s, 1.2°C / s, 1.6°C / s, 2.0°C / s, 2.4°C / s, 2.8°C / s, or 3.2°C / s; the slow cooling rate is typically, but not limited to, for example, 1.2°C / s, 2°C / s, 3°C / s, 4°C / s, 5°C / s, 6°C / s, 7°C / s, or 7.8°C / s; and the rapid cooling rate is typically, but not limited to, for example, 15°C / s, 20°C / s, 25°C / s, 30°C / s, or 33°C / s.

[0063] In some embodiments, the unit speed gradually decreases as the thickness of the cold-rolled strip increases. Preferably, the unit speed decreases by 10 m / min for every 0.3 mm increase in the thickness of the cold-rolled strip. This facilitates speed control based on product thickness specifications, resulting in better product microstructure stability through flexible control. More preferably, when the thickness of the cold-rolled strip is 0.9 mm, the unit speed is 120 m / min, and thereafter, the unit speed decreases by 10 m / min for every 0.3 mm increase in the thickness of the cold-rolled strip.

[0064] In some embodiments, the leveling elongation gradually decreases as the thickness of the cold-rolled strip increases. Preferably, the leveling elongation decreases by 0.05% for every 0.4 mm increase in the thickness of the cold-rolled strip. This helps to control the leveling elongation according to the product thickness specifications, resulting in better stability of the product's microstructure through flexible control. More preferably, when the thickness of the cold-rolled strip is 0.9 mm, the leveling elongation is 0.50%, and based on this, the leveling elongation decreases by 0.05% for every 0.4 mm increase in the thickness of the cold-rolled strip.

[0065] In some embodiments, in step (d), the cold-rolled thin steel strip is first heated to 300°C at a heating rate of 8–20°C / s, then heated to 700°C at a heating rate of 3–10°C / s, and then heated to 820–860°C at a heating rate of 0.5–3.2°C / s; after being held at a uniform temperature for 70–150 s, it is slowly cooled to 710–750°C at a rate of 1.2–7.8°C / s, and then rapidly cooled to 440–470°C at a rate of 15–33°C / s, followed by a period of uniform holding. After a certain time, the strip enters a zinc bath for galvanizing. After galvanizing, the strip is first cooled to 390–420°C by an air knife, and then rapidly heated at a rate of 15–40°C / s for alloying treatment. The alloying temperature is 510–560°C, and the alloying holding time is 6–36 seconds. The alloyed strip is then cooled to room temperature using a fan (cooling rate ≥5°C / s). The plating solution contains 0.15%–0.30% Al, with the remainder being Zn and unavoidable impurities. The zinc layer weight per unit area is 60–110 g / cm². The unit speed is 80–120 m / min, gradually decreasing as the cold-rolled material thickness increases. For every 0.3 mm increase in the thickness of the cold-rolled thin strip, the unit speed is adjusted accordingly, decreasing by 10 m / min. The leveling elongation ranges from 0.30% to 0.50%, decreasing by 0.05% for every 0.4 mm increase in material thickness.

[0066] According to a second aspect of the present invention, a 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel is provided, which is prepared by means of the method described in the first aspect of the present invention.

[0067] The yield strength of 980MPa grade low-carbon equivalent alloyed hot-dip galvanized duplex steel is 650-745MPa, the tensile strength is 1015-1083MPa, and the elongation is A 80 The value is 11.0%–14.5%, and the porosity is 44%–54%.

[0068] The microstructure of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized dual-phase steel consists of 30-40% ferrite matrix and 60-65% blocky martensite.

[0069] The present invention will now be described in detail through specific embodiments.

[0070] Example

[0071] The following provides eight examples of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steels, whose chemical compositions are shown in Table 1.

[0072] Table 1 Chemical composition (wt.%) of 980MPa grade low-spot welding carbon equivalent alloyed hot-dip galvanized duplex steel

[0073] serial number C Si Mn P S Cr Nb Ti B Als Example 1 0.085 0.48 1.95 0.012 0.005 0.58 0.033 0.042 0.0028 0.045 Example 2 0.090 0.45 2.05 0.010 0.003 0.55 0.028 0.038 0.0025 0.038 Example 3 0.093 0.40 1.90 0.008 0.006 0.60 0.030 0.040 0.0024 0.040 Example 4 0.095 0.42 2.08 0.010 0.004 0.62 0.032 0.035 0.0026 0.035 Example 5 0.05 0.6 1.80 0.018 0.008 0.40 0.015 0.025 0.0020 0.065 Example 6 0.12 0.2 2.30 0.015 0.007 0.80 0.035 0.055 0.0030 0.010 Example 7 0.08 0.50 2.15 0.012 0.005 0.45 0.01 0.045 0.0010 0.055 Example 8 0.10 0.35 1.85 0.010 0.009 0.65 0.04 0.030 0.0050 0.025

[0074] The preparation method of the above-mentioned 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel plate is as follows:

[0075] A. Smelting process: Through smelting process, prepare 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel billets with the chemical composition shown in Table 1.

[0076] B. Hot rolling process: The slab is heated, descaled, hot rolled and laminar cooled to obtain a hot rolled coil. The specific hot rolling process parameters are shown in Table 2.

[0077] Table 2 Main process parameters for hot rolling of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel

[0078]

[0079] The winding temperature is 520-580℃, and "U-shaped winding" is adopted (the winding temperature of the first 80m of the belt head is 570-630℃, and the winding temperature of the last 100m of the belt tail is 580-640℃). Laminar flow cooling adopts the front-end cooling method.

[0080] C. Pickling and rolling process: After pickling the hot-rolled coil, it is cold-rolled into thin strip steel. The thickness of the hot-rolled plate, the thickness of the cold-rolled coil, and the cold-rolling reduction rate are shown in Table 3.

[0081] Table 3. Raw materials, finished products, and reduction rates of cold-rolled 980MPa grade low-spot welding carbon equivalent alloyed hot-dip galvanized duplex steel.

[0082] serial number Cold-rolled sheet thickness / mm Cold rolling reduction rate / % Example 1 1.2 57 Example 2 0.9 60 Example 3 1.8 51 Example 4 2.1 48 Example 5 0.9 60 Example 6 1.3 56 Example 7 2.1 48 Example 8 2.5 44

[0083] D. Hot-dip galvanizing process: Cold-rolled thin steel strips are first heated in sections to 300℃, 700℃, and 820-860℃ at heating rates of 8-20℃ / s, 3-10℃ / s, and 0.5-3.2℃ / s, respectively. After being held at the same temperature for 70-150s, they are then slowly cooled to 710-750℃ at rates of 1.2-7.8℃ / s and 15-33℃ / s, respectively, and then rapidly cooled to 440-470℃. After being held at the same temperature for a period of time, they are then placed in a zinc bath for galvanizing. After galvanizing, the strip steel is first cooled to 390–420℃ by an air knife, and then rapidly heated at a rate of 15–40℃ / s to undergo alloying treatment. The alloying temperature is 510–560℃, and the alloying holding time is 6–36s. After alloying, the strip steel is cooled to room temperature using a fan (cooling rate ≥5℃ / s). The plating solution contains 0.15%–0.30% Al, with the remainder being Zn and unavoidable impurities. The zinc layer weight per unit area is 60–110 g / cm². The unit speed is 80–120 m / min, gradually decreasing as the cold-rolled material thickness increases. For every 0.3 mm increase in the thickness of the cold-rolled thin strip steel, the unit speed is adjusted accordingly, decreasing by 10 m / min. The leveling elongation range is 0.30–0.50%, decreasing by 0.05% for every 0.4 mm increase in material thickness. Specific hot-dip galvanizing process parameters are shown in Table 4.

[0084] Table 4. Main process parameters of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel

[0085]

[0086] Comparative Example

[0087] The corresponding products were prepared as comparative examples according to the processes disclosed in documents CN 108486501 A, CN 109402525 A, and CN 109097576 A.

[0088] Performance Characterization

[0089] The microstructure of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel (corresponding to Example 1) prepared by the process of the present invention is as follows: Figures 1 to 2 As shown. From Figure 1 and Figure 2 As can be seen from the data, the microstructure of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel prepared by this invention consists of ferrite and blocky martensite, and the microstructure is uniform.

[0090] The mechanical properties of the above-mentioned 980MPa grade low-spot weld carbon equivalent alloyed hot-dip galvanized duplex steel were tested according to GB / T228-2010 "Metallic materials - Tensile testing at room temperature", and the hole-expanding performance of the above-mentioned 980MPa grade low-spot weld carbon equivalent alloyed hot-dip galvanized duplex steel was tested according to GB / T 24524-2009 "Metallic materials - Plate and strip hole expansion test method". See Table 5 for details.

[0091] Table 5. Properties of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel

[0092]

[0093] As can be seen from Table 5, the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized dual-phase steel prepared by the present invention, compared with the steel in the prior art, has excellent hole expansion performance while ensuring high yield strength, tensile strength and elongation.

Claims

1. A method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, characterized in that, Includes the following steps: (a) Smelting process: Smelting is carried out according to the chemical composition of 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, and casting it into slabs. The chemical composition of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel includes, by mass percentage: C: 0.05%~0.12%, Si: 0.42%~0.60%, Mn: 1.80%~2.08%, P≤0.018%, S≤0.009%, Als: 0.010%~0.065%, N≤0.0065%, Cr: 0.40%~0.80%, Nb: 0.01%~0.04%, Ti: 0.038%~0.055%, B: 0.0010%~0.0050%, with the balance being Fe and unavoidable impurities; (b) Hot rolling process: The slab obtained in step (a) is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled coil; (c) Pickling and rolling process: The hot-rolled coil obtained in step (b) is pickled and then cold-rolled into cold-rolled strip steel; (d) Hot-dip galvanizing process: The cold-rolled strip steel obtained in step (c) is heated to the uniform heating temperature by a segmented heating method and then held for uniform heating. After reaching the uniform heating holding time, it is cooled to 440~470℃ by a segmented cooling method. After holding for a period of time, it is put into the zinc bath for galvanizing treatment. After galvanizing, it is cooled to 390~420℃, and then heated to the alloying temperature for alloying treatment. After reaching the alloying holding time, it is cooled to room temperature to obtain 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel with a microstructure consisting of 30~40% ferrite matrix and 60~65% blocky martensite. The uniform heating temperature is 840~860℃, the uniform heating holding time is 100~150s, the alloying temperature is 510~560℃, and the alloying holding time is 6~36s.

2. The method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel according to claim 1, characterized in that, The chemical composition of the 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, by mass percentage, includes: C: 0.08%~0.10%, Si: 0.42%~0.50%, Mn: 1.85%~2.08%, P≤0.015%, S≤0.008%, Als: 0.025%~0.055%, N≤0.0060%, Cr: 0.45%~0.65%, Nb: 0.015~0.035%, Ti: 0.038~0.045%, B: 0.0020%~0.0030%, with the balance being Fe and unavoidable impurities.

3. The method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel according to claim 1, characterized in that, Includes one or more of the following: In step (b), the heating temperature is 1215-1250℃, the finishing rolling start temperature is 1035-1130℃, the finishing rolling finish temperature is 850-920℃, the laminar flow cooling adopts the front-stage cooling method, the upper and lower surface cooling rates are 25% and 50% respectively, the coiling temperature is 520-580℃, and the coiling method adopts "U-shaped coiling"; In step (c), the cold rolling reduction rate is 44%-60%; In step (d), the zinc plating solution contains 0.15% to 0.30% Al, with the remainder being Zn and unavoidable impurities. The unit speed is 80-120 m / min, and the flattening elongation range is 0.30-0.50%.

4. The method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel according to claim 1 or 3, characterized in that, In step (c), as the thickness of the cold-rolled strip increases, the cold rolling reduction rate gradually decreases. For every 0.4 mm increase in the thickness of the cold-rolled strip, the cold rolling reduction rate decreases by 3-5%.

5. The method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel according to claim 1, characterized in that, In step (d): The segmented heating method is as follows: first, heat to 300℃ at a heating rate of 8~20℃ / s, then heat to 700℃ at a heating rate of 3~10℃ / s, and then heat to the homogenization temperature at a heating rate of 0.5~3.2℃ / s. The segmented cooling method is as follows: first, cool to 710-750℃ at a cooling rate of 1.2-7.8℃ / s, and then cool to 440-470℃ at a cooling rate of 15-33℃ / s.

6. The method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel according to claim 3, characterized in that, In step (d): As the thickness of cold-rolled strip increases, the speed of the unit gradually decreases. For every 0.3 mm increase in the thickness of cold-rolled strip, the speed of the unit decreases by 10 m / min. As the thickness of cold-rolled strip increases, the flat elongation gradually decreases. For every 0.4 mm increase in the thickness of cold-rolled strip, the flat elongation decreases by 0.05%.

7. The method for preparing 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel according to claim 1 or 3, characterized in that, The thickness of the hot-rolled plate obtained in step (b) is 2.25-4.50 mm, and the thickness of the cold-rolled strip obtained in step (c) is 0.9-2.5 mm.

8. A 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel, characterized in that, It is prepared using the method described in any one of claims 1-7.

9. The 980MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel according to claim 8, characterized in that, The 980MPa grade low-spot welding carbon equivalent alloyed hot-dip galvanized duplex steel has a yield strength of 650~745MPa, a tensile strength of 1015~1083MPa, and an elongation A. 80 The value is 11.0~14.5%, and the porosity is 44~54%.

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