A low-cost low-point welding carbon equivalent 780mpa grade hot-dip galvanized dual-phase steel and a preparation method thereof

Abstract

The present application relates to a kind of low-cost low spot welding carbon equivalent 780MPa grade hot-dip galvanizing dual-phase steel and its preparation method, the low-cost low spot welding carbon equivalent 780MPa grade hot-dip galvanizing dual-phase steel includes the following mass percentage of component: C:0.04%~0.09%, Si:0.25%~0.70%, Mn:1.55%~2.00%, P≤0.010%, S≤0.003%, Als:0.01%~0.070%, N≤0.0050%, Nb:0.010~0.040%, Cr:0.30~0.70% the rest element is Fe and inevitable impurity.The present application can reduce spot welding carbon equivalent, improve welding performance, optimize stamping and reaming performance by strictly controlling each component ratio;The present application can also be adjusted according to product thickness specification to each process, and the product organization performance stability is better by flexible control.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of cold-rolled plate strip production, in particular to a low-cost low-spot welding carbon equivalent 780MPa grade hot-dip galvanized dual-phase steel suitable for production by a hot-dip galvanizing unit, and further relates to a preparation method of the low-cost low-spot welding carbon equivalent 780MPa grade hot-dip galvanized dual-phase steel. BACKGROUND

[0002] With the gradual improvement of people's awareness of energy saving and material service safety, many automobile manufacturers choose high-strength steel as automobile materials. The automobile industry adopts hot-dip galvanized high-strength steel plate to reduce the thickness of the steel plate, which can improve the corrosion resistance, dent resistance, durability, large deformation impact strength and safety of the automobile, and therefore the automobile steel plate will develop towards the direction of high strength, high toughness, corrosion resistance and easy forming processing. With the increasing requirement of automobile body on corrosion resistance, galvanized automobile plate is more and more used in automobile production. Influenced by the low resistivity and hardness of the plated layer, the increase of the contact surface between the spot welding samples and the decrease of the contact resistance, the hot-dip galvanizing welding performance is lower than that of the continuous annealing product of the same strength. At the same time, since the martensite of the hot-dip galvanized dual-phase steel is formed in the air cooling stage after galvanizing, the martensite has high quenching hardness, which leads to a large difference in hardness between the soft (ferrite) and hard (martensite) phases, and is not conducive to obtaining good hole expansion performance. Through literature retrieval, there are similar documents on the preparation method of low-cost low-spot welding carbon equivalent 780MPa grade galvanized dual-phase steel:

[0003] CN 109825768 A discloses a 780 MPa grade ultra-thin specification hot-dip galvanized dual-phase steel and a preparation method thereof. The chemical composition of the steel is as follows in terms of weight percentage: C: 0.03% to 0.07%, Si: 0.001% to 0.10%, Mn: 1.00% to 1.80%, P≤0.012%, S≤0.006%, Al: 0.60 to 1.20%, Nb: 0.010 to 0.050%, Ti: 0.010 to 0.050%, Cr: 0.10 to 0.30%, Mo: 0.20 to 0.40%, N≤0.004%, Ni≤0.20%, Cu≤0.20%, and the rest is Fe and inevitable impurities; and C-0.003×Si+0.014×Mn-0.040×P-0.222×S+0.023×Ni+0.003×Cu-0.004×Mo≤0.085; Mo+Cr≥0.30. The hot rolling final rolling temperature is controlled at 820 to 900°C, and the thickness of the hot rolling plate is controlled at 0.6 to 1.6 mm; the average cooling rate of the strip steel is controlled at≥20°C / s, the coiling temperature is 550 to 650°C, and the cold rolling reduction rate is controlled at 30% to 70%; the hot-dip galvanizing soaking temperature is 780 to 850°C, the soaking time is 30 to 200 s, the strip steel after soaking is cooled at a cooling rate of≥30°C / s, the dew point in the annealing furnace is -15 to -60°C, the hydrogen content H2 in the furnace is 1 to 10%, the temperature of the strip steel when entering the zinc pot is 450 to 500°C, the zinc liquid temperature is 450 to 470°C, the aluminum content of the zinc liquid is 0.15 to 0.25%, and the strip steel is cooled at a cooling rate of≥15°C / s to below 200°C during cooling after hot-dip galvanizing. The steel contains a relatively high Al content (0.60 to 1.20%), which easily causes the molten steel to become sticky and easily block the water hole, and the high Al2O3 inclusions lead to a decrease in the purity of the molten steel. In addition, the high Ti (0.010 to 0.050%) and Nb (0.010 to 0.050%) elements lead to a high hot-rolled state performance of the strip steel, and the load of the pickling mill set is heavy. The high Nb and Ti elements lead to an increase in the recrystallization temperature, and a higher zinc soaking process is required for hot-dip galvanizing, which is high in cost. The addition of the noble alloy Mo element leads to a substantial increase in the alloy cost.

[0004] CN 109097705 A discloses an 800MPa grade cold-rolled hot-dip galvanized duplex steel and its production method. Its chemical composition by weight percentage is as follows: C: 0.05%-0.10%, Mn: 1.60%-2.30%, Als: 0.010-1.0%, Si: 0.10%-0.60%, Nb: 0.010-0.050%, Cr: 0.05-0.30%, Mo: 0.05-0.30%, P≤0.015%, S≤0.010%, N≤0.008%, with the remainder being Fe and unavoidable impurities. The precious metal elements Cr and Mo satisfy the following relationship: 0.05≤Cr+Mo≤0.30, and C+Si / 30+Mn / 20≤0.22. The superheat during steel casting is 15-30℃, the slab exit temperature is 1180-1300℃, the heating time is 150min-300min, the hot rolling final rolling temperature is 850-950℃, the strip temperature is ≥620℃, the average cooling rate of the strip is ≥15℃ / s, and the coiling temperature is 500-620℃. The cold rolling reduction rate of the strip steel is controlled at 40%-70%, the hot-dip galvanizing temperature is 760-840℃, the heating rate at temperatures ≤760℃ is ≤5℃ / s, the holding time at 760-840℃ is 60-300s, the cooling rate is ≥15℃ / s, the dew point in the annealing furnace is 0~-40℃, the H2 content in the furnace is 1-5%, H2O / H2≤1.0, the strip steel temperature entering the zinc pot is 440-500℃, the zinc liquid temperature is 450-470℃, and the aluminum content in the zinc liquid is 0.15-0.25%. This patent contains a relatively large amount of precious metal elements such as Mo and Nb, which increases the alloy cost. The high Al content (0.60~1.20%) easily makes the molten steel sticky and prone to clogging the nozzle. At the same time, the high Al2O3 inclusions lead to a decrease in the purity of the molten steel.

[0005] CN 109943765A discloses an 800MPa grade high yield strength ratio cold-rolled duplex steel and its preparation method. The chemical composition by mass percentage is: C: 0.08-0.10%, Si: 0.6-0.8%, Mn: 1.8-2.0%, Cr: 0.6-0.8%, Als: 0.03-0.06%, Nb: 0.04-0.06%, P≤0.02%, S≤0.01%, with the balance being Fe and unavoidable impurities. The billet or ingot is heated to a temperature range of 1180-1260℃ and rolled 5-10 times by a roughing mill. After rough rolling, the thickness of the billet or ingot is 30-50mm. It is then rolled 5-7 times by a hot continuous rolling mill. After rolling to the required thickness, the steel is coiled within a temperature range of T1 (540-620℃). The cold rolling reduction rate is 50-75%. The cold-rolled steel, after pickling and cold rolling, is first slowly heated to 170℃, then rapidly heated to a temperature range of T2 and held for 90-160 seconds. It is then cooled to a temperature range of T3 at a rate of V1, and then rapidly cooled to a temperature range of T4 at a cooling rate of V2. After aging for 350-700 seconds, it is cooled to room temperature. The temperature ranges are: T2 (830-850℃), T3 (640-700℃), and T4 (300-340℃). The values ​​of V1 and V2 are 5-7℃ / s and 36-60℃ / s, respectively. This patent has a high Si content, which is detrimental to obtaining good surface quality. Furthermore, it is a cold-rolled continuous annealed product, which differs significantly from hot-dip galvanized products.

[0006] Therefore, existing technologies still need improvement. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention strictly controls the C content (0.04%–0.08%), P content (≤0.010%), and S content (≤0.003%), which helps reduce the carbon equivalent of spot welding (Ceq(spot) = C + Si / 30 + Mn / 20 + 2P + 4S), thus improving welding performance. Simultaneously, by reducing the C content and using appropriate rolling and hot-dip galvanizing processes, a larger quantity of martensite with lower hardness is obtained (reducing the C and alloy content per unit austenite before transformation), ensuring strength while reducing the hardness difference between soft and hard phases, resulting in good hole-expanding performance. A lower Si content minimizes the impact on coating quality, while appropriate amounts of Mn and Cr elements improve hardenability and ensure strength. The absence of expensive alloying elements such as Mo and Ni helps control costs.

[0008] Specifically, according to one aspect of the present invention, a low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel is provided, comprising the following components by mass percentage: C: 0.04%–0.09%, Si: 0.25%–0.70%, Mn: 1.55%–2.00%, P≤0.010%, S≤0.003%, Als: 0.01%–0.070%, N≤0.0050%, Nb: 0.010–0.040%, Cr: 0.30–0.70%, with the remainder being Fe and unavoidable impurities.

[0009] In embodiments of the present invention, the low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel comprises the following components by mass percentage: C: 0.05%–0.08%, Si: 0.35%–0.50%, Mn: 1.65%–1.90%, Nb: 0.015%–0.030%, Al: 0.025%–0.055%, Cr: 0.35%–0.55%, P≤0.008%, S≤0.002%, N≤0.0035%, with the balance being Fe and unavoidable impurities.

[0010] According to another aspect of the present invention, a method for preparing low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel is provided, comprising the following steps: a. Smelting process: smelting the chemical composition of the low-cost, low-spot-welding carbon equivalent 780MPa grade galvanized duplex steel comprising the following mass percentages to obtain a slab: C: 0.04%~0.09%, Si: 0.25%~0.70%, Mn: 1.55%~2.00%, P≤0.010%, S≤0.003%, Als: 0.01%~0.070%, N≤0.0050%, Nb: 0.010~0.00%. 0.40%, Cr: 0.30-0.70%, the remaining elements are Fe and unavoidable impurities; b. Hot rolling process: the slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled coil; c. Pickling process: the hot rolled coil is pickled and then cold rolled into a thin strip of predetermined thickness; d. Hot-dip galvanizing process: the cold rolled thin steel strip is heated in sections, and after a first predetermined time of uniform heat preservation, it is slowly cooled and then rapidly cooled in sequence, and after a second predetermined time of uniform heat preservation, it is placed in a zinc bath for galvanizing treatment for a third predetermined time. After exiting the zinc bath, it is cooled to room temperature to obtain a low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel.

[0011] In an embodiment of the present invention, step b includes: heating the slab to 1230±20℃ for austenitization, holding it at that temperature for 5.0h, then descaling and rough rolling, with an intermediate slab thickness of 38±2mm; the finishing rolling start temperature is ≥1030℃, and the finishing rolling temperature is ≥845℃; laminar flow cooling is performed using a front-stage cooling method, with a coiling temperature of 480~560℃ and a hot-rolled thickness of 3.00~6.00mm.

[0012] In an embodiment of the present invention, in step c, the predetermined thickness is 0.7-2.5 mm, and the cold rolling reduction rate is 60%-78%.

[0013] In the embodiments of the present invention, the cold rolling reduction rate gradually decreases as the cold rolling thickness of the material increases. For every 0.3 mm increase in the thickness specification of cold-rolled thin strip steel, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by about 3%.

[0014] In an embodiment of the present invention, step d includes: first heating the cold-rolled thin steel strip in sections to 700°C and 800-840°C respectively at a first heating rate and a second heating rate; after heat homogenization and heat preservation for the first predetermined time, slowly cooling it to 700-735°C and rapidly cooling it to 450-470°C respectively at a first cooling rate and a second cooling rate; after heat homogenization and heat preservation for the second predetermined time, entering the zinc bath for galvanizing treatment for the third predetermined time; and after exiting the zinc bath, cooling it to room temperature at a rate of ≥5°C / s.

[0015] In an embodiment of the present invention, the first predetermined time is 40-110s, the second predetermined time is 60-120s, the third predetermined time is 8-25s, the first heating rate is 5-20℃ / s, the second heating rate is 0.3-4℃ / s, the first cooling rate is 0.8-6.5℃ / s, and the second cooling rate is 12-30℃ / s.

[0016] In an embodiment of the present invention, the unit speed is 70-130 m / min, and the flattening elongation range is 0.20-0.50%.

[0017] In an embodiment of the present invention, the unit speed gradually decreases as the cold-rolled material thickness increases. For every 0.3 mm increase in the thickness specification of the cold-rolled thin strip steel, the unit speed is adjusted accordingly, and the unit speed is reduced by 10 m / min. For every 0.3 mm increase in material thickness, the flattening elongation rate decreases by 0.05%.

[0018] This invention, through strict control of C content (0.04%–0.08%), P content (≤0.010%), and S content (≤0.003%), helps to reduce the carbon equivalent of spot welding (Ceq(spot) = C + Si / 30 + Mn / 20 + 2P + 4S), improve welding performance, and optimize stamping and hole-expanding performance. By using a lower Si content, the invention reduces the impact on coating quality, while appropriate amounts of Mn and Cr elements improve hardenability and ensure strength. This invention does not involve the addition of expensive alloying elements such as Mo and Ni, which helps control costs. It exhibits excellent stamping, hole-expanding, and flanging performance, and by adjusting each process according to the product thickness specifications, flexible control results in better product microstructure and performance stability. Attached Figure Description

[0019] Figure 1 The diagram shows a flow chart of a low-cost, low-spot-welding carbon equivalent 780MPa hot-dip galvanized duplex steel preparation method provided by the present invention.

[0020] Figure 2 An optical microscope image of the low-cost, low-spot-welding carbon equivalent 780MPa hot-dip galvanized duplex steel provided by the present invention is shown; and

[0021] Figure 3 The image shown is a scanning electron microscope image of the low-cost, low-spot-welding carbon equivalent 780MPa hot-dip galvanized duplex steel provided by the present invention. Detailed Implementation

[0022] It should be understood that the embodiments of the invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in this invention, those skilled in the art will readily recognize that various modifications are possible without substantially departing from the teachings of the invention. Accordingly, all such modifications should be included within the scope of the invention. Other substitutions, modifications, variations, and deletions can be made to the design, operating conditions, and parameters of the following exemplary embodiments without departing from the spirit of the invention.

[0023] According to one aspect of the present invention, a low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel is provided, comprising the following components by mass percentage: C: 0.04%–0.09%, Si: 0.25%–0.70%, Mn: 1.55%–2.00%, P≤0.010%, S≤0.003%, Als: 0.01%–0.070%, N≤0.0050%, Nb: 0.010–0.040%, Cr: 0.30–0.70%, with the remainder being Fe and unavoidable impurities. Wherein, Als refers to effective Al dissolved in the iron matrix.

[0024] In embodiments of the present invention, the low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel comprises the following components by mass percentage: C: 0.05%–0.08%, Si: 0.35%–0.50%, Mn: 1.65%–1.90%, Nb: 0.015%–0.030%, Al: 0.025%–0.055%, Cr: 0.35%–0.55%, P≤0.008%, S≤0.002%, N≤0.0035%, with the balance being Fe and unavoidable impurities.

[0025] The aforementioned low-cost, low-spot-welding carbon equivalent 780MPa grade galvanized duplex steel has a yield strength of 450-510MPa, a tensile strength of 790-835MPa, an elongation (A80) of 16.0-21.0%, a yield strength ratio of 0.53-0.65, and a hole expansion rate of 35%-50%. Its microstructure consists of 35%-40% ferrite (average grain size of 2.0μm) + 60%-65% martensite.

[0026] According to another aspect of the present invention, a method for preparing low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel is provided, such as... Figure 1 As shown, it includes the following steps:

[0027] a. Smelting process: The chemical composition of low-cost, low-spot welding carbon equivalent 780MPa grade galvanized duplex steel, including the following mass percentages, is smelted to obtain a slab: C: 0.04%~0.09%, Si: 0.25%~0.70%, Mn: 1.55%~2.00%, P≤0.010%, S≤0.003%, Als: 0.01%~0.070%, N≤0.0050%, Nb: 0.010~0.040%, Cr: 0.30~0.70%, with the remainder being Fe and unavoidable impurities;

[0028] b. Hot rolling process: The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot rolled coil;

[0029] c. Pickling and rolling process: After pickling, the hot-rolled coil is cold-rolled into a thin strip of steel of a predetermined thickness;

[0030] d. Hot-dip galvanizing process: Cold-rolled thin steel strip is heated in sections, and after a first predetermined time of uniform heat preservation, it is slowly cooled and then rapidly cooled in sequence. After a second predetermined time of uniform heat preservation, it is placed in a zinc bath for galvanizing treatment for a third predetermined time. After exiting the zinc bath, it is cooled to room temperature to obtain low-cost, low-spot-weld carbon equivalent 780MPa grade hot-dip galvanized duplex steel.

[0031] In an embodiment of the present invention, in step a, the galvanized duplex steel with a low cost and low spot welding carbon equivalent of 780MPa is smelted according to its chemical composition and then cast into a slab.

[0032] In an embodiment of the present invention, in step b, the slab is heated, descaled, rough rolled, finish rolled, and laminar flow cooled to obtain a hot-rolled coil; it is heated to 1230±20℃ for austenitization, held for 5.0h, and then descaled and rough rolled, with an intermediate slab thickness of 38±2mm; the finishing rolling start temperature is ≥1030℃, and the finishing rolling temperature is ≥845℃; the laminar flow cooling adopts a front-stage cooling method, the coiling temperature is 480~560℃, and the hot-rolled thickness is 3.00~6.00mm.

[0033] In an embodiment of the present invention, in step c, the hot-rolled coil is pickled and then cold-rolled into a thin strip steel of 0.7-2.5mm, with a cold rolling reduction rate of 60%-78%. As the cold rolling thickness of the material increases, the cold rolling reduction rate gradually decreases. For every 0.3mm increase in the thickness specification of the cold-rolled thin strip steel, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by about 3%.

[0034] In an embodiment of the present invention, in step d, the cold-rolled thin steel strip is first heated in sections to 700°C and 800-840°C at heating rates of 5-20°C / s and 0.3-4°C / s, respectively; after being held at the same temperature for 40-110 seconds, it is then slowly cooled to 700-735°C and rapidly cooled to 450-470°C at rates of 0.8-6.5°C / s and 12-30°C / s, respectively. After being held at the same temperature for a period of time, it is then placed in a zinc bath for galvanizing treatment for 8-25 seconds. After exiting the zinc bath, it is cooled to room temperature at a rate of ≥5°C / s. The unit speed is 70-130 m / min, and the unit speed gradually decreases as the cold-rolled thickness of the material increases. For every 0.3 mm increase in the thickness of the cold-rolled thin steel strip, the unit speed is adjusted accordingly, decreasing by 10 m / min. The flat elongation ranges from 0.20% to 0.50%, and decreases by 0.05% for every 0.3mm increase in material thickness.

[0035] This invention, through strict control of the content of each component, enables the production of low-cost, low-spot-weld carbon equivalent 780MPa grade hot-dip galvanized duplex steel with excellent performance, low cost, and good stability. The various alloying elements play different roles in this low-cost, low-spot-weld carbon equivalent 780MPa grade galvanized duplex steel, as detailed below:

[0036] 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 0.04%–0.09%, preferably 0.05%–0.08%.

[0037] 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 0.25%–0.70%, preferably 0.35%–0.50%.

[0038] 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 1.55%–2.00%, preferably 1.65%–1.90%.

[0039] Cr can replace Mn, increasing the strength of steel and reducing segregation. It can also inhibit pearlite transformation, significantly improving the hardenability of steel materials and facilitating the acquisition of a sufficient amount of martensite to ensure strength. Furthermore, the addition of a certain amount of Cr in this invention can 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 iron oxide scale and the formation of AlN, and improve the adhesion of iron oxide scale, thereby effectively reducing pitting defects caused by iron oxide scale indentation. Therefore, in this invention, the Cr content is 0.30–0.70%, preferably 0.35–0.55%.

[0040] Al is a common deoxidizer in steel and can also form AlN pinning 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% to 0.070%, preferably 0.025% to 0.055%.

[0041] 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 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 ferrite will reprecipitate, resulting in significant precipitation strengthening. Therefore, the Nb content is 0.01–0.04%, preferably 0.015–0.030%. Phosphorus (P) is an impurity element in steel, which tends to segregate at grain boundaries, weakening intergranular bonding. During rapid solidification, a high P content can easily lead to billet cracking. Furthermore, P significantly reduces spot welding performance; therefore, the P content must be strictly controlled to obtain good spot welding results. Therefore, in this invention, the P content is ≤0.010%, preferably ≤0.008%.

[0042] Sulfur (S) is an impurity element in steel, which 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 leading to fine microcracks on the surface of the cast billet during rapid solidification. At the same time, S significantly reduces spot welding performance; therefore, the S content must be strictly controlled to obtain good spot welding results. Thus, the S content in this invention is ≤0.003%, preferably ≤0.002%.

[0043] Nitrogen (N), an impurity element in steel, readily enters the interstitial spaces of iron elements due to its small atomic size, significantly increasing lattice distortion and thus greatly enhancing strength. However, it also noticeably deteriorates the steel's ductility and toughness. Furthermore, N readily combines with Al, B, and Ti in steel to form second-phase compounds such as AlN, BN, and TiN, which, while strengthening the steel, also worsen its ductility and toughness. Especially when the N content is too high, the combined NiN with Ti can precipitate to micrometer-sized particles, failing to provide strengthening and instead causing stress concentration and crack initiation. Therefore, the N content must be strictly controlled. Thus, in this invention, the N content is ≤0.0050%, preferably 0.0035%.

[0044] The present invention is further illustrated below through specific embodiments:

[0045] This embodiment provides four sets of low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steels, the chemical composition of which is shown in Table 1:

[0046] Table 1. Chemical composition (wt.%) of low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel

[0047] No. C Si Mn P S Cr Nb Als Example 1 0.055 0.45 1.80 0.005 0.002 0.50 0.028 0.043 Example 2 0.065 0.40 1.70 0.004 0.001 0.40 0.023 0.045 Example 3 0.060 0.43 1.75 0.005 0.001 0.48 0.025 0.035 Example 4 0.070 0.38 1.68 0.005 0.002 0.38 0.020 0.044

[0048] The preparation method of the above-mentioned low-cost spot-welding carbon equivalent 780MPa hot-dip galvanized duplex steel sheet is as follows:

[0049] A. Smelting process: Through smelting process, prepare 780MPa grade low spot welding carbon equivalent alloyed zinc duplex steel billets with the chemical composition shown in Table 1.

[0050] 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.

[0051] Its final rolling temperature is ≥845℃, laminar flow cooling adopts front-stage cooling method, and the coiling temperature is 480~560℃.

[0052] 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.

[0053] D. Hot-dip galvanizing process: The cold-rolled thin steel strip is first heated in sections to 700℃ and 800-840℃ at heating rates of 5-20℃ / s and 0.3-4℃ / s respectively. After heat homogenization and holding for 40-110s, it is slowly cooled to 700-735℃ and rapidly cooled to 450-470℃ at rates of 0.8-6.5℃ / s and 12-30℃ / s respectively. After a period of even heat homogenization and holding, it enters the zinc bath for galvanizing treatment for 8-25s. After exiting the zinc bath, it is cooled to room temperature at a rate of ≥5℃ / s. The unit speed is 70-130m / min. As the cold-rolled thickness of the material increases, the unit speed gradually decreases. For every 0.3mm increase in the thickness of the cold-rolled thin steel strip, the unit speed is adjusted accordingly, decreasing by 10m / min. The flat elongation ranges from 0.20% to 0.50%, and decreases by 0.05% for every 0.3mm increase in material thickness. Specific hot-dip galvanizing process parameters are shown in Table 4.

[0054] Table 2. Main process parameters for hot-rolling of low-cost, spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel.

[0055] No. Tapping temperature / °C Billet thickness / mm Opening temperature / °C Finishing temperature / °C Coiling temperature / °C Hot rolling thickness / mm Example 1 1235 39.0 1087 875 508 5.90 Example 2 1220 38.0 1069 887 523 5.30 Example 3 1233 37.5 1055 877 533 4.65 Example 4 1228 38.5 1078 868 515 5.60

[0056] Table 3 Low-cost spot welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel cold-rolled raw materials, finished products, and reduction rates

[0057] No. Hot rolling plate thickness / mm Cold rolling plate thickness / mm Cold rolling reduction / % Example 1 5.90 2.20 63 Example 2 5.30 1.60 69 Example 3 4.65 1.30 72 Example 4 5.60 1.90 66

[0058] Table 4. Main process parameters for low-cost spot welding of 780MPa grade hot-dip galvanized duplex steel.

[0059] No. Soaking temperature / °C Temper end temperature / °C Galvanizing (equalization) temperature / °C Line speed m / min Temper elongation / % Example 1 825 725 465 80 0.25 Example 2 815 715 455 100 0.35 Example 3 810 710 450 110 0.40 Example 4 820 720 460 90 0.30

[0060] The microstructure of the low-cost spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel prepared by the above process is as follows: Figure 2 to Figure 3As shown in Table 5, the mechanical properties of the above-mentioned low-cost spot weld carbon equivalent 780MPa grade hot-dip galvanized duplex steel were tested according to GB / T228-2010 "Metallic Materials - Tensile Testing at Room Temperature". The spot weld carbon equivalent was calculated according to Ceq(spot)=C+Si / 30+Mn / 20+2P+4S.

[0061] Table 5 Low-cost spot welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel

[0062] No. Yield strength / MPa Tensile strength / MPa Elongation A 80 %]] Yield ratio Ceq(spot) Example 1 463 812 19.5 0.570 0.178 Example 2 476 820 18.0 0.580 0.175 Example 3 484 825 17.5 0.587 0.176 Example 4 470 818 19.0 0.575 0.185 CN 109825768 A 502 785 17.0 0.603 CN 109097705 A 486 830 18.0 0.631 CN 109943765 A 545 836 16.5 0.652

[0063] Therefore, it can be seen that the present invention can obtain low-cost, low-spot-welding carbon equivalent 780MPa hot-dip galvanized duplex steel.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Any modifications or equivalent substitutions made to the present invention without departing from the spirit and scope thereof should be covered within the protection scope of the claims of the present invention.

Claims

1. A method for preparing low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel, characterized in that, Includes the following steps: a. Smelting process: The chemical composition of low-cost, low-spot welding carbon equivalent 780MPa grade galvanized duplex steel, including the following mass percentages, is smelted to obtain a slab: C: 0.04%~0.09%, Si: 0.25%~0.70%, Mn: 1.55%~2.00%, P≤0.010%, S≤0.003%, Als: 0.01%~0.070%, N≤0.0050%, Nb: 0.010~0.040%, Cr: 0.30~0.70%, with the remainder being Fe and unavoidable impurities; b. Hot rolling process: The slab is heated, descaled, rough rolled, finish rolled, and laminar flow cooled to obtain a hot-rolled coil. Specifically, the process includes: heating the slab to 1230±20℃ for austenitization, holding at that temperature for 5.0h, then descaling and rough rolling, with an intermediate slab thickness of 38±2mm; the finish rolling start temperature is ≥1030℃, and the finish rolling temperature is ≥845℃; laminar flow cooling adopts a front-stage cooling method, with a coiling temperature of 480~533℃ and a hot-rolled thickness of 3.00~6.00mm. c. Pickling and rolling process: After pickling, the hot-rolled coil is cold-rolled into a thin strip of steel of a predetermined thickness; d. Hot-dip galvanizing process: First, the cold-rolled thin steel strip is heated in sections to 700℃ and 800-840℃ at heating rates of 5-20℃ / s and 0.3-4℃ / s respectively; after a first predetermined time of heat soaking, it is slowly cooled to 700-735℃ and rapidly cooled to 450-470℃ at cooling rates of 0.8-6.5℃ / s and 12-30℃ / s respectively; after a second predetermined time of heat soaking, it is placed in a zinc bath for galvanizing treatment for a third predetermined time. After exiting the zinc bath, it is cooled to room temperature at a rate of 5℃ / s to obtain a low-cost, low-spot-weld carbon equivalent 780MPa hot-dip galvanized duplex steel. Among them, the unit speed gradually decreases as the cold-rolled material thickness increases. For every 0.3mm increase in the thickness specification of cold-rolled thin strip steel, the unit speed is adjusted accordingly, and the unit speed is reduced by 10m / min; for every 0.3mm increase in material thickness, the flattening elongation rate is reduced by 0.05%.

2. The method for preparing low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel according to claim 1, characterized in that, In step c, the predetermined thickness is 0.7-2.5 mm, and the cold rolling reduction rate is 60%-78%.

3. The method for preparing low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel according to claim 1, characterized in that, The cold rolling reduction rate gradually decreases as the cold rolling thickness of the material increases. For every 0.3 mm increase in the thickness specification of cold-rolled thin strip steel, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by about 3%.

4. The method for preparing low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel according to claim 1, characterized in that, The first predetermined time is 40~110s, the second predetermined time is 60~120s, and the third predetermined time is 8~25s.

5. The method for preparing low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel according to claim 1, characterized in that, The unit speed is 70-130m / min, and the flattening elongation range is 0.20~0.50%.

6. A low-cost, low-spot-welding carbon equivalent 780MPa hot-dip galvanized duplex steel prepared by the method of any one of claims 1-5, characterized in that, The composition includes the following percentages by mass: C: 0.04%~0.09%, Si: 0.25%~0.70%, Mn: 1.55%~2.00%, P≤0.010%, S≤0.003%, Als: 0.01%~0.070%, N≤0.0050%, Nb: 0.010~0.040%, Cr: 0.30~0.70%, with the remainder being Fe and unavoidable impurities.

7. The low-cost, low-spot-welding carbon equivalent 780MPa grade hot-dip galvanized duplex steel according to claim 6, characterized in that, The composition includes the following percentages by mass: C: 0.05%–0.08%, Si: 0.35%–0.50%, Mn: 1.65%–1.90%, Nb: 0.015%–0.030%, Al: 0.025%–0.055%, Cr: 0.35%–0.55%, P≤0.008%, S≤0.002%, N≤0.0035%, with the balance being Fe and unavoidable impurities.

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