A 780MPa grade hot-dip galvanized composite steel and its preparation method

Abstract

The present invention provides a 780MPa grade hot-base galvanized complex phase steel, wherein the composition of the 780MPa grade hot-base galvanized complex phase steel comprises the following chemical components in mass percentage: C: 0.06-0.12%, Si: 0.10-0.40%, Mn: 1.50-1.90%, P≤0.025%, S≤0.015%, Mo: 0.20-0.33%, Nb: 0.020-0.05%, Ti: 0.030-0.080%, Als: 0.010-0.070%, N≤0.0050%, and the remaining elements are Fe and inevitable impurities. The present invention can prepare a 780MPa grade hot-base galvanized complex phase steel with required thickness specifications, mechanical properties that meet requirements, and good surface quality to meet user requirements.

Description

Technical Field

[0001] The present invention relates to the technical field of cold-rolled strip production, and in particular to a 780MPa grade hot-dip galvanized complex phase steel and a preparation method thereof. Background Art

[0002] As energy and environmental issues become increasingly prominent, energy conservation and environmental protection have become inevitable issues in the development of the automotive industry. At present, steel is the metal material with the largest application volume in automobiles. Increasing the application ratio of high-strength steel in automobiles and achieving lightweight automobiles can significantly reduce fuel consumption and exhaust emissions. The types, proportions and strength levels of advanced high-strength steels used in car bodies are gradually increasing. Among these high-strength steels, complex phase steels are widely used in the manufacture of various parts with high requirements for local forming capabilities, such as seat rails, chassis structural parts and suspension parts, due to their high strength and plasticity and excellent flanging, bending and hole expansion properties. Hot-base galvanizing is a process in which hot-rolled strip steel is pickled without cold rolling and calendering, and directly reaches the galvanized sheet of the same specifications after cold rolling. It has the characteristics of short process, environmental protection and energy saving. The hot-base galvanizing production method reduces product costs and stabilizes product quality. 780MPa grade hot-base duplex steel has good hole expansion performance and high strength, which is favored by steel enterprises and automobile manufacturers. Through the query of relevant patents, the patents similar to 780MPa grade hot-base duplex steel with the same composition are as follows:

[0003] CN 113249648 A discloses a 800MPa grade hot-base zinc-aluminum-magnesium coated complex phase steel and its preparation method, wherein the chemical composition mass percentage is: C: 0.05%-0.12%, Si: 0.05%-0.85%, Mn: 1.4%-2.0%, Al: 0.01%-0.08%, P: 0-0.008%, S: 0-0.001%, N: 0-0.008%, component A, component B, component C, and the rest is Fe and inevitable impurities. A includes at least one of the following components: Cr: 0.05%-0.5%, Mo: 0.005%-0.3%; B includes at least one of the following components: Cu: 0.01%-0.05%, Ni: 0.01%-0.05%; C includes at least one of the following components: Nb: 0.001%-0.04%, Ti: 0.01%-0.12%. The thickness of the slab is 220mm-250mm; the temperature of heating before rolling is 1230℃~1270℃, the heating time is 210min~260min, the end temperature of rough rolling is 1040℃~1100℃, the final rolling temperature of the finishing rolling is 880℃~940℃, the coiling temperature is 450℃~500℃, and the thickness of the hot rolled coil is 2.0~3.5mm; the cooling after rolling adopts the front section water cooling + rear section air cooling mode, the front section water cooling cooling rate is 30℃ / s~60℃ / s, the front section water cooling end temperature is 550℃~600℃, the rear section air cooling cooling rate is 10℃ / s~20℃ / s, and the air cooling end temperature is 450℃~500℃. The scale breaking elongation during uncoiling pickling is 0.5%~0.7%, and the leveling rolling force is 2000N~4000N. Preheat the pickled plate to 210℃~230℃, then heat it to 610℃~660℃ at a heating rate of 15℃ / s~25℃ / s, and then soak and keep it warm for 30s~50s; after soaking, cool it to 420℃~440℃ at a cooling rate of 8℃ / s~16℃ / s, and then galvanize with aluminum and magnesium, and control the coating thickness to 7μm~17μm. The high Si content added is not conducive to obtaining good surface quality, and the high Ti content is easy to produce liquid TiN, which is not conducive to obtaining good plasticity. The low soaking temperature is not conducive to the connection with other strip steels.

[0004] CN 113025882 B discloses a hot-dip galvanized ferrite bainite high-strength steel plate and a preparation method thereof, wherein the chemical composition by mass percentage is: C: 0.05%-0.12%, Si: 0.20%-0.70%, Mn: 0.7%-1.5%, Al: 0.01%-0.50%, Nb: 0.02%-0.06%, Cr: 0-0.03%, Cu: 0-0.01%, Ni: 0-0.01%, Ti: 0-0.006%, B: 0-0.002%, P: 0-0.01%, S: 0-0.003%, and the rest is Fe and unavoidable impurities. The slab is discharged from the furnace at a temperature of 1220°C to 1260°C, and then rough rolling, finish rolling and coiling are performed, and then air cooling is performed to obtain a hot-rolled plate; the starting rolling temperature of the rough rolling is 1090°C to 1160°C, and the ending temperature of the rough rolling is 1030°C to 1100°C; the starting rolling temperature of the finish rolling is 980°C to 1040°C, and the temperature of the final rolling is 840°C to 920°C; the coiling temperature is 420°C to 480°C; the finish rolled plate is cooled to 550°C to 650°C at a rate of 40°C / s to 80°C / s to obtain a first cooling plate; the air cooling stage is: cooling the first cooling plate to 42 0℃~480℃, the post-rolling cooling plate is obtained, the galvanized substrate is preheated to 210℃~230℃, then heated to 640℃~660℃ at a rate of 2℃ / s~3℃ / s, and kept warm for 15s~20s for pre-oxidation; then continued to heat to 660℃~720℃ at a rate of 0~0.5℃ / s and uniformly heated for 100s~140s, then slowly cooled to 600℃~640℃ at a rate of 1℃ / s~3℃ / s, and then quickly cooled to 450℃~470℃ at a rate of 10℃ / s~20℃ / s, and then cooled to room temperature after galvanizing in a zinc pot, and the skin-clearing elongation is 1.8%~2.2%. Its strength level is much lower than 780MPa, and the tensile straightening and scale breaking, pickling process, etc. are not described, which is not conducive to obtaining good surface quality.

[0005] CN108486500 A discloses a cold-rolled hot-dip galvanized complex phase steel and a preparation method thereof, wherein the chemical composition by mass percentage is: C: 0.05-0.13%, Si: 0.1-0.5%, Mn: 1.5-2.5%, P≤0.015%, S≤0.015%, Al: 0.1-0.6%, Cr: 0.2-0.6%, Mo: 0.1-0.5%, Nb: 0.01-0.06%, Ti: 0.01-0.06%, and the rest is Fe and unavoidable impurities. The heating temperature of the steel billet is 1100-1300°C; the hot rolling final rolling temperature of the strip is 850-910°C; the hot rolling coiling temperature is 620-670°C, the cold rolling reduction rate is 50%-75%, preheating to 220°C, and then heating to 760°C-820°C for insulation; the strip after insulation is slowly cooled to 660°C-740°C, and the cooling rate is 10-20°C / s. The strip after slow cooling is quickly cooled to a galvanizing temperature of 450-460°C by high hydrogen cooling, and the cooling rate is 20-40°C / s. After galvanizing, it is cooled to 410-430°C; the strip is air-cooled after galvanizing, and the temperature of the cooling tower top roller is 250-300°C. The high Al content (0.1-0.6%) added will increase the viscosity of the steel, block the water outlet during casting and cause smelting difficulties, and also increase the Al2O3 inclusions and reduce the surface quality. In addition, the thickness of the cold-based and hot-based galvanized complex phase steel of the present invention is quite different from that of the hot-based galvanized complex phase steel, and the process is also significantly different. Summary of the invention

[0006] According to the technical problems raised above, a 780MPa grade hot-dip galvanized complex phase steel and a preparation method thereof are provided. The present invention mainly prepares a complex phase steel structure by hot rolling, then changes the surface quality of the strip after drawing and leveling, and pickling, and then adopts a relatively low soaking temperature, and uses a hot-dip galvanizing production line to prepare a 780MPa grade hot-dip galvanized complex phase steel with required thickness specifications, mechanical properties that meet the requirements, and good surface quality while maintaining the hot rolling structure as much as possible, so as to meet user requirements.

[0007] The technical means adopted by the present invention are as follows:

[0008] A 780MPa grade hot-dip galvanized complex phase steel, wherein the composition of the 780MPa grade hot-dip galvanized complex phase steel comprises the following chemical components in mass percentage:

[0009] C: 0.06~0.12%, Si: 0.10~0.40%, Mn: 1.50~1.90%, P≤0.025%, S≤0.015%, Mo: 0.20~0.33%, Nb: 0.020~0.05%, Ti: 0.030~0.080%, Als: 0.010~0.070%, N≤0.0050%, and the remaining elements are Fe and unavoidable impurities.

[0010] Furthermore, the composition of the 780MPa grade hot-dip galvanized complex phase steel includes the following chemical components in mass percentage:

[0011] C: 0.07~0.10%, Si: 0.15~0.30%, Mn: 1.65~1.80%, Mo: 0.20~0.28%, Nb: 0.025~0.040%, Ti: 0.030~0.055%, Als: 0.030~0.060%, P≤0.020%, S≤0.010%, N≤0.0045%, the balance is Fe and unavoidable impurities.

[0012] Furthermore, the microstructure of the 780MPa grade hot-dip galvanized composite steel is composed of 25-30% ferrite, 15-20% martensite and 50-60% bainite, and its yield strength is 570-720MPa, tensile strength is 810-890MPa, and elongation A 80 The value is 13.5~18.0%, and the hole expansion rate is 65~85%.

[0013] The present invention also provides a method for preparing 780MPa grade hot-dip galvanized complex phase steel, comprising the following steps:

[0014] Step 1, smelting process: smelting according to the chemical composition of 780MPa grade hot-dip galvanized complex phase steel, and obtaining slabs by casting;

[0015] Step 2, hot rolling process: the slab is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling to obtain a hot-rolled coil, i.e., hot-rolled strip steel;

[0016] Step 3, pickling process: after the hot-rolled strip is uncoiled, stretched, straightened, scaled and the plate shape is adjusted, shallow turbulent pickling is performed to obtain pickled strip;

[0017] Step 4: Hot-dip galvanizing process: The pickled strip is hot-dip galvanized to obtain 780MPa grade hot-base galvanized complex phase steel.

[0018] Furthermore, in the step 2, the heating temperature of the heating process is 1210-1255°C; the total furnace time is 280-400 min, the finishing rolling start temperature is 1040-1130°C, the final rolling temperature is 860-930°C, and the hot rolling thickness is 2.5-4.5 mm; the head and tail of the strip are adjusted by using a hot coil box before entering the finishing mill;

[0019] High-pressure water descaling treatment was carried out at the roughing and finishing entrances, with the pressure of high-pressure water being 18-22 MPa, and the descaling water volume at the roughing and finishing entrances being 540 m3 respectively. 3 / h and 450m 3 / h, the distances between the nozzle and the slab / strip are 120mm and 90mm respectively.

[0020] Furthermore, in the step 2, the laminar cooling adopts a sparse cooling method, the intermediate temperature is 580-650°C, the coiling temperature is 460-530°C, and the "U-type coiling" method is adopted. The "U-type coiling" means that the coiling temperature of the first 80m of the strip head is 490-560°C, and the coiling temperature of the last 100m of the strip tail is 500-570°C;

[0021] The laminar cooling type is upper and lower tube laminar cooling, and the maximum water volume of the cooling system is 18000m 3 / h; the laminar cooling water pressure is 0.05~0.08Mpa; the strip is subjected to side water spraying and blowing treatment, wherein the side spraying water pressure is 1.2~1.6MPa, and the maximum side spraying water volume is 350m 3 / h, the side jet pressure is 0.4~0.6MPa, and the total side jet volume is 60m 3 / h.

[0022] Furthermore, in the step three, during the uncoiling process, the uncoiler threading speed is 50-70 m / min, the tail swinging speed is 100-140 m / min, and the rest of the uncoiling speed is maintained at 150-200 m / min; a six-roll straightening machine is used to pull the strip head out of the uncoiler, and the strip head and tail are straightened to make the strip easy to thread, the six-roll straightening machine is maintained at a speed of 150-200 m / min for descaling treatment, the elongation is maintained at 1.2-2.5%, and the tension of the straightening machine is 40-50 tons.

[0023] Furthermore, in the step 3, the pickling process uses an acid solution to remove the oxide scale on the surface of the hot-rolled coil to obtain a pickled plate, which adopts low acid consumption and high efficiency immersion acid tank pickling, including three-stage pickling and four-stage rinsing, wherein the acid solution temperature of the three-stage pickling process is 75-93°C, the free HCl concentration of the three-stage pickling is 2.5-5.0%, 6-12%, 9-15%, respectively, and the Fe 2+ The four-stage rinsing temperatures are 30-50℃, 40-60℃, 50-70℃, 60-80℃, and the pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, 5.0-6.5, respectively. -1 The concentration is required to be ≤4; the strip after rinsing is dried in a dryer, and the air supply temperature of the dryer is 80-140℃.

[0024] Furthermore, in the step 4, the pickled strip is heated to 300°C and 680-730°C at a speed of 5-10°C / s and 0.2-2.0°C / s, respectively, and after being evenly heated and kept warm for 40-100s, it is cooled to 530-580°C at a speed of 1-7°C / s, and then cooled to 440-480°C at a speed of 6-25°C / s, and then evenly heated and kept warm for 20-55s, and then immersed in a zinc pot for hot-dip galvanizing, and after exiting the zinc pot, it is cooled to room temperature at a speed of 4-12°C / s to obtain 780MPa grade hot-dip galvanized complex phase steel; the unit speed is 60-120m / min, and the unit speed is reduced by 15m / min for every 0.5mm increase in the strip thickness; the skin-clearing elongation is 0.4-0.8%, and the skin-clearing elongation is reduced by 0.1% for every 0.5mm increase in the strip thickness.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] The 780MPa grade hot-dip galvanized complex phase steel and the preparation method thereof provided by the present invention prepare the complex phase steel structure by hot rolling, then change the surface quality of the strip by drawing and leveling to break the scale and pickling, and then adopt a relatively low soaking temperature to prepare the 780MPa grade hot-dip galvanized complex phase steel with the required thickness specification, mechanical properties that meet the requirements and good surface quality by using a hot-dip galvanizing production line while keeping the hot-rolled structure as much as possible, so as to meet the user requirements.

[0027] In summary, the technical solution of the present invention can solve the following problems in the prior art: (1) the problem of poor surface quality due to high added Si content; (2) the problem of poor plasticity due to high Ti content (easy to produce liquid TiN); (3) the problem of difficult transition due to low soaking temperature; (4) the problem of no description of the straightening and descaling, pickling process, etc., which is not conducive to obtaining good surface quality; (5) the problem of difficult smelting due to high added Al content (0.1-0.6%) and Al 2 O 3 A series of problems such as increased inclusions and poor surface quality are solved, thus obtaining 780MPa grade hot-dip galvanized complex phase steel with good surface quality and excellent mechanical properties.

[0028] Based on the above reasons, the present invention can be widely promoted in the fields of cold-rolled strip production and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0030] Figure 1 It is a schematic diagram of the process of the present invention.

[0031] Figure 2 This is a scanned photo of the 780MPa grade hot-dip galvanized complex phase steel prepared in Example 1 of the present invention.

[0032] Figure 3 for Figure 2 Magnified image of . DETAILED DESCRIPTION

[0033] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0034] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, 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. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0036] Unless otherwise specifically stated, the relative arrangement, numerical expressions and numerical values ​​of the parts and steps described in these embodiments do not limit the scope of the present invention. At the same time, it should be clear that, for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The technology, methods and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be regarded as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0037] As energy and environmental issues become increasingly prominent, energy conservation and environmental protection have become inevitable issues in the development of the automotive industry. Lightweighting of automobiles can simultaneously solve the requirements of energy consumption, emissions and safety. The types, proportions and strength levels of advanced high-strength steels used in car bodies are gradually increasing. Among these high-strength steels, complex phase steels are widely used in the manufacture of various parts with high requirements for local forming capabilities due to their high strength and plasticity as well as excellent flanging, bending and hole expansion properties. Hot-base galvanizing is the process of directly achieving galvanized sheets of the same specifications after cold-base calendering after hot-rolled strip steel is pickled without the need for cold rolling. It has the characteristics of short process, environmental protection and energy saving. 780MPa grade hot-base galvanized complex phase steel with specifications, mechanical properties and surface quality that meet the requirements is prepared through conventional hot rolling, pickling and hot-dip galvanizing units to meet user requirements and produce significant economic and social benefits.

[0038] The invention provides a 780MPa grade hot-base galvanized complex phase steel and a preparation method thereof, namely a production process of a 780MPa grade hot-base galvanized complex phase steel and a preparation method thereof, belonging to the technical field of cold-rolled plate and strip production.

[0039] This scheme is to prepare a complex phase steel structure by hot rolling, then change the surface quality of the strip after stretching and descaling, pickling, and then use a relatively low soaking temperature to use the hot-dip galvanizing production line to prepare 780MPa grade hot-dip galvanized complex phase steel with required thickness specifications, mechanical properties that meet the requirements, and good surface quality while maintaining the hot-rolled structure as much as possible to meet user requirements.

[0040] The invention discloses a 780MPa grade hot-dip galvanized complex phase steel, whose chemical composition ranges as follows: C: 0.06-0.12%, Si: 0.10-0.40%, Mn: 1.50-1.90%, P≤0.025%, S≤0.015%, Mo: 0.20-0.33%, Nb: 0.020-0.05%, Ti: 0.030-0.080%, Als: 0.010-0.070%, N≤0.0050%, and the remaining elements are Fe and inevitable impurities.

[0041] A preferred 780MPa grade hot-dip galvanized complex phase steel has the following chemical compositions by weight percentage: C: 0.07-0.10%, Si: 0.15-0.30%, Mn: 1.65-1.80%, Mo: 0.20-0.28%, Nb: 0.025-0.040%, Ti: 0.030-0.055%, Als: 0.030-0.060%, P≤0.020%, S≤0.010%, N≤0.0045%, and the remainder is Fe and unavoidable impurities.

[0042] Among them, there is a 780MPa grade hot-dip galvanized composite steel, the microstructure of which is composed of 25-30% ferrite + 15-20% martensite + 50-60% bainite, and its yield strength is 570-720MPa, tensile strength is 810-890MPa, and elongation A 80 The value is 13.5~18.0%, and the hole expansion rate is 65~85%.

[0043] like Figure 1 As shown, the present invention also provides a method for preparing 780MPa grade hot-dip galvanized complex phase steel, and the production steps include:

[0044] (a) Smelting process: Smelting according to the chemical composition of 780MPa grade hot-dip galvanized complex phase steel and forming slabs by casting;

[0045] (b) Hot rolling process: The slab is heated, descaled, rough rolled, finished rolled and laminar cooled to obtain hot rolled coils; the heating temperature is 1210-1255°C, the total furnace time is 280-400 min to ensure uniform chemical composition of each part of the strip, and the finishing rolling start temperature is 1040-1130°C. The final rolling temperature is 860-930°C, the hot rolling thickness is 2.5-4.5 mm, and in order to ensure the stability of the longitudinal structure and performance of the strip, the head and tail of the strip are adjusted by using a hot coil box before entering the finishing mill. The laminar cooling adopts a sparse cooling method, the intermediate temperature is 580-650°C, the coiling temperature is 460-530°C, and "U-type coiling" is adopted (the coiling temperature of the first 80m of the strip is 490-560°C, and the coiling temperature of the last 100m of the strip is 500-570°C).

[0046] In order to obtain good surface quality, high-pressure water descaling treatment was carried out at the rough rolling entrance and the finishing rolling entrance. The pressure of the high-pressure water was 18-22 MPa, and the descaling water volume at the rough rolling entrance and the finishing rolling entrance was 540 m3 / s, respectively. 3 / h and 450m 3 / h, the distances between the nozzle and the slab / strip are 120mm and 90mm respectively.

[0047] The laminar cooling type is upper and lower tube laminar cooling, and the maximum water volume of the cooling system is 18000m 3 / h; the laminar cooling water pressure is 0.05~0.08MPa. In order to prevent water accumulation on the surface of the steel plate, the strip is subjected to side water spraying and blowing treatment, wherein the side spraying water pressure is 1.2~1.6MPa, and the maximum side spraying water volume is 350m 3 / h, side jet pressure is 0.4~0.6MPa, total side jet volume: 60m 3 / h.

[0048] (c) Pickling process: After the hot-rolled strip is uncoiled, straightened, scaled, and the strip shape is adjusted, i-BOX (shallow turbulent pickling) is used. The uncoiler threading speed is 50-70 m / min, the tail swinging speed is 100-140 m / min, and the rest is kept at a speed of 150-200 m / min. The six-roll straightening machine is used to pull the strip head out of the uncoiler and straighten the strip head and tail to make the strip easy to thread. The six-roll straightening machine is kept at a speed of 150-200 m / min for scale breaking, the elongation is kept at 1.2-2.5%, and the tension of the straightening machine is 40-50 tons. The pickled plate is obtained by removing the oxide scale on the surface of the hot-rolled coil using an acid solution. The pickling adopts a low-acid consumption, high-efficiency immersion acid tank (i-Box) pickling, three-stage pickling, and four-stage rinsing. The acid solution temperature of the three-stage pickling process is 75-93° C., and the free HCl concentration of the three-stage pickling is 2.5-5.0%, 6-12%, and 9-15%, respectively. The Fe 2+ The four-stage rinsing temperatures are 30-50°C, 40-60°C, 50-70°C, and 60-80°C, respectively. The pH values ​​are controlled at 1.0-2.5, 3.0-4.0, 4.0-5.5, and 5.0-6.5, respectively. -1 The concentration is required to be ≤ 4. The strip steel after rinsing is dried in a dryer (the air supply temperature of the dryer is 80-140°C).

[0049] (d) Hot-dip galvanizing process: The pickled strip is hot-dip galvanized to produce 780MPa grade hot-dip galvanized composite steel. The 780MPa grade hot-dip galvanized complex phase steel is heated to 300°C and 680-730°C at a speed of 5-10°C / s and 0.2-2.0°C / s respectively, cooled to 530-580°C at a speed of 1-7°C / s after being evenly heated and kept warm for 40-100s, then cooled to 440-480°C at a speed of 6-25°C / s, evenly kept warm for 20-55s, immersed in a zinc pot for hot-dip galvanizing, and cooled to room temperature at a speed of 4-12°C / s after being taken out of the zinc pot to obtain 780MPa grade hot-dip galvanized complex phase steel; the unit speed is 60-120m / min, and the unit speed is reduced by 15m / min for every 0.5mm increase in strip steel thickness; the skin-pass elongation is 0.4-0.8%, and the skin-pass elongation is reduced by 0.1% for every 0.5mm increase in strip steel thickness.

[0050] C determines the strength, plasticity and formability of steel plates. It is an effective and low-cost strengthening element, and directly affects the volume fraction of bainite, martensite, etc. in complex phase steel after critical zone treatment. However, it is not easy to obtain hard phase structure with low C content (<0.06%), and it is difficult to achieve the required strength level. It is necessary to increase the content of alloying elements such as Mn and Cr to ensure strength, which leads to increased costs. If the C content is too high, the hardenability of the steel is enhanced, and a large amount of martensite structure can be obtained at a lower cooling rate, but no bainite structure can be obtained; at the same time, too high C content will also lead to decreased welding and forming performance. Therefore, the C content of the present invention is 0.06-0.12%, preferably 0.07-0.10%.

[0051] Si can expand Fe~Fe 3 The (α+γ) region in the C phase diagram widens the temperature range of critical zone treatment, improves the process performance of complex phase steel, and increases the A 3 The inclination of the line increases the flexibility of the heat treatment process, which is beneficial to maintaining the stability of the properties of the complex phase steel, such as strength. Si can inhibit the formation of cementite in steel, and at the same time increase the activity of carbon in austenite, hindering the nucleation and growth of cementite. In addition, Si increases the activity of carbon, making the untransformed austenite further carbon-enriched, thereby improving its hardenability. In addition, Si can be dissolved in ferrite and austenite to increase the strength of steel. Its effect is second only to C and P, and is stronger than elements such as Mn, Cr, Ti and Ni; however, when the Si content is too high, the surface oxide scale formed by Si in the heating furnace is difficult to remove, which increases the difficulty of dephosphorization; at the same time, it is easy to enrich on the surface to form SiO during annealing. 2 , thus causing surface defects such as missed plating. Therefore, the Si content of the present invention is 0.10-0.40%, preferably 0.15-0.30%.

[0052] Mn is a typical austenite stabilizing element, which significantly improves the hardenability of steel and reduces the critical cooling rate of bainite and martensite formation, thereby effectively reducing the cooling rate of the rapid cooling section during annealing, which is beneficial to obtain bainite or martensite structure. Mn is a good deoxidizer and desulfurizer, and is also a commonly used solid solution strengthening element in steel. Manganese reduces the γ-α phase transition temperature through solid solution strengthening to promote grain refinement and change the microstructure after phase transformation. At the same time, Mn easily combines with S to form a high melting point compound MnS, thereby eliminating or weakening the hot brittleness caused by FeS and improving the hot processing properties of steel. In addition, manganese, as an element that expands the γ phase region, will reduce A 3 , A 1Critical point, but high manganese content (> 2.0%) delays pearlite transformation and bainite transformation, making the "process window" smaller, the bainite zone shifts to the right, and the sensitivity of steel to process conditions becomes greater, which is not conducive to stable mass production. Too low manganese content is easy to cause pearlite transformation, and it is difficult to form a sufficient amount of bainite in the organization. Therefore, the Mn content of the present invention is 1.50-1.90%, preferably 1.65-1.80%.

[0053] Al is a strong deoxidizing element. In order to ensure that the oxygen content in the steel is as low as possible, a certain amount of aluminum needs to be added. At the same time, soluble aluminum is often used as a microalloying element to combine nitrogen in the steel. Very finely dispersed AlN prevents the growth of austenite grains. During the γ-α transformation process, AlN plays a nucleation role, thereby accelerating the austenite transformation. The most important function of AlN is to refine the grains and obtain aging resistance. When the Als content is less than 0.010%, its effect cannot be exerted; but adding a large amount of aluminum is likely to form alumina lumps. This leads to an increase in the viscosity of the mold slag, resulting in improper heat transfer and lubrication during casting. Therefore, the Als content of the present invention is 0.010-0.070%, preferably 0.030-0.060%.

[0054] Mo significantly delays the transformation of pearlite and bainite, thereby obtaining a high volume fraction of martensite to ensure strength. In addition, the Gibbs free energy of Mo oxide is equivalent to that of Fe oxide, so Mo will not affect the surface galvanizing quality of dual-phase steel, but its price is relatively expensive. Therefore, the Mo content of the present invention is 0.20-0.33, preferably 0.20-0.28%.

[0055] Nb exists mainly in the form of NbC in complex phase steel, which has significant grain refinement and dispersion precipitation strengthening effects. The strain-induced precipitation of niobium carbides during hot rolling can hinder the recovery and recrystallization of deformed austenite, and the deformed austenite structure after controlled rolling and controlled cooling obtains fine phase transformation products. At the same time, during the annealing process, the precipitation of fine niobium carbonitride can play a role in precipitation strengthening. During the hot-dip galvanizing annealing heating process, undissolved NbC particles can pin the ferrite grain boundaries, thereby playing a role in grain refinement; when the annealing temperature increases to the two-phase region, the NbC dissolution temperature is low, so it is fully dissolved in the matrix, and at the same time, the solid solution C atoms are enriched in the austenite to improve its stability; during the cooling process, the NbC in the ferrite will re-precipitate, thereby producing obvious precipitation strengthening. Therefore, the Nb content of the present invention is 0.020-0.050, preferably 0.025-0.040%.

[0056] Ti inhibits austenite coarsening during heating by forming TiN, and forms TiC to pin grain boundaries during cooling, thereby refining grains and reducing aging and cold brittleness. If the Ti content is too low, the strength is insufficient and the performance cannot be met. If the Ti content is too high, the strength will be significantly increased, affecting the performance, and in severe cases, it will cause cracking of stamping parts. Therefore, the Ti content of the present invention is 0.030-0.080, preferably 0.030-0.055.

[0057] like Figure 2-3 Shown is a scanned photo of 780MPa grade hot-dip galvanized complex phase steel, which is composed of 25-30% ferrite + 15-20% martensite + 50-60% bainite.

[0058] Example 1

[0059] This embodiment provides two groups of 780MPa grade hot-dip galvanized complex phase steels, whose chemical compositions are shown in Table 1;

[0060] Table 1 Chemical composition (wt.%) of 780MPa grade hot-dip galvanized composite steel in the embodiment

[0061] serial number C Si Mn P S Mo Nb Ti Als 1 0.080 0.28 1.75 0.009 0.003 0.26 0.033 0.035 0.045 2 0.095 0.22 1.68 0.007 0.004 0.22 0.036 0.045 0.033

[0062] The preparation method of the above 780MPa grade hot-dip galvanized composite steel has the following specific process:

[0063] A. Smelting process: After the smelting process, a complex phase steel slab with the chemical composition shown in Table 1 is prepared;

[0064] B. Hot rolling process: The slab is subjected to heating, dephosphorization, hot rolling and laminar cooling to obtain a hot rolled coil. The specific hot rolling process parameters are shown in Table 2;

[0065] Table 2 Main process parameters for hot rolling of 780MPa grade hot-dip galvanized composite steel in the embodiment

[0066]

[0067] C. Pickling process: The hot-rolled strip is subjected to straightening, descaling and pickling to obtain good surface quality. The specific process is shown in Tables 3 to 5;

[0068] Table 3 Parameters of the 780MPa grade hot-dip galvanized composite steel straightening process in the embodiment

[0069]

[0070] Table 4 Parameters of the pickling process for 780MPa grade hot-dip galvanized composite steel in the embodiment

[0071]

[0072] Table 5 780MPa grade hot-dip galvanized composite steel rinsing process parameters in the embodiment

[0073]

[0074]

[0075] The pickled and rinsed strips are dried to 90°C and 87°C respectively to obtain pickled plates with good surface quality;

[0076] D. Hot-dip galvanizing process: The pickled strip is processed by hot-dip galvanizing to make the required product. The strip is first slowly heated to the absorptive temperature and kept warm for a period of time. It is different from the usual cold-based galvanized complex phase steel. Its absorptive temperature is relatively low so that it can keep the complex phase structure of the hot-rolled strip as much as possible. Under the action of heating, the recovery and dislocation density are reduced, and the strength is slightly reduced. After that, the strip is slowly cooled, quickly cooled, and evenly kept warm, and then immersed in the zinc pot for galvanizing. After leaving the zinc pot, it is air-cooled to room temperature, and the strip shape is adjusted through the skin pass machine. The specific hot-dip galvanizing process parameters are shown in Table 6:

[0077] Table 6 Main galvanizing process parameters of 780MPa grade hot-dip galvanized composite steel in the embodiment

[0078]

[0079] The microstructure of the 780MPa grade hot-dip galvanized composite steel Case 1 prepared by the above process is as follows: Figure 2 and Figure 3 As shown, the performance of the above 780MPa grade hot-dip galvanized composite steel was tested according to GB / T228-2010 "Room Temperature Tensile Test Method for Metallic Materials", and its mechanical properties are shown in Table 7 below:

[0080] Table 7 Mechanical properties of 780MPa grade hot-dip galvanized composite steel in the embodiment

[0081] serial number Yield strength / MPa Tensile strength / MPa <![CDATA[Elongation after fracture A 80 %]]> Yield-to-strength ratio Hole expansion rate / % 1 668 846 16.5 0.790 73 2 647 827 16.0 0.782 69 CN 113249648 A 713 812 16.4 0.878 70 CN 113025882 B 530 622 22.0 0.852 91 CN 108486500 A 593 832 13.0 0.713

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing 780MPa grade hot-dip galvanized complex phase steel, characterized in that: The composition of the 780MPa grade hot-dip galvanized complex phase steel includes the following chemical components in mass percentage: C: 0.06~0.12%, Si: 0.10~0.40%, Mn: 1.50~1.90%, P≤0.025%, S≤0.015%, Mo: 0.20~0.33%, Nb: 0.020~0.05%, Ti: 0.030~0.080%, Als: 0.010~0.070%, N≤0.0050%, the remaining elements are Fe and unavoidable impurities; The microstructure of the 780MPa grade hot-dip galvanized composite steel is composed of 25-30% ferrite, 15-20% martensite and 50-60% bainite, and its yield strength is 570-720MPa, tensile strength is 810-890MPa, and elongation A 80 The value is 13.5~18.0%, and the hole expansion rate is 65~85%; The preparation method of the 780MPa grade hot-dip galvanized complex phase steel comprises the following steps: Step 1, smelting process: smelting according to the chemical composition of 780MPa grade hot-dip galvanized complex phase steel, and obtaining slabs by casting; Step 2, hot rolling process: the slab is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling to obtain a hot-rolled coil, i.e., hot-rolled strip steel; Step 3, pickling process: after the hot-rolled strip is uncoiled, stretched, straightened, scaled and the plate shape is adjusted, shallow turbulent pickling is performed to obtain pickled strip; Step 4: Hot-dip galvanizing process: The pickled steel strip is hot-dip galvanized to obtain 780MPa grade hot-base galvanized complex phase steel; In the step 2, the laminar cooling adopts a sparse cooling method, the intermediate temperature is 580-650°C, the coiling temperature is 460-530°C, and the "U-type coiling" method is adopted. The "U-type coiling" means that the coiling temperature of the first 80m of the strip head is 490-560°C, and the coiling temperature of the last 100m of the strip tail is 500-570°C; The laminar cooling type is upper and lower tube laminar cooling, and the maximum water volume of the cooling system is 18000m 3 / h; the laminar cooling water pressure is 0.05~0.08MPa; the strip is subjected to side water spraying and blowing treatment, wherein the side spraying water pressure is 1.2~1.6MPa, and the maximum side spraying water volume is 350m 3 / h, the side jet pressure is 0.4~0.6MPa, and the total side jet volume is 60m 3 / h; In the step 4, the pickled strip is heated to 300°C and 680°C to 730°C at a speed of 5-10°C / s and 0.2-2.0°C / s, respectively, and after being evenly heated and kept for 40-100s, it is cooled to 530-580°C at a speed of 1-7°C / s, and then cooled to 440-480°C at a speed of 6-25°C / s, and then evenly heated and kept for 20-55s, and then immersed in a zinc pot for hot-dip galvanizing, and after exiting the zinc pot, it is cooled to room temperature at a speed of 4-12°C / s to obtain 780MPa grade hot-dip galvanized complex phase steel; the unit speed is 60-120m / min, and the unit speed is reduced by 15m / min for every 0.5mm increase in the strip thickness; the skin-pass elongation is 0.4-0.8%, and the skin-pass elongation is reduced by 0.1% for every 0.5mm increase in the strip thickness.

2. The method for preparing 780MPa grade hot-dip galvanized complex phase steel according to claim 1, characterized in that: The composition of the 780MPa grade hot-dip galvanized complex phase steel includes the following chemical components in mass percentage: C: 0.07~0.10%, Si: 0.15~0.30%, Mn: 1.65~1.80%, Mo: 0.20~0.28%, Nb: 0.025~0.040%, Ti: 0.030~0.055%, Als: 0.030~0.060%, P≤0.020%, S≤0.010%, N≤0.0045%, the balance is Fe and unavoidable impurities.

3. The method for preparing 780MPa grade hot-dip galvanized complex phase steel according to claim 1, characterized in that: In the step 2, the heating temperature of the heating process is 1210-1255°C; the total furnace time is 280-400 min, the finishing rolling start temperature is 1040-1130°C, the final rolling temperature is 860-930°C, and the hot rolling thickness is 2.5-4.5 mm; the head and tail of the strip are adjusted by using a hot coil box before entering the finishing rolling unit; High-pressure water descaling treatment was carried out at the roughing and finishing entrances, with the pressure of high-pressure water being 18-22 MPa and the descaling water volume of the roughing and finishing entrances being 540 m3 respectively. 3 / h and 450m 3 / h, the distances between the nozzle and the slab / strip are 120mm and 90mm respectively.

4. The method for preparing 780MPa grade hot-dip galvanized complex phase steel according to claim 1, characterized in that: In the step three, during the uncoiling process, the uncoiler threading speed is 50-70 m / min, the tail swinging speed is 100-140 m / min, and the rest of the uncoiling speed is maintained at 150-200 m / min; a six-roll straightening machine is used to pull the strip head out of the uncoiler, and the strip head and tail are straightened to make the strip easy to thread, the six-roll straightening machine is maintained at a speed of 150-200 m / min for descaling treatment, the elongation is maintained at 1.2-2.5%, and the tension of the straightening machine is 40-50 tons.

5. The method for preparing 780MPa grade hot-dip galvanized complex phase steel according to claim 1 or 4, characterized in that: In the step 3, the pickling process uses an acid solution to remove the oxide scale on the surface of the hot-rolled coil to obtain a pickled plate, which adopts low acid consumption and high efficiency immersion acid tank pickling, including three-stage pickling and four-stage rinsing, wherein the acid solution temperature of the three-stage pickling process is 75-93°C, the free HCl concentration of the three-stage pickling is 2.5-5.0%, 6-12%, and 9-15%, respectively, and the Fe 2+ The four-stage rinsing temperatures are 30~50℃, 40~60℃, 50~70℃, 60~80℃, and the pH values ​​are controlled at 1.0~2.5, 3.0~4.0, 4.0~5.5, 5.0~6.5, respectively. -1 The concentration is required to be ≤4; the strip after rinsing is dried in a dryer, and the air supply temperature of the dryer is 80~140℃.

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