590MPa-grade hot-dip galvanized dual-phase steel as well as preparation method and application thereof

Abstract

The invention provides 590MPa-grade hot-dip galvanized dual-phase steel, a preparation method and application, and the hot-dip galvanized dual-phase steel comprises the following chemical components in percentage by mass: 0.07 to 0.09 percent of C, 0.20 to 0.30 percent of Si, 1.5 to 1.7 percent of Mn, 0.02 to 0.05 percent of Al, 0.30 to 0.40 percent of Cr, 0.015 to 0.025 percent of Nb, 0.015 to 0.025 percent of Ti, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.008 percent of N and the balance of Fe and inevitable impurities. The thickness of the hot-dip galvanized dual-phase steel is 0.8-2.3 mm, and the structure of the hot-dip galvanized dual-phase steel is ferrite and martensite with the volume content being 20-30%. A casting blank with the components and the content is subjected to heating, hot rolling, acid rolling and galvanizing procedures. When parts are punched, the edge quality can be obviously improved, the technological method is simple, and control is convenient and fast.

Description

Technical Field

[0001] The present invention relates to the field of metallurgical technology, and particularly to a 590 MPa grade hot-dip galvanized dual-phase steel, a preparation method and an application thereof. Background Art

[0002] With the development of automotive lightweighting, the increasing requirements of users for automotive anti-corrosion and safety, and the environmental protection issues faced by the automotive industry, the application of galvanized advanced high-strength steels in automobiles is increasing day by day. As a material with good comprehensive cost and performance in automotive structural parts, galvanized dual-phase steel has the advantages of good anti-corrosion ability, high initial work-hardening rate, high collision energy absorption ability, good welding performance, and good strength and ductility matching, and its application in automobiles is gradually increasing.

[0003] However, for dual-phase steels with a tensile strength of 590 MPa or above, a large amount of alloying elements are often required to ensure mechanical properties. The defects caused by more alloying elements make it difficult to achieve stable production of high-strength steels above 590 MPa level in continuous dies and mechanical parts dies.

[0004] At present, the high-strength steels with a tensile strength below 590 MPa, which are widely used in automobiles, can no longer fully meet the development of automotive lightweighting. The application of higher-strength materials has become the direction of efforts of major automotive companies. Therefore, the application ratio of galvanized dual-phase steel with a tensile strength of 590 MPa and above in automobiles will gradually increase. There is an urgent need for domestic automotive supporting factories to put forward higher requirements for the stamping performance of galvanized dual-phase steel with a tensile strength of 590 MPa and above. Summary of the Invention

[0005] The main purpose of the present invention is to provide a 590 MPa grade hot-dip galvanized dual-phase steel, a preparation method and an application thereof, aiming to solve the technical problem of unstable stamping performance of the steel plates obtained by the existing technology.

[0006] To achieve the above object, the present invention provides a 590 MPa grade hot-dip galvanized dual-phase steel. In terms of mass percentage, the chemical composition of the hot-dip galvanized dual-phase steel is: C: 0.07 - 0.09%, Si: 0.20 - 0.30%, Mn: 1.5 - 1.7%, Al: 0.02 - 0.05%, Cr: 0.30 - 0.40%, Nb: 0.015 - 0.025%, Ti: 0.015 - 0.025%, P ≤ 0.015%, S ≤ 0.015%, N ≤ 0.008%, and the balance is Fe and unavoidable impurities; the structure of the hot-dip galvanized dual-phase steel is ferrite and martensite, and the volume content of martensite is 20 - 30%.

[0007] According to an embodiment of the present application, the yield strength of the hot-dip galvanized dual-phase steel is 340 - 430 MP a, the tensile strength is 590 - 700 MPa, and the elongation after fracture is ≥ 22%.

[0008] According to an embodiment of the present application, the thickness of the hot-dip galvanized dual-phase steel is 0.8 - 2.3 mm.

[0009] According to an embodiment of the present application, the thickness of the hot-dip galvanized dual-phase steel is 0.8 - 2.0 mm.

[0010] The present invention also provides a method for preparing the above-mentioned 590 MPa grade hot-dip galvanized dual-phase steel, comprising the following steps:

[0011] The billet corresponding to the components of the above-mentioned 590 MPa grade hot-dip galvanized dual-phase steel is successively subjected to hot rolling, pickling and hot-dip galvanizing processes to obtain the hot-dip galvanized dual-phase steel.

[0012] According to an embodiment of the present application, the hot rolling includes:

[0013] The billet is heated at 1180 - 1250 °C for 150 - 400 min, and then successively subjected to rough rolling, finish rolling and coiling to obtain a hot-rolled steel coil.

[0014] Among them, the temperature of the rough rolling is 1050 - 1300 °C; the temperature of the final finish rolling is 860 - 920 °C, and the temperature of the coiling is 530 - 620 °C.

[0015] According to an embodiment of the present application, the thickness of the intermediate billet after rough rolling is 40 mm.

[0016] According to an embodiment of the present application, the pickling and rolling includes:

[0017] In an acid solution, the hot-rolled steel coil obtained by hot rolling is pickled to obtain a steel plate.

[0018] The steel plate is cold-rolled and coiled to obtain a cold-hardened coil.

[0019] Among them, the concentration of sulfuric acid in the acid solution is 160 - 175 g / L; the temperature of the pickling is 82 - 85 °C.

[0020] The pickling speed ≤ 220 m / min; the total cold rolling reduction rate is 52 - 70%.

[0021] According to an embodiment of the present application, the galvanizing process includes:

[0022] The cold-hardened coil is annealed and then galvanized to obtain the 590 MPa grade hot-dip galvanized dual-phase steel.

[0023] The annealing process steps successively include a preheating section, a pre-oxidation section, a heating section, a slow cooling section and a rapid cooling section.

[0024] Among them, the traveling speed of the chilled coil in the annealing process step is 60 - 90 m / min.

[0025] The temperature of the preheating section is 560 - 660 °C; the temperature of the pre-oxidation section is 580 - 680 °C, and the oxygen content is 0.2 - 2.0%; the temperature of the heating section is 800 - 820 °C; the temperature of the slow cooling section is 680 - 720 °C; the temperature of the rapid cooling section is 450 - 470 °C.

[0026] According to an embodiment of the present application, the steps after galvanizing further include skin pass rolling and tension leveling to obtain the 590 MPa grade hot-dip galvanized dual-phase steel.

[0027] Among them, the elongation after fracture of the skin pass rolling is 0.3 - 0.5%; the skin pass rolling force is 260 - 295 t; the elongation after fracture of the tension leveling is 0.05 - 0.15%.

[0028] Application of the 590 MPa grade hot-dip galvanized dual-phase steel prepared according to the above 590 MPa grade hot-dip galvanized dual-phase steel or the above preparation method in improving the edge quality of stamping parts.

[0029] The beneficial effects of the present invention are:

[0030] The present invention prepares a 590 MPa grade hot-dip galvanized dual-phase steel with a yield strength of 340 - 430 MP by selecting a suitable chemical composition and content of the continuous casting billet a , a tensile strength of 590 - 700 MPa, and an elongation after fracture of ≥22%. By adding low-cost alloys such as Si and Mn, adding a small amount of Cr element, and adding trace amounts of alloys such as Nb and Ti, the product performance can be guaranteed while reducing the cost.

[0031] The present invention controls the acid rolling process, controls the total cold rolling reduction rate to be 52 - 70%, and can ensure the pickling quality. By adopting a relatively high galvanizing annealing temperature, increasing the transformation amount of austenite during the heating process, controlling the transformation mode and transformation amount of austenite during the cooling process, and performing skin pass rolling and tension leveling on the galvanized steel sheet, a 590 MPa grade hot-dip galvanized dual-phase steel with high strength, good stability, and uniformity is obtained.

[0032] The above process method has a simple process and convenient control. The thickness of the steel sheet of the 590 MPa grade hot-dip galvanized dual-phase steel prepared is 0.8 - 2.3 mm, which can meet the requirements of the automotive steel sheet field. Description of the Drawings

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0034] Figure 1 SEM image of the 590MPa grade hot-dip galvanized dual-phase steel product of the present invention;

[0035] Figure 2 Micro-cracking phenomenon at the edge when the dual-phase steel material of conventional 590MPa grade and above is stamped into parts;

[0036] Figure 3 Effect diagram of improving the edge quality when the 590MPa grade hot-dip galvanized dual-phase steel of the present invention is stamped into parts.

[0037] The realization, functional features and advantages of the object of the present invention will be further described in conjunction with the embodiments with reference to the drawings. Specific embodiments

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0039] It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other. It should also be understood that the terms used in the embodiments of the present invention are for the purpose of describing specific embodiments, rather than limiting the scope of protection of the present invention.

[0040] To achieve the above object, the present invention provides a 590MPa grade hot-dip galvanized dual-phase steel. In terms of mass percentage, the chemical composition of the hot-dip galvanized dual-phase steel is: C: 0.07 - 0.09%, Si: 0.20 - 0.30%, Mn: 1.5 - 1.7%, Al: 0.02 - 0.05%, Cr: 0.30 - 0.40%, Nb: 0.015 - 0.025%, Ti: 0.015 - 0.025%, P ≤ 0.015%, S ≤ 0.015%, N ≤ 0.008%, and the rest is Fe and unavoidable impurities; the structure of the hot-dip galvanized dual-phase steel is ferrite and martensite, and the volume content of martensite is 20 - 30%.

[0041] Among them, ferrite has good plasticity and toughness. Martensite itself has high strength and hardness, and its hardness increases with the increase of carbon content. Dual-phase steel containing 20-30% martensite has good work-hardening behavior during plastic deformation, which can improve the processing performance of the material. Low-carbon martensite has good plasticity and toughness. An appropriate amount of martensite content can balance the hardness and plasticity of the material, enabling the material to have good plasticity and toughness while maintaining high strength. It can also improve the fatigue resistance of the material. Ferrite and martensite together form dual-phase steel, which can provide good plasticity and toughness while ensuring high strength, thus showing better performance during forming and use. In addition, ferrite-martensite dual-phase steel can partially accept quenching strengthening, obtain high mechanical properties, and have good stress corrosion resistance.

[0042] In 590 MPa grade hot-dip galvanized dual-phase steel, the functions of each element are as follows:

[0043] C: Carbon is an economical strengthening element in steel and an important component of dual-phase steel, which determines the strength, plasticity and forming performance of the steel plate. If the C content is too low, the stability of austenite and the hardenability of martensite will decrease. If the C content is too high, the plasticity and weldability of dual-phase steel will decrease. The C content of the present invention is in the range of 0.07-0.10%, which can ensure strength while adding less precious alloying elements.

[0044] Si: Silicon can dissolve in ferrite and austenite to improve the strength of the material. The higher the silicon content, the higher the material strength. However, when the silicon content is high, it is easy to generate scale that is difficult to pickling, and it will also cause missing plating after galvanizing, affecting the surface quality of the steel plate. Therefore, Si should be controlled within 0.20-0.30%.

[0045] Mn: Manganese is a commonly used solid solution strengthening element in steel. Usually, no less than 1.2% of manganese is added to dual-phase steel to improve the hardenability of the steel plate and ensure the strength of the steel plate. Adding a certain amount of manganese can combine with sulfur to form MnS, avoiding the occurrence of hot brittleness quality problems and improving the hot working performance of the steel. Manganese can also improve the stability of austenite, shift the C curve to the right, and reduce the critical cooling rate of the steel. However, when the manganese content is too high, it is easy to generate oxides on the surface that are difficult to pickling during hot rolling, and external oxidation is likely to occur during galvanizing annealing, forming a large amount of oxides, and even causing missing plating on the galvanized surface in severe cases. Therefore, Mn should be controlled within 1.5-2.0%.

[0046] Cr: Chromium can significantly delay the transformation of pearlite and bainite, so that austenite can be fully transformed into martensite structure. And chromium has a certain cost advantage. Therefore, the added Cr in the present invention should be controlled within 0.30-0.40%.

[0047] Al: Aluminum is added as a deoxidizer during steelmaking. Meanwhile, it can form AlN to pin the grain boundaries, thus playing a role in refining the grains. Therefore, the content of Al should be controlled within 0.02 - 0.05%.

[0048] Nb: Niobium plays a role in solution strengthening in steel. When dissolved in austenite, it significantly improves the hardenability of steel. The roles of niobium in steel mainly include grain refinement, precipitation strengthening, improving strength and hot strength. Niobium is a strong carbide - forming element, forming stable NbC or Nb4C3 in steel, which are finely and dispersedly distributed on the matrix, playing a role in precipitation strengthening. Niobium can also refine grains, reduce the overheating sensitivity and aging sensitivity of steel, and improve strength. Therefore, the content of Nb should be controlled within 0.015 - 0.025%.

[0049] Ti: In steel, in addition to combining with carbon and nitrogen to form Ti[C, N] compounds to refine ferrite grains for the purpose of strength and toughness, Ti also has a strong affinity with oxygen and sulfur in steel, improving the morphology of sulfides and forming spherical Ti4C2S2 that is not easily plastically deformed, significantly improving the toughness of steel, and improving the properties of the heat - affected zone of welding and fatigue properties. Therefore, the content of Ti should be controlled within 0.015 - 0.025%.

[0050] In the present invention, by reasonably selecting the chemical element composition and content, mainly by controlling the reasonable contents of C, Si, Mn, Cr, Nb, and Ti. Among them, by adding a higher content of Mn, the hardenability and performance stability of the hot - dip galvanized dual - phase steel are improved, and the requirement for the cooling capacity after plating is reduced; adding a small amount of Cr element plays a role in reducing the critical cooling rate of steel, improving hardenability and strength; adding Nb and Ti plays a role in grain refinement and strengthening. Controlling the alloying elements and their contents within a reasonable range can not only ensure the product performance and process feasibility but also reduce the alloy cost of the enterprise, meeting the usage requirements of automotive steel sheets.

[0051] In some embodiments, the yield strength of the hot - dip galvanized dual - phase steel is 340 - 430MP a , the tensile strength is 590 - 700MPa, and the elongation after fracture is ≥22%.

[0052] In some specific embodiments, the yield strength of the hot - dip galvanized dual - phase steel is 355 - 387MP a , the tensile strength is 611 - 635MPa, and the elongation after fracture is 25.5 - 29%.

[0053] In some more specific embodiments, the yield strength of the hot - dip galvanized dual - phase steel is 387MPa, the tensile strength is 635MPa, and the elongation after fracture is 29%.

[0054] In some embodiments, the thickness of the hot - dip galvanized dual - phase steel is 0.8 - 2.3mm.

[0055] In some specific embodiments, the thickness of the hot-dip galvanized dual-phase steel is 0.8 - 2.0 mm.

[0056] The present invention provides a method for preparing the above-mentioned 590 MPa grade hot-dip galvanized dual-phase steel, comprising the following steps:

[0057] The billet of the corresponding components of the above-mentioned 590 MPa grade hot-dip galvanized dual-phase steel is successively subjected to hot rolling, pickling rolling and hot-dip galvanizing processes to obtain the hot-dip galvanized dual-phase steel.

[0058] Among them, after the hot rolling, a hot-rolled steel coil is obtained.

[0059] After the pickling rolling, a cold-hardened coil is obtained.

[0060] After the hot-dip galvanizing process, the hot-dip galvanized dual-phase steel is obtained.

[0061] In some embodiments, the billet of the corresponding components of the above-mentioned 590 MPa grade hot-dip galvanized dual-phase steel is heated at 1180 - 1250 °C for 150 - 400 min to obtain a heated slab. Among them, the thickness of the billet is 230 mm. After rough rolling, the thickness of the intermediate slab is 40 mm.

[0062] In some embodiments, by monitoring the thickness of the intermediate slab, it is confirmed whether rough rolling is completed. If the thickness of the intermediate slab is too thick and the impurity content is high, it will affect the shape and thickness of the finished steel plate. Monitor the thickness of the intermediate slab. When the measured thickness of the intermediate slab is 40 mm, it enters the finish rolling for rolling; if the thickness of the intermediate slab is greater than 40 mm, it continues to return to rough rolling for rolling until the thickness of the intermediate slab is appropriate and then enters the finish rolling.

[0063] In some embodiments, the hot rolling includes:

[0064] After heating the billet at 1180 - 1250 °C for 150 - 400 min, it is successively subjected to rough rolling, finish rolling and coiling to obtain a hot-rolled steel coil.

[0065] Among them, the temperature of the rough rolling is 1050 - 1300 °C, the temperature of the final finish rolling is 860 - 920 °C, and the temperature of the coiling is 530 - 620 °C.

[0066] In some embodiments, the finish rolling final rolling temperature is rolled at an austenite region temperature higher than Ar3. If the temperature is too high, the scale is seriously difficult to pickling, affecting the surface quality of the steel plate; if the temperature is too low, material mixed crystals will appear during two-zone rolling, and snowflake points, microcracks and hidden cracks will occur during stamping. Therefore, the final finish rolling temperature is controlled at 860 - 920 °C.

[0067] In some embodiments, a high coiling temperature is beneficial to grain growth, reducing strength and improving formability. However, if the annealing temperature is too high, the scale on the steel plate surface will be severe and difficult to pickling, affecting the quality of the steel plate. Therefore, the coiling temperature is controlled at 530 - 620 °C.

[0068] In some embodiments, the final finish rolling temperature is controlled at 860 - 920 °C to reduce the thickness of the steel after finish rolling.

[0069] In some embodiments, the thickness of the intermediate billet after rough rolling is 40 mm.

[0070] In some embodiments, the acid rolling includes:

[0071] Pickling the hot-rolled steel coil obtained by hot rolling in an acid solution to obtain a steel plate.

[0072] Cold rolling and coiling the steel plate to obtain a cold-hardened coil.

[0073] Among them, the concentration of sulfuric acid in the acid solution is 160 - 175 g / L; the temperature of pickling is 82 - 85 °C.

[0074] The pickling speed ≤ 220 m / min; the total cold rolling reduction rate is 52 - 70%.

[0075] In some embodiments, the cold-rolled sheet is obtained after uncoiling the hot-rolled steel coil and pickled. Among them, the pickling speed is related to the thickness of the cold-rolled sheet; when the thickness of the cold-rolled sheet ≤ 1.5 mm, the pickling speed ≤ 220 m / min; when the thickness of the cold-rolled sheet > 1.5 mm, the pickling speed ≤ 180 m / min. If the pickling speed is too fast, pickling is insufficient and impurities on the cold-rolled sheet cannot be removed. If the pickling speed is too slow, the pickling solution may corrode the cold-rolled sheet.

[0076] In some embodiments, a larger cold rolling deformation amount can reduce the recrystallization temperature and phase transformation temperature, ensuring product performance; however, if the deformation amount is too large, the rolling difficulty will increase. Therefore, according to the actual situation of the rolling mill, the total cold rolling reduction rate is controlled between 52 - 70%.

[0077] In some embodiments, the galvanizing process includes:

[0078] Annealing and galvanizing the cold-hardened coil to obtain the 590 MPa grade hot-dip galvanized dual-phase steel.

[0079] The annealing process steps sequentially include a preheating section, a pre-oxidation section, a heating section, a slow cooling section, and a rapid cooling section. Among them, the traveling speed of the cold-hardened coil in the annealing process steps is 60 - 90 m / min.

[0080] The temperature of the preheating section is 560 - 660 °C; the temperature of the pre-oxidation section is 580 - 680 °C, and the oxygen content is 0.2 - 2.0%; the temperature of the heating section is 800 - 820 °C; the temperature of the slow cooling section is 680 - 720 °C; the temperature of the rapid cooling section is 450 - 470 °C.

[0081] In some embodiments, the temperature of the preheating section can ensure that the material entering the heating section rapidly heats up to the two-phase region. The relatively high temperature of the heating section can ensure the production of an appropriate amount of austenite in the annealing furnace. The temperature of the slow cooling section can ensure that a small amount of austenite transforms into ferrite during cooling, which not only ensures the strength of the product but also improves the stability of austenite. The temperature of the rapid cooling section can ensure that the material rapidly passes through the bainite transformation region, ensuring that austenite transforms into martensite after galvanizing. A suitable annealing process can also offset the influence of the strength reduction of the substrate caused by batch annealing.

[0082] In some embodiments, after the cold-rolled coil is uncoiled and annealed to obtain a substrate, it is then galvanized; the advancing speed of the annealing process steps is controlled to be 60 - 90 m / min. The pre-oxidation section process uses oxygen content control, and the oxygen content is 0.2 - 2.0%; the galvanized steel sheet is skin-passed and stretcher-leveled to obtain a hot-dip galvanized dual-phase steel sheet; among them, the post skin-pass elongation is 0.3 - 0.5%, the skin-pass rolling force is above 200 t, and the post stretcher-leveling elongation is 0.05 - 0.15%.

[0083] In some embodiments, if the advancing speed of the annealing process steps is too fast, the particle size in the cold-rolled coil will be damaged, thereby deteriorating the properties of the steel.

[0084] In some embodiments, the steps after galvanizing further include skin-passing and stretcher-leveling to obtain the 590 MPa grade hot-dip galvanized dual-phase steel.

[0085] Among them, the post skin-pass elongation is 0.3 - 0.5%; the skin-pass rolling force is 260 - 295 t; the post stretcher-leveling elongation is 0.05 - 0.15%.

[0086] In some embodiments, the purpose of skin-passing is to eliminate the yield plateau of the steel sheet, avoid the generation of tensile strain mark defects during stamping, improve the sheet shape, and improve the surface quality of galvanizing. The characteristics of the dual-phase steel product are that there is no yield plateau. The main purpose of skin-passing the dual-phase steel is to improve the sheet shape and stamping quality. Therefore, the control range of the post skin-pass elongation is between 0.3 - 0.5%.

[0087] In some embodiments, the purpose of stretcher-leveling is to adjust and optimize the sheet shape and avoid edge waves and center waves in the material. If the post stretcher-leveling elongation is too low, the sheet shape is not good; if the post stretcher-leveling elongation is too high, stretcher marks are likely to appear. Therefore, the control range of the post stretcher-leveling elongation is between 0.05 - 0.15%.

[0088] The process corresponding to the preparation method provided by the present invention is simple and convenient to control. The acid rolling process plays an important role in achieving the strength level of 590 MPa. Rolling through the acid rolling mill can ensure that the cold hard substrate has a high cold rolling reduction rate, guarantee high strength, stable performance and uniform structure in galvanizing annealing, and at the same time ensure the pickling quality. Selecting the appropriate annealing temperature for galvanizing plays an important role in achieving high stamping performance and the strength level of 590 MPa. By adopting a higher annealing temperature, controlling the transformation amount of austenite during the heating process, the grain size, and the transformation mode and amount of austenite during the cooling process, the strength of the finished product can be guaranteed. The thickness of the steel plate with a strength level of 590 MPa is 0.8 - 2.3 mm, the yield strength is 340 - 430 MPa, the tensile strength is 590 - 700 MPa, and the elongation after fracture is 25.5 - 29%.

[0089] Application of the 590 MPa grade hot-dip galvanized dual-phase steel prepared according to the above 590 MPa grade hot-dip galvanized dual-phase steel or the above preparation method in improving the edge quality of stamping parts. A 590 MPa grade hot-dip galvanized dual-phase steel with low cost, good yield strength, tensile strength and elongation after fracture is provided, which can play a role in improving the edge quality when stamped into parts, solve the problem of brittleness of high-strength steel plates, and realize stable production of stamping.

[0090] For a further understanding of the present invention, the following is an example for illustration:

[0091] A 590 MPa grade hot-dip galvanized dual-phase steel, its chemical composition by weight percentage is C: 0.07 - 0.10%, Si: 0.20 - 0.30%, Mn: 1.5 - 2.0%, Al: 0.02 - 0.05%, Cr: 0.30 - 0.40%, Nb: 0.015 - 0.025%, Ti: 0.015 - 0.025%, P ≤ 0.015%, S ≤ 0.015%, N ≤ 0.008%, and the rest is Fe and unavoidable impurities.

[0092] Among them, Table 1 shows the chemical composition (%) of the continuous casting billets of Examples 1 - 6; Table 2 shows the hot rolling process parameters of Examples 1 - 6; Table 3 shows the pickling process parameters of Examples 1 - 6; Table 4 shows the acid rolling process parameters of Examples 1 - 6; Table 5 shows the galvanizing process parameters of Examples 1 - 6; Table 6 shows the mechanical property test results of the steel of Examples 1 - 6.

[0093] Table 1 Chemical composition (%) of the continuous casting billets of Examples 1 - 6

[0094] Example C Si Mn Als Cr Nb Ti P S N 1 0.0863 0.2424 1.6683 0.0348 0.3389 0.0216 0.0225 0.0114 0.0021 0.0046 2 0.0864 0.2372 1.6497 0.0328 0.3451 0.0198 0.0217 0.0108 0.0024 0.0048 3 0.0865 0.2303 1.6924 0.0337 0.3439 0.0209 0.0233 0.0115 0.0023 0.0049 4 0.0865 0.2262 1.6759 0.0369 0.3587 0.0213 0.0241 0.0113 0.0022 0.0043 5 0.0875 0.2429 1.6499 0.0322 0.3493 0.0204 0.0228 0.0107 0.0025 0.0045 6 0.0878 0.2437 1.6711 0.0323 0.3672 0.0221 0.0209 0.0103 0.0024 0.0045

[0095] The above-mentioned continuous casting billet is subjected to hot rolling, pickling cold rolling, and hot dip galvanizing processes to obtain the hot dip galvanized dual-phase steel of the present invention with a thickness of 0.8 - 2.3 mm, a yield strength of 340 - 430 MPa, a tensile strength of 590 - 700 MPa, and an elongation after fracture of 25.5 - 29%.

[0096] The continuous casting billet is heated in a heating furnace, and the tapping temperature is controlled at 1180 - 1250 °C for 150 - 400 min. After rough rolling, the thickness of the intermediate billet is 40 mm. The finish rolling temperature is controlled at 860 - 920 °C, and the coiling temperature is controlled at 530 - 620 °C to obtain a hot rolled steel coil.

[0097] The hot rolled steel coil is uncoiled to obtain a cold rolled sheet and pickled; wherein, the pickling temperature is 82 - 85 °C, the concentration of sulfuric acid in the acid solution is 160 - 175 g / L, when the thickness of the cold rolled sheet ≤ 1.5 mm, the pickling speed ≤ 220 m / min, when the thickness of the cold rolled sheet > 1.5 mm, the pickling speed ≤ 180 m / min; the pickled steel sheet is cold rolled and coiled to obtain a cold rolled and hardened coil, wherein, the total cold rolling reduction rate is 52 - 70%.

[0098] The cold rolled and hardened coil is galvanized after continuous annealing. The temperature of the preheating section is 560 - 660 °C, the temperature of the pre-oxidation section is 580 - 680 °C, the temperature of the heating section is 800 - 820 °C, the temperature of the slow cooling section is 680 - 720 °C, the temperature of the rapid cooling section is 450 - 470 °C, and the speed of the process section ≥ 60 m / min; the process of the pre-oxidation section adopts oxygen content control, and the oxygen content is 0.2 - 2.0%; after galvanizing in a zinc pot, it enters the skin pass and tension leveling section. The elongation after fracture of the skin pass is controlled at 0.3 - 0.5%, the skin pass rolling force ≥ 200 t, the elongation after fracture of the tension leveling is controlled at 0.05 - 0.15%, and finally, the hot dip galvanized steel coil with a thickness of 0.8 - 2.3 mm is obtained through outlet coiling and splitting. By controlling the galvanizing annealing process, the purpose of ensuring high strength and high stamping performance of the finished product is achieved.

[0099] Table 2 Hot rolling process parameters of Examples 1 - 6

[0100]

[0101] Table 3 Pickling process parameters of Examples 1 - 6

[0102]

[0103] Table 4 Pickling cold rolling process parameters of Examples 1 - 6

[0104]

[0105] Table 5 Galvanizing process parameters of Examples 1 - 6

[0106]

[0107] Table 6 Mechanical property test results of the steel in Examples 1 to 6

[0108]

[0109] In the present invention, the mechanical properties of the products in Examples 1 to 6 were tested according to the method of GB / T 228.

[0110] As can be seen from Examples 1 to 6, the present invention obtains a 590 MPa grade hot-dip galvanized dual-phase steel by controlling the key process parameters and component ratios of each process. The yield strength of the hot-dip galvanized dual-phase steel is 340 - 430 MPa, the tensile strength is 590 - 700 MPa, the elongation after fracture is ≥ 22%, and the surface quality reaches the FB level.

[0111] The mechanical property test results of the steel in each example of the present invention are respectively listed in Table 6. From the performance data listed in Table 6, the performance can be stably controlled. It is found through Table 6 that for the 590 MPa grade hot-dip galvanized dual-phase steel prepared in Examples 1 to 6, taking Example 1 as an example, when the sampling positions are the head, middle or tail, the corresponding thickness and width are 0.8 mm and 1100 mm respectively, the yield strengths are 355 MPa, 372 MPa and 367 MPa respectively, the tensile strengths are 620 MPa, 625 MPa and 618 MPa respectively, and the elongations after fracture are 26.5%, 28% and 26% respectively. It can be seen that the mechanical property parameters corresponding to different sampling positions vary little. Combining with Figure 1 the SEM images of the 590 MPa grade hot-dip galvanized dual-phase steel product of the present invention in [reference], in which the microstructure is ferrite and martensite, and the volume content of martensite is 20 - 30%. It further proves that the hot-dip galvanized dual-phase steel material is uniform and the performance is stable.

[0112] Figure 2 is a diagram of the micro-cracking phenomenon at the edge when the conventional dual-phase steel material of 590 MPa grade and above is stamped into parts, Figure 3 is the effect diagram of improving the edge quality when the 590 MPa grade hot-dip galvanized dual-phase steel of the present invention is stamped into parts. By comparison, it is found that when the dual-phase steel material of 590 MPa grade and above is stamped into parts before improvement, micro-cracking occurs at the edge, while the 590 MPa grade hot-dip galvanized dual-phase steel of the present invention can significantly improve the edge quality when stamped into parts, and can effectively solve the problems of micro-cracking and hidden cracking generated at the edge of the parts during the stamping process.

[0113] The present invention controls the alloy within a reasonable range by reasonably selecting the chemical element composition and content, mainly by controlling the reasonable contents of C, Si, Mn, Cr, Nb, and Ti. Among them, adding a higher Mn content improves the hardenability and performance stability of the galvanized dual-phase steel, reduces the requirement for the post-plating cooling capacity, adding a small amount of Cr element plays a role in reducing the critical cooling rate of the steel, improving the hardenability and strength, and adding Nb and Ti alloys plays a role in refining grains and strengthening.

[0114] Among them, each chemical component and ratio cooperate with each other to obtain a continuous casting billet through steelmaking. The preparation method of the hot-dip galvanized dual-phase steel includes subjecting the continuous casting billet to hot rolling, pickling rolling, and hot-dip galvanizing processes. By reasonable composition design, the performance of the finished product is ensured, and a suitable pickling process for pickling rolling is coordinated to ensure the surface pickling quality, the cold rolling reduction rate, and the pre-oxidation technology and annealing process in the galvanizing furnace, so as to obtain a steel coil with controllable cost, stable mechanical properties, high stamping performance, and surface quality reaching the FB level, effectively solving the problems of micro-cracks and hidden cracks generated at the edges of parts during stamping.

[0115] In the present invention, the steelmaking composition, pickling rolling process, galvanizing pre-oxidation process, and annealing process play an important role in obtaining a 590 MPa grade hot-dip galvanized dual-phase steel. Among them, the component ratio plays an important role in the stamping performance. By controlling the process parameters, the tissue segregation can be reduced, the grain refinement can be improved, and at the same time, the problem of brittleness of high-strength steel plates can be solved to achieve stable production of stamping. Rolling through the pickling rolling mill can ensure that the cold-hardened substrate has a high cold rolling reduction rate, ensure that the galvanizing annealing obtains high strength, stable performance, and uniform structure, and at the same time, the pickling quality can be ensured. By adopting a higher annealing temperature, controlling the transformation amount of austenite during the heating process, the grain size, and the transformation mode and transformation amount of austenite during the cooling process, the strength of the finished product can be ensured.

[0116] Through the above process method, the process is simple, the control is convenient, and a steel plate with a strength level of 590 MPa is prepared, with a thickness of 0.8 - 2.3 mm, which can meet the requirements of the automotive steel plate field.

[0117] In summary, in the above technical solutions of the present invention, the above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structural transformation made under the technical concept of the present invention by using the content of the specification and drawings of the present invention, or directly / indirectly applied in other related technical fields, is included in the patent protection scope of the present invention.

Claims

1. A 590 MPa grade hot-dip galvanized dual-phase steel, characterized in that, In terms of mass percentage, the chemical composition of the hot-dip galvanized dual-phase steel is as follows: C: 0.07 - 0.09%, Si: 0.20 - 0.30%, Mn: 1.5 - 1.7%, Al: 0.02 - 0.05%, Cr: 0.30 - 0.40%, Nb: 0.015 - 0.025%, Ti: 0.015 - 0.025%, P ≤ 0.015%, S ≤ 0.015%, N ≤ 0.008%, and the balance is Fe and unavoidable impurities; The structure of the hot-dip galvanized dual-phase steel is ferrite and martensite, and the volume content of martensite is 20 - 30%; The thickness of the hot-dip galvanized dual-phase steel is 0.8 - 2.3 mm.

2. The 590 MPa grade hot-dip galvanized dual-phase steel according to claim 1, characterized in that, The yield strength of the hot-dip galvanized dual-phase steel is 340 - 430 MPa a , the tensile strength is 590 - 700 MPa, and the elongation after fracture is ≥22%.

3. The 590 MPa grade hot-dip galvanized dual-phase steel according to claim 1, wherein The thickness of the hot-dip galvanized dual-phase steel is 0.8 - 2.0 mm.

4. A preparation method of a 590 MPa grade hot-dip galvanized dual-phase steel according to any one of claims 1 to 3, characterized in that, It includes the following steps: The billets corresponding to the components of the 590 MPa grade hot-dip galvanized dual-phase steel as described in any one of claims 1 - 3 are successively subjected to hot rolling, pickling rolling, and hot-dip galvanizing processes to obtain the hot-dip galvanized dual-phase steel.

5. The preparation method of the 590MPa grade hot-dip galvanized dual-phase steel according to claim 4, characterized in that, The hot rolling includes: After heating the billets at 1180 - 1250 °C for 150 - 400 min, rough rolling, finish rolling, and coiling are successively carried out to obtain a hot-rolled steel coil; Among them, the temperature of the rough rolling is 1050 - 1300 °C; the temperature of the final finish rolling is 860 - 920 °C, and the temperature of the coiling is 530 - 620 °C.

6. The preparation method of the 590 MPa grade hot-dip galvanized dual-phase steel according to claim 5, characterized in that, The thickness of the intermediate billet after rough rolling is 40 mm.

7. The preparation method of the 590 MPa grade hot-dip galvanized dual-phase steel according to claim 4, characterized in that, The pickling rolling includes: In an acid solution, the hot-rolled steel coil obtained by hot rolling is pickled to obtain a steel plate; The steel plate is cold-rolled and coiled to obtain a cold-rolled and hardened coil; Among them, the concentration of sulfuric acid in the acid solution is 160 - 175 g / L; the temperature of the pickling is 82 - 85 °C; The pickling speed ≤ 220 m / min; the total cold-rolling reduction ratio is 52 - 70%.

8. The preparation method of the 590 MPa grade hot-dip galvanized dual-phase steel according to claim 4, characterized in that, The galvanizing process includes: The cold-rolled and hardened coil is annealed and then galvanized to obtain the 590 MPa grade hot-dip galvanized dual-phase steel; The annealing process steps successively include a preheating section, a pre-oxidation section, a heating section, a slow cooling section, and a rapid cooling section; Among them, the traveling speed of the cold-rolled and hardened coil in the annealing process steps is 60 - 90 m / min; The temperature of the preheating section is 560 - 660 °C; the temperature of the pre-oxidation section is 580 - 680 °C, and the oxygen content is 0.2 - 2.0%; the temperature of the heating section is 800 - 820 °C; the temperature of the slow cooling section is 680 - 720 °C; the temperature of the rapid cooling section is 450 - 470 °C.

9. The preparation method of the 590 MPa grade hot-dip galvanized dual-phase steel according to claim 8, characterized in that, The steps after galvanizing further include skin pass rolling and tension leveling to obtain the 590 MPa grade hot-dip galvanized dual-phase steel; Among them, the elongation after skin pass rolling is 0.3 - 0.5%; the skin pass rolling force is 260 - 295 t; the elongation after tension leveling is 0.05 - 0.15%.

10. Application of the 590 MPa grade hot-dip galvanized dual-phase steel as described in any one of claims 1 - 3 or the 590 MPa grade hot-dip galvanized dual-phase steel prepared by the preparation method as described in any one of claims 4 - 9 in improving the edge quality of stamping parts.

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