Hot-dip galvanized dual-phase steel with tensile strength of 780 MPa and preparation method thereof

Abstract

The invention discloses hot-dip galvanized dual-phase steel with tensile strength of 780 MPa and a preparation method of the hot-dip galvanized dual-phase steel. The preparation method comprises the steps that cold and hard strip steel is subjected to annealing galvanizing treatment, the hot-galvanized dual-phase steel with the tensile strength of 780 MPa is obtained, and annealing galvanizing treatment comprises a preheating section, a pre-oxidation section, a heating section, a slow cooling section and a rapid cooling section; the temperature of the preheating section is 550-670 DEG C; the temperature of the pre-oxidation section is 570-690 DEG C; the temperature of the heating section is 790-830 DEG C; the temperature of the slow cooling section is 670-730 DEG C; and the temperature of the rapid cooling section is 440-480 DEG C. According to the preparation method, the tensile strength of the hot-dip galvanized dual-phase steel can be improved, and stable production of the hot-dip galvanized dual-phase steel with the tensile strength being 780 MPa or above is achieved.

Description

Technical Field

[0001] The present application belongs to the field of metal material processing, and in particular relates to a hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa and a preparation method thereof. Background Art

[0002] As the concepts of automobile weight reduction, energy saving, environmental protection and safety continue to deepen, advanced high-strength steels such as dual-phase steel are gaining more and more attention. Dual-phase steel has gradually become an important material for automobile manufacturing due to its good strength and ductility matching, high initial work hardening rate, high collision absorption energy and low yield strength ratio.

[0003] At present, the galvanized duplex steel with a tensile strength of 450-590MPa, which is the most widely used in automobiles, can no longer fully meet the development needs of the lightweight automobile industry. The galvanized duplex steel with a tensile strength of 780MPa and above has gradually become the main steel used in automobiles. In the existing technology, a large amount of alloy elements are added to the molten steel to improve the tensile strength of the duplex steel. However, this method has high production costs and is difficult to produce. The surface of the galvanized steel is prone to defects such as missing plating and pitting, and it is impossible to achieve the stable production of hot-dip galvanized duplex steel with a tensile strength of 780MPa and above. At present, it is urgent to develop a new preparation process to produce this high tensile strength galvanized duplex steel. Summary of the invention

[0004] The embodiment of the present application provides a hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa and a preparation method thereof, which can effectively improve the tensile strength of the hot-dip galvanized dual-phase steel and realize the stable production of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa and above.

[0005] In a first aspect, an embodiment of the present application provides a method for preparing hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, comprising:

[0006] The cold hardened strip steel is subjected to annealing and galvanizing treatment to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, wherein the annealing and galvanizing treatment includes a preheating section, a pre-oxidation section, a heating section, a slow cooling section and a fast cooling section;

[0007] The temperature of the preheating section is 550℃~670℃;

[0008] The temperature of the pre-oxidation section is 570℃~690℃;

[0009] The temperature of the heating section is 790℃~830℃;

[0010] The temperature of the slow cooling section is 670℃~730℃;

[0011] The temperature of the rapid cooling section is 440℃~480℃.

[0012] According to an embodiment of the first aspect of the present application, the step of annealing and galvanizing the chilled steel strip to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa includes controlling the oxygen content of the treatment environment in the pre-oxidation section to be 0.2% to 2.5%; and / or,

[0013] During the annealing and galvanizing process, the speeds of the preheating section, pre-oxidation section, heating section, slow cooling section and fast cooling section are all 50 to 200 m / min.

[0014] According to the embodiment of the first aspect of the present application, the step of annealing and galvanizing the chilled strip to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa includes pickling, hood annealing and acid rolling the continuous casting slab to obtain a chilled coil.

[0015] According to an embodiment of the first aspect of the present application, before the step of pickling the continuously cast slab, the step further includes hot rolling the continuously cast slab to obtain a hot rolled steel ring;

[0016] The continuous casting slab includes, by mass percentage, C: 0.13%-0.20%, Si: 0.23%-0.37%, Mn: 2.0%-2.7%, Als: 0.01%-0.06%, Cr: 0.23%-0.37%, P≤0.017%, S≤0.017%, N≤0.009%, and the rest is Fe and unavoidable impurities.

[0017] According to an embodiment of the first aspect of the present application, the step of hot rolling the continuous casting slab to obtain the hot rolled steel ring includes heating, rough rolling, finish rolling, and coiling the continuous casting slab to obtain the hot rolled steel ring;

[0018] In the step of pickling, bell annealing and pickling rolling the continuous casting slab to obtain a chilled coil, the pickling includes uncoiling, pickling and coiling the hot-rolled steel coil to obtain a pickled coil;

[0019] Hood annealing includes hood annealing the pickled coil to obtain the pickled coil after hood annealing;

[0020] Pickling rolling includes uncoiling, pickling and coiling the pickled coil after the cover is withdrawn to obtain a cold hard coil.

[0021] According to the embodiment of the first aspect of the present application, after the step of annealing and galvanizing the cold-hardened strip steel to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, the step includes performing skin pass, straightening and coiling on the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa to obtain a hot-dip galvanized dual-phase steel coil with a tensile strength of 780 MPa.

[0022] According to the embodiment of the first aspect of the present application, in the step of heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot-rolled steel ring, the heating temperature is 1170°C to 1260°C, the heating time is 140min to 410min, the final rolling temperature of the finish rolling is 850°C to 930°C, and the coiling temperature is 570°C to 650°C;

[0023] In the step of uncoiling, pickling and coiling the hot-rolled steel coil to obtain the pickled coil, the temperature of the pickling solution used for pickling is 70° C. to 95° C., the mass concentration of the pickling solution used for pickling is 110 g / L to 180 g / L, and the pickling speed is 20 m / min to 110 m / min;

[0024] In the step of performing hood annealing on the pickled coil to obtain the hood annealed pickled coil, the hood annealing temperature is 550° C. to 650° C., and the hood annealing time is 8 hours to 20 hours;

[0025] In the step of uncoiling, pickling and coiling the pickled coil after the cover is withdrawn to obtain a chilled coil, the reduction rate of the pickling rolling is 50% to 75%.

[0026] According to an embodiment of the first aspect of the present application, in the step of skin-passing, straightening and coiling hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa to obtain a hot-dip galvanized dual-phase steel coil with a tensile strength of 780 MPa, the skin-pass elongation is 0.2% to 0.6%, the skin-pass rolling force is 200t to 350t, and the straightening elongation is 0.05% to 0.18%.

[0027] In the second aspect, an embodiment of the present application provides a hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, which is prepared according to the above-mentioned preparation method of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, and the thickness of the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 0.6 to 2.2 mm.

[0028] According to the embodiment of the second aspect of the present application, the yield strength of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 410 MPa to 550 MPa; and the elongation after fracture is 16% to 25%.

[0029] The preparation method of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa provided in the present application adds a pre-oxidation process during the annealing and galvanizing treatment of the cold-hardened strip steel, which can transform the external oxidation of the alloy elements in the cold-hardened strip steel into internal oxidation; and, the temperature of each process section in the annealing and galvanizing is controlled at a high level, such as setting the temperatures of the preheating section, pre-oxidation section, heating section, slow cooling section, and fast cooling section to 550℃~670℃, 570℃~690℃, 790℃~830℃, 670℃~730℃, and 440℃~480℃, respectively, which can effectively increase the transformation amount of austenite during the heating process and control the transformation mode and transformation amount of austenite during the cooling process, thereby ensuring that the prepared hot-dip galvanized dual-phase steel can have excellent mechanical properties, and the tensile strength can be as high as 780MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solution of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0031] Figure 1 This is a scanning electron microscope image of the 780MPa grade hot-dip galvanized dual-phase steel in Example 2 of the present application.

[0032] in:

[0033] F stands for ferrite; M stands for martensite. DETAILED DESCRIPTION

[0034] The features and exemplary embodiments of various aspects of the present application will be described in detail below. In the detailed description below, many specific details are proposed to provide a comprehensive understanding of the present application. However, it is obvious to those skilled in the art that the present application can be implemented without the need for some of these specific details. The following description of the embodiments is only to provide a better understanding of the present application by illustrating examples of the present application. In the accompanying drawings and the following description, at least some of the known structures and technologies are not shown to avoid unnecessary ambiguity in the present application; and, for clarity, the size of some structures may be exaggerated. In addition, the features, structures or characteristics described below may be combined in one or more embodiments in any suitable manner.

[0035] It should be noted that, in this application, the term "comprises" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprises..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0036] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be measured by various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application). Unless otherwise specified, the test temperature of the parameters mentioned in this application is 25° C. and the test pressure is standard atmospheric pressure.

[0037] C: Carbon is an economical strengthening element in steel and one of the important components of dual-phase steel, which determines the strength, plasticity and formability 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 welding performance of the dual-phase steel will decrease. The C content in this application is higher than that in other patent documents, which can ensure strength while adding less precious alloy elements. Therefore, the C content is controlled at 0.15% to 0.18%.

[0038] Si: Silicon can be dissolved in ferrite and austenite to improve the strength of the material. As the Si content increases, the material strength also increases. However, when the Si content is high, it is easy to generate iron oxide scale that is difficult to pickle, and it will also cause leakage after galvanizing, affecting the surface quality of the steel plate. Therefore, the Si content is controlled at 0.25% to 0.35%.

[0039] Mn: Manganese is a commonly used solid solution strengthening element in steel. Duplex steel usually adds no less than 1.2% manganese 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, which can avoid hot brittleness quality problems and improve the hot processing performance of steel. Manganese can also improve the stability of austenite, shift the C curve to the right, and reduce the critical cooling rate of steel. However, when the manganese content is too high, it is easy to generate oxides that are difficult to pickle on the surface during hot rolling, and it is easy to cause external oxidation during galvanizing annealing, forming a large amount of oxides, which can cause leakage of galvanized surface in severe cases. Therefore, the Mn content is controlled at 2.2% to 2.5%.

[0040] Cr: Chromium can significantly delay the transformation of pearlite and bainite, thereby allowing austenite to fully transform into martensite structure, and chromium has obvious cost advantages over molybdenum and niobium. Therefore, chromium is added in this application, and the Cr content is controlled at 0.25% to 0.35%.

[0041] Als: Aluminum is added as a deoxidizer during steelmaking. It can also form AlN to pin the grain boundaries, thereby refining the grains. Therefore, the Als content is controlled at 0.02% to 0.05%.

[0042] As mentioned in the background technology, the prior art mostly adopts the method of adding a large amount of alloy elements into molten steel to improve the tensile strength of duplex steel, but this method will cause problems such as high strength of hot-rolled coil, difficulty in acid rolling, and difficulty in pickling the surface. The production is difficult, and defects such as missing plating and pitting are prone to occur on the surface of galvanized steel. It is impossible to achieve stable production of hot-dip galvanized duplex steel with a tensile strength of 780MPa and above. The inventors have found that by adding a pre-oxidation process during the annealing and galvanizing treatment of the cold-hardened strip, the external oxidation of the alloy elements in the cold-hardened strip can be converted into internal oxidation; and by controlling the temperature of each process section in the annealing and galvanizing at a high level, such as setting the temperatures of the preheating section, pre-oxidation section, heating section, slow cooling section, and fast cooling section at 550°C to 670°C, 570°C to 690°C, 790°C to 830°C, 670°C to 730°C, and 440°C to 480°C, respectively, the transformation amount of austenite during the heating process can be effectively increased, and the transformation mode and transformation amount of austenite during the cooling process can be controlled, thereby ensuring that the prepared hot-dip galvanized dual-phase steel can have excellent mechanical properties, and the tensile strength can be as high as 780MPa.

[0043] In order to solve the problems of the prior art, the embodiments of the present application provide a hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa and a preparation method thereof.

[0044] The present application provides a method for preparing hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, comprising:

[0045] The cold hardened strip steel is subjected to annealing and galvanizing treatment to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, wherein the annealing and galvanizing treatment includes a preheating section, a pre-oxidation section, a heating section, a slow cooling section and a fast cooling section;

[0046] The temperature of the preheating section is 550℃~670℃;

[0047] The temperature of the pre-oxidation section is 570℃~690℃;

[0048] The temperature of the heating section is 790℃~830℃;

[0049] The temperature of the slow cooling section is 670℃~730℃;

[0050] The temperature of the rapid cooling section is 440℃~480℃.

[0051] Preferably, the temperature of the preheating section is 560°C to 660°C;

[0052] The temperature of the pre-oxidation section is 580℃~680℃;

[0053] The temperature of the heating section is 800℃~820℃;

[0054] The temperature of the slow cooling section is 680℃~720℃;

[0055] The temperature of the rapid cooling section is 450℃~470℃.

[0056] More preferably, the temperature of the preheating section is 610°C to 650°C;

[0057] The temperature of the pre-oxidation section is 625℃~670℃;

[0058] The temperature of the heating section is 803℃~820℃;

[0059] The temperature of the slow cooling section is 685℃~716℃;

[0060] The temperature of the rapid cooling section is 452℃~470℃.

[0061] The embodiment of the present application introduces a pre-oxidation process during the annealing and galvanizing treatment of the cold-hardened steel strip, which can transform the external oxidation of the alloy elements in the cold-hardened steel strip into internal oxidation, thereby improving the surface quality and mechanical properties of the galvanized dual-phase steel; the temperature of the preheating section during the annealing and galvanizing treatment is controlled at a high level, which can ensure that the cold-hardened steel strip can quickly enter the heating section and then heat up to the two-phase zone; the temperature of the pre-oxidation section is controlled at a high level, which can ensure that the external oxidation of the alloy elements is fully transformed into internal oxidation; the temperature of the heating section is controlled at a high level, which can ensure that a certain amount of austenite structure is generated in the steel strip; the temperature of the slow cooling section is controlled at a high level, which can allow a small amount of austenite structure to be transformed into ferrite structure during the cooling process of the steel, which not only ensures the strength of the steel but also improves the stability of the austenite structure; the temperature of the fast cooling section is controlled at a high level, which can enable the steel to quickly pass through the bainite transformation zone, which can ensure that the galvanized steel can be transformed from austenite structure to martensite structure. By controlling the temperatures of the preheating section, pre-oxidation section, heating section, slow cooling section and rapid cooling section in the annealing and galvanizing treatment at appropriate levels, the amount of austenite generated in the strip during the heating process can be effectively increased, and the transformation mode and amount of austenite in the strip during the cooling process can be controlled, thereby ensuring that the prepared hot-dip galvanized dual-phase steel has excellent mechanical properties.

[0062] In some optional embodiments, the step of annealing and galvanizing the chilled steel strip to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa includes controlling the oxygen content of the treatment environment in the pre-oxidation section to be 0.2% to 2.5%.

[0063] Exemplarily, the oxygen content of the controlled processing environment in the pre-oxidation stage can be 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.8%, 0.9%, 1.0%, 1.5%, 2.0%, or 2.5%.

[0064] In the embodiment of the present application, the oxygen content of the treatment environment in the pre-oxidation stage is controlled to be 0.2% to 2.5%, thereby avoiding defects such as plating leakage and pitting in duplex steel caused by external oxidation or overoxidation of alloy elements.

[0065] In some optional embodiments, during the annealing and galvanizing process, the speeds of the preheating section, the pre-oxidation section, the heating section, the slow cooling section and the fast cooling section are all 50 to 200 m / min.

[0066] Preferably, during the annealing and galvanizing process, the speeds of the preheating section, the preoxidation section, the heating section, the slow cooling section and the fast cooling section are all 60 to 200 m / min.

[0067] Exemplarily, the speeds of the preheating section, pre-oxidation section, heating section, slow cooling section and fast cooling section are all 50m / min, 55m / min, 60m / min, 65m / min, 70m / min, 75m / min, 80m / min, 85m / min, 90m / min, 95m / min, 100m / min, 120m / min, 150m / min, 180m / min and 200m / min.

[0068] The preheating section, pre-oxidation section, heating section, slow cooling section and fast cooling section in the annealing and galvanizing process can be collectively referred to as the furnace process section. In the embodiment of the present application, by controlling the speed of the furnace process section to 50-200 m / min, it can be ensured that the strip obtains the required austenite content under sufficient heating conditions, and cooperates with the cooling process to obtain the appropriate martensite and ferrite contents. At the same time, the minimum speed is specified to ensure the surface quality of the galvanizing.

[0069] In some optional embodiments, the step of annealing and galvanizing the chilled steel strip to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa includes pickling, hood annealing and acid rolling the continuous casting slab to obtain a chilled coil.

[0070] In some optional embodiments, before the step of annealing and galvanizing the chilled steel strip to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, the step includes uncoiling the chilled coil to obtain the chilled steel strip.

[0071] The pickling process in the embodiment of the present application can not only ensure the pickling quality of the substrate, but also reduce the pickling difficulty of pickling, and improve the surface quality of the steel after galvanizing; the hood annealing process can reduce the difficulty of pickling, improve the problem of high-strength steel plate shape difference, and ensure the stability of pickling; the pickling process can ensure a high cold rolling reduction rate for the cold hardened substrate, thereby ensuring that the hot-dip galvanized dual-phase steel has high tensile strength, stable mechanical properties, and high uniformity of the organization.

[0072] In some optional embodiments, before the step of pickling the continuously cast slab, the step also includes hot rolling the continuously cast slab to obtain a hot-rolled steel ring; the continuously cast slab includes, in terms of mass percentage: C: 0.13% to 0.20%, Si: 0.23% to 0.37%, Mn: 2.0% to 2.7%, Als: 0.01% to 0.06%, Cr: 0.23% to 0.37%, P≤0.017%, S≤0.017%, N≤0.009%, and the rest is Fe and unavoidable impurities.

[0073] Preferably, the continuous casting slab comprises, by mass percentage, the following: C: 0.15%-0.18%, Si: 0.25%-0.37%, Mn: 2.0%-2.5%, Als: 0.02%-0.05%, Cr: 0.25%-0.35%, P≤0.015%, S≤0.015%, N≤0.008%, and the rest is Fe and unavoidable impurities.

[0074] More preferably, the continuous casting slab comprises, in mass percentage, the following: C: 0.15% to 0.17%, Si: 0.30% to 0.37%, Mn: 2.1% to 2.5%, Als: 0.02% to 0.03%, Cr: 0.30% to 0.35%, P≤0.015%, S≤0.005%, N≤0.006%, and the rest is Fe and unavoidable impurities.

[0075] In the embodiments of the present application, low-cost alloys such as Si and Mn are added to improve the hardenability and performance stability of the galvanized dual-phase steel, and the requirements of the galvanized steel plate for the post-plating cooling capacity are reduced; a small amount of Cr element is added to reduce the critical cooling rate of the steel and improve the hardenability and strength of the galvanized dual-phase steel; avoiding the addition of a large amount of precious alloys such as Nb and Mo can reduce the production cost of the enterprise.

[0076] In some optional embodiments, the step of hot rolling the continuous casting slab to obtain the hot rolled steel ring includes heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot rolled steel ring.

[0077] In some optional embodiments, the continuous casting slab is pickled, hood annealed and pickled to obtain a cold hard coil, and the pickling includes uncoiling, pickling and coiling the hot-rolled steel coil to obtain the pickled coil.

[0078] In some optional embodiments, the hood annealing includes hood annealing the pickled coil to obtain a hood annealed pickled coil.

[0079] In some optional embodiments, the pickling rolling includes uncoiling, pickling and coiling the pickled coil after the hood is withdrawn to obtain a cold hard coil.

[0080] In some optional embodiments, after the step of annealing and galvanizing the cold-hardened strip steel to obtain hot-dip galvanized duplex steel with a tensile strength of 780 MPa, the step includes performing skin pass, straightening and coiling on the hot-dip galvanized duplex steel with a tensile strength of 780 MPa to obtain a hot-dip galvanized duplex steel coil with a tensile strength of 780 MPa.

[0081] In some optional embodiments, in the steps of heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot-rolled steel ring, the heating temperature is 1170°C to 1260°C.

[0082] Preferably, the heating temperature is 1180°C to 1250°C.

[0083] Exemplarily, the heating temperature can be 1170°C, 1175°C, 1180°C, 1185°C, 1190°C, 1195°C, 1200°C, 1210°C, 1220°C, 1221°C, 1222°C, 1223°C, 1224°C, 1225°C, 1226°C, 1227°C, 1228°C, 1229°C, 1230°C, 1231°C, 1232°C, 1233°C, 1234°C, 1235°C, 1240°C, 1245°C, 1246°C, 1247°C, 1248°C, 1250°C.

[0084] In some optional embodiments, in the steps of heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot-rolled steel ring, the heating time is 140 minutes to 410 minutes.

[0085] Preferably, the heating time is 150 min to 400 min.

[0086] More preferably, the heating time is 180 min to 280 min.

[0087] Exemplarily, the heating time can be 180 min, 185 min, 190 min, 191 min, 192 min, 193 min, 195 min, 200 min, 210 min, 214 min, 215 min, 216 min, 218 min, 220 min, 221 min, 222 min, 223 min, 224 min, 225 min, 226 min, 227 min, 228 min, 229 min, 230 min, 235 min, 240 min, 250 min, 255 min, 257 min, 258 min, 259 min, 260 min, 265 min, 270 min, 275 min, 276 min, 277 min, 278 min, 279 min, 280 min.

[0088] In some optional embodiments, in the steps of heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot rolled steel ring, the final rolling temperature of the finish rolling is 850°C to 930°C.

[0089] Preferably, the final rolling temperature of the finish rolling is 860°C to 920°C.

[0090] More preferably, the final rolling temperature of the finish rolling is 890°C to 915°C.

[0091] Exemplarily, the final rolling temperature of the finishing rolling can be 880℃, 885℃, 890℃, 895℃, 900℃, 901℃, 902℃, 903℃, 904℃, 905℃, 906℃, 907℃, 908℃, 909℃, 910℃, 911℃, 912℃, 913℃, 914℃, and 915℃.

[0092] In the embodiment of the present application, the final rolling temperature of the fine rolling adopts an austenite zone temperature higher than Ar3, so as to avoid the serious iron oxide scale on the surface of the steel plate due to the excessively high rolling temperature, which makes it difficult to pickle and affects the surface quality of the steel plate; or to avoid the mixed crystal of the material when the ingot enters the two zones for rolling due to the excessively low rolling temperature, and the generation of snowflake point defects during stamping, which affects the surface quality of the galvanized steel.

[0093] In some optional embodiments, in the steps of heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot-rolled steel ring, the coiling temperature is 570°C to 650°C.

[0094] Preferably, the coiling temperature is 580°C to 640°C.

[0095] More preferably, the coiling temperature is 590°C to 620°C.

[0096] Exemplarily, the coiling temperature can be 590°C, 595°C, 596°C, 597°C, 598°C, 599°C, 600°C, 601°C, 602°C, 603°C, 604°C, 605°C, 606°C, 607°C, 608°C, 609°C, 610°C, 611°C, 612°C, 613°C, 614°C, 615°C, 616°C, 617°C, 618°C, 619°C, and 620°C.

[0097] In the embodiment of the present application, the coiling temperature is controlled at 570°C to 650°C, which is beneficial to grain growth, reduces the strength of the steel, improves the formability of the steel plate, and avoids the generation of severe iron oxide scale on the surface of the steel plate due to excessively high coiling temperature, which makes pickling difficult and affects the quality of the steel plate.

[0098] In some optional embodiments, in the steps of uncoiling, pickling and coiling the hot-rolled steel coil to obtain the pickled coil, the temperature of the pickling solution used for pickling is 70°C to 95°C, the mass concentration of the pickling solution used for pickling is 110g / L to 180g / L, and the pickling speed is 20m / min to 110m / min.

[0099] Preferably, the temperature of the pickling solution used for pickling is 75° C. to 85° C., the mass concentration of the pickling solution used for pickling is 145 g / L to 170 g / L, and the pickling speed is 50 m / min to 70 m / min.

[0100] In some optional embodiments, the pickling solution is a hydrochloric acid solution.

[0101] In the embodiment of the present application, the pickling process is controlled to avoid oxide intrusion and over-pickling, thereby ensuring the surface pickling quality of the substrate and achieving high surface quality of the galvanized finished product.

[0102] In some optional embodiments, in the step of performing hood annealing on the pickled coil to obtain the hood-annealed pickled coil, the annealing temperature of the hood annealing is 550°C to 650°C.

[0103] Preferably, the annealing temperature of the hood annealing is 560°C to 620°C.

[0104] More preferably, the annealing temperature of the bell annealing is 570°C to 610°C.

[0105] Exemplarily, the annealing temperature of the bell annealing may be 570°C, 575°C, 580°C, 585°C, 590°C, 595°C, 600°C, 605°C, or 610°C.

[0106] In some optional embodiments, the annealing time of the hood annealing is 8 hours to 20 hours.

[0107] Preferably, the annealing time of the hood annealing is 10 hours to 20 hours.

[0108] More preferably, the annealing time of the bell annealing is 10 hours to 15 hours.

[0109] Exemplarily, the annealing time of the hood annealing may be 10 h, 11 h, 12 h, 13 h, 14 h, or 15 h.

[0110] The embodiment of the present application can reduce the strength of the substrate by 200-300 MPa by controlling the process parameters of the hood annealing, reduce the difficulty of pickling production, and solve the problem of high-strength steel plate shape difference, thereby achieving stable production of the pickling unit.

[0111] In some optional embodiments, in the step of uncoiling, pickling and coiling the pickled coil after hood annealing to obtain a chilled coil, the reduction rate of pickling is 50% to 75%.

[0112] Preferably, the reduction ratio of the acid rolling is 55% to 70%.

[0113] More preferably, the reduction ratio of the acid rolling is 55% to 68%.

[0114] In the embodiment of the present application, the pickled coil after hood annealing is uncoiled and then pickled and rolled to obtain a cold hard coil, wherein the pickling rolling unit adopts a six-roller five-stand continuous rolling mill, and the rolling reduction rate of the pickling is 50% to 75%, which can reduce the recrystallization temperature and phase transformation temperature of the steel and ensure the product performance of the dual-phase steel.

[0115] In some optional embodiments, in the step of skin-passing, straightening and coiling hot-dip galvanized duplex steel with a tensile strength of 780 MPa to obtain a hot-dip galvanized duplex steel coil with a tensile strength of 780 MPa, the skin-pass elongation is 0.2% to 0.6%, the skin-pass rolling force is 200t to 350t, and the straightening elongation is 0.05% to 0.18%.

[0116] Preferably, the elongation of skin-pass is 0.3% to 0.5%, the skin-pass rolling force is 250 t to 350 t, and the elongation of tension leveling is 0.05% to 0.15%.

[0117] More preferably, the elongation of skin-pass is 0.35% to 0.45%, the skin-pass rolling force is 250 t to 300 t, and the elongation of tension leveling is 0.08% to 0.13%.

[0118] In the embodiment of the present application, the galvanized duplex steel is subjected to skin-smoothing and straightening, which can improve the straightness and flatness of the galvanized duplex steel sheet and improve the sheet shape; at the same time, the zinc particles on the surface of the steel sheet can be flattened to make the surface of the steel sheet smooth and improve the surface quality of the duplex steel. Among them, the straightening treatment can also improve the yield strength of the steel by cold working strengthening. The straightening treatment includes processes such as cold drawing, cold drawing or cold rolling, which can cause plastic deformation of the steel, thereby increasing its yield strength.

[0119] The embodiment of the present application provides a hot-dip galvanized duplex steel with a tensile strength of 780 MPa, which is prepared according to the above-mentioned method for preparing hot-dip galvanized duplex steel with a tensile strength of 780 MPa. The thickness of the hot-dip galvanized duplex steel with a tensile strength of 780 MPa is 0.6 mm to 2.2 mm.

[0120] Preferably, the thickness of the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 0.8 mm to 2.0 mm.

[0121] Exemplarily, the thickness of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa can be 0.8 mm, 0.9 mm, 1.0 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.7 mm, 1.8 mm, 1.9 mm, or 2.0 mm.

[0122] In some optional embodiments, the yield strength of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 410 MPa to 550 MPa.

[0123] Preferably, the yield strength of the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 430 MPa to 490 MPa.

[0124] Exemplarily, the yield strength of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 410 MPa, 415 MPa, 420 MPa, 425 MPa, 430 MPa, 435 MPa, 440 MPa, 445 MPa, 450 MPa, 455 MPa, 460 MPa, 465 MPa, 470 MPa, 475 MPa, 485 MPa, 490 MPa, 495 MPa, 500 MPa, 505 MPa, 510 MPa, 515 MPa, 520 MPa, 530 MPa, 540 MPa, and 550 MPa.

[0125] In some optional embodiments, the elongation after fracture of hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 16% to 25%.

[0126] Preferably, the elongation after fracture of the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 17% to 23%.

[0127] Exemplarily, the elongation after fracture of hot-dip galvanized duplex steel with a tensile strength of 780 MPa is 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%, 20.5%, 21%, 21.5%, 22%, 22.5%, and 23%.

[0128] Example

[0129] The following examples describe the disclosure of the present application in more detail, and these examples are for illustrative purposes only, as various modifications and variations within the scope of the disclosure of the present application are apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further treatment, and the instruments used in the examples are commercially available.

[0130] Examples 1-6

[0131] 1. The continuous casting slab is heated in a heating furnace, rough rolled, finish rolled and coiled to obtain a hot rolled steel coil, wherein the thickness of the continuous casting slab is 230 mm, the mass percentage of the chemical composition in the continuous casting slab is shown in Table 1, and the heating temperature, heating time, final rolling temperature of finish rolling and coiling temperature are all shown in Table 2.

[0132] 2. The hot-rolled steel coil in step 1 is uncoiled and pickled, and the pickled steel plate coil is obtained as a pickled coil, wherein the pickling temperature, acid solution concentration and pickling speed are shown in Table 3.

[0133] 3. The pickled coil in step 2 is subjected to hood annealing to obtain a pickled coil after hood annealing, wherein the annealing temperature and annealing time are shown in Table 4.

[0134] 4. The pickled coil after the cover is withdrawn in step 3 is uncoiled, pickled and rolled to obtain a cold hardened coil, wherein the pickling rolling unit adopts a six-roller five-stand continuous rolling mill, and the pickling rolling reduction rate is shown in Table 5.

[0135] 5. The chilled coil in step 4 is uncoiled and then galvanized after continuous annealing, wherein the preheating section temperature, preoxidation section temperature, heating section temperature, slow cooling section temperature and fast cooling section temperature are as shown in Table 6, and the furnace process section speed is ≥60m / min; the preoxidation section process adopts oxygen content control, and the oxygen content of the treatment environment in the preoxidation section is controlled as shown in Table 6; the galvanized steel sheet is obtained after zinc pot galvanizing, and the galvanized steel sheet is subjected to skin pass and straightening to obtain a hot-dip galvanized dual-phase steel sheet, and finally the hot-dip galvanized steel coil is obtained by outlet coiling and splitting, wherein the skin pass elongation, skin pass rolling force and straightening elongation are as shown in Table 6.

[0136] Table 1

[0137] Example C Si Mn Als Cr P S N 1 0.1645 0.3321 2.2123 0.0235 0.3206 0.0110 0.0034 0.0045 2 0.1589 0.3229 2.3025 0.0278 0.3398 0.0130 0.0024 0.0059 3 0.1580 0.3441 2.3985 0.0250 0.3169 0.0124 0.0019 0.0048 4 0.1599 0.3501 2.2198 0.0265 0.3260 0.0106 0.0043 0.0048 5 0.1627 0.3511 2.4067 0.0231 0.3255 0.0127 0.0025 0.0037 6 0.1592 0.3625 2.3845 0.0288 0.3172 0.0145 0.0029 0.0035

[0138] Table 2

[0139]

[0140] Table 3

[0141]

[0142] Table 4

[0143]

[0144] Table 5

[0145]

[0146] Table 6

[0147]

[0148] Data analysis

[0149] The substrate properties of the pickled coils of Examples 1-6 after hood annealing are shown in Table 7.

[0150] Table 7

[0151]

[0152] It can be seen from Table 7 that by controlling the annealing temperature of the hood annealing to 550℃~650℃ and the annealing time of the hood annealing to 8h~20h, the strength of the substrate can be generally reduced by 200~300MPa, thereby reducing the production difficulty of pickling and solving the problem of poor shape of high-strength steel plates, thus achieving stable production of the pickling unit.

[0153] The hot-dip galvanized dual-phase steel coils finally prepared in Examples 1-6 were subjected to mechanical property tests according to the GB / T 228 method. The specific results are shown in Table 8.

[0154] Table 8

[0155]

[0156] As can be seen from Table 8, the thickness of the galvanized steel strip prepared in Examples 1-6 of the present application is 0.8 mm to 2.0 mm, the yield strength can be as high as 488 MPa, the tensile strength can be as high as 834 MPa, and the elongation after fracture can be as high as 22.5%; Figure 1This is an electron microscope image of the product structure of the 780MPa grade hot-dip galvanized dual-phase steel prepared in Example 2 of the present application, in which the structure is ferrite + martensite, and the martensite content is 30% to 40%. In summary, the 780MPa grade hot-dip galvanized dual-phase steel prepared by the method of the present application can meet the requirements of the automotive steel plate field.

Claims

1. A method for preparing hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, characterized in that: include: The cold-hardened strip steel is subjected to annealing and galvanizing treatment to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, wherein the annealing and galvanizing treatment includes a preheating section, a pre-oxidation section, a heating section, a slow cooling section and a fast cooling section; The temperature of the preheating section is 550°C to 670°C; The temperature of the pre-oxidation section is 570°C to 690°C; The temperature of the heating section is 790°C to 830°C; The temperature of the slow cooling section is 670°C to 730°C; The temperature of the rapid cooling section is 440°C to 480°C.

2. The preparation method according to claim 1, characterized in that: The step of annealing and galvanizing the cold-hardened strip steel to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa comprises controlling the oxygen content of the treatment environment in the pre-oxidation section to be 0.2% to 2.5%; and / or, During the annealing and galvanizing treatment process, the speeds of the preheating section, preoxidation section, heating section, slow cooling section and fast cooling section are all 50-200 m / min.

3. The preparation method according to claim 1, characterized in that: The step of annealing and galvanizing the chilled steel strip to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa includes pickling, hood annealing and acid rolling the continuous casting slab to obtain a chilled coil.

4. The preparation method according to claim 3, characterized in that: Before the step of pickling the continuous casting slab, the step further includes hot rolling the continuous casting slab to obtain a hot rolled steel ring; The continuous casting slab comprises, by mass percentage, C: 0.13% to 0.20%, Si: 0.23% to 0.37%, Mn: 2.0% to 2.7%, Als: 0.01% to 0.06%, Cr: 0.23% to 0.37%, P≤0.017%, S≤0.017%, N≤0.009%, and the rest is Fe and unavoidable impurities.

5. The preparation method according to claim 4, characterized in that: The step of hot rolling the continuous casting slab to obtain the hot rolled steel ring comprises heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot rolled steel ring; In the step of pickling, bell annealing and pickling the continuous casting slab to obtain a chilled coil, the pickling comprises uncoiling, pickling and coiling the hot-rolled steel coil to obtain a pickled coil; The hood annealing includes hood annealing the pickled coil to obtain a hood annealed pickled coil; The pickling rolling includes uncoiling, pickling and coiling the pickled coil after the cover is withdrawn to obtain a cold hard coil.

6. The preparation method according to claim 1, characterized in that: The step of annealing and galvanizing the cold-hardened strip steel to obtain hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa includes performing skin pass, straightening and coiling on the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa to obtain a hot-dip galvanized dual-phase steel coil with a tensile strength of 780 MPa.

7. The preparation method according to claim 5, characterized in that: In the step of heating, rough rolling, finish rolling and coiling the continuous casting slab to obtain the hot rolled steel ring, the heating temperature is 1170°C to 1260°C, the heating time is 140min to 410min, the final rolling temperature of the finish rolling is 850°C to 930°C, and the coiling temperature is 570°C to 650°C; In the step of uncoiling, pickling and coiling the hot-rolled steel coil to obtain the pickled coil, the temperature of the pickling solution used for pickling is 70° C. to 95° C., the mass concentration of the pickling solution used for pickling is 110 g / L to 180 g / L, and the pickling speed is 20 m / min to 110 m / min; In the step of performing hood annealing on the pickled coil to obtain the hood annealed pickled coil, the hood annealing temperature is 550° C. to 650° C., and the hood annealing time is 8 h to 20 h; In the step of uncoiling, pickling and coiling the pickled coil after the cover is withdrawn to obtain a cold hard coil, the rolling reduction rate of the pickling is 50% to 75%.

8. The preparation method according to claim 6, characterized in that: In the step of performing skin pass, straightening and coiling on the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa to obtain a hot-dip galvanized dual-phase steel coil with a tensile strength of 780 MPa, the skin pass elongation is 0.2% to 0.6%, the skin pass rolling force is 200t to 350t, and the straightening elongation is 0.05% to 0.18%.

9. A hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa, characterized in that: The hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is prepared according to the preparation method of any one of claims 1 to 8, wherein the thickness of the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 0.6 to 2.2 mm.

10. The hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa according to claim 9, characterized in that: The yield strength of the hot-dip galvanized dual-phase steel with a tensile strength of 780 MPa is 410 MPa to 550 MPa, and the elongation after fracture is 16% to 25%.

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