A hot-dip galvanized low-alloy high-strength steel and its preparation method

Abstract

The present application relates to a hot-dip galvanized low-alloy high-strength steel and a preparation method thereof, belonging to the field of metallurgy. The high-strength steel comprises a steel matrix and a zinc coating attached to at least part of the surface of the steel matrix. The chemical composition of the steel matrix comprises: C: 0.07 wt% - 0.09 wt%, Si: ≤0.05 wt%, Mn: 1.3 wt% - 1.5 wt%, P: ≤0.018 wt%, S: ≤0.01 wt%, Alt: 0.020 wt% - 0.050 wt%, Nb: 0.040 wt% - 0.06 wt%, Ti: 0.030 wt% - 0.050 wt%, and the balance is Fe and inevitable impurities. By adding appropriate amounts of Nb and Ti microalloying elements, the high-strength steel of the present application has excellent mechanical properties while maintaining a thin gauge, meeting the requirements of the automotive industry for weight reduction, safety, energy conservation, environmental protection, corrosion resistance, etc., and effectively solving the technical problem of insufficient strength existing in the thin-gauge hot-dip galvanized low-alloy high-strength steel for automobiles in the prior art.

Description

Technical Field

[0001] This application relates to the field of metallurgy, and particularly relates to a hot-dip galvanized low-alloy high-strength steel and a preparation method thereof. Background Art

[0002] With the development of energy conservation, emission reduction and lightweight in the automotive industry, in order to reduce self-weight and cost, automotive manufacturing enterprises usually use thin-gauge and high-strength steel plates for automotive structural parts. Using low-alloy high-strength steel can not only reduce the weight of the vehicle but also improve the safety of the vehicle, and it is increasingly widely used as internal structural parts of vehicles.

[0003] Hot-dip galvanized low-alloy high-strength steel for automotive use with high strength levels has comprehensive properties such as high yield strength and yield ratio, excellent deformation resistance, good welding performance, and corrosion resistance, which well meet the requirements of vehicles for weight reduction, safety, energy conservation, environmental protection, corrosion resistance, etc.

[0004] At present, the strength of commercially available hot-dip galvanized low-alloy high-strength steel for automotive use is mainly 260 MPa - 500 MPa, belonging to the low-strength level. After the thickness of the low-strength-level hot-dip galvanized low-alloy high-strength steel is thinned, it is more conducive to reducing the weight of the vehicle body, being more energy-saving and environmentally friendly. However, after the thickness is thinned, the ability to resist deformation is weakened, resulting in safety problems for the vehicle. Therefore, low-strength-level hot-dip galvanized low-alloy high-strength steel is difficult to meet the requirements of the automotive industry for weight reduction, safety, energy conservation, environmental protection, etc. Summary of the Invention

[0005] This application provides a hot-dip galvanized low-alloy high-strength steel and a preparation method thereof to solve the technical problems that the thin-gauge hot-dip galvanized low-alloy high-strength steel for automotive use in the prior art has difficulty in meeting the requirements of the automotive industry for weight reduction, safety, energy conservation, and environmental protection.

[0006] In a first aspect, this application provides a hot-dip galvanized low-alloy high-strength steel, the high-strength steel includes a steel matrix, and the chemical composition of the steel matrix includes: C: 0.07 wt% - 0.09 wt%, Si: ≤ 0.05 wt%, Mn: 1.3 wt% - 1.5 wt%, P: ≤ 0.018 wt%, S: ≤ 0.01 wt%, Alt: 0.020 wt% - 0.050 wt%, Nb: 0.040 wt% - 0.06 wt%, Ti: 0.030 wt% - 0.050 wt%, and the rest is Fe and inevitable impurities.

[0007] Optionally, the structure of the steel matrix mainly includes equiaxed ferrite and globular pearlite.

[0008] Optionally, the high-strength steel further includes a zinc coating attached to at least a part of the surface of the steel substrate, and the mechanical properties of the high-strength steel satisfy at least one of the following: 550 MPa ≤ yield strength ≤ 650 MPa, tensile strength ≥ 610 MPa, elongation A80 ≥ 13%.

[0009] Optionally, the thickness of the high-strength steel ≤ 1.9 mm.

[0010] In a second aspect, the present application provides a method for preparing a hot-dip galvanized low-alloy high-strength steel, which is used to prepare the hot-dip galvanized low-alloy high-strength steel described in the first aspect. The method includes:

[0011] Obtaining a continuous casting billet with a set chemical composition;

[0012] Heating, hot rolling, cooling, and coiling the continuous casting billet to obtain a hot-rolled strip;

[0013] Pickling and cold rolling the hot-rolled strip to obtain a cold-rolled strip;

[0014] Continuously annealing, hot-dip galvanizing, skin pass rolling, and tension leveling the cold-rolled strip to obtain a hot-dip galvanized low-alloy high-strength steel;

[0015] Among them, the tapping temperature of the heating is 1240 °C - 1280 °C.

[0016] Optionally, the cooling mode is an ultra-fast cooling medium pressure mode and a front-end intensive cooling mode, and the cooling rate is 25 °C / s - 35 °C / s.

[0017] Optionally, the coiling temperature is 430 °C - 550 °C.

[0018] Optionally, the total reduction rate of the cold rolling is 56% - 72%.

[0019] Optionally, the heating temperature and soaking temperature of the continuous annealing are 780 °C - 830 °C respectively, the slow cooling temperature is 590 °C - 630 °C, and the fast cooling temperature is 455 °C - 475 °C.

[0020] Optionally, the elongation of the skin pass rolling is 1.2% - 1.6%, and the elongation of the tension leveling is 0.06% - 0.12%.

[0021] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0022] A hot-dip galvanized low-alloy high-strength steel and a preparation method thereof provided by an embodiment of the present application can fully exert the precipitation and grain refinement effects of micro-alloying elements by adding appropriate amounts of Nb and Ti micro-alloying elements, so that the hot-dip galvanized low-alloy high-strength steel can have a relatively high strength level and relatively high plasticity, effectively solving the technical problems of weight reduction, safety, energy conservation, and environmental protection in the automotive industry that are difficult to meet by thin-gauge hot-dip galvanized low-alloy high-strength steel in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings herein are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present application and, together with the specification, are used to explain the principles of the present application.

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1 It is a flowchart of a preparation method of a hot-dip galvanized low-alloy high-strength steel provided by an embodiment of the present application;

[0026] Figure 2 It is an electron microscope micrograph of the steel matrix in the hot-dip galvanized low-alloy high-strength steel provided by Embodiment 1 of the present application;

[0027] Figure 3 It is a metallographic micrograph of the steel matrix in the hot-dip galvanized low-alloy high-strength steel provided by Embodiment 1 of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0029] Various embodiments of the present application may exist in the form of a range. It should be understood that the description in the form of a range is only for convenience and brevity, and should not be construed as a rigid limitation on the scope of the present application. Therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and individual values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and individual numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Additionally, whenever a numerical range is indicated in the present application, it means including any cited numbers (fractions or integers) within the indicated range. Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present application can be obtained through market purchases or can be prepared by existing methods.

[0030] In the present application, unless otherwise stated, the orientation terms such as "upper" and "lower" specifically refer to the drawing directions in the figures. Additionally, in the present application, terms such as "including" and "comprising" mean "including but not limited to". In the present application, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. In the present application, "and / or" describes the association relationship of associated objects and indicates that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one kind", "at least one of the following items (pieces)" or similar expressions refer to any combination of these items, including any combination of single items (pieces) or plural items (pieces). For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0031] In a first aspect, an embodiment of the present application provides a hot-dip galvanized low-alloy high-strength steel. The high-strength steel includes a steel matrix, and the chemical composition of the steel matrix includes: C: 0.07 wt% - 0.09 wt%, Si: ≤0.05 wt%, Mn: 1.3 wt% - 1.5 wt%, P: ≤0.018 wt%, S: ≤0.01 wt%, Alt: 0.020 wt% - 0.050 wt%, Nb: 0.040 wt% - 0.06 wt%, Ti: 0.030 wt% - 0.050 wt%, and the balance is Fe and unavoidable impurities.

[0032] Positive effects of controlling the C content to be 0.07 wt% - 0.09 wt%: As an interstitial solid solution element, C can improve the strength of steel. C is the main element second only to Fe. The basic structure of steel is mainly ferrite and pearlite. The content of C directly determines the proportion of pearlite and directly affects the strength, plasticity and weldability of steel. When the content of C > 0.09 wt%, it will enter the peritectic steel range, which is likely to cause peritectic steel crack defects during the preparation process. When the content of C < 0.07 wt%, insufficient pearlite content cannot be obtained, thus reducing the strength. Exemplarily, the content of C can be 0.07 wt%, 0.075 wt%, 0.08 wt%, 0.085 wt%, 0.09 wt%.

[0033] Positive effects of controlling the Si content to be ≤ 0.05 wt%: When the Si content is as low as possible, the product can obtain better surface quality after galvanizing. Si can dissolve in ferrite and austenite to improve the hardness and strength of steel. However, if the content is too high, it is easy to cause the furnace scale generated during the heating process of the hot rolling process to adhere to the surface of the product and is not easy to remove, resulting in defects such as missing plating and uneven zinc coating. At the same time, oxidation reaction will occur during the continuous annealing heating process, affecting the surface quality of the galvanized coating. Exemplarily, the content of Si can be 0.01 wt%, 0.015 wt%, 0.02 wt%, 0.025 wt%, 0.03 wt%, 0.035 wt%, 0.04 wt%, 0.045 wt%, 0.05 wt%.

[0034] Positive effects of controlling the Mn content to be 1.3 wt% - 1.5 wt%: Mn has the effect of solid solution strengthening. If the content of Mn is too low, the strength of steel cannot be improved. If the content of Mn is too high, it will reduce the plasticity and weldability of steel, and at the same time cause banded segregation to a certain extent, deteriorating the mechanical properties of steel. Exemplarily, the content of Mn can be 1.3 wt%, 1.33 wt%, 1.35 wt%, 1.37 wt%, 1.4 wt%, 1.43 wt%, 1.45 wt%, 1.47 wt%, 1.5 wt%.

[0035] Positive effects of controlling the P content to be ≤ 0.018 wt%: When the content of P is on the high side, it can increase the strength and hardness of steel, but cause a significant decrease in plasticity and impact toughness. Especially at low temperatures, it makes the steel significantly brittle. Cold brittleness makes the cold working and weldability of steel worse. The higher the P content, the greater the cold brittleness, and cold brittleness defects are likely to appear. Therefore, the upper limit content of P is set to 0.018 wt%. Exemplarily, the content of P can be 0.005 wt%, 0.008 wt%, 0.01 wt%, 0.012 wt%, 0.014 wt%, 0.016 wt%, 0.018 wt%.

[0036] Positive effects of controlling the S content to ≤ 0.01 wt%: S exists in the steel in the form of FeS, and FeS and Fe form a low-melting-point (985 °C) compound. The hot working temperature of the steel is generally above 1150 °C - 1200 °C. Therefore, when the S content is too high, cracking of the workpiece will occur due to the premature melting of FeS during hot working of the steel, reducing the ductility and toughness of the steel. Thus, the upper limit content of S is set to 0.01 wt%. Exemplarily, the S content can be 0.002 wt%, 0.004 wt%, 0.006 wt%, 0.008 wt%, 0.01 wt%.

[0037] Positive effects of controlling the Alt content to 0.020 wt% - 0.050 wt%: Alt has the effect of refining grains. If the content is too low, the grains are not refined enough to improve the strength of the steel. If the content is too high, it will lead to an increase in production cost. Exemplarily, the Alt content can be 0.020 wt%, 0.025 wt%, 0.030 wt%, 0.035 wt%, 0.040 wt%, 0.045 wt%, 0.050 wt%.

[0038] Positive effects of controlling the Nb content to 0.040 wt% - 0.06 wt%: Nb has the effects of precipitation strengthening and fine grain strengthening. If the content is too low, the amount of precipitates is too small and the grains are not refined enough to improve the strength of the steel. If the content is too high, the grains become finer and the precipitates become more dispersed, thus affecting the plasticity or toughness of the steel. At the same time, Nb is also a precious metal, which is not conducive to controlling the production cost. Exemplarily, the Nb content can be 0.040 wt%, 0.045 wt%, 0.050 wt%, 0.055 wt%, 0.06 wt%.

[0039] Positive effects of controlling the Ti content to 0.030 wt% - 0.050 wt%: Ti has the effects of precipitation strengthening and fine grain strengthening. If the content is too low, the amount of precipitates is too small and the grains are not refined enough to improve the strength of the steel. If the content is too high, the precipitates of Ti are fine and dispersed, thus affecting the plasticity or toughness of the steel. Exemplarily, the Ti content can be 0.030 wt%, 0.032 wt%, 0.035 wt%, 0.037 wt%, 0.040 wt%, 0.042 wt%, 0.045 wt%, 0.047 wt%, 0.050 wt%.

[0040] In some embodiments, the structure of the steel matrix mainly includes equiaxed ferrite and globular pearlite.

[0041] In some embodiments, the high-strength steel further includes a zinc coating attached to at least a part of the surface of the steel matrix, and the mechanical properties of the high-strength steel satisfy at least one of the following: 550 MPa ≤ yield strength ≤ 650 MPa, tensile strength ≥ 610 MPa, elongation A80 ≥ 13%.

[0042] Exemplarily, the yield strength of the high-strength steel may be 550 MPa, 560 MPa, 570 MPa, 580 MPa, 590 MPa, 600 MPa, 610 MPa, 620 MPa, 630 MPa, 640 MPa, 650 MPa, and the tensile strength may be 610 MPa, 620 MPa, 630 MPa, 640 MPa, 650 MPa, 660 MPa, 670 MPa, 680 MPa, 690 MPa, 700 MPa, 710 MPa, 720 MPa, and the elongation A80 may be 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%.

[0043] In some embodiments, the thickness of the high-strength steel is ≤ 1.9 mm.

[0044] Exemplarily, the thickness of the high-strength steel may be 1.9 mm, 1.7 mm, 1.5 mm, 1.3 mm, 1.1 mm, 0.9 mm, 0.7 mm.

[0045] The high-strength steel has excellent mechanical properties and a relatively thin thickness, meeting the requirements of the automotive field for weight reduction, safety, energy conservation, and environmental protection.

[0046] In a second aspect, as Figure 1 ]>shown, based on a general inventive concept, an embodiment of the present application further provides a method for preparing a hot-dip galvanized low-alloy high-strength steel, the method being used to prepare the hot-dip galvanized low-alloy high-strength steel described in the first aspect, and the method includes:

[0047] Obtaining a continuous casting billet with a set chemical composition;

[0048] Heating, hot rolling, cooling, and coiling the continuous casting billet to obtain a hot-rolled strip;

[0049] Pickling and cold rolling the hot-rolled strip to obtain a cold-hardened strip;

[0050] Continuously annealing, hot-dip galvanizing, temper rolling, and tension leveling the cold-hardened strip to obtain a hot-dip galvanized low-alloy high-strength steel;

[0051] Among them, the tapping temperature of the heating is 1240°C - 1280°C.

[0052] The tapping temperature is lower than 1240 °C, which is not conducive to the re-solution of alloying elements during heating, and is also not conducive to the temperature uniformity of the slab during heating; when the heating temperature is higher than 1280 °C, the slab has excessive burning loss, and the gas consumption is high, which is not conducive to low-cost preparation and is also not conducive to the control of the surface quality of hot-rolled strip steel; preparing in the range of 1240 °C - 1280 °C of the tapping temperature is conducive to the re-solution of alloying elements, and is more conducive to the hot-dip galvanized low-alloy high-strength steel to obtain higher strength. Exemplarily, the tapping temperature can be 1240 °C, 1250 °C, 1260 °C, 1270 °C, 1280 °C.

[0053] The hot rolling includes: rough rolling the heated slab using a 2-stand rolling mill in the 1+5 mode to obtain an intermediate slab; then using a 6-stand continuous rolling mill to finish rolling the intermediate slab; wherein, the starting rolling temperature of the finish rolling is 1030 °C - 1090 °C, and the final rolling temperature of the finish rolling is 870 °C - 910 °C.

[0054] The starting rolling temperature of the finish rolling is 1030 °C - 1090 °C. If the starting rolling temperature is too low, the rolling area will be in the two-phase region for rolling, which is not conducive to the formation of a uniform structure; if the starting rolling temperature is too high, the scale on the surface of the hot-rolled strip steel will be too thick, which is not conducive to the control of the surface quality. Exemplarily, the starting rolling temperature of the finish rolling can be 1030 °C, 1040 °C, 1050 °C, 1060 °C, 1070 °C, 1080 °C, 1090 °C.

[0055] The final rolling temperature of the finish rolling is 870 °C - 910 °C. If the final rolling temperature is too low, the rolling area will be in the two-phase region for rolling, which is not conducive to the formation of a uniform structure; if the final rolling temperature is too high, the scale on the surface of the hot-rolled strip steel will be too thick, which is not conducive to the control of the surface quality. Exemplarily, the final rolling temperature of the finish rolling can be 870 °C, 880 °C, 890 °C, 900 °C, 910 °C.

[0056] By controlling the hot rolling, cold rolling, continuous annealing and tempering processes, the precipitation and grain refinement effects of micro-alloying elements are fully exerted, so that the hot-dip galvanized low-alloy high-strength steel can simultaneously have a higher strength level and higher plasticity. On the other hand, ultra-fast cooling and low-temperature coiling after hot rolling can obtain a finer hot-rolled structure, and at the same time inhibit the precipitation of the second phase during the coiling process after hot rolling, so that the micro-alloying elements are maintained in a solid solution state, which is more conducive to improving the strength of the hot-dip galvanized low-alloy high-strength steel. In addition, the fine hot-rolled structure is inherited to the cold rolling and continuous annealing processes, making the annealed structure finer, thereby improving the strength of the hot-dip galvanized low-alloy high-strength steel.

[0057] In some embodiments, the cooling mode is the ultra-fast cooling medium-pressure mode and the front-end intensive cooling mode, and the cooling rate is 25 °C / s - 35 °C / s.

[0058] The above cooling mode and speed can appropriately increase the cooling speed of the hot-rolled strip steel, enabling the hot-rolled strip steel to avoid the temperature range in which precipitates precipitate, while obtaining a finer and more uniform hot-rolled structure, so that the hot-dip galvanized low-alloy high-strength steel obtains a finer structure and more dispersed precipitates, thereby obtaining higher strength. Exemplarily, the cooling speed can be 25°C / s, 27°C / s, 29°C / s, 31°C / s, 33°C / s, 35°C / s.

[0059] In some embodiments, the coiling temperature is 430°C - 550°C.

[0060] A coiling temperature higher than 550°C will cause precipitates to precipitate and decompose during the continuous annealing process, which is not conducive to improving the performance of the hot-dip galvanized low-alloy high-strength steel; if the temperature is too low, it will lead to excessive water flow during cooling, resulting in the warping defect of the hot-rolled strip steel, which is not conducive to controlling the plate shape and is not conducive to cold rolling preparation. Exemplarily, the coiling temperature can be 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C.

[0061] Pickling the hot-rolled strip steel aims to remove the scale layer on the surface of the hot-rolled strip steel and reduce defects such as missing plating during the hot-dip galvanizing process.

[0062] In some embodiments, the total reduction ratio of cold rolling is 56% - 72%.

[0063] If the total reduction ratio is too small, the distortion energy of the structure is too small, which is not conducive to recovery and recrystallization during the continuous annealing process, and is not conducive to the hot-dip galvanized low-alloy high-strength steel obtaining a fine structure, thereby reducing the strength; if the total reduction ratio is too large, it exceeds the preparation capacity of cold rolling. Exemplarily, the total reduction ratio of cold rolling can be 56%, 58%, 60%, 62%, 64%, 66%, 68%, 70%, 72%.

[0064] In some embodiments, the heating temperature and soaking temperature of the continuous annealing are 780°C - 830°C respectively, the slow cooling temperature is 590°C - 630°C, and the rapid cooling temperature is 455°C - 475°C.

[0065] The heating temperature and soaking temperature being 780°C - 830°C respectively is conducive to the precipitation of precipitates during the continuous annealing process. If the heating temperature or soaking temperature is too low, it is not conducive to the precipitation of precipitates during the continuous annealing process; if the heating temperature or soaking temperature is too high, it will lead to the growth and dissolution of precipitates, and fine and dispersed precipitates cannot be obtained, thereby reducing the strength of the hot-dip galvanized low-alloy high-strength steel. Exemplarily, the heating temperature and soaking temperature can be 780°C, 790°C, 800°C, 810°C, 820°C, 830°C respectively.

[0066] A slow cooling temperature of 590°C - 630°C and a rapid cooling temperature of 455°C - 475°C are beneficial to improving the preparation efficiency. Exemplarily, the slow cooling temperature can be 590°C, 600°C, 610°C, 620°C, 630°C, and the rapid cooling temperature can be 455°C, 460°C, 465°C, 470°C, 475°C.

[0067] In some embodiments, the temperature of the hot-dip galvanizing when entering the zinc pot is 455°C - 465°C.

[0068] The temperature of entering the zinc pot being 455°C - 465°C can reduce the probability of zinc slag formation on the surface of the cold-rolled strip during hot-dip galvanizing. Exemplarily, the temperature of entering the zinc pot can be 455°C, 460°C, 465°C.

[0069] In some embodiments, the elongation of skin pass is 1.2% - 1.6%, and the elongation of tension leveling is 0.06% - 0.12%.

[0070] The elongation of skin pass being 1.2% - 1.6%, if the elongation is too low, the yield plateau of the hot-dip galvanized low-alloy high-strength steel will be too long, and orange peel defects are likely to occur during stamping; if the elongation is too high, it will exceed the upper limit of the rolling force of the skin pass mill. Exemplarily, the elongation of skin pass can be 1.2%, 1.3%, 1.4%, 1.5%, 1.6%.

[0071] The elongation of tension leveling being 0.06% - 0.12% is beneficial to improving the sheet shape of the hot-dip galvanized low-alloy high-strength steel and simultaneously increasing the yield strength. Exemplarily, the elongation of tension leveling can be 0.06%, 0.08%, 0.10%, 0.12%.

[0072] The following further elaborates on this application in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate this application and not to limit the scope of this application. For the experimental methods without specific conditions noted in the following embodiments, they are usually determined according to national standards. If there is no corresponding national standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0073] Example 1

[0074] The chemical composition and content of the steel matrix in the hot-dip galvanized low-alloy high-strength steel are shown in Table 1.

[0075] The specific preparation method of the hot-dip galvanized low-alloy high-strength steel in this example is as follows:

[0076] A continuous casting billet with a set chemical composition is obtained; the continuous casting billet is heated, and the tapping temperature of the continuous casting billet is 1275 °C, and then hot rolling is carried out. Specifically, the hot rolling is carried out by using a 2-stand rolling mill in the 1+5 mode to rough roll the heated continuous casting billet to obtain an intermediate billet, and then a 6-stand continuous rolling mill is used to finish roll the intermediate billet. The starting rolling temperature of the finish rolling is 1085 °C, and the final rolling temperature is 903 °C. Then, it is cooled at a speed of 35 °C / s and coiled at 435 °C to obtain a hot-rolled strip steel; the hot-rolled strip steel is pickled to remove the scale layer on the surface of the hot-rolled strip steel, and then cold rolling is carried out and the total cold rolling reduction rate is controlled to be 71.5%. After the second coiling, a cold-rolled and hardened strip steel is obtained; the cold-rolled and hardened strip steel is continuously annealed. The heating temperature of the continuous annealing is 801 °C, the soaking temperature is 802 °C, the slow cooling temperature is 619 °C, and the rapid cooling temperature is 466 °C. Then, hot-dip galvanizing is carried out at the temperature of entering the zinc pot of 460 °C. After galvanizing, temper rolling and tension leveling are carried out to obtain a hot-dip galvanized low-alloy high-strength steel. Among them, the elongation of the temper rolling is 1.25%, and the elongation of the tension leveling is 0.1%.

[0077] As Figure 2 shown, the SEM microstructure of the steel matrix in the hot-dip galvanized low-alloy high-strength steel provided in Example 1 mainly includes equiaxed ferrite and globular pearlite, without banded structure. The average size of the ferrite grains is 2 μm. The ferrite grains are fine, uniform and equiaxed, and the pearlite is evenly distributed after spheroidization. Fine and dispersed Nb-Ti second-phase particle precipitates are obtained, so as to obtain higher strength, better plasticity, toughness and welding performance, obtain a higher yield ratio, and have better formability than the same-level dual-phase steel.

[0078] As Figure 3 shown, the metallographic microstructure of the steel matrix in the hot-dip galvanized low-alloy high-strength steel provided in Example 1 mainly includes ferrite and pearlite. The ferrite is equiaxed, the average size of the ferrite grains is 2 μm, it is fine and uniform, without banded structure, and the pearlite is evenly distributed after spheroidization. It is beneficial for the hot-dip galvanized low-alloy high-strength steel to obtain higher strength, better plasticity, toughness and welding performance.

[0079] The differences between Examples 2-4 and Comparative Examples 1-2 and Example 1 are as follows: the chemical composition and content of the steel matrix in the hot-dip galvanized low-alloy high-strength steel and the preparation process parameters are different. For details, see Tables 1-4. The remaining preparation steps are the same as those in Example 1.

[0080] Table 1 Chemical composition and content (wt%) of the steel matrix in the hot-dip galvanized low-alloy high-strength steel

[0081]

[0082]

[0083] Table 2 Preparation process parameters of the hot-rolled strip steel

[0084]

[0085] Table 3 Preparation Process Parameters of Chilled Strip Steel

[0086] Group Total reduction ratio of cold rolling Example 1 71.5％ Example 2 65.7％ Example 3 65.7％ Example 4 57.2％ Comparative Example 1 65.7％ Comparative Example 2 63.5％

[0087] Table 4 Preparation Process Parameters of Hot-Dip Galvanized Low-Alloy High-Strength Steel

[0088]

[0089] Table 5 Mechanical Properties and Thickness of Hot-Dip Galvanized Low-Alloy High-Strength Steel

[0090]

[0091]

[0092] As shown in Table 5, for the mechanical properties and thickness of the hot-dip galvanized low-alloy high-strength steel provided in Examples 1-4 and Comparative Examples 1-2, the hot-dip galvanized low-alloy high-strength steel obtained by the present invention through a reasonable chemical composition design and controlling processes such as hot rolling, cold rolling, continuous annealing, and temper rolling has high strength (yield strength) and yield ratio, excellent anti-deformation ability, and a relatively thin thickness, meeting the requirements of the automotive industry for lightweighting, and can effectively solve the technical problems in the prior art that the thin-gauge hot-dip galvanized low-alloy high-strength steel for automobiles is difficult to meet the automotive industry's requirements for weight reduction, safety, energy conservation, and environmental protection. At the same time, the alloy of the present invention has a low cost and a simple preparation method, and is suitable for industrial preparation.

[0093] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but will conform to the widest scope consistent with the principles and novel features claimed in the present application.

Claims

1. A hot-dip galvanized low-alloy high-strength steel, characterized in that, The high-strength steel includes a steel matrix, and the chemical composition of the steel matrix consists of the following components: C: 0.07 wt% - 0.09 wt%, Si: ≤0.05 wt%, Mn: 1.3 wt% - 1.5 wt%, P: ≤0.018 wt%, S: ≤0.01 wt%, Alt: 0.020 wt% - 0.050 wt%, Nb: 0.040 wt% - 0.06 wt%, Ti: 0.030 wt% - 0.050 wt%, and the rest is Fe and inevitable impurities; the structure of the steel matrix mainly includes equiaxed ferrite and globular pearlite, without banded structure. The high-strength steel also includes a zinc coating attached to at least part of the surface of the steel matrix. The mechanical properties of the high-strength steel meet at least one of the following: 550 MPa ≤ yield strength ≤ 650 MPa, tensile strength ≥ 610 MPa, elongation A80 ≥ 13%, and the thickness of the high-strength steel ≤ 1.9 mm.

2. A preparation method of hot-dip galvanized low-alloy high-strength steel, characterized in that, The method is used to prepare the hot-dip galvanized low-alloy high-strength steel according to claim 1, and the method includes: Obtaining a continuous casting billet with a set chemical composition; Heating, hot rolling, cooling and coiling the continuous casting billet to obtain a hot-rolled strip; Pickling and cold rolling the hot-rolled strip to obtain a cold-rolled and hardened strip; Performing continuous annealing, hot-dip galvanizing, skin pass rolling and tension leveling on the cold-rolled and hardened strip to obtain a hot-dip galvanized low-alloy high-strength steel; Among them, the tapping temperature of the heating is 1240°C - 1280°C, and the coiling temperature is 430°C - 550°C.

3. The method according to claim 2, wherein The cooling mode is an ultra-fast cooling medium-pressure mode and a front-end intensive cooling mode, and the cooling rate is 25°C / s - 35°C / s.

4. The method according to claim 2, wherein The total reduction rate of the cold rolling is 56% - 72%.

5. The method according to claim 2, wherein The heating temperature and soaking temperature of the continuous annealing are 780°C - 830°C respectively, the slow cooling temperature is 590°C - 630°C, and the fast cooling temperature is 455°C - 475°C.

6. The method according to claim 2, characterized in that, The elongation of the skin pass rolling is 1.2% - 1.6%, and the elongation of the tension leveling is 0.06% - 0.12%.

Images