A 700MPa grade low alloy high strength coated steel plate and its preparation method

Abstract

The present application relates to the field of high-strength steel preparation, and in particular to a 700MPa-grade low-alloy high-strength coated steel plate and a preparation method thereof; the coated steel plate comprises a steel substrate, the chemical composition of the steel substrate comprises: C, Si, Mn, Cr, Al, P, S, N, and the rest are solid solution metal elements, Fe and inevitable impurities; wherein the solid solution metal elements comprise at least one of V and Ti; the method comprises: heating the ingot, then performing rough rolling, finish rolling and cooling after rolling, and then coiling to obtain a hot-rolled coil; uncoiling the hot-rolled coil, and then performing pickling to obtain a pickled plate; annealing the pickled plate in a continuous hot-dip manner, and then cooling and then performing skinning to obtain a low-alloy high-strength coated steel plate; by controlling the solid solution metal elements V and Ti, and then controlling the Si content, the strength and hardness of ferrite in the metallographic structure can be improved, and the difference in the transverse and longitudinal mechanical properties of the low-alloy high-strength steel can be reduced.

Description

Technical Field

[0001] The present application relates to the field of high-strength steel preparation, and in particular to a 700MPa-grade low-alloy high-strength coated steel plate and a preparation method thereof. Background Art

[0002] Low-alloy high-strength steel is widely used in the manufacture of automotive structural parts and reinforcements due to its low cost, high yield-to-strength ratio, and good forming and welding properties, such as seat rails, fender support plates, and bumper reinforcement plates. At present, the widely used types of low-alloy high-strength steel include cold-rolled annealed plates, cold-rolled galvanized plates, hot-rolled bare plates, and hot-rolled pickled plates.

[0003] As the automotive industry's requirements for lightweight and collision safety continue to increase, the demand for the strength level of low-alloy high-strength steel is also gradually increasing. Therefore, in order to meet the demand for the strength level of low-alloy high-strength steel, it is necessary to increase the amount of alloy. However, if the cold rolling process is used to produce low-alloy high-strength steel, excessive alloy content and too low annealing temperature will lead to the common problems of insufficient recrystallization and fiberization of the structure along the rolling direction of low-alloy high-strength steel, resulting in obvious differences in transverse and longitudinal mechanical properties, especially yield strength, which seriously affects subsequent processing and use.

[0004] Therefore, how to reduce the difference in the transverse and longitudinal mechanical properties of low-alloy high-strength steel is a technical problem that needs to be solved urgently. Summary of the invention

[0005] The present application provides a 700MPa grade high-strength coated steel plate and a preparation method thereof, so as to solve the technical problem that the difference in the transverse and longitudinal mechanical properties of low-alloy high-strength steel in the prior art is too large.

[0006] In a first aspect, the present application provides a 700MPa grade low alloy high strength coated steel plate, the coated steel plate comprising a steel substrate, and the chemical composition of the steel substrate comprises, by mass fraction:

[0007] C: 0.05% ~ 0.1%, Si: 0.02% ~ 0.1%, Mn: 1.5% ~ 2.2%, Cr: 0.02% ~ 0.1%, Al: 0.1% ~ 0.5%, P: 0 ~ 0.01%, S: 0 ~ 0.001%, N: 0 ~ 0.005%, the rest are solid solution metal elements, Fe and unavoidable impurities;

[0008] Wherein, the solid solution metal element includes at least one of V and Ti.

[0009] Optionally, the solid solution metal element includes V, and the mass fraction of V is 0.01% to 0.08%;

[0010] And / or, the solid solution metal element includes Ti, and the mass fraction of Ti is 0.02% to 0.08%;

[0011] And / or, the solid solution metal elements include V and Ti, the mass fraction of V is 0.01% to 0.08%, and the mass fraction of Ti is 0.02% to 0.08%.

[0012] Optionally, the coated steel plate includes a coating, the coating includes a zinc coating and / or a zinc-aluminum-magnesium coating, and the thickness of the coating is 7 μm to 20 μm.

[0013] Optionally, the metallographic structure of the coated steel plate comprises, by area ratio, ferrite: 80% to 95% and pearlite: 5% to 20%;

[0014] The equivalent grain diameter of the ferrite is less than 8 μm, and the equivalent grain diameter of the pearlite is less than 2 μm.

[0015] In a second aspect, the present application provides a method for preparing the coated steel sheet according to the first aspect, the method comprising:

[0016] The ingot is heated, and then rough rolled, finish rolled, cooled after rolling, and coiled to obtain a hot rolled coil;

[0017] The hot-rolled coil is uncoiled and then pickled to obtain a pickled plate;

[0018] The pickled plate is annealed by continuous hot-dip annealing, then cooled, and then smoothed to obtain a low-alloy high-strength coated steel plate with uniform transverse and longitudinal mechanical properties;

[0019] Wherein, the ingot contains the chemical composition of the steel substrate in the coated steel plate described in the first aspect.

[0020] Optionally, the terminal temperature of the heating is 1240° C. to 1270° C., the final rolling temperature of the finish rolling is 900° C. to 940° C., and the thickness of the hot-rolled coil is 1.6 mm to 6.0 mm.

[0021] Optionally, the post-rolling cooling includes an air cooling section and a water cooling section, the air cooling rate of the air cooling section is 5°C / s to 20°C / s, the water cooling rate of the water cooling section is 15°C / s to 30°C / s, and the coiling temperature is 620°C to 700°C.

[0022] Optionally, the annealing treatment includes preheating treatment, heating treatment, soaking treatment and cooling treatment. The end temperature of the preheating treatment is 210℃~230℃, the end temperature of the heating treatment is 610℃~720℃, the heating rate of the heating treatment is 10℃ / s~30℃ / s, the time of the soaking treatment is 30s~75s, the end temperature of the cooling treatment is 430℃~460℃, and the cooling rate of the cooling treatment is 5℃ / s~20℃ / s.

[0023] Optionally, the pickling strip running speed is ≤150m / min, and the pickling flat rolling force is 1800kN~2500kN.

[0024] Optionally, the elongation of the finishing is 0.8% to 1.6%.

[0025] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0026] A 700MPa-grade low-alloy high-strength coated steel plate provided in an embodiment of the present application controls the chemical composition of a steel substrate in the coated steel plate. Since carbon is the main strengthening element for improving the hardness and strength of ferrite in the metallographic structure of the steel substrate, controlling the carbon content can make the strength of the steel substrate meet expectations. Since vanadium or titanium is a strong carbonitride-forming element, it can improve the strength and hardness of ferrite in the metallographic structure, which can not only ensure the strength of the coated steel plate, but also reduce the hardness difference between ferrite and pearlite. At the same time, since silicon solid solution strengthening can improve the hardness and strength of ferrite, controlling the silicon content can further reduce the hardness difference between ferrite and pearlite in the metallographic structure of the steel substrate, and not adding Nb elements that can reduce the recrystallization temperature is beneficial to avoid the formation of fibrous tissue in the coated steel plate along the rolling direction during the hot rolling process, thereby reducing the difference between the transverse and longitudinal tissues of the low-alloy high-strength steel, reducing the difference in the transverse and longitudinal mechanical properties of the low-alloy high-strength steel, and obtaining a steel plate with good uniformity of tissue performance and hole expansion performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0029] Figure 1 A schematic diagram of a process for a method provided in an embodiment of the present application;

[0030] Figure 2 This is a result diagram of the metallographic structure of the steel substrate in the coated steel plate provided in the embodiment of the present application. DETAILED DESCRIPTION

[0031] The present invention will be described in detail below in conjunction with specific implementations and examples, and the advantages and various effects of the present invention will be more clearly presented. It should be understood by those skilled in the art that these specific implementations and examples are for illustrating the present invention, rather than limiting the present invention.

[0032] Throughout the specification, unless otherwise specifically stated, the terms used herein should be understood as meanings commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art to which the present invention belongs. In the event of a conflict, the present specification takes precedence.

[0033] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0034] The creative thinking of this application is:

[0035] While solving the differences in the transverse and longitudinal mechanical properties of low-alloy high-strength steel, since the differences in the transverse and longitudinal mechanical properties of hot-rolled low-alloy high-strength steel are smaller than those of cold-rolled products, it is mainly used in the form of hot-rolled bare plates and hot-rolled pickled plates, which makes it difficult to meet the corrosion resistance requirements of body and chassis parts. Therefore, while solving the differences in the transverse and longitudinal mechanical properties, it is also necessary to improve the corrosion resistance of steel products.

[0036] The technical solution provided by the embodiment of the present invention is to solve the above technical problems, and the overall idea is as follows:

[0037] In one embodiment of the present application, a 700 MPa grade low alloy high strength coated steel plate is provided, wherein the coated steel plate comprises a steel substrate, and the chemical composition of the steel substrate comprises, by mass fraction:

[0038] C: 0.05% ~ 0.1%, Si: 0.02% ~ 0.1%, Mn: 1.5% ~ 2.2%, Cr: 0.02% ~ 0.1%, Al: 0.1% ~ 0.5%, P: 0 ~ 0.01%, S: 0 ~ 0.001%, N: 0 ~ 0.005%, the rest are solid solution metal elements, Fe and unavoidable impurities;

[0039] Wherein, the solid solution metal element includes at least one of V and Ti.

[0040] In the embodiment of the present application, the positive effect of the mass fraction of C being 0.05% to 0.1% is that within this mass fraction range, since C is an interstitial solid solution atom, it is also the main strengthening element for improving the matrix structure of low-alloy high-strength steel, especially the hardness and strength of ferrite. In addition, C can form carbides with micro-alloy elements such as V and Ti, and the formed carbides will precipitate on the ferrite matrix, thereby playing a role in precipitation strengthening; when the mass fraction value is greater than the maximum value of the endpoint of the range, the forming performance and welding performance of the steel will be damaged; when the mass fraction value is less than the minimum value of the endpoint of the range, since C is the main element for maintaining the strength of the steel, if the C content is insufficient, the strength of the steel will be insufficient.

[0041] The positive effect of Si mass fraction of 0.02% to 0.1% is that within the range of this mass fraction, since Si is a ferrite-forming element and has a strong solid solution strengthening effect, it can improve the hardness and strength of ferrite, thereby reducing the hardness difference between ferrite and pearlite, which is beneficial to improving the hole expansion performance of steel; when the mass fraction value is greater than the maximum value of the endpoint of this range, it will cause iron oxide scale to appear on the surface of the hot-rolled coil, which is difficult to clean in the pickling stage, and is easy to cause plating leakage, dezincification and zinc flow defects. When the mass fraction value is less than the minimum value of the endpoint of this range, it will lead to insufficient Si content and cannot effectively reduce the hardness difference between ferrite and pearlite.

[0042] The positive effect of Mn mass fraction of 1.5% to 2.2% is due to the fact that Mn is a solid solution strengthening element, which is essential for ensuring that low-alloy high-strength steel obtains a yield strength and tensile strength of more than 700MPa; when the mass fraction value is greater than the maximum value at the endpoint of the range, the hardenability of the steel will be improved, and hard phase structures such as bainite and martensite will be easily generated, and segregation will be easily formed, resulting in uneven metallographic structure of the steel, thereby damaging the formability of the steel; when the mass fraction value is less than the minimum value at the endpoint of the range, the yield strength and tensile strength of the steel cannot be made to meet the requirements.

[0043] The positive effect of Cr mass fraction of 0.02% to 0.1% is due to the fact that Cr is an element that improves the hardenability of steel products and is also a strengthening element, which is beneficial to improving the strength of low-alloy high-strength steel; when the mass fraction value is greater than the maximum value of the endpoint of the range, during the annealing and galvanizing process, Cr tends to be externally oxidized on the surface of the steel plate, which is easy to cause surface leakage and affect the distribution of the coating; when the mass fraction value is less than the minimum value of the endpoint of the range, the strength of the low-alloy high-strength steel will not meet expectations.

[0044] The positive effect of Al's mass fraction of 0.1% to 0.5% is that within this mass fraction range, since Al is a non-carbide forming element, it can inhibit the precipitation of a large amount of cementite during annealing and galvanizing, so as to avoid excessively high yield strength and ensure a lower hardness difference between the final ferrite and pearlite. At the same time, since part of Al can replace Si, better surface quality of hot-rolled and pickled steel products can be obtained; when the mass fraction value is greater than the maximum value of the endpoint of the range, too high Al will increase the viscosity of the molten steel, which is easy to cause blockage of the continuous casting nozzle and affect the casting of the steel; when the mass fraction value is less than the minimum value of the endpoint of the range, the Al content is insufficient, and the precipitation of a large amount of cementite during annealing and galvanizing cannot be effectively inhibited, resulting in excessively high yield strength of the steel product, affecting the quality of the steel product.

[0045] The positive effect of the P mass fraction of 0 to 0.01% is that within this mass fraction range, since P can act as an interstitial solid solution atom, the strength of the steel plate can be appropriately improved; when the mass fraction value is greater than the maximum value at the end point of this range, the plasticity and formability of the steel will deteriorate because P is easily segregated at the grain boundary.

[0046] The mass fraction of S is 0 to 0.001%. The positive effect is that within this mass fraction range, S is easily combined with Mn to form coarse MnS inclusions, and the formed MnS inclusions will deteriorate the forming properties of the steel plate such as hole expansion and flanging, affecting the forming properties of the steel plate.

[0047] The mass fraction of N is 0 to 0.005%. The positive effect is that within this mass fraction range, since N is inevitably present in steel and is usually a harmful impurity element, and the binding force between N and Ti is stronger than that between C, an appropriate amount of N will form nitrides with V or Ti, thereby refining the grains and improving the strength and hardness of ferrite in the metallographic structure.

[0048] In some optional embodiments, the solid solution metal element includes V, and the mass fraction of V is 0.01% to 0.08%;

[0049] And / or, the solid solution metal element includes Ti, and the mass fraction of Ti is 0.02% to 0.08%.

[0050] In the embodiment of the present application, the positive effect of the mass fraction of V being 0.01% to 0.08% is that within this mass fraction range, since V is a strong carbonitride-forming element, it precipitates in the form of carbonitrides during hot rolling coiling and annealing galvanizing, which can improve the hardness and strength of the ferrite structure, not only ensuring that the strength of the steel product meets expectations, but also reducing the hardness difference between the ferrite and pearlite structures, improving the local forming properties of the steel such as hole expansion and bending, and V will not significantly increase the recrystallization temperature, and can avoid or reduce the formation of fibrous structures along the rolling direction during hot rolling, which is beneficial to reducing the differences in transverse and longitudinal structures and the differences in mechanical properties of the steel; when the mass fraction value is greater than the maximum value of the endpoint of the range, due to the excessively high V content, the precipitates formed will reach saturation, and the cost of the steel product will also increase; when the mass fraction value is less than the minimum value of the endpoint of the range, V will not be able to form sufficient carbonitrides, thereby affecting the performance of the steel.

[0051] The positive effect of Ti mass fraction of 0.02% to 0.08% is that within this mass fraction range, since Ti is a strong carbonitride-forming element, it precipitates in the form of carbonitrides during hot rolling coiling and annealing galvanizing, which can improve the hardness and strength of the ferrite structure, not only ensuring that the strength of the steel product meets expectations, but also reducing the hardness difference between the ferrite and pearlite structures, improving the local forming properties of the steel such as hole expansion and bending, and Ti will not significantly increase the recrystallization temperature, which can avoid or reduce the formation of fibrous structure along the rolling direction during hot rolling, which is beneficial to reducing the difference in transverse and longitudinal structures and the difference in mechanical properties of steel; when the mass fraction value is greater than the maximum value of the endpoint of the range, due to the excessively high Ti content, the precipitates formed will reach saturation, and the cost of the steel product will also increase; when the mass fraction value is less than the minimum value of the endpoint of the range, V will not be able to form sufficient carbonitrides, thereby affecting the performance of the steel.

[0052] In some optional embodiments, the coated steel plate includes a coating, the coating includes a zinc coating and / or a zinc-aluminum-magnesium coating, and the coating has a thickness of 7 μm to 20 μm.

[0053] In the embodiment of the present application, the positive effect of the coating thickness being 7μm to 20μm is that within this thickness range, it can ensure that the coating of the coated steel can resist corrosion from the atmospheric environment, thereby ensuring the corrosion resistance of the coated steel plate; when the thickness value is greater than the maximum value of the endpoint of the range, the coating thickness is too large, affecting the overall cost of the steel product; when the thickness value is less than the minimum value of the endpoint of the range, the coating thickness is insufficient, resulting in insufficient anti-corrosion life of the steel product, affecting the use of parts made from the steel product.

[0054] In some optional embodiments, the metallographic structure of the coated steel plate includes, by area ratio: ferrite: 80% to 95% and pearlite: 5% to 20%;

[0055] The equivalent grain diameter of the ferrite is less than 8 μm, and the equivalent grain diameter of the pearlite is less than 2 μm.

[0056] In the embodiment of the present application, the positive effect of the ferrite volume fraction being 80% to 95% is that within this volume fraction range, since in low-alloy high-strength steel, ferrite is the matrix structure, has the characteristics of low strength, low hardness, and easy deformation, it is an important component phase to ensure the plasticity and formability of low-alloy high-strength steel. Therefore, ensuring that the ferrite volume fraction is within the range can ensure the performance advantages of the steel plate.

[0057] The positive effect of pearlite volume fraction of 5% to 20% is that within this volume fraction range, since pearlite is a mechanical mixture of ferrite and cementite, its strength and hardness are significantly higher than those of ferrite, while its plasticity and toughness are worse than those of ferrite. Therefore, pearlite with a suitable volume fraction can ensure the plasticity and formability of steel.

[0058] Control the equivalent grain diameter of ferrite and the equivalent grain diameter of pearlite. Since grain refinement can not only improve the strength of the steel plate, but also improve the plasticity, in addition, the finer the grains, the more grain boundaries there are, the stronger the inhibition of crack propagation during deformation, and the better the local forming performance of the steel plate such as hole expansion and bending. Therefore, it is necessary to control the equivalent grain diameter of ferrite below 8μm, and the equivalent grain diameter of pearlite below 2μm.

[0059] In one embodiment of the present application, Figure 2 As shown, a method for preparing a 700MPa grade low alloy high strength coated steel plate is provided, the method comprising:

[0060] S1. The ingot is heated, and then rough rolled, finish rolled and cooled after rolling, and then coiled to obtain a hot rolled coil;

[0061] S2. uncoiling the hot-rolled coil and then pickling it to obtain a pickled plate;

[0062] S3. The pickled plate is annealed by continuous hot-dip annealing, cooled, and then smoothed to obtain a low-alloy high-strength coated steel plate having uniform mechanical properties in the transverse and longitudinal directions;

[0063] Wherein, the ingot contains the chemical composition of the steel substrate in the coated steel plate described in the first aspect.

[0064] In some optional embodiments, the terminal temperature of the heating is 1240° C. to 1270° C., the final rolling temperature of the finish rolling is 900° C. to 940° C., and the thickness of the hot-rolled coil is 1.6 mm to 6.0 mm.

[0065] In the embodiment of the present application, the positive effect of the heating end temperature being 1240°C to 1270°C is that within this temperature range, due to the high content of microalloying elements such as V and Ti in the coated steel plate, it is necessary to ensure the precipitation effect of the microalloying elements and to ensure the local forming performance of the steel; when the temperature value is greater than or less than the endpoint value of the range, if the heating temperature is too high, the grains of the steel plate will be coarse, which will affect the local forming performance of the steel plate; if the heating temperature is too low, the microalloying elements will not be fully dissolved, and the precipitation strengthening effect of the microalloying elements during the hot rolling coiling process will be weak, affecting the strength of the steel.

[0066] The positive effect of the final rolling temperature of finishing rolling being 900℃~940℃ is that the main reason for the difference in transverse and longitudinal properties and poor hole expansion performance of low-alloy high-strength steel is the uneven organization. In order to avoid hot rolling entering the non-recrystallization zone and forming a fibrous organization along the rolling direction, it is necessary to control the final rolling temperature of finishing rolling. Within this temperature range, the metallographic organization of the steel can be made uniform, and the rolling load of hot rolling can be ensured to be within an appropriate range; when the temperature value is greater than or less than the end value of the range, if the final rolling temperature is too large, the austenite grains will be coarse, and the ferrite and pearlite grain sizes formed during cooling and coiling will increase, which is not conducive to the strength, plasticity and hole expansion performance of the steel plate. If the final rolling temperature is too small, the deformation resistance of the hot-rolled plate will increase, which will increase the rolling load of the hot rolling mill, and at the same time cause uneven organization, and fibrous organization will be formed along the rolling direction, affecting the performance of the steel.

[0067] In some optional embodiments, the post-rolling cooling includes an air cooling section and a water cooling section, the air cooling rate of the air cooling section is 5°C / s to 20°C / s, the water cooling rate of the water cooling section is 15°C / s to 30°C / s, and the coiling temperature is 620°C to 700°C.

[0068] In the embodiment of the present application, the air cooling speed of the air cooling section is 5°C / s to 20°C / s. The positive effect is that within this speed range, the hot-rolled steel coils of 1.6mm to 6.0mm can be cooled so that the shape of the steel coils meets the standard; when the air cooling speed is greater than or less than the endpoint value of the range, if the air cooling speed is too large, water cooling will be performed immediately in the high temperature zone, affecting the shape of the hot-rolled plate. If the air cooling speed is too small, the cooling will be too slow, affecting the shape of the hot-rolled plate.

[0069] The positive effect of the water cooling speed of the water cooling section being 15℃ / s to 30℃ / s is that within this water cooling speed range, the hot rolled coil after hot rolling is prompted to enter the pearlite transformation zone, so that the volume fraction of pearlite in the metallographic structure of the steel plate meets the expected target; when the water cooling speed is greater than or less than the end value of the range, if the water cooling speed is too large, the coiling temperature cannot be accurately controlled and the plate shape of thin gauge steel plates is prone to poor. If the water cooling speed is too small, the hot rolled structure tends to grow, which is not conducive to the control of the equivalent grains of pearlite.

[0070] The positive effect of the coiling temperature of 620℃~700℃ is that since the coiling temperature will affect the structure and proportion of the steel plate, within the end temperature range, the proportion of ferrite and pearlite in the metallographic structure of the steel plate can be within the expected range; when the end temperature value is greater than or less than the end value of the range, if the end temperature value is too high, the ferrite proportion is too high and the pearlite proportion is insufficient, which will lead to low strength of the steel plate. In addition, too high coiling temperature will also cause the carbides of V and Ti to grow and coarsen, affecting the yield strength.

[0071] In some optional embodiments, the annealing treatment includes preheating treatment, heating treatment, soaking treatment and cooling treatment. The end temperature of the preheating treatment is 210℃~230℃, the end temperature of the heating treatment is 610℃~720℃, the heating rate of the heating treatment is 10℃ / s~30℃ / s, the time of the soaking treatment is 30s~75s, the end temperature of the cooling treatment is 430℃~460℃, and the cooling rate of the cooling treatment is 5℃ / s~20℃ / s.

[0072] In the embodiment of the present application, the endpoint temperature of the preheating treatment is 210°C to 230°C. The positive effect is that within this temperature range, direct rapid heating can be avoided to cause internal stress in the steel plate, resulting in poor plate shape; if the preheating temperature is greater than 230°C, the steel plate will recover and soften significantly; if the preheating temperature is less than 210°C, the preheating effect is not good.

[0073] The positive effect of the end point temperature of the heating treatment being 610℃~720℃ is that the microstructure of the steel product can be regulated in the annealing stage after hot rolling, and the type and proportion of the organization will not be changed during the galvanizing process, only the surface state of the steel plate will be changed; when the temperature value is greater than or less than the end point value of the range, if the temperature value is too large, the steel plate will enter the two-phase region, and the metallographic organization type and grain size of the steel product will change, thereby causing changes in mechanical properties; if the temperature value is too small, the chemical reaction between the plating solution and the steel plate surface will be relatively weak, and surface defects such as missed plating and zinc flow lines will easily occur.

[0074] The positive effect of the heating rate of the heating treatment is 10℃ / s to 30℃ / s. Within this heating rate range, the slow heating structure recovery softening can be avoided and the production efficiency can be improved. When the heating rate is less than 10℃ / s, the structure of the pickled board will recover and soften significantly, and the production efficiency is low. When the heating rate is greater than 30℃ / s, energy consumption will increase and it will be difficult to control the heating end temperature.

[0075] The positive effect of the soaking treatment time of 30s to 75s is that within this time range, the galvanizing effect of the steel product is guaranteed, and at the same time, the strength of the steel product is guaranteed to meet expectations; when the time value is greater than or less than the endpoint value of this range, if the time is too long, the V and Ti carbonitrides precipitated by hot rolling will grow and coarsen, which is not conducive to the improvement of yield strength; if the time is too short, the surface reduction effect of the strip is poor, affecting the subsequent galvanizing effect.

[0076] The positive effect of the cooling treatment end temperature of 430℃~460℃ is that within this temperature range, good galvanized surface quality can be guaranteed; if the cooling end temperature is lower than 430℃, the bonding between the steel plate and the coating is poor, which can easily lead to dezincification; if the cooling end temperature is higher than 460℃, it can easily lead to zinc flow defects.

[0077] The positive effect of the cooling rate of the cooling treatment is 5℃ / s~20℃ / s. Within this cooling rate range, the production efficiency and the surface quality of the strip can be guaranteed; if the cooling rate is less than 5℃ / s, the production efficiency will be reduced; if the cooling rate is greater than 20℃ / s, it is difficult to control the cooling end temperature, which will lead to poor surface quality of the strip.

[0078] In some optional embodiments, the strip running speed of the acid rolling is ≤150m / min, and the flat rolling force of the acid rolling is 1800kN to 2500kN.

[0079] In the embodiment of the present application, the positive effect of the acid-rolled strip running speed of ≤150m / min is that within the range of this running speed, the strip can be kept in the pickling tank for the longest time, so as to eliminate the residual iron oxide and red rust on the surface of the hot-rolled strip to the greatest extent, thereby improving the coating effect of continuous hot-dip galvanizing.

[0080] The positive effect of the flattening rolling force of 1800kN to 2500kN is that within the range of the flattening force, the shape and surface quality of the steel plate can be guaranteed; when the flattening rolling force is greater than or less than the end point value of the range, if the flattening rolling force is too large, transverse roller marks will appear on the surface of the strip, and the appearance quality of the final coated steel plate will deteriorate; if the flattening rolling force is too small, the shape of the strip will be poor, and the existing wave-shaped defects will cause the strip to deviate.

[0081] In some optional embodiments, the elongation of the finishing is 0.8% to 1.6%.

[0082] In the embodiment of the present application, the positive effect of the smoothing elongation of 0.8% to 1.6% is due to the fact that the characteristic of low-alloy high-strength steel is that the tensile curve often has a yield platform, and the yield platform will cause a Luders band to appear on the surface of the steel plate during stamping. Within this elongation range, the yield platform can be effectively eliminated. In addition, increasing the smoothing elongation can also improve the uniformity of the coating; when the elongation value is greater than or less than the end point value of the range, if the elongation value is too large, it will cause severe work hardening of the steel plate and excessive yield strength. If the elongation value is too small, the yield platform cannot be completely eliminated.

[0083] The chemical composition of the steel substrate in the coated steel sheets of the embodiments and comparative examples is shown in Table 1

[0084] Table 1 Chemical composition percentage of steel substrate in coated steel sheets of various embodiments and comparative examples

[0085]

[0086]

[0087] The preparation process parameters of the hot-rolled coils of various embodiments and comparative examples are shown in Table 2.

[0088] Table 2 Preparation process parameters of hot rolled coils of various embodiments and comparative examples

[0089]

[0090]

[0091] The process parameters of continuous hot-dip galvanizing in each embodiment and comparative example are shown in Table 3.

[0092] Table 3 Process parameters of continuous hot-dip galvanizing in various embodiments and comparative examples

[0093]

[0094]

[0095] Related tests:

[0096] The metallographic structure and mechanical properties of the coated steel sheets obtained in each embodiment and comparative example were tested, and the results are shown in Table 4.

[0097] Related test methods:

[0098] Metallographic structure detection: Cut the metallographic specimens from the coated steel plate, corrode with 4% by volume nitric acid alcohol solution, observe and obtain images under a color metallographic microscope, in which ferrite appears light green and pearlite appears black, and the area ratios of ferrite and pearlite are calculated; based on the metallographic photographs, use professional image analysis software to calculate the equivalent grain diameters of ferrite and pearlite.

[0099] Mechanical properties testing: Using ZWICK / Roell Z100 tensile testing machine, the transverse and longitudinal yield strength, tensile strength and elongation after fracture of the steel plate were tested in accordance with GB / T228.1-2010 standard; using ZWICK BUP1000 forming testing machine, the hole expansion rate was tested in accordance with ISO16630-2003 standard.

[0100] Table 4 Metallographic structure and mechanical properties of the coated steel sheets of the embodiments and comparative examples

[0101]

[0102]

[0103]

[0104] Detailed analysis of Table 4:

[0105] Yield strength (Rp0.2) refers to the elongation strength of steel when the non-proportional elongation is 0.2%. The more the yield strength meets the standard, the better the mechanical properties of the steel.

[0106] Tensile strength (Rm) refers to the maximum stress per unit area before the specimen is broken. The closer the tensile strength meets the standard, the better the mechanical properties of the steel.

[0107] Elongation after fracture A 80 It refers to the percentage of the total deformation ΔL of the gauge section after the specimen is tensile fractured to the original gauge length L. The more the total elongation conforms to the standard, the better the mechanical properties of the steel.

[0108] The hole expansion rate is an indicator to express the flanging forming performance of parts, and the hole expansion rate is related to the mechanical properties of steel.

[0109] From the data of Examples 1-8, it can be seen that:

[0110] By controlling the chemical composition and preparation method of the steel substrate in the coated steel plate, good uniformity of organizational properties and hole expansion performance can be obtained. Since carbon is the main strengthening element for improving the hardness and strength of ferrite in the metallographic structure of the steel substrate, controlling the carbon content can make the strength of the steel substrate meet expectations; since vanadium or titanium is a strong carbonitride forming element, it can improve the strength and hardness of ferrite in the metallographic structure, which can not only ensure the strength of the coated steel plate, but also reduce the hardness difference between ferrite and pearlite; at the same time, since silicon solid solution strengthening can improve the hardness and strength of ferrite, controlling the silicon content can further reduce the hardness difference between ferrite and pearlite in the metallographic structure of the steel substrate. Through the above treatment, a coated steel plate with expected strength and excellent hole expansion performance can be obtained. The present invention controls the final rolling temperature ≥900℃, and does not add Nb elements that will reduce the recrystallization temperature, which is conducive to avoiding the formation of fibrous tissue in the hot rolling process of the coated steel plate along the rolling direction, thereby reducing the difference between the transverse and longitudinal tissues of low-alloy high-strength steel and reducing the difference in transverse and longitudinal mechanical properties of low-alloy high-strength steel.

[0111] like Figure 1 As shown, by using the chemical composition and preparation method described in Example 1, the metallographic structure of the obtained coated steel sheet is fine and uniform, and is composed of ferrite and pearlite.

[0112] From the data of Comparative Examples 1-4, it can be seen that:

[0113] In Comparative Example 1, if the C content is not within the range of the embodiment of the present invention, the longitudinal yield strength of the steel plate obtained is only 664 MPa, and the hole expansion rate is only 72%;

[0114] In Comparative Example 2, if the V and Ti contents are not within the range of the embodiment of the present invention, the ferrite equivalent grain diameter of the obtained steel plate is greater than 8 μm, the longitudinal yield strength is only 618 MPa, and the hole expansion rate is only 68%;

[0115] In Comparative Example 3, if the final rolling temperature is 850°C, which is not within the range of the embodiment of the present invention, the difference in the transverse and longitudinal yield strength of the obtained steel plate reaches 38 MPa;

[0116] In Comparative Example 4, if the soaking temperature is 750°C, which is not within the scope of the embodiments of the present invention, the microstructure type of the steel plate obtained changes, contains no pearlite, contains 16% bainite, and the equivalent grain diameter of ferrite exceeds 8 μm, and the hole expansion rate of the steel plate is only 48%.

[0117] One or more technical solutions in the embodiments of the present application also have at least the following technical effects or advantages:

[0118] (1) The coated steel plate provided in the embodiment of the present application has good uniformity of organizational performance and hole expansion performance by controlling the chemical composition and preparation method of the steel substrate in the coated steel plate. Since carbon is the main strengthening element for improving the hardness and strength of ferrite in the metallographic structure of the steel substrate, controlling the carbon content can make the strength of the steel substrate meet expectations; since vanadium or titanium is a strong carbonitride forming element, it can improve the strength and hardness of ferrite in the metallographic structure, which can not only ensure the strength of the coated steel plate, but also reduce the hardness difference between ferrite and pearlite; at the same time, since silicon solid solution strengthening can improve the hardness and strength of ferrite, controlling the silicon content can further reduce the hardness difference between ferrite and pearlite in the metallographic structure of the steel substrate. Through the above treatment, a coated steel plate with expected strength and excellent hole expansion performance can be obtained. The present invention controls the final rolling temperature to be ≥900°C and does not add Nb elements that will lower the recrystallization temperature, which is beneficial to avoiding the formation of fibrous tissue along the rolling direction of the coated steel plate during the hot rolling process, thereby reducing the difference between the transverse and longitudinal tissues of the low-alloy high-strength steel and reducing the difference in the transverse and longitudinal mechanical properties of the low-alloy high-strength steel.

[0119] (2) The coated steel plate provided in the embodiment of the present application has a yield strength ≥700MPa, a difference in yield strength between the transverse and longitudinal directions ≤25MPa, and a hole expansion rate ≥75%, which can meet the manufacturing requirements of parts with multiple thickness specifications, high strength, and high local formability.

[0120] (3) The coated steel plate provided in the embodiment of the present application has a uniform microstructure and a difference in yield strength between the transverse and longitudinal directions of ≤25 MPa.

[0121] (4) The coated steel plate provided in the embodiment of the present application has a coating thickness of 7 μm to 20 μm. Compared with uncoated hot-rolled or hot-rolled pickled low-alloy high-strength steel, the coated steel plate has outstanding corrosion resistance and greatly improves the service life of the formed parts.

[0122] (5) Compared with the cold-rolled low-alloy high-strength steel of the same grade, the coated steel plate provided by the embodiment of the present application is less prone to cracking during the processing and forming process, and has a high yield rate.

[0123] (6) The method provided in the embodiment of the present application does not require cold rolling and directly performs coating and plating on the surface of the pickled hot-rolled steel plate, which has a short process flow, low energy consumption and less emissions.

[0124] It should be noted that, in this article, 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 such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are 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 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 "include..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0125] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0126] The foregoing is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a coated steel sheet, characterized in that: The method comprises: The ingot is heated, and then rough rolled, finish rolled, cooled after rolling, and coiled to obtain a hot rolled coil; The hot-rolled coil is uncoiled and then pickled to obtain a pickled plate; Without cold rolling, the pickled plate is directly annealed by continuous hot-dip annealing, then cooled, and then smoothed to obtain a low-alloy high-strength coated steel plate with uniform transverse and longitudinal mechanical properties; Wherein, the chemical composition of the ingot, measured by mass fraction, includes: C: 0.05% ~ 0.1%, Si: 0.02% ~ 0.1%, Mn: 1.5% ~ 2.2%, Cr: 0.02% ~ 0.1%, Al: 0.1% ~ 0.5%, P: 0 ~ 0.01%, S: 0 ~ 0.001%, N: 0 ~ 0.005%, the rest are solid solution metal elements, Fe and unavoidable impurities; Wherein, the solid solution metal element includes at least one of V and Ti, the mass fraction of V is 0.01% to 0.08%, and the mass fraction of Ti is 0.02% to 0.08%; Calculated by area ratio, the metallographic structure of the coated steel plate includes: ferrite: 80% to 95% and pearlite: 5% to 20%; the equivalent grain diameter of the ferrite is less than 8 μm, and the equivalent grain diameter of the pearlite is less than 2 μm; The end temperature of the heating is 1240°C to 1270°C, the final rolling temperature of the finish rolling is 900°C to 940°C, and the thickness of the hot-rolled coil is 1.6mm to 6.0mm; The post-rolling cooling includes an air cooling section and a water cooling section, the air cooling speed of the air cooling section is 5°C / s to 20°C / s, the water cooling speed of the water cooling section is 15°C / s to 30°C / s, and the coiling temperature is 620°C to 700°C; The annealing treatment includes preheating treatment, heating treatment, soaking treatment and cooling treatment. The end temperature of the preheating treatment is 210°C to 230°C, the end temperature of the heating treatment is 610°C to 720°C, the heating rate of the heating treatment is 10°C / s to 30°C / s, the time of the soaking treatment is 30s to 75s, the end temperature of the cooling treatment is 430°C to 460°C, and the cooling rate of the cooling treatment is 5°C / s to 20°C / s.

2. The method according to claim 1, characterized in that The pickling strip running speed is ≤150m / min, and the pickling leveling rolling force is 1800kN-2500kN.

3. The method according to claim 1, characterized in that The elongation of the finishing is 0.8% to 1.6%.

4. The coated steel sheet prepared by the method according to any one of claims 1 to 3, characterized in that: The coated steel plate comprises a coating, wherein the coating comprises a zinc coating and / or a zinc-aluminum-magnesium coating, and the thickness of the coating is 7 μm to 20 μm.

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