590MPa-grade hot-dip galvanized DH steel and preparation method thereof

Abstract

The invention relates to the technical field of metallurgy, and discloses 590 MPa-grade hot-dip galvanized DH steel and a preparation method thereof.The 590 MPa-grade hot-dip galvanized DH steel comprises, by mass, 0.12%-0.14% of C, 0.3%-0.5% of Si, 1.5%-1.7% of Mn, 0.4%-0.6% of Al, 0.15%-0.25% of Cr, smaller than or equal to 0.02% of P, smaller than or equal to 0.01% of S, smaller than or equal to 0.008% of N and the balance Fe and inevitable impurities. The preparation method of the 590MPa-grade hot-dip galvanized DH steel comprises the procedures of steelmaking, hot rolling, acid rolling and hot-dip galvanizing, and the hot-dip galvanized DH steel with stable mechanical properties and good surface quality is obtained by optimizing and adjusting chemical components and a production method.

Description

Technical Field

[0001] This application relates to the field of metallurgical technology, specifically to a 590MPa grade hot-dip galvanized DH steel and its preparation method. Background Technology

[0002] Against the backdrop of "dual-carbon" policies and escalating energy consumption, the automotive industry urgently needs to reduce carbon emissions. Automotive lightweighting, as a core pathway to achieving energy conservation and emission reduction goals, requires automotive steel to possess high strength to ensure safety, while also exhibiting good plasticity and formability to meet the demands of vehicle weight reduction and the forming of complex components. Traditional dual-phase steel (DP steel) has good strength and plasticity and low cost, making it widely used in the automotive industry. However, its poor flanging and bending properties limit its application, making it difficult to manufacture complex components. To address these issues, reinforced formability dual-phase steel (DH steel) has gradually entered the market. DH steel introduces a certain amount of retained austenite into the matrix of DP steel, resulting in a transformation-induced plasticity effect (TRIP effect) during deformation. This gives DH steel excellent formability and a higher work hardening index, allowing it to be used to manufacture more complex automotive structural parts and significantly improve the collision energy absorption of automotive structural parts. It can be applied to automotive structural parts and reinforcement components, and has broad market prospects.

[0003] Currently, most of the 590MPa grade hot-dip galvanized DH steel produced by major steel companies in China uses Nb, Mo and other precious metals in existing publicly available technologies. The extensive use of Nb, Mo and other precious alloys has led to increased costs for steel companies. Therefore, developing low-cost, high-surface-quality hot-dip galvanized DH steel is of great significance for the development of steel companies and cost reduction in the automotive industry. Summary of the Invention

[0004] The purpose of this application is to provide a 590MPa grade hot-dip galvanized DH steel and its preparation method, so as to solve the technical problem of the lack of low-cost, high-surface-quality hot-dip galvanized DH steel in the prior art.

[0005] To achieve the above objectives, this application provides a 590MPa grade hot-dip galvanized DH steel, which is produced by sequentially passing steelmaking, hot rolling, pickling, and hot-dip galvanizing processes to obtain a thickness of 0.7~2.3mm. The chemical composition of the steel billet corresponding to the 590MPa grade hot-dip galvanized DH steel, by weight percentage, is C: 0.12%~0.14%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, Al: 0.4%~0.6%, Cr: 0.15%~0.25%, P≤0.02%, S≤0.01%, N≤0.008%, with the remainder being Fe and unavoidable impurities.

[0006] As a preferred option, the steel plates or strips corresponding to the 590MPa grade hot-dip galvanized DH steel have a performance range of 340~440MPa, tensile strength of 590~700MPa, and elongation of ≥27%.

[0007] To achieve the above objectives, this application also provides a method for preparing 590MPa grade hot-dip galvanized DH steel, applied to the preparation of the 590MPa grade hot-dip galvanized DH steel as described above. The chemical composition of the continuous casting billet corresponding to the 590MPa grade hot-dip galvanized DH steel, by weight percentage, includes: C: 0.12%~0.14%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, Al: 0.4%~0.6%, Cr: 0.15%~0. The continuous casting billet contains 25% P, ≤0.02% S, ≤0.01% N, and ≤0.008% Fe and unavoidable impurities. The billet is heated and then hot-rolled and coiled sequentially. The coiling process uses a U-shaped coiling method, with the head and tail coiling temperatures at 590~620℃ and the middle section coiling temperature at 540~580℃, resulting in a hot-rolled steel coil. The hot-rolled steel coil is then subjected to pickling, cold rolling, annealing, hot-dip galvanizing, and finishing treatments to obtain 590MPa grade hot-dip galvanized DH steel.

[0008] Preferably, the specific process parameters corresponding to the hot rolling are as follows: The thickness of the continuously cast slab is 230 mm. The slab is heated at 1200~1280℃ for 150~350 min. The heated slab is then rough-rolled, finish-rolled, and coiled to obtain the hot-rolled steel coil. The final finishing temperature is 880~920℃, and a U-shaped coiling process is used. The coiling temperature for the first 20 meters and the last 20 meters is 590~620℃, and the coiling temperature for the middle section is 540~580℃.

[0009] Preferably, the specific process parameters corresponding to the pickling and rolling are as follows: After the hot-rolled steel coil is uncoiled, it is pickled, wherein the pickling temperature is ≥82℃, the acid concentration is ≥100g / L, the cold-rolled plate thickness is ≤1.5mm, the pickling speed is ≤220m / min, and the cold-rolled plate thickness is >1.5mm, the pickling speed is ≤180m / min; the pickled steel plate is then cold-rolled and coiled to obtain a cold-hardened coil, wherein the total cold-rolling reduction rate is 52~70%.

[0010] Preferably, the specific process parameters corresponding to the annealing hot-dip galvanizing are as follows: After the cold-rolled steel sheet is uncoiled, it is annealed and then galvanized. The preheating temperature is 560~660℃, the pre-oxidation temperature is 580~680℃, the heating temperature is 790~810℃, the slow cooling temperature is 680~720℃, the rapid cooling temperature is 460~480℃, and the process speed is ≥60m / min. The oxygen content in the pre-oxidation process is controlled to be 0.2~2.0%. The galvanized steel sheet is then finished and stretched to obtain hot-dip galvanized steel sheet. The finishing elongation is 0.4~0.6%, and the finishing rolling force is above 200t.

[0011] Beneficial effects: The 590MPa grade hot-dip galvanized DH steel and its preparation method of this application adopt low-cost alloys such as C, Si, Mn, and Al, and add trace amounts of Cr, avoiding the addition of large amounts of expensive alloys such as Nb and Mo. This ensures product performance and process feasibility while reducing enterprise production costs and meeting the requirements for automotive steel sheets. The U-shaped low-temperature coiling process ensures the stability of the coiled performance and reduces the formation of surface iron oxide scale and the difficulty of pickling, improving the surface quality of the galvanized product. By controlling the pickling process, the pickling effect of the steel coil surface is guaranteed. The pre-oxidation process, by reasonably controlling the oxidation amount in the pre-oxidation section, transforms the external oxidation of alloy elements in the strip steel into internal oxidation, ensuring the surface quality of galvanization. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 The microstructure of the 590MPa grade hot-dip galvanized DH steel provided in the embodiments of this application (1); Figure 2 The microstructure of the 590MPa grade hot-dip galvanized DH steel provided in the embodiments of this application (2).

[0014] The implementation, functional features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0015] The technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0016] In this document, the term "comprising" is intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0017] The purpose of this embodiment is to provide a 590MPa grade hot-dip galvanized DH steel and its preparation method. By reasonably controlling the content of C, Si, Mn, Al and Cr, combined with the U-shaped low-temperature coiling process, and with appropriate pickling concentration and pickling speed and pre-oxidation technology in the galvanizing furnace, steel coils with stable mechanical properties and high surface quality are manufactured.

[0018] This embodiment discloses a 590MPa grade hot-dip galvanized DH steel, which is produced by sequentially passing steelmaking, hot rolling, pickling, and hot-dip galvanizing processes to obtain a thickness of 0.7~2.3mm. The chemical composition of the steel billet corresponding to the 590MPa grade hot-dip galvanized DH steel, by weight percentage, is C: 0.12%~0.14%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, Al: 0.4%~0.6%, Cr: 0.15%~0.25%, P≤0.02%, S≤0.01%, N≤0.008%, with the remainder being Fe and unavoidable impurities.

[0019] Specifically, the performance range of the steel plate or strip corresponding to the 590MPa grade hot-dip galvanized DH steel is 340~440MPa, tensile strength 590~700MPa, and elongation ≥27%.

[0020] In the specific application of this embodiment, by rationally selecting the chemical element composition and content, mainly by controlling the reasonable content of C, Si, Mn, Al, and Cr, the alloying elements are kept within a reasonable range. The addition of appropriate Mn content ensures the hardenability and performance stability of the steel coil and reduces the requirements for post-plating cooling capacity. The addition of trace amounts of Cr element reduces the critical cooling rate of the steel and improves hardenability and strength. Controlling the content of each element within the range reduces the impact on the weldability and surface quality of galvanization. The U-shaped low-temperature coiling process ensures the performance of the substrate and reduces the formation of surface iron oxide scale and pickling difficulty. Pickling and rolling control the pickling process to ensure the surface pickling quality. The galvanization adopts a pre-oxidation process to reduce the external oxidation of alloying elements, ultimately ensuring the acquisition of low-cost steel coils with stable mechanical properties and high surface quality.

[0021] This embodiment discloses a method for preparing 590MPa grade hot-dip galvanized DH steel, applied to the preparation of the 590MPa grade hot-dip galvanized DH steel as described above. The chemical composition of the continuous casting billet corresponding to the 590MPa grade hot-dip galvanized DH steel, by weight percentage, includes: C: 0.12%~0.14%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, Al: 0.4%~0.6%, Cr: 0.15%~0.25%. P≤0.02%, S≤0.01%, N≤0.008%, with the remainder being Fe and unavoidable impurities; the continuously cast billet is heated and then hot-rolled and coiled sequentially. The coiling adopts a U-shaped coiling process, with the head and tail coiling temperature at 590~620℃ and the middle section coiling temperature at 540~580℃, to obtain a hot-rolled steel coil; the hot-rolled steel coil is then subjected to pickling, cold rolling, annealing, hot-dip galvanizing, and finishing treatment to obtain 590MPa grade hot-dip galvanized DH steel.

[0022] Specifically, the process parameters corresponding to the hot rolling are as follows: The thickness of the continuously cast slab is 230 mm. The slab is heated at 1200~1280℃ for 150~350 min. The heated slab is then rough-rolled, finish-rolled, and coiled to obtain the hot-rolled steel coil. The final finishing temperature is 880~920℃, and a U-shaped coiling process is used. The coiling temperature for the first 20 meters and the last 20 meters is 590~620℃, and the coiling temperature for the middle section is 540~580℃.

[0023] Specifically, the process parameters corresponding to the pickling and rolling are as follows: After the hot-rolled steel coil is uncoiled, it is pickled, wherein the pickling temperature is ≥82℃, the acid concentration is ≥100g / L, the cold-rolled plate thickness is ≤1.5mm, the pickling speed is ≤220m / min, and the cold-rolled plate thickness is >1.5mm, the pickling speed is ≤180m / min; the pickled steel plate is then cold-rolled and coiled to obtain a cold-hardened coil, wherein the total cold-rolling reduction rate is 52~70%.

[0024] Specifically, the process parameters corresponding to the annealing hot-dip galvanizing are as follows: After the cold-rolled steel sheet is uncoiled, it is annealed and then galvanized. The preheating temperature is 560~660℃, the pre-oxidation temperature is 580~680℃, the heating temperature is 790~810℃, the slow cooling temperature is 680~720℃, the rapid cooling temperature is 460~480℃, and the process speed is ≥60m / min. The oxygen content in the pre-oxidation process is controlled to be 0.2~2.0%. The galvanized steel sheet is then finished and stretched to obtain hot-dip galvanized steel sheet. The finishing elongation is 0.4~0.6%, and the finishing rolling force is above 200t.

[0025] In this embodiment, the steelmaking composition, hot rolling coiling temperature, pickling and pickling process, galvanizing pre-oxidation process, and annealing process play important roles in obtaining 590MPa grade hot-dip galvanized DH steel. The above-described process is simple, easy to control, and can produce steel plates with a strength of 590MPa and a thickness of 0.7~2.3mm, meeting the requirements of the automotive steel plate industry.

[0026] The key chemical composition ratios and key process steps in this embodiment will now be described in detail.

[0027] Regarding the key chemical components, their proportions and functions, specifically: C: Carbon is a stabilizing element in austenite. The C content significantly affects all phase transformation processes and controls the final microstructure and mechanical properties. The C content in retained austenite affects its stability, and the content of retained austenite is a key factor determining the properties of DH steel. Generally, a high C content in retained austenite results in high stability, but excessively high C content is detrimental to mechanical properties and affects weldability. Conversely, a low C content leads to lower stability of retained austenite. Therefore, in this embodiment, C should be controlled within the range of 0.12% to 0.14%.

[0028] Si: Silicon is a solid solution strengthening element for ferrite. It can accelerate the segregation of carbon into austenite, improve the hardenability of the material, further purify the ferrite, and expand the temperature range of the ferrite and austenite two-phase region, thus improving the microstructure and properties of DH steel and helping to maintain the stability and reproducibility of its properties. However, when the Si content is high, it is easy to oxidize on the surface of the steel strip during annealing, reducing the wettability of zinc plating and causing defects such as incomplete plating, and also deteriorating the weldability. Therefore, in this embodiment, Si should be controlled at 0.3%~0.5%.

[0029] Mn: Manganese expands the austenite region and can delay the formation of pearlite and bainite during the slow cooling stage, improving the hardenability of steel and thus promoting the formation of martensite during the cooling process after slow cooling. However, when the Mn content is low, the austenite becomes unstable and easily transforms into bainite during cooling, failing to obtain the ferrite, martensite, and retained austenite structure required for DH steel. If the Mn content is too high, it will excessively stabilize the austenite, inhibiting the effect of Si in promoting ferrite precipitation, resulting in less ferrite precipitation and a decrease in the carbon content of the retained austenite, which in turn reduces the hardenability of the steel after slow cooling. At the same time, when the Mn content is too high, Mn will be oxidized or deposited on the surface of the steel strip during annealing, deteriorating the wettability of galvanized steel and, in severe cases, causing incomplete galvanization. Therefore, in this embodiment, the Mn content should be controlled at 1.5%~1.7%.

[0030] Al: The main role of aluminum in steel is to refine grains and fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel and reducing its tendency for cold brittleness and aging. The role of Al in DH steel is similar to that of Si; it can affect the austenite morphology during critical heating. Al can also form AlN precipitation, further refining the grains. Since excessive Si is not suitable for galvanized DH steel, and Al's role is similar to Si, Al can be used to replace Si in DH steel. However, the Al content should not be too high, as this will not only increase production costs but also lead to difficulties in continuous casting. Therefore, in this embodiment, Al should be controlled at 0.4%~0.6%.

[0031] Cr: Chromium can significantly delay the transformation of pearlite and bainite, improve the hardenability of austenite formed during critical zone annealing, and obtain a high martensite volume fraction at a lower cooling rate, thereby enabling austenite to fully transform into martensite. Furthermore, Cr has a significant cost advantage over Mo and Nb. Therefore, Cr is added in this embodiment, and the Cr content should be controlled at 0.15% to 0.25%.

[0032] The key process steps and their functions are as follows: Regarding the final rolling temperature: the hot rolling final rolling temperature is set at an austenitic zone temperature higher than Ar3. If the temperature is too high, the iron oxide scale will be severe and difficult to pickle, affecting the surface quality of the steel plate; if the temperature is too low, entering the two-zone rolling process will result in mixed crystals in the material, producing snowflake-like defects during stamping, affecting the galvanized surface quality. Therefore, in this embodiment, the final rolling temperature is generally controlled between 880 and 920°C.

[0033] Regarding the coiling temperature: a high coiling temperature is beneficial for grain growth, reduces strength and improves formability, but an excessively high annealing temperature will cause severe iron oxide scale on the steel plate surface, making it difficult to pickle and affecting the quality of the steel plate. At the same time, in order to ensure the stability of the continuous coiling performance, U-shaped coiling is adopted. Therefore, the coiling temperature control range in this embodiment is: using U-shaped coiling process, the coiling temperature of the first 20 meters and the last 20 meters is 590~620℃, and the coiling temperature of the middle section is 540~580℃.

[0034] For the pickling and rolling process: Hot-rolled coils inevitably generate iron oxide scale. While the scale is controllable, an improper pickling process can lead to incomplete removal of the surface iron oxide scale, resulting in oxide intrusion and over-pickling. Therefore, the pickling process in this embodiment is controlled as follows: pickling temperature ≥ 82℃, acid concentration ≥ 100g / L, cold-rolled plate thickness ≤ 1.5mm, pickling speed ≤ 220m / min; cold-rolled plate thickness > 1.5mm, pickling speed ≤ 180m / min. The pickled steel plate is then cold-rolled and coiled to obtain a cold-hardened coil, with a total cold-rolling reduction rate of 52~70%. A larger cold-rolling deformation can reduce the recrystallization temperature and phase transformation temperature; however, excessive deformation increases rolling difficulty. Therefore, the cold-rolling deformation is controlled between 52~70% based on the actual conditions of the rolling mill.

[0035] Regarding the pre-oxidation process: By controlling the pre-oxidation process, the external oxidation of alloying elements in the strip steel can be transformed into internal oxidation, ensuring the surface quality of the galvanized product. If the oxygen content is too low, the alloying elements will undergo external oxidation, resulting in incomplete galvanization. If the oxygen content is too high, over-oxidation will occur, leading to defects such as pitting in the galvanized product. Therefore, in this embodiment, the pre-oxidation stage process uses oxygen content control, maintaining the oxygen content between 0.2% and 2.0%. Regarding the galvanizing annealing temperature: the preheating zone temperature ensures rapid heating to the two-phase region upon entering the heating zone; a suitable heating zone temperature ensures the production of a suitable amount of austenite within the annealing furnace; the slow cooling zone temperature ensures that a small amount of austenite transforms into ferrite during cooling, thus guaranteeing both product strength and austenite stability; and the rapid cooling zone temperature ensures that the material quickly passes through the bainite transformation zone, guaranteeing the transformation of austenite into martensite after galvanizing. Therefore, in this embodiment, the galvanizing annealing temperature is controlled as follows: preheating zone temperature 560~660℃, pre-oxidation zone temperature 580~680℃, heating zone temperature 790~810℃, slow cooling zone temperature 680~720℃, rapid cooling zone temperature 460~480℃, and process speed ≥60m / min.

[0036] Regarding the flattening elongation: The purpose of flattening is to eliminate the yield plateau of the steel plate, avoid tensile strain mark defects during the stamping process, improve the plate shape, and enhance the surface quality of galvanization. DH steel products are characterized by the absence of a yield plateau; therefore, the main purpose of DH steel finishing is to improve the plate shape and surface quality. Thus, in this embodiment, the flattening elongation is controlled within the range of 0.3% to 0.5%.

[0037] The specific application of this embodiment will now be described in further detail.

[0038] A 590MPa grade hot-dip galvanized DH steel and its preparation method are disclosed. The chemical composition of the steel billet, by weight percentage, is: C: 0.12%~0.14%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, Al: 0.4%~0.6%, Cr: 0.15%~0.25%, P≤0.02%, S≤0.01%, N≤0.008%, with the balance being Fe and unavoidable impurities. Refer to Table 1, which shows the chemical composition (%) of the billets in Examples 1-6.

[0039] Table 1 By subjecting the above-mentioned slab to heating, hot rolling, cold rolling, and galvanizing processes, the 590MPa grade hot-dip galvanized DH steel with a thickness of 0.7~2.3mm disclosed in this embodiment can be obtained.

[0040] The billet is heated in a furnace, with the exit temperature controlled at 1200~1280℃. After seven passes of rough rolling, the intermediate billet thickness is 40mm. The final rolling temperature is controlled at 880~920℃. A U-shaped coiling process is adopted, with the first 20 meters and the last 20 meters coiled at 590~620℃, and the middle section coiled at 540~580℃. Through high-temperature heating, high-temperature rolling, and low-temperature coiling, the thickness and structure of the iron oxide scale on the steel plate surface are effectively controlled, and the U-shaped coiling process ensures the stability of the continuous coiling performance.

[0041] After the hot-rolled coil is dephosphorized by tension leveling, it is pickled. The pickling temperature is ≥82℃, the HCl concentration in the pickling bath is ≥100g / L, the thickness of the cold-rolled plate is ≤1.5mm, and the pickling speed is ≤220m / min. For the thickness of the cold-rolled plate >1.5mm, the pickling speed is ≤180m / min. After pickling, it is rolled into cold-hardened coils with a thickness of 0.7-2.3mm by 5 stands. The total reduction rate of cold rolling is 52~70%. The surface pickling effect is ensured by controlling the pickling process. After continuous annealing, the cold-rolled coils are galvanized. The preheating temperature is 560~660℃, the pre-oxidation temperature is 580~680℃, the heating temperature is 790~810℃, the slow cooling temperature is 680~720℃, and the rapid cooling temperature is 460~480℃. The process speed is ≥60m / min. The oxygen content in the pre-oxidation process is controlled at 0.2~2.0%. After galvanizing in a zinc pot, the coils enter the finishing and straightening section. The finishing elongation is controlled at 0.3~0.5%, and the finishing rolling force is ≥200t. Finally, the coils are coiled and slit at the exit to obtain hot-dip galvanized steel coils with a thickness of 0.7~2.3mm. By controlling the galvanizing and annealing processes and the pre-oxidation process, the high strength and high surface quality of the finished product are guaranteed.

[0042] Refer to Table 2, which shows the hot rolling process parameters for Examples 1-6.

[0043] Table 2 Refer to Table 3, which shows the pickling and rolling process parameters for Examples 1-6.

[0044] Table 3 Refer to Table 4, which shows the galvanizing process parameters for Examples 1-6.

[0045] Table 4 The mechanical property test results of the steel in each embodiment of this example are listed in Table 5. From the performance data listed in Table 5, it can be seen that the performance of the head, middle and tail can be stably controlled.

[0046] Table 5 In the specific application of this embodiment, the mechanical properties of the products in Examples 1-6 were tested according to the GB / T 228 method.

[0047] Reference Figure 1 , Figure 1 The microstructure of the 590MPa grade hot-dip galvanized DH steel provided in this application embodiment (1). Refer to... Figure 2 , Figure 2 The microstructure of the 590MPa grade hot-dip galvanized DH steel provided in the embodiments of this application (2).

[0048] like Figure 1 and Figure 2 As shown, the microstructure of the 590MPa grade hot-dip galvanized DH steel in this embodiment is ferrite + martensite + a small amount of retained austenite, and the microstructure is uniform.

[0049] Compared with the prior art, the 590MPa grade hot-dip galvanized DH steel and its preparation method in this embodiment have the following significant advantages: This embodiment uses low-cost alloys such as C, Si, Mn, and Al, and adds trace amounts of Cr, avoiding the addition of large amounts of expensive alloys such as Nb and Mo. This ensures product performance and process feasibility while reducing enterprise production costs and meeting the requirements for automotive steel sheets.

[0050] This embodiment adopts a U-shaped low-temperature winding process, which not only ensures the performance stability of the continuous winding, but also reduces the generation of surface iron oxide scale and the difficulty of pickling, thereby improving the surface quality of the galvanized product.

[0051] This embodiment ensures the pickling effect on the surface of the steel coil by controlling the pickling process.

[0052] This embodiment employs a pre-oxidation process. By rationally controlling the amount of oxidation in the pre-oxidation stage, the external oxidation of alloying elements in the strip steel is transformed into internal oxidation, thus ensuring the surface quality of the galvanized steel.

[0053] Finally, it should be noted that the above description is only a preferred embodiment of this application and is not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A 590MPa grade hot-dip galvanized DH steel, characterized in that, 590MPa grade hot-dip galvanized DH steel with a thickness of 0.7~2.3mm is produced by sequentially processing steelmaking, hot rolling, pickling, and hot-dip galvanizing. The chemical composition of the steel billet corresponding to the 590MPa grade hot-dip galvanized DH steel, by weight percentage, is C: 0.12%~0.14%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, Al: 0.4%~0.6%, Cr: 0.15%~0.25%, P≤0.02%, S≤0.01%, N≤0.008%, with the remainder being Fe and unavoidable impurities.

2. The 590MPa grade hot-dip galvanized DH steel according to claim 1, characterized in that, The performance range of the steel plate or strip corresponding to the 590MPa grade hot-dip galvanized DH steel is 340~440MPa, tensile strength 590~700MPa, and elongation ≥27%.

3. A method for preparing 590MPa grade hot-dip galvanized DH steel, applied to the preparation of the 590MPa grade hot-dip galvanized DH steel as described in claim 1, characterized in that, The chemical composition of the continuous casting billet corresponding to the 590MPa grade hot-dip galvanized DH steel, by weight percentage, includes: C: 0.12%~0.14%, Si: 0.3%~0.5%, Mn: 1.5%~1.7%, Al: 0.4%~0.6%, Cr: 0.15%~0.25%, P≤0.02%, S≤0.01%, N≤0.008%, with the remainder being Fe and unavoidable impurities. The continuous casting billet is heated and then sequentially hot-rolled and coiled. The coiling adopts a U-shaped coiling process, with the head and tail coiling temperatures at 590~620℃ and the middle section coiling temperature at 540~580℃, resulting in a hot-rolled steel coil. The hot-rolled steel coil is then sequentially pickled, cold-rolled, annealed, hot-dip galvanized, and finished to obtain the 590MPa grade hot-dip galvanized DH steel.

4. The method for preparing 590MPa grade hot-dip galvanized DH steel according to claim 3, characterized in that, The specific process parameters corresponding to the hot rolling are as follows: The thickness of the continuously cast slab is 230 mm. The slab is heated at 1200~1280℃ for 150~350 min. The heated slab is then rough-rolled, finish-rolled, and coiled to obtain the hot-rolled steel coil. The final finishing temperature is 880~920℃, and a U-shaped coiling process is used. The coiling temperature for the first 20 meters and the last 20 meters is 590~620℃, and the coiling temperature for the middle section is 540~580℃.

5. The method for preparing 590MPa grade hot-dip galvanized DH steel according to claim 3, characterized in that, The specific process parameters corresponding to the pickling and rolling are as follows: After the hot-rolled steel coil is uncoiled, it is pickled, wherein the pickling temperature is ≥82℃, the acid concentration is ≥100g / L, the cold-rolled plate thickness is ≤1.5mm, the pickling speed is ≤220m / min, and the cold-rolled plate thickness is >1.5mm, the pickling speed is ≤180m / min; the pickled steel plate is then cold-rolled and coiled to obtain a cold-hardened coil, wherein the total cold-rolling reduction rate is 52~70%.

6. The method for preparing 590MPa grade hot-dip galvanized DH steel according to claim 5, characterized in that, The specific process parameters corresponding to the annealing hot-dip galvanizing are as follows: After the cold-rolled steel sheet is uncoiled, it is annealed and then galvanized. The preheating temperature is 560~660℃, the pre-oxidation temperature is 580~680℃, the heating temperature is 790~810℃, the slow cooling temperature is 680~720℃, the rapid cooling temperature is 460~480℃, and the process speed is ≥60m / min. The oxygen content in the pre-oxidation process is controlled to be 0.2~2.0%. The galvanized steel sheet is then finished and stretched to obtain hot-dip galvanized steel sheet. The finishing elongation is 0.4~0.6%, and the finishing rolling force is above 200t.

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