A low-carbon hot-rolled substrate galvannealed dual-phase steel and a method for producing the same

Abstract

The present application belongs to the field of metallurgy, and particularly relates to a low-carbon hot-rolled substrate hot-dip galvanized dual-phase steel and a production method thereof. The chemical composition of the hot-dip galvanized dual-phase steel includes, in percentage by weight, C: 0.03-0.06%, Si: 0.1-0.6%, Mn: 1.4-2.0%, P≤0.02%, S≤0.01%, Al: 0.02-0.06%, Cr: 0.2-0.6%, and the balance of Fe and inevitable impurities, and satisfies 25≤(Mn+Cr) / C≤90; the mechanical properties satisfy yield strength of 300-430 MPa, tensile strength of 590-700 MPa, yield strength ratio of 0.50-0.65, and elongation of more than 20%. The production method of the present application does not include a cold rolling process, and through hot rolling and low-temperature coiling, phase change energy storage is used to replace deformation energy storage, thereby saving energy.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and in particular relates to a low-carbon hot-rolled substrate hot-dip galvanized duplex steel and its production method. Background Technology

[0002] Galvanized duplex steel (DP steel) is an advanced high-strength steel with a microstructure characterized by martensite islands distributed on a ferrite matrix. This structure endows it with low yield strength, high tensile strength, high work hardening rate, and good formability. Combined with the corrosion protection of the galvanized layer, it is widely used in automotive structural parts, crash barriers, and other safety components.

[0003] The traditional production process for galvanized duplex steel is as follows: converter steelmaking → LF furnace refining → slab continuous casting → hot rolling → pickling → cold rolling → continuous hot-dip galvanizing / alloying → leveling. Among these, the cold rolling process is considered an indispensable core step, its main functions including: ① thinning the hot-rolled plate to the target thickness; ② introducing high-density dislocations through large plastic deformation, i.e., deformation energy storage, providing the driving force for recrystallization in the subsequent continuous annealing / galvanizing process, thereby obtaining a fine ferrite + martensite duplex structure; ③ generating deformation-induced precipitation, strengthening the matrix.

[0004] For example, patent CN202211277583 discloses a low-cost, high-formability alloyed hot-dip galvanized duplex steel sheet and its manufacturing method, with the process flow explicitly including a "pickling and cold rolling" step. Patent CN202411603977.2 discloses a low-cost, 590MPa-grade hot-dip galvanized duplex steel and its preparation method, with a chemical composition including C: 0.06%–0.10%, and the process flow also includes a "pickling and rolling" step. The above-mentioned prior art reflects the technical consensus in this field: the production of high-performance galvanized duplex steel must rely on a cold rolling process.

[0005] However, the cold rolling process has the following inherent drawbacks: ① High equipment investment, long production lines, and high energy consumption; ② Complex processes and long production cycles, reducing production efficiency; ③ The cold rolling process requires a large amount of rolls and emulsion, increasing production costs; ④ The cold-rolled steel sheet needs to undergo pretreatment such as degreasing, increasing process complexity; ⑤ Cold rolling and pickling reductions of >50% are necessary to obtain the required specifications and the accumulation of deformation energy to achieve the target microstructure.

[0006] With advancements in thin slab continuous casting and rolling technology, hot-rolled strips with thicknesses of 1.0 mm and above can now be stably produced, making it possible to cool hot-rolled strips and omit the cold rolling process. However, directly using hot-rolled plates for galvanizing and obtaining the desired duplex microstructure faces fundamental technical obstacles:

[0007] First, the lack of deformation energy storage introduced by cold rolling significantly reduces the recrystallization driving force of hot-rolled plates during galvanizing and annealing, resulting in coarsening of ferrite grains (usually >15μm), making it difficult to balance the toughness and strength of the material.

[0008] Secondly, martensite formation is difficult. Traditional composition design relies on a high carbon content (usually C≥0.07%) to ensure martensite formation and strength. However, without cold rolling, simply increasing the overall content of alloying elements to enhance hardenability will lead to a significant increase in cost, increased segregation, decreased weldability, and difficulty in obtaining an ideal duplex microstructure.

[0009] Third, the yield strength ratio of DP590 galvanized duplex steel produced by existing technology is typically ≥0.60. A lower yield strength ratio (<0.60) means that the material can enter the plastic deformation stage earlier under impact loads and absorb more energy, which is a key performance indicator for automotive safety components. However, how to further reduce the yield strength ratio without cold rolling has been a long-standing technical challenge in this field. Summary of the Invention

[0010] The purpose of this invention is to provide a low-carbon hot-rolled substrate hot-dip galvanized duplex steel and its production method, so as to solve the problems existing in the prior art.

[0011] The technical solution adopted by this invention to solve its technical problem is:

[0012] A low-carbon hot-rolled substrate hot-dip galvanized duplex steel, the chemical composition of the hot-dip galvanized duplex steel by weight percentage includes: C: 0.03~0.06%, Si: 0.1~0.6%, Mn: 1.4~2.0%, P≤0.02%, S≤0.01%, Al: 0.02~0.06%; Cr: 0.2~0.6%, with the balance being Fe and unavoidable impurities, and satisfying: 25≤(Mn+Cr) / C≤90; its mechanical properties satisfy: yield strength of 300~430MPa, tensile strength of 590~700MPa, yield-to-tensile ratio of 0.50~0.65, and elongation of ≥20%.

[0013] The design principles and reasons for limiting the content range of the chemical composition of this invention are as follows:

[0014] Carbon (C): 0.03~0.06%. C is the element with the most significant solid solution strengthening effect in steel materials and also the element that expands the austenite region, directly affecting the ferrite and martensite content in duplex steel. Traditionally, it is believed that to obtain sufficient martensite, the C content usually needs to be ≥0.06%. This invention breaks through this technical bias and adopts a low-carbon design, mainly based on the following considerations: ① Reducing the C content can reduce the degree of solid solution strengthening of ferrite, which is beneficial to obtaining low yield strength; ② This invention uses an in-situ element enrichment mechanism to enrich C in local austenite, forming high-carbon martensite even with a low average C content; ③ Low carbon content is beneficial to improving weldability and formability. When C < 0.03%, it is difficult to form sufficient martensite even through enrichment; when C > 0.06%, the average carbon content is too high, which is not conducive to reducing the yield strength ratio.

[0015] Silicon (Si): 0.1~0.6%. Si can dissolve in ferrite and austenite to increase the strength of steel and promotes ferrite formation, making it an important element for obtaining a ferrite + martensite dual-phase structure. However, when the Si content is too high, the surface iron oxide scale is difficult to control, affecting the quality of galvanizing.

[0016] Manganese (Mn): 1.4~2.0%. Mn significantly improves the hardenability of steel, reduces the critical cooling rate required to obtain martensite, and also has a solid solution strengthening effect. Mn works synergistically with Cr to further delay the transformation of pearlite and bainite. More importantly, this invention is the first to discover that Mn exhibits a similar exclusion and enrichment behavior to C during ferrite formation, enabling local enrichment of Mn in retained austenite, further improving the hardenability and strength of martensite. When the Mn content is below 1.4%, the enrichment effect is insufficient; above 2.0%, it leads to increased segregation and higher costs.

[0017] Chromium (Cr): 0.2~0.6%. Cr is a medium-strong carbide-forming element that can improve the hardenability of steel and delay the pearlite and bainite transformations. Cr has a synergistic effect with Mn and is relatively inexpensive. This invention controls the Cr content at 0.2~0.6% to synergistically improve hardenability with Mn.

[0018] (Mn+Cr) / C ratio: 25~90. This parameter reflects the ratio of alloying elements to carbon, characterizing the potential for in-situ enrichment of elements. When the ratio is below 25, the total amount of alloying elements is relatively insufficient, the enrichment effect is limited, and it is difficult to obtain sufficiently stable retained austenite. When the ratio is above 90, the alloying elements are excessive, increasing costs and the risk of segregation, and may also lead to excessive martensite content and increased yield strength ratio. By controlling this ratio within the range of 25~90, it is possible to ensure the effective enrichment of C and Mn during ferrite formation, achieving the microstructure control target.

[0019] A method for producing low-carbon hot-rolled galvanized duplex steel substrate includes the following steps, but does not include any cold rolling process:

[0020] S1. After being smelted in a converter and refined in an LF furnace, molten steel with the chemical composition of claim 1 is obtained, and then continuously cast into a billet.

[0021] S2. After heating the continuously cast billet, hot rolling is performed to the target thickness. After rolling, laminar cooling is performed and the billet is coiled to obtain a hot-rolled plate.

[0022] S3. The hot-rolled plate is fed into the continuous hot-dip galvanizing production line and undergoes pickling, heat treatment, galvanizing, post-galvanizing cooling, and leveling in sequence to obtain hot-dip galvanized duplex steel.

[0023] Furthermore, in step S2, the target thickness is 1.0~3.0 mm; in hot continuous rolling, the finishing rolling temperature is controlled at 780~900℃, ensuring that the austenite is above the crystallization temperature, guaranteeing the smooth progress of hot rolling deformation, and providing a suitable austenite grain size for subsequent cooling transformation. The laminar cooling rate is ≥15℃ / s. Rapid cooling is to suppress high-temperature transformations, such as pearlite transformation, and obtain a low-temperature phase transformation structure dominated by lath bainite / martensite. Laminar cooling is performed to 100~300℃ for coiling. This coiling temperature is lower than that of traditional processes, and its purpose is: ① to obtain a bainite / martensite structure with high-density lath interfaces by rapidly cooling to a low temperature; ② these lath interfaces can serve as genetic templates for ferrite nucleation in subsequent heat treatment, inducing explosive nucleation; ③ the interfacial energy stored in the lath structure can partially replace the deformation energy stored in traditional cold rolling, providing a driving force for subsequent recrystallization. When the winding temperature is below 100℃, the production difficulty increases and the residual stress is too large; when it is above 300℃, the lath structure is tempered and the genetic effect is weakened.

[0024] Further, in step S3, the specific method of heat treatment is as follows: the pickled hot-rolled plate is first heated to 750~830℃ at a heating rate of ≥8℃ / s. The purpose of rapid heating is to retain the lath interface characteristics formed in step S2 and avoid excessive recovery of the interface during slow heating. When the heating temperature is below 750℃, austenitization is insufficient; when it is above 830℃, ferrite grains tend to coarsen and energy consumption increases. The temperature is then reduced to 650℃ in 30~150 seconds. The purpose of this is: ① During the cooling process, the lath interface continuously induces explosive nucleation of ferrite, forming a large number of fine equiaxed ferrite grains in a very short time; ② As the temperature decreases, ferrite continues to form, and C and Mn atoms are squeezed into the untransformed retained austenite, so that the local C content in the retained austenite reaches 0.15-0.25wt%, and the local Mn content reaches 2.5-3.5wt%. This cooling enrichment mechanism avoids element homogenization. If the cooling time is too short, the element enrichment will be insufficient; if the cooling time is too long, the grains will coarsen and the production efficiency will decrease. Subsequently, the temperature is rapidly cooled to 440~465℃ at a cooling rate of ≥10℃ / s before immersion in a zinc bath for hot-dip galvanizing. Rapid cooling to the galvanizing temperature aims to suppress pearlite or bainite transformation during cooling, retaining highly enriched residual austenite before it enters the zinc bath.

[0025] Furthermore, the cooling time is determined based on the thickness of the hot-rolled plate:

[0026] When the thickness of the hot-rolled plate is 1.0~1.5mm: cooling time is 30~60 seconds;

[0027] When the thickness of the hot-rolled plate is 1.5~2.5mm: cooling time is 60~120 seconds;

[0028] When the thickness of the hot-rolled plate is 2.5~3.0mm: cooling time is 120~150 seconds.

[0029] Adjusting the cooling time according to the thickness is to ensure that the core tissue reaches the target temperature and completes element diffusion.

[0030] Furthermore, in step S3, the post-galvanizing cooling is as follows: after the hot-rolled plate exits the zinc pot, the cooling rate is controlled according to the target strength requirements to cool the galvanized hot-rolled plate to below 150°C, thereby ensuring that the residual austenite enriched with C and Mn elements is completely transformed into martensite, and avoiding excessive retention of residual austenite at room temperature, which would affect the strength stability.

[0031] Furthermore, the target strength is controlled by the rapid cooling endpoint temperature during heat treatment and the post-plating cooling rate, specifically:

[0032] When the target tensile strength is 590~630MPa, the rapid cooling endpoint temperature is 460~465℃, and the post-plating cooling rate is 1~8℃ / s.

[0033] When the target tensile strength is 630~670MPa, the rapid cooling endpoint temperature is 450~460℃, and the post-plating cooling rate is 8~15℃ / s.

[0034] When the target tensile strength is 670~700MPa, the rapid cooling endpoint temperature is 440~450℃, and the post-plating cooling rate is 15~30℃ / s.

[0035] Its mechanism of action is as follows: the lower the endpoint temperature of rapid cooling, the higher the enrichment degree of C and Mn in austenite, and the stronger the hardenability; the higher the cooling rate after plating, the more complete the martensitic transformation. Through the synergistic regulation of the two, performance differentiation within the strength range of 590~700MPa can be achieved without changing the steel composition.

[0036] Furthermore, in step S3, the reduction rate of the leveling machine during the leveling process is 0.1~0.5%. The purpose of slight leveling is to eliminate the yield plateau, improve the plate shape and surface quality, and at the same time avoid excessive cold deformation from affecting the mechanical properties.

[0037] The present invention has the following beneficial effects:

[0038] 1. This invention constructs an initial lath bainite / martensite structure by low-temperature coiling, uses its interface as a genetic template to induce explosive nucleation of ferrite, replaces deformation energy storage with phase transformation energy storage, and utilizes the element displacement effect during the cooling process to achieve in-situ enrichment of C and Mn, thus obtaining an ultrafine-grained dual-phase structure without completely omitting the cold rolling process.

[0039] 2. The hot-rolled galvanized duplex steel with a substrate prepared by this invention has a low carbon content, allowing for control of the martensite ratio to ≤10%. This reduces the strength of ferrite on one hand, and improves the strength of martensite through alloying elements and carbon enrichment on the other. The yield strength ratio can be controlled within the range of 0.50 to 0.65, significantly better than the 0.60 to 0.65 of traditional DP590. A low yield strength ratio means that the material can enter the plastic deformation stage earlier under dynamic loads (such as collisions and explosions), absorbing more energy, making it an ideal material for automotive safety components. Simultaneously, the elongation is ≥20%, meeting forming requirements.

[0040] 3. By controlling the synergistic effect of rapid cooling endpoint temperature and post-plating cooling rate, performance differentiation can be achieved within a wide strength range of 590–700 MPa under the same composition system, meeting the diverse needs of customers for products with the same strength level but different yield strength ratios and elongation characteristics.

[0041] 4. The cold rolling process and related equipment are completely eliminated, shortening the process flow by more than 30%, reducing energy consumption by more than 25%, and significantly lowering production costs. Furthermore, the use of a low-carbon alloy design results in lower alloy costs compared to traditional high-carbon solutions.

[0042] 5. It ensures that martensitic structure is obtained even during the galvanizing process and subsequent slow cooling, breaking the limitation that conventional processes require rapid cooling to obtain martensite.

[0043] 6. This invention eliminates the consumption of rolling oil and emulsion in the cold rolling process, reduces the burden of wastewater treatment, and conforms to the development direction of green manufacturing. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0045] Example 1:

[0046] A method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel, taking the production of 1.5mm diameter hot-dip galvanized duplex steel as an example, includes the following steps:

[0047] S1. The steel obtained after converter smelting and LF furnace refining has the following composition:

[0048] By weight percentage, C: 0.042%, Si: 0.23%, Mn: 1.76%, P: 0.012%, S: 0.0012%, Al: 0.03%, Cr: 0.43%, with the balance being Fe and unavoidable impurities. The ratio (Mn+Cr) / C is approximately 52.1. Molten steel is then continuously cast to obtain continuously cast billets.

[0049] S2. Heat the continuously cast billet to 1230℃, hold it at that temperature, and then perform hot continuous rolling to achieve a finished thickness of 1.5mm. Control the finishing rolling temperature to 800℃, and then perform laminar cooling at a cooling rate of 25℃ / s after rolling. Cool to 180℃ and then coil to obtain a hot-rolled plate with an initial microstructure mainly composed of lath bainite / martensite.

[0050] S3. Hot-rolled sheets are fed into a continuous hot-dip galvanizing production line. After pickling, they undergo heat treatment. During heat treatment, the sheets are first heated to 800°C at a heating rate of 10°C / s, then cooled for 60 seconds to reduce the temperature to 650°C. Subsequently, they are rapidly cooled to 455°C at a cooling rate of 15°C / s and immersed in a zinc bath for hot-dip galvanizing. After exiting the zinc bath, they are cooled to below 150°C at a cooling rate of 5°C / s. The galvanized sheets are then leveled, with a leveling machine reduction rate of 0.4%, resulting in hot-dip galvanized duplex steel.

[0051] Testing revealed that the microstructure of the hot-dip galvanized duplex steel exhibited an average ferrite grain size of 7.5 μm, a martensite island volume fraction of approximately 8%, and an average martensite island size of approximately 1.8 μm, dispersed within the ferrite matrix. The mechanical properties of the hot-dip galvanized duplex steel were: yield strength 330 MPa, tensile strength 630 MPa, elongation after fracture 28%, and yield strength ratio 0.524. The coating quality was uniform, free from defects such as incomplete coating or bubbles.

[0052] Example 2:

[0053] A method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel, taking the production of 1.2mm diameter hot-dip galvanized duplex steel as an example, includes the following steps:

[0054] S1. The steel obtained after converter smelting and LF furnace refining has the following composition:

[0055] By weight percentage, C: 0.038%, Si: 0.28%, Mn: 1.82%, P: 0.013%, S: 0.0015%, Al: 0.04%, Cr: 0.48%, with the balance being Fe and unavoidable impurities. The ratio (Mn+Cr) / C is approximately 60.5. Molten steel is then continuously cast to obtain continuously cast billets.

[0056] S2. After heating the continuously cast billet, hot rolling is performed to the target thickness of 1.2 mm. The finishing rolling temperature is controlled at 820℃. After rolling, laminar cooling is performed at a cooling rate of 30℃ / s. The billet is then coiled at 160℃ to obtain a hot-rolled plate with an initial microstructure mainly composed of lath bainite / martensite.

[0057] S3. Hot-rolled sheets are fed into a continuous hot-dip galvanizing production line. After pickling, they undergo heat treatment. During heat treatment, the sheets are first heated to 790°C at a heating rate of 12°C / s, then cooled for 45 seconds to reduce the temperature to 650°C. They are then rapidly cooled to 460°C and immersed in a zinc bath for hot-dip galvanizing. After exiting the zinc bath, they are cooled to below 150°C at a cooling rate of 8°C / s. The galvanized sheets are then leveled, with a leveling machine reduction rate of 0.3%, resulting in hot-dip galvanized duplex steel.

[0058] The mechanical properties of hot-dip galvanized duplex steel, as tested, are: yield strength 320 MPa, tensile strength 615 MPa, elongation 29%, and yield strength ratio 0.520. The coating quality is uniform, without defects such as missed coatings or bubbles.

[0059] Example 3:

[0060] A method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel, taking the production of 2.0mm diameter hot-dip galvanized duplex steel as an example, includes the following steps:

[0061] S1. The steel obtained after converter smelting and LF furnace refining has the following composition:

[0062] By weight percentage, C: 0.055%, Si: 0.35%, Mn: 1.90%, P: 0.014%, S: 0.0018%, Al: 0.05%, Cr: 0.55%, with the balance being Fe and unavoidable impurities. The ratio (Mn+Cr) / C is approximately 44.5. Molten steel is then continuously cast to obtain continuously cast billets.

[0063] S2. After heating the continuously cast billet, hot rolling is performed to the target thickness of 2.0 mm. The finishing rolling temperature is controlled at 850℃. After rolling, laminar cooling is performed at a cooling rate of 22℃ / s. The billet is then coiled at 200℃ to obtain a hot-rolled plate with an initial microstructure mainly composed of lath bainite / martensite.

[0064] S3. Hot-rolled sheets are fed into a continuous hot-dip galvanizing production line. After pickling, they undergo heat treatment. During heat treatment, the sheets are first heated to 820°C at a heating rate of 9°C / s, then cooled for 100 seconds to reduce the temperature to 650°C. Subsequently, they are rapidly cooled to 450°C and immersed in a zinc bath for hot-dip galvanizing. After exiting the zinc bath, they are cooled to below 150°C at a cooling rate of 12°C / s. The galvanized sheets are then leveled, with a leveling machine reduction rate of 0.5%, resulting in hot-dip galvanized duplex steel.

[0065] The mechanical properties of hot-dip galvanized duplex steel, as tested, are: yield strength 350 MPa, tensile strength 650 MPa, elongation 26%, and yield strength ratio 0.538. The coating quality is uniform, without defects such as missed coatings or bubbles.

[0066] Example 4:

[0067] This embodiment provides a method for producing hot-dip galvanized duplex steel with a low-carbon hot-rolled substrate. The method steps are basically the same as those in Embodiment 1. The difference is that this embodiment produces high-strength hot-dip galvanized duplex steel with a strength of over 680 MPa. In the heat treatment, it is rapidly heated to 800°C at a rate of 10°C / s; rapidly cooled to 445°C before galvanizing; and the cooling rate after galvanizing is 20°C / s.

[0068] The mechanical properties of hot-dip galvanized duplex steel, as tested, are: yield strength 410 MPa, tensile strength 685 MPa, elongation 23%, and yield strength ratio 0.599.

[0069] Example 5:

[0070] This embodiment provides a method for producing hot-dip galvanized duplex steel with a low-carbon hot-rolled substrate. The method steps are basically the same as those in Embodiment 1. The difference is that this embodiment produces hot-dip galvanized duplex steel with a low yield strength ratio. In the heat treatment, it is cooled to 463°C at a cooling rate of 15°C / s before galvanizing. The cooling rate after galvanizing is 3°C / s.

[0071] The mechanical properties of hot-dip galvanized duplex steel, as tested, are: yield strength 310 MPa, tensile strength 605 MPa, elongation 30%, and yield strength ratio 0.512.

[0072] Example 6:

[0073] This embodiment provides a method for producing hot-dip galvanized duplex steel with a low-carbon hot-rolled substrate. The method steps are basically the same as in Embodiment 1, except that this embodiment produces 2.8mm hot-dip galvanized duplex steel. The finishing rolling temperature in the hot continuous rolling process is 820℃, and after rolling, it is cooled to 220℃ at a rate of 20℃ / s and coiled, resulting in a finished product thickness of 2.8mm. In the heat treatment, the steel is first rapidly heated to 790℃ at a heating rate of 8℃ / s, then cooled for 140 seconds to reduce the hot-rolled plate temperature to 650℃, followed by rapid cooling to 455℃ at a rate of 12℃ / s for galvanizing. After galvanizing, the cooling rate is 8℃ / s, and the temperature is cooled to below 150℃. The leveling mill reduction rate is 0.3%.

[0074] The mechanical properties of hot-dip galvanized duplex steel, as tested, are: yield strength 340 MPa, tensile strength 625 MPa, elongation 26%, and yield strength ratio 0.544.

[0075] Comparative Example 1:

[0076] It uses a standard DP590 composition: C 0.075%, Si 0.40%, Mn 1.70%, and Cr 0.35%. It is produced using a cold rolling process.

[0077] Process flow: Hot rolling (final rolling temperature 860℃, coiling temperature 600℃) → pickling → 50% cold rolling → annealing on continuous galvanizing line → leveling.

[0078] Performance testing: Yield strength 385MPa, tensile strength 615MPa, elongation 26%, yield strength ratio 0.626.

[0079] Comparative Example 2:

[0080] The steel composition in this comparative example is the same as in Example 1, and the production method involves high-temperature coiling at 600°C, omitting cold rolling. The galvanizing process is the same as in Example 1.

[0081] Results: The microstructure is coarse and uneven, with ferrite grains exceeding 18 μm and martensite islands approximately 5 μm in size. Properties: Yield strength 365 MPa, tensile strength 595 MPa, elongation 20%, yield strength ratio 0.613. Properties are unstable, with pearlite appearing in localized areas.

[0082] Comparative Example 3:

[0083] The method and steps of this comparative example are basically the same as those of Example 1, except that a slow heating rate of 3℃ / s is used in the heat treatment.

[0084] Results: Ferrite grain size reached 12 μm, and martensite island size was approximately 3 μm. Properties: Yield strength 350 MPa, tensile strength 610 MPa, elongation 23%, yield strength ratio 0.574. The results were inferior to Example 1, indicating that rapid heating is crucial for preserving the genetic interface.

[0085] Comparative Example 4: (High-carbon scheme, attempting without cold rolling)

[0086] The method and steps of this comparative example are basically the same as those of Example 1, except that the molten steel contains: C 0.08%, Si 0.35%, Mn 1.80%, and Cr 0.45%.

[0087] Results: With a martensite content of 25%, the yield strength increased significantly to 410 MPa, the tensile strength to 640 MPa, the elongation to 19%, and the yield-to-tensile ratio to 0.641. Performance deteriorated, with the yield-to-tensile ratio increasing instead of decreasing.

[0088] Comparative Example 5:

[0089] The method and steps of this comparative example are basically the same as those of Example 1. The difference is that the composition of the molten steel is: C 0.06%, Mn 1.3%, Cr 0.1%, then (Mn+Cr) / C=23.3<25.

[0090] Results: The martensite content was approximately 6%, but the martensite island size was approximately 4 μm, and the ferrite grain size was approximately 11 μm. Properties: Yield strength 340 MPa, tensile strength 595 MPa, elongation 22%, yield strength ratio 0.571, and slightly poor microstructure uniformity.

[0091] Comparative Example 6:

[0092] The method and steps of this comparative example are basically the same as those of Example 1. The difference is that the composition of the molten steel is: C 0.028%, Mn 2.0%, Cr 0.6%, then (Mn+Cr) / C=92.9>90.

[0093] Results: The martensite content reached 12%, the martensite island size was approximately 2 μm, and the ferrite grain size was approximately 8 μm. Properties: Yield strength 380 MPa, tensile strength 630 MPa, elongation 21%, yield strength ratio 0.603, which is relatively high.

[0094] The effects of the above embodiments and comparative examples are compared, as shown in Table 1 below:

[0095] Table 1: Comparison of Results

[0096] project Example 1 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 cold rolling process none none none have none none none none none C content (wt%) 0.042 0.042 0.042 0.075 0.042 0.042 0.08 0.06 0.028 (Mn+Cr) / C 52.1 52.1 52.1 - 52.1 52.1 28.1 23.3 92.9 Low temperature winding ℃ 180 180 180 600 600 180 180 180 180 Heating rate ℃ / s 10 10 10 - 10 3 10 10 10 Final temperature of rapid cooling (°C) 455 445 463 460 455 455 455 455 455 Cooling rate after plating (°C / s) 5 20 3 Quick-cooling 5 5 5 5 5 Ferrite grain size (μm) 7.5 8.0 7.0 9.5 18 12 10 11 8 Martensite content (%) 8 10 6 25 10 12 25 6 12 Yield strength (MPa) 330 410 310 385 365 350 410 340 380 Tensile strength (MPa) 630 685 605 615 595 610 640 595 630 Elongation (%) 28 23 30 26 20 23 19 22 21 The ratio of yield strength 0.524 0.599 0.512 0.626 0.613 0.574 0.641 0.571 0.603

[0097] As shown in Table 1 above, compared with Comparative Example 1, this invention, by completely eliminating the cold rolling process and using low carbon content, not only achieves a tensile strength of DP590, but also significantly reduces the yield strength ratio from 0.626 to 0.524, a reduction of 16.3%, while also exhibiting higher elongation. This demonstrates that the low-carbon, cold-rolling-free process of this invention has significant advantages over the traditional cold rolling process. Comparative Example 2 shows that low-temperature coiling is a key condition for obtaining a lath-like genetic interface; otherwise, it leads to coarsening of the microstructure and an increase in the yield strength ratio to 0.613. Comparative Example 3 shows that slow heating causes interface recovery, weakens the genetic effect, and degrades performance. Comparative Example 4 shows that the traditional approach of simply increasing carbon content leads to a sharp deterioration in the yield strength ratio to 0.641 under the cold-rolling-free process, further demonstrating the significant advantages of the process of this invention. Comparative examples 5 and 6 show that when the (Mn+Cr) / C ratio is <25, the alloying elements are relatively insufficient, the enrichment effect is limited, and the microstructure uniformity is slightly poor; when the ratio is >90, the martensite content increases, the yield strength ratio increases, and the performance deteriorates.

[0098] The above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the design concept of the present invention should fall within the protection scope of the present invention.

[0099] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A low-carbon hot-rolled substrate hot-dip galvanized duplex steel, characterized in that, The chemical composition of the hot-dip galvanized duplex steel, by weight percentage, includes: C: 0.03~0.06%, Si: 0.1~0.6%, Mn: 1.4~2.0%, P≤0.02%, S≤0.01%, Al: 0.02~0.06%; Cr: 0.2~0.6%, with the balance being Fe and unavoidable impurities, and satisfying: 25≤(Mn+Cr) / C≤90; its mechanical properties satisfy: yield strength of 300~430MPa, tensile strength of 590~700MPa, yield-to-tensile ratio of 0.50~0.65, and elongation of ≥20%.

2. The method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel according to claim 1, characterized in that, Includes the following steps, but does not include any cold rolling process: S1. After being smelted in a converter and refined in an LF furnace, molten steel with the chemical composition described in claim 1 is obtained, and then continuously cast into a billet. S2. After heating the continuously cast billet, hot rolling is performed to the target thickness. After rolling, laminar cooling is performed and the billet is coiled to obtain a hot-rolled plate. S3. The hot-rolled plate is fed into the continuous hot-dip galvanizing production line and undergoes pickling, heat treatment, galvanizing, post-galvanizing cooling, and leveling in sequence to obtain hot-dip galvanized duplex steel.

3. The method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel according to claim 2, characterized in that, In step S2, the target thickness is 1.0~3.0 mm; In hot continuous rolling, the finishing rolling temperature is controlled at 780~900℃; the cooling rate of laminar flow cooling is ≥15℃ / s, and the coiling is carried out after laminar flow cooling to 100~300℃.

4. The method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel according to claim 2, characterized in that, In step S3, the specific method of heat treatment is as follows: first, the pickled hot-rolled plate is heated to 750~830℃ at a heating rate of ≥8℃ / s, then cooled to 650℃ in 30~150 seconds, and then rapidly cooled to 440~465℃ at a cooling rate of ≥10℃ / s before being immersed in a zinc pot for hot-dip galvanizing.

5. The method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel according to claim 4, characterized in that, The cooling time is determined based on the thickness of the hot-rolled plate: When the thickness of the hot-rolled plate is 1.0~1.5mm: cooling time is 30~60 seconds; When the thickness of the hot-rolled plate is 1.5~2.5mm: cooling time is 60~120 seconds; When the thickness of the hot-rolled plate is 2.5~3.0mm: cooling time is 120~150 seconds.

6. The method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel according to claim 4, characterized in that, In step S3, the post-galvanizing cooling is as follows: after the hot-rolled plate exits the zinc pot, the cooling rate is controlled according to the target strength requirements to cool the galvanized hot-rolled plate to below 150°C.

7. The method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel according to claim 6, characterized in that, The target strength is controlled by the rapid cooling endpoint temperature during heat treatment and the post-plating cooling rate, specifically: When the target tensile strength is 590~630MPa, the rapid cooling endpoint temperature is 460~465℃, and the post-plating cooling rate is 1~8℃ / s. When the target tensile strength is 630~670MPa, the rapid cooling endpoint temperature is 450~460℃, and the post-plating cooling rate is 8~15℃ / s. When the target tensile strength is 670~700MPa, the rapid cooling endpoint temperature is 440~450℃, and the post-plating cooling rate is 15~30℃ / s.

8. The method for producing low-carbon hot-rolled substrate hot-dip galvanized duplex steel according to claim 2, characterized in that, In step S3, the leveling machine reduction rate during the leveling process is 0.1~0.5%.