A high-strength, corrosion-resistant structural aluminum-coated steel strip with substrate and its manufacturing method

Abstract

A high-strength, corrosion-resistant structural aluminum-clad laminate substrate steel and its production method are disclosed. The chemical composition by weight percentage is as follows: C 0.04–0.09%, 0 < Si ≤ 0.005%, Mn 0.4–0.8%, P 0.008–0.020%, S ≤ 0.008%, 0 < Al ≤ 0.005%, N 0.012–0.030%, 0 < O ≤ 0.01%, B 0.0005–0.002%, with selective addition of Ti 0.01–0.06%, Nb 0.01–0.03%, Cu 0.01–0.15%, Cr 0.03–0.60%, and Ca. It contains one or more of the following components from 0.001% to 0.004%, with the balance including Fe and other unavoidable impurities; and simultaneously satisfies the following conditions: 0.5C + 3.83N + O - 0.89Al - 1.15Si ≥ 0.08; 150(C - 0.01) + 60P + 300N + 5Mn ≥ 13.0. Its yield strength is ≥ 300MPa, tensile strength is ≥ 360MPa, elongation is ≥ 30%, it possesses excellent plasticity, aluminum cladding properties, and high strength, with good surface quality, eliminating defects such as surface peeling and slag inclusions. Aluminum-clad sheets and strips produced using this material have excellent surface quality and high strength.

Description

Technical Field

[0001] This invention belongs to the field of low alloy steel manufacturing, specifically relating to a high-strength, corrosion-resistant structural aluminum-clad steel strip substrate and its production method. Background Technology

[0002] Aluminum-clad steel sheets and strips are composite strip materials formed by rolling and deforming steel strips at room temperature to coat them with an aluminum layer. Structurally, they can be steel-aluminum, steel-aluminum-steel, or aluminum-steel-aluminum. Aluminum-clad steel sheets and strips combine the strength of steel with the excellent heat dissipation, corrosion resistance, light weight, and aesthetic appeal of aluminum. Compared to pure aluminum, they significantly reduce costs, leading to their widespread use in radiator fins, corrosion-resistant pipes, and appliance panels. Their applications are also expanding into automotive engine guards, new energy vehicle components, atmospheric corrosion resistant materials, cookware, and construction.

[0003] To extend service life, existing container panels mostly use weathering steel, which requires the addition of numerous corrosion-resistant alloying elements to its composition. Simultaneously, to achieve high strength and weight reduction, reinforcing elements such as Ti and Nb are also added, significantly increasing costs. Furthermore, weathering steel provides overall corrosion resistance, while corrosion in steel structures primarily occurs on the material surface. The numerous corrosion-resistant alloys added internally to the steel plate cannot fully function, representing a waste of resources.

[0004] Aluminum is not only lightweight, aesthetically pleasing, and has good heat dissipation, but it also exhibits excellent resistance to atmospheric corrosion. As a reactive metal, aluminum readily reacts with oxygen in the air to form aluminum oxide, creating a dense film on the surface at a thickness of only a few micrometers, which hinders further corrosion. In industrial atmospheric environments, the corrosion rate of aluminum is approximately 1 μm / a, and its corrosion resistance is about 10 times that of conventional atmospheric corrosion-resistant steel. Furthermore, its aesthetically pleasing surface eliminates the need for painting, reducing painting processes, lowering costs, and making it more energy-efficient and environmentally friendly.

[0005] With the development and application of steel for aluminum-clad sheets and strips, numerous patents have been created for composite sheets and strips and the steel used as their substrates.

[0006] Chinese patent CN102019727 discloses "aluminum-coated steel strip for coolers and its preparation method and the steel strip and aluminum alloy strip used therein". The patent introduces a method for producing composite strips. Although it mentions the substrate used, it is mainly used to produce aluminum-coated strips with a thickness of about 1.5 mm, which has a low yield strength.

[0007] The steel used in the production of aluminum-clad sheets and strips deforms together with the aluminum cladding during the manufacturing process. Therefore, it is required to have strength and ductility comparable to aluminum, especially excellent ductility and good steel-aluminum bonding performance, with specific requirements on its composition. As a result, existing conventional low-strength steels are difficult to use in the production of aluminum-clad sheets and strips.

[0008] Japanese Patent JP2005281806 discloses "Low Yield Point Steel with Excellent Toughness and its Production Method." The steel disclosed is a low-alloy structural steel with low yield strength and high elongation, typically around 200 MPa. The resulting product is generally a thick plate. In terms of composition design, it uses a low carbon (C)-silicon (Si)-manganese (Mn) base with the addition of one or more alloying elements such as chromium (Cr), molybdenum (Mo), nickel (Ni), copper (Cu), and boron (B). The resulting steel has low strength and does not mention aluminum cladding properties; it is mainly used in the production of seismic dampers.

[0009] Chinese patent CN111349869A discloses a "high-strength steel for aluminum-clad substrates and its production method," which yields a steel with a yield strength of 210-290 MPa, a tensile strength ≥320 MPa, and an elongation ≥40%, but its strength is lower than that of this invention. By adding a higher content of phosphorus (P), the diffusion of aluminum (Al) is suppressed, thereby improving the steel-aluminum bonding performance. However, when this steel is used in the production of aluminum-clad materials, a layer of iron-aluminum compounds with a thickness not exceeding 5 μm still exists at the steel-aluminum interface. Furthermore, excessively high P content easily leads to cold brittleness, which is detrimental to the forming and welding performance of the steel plate. Moreover, the high oxygen (O) content in the steel composition results in numerous defects such as porosity and inclusions inside and on the surface of the cast billet, increasing the difficulty of billet cleaning and making the strip surface prone to peeling defects, thus reducing the yield.

[0010] Chinese patent CN107881426A discloses a "high-strength aluminum-coated steel strip and its manufacturing method," which ensures the bonding performance of steel and aluminum by controlling N and O and Si and Al, and has a yield strength of 280-400 MPa. It is mainly used in the production of radiator fins, home appliance panels, etc. However, the high O (oxygen) content in the steel composition will lead to more defects such as porosity and slag inclusions inside and on the surface of the billet, increasing the difficulty of billet cleaning, making the strip surface prone to peeling defects, and reducing the yield.

[0011] Chinese patent CN114250411A discloses a steel for aluminum-coated steel sheets with substrates for kitchenware and its production method. While the steel sheet involves a nitriding process, its yield strength and tensile strength do not exceed 280 MPa and 380 MPa, respectively. Simultaneously, the patent requires controlling the carbon content to below 0.005% and limits the P and S contents to no more than 0.012% and 0.006%, respectively, which increases the difficulty of steelmaking and production costs. This patent primarily improves the strength during nitriding through V and Mo precipitation strengthening. Functionally, this steel is limited to the processing of aluminum-coated materials for woks and is not suitable for the production of other aluminum-coated materials with high strength requirements.

[0012] A comparison with existing patents reveals that the steel used in existing aluminum-clad laminate substrates either has low strength, poor steel-aluminum bonding performance, or poor surface quality of the cast billet, which increases the difficulty of cleaning the cast billet. Summary of the Invention

[0013] The purpose of this invention is to provide a high-strength, corrosion-resistant structural aluminum-clad sheet / strip base steel and its production method. The steel has a yield strength ≥300MPa, tensile strength ≥360MPa, and elongation ≥30%. It has excellent plasticity, aluminum cladding performance, high strength, and high surface quality, eliminating defects such as surface peeling and slag inclusions. The aluminum-clad sheet / strip produced using this steel has excellent surface quality and high strength, and is mainly used in the production of high-strength, corrosion-resistant structural aluminum-clad sheet / strip.

[0014] To achieve the above objectives, the technical solution of the present invention is as follows:

[0015] A high-strength, corrosion-resistant structural steel for aluminum-clad laminates and strips has the following chemical composition by weight percentage: C: 0.04–0.09%, 0 < Si ≤ 0.005%, Mn: 0.4–0.8%, P: 0.008–0.020%, S ≤ 0.008%, 0 < Al ≤ 0.005%, N: 0.012–0.030%, 0 < O ≤ 0.01%, B: 0.0005–0.002%, and selectively adding one or more of the following: Ti: 0.01–0.06%, Nb: 0.01–0.03%, Cu: 0.01–0.15%, Cr: 0.03–0.60%, Ca: 0.001–0.004%, with the balance including Fe and other unavoidable impurities; and simultaneously satisfying the following conditions.

[0016] 0.5C+3.83N+O-0.89Al-1.15Si≥0.08;

[0017] 150(C-0.01)+60P+300N+5Mn≥13.0.

[0018] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0019] The microstructure of the steel used for the aluminum-clad laminate substrate of the present invention is a uniform ferrite structure with a small amount of bainite, wherein the bainite content is ≤20%.

[0020] The steel used for the aluminum-clad sheet substrate of the present invention has a yield strength ≥300MPa, a tensile strength ≥360MPa, and an elongation ≥30%.

[0021] The steel used in the high-strength corrosion-resistant structural aluminum-clad sheet and strip of this invention requires a yield strength and tensile strength of over 300MPa and 360MPa respectively, an elongation of over 30%, good steel-aluminum bonding performance, excellent plasticity, aluminum cladding performance and high strength, good surface quality, and eliminates defects such as surface peeling and slag inclusion. Therefore, it is necessary to limit the content of specific alloying elements as much as possible.

[0022] In the design of the chemical composition of the steel used as the substrate for the aluminum-clad laminate of this invention:

[0023] Carbon (C) strengthens yield strength through solid solution and phase transformation, while simultaneously inhibiting the diffusion of Al in steel and the formation of iron-aluminum compounds. Excessive C content is detrimental to the subsequent processing properties of the material, such as stamping and deep drawing; however, higher C content tends to agglomerate at dislocation and other defect sites, negatively impacting surface quality. Based on actual steelmaking processes, this invention controls the C content to be between 0.04% and 0.09%.

[0024] Si is a deoxidizing element and a solid solution strengthening element, which increases yield strength and decreases elongation. Although an appropriate amount of Si is beneficial to improving the bonding performance of the steel-aluminum interface, it will also weaken the inhibitory effect of O (oxygen) on the formation of brittle compound layer at the steel-aluminum interface. Therefore, this invention controls 0 < Si ≤ 0.005%.

[0025] Mn is also a common strengthening element in steel. It increases yield strength through solid solution strengthening, but reduces elongation. Appropriate amounts of Mn can combine with sulfur in steel to form MnS, reducing the steel's hot brittleness. However, excessive Mn solid solution strengthens the microstructure, inhibits grain growth, and reduces γ-texture (ND∥). <111> The high strength of the material is very detrimental to the stamping and forming of steel plates and will also increase costs. Therefore, this invention controls its content to be between 0.40% and 0.80%.

[0026] In steel, phosphorus (P) is generally controlled as an impurity element, with its addition permitted only in a few special-purpose steels. For example, in sulfur-containing free-machining steels, adding no more than 0.15% P allows it to dissolve in ferrite, causing strengthening and embrittlement to improve its machinability. Additionally, P is a major corrosion-resistant element in traditional atmospheric corrosion-resistant steels; P in steel can accelerate the uniform dissolution of Fe. 2+ The oxidation rate helps to form a uniform FeOOH rust layer on the steel surface and promotes the formation of amorphous iron hydroxide FeO. x (OH) 3-2xA dense protective film effectively improves the atmospheric corrosion resistance of steel. Generally, the optimal corrosion resistance is achieved when the phosphorus (P) content is between 0.08% and 0.15%, so early atmospheric corrosion resistant steels used P as a corrosion-resistant element. However, excessive P causes "cold brittleness" in steel, reducing plasticity and impact toughness, and worsening weldability and cold bending performance. Therefore, the amount of P added is strictly limited for steels requiring low-temperature impact toughness or high formability. In recent years, some atmospheric corrosion resistant steels have also reduced the P content and chosen other alternative corrosion-resistant elements to ensure corrosion resistance, even controlling it as an impurity element with a content ≤0.015%. However, controlling the P content at such a low level increases the difficulty of steelmaking and raises manufacturing costs. This invention considers that P can improve the strength of steel for aluminum-clad laminates and strips through solid solution strengthening and phase transformation strengthening, while also improving the steel-aluminum bonding performance. Therefore, this invention adds 0.008% to 0.020% P.

[0027] Sulfur (S) is detrimental to the properties of steel, easily causing "hot brittleness," reducing the low-temperature toughness of steel, and worsening the bonding performance at the steel-aluminum interface. The addition of manganese (Mn) can form MnS with S, and increasing the Mn / S ratio can improve thermal ductility. It is necessary to control its content at the lowest possible level. Considering that excessively low content increases the difficulty of steelmaking and production costs, this invention requires that the S content be controlled below 0.008%.

[0028] Al is an important deoxidizing element in steel, but high levels of Al can easily diffuse to the steel-aluminum interface, worsening the interfacial bond strength. Therefore, its content must be controlled within a certain range; hence, this invention controls 0 < Al ≤ 0.005%.

[0029] Boron (B) is an extremely reactive element in steel, readily forming compounds with carbon (C) and nitrogen (N). Simultaneously, B readily segregates at austenite grain boundaries and dislocations, thus suppressing the segregation of other interstitial atoms at these locations. Furthermore, B segregation does not hinder dislocation movement, resulting in uniform strain, improved material plasticity, and beneficial effects on aluminum-coated rolling performance. B significantly improves hardenability; even trace amounts of B can effectively increase the strength of steel. However, excessively high B content can promote increased hardenability, leading to excessively high strength and reduced stamping performance. Therefore, this invention controls its content to 0.0005–0.002%.

[0030] N can form AlN particles with Al in steel, thereby binding Al and limiting its diffusion in steel. At the same time, similar to C, N tends to agglomerate at dislocations to form Cotillard atmospheres, which leads to strain concentration and causes uneven strain in aluminum-clad materials during processing, affecting surface quality. Moreover, excessive N also deteriorates impact toughness. Therefore, this invention controls its content to be 0.012-0.030%.

[0031] Oxygen (O) can suppress the adverse effects of Al in steel on the properties of aluminum cladding, so an appropriate amount of O is required. However, excessive oxygen can cause defects such as subcutaneous bubbles and porosity, and exacerbate the hot brittleness of sulfur, making the strip surface prone to defects such as peeling and slag inclusions. During the solidification process of steel, oxygen will precipitate in large quantities in the form of oxides, reducing the plasticity and impact toughness of the steel. Therefore, the O content should be controlled to be ≤0.01%.

[0032] In addition to the elements mentioned above, this invention also requires the selective addition of one or more of the following: Ti: 0.01–0.06%, Nb: 0.01–0.03%, Cu: 0.01–0.15%, Cr: 0.03–0.60%, and Ca: 0.001–0.004%, thereby further improving the room temperature strength and the strength after heat treatment. Wherein:

[0033] In steel, Cr forms a continuous solid solution with Fe, resulting in solid solution strengthening. It also forms various types of carbides with C, such as M3C, M7C3, and M... 23 C6 and other elements produce a secondary strengthening effect. Simultaneously, the addition of Cr helps suppress Al diffusion and improves the bonding performance between steel and aluminum. Higher Cr content is detrimental to toughness, increases welding difficulty, and is a valuable alloying element; therefore, this invention controls its content to 0.03–0.6%.

[0034] Cu has solid solution and precipitation strengthening effects. At higher content, it exhibits a secondary hardening effect during tempering at appropriate temperatures, thereby increasing strength. Simultaneously, the addition of Cu also improves the bonding properties between steel and aluminum. Excessive Cu content causes cracks in the steel billet during heating and hot rolling, deteriorating surface properties. Therefore, this invention controls the Cu content to be between 0.01% and 0.15%.

[0035] Nitrogen (Nb) is a strong nitride-carbide-forming element. During post-rolling cooling, it can combine with carbon and nitrogen in steel to form intermediate phases such as NbC, Nb(CN), and NbN. The resulting fine carbide particles refine the microstructure, producing fine-grain strengthening and precipitation strengthening effects, significantly improving the strength of the steel. Furthermore, Nb can inhibit the expansion of austenite interfaces and increase the recrystallization temperature of steel, allowing for rolling in the non-recrystallization zone at higher temperatures. Therefore, adding an appropriate amount of Nb to steel is beneficial for strength improvement, but the effect no longer increases strength beyond 0.06%. Higher Nb content leads to the formation of coarse carbonitride particles at grain boundaries, deteriorating impact toughness. Therefore, Nb can be selectively added, but its content should be controlled between 0.01% and 0.03%.

[0036] Ti has high chemical reactivity, readily forming compounds with C, N, O, S, etc., and precipitating during rolling and cooling to increase strength. Simultaneously, the fine precipitates can pin grain boundaries, thereby refining austenite grains and inhibiting grain growth in the weld heat-affected zone, improving the weldability of the steel. The combined addition of Ti and B can significantly reduce temper brittleness. However, excessive Ti content can lead to the growth and agglomeration of titanium nitride particles at high temperatures, impairing the steel's plasticity and toughness. Therefore, its content needs to be controlled between 0.01% and 0.06%.

[0037] Adding Ca to steel can alter the shape of sulfides, suppress the hot brittleness of S, and improve toughness. Furthermore, when excessive Ti is present in the steel, it can form titanium sulfide or titanium carbosulfide with sulfides. Too low a Ca content has little effect, while a content exceeding 0.005% results in excessively large Ca(O,S) assemblies, increasing brittleness and potentially becoming crack initiation points. It also reduces steel purity and worsens the toughness of the weld heat-affected zone. Therefore, this invention limits its content to 0.001–0.004%.

[0038] The steel used for the aluminum-clad steel substrate described in this invention requires high surface quality, high strength, and good steel-aluminum bonding performance to meet the needs of a wider range of applications. It requires a yield strength ≥300MPa and a tensile strength ≥360MPa. To meet these performance requirements, this invention strictly designs the composition based on the influence of different alloying elements on these properties, achieving the performance requirements through the combined effect of multiple elements. The addition of carbon (C) has both solid solution strengthening and phase transformation strengthening effects. Existing aluminum-clad steels (such as Chinese patent CN114250411A) generally control C ≤0.005% to ensure plasticity, which greatly increases the difficulty of steelmaking. This invention, to ensure high strength after nitriding treatment and simultaneously require a yield strength of over 300MPa for the substrate steel, uses 0.04–0.09% C. This serves two purposes: firstly, it provides solid solution strengthening; secondly, during steel production, controlled rolling and post-rolling cooling form a ferrite + small amount of bainite microstructure in the steel, resulting in phase transformation strengthening and increased strength.

[0039] Because aluminum-clad laminates undergo annealing heat treatment during production, conventional fine-grain strengthening methods are not very effective after heat treatment. However, solid solution strengthening is not limited by heat treatment, so the higher the steel strength, the higher the strength of the finished aluminum-clad laminate. Therefore, this invention adds 0.4–0.8% Mn to ensure steel strength through Mn solid solution strengthening.

[0040] In this invention, the phosphorus (P) content is controlled at 0.008–0.020%. P is typically controlled as an impurity element in steel, so a lower content is desirable, generally below 0.015% or even lower. However, lower P content increases steelmaking costs and difficulty. Since P belongs to the same group as nitrogen (N), appropriate P content is beneficial to steel-aluminum bonding performance. However, research in this invention has found that its effect on improving steel-aluminum bonding performance is not linear; P content exceeding 0.020% begins to deteriorate steel-aluminum bonding performance. Therefore, considering the difficulty, cost, and performance requirements of steelmaking, the upper limit for P control is relaxed to 0.020%. This ensures steel-aluminum bonding performance while simultaneously improving strength and reducing steelmaking costs.

[0041] In the steel of this invention, Cr not only provides solid solution strengthening but also has a secondary strengthening effect. Cu also provides solid solution strengthening and can precipitate during heat treatment to enhance strength. Nb, V, and Ti precipitate fine carbonitrides during rolling and cooling, providing precipitation strengthening to the matrix.

[0042] Based on the strengthening effect of the above elements on steel, it is required to control the content of C, P, N and Mn in steel to meet the relationship 1: 150(C-0.01)+60P+300N+5Mn≥13.0. If the value of the relationship is lower than 13.0, the strength is likely to be low.

[0043] The key to improving the bonding performance of steel and aluminum lies in controlling the formation of brittle iron-aluminum compounds at the steel-aluminum interface in the aluminum-clad material. The formation of these compounds is closely related to the diffusion of Al in the steel; controlling Al diffusion can inhibit the formation of these compounds within a certain temperature range. This invention demonstrates that the synergistic effect of multiple elements such as C, Cu, O, and N can further improve the bonding performance of steel and aluminum. O (oxygen) and N can inhibit Al diffusion and reduce the formation of brittle iron-aluminum compounds, thereby improving the bonding performance. However, excessive O content will lead to subcutaneous bubbles and porosity in the cast billet, and exacerbate the hot brittleness caused by sulfur, making the steel surface prone to defects such as peeling and slag inclusions. Therefore, this invention limits the O content to ≤0.01%, while using 0.012–0.03% N to ensure the bonding performance of steel and aluminum. Si and Al, as deoxidizing elements, hinder the improvement effect of O on the bonding performance of steel and aluminum, so their content should be limited.

[0044] Therefore, this invention achieves excellent steel-aluminum bonding performance through the synergistic effect of multiple elements such as C, Mn, Si, Al, O, and N. Based on the relative strengths of the effects of C, N, O, Al, and Si on the steel-aluminum bonding performance, their contents are required to satisfy the relationship 2: 0.5C + 3.83N + O - 0.89Al - 1.15Si ≥ 0.08, thereby ensuring excellent steel-aluminum bonding performance. If the value of this relationship is lower than 0.08, steel-aluminum delamination is likely to occur.

[0045] Overall, the present invention adopts the above-designed composition system, which comprehensively considers the requirements of surface quality, steel-aluminum bonding performance and high strength. Through the combined effect of each element, the invention steel grade that meets the performance requirements can be obtained.

[0046] The method for producing high-strength, corrosion-resistant structural aluminum-clad laminate strip substrate steel according to the present invention includes the following steps:

[0047] 1) Smelting

[0048] The above-mentioned components are processed through molten iron pretreatment, converter blowing, ladle refining, and casting into slabs;

[0049] 2) Reheating and controlled rolling

[0050] The thickness of the rolled steel strip is >2.5mm, the heating temperature is ≥1230℃, and the roughing rolling end temperature is above 1050℃; the finishing rolling start temperature is above 980℃, and the finishing rolling end temperature is 860~900℃.

[0051] The thickness of the rolled steel strip is ≤2.5mm, the heating temperature is ≥1100℃, the roughing rolling end temperature is ≥900℃, and the cumulative deformation during the roughing rolling stage is ≥80%; the finishing rolling start temperature is 790~850℃, and the finishing rolling end temperature is 710~750℃.

[0052] 3) Control the cooling rate to be above 20℃ / s;

[0053] 4) Winding, winding temperature 550~610℃.

[0054] Preferably, the slab obtained in step 1) is machine-cleaned.

[0055] Preferably, in step 2), the side pressure during the roughing stage of the rolling process is controlled to be within 50 mm.

[0056] In the production method described in this invention:

[0057] Hot metal pretreatment removes phosphorus and sulfur (P and S) to ensure low P and S content in the steel; top and bottom combined blowing in the converter controls carbon content.

[0058] The purpose of machine cleaning after continuous casting is to eliminate surface defects of the billet, reduce defects such as slag inclusions, peeling, pits, and scars on the surface of the strip, improve the surface quality of the strip, and reduce surface defects of the finished aluminum-coated material.

[0059] This invention develops a rolling process based on the phase transformation temperature curve of the steel grade and in conjunction with target properties, determining the temperature control parameters for the rolling process. This invention relates to steel grades with low alloy content, whose continuous cooling curves are as follows: Figure 1As shown in the figure, the ferrite phase transformation begins at 852℃, and pearlite begins to form after the temperature drops to 698℃. The bainite phase transformation begins at 653℃, and when the cooling rate is above 1℃ / s, only ferrite and bainite form.

[0060] For different thickness specifications, this invention designs two rolling processes for the steel grades. For steel strips with a thickness greater than 2.5 mm, a conventional austenitic rolling process is used; while for steel strips with a thickness less than 2.5 mm, a ferritic rolling process is selected to obtain better strip shape and higher strength.

[0061] Based on the composition calculation of the steel grade according to this invention, the Ar3 temperature of the steel grade is 857℃, and the ferrite phase transformation initiation temperature is 852℃. Therefore, during the austenitic rolling process, it is required to control the finishing rolling end temperature above 860℃. If the finishing rolling end temperature is too high, a higher heating temperature is required, increasing energy consumption and strip oxidation. Therefore, the upper limit of the finishing rolling end temperature is set to no more than 900℃. Correspondingly, the finishing rolling inlet temperature is required to be above 980℃, and the roughing rolling end temperature is required to be above 1050℃. To ensure the roughing rolling end temperature, and considering the dissolution behavior of microalloying element carbonitrides in austenite and the austenite grain growth behavior during heating, it is required to control the heating temperature above 1230℃.

[0062] When the strip thickness is below 2.5mm, the strip's deformation resistance is low in the high-temperature austenitic region (860-900℃), making it prone to strip breakage, localized stretching, and lateral bending at the finishing mill exit. Therefore, this invention employs a ferritic rolling process. At lower temperatures, the strip strength is higher, and under the same coiling tension, it is less prone to deformation, thus reducing narrowing and strip breakage during rolling. Simultaneously, the higher strength allows for higher coiling tension, improving strip shape and reducing lateral bending. Ferritic rolling requires the entire finishing deformation to occur within the ferritic region; therefore, the finishing rolling start temperature must not exceed 850℃, and the finish temperature must not be lower than 710℃. If the finishing rolling start temperature is too low, the hot deformation resistance is high, increasing the rolling load on the last stand and leading to increased energy consumption; if it is too high, it easily enters the austenitic-ferrite two-phase region, easily forming mixed crystals in the matrix, causing rolling force fluctuations, which is detrimental to strip thickness control and strip shape. Therefore, in the ferritic rolling process, the starting temperature for finishing rolling is set to 790–850℃, and the ending temperature is set to 710–750℃. Based on the starting temperature for finishing rolling, and considering the temperature drop of the intermediate slab on the roller table, the ending temperature for rough rolling is required to be above 900℃, and the corresponding slab heating temperature is controlled above 1100℃. Furthermore, to ensure the grain refinement effect of recrystallization, the cumulative deformation during the rough rolling stage is required to be ≥80%.

[0063] To achieve high strength, the steel grade of this invention incorporates one or more of Cu, Cr, Nb, V, and Ti. Copper has limited solid solubility in steel, which decreases rapidly with decreasing temperature. At high temperatures, Cu dissolved in austenite precipitates as fine ε-Cu particles and disperses throughout the matrix. Its twinned structure and dislocation pinning effect enhance the steel's strength. Studies show that Cu precipitates form as early as 650℃, with the precipitate size decreasing as the temperature decreases. The optimal precipitation strengthening effect is observed at 500℃; therefore, the lower limit of the coiling temperature should not be lower than 500℃. Figure 2 The TTT curves show that 581℃ is the temperature at which bainite transformation is fastest. The microstructure of the steel grade of this invention is mainly equiaxed ferrite. To obtain higher strength, a certain amount of bainite in the matrix is ​​desired, so the coiling temperature is controlled at around 580℃. The carbonitrides of Nb, V, and Ti added also have good precipitation strengthening effects near this temperature. Therefore, considering both precipitation strengthening and phase transformation strengthening effects, the coiling temperature is set at 550–610℃.

[0064] To achieve the desired performance, the steel's matrix structure is controlled to be a uniform equiaxed ferrite structure, while allowing a small amount of bainite to form. From Figure 1 As you can see, a ferrite + a small amount of bainite structure can be obtained when the cooling rate is above 1℃ / s; considering that rapid cooling can refine the structure and increase the phase transformation rate, the present invention relates to controlling the post-rolling cooling rate of steel grades to above 20℃ / s.

[0065] To achieve good surface quality, the side pressure during the rough rolling stage should be controlled within 50 mm. Simultaneously, the edges and corners of the cast billet must be free of defects such as porosity and scars.

[0066] To suppress abnormal ferrite grain growth in the steel after final rolling and ensure a uniform equiaxed ferrite + bainite microstructure, a pre-cooling method is required for post-rolling cooling. This means the steel is intensively cooled immediately after exiting the hot continuous rolling mill. The process of this invention can obtain a uniform ferrite + small amount of bainite microstructure, specifically requiring a bainite content ≤20%. Excessive bainite content will lead to insufficient plasticity and increase the difficulty of aluminum cladding rolling.

[0067] The present invention has the following advantages:

[0068] 1. This invention improves the strength of steel through solid solution strengthening and phase transformation strengthening. In terms of composition design, it adopts a simple C-Mn design and controls the contents of C, P, N, and Mn to satisfy the relationship: 150(C-0.01)+60P+300N+5Mn≥13.0, thus achieving higher strength. Its yield strength is ≥300MPa and tensile strength is ≥360MPa, and it is mainly used in the production of high-strength, corrosion-resistant structural aluminum-clad steel sheets and strips.

[0069] 2. In order to obtain high steel-aluminum bonding performance, the present invention requires that the contents of C, Mn, Si, Al, O, and N meet the following relationship in the composition design: 0.5C + 3.83N + O - 0.89Al - 1.15Si ≥ 0.08, based on the effects of C, Mn, Si, Al, O, and N on the steel-aluminum bonding performance. This inhibits the formation of brittle iron-aluminum compounds at the steel-aluminum interface, thereby ensuring that the steel plate obtains good steel-aluminum bonding performance.

[0070] 3. This invention adopts a low-oxygen, high-nitrogen composition design, limiting the O content to ≤0.01%, and using 0.012 to 0.030% N to compensate for the steel-aluminum bonding performance, thereby solving the defects such as subcutaneous bubbles and porosity in high-oxygen cast billets, reducing the occurrence of defects such as steel surface defects, peeling, and slag inclusions, and resulting in better surface quality.

[0071] 4. By controlling the P content to 0.008-0.020%, this invention utilizes the solid solution strengthening effect of P and its influence on the steel-aluminum bonding performance to further improve the steel-aluminum bonding performance, while reducing the difficulty and cost of steelmaking and increasing production efficiency.

[0072] 5. In terms of process control, this invention employs austenitic rolling and ferritic rolling processes respectively for different billet thicknesses obtained after rolling. This allows the steel plate to achieve high strength while significantly improving the shape of thin-gauge steel strips, reducing problems such as strip narrowing, breakage, and lateral bending during rolling. The production process is simple, low-cost, and suitable for large-scale production in enterprises.

[0073] 6. The process of this invention employs different controlled rolling processes, combined with a coiling temperature of 550-610℃ and a post-rolling cooling rate of ≥20℃ / s, to obtain a steel plate with a uniform ferrite + a small amount of bainite structure, wherein the bainite content is ≤20%. The obtained steel plate has good deformation capacity, which can meet the deformation requirements of more than 80% for aluminum cladding rolling, as well as the deformation of subsequent stamping and deep drawing processes. No intermediate annealing is required, reducing production steps and improving production efficiency. Attached Figure Description

[0074] Figure 1 This is a schematic diagram of the static CCT curve (calculated) of the steel grade of this invention.

[0075] Figure 2 This is a schematic diagram of the static TTT curve (calculation) of the steel grade of this invention.

[0076] Figure 3 This is a photograph of the typical matrix structure of the steel in Example 1 of the present invention. Detailed Implementation

[0077] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0078] In this embodiment of the invention, steelmaking is carried out in a 500kg vacuum induction furnace. The chemical composition of the steel in the embodiment is shown in Table 1, the specific process parameters are shown in Table 2, and the mechanical properties of the steel in the embodiment are shown in Table 3.

[0079] Depend on Figure 3 It can be seen that the microstructure of the steel plate obtained by using the composition and process described in this invention is a uniform ferrite + a small amount of bainite, wherein the bainite content is ≤20%.

[0080] A comparison was made with similar steel grades in terms of composition, production process, and performance. Among them,

[0081] Comparison steel 1 is Chinese patent CN101514426A, "Low yield point steel for seismic resistance of buildings with a yield strength of 100MPa and its production method", which has a high Si and Al content and does not contain O.

[0082] Compared to steel 2, which is the subject of Chinese patent CN101525720 "A novel special substrate for preparing aluminum-coated steel strips", the alloy content is too high.

[0083] Compared with steel 3, which is Chinese patent CN111349869A "a high-strength steel for aluminum-clad substrate and its production method", 0.018-0.03% P was added to its composition. An iron-aluminum compound layer with a thickness of no more than 5μm appeared at the steel-aluminum interface, which reduced the steel-aluminum bonding performance and the strength was low. In addition, its composition contained a high oxygen content, which was detrimental to the surface quality of the billet and strip steel, increased the production difficulty, and affected the yield.

[0084] Compared to steel 4, which is covered by Chinese patent CN107881426A "a high-strength aluminum-coated sheet and strip and its manufacturing method", its high oxygen content is detrimental to the surface quality of the billet and strip steel, increases the production difficulty, and affects the yield.

[0085] The steel grade of this invention differs significantly in composition from the four comparative steels. To ensure excellent steel-aluminum bonding performance, high strength, and good surface quality, the composition system of the steel grade of this invention has been specifically designed.

[0086] Of the four comparative steels mentioned above, comparative steels 1 and 2 contain high levels of Al and extremely high levels of P, S, and N, without considering the steel-aluminum bonding performance; comparative steel 3 explicitly contains P, which is detrimental to the steel-aluminum bonding performance and results in lower strength; comparative steel 4 involves an aluminum-coated strip, without considering the influence of O content on surface quality. Therefore, the comparative steels differ significantly from the steels of this invention in terms of composition.

[0087] In terms of performance, comparative steels 1 and 2 only require lower yield strength and higher elongation, while the steel grade of this invention, in addition to the above requirements, also explicitly requires the steel grade to have good aluminum coating properties, suitable for the production of aluminum-coated strip steel, without the need for intermediate annealing, which is something that comparative steel 2 does not possess.

[0088] While steels 3 and 4 also require aluminum cladding properties, they are primarily used in the production of aluminum-clad sheets and strips for radiator fins, appliance panels, etc., with an aluminum layer thickness not exceeding 0.1 mm. The steel involved in this invention has a wider range of applications, mainly used in the production of aluminum-clad sheets and strips for containers and other structures requiring high strength and corrosion resistance. Therefore, the steel of this invention differs significantly from the comparative steels in terms of performance.

[0089] As can be seen from Table 3, the steel of the present invention has a stable yield strength. Under different rolling processes, the yield and tensile strength of various specifications of steel plates are above 300MPa and 360MPa, respectively, and it has a high elongation. It is superior to the comparative steel in terms of composition design and function, thus meeting the production requirements of aluminum-clad sheets and strips with high elongation, good aluminum cladding performance, high surface quality and strong deformation capacity.

[0090] Furthermore, the embodiments are merely illustrative examples, and the thickness of the steel grade described in this invention is not limited thereto. Thinner or thicker strip steel can be produced within the limits allowed by the equipment.

[0091]

[0092]

[0093]

Claims

1. A high-strength, corrosion-resistant structural steel for use with aluminum-clad laminates and strips, comprising the following chemical composition by weight percentage: C: 0.04–0.09%, 0 < Si ≤ 0.005%, Mn: 0.4–0.8%, P: 0.008–0.020%, S ≤ 0.008%, 0 < Al ≤ 0.005%, N: 0.0212–0.030%, 0 < O ≤ 0.01%, B: 0.0005–0.002%, and optionally adding one or more of the following: Ti: 0.01–0.06%, Nb: 0.01–0.03%, Cu: 0.01–0.15%, Cr: 0.03–0.60%, Ca: 0.001–0.004%, with the balance being Fe and other unavoidable impurities; and simultaneously satisfying the following conditions: 0.5C+3.83N+O-0.89Al-1.15Si≥0.08; 150(C-0.01)+60P+300N+5Mn≥13.

0.

2. The steel for high-strength, corrosion-resistant structural aluminum-clad steel strip substrate as described in claim 1, characterized in that, The balance consists of Fe and other unavoidable impurities.

3. The steel for high-strength, corrosion-resistant structural aluminum-coated steel sheets and substrates as described in claim 1 or 2, characterized in that, The microstructure of the steel used for the aluminum-clad laminate substrate is uniform ferrite with a small amount of bainite, wherein the bainite content is ≤20%.

4. The steel for high-strength, corrosion-resistant structural aluminum-coated sheet / strip substrate as described in claim 1 or 2, characterized in that, The steel used for the aluminum-clad sheet substrate has a yield strength ≥300MPa, a tensile strength ≥360MPa, and an elongation ≥30%.

5. The steel for high-strength, corrosion-resistant structural aluminum-coated sheet / strip substrate as described in claim 3, characterized in that... The steel used for the aluminum-clad sheet substrate has a yield strength ≥300MPa, a tensile strength ≥360MPa, and an elongation ≥30%.

6. The method for producing high-strength, corrosion-resistant structural aluminum-clad laminate / strip steel as described in any one of claims 1 to 5, characterized in that, Includes the following steps: 1) Smelting The components described in claim 1 or 2 are pretreated with molten iron, smelted in a converter, refined outside the ladle, and cast into slabs. 2) Reheating and controlled rolling The thickness of the rolled steel strip is >2.5mm, the heating temperature is ≥1230℃, and the roughing rolling end temperature is above 1050℃; the finishing rolling start temperature is above 980℃, and the finishing rolling end temperature is 860~900℃. The thickness of the rolled steel strip is ≤2.5mm, the heating temperature is ≥1100℃, the roughing rolling end temperature is ≥900℃, and the cumulative deformation during the roughing rolling stage is ≥80%; the finishing rolling start temperature is 790~850℃, and the finishing rolling end temperature is 710~750℃. 3) Control the cooling process, ensuring a cooling rate of 20℃ / s or higher; 4) Winding, winding temperature 550~610℃.

7. The production method as described in claim 6, characterized in that, The slab obtained in step 1) is machine cleaned.

8. The production method as described in claim 6, characterized in that, Step 2) During the rolling process, the side pressure during the roughing stage should be controlled within 50 mm.

9. The production method as described in claim 6, characterized in that, The microstructure of the steel used for the aluminum-clad laminate substrate is uniform ferrite with a small amount of bainite, wherein the bainite content is ≤20%.

10. The production method as described in claim 6 or 9, characterized in that, The steel used for the aluminum-clad sheet substrate has a yield strength ≥300MPa, a tensile strength ≥360MPa, and an elongation ≥30%.

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