A high-strength, easily nitrided aluminum-coated sheet and strip and its production method

Abstract

A high-strength, easily nitrided aluminum-coated steel strip and its production method are disclosed. The strip comprises a steel strip and an aluminum strip. The steel strip has the following chemical composition by weight percentage: C 0.01–0.07%, 0 < Si ≤ 0.005%, Mn 0.40–1.20%, P 0.008–0.020%, S ≤ 0.008%, 0 < Al ≤ 0.005%, Cr 0.03–0.60%, N 0.003–0.010%, O 0.011–0.05%, Cu 0.01–0.15%, Ca 0.001–0.004%, and selectively added Ti 0.01–0.06%, Nb 0.01–0.03%, V... One or more of the following components are present in the range of 0.01% to 0.03%, with the balance including Fe and other unavoidable impurities; and must simultaneously satisfy the following: 0.5C + 6.86N + O + 0.05Cu + 0.047Cr - 0.89Al - 1.15Si ≥ 0.085; 150(C - 0.01) + 60P + 300N + 5Mn + 3Cr ≥ 9.0. The yield strength of the aluminum-clad sheet / strip is ≥ 330MPa, the tensile strength is ≥ 400MPa, the elongation A50 is ≥ 18%, and it retains high strength after high-temperature nitriding treatment, while maintaining good steel-aluminum bonding performance and cold bending performance.

Description

Technical Field

[0001] This invention belongs to the field of low alloy steel manufacturing, specifically relating to a high-strength, easily nitrided aluminum-coated sheet and strip and its production method. Background Technology

[0002] Aluminum-clad steel sheets and strips are composite strip materials formed by rolling an aluminum layer onto the surface of steel strip at room temperature. 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 application in radiator fins, corrosion-resistant pipes, and appliance panels. Their use is also expanding into automotive engine guards, new energy vehicle components, atmospheric corrosion resistant materials, cookware, and construction.

[0003] In addition to requiring excellent steel-aluminum bonding properties, aluminum-clad sheets and strips also demand high surface strength and high surface finish. In the kitchenware industry, new woks employing a steel-aluminum-steel structure combine the strength of steel with the excellent heat dissipation of aluminum, thus avoiding the uneven heat dissipation and susceptibility to scorching issues common in iron woks, and are also lighter. Furthermore, when producing products like woks using aluminum-clad sheets and strips, nitriding treatment is necessary to achieve high surface hardness. However, conventional aluminum-clad sheets undergoing high-temperature nitriding suffer from reduced strength and the high temperature provides an impetus for the formation of brittle phases at the steel-aluminum interface, leading to decreased steel-aluminum bonding properties and even delamination.

[0004] Chinese patent CN101660087 discloses "an aluminum-steel-aluminum composite material and its preparation method," which involves surface-treating aluminum and steel, then cold-rolling them into high-precision aluminum strips and steel strips respectively, followed by another cold rolling into a high-precision aluminum-steel-aluminum composite strip, and annealing it at 650–850℃ for 1–4 hours. Since aluminum's melting point is approximately 640℃, annealing at such a high temperature severely deteriorates the interfacial bonding strength between steel and aluminum. This fails to meet the performance and surface quality requirements of current applications of aluminum-clad laminates and strips.

[0005] Chinese patent CN102019727 discloses "Aluminum-coated steel strip for coolers and its preparation method, as well as the steel strip and aluminum alloy strip used therein." This patent mainly relates to aluminum-coated steel strips used for heat dissipation and the substrates used therein, with a single-pass reduction rate of 58-62%. Its structure is steel-aluminum, with a thickness of approximately 1.5 mm, of which the aluminum film thickness is 50-80 μm. This patent does not cover the performance of the aluminum-coated strip after high-temperature nitriding treatment.

[0006] Chinese patents CN107881410A and CN108796384A, which disclose "An Aluminum-Clad Strip with Excellent Heat Dissipation and Its Production Method," and CN108796384A, which disclose "An Aluminum-Clad Strip with High Surface Quality that is Easy to Stamp and Its Production Method," involve aluminum-clad materials mainly used in the production of radiator fins and appliance panels, and do not involve nitriding treatment during the production process. Conventional aluminum-clad strips experience a significant decrease in strength after nitriding treatment, which can easily lead to deformation of cookware during the flanging process. Therefore, these two patents differ significantly from this invention in terms of function and application.

[0007] Chinese patent CN107781426A discloses a "high-strength aluminum-clad sheet and strip and its manufacturing method," which ensures the steel-aluminum bonding performance of the aluminum-clad sheet and strip by controlling N, O, Si, and Al, and achieves a yield strength of 280-400 MPa. However, this is the tensile performance after annealing at a relatively low temperature, and it does not involve high-temperature nitriding treatment. The performance of the aluminum-clad sheet and strip after high-temperature treatment is still unknown. The aluminum-clad material involved is mainly used in the production of heat sink fins and appliance panels, and does not meet the requirements for applications requiring high-temperature heat treatment.

[0008] Chinese patent CN108796384A discloses "A high surface quality aluminum-clad sheet and strip that is easy to stamp and process and its production method". In this patent, the P, S, Si, Al and O, N composition in the substrate is strictly controlled to ensure that the substrate and aluminum have good interfacial bonding characteristics. However, it does not involve high-temperature nitriding treatment, and the performance of the aluminum-clad sheet and strip after high temperature is still unknown.

[0009] Chinese patent CN114250411A discloses "An Aluminum-Clad Plate and Strip for Kitchenware and Its Production Method." The aluminum-clad plate and strip used in wok manufacturing involves nitriding during processing and specifies strength requirements after nitriding. However, this patent only requires a tensile strength ≥320MPa, while the yield strength and tensile strength of the substrate used are ≤280MPa and 380MPa respectively, significantly lower than those of this invention. Furthermore, the steel strip involved in this patent requires the carbon content to be controlled below 0.005%, and limits the P and S contents to no more than 0.012% and 0.006% respectively, increasing the difficulty of steelmaking and production costs. In terms of application, this application is limited to the processing of aluminum-clad materials for woks, with the aluminum layer thickness accounting for approximately 40-60% of the total thickness; it cannot be used for the production of other aluminum-clad materials with high strength requirements.

[0010] The comparison results show that the existing steel-aluminum composite strips either have poor steel-aluminum bonding performance or do not meet the requirements of nitriding treatment and high strength. The thickness of the aluminum layer is mostly less than 100μm, and they are mainly used in the fields of heat sinks, home appliance panels, food trays, etc., mainly taking advantage of the high heat dissipation coefficient of aluminum. Summary of the Invention

[0011] The purpose of this invention is to provide a high-strength, easily nitrided aluminum-coated sheet and strip, and its production method. The aluminum-coated sheet and strip possess excellent steel-aluminum bonding performance, stamping and deep-drawing performance, and surface quality. Its yield strength is ≥330MPa, tensile strength is ≥400MPa, and elongation (A50) is ≥18%. After high-temperature nitriding or other high-temperature treatments, its yield strength remains above 330MPa, while maintaining good steel-aluminum bonding performance and cold-bending properties, meeting the requirements of D=1a and 180° cold bending. It is suitable for the production and processing of products such as woks or other products with high strength requirements, and features high strength, lightweight, corrosion resistance, aesthetic appeal, good heat dissipation, and ease of cold forming.

[0012] To achieve the above objectives, the technical solution of the present invention is as follows:

[0013] A high-strength, easily nitrided aluminum-coated sheet / strip is composed of a steel strip and an aluminum strip. The chemical composition (by weight percentage) of the steel strip is as follows: C: 0.01–0.07%, 0 < Si ≤ 0.005%, Mn: 0.4–1.2%, P: 0.008–0.020%, S ≤ 0.008%, 0 < Al ≤ 0.005%, Cr: 0.03–0.60%, N: 0.003–0.010%, O: 0.0 11–0.05%, Cu: 0.01–0.15%, Mo: 0.01–0.10%, B: 0.0005–0.002%, Ca: 0.001–0.004%, and optionally add one or more of Ti: 0.01–0.06%, Nb: 0.01–0.03%, V: 0.01–0.03%, with the balance containing Fe and other unavoidable impurities; and simultaneously satisfying the following:

[0014] 0.5C+6.86N+O+0.05Cu+0.047Cr-0.89Al-1.15Si≥0.085;

[0015] 150(C-0.01)+60P+300N+5Mn+3Cr+15Mo≥9.0.

[0016] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0017] The steel strip of this invention has a yield strength ≥300MPa, a tensile strength ≥380MPa, and an elongation ≥25%.

[0018] The aluminum-coated sheet and strip of the present invention have a yield strength ≥330MPa, a tensile strength ≥400MPa, and an elongation A50 ≥18%.

[0019] The aluminum-coated sheet and strip of the present invention are steel-aluminum, aluminum-steel-aluminum, or steel-aluminum-steel structures.

[0020] In the design of the chemical composition of the steel strip for the aluminum-coated steel strip with excellent nitriding performance of this invention:

[0021] Carbon (C) strengthens through solid solution and phase transformation, thereby increasing yield strength and 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, which is detrimental to surface quality. Based on actual steelmaking processes, the C content is controlled at 0.01–0.07%.

[0022] Si is a deoxidizing element and a solid solution strengthening element, which increases yield strength and decreases elongation. An appropriate amount of Si is beneficial to improving the interfacial bonding performance of steel and aluminum, but at the same time it weakens the inhibitory effect of O (oxygen) on the formation of brittle compound layers at the steel-aluminum interface. Therefore, 0 < Si ≤ 0.005% should be controlled.

[0023] 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 steel strip is detrimental to the stamping and forming process and increases the cost, so its content is controlled between 0.40% and 1.20%.

[0024] 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) 3-2x A 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 improves the strength of the substrate for aluminum-clad laminates and strips and the strength after nitriding through solid solution strengthening and phase transformation strengthening, while also improving the steel-aluminum bonding performance. Therefore, this invention adds 0.008–0.020% P.

[0025] 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 hot 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, the S content should be controlled below 0.008%.

[0026] Al is an important deoxidizing element in steel, but high levels of Al can easily diffuse to the steel-aluminum interface, worsening the interfacial bonding strength. Therefore, its content must be controlled within a certain range. In this invention, it is controlled as an impurity element, with 0 < Al ≤ 0.005%.

[0027] 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 alloying elements produce a secondary strengthening effect. Simultaneously, the addition of Cr helps suppress Al diffusion and improves the bonding performance between steel and aluminum. Furthermore, Cr is an effective nitriding element, simultaneously increasing surface hardness and nitriding layer depth. Conventional nitriding steels typically contain up to 3% Cr, which significantly increases costs; higher Cr content is detrimental to toughness, increases welding difficulty, and, being a valuable alloying element, Cr content is controlled to 0.03–0.6% in this invention.

[0028] Cu has solid solution and precipitation strengthening effects. When its content is high, it exhibits a secondary hardening effect during tempering at appropriate temperatures, thereby increasing strength and facilitating strength retention during nitriding at high temperatures. Simultaneously, the addition of Cu also improves the bonding performance between steel and aluminum. However, excessive Cu content can cause cracks in the steel billet during heating and hot rolling, deteriorating surface properties. Therefore, this invention controls its content to be between 0.01% and 0.15%.

[0029] Mo can exist in steel in solid solution, resulting in solid solution strengthening. Simultaneously, Mo can increase the solubility of Nb, V, and Ti, promoting precipitation strengthening. In low-alloy high-strength steels, the strength of the steel increases significantly with increasing Mo content. In this invention, the addition of Mo improves the tempering resistance during nitriding, thus maintaining high strength after nitriding heat treatment. However, higher Mo content is detrimental to weldability and increases cost; therefore, this invention controls its content to 0.01–0.10%.

[0030] 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 in these areas. 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%.

[0031] In steel, nitrogen (N) can combine with aluminum (Al) to form AlN particles, thereby binding Al and limiting its diffusion in the steel. At the same time, similar to carbon (C), nitrogen tends to agglomerate at dislocations to form Cotillard atmospheres, which leads to strain concentration and causes uneven strain in the aluminum-clad material during processing, affecting surface quality. Moreover, excessive nitrogen also deteriorates impact toughness. Therefore, this invention controls its content to be 0.003 to 0.010%.

[0032] Oxygen (O) can suppress the adverse effects of Al in steel on the properties of aluminum cladding, so it is necessary to add an appropriate amount of O. However, excessive oxygen can cause defects such as subcutaneous bubbles and porosity, and exacerbate the hot brittleness caused by sulfur. 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, this invention controls its content to be 0.011-0.05%.

[0033] 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. This also reduces steel purity and worsens the toughness of the weld heat-affected zone. Therefore, this invention controls its content to be between 0.001% and 0.004%.

[0034] In addition to the elements mentioned above, this invention also requires the addition of one or more of the following: Ti: 0.01–0.06%, Nb: 0.01–0.03%, and V: 0.01–0.03%, to further improve room temperature strength and strength after nitriding. Wherein:

[0035] Ti has high chemical reactivity and readily forms compounds with C, N, O, S, etc., precipitating during rolling and cooling. These fine precipitates can pin grain boundaries, refine austenite grains, and simultaneously inhibit grain growth in the weld heat-affected zone, thus improving the weldability of the steel strip. Furthermore, Ti has a strong affinity for N, increasing surface hardness and nitriding depth, raising the nitriding temperature, and significantly shortening the nitriding time, making it an important additive element for rapid nitriding steel. 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, in this invention, the Ti content is controlled at 0.01–0.06%.

[0036] Nitrogen (Nb) is a strong nitrogen-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 strip. However, higher Nb content leads to the formation of coarse carbonitride particles at grain boundaries, deteriorating impact toughness. Therefore, in this invention, the Nb content is controlled between 0.01% and 0.03%.

[0037] V is a strong carbonitriding element that can precipitate during phase transformation. The V-formed carbonitride V4C3 has a lower precipitation temperature than the Ti-formed carbonitride, hindering dislocation movement, inhibiting grain boundary movement, and suppressing grain growth. V, along with C, N, and other microalloying elements in steel, exhibits excellent precipitation strengthening effects during medium- and low-temperature heat treatment. In particular, the precipitation temperature of V(C,N) generally does not exceed 700℃, and the lower the temperature, the finer the precipitation and the better the strengthening effect. The nitriding temperature is generally between 550-580℃, at which temperature a large amount of V carbonitrides precipitate, thus ensuring that the aluminum-clad steel strip retains good strength after nitriding. N preferentially forms BN and AlN with B and Al in steel. The addition of V forms VN with the remaining N in the steel, while simultaneously utilizing the precipitation strengthening of V's C and N compounds, thus ensuring high strength during nitriding. At the same time, the addition of V significantly increases the hardness and depth of the nitrided layer, and provides a good balance of strength and toughness, resulting in good impact resistance. High V content leads to the formation of coarse carbonitride particles, significantly deteriorating the impact toughness of the weld heat-affected zone. Therefore, this invention controls its content to be between 0.01% and 0.03%.

[0038] The steel strip used in the aluminum-coated sheet and strip of this invention has high strength, is suitable for nitriding, and has good steel-aluminum bonding performance to meet the needs of more application fields. It is required that the yield strength of the steel strip be ≥300MPa and the tensile strength be ≥380MPa.

[0039] To meet the aforementioned performance requirements, this invention rigorously 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) provides both solid solution strengthening and phase transformation strengthening. Existing aluminum-coated steel patents (such as Chinese patent CN114250411A) generally control C ≤ 0.005% to ensure plasticity, which significantly increases the difficulty of steelmaking. This invention, to ensure high strength after nitriding treatment and require a yield strength of over 300 MPa for the steel strip, uses 0.01–0.07% C. This serves two purposes: firstly, it provides solid solution strengthening; secondly, during strip steel production, controlled rolling and post-rolling cooling create a ferrite + small amount of bainite microstructure in the matrix, thereby improving strength.

[0040] Because the aluminum-clad steel strip undergoes annealing heat treatment during production, and the aluminum-clad steel strip involved in this invention also undergoes subsequent nitriding heat treatment, conventional grain refinement strengthening methods are not effective after heat treatment, while solid solution strengthening is not limited by heat treatment. Therefore, the higher the strength of the steel strip, the higher the strength of the finished aluminum-clad material. To address this, this invention adds 0.4–1.2% Mn, ensuring the steel strip strength through Mn's solid solution strengthening. Mo also produces a solid solution strengthening effect in the steel and promotes the precipitation strengthening effect of Nb, V, and Ti on the steel strip, and this strengthening effect persists even after heat treatment. Furthermore, Mo can also improve the strength after nitriding heat treatment.

[0041] In this invention, the phosphorus (P) content is controlled at 0.008–0.020%. This is due to both the solid solution strengthening effect of P and its impact on the steel-aluminum bonding performance and production costs. 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 makes the process more difficult. Since P belongs to the same group as nitrogen (N), appropriate P content is beneficial to the steel-aluminum bonding performance. However, research in this invention has found that its effect on improving the steel-aluminum bonding performance is not linear; the bonding performance begins to deteriorate when the P content exceeds 0.020%. Therefore, considering the difficulty and cost of steelmaking, as well as the performance requirements of this invention, the upper limit for P control is relaxed to 0.020%, which ensures both the steel-aluminum bonding performance and improves strength while reducing steelmaking costs.

[0042] The steel strip involved in this invention is required to have nitriding properties, and it is desirable that the steel strip easily forms a nitrided layer during the nitriding process. In addition to solid solution strengthening, Cr in steel also has a secondary strengthening effect. However, in this invention, adding 0.03-0.6% Cr not only improves strength but also enhances the nitriding performance of the steel. However, current nitriding steels such as 35MoCrAl and 42CrMo generally have a Cr content of over 3%. Cr is a precious alloying element, and such a high content obviously increases costs. Research in this invention shows that Cr can promote the diffusion of N in the matrix, rapidly forming a nitrided layer, thereby giving the steel strip good nitriding properties. The aluminum-clad steel strip of this invention also has an appropriate amount of N added to improve the steel-aluminum bonding performance. The N in the steel, combined with Cr, further promotes the formation of the nitrided layer and strengthens the steel strip, increasing its strength. Since the aluminum-clad substrate only requires a nitrided layer hardness of approximately 500 Hv and a nitrided layer thickness not exceeding 100 μm, adding 0.03-0.6% Cr to the steel strip achieves the required nitrided layer thickness and hardness. Higher Cr content would be redundant and would increase costs.

[0043] While Cu also exhibits solid solution strengthening in steel and can precipitate during heat treatment to enhance strength, this invention primarily utilizes the addition of Cu to further improve the steel-aluminum bonding performance. This is because the aluminum-clad steel sheet / strip described in this invention undergoes annealing and nitriding heat treatments during production and use. The high temperatures during these heat treatments easily promote the formation of brittle iron-aluminum compounds, deteriorating the steel-aluminum bonding performance. Therefore, the steel grade described in this invention has higher requirements for steel-aluminum bonding performance than existing aluminum-clad substrates. Adding an appropriate amount of Cu is beneficial for improving the steel-aluminum bonding performance and increasing the strength after nitriding treatment.

[0044] Based on the strengthening effect of the above elements on the matrix, it is required to control the content of C, P, N and Mn, Cr and Mo in the steel to meet the relationship 1: 150(C-0.01)+60P+300N+5Mn+3Cr+15Mo≥9.0. If the value of the relationship is lower than 9.0, the strength is likely to be low.

[0045] The formation of iron-aluminum compounds is highly temperature-sensitive. The high temperatures during nitriding provide a driving force for their formation, making them more likely to form and leading to iron-aluminum delamination. Therefore, the steel strip of this invention has higher requirements for steel-aluminum bonding performance. The key to this bonding performance lies in controlling the formation of brittle iron-aluminum compounds at the steel-aluminum interface in the aluminum-clad material. Research in this invention shows that the formation of iron-aluminum compounds is closely related to the diffusion of Al in steel. Controlling Al diffusion can inhibit the formation of iron-aluminum compounds within a certain temperature range. Therefore, the addition of C, Cr, and Cu has a certain improving effect on steel-aluminum bonding performance, while O (oxygen) and N can inhibit Al diffusion, reduce the formation of brittle iron-aluminum compounds, and thus improve steel-aluminum bonding performance. However, Si and Al, as deoxidizing elements, hinder the improving effect of O on steel-aluminum bonding performance, so their content should be limited.

[0046] This invention achieves excellent steel-aluminum bonding performance through the synergistic effect of multiple elements such as C, Cr, Cu, O, and N. Based on the relative strengths of the effects of C, N, O, Cu, Cr, Al, and Si on steel-aluminum bonding performance, their contents are required to satisfy the relationship 2: 0.5C + 6.86N + O + 0.05Cu + 0.047Cr - 0.89Al - 1.15Si ≥ 0.085. This ensures that the steel strip has excellent steel-aluminum bonding performance. If the value of this relationship is lower than 0.085, the aluminum-coated sheet prepared from this steel strip is prone to steel-aluminum delamination during subsequent nitriding treatment.

[0047] Overall, this invention adopts the composition system required above, comprehensively considering the steel-aluminum bonding performance, strength and nitriding characteristics. Through the combined effect of each element, a steel strip with aluminum-coated sheet and strip that meets the requirements can be obtained.

[0048] The method for producing the aluminized sheet and strip with excellent nitriding performance according to the present invention includes the following steps:

[0049] 1) Cleaning and polishing of the surfaces of aluminum strip and steel strip composite;

[0050] 2) Rolling

[0051] Aluminum strip and steel strip are laminated and rolled at room temperature to form aluminum-coated sheet and strip, with a single-pass rolling deformation of 50-85%.

[0052] 3) Annealing

[0053] The coil is annealed in a bell-type furnace. The annealing temperature is 480-530℃, the annealing holding time is t = 120 + 3H, where t is in min and H is the thickness of the aluminum-coated steel coil in cm. After annealing, the coil is water-cooled to below 200℃ and then cooled in the furnace to below 80℃ before being removed from the furnace.

[0054] or,

[0055] Continuous annealing and winding; wherein, the annealing temperature is 480~530℃, the continuous annealing time t0=(h+1)×1.5±1min, h is the thickness of the aluminum-clad sheet / strip in mm, h≥1mm; h<1mm, annealing time≥2min;

[0056] 4) Finishing.

[0057] Preferably, in step 1), the surface cleaning includes pickling, degreasing, and drying.

[0058] Preferably, in step 1), the surface cleaning includes sandblasting or shot blasting.

[0059] Preferably, in step 1), the grinding is performed using a sanding belt or a grinding wheel, and the grinding direction is parallel to the rolling direction.

[0060] Preferably, in step 2), the rolling is performed in one or two passes.

[0061] In the production process of the aluminum-coated sheet and strip described in this invention:

[0062] Surface cleaning of aluminum and steel strips includes necessary pickling, degreasing and drying, or direct sandblasting and shot blasting to remove oil, rust and oxide scale from the surface of the aluminum and steel strips.

[0063] Grinding is used to increase the roughness of the surfaces to be laminated and expose fresh base metal, thereby improving the mechanical bonding strength between steel and aluminum during the cladding rolling process. Grinding is preferably performed using abrasive belts or grinding wheels, which creates a rough surface, improving the mechanical interlocking of aluminum and steel during cladding rolling and resulting in better steel-aluminum bonding strength. Furthermore, grinding creates a microscopic work-hardened layer on the surfaces to be laminated. This hardened layer is broken up during the cladding rolling process, exposing fresh metal matrix and achieving point contact bonding between steel and aluminum, further enhancing the steel-aluminum bonding strength.

[0064] When rolling steel and aluminum at room temperature, the deformation per single pass should be controlled within the range of 50% to 85%. Depending on the requirements, steel and aluminum strips of different specifications are rolled into aluminum-clad sheets and strips of the desired thickness and structure in 1-2 passes. Lower deformation results in more rolling passes, requiring repeated feeding and threading, extending production time, reducing efficiency, and increasing costs. Furthermore, insufficient interfacial bonding strength due to excessively low deformation can lead to direct delamination of the steel and aluminum. While excessive deformation per pass increases rolling efficiency, it also results in high rolling loads and demanding equipment requirements. The choice between 1 or 2 passes depends on equipment capacity and finished product specifications.

[0065] After aluminum-coated rolling, the strip needs to be annealed. The purpose of annealing is twofold: firstly, to eliminate the work hardening generated during room temperature rolling and restore the plasticity of the aluminum-coated material, facilitating subsequent product forming and processing; secondly, to allow the atoms at the steel-aluminum interface to diffuse into each other during annealing, transitioning from a mechanical bond during rolling to a metallurgical bond, further improving the interfacial bonding strength.

[0066] Since the melting point of aluminum is around 640℃ (fluctuating depending on the content of other alloys in the aluminum), the corresponding recrystallization temperature is approximately 250℃. Generally, the annealing temperature is controlled to be at least 100-200℃ above the recrystallization temperature. Lower annealing temperatures will significantly increase annealing time, reduce production efficiency, and increase production costs; while excessively high annealing temperatures will increase energy consumption and production costs. Furthermore, higher temperatures provide the impetus for the formation of brittle phases at the steel-aluminum interface, reducing the bonding strength of the steel-aluminum interface. Therefore, this invention controls the annealing temperature to be between 480 and 530℃, specifically determined based on the size specifications and finished product performance of the aluminum-clad sheet / strip.

[0067] There are generally two annealing methods: conventional bell-type furnace annealing and continuous annealing. The annealing holding time t for aluminum-clad sheets and strips is determined comprehensively based on the thickness h (mm) of the aluminum-clad sheets and strips, the furnace temperature T (°C), and the annealing method.

[0068] When continuous annealing is used, the annealing time t0 = (h+1)×1.5±1min, where h≥1mm. If h<1mm, the annealing time shall not be less than 2min.

[0069] When using a bell-type furnace for annealing, the annealing time includes three stages: heating, holding, and cooling. The annealing time is closely related to the thickness of the aluminized sheet / strip, and for aluminized steel coils, it corresponds to the difference between the inner and outer radii of the coil. Generally, this is a linear relationship, but the specific ratio may vary depending on the actual situation. The annealing time t (min) during the holding stage is controlled based on the thickness H of the aluminized steel coil (the difference between its inner and outer radii), requiring the following condition: t = 120 + 3H, where H is in cm. Based on the bell-type furnace's inner diameter, the outer diameter of the aluminized coil is generally below 1800 mm, and the inner diameter is approximately 560 mm. Therefore, the holding time for the aluminized steel coil in the bell-type furnace should not exceed 306 min, approximately 5 hours. In existing technologies, when annealing aluminized coils in a bell-type furnace, the holding time is set based on the operator's experience, typically ranging from 15 to 18 hours. Additionally, heating requires 6 to 8 hours, and cooling requires approximately 6 hours.

[0070] This invention clarifies the linear relationship between annealing holding time and steel coil specifications, making it more operable. Compared with existing processes, the annealing process of this invention significantly shortens heat treatment time, reduces energy consumption, improves production efficiency, and lowers production costs.

[0071] Annealed aluminum-clad coils need to be finished, partly to improve the shape and trim the edges, and partly to further improve the surface quality through micro-pressure.

[0072] The present invention has the following advantages:

[0073] 1. Based on the characteristic that the solid solution strengthening and phase transformation strengthening effects are not affected by the subsequent annealing and nitriding heat treatment of the steel strip described in this invention, the present invention adopts a simple C-Mn design in terms of composition design, and controls the contents of C, P, N and Mn, Cr, Mo to satisfy the relationship: 150(C-0.01)+60P

[0074] +300N+5Mn+3Cr+15Mo≥9.0, achieving higher strength. The yield strength of the steel strip is ≥

[0075] 300MPa, tensile strength ≥380MPa, thus ensuring that the yield strength of the obtained aluminum-clad sheet / strip is ≥

[0076] 330MPa, tensile strength ≥400MPa; at the same time, the selective addition of Mo, Cu and Nb, V and Ti ensures high strength after heat treatment to meet the needs of more application fields.

[0077] 2. The steel described in this invention must meet the requirements for subsequent nitriding treatment; therefore, the steel-aluminum bonding performance is subject to higher requirements. In the composition design, based on the effects of C, N, O, Cu, Cr, Al, and Si on the steel-aluminum bonding performance, their contents are required to satisfy the following relationship: 0.5C + 6.86N + O + 0.05Cu + 0.047Cr

[0078] The -0.89Al-1.15Si ≥ 0.085, thus ensuring excellent steel-aluminum bonding performance of the steel strip. The resulting aluminized strip can withstand the subsequent high-temperature nitriding process without the formation of iron-aluminum compounds, and there will be no steel-aluminum delamination or strength reduction. Even after high-temperature nitriding, it retains high strength while maintaining good steel-aluminum bonding performance.

[0079] 3. In this invention, based on the influence of Cr on nitriding and considering the nitriding layer requirements of the steel strip used in aluminum-clad steel sheets, a small amount of Cr (0.03-0.6%) is added to promote the diffusion of N in the matrix and rapidly form a nitriding layer. This results in the steel strip possessing excellent nitriding properties, and the obtained aluminum-clad steel sheet retains high strength after nitriding treatment, meeting the requirements for subsequent processing and use. Compared to existing nitriding steels, this method saves on the use of expensive alloying elements, reduces costs, and achieves excellent nitriding properties.

[0080] 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. The aluminum-coated strip still has high steel-aluminum bonding performance after high-temperature nitriding treatment, while reducing the difficulty and cost of steelmaking and increasing production efficiency.

[0081] 5. This invention can employ either bell-type annealing or continuous annealing during the annealing stage, and establishes a linear relationship between annealing holding time and steel coil specifications. The annealing time is significantly shortened compared to the traditional bell-type annealing process, reducing energy consumption, improving production efficiency, lowering production costs, and enhancing operability. Attached Figure Description

[0082] Figure 1 This is a schematic diagram of the aluminum-coated sheet and strip of the present invention.

[0083] Figure 2 This is a schematic diagram of the aluminum-coated sheet and strip of the present invention.

[0084] Figure 3 This is a schematic diagram of the aluminum-coated sheet and strip of the present invention. Detailed Implementation

[0085] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0086] See Figure 1 , Figure 2 and Figure 3 The diagram shown is a structural schematic of an embodiment of the aluminum-coated strip of the present invention, wherein 1 is a steel strip and 2 is an aluminum strip.

[0087] According to the chemical composition requirements of steel strips for aluminum-clad sheets and strips involved in this invention, steel strips 1 and aluminum strips 2 of different specifications are selected, and after surface cleaning and grinding, aluminum-clad sheets and strips are obtained by aluminum-clad rolling and annealing. The finished aluminum-clad sheets and strips have a thickness of 0.5 to 2.0 mm.

[0088] The chemical composition of the steel strip in this embodiment of the invention is shown in Table 1, the specific production process parameters are shown in Table 2, and the performance of the finished product is shown in Table 3.

[0089] Although the steel strips used in Comparative Examples 1-4 also employed a lower Si-Al design and added certain amounts of O and N, their overall composition differed from that of the present invention. The resulting performance also differed significantly from that of the present invention, as detailed in Table 3.

[0090] The values ​​of Equation 2 and Equation 1 in Comparative Examples 1 and 2 are both lower than those in the embodiments of the present invention; although the value of Equation 1 in Comparative Example 3 meets the requirements of the present invention, the value of Equation 2 is lower than that in the embodiments of the present invention; the value of Equation 1 in Comparative Example 4 is lower than that in the embodiments of the present invention, and the value of Equation 1 barely meets the requirements of this application, but the P content in Comparative Example 4 is higher than that in the present invention.

[0091] As can be seen from the results in Table 3, the strength of Comparative Examples 1 and 2 is significantly lower than that of the present invention, and the aluminum coating performance is poor after simulated nitriding high-temperature heat treatment.

[0092] Although Comparative Example 3 has higher strength, iron-aluminum compounds are present on the steel-aluminum interface after simulated nitriding high-temperature heat treatment.

[0093] Comparative Example 4 has low strength, and iron-aluminum compounds also appeared on the steel-aluminum interface after simulated nitriding, which does not meet the requirements of this invention.

[0094] The aluminum-clad sheet and strip prepared using the components and process described in this invention exhibits a yield strength ≥330 MPa, tensile strength ≥400 MPa, and elongation A50 ≥18%; it also demonstrates excellent steel-aluminum interfacial bonding (no iron-aluminum compounds are formed). To verify the strength of the aluminum-clad material after high-temperature nitriding, the aluminum-clad sheet and strip were subjected to a simulated nitriding process at 550℃ for 4 hours, and its tensile properties were tested. The results are shown in Table 3, indicating that the strength of the sample decreased only slightly after heat treatment. This demonstrates that the finished aluminum-clad material retains high strength even after high-temperature heat treatment during subsequent processing.

[0095]

[0096]

[0097]

Claims

1. A high-strength, easily nitrided aluminum-coated sheet / strip, comprising a steel strip and an aluminum strip, wherein the steel strip has the following chemical composition by weight percentage: C: 0.021–0.07%, 0 < Si ≤ 0.005%, Mn: 0.4–1.2%, P: 0.008–0.020%, S ≤ 0.008%, 0 < Al ≤ 0.005%, Cr: 0.03–0.60%, N: 0.003–0.010%, O:

0. 0.011–0.05%, Cu: 0.01–0.15%, Mo: 0.01–0.10%, B: 0.0005–0.002%, Ca: 0.001–0.004%, and optionally add one or more of Ti: 0.01–0.06%, Nb: 0.01–0.03%, V: 0.01–0.03%, with the balance containing Fe and other unavoidable impurities; and the following conditions must be met simultaneously: 0.5C+6.86N+O+0.05Cu+0.047Cr-0.89Al-1.15Si≥0.085; 150(C-0.01)+60P+300N+5Mn+3Cr+15Mo≥9.

0.

2. The high-strength, easily nitrided aluminum-coated sheet and strip as described in claim 1, characterized in that, The balance consists of Fe and other unavoidable impurities.

3. The high-strength, easily nitrided aluminum-coated sheet and strip as described in claim 1 or 2, characterized in that, The steel strip has a yield strength ≥300MPa, a tensile strength ≥380MPa, and an elongation ≥25%.

4. The high-strength, easily nitrided aluminum-coated sheet and strip as described in claim 1 or 2, characterized in that, The aluminum-clad sheet / strip has a yield strength ≥330MPa, a tensile strength ≥400MPa, and an elongation A50 ≥18%.

5. The high-strength, easily nitrided aluminum-coated sheet and strip as described in claim 3, characterized in that, The aluminum-clad sheet / strip has a yield strength ≥330MPa, a tensile strength ≥400MPa, and an elongation A50 ≥18%.

6. The high-strength, easily nitrided aluminum-coated sheet and strip as described in claim 1, 2, or 5, characterized in that, The aluminum-clad sheet / strip has a steel-aluminum, aluminum-steel-aluminum, or steel-aluminum-steel structure.

7. The high-strength, easily nitrided aluminum-coated sheet and strip as described in claim 3, characterized in that, The aluminum-clad sheet / strip has a steel-aluminum, aluminum-steel-aluminum, or steel-aluminum-steel structure.

8. The high-strength, easily nitrided aluminum-coated sheet and strip as described in claim 4, characterized in that, The aluminum-clad sheet / strip has a steel-aluminum, aluminum-steel-aluminum, or steel-aluminum-steel structure.

9. The method for producing high-strength, easily nitrided aluminum-coated sheet and strip as described in any one of claims 1 to 8, characterized in that, Includes the following steps: 1) Cleaning and polishing of the surfaces of aluminum strip and steel strip composite; 2) Rolling Aluminum strip and steel strip are laminated and rolled at room temperature to form aluminum-coated sheet and strip, with a single-pass rolling deformation of 50-85%. 3) Annealing The coil is annealed in a bell-type furnace; the annealing temperature is 480-530℃, the annealing holding time is t=120+3H, t is in min, and H is the thickness of the aluminum-coated steel coil in cm; after annealing, it is water-cooled to below 200℃, and finally cooled in the furnace to below 80℃ before being taken out of the furnace. or, Continuous annealing and winding; wherein, the annealing temperature is 480~530℃, and the continuous annealing time t0=(h+1)×1.5±1min, where h is the thickness of the aluminum-clad sheet / strip in mm, and h≥1mm; h < 1 mm, annealing time ≥ 2 min; 4) Finishing.

10. The production method as described in claim 9, characterized in that, In step 1), the surface cleaning includes pickling, degreasing and drying.

11. The production method as described in claim 9, characterized in that, In step 1), the surface cleaning includes sandblasting and shot blasting.

12. The production method as described in claim 9, characterized in that, In step 1), the grinding is performed using a sanding belt or a grinding wheel, and the grinding direction is parallel to the rolling direction.

13. The production method as described in claim 9, characterized in that, In step 2), the rolling process is either a single-pass rolling process or a two-pass rolling process.

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