A steel with a yield strength of 300 MPa for use in aluminum-clad laminates and strips and its production method.

Abstract

A steel for aluminum-clad substrates with a yield strength of 300 MPa and its production method are disclosed. The steel's chemical composition by weight percentage 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 0.011–0.05%, Cu 0.01–0.15%, Mo 0.01–0.10%, B 0.0005–0.002%, Ca 0.001–0.004%, with selective addition of Ti 0.01–0.06%, Nb 0.01–0.03%, and 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 unavoidable impurities; and simultaneously satisfying the following conditions: 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. Its yield strength is ≥ 300MPa, tensile strength is ≥ 380MPa, and elongation is ≥ 25%. It possesses high strength, excellent plasticity, and aluminized properties. Aluminized sheets and strips produced using this material have excellent surface quality and high strength, meeting the requirements of subsequent high-temperature nitriding processes. It is mainly used in the production of aluminized sheets and strips for woks or other applications requiring high strength.

Description

Technical Field

[0001] This invention belongs to the field of low alloy steel manufacturing, specifically relating to a steel for aluminum-clad laminate substrate with a yield strength of 300MPa 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] The substrate for aluminum cladding not only requires excellent steel-aluminum bonding performance but also high surface finish and high strength. In the cookware industry, new woks with 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, while also being lighter. To extend service life, the material surface is typically nitrided to increase hardness. This requires the substrate to be able to withstand nitriding, forming a high-hardness nitrided layer on the surface, while maintaining high strength after high-temperature treatment.

[0004] With the development of aluminum-clad substrates and the application of aluminum-clad sheets and strips, numerous patents have been created for composite sheets and strips and the steel used in their substrates.

[0005] 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 1.5mm. Its yield strength is low and it does not meet the requirements of nitriding processing.

[0006] The substrate used in the production of aluminum-clad steel strip deforms together with the aluminum layer during the composite strip production 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 composition. Therefore, existing conventional low-strength steel is difficult to use in the production of aluminum-clad steel strip.

[0007] 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.

[0008] Chinese patent CN111349869A, which discloses "a high-strength steel for aluminum-clad substrates and its production method," yields a steel with a yield strength of 210-290 MPa, a tensile strength ≥320 MPa, and an elongation ≥40%, which is lower than the strength of this invention. While adding a higher phosphorus (P) content to suppress Al diffusion and improve the steel-aluminum bonding performance, the steel used in this invention still has an iron-aluminum compound layer with a thickness not exceeding 5 μm at the steel-aluminum interface. Furthermore, excessively high P content can lead to cold brittleness, which is detrimental to the forming and welding performance of the steel plate. Moreover, this patent does not mention nitriding treatment, thus lacking the characteristic of maintaining high strength after nitriding.

[0009] Chinese patent CN107881426A discloses "a high-strength aluminum-clad sheet and 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 280-400MPa. It is mainly used in the production of heat sinks, home appliance panels, etc. However, this application does not have nitriding characteristics, and the performance after high-temperature nitriding is unknown.

[0010] Chinese patent CN114250411A discloses a steel for aluminized sheet with substrate for kitchenware and its production method. While the steel involves a nitriding process, its yield strength and tensile strength do not exceed 280 MPa and 380 MPa, respectively. Furthermore, 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, without mentioning the nitriding adaptability of the substrate. Functionally, this steel is limited to the processing of aluminized materials for woks and is not suitable for the production of other aluminized materials with high strength requirements.

[0011] A comparison with existing patents reveals that the steel used in existing aluminum-clad laminates and strips either has low strength, poor steel-aluminum bonding performance, or does not meet the requirements for nitriding treatment. Summary of the Invention

[0012] The purpose of this invention is to provide a steel for aluminum-clad sheet substrate with a yield strength of 300MPa and its production method. The steel has a yield strength ≥300MPa, tensile strength ≥380MPa, and elongation ≥25%. It has excellent plasticity, aluminum cladding performance and high strength. The aluminum-clad sheet produced by it has excellent surface quality and high strength, and can meet the requirements of subsequent high-temperature nitriding treatment process. It is mainly used for the production of aluminum-clad sheets for woks or other products with high-temperature heat treatment and higher strength requirements.

[0013] To achieve the above objectives, the technical solution of the present invention is as follows:

[0014] A steel for use as a substrate in aluminum-clad laminates and strips with a yield strength of 300 MPa has the following chemical composition by weight percentage: 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: 0.011–0.05%, Cu: 0.01–0.15%, Mo: 0.01–0.10%, B: 0.0005–0.002%, Ca: 0.001–0.004%, and optionally one or more of Ti: 0.01–0.06%, Nb: 0.01–0.03%, and V: 0.01–0.03%, with the balance containing Fe and other unavoidable impurities; and simultaneously satisfying the following conditions:

[0015] 0.5C+6.86N+O+0.05Cu+0.047Cr-0.89Al-1.15Si≥0.085;

[0016] 150(C-0.01)+60P+300N+5Mn+3Cr+15Mo≥9.0.

[0017] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0018] 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%.

[0019] The steel used for the aluminum-clad sheet substrate of the present invention has a yield strength ≥300MPa, a tensile strength ≥380MPa, and an elongation ≥25%.

[0020] The steel used for the substrate of the aluminum-clad sheet and strip of the present invention with a yield strength of 300MPa is required to have a yield strength and tensile strength of 300MPa and 380MPa or higher, respectively, an elongation of 25% or higher, and good steel-aluminum bonding performance. The aluminum-clad material prepared with it still has high strength after high-temperature nitriding heat treatment. Therefore, it is necessary to limit the content of specific alloying elements as much as possible.

[0021] In the design of the chemical composition of the steel used as the substrate for the aluminum-clad laminate of this invention:

[0022] Carbon (C) strengthens the 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 0.01% and 0.07%.

[0023] 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%.

[0024] 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 1.20%.

[0025] 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 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% to 0.020% P.

[0026] 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%.

[0027] 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%.

[0028] 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 alloys 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; higher Cr content is detrimental to toughness, increases welding difficulty, and, being a valuable alloying element, its content is controlled to 0.03–0.6% in this invention.

[0029] 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 high-temperature nitriding. 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%.

[0030] Mo can exist in steel in solid solution form, 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 of the steel 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%.

[0031] 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%.

[0032] 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 0.003 to 0.010%.

[0033] Oxygen (O) can suppress the adverse effects of Al on the aluminum cladding properties of steel, 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 limits its content to 0.011-0.05%.

[0034] 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%.

[0035] 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%, and V: 0.01–0.03%, to further improve room temperature strength and strength after heat treatment. Wherein:

[0036] 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. However, higher Nb content can lead 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 to 0.01–0.03%.

[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 and 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, and simultaneously utilizes 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 of the nitrided layer, deepens the nitrided layer, and has a good combination 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, its composition is limited to 0.01–0.03%.

[0038] Ti has high chemical reactivity, readily forming compounds with C, N, O, and S, which precipitate during rolling and cooling to enhance strength. Simultaneously, these fine precipitates can pin grain boundaries, refining austenite grains and inhibiting grain growth in the weld heat-affected zone, thus improving the steel's weldability. Ti has a strong affinity for N, increasing surface hardness and nitriding depth, raising the nitriding temperature, and significantly shortening nitriding time, making it an important additive element for rapid nitriding steel. However, excessively high 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 must be controlled between 0.01% and 0.06%.

[0039] The steel used for the aluminum-clad steel strip substrate described in this invention requires high strength, suitability for nitriding, 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 ≥380MPa. 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 steel patents (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, uses 0.01–0.07% C. This serves two purposes: firstly, it provides solid solution strengthening; secondly, during strip production, controlled rolling and post-rolling cooling form a ferrite + small amount of bainite microstructure in the matrix, resulting in phase transformation strengthening and increased strength.

[0040] Because aluminum-clad laminates undergo annealing and subsequent nitriding heat treatments during production, conventional grain refinement strengthening methods are ineffective after heat treatment. Solid solution strengthening, however, is not limited by heat treatment; therefore, the higher the strength of the substrate steel, the higher the strength of the finished aluminum-clad laminate. To address this, this invention adds 0.4–1.2% Mn. Mn's solid solution strengthening ensures the strength of the substrate steel. Mo also provides solid solution strengthening in the steel and promotes the precipitation strengthening effect of Nb, V, and Ti on the matrix. This strengthening effect persists even after heat treatment. Furthermore, Mo can also increase the strength of the steel after heat treatment.

[0041] 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.

[0042] The steel of this invention requires nitriding properties, and it is desirable that the steel grade easily forms a nitrided layer during the nitriding process. Besides 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. 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 excellent nitriding properties. The steel of this invention also adds an appropriate amount of N to improve the steel-aluminum bonding performance. The N in the steel, in combination with Cr, further promotes the formation of the nitrided layer and strengthens the matrix steel, increasing the steel's strength. Since the steel used for aluminum-clad substrates only requires a nitriding layer hardness of about 500 Hv and a nitriding layer thickness of no more than 100 μm, adding 0.03 to 0.6% Cr to the steel used for aluminum-clad substrates can achieve the required nitriding 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 steel 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 described in this invention has higher requirements for steel-aluminum bonding performance than existing steels used in 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 steel, it is required to control the content of C, P, N and Mn, Cr and Mo in 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 the impetus for their formation, making them more likely to form and leading to iron-aluminum delamination. Therefore, the steel of this invention has higher requirements for steel-aluminum bonding performance than existing steels used in aluminum-clad substrates. 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 the 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] Therefore, 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 the steel-aluminum bonding performance, their contents are required to satisfy the relationship 2: 0.5C + 3.83N + O + 0.05Cu + 0.047Cr - 0.89Al - 1.15Si ≥ 0.085, thereby ensuring excellent steel-aluminum bonding performance. If the value of this relationship is lower than 0.085, the aluminum-coated sheet / strip prepared from this steel is prone to steel-aluminum delamination during subsequent nitriding treatment.

[0047] Overall, the present invention adopts the above-designed composition system, which comprehensively considers the steel-aluminum bonding performance, strength and nitriding characteristics. Through the combined effect of each element, the invention steel grade that meets the performance requirements can be obtained.

[0048] The method for producing steel for aluminum-clad laminates and strips with a yield strength of 300 MPa as described in this invention includes the following steps:

[0049] 1) Smelting

[0050] The above-mentioned components are processed through molten iron pretreatment, converter blowing, ladle refining, and casting into slabs;

[0051] 2) Reheating and controlled rolling

[0052] 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 870~910℃.

[0053] 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 800~860℃, and the finishing rolling end temperature is 710~770℃.

[0054] 3) Control the cooling rate to be above 20℃ / s;

[0055] 4) Winding, winding temperature 530~590℃.

[0056] Preferably, in step 1), the obtained slab is machine-cleaned.

[0057] Preferably, in step 2), the side pressure during the roughing stage of the rolling process is controlled to be within 50 mm.

[0058] In the production method described in this invention:

[0059] 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.

[0060] 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.

[0061] 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 1 As shown in the figure, the ferrite transformation begins at 864℃, and pearlite begins to form after the temperature drops to 709℃. The bainite transformation begins at 656℃, and when the cooling rate is above 2℃ / s, only ferrite and bainite form.

[0062] 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.

[0063] Based on the composition calculation of the steel grade according to this invention, the Ar3 temperature of the steel grade is 872℃, and the ferrite phase transformation initiation temperature is 864℃. Therefore, during the austenitic rolling process, it is required to control the finishing rolling end temperature above 870℃. 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 910℃. 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℃.

[0064] When the strip thickness is below 2.5mm, the strip's deformation resistance is low in the high-temperature austenitic region (870-910℃), making it prone to strip breakage, localized stretching, and lateral bending at the finishing mill exit. Therefore, this invention employs a ferritic rolling process. On one hand, the strip strength is higher at lower temperatures, making it less prone to deformation under the same coiling tension, 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 860℃, 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 800–860℃, and the ending temperature is set to 710–770℃. 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%.

[0065] 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 strength of the steel. Studies show that Cu precipitates form as early as 650℃, with the precipitates becoming smaller at lower temperatures; 500℃ exhibits the best precipitation strengthening effect, therefore the lower limit of the coiling temperature should not be lower than 500℃. Figure 2 The TTT curves show that 567℃ is the temperature at which bainite transformation is fastest. The steel matrix of this invention is mainly equiaxed ferrite. To obtain higher strength, a certain amount of bainite is desired in the matrix; therefore, the coiling temperature is controlled at around 560℃. The added Nb, V, and Ti carbonitrides also exhibit good precipitation strengthening effects near this temperature. Therefore, considering both precipitation strengthening and phase transformation strengthening effects, the coiling temperature is set at 530–590℃, and the post-rolling cooling rate is controlled to be ≥5℃ / s.

[0066] 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 2℃ / 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.

[0067] To achieve good strip surface quality, the lateral pressure during the roughing 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, or require surface cleaning.

[0068] To suppress abnormal ferrite grain growth in the strip after final rolling and ensure a uniform equiaxed ferrite + bainite microstructure, a pre-cooling method is required for post-rolling cooling. This means the strip 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, the bainite content must be ≤20%. Excessive bainite content will lead to insufficient plasticity and increase the difficulty of aluminum cladding rolling.

[0069] The present invention has the following advantages:

[0070] 1. Based on the characteristic that solid solution strengthening and phase transformation strengthening effects are not affected by subsequent annealing and nitriding heat treatment of steel plates, this 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 meet the relationship: 150(C-0.01)+60P+300N+5Mn+3Cr+15Mo≥9.0, thereby obtaining higher strength. The yield strength of the steel plate is ≥300MPa and the tensile strength is ≥380MPa. At the same time, the selective addition of Mo, Cu, Nb, V and Ti ensures high strength after nitriding heat treatment, so as to meet the use requirements of products such as woks or other products with high strength requirements.

[0071] 2. The steel described in this invention must meet the requirements of 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 meet the following relationship: 0.5C + 6.86N + O + 0.05Cu + 0.047Cr - 0.89Al - 1.15Si ≥ 0.085, inhibiting the formation of brittle iron-aluminum compounds at the steel-aluminum interface, thereby ensuring good steel-aluminum bonding performance of the steel plate. The finished aluminum-clad material exhibits a clean interface and no iron-aluminum compounds form during subsequent high-temperature nitriding treatment and processing.

[0072] 3. In this invention, based on the influence of Cr on nitriding and combined with the requirements of the nitriding layer of the substrate steel, a small amount of Cr (0.03-0.6%) is added to promote the diffusion of N in the matrix and quickly form a nitriding layer. This gives the substrate steel good nitriding characteristics, while improving the strength of the steel plate. The performance of the steel plate meets the requirements of subsequent processing and use. Compared with existing nitriding steels, it saves the use of expensive alloying elements, reduces costs, and obtains good nitriding characteristics.

[0073] 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.

[0074] 5. In terms of process control, this invention employs austenitic rolling and ferritic rolling processes respectively for different billet thicknesses obtained after rolling. This significantly improves the strip shape of thin-gauge steel strips while achieving high strength, 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.

[0075] 6. The process of this invention employs different controlled rolling processes, combined with a coiling temperature of 530–590℃ 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 resulting steel plate has good deformation capacity, meeting the deformation requirements of more than 80% for aluminum cladding rolling, as well as the deformation during subsequent stamping and deep drawing processes. No intermediate annealing is required, reducing production steps and improving production efficiency. Attached Figure Description

[0076] Figure 1 This is a schematic diagram of the static CCT curve (calculated) of the steel grade of this invention.

[0077] Figure 2 This is a schematic diagram of the static TTT curve (calculation) of the steel grade of this invention.

[0078] Figure 3 This is a photograph of the typical matrix structure of the steel in Example 1 of the present invention. Detailed Implementation

[0079] The invention will now be further described with reference to the accompanying drawings.

[0080] 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.

[0081] 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%.

[0082] A comparison was made with similar steel grades in terms of composition, production process, and performance. Among them,

[0083] 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.

[0084] 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.

[0085] Compared with steel 3, which is Chinese patent CN111349869A "a high-strength steel for aluminum-clad substrate and its production method", 0.018 to 0.03% P was added to the composition. An iron-aluminum compound layer with a thickness of no more than 5 μm appeared at the steel-aluminum interface, which reduced the bonding performance between steel and aluminum and also resulted in lower strength.

[0086] The comparison steel 4 is Chinese patent CN107881426A "A high-strength aluminum-coated sheet and strip and its manufacturing method", which does not take into account the nitriding characteristics and the strength reduction problem during nitriding.

[0087] The steel grade of this invention differs significantly in composition from the four comparative steels. To ensure good steel-aluminum bonding performance and high-temperature strength after nitriding treatment, the composition system of the steel grade of this invention has been specifically designed.

[0088] 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 sheet / strip, but the strength reduction during nitriding is not considered, so like comparative steel 3, high-temperature strength is not taken into account in its composition. Therefore, the comparative steels differ significantly from the steels of this invention in terms of composition.

[0089] As shown in Table 3, the steel of this invention possesses stable yield strength. Under different rolling processes, the yield and tensile strengths of various specifications of steel plates are all above 300 MPa and 380 MPa, respectively, and it also exhibits high elongation. Its composition and functionality are superior to comparative steels, resulting in aluminum-coated sheets and strips produced from this invention exhibiting excellent stamping and deep-drawing performance, making it suitable for processing components requiring high-temperature heat treatment such as nitriding and high strength.

[0090] 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, explicitly requires good aluminizing properties, making it suitable for the production of aluminized strip steel without intermediate annealing, which is something comparative steel 2 lacks. Although comparative steels 3 and 4 also require aluminizing properties, they are mainly used for the production of aluminized sheets and strips for radiator fins, appliance panels, etc., with an aluminum layer thickness not exceeding 0.1 mm. The steel grade of this invention has a wider range of applications, and comparative steels 3 and 4 do not meet the requirements for nitriding treatment. Therefore, the steel of this invention also differs significantly from the comparative steels in terms of performance.

[0091] In summary, the steel of this invention adopts a C-Mn design, supplemented with suitable C, N, V, O, and components such as Cu, Cr, and Mo, to create an aluminum-clad substrate with yield strength and tensile strength exceeding 300 MPa and 380 MPa respectively. This meets the requirements for high elongation, good aluminum cladding performance, strong deformation capacity, and suitability for nitriding treatment. It can also be used in the production of other aluminum-clad materials with high strength requirements. Furthermore, the production cycle is short, the process is simple, and the performance meets or exceeds the requirements of comparative steel grades.

[0092] 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.

[0093]

[0094]

[0095]

Claims

1. A steel for use as a substrate in aluminum-clad laminates and strips with a yield strength of 300 MPa, wherein the chemical composition by weight percentage is: 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: 0.011–0.05%, Cu: 0.01–0.15%, Mo: 0.01–0.10%, B: 0.0005–0.002%, Ca: 0.001–0.004%, and optionally adding one or more of Ti: 0.01–0.06%, Nb: 0.01–0.03%, and V: 0.01–0.03%, with the balance containing Fe and other unavoidable impurities; and simultaneously satisfying the following conditions: 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 steel for aluminum-clad laminates and strips with a yield strength of 300 MPa as described in claim 1, characterized in that, The balance consists of Fe and other unavoidable impurities.

3. The steel for aluminum-clad laminates and strips with a yield strength of 300 MPa 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 aluminum-clad laminates and strips with a yield strength of 300 MPa 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 ≥380MPa, and an elongation ≥25%.

5. The steel for aluminum-clad laminates and strips with a yield strength of 300 MPa 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 ≥380MPa, and an elongation ≥25%.

6. The method for producing steel for aluminum-clad laminates and strips with a yield strength of 300 MPa 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 870~910℃. 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 800~860℃, and the finishing rolling end temperature is 710~770℃. 3) Control the cooling process, ensuring a cooling rate of 20℃ / s or higher; 4) Winding, winding temperature 530~590℃.

7. The method for producing steel for aluminum-clad laminates and strips with a yield strength of 300 MPa as described in claim 6, characterized in that, In step 1), the obtained slab is machine-cleaned.

8. The method for producing steel for aluminum-clad laminates and strips with a yield strength of 300 MPa 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 method for producing steel for aluminum-clad laminates and strips with a yield strength of 300 MPa 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 method for producing steel for aluminum-clad laminates and strips with a yield strength of 300 MPa 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 ≥380MPa, and an elongation ≥25%.

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