390mpa grade hot-rolled strip for building structures resistant to corrosion in splash zone and method of manufacturing the same

Abstract

390MPa grade hot-rolled strip for building structure resisting splash zone corrosion of sea waves and its manufacturing method, the hot-rolled strip includes base layer, corrosion resistant layer and interface transition layer between the base layer and the corrosion resistant layer; the base layer component mass percentage is: C 0.03~0.10%, Si 0.1~0.3%, Mn 1.00~1.50%, P 0.0005~0.003%, S 0.0005~0.01%, Cr 0.02~0.15%, Ni 0.01~0.1%, Cu 0.002~0.020%, Al 0.015~0.03%, Ti 0.008~0.012%, Nb 0.02~0.045%, N 0.0005~0.005%, V 0.05~0.2%, the balance includes Fe and inevitable impurities; the corrosion resistant layer uses industrial pure titanium.The yield strength of the hot-rolled strip for building structure is greater than or equal to 390MPa, the tensile strength is greater than or equal to 515MPa, the yield strength ratio is less than or equal to 0.75, the impact energy at-40 DEG C is greater than or equal to 190J, the corrosion rate of sea wave splash is less than or equal to 0.006mm / year, the interface transition layer thickness is less than or equal to 8μm, and the interface shear strength is greater than or equal to 260MPa.

Description

Technical Field

[0001] This invention relates to the field of steel technology for building structures, and in particular to a hot-rolled strip steel for building structures that is resistant to corrosion from sea splash zones and its manufacturing method. Background Technology

[0002] The ocean is an extremely harsh and complex corrosive environment. Seawater is a strong electrolyte solution containing a high concentration of chloride ions. Steel structures, which form the main body of marine engineering facilities, are highly susceptible to electrochemical reactions with the surrounding medium, resulting in severe corrosion and significantly reducing their service life. Particularly in the splash zone, the most demanding corrosive area of ​​the marine environment, various facilities are subjected to alternating wet and dry conditions, seawater spray, sunlight, corrosive components in the atmosphere, and oxygen, leading to particularly severe corrosion of the materials.

[0003] Surveys show that steel piles for facilities such as seaports and offshore oil platforms in my country suffer from severe corrosion in this area, typically 3 to 10 times that of fully submerged seawater areas. Severe localized corrosion in this region can significantly reduce the load-bearing capacity of the entire facility, shorten its service life, impact production safety, and even lead to premature decommissioning.

[0004] In the splash zone, due to the alternating wet and dry conditions and ample oxygen supply, the resulting corrosion products offer no protection. Furthermore, the spray from the seawater can directly impact metal surfaces, causing severe corrosion. Corrosion tests and investigations indicate that, under normal conditions, the average corrosion rate for ordinary carbon steel and low-alloy steel in the marine atmosphere is approximately 0.03–0.08 mm / year, while in the splash zone it is 0.3–0.5 mm / year. This splash zone is highly susceptible to severe corrosion damage, significantly reducing the load-bearing capacity of the entire steel structure, impacting safe production, shortening its service life, and forcing premature scrapping.

[0005] Based on the aforementioned operating conditions, industrial pure titanium was selected as the corrosion-resistant layer. Titanium has high chemical reactivity and readily reacts with oxygen in the air to form oxides. The oxides on the surface of titanium metal are dense, stable, and possess strong self-healing capabilities. The self-healing ability of titanium oxides mainly refers to the ability of a new titanium oxide film to rapidly form after damage to a certain area of ​​the titanium surface, preventing further contact between corrosive media and titanium.

[0006] For steel used in marine structural engineering, in addition to meeting corrosion resistance requirements, good mechanical properties are also essential. Among these, yield strength ratio and low-temperature impact toughness are increasingly becoming key performance indicators for structural steel. The yield strength ratio is the ratio of a steel's yield strength to its tensile strength, reflecting its ability to prevent strain concentration during plastic deformation. A lower yield strength ratio allows for a more uniform distribution of plastic deformation over a wider range. Steel structures made from low yield strength ratio steel exhibit more uniform plastic deformation distribution under seismic forces; conversely, high yield strength ratio materials may experience strain concentration, reducing the overall plastic deformation capacity of the steel and leading to brittle failure, potentially causing structural deterioration and sudden collapse. Steel undergoes a brittle-ductile transition at low temperatures, changing its fracture mode from ductile to brittle. This is significant in engineering applications because it prevents brittle fracture of components when the steel is used at temperatures above this threshold. Therefore, structural steel typically requires specific low-temperature impact performance requirements based on its service environment. Ocean temperatures vary considerably across latitudes; for example, near the Bohai Bay in China, winter temperatures can drop below -20°C. This necessitates building materials with impact performance capable of reaching -40°C to prevent brittle fracture. If improving the tensile strength while simultaneously enhancing the ductility and toughness of a material results in a relatively small increase in the yield-to-tensile ratio, making it difficult to control a low yield-to-tensile ratio.

[0007] Chinese patent CN201210260231.7 discloses a method for preparing a titanium-steel-titanium double-sided composite plate. The method involves stacking four titanium plates and three steel plates in a specific order within a closed frame formed by welding the outermost two steel plates. A separating agent composed of 1 part by weight of active α-Al₂O₃ and 1.5 parts by weight of a 4% polyvinyl alcohol aqueous solution is added between the titanium plates. A nickel-based alloy is used as a transition layer between the titanium and steel plates. The plate is heated to 500℃~630℃ and then evacuated, held at this temperature for 1~2 hours, achieving a vacuum degree of 20~200Pa. Its key feature is... After the billets are assembled, they are welded first, and then a vacuum is drawn. Ordinary electric arc welding and submerged arc welding are sufficient for the welding conditions. Compared with welding under vacuum conditions, the welding conditions are less demanding, the cost is lower, and there is no need to build a separate vacuum chamber. Subsequently, the assembled billets are rolled and composited in a conventional heating furnace at a rolling temperature of 700℃~900℃. By sealing the outermost steel plate and drawing a vacuum, the carbon in the gas can be blocked. At the same time, a nickel-based alloy isolation layer is added to prevent the formation of TiC at the interface, resulting in a titanium-steel composite plate with a shear strength of 230~260MPa and an interface bonding rate of 99.6%~100%.

[0008] Chinese patent CN201710769999.X discloses a method for preparing a titanium-steel composite plate. The method involves selecting the surfaces of the titanium and steel billets that are in contact with each other, applying a high-temperature resistant, carburizing, and nitriding-resistant isolation coating to the titanium surfaces, and then drying them at room temperature. After drying, the titanium billets are stacked in pairs, with a steel billet placed in the middle to complete the assembly and obtain the composite billet. The titanium plate used has a thickness greater than 2 mm, and the steel plate has a thickness greater than 5 mm. The composite billet is then sealed around its perimeter, leaving a certain size of unwelded area. The billet is then vacuumed to a depth of 100°C. -2 ~10 -3 After Pa, welding is performed; the slab is heated to 500-700℃ and rolled, with the first pass reduction exceeding 25% and the last pass reduction not exceeding 15%, for a total reduction of 60-70%, and a rolling speed of 0.1-1.0 mm / s. This patent utilizes a coating that provides high-temperature impermeability protection, preventing the diffusion and oxidation of other impurity elements at high temperatures, and blocking the diffusion of elements such as C and N. In its embodiment, Q235 is combined with TA1, producing steel plates with shear strengths reaching 176 MPa, 181 MPa, and 182 MPa.

[0009] The two patents above mainly avoid the formation of brittle Ti compounds by adding an additional nickel-based alloy isolation layer between titanium and carbon steel.

[0010] Chinese patent CN201811327623.4 discloses a titanium-steel-titanium composite plate and its preparation method. The method involves fixing a carbon steel sheet between two titanium plates of the same size and performing a hot composite rolling process using an irreversible high-pressure hot rolling mill to fuse the three layers of strip into a single unit. After rolling, the composite plate undergoes heat treatment, including an initial annealing at 500–600°C for 20–60 minutes and a recrystallization annealing at 680–700°C for 30–120 minutes. Finally, the product is obtained through straightening, leveling, shearing, and shaping. This patent mainly describes a method for preparing a non-hot-rolled composite titanium-steel plate. Due to the use of an irreversible rolling mill, only single-pass rolling production is possible, and heat treatment is also required. The embodiments mainly involve the steel strip production method, and the performance of the composite is not mentioned.

[0011] Chinese patent CN201510543767.3 discloses a method for preparing titanium-steel composite plates. The titanium-steel composite plates obtained by this method have high bonding strength. This patent involves fixing a titanium plate between two ordinary carbon steel plates or steel billets, welding the billet around its perimeter in a vacuum environment, heating the combined billet to 850-900℃ for 120-360 minutes, controlling the initial rolling temperature above 800℃ and the final rolling temperature below 700℃, and controlling the deformation per pass to 20-30%, with a total rolling deformation ≥90%. A large reduction is performed during rolling to break up the brittle phase compounds generated at the interface and reduce their impact on the bonding surface. A titanium-steel composite plate with a bonding strength greater than 240MPa is obtained. However, this patent requires a very high reduction per pass and a high total deformation, which can easily cause cracking of the edge welds during the rolling process, compromise the vacuum level, and hinder interface bonding.

[0012] Chinese patent CN201610994234.1 discloses a production method for titanium-steel composite plates, involving an annealing technology production method for titanium-steel plates. First, titanium plates and steel plates are assembled into a symmetrical multi-layer composite billet consisting of steel plate-titanium plate-release agent-titanium plate-steel plate. The composite is then performed through rolling or explosive bonding. The composite billet is annealed and pickled using a continuous annealing and pickling line. It is first heated to 500–750°C to recrystallize the core titanium plate, and then heated to 950–1050°C to recrystallize the base steel plate. The patent aims to simultaneously obtain the properties of both the composite and the base material through two-stage heat treatment. However, this two-stage heat treatment can lead to excessive diffusion of titanium, iron, and carbon elements, resulting in brittleness from intermetallic compounds of iron and titanium and titanium carbide, thus deteriorating the interfacial shear strength.

[0013] Chinese patent CN201710996925.X discloses a thin-layered double-sided titanium-steel composite plate and its preparation method. Through thick-walled billet assembly and high-reduction rolling technology, a good composite between titanium and steel is achieved. This patent describes a double-sided titanium composite plate composed of a titanium cladding layer, a base layer, and another titanium cladding layer. The titanium cladding layer is made of TA2 material, with a thickness of 0.2–1 mm. The billets are stacked in the following order from top to bottom: cover plate, titanium composite material, carbon steel base material, titanium composite material, and cover plate again. After vacuum extraction in a vacuum chamber, the gaps around the plate are vacuum-sealed with electron beam welding at a vacuum level of 1.0 × 10⁻⁶. -2 ~4.5×10 -2Pa, the composite billet after sealing and welding is heated to 900-920℃ and held for 1 min / mm × total thickness of the composite billet. The initial rolling temperature is 880-900℃, the final rolling temperature is above 800℃, and it is air-cooled to room temperature. The single-pass reduction rate is ≥15%, and the reduction rate of the first three passes is ≥20%, with a total reduction rate of ≥80%. The composite plate obtained after rolling is trimmed, separated, and surface-ground to obtain a double-sided titanium-steel composite plate. This patent uses surface cleaning treatment of the composite billet, the air isolation effect of the cover plate, control of rolling temperature, and the use of large reduction to break down and refine the titanium iron and titanium carbon compounds generated at the composite interface, and disperse them in the composite interface, improving the distribution state of the compounds and further ensuring the composite quality and performance stability. The shear strength reaches 241 MPa.

[0014] Chinese patent CN201710983322.6 discloses a thin-layered titanium-steel composite plate and its preparation method. It employs a double-layered structure of titanium and carbon steel. The billet assembly method and heating process are similar to those of Chinese patent CN201710996925.X. The initial rolling temperature is 880–900℃, the single-pass reduction rate is 25–30%, and the total reduction rate is ≥85%. While controlling the single-pass and total reduction rates, the thickness of the titanium-steel composite plate is limited to 3–16 mm. The final rolling temperature is above 800℃, followed by air cooling to room temperature. The titanium-steel composite plate is obtained through surface treatment, with a titanium cladding thickness ≤1 mm. This patent improves the composite quality by using a symmetrical billet assembly method and welding titanium into the carbon steel plate. After rolling, the shear strength of the steel plate reaches above 238 MPa, and the composite interface bonding rate is 100%. The carbon steel layer meets the national standard requirements for Q345 grade carbon steel.

[0015] The two patents mentioned above do not mention the detailed design of the cladding and the base layer, but only describe the tensile properties and shear strength. The required reduction rate per pass and the total reduction rate for the cladding are too high, and there is no control over the corrosion resistance of the material, the low-temperature impact performance of the base material, the yield strength ratio and other performance indicators, which do not meet the requirements for steel for building structures.

[0016] In summary, the above patents mainly describe the preparation method of composite steel plates. The specific embodiments mainly provide a brief description of the interfacial shear strength and tensile properties in terms of performance. Steel used in steel structures in the wave splash zone must not only be resistant to corrosion in the wave splash zone, but also ensure that it has the necessary structural steel performance requirements, such as the aforementioned low yield strength ratio and corresponding low temperature impact performance to ensure structural safety. However, the above patents have not made relevant component and process designs for corrosion rate, yield strength ratio, and low temperature impact of the corrosion-resistant layer, and cannot guarantee that they can meet the requirements for high corrosion resistance steel plates for steel structures in the wave splash zone environment. Summary of the Invention

[0017] The purpose of this invention is to provide a hot-rolled strip steel for building steel structures with a 390MPa grade resistant to corrosion from sea splash zones and its manufacturing method. Without reducing the corrosion resistance of the resistant layer itself, the mechanical properties of its base layer (carbon steel) also meet the corresponding strength level requirements, and the base layer possesses excellent yield strength ratio and low-temperature impact toughness. The hot-rolled strip steel for building structures has a yield strength ≥390MPa, tensile strength ≥515MPa, yield strength ratio ≤0.75, impact energy at -40℃ ≥190J, sea splash corrosion rate ≤0.006mm / year, interface transition layer thickness ≤8μm, and interface shear strength ≥260MPa. It can meet the corrosion resistance requirements in sea splash zone environments, possesses good mechanical properties resistant to sea splash corrosion, and is highly economical. It is applicable to steel structural components such as steel piles for facilities such as harbor terminals and offshore oil platforms.

[0018] To achieve the above objectives, the technical solution of the present invention is as follows:

[0019] This invention employs a low-carbon microalloying composition design. By reducing carbon content, it decreases the formation of TiC compounds in the interfacial transition layer and the formation of carbonitrides in the base layer, thus hindering grain growth and improving the low-temperature impact toughness of the base layer. Simultaneously, the addition of microalloying elements, combined with the rolling cooling process, solves the problem of low material strength under low-carbon conditions. Furthermore, without adding a metal isolation layer, it achieves an excellent combination of titanium and carbon steel. At the same time, by controlling the thickness of the interfacial transition layer, the mechanical properties of the base layer (carbon steel) can meet the corresponding strength level requirements without reducing the corrosion resistance of the corrosion-resistant layer itself. Moreover, the base layer possesses excellent yield strength ratio and low-temperature impact toughness.

[0020] Specifically, the 390MPa grade hot-rolled strip steel for building structures resistant to sea wave splash corrosion described in this invention includes a base layer, a corrosion-resistant layer, and an interface transition layer between the base layer and the corrosion-resistant layer.

[0021] The chemical composition of the base layer, by mass percentage, is as follows: C 0.03–0.10%, Si 0.1–0.3%, Mn 1.00–1.50%, P 0.0005–0.003%, S 0.0005–0.01%, Cr 0.02–0.15%, Ni 0.01–0.1%, Cu 0.002–0.020%, Al 0.015–0.03%, Ti 0.008–0.012%, Nb 0.02–0.045%, N 0.0005–0.005%, V 0.05–0.2%, with the balance including Fe and other unavoidable impurities.

[0022] The corrosion-resistant layer is made of industrial pure titanium;

[0023] The hot-rolled strip steel used in the building structure has a yield strength ≥390MPa, tensile strength ≥515MPa, yield-to-yield ratio ≤0.75, impact energy at -40℃ ≥190J, sea wave splash corrosion rate ≤0.006mm / year, interface transition layer thickness ≤8μm, and interface shear strength ≥260MPa.

[0024] Preferably, the chemical composition of the base layer also satisfies the following relationship:

[0025] 0.02% ≤ Cu + Ni ≤ 0.10%;

[0026] 2(C+N)≤Ti+Nb+Cr+V≤0.35%.

[0027] Furthermore, the remainder of the base layer components is Fe and other unavoidable impurities.

[0028] The microstructure of the base layer described in this invention is ferrite + bainite + martensite, with the bainite + martensite content being 5-15% and the average grain size of ferrite being ≥8.5.

[0029] The base layer of the present invention has a yield strength ≥390MPa, a tensile strength ≥515MPa, a yield-to-tensile ratio ≤0.75, and an impact energy ≥190J at -40℃.

[0030] Preferably, the corrosion-resistant layer is TA1, TA2, TA3 or TA4.

[0031] The microstructure of the corrosion-resistant layer is a single, equiaxed α-Ti.

[0032] The corrosion resistance rate of the corrosion-resistant layer is ≤0.006 mm / year.

[0033] Preferably, the interface transition layer achieves 100% metallurgical bonding, has a coherent atomic height, a thickness ≤8μm, and an interface shear strength ≥260MPa.

[0034] Preferably, the interfacial transition layer has fine grains with an average grain size of 15–40 μm and contains (Ti, Nb) C precipitates smaller than 120 nm.

[0035] Preferably, the thickness of the hot-rolled strip steel used in the building structure is 1.0 to 20 mm.

[0036] In the base composition design of the hot-rolled strip steel for building steel structures described in this invention:

[0037] Carbon (C) plays a role in solid solution strengthening in steel, significantly improving its strength. However, excessively high C content is detrimental to weldability and toughness. More importantly, high C content can diffuse to the composite interface, forming a large number of large-particle TiC hard phases in the interface transition layer, reducing the composite interface strength. To ensure the shear strength of the interface, this invention uses a low C content. The effect of C content variation on the yield strength of steel is less than its effect on tensile strength. Under the premise of ensuring product formability and weldability, appropriately increasing the C content is beneficial to reducing the yield strength ratio of the steel. Based on this, the C content in the base layer composition described in this invention is controlled at 0.03–0.10%.

[0038] Si: Adding Si to steel can effectively deoxidize and improve the purity of the steel. In addition, Si can play a solid solution strengthening role in steel, improving its strength and hardness; however, Si is detrimental to the weldability of the material. Therefore, in the base layer composition described in this invention, the Si content is controlled at 0.10–0.30%.

[0039] Mn: Mn is the cheapest matrix strengthening element. It can lower the austenite transformation temperature, delay the pearlite transformation, refine ferrite grains, and improve the strength of steel. Simultaneously, Mn can also eliminate the influence of sulfur (S) on steel. However, excessively high Mn content can easily lead to segregation bands and martensitic structures, which are detrimental to the toughness of the steel. Therefore, in the matrix composition described in this invention, the Mn content is controlled at 1.0% to 1.5%.

[0040] Al: Al is mainly added in excess to steel as a de-oxidizing element to ensure that the O content in the steel is as low as possible. After de-oxidation, the excess Al combines with the N element in the steel to form AlN precipitates. During heating, AlN hinders the growth of austenite grains, refines the austenite grains, and improves the strength and toughness of the matrix. At the same time, the formation of AlN fixes some of the N in the matrix, reducing the diffusion of interstitial N atoms from the carbon steel base layer to the composite interface, forming hard TiN in the interface transition layer, which deteriorates the interfacial shear strength of the composite plate. It can also reduce the amount of Ti and Nb added, reducing the overall cost. Based on this, the Al content in the base layer composition of the present invention is controlled at 0.015-0.03%.

[0041] Ti: At high temperatures, Ti forms stable TiN or Ti(N,C), which acts to solidify C and N, preventing interstitial C and N atoms in the carbon steel base layer from diffusing to the interface. Hard TiN or Ti(N,C) precipitates in the interface transition layer, resulting in a composite plate with high interfacial shear strength. Simultaneously, during heating, TiN hinders austenite growth, refining austenite grains and improving matrix strength and toughness. In subsequent welding, especially in the heat-affected zone (HAZ) close to the weld melt boundary, austenite grain growth is suppressed, thereby improving the toughness of the weld HAZ and meeting the needs of high welding heat input processes. To improve the strength of the low-carbon matrix and reduce the diffusion of C and N to the interface, a composite plate with high interfacial shear strength is obtained. Based on this, the Ti content in the base layer composition of this invention is controlled at 0.008–0.012%.

[0042] Nb exists in steel in the form of solid-solution Nb and Nb(C,N), playing a role in solid-solution dragging and precipitation pinning during recrystallization. Adding a small amount of Nb to the base carbon steel is mainly to increase the recrystallization temperature, resulting in grain refinement after rolling in both the recrystallized and non-recrystallized regions, which is beneficial for improving the low-temperature impact toughness of the base carbon steel. Due to the effect of the Nb(C,N) precipitates, the original austenite grains become finer, thereby promoting the formation of even finer recrystallized grains and achieving an ideal combination of high strength and high toughness. Simultaneously, Nb can fix interstitial C and N atoms in the matrix, reducing the diffusion of C and N to the interface, resulting in a composite plate with high interfacial shear strength. Based on this, the Nb content in the base layer described in this invention is controlled at 0.02–0.045%.

[0043] Cu: Cu plays a role in solid solution strengthening, and as the Cu content increases, the room temperature impact toughness of the steel increases slightly. Therefore, in the base layer composition described in this invention, the Cu content is controlled at 0.002 to 0.02%.

[0044] Nitrogen (N): Nitrogen can react with Ti and Al to form second-phase particles, refining austenite grains and improving the strength and toughness of the matrix. However, when the N content is too high, the generated TiN content is excessive and the particles are too coarse, which will affect the plasticity and toughness of the carbon steel base layer of this invention. Based on this, the N content in the base layer of this invention is controlled at 0.0005% to 0.005%.

[0045] Ni: Ni is an element that stabilizes austenite and plays a role in improving toughness and strength. Adding Ni to steel can significantly improve its low-temperature impact toughness. However, nickel is expensive, and excessive addition will increase the cost of composite plates. Based on this, an appropriate amount of Ni is added to the base layer composition described in this invention, and the Ni content is controlled to be between 0.01% and 0.10%.

[0046] Cr: Cr is a strong carbide-forming element with a low diffusion rate in austenite, while also hindering the diffusion of C. During low-temperature processes, it forms fine carbides, playing a role in precipitation strengthening. Simultaneously, it can fix interstitial C and N atoms in the matrix, reducing the diffusion of C and N to the interface, resulting in a composite plate with high interfacial shear strength. While Cr increases the matrix strength in steel, it reduces toughness. To achieve the optimal balance between strength and toughness, the Cr content in the base layer composition described in this invention is controlled at 0.02–0.15%.

[0047] V: V is a strong carbonitride forming element. When added to steel in combination with Ti and Nb, it can form fine, complex carbonitrides with a wider precipitation temperature range. This effectively inhibits austenite grain growth and recrystallization, improving the strength and toughness of the base carbon steel. Simultaneously, V carbonitrides have a relatively low precipitation temperature, effectively inhibiting ferrite grain growth during phase transformation and strengthening the ferrite matrix. Therefore, an appropriate amount of V is added to the base steel composition described in this invention, with the V content controlled between 0.05% and 0.20%.

[0048] S and P are both unavoidable impurity elements, and their content should be as low as possible. Considering the actual steelmaking level of steel plants, the content of S and P in this invention is controlled as follows: S≤0.010%; P≤0.003%.

[0049] The corrosion-resistant layer of the present invention is made of industrial pure titanium, preferably TA1, TA2, TA3 and TA4, and its composition meets the GB / T3620.1-2016 standard "Titanium and Titanium Alloy Grades and Chemical Composition".

[0050] Furthermore, in the design of the base chemical composition of the hot-rolled strip steel for building steel structures described in this invention:

[0051] Since both Cu and Ni can improve the toughness of the substrate, and the combined addition effect is particularly significant, while the addition of Ni can reduce the diffusion rate of C in steel and reduce the diffusion of C to the interface, this invention controls 0.02% ≤ Cu + Ni ≤ 0.10%, and can control the interface transition layer to within 8 μm.

[0052] Since Ti, Nb, Cr, and V are all strong carbonitride-forming elements, they will form corresponding carbonitrides in the base carbon steel. This can fix interstitial atoms in the base layer and prevent C and N interstitial atoms from diffusing to the interface to form large, aggregated carbonitride particles in the interface transition layer. This can control the interface transition layer to within 8 μm, thereby improving the interfacial shear strength. Simultaneously, Ti, Nb, and Cr can refine the grains of the base carbon steel and improve its toughness at different stages of hot rolling. Therefore, this invention controls: 2(C+N)≤Ti+Nb+Cr+V≤0.35%.

[0053] The manufacturing method of the 390MPa grade hot-rolled strip steel for building structures resistant to sea wave splash corrosion according to the present invention includes the following steps:

[0054] 1) Smelting and casting

[0055] The base layer and corrosion-resistant layer are smelted and cast into billets according to their respective compositions;

[0056] 2) Billet assembly

[0057] The base layer and corrosion-resistant layer blanks are ground and polished, and the bonding surfaces of the blanks are welded and sealed around the perimeter to form a composite blank; the bonding surfaces after welding and sealing are vacuumed.

[0058] 3) Heating

[0059] The composite preform is heated to 900–1000°C;

[0060] 4) Rolling

[0061] The roughing temperature is controlled above 860℃; the finishing temperature is controlled between 780℃ and 850℃, the reduction rate per pass is controlled between 5% and 20%, and the cumulative reduction rate is ≥85%.

[0062] 5) Cooling

[0063] In the first stage, after the steel strip exits the rolling mill stand, it is cooled to 700-750℃ at a cooling rate of ≤10℃ / s.

[0064] The temperature is ℃, and then in the second stage, it is cooled to 400-500℃ at a cooling rate of 30-50℃ / s before winding.

[0065] Preferably, in step 1), the thickness of the corrosion-resistant layer is 0.5% to 20% of the total thickness of the composite blank.

[0066] Preferably, the reduction rate used in step 4) rolling is 10-15%.

[0067] In the manufacturing method described in this invention:

[0068] 1) Smelting: P and S elements can worsen the fracture toughness of steel, so low P and low S control is necessary during the smelting process to improve billet quality. Adopting clean steel production technology reduces the content of gases and inclusions in steel, improving the overall performance of the steel, especially its resistance to lamellar tearing.

[0069] 2) Billet Assembly: The thickness of the corrosion-resistant layer should be 0.5% to 20% of the total thickness of the composite billet. Pre-treat the carbon steel billets for the corrosion-resistant layer and base layer, and seal the bonding surfaces of the billets with welding around the perimeter. Vacuum treatment is then applied to the welded and sealed surfaces. Vacuum treatment protects the surface of the corrosion-resistant layer from oxidation, which is crucial for ensuring its resistance to corrosion from the splash zone. It also prevents edge cracking and roll-out of the corrosion-resistant layer during subsequent large cumulative reduction deformation.

[0070] 3) Heating: For single carbon steel, the slab heating temperature is generally controlled between 1000 and 1250℃, which is conducive to the dissolution and full diffusion of precipitates in the steel, promotes the homogenization of elements in the slab, and gives full play to the strengthening effect of microalloying elements in the steel. For single industrial pure titanium plates, the heating temperature is generally controlled between 850 and 1000℃. Excessive heating temperature will produce β phase transformation, and the β phase will grow rapidly, deteriorating the performance of industrial pure titanium. This is because excessive heating temperature will also allow elements to diffuse fully, promoting the subsequent achievement of 100% metallurgical bonding at the interface. However, higher heating temperature will increase the tendency of austenite grains to coarsen, increasing the difficulty of subsequent controlled rolling. Most importantly, it will accelerate the diffusion of C, N, Ti, and Fe to the interface, forming thicker brittle precipitates and intermetallic compounds at the interface, forming a thick interface transition layer, which deteriorates the interface shear strength. Preferably, a lower heating temperature than that used in traditional carbon steel production is adopted, with the heating temperature set between 900 and 1000℃.

[0071] 4) Rolling: The roughing temperature is controlled above 860℃. A large reduction rate is applied in the high-temperature roughing zone to allow for sufficient recrystallization, refining the grains and improving the material's strength and toughness. Controlled rolling is then performed in the non-recrystallized finishing zone, where austenite recrystallization no longer occurs. By using a reasonable reduction rate and final rolling temperature, deformation energy and deformation dislocations are accumulated, forming high-density deformation bands within the austenite grains. This increases the deformation nucleation points of the ferrite phase, further refining the grain size after the matrix phase transformation and improving the material's strength and toughness. Simultaneously, deformation induces the precipitation of carbonitrides of Nb, Ti, Cr, and V during this stage, increasing the matrix strength, inhibiting C diffusion to the interface, and preventing the formation of excessively thick TiC at the interface, which would degrade the interfacial shear strength. The reduction rate per pass is guaranteed to be 5-20%, and the cumulative reduction rate is ≥85%. Preferably, the finishing rolling temperature is controlled at 780-850℃ to ensure the corrosion resistance of titanium while avoiding rolling the base layer in the two-phase region, and to obtain ferrite with an average grain size of ≥8.5 and bainite + martensite with a content of 5-15%.

[0072] 5) Cooling: By controlling the initial cooling, final cooling, and cooling rate, the type, size, and content of various components of the post-rolled microstructure can be controlled. Excessive cooling rate will form a large amount of bainite and martensite. Since martensite has low toughness and a high yield strength ratio, it is detrimental to the performance of the steel plate. Excessive cooling rate will lead to the formation of a large amount of coarse ferrite. Coarse microstructure promotes crack propagation and reduces impact performance. Therefore, the cooling rate should be reasonably controlled. Controlling the final rolling temperature can prevent abnormally coarse microstructure from forming during rolling in the two-phase region. Simultaneously, rapid cooling to the phase transformation temperature after rolling further inhibits microstructure growth, improves material strength and low-temperature impact toughness by refining the grains, and forms a hard phase microstructure of 5-15% or more to ensure strength.

[0073] Preferably, a two-stage cooling method is adopted. After exiting the mill stand, the temperature is first cooled to the ferrite phase transformation temperature at a cooling rate of ≤10℃ / s, and then rapidly cooled to 400-500℃ at a cooling rate of 30-50℃ / s for coiling to obtain a small amount of bainite and martensite structure. During the coiling process, the structure is restored and the precipitation of V carbonitrides is promoted, ensuring that the base layer has a low yield strength ratio and high low-temperature impact toughness.

[0074] Preferably, if the corrosion-resistant layer is too thick, it will affect the mechanical properties of the material and the production cost; while if the corrosion-resistant layer is too thin, it will reduce the corrosion resistance and service life of the material. Therefore, the ratio of the corrosion-resistant layer to the total thickness of the composite billet in the above-mentioned billet assembly process is preferably 0.5% to 20%.

[0075] This invention combines a corrosion-resistant layer with a base layer, and through composition design and thickness ratio design, forms a corrosion-resistant layer on the surface of the base layer, i.e., carbon strip steel, which is resistant to corrosion in the splash zone of sea waves through a rolling process. The final product is a strip steel that combines resistance to corrosion in the splash zone of sea waves, good mechanical properties, and high economic efficiency. The strip steel can then be processed into structural components that can be effectively used in steel structural components used in the splash zone of sea waves.

[0076] Compared with the prior art, the present invention has the following beneficial effects:

[0077] This invention employs a low-carbon micro-alloying composition design, achieving an excellent combination of titanium and carbon steel without adding a metal isolation layer. At the same time, it controls the thickness of the interface transition layer, ensuring that the mechanical properties of the base layer (carbon steel) meet the corresponding strength level requirements without reducing the corrosion resistance of the corrosion-resistant layer itself. Furthermore, the base layer possesses excellent yield strength ratio and low-temperature impact toughness.

[0078] This invention reduces the formation of TiC compounds in the interfacial transition layer and the formation of carbonitrides in the substrate by reducing carbon content, thereby hindering grain growth and improving the low-temperature impact toughness of the substrate. At the same time, the addition of microalloying elements solves the problem of low material strength under low carbon conditions. The yield strength is ≥390MPa, the tensile strength is ≥515MPa, and the yield-to-tensile ratio is ≤0.75. The impact energy at -40℃ is ≥190J, all of which are higher than the performance requirements in the national standard GB / T 19879-2015 "Steel Plates for Building Structures".

[0079] Chinese patent CN201210260231.7 does not specify the heating temperature, but by adding a nickel plate as an isolation layer between the cladding and the base layer to prevent the formation of TiC at the interface, the resulting titanium-steel composite plate has an interface bonding rate of 99.6%–100%. In contrast, this invention specifies a composite billet heating temperature of 900–1000℃. Through low-carbon and microalloying design, the addition of Ni is reduced, lowering production costs. Furthermore, through optimized processes such as heating and rolling, a transition layer of a certain thickness can be formed, reducing the formation of brittle TiC phase in the interface transition layer, thus achieving a 100% metallurgical bonding rate.

[0080] Chinese patent CN201710769999.X describes a slab heating temperature of 500–700℃ and a total reduction rate of 60–70%, producing strips with a maximum interfacial shear strength of 182 MPa. This invention, however, fully considers the impact of the high-temperature phase transformation of industrial pure titanium in the corrosion-resistant layer on corrosion resistance, as well as the strength and toughness control of the base carbon steel. Combined with low-carbon microalloying design and a global processing technology design, the heating temperature of the composite billet is set at 900–1000℃. At this temperature, the corrosion-resistant layer does not undergo phase transformation, and the precipitates in the base carbon steel are fully dissolved. During controlled rolling, this refines the base grains, improving the base strength and toughness. Combined with a cumulative reduction rate ≥85%, the brittle phases in the interfacial transition layer are broken up, increasing the interfacial shear strength.

[0081] The two patents mentioned above mainly avoid the formation of brittle Ti compounds by adding an additional nickel-based alloy isolation layer between titanium and carbon steel. However, this invention does not add an isolation layer through composition and process design. Furthermore, it differs from the two patents in its billet assembly method and the material of the base carbon steel.

[0082] Under the process conditions of this invention, both the original corrosion resistance of industrial pure titanium in the corrosion-resistant layer and the mechanical properties of the base layer are guaranteed, solving the problem that the processing windows of traditional titanium and carbon steel are too different and cannot be balanced. At the same time, the formation of an interface transition layer with a diameter of no more than 10 μm is controlled to fully diffuse the elements between the base layer and the corrosion-resistant layer. This layer has fine grains with an average grain size of 15-40 μm and contains (Ti, Nb) C precipitates smaller than 120 nm, which strengthens the interfacial bonding performance and ensures an interfacial shear strength ≥260 MPa, which is higher than the interfacial shear strength of 182 MPa in Chinese patent CN201710769999.X.

[0083] Chinese patent CN201811327623.4 describes a non-hot-rolled composite titanium strip steel preparation method that achieves composite bonding through warm rolling, followed by two-stage heat treatment operations, including a primary annealing at 500-600℃ for 20-60 minutes and a recrystallization annealing at 680-700℃ for 30-120 minutes. This method is completely different from the manufacturing method of this invention.

[0084] Chinese patent CN201510543767.3 specifies a heating temperature of 850–900℃, a final rolling temperature below 700℃, and controls the deformation per pass to 20–30%, with a total rolling deformation ≥90%, and a shear strength greater than 240MPa for the titanium-steel composite plate. This patent requires very high reduction per pass and total deformation, which easily leads to edge weld cracking during rolling, disrupting the vacuum, hindering composite bonding, and resulting in poor rolling stability. The present invention controls the single-pass reduction rate to 5–20%, effectively preventing weld cracking during rolling, ensuring the vacuum level inside the slab, and improving interfacial shear strength, rolling stability, and success rate.

[0085] Chinese patent CN201610994234.1 discloses a production method for annealing titanium steel plates. First, titanium and steel plates are assembled into a symmetrical multi-layer composite billet consisting of a steel plate, a titanium plate, a release agent, and another titanium plate. This composite is then formed through rolling or explosive bonding. The resulting billet is annealed and pickled using a continuous annealing and pickling line. The initial heating to 500–750°C causes recrystallization of the core titanium plate, followed by heating to 950–1050°C to recrystallize the base steel plate. The rolling process and the corrosion and structural properties of the resulting steel plate are not specifically described. This invention differs significantly from the present invention in its manufacturing process. The present invention does not require two-stage heat treatment, and two-stage heat treatment can lead to excessive diffusion of titanium, iron, and carbon elements, resulting in brittle intermetallic compounds of iron and titanium and titanium carbide, which deteriorates the interfacial shear strength.

[0086] The corrosion-resistant layer material of Chinese patent CN201710996925.X is TA2, with a titanium cladding thickness of 0.2-1mm. It is heated to 900-920℃ and held at that temperature, with an initial rolling temperature of 880-900℃ and a final rolling temperature above 800℃, followed by air cooling to room temperature, achieving a shear strength of 241MPa. This invention uses a heating temperature of 900-1000℃, a finishing rolling temperature of 750-850℃, and a two-stage cooling method. This process can produce hot-rolled plates with TA1, TA2, TA3, and TA4 as corrosion-resistant layers, accounting for 0.5-20% of the total thickness of the composite billet, and with an interfacial shear strength ≥260MPa.

[0087] The billet assembly method and heating process of Chinese Patent CN201710983322.6 are similar to those of Chinese Patent CN201710996925.X, with a single-pass reduction rate of 25-30% and a total reduction rate ≥85%. While controlling the single-pass and total reduction rates, the thickness of the titanium-steel composite plate is limited to 3-16 mm, the final rolling temperature is above 800℃, air-cooled to room temperature, and surface-treated to obtain the titanium-steel composite plate, with a titanium cladding thickness ≤1 mm. This invention uses a single-pass reduction rate of 5-20%, controls rolling stability, and employs a two-stage cooling method to ensure the material's microstructure, thereby controlling the yield strength ratio and toughness. There are also significant differences in the thickness of the corrosion-resistant layer and the total thickness of the composite strip.

[0088] The 390MPa-grade hot-rolled strip steel for building structures resistant to sea splash corrosion described in this invention can solve the inherent pain points of stainless steel or carbon steel used in sea splash environments. This 390MPa-grade hot-rolled strip steel for building structures resistant to sea splash corrosion can be effectively applied to the manufacture of steel structural components used in sea splash environments, such as steel structural components of facilities like seaports and offshore oil platforms. It can meet the requirements of these components for sea splash corrosion resistance and mechanical properties, greatly improving the applicability, safety, and durability of these components, and has significant economic and social benefits. Attached Figure Description

[0089] Figure 1 This is a schematic diagram of an interlayer structure of the hot-rolled strip steel for building structures with a 390MPa grade resistance to corrosion in the splash zone of ocean waves, as described in this invention.

[0090] Figure 2 This is a schematic diagram of another interlayer structure for the hot-rolled strip steel for building structures with a 390MPa grade resistant to corrosion in the splash zone of ocean waves, as described in this invention.

[0091] Figure 3 This is a microstructure photograph of the corrosion-resistant layer in Embodiment 3 of the present invention.

[0092] Figure 4This is a scanned image of the interface transition layer where the base layer and the corrosion-resistant layer are combined, as shown in Embodiment 3 of the present invention.

[0093] Figure 5 This is a photograph of the basic microstructure of Embodiment 3 of the present invention. Detailed Implementation

[0094] The technical solution of the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. It should be understood that the following embodiments are only used to describe specific implementations of the present invention and do not constitute any limitation on the scope of protection of the present invention.

[0095] See Figure 1 , Figure 2 , Figure 4 The diagram shows two interlayer structures of hot-rolled strip steel for building structures according to the present invention, wherein 1 is the base layer, 2 is the corrosion-resistant layer, and 3 is the interface transition layer.

[0096] The composition of the base layer of the hot-rolled strip steel (composite steel plate) for building structures described in this invention is shown in Table 1, with the balance being Fe and unavoidable impurities. Table 2 shows the manufacturing process parameters of the composite steel plate embodiments of this invention. Table 3 shows the metallographic structure and mechanical properties of the base layer and corrosion-resistant layer, and the thickness of the interface transition layer in the composite steel plates of the embodiments and comparative examples.

[0097] The yield strength and tensile strength of the composite steel plate were measured in accordance with GB / T 6396-2008 "Mechanical and Technological Properties of Composite Steel Plates" and GB / T 228-2010 "Metallic Materials - Tensile Testing at Room Temperature".

[0098] The impact energy KV2 / J (longitudinal) of the base carbon steel at -40℃ was measured in accordance with GB / T 6396-2008 "Mechanical and Technological Properties of Composite Steel Plates" and GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials".

[0099] Grain size rating is performed as follows: according to GB / T 6394-2017 "Method for determination of average grain size of metals", the intercept point method is used to rate the grain size of ferrite structure in stainless steel and carbon steel respectively.

[0100] The comparative examples were prepared using essentially the same steps as those in the embodiments of the present invention, except that the composition of the base carbon steel and certain process parameters used during the rolling or cooling steps did not meet the requirements of the present invention.

[0101] The metallographic structure of the corrosion-resistant layer in Example 3 is shown below. Figure 3 The image shows a single, equiaxed α-Ti with an average grain size of 103.4 μm.

[0102] See the interface transition layer in Example 3. Figure 4 The thickness of the interface transition layer 3 is 7.4 μm, in which the discontinuous fine particles are TiC with a size of less than 120 nm.

[0103] See the base metallographic structure of Example 3. Figure 5 The microstructure of the base carbon steel is ferrite + bainite + martensite, with a bainite + martensite volume fraction of 7.8% and a ferrite grain size of ≥8.5.

[0104] Table 4 shows the corrosion of the composite steel plate samples from Examples 1-8 and Comparative Examples 1-4 in the South China Sea splash zone after 6 months. The observations show that, except for Comparative Example 4, the corrosion rates of the other examples and comparative examples are all ≤0.006 mm / year.

[0105] Comparative Examples 1-4, due to the use of unsuitable composition design requirements and heat treatment process conditions, resulted in certain properties of the composite steel plates failing to meet usage requirements (performance parameters are not within the scope defined by the invention). Among them:

[0106] Comparative Example 1, due to its chemical composition being free of Ti, Nb, and Cr, and the rolling reduction rate being outside the range defined in this invention, cannot meet the requirements for yield strength, yield ratio, impact performance, and interfacial shear strength.

[0107] Comparative Example 2 does not meet the requirements for yield strength ratio and impact performance because its chemical composition does not contain Ni, its finishing rolling temperature and second-stage cooling temperature are not within the range specified in this invention.

[0108] Comparative Example 3, due to the addition of less than 0.02% Cu+Ni in its chemical composition and the fact that the first and second stage cooling temperatures were not within the range specified in this invention, had a metallographic structure with a high content of bainite and martensite, and its mechanical properties, yield strength ratio, and impact performance could not meet the requirements.

[0109] In Comparative Example 4, the heating temperature, finishing rolling temperature, and first-stage cooling temperature were not within the ranges defined in this invention. Therefore, the thickness of the interface transition layer was too thick, resulting in the shear strength not meeting the performance requirements. Due to the excessively high heating temperature, β-Ti could not be completely eliminated during subsequent processing and cooling, leading to a high corrosion rate.

[0110] Through the preparation method of the present invention, especially the control of heating, rolling and cooling processes, the base layer in the strip steel exhibits a low yield strength ratio and good low-temperature impact toughness, while the coating has excellent corrosion resistance and high bonding strength. Its yield strength is 391-481 MPa, tensile strength is 551-668 MPa, yield strength ratio is ≤0.75, impact energy at -40℃ is above 190 J, and interfacial shear strength is greater than 260 MPa.

[0111] It should be noted that all technical features described in this invention can be freely combined or integrated in any manner, unless they contradict each other. Various modifications and variations can be made to this invention without departing from its scope, as will be apparent to those skilled in the art. For example, features shown or described as part of one embodiment can be used with another embodiment to produce yet another embodiment. Therefore, this invention is intended to cover these modifications that fall within the scope of the appended claims and their equivalents.

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Claims

1. 390MPa grade hot-rolled strip steel for building structures resistant to corrosion in the splash zone of sea waves, including base layer, corrosion-resistant layer and interface transition layer between base layer and corrosion-resistant layer; The chemical composition of the base layer, by mass percentage, is as follows: C 0.03~0.10%, Si 0.1~0.3%, Mn 1.00~1.50%, P 0.0005~0.003%, S 0.0005~0.01%, Cr 0.02~0.15%, Ni 0.01~0.1%, Cu 0.002~0.020%, Al 0.015~0.03%, Ti 0.008~0.012%, Nb 0.02~0.045%, N 0.0005~0.005%, V 0.05~0.2%, with the balance being Fe and other unavoidable impurities. The corrosion-resistant layer is made of industrial pure titanium; The hot-rolled strip steel used in the building structure has a yield strength ≥390MPa, tensile strength ≥515MPa, yield-to-tensile ratio ≤0.75, impact energy at -40℃ ≥190J, sea wave splash corrosion rate ≤0.006mm / year, interface transition layer thickness ≤8μm, and interface shear strength ≥260MPa.

2. The hot-rolled strip steel for building structures as described in claim 1, characterized in that, The chemical composition of the base layer also satisfies the following relationship: 0.02%≤Cu+Ni≤0.10%; 2(C+N)≤Ti+Nb+Cr+V≤0.35%.

3. The hot-rolled strip steel for building structures as described in claim 1, characterized in that, The industrial pure titanium is TA1, TA2, TA3 or TA4.

4. The hot-rolled strip steel for building structures as described in claim 1 or 2, characterized in that, The microstructure of the base layer is ferrite + bainite + martensite, with bainite + martensite content of 5%~15% and average ferrite grain size ≥8.

5.

5. The hot-rolled strip steel for building structures as described in claim 1 or 2, characterized in that, The base layer has a yield strength ≥390MPa, tensile strength ≥515MPa, yield-to-tensile ratio ≤0.75, and impact energy at -40℃ ≥190J.

6. The hot-rolled strip steel for building structures as described in claim 4, characterized in that, The base layer has a yield strength ≥390MPa, tensile strength ≥515MPa, yield-to-tensile ratio ≤0.75, and impact energy at -40℃ ≥190J.

7. The hot-rolled strip steel for building structures as described in claim 1, characterized in that, The microstructure of the corrosion-resistant layer is a single, equiaxed α-Ti.

8. The hot-rolled strip steel for building structures as described in claim 1 or 7, characterized in that, The corrosion resistance rate of the corrosion-resistant layer is ≤0.006 mm / year.

9. The hot-rolled strip steel for building structures as described in claim 1, characterized in that, The interface transition layer achieves 100% metallurgical bonding, with atomically coherent structure. The thickness of the interface transition layer is ≤8μm. The layer has fine grains with an average grain size of 15~40μm and contains (Ti, Nb) C precipitates smaller than 120nm. The interfacial shear strength is ≥260MPa.

10. The hot-rolled strip steel for building structures as described in claim 1, characterized in that, The thickness of the hot-rolled strip steel used in the building structure is 1.0~20mm.

11. The method for manufacturing 390MPa grade hot-rolled strip steel for building structures resistant to seawater splash corrosion as described in any one of claims 1 to 10, characterized in that, Includes the following steps: 1) Smelting and casting The base layer and corrosion-resistant layer as described in claim 1 or 2 are smelted and cast into billets respectively; 2) Billet assembly The base layer and corrosion-resistant layer blanks are ground and polished, and the bonding surfaces of the blanks are welded and sealed around the perimeter to form a composite blank; the bonding surfaces after welding and sealing are vacuumed. 3) Heating The composite preform is heated to 900~1000℃; 4) Rolling The roughing rolling temperature is controlled above 860℃; the finishing rolling temperature is controlled between 780 and 850℃; the reduction rate per pass is controlled between 5% and 20%; and the cumulative reduction rate is ≥85%. 5) Cooling In the first stage, the steel strip is cooled to 700~750℃ at a cooling rate of ≤10℃ / s after exiting the rolling mill stand. Then, in the second stage, it is cooled to 400~500℃ at a cooling rate of 30~50℃ / s before being coiled.

12. The manufacturing method as described in claim 11, characterized in that, Step 1) The thickness of the corrosion-resistant layer is 0.5~20% of the thickness of the composite blank.

13. The manufacturing method as described in claim 11, characterized in that, In step 4), the pass reduction rate is 10-15%.

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