High-strength hot-rolled strip steel with high weather resistance and method for producing the same

Abstract

The present invention provides a hot-rolled strip steel and a manufacturing method thereof. This hot-rolled strip steel has the following components, and the mass percentages of the components are: C is 0.04 to 0.15%, Si is 0.50% or less, Mn is 0.30 to 2.00%, Cr is 1.5 to 4.5%, Cu is 0.10 to 0.60%, P is 0.03% or less, S is 0.01% or less, and Al is 0.01 to 0.60%. The balance is Fe and inevitable impurities, and further: 2.5% ≤ 2Mn + Cr ≤ 6.0% is satisfied, and the elemental symbols are calculated by replacing with the mass percentages of the corresponding elements in the hot-rolled strip steel. The hot-rolled strip steel of the present invention exhibits excellent atmospheric corrosion resistance and mechanical properties, and can be directly used for supporting bracket structural parts such as guard rails, mast towers, and solar power generation equipment without applying a coating on the surface.

Description

Technical Field

[0001] The present invention relates to the technical field of low alloy steels, and particularly to high-strength hot-rolled strip steel with high weather resistance and a manufacturing method thereof.

Background Art

[0002] Currently, weathering steels are widely used in the production of outdoor steel structures such as containers, railway vehicles, and bridges that require high atmospheric corrosion resistance. In recent years, with the increasing requirements for green, low-carbon, and environmentally friendly production, the application scenarios of atmospheric corrosion-resistant steels have also expanded. By utilizing the weather resistance of atmospheric corrosion-resistant steels, structural profiles used in atmospheric environments such as guardrails, mast towers, support brackets, and solar power generation brackets are produced. The surfaces of these steel structures can be either directly exposed or lightly coated on the surface before use, and since very high weather resistance can be achieved, it is possible to replace some of the traditional steel surface corrosion protection processes such as pre-galvanizing, zinc-aluminum-magnesium, and post-galvanizing. The use of atmospheric corrosion-resistant steels can not only reduce the energy consumption and pollution caused by the metal coating process, but also improve the service life of steel structures and reduce the later corrosion maintenance costs.

[0003] Solutions involving high-strength weathering steel have been provided in the prior art. For example, Chinese Patent CN202011384068.6 discloses a low-alloy structural steel with high strength and high weather resistance for highway guardrails, and its yield strength is approximately 500 MPa. The main design concept is to increase the content of P element to 0.07 - 0.12% and increase the content of Cr element to 0.30 - 1.25%, so as to form a layer rich in P and Cr on the surface of the rust layer, making the rust layer stable and dense and improving the weather resistance of the structural steel. This steel has a fine structure of ferrite and pearlite, and the volume content of pearlite is 5% - 25%. However, in the case of structural steel, P is an impurity element in the steel. When P is contained in excess, central segregation and grain boundary segregation will occur, affecting the formability and toughness of the steel and not contributing to the processing performance and service safety of the steel.

[0004] Chinese Patent CN202010116991.5 discloses a high-strength weathering steel mainly used for railway vehicles. It also aims to provide a high-strength weathering steel that solves the problem that prior art steels cannot achieve high strength and high weather resistance. This steel mainly has the following components: C: 0.06 - 0.07%, Si: 0.23 - 0.26%, Mn: 1.40 - 1.50%, Ni: 0.0 - 0.19%, Cr: 0.0 - 0.51%, Cu: 0.31 - 0.33%, Ti: 0.110 - 0.12%, Nb: 0.030 - 0.036%, Sb: 0.0 - 0.09%. Such steel adopts a nearly complete ferrite structure, and the content of pearlite in the steel is only 2% or less. The yield strength of this steel is 636 - 710 MPa, and the tensile strength is 698 - 775 MPa. This invention achieves high strength through the combined precipitation strengthening of Nb and Ti. However, in this solution, since the Cr content in the steel is as low as 0.51% or less, there is a drawback that the weather resistance of the steel remains at the level of ordinary weathering steel. Also, in this invention, although it mentions using Sb to improve corrosion resistance, Sb is a harmful element in the steel and may deteriorate the properties of the steel, especially the low-temperature toughness. On the other hand, since Sb has a low melting point, it is difficult to control the yield after smelting. Therefore, it is difficult to actually apply this solution.

[0005] Chinese Patent CN201810154871.7 discloses a high-strength weathering steel with a yield strength of 550 MPa grade. This steel is strengthened with 0.05 - 0.09 wt% of Ti and contains 0.30 - 0.60 wt% of Cr. CN202110398903.X discloses a 700 MPa grade high-strength weathering steel plate resistant to atmospheric corrosion. This steel plate is also strengthened with 0.100% - 0.140% of Ti and contains 0.60% - 1.00% of Cr. However, these patent inventions aim to obtain high-strength weathering steel, and the weather resistance of the products is still based on general designs.

[0006] Chinese Patent CN200910180491.1 discloses a high-strength low-alloy hot-rolled ferritic / bainitic weathering steel with a yield strength of 450 MPa grade. This steel mainly contains 1.00 - 1.50% manganese, 0.50 - 0.70% chromium, 0.20 - 0.30% nickel, 0.20 - 0.40% copper, 0.01 - 0.025% titanium and 0.03 - 0.05% niobium. Such steel has a uniform acicular structure of ferrite + bainite, so weathering steel with high strength, low yield-tensile ratio, high weather resistance and excellent low-temperature toughness can be produced at low cost. However, since a one-step cooling from 550 to 590 °C after rolling is adopted in its manufacturing process, as a result, the obtained structure is acicular ferrite + bainite, and the weather resistance remains at an ordinary level.

Summary of the Invention

[0007] The object of the present invention is to provide a hot-rolled high-strength steel strip with high weather resistance and a manufacturing method thereof. The hot-rolled steel strip of the present invention has a corrosion rate of 30% or less of Q355B ordinary structural steel, and the weather resistance of the hot-rolled steel strip of the present invention is more than twice higher than that of ordinary weathering steel (such as Q450NQR1 steel). The steel strip of the present invention has the following characteristics: the corrosion rate rapidly decays over time, and the corrosion depth during the 25-year service cycle in a general service environment is 0.1 mm or less, and it has high strength and good formability, with a yield strength of 550 MPa or more, a tensile strength of 650 MPa or more, an elongation rate of 20% or more, and qualified cold bending performance at 180° and D = 1t. Therefore, the hot-rolled steel strip of the present invention can achieve the processing of profile components with complex cross-sections, can replace corrosion protection by post-galvanizing, and can be directly used to support bracket structure components such as guardrails, mast towers, and solar power generation equipment without applying a coating on the surface.

[0008] The present invention achieves the above technical objectives by optimizing the chemical composition of hot-rolled strip steel. Specifically, in the present invention, a high Cr content of 1.5% to 4.5% is adopted in the composition design of hot-rolled strip steel to promote the formation of a uniform and dense rust layer on the surface of the hot-rolled strip steel during use. Cr can rapidly accumulate in the thin rust layer, and the concentration at the interface between the rust layer and the substrate is 12% or more. Therefore, by significantly increasing the corrosion potential and electrochemical impedance and blocking the continuous occurrence of corrosion, extremely high atmospheric corrosion resistance can be achieved (the hot-rolled strip steel of the present invention is used in an environment with a corrosion level of C1 to C3 specified in GB / T 19292.1-2018). On the other hand, by utilizing the composition design of C, Mn, and high Cr, this hot-rolled strip steel has a multiphase structure of polygonal ferrite + MA-containing bainite. Furthermore, the steel of the present invention exhibits high strength and high formability by utilizing mechanisms such as the high plasticity of ferrite and the strengthening effect of the bainite structure.

[0009] Specifically, the above hot-rolled strip steel has the following components, and the mass percentage of the components is: C is 0.04 to 0.15%, Si is 0.50% or less, Mn is 0.30 to 2.00%, Cr is 1.5 to 4.5%, Cu is 0.10 to 0.60%, P is 0.03% or less, S is 0.01%, and Al: 0.01 to 0.60%. The balance is Fe and inevitable impurities. At the same time, this hot-rolled strip steel satisfies 2.5% ≤ 2Mn + Cr ≤ 6.0%. The element symbols are replaced with the mass percentages of the corresponding elements in the hot-rolled strip steel for calculation. For example, when the Mn content in the steel is 0.10%, the numerical value 0.10% is substituted for calculation.

[0010] Preferably, this hot-rolled strip steel further has Ni, and the composition of this hot-rolled strip steel satisfies Ni ≤ 0.40% and Si + 2Ni ≥ 0.10%, and the influence of Cu embrittlement can be reduced. The element symbols are replaced with the mass percentages of the corresponding elements in the hot-rolled strip steel for calculation.

[0011] Preferably, the hot-rolled strip steel of the present invention further has at least one selected from the following components, and the components are: Ti of 0.15% or less, Nb of 0.06% or less, V of 0.15% or less, Mo of 0.40% or less, and B of 0.002% or less, and the strength can be further improved.

[0012] Preferably, the above composition further has at least one selected from the following components, and the components are: Sb of 0.15% or less, Re of 0.15% or less, Ca of 0.015% or less, and Mg of 0.015% or less. Thereby, the corrosion resistance is further improved.

[0013] Preferably, the hot-rolled strip steel of the present invention has a microstructure that is a multiphase structure of polygonal ferrite + MA-containing bainite. The ferrite has a grain size of grade 9 or more, preferably grade 11 or more. In the microstructure, the content of polygonal ferrite is 40 vol% to 70 vol%, the content of bainite is 20 vol% to 60 vol%, preferably 30 vol% to 60 vol%, and the bainite structure contains fine granular MA accounting for 30% or more of the bainite structure, preferably 30 vol% to 50 vol%. Furthermore, the content of pearlite or carbide(s) in the microstructure is 15 vol% or less. Unless otherwise specified, the content of the steel microstructure in the present invention means the volume fraction.

[0014] Preferably, the hot-rolled strip steel of the present invention has a yield strength of 550 MPa or more, preferably 650 MPa or more, a tensile strength of 650 MPa or more, preferably 800 MPa or more, a yield-tensile ratio of 0.85 or less, preferably 0.80 or less, an elongation at break of 20% or more, preferably 23% or more, the cold bending performance meets the qualification when bent at 180° and D = 1t (D is the bending diameter and t is the thickness of the steel plate), and the impact energy at -40 °C is 80 J or more, preferably 95 J or more.

[0015] Preferably, the hot-rolled strip steel of the present invention exhibits excellent weather resistance, with a corrosion rate of 30% or less of that of Q355B steel, that is, its weather resistance is more than three times that of Q355B steel and more than twice that of ordinary weather-resistant steel (such as Q450NQR1 steel). Furthermore, the corrosion rate of the hot-rolled strip steel of the present invention rapidly decreases as the corrosion depth increases, and this hot-rolled strip steel has a corrosion depth of 0.1 mm or less after 25 years as shown by the simulated corrosion test results.

[0016] The hot-rolled strip steel of the present invention has high strength and good formability, with a yield strength of 550 MPa or more, a tensile strength of 650 MPa or more, an elongation at break of 20% or more, and has cold bending performance that meets the requirements when bent at 180° and D = 1t (the cold bending performance is evaluated according to the GB / T 232-2010 standard, D is the bending diameter, and t is the thickness of the steel plate). Due to the presence of a bainite-based structure in which a large amount of MA is dispersed in the microstructure, the yield-tensile ratio is even lower at 0.85 or less, which extremely contributes to the dimensional stability of processing and forming. It exhibits excellent low-temperature impact toughness, and the impact energy at -40°C is 80 J or more.

[0017] In the design of the high-strength hot-rolled strip steel according to the present invention, the roles of each element are as follows:

[0018] C, which is an effective strengthening element in steel, in addition to solid solution strengthening, forms nano-scale second-phase precipitation particles with fine alloying elements such as Ti and Nb, and plays a role in precipitation strengthening and microstructure refinement. This is also a common method to improve the strength of steel. When the steel has high contents of Mn and Cr, C can transform into bainite or martensite hard phases at a low critical cooling rate, and can significantly improve the tensile strength of the material. As the most economical strengthening element, the C content in the present invention is 0.04% or more. However, excessive C not only forms more carbide or bainite hard phase structures in the steel, reducing the toughness and formability of the material, but also reduces the welding performance of the steel. Therefore, the C content in the present invention is 0.15% or less. Note that the element contents in the hot-rolled strip steel of the present invention mean mass fractions unless otherwise specified.

[0019] Si, which is often used as a deoxidizing element in steel, also has a solid-solution strengthening effect on steel. Si can also improve the corrosion resistance of the material and has a certain effect of reducing Cu embrittlement. However, when the Si content becomes higher, hematite scale defects are formed on the surface of the hot-rolled strip steel, which seriously affects the surface quality of the strip steel. At the same time, the welding performance of the material deteriorates, and the toughness of the area affected by the welding heat deteriorates. Therefore, in the present invention, the Si content is 0.50% or less, preferably 0.08% ≤ Si ≤ 0.50%.

[0020] Mn, an important strengthening and toughness-improving element in steel, has a solid-solution strengthening effect, and can lower the transformation temperature of supercooled austenite, lower the ferrite transformation temperature, and promote the refinement of the structure. As a result, the strength and toughness of the material can be improved. However, when the Mn content becomes excessive, the transformation of ferrite to bainite is significantly inhibited, and the plasticity and cold formability of the material deteriorate. Therefore, the Mn content in the present invention is 0.3% - 2.0%.

[0021] Cr is an important element for improving the weather resistance of steel plates. The main mechanisms for improving the weather resistance of weathering steel are as follows. On the one hand, by adding corrosion-resistant elements to increase the corrosion potential of the substrate, the electrochemical impedance can be increased and the corrosion rate can be reduced. On the other hand, Cr promotes the formation of a dense rust layer on the surface, physically blocking the corrosion medium and changing the corrosion environment of the substrate. As the corrosion depth increases, the corrosion gradually slows down. When the Cr content in the steel exceeds 1.5%, a uniform and dense rust layer is formed on the substrate surface due to the combined action of Cr, Cu and other elements. When the Cr content becomes even higher, combined with the action of Cu and other elements, α-FeOOH in the rust layer becomes very fine, preventing further penetration of electrochemical corrosion media such as water, and contributing to improving the electrochemical impedance. At the same time, due to the high Cr concentration, when the thickness of the rust layer increases, Cr accumulates in front of the rust layer, and the Cr concentration in the rust layer rises rapidly. Before the thickness of the rust layer reaches 0.1 mm, the Cr concentration at the interface between the rust layer and the substrate reaches 12% or more. When the Cr concentration is concentrated to 12% or more, the hot-rolled strip steel of the present invention produces an effect similar to that of stainless steel. That is, the corrosion potential at the interface between the rust layer in front of the corrosion front and the substrate becomes very high. Combined with the barrier effect of the low corrosion medium of the dense rust layer, the electrochemical impedance of the strip steel surface becomes very high, and the corrosion reaction is basically interrupted. However, the Cr content in the substrate should not be too high. When the Cr content in the substrate increases, the corrosion potential of the substrate will rise. When the Cr content exceeds 4.5%, selective corrosion is promoted at the initial stage of rust formation, and the uniformity of the thickness of the rust layer will deteriorate. That is, the chemical impedance becomes non-uniform due to the Cr concentration in front of the rust layer and the corrosion environment, the corrosion potential difference in front of the corrosion layer becomes large, the galvanic effect becomes stronger, and thus the corrosion inhibition effect will deteriorate. In this case, although the relative corrosion rate decreases under limited test conditions, the corrosion depth does not decrease over a long period of time, and the effect of improving the weather resistance during long-term use cannot be achieved. Therefore, the Cr content in the present invention is 1.5% to 4.5%, preferably 2.00% to 3.50%.

[0022] Referring to FIGS. 1 and 2, FIG. 1 shows the effect of the Cr content in the steel on the relative corrosion rate of the hot-rolled strip in the repeated immersion test, and FIG. 2 shows the effect of the Cr content in the steel on the long-term corrosion depth of the hot-rolled strip.

[0023] As shown in FIG. 2, the corrosion rate of the hot-rolled strip of the present invention rapidly decays over time, and the simulated corrosion depth for 25 years is estimated to be 0.1 mm or less when the Cr content in the steel is, for example, 2%. When the Cr content in the steel is 5% or more, the simulated corrosion depth for 25 years is estimated to be about 0.12 mm, and the corrosion resistance actually decreases.

[0024] Furthermore, Cr is also an element that enhances the hardenability of the steel. A higher Cr content can cause the steel to form air-cooled bainite or air-cooled martensite at a lower air-cooling rate, thereby significantly improving the tensile strength of the material and reducing the yield-tensile ratio of the material, which is beneficial for reducing springback during forming and improving the dimensional stability of the formed part. In the present invention, by combining a design with a high Cr content, the effect of Cr can be fully utilized and combined with the strengthening effects of elements such as C and Mn to further improve the strength of the steel.

[0025] Cu is also one of the important corrosion-resistant elements, and its effect becomes even more remarkable when added together with Cr. Cu can promote the formation of a dense rust layer on the surface of the steel. By adding 0.10% or more of Cu, the atmospheric corrosion resistance of the steel can be significantly improved. However, since Cu is a metal with a low melting point, strip steel containing a large amount of Cu is likely to have embrittlement of copper and surface warping defects during hot rolling, deteriorating the surface quality of the steel. At the same time, Cu is also a precious element. For the above reasons, in the present invention, the Cu content is designed to be 0.10 - 0.60%.

[0026] P is often added as a corrosion-resistant element in traditional atmospheric corrosion-resistant steels, which can promote the formation of a surface protective rust layer and effectively improve the atmospheric corrosion resistance of the steel. However, P is also a harmful impurity element in steel and is prone to segregation at the center of the thickness during the continuous casting of steel slabs. At the same time, P is prone to segregation at grain boundaries, reducing the grain boundary binding energy and thus reducing the toughness and plasticity of the steel. Based on the same principle, P is also very harmful to the welding performance of steel. Therefore, in the present invention, it is an object not to use an atmospheric corrosion-resistant steel with a high P content and to minimize the P content in the steel, and it is required that the P content be 0.03% or less.

[0027] S, which is a common harmful impurity element in steel, has an adverse effect on properties such as low-temperature toughness, welding performance, and cold-forming performance. The S content in the steel of the present invention is 0.01% or less.

[0028] Al is a very effective deoxidizing element, and at the same time, Al is beneficial to grain refinement and the improvement of the strength and toughness of steel materials. At the same time, Al can promote the formation of ferrite, suppress the transformation of pearlite, and also promote the transformation of a ferrite-bainite two-phase structure. However, when the Al content becomes higher, it does not contribute to smooth injection and is prone to blocking the water outlet. Therefore, in the present invention, it is required that the Al content in the steel be 0.01 - 0.60%.

[0029] Ti is a strong carbide and nitride forming element and can precipitate in the form of extremely fine TiC or Ti(C,N) second-phase particles, thereby significantly improving the strength of the material. Ti is a very effective strengthening element. However, in the present invention, since phase transformation strengthening can be controlled by the process and Cr and Mn elements, Ti is not an essential strengthening element. When Ti is added in an excessive high content, the precipitation strengthening effect of Ti will gradually weaken and will begin to affect the low-temperature toughness of the steel. Therefore, in the present invention, the Ti content is 0.15% or less, preferably 0.05% or more and 0.10% or less.

[0030] Nb is also a strong carbonitride-forming element, capable of forming NbC and Nb(CN) carbide particles as second-phase precipitates and exerting a precipitation strengthening effect. However, since the cost of Nb is much higher than that of Ti, it is not economical to add Nb to increase strength compared to Ti. At the same time, an excessive Nb content also affects the quality of the casting billet during the cooling process of continuous strip casting, causing defects such as surface cracks and corner cracks. Therefore, the Nb content in the present invention is 0.06% or less.

[0031] V is a strong carbide-forming element and can exert a strong precipitation strengthening effect. VC has a lower precipitation temperature than TiC and generally exhibits a better precipitation strengthening effect between 500 and 550 °C. Therefore, when the strip is coiled at a low temperature, the strength can be improved by adding V. However, the economy of V in terms of strength improvement is inferior to that of Ti, and if the V content is too high, the weld toughness of the steel will also decrease. Therefore, the V content in the present invention is 0.15% or less.

[0032] Mo is an alloying element commonly used in steel and has functions such as improving hardenability, suppressing ferrite transformation, refining the structure, and enhancing the contribution of TiC precipitation strengthening. Adding a small amount of Mo to the steel is beneficial for improving the properties of the steel. However, Mo is a noble metal element, and its excessive content does not contribute to production economy. Therefore, in the present invention, Mo ≤ 0.40%.

[0033] B has a strong effect of improving hardenability, significantly suppressing ferrite transformation, and can improve the strength of the steel by obtaining a bainite structure. However, an excessive B content will reduce the plasticity and processing performance of the material. Therefore, in the present invention, B ≤ 0.002%.

[0034] Ni can improve the corrosion resistance of steel and also mitigate the problem of surface Cu embrittlement caused by Cu. However, the price of Ni is very high, and the excessive addition of Ni significantly increases the alloy cost of the material. Therefore, in the present invention, the Ni content is designed to be 0.40% or less, preferably 0.05% - 0.25%.

[0035] Sb can be added as an element to improve corrosion resistance. However, Sb is also a harmful element in steel and will deteriorate the performance of steel, especially its low-temperature toughness. Therefore, in the present invention, the Sb content is limited to 0.15% or less.

[0036] Re (rare earth element) has the effect of improving corrosion resistance and enhancing the toughness of the material. However, it is difficult to control the yield of the Re element in the smelting process, and its excessive addition will reduce the economy of the steel. Therefore, in the present invention, the Re content is limited to 0.15% or less.

[0037] Ca can form spherical and dispersed CaS together with S, thereby improving the distribution of sulfide inclusions in steel and being beneficial to the improvement of the corrosion uniformity and toughness of the material. However, when the Ca content in steel becomes excessive, calcium oxide inclusions will increase. Therefore, in the present invention, the Ca content is limited to 0.015% or less.

[0038] Mg can combine with O to form fine MgO that serves as the nucleation center for other inclusions, thereby refining the size of the inclusions and improving their dispersion and distribution. This can also improve the uniform corrosion of the material and enhance the toughness of the material. However, when the Mg content in steel becomes excessive, oxide inclusions will increase. Therefore, in the present invention, the Mg content is limited to 0.015% or less.

[0039] Furthermore, the composition design of the high-strength hot-rolled strip steel of the present invention also needs to meet the following requirements:

[0040] This requirement is 2.5% ≤ 2Mn + Cr ≤ 6.0%. Both Mn and Cr can improve the hardenability of the material, form bainite or martensite at a lower critical cooling rate, and achieve higher strength. The inhibitory effect of Mn on ferrite transformation is more than twice that of Cr. According to thermal simulation and CCT calculation, under the condition of 2Mn + Cr ≥ 2.5%, the free C in the steel can transform into a bainite structure containing MA instead of carbide during the natural cooling process after the strip is coiled, thereby significantly improving the strength of the material. Therefore, the steel of the present invention not only significantly improves the corrosion resistance by using a high Cr content, but also achieves a significant improvement in strength by combining the effect and process of the high Cr content on phase transformation. However, if the contents of Mn and Cr are excessively high, the transformation of ferrite is significantly inhibited, resulting in an overly long transformation time and difficulty in precipitating sufficient ferrite during the laminar flow cooling control process of the strip. Based on the above, in the present invention, 2.5% ≤ 2Mn + Cr ≤ 6.0%. Preferably, 3.2% ≤ 2Mn + Cr ≤ 5.0%.

[0041] Preferably, the composition of the hot-rolled strip of the present invention satisfies Si + 2Ni ≥ 0.10%. Cu is likely to form copper embrittlement defects on the strip surface, but both Si and Ni have the effect of improving copper embrittlement defects and can complement each other. Although Ni has a better effect on improving Cu embrittlement than Si, the cost of Ni is even higher. If the content of Si + 2Ni is 0.10% or more, the copper embrittlement defects can be improved. Therefore, the hot-rolled strip of the present invention may contain one or both of the Si element and the Ni element. In the steel, if Si ≤ 0.50%, Ni ≤ 0.40%, and Si + 2Ni ≥ 0.10%, the balance among the economy of material design, the surface quality of the material, and the copper embrittlement problem can be adjusted by utilizing the complementary relationship between the two, and the copper embrittlement problem can be made more economical and controllable. Preferably, Si + 2Ni ≥ 0.4%.

[0042] Another aspect of the present invention provides a manufacturing method of the above hot-rolled strip steel, which has the following steps: 1) Smelting and casting Smelt molten steel according to the above composition, and then cast it into slabs; 2) Heating Transfer the slabs to a heating furnace of a conventional hot rolling production line for heating, or transfer them to a soaking furnace of a thin slab continuous casting and rolling production line for soaking. Rapidly heat the slabs so that the slab surface temperature rises from 1050 °C to 1150 °C in 15 minutes. Set the heating time of the slabs in the soaking section to 20 - 60 minutes, and set the tapping temperature of the slabs to 1180 - 1230 °C; 3) Rolling After taking out the slabs from the heating furnace and adjusting their sizes, perform rough rolling. During the rough rolling stage, perform scale removal. The pressure of the high-pressure water for scale removal is 15 MPa or more, preferably 20 MPa or more, and the temperature at the rough rolling exit is 1040 °C or less. Then, perform final rolling on the rough-rolled strip steel. Use multi-stand continuous rolling for the final rolling, and set the final rolling temperature to 820 - 880 °C; 4) Cooling Adopt laminar flow cooling for cooling, in combination with two-stage cooling. Perform the first-stage cooling to rapidly cool the strip steel from 150 to 350 °C / s to 640 - 690 °C, then air-cool for 7 - 14 seconds, and perform the second-stage cooling to cool the strip steel from 60 to 300 °C / s to 480 - 560 °C, and then wind it up in a coil shape.

[0043] Preferably, in step 3), the temperature at the rough rolling exit is 1000 - 1040 °C.

[0044] Preferably, in step 3), in the final rolling, the final rolling temperature is 860 - 880 °C when the thickness of the final hot-rolled strip is 3 mm or less, 840 - 860 °C when the thickness of the final hot-rolled strip is 3 - 5 mm, and 820 - 840 °C when the thickness of the final hot-rolled strip is 5 mm or more.

[0045] In the manufacturing method of the high weathering steel according to the present invention:

[0046] In order to reduce the influence of Cu embrittlement, the manufacturing method according to the present invention optimizes the heating curve of the steel slab in the heating furnace and adopts a high-temperature high-speed combustion and low furnace outlet temperature process to suppress surface Cu embrittlement. The heating time in the surface temperature range of 1050 - 1150 °C is controlled within 15 minutes to quickly pass through the sensitive temperature range where Cu embrittlement occurs. The steel slab is heated in the soaking zone for 20 - 60 minutes, and a lower furnace outlet temperature of 1180 - 1230 °C is used.

[0047] Rough rolling is carried out after the steel slab is taken out of the heating furnace and sized. During the rough rolling stage, it is necessary to ensure sufficient descaling pressure to achieve a good descaling effect. It has been confirmed through production practice that high-pressure water of 15 MPa or more has a good crushing and removal effect on the dense primary oxide scale on the surface of the steel slab with a high Cr content. The removal of the oxide scale has a significant improvement effect in suppressing Cu embrittlement caused by Cu concentrated on the surface. Therefore, in the manufacturing process of the present invention, it is necessary to set the pressure of the high-pressure water for descaling to 15 MPa or more.

[0048] Also, in order to suppress the occurrence of surface Cu embrittlement, the temperature at the outlet of rough rolling is 1040 °C or lower, preferably 1000 - 1040 °C.

[0049] After rough rolling, the strip enters the final rolling process using a multi-stand continuous rolling process. Depending on the thickness of the rolled strip, the final rolling temperature is controlled at 820 - 880°C. Compared with a general rolling process, the steel of the present invention requires an even lower final rolling temperature. For example, when the final rolling temperature is 860 - 880°C, it is the case where the thickness of the final hot-rolled strip is 3 mm or less; when the final rolling temperature is 840 - 860°C, it is the case where the thickness of the final hot-rolled strip is 3 - 5 mm; and when the final rolling temperature is 820 - 840°C, it is the case where the thickness of the final hot-rolled strip is 5 mm or more. The purpose of controlling the final rolling temperature is to increase the rolling deformation energy and, importantly, to promote the transformation of fine-grained ferrite after laminar flow cooling of the strip in order to improve the plasticity of the steel according to the present invention.

[0050] In the steel composition of the present invention, due to the high contents of Mn and Cr, that is, 2Mn + Cr ≥ 2.5%, it plays an important role in suppressing the phase transformation of ferrite and improving the plasticity and formability of the material. Therefore, during the laminar flow cooling stage, the structure of the steel strip of the present invention needs to be precisely controlled by a two-stage cooling process. The advantage of the two-stage cooling process is that by the rapid cooling in the first stage, the supercooling degree of the phase transformation of the material can be significantly improved, the nucleation points of the phase transformation can be increased, and it can be cooled to the temperature interval of 640 - 690°C, which is the optimal temperature interval for the ferrite phase transformation of the material. During air cooling at such a temperature for 7 - 14 seconds, a large amount of ferrite nucleates, transforms, and grows, resulting in an even higher content of uniform and fine ferrite, and the plasticity and formability of the material can be greatly improved.

[0051] When fine alloying elements such as Nb and Ti are added to the material, a large number of precipitates also occur during this period, thereby significantly improving the strength of the material. On the other hand, during the transformation of ferrite, the supersaturated C element in the ferrite phase accumulates in the untransformed austenite, and as a result, the C concentration in the remaining phase will increase. The air cooling time in the first cooling stage is controlled to be 7 - 14 seconds. If the time is too short, the transformation of the ferrite phase will be insufficient, and the content of ferrite will decrease. If the time is too long, the transformation of pearlite will occur, pearlite or carbide(s) will be formed in the material, and its strength will decrease.

[0052] After entering the second cooling stage, by cooling the strip at a cooling rate of 60°C / s or more, preferably 60 - 300°C / s, to 480 - 560°C, the austenite in the strip that has not transformed and is concentrated with a high concentration of C can be transformed into bainite. Furthermore, due to the enrichment of C and the high contents of Mn and Cr, the transformation of pearlite is suppressed, the bainite transformation point is significantly reduced, and the critical cooling rate of bainite / martensite is also significantly reduced. Therefore, during the second-stage cooling and slow cooling after coiling, a considerable amount of MA structure transformation will occur, which will greatly contribute to the improvement of the material strength.

[0053] By controlling the above cooling process, in the hot-rolled strip, a multiphase structure of polygonal ferrite + bainite (MA occupies more than 30 vol% of the bainite structure) can be obtained. The ferrite structure is very fine, and the grain size is above grade 9. Preferably, in the hot-rolled strip of the present invention, the content of polygonal ferrite is 40 vol% - 70 vol%, the content of bainite is 20 vol% - 60 vol%, preferably 30 vol% - 60 vol%, and it contains a large amount of fine granular MA in the bainite structure (MA occupies more than 30 vol%, for example, 30 vol% - 50 vol% of the bainite structure), and contains a small amount of pearlite or carbide(s) (pearlite content + carbide content ≤ 15 vol%).

[0054] Since there exists a bainite structure in the fine structure of steel in which a large amount of MA is distributed, this material exhibits both high strength and a low yield-tensile ratio. Specifically, the yield strength is 550 MPa or more, the tensile strength is 650 MPa or more, and the yield-tensile ratio is 0.85 or less. The steel of the present invention has a high content of polygonal ferrite, imparting high plasticity to the material. Therefore, it has an elongation at break of 20% or more, and has qualified cold bending performance when bent at 180° and D = 1t, showing high cold forming performance.

[0055] The steel of the present invention has an extremely small ferrite structure and a grain size of grade 9 or more, and further grade 11 or more, so it is excellent in low-temperature impact toughness, and the impact energy at -40°C is 80 J or more.

[0056] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0057] First, the present invention utilizes components such as C-Mn-Cu and adds a relatively high content of Cr, thereby promoting the formation of a uniform and dense rust layer on the surface of the hot-rolled steel strip when used in the atmosphere, and enabling the rapid concentration of the Cr concentration to 12% or more at the interface between the thin rust layer with a thickness of 0.1 mm or less and the strip substrate. Due to the uniform and dense rust layer and the high Cr concentration brought about by the Cr concentration in the rust layer, the corrosion potential and electrochemical impedance of the surface of the steel strip substrate are significantly improved, and the continuous occurrence of surface corrosion is inhibited. Therefore, the hot-rolled steel strip of the present invention has extremely high atmospheric corrosion resistance. Repeated immersion tests were carried out on Q355B steel and the hot-rolled steel strip of the present invention. As a result, the corrosion rate of the hot-rolled steel strip of the present invention was 30% or less of that of Q355B steel, that is, its weather resistance was more than three times that of ordinary structural steel Q355B and more than twice that of ordinary weathering steel (Q450NQR1). In addition, the corrosion rate in the atmosphere rapidly decays as the corrosion depth of the strip surface increases. As a result of simulating the corrosion of the steel strip of the present invention for 25 years, the corrosion depth in the outside air for 25 years was 0.1 mm or less. Therefore, the hot-rolled steel strip of the present invention can meet the requirements for the bare use of structural components such as brackets for solar power generation without applying a coating on the surface, and its service life can be extended to 25 years or more.

[0058] Second, the present invention controls the Cr and Mn contents in the steel (2.5% ≤ 2Mn + Cr ≤ 6.0%), and precisely adjusts the material structure by the segmented cooling process, so that the polygonal ferrite content of the obtained hot-rolled strip steel is 40 vol% to 70 vol%, and the bainite content is 20 vol% to 60 vol%, preferably 30 vol% to 60 vol%. At the same time, by utilizing the enrichment of abundant C element in austenite after ferrite transformation and controlling to satisfy 2Mn + Cr ≥ 2.5%, pearlite can be significantly suppressed, and the generation of bainite / martensite can be promoted. By utilizing the low-temperature transformation of bainite / martensite, a bainite structure containing a large amount of finely dispersed MA (MA occupies more than 30 vol% of the bainite structure) can be obtained, and the steel of the present invention has high strength and good formability.

[0059] In the present invention, it is preferably satisfied that Si + 2Ni ≥ 0.10%, thereby achieving a balance among the economy of material design, the surface quality of the material, and the problem of Cu embrittlement.

[0060] At the same time, the manufacturing method of the present invention optimizes the heating curve of the steel slab in the heating furnace, adopts a high-temperature high-speed combustion and low outlet temperature process to suppress surface Cu embrittlement, and combines high-pressure water descaling with a pressure of 15 MPa or more, preferably 20 MPa or more in the rough rolling section to reduce the occurrence of Cu embrittlement problems.

[0061] The steel of the present invention does not require additional addition of strengthening elements. Instead, the present invention only utilizes C, Mn, and a relatively high Cr content, controls it such that 2.5% ≤ 2Mn + Cr ≤ 6.0%, and in combination with a divided cooling process, adjusts the phase transformation and structure of the steel so that the hot-rolled strip has a multiphase structure of polygonal ferrite + bainite (MA occupies more than 30% of the bainite structure), thereby economically achieving high strength and high formability of the steel. The steel of the present invention has the characteristics of high strength, low yield-tensile ratio, high plasticity, and high low-temperature toughness. Specifically, the yield strength is 550 MPa or more, the tensile strength is 650 MPa or more, the yield-tensile ratio is 0.85 or less, the elongation at break is 20% or more, and it has cold bending performance satisfying the qualification under the bending condition of 180° and D = 1t (D is the bending diameter and t is the thickness of the steel plate), and has low-temperature impact toughness, that is, the impact energy at -40°C is 80 J or more. It has good formability and can process profile components with complex cross-sections. The lower yield-tensile ratio is advantageous for reducing the rebound of components during rolling forming processing and contributes significantly to the stability of processing dimensions. Furthermore, the hot-rolled strip steel of the present invention exhibits excellent low-temperature impact toughness, enabling wide application of the products.

Brief Description of the Drawings

[0062]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Best Mode for Carrying Out the Invention

[0063] Detailed Description The technical solution of the present invention will be further described in conjunction with embodiments and the accompanying drawings.

[0064] The main processes for manufacturing the hot-rolled strip steel in Examples 1 to 14 of the present invention were as follows: 1) According to the chemical composition shown in Table 1, converter steelmaking and refining were carried out, and then a steel slab was obtained by continuous casting. 2) The steel slab was heated in a heating furnace. During the heating process, the slab was heated so that the surface temperature of the slab increased from 1050 °C to 1150 °C in 15 minutes, heated for 20 to 60 minutes in the soaking section, and tapped at a temperature of 1180 to 1230 °C. 3) The steel slab was subjected to descaling for rough rolling, sizing, rough rolling, flying shear, descaling for final rolling, and final rolling to obtain a strip steel. In the rough rolling stage, high-pressure water descaling with a pressure of 15 MPa or more, preferably 20 MPa or more, was used, and the temperature at the rough rolling outlet was set to 1040 °C or less. Then, final rolling was performed on the rough-rolled strip steel. Multi-stand continuous rolling was used for the final rolling, and the final rolling was carried out at a temperature of 820 to 880 °C. 4) After the final rolling of the strip steel, laminar cooling and coiling were carried out to obtain the hot-rolled strip steel of the present invention. Cooling was carried out by combining laminar cooling and two-stage cooling. In the first-stage cooling, the strip steel was rapidly cooled to 640 to 690 °C at a cooling rate of 150 °C / s or more, preferably 150 to 350 °C / s, then air-cooled for 7 to 14 seconds, and in the second-stage cooling, the strip steel was cooled to 480 to 560 °C at a cooling rate of 60 °C / s or more, preferably 60 to 300 °C / s, and then coiled.

[0065] Table 2 shows the specific production process parameters used in Examples 1 to 14. After obtaining the hot-rolled strip steel according to the above steps 1 to 4, a performance test of the hot-rolled strip steel was carried out. Table 3 shows the specific performance parameters of the strip steel. The microstructures of Examples 1 to 14 are also shown in Table 3. The contents of ferrite and bainite in the steel are shown in Table 3, and the remainder of the microstructure was a small amount of pearlite and / or carbide(s).

[0066] In accordance with the standard TB / T 2375 "Test Method for Repeated Immersion Corrosion of Weathering Steel for Railways", the corrosion resistance of the hot-rolled strip steels of Examples 1 to 14 and the steels of Comparative Examples 1 to 2 was tested. The "relative corrosion rate" in Table 3 represents the corrosion rates of the hot-rolled strip steels of Examples 1 to 14 and Q450NQR1 steel with respect to Q345B steel.

[0067] The yield strength, tensile strength and elongation at break of the hot-rolled strip steels of Examples 1 to 14 were tested in accordance with the standard GB / T 228.1-2021 "Metallic materials-Tensile testing-Part 1: Method of test at room temperature"; and the cold bending performance was tested in accordance with the standard GB / T 232-2010 "Metallic materials-Bend test".

[0068] The grain size of ferrite in Examples 1 to 14 was measured in accordance with the standard GB / T 6394-2017 "Determination method for estimating the average grain size of metals".

[0069] Figures 3 to 5 show the metallographic photographs of the hot-rolled strip steels of Example 13, Example 5 and Example 2 of the present invention, respectively. As can be seen from Figures 3 to 5, the metallographic structures of the hot-rolled strip steels of Example 13, Example 5 and Example 2 of the present invention are mainly polygonal ferrite + bainite, the content of ferrite is 70 vol% (Figure 3) to 40 vol% (Figure 5), and the remainder is a structure mainly composed of bainite, and the proportion of bainite in the steel was 20 vol% to 60 vol%. The bainite in the present invention was actually granular bainite in which a large amount of MA was distributed in the bainite structure. Specifically, the proportion of MA in the bainite structure was 30 vol% or more, generally 30 vol% to 50 vol%. Furthermore, a small amount of pearlite or carbide(s) was present in the hot-rolled strip steel.

[0070] In the present invention, existing Q355B steel and Q450NQR1 steel were used as comparative examples. The components of Comparative Example 1 (Q355B) and Comparative Example 2 (Q450NQR1) are also shown in Table 1.

[0071] As can be seen from the results in Table 3, the hot-rolled strip steel of the present invention exhibited excellent corrosion resistance.

[0072]

Table 1

[0073]

Table 2

[0074]

Table 3

Claims

1. A hot-rolled strip steel having the following components, wherein the mass percentages of the components are: C is 0.04 to 0.15%, Si is 0.50% or less, Mn is 0.30 to 2.00%, Cr is 1.5 to 4.5%, Cu is 0.10 to 0.60%, P is 0.03% or less, S is 0.01%, and Al is 0.01 to 0.60%, the balance being Fe and unavoidable impurities, and further: 2.5% ≤ 2Mn + Cr ≤ 6.0% is satisfied, and the elemental symbols are calculated by replacing them with the mass percentages of the corresponding elements in the hot-rolled strip steel. The hot-rolled strip steel.

2. The hot-rolled strip steel further has Ni, and the composition of the hot-rolled strip steel satisfies the following items: Ni ≤ 0.40% and Si + 2Ni ≥ 0.10%, and the elemental symbols are calculated by replacing them with the mass percentages of the corresponding elements in the hot-rolled strip steel. The hot-rolled strip steel according to Claim 1.

3. The hot-rolled strip steel further has at least one selected from the following components: Ti of 0.15% or less, Nb of 0.06% or less, V of 0.15% or less, Mo of 0.40% or less, and B of 0.002% or less. The hot-rolled strip steel according to Claim 1 or 2.

4. The hot-rolled strip steel further has at least one selected from the following components: Sb of 0.15% or less, Re of 0.15% or less, Ca of 0.015% or less, or Mg of 0.015% or less. The hot-rolled strip steel according to any one of Claims 1 to 3.

5. The hot-rolled strip steel has a microstructure of a polygonal ferrite + MA-containing bainite multiphase structure, and the grain size of the ferrite is grade 9 or higher, preferably grade 11 or higher; in the microstructure, the polygonal ferrite has a content of 40 vol% to 70 vol%, the bainite has a content of 20 vol% to 60 vol%, preferably 30 vol% to 60 vol%, and the fine granular MA in the bainite structure occupies 30 vol% or more of the bainite; and the microstructure has pearlite or carbide with a content of 15 vol% or less. The hot-rolled strip steel according to any one of Claims 1 to 4.

6. The hot-rolled strip has a corrosion rate of 30% or less of Q355B steel; and / or the hot-rolled strip has a corrosion depth of 0.1 mm or less during a simulated service cycle of 25 years; and / or the hot-rolled strip has a yield strength of 550 MPa or more, a tensile strength of 650 MPa or more, a yield-tensile ratio of 0.85 or less, an elongation at break of 20% or more, cold bending performance satisfying the qualification under bending conditions of 180° and D = 1t, and an impact energy at -40 °C of 80 J or more, the hot-rolled strip according to any one of claims 1 to 5.

7. A method for manufacturing a hot-rolled strip according to any one of claims 1 to 6, having the following steps, the steps being: 1) A smelting and casting step, smelting molten steel according to the composition according to any one of claims 1 to 4, and then casting it into a slab; 2) A heating step, transferring the slab to a heating furnace or soaking furnace for soaking; heating the slab so that the slab surface temperature rises from 1050 °C to 1150 °C in 15 minutes; here, the heating time of the slab in the soaking section is 20 to 60 minutes, and the tapping temperature of the slab is 1180 to 1230 °C; 3) A rolling step, taking out the slab from the heating furnace, sizing it, and then performing rough rolling. In the rough rolling stage, high-pressure water descaling with a pressure of 15 MPa or more, preferably 20 MPa or more, is used, and the temperature at the rough rolling exit is set to 1040 °C or less. Then, finish rolling is performed on the rough-rolled strip. Multi-stand continuous rolling is used for the finish rolling, and the finish rolling temperature is 820 to 880 °C; 4) A cooling step, adopting laminar flow cooling for cooling, in combination with two-stage cooling, performing the first-stage cooling to rapidly cool the strip to 640 to 690 °C at a cooling rate of 150 °C / s or more, preferably 150 to 350 °C / s, then air-cooling for 7 to 14 seconds, and performing the second-stage cooling to cool the strip to 480 to 560 °C at a cooling rate of 60 °C / s or more, preferably 60 to 300 °C / s, and then winding it into a coil. The manufacturing method.

8. The manufacturing method according to claim 7, wherein in step 3), the temperature at the rough rolling exit is 1000 to 1040 °C.

9. In step 3), in the final rolling, when the thickness of the final hot-rolled strip is 3 mm or less, the final rolling temperature is set to 860 to 880°C; when the thickness of the final hot-rolled strip is 3 to 5 mm, the final rolling temperature is set to 840 to 860°C; and when the thickness of the final hot-rolled strip is 5 mm or more, the final rolling temperature is set to 820 to 840°C. The manufacturing method according to claim 7 or 8.

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