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

Abstract

The present invention discloses 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: 0.04 to 0.09%, Si ≤ 0.50%, Mn: 0.10 to 1.50%, P ≤ 0.03%, S ≤ 0.01%, Al ≤ 0.60%, Cr: 1.5 to 4.5%, Cu: 0.10 to 0.60%, Ti: 0.05 to 0.18%, Ni ≤ 0.30%, Nb ≤ 0.06%, N ≤ 0.008%, and the balance is Fe and inevitable impurities, and this hot-rolled strip steel further satisfies: 2Mn + Cr ≤ 6%, and the element symbols are calculated by replacing them with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. The present invention exhibits excellent atmospheric corrosion resistance and mechanical properties.

Description

Technical Field

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

Background Art

[0002] Background Weather-resistant steel is widely used in the production of outdoor steel structures that require weather resistance, such as containers, railway vehicles, and bridges. With the increasing demand for green, low-carbon, and environmentally friendly materials, the application scenarios of weather-resistant steel are also expanding. Using the weather resistance of weather-resistant steel, some structural profiles used in the atmospheric environment, such as guardrails, mast towers, support brackets, and solar power generation brackets, have been made. These steel structures can be used directly exposed, or their surfaces can be lightly coated before use, achieving very high weather resistance. Therefore, some traditional steel surface corrosion protection processes, such as pre-galvanizing, pre-treatment of plating using zinc-aluminum-magnesium coating, and post-galvanizing, can be replaced. The use of weather-resistant steel can not only reduce energy consumption and pollution caused by the metal coating process, but also improve the service life of steel structures and reduce the later corrosion protection maintenance cost.

[0003] Solutions involving high-strength weather-resistant steel are provided, for example, in the prior art:

[0004] Chinese Patent CN202011384068.6 discloses a low-alloy structural steel with high strength and high weather resistance for highway guardrails, and the yield strength is about 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 that a layer rich in P and rich in Cr is formed on the surface of the rust layer, thereby 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, for structural steel, P is an impurity element in the steel. Excessive content of P causes central segregation and grain boundary segregation of P, affects the formability and toughness of the steel, and does not contribute to the processing performance and service safety of the steel.

[0005] 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 to solve the problem that the steel existing in the prior art cannot achieve high strength and high weather resistance at the same time. This steel mainly contains the following chemical 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%, and Sb: 0.0 - 0.09%. Such steel mainly adopts a fine structure of ferrite, and the pearlite content in the steel is only 2% or less. The yield strength is 636 MPa - 710 MPa, and the tensile strength is 698 MPa - 775 MPa. In this invention, higher strength is achieved by the composite precipitation strengthening of Nb and Ti. However, since the Cr content in the steel is as low as 0.51% or less, it has the drawback that its weather resistance remains at the level of ordinary weathering steel. In addition, this invention mentions using Sb to improve corrosion resistance, but Sb is a harmful element in the steel and will reduce the performance 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, and the actual application is difficult.

[0006] 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. Chinese Patent CN202110398903.X discloses a high-strength weathering steel plate with a 700 MPa grade of atmospheric corrosion resistance. 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.

Summary of the Invention

[0007] The object of the present invention is to provide a high-strength and high-plasticity hot-rolled strip steel with high weather resistance and a manufacturing method thereof. The hot-rolled strip steel of the present invention is based on high-strength weathering steel, and the atmospheric corrosion resistance of the material is significantly improved. The corrosion rate is 30% or less of that of ordinary structural steel Q355B, that is, it has weather resistance three times or more that of steel Q355B, and its weather resistance is also one time or more that of ordinary weathering steel (such as Q450NQR1 steel). The hot-rolled strip steel of the present invention can replace the corrosion protection after galvanization, and can be used directly on guardrails, mast towers, solar power generation equipment and other supporting structural parts without applying a coating on the surface. Furthermore, the steel of the present invention has a high strength with a yield strength of 600 MPa or more and a tensile strength of 700 MPa or more, while maintaining a high plasticity with an elongation rate of 24% or more. It has cold bending performance that meets the qualification under the bending conditions of 180° and D = 0.5t, preferably cold bending performance that meets the qualification under the conditions of 180° and D = 0t. It has extremely high cold processing and forming performance, can be completely bent without cracking, and can meet the requirements of rolling and forming of profile components with complex cross-sections.

[0008] The present invention realizes the technical object by optimizing the chemical composition of the hot-rolled strip steel.

[0009] Specifically, by using a high Cr content of 1.5 to 4.5%, the formation of a uniform and dense rust layer on the surface of the hot-rolled strip steel during use can be promoted, and Cr can be rapidly concentrated in the thin rust layer. The Cr concentration at the interface between the rust layer and the substrate can be 12% or more, significantly improving the corrosion potential and electrochemical impedance, and by preventing the continuation of corrosion, extremely high atmospheric corrosion resistance can be obtained. On the other hand, with a high Cr content, a microstructure mainly composed of fine ferrite of 70% by volume or more can be obtained, and since a large amount of nano-scale precipitation phases of TiC with a diameter of 10 nm or less exist in the ferrite, hot-rolled strip steel having high strength, high plasticity, high weather resistance, and excellent formability can be economically obtained.

[0010] Specifically, the hot-rolled strip steel of the present invention has the following components, and the mass percentages of the components are: C: 0.04 to 0.09%, Si ≤ 0.50%, Mn: 0.10 to 1.50%, P ≤ 0.03%, S ≤ 0.01%, Al ≤ 0.60%, Cr: 1.5 to 4.5%, Cu: 0.10 to 0.60%, Ti: 0.05 to 0.18%, Ni ≤ 0.30%, Nb ≤ 0.06%, N ≤ 0.008%, and the balance is Fe and inevitable impurities, and this hot-rolled strip steel further satisfies 2Mn + Cr ≤ 6%, and the element symbols are calculated by replacing them with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. For example, when the Mn content in the steel is 0.10%, the numerical value 0.10% is substituted for calculation. Preferably, the composition of this hot-rolled strip steel satisfies Ti - 3N ≥ 0.04%, and the element symbols are calculated by replacing them with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. In this case, the strengthening effect of Ti can be fully exerted.

[0011] Preferably, the composition of this hot-rolled strip steel satisfies Si + 2Ni ≥ 0.10%, and the element symbols are calculated by replacing them with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. In this case, the influence of copper embrittlement can be reduced.

[0012] Preferably, the hot-rolled strip steel of the present invention further has at least one selected from the following components, and the components are: Ca of 0.015% or less, Mg of 0.015% or less, B of 0.003% or less, Mo of 0.30% or less, V of 0.15% or less, and Re of 0.015% or less.

[0013] Preferably, the hot-rolled strip steel of the present invention has a fine structure of polygonal ferrite + a small amount of pearlite + bainite mainly in the form of MA, and the grain size of ferrite is 8 grades or more, preferably 9 grades or more. The content of polygonal ferrite is 70-90%, and the content of pearlite is 5-30%. In the steel of the present invention, the content of pearlite is more preferably 15% or more and 25% or less. The content of bainite is 5-15%, and the MA structure accounts for about 20-70%, preferably 30-70% of bainite. Ferrite contains a large amount of nano-scale precipitation phases of TiC with a diameter of 10 nm or less, and the content of nano-scale precipitation phases of TiC in ferrite is about 0.005%-0.03%. Unless otherwise specified, the content of the fine structure of the steel in the present invention means the volume fraction.

[0014] The hot-rolled strip steel of the present invention has very high weather resistance, and the corrosion rate of this hot-rolled strip steel is 30% or less of that of Q355B steel, that is, its weather resistance is more than 3 times that of Q355B steel and more than 2 times 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 with the increase of the corrosion depth, and as a result of the simulated corrosion test, during the 25-year simulated service cycle, the corrosion depth of this hot-rolled strip steel is 0.1 mm or less.

[0015] The hot-rolled strip steel of the present invention has a yield strength of 600 MPa or more, preferably 650 - 800 MPa; a tensile strength of 700 MPa or more, preferably 750 - 850 MPa; an elongation at break of 20% or more, preferably 24% or more, more preferably 26% or more; and has cold bending performance satisfying the qualification under the bending conditions of 180° and D = 0.5t, preferably cold bending performance satisfying the qualification under the bending conditions of 180° and D = 0t (D is the bending diameter and t is the thickness of the steel plate); it has excellent formability that can be completely bent without cracks, and can meet the requirements of rolling forming processing of profile components with complex cross-sections.

[0016] In the composition of the hot-rolled strip steel of the present invention, the functions of each element are as follows:

[0017] C is an effective strengthening element in steel. In addition to solid solution strengthening, C can also form nano-scale second-phase precipitation particles with Ti, Nb and other fine alloying elements, and play a role in precipitation strengthening and refinement of the structure. As the most economical strengthening element, the C content of the present invention is 0.04% or more. However, if C is too much, it will form too much carbide or bainite hard phase structure in the steel, not only reducing the toughness and formability of the material, but also causing the galvanic cell effect to reduce the corrosion resistance of the steel and deteriorating the welding performance of the steel. Therefore, the C content is 0.09% or less. The element content in the hot-rolled strip steel of the present invention means mass fraction unless otherwise specified.

[0018] Si is generally used as a deoxidizing element in steel, and also has a solid solution strengthening effect on steel. In addition, it can improve the corrosion resistance of the material and has a certain effect of reducing the embrittlement of copper. However, when the content of Si is high, scale defects will occur on the surface of the hot-rolled strip steel, which will have a significant impact on the surface quality of the strip steel, deteriorate the welding performance of the material, and as a result, deteriorate the toughness of the heat-affected zone of welding. Therefore, the content of Si in the present invention is 0.50% s or less, preferably 0.06% ≤ Si ≤ 0.50%.

[0019] Mn is an important strengthening element in steel. It has a solid-solution strengthening effect, can lower the transformation temperature of the supercooled austenite phase, and can also lower the transformation temperature of the ferrite phase, contributing to the refinement of the microstructure, thus improving the strength and toughness of the material. However, if the Mn content is excessive, the transformation from ferrite to bainite is significantly inhibited, and as a result, the plasticity and cold formability of the material decrease. Therefore, the Mn content in the present invention is 0.1 to 1.5%, preferably 0.3 to 1.2%, and more preferably 0.5 to 1.0%.

[0020] Cr is an important element in improving the weather resistance of steel plates. The main mechanisms for improving the corrosion resistance of weathering steel are as follows. On the one hand, by adding corrosion-resistant elements, the corrosion potential of the substrate is increased, and by increasing the electrochemical impedance, the corrosion rate is reduced. On the other hand, Cr promotes the formation of a dense rust layer on the surface and changes the corrosion environment of the substrate, enabling a physical barrier against the corrosive medium and gradually slowing down the corrosion as the corrosion depth increases. When the Cr content in the steel exceeds 1.5%, a uniform and dense rust layer can be formed on the substrate surface due to the combined action of Cr, Cu, and other elements. As the Cr content becomes even higher, in combination with Cu and other elements, α-FeOOH in the rust layer becomes extremely fine, which prevents further penetration of electrochemical corrosion media such as water and contributes to improving the electrochemical impedance. At the same time, due to the high concentration of Cr, as the thickness of the rust layer increases, Cr is concentrated in front of the rust layer, and the Cr concentration in the rust layer increases 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 can reach 12% or more. When the concentrated Cr concentration reaches 12% or more, the hot-rolled strip steel of the present invention has the same effect as stainless steel, that is, the corrosion potential at the interface between the rust layer in front of the corrosion and the substrate becomes very high, and due to the blocking effect of the dense rust layer on the low-corrosion medium, the electrochemical impedance of the strip surface becomes very high, and the corrosion reaction is basically interrupted. However, the Cr content in the substrate should not be too high. As the Cr content in the substrate increases, the corrosion potential of the substrate will increase. For example, when the Cr content exceeds 4.5%, selective corrosion at the initial stage of rust layer formation is promoted, and the uniformity of the thickness of the rust layer will deteriorate. That is, the Cr concentration in front of the rust layer and the corrosion environment make the chemical impedance non-uniform, the corrosion potential difference in front of the corrosion increases, the galvanic effect becomes stronger, and thus the corrosion inhibition effect deteriorates. In such a case, although the relative corrosion rate decreases under limited test conditions, the corrosion depth does not decrease over a long period, 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% - 4.5%, preferably 2.27% - 3.68%.

[0021] Referring to FIGS. 1 and 2, FIG. 1 shows the influence of the Cr content in the steel on the relative corrosion rate of the hot-rolled strip in the 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.

[0022] From FIG. 2, it can be seen that the corrosion rate of the hot-rolled strip of the present invention decreases rapidly over time. For example, when the Cr content in the steel is 2%, the estimated corrosion depth during the 25-year simulation period may be 0.1 mm or less. However, when the Cr content in the steel is 5%, the estimated corrosion depth during the 25-year simulation period is about 0.12 mm, indicating a decrease in corrosion resistance. Furthermore, Cr is also an element that improves the hardenability of the steel, and the higher the Cr content, the more likely it is to form air-cooled bainite or air-cooled martensite in the steel at a lower air-cooling rate, which can significantly improve the tensile strength of the material and reduce the yield-tensile ratio of the material, contributing to reducing springback and improving the stability of the forming dimensions. The present invention adopts a design with a high Cr content, fully utilizes this function of Cr, and combines the strengthening effects of C, Mn and other elements to further improve the strength of the steel.

[0023] Cu is also one of the important corrosion-resistant elements, and its effect becomes more prominent when added together with Cr. Cu can promote the formation of a dense rust layer on the steel surface. When 0.10% or more of Cu is added, the atmospheric corrosion resistance of the steel can be significantly improved. However, Cu is a metal with a low melting point, and strip steel containing more Cu is likely to have defects such as copper embrittlement network cracking and warping on the surface during the hot rolling process, deteriorating the surface quality of the steel. At the same time, Cu is also an expensive element. For the above reasons, the Cu content in the present invention is 0.10 - 0.60%, preferably 0.2 - 0.5%, and more preferably 0.25 - 0.38%.

[0024] P is often added as a corrosion-resistant element in traditional atmospheric corrosion-resistant steels, which can promote the formation of a protective rust layer on the surface 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 continuous casting of slabs. At the same time, P is prone to segregation at grain boundaries, reducing the grain boundary binding energy, thus reducing the toughness and plasticity of the steel. For the same principle, P is also very adverse to the welding performance of the steel. Therefore, in the present invention, an atmospheric corrosion-resistant steel with a high P content is not adopted, and the content of P in the steel is minimized, and the content of P is required to be 0.03% or less.

[0025] S is a general harmful impurity element in steel, which has an adverse effect on low-temperature toughness, welding performance, cold-forming performance, etc. The content of S in the steel of the present invention is 0.01% or less.

[0026] Al is a very effective deoxidizing element, and Al contributes to the refinement of grains, and improves the strength and toughness of the steel. At the same time, Al can promote the formation of ferrite, suppress the pearlite transformation, and can also promote the transformation of the ferrite-bainite two-phase structure. However, when the content of Al is high, it does not contribute to smooth pouring and the water outlet is prone to blockage. Therefore, in the present invention, the content of Al in the steel is 0.60% or less, preferably 0.011 to 0.56%, and more preferably 0.02 to 0.30%.

[0027] Ti is an element that forms strong carbonitrides and can precipitate as second-phase particles in the form of extremely fine TiC or Ti(C,N), thus significantly improving the strength of the material, and it is a very effective strengthening element. If the Ti content is 0.05% or more, it will play a role in strong precipitation strengthening. At the same time, due to the precipitation of TiC, the free C is significantly reduced, forming larger carbides or pearlite, thereby reducing the galvanic effect of heterogeneous phases in the corrosion process and improving the material's resistance to intergranular corrosion, improving the strength of the material while improving the corrosion resistance of the material. However, if the added Ti content is too high, the precipitation strengthening effect of Ti gradually weakens and begins to affect the low-temperature toughness of the steel. Therefore, the Ti content of the present invention is 0.05 to 0.18%, preferably 0.072 to 0.160%.

[0028] Ni can improve the corrosion resistance of steel and can also improve the surface embrittlement caused by Cu. However, the price of Ni is very high, and if the addition amount is too much, the alloy cost of the material will increase significantly. Therefore, the Ni content in the present invention is 0.30% or less, preferably 0.1% or more and 0.25% or less, and more preferably 0.20% or less.

[0029] Nb is also an element that forms strong carbonitrides, and it can also precipitate as a second phase in the form of NbC and Nb(CN) carbide particles, which can also bring about precipitation strengthening. However, since the cost of Nb is much higher than that of Ti, adding Nb to increase strength is not as economical as Ti. At the same time, too high a Nb content also affects the quality of the slab during the cooling process for strip casting, resulting in surface cracks, corner cracks and other defects. Therefore, the Nb content in the present invention is 0.06% or less.

[0030] N is an impurity element in steel. In Ti-containing steel, N combines with Ti and is likely to precipitate during steelmaking to form coarse TiN inclusions. On the one hand, TiN inclusions impair the toughness of steel, and on the other hand, they also reduce the effective Ti content in steel. Therefore, the N content in the present invention is 0.008% or less.

[0031] Furthermore, the composition design of the hot-rolled strip steel of the present invention needs to satisfy 2Mn + Cr ≤ 6%. Mn and Cr can shift the C curve of ferrite transformation to the right, significantly inhibit ferrite transformation, and further lengthen the transformation time. According to thermal simulation and CCT calculation, when 2Mn + Cr > 6%, it is difficult for the strip steel to undergo sufficient ferrite transformation during laminar flow cooling and after coil winding, and sufficient ferrite transformation is very important for the properties of steel. First, the amount of ferrite transformation directly affects the ductility of the steel of the present invention, that is, the plasticity of the material. If the elongation is insufficient, it becomes difficult for the material to meet the forming requirements of parts with complex cross-sections. Second, the ferrite transformation also has an important influence on the precipitation of the nano-scale phase of TiC. Since the diffusion coefficient of Ti in the α-ferrite phase is high and the solubility of C in the α-ferrite phase is very low, with the occurrence of the γ-phase → α-phase transformation, TiC can precipitate rapidly in the form of interphase precipitates or dispersed precipitates to form a nano-scale precipitation phase, resulting in a significant improvement in strength. If the contents of Mn and Cr are too high, the ferrite phase will not transform, and it will be difficult for TiC to precipitate quickly. As the temperature decreases, a large amount of the structure transforms into bainite, and the free C element in the steel precipitates in the form of carbide, losing the opportunity to combine with Ti to form TiC, and not obtaining a sufficient strengthening effect. Therefore, the complete transformation of ferrite has an important influence on the strength and plasticity of steel. Therefore, in the present invention, it is necessary to satisfy 2Mn + Cr ≤ 6%, and the element symbols are calculated by replacing them with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. Preferably, 3% ≤ 2Mn + Cr ≤ 5%.

[0032] Preferably, the composition of the hot-rolled strip steel of the present invention satisfies Ti-3N≥0.04%, and the element symbols are calculated by replacing them with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. When N in the steel preferentially combines with Ti to form TiN inclusions, the contribution of Ti to the strength of the steel will be weakened. In the present invention, the effective content of Ti is defined as Ti-3N. If Ti-3N≥0.04%, sufficient Ti can be ensured to combine with C to form TiC precipitation particles for precipitation strengthening. Preferably, Ti-3N≥0.06%, and more preferably, Ti-3N≥0.09%.

[0033] Preferably, the composition of the hot-rolled strip steel of the present invention satisfies Si+2Ni≥0.10%, and the element symbols are calculated by replacing them with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. Cu is likely to form embrittlement defects on the strip steel surface, while Si and Ni have the effect of improving embrittlement defects, and they can complement each other. Compared with Si, Ni has a higher effect of improving copper embrittlement, but the cost is also higher. If the content of Si+2Ni is 0.10% or more, the effect of improving copper embrittlement defects can be achieved. Therefore, the hot-rolled strip steel of the present invention can contain one or both of the elements Si and Ni. When Si in the steel is 0.50% or less, Ni is 0.30% or less, and Si+2Ni≥0.10%, the balance among material design economy, material surface quality, and copper embrittlement can be adjusted by utilizing the complementary relationship between the two elements, and the copper embrittlement problem can be economically suppressed. Preferably, Si+2Ni≥0.30%.

[0034] Another aspect of the present invention provides a method for manufacturing the above-mentioned hot-rolled strip steel, which includes the following steps: 1) Smelting and casting Smelt molten steel according to the above composition, and then cast it into slabs; 2) Heating of slabs Rapidly heat the slabs to exceed the slab temperature range of 1050-1150°C within 15 minutes, keep the furnace temperature in the soaking section at 1230-1290°C, soak the slabs for 30-90 minutes, and tap the slabs at a temperature of 1230-1290°C; 3) Rolling After sizing the slab exiting the reheating furnace, rough rolling is carried out. During the rough rolling stage, high-pressure water of 15 MPa or more, preferably 20 MPa or more, is used for scale removal, and the temperature at the rough rolling exit is set to 1080 °C or less. Then, finish rolling is carried out on the rough rolled strip steel. Multi-stand continuous rolling is used for finish rolling, and the finish rolling temperature is controlled to 820 - 880 °C; 4) Cooling and coiling Laminar flow cooling is adopted for cooling, and the strip steel is coiled at a temperature of 630 - 680 °C. After the laminar flow cooling is completed, the steel coil is slowly cooled through hot coil stacking, a slow cooling wall or an insulating cover, and the steel coil at a temperature of 530 °C or more is cooled at a cooling rate of 1 °C / min or less.

[0035] Preferably, the temperature at the exit of rough rolling is 1040 - 1080 °C.

[0036] Preferably, in the finish rolling stage of step 3), when the thickness of the final hot rolled strip steel is 3 mm or less, the finish rolling temperature is 860 - 880 °C; when the thickness is 3 - 5 mm, the finish rolling temperature is 840 - 860 °C; and when the thickness is 5 mm or more, the finish rolling temperature is 820 - 840 °C.

[0037] In the method for manufacturing hot rolled strip steel of the present invention:

[0038] In the case of Cu-containing steel, in order to avoid copper embrittlement, low-temperature heating is usually used. However, in the case of Ti-containing steel, high-temperature heating is usually required to completely dissolve Ti and provide the conditions for precipitation strengthening of TiC. Since the hot rolled strip steel of the present invention contains both Cu and Ti, there is a contradiction in the heating system.

[0039] The manufacturing process of the present invention optimizes the heating curve of the slab during the heating process. By burning at high temperature and high speed, the surface temperature of the slab can quickly exceed the sensitive temperature range of 1050 - 1150°C where copper embrittlement occurs. And by controlling the heating time within 15 minutes, the molten Cu is absorbed by the newly formed oxide film on the substrate surface, and it can avoid the penetration of Cu into the slab substrate, thus suppressing the occurrence of copper embrittlement.

[0040] In the soaking stage, the temperature is maintained at 1230 - 1290°C, and the soaking time is controlled within 30 - 90 minutes to ensure that Ti is fully dissolved. The tapping temperature is also controlled at 1230 - 1290°C. The technical scheme of the present invention overcomes the contradiction that Cu-containing steel needs to be heated at a low temperature to suppress the occurrence of copper embrittlement, while steel with a high Ti content needs to be heated at a high temperature for sufficient solid solution. It can not only ensure the sufficient solid solution of Ti and provide conditions for subsequent Ti precipitation strengthening, but also reduce the occurrence of copper embrittlement and improve the surface quality of the hot-rolled strip steel.

[0041] After the slab is taken out of the heating furnace and sized, rough rolling is carried out. In the rough rolling stage, in order to obtain a good scale removal effect, it is necessary to ensure a sufficiently high scale removal pressure. It has been proven in actual production that high-pressure water of 15 MPa or more, preferably 20 MPa or more, has excellent crushing and removal effects on the dense primary oxide film on the surface of the slab with a high Cr content. The removal of the oxide film has a significant improvement effect in reducing copper embrittlement caused by surface Cu enrichment. Therefore, in the manufacturing process of the present invention, it is necessary that the high-pressure water used for scale removal is 15 MPa or more.

[0042] Also, in order to suppress the occurrence of surface copper embrittlement and considering the effect of Ti precipitation strengthening, the temperature at the rough rolling exit is 1080°C or less, preferably 1040 - 1080°C.

[0043] After rough rolling, the strip steel is subjected to final rolling that adopts a multi-stand continuous rolling process. According to the thickness of the rolled strip steel, the final rolling temperature is controlled at 820 - 880°C. The steel of the present invention adopts a relatively lower final rolling temperature compared with the rolling process of general steel. For example, when the thickness of the final hot-rolled strip steel is 3 mm or less, the final rolling temperature is 860 - 880°C; when the thickness of the final hot-rolled strip steel is 3 - 5 mm, the final rolling temperature is 820 - 840°C; and when the thickness of the final hot-rolled strip steel is 5 mm or more, the final rolling temperature is 820 - 840°C. The purpose of controlling the final rolling temperature is to increase the rolling deformation energy and promote the fine-grained ferrite transformation after laminar flow cooling of the strip steel. In addition, the precipitation of ferrite can promote the precipitation of TiC and significantly improve the precipitation strengthening effect. Therefore, the use of final rolling at a low temperature is very important for improving the plasticity and strength of the hot-rolled strip steel of the present invention.

[0044] Since the steel of the present invention has a high content of Mn and Cr, the ferrite phase transformation will be suppressed. However, the ferrite phase transformation plays an important role in improving the plasticity and formability of the material and promoting the precipitation of TiC to improve the strength of the steel. Therefore, while reducing the final rolling temperature during the hot rolling stage to obtain more transformation force, the coil winding temperature after laminar flow cooling is increased to give a relatively higher ferrite phase transformation temperature and a relatively sufficient transformation time to ferrite. Thus, in the present invention, the coil winding temperature of the strip steel is set at 630 - 680°C. Also, after the laminar flow cooling is completed, it is necessary for the strip steel to slowly cool the steel coil through hot coil stacking, slow cooling wall or insulating cover, etc., thereby cooling the steel coil at a temperature of 530°C or above at a cooling rate of 1°C / min or less to ensure sufficient occurrence of ferrite phase transformation and sufficient precipitation of TiC particles, and improve the plasticity and strength of the material. In addition, increasing the coil winding temperature in the manufacturing process of the strip steel also contributes to the shape control of the strip steel.

[0045] By controlling the hot rolling process described above, the hot-rolled strip steel of the present invention has a structure mainly composed of polygonal ferrite, and the grain size of the ferrite reaches grade 8 or above, or even grade 11. The content of polygonal ferrite in the microstructure of the hot-rolled strip steel is 70-90%, and a large amount of precipitation phases of TiC with a diameter of 10 nm or less are dispersed in the ferrite, which significantly contributes to the high strength of the material. Furthermore, this hot-rolled strip steel also contains 5-30%, preferably 15-30%, more preferably 15-25% of pearlite, and a small amount, i.e., 5-15% of bainite mainly in the form of MA, which can further improve the strength of the steel. Preferably, the bainite contains 30-70% of the MA structure. A large amount of nano-scale precipitation phases of TiC exist in the steel, which endows the material with high strength. Specifically, the yield strength is 600 MPa or more, and the tensile strength is 700 MPa or more. Since the microstructure of the hot-rolled strip steel of the present invention is mainly composed of ferrite, it has high plasticity. In particular, the elongation at break is 24% or more, and it has cold bending performance that meets the qualification under the bending conditions of 180° and D = 0.5t, and preferably has cold bending performance that meets the qualification under the bending conditions of 180° and D = 0t. That is, the hot-rolled strip steel of the present invention has high strength and high plasticity.

[0046] Compared with the prior art, the advantages of the present invention are as follows:

[0047] First, in the present invention, by using a high Cr addition content of 1.5 to 4.5% together with the addition of Cu and other components, when the hot-rolled strip steel of the present invention is used in the atmosphere, the formation of a uniform and dense rust layer on its surface is promoted, and it is possible to rapidly concentrate the Cr concentration to 12% or more at the interface between the thin rust layer of 0.1 mm or less and the strip steel substrate. Through the formation of a uniform and dense rust layer on the strip steel surface and the high concentration of Cr brought about by the Cr concentration in the rust layer, a significant increase in the corrosion potential and electrochemical impedance of the strip steel substrate surface is realized, and the continuation of surface corrosion is inhibited. Therefore, the hot-rolled strip steel of the present invention has extremely high atmospheric corrosion resistance (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). Alternate immersion tests were carried out on Q355B steel and the hot-rolled strip steel of the present invention respectively. As a result, the corrosion rate of the hot-rolled strip steel of the present invention is 30% or less of that of Q355B steel, that is, its corrosion resistance is more than 3 times that of steel Q355B, and more than 2 times that of general weathering steel (Q450NQR1), and as the corrosion depth on the strip steel surface increases, its corrosion rate in the atmosphere rapidly decreases. As a result of simulating the corrosion of the strip steel of the present invention for 25 years, the corrosion depth of the strip steel in the atmosphere for 25 years is 0.1 mm or less.

[0048] Second, the hot-rolled strip steel of the present invention utilizes relatively economical precipitation strengthening of Ti, controls the contents of Mn and Cr in the steel, and requires that 2Mn + Cr ≤ 6%, whereby good corrosion resistance can be obtained for this strip steel. Furthermore, through specific high-temperature coil winding and the cooling process after coil winding, this hot-rolled strip steel contains 70 - 90% by volume, preferably 75 - 90% by volume of ferrite, and contains a large number of nano-scale precipitation phases of TiC distributed in the ferrite. Therefore, the material has high strength and high plasticity. Specifically, the yield strength is 600 MPa or more, the tensile strength is 700 MPa or more, the elongation at break is 24% or more, and it has cold bending performance that meets the qualification under the bending conditions of 180° and D = 0.5t, preferably cold bending performance that meets the qualification under the bending conditions of 180° and D = 0t (the cold bending performance is evaluated according to the standard GB / T 232 - 2010 "Metallic materials - Bend test", D is the bending diameter, and t is the thickness of the steel plate). It has excellent forming performance, can be completely bent without cracking, and can meet the requirements of rolling and forming processing of profile components with complex cross-sections.

[0049] Also, in the present invention, a composition design satisfying Si + 2Ni ≥ 0.10% is preferred, whereby a balance can be achieved among the economy of material design, the surface quality of the material, and the problem of copper brittleness.

[0050] At the same time, in terms of the heating system, the manufacturing method of the present invention overcomes the contradiction between the Cu-containing steel that needs to be heated at a low temperature to suppress copper embrittlement and the steel with a high Ti content that needs to be heated at a high temperature for sufficient solid solution. By controlling the heating rate of the slab, high-speed combustion at a high temperature enables the slab surface temperature to quickly exceed 1050 - 1150°C, which is sensitive to copper embrittlement. Next, by controlling the heating time and heating temperature in the soaking section, sufficient solid solution of Ti at a high temperature is ensured, providing the conditions for precipitation strengthening of Ti. On the other hand, by using high-pressure water descaling of 15 MPa or more in the rough rolling section, the occurrence of copper embrittlement of the steel can be reduced, and the surface quality of the strip steel can be guaranteed.

Brief Description of the Drawings

[0051]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0052] Detailed Description Hereinafter, the present invention will be further described with reference to examples and the accompanying drawings.

[0053] The main manufacturing processes of the hot-rolled strip steels of Examples 1 to 8 of the present invention were as follows: 1) After performing converter smelting and refining according to the chemical composition shown in Table 1, continuous casting was performed to obtain a slab. 2) The slab was heated in a heating furnace. In the heating process, the slab heating process was controlled so that the slab was heated beyond the range of 1050 to 1150 °C within 15 minutes, the furnace temperature in the soaking section was maintained at 1230 to 1290 °C, and the slab was soaked for 30 to 90 minutes, and the slab was discharged at a temperature of 1230 to 1290 °C. 3) The 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. During the rough rolling stage, high-pressure water of 15 MPa or more was used for descaling, and the temperature at the roughing exit was set to 1080 °C or less. Final rolling was performed on the rough-rolled strip steel, a multi-stand continuous rolling process was adopted for the final rolling, and the final rolling temperature was controlled at 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. The coiling of the strip steel was carried out at a temperature of 630 - 680°C. After the laminar cooling was completed, the steel coil was slowly cooled through hot coil stacking, a slow cooling wall or an insulating cover, and the steel coil at a temperature of 530°C or higher was cooled at a cooling rate of 1°C / min or less.

[0054] The specific process parameters adopted in the production processes of Examples 1 - 8 are shown in Table 2. After obtaining the hot-rolled strip steel according to the above steps 1 - 4, the performance of the hot-rolled strip steel was tested, and the specific performance parameters of the strip steel are shown in Table 3. The microstructures of Examples 1 - 8 are also shown in Table 3.

[0055] The corrosion resistance of the hot-rolled strip steels of Examples 1 - 8 and the steels of Comparative Examples 1 - 2 was tested according to TB / T 2375 "Method for Alternate Immersion Corrosion Test of Weathering Steels for Railways", and the "Relative Corrosion Rate" in Table 3 was the corrosion rate of the hot-rolled strip steels of Examples 1 - 8 and Q450NQR1 steel with respect to Q345B steel.

[0056] The yield strength, tensile strength and elongation at break of the steels of Examples 1 - 8 were tested in accordance with 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 GB / T 232 - 2010 "Metallic materials - Bend test".

[0057] The grain size of ferrite in Examples 1 - 8 was measured in accordance with GB / T 6394 - 2017 "Determination methods for estimating the average grain size of metals".

[0058] Figures 3 - 4 show the metallographic structures of the hot-rolled strip steels of Examples 2 and 6 of the present invention respectively. From Figures 3 - 4, it can be seen that the hot-rolled strip steels of Examples 2 and 6 of the present invention have a fine structure mainly composed of ferrite, with ferrite accounting for 80% or more, a small amount of non-typical pearlite structure also existing in the steel, and in fact, some bainite is mixed in the pearlite structure, and MA structure is observed in the bainite.

[0059] Figure 5 shows the dark field morphology observed by transmission electron microscope (TEM) of the hot-rolled strip steel in Example 2. It can be seen that a large number of TiC precipitates with a size of 10 nm or less are distributed at the ferrite grain boundaries and within the grains in the steel, and these extremely fine TiC particles strongly contribute to the strength of the material.

[0060] In the present invention, the existing Q355B steel and Q450NQR1 steel were used as comparative examples. As can be seen from the results in Table 3, the hot-rolled strip steel of the present invention showed excellent corrosion resistance.

[0061]

Table 1

[0062]

Table 2

[0063]

Table 3

Claims

1. A hot-rolled strip steel having the following components, wherein the mass percentages of the components are: C: 0.04 - 0.09%, Si ≤ 0.50%, Mn: 0.10 - 1.50%, P ≤ 0.03%, S ≤ 0.01%, Al ≤ 0.60%, Cr: 1.5 - 4.5%, Cu: 0.10 - 0.60%, Ti: 0.05 - 0.18%, Ni ≤ 0.30%, Nb ≤ 0.06%, N ≤ 0.008%, and the balance is Fe and inevitable impurities, and the hot-rolled strip steel further satisfies: 2Mn + Cr ≤ 6%, and the element symbols are calculated by replacing with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel. The hot-rolled strip steel.

2. The hot-rolled strip steel according to Claim 1, wherein Ti - 3N ≥ 0.04%, and the element symbols are calculated by replacing with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel.

3. The hot-rolled strip steel according to Claim 1 or 2, wherein Si + 2Ni ≥ 0.10%, and the element symbols are calculated by replacing with the mass percentages of the corresponding chemical elements in the hot-rolled strip steel.

4. The hot-rolled strip steel according to any one of Claims 1 to 3, wherein the hot-rolled strip steel further has at least one selected from the following components: Ca of 0.015% or less, Mg of 0.015% or less, B of 0.003% or less, Mo of 0.30% or less, V of 0.15% or less, and Re of 0.015% or less.

5. The hot-rolled strip steel according to any one of Claims 1 to 4, wherein the hot-rolled strip steel has a fine structure of polygonal ferrite + a small amount of pearlite + bainite mainly in the form of MA, and the ferrite has a grain size of 8 grades or more, preferably 9 grades or more; the hot-rolled strip steel has, by volume%, a polygonal ferrite content of 70 - 90%, a pearlite content of 5 - 30%, and a bainite content of 5 - 15%; and the ferrite contains a nano-scale precipitation phase of TiC with a diameter of 10 nm or less at a volume ratio of 0.005% - 0.03%.

6. The corrosion rate of the hot-rolled strip steel is 30% or less of that of Q355B steel; the corrosion depth during the simulated service cycle of 25 years of the hot-rolled strip steel is 0.1 mm or less; and / or the hot-rolled strip steel has a yield strength of 600 MPa or more, a tensile strength of 700 MPa or more, an elongation at break of 20% or more, and cold bending performance satisfying the qualification under bending conditions of 180° and D = 0.5t, preferably cold bending performance satisfying the qualification under bending conditions of 180° and D = 0t. The hot-rolled strip steel according to any one of claims 1 to 5.

7. A method for manufacturing the hot-rolled strip steel according to any one of claims 1 to 6, comprising the following steps: 1) A smelting and casting step, Smelt molten steel according to the composition according to any one of claims 1 to 4, and then cast it into a slab; 2) A slab heating step, Heat the slab so that the surface temperature of the slab exceeds the range of 1050 - 1150°C within 15 minutes, keep the furnace temperature of the soaking section at 1230 - 1290°C, soak the slab for 30 - 90 minutes, and discharge the slab at a temperature of 1230 - 1290°C; 3) A rolling step, After sizing the slab exiting the heating furnace, perform rough rolling. During the rough rolling stage, high-pressure water of 15 MPa or more, preferably 20 MPa or more, is used for scale removal, and the temperature at the exit of the rough rolling is set to 1080°C or less. Perform final rolling on the rough-rolled strip steel. Multi-stand continuous rolling is used for the final rolling, and the final rolling temperature is controlled at 820 - 880°C; 4) A cooling and coiling step, Adopt laminar flow cooling for cooling, and wind the strip steel into a coil at a temperature of 630 - 680°C. After the laminar flow cooling is completed, slowly cool the steel coil through hot coil stacking, a slow cooling wall or an insulating cover, and cool the steel coil at a temperature of 530°C or more at a cooling rate of 1°C / min or less. The above method.

8. The method for manufacturing the hot-rolled strip steel according to claim 7, wherein the temperature at the exit of the rough rolling is 1040 - 1080°C.

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

10. The method for manufacturing the hot-rolled strip steel according to claim 7, wherein in step 2), the slab is heated in a heating furnace or a soaking furnace for soaking.

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