A 350MPa grade high corrosion resistant steel based on CSP process and its manufacturing method

Abstract

The invention discloses a 350MPa grade high corrosion resistant steel based on CSP process and a manufacturing method thereof, belonging to the field of steel material production. The chemical composition and weight percentage content (wt%) of the steel plate of the invention are: C: 0.08-0.10%; Si: 0.1-0.3%; Mn: 0.50-0.75%; P: 0.02-0.03%; S: ≤0.003%; Cr: 5.6-7.0%; Ni: 0.05-0.10%; Cu: 0.10-0.30%; Alt: 0.020-0.045%; the balance is Fe and unavoidable inclusions. The present invention is based on key technical points such as key component design, steelmaking and rolling, cooling process control, etc., to realize the manufacture of high-corrosion-resistant steel with high strength, high corrosion resistance, high mechanical property stability and good surface quality based on the CSP process. The corrosion-resistant steel plate does not need to add micro-alloying elements such as Nb and Ti, and is mainly used in industries such as railway vehicles, bridges, and photovoltaic brackets.

Description

Technical Field

[0001] The present invention relates to the technical field of steel materials and production thereof, and more specifically to a 350MPa grade high corrosion resistant steel based on a CSP process and a manufacturing method thereof. Background Art

[0002] High corrosion-resistant steel has extremely excellent corrosion resistance, and its atmospheric corrosion resistance is more than doubled compared to ordinary corrosion-resistant steel, so its service life can be significantly improved. In recent years, it has been used in large quantities in the manufacturing of new-generation railway passenger cars, and is expected to be widely used in industries such as bridges and photovoltaic brackets in the future. However, the existing high-corrosion-resistant steel manufacturing is mostly based on conventional process production lines. Under the background of "dual carbon" and the general trend of green environmental protection, CSP (thin slab continuous casting and rolling) technology is a short-process, low-energy consumption, and thin-gauge steel manufacturing technology. Compared with conventional process hot-rolled strip production technology, it has better economy and high efficiency. Therefore, if the composition design and manufacturing of high-performance and stable high-corrosion-resistant hot-rolled steel can be achieved in the CSP process, it will be a technical breakthrough and innovation.

[0003] At present, there are a large number of patents on high corrosion-resistant steel in China. For example, the invention patent with the patent publication number CN 106906426A discloses a high corrosion-resistant weathering steel and its manufacturing method, whose chemical composition is: C≤0.03%; Si≤0.10%; Mn: 0.1-0.5%; P≤0.015%; S≤0.005%; Nb: 0.03-0.05%; Ti: 0.02-0.05%; Cr: 4.0-6.0%; Ni: 0.4-0.6%; Cu: 0.4-0.6%; Al: 0.02-0.06%. The manufacturing method is based on conventional processes, such as continuous casting into 230mm or 250mm ingots, heating in a heating furnace for 3-4h, and using two-stage rolling of rough rolling + finishing rolling, which is significantly different from the CSP production line, and adding micro-alloying elements such as Nb and Ti.

[0004] In addition, there are also many patents on the manufacture of weathering steel based on CSP lines. For example, the invention patent with the patent publication number CN109338212A discloses a thin-gauge weathering steel and method with a yield strength of ≥550MPa produced by a short process, and its chemical composition is: C: 0.030-0.065%; Si: 0.10-0.30%; Mn: 0.7-1.3%; Cu: 0.10-0.40%; Cr: 0.20-0.55%; Ni: 0.15-0.40%; Ti: 0.06-0.10%; Nb: 0.02 -0.06%; Als: 0.01-0.06%; P≤0.020%; S≤0.008%; N≤0.008%. Although this type of steel is based on the CSP process, it is significantly different from high corrosion-resistant steel in terms of composition design. Summary of the invention

[0005] 1. Technical problem to be solved by the invention

[0006] High corrosion-resistant steel, due to the addition of high Cr content, high crack sensitivity, high hardenability, high oxide scale viscosity, etc. are the difficulties in realizing stable mass industrial production. The present invention intends to provide a 350MPa grade high corrosion-resistant steel based on the CSP process and a manufacturing method thereof. Based on key technical points such as key component design, steelmaking and rolling, and cooling process control, a high corrosion-resistant steel with high strength, high corrosion resistance, high mechanical property stability, and good surface quality based on the CSP process is manufactured. The corrosion-resistant steel does not require the addition of micro-alloying elements such as Nb and Ti, and is mainly used in industries such as railway vehicles, bridges, and photovoltaic brackets.

[0007] 2. Technical solution

[0008] In order to achieve the above object, the technical solution provided by the present invention is:

[0009] The present invention discloses a 350MPa grade high corrosion resistant steel based on CSP process, wherein the chemical composition and weight percentage content (wt%) of the steel plate are as follows: C: 0.08-0.10%; Si: 0.1-0.3%; Mn: 0.50-0.75%; P: 0.02-0.03%; S: ≤0.003%; Cr: 5.6-7.0%; Ni: 0.05-0.10%; Cu: 0.10-0.30%; Alt: 0.020-0.045%; the balance is Fe and unavoidable inclusions. More optimally, the content ratio of Cu to Ni is Cu:Ni=2-3. The present invention adopts a higher C content design of 0.08-0.10%, replacing Nb and Ti with C. The microstructure type of the steel plate is ferrite + fine pearlite. The obtained fine pearlite is dispersed in a spherical or elliptical shape. The fine pearlite accounts for 6-10%, and the rest is ferrite, which is more conducive to achieving a good strength-toughness match.

[0010] The main chemical composition design ideas of the 350MPa high corrosion-resistant steel produced based on the CSP process are:

[0011] C: An element with good economic efficiency and significant strengthening effect. Unlike the ultra-low C design of less than 0.07% used in most of the current high weathering steel patents, the present invention adopts a medium-low carbon content design of 0.08-0.10%. Through the solid solution strengthening effect of relatively high C, its strength and low yield ratio are guaranteed, and there is no need to add micro-alloying elements such as niobium and titanium. By forming ferrite + fine pearlite structure, good strength-toughness matching and forming performance are achieved; but when the C content exceeds 0.10%, the crack sensitivity of the ingot increases significantly, and longitudinal cracks and ingot corner cracks are easily generated. The C content of the present invention is strictly controlled at 0.08-0.10%.

[0012] Si: Si can improve strength through solid solution strengthening and is also an economical element for improving corrosion resistance. However, a higher content of Si can easily lead to strip-shaped oxygen compression defects on the surface of corrosion-resistant steel plates. This surface defect is difficult to remove during the descaling process and is particularly prominent in high-Cr steel. Therefore, the present invention adopts a low Si design and controls Si at 0.1-0.3%.

[0013] Mn: High Mn content will cause banded structure in the microstructure. This unevenly distributed banded structure is one of the reasons for the decrease in low-temperature toughness. At the same time, a higher content of Mn will form MnS inclusions at the center of the thickness of the ingot, thereby deteriorating the impact toughness, especially the low-temperature impact toughness. In order to reduce the generation of banded structure and MnS inclusions, the Mn content is controlled at 0.50-0.75%.

[0014] Cr, Ni, and Cu are added in combination: Cr: 5.6-7.0%; Ni: 0.05-0.10%; Cu: 0.10-0.30%, and Cu:Ni=2-3. Cr is the most important corrosion-resistant element in the present invention. A high content of Cr forms dense Cr2O3 on the surface, which significantly improves the passivation ability and hinders further corrosion of the matrix; but an excessively high content of Cr will be detrimental to welding performance and toughness, and the cost will increase significantly; Cu is also an element that significantly improves corrosion resistance, but Cu, as a low-melting-point element, is prone to cause grain boundary segregation, resulting in the appearance of "copper brittleness" defects. Therefore, the present invention adopts a low-Cu design, and at the same time adds an appropriate amount of Ni to inhibit the precipitation of Cu, and improves the low-temperature impact performance, so as to meet its use in high-cold environments. The synergistic effect of the above three corrosion-resistant elements enriches on the surface to form a dense and stable rust layer, which prevents the corrosive medium from diffusing into the matrix. The atmospheric corrosion resistance of the present invention is more than 2 times higher than that of conventional weathering steel. Based on the above comprehensive considerations, Cr is controlled at 5.6-7.0%, Ni at 0.05-0.10%, and Cu at 0.10-0.30%.

[0015] The manufacturing method of the 350MPa grade high corrosion resistant steel based on the CSP process of the present invention comprises the following steps: molten iron pretreatment deep desulfurization → converter smelting → LF → RH → thin slab continuous casting → heating → high pressure water descaling → CSP hot rolling → coiling.

[0016] Specifically, the smelting, refining and continuous casting processes include:

[0017] After deep desulfurization of molten iron to ≤0.005%, LF+RH double refining is used to reach the required composition control range; during continuous casting, the ingot thickness is 60-80mm, the ingot pulling speed is 3.0-4.5m / min, and special protective slag for weathering steel is used, the protective slag basicity is 1.0-1.3, and the viscosity is 0.9-1.6Pa·S.

[0018] The slab heating and rolling process specifically includes:

[0019] During the heating stage, the ingot is sent to a roller bottom heating furnace for low-temperature heating. The heating time is 25 to 35 minutes, and the ingot exit temperature is 1080 to 1120°C. The air-to-coal ratio of the heating furnace is 1.9 to 2.3, the excess air coefficient is 1.1 to 1.5, and during the high-pressure water descaling process, the descaling water pressure is controlled at 25 to 30 MPa.

[0020] CSP hot rolling adopts 7-stand hot rolling mill, with the starting rolling temperature of 1000~1050℃ and the final rolling temperature of 800~850℃.

[0021] In the coiling stage, the hot coil is controlled by "L-type cooling" with low temperature at the front and high temperature at the back, that is, the coiling temperature of the tail of the hot coil is higher than that of the head and middle of the hot coil. Specifically, the coiling temperature of the head and middle of the hot coil is 620-660℃, and the temperature compensation is performed on the tail of the hot coil. The coiling temperature of the 20-40m area of ​​the tail of the hot coil is 20-40℃ higher than that of the head and middle. After coiling, the surface of the steel saddle on the steel coil transportation line adopts insulation design. Preferably, the thermal conductivity of the insulation material is less than 0.2W / (m·K).

[0022] The present invention obtains a 350MPa grade high corrosion resistant steel based on the CSP process by optimizing the composition design and the controlled rolling and controlled cooling process, wherein:

[0023] (1) The “medium C + high Cr” design is mainly adopted, without the need to add Nb and Ti microalloying elements, nor the need for the “RH + LF” reverse duplex design in the conventional process to perform strict decarburization control. The CSP short-process production line can achieve efficient, more economical, and strong and tough 350MPa grade high corrosion-resistant steel production.

[0024] (2) In the CSP process, the continuous casting slab enters the roller bottom soaking furnace for heating immediately after cutting. Compared with the conventional process, the CSP short process reduces the steps of cooling the slab down to the floor, stacking, and hoisting into the furnace, thus significantly reducing the risk of cracks or even breakage during the landing, stacking, and hoisting of high Cr steel slabs.

[0025] (3) When high corrosion-resistant steel is in contact with air or other media with good thermal conductivity, it exhibits extremely strong hardenability relative to ordinary steel grades. The difference in thermal conductivity at different parts causes significant differences in microstructure, resulting in extremely poor performance stability. The present invention adopts an "L-type cooling" process for temperature compensation at the tail of the hot coil and a saddle insulation design to improve the stability of the organization and mechanical properties of the hot coil in the longitudinal direction. For example, the performance at the tail does not increase compared with other positions in the longitudinal direction, and the fluctuation of the through-coil performance is controlled to be less than 50MPa.

[0026] (4) While high Cr steel generates Cr2O3 in the matrix, which makes it have excellent atmospheric corrosion resistance, it also greatly increases the difficulty of removing the oxide scale during hot rolling. The present invention only heats for 25 to 30 minutes in the CSP production line, which is 130 to 170 minutes less than the conventional process, and adopts a low-temperature heating process of 1080 to 1120°C and a high pressure value of 25 to 30 MPa for descaling water, which effectively reduces the stickiness of the oxide scale.

[0027] The steel plate prepared according to the present invention has outstanding comprehensive properties, and its mechanical properties are: ReL≥350MPa, Rm≥510MPa, A≥30%; the fluctuation of mechanical properties in the length direction is <50MPa; -60℃ KV2 impact energy is ≥80J; after 180° cold bending (d=0a), no cracks appear on the outer side of the bent sample; compared with Q345B under 72h immersion corrosion test, the relative corrosion rate is <25%.

[0028] 3. Beneficial effects

[0029] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0030] (1) The 350MPa grade high corrosion resistant steel manufactured based on the CSP process of the present invention adopts the "medium C + high Cr" design, and its production process is matched, the obtained product has excellent comprehensive performance, mechanical properties: ReL ≥ 350MPa, Rm ≥ 510MPa, A ≥ 30%; and has good through-rolling performance stability, the fluctuation of mechanical properties in the length direction is <50MPa; -60℃ KV2 impact energy ≥ 80J (impact sample size 5*10*55mm);

[0031] Excellent cold forming performance and corrosion resistance: after 180° cold bending (d=0a), no cracks appeared on the outer side of the bent specimen; compared with Q345B in a 72h circumferential immersion corrosion test, the relative corrosion rate is <25%.

[0032] (2) The manufacturing method of 350MPa grade high corrosion resistant steel based on CSP process of the present invention reduces the steps of cooling down the ingots, dropping them off the production line, stacking them, and hoisting them into the furnace, thereby significantly reducing the risk of cracks or even breakages in the process of dropping, stacking, and hoisting high Cr steel ingots; and it can be realized by using CSP short process production line, which is more efficient and economical.

[0033] (3) The manufacturing method of 350MPa grade high corrosion resistant steel based on CSP process of the present invention adopts the "L-type cooling" process and saddle insulation design of the temperature compensation of the tail of the hot coil to improve the stability of the organization and mechanical properties of the hot coil in the longitudinal direction, and there is no increase in the performance of the tail compared with other positions in the longitudinal direction. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 The optical microstructure morphology of samples taken at the middle position (200m at the tail) and 3m at the tail in the length direction of Example 1 of the present invention is ferrite + fine pearlite, and the grain size is 12 to 12.5. The same microstructure morphology in the length direction of the hot coil ensures the stability of the mechanical properties.

[0035] Figure 2 The optical microstructure morphology of samples taken at the middle position (200m at the tail) and 3m at the tail in the length direction of Comparative Example 8, the microstructure type at the tail 3m is bainite + ferrite, and the grain size is level 12; the microstructure morphology at the tail 200m is the same as that of Example 1.

[0036] Figure 3 This is the stripe defect on the hot coil surface of Comparative Example 2 (Si content is 0.35%).

[0037] Figure 4 The yield strength (R eL ) and tensile strength (R m ) along the distribution curve of the length direction of the hot coil, under the ordinary cooling method, the through-coil strength fluctuation is as high as 170MPa, while after the implementation of "L-type cooling + saddle insulation", the yield strength fluctuation in the length direction is only 47MPa, and the tensile strength fluctuation is only 43MPa. The fluctuations of the mechanical properties of the two are both less than 50MPa. Under the slow cooling condition of "L-type cooling + saddle insulation", the cooling rates of the tail and the middle of the hot coil are similar, and the through-coil is all ferrite + fine pearlite microstructure, which effectively weakens the high hardenability effect of high Cr and high corrosion resistant steel, so that it has good performance stability. DETAILED DESCRIPTION

[0038] In order to further understand the content of the present invention, the present invention is described in detail in conjunction with the accompanying drawings.

[0039] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0040] The present invention will be further described below in conjunction with the embodiments.

[0041] Example

[0042] The chemical compositions of the examples and comparative examples of the present invention are shown in Table 1.

[0043] Table 1 Chemical compositions of the embodiments of the present invention and the comparative examples

[0044]

[0045] The main preparation process parameters of the above examples are shown in Table 2 below, and the main preparation process parameters of the comparative examples are shown in Table 3 below.

[0046] Table 2 Main process parameters of the preparation process of the embodiment

[0047]

[0048] Table 3 Main process parameters of the comparative example preparation process

[0049]

[0050] The mechanical properties and process properties of the hot coil tail 3m of the above embodiments and comparative examples are shown in Table 4, wherein the corrosion test was carried out in accordance with TB / T 2375 for 72h cycle immersion corrosion test, with Q345B low alloy steel as the comparative sample, and the test results are shown in Table 4.

[0051] Table 4 Mechanical properties and process properties of examples and comparative examples

[0052]

[0053] The above is a schematic description of the present invention and its implementation mode, which is not restrictive and is only one of the implementation modes of the present invention, and is not actually limited thereto. Therefore, if a person skilled in the art is inspired by it and, without departing from the purpose of the invention, designs a structure and an implementation mode similar to the technical solution without creativity, they shall all fall within the protection scope of the present invention.

Claims

1. A method for producing 350MPa grade high corrosion resistant steel based on CSP process, characterized in that: The chemical composition and weight percentage content (wt%) of the high corrosion resistant steel are: C: 0.08-0.10%; Si: 0.1-0.3%; Mn: 0.50~0.75%; P：0.02~0.03％； S: ≤0.003%; Cr: 5.6~7.0%; Ni: 0.05~0.10%; Cu: 0.10~0.30%; Alt: 0.020~0.045%; the balance is Fe and unavoidable inclusions; The ratio of Cu to Ni in the steel plate composition is Cu:Ni=2~3; the mechanical properties of the steel plate are: ReL≥350MPa, Rm≥510MPa, A≥30%; the fluctuation of mechanical properties in the length direction is <50MPa; -60℃ KV2 impact energy is ≥80J; after 180° cold bending d=0a, no cracks appear on the outer side of the bent specimen; compared with Q345B under 72h circumferential immersion corrosion test, the relative corrosion rate is <25%; the microstructure type of the steel plate is ferrite + fine pearlite, the fine pearlite is spherical or elliptical dispersed, and the fine pearlite accounts for 6~10%; The manufacturing method of the high corrosion-resistant steel comprises the following steps: hot metal pretreatment deep desulfurization → converter smelting → LF → RH → thin slab continuous casting → heating → high-pressure water descaling → CSP hot rolling → coiling; After the molten iron is deeply desulfurized to ≤0.005%, LF+RH double refining is used to reach the required composition control range; during the continuous casting process, the ingot thickness is 60~80mm, the ingot pulling speed is 3.0~4.5m / min, and special protective slag for weathering steel is used, with a protective slag basicity of 1.0~1.3 and a viscosity of 0.9~1.6Pa•S; In the heating stage, the ingot is sent to the roller bottom heating furnace for low temperature heating. The heating time is 25-35 minutes, and the ingot out-of-furnace temperature is 1080-1120℃. The air-to-coal ratio of the heating furnace is 1.9-2.3, the excess air coefficient is 1.1-1.5, and the descaling water pressure is controlled at 25-30MPa during the high-pressure water descaling process. The starting rolling temperature of CSP hot rolling is 1000~1050℃, and the final rolling temperature is 800~850℃; During the coiling stage, the hot coil is controlled by "L-type cooling" with low temperature at the front and high temperature at the back: the coiling temperature at the tail of the hot coil is higher than that at the head and middle of the hot coil; specifically, the coiling temperature at the head and middle of the hot coil is 620-660℃ during the coiling stage, and the coiling temperature in the 20-40m area at the tail of the hot coil is 20-40℃ higher than that at the head and middle; after coiling, the surface of the steel saddle on the steel coil transportation line adopts insulation design, and the thermal conductivity of the insulation material is <0.2W / (m·K).

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