Easily-formed corrosion-resistant and wear-resistant steel plate for conveying pipeline and manufacturing method thereof

Abstract

The invention relates to an easy-to-form corrosion-resistant and wear-resistant steel plate for a conveying pipeline and a manufacturing method thereof, and belongs to the field of pipeline steel. The surface Brinell hardness of the wear-resistant steel plate is 370-430 HB, the ductility is larger than or equal to 10%, the Charpy V-shaped impact energy at the temperature of-40 DEG C is larger than or equal to 30 J, d is equal to 4 t, and no crack exists after 180-degree cold bending; the martensite structure is uniform and fine, the grain size is larger than or equal to 7 grades, and the total grade of A, B, C and D fine inclusions in the structure is smaller than or equal to 2.0. And under the working condition that the pH value is 6.0-8.5, the corrosion resistance of the steel can be improved by 40% or above compared with that of Q355 structural steel. A high-cleanliness steelmaking continuous casting process, plate blank hydrogen diffusion, two-stage controlled rolling, high-temperature normalizing pretreatment, off-line quenching and tempering are adopted, control is conducted from the aspects of chemical component design, base metal structure and the like, and the low-temperature toughness, formability, corrosion resistance and weldability of the wear-resistant steel plate for the conveying pipeline are guaranteed; and a solution is provided for the application of the long-service-life wear-resistant pipeline.

Description

Technical Field

[0001] This invention belongs to the field of iron-based alloys, and specifically relates to a pipeline steel. Background Technology

[0002] Pipelines are crucial auxiliary equipment in road construction, mining, and dredging. Affected by the transported media (coarse sand, ore, slurry, etc.), pipelines suffer severe wear and corrosion, significantly reducing their service life and increasing construction maintenance and material consumption. Currently, most pipelines are made from Q355 low-alloy structural steel plates, which have low strength, good plasticity, good formability, high rolling efficiency, and low cost. However, due to their low hardness (≤200HB), they have poor wear resistance and short service life. Furthermore, the low alloy content of the structural steel and the lack of corrosion resistance design lead to severe corrosion failure when used in humid environments such as mines and oceans. The development of corrosion-resistant, easily formable, and highly wear-resistant pipelines has become a key technical challenge urgently needing to be solved in the metallurgical and wear-resistant industries.

[0003] Chinese Patent Publication No. CN113774275A discloses an easily formable, high-wear-resistant steel plate for sediment transport pipelines and its preparation method. The steel plate has low hardness (280-360 HB), failing to compensate for insufficient wear resistance. It employs a high Ti (Ti: 0.40-0.80%) composition design; however, the excessively high Ti content leads to the precipitation of a large number of TiC ultra-hard particles, significantly reducing the steel plate's ductility and toughness. The steel plate's room temperature impact resistance is only 20 J. Furthermore, this patent does not address the corrosion resistance of the steel plate.

[0004] Chinese Patent Publication No. CN116179964A discloses a low-cost, high-hardness steel plate for dredging pipelines and its production method. The steel plate has high hardness (≥430HB). Excessive hardness leads to increased stress in the steel plate, causing deformation of the straight seam edges during pipe rolling and making centering welding difficult. To reduce costs, this patent uses a high C / Mn alloy design, with C: 0.16-0.20% and Mn: 1.30-1.50%. C and Mn are easily segregated elements, which increase the degree of segregation at the center of the steel plate, leading to uneven performance and cracking during pipe rolling, while also reducing the steel plate's corrosion resistance.

[0005] In summary, existing technologies mainly address the wear resistance of pipelines by increasing hardness or precipitating superhard particles, but there is limited research on solutions to problems related to deformation during pipeline forming, precision control, and corrosion resistance in subsequent applications. Summary of the Invention

[0006] The purpose of this invention is to provide a 4-20mm thick, 400HB grade easily formable, corrosion-resistant, and wear-resistant steel plate and its manufacturing method. This wear-resistant steel plate has a surface Brinell hardness of 370-430HB, elongation ≥10%, Charpy V-impact energy ≥30J at -40℃, and exhibits no cracking after 180° cold bending at d=4t. The martensitic structure is uniform and fine, with a grain size ≥7, and the total grade of A, B, C, and D fine inclusions in the structure is ≤2.0. Under working conditions with a pH of 6.0-8.5, its corrosion resistance is more than 40% higher than that of Q355 structural steel.

[0007] To achieve the above objectives, the chemical composition of the present invention, by mass percentage, is as follows: C: 0.12–0.15%, Si: ≤0.25%, Mn: 0.30–0.50%, Nb: 0.010–0.040%, Ti: ≤0.008%, Al: 0.05–0.07%, Cr: 0.80–1.00%, B: 0.0015–0.005%, Ca: 0.0010–0.0050%, P: ≤0.010%, S: ≤0.0015%, O: ≤0.0015%, N: ≤0.0035%, H: ≤0.0002%, with the balance being Fe and unavoidable impurity elements, and satisfying a carbon equivalent (CEV) ≤0.45%.

[0008] The rationale for limiting the steel composition in this invention is explained as follows: C: Carbon content determines the hardness of the steel plate. Low carbon content results in low hardness, good toughness, and excellent weldability; high carbon content leads to complete martensitic transformation during quenching, resulting in high strength, high hardness, and good wear resistance, but also reduced plasticity and toughness, and poor weldability. Based on the requirements for steel plate hardness, wear resistance, and weldability, the carbon content in this invention is controlled at 0.12–0.15%.

[0009] Si: Silicon dissolved in ferrite and austenite improves strength and hardness. Excessive content deteriorates the toughness and weldability of martensitic steel, while also reducing surface quality. To ensure pipe surface quality and improve weld performance, this invention controls the Si content to a low level of ≤ 0.25%.

[0010] Manganese (Mn) is an effective element for increasing austenite stability and improving the hardenability of steel. However, excessive amounts can lead to grain coarsening, segregation in continuously cast billets, poor toughness, and reduced weldability. Mn has poor corrosion resistance and will preferentially corrode in pipelines transporting corrosive media such as mud and sea sand; therefore, its content must be controlled to avoid excessive levels. This invention limits the added manganese content to the range of 0.30% to 0.50%.

[0011] Cr: An essential element for improving the corrosion resistance of steel plates, it also reduces the critical cooling rate for martensitic transformation and improves hardenability. Cr can also form various carbides in steel, increasing its strength, hardness, and wear resistance, as well as its resistance to high-temperature tempering. However, excessive Cr content reduces the weldability of the steel plate and makes it prone to cracking during quenching. This invention investigates the effect of different Cr contents on the corrosion rate of steel plates, see [link to relevant documentation]. Figure 1 When the Cr content is ≥0.80%, the corrosion rate of the steel plate decreases significantly, demonstrating good corrosion resistance. Considering the balance between corrosion resistance and weldability, the chromium content in this invention is controlled between 0.80% and 1.00%.

[0012] Nb: The solute dragging effect of Nb and the pinning effect of nano-sized precipitates Nb(C,N) on austenite grain boundaries inhibit austenite grain growth during heating. The effect is not significant when the addition amount is less than 0.010%, and decreases toughness when it exceeds 0.040%, leading to surface cracks in the continuously cast billet. Therefore, this invention specifies that the niobium content should be within the range of 0.010% to 0.040%.

[0013] Ti has a strong affinity for N. Trace amounts of Ti are typically added to B-containing steels to promote TiN formation and prevent BN grain boundary precipitation, ensuring the hardenability of the steel plate by controlling the Ti / N ratio to ≥3.42. However, TiN forms at temperatures above 1400℃, usually precipitates in the liquid phase, and is relatively large, easily reaching the micrometer scale. Furthermore, the precipitates are hard, often with sharp corners, and are difficult to deform, leading to microcracks during the bending process of the steel plate and pipe. Therefore, this invention does not allow the addition of Ti and explicitly controls the Ti content to ≤0.008%.

[0014] Al: A strong deoxidizing element, and also has a strong affinity for nitrogen (N), which can eliminate the aging sensitivity caused by N. The precipitation of nitrogen compounds refines the austenite grains and protects the hardenability of boron (B). In this invention, the Al content is specified to be between 0.050% and 0.070%.

[0015] B: The present invention adds 0.0015% to 0.0050% trace amounts of B, the main purpose of which is to improve the hardenability of the steel plate, thereby reducing the amount of other precious metals added and lowering costs. B exceeding 0.005% easily causes segregation, forming borides, which severely deteriorates the toughness of the steel plate and reduces hardenability.

[0016] Ca: Ca treatment is typically used to modify inclusions, changing elongated inclusions such as MnS into spherical inclusions such as CaS, thereby reducing the anisotropy of the steel plate and improving its overall performance. In this invention, the Ca content is controlled at 0.0010–0.0050%.

[0017] P: A harmful element that adversely affects the plasticity and toughness of materials. This invention aims for ultra-pure steel, strictly controlling the P content to ≤0.01%.

[0018] Sulfur (S): A harmful element in steel, it adversely affects the material's plasticity and toughness. High S content easily leads to the formation of long, thin inclusions such as MnS, causing anisotropy in the steel plate and making it prone to delamination and cracking. This invention requires S content to be ≤0.0015%.

[0019] O, N, and H: Harmful gaseous elements. High content leads to numerous inclusions, easily causing white spots, significantly reducing the plasticity and toughness of steel plates, and resulting in delayed-cutting cracks. This invention strictly controls the O content to no more than 0.0015%; the N content to no more than 0.0035%; and the H content to ≤0.0002%.

[0020] CEV: This invention adopts the carbon equivalent formula CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; carbon equivalent has a significant impact on the strength, cutting crack resistance, and weldability of steel. High CEV results in high strength but reduced weldability and increased susceptibility to cracking; typically, when CEV>0.45%, preheating welding is required to reduce welding crack sensitivity; to improve the welding efficiency of straight and circumferential seams in pipelines and achieve welding without preheating, this invention controls CEV≤0.45%.

[0021] The present invention also provides a method for preparing the above-mentioned easily formable, corrosion-resistant, and wear-resistant steel plate for conveying pipelines, the specific process of which is as follows: Smelting and continuous casting processes: Molten iron undergoes pretreatment, smelting is carried out in an electric furnace or converter, and then refined in an LF refining furnace, followed by VD or RH vacuum treatment. After degassing, the molten steel undergoes trace Ca treatment. To ensure the ductility and toughness of the steel plate, the total grade of inclusions (A, B, C, and D) is controlled to ≤2.0. Continuous casting employs low superheat pouring, argon protection throughout the process, and dynamic light reduction control. The superheat of the molten steel is controlled between 5 and 25°C, and the central segregation does not exceed grade C1.0.

[0022] Hydrogen diffusion treatment process: Due to the high hardness and strength of wear-resistant pipes, hydrogen atoms can easily cause delayed cracking under acidic or alkaline conditions. To solve this problem, this invention requires that the continuously cast billet undergo slow cooling treatment in a pit or under a cover after it comes off the production line. The initial temperature of the treatment should be controlled at 650-700℃, the treatment time should not be less than 60 hours, and the slow cooling rate should be ≤5℃ / h.

[0023] Heated rolling process: The billet is heated to 1180-1250℃, and held at that temperature for at least 30 minutes once the core temperature reaches the surface temperature. After descaling, the billet undergoes a two-stage controlled rolling process of roughing and finishing to refine the grains and improve strength and toughness. The initial rolling temperature for roughing is between 1000-1100℃, with a minimum thickness of ≥4H (where H is the finished thickness). The initial rolling temperature for finishing is between 820-920℃. After rolling, the steel plate is slowly cooled by air cooling. Using a large intermediate billet thickness and a suitable finishing temperature ensures austenite deformation and accumulates deformation energy. Furthermore, it promotes the induced precipitation of fine Nb precipitates, pins grain boundaries and dislocations, increases nucleation sites, and refines the grains.

[0024] Normalizing heat treatment process: To reduce quenching stress in steel plates, increase the uniformity of steel plate microstructure, and improve tube manufacturing accuracy and efficiency, high-temperature normalizing pretreatment is performed on the steel plates before quenching. The normalizing temperature is 920-950℃, with a temperature control accuracy of ±10℃, and the holding time after reaching the furnace temperature is 30-60 minutes. High-temperature normalizing can promote the full equiaxing of anisotropic grain structure after rolling; on the other hand, it can further homogenize micro-segregated elements; thereby reducing quenching stress and deformation in subsequent tube manufacturing processes, and improving tube manufacturing accuracy and efficiency.

[0025] Quenching heat treatment process: After normalizing, the steel plate undergoes offline quenching. The quenching temperature is 880–920℃, and the holding time after reaching the furnace temperature is 30–60 minutes. To ensure the uniformity of the steel plate, the temperature control accuracy is ±10℃ to ensure a uniform quenched martensitic structure.

[0026] Tempering heat treatment process: After quenching, the steel plate needs to be stress-relieved tempered at 150-250℃. After the core of the steel plate reaches the temperature, the tempering holding time is 60-120 minutes. The steel plate obtains a stable low-stress tempered martensitic structure, with excellent low-temperature impact toughness, while significantly reducing the risk of deformation and cracking during steel plate rolling.

[0027] Compared with the prior art, the advantages of the present invention are as follows: This invention adopts a low Mn, Ti-free, and medium Cr design, with low alloy content in the steel plate and CEV≤0.45%. While ensuring the wear resistance of the steel plate, it also has excellent toughness, corrosion resistance and weldability, enabling wear-resistant conveying pipelines to be welded without preheating.

[0028] In terms of production process, this invention adopts two-stage controlled rolling, with an intermediate billet thickness ≥4H, where H is the target thickness, to ensure austenite deformation and accumulate deformation energy, promoting the refinement of the original austenite grains; after rolling and before quenching, a high-temperature normalizing pretreatment is performed, which on the one hand promotes the full equiaxing of the anisotropic grain structure after rolling; on the other hand, it promotes the further homogenization of micro-segregated elements; it can significantly reduce quenching stress, reduce deformation in the subsequent tube making process, and improve tube making accuracy and efficiency.

[0029] In terms of product performance, the steel plate of this invention has a surface Brinell hardness of 370-430 HB; tensile strength ≥1100 MPa, elongation ≥10%, and Charpy V-type impact energy ≥30 J at -40℃, exhibiting good low-temperature toughness and strength; the steel plate can be bent 180° without cracking when d=4t (t is the thickness of the steel plate), exhibiting good formability; under working conditions with a pH value of 6.0-8.5, its corrosion resistance can be improved by 40% compared with Q355 structural steel.

[0030] The easily formable wear-resistant steel plate for conveying pipelines of the present invention has excellent comprehensive performance. Compared with existing steel grades, it has low alloy content, and the added elements are all conventional elements. The production and manufacturing process is simple to control, which is an inevitable trend for the development of social economy and steel industry. Attached Figure Description

[0031] Figure 1 This relates to the corrosion rate of wear-resistant steel plates and the Cr content. Figure 2 Metallographic images (500×) of typical microstructure of the test steel in Example 1 of this invention; Figure 3 The test steel of Example 1 was subjected to a 180° bend in the laboratory according to the national standard GB / T232. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0033] The production process of the high-temperature wear-resistant steel of this invention is as follows: converter steelmaking -> LF refining -> VD or RH high vacuum degassing -> continuous casting -> billet hydrogen diffusion treatment -> heating -> rolling -> normalizing -> quenching -> medium-temperature tempering The method for producing the easily formable wear-resistant steel plate according to Embodiment 1 of the present invention includes the following steps: (1) Smelting: Smelting is carried out in an electric furnace or converter, and then the mixture is sent to an LF refining furnace for refining and then subjected to VD or RH vacuum treatment. The total level of inclusions in categories A, B, C and D is controlled to be ≤2.0.

[0034] (2) Continuous casting: The molten steel is cast into a 150mm thick continuous casting billet. The casting temperature is controlled at 5-25℃ above the liquidus. Dynamic light pressure is implemented during the casting process, and the center segregation is not higher than C1.0 grade.

[0035] (3) Hydrogen diffusion treatment of slabs: After the slabs are cast, they are placed in the pit for slow cooling treatment. The initial temperature of the treatment is controlled at 650-700℃, the time is ≥60 hours, and the slow cooling rate is ≤5℃ / h.

[0036] (4) Heating and rolling: The continuously cast billet obtained in step (3) is placed into a walking beam furnace and heated to 1180-1250℃. When the core temperature reaches the surface temperature, the billet is held for at least 30 minutes. After the billet exits the furnace, it undergoes two-stage controlled rolling: roughing and finishing. The initial rolling temperature for roughing is between 1000-1100℃. The thickness of the intermediate billet after roughing is controlled to be ≥4H, where H is the finished product thickness. The initial rolling temperature for finishing is between 820-920℃. To reduce residual stress, the steel plate is slowly cooled by air cooling after rolling.

[0037] (5) Normalizing: The normalizing temperature of the steel plate is 940±10℃ and the holding time is 30~60min.

[0038] (6) Quenching: The quenching temperature of the steel plate is 920±10℃, the holding time is 30~60min, and the quenching medium is water.

[0039] (7) Tempering: Quenched steel plates are placed in a tempering furnace for high-temperature tempering at a temperature of 150-250°C and a holding time of 60-120 minutes.

[0040] Specific components and process parameters are shown in Tables 1 to 4. The performance of each example sample is shown in Table 5.

[0041] Figure 1 This study investigates the effect of different Cr contents on the corrosion rate of steel plates under accelerated salt spray corrosion tests. In Example 1, the Cr content of the tested steel was 0.95%, compared to 0.10% for conventional Q355 structural steel plates used in pipelines. It is evident that the annual corrosion rate of the tested steel in Example 1 was reduced by approximately 50% compared to Q355, demonstrating excellent corrosion resistance.

[0042] Figure 2 Microscopic photographs of the experimental steel from Example 1 are provided. The microstructure of the finished steel plate is a uniform martensitic structure. It can be seen that after two-stage controlled rolling, normalizing, and quenching multi-stage heat treatment, the steel plate has fine and uniform grains with a grain size ≥ 7, which fully guarantees the low-temperature impact toughness and formability of the steel plate while meeting the strength requirements.

[0043] Figure 3The morphology of the test steel plate from Example 1 after being bent 180° in the laboratory according to the national standard GB / T232 is shown. The bending diameter d = 4t. No cracks were observed after bending, and the surface was good, demonstrating good formability.

[0044] The invention employs a high-purity steelmaking continuous casting process, slab hydrogen expansion, two-stage controlled rolling, high-temperature normalizing pretreatment, offline quenching, and tempering. It controls the process from the perspectives of chemical composition design and base material microstructure, ensuring the low-temperature toughness, formability, corrosion resistance, and weldability of wear-resistant steel plates for pipelines, and providing a solution for the application of long-life wear-resistant pipelines.

[0045] Table 1. Chemical composition (wt%) of wear-resistant steel plates in the examples.

[0046] Table 2 Continuous Casting Process Control

[0047] Table 3 Rolling Process Control

[0048] Table 4 Heat Treatment Process Control

[0049] Table 5. Hardness, tensile strength, impact strength, and bending strength of embodiments of the present invention.

Claims

1. A steel plate for conveying pipelines, characterized in that: It is composed of the following components in weight percentage: C: 0.12-0.15%, Si: ≤ 0.25%, Mn: 0.30-0.50%, Nb: 0.010-0.040%, Ti: ≤0.008%, Al: 0.05-0.07%, Cr: 0.80-1.00%, B: 0.0015-0.005%, Ca: 0.0010-0.0050%, P: ≤0.010%, S: ≤0.0015%, O: ≤0.0015%, N: ≤0.0035%, H: ≤0.0002%, with the balance being Fe and unavoidable impurity elements, and satisfying a carbon equivalent (CEV) ≤0.45%.

2. The steel plate according to claim 1, characterized in that: The product has a thickness of 4-20mm, a microstructure of tempered martensite, a grain size of ≥7, and a total grade of A, B, C, and D fine inclusions in the microstructure of ≤2.

0.

3. The steel plate according to claim 1, characterized in that: Surface Brinell hardness 370~430HB, elongation ≥10%, Charpy V-type impact energy ≥30J at -40℃, no cracks when cold bent at 180° with d=4t.

4. A method for manufacturing a conveying pipeline, characterized in that: Including steps (1) Smelting: Smelting is carried out in an electric furnace or converter according to the chemical composition, and further fine-tuning is carried out by ladle refining and vacuum treatment; (2) Continuous casting: The molten steel is poured into a continuous casting billet using the continuous casting process; (3) Hydrogen diffusion: After the billet comes off the production line, it is placed in a pit or covered with a cover for slow cooling and hydrogen diffusion treatment; (4) Heating and rolling: The billet is heated to 1180-1250℃. When the core temperature reaches the surface temperature, the billet is held for at least 30 minutes. After the billet is removed from the furnace, it is descaled and then subjected to two-stage controlled rolling: rough rolling and finish rolling. The starting temperature of rough rolling is 1000-1100℃. The thickness after the rough rolling is ≥4H, where H is the finished thickness. The starting temperature of finish rolling is 820-920℃. After the steel plate is rolled, it is slowly cooled by air cooling. (5) Normalizing: The rolled steel plate is normalized; (6) Quenching: The steel plate is quenched after normalizing; (7) Tempering: The steel plate is tempered after quenching.

5. The method according to claim 4, characterized in that: Step (2): During continuous casting, the superheat of molten steel is controlled at 5-25℃, and the center segregation is not higher than C1.0 grade.

6. The method according to claim 4, characterized in that: Step (3): The starting temperature of the hydrogen diffusion treatment is controlled at 650-700℃, the time is not less than 60 hours, and the slow cooling rate is ≤5℃ / h.

7. The method according to claim 4, characterized in that: Step (5), normalizing temperature: 920-950℃, temperature control accuracy within ±10℃, hold the furnace at the normalizing temperature for 30-60 minutes.

8. The method according to claim 4, characterized in that: Step (6), quenching temperature: 880~920℃, temperature control accuracy within ±10℃, hold for 30~60min after the furnace reaches the temperature.

9. The method according to claim 4, characterized in that: Step (7), tempering temperature: 150~250℃, temperature control accuracy within ±10℃, hold the furnace at the temperature for 60~120min after reaching the temperature.

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