A weather-resistant steel for coastal light poles produced by a short process and a production method thereof

Abstract

A type of weathering steel for coastal light poles, produced using a short-process method, has the following composition and wt%: C: 0.03–0.06%, Si: 0.40–0.80%, Mn: 0.20–0.45%, Cr: 1.05–1.49%, Cu: 0.30–0.80%, P: 0.07–0.15%, S≤0.005%, Ti: 0.02–0.08%, N≤0.006%, Ca: 0.0010–0.0025%. Production method: conventional smelting and casting into billets; heating the billets; precision rolling; post-cooling; coiling; ready for use. This invention provides a high-strength, atmospheric-corrosion-resistant hot-rolled strip steel manufactured through a short-process technology, ensuring a yield strength ≥420MPa, tensile strength ≥500MPa, yield-to-tensile ratio ≤0.80, elongation at break ≥20%, and low-temperature impact toughness with an impact energy of over 60J at -20℃. The weather resistance, particularly in marine climates, is improved from a corrosion resistance index (I) of no more than 7.5 to no less than 8, more than double that of ordinary weathering steel. This meets the requirements for use in C4-C5 environments.

Description

Technical Field

[0001] This invention relates to weathering steel and its production method, specifically to a method for producing weathering steel for coastal light poles using a short-process production line, which includes multiple production lines such as CSP, ESP, and MCCR. Background Technology

[0002] Short-process manufacturing is gaining increasing attention due to its simple, efficient, and energy-saving manufacturing process, as well as its high dimensional accuracy and high-performance uniformity. Depending on the production line layout, it has formed various forms such as CSP, ESP, and MCCR, but in essence, they are all continuous casting and rolling.

[0003] Coastal environments are typically C4-C5 environments, where lampposts are usually made of thick galvanized steel. However, galvanized steel suffers from a series of problems, including susceptibility to damage, difficult maintenance, and pollution from discarded parts. Therefore, under the requirements of a green, low-carbon, and environmentally friendly era, replacing galvanized steel with weathering steel has become a feasible solution. Weathering steel is already widely used in the production of outdoor steel structures requiring resistance to atmospheric corrosion, such as containers, railway vehicles, and bridges, and in some cases, it can be used without coating. Currently, the manufacture of high-strength weathering steel using short-process production lines also involves some invention patents, with products mainly used in containers, railway vehicles, and derrick structures, with a product lifespan of ≤10 years. (As searched:)

[0004] Chinese patent application CN201811122091.0 discloses "A thin-gauge weathering steel with a tensile strength ≥800MPa and a method thereof produced using a short process," with a composition of Cr: 0.35–0.50% and Cu: 0.05–0.30%. However, it suffers from a low content of corrosion-resistant alloys, achieving only a general level of weather resistance. Furthermore, due to its C: 0.04–0.09% and Mn: 1.45–1.75%, it readily enters the peritectic region, leading to abnormal slab quality and even steel leakage accidents during high-speed continuous casting due to peritectic reactions.

[0005] Chinese patent application CN201811122365.6 discloses "A method for producing thin-gauge weathering steel with a yield strength ≥550MPa using a short process." The main technical components are: 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%. Its disadvantages are that due to the low Cu and Cr content, it only meets the requirements of general weathering steel, and the presence of Ni (0.15–0.40%) results in higher manufacturing costs.

[0006] Chinese patent application CN201610766086.8 discloses a method for producing weathering steel based on ESP thin slab continuous casting and rolling process. The raw materials in this document, by mass percentage, include: 0.01–0.06% C, 0.30–0.60% Si, 0.30–0.60% Mn, 0.05–0.12% P, 0.25–0.45% Cu, 0.30–0.80% Cr, ≤0.30% Ni, and ≤0.01% S. Casting is performed at a casting speed of not less than 4.5 m / min, and the temperature at the finishing mill exit is not less than 820°C. The hot-rolled strip is then cooled to 500–600°C via laminar flow cooling before being coiled and stored in a coiler. This literature is still geared towards the application needs of general weathering steel, intentionally controlling the Cu and Cr content, and does not add Ti alloying element. The Mn content is only 0.30-0.60%, resulting in a low overall strength level.

[0007] A comparison with existing patents reveals that current weathering steel is mainly designed for railway vehicles, containers, bridges, and other applications, primarily in C1-C3 environments, and cannot meet the requirements for long-term outdoor fixed environments in coastal C4-C5 environments. Summary of the Invention

[0008] This invention aims to overcome the shortcomings of existing technologies, such as the inability to meet the requirements of long-term outdoor fixed environment use in coastal C4-C5 environments and the low corrosion resistance index I. It provides a high-strength, atmospheric-corrosion-resistant hot-rolled strip steel manufactured through a short-process technology, ensuring a yield strength ≥420MPa, tensile strength ≥500MPa, yield-to-tensile ratio ≤0.80, elongation at break ≥20%, and low-temperature impact toughness (-20℃ impact energy can reach above 60J). The weather resistance, especially in marine climates, is improved from a corrosion resistance index I of no more than 7.5 to no less than 8, more than double that of ordinary weathering steel. This invention meets the usage requirements of steel grades in C4-C5 environments and is a short-process production method for weathering steel for coastal light poles.

[0009] Measures to achieve the above objectives:

[0010] A type of weathering steel for coastal light poles, produced using a short-process manufacturing method, comprises the following components and weight percentages: C: 0.03–0.06%, Si: 0.40–0.80%, Mn: 0.20–0.45%, Cr: 1.05–1.49%, Cu: 0.30–0.80%, P: 0.07–0.15%, S≤0.005%, Ti: 0.02–0.08%, N≤0.006%, Ca: 0.0010–0.0025%, with the remainder being Fe and impurities. The steel also satisfies the following two conditions: C + Mn / 6 + Cr / 10 + Cu / 5 + P / 15 not exceeding 0.50% and I = 26.01%Cu + 1.20%Cr + 1.49%Si + 17.28%P - 33.29%Cu. 2 ≥8; The metallographic structure consists of ferrite and Mao islands, wherein the volume content of ferrite is 65-80% and the volume content of Mao islands is 20-35%.

[0011] Preferably, the weight percentage content of Cr is 1.05% to 1.35%.

[0012] Preferably, the weight percentage content of Cu is 0.33% to 0.53%.

[0013] Preferably, the weight percentage content of P is 0.085 to 0.130%.

[0014] Preferably, the concentration of C+Mn / 6+Cr / 10+Cu / 5+P / 15 is between 0.30 and 0.40.

[0015] A method for producing weather-resistant steel for coastal light poles using a short-process production method, comprising the following steps:

[0016] 1) Conventional smelting and casting of billets, during which: the casting speed is controlled to be no less than 3.8m / min, the superheat of molten steel is controlled to be ≤15℃, the light reduction rate of continuous casting slabs is controlled to be no less than 20%, and the thickness of the billet is controlled to be 62~72mm;

[0017] 2) Heat the billet to a temperature of 1200-1250℃, and keep it at this temperature for 20-30 minutes.

[0018] 3) Perform finishing rolling to a thickness of 1.5-4mm with a cumulative deformation rate of not less than 94%; during this period: high-pressure water descaling between the rolling mills is not required, and the temperature difference between the middle and the edge of the steel strip is controlled to be less than 30℃.

[0019] 4) Perform post-cooling: first air cool for 5-10 seconds, then cool to the winding temperature at a cooling rate of not less than 80℃ / s.

[0020] 5) Perform winding, controlling the winding temperature between 580 and 660℃;

[0021] 6) To be used.

[0022] Preferably, the temperature difference between the middle and the edge of the steel strip is controlled to be less than 25°C.

[0023] The role and mechanism of each component and main process in this invention

[0024] C: C is an effective strengthening element in steel. Besides solid solution strengthening, it can also form nanoscale second-phase precipitates with microalloying elements such as Ti and Nb, playing a role in precipitation strengthening and microstructure refinement. Simultaneously, in high-Cr content systems, C can transform into bainitic or martensitic hard phases at relatively low critical cooling rates, significantly improving the tensile strength of the material. However, excessive C will form more carbides in the steel, reducing the material's toughness and formability, and also forming galvanic cells that reduce the steel's corrosion resistance and weldability. Considering the impact of peritectic reactions on the surface quality of the billet during high-speed continuous casting, this invention designs the C content to be 0.03-0.06%.

[0025] Si: Si is a commonly used deoxidizing element in steel. It also has a solid solution strengthening effect on steel and can improve the corrosion resistance of materials. However, excessive Si content will reduce the weldability of materials and cause the toughness of the weld heat-affected zone to deteriorate. Therefore, the designed Si content is 0.40-0.80%.

[0026] Mn: Mn is an important strengthening and toughening element in steel, playing a role in solid solution strengthening. It can also lower the transformation temperature of supercooled austenite and the transformation temperature of ferrite, which is beneficial for microstructure refinement and improves the strength and toughness of the material. However, excessive Mn content will inhibit ferrite transformation, causing the microstructure to transform into bainite, thus reducing the plasticity and cold formability of the material. Therefore, the Mn content in this invention is designed to be 0.2%-0.45%.

[0027] Cr: Cr is an important element for improving the weather resistance of steel plates. On the one hand, it increases the corrosion potential of the matrix, thereby reducing the corrosion rate by increasing electrochemical impedance; more importantly, it can promote the formation of a dense rust layer on the surface, which physically blocks the corrosive medium and changes the corrosive environment at the matrix location. This invention requires a Cr content of 1.05%-1.49%.

[0028] Cu: Cu is also an important corrosion-resistant element, and its effect is more pronounced when added together with Cr. Cu can promote the formation of a dense rust layer on the steel surface; adding more than 0.10% Cu can significantly improve the atmospheric corrosion resistance of steel. However, Cu is a metal with a low melting point. During the heating of strip steel, due to selective oxidation, the low-melting-point copper-rich phase tends to accumulate on the matrix surface and easily forms copper brittle cracks and peeling defects on the strip surface during rolling, deteriorating the surface quality. At the same time, Cu is also a valuable element; the Cu content in this invention is designed to be 0.30%-0.80%.

[0029] P: P can promote the formation of a protective rust layer on the surface and effectively improve the atmospheric corrosion resistance of steel. However, P is also a harmful impurity element in steel and is prone to segregation in the center of the thickness during continuous casting of steel billets. This invention requires a P content of 0.07-0.15%.

[0030] S: S is a common harmful impurity element in steel, which has an adverse effect on weather resistance, low temperature toughness, weldability, cold forming performance, etc. Therefore, the S content is required to be ≤0.005%.

[0031] Ti: Ti is a strong carbonitride forming element that can precipitate as extremely fine TiC or Ti(C,N) second-phase particles, significantly improving the strength of materials. Ti is relatively inexpensive compared to microalloying elements such as Nb, therefore it is added as an important strengthening element in this patent. Simultaneously, the precipitation of TiC significantly reduces the formation of larger carbides or pearlite from free C, thereby reducing the galvanic cell effect of heterogeneous phases during corrosion, improving the material's resistance to intergranular corrosion, resulting in uniform corrosion, improved rust layer quality, and enhanced strength while also improving corrosion resistance. However, excessive addition of Ti gradually weakens the precipitation strengthening effect and begins to significantly affect the low-temperature toughness of the steel. Therefore, the designed Ti content is 0.02-0.08%.

[0032] N: N is an impurity element in steel. N and Ti can combine in molten steel to form large-sized TiN inclusions. This will reduce the effective content of Ti on the one hand, and TiN inclusions will also significantly impair the toughness of steel on the other hand. Therefore, the content of N should be controlled as low as possible. This invention requires the N content to be ≤0.006%.

[0033] Ca: Ca can form spherical dispersed CaS with S, thereby improving the distribution of sulfide inclusions in steel, improving uniform corrosion and increasing the toughness of the material. The Ca content is generally limited to 0.0005-0.0025%.

[0034] C+Mn / 6+Cr / 10+Cu / 5+P / 15≤0.5: This design aims to fully utilize the strengthening and corrosion-resistant properties of C, Mn, and Cr while taking into account the impact of peritectic reactions in high-speed continuous casting on the manufacturing process. Mn and Cr both improve the hardenability of materials, allowing them to form bainite or martensite at lower critical cooling rates, thus achieving higher strength and a lower yield strength ratio. However, excessively high Mn and Cr content can promote entry into the peritectic region, causing surface shrinkage of the billet shell during short-process high-speed continuous casting, leading to surface crack defects, and even, in severe cases, billet shell cracking resulting in leaks.

[0035] The reason this invention controls the casting speed to be no less than 3.8 m / min, the superheat of molten steel to be ≤15℃, and the light reduction rate of the continuously cast slab to be no less than 20% is because the composition design includes a high content of phosphorus (P). P segregation leads to overall performance degradation. Therefore, during continuous casting, a low superheat is required, combined with slab reduction, to reduce or avoid selective solidification of P during slab solidification. With high superheat, the center of the slab remains liquid during solidification, making it prone to elemental segregation due to selective solidification. A certain amount of slab reduction can displace the unsolidified liquid. Combining low superheat and slab reduction effectively solves the P segregation problem.

[0036] The reason this invention controls the furnace entry temperature of the billet to be no less than 900℃, and the billet heating temperature to be between 1200 and 1250℃, holding it at this temperature for 20 to 30 minutes, is because the billet composition is designed to contain Ti and Cu. To ensure the solid solution of Ti, a high heating temperature is required, while Cu has a low melting temperature. Therefore, there is a contradiction between the two during the heating stage. This patent design uses a high furnace entry temperature for the billet, rapidly heating it to the Ti solid solution temperature range, and holding it for a short time to limit the melting effect of Cu, thus reducing the occurrence of copper embrittlement defects caused by Cu melting on the surface.

[0037] The reason this invention controls the cumulative deformation rate of the finishing mill to no less than 94% when rolling down to a thickness of 1.5-4mm, without requiring high-pressure water descaling between mills, and controls the temperature difference between the center and edge of the strip to be below 30℃, is because this invention is designed with a higher Cr content, which greatly improves the hardenability of the strip. When the temperature deviation of the strip is large, differences in microstructure and properties are prone to occur, and abnormal strip shape can easily result from these differences in microstructure. However, during the rolling process, the temperature drop at the edge of the strip is generally greater than that at the center, and can be controlled within 30℃ without the use of various cooling water systems between mill stands.

[0038] The reason why this invention first air-cools for 5-10 seconds in the later cooling stage and then cools to the coiling temperature at a cooling rate of not less than 80℃ / s is that ferrite is the microstructure with the best relative corrosion resistance. By using the later cooling stage and air-cooling for 5-10 seconds after final rolling, the rolled microstructure can be fully restored and homogenized. With a cooling rate of not less than 80℃ / s, a fine and uniform ferrite microstructure with Mao islands can be obtained.

[0039] The reason why the winding temperature is controlled at 580-660℃ in this invention is that the Ti precipitation strengthening effect is best within this temperature range.

[0040] Compared with the prior art, the present invention provides high-strength, atmospheric corrosion-resistant hot-rolled strip steel manufactured through a short-process technology, under the conditions of ensuring yield strength ≥420MPa, tensile strength ≥500MPa, yield ratio ≤0.80, elongation at break ≥20%, and low-temperature impact toughness with an impact energy of over 60J at -20℃. The weather resistance, especially the corrosion resistance index I in marine climates, is improved from no more than 7.5 to no less than 8, and the weather resistance is more than twice that of ordinary weathering steel, which can meet the requirements of steel grades in C4-C5 environments. Attached Figure Description

[0041] Figure 1 This is an image of the metallographic structure of the present invention at a winding temperature of 580°C, magnified 500 times.

[0042] Figure 2 This is an image of the metallographic structure of the present invention at a winding temperature of 660°C, magnified 500 times. Detailed Implementation

[0043] The present invention will now be described in detail:

[0044] Table 1 is a list of chemical components of the various embodiments and comparative examples of the present invention;

[0045] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0046] Table 3 is a list of performance test results for each embodiment and comparative example of the present invention.

[0047] The various embodiments of the present invention are produced according to the following steps.

[0048] 1) Conventional smelting and casting of billets, during which: the casting speed is controlled to be no less than 3.8m / min, the superheat of molten steel is controlled to be ≤15℃, the light reduction rate of continuous casting slabs is controlled to be no less than 20%, and the thickness of the billet is controlled to be 62~72mm;

[0049] 2) Heat the billet to a temperature of 1200-1250℃, and keep it at this temperature for 20-30 minutes.

[0050] 3) Perform finishing rolling to a thickness of 1.5-4mm with a cumulative deformation rate of not less than 94%; during this period: high-pressure water descaling between the rolling mills is not required, and the temperature difference between the middle and the edge of the steel strip is controlled to be less than 30℃.

[0051] 4) Perform post-cooling: first air cool for 5-10 seconds, then cool to the winding temperature at a cooling rate of not less than 80℃ / s.

[0052] 5) Perform winding, controlling the winding temperature between 580 and 660℃;

[0053] 6) To be used.

[0054] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0055]

[0056] Continued from Table 1

[0057]

[0058] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0059]

[0060] Table 3. Performance of each embodiment of the present invention and relative corrosion rate test results compared with the comparative patent.

[0061]

[0062]

[0063] Note: The experimental results of corrosion rate in this invention were obtained in accordance with TB / T2375 "Cyclic Immersion Corrosion Test Method for Weathering Steel for Railway".

[0064] As can be seen from Table 3, the short-process corrosion-resistant steels introduced in the comparative examples either focus on high strength while taking into account certain weather resistance, or focus on high weather resistance at the expense of material strength. Neither can achieve the high strength and ultra-high weather resistance that can be achieved by the design of this patent, so as to meet the high strength and lightweight usage requirements in C4-C5 environments.

[0065] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A method for producing weathering steel for coastal light poles using a short-process production method, wherein the composition and weight percentage content of the weathering steel for coastal light poles produced using the short-process production method are as follows: C: 0.03~0.06%, Si: 0.54~0.80%, Mn: 0.20~0.45%, Cr: 1.05~1.49%, Cu: 0.30~0.80%, P: 0.07~0.85% or P: 0.127~0.15%, S≤0. 0.005%, Ti: 0.02~0.045%, N≤0.006%, Ca: 0.0010~0.0025%, the remainder being Fe and impurities; and the following two formulas must be satisfied simultaneously: C+Mn / 6+Cr / 10+Cu / 5+P / 15 not exceeding 0.50% and I=26.01(%Cu)+1.20(%Cr)+1.49(%Si)+17.28(%P)-33.29(%Cu) 2 ≥8; the metallographic structure consists of ferrite and Mao islands, wherein: Ferrite volume content is 65-80%, and Mao island volume content is 20-35%; The steps are as follows: 1) Conventional smelting and casting of billets, during which: the casting speed is controlled to be no less than 3.8m / min, the superheat of molten steel is controlled to be ≤15℃, the light reduction rate of continuous casting slab is controlled to be no less than 20%, and the thickness of the billet is controlled to be 62~72mm; 2) Heat the billet to a temperature of 916~920℃ when it enters the furnace, and keep it at 1200~1250℃ for 20~30 minutes. 3) Perform finishing rolling to a thickness of 1.5-4mm with a cumulative deformation rate of not less than 94%; during this period: high-pressure water descaling between the rolling mills is not required, and the temperature difference between the middle and the edge of the steel strip is controlled to be less than 30℃; 4) Perform post-cooling: first air cool for 5-10 seconds, then cool to the winding temperature at a cooling rate of 81℃ / s. 5) Perform winding, controlling the winding temperature at 580~660℃; 6) To be used.

2. The method for producing weathering steel for coastal light poles using a short process as described in claim 1, characterized in that: C+Mn / 6+Cr / 10+Cu / 5+P / 15 is between 0.30~0.

40.

3. The method for producing weathering steel for coastal light poles using a short process as described in claim 1, characterized in that: The temperature difference between the middle and edge of the steel strip should be kept below 25°C.

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