High-strength low-temperature-resistant acid-resistant hot-rolled strip steel and production method thereof

Abstract

This invention relates to the field of metallurgical technology, specifically to a high-strength, low-temperature resistant, and acid-corrosion-resistant hot-rolled strip steel and its production method. The main chemical composition and mass percentage of the strip steel are: C: 0.040%-0.060%, Mn: 1.15%-1.25%, P≤0.010%, S≤0.0012%, Nb: 0.034%-0.044%, Ti: 0.008%-0.018%, Cu: 0.08%-0.15%, Cr: 0.20%-0.25%, Ni: 0.08%-0.15%. This invention employs optimized element ratios and a controlled rolling and cooling process to obtain a hot-rolled strip steel that combines high strength, high and low temperature toughness, and excellent acid corrosion resistance, while simultaneously reducing the difficulty of production control. This invention is applicable to the manufacture of oil and gas pipelines operating in harsh environments such as low temperature, high pressure, and corrosion.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a high-strength, low-temperature resistant, acid-corrosion resistant hot-rolled strip steel and its production method. Background Technology

[0002] Pipeline steel is mainly used in oil and gas pipeline transportation projects. To improve oil and gas transportation efficiency and reduce pipeline construction costs, the required transmission pressure for oil and gas pipelines is constantly increasing. Simultaneously, with the deepening of oil and gas exploration and development, the number of oil and gas fields containing acidic media containing hydrogen sulfide (H2S corrosion damage is one of the main forms of pipeline corrosion damage) is significantly increasing, and new resource extraction is expanding into areas with harsh geological conditions such as oceans, polar regions, and permafrost. Therefore, pipeline steel with high strength, high and low temperature crack arrest toughness, and good resistance to HIC (hydrogen-induced cracking) and SSC (sulfide stress cracking) has become a current research hotspot.

[0003] Pipeline steel commonly used in oil and gas pipelines includes spiral submerged arc welded pipes, straight seam submerged arc welded pipes, resistance welded pipes, and seamless steel pipes. Spiral submerged arc welded pipes, in particular, are made from hot-rolled strip steel, spirally formed, and welded using double-sided submerged arc welding. This allows for the production of large-diameter pipes from relatively narrow strip steel. Due to their cost-effectiveness and high production flexibility, spiral submerged arc welded pipes are widely used in the laying of pipelines transporting oil and natural gas. Higher strength grades and thicker specifications of pipeline steel not only make it difficult to guarantee the steel's low-temperature crack arrest toughness but also significantly increase its susceptibility to HIC and SSC (sulfur-induced cracking and corrosion). Therefore, developing high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel for pipelines is of great significance for coping with complex and harsh service environments, ensuring the service life and safe operation of pipelines, and promoting the development of oil and gas resources.

[0004] Patent application number 202010096717.6 discloses a manufacturing method for X65MS acid-resistant pipeline steel. This method uses a high manganese content in the pipeline steel. While the high manganese content improves the strength of the pipeline steel to some extent, it also increases the risk of corrosion and cracking. In terms of manufacturing process, it employs low-temperature finishing rolling and low-temperature coiling processes. The rolling mill and coiling machine are under high load, making production control difficult. The finishing rolling reduction rate and cooling rate are low, and the low-temperature crack arrest toughness of the pipeline steel only meets the requirement of ≥93% drop hammer strength at -15℃, failing to meet the toughness requirements at even lower temperatures. Summary of the Invention

[0005] To address the technical problems of high-strength, acid-resistant hot-rolled strip steel for pipelines being difficult to control in production processes and unable to meet toughness requirements at lower temperatures in existing technologies, this invention provides a high-strength, low-temperature acid-resistant hot-rolled strip steel and its production method. This results in high-strength hot-rolled strip steel that combines excellent acid resistance with high crack-arresting toughness at frigid temperatures, while also reducing the difficulty of controlling the production process of hot-rolled strip steel.

[0006] In a first aspect, the present invention provides a high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel, the chemical composition and mass percentage of which are C: 0.040%-0.060%, Si: 0.10%-0.20%, Mn: 1.15%-1.25%, P≤0.010%, S≤0.0012%, Nb: 0.034%-0.044%, Ti: 0.008%-0.018%, Cu: 0.08%-0.15%, Cr: 0.20%-0.25%, Ni: 0.08%-0.15%, Als: 0.020%-0.050%, Ca: 0.0010%-0.0040%, O≤0.0028%, N≤0.0040%, H≤0.0002%, with the remainder being Fe and unavoidable impurities.

[0007] For non-standard hot-rolled strip steel specimens with a thickness greater than or equal to 10 mm and less than 12 mm, the impact energy at -60℃ is ≥280 J, the shear area of ​​the impact fracture surface is 100%, and the shear area of ​​the fracture surface in the drop hammer test at -40℃ is 100%. For standard hot-rolled strip steel specimens with a thickness greater than or equal to 12 mm and not greater than 16 mm, the impact energy at -60℃ is ≥360 J, the shear area of ​​the impact fracture surface is 100%, and the shear area of ​​the fracture surface in the drop hammer test at -40℃ is ≥98%.

[0008] The yield strength of hot-rolled strip steel is 485-520MPa, the tensile strength is 565-600MPa, the elongation is ≥35%, the yield strength ratio is ≤0.90, and the hardness is HV10≤200.

[0009] Hot-rolled strip steel was immersed in solution A for 96 hours according to standard NACE TM0284-2016, with a crack length rate of 0%, a crack thickness rate of 0%, and a crack sensitivity rate of 0%. Hot-rolled strip steel was tested for 720 hours according to ASTM G39-99-2011 and NACE TM0177-2016 standards, with a loading stress of 90% of the specified minimum yield strength. Under 10x magnification using a low-power microscope, no cracks or fissures were found on the tensile surface.

[0010] The grain size of hot-rolled strip steel is grade 12-13. Hot-rolled strip steel has no banded segregation structure. The microstructure types include acicular ferrite and quasi-polygonal ferrite.

[0011] The high-strength, low-temperature resistant, and acid-corrosion-resistant hot-rolled strip steel provided by this invention has the following roles in its chemical composition design:

[0012] Carbon (C) is a crucial element determining the properties of strip steel, exhibiting solid solution strengthening. Increasing the carbon content improves the yield strength and tensile strength of the strip steel, but reduces its plasticity and impact toughness. Furthermore, high carbon content negatively impacts the weldability, low-temperature toughness, and acid corrosion resistance of the strip steel. This invention controls the carbon content to 0.040%-0.060%, thereby reducing the adverse effects of carbon on the low-temperature toughness of the strip steel while simultaneously improving its yield strength and tensile strength.

[0013] Niobium (Nb) is a microalloying element that can increase the austenite recrystallization temperature of strip steel and expand the rolling range of the non-recrystallized zone. Niobium can interact with carbon and nitrogen to form carbides or carbonitrides, which precipitate at dislocations and grain boundaries, inhibiting austenite recrystallization and preventing grain growth, thus refining the grains. It also promotes the formation of acicular ferrite, a chain-like structure that effectively prevents crack propagation and improves the toughness of the strip steel. The fine, dispersed precipitates formed with niobium can also increase H traps, effectively reducing the H diffusion rate and hindering H diffusion, which is beneficial for improving the strip steel's resistance to HIC and SSC. Since the formation of precipitates with niobium is affected by carbon content, this invention controls the niobium content at 0.034%-0.044%, which is beneficial for fully utilizing the role of niobium and reducing the manufacturing cost of the strip steel.

[0014] Manganese (Mn) has a significant solid solution strengthening effect, and can also increase the hardenability of strip steel, reduce the phase transformation temperature of strip steel, refine the microstructure of strip steel, and compensate for the loss of yield strength and tensile strength caused by the low carbon content of strip steel. However, manganese is an element that is prone to segregation. When the manganese content is too high, the low temperature toughness, weldability and acid corrosion resistance of strip steel will decrease. Therefore, this invention controls the manganese content between 1.15% and 1.25%. By reducing the manganese content, the loss of low temperature toughness, weldability and acid corrosion resistance of strip steel is effectively reduced.

[0015] Phosphorus (P) easily leads to banded structure and central segregation, which deteriorates the low-temperature toughness, weldability and acid corrosion resistance of strip steel. This invention controls the phosphorus content to ≤0.010% to reduce the impact of phosphorus on the performance of strip steel.

[0016] Sulfur (S) is a harmful element that reduces the ductility and toughness of strip steel. At the same time, S and Mn easily combine to form strip-shaped MnS inclusion defects, and H tends to accumulate on the inclusion defects, which is extremely detrimental to the acid corrosion resistance. This invention controls the sulfur content to below 0.0012% to improve the acid corrosion resistance of strip steel.

[0017] Titanium (Ti) is a microalloying element that can combine with nitrogen at high temperatures to form nanoscale nitrides, refining austenite grains and improving the toughness of the weld heat-affected zone. However, the titanium content should not be too high, otherwise it is easy to form micron-sized nitrides, which will deteriorate the low-temperature toughness and acid corrosion resistance of the strip steel, especially the thick strip steel. In this invention, the titanium content is controlled at 0.008%-0.018%, which promotes the formation of nanoscale nitrides.

[0018] Chromium (Cr) is a low-cost element with significant alloying effects. Chromium has strong solid solution strengthening properties, which can improve the hardenability of strip steel, effectively improve the uniformity of the microstructure in the thickness direction of the strip steel, and inhibit the pearlite transformation. It can also form a passivation film on the surface of the strip steel to inhibit the entry of harmful gases and improve the strength, toughness and corrosion resistance of the strip steel. However, excessive chromium content will increase the sensitivity to welding cracks. In this invention, the chromium content is controlled at 0.20%-0.25%, which is beneficial to reduce the production cost of strip steel, improve the strength, toughness and corrosion resistance of the strip steel, and reduce the crack sensitivity rate.

[0019] Copper (Cu) can form a protective film on the surface of strip steel, reducing the adsorption and absorption of H and improving the resistance to HIC and SSC. However, copper increases the hot brittleness tendency of steel. Therefore, this invention controls the copper content at a low level of 0.08%-0.15% to minimize the impact of copper on the impact value of strip steel and improve the resistance to HIC and SSC.

[0020] Nickel (Ni) can suppress the tendency of Cu to cause hot cracking and is beneficial to the yield strength, tensile strength and toughness of strip steel. Since nickel-iron alloy is expensive, this invention controls the nickel content at 0.08%-0.15% to balance the relationship between strip steel production cost and strip steel performance.

[0021] Furthermore, the chemical composition and mass percentages are as follows: C: 0.057%, Si: 0.19%, Mn: 1.23%, P: 0.010%, S: 0.0009%, Nb: 0.034%, Ti: 0.012%, Cu: 0.10%, Cr: 0.22%, Ni: 0.11%, Als: 0.020%-0.050%, Ca: 0.0013%, O: 0.0018%, N: 0.0035%, H: 0.0001%, with the remainder being Fe and unavoidable impurities.

[0022] Furthermore, the chemical composition and mass percentages are as follows: C: 0.042%, Si: 0.16%, Mn: 1.16%, P: 0.009%, S: 0.0008%, Nb: 0.038%, Ti: 0.015%, Cu: 0.12%, Cr: 0.21%, Ni: 0.13%, Als: 0.020%-0.050%, Ca: 0.0017%, O: 0.0024%, N: 0.0039%, H: 0.0001%, with the remainder being Fe and unavoidable impurities.

[0023] Furthermore, the thickness of hot-rolled strip steel is 10-16mm.

[0024] Secondly, the present invention provides a method for producing the above-mentioned high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel, comprising the following steps:

[0025] Step 1: Prepare the billet. The raw material molten iron is smelted and refined in sequence to obtain molten steel. The obtained molten steel is continuously cast to obtain the billet. The thickness of the billet is 230mm.

[0026] Step 2: Heating. The billet obtained in Step 1 is heated for a total duration of 160-230 minutes, with the soaking temperature in the soaking zone being 1190-1240℃ and the soaking time being 35-55 minutes. The billet is then removed from the furnace at a temperature of 1180-1230℃.

[0027] Step 3: Rolling, including roughing and finishing. Roughing adopts a 1+5 rolling mode, first entering a two-high reversible roughing mill for one pass, and then entering a four-high reversible roughing mill for five passes of reciprocating rolling. The finishing temperature of roughing is 1010-1050℃. Finishing adopts a seven-stand continuous rolling process, with a finishing mill opening temperature of 950-990℃, a finishing mill exit temperature of 830-870℃, and a total finishing reduction rate of 73%-83%.

[0028] Step 4: Cooling, including ultra-fast cooling and laminar flow cooling. The cooling outlet temperature of ultra-fast cooling is 580-630℃, the cooling rate is 30-50℃ for 5 seconds, and the cooling water pressure is ≥0.35MPa. The cooling rate of laminar flow cooling is 15-20℃ for 5 seconds.

[0029] Step 5: Coiling. The thickness of the hot-rolled strip to be coiled is 10-16mm. The coiling temperature is 490-540℃. After coiling, air cool to room temperature.

[0030] Furthermore, in step one, the raw material molten iron has a S ≤ 0.003%; smelting includes converter smelting, with a converter endpoint C ≤ 0.05% and P ≤ 0.008%; refining includes LF refining and RH refining, where LF refining includes calcium treatment, and when S > 0.0010%, Ca5S ≥ 1.7, and the LF refining treatment cycle is ≥ 15 min; the vacuum degassing time for RH refining is 10-20 min, and the soft blowing time is 15-25 min; continuous casting adopts full-process protective casting, with the tundish superheat controlled at 20-30℃, and continuous casting adopts constant casting speed control, with the billet casting speed at 1.1-1.4 m / 5 min; low-carbon alloy steel protective slag is used in continuous casting, and the billet slow cooling time after continuous casting is 48-72 h. This invention employs a converter smelting and LF+RH dual refining process to obtain molten steel. Converter smelting achieves decarburization, dephosphorization, and alloying. LF and RH refining processes are optimized to ensure the purity of the molten steel, controlling the content of harmful elements such as P, S, O, N, and H to low levels. Calcium treatment modifies non-metallic inclusions such as MnS, which improves the acid corrosion resistance of the strip steel. In the continuous casting process, full-process protective casting is used to prevent secondary oxidation of the molten steel. Constant casting speed control ensures stable billet quality, while limiting the superheat of the tundish is mainly to ensure billet quality and avoid or reduce segregation. Slow cooling of the billet uses a stacking slow cooling process, allowing hydrogen to escape from the inside of the billet, improving acid corrosion resistance.

[0031] Furthermore, in step three, the third pass of the finishing mill is passed without load using an F3 stand, optimizing the rolling load distribution of the other six passes and increasing the reduction rate of the fourth, fifth, and sixth passes. This ensures that the reduction rate of the seventh pass is 12%-18%, achieving sufficient refinement and homogenization of the austenite grains.

[0032] Furthermore, the coiling temperature for hot-rolled strip steel with a thickness of less than 12 mm is 500-540℃; the coiling temperature for hot-rolled strip steel with a thickness of 12 mm or greater is 490-530℃.

[0033] The beneficial effects of this invention are as follows:

[0034] This invention achieves a microstructure of low-carbon, low-manganese, niobium-titanium composite microalloying, appropriate addition of copper, chromium, and nickel, and suitable element content ratios. Combined with clean steel smelting and continuous casting processes, it reduces the content of harmful elements such as P and S, controls the content and morphology of inclusions, and employs controlled rolling and cooling processes adapted to the composition design. This results in a microstructure of fine, uniform acicular ferrite and quasi-polygonal ferrite without banded segregation. The hot-rolled strip produced not only meets the requirements of high strength and high and low temperature toughness but also has excellent acid and corrosion resistance, making it suitable for manufacturing oil and gas pipelines that operate in harsh environments such as high pressure, low temperature, and acidity.

[0035] This invention designs appropriate roughing and finishing rolling process temperatures based on the influence of niobium, carbon, and other elemental contents on the austenite recrystallization temperature. The roughing finishing temperature (R2DT) is 1010-1050℃, ensuring the roughing process is entirely within the recrystallization zone. Through multiple passes, the austenite repeatedly recrystallizes, refining the grain size. The finishing rolling starting temperature (FET) is 950-990℃, eliminating the need for steel placement before the finishing mill and ensuring the finishing process enters the non-recrystallization zone, preventing mixed grain formation. Finishing rolling employs a 7-stand continuous rolling process, with the F3 mill skipped to increase the reduction rate of other passes. By controlling the finishing reduction rate, especially ensuring the reduction rate of the final finishing pass, the austenite grains are fully broken up during the rolling process, achieving austenite grain refinement and homogenization.

[0036] Cooling employs ultrafast cooling and laminar flow cooling. Ultrafast cooling provides concentrated cooling, while laminar flow cooling provides sparse cooling. By comprehensively controlling the ultrafast cooling rate, water pressure, outlet temperature, and laminar flow cooling rate, and in conjunction with an appropriate coiling temperature, a complex structure of fine and uniform acicular ferrite and quasi-polygonal ferrite is obtained. The numerous dispersed fine precipitates and the entangled dislocations pinned by the precipitates within the matrix structure can all serve as irreversible H traps. Irregular grain boundaries can effectively limit crack propagation, thereby improving the steel's strength, toughness, and acid corrosion resistance.

[0037] The production method of this invention is simple and easy to implement. In particular, the rolling and coiling temperatures are relatively high, the rolling mill and coiler loads are low, the process control is easy, the production is more stable, the cost performance is high, and it is suitable for large-scale production applications. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 This is a microstructure diagram of the hot-rolled strip steel obtained in Example 4 of the present invention.

[0040] Figure 2 This is a photograph of the fracture surface of the hot-rolled strip steel obtained in Example 4 of the present invention during a drop hammer test at -40℃.

[0041] Figure 3 This is a photograph of the hot-rolled strip steel before the HIC test obtained in Example 4 of this invention.

[0042] Figure 4 This is a photograph of the hot-rolled strip steel after the HIC test obtained in Example 4 of the present invention.

[0043] Figure 5This is a photograph of the hot-rolled strip steel before the SSC test obtained in Example 4 of this invention.

[0044] Figure 6 This is a photograph of the hot-rolled strip steel after the SSC test obtained in Example 4 of this invention. Detailed Implementation

[0045] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0046] Example 1

[0047] A high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel has the following chemical composition and mass percentage: C: 0.045%, Si: 0.17%, Mn: 1.17%, P: 0.008%, S: 0.0012%, Nb: 0.038%, Ti: 0.016%, Cu: 0.09%, Cr: 0.23%, Ni: 0.08%, Als: 0.028%, Ca: 0.0027%, O: 0.0021%, N: 0.0036%, H: 0.0001%, with the remainder being Fe and unavoidable impurities.

[0048] The hot-rolled strip has a thickness of 10.30 mm and a grain size of grade 13. It has no banded segregation structure and its microstructure includes acicular ferrite and quasi-polygonal ferrite.

[0049] The production method of the above-mentioned high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel includes the following steps:

[0050] Step 1: Prepare the billet. The raw molten iron is fed into a converter for smelting, controlling the S content of the incoming molten iron to be ≤0.003%, the final C content to be ≤0.05%, and the P content to be ≤0.008%. After converter smelting, the steel is sequentially refined in an LF furnace and an RH furnace to reduce the content of harmful elements, yielding molten steel. The LF furnace refining includes calcium treatment combined with Ca modification treatment to effectively remove or spheroidize MnS inclusions in the steel. When S > 0.0010%, Ca5S ≥ 1.7. The processing cycle of the LF furnace refining is [not specified]. The vacuum degassing time for RH furnace refining is 12 min, and the soft blowing time is 17 min. The resulting molten steel is continuously cast to obtain a billet with a thickness of 230 mm. The continuous casting adopts full-process protective casting, and the superheat of the tundish is controlled at 21℃. The continuous casting adopts constant casting speed control, and the billet casting speed is 1.1 m 5 min. Low carbon alloy steel protective slag is used for continuous casting. The billet is stacked and slowly cooled for 48 h after continuous casting to allow hydrogen gas to fully escape from the inside of the billet and reduce the probability of hydrogen-induced cracking.

[0051] Step 2: Heating. The billet obtained in Step 1 is heated for a total of 189 minutes, with the soaking temperature at 1220℃ and the soaking time at 43 minutes. The billet exits the furnace at 1201℃ to ensure that it is thoroughly and evenly heated.

[0052] Step 3: Rolling, including roughing and finishing. Roughing adopts a 1+5 rolling mode, first entering a two-high reversible roughing mill for one pass, and then entering a four-high reversible roughing mill for five reciprocating passes. The finishing temperature (R2DT) is 1016℃. Finishing adopts a seven-stand continuous rolling process. The third finishing pass uses an F3 stand for idle rolling. The finishing opening temperature (FET) is 958℃ to ensure that finishing is carried out in the non-recrystallization zone. The finishing exit temperature (FDT) is 852℃. The total reduction rate of finishing is 81.6%, and the reduction rate of the seventh finishing pass is 16.4%.

[0053] Step 4: Cooling. Immediately after finishing rolling, ultra-fast cooling and laminar flow cooling are performed. Ultra-fast cooling is concentrated ultra-fast cooling, while laminar flow cooling is sparse laminar flow cooling. The cooling outlet temperature (MT1) for ultra-fast cooling is 619℃, the cooling rate is 48℃ for 5 seconds, and the cooling water pressure is ≥0.35MPa. The cooling rate for laminar flow cooling is 15-20℃ for 5 seconds.

[0054] Step 5: Coiling. The thickness of the hot-rolled strip to be coiled is 10.30 mm. The coiling temperature (CT) is 537℃. After coiling, air cool to room temperature.

[0055] Example 2

[0056] A high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel has the following chemical composition and mass percentage: C: 0.057%, Si: 0.19%, Mn: 1.23%, P: 0.010%, S: 0.0009%, Nb: 0.034%, Ti: 0.012%, Cu: 0.10%, Cr: 0.22%, Ni: 0.11%, Als: 0.036%, Ca: 0.0013%, O: 0.0018%, N: 0.0035%, H: 0.0001%, with the remainder being Fe and unavoidable impurities.

[0057] The hot-rolled strip has a thickness of 11.30 mm and a grain size of 12.5. It has no banded segregation structure and its microstructure includes acicular ferrite and quasi-polygonal ferrite.

[0058] The production method of the above-mentioned high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel includes the following steps:

[0059] Step 1: Prepare the billet. The raw molten iron is fed into a converter for smelting, controlling the S content of the incoming molten iron to be ≤0.003%, the final C content to be ≤0.05%, and the P content to be ≤0.008%. After converter smelting, the steel is sequentially refined in an LF furnace and an RH furnace to reduce the content of harmful elements, yielding molten steel. The LF furnace refining includes calcium treatment combined with Ca modification treatment to effectively remove or spheroidize MnS inclusions in the steel. When S > 0.0010%, Ca5S ≥ 1.7. The processing cycle of the LF furnace refining is [not specified]. The vacuum degassing time for RH furnace refining is 14 min, and the soft blowing time is 15 min. The resulting molten steel is continuously cast to obtain a billet with a thickness of 230 mm. The continuous casting adopts full-process protective casting, and the superheat of the tundish is controlled at 26℃. The continuous casting adopts constant casting speed control, and the billet casting speed is 1.3 m 5 min. Low carbon alloy steel protective slag is used in continuous casting. The billet is stacked and slowly cooled for 56 h after continuous casting to allow hydrogen gas to fully escape from the inside of the billet and reduce the probability of hydrogen-induced cracking.

[0060] Step 2: Heating. The billet obtained in Step 1 is heated for a total duration of 197 minutes, with a soaking temperature of 1218℃ and a soaking time of 36 minutes. The billet is removed from the furnace at a temperature of 1210℃ to ensure thorough and uniform heating.

[0061] Step 3: Rolling, including roughing and finishing. Roughing adopts a 1+5 rolling mode, first entering a two-high reversible roughing mill for one pass, and then entering a four-high reversible roughing mill for five passes of reciprocating rolling. The finishing temperature (R2DT) is 1029℃. Finishing adopts a seven-stand continuous rolling process. The third pass of finishing uses an F3 stand for idle rolling. The finishing opening temperature (FET) is 983℃, so that finishing is carried out in the non-recrystallization zone. The finishing exit temperature (FDT) is 847℃. The total reduction rate of finishing is 79.8%, and the reduction rate of the seventh pass of finishing is 15.3%.

[0062] Step 4: Cooling. Immediately after finishing rolling, ultra-fast cooling and laminar flow cooling are performed. Ultra-fast cooling is concentrated ultra-fast cooling, while laminar flow cooling is sparse laminar flow cooling. The cooling outlet temperature (MT1) for ultra-fast cooling is 622℃, the cooling rate is 45℃ for 5 seconds, and the cooling water pressure is ≥0.35MPa. The cooling rate for laminar flow cooling is 15-20℃ for 5 seconds.

[0063] Step 5: Coiling. The thickness of the hot-rolled strip to be coiled is 11.30 mm. The coiling temperature (CT) is 523℃. After coiling, air cool to room temperature.

[0064] Example 3

[0065] A high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel has the following chemical composition and mass percentage: C: 0.053%, Si: 0.14%, Mn: 1.18%, P: 0.008%, S: 0.0012%, Nb: 0.043%, Ti: 0.017%, Cu: 0.13%, Cr: 0.24%, Ni: 0.13%, Als: 0.031%, Ca: 0.0021%, O: 0.0028%, N: 0.0024%, H: 0.0002%, with the remainder being Fe and unavoidable impurities.

[0066] The hot-rolled strip has a thickness of 14.27 mm and a grain size of 12.5. It has no banded segregation structure and its microstructure includes acicular ferrite and quasi-polygonal ferrite.

[0067] The production method of the above-mentioned high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel includes the following steps:

[0068] Step 1: Prepare the billet. The raw molten iron is fed into a converter for smelting, controlling the S content of the incoming molten iron to be ≤0.003%, the final C content to be ≤0.05%, and the P content to be ≤0.008%. After converter smelting, the steel is sequentially refined in an LF furnace and an RH furnace to reduce the content of harmful elements, yielding molten steel. The LF furnace refining includes calcium treatment combined with Ca modification treatment to effectively remove or spheroidize MnS inclusions in the steel. When S > 0.0010%, Ca5S ≥ 1.7. The processing cycle of the LF furnace refining is [not specified]. The vacuum degassing time for refining in the RH furnace is 15 min, and the soft blowing time is 18 min. The resulting molten steel is continuously cast to obtain a billet with a thickness of 230 mm. The continuous casting adopts full-process protective casting, and the superheat of the tundish is controlled at 23℃. The continuous casting adopts constant casting speed control, and the billet casting speed is 1.2 m 5 min. Low carbon alloy steel protective slag is used in continuous casting. The billet is stacked and slowly cooled for 54 h after continuous casting to allow hydrogen gas to fully escape from the inside of the billet and reduce the probability of hydrogen-induced cracking.

[0069] Step 2: Heating. The billet obtained in Step 1 is heated for a total of 203 minutes, with the soaking temperature at 1211℃ and the soaking time at 42 minutes. The billet exiting the furnace is at 1196℃ to ensure that the billet is thoroughly and evenly heated.

[0070] Step 3: Rolling, including roughing and finishing. Roughing adopts a 1+5 rolling mode, first entering a two-high reversible roughing mill for one pass, and then entering a four-high reversible roughing mill for five reciprocating passes. The finishing temperature (R2DT) is 1019℃. Finishing adopts a seven-stand continuous rolling process. The third finishing pass uses an F3 stand for idle rolling. The finishing opening temperature (FET) is 975℃, so that finishing is carried out in the non-recrystallization zone. The finishing exit temperature (FDT) is 855℃. The total reduction rate of finishing is 76.2%, and the reduction rate of the seventh finishing pass is 14.1%.

[0071] Step 4: Cooling. Immediately after finishing rolling, ultra-fast cooling and laminar flow cooling are performed. Ultra-fast cooling is concentrated ultra-fast cooling, while laminar flow cooling is sparse laminar flow cooling. The cooling outlet temperature (MT1) for ultra-fast cooling is 586℃, the cooling rate is 40℃ for 5 seconds, and the cooling water pressure is ≥0.35MPa. The cooling rate for laminar flow cooling is 15-20℃ for 5 seconds.

[0072] Step 5: Coiling. The thickness of the hot-rolled strip to be coiled is 14.27 mm. The coiling temperature (CT) is 516℃. After coiling, air cool to room temperature.

[0073] Example 4

[0074] A high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel has the following chemical composition and mass percentage: C: 0.042%, Si: 0.16%, Mn: 1.16%, P: 0.009%, S: 0.0008%, Nb: 0.038%, Ti: 0.015%, Cu: 0.12%, Cr: 0.21%, Ni: 0.13%, Als: 0.040%, Ca: 0.0017%, O: 0.0024%, N: 0.0039%, H: 0.0001%, with the remainder being Fe and unavoidable impurities.

[0075] The hot-rolled strip has a thickness of 15.88 mm and a grain size of grade 12. It has no banded segregation structure and its microstructure includes acicular ferrite and quasi-polygonal ferrite.

[0076] The production method of the above-mentioned high-strength, low-temperature resistant, and acid-corrosion resistant hot-rolled strip steel includes the following steps:

[0077] Step 1: Prepare the billet. The raw molten iron is fed into a converter for smelting, controlling the S content of the incoming molten iron to be ≤0.003%, the final C content to be ≤0.05%, and the P content to be ≤0.008%. After converter smelting, the steel is sequentially refined in an LF furnace and an RH furnace to reduce the content of harmful elements, yielding molten steel. The LF furnace refining includes calcium treatment combined with Ca modification treatment to effectively remove or spheroidize MnS inclusions in the steel. When S > 0.0010%, Ca5S ≥ 1.7. The processing cycle of the LF furnace refining is [not specified]. The vacuum degassing time for RH furnace refining is 13 min, and the soft blowing time is 16 min. The resulting molten steel is continuously cast to obtain a billet with a thickness of 230 mm. The continuous casting adopts full-process protective casting, and the superheat of the tundish is controlled at 29℃. The continuous casting adopts constant casting speed control, and the billet casting speed is 1.2 m 5 min. Low carbon alloy steel protective slag is used for continuous casting. The billet is stacked and slowly cooled for 62 h after continuous casting to allow hydrogen gas to fully escape from the inside of the billet and reduce the probability of hydrogen-induced cracking.

[0078] Step 2: Heating. The billet obtained in Step 1 is heated for a total duration of 211 minutes, with a soaking temperature of 1209℃ and a soaking time of 38 minutes. The billet is removed from the furnace at a temperature of 1217℃ to ensure thorough and uniform heating.

[0079] Step 3: Rolling, including roughing and finishing. Roughing adopts a 1+5 rolling mode, first entering a two-high reversible roughing mill for one pass, and then entering a four-high reversible roughing mill for five reciprocating passes. The finishing temperature (R2DT) is 1037℃. Finishing adopts a seven-stand continuous rolling process. The third finishing pass uses an F3 stand for idle rolling. The finishing opening temperature (FET) is 989℃, so that finishing is carried out in the non-recrystallization zone. The finishing exit temperature (FDT) is 840℃. The total reduction rate of finishing is 73.5%, and the reduction rate of the seventh finishing pass is 12.5%.

[0080] Step 4: Cooling. Immediately after finishing rolling, ultra-fast cooling and laminar flow cooling are performed. Ultra-fast cooling is concentrated ultra-fast cooling, while laminar flow cooling is sparse laminar flow cooling. The cooling outlet temperature (MT1) for ultra-fast cooling is 595℃, the cooling rate is 36℃ for 5 seconds, and the cooling water pressure is ≥0.35MPa. The cooling rate for laminar flow cooling is 15-20℃ for 5 seconds.

[0081] Step 5: Coiling. The thickness of the hot-rolled strip to be coiled is 15.88 mm. The coiling temperature (CT) is 494℃. After coiling, air cool to room temperature.

[0082] The microstructure of the hot-rolled strip obtained in this embodiment is shown in the figure below. Figure 1 As shown.

[0083] The hot-rolled strip steels obtained in Examples 1-4 were subjected to mechanical property tests and acid resistance tests, respectively. The mechanical property test data of the hot-rolled strip steels obtained in Examples 1-4 are shown in Table 1. The fracture surface of the hot-rolled strip steel obtained in Example 4 under the -40℃ drop hammer test is shown in Table 1. Figure 2 As shown in Table 2; the acid resistance test data are shown in Table 4, and the photos of the hot-rolled strip steel before and after the HIC test in Example 4 are shown in Table 5. Figure 3 , Figure 4 As shown, the before and after SSC test photos of the hot-rolled strip steel obtained in Example 4 are as follows. Figure 5 , Figure 6 As shown, the acid resistance test was conducted according to the standard NACE TM0284-2016 (HIC), the 4-point bending method according to the ASTM G39-99 (2011) standard, and the sulfide stress cracking (SSC) test according to the NACE TM0177-2016 standard.

[0084] Table 1. Mechanical property test data of hot-rolled strip steel obtained in Examples 1-4

[0085]

[0086] The hot-rolled strip obtained by the production method disclosed in the present application has a thickness of 10-16 mm, a grain size of 12-13 grades, no banded structure, and non-metallic inclusions ≤ 0.5 grades. As can be seen from Table 1, the yield strength of the hot-rolled strip with a thickness of 10-16 mm is 489-516 MPa, the tensile strength is 576-592 MPa, the elongation rate ≥ 37%, the yield ratio ≤ 0.87, and the hardness HV10 ≤ 195; the -60 °C impact energy of the hot-rolled strip with a thickness of [10-12) mm ≥ 283 J (354 size), the impact fiber cross-section rate is 100%, and the shear area of the fracture in the -40 °C drop weight test is 100%; the -60 °C impact energy of the strip with a thickness of [12-16] mm ≥ 362 J (full size), the impact fiber cross-section rate is 100%, and the shear area of the fracture in the -40 °C drop weight test ≥ 98%.

[0087] Table 2 Detection data of the acid resistance of the hot-rolled strips obtained in Examples 1-4

[0088]

[0089] As can be seen from Table 2, according to the standard NACE TM0284-2016 (HIC) test, after 96 h of immersion test in a saturated H2S solution, no hydrogen blisters appeared on the surfaces of all specimens in Examples 1-4; there were no cracks in the cross-sections of all specimens in Examples 1-4 under a microscope with a magnification of 100 times. According to the four-point bending method of ASTM G39-99 (2011) standard and the sulfide stress cracking (SSC) test of NACE TM0177-2016 standard, the applied stress of all specimens in Examples 1-4 was 90% of the specified minimum yield strength. After 720 h of test, the tensile surface of the test was inspected under a low-power microscope with a magnification of 10 times, and no cracking or cracks were found in all specimens, and the specimens were qualified.

[0090] Although the present invention has been described in detail by referring to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, those of ordinary skill in the art can make various equivalent modifications or substitutions to the embodiments of the present invention, and all such modifications or substitutions should be within the scope of the present invention. Any person familiar with the technical field of the present invention can easily think of changes or substitutions within the technical scope disclosed by the present invention, and all should be covered within the protection scope of the present invention.

Claims

1. A high-strength low-temperature-resistant acid corrosion-resistant hot-rolled strip steel, characterized by, The chemical composition and mass percentage are as follows: C: 0.040%-0.060%, Si: 0.10%-0.20%, Mn: 1.15%-1.25%, P≤0.010%, S≤0.0012%, Nb: 0.034%-0.044%, Ti: 0.008%-0.018%, Cu: 0.08%-0.15%, Cr: 0.20%-0.25%, Ni: 0.08%-0.15%, Als: 0.020%-0.050%, Ca: 0.0010%-0.0040%, O≤0.0028%, N≤0.0040%, H≤0.0002%, with the remainder being Fe and unavoidable impurities. For hot-rolled strip steel with a thickness of less than 12mm, the impact energy at -60℃ is ≥280J, and the shear area of ​​the impact fracture surface is 100%; for hot-rolled strip steel with a thickness of ≥12mm, the impact energy at -60℃ is ≥360J, and the shear area of ​​the impact fracture surface is 100%; for hot-rolled strip steel with a thickness of ≥12mm, the impact energy at -60℃ is ≥360J, and the shear area of ​​the impact fracture surface is 100%; for hot-rolled strip steel with a thickness of ≥12mm, the shear area of ​​the impact fracture surface is ≥98%; for hot-rolled strip steel with a thickness of ≥12mm, the impact energy at -60℃ is ≥360J, and the shear area of ​​the impact fracture surface is 100%; for hot-rolled strip steel with a thickness of ≥12mm, the impact energy at -60℃ is ≥360J, and the shear area of ​​the impact fracture surface is ≥98 ... The yield strength of hot-rolled strip steel is 485-520MPa, the tensile strength is 565-600MPa, the elongation is ≥35%, the yield strength ratio is ≤0.90, and the hardness is HV10≤200. Hot-rolled strip steel was immersed in solution A for 96 hours according to standard NACE TM0284-2016, with a crack length rate of 0%, a crack thickness rate of 0%, and a crack sensitivity rate of 0%. Hot-rolled strip steel was tested for 720 hours according to ASTM G39-99-2011 and NACE TM0177-2016 standards, with a loading stress of 90% of the specified minimum yield strength. Under 10x magnification using a low-power microscope, no cracks or fissures were found on the tensile surface. The grain size of hot-rolled strip steel is grade 12-13. Hot-rolled strip steel has no banded segregation structure. The microstructure types include acicular ferrite and quasi-polygonal ferrite. The hot-rolled strip steel production method is as follows: Step 1: Prepare the billet. The raw material molten iron is smelted and refined in sequence to obtain molten steel. The obtained molten steel is continuously cast to obtain the billet. The thickness of the billet is 230mm. Step 2: Heating. The billet obtained in Step 1 is heated for a total duration of 160-230 minutes, with the soaking zone temperature at 1190-1240℃ and the soaking time at 35-55 minutes. The billet is then removed from the furnace at a temperature of 1180-1230℃. Step 3: Rolling, including roughing and finishing. Roughing adopts a 1+5 rolling mode, first entering a two-high reversible roughing mill for one pass, and then entering a four-high reversible roughing mill for five passes. The finishing temperature is 1010-1050℃. Finishing uses a seven-stand continuous rolling process, with a finishing mill opening temperature of 950-990℃, a finishing mill exit temperature of 830-870℃, and a total finishing reduction rate of 73%-83%. Step 4: Cooling, including ultra-fast cooling and laminar flow cooling. The cooling outlet temperature of ultra-fast cooling is 580-630℃, the cooling rate is 30-50℃ / s, and the cooling water pressure is ≥0.35MPa; the cooling rate of laminar flow cooling is 15-20℃ / s. Step 5: Coiling. The thickness of the hot-rolled strip to be coiled is 10-16mm. The coiling temperature is 490-540℃. After coiling, air cool to room temperature.

2. The high-strength, low-temperature resistant, acid-corrosion resistant hot-rolled strip steel as described in claim 1, characterized in that, The chemical composition and mass percentage are as follows: C: 0.057%, Si: 0.19%, Mn: 1.23%, P: 0.010%, S: 0.0009%, Nb: 0.034%, Ti: 0.012%, Cu: 0.10%, Cr: 0.22%, Ni: 0.11%, Als: 0.020%-0.050%, Ca: 0.0013%, O: 0.0018%, N: 0.0035%, H: 0.0001%, with the remainder being Fe and unavoidable impurities.

3. The high-strength, low-temperature resistant, acid-corrosion resistant hot-rolled strip steel as described in claim 1, characterized in that, The chemical composition and mass percentage are as follows: C: 0.042%, Si: 0.16%, Mn: 1.16%, P: 0.009%, S: 0.0008%, Nb: 0.038%, Ti: 0.015%, Cu: 0.12%, Cr: 0.21%, Ni: 0.13%, Als: 0.020%-0.050%, Ca: 0.0017%, O: 0.0024%, N: 0.0039%, H: 0.0001%, with the remainder being Fe and unavoidable impurities.

4. The high-strength, low-temperature resistant, acid-corrosion resistant hot-rolled strip steel as described in claim 1, characterized in that, In step one, the raw material molten iron has a sulfur content of ≤0.003%; smelting includes converter smelting, with a final converter concentration of C ≤0.05% and P ≤0.008%; refining includes LF refining and RH refining, where LF refining includes calcium treatment. When S > 0.0010%, Ca / S ≥ 1.7, and the LF refining cycle is ≥15 min; the vacuum degassing time for RH refining is 10-20 min, and the soft blowing time is 15-25 min; continuous casting adopts full-process protective casting, with the tundish superheat controlled at 20-30℃, and continuous casting adopts constant casting speed control, with the billet casting speed at 1.1-1.4 m / min. Low-carbon alloy steel protective slag is used in continuous casting, and the billet slow cooling time after continuous casting is 48-72 h.

5. The high-strength, low-temperature resistant, acid-corrosion resistant hot-rolled strip steel as described in claim 1, characterized in that, In step three, the third pass of the finishing mill is passed in the air using the F3 mill, and the reduction rate of the seventh pass is 12%-18%.

6. The high-strength, low-temperature resistant, acid-corrosion-resistant hot-rolled strip steel as described in claim 1, characterized in that, The coiling temperature for hot-rolled strip steel with a thickness of less than 12mm is 500-540℃; the coiling temperature for hot-rolled strip steel with a thickness of 12mm or greater is 490-530℃.

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