A 300-350 mm high-strength structural steel s460nlz35 and a method for producing the same

Abstract

This invention discloses a 300-350mm high-strength structural steel S460NLZ35 and its production method. The steel plate has the following chemical composition (wt%): C: 0.09-0.11, Si: 0.06-0.10, Mn: 1.60-1.70, P: ≤0.006, S: ≤0.004, Ni: 0.30-0.40, Nb: 0.035-0.045, Cr: 0.20. The steel plate contains approximately 0.30 g / L, V: 0.060–0.075 g / L, Als: 0.030–0.040 g / L, with the remainder being Fe and residual elements. After water-cooled casting, heating, rolling, cooling, and heat treatment, the resulting steel plate meets the requirements of EN10160-1999S3E4 in terms of flaw detection. Its yield strength is ≥370 Pa, tensile strength is 530–710 MPa, elongation is ≥17%, impact resistance at -50℃ is ≥150 J, and resistance to lamellar tearing reaches Z45. The uniform strength and toughness across the entire thickness section meets the high-performance requirements of various types of bridges, heavy machinery, offshore platforms, hydroelectric power plant applications, and other complex environments.

Description

Technical Field

[0001] This invention relates to the field of extra-thick plate production technology, specifically to a 300-350mm thick S460NLZ35 high-strength structural steel and its production method. Background Technology

[0002] With the rapid development of global marine engineering, pumped storage, and bridge structure construction, especially the continuous advancement in offshore wind power generation, deep-sea resource development, and offshore platforms, the demand for S460NLZ35 grade marine structural steel continues to grow. In particular, for the upgrading of heavy equipment, for steel plates with a thickness greater than 300mm, due to the difficulty in internal flaw detection quality control and performance, forgings are generally required to meet the usage requirements.

[0003] The publication CN107267861A, titled "Production of Normalized High-Strength Q460NL Steel Plate from Continuous Casting Billets and Its Production Method," describes a 100-150mm thick Q460NL steel plate produced by a continuous casting billet rolling process. However, the plate thickness is relatively small, and the lamellar tear performance and internal flaw detection quality have not been tested, which cannot meet the current requirements for large-thickness applications.

[0004] The patent application CN109536832A, entitled "A 150-200mm thick high-strength steel S460NL plate and its production method", uses a die-cast billet to roll a steel plate with a thickness of 150-200mm. However, the production process using the quenching and tempering process is long and the alloy cost is high, which does not give it a competitive advantage.

[0005] Therefore, there is an urgent need to provide an effective solution for the efficient and low-cost production of high-strength structural steel with greater thickness and balanced performance across all parameters. Summary of the Invention

[0006] To meet the above technical requirements, the purpose of this invention is to provide a 300-350mm high-strength structural steel S460NLZ35. This steel plate has a uniform strength and toughness match throughout its entire thickness section, which can meet the high-performance requirements of various types of bridges, heavy machinery, offshore platforms, hydroelectric power plants and other complex environments.

[0007] Another objective of this invention is to provide a method for producing 300-350mm high-strength structural steel S460NLZ35.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a 300-350mm high-strength structural steel S460NLZ35, wherein the steel plate has the following chemical composition (unit, wt%) by mass fraction: C: 0.09-0.11, Si: 0.06-0.10, Mn: 1.60-1.70, P: ≤0.006, S: ≤0.004, Ni: 0.30-0.40, Nb: 0.035-0.045, Cr: 0.20-0.30, V: 0.060-0.075, Als: 0.030-0.040, with the remainder being Fe and residual elements.

[0009] Regarding the ingredient design, it should be noted that:

[0010] Carbon: Carbon is the most important strengthening element in this invention, which can improve the strength of steel. According to the performance characteristics of this steel, the material is required to have good low-temperature toughness while meeting the strength requirements. If the carbon content is too high, the strength will increase but the low-temperature impact resistance will be poor. If the carbon content is too low, the tensile strength will not meet the standard requirements. Therefore, the carbon content is limited to 0.09 to 0.11.

[0011] Silicon: Silicon is a solid solution strengthening element that can improve the strength and durability of steel plates. As the content of silicon increases, the recrystallization temperature of steel increases, which is detrimental to the control of the rolling process. Therefore, the limit range of silicon content is 0.06 to 0.10.

[0012] Manganese: Manganese is one of the important toughening elements in this invention and increases the hardenability of steel. Its alloy cost is much lower than that of other alloying elements. Therefore, the range of manganese is limited to 1.60 to 1.70.

[0013] Aluminum: Aluminum has strong deoxidizing and nitrogen-fixing effects, which can refine the grains of steel, thereby inhibiting the aging of low-carbon steel and improving the toughness of steel at low temperatures. However, when the Al content is >0.040, it will deteriorate the low-temperature impact performance of steel. Therefore, the limit range for aluminum content is 0.030 to 0.040.

[0014] Vanadium: Vanadium is one of the important strong carbonitride forming elements in this invention. Its V(C,N) content, which easily precipitates at low temperatures, has a significant precipitation effect. Considering the solid solubility product of V and related elements in steel, as well as the content of N, C, and other elements in the steel, and combining the influence of other alloying elements on the strength of the steel plate, the reasonable range for the V content in the bainitic steel of this invention is determined to be 0.060–0.075. If the vanadium content is less than 0.060%, the strengthening effect of second-phase precipitation is not significant, and the strength of the steel plate will not meet the standard requirements; if the vanadium content is higher than 0.075%, it will increase production costs.

[0015] Nickel: Nickel is one of the important strengthening and toughening elements in this invention. Adding Ni to steel can strengthen it through solid solution and improve its hardenability. Furthermore, Ni can improve the low-temperature impact toughness of the steel. Based on the performance characteristics of this steel grade, the steel plate is required to possess low-temperature impact toughness at -50℃ while meeting high strength requirements. If the nickel content is less than 0.30%, the low-temperature toughness of the steel plate will not meet the standard requirements; if the nickel content is greater than 0.40%, it will have little impact on the hardenability of the steel plate while increasing production costs. Therefore, the nickel content is limited to the range of 0.30% to 0.40%.

[0016] Chromium: Chromium can improve the hardenability of steel and increase its strength. With the increase of chromium, the wear resistance and hardness of steel are significantly improved, while ensuring the strength and toughness of the steel plate are matched. The chromium content is limited to 0.20 to 0.30.

[0017] Niobium: Niobium is one of the important strengthening and toughening elements in this invention. Adding Nb to steel can improve hardenability and refine grains, which can significantly improve the strength of steel. However, as the amount of niobium increases, the impact energy of the alloy decreases. Therefore, the range of Nb is limited to 0.035 to 0.045.

[0018] The above-mentioned production method of 300-350mm high-strength structural steel S460NLZ35

[0019] This includes ingot casting, ingot heating, rolling, cooling, and heat treatment, as detailed below:

[0020] a. Steel ingot casting: The steel ingot adopts an ingot mold with a taper of 950-1050mm. The casting temperature is controlled at 1555-1565℃ and the casting time is 19-21min to allow the inclusions in the steel to float to the surface and ensure that the molten steel is fully fed during the casting process.

[0021] b. Ingot Heating: The ingot is preheated and charged, with a preheating temperature ≥350℃ and a furnace entry temperature >300℃. The ingot is then heated for 3-5 hours. The heating rate in the low-temperature section (<900℃) is ≤60℃ / h to reduce the temperature difference between the core and surface of the billet. The purpose of this preheating is twofold: first, to reduce energy consumption by preheating the ingot; and second, to ensure the billet reaches the same internal and external temperatures as the furnace temperature before heating, suppressing the formation of martensite at the corners of the ingot due to rapid cooling in the mold. This preheating process transforms some of the martensite, preventing further cooling and the resulting increase in low-temperature martensite, which could lead to volume expansion and stress cracking. The ingot is then rapidly heated to the soaking zone, with a heating temperature of 1200±10℃ and a holding time of 7-8.5 min / cm. This ensures the ingot is thoroughly heated for high-reduction rolling processes and prevents the growth of original austenite grains.

[0022] c. Rolling: High-temperature controlled rolling, with an initial rolling temperature ≥1100℃, the first pass eliminates taper, and subsequent passes reduce the thickness by ≥60mm, directly rolling to the finished thickness, and a release temperature ≥940℃. By controlling the temperature and using a large reduction rolling process, the rolling force is fully penetrated into the core of the billet, causing the grains in half of the billet to flatten and elongate, breaking up the original austenite that has been coarsened by heating. The Nb and V elements in the steel prevent the growth of the already broken original austenite grains. On the other hand, through dislocation strengthening and large reduction rolling process control, the original resistance to lamellar tearing and the quality of internal flaw detection of the steel plate are ensured.

[0023] d. Cooling: After rolling, the steel plate is quickly fed into the ACC for repeated cooling. The ACC opens all the manifolds, the roller speed is 0.3~0.5m / s, and the red-hot temperature is ≤530℃, so that the original structure of the steel plate is uniform and refined.

[0024] e. Heat Treatment: A normalizing and rapid cooling process is employed, with a heating rate of ≥70℃ / h, reaching 910±10℃ and holding for 2.0~2.2min / mm. After holding, the steel is rapidly removed and immersed in water for cooling within 30 seconds (water temperature ≤10℃), with a cooling rate controlled at ≥1.5℃ / s. The red-hot temperature is controlled at 480~500℃, followed by air cooling to room temperature. Rapid heating to approximately 910℃ ensures complete austenitization of the steel plate's internal structure while preventing grain growth in the high-temperature region. Rapid low-temperature cooling rapidly transforms the internal structure of the steel plate into a B+P+F structure. After this transformation, air cooling allows the core temperature to fully dissipate to the surface. The precipitation of hardenability-enhancing elements such as Ni and Cr, and grain-refining elements such as Nb and V, strengthens the overall strength of the steel plate. The red-hot temperature is controlled at 480~500℃. This also prevents the steel plate temperature from entering the martensitic region, thus inhibiting martensite formation. The steel plate achieves a balance between strength and toughness through a low-temperature rapid cooling and reddening process after normalizing, which reduces one tempering process compared to conventional heat treatment processes (quenching and tempering) for extra-thick plates.

[0025] The extra-thick plates obtained according to this scheme meet the requirements of EN10160-1999 S3E4 in terms of flaw detection. Their yield strength is ≥370Pa, tensile strength is 530~710MPa, elongation is ≥17%, impact resistance at -50℃ is ≥150J, and resistance to lamellar tearing reaches Z45. The uniform strength and toughness matching across the entire thickness section can meet the high-performance requirements of various types of bridges, heavy machinery, offshore platforms, hydroelectric power plant applications, and other complex environments. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0027] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0028] Example 1:

[0029] S460NLZ35 steel plate with a thickness of 300mm (rolled)

[0030] Through hot metal pretreatment, converter smelting, and ladle refining, molten steel with the following chemical composition (in wt%) was obtained: C: 0.09, Si: 0.1, Mn: 1.65, P: 0.004, S: 0.002, Cr: 0.22, Ni: 0.35, Nb: 0.035, V: 0.065, Als: 0.030, CEV: 0.51, with the remainder being Fe and residual elements.

[0031] a. The steel ingot adopts a 950 / 1050mm taper and 41-ton ingot design to ensure a 3.5 times compression ratio and a casting time of 19-21 minutes, so that the inclusions in the steel can float to the surface and the molten steel can be fully fed back during the casting process.

[0032] b. Ingot heating: The ingot cleaning temperature is 356℃, the furnace entry temperature is 310℃, and the ingot is left to simmer for 4 hours. The heating rate is 50℃ / h until the ingot reaches 900℃, then the temperature is increased at the maximum rate to the soaking zone, where the soaking zone temperature is 1200±

[0033] The temperature is 10℃, and the holding time in the soaking zone is controlled at 13.5 hours to ensure that the steel ingot is thoroughly heated, which is conducive to the execution of the high-pressure rolling process, while avoiding the growth of the original austenite grains.

[0034] c. Rolling: High-temperature controlled rolling, with an initial rolling temperature of 1180℃, the first pass eliminating the taper and rolling to 960mm, and subsequent passes reducing by 60mm (pass rolling data are shown in Table 1), and the release temperature of 947℃.

[0035] Table 1

[0036]

[0037] d. Cooling: The steel plate enters the ACC within 26 seconds after rolling and adopts a rapid multiple cooling process. The ACC opens 22 sets of manifolds and the roller speed is 0.5m / s to cool 8 times, with a red-hot temperature of 503℃, so that the original microstructure of the steel plate is uniform and refined.

[0038] e. Heat treatment: The normalizing and rapid cooling process is adopted. First, the temperature is raised to 910±10℃ at a rate of 75℃ / h and held for 600 minutes. After the holding time is completed, the temperature is cooled in water within 30 seconds at a water temperature of 7℃. The temperature of the red glow is 496℃. Then, the temperature is cooled to room temperature by air.

[0039] The 300mm thick S460NLZ35 steel plate produced by the above method was subjected to 100% full coverage flaw detection according to the flaw detection requirements of EN10160-1999 and met the flaw detection requirements of S3E4. The actual properties of the steel plate are shown in Table 2.

[0040] Table 2

[0041]

[0042] Example 2:

[0043] S460NLZ35 steel plate with a thickness of 320mm rolled plate

[0044] Through hot metal pretreatment, converter smelting, and ladle refining, molten steel with the following chemical composition (in wt%) was obtained: C: 0.10, Si: 0.06, Mn: 1.67, P: 0.003, S: 0.003, Cr: 0.23, Ni: 0.32, Nb: 0.041, V: 0.071, Als: 0.030, CEV: 0.46, with the remainder being Fe and residual elements.

[0045] a. The steel ingot adopts a 950 / 1050mm taper, 43-ton ingot design, 3.28 times compression ratio, and a casting time of 20 minutes;

[0046] b. Ingot heating: The ingot cleaning temperature is 358℃, the furnace entry temperature is 314℃, and the ingot is kept in the furnace for 4 hours. The heating rate is 53℃ / h before reaching 900℃, and then the temperature is increased at the maximum rate to the soaking zone. The soaking zone heating temperature is 1200±

[0047] At 10℃, the heat preservation time of the heat spreader is controlled at 14 hours.

[0048] c. Rolling: High-temperature controlled rolling, with an initial rolling temperature of 1190℃, the first pass eliminating the taper and rolling to 980mm, and subsequent passes reducing by 60mm (pass rolling data are shown in Table 3), and the release temperature of 950℃.

[0049] Table 3

[0050]

[0051] d. Cooling: The steel plate enters the ACC 23s after rolling and adopts a rapid multiple cooling process. The ACC opens 22 sets of manifolds and the roller speed is 0.5m / s to cool 8 times, with a red-hot temperature of 512℃, so that the original microstructure of the steel plate is uniform and refined.

[0052] e. Heat treatment: The normalizing and rapid cooling process is adopted. The temperature is first raised to 910±10℃ at a rate of 77℃ / h, and held for 672 minutes. After the holding time is completed, the water is cooled within 25 seconds at a water temperature of 9℃. The red temperature is 483℃. Then, the water is air cooled to room temperature.

[0053] The 320mm thick S460NLZ35 steel plate produced by the above method was subjected to 100% full coverage flaw detection according to the flaw detection requirements of EN10160-1999 and met the flaw detection requirements of S3E4. The actual properties of the steel plate are shown in Table 4.

[0054] Table 4

[0055]

[0056] Example 3:

[0057] S460NLZ35 steel plate with a thickness of 350mm (rolled)

[0058] Through hot metal pretreatment, converter smelting, and ladle refining, molten steel with the following chemical composition (in wt%) was obtained: C: 0.11, Si: 0.08, Mn: 1.70, P: 0.003, S: 0.002, Cr: 0.28, Ni: 0.36, Nb: 0.043, V: 0.074, Als: 0.032, CEV: 0.49, with the remainder being Fe and residual elements.

[0059] a. The steel ingot adopts a 950 / 1050mm taper and a 43-ton ingot design to ensure a 3 times compression ratio and a casting time of 20 minutes;

[0060] b. Steel ingot heating: The cleaning temperature of the steel ingot is 60℃, the furnace temperature is 317℃, and the steel is steamed for 5 hours. The heating rate is 57℃ / h before the low temperature section is <900℃, and then the temperature is increased to the soaking section at the maximum rate. The heating temperature of the soaking section is 1200±10℃, and the holding time of the soaking section is controlled at 15 hours.

[0061] c. Rolling: High-temperature controlled rolling, with an initial rolling temperature of 1197℃, the first pass eliminating the taper and rolling to a thickness of 1010mm, and subsequent passes reducing by 60mm (pass rolling data are shown in Table 5), and a release temperature of 943℃.

[0062] Table 5

[0063]

[0064] d. Cooling: The steel plate enters the ACC 25s after rolling and adopts a rapid multiple cooling process. The ACC opens 22 sets of manifolds and the roller speed is 0.5m / s to cool 9 times, with a red-hot temperature of 503℃, so that the original microstructure of the steel plate is uniform and refined.

[0065] e. Heat treatment: The normalizing and rapid cooling process is adopted. The temperature is raised to 910±10℃ at a rate of 80℃ / h, and held for 735 minutes. After the holding time is completed, the water is cooled within 23 seconds at a water temperature of 6℃. The red temperature is 469℃. Then, the water is air cooled to room temperature.

[0066] The 350mm thick S460NLZ35 steel plate produced by the above method was subjected to 100% full coverage flaw detection according to the flaw detection requirements of EN10160-1999 and met the flaw detection requirements of S3E4. The actual properties of the steel plate are shown in Table 6.

[0067] Table 6

[0068]

[0069] In summary, the 300-350mm thick S460NLZ35 high-strength steel plate obtained using this method undergoes 100% full coverage flaw detection according to EN10160-1999 requirements, meets S3E4 flaw detection requirements, exhibits uniform and stable performance and good strength and toughness matching between the 1 / 2 and 1 / 4 sections, and demonstrates excellent resistance to lamellar tearing. It meets the quality requirements of various types of bridges, heavy machinery, offshore platforms, hydroelectric power plant rings, etc., and is suitable for mass production.

Claims

1. A method for producing 300-350mm high-strength structural steel S460NLZ35, characterized in that, The steel has the following chemical composition by mass percentage: C: 0.09~0.11, Si: 0.06~0.10, Mn: 1.60~1.70, P: ≤0.006, S: ≤0.004, Ni: 0.30~0.40, Nb: 0.035~0.045, Cr: 0.20~0.30, V: 0.060~0.075, Als: 0.030~0.040, with the remainder being Fe and residual elements. The steel's flaw detection conforms to EN10160-1999 S3E4 requirements, and its yield strength ≥370Pa, tensile strength 530~710MPa, elongation ≥17%, impact resistance at -50℃ ≥150J, and resistance to lamellar tearing reaching Z45. The production method of the aforementioned 300-350mm high-strength structural steel S460NLZ35 includes ingot casting, ingot heating, rolling, cooling, and heat treatment, as detailed below: a. Steel ingot casting: The steel ingot is cast using an ingot mold with a taper of 950-1050mm. The casting temperature is controlled at 1555-1565℃ and the casting time is 19-21 minutes. b. Steel ingot heating: Steel ingots are cleaned and charged at a temperature of ≥350℃, and the furnace temperature is >300℃. The steel is then steamed for 3-5 hours. The heating rate in the low-temperature section <900℃ is ≤60℃ / h to reduce the temperature difference between the core and the surface of the billet. Then, the temperature is rapidly increased to the soaking section. The heating temperature in the soaking section is 1200±10℃, and the holding time in the soaking section is controlled at 7-8.5min / cm. c. Rolling: High-temperature controlled rolling, with an initial rolling temperature ≥1100℃, eliminating taper in the first pass, and subsequent passes having a reduction of ≥60mm, rolling directly to the finished thickness, and a release temperature ≥940℃; d. Cooling: After rolling, the steel plate is quickly fed into the ACC for repeated cooling. The ACC is fully opened, the roll speed is 0.3~0.5m / s, and the reddening temperature is ≤530℃. e. Heat treatment: The normalizing and rapid cooling process is adopted. The temperature is raised to 910±10℃ at a heating rate of ≥70℃ / h and held for 2.0~2.2min / mm. After the holding time is completed, the temperature is cooled in water within 30s with a water temperature ≤10℃ and a reddening temperature of 480~500℃. Then, the temperature is cooled to room temperature by air.

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