Production method of 890MPa-grade fire-resistant and weathering-resistant steel with thickness of 20-60mm

Abstract

The invention discloses a production method of 890MPa-grade fire-resistant and weathering-resistant steel with the thickness of 20-60mm. The 890MPa-grade fire-resistant and weathering-resistant steel with the thickness of 20-60mm is obtained by adopting a low-C and low-Mn component design, adding a proper amount of Cr, Ni, Mo and Cu alloy elements, ensuring relatively low carbon equivalent and welding crack sensitivity coefficient, ensuring grain refinement of a steel plate and stabilizing the yield strength in a required range through a reasonable production process, and ensuring that the 890MPa-grade fire-resistant and weathering-resistant steel with the thickness of 20-60mm has the advantages of high strength, high strength, high strength and the like. The yield strength at room temperature is larger than or equal to 890 MPa, the impact energy at-40 DEG C is larger than or equal to 150 J, the weather resistance index I is larger than or equal to 7.0, the high-temperature strength at 600 DEG C is larger than or equal to 2 / 3 of the room-temperature strength, and the obtained steel has good fire resistance, weather resistance and comprehensive mechanical performance. The application requirements of 890MPa-grade fire-resistant and weather-resistant steel are completely met, and the 890MPa-grade fire-resistant and weather-resistant steel can be widely applied to the fields of buildings, bridges, ships

Description

Technical Field

[0001] The present invention relates to the technical field of steel materials, and in particular to a production method of 890 MPa-grade fire-resistant and weathering steel with a thickness of 20 to 60 mm. Background Art

[0002] With the rapid development of industries such as construction, bridges, and ships, the performance requirements for steel are becoming increasingly stringent. Steel is not only required to have high strength and toughness, but also to have good fire resistance and weather resistance. Fire resistance refers to the ability of steel to maintain a certain strength and stability at high temperatures, while weather resistance refers to the ability of steel to resist corrosion in the natural environment.

[0003] Currently, the most common fire-resistant and weathering steels on the market are S355 and S460 grades, which have low strength and cannot meet the strength requirements of high-rise buildings, long-span bridges, and other applications. For example, Chinese patent CN103361568 discloses "a fire-resistant and weathering building steel and its manufacturing method." The steel obtained using this method has a yield strength of 350.00 MPa at 600°C, reaching 72.3% of the room temperature yield strength, exceeding 2 / 3 of the room temperature yield strength, meeting the fire resistance requirements. However, the finished product is only 0.5 to 6 mm thick, which limits its application range.

[0004] There has been no report on how to produce higher-grade and thicker fire-resistant and weathering steel on a large scale and stably due to its high fire resistance requirements and great technical difficulty.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The object of the present invention is to provide a production method for 890MPa grade high-refractory and weathering steel with a thickness of 20 to 60mm. The method adopts a low-C and low-Mn composition design, adds appropriate amounts of Cr, Ni, Mo, and Cu alloy elements, ensures a low carbon equivalent and welding crack sensitivity coefficient, and ensures that the steel plate grain is refined and the yield strength is stable within the required range through a reasonable production process. The obtained steel has good fire resistance, weather resistance and comprehensive mechanical properties.

[0007] To achieve the above object, the present invention adopts the following technical solutions: A method for producing 890 MPa grade fire-resistant and weathering steel, comprising the following chemical composition by mass percentage (unit: wt%): C < 0.10, Si: 0.20-0.40, Mn: 1.20-1.50, P ≤ 0.015, S ≤ 0.005, Cr: 0.50-0.80, Mo: 0.20-0.30, Ni: 0.50-0.80, Cu: 0.20-0.40, V: 0.05-0.10, Nb: 0.02-0.05, Ti: 0.01-0.03, Als: 0.020-0.035, B: 0.001-0.003, with the remainder being Fe and unavoidable impurities, wherein the steel has a crack sensitivity index Pcm ≤ 0.25% and a weathering index I ≥ 7.0. Regarding the composition design, it should be noted that: C: Carbon is the most important and economical strengthening element in steel. Generally, increasing carbon content increases the amount of pearlite and the strength of the steel, but this has a negative impact on the toughness, ductility, and weldability of the steel. When carbon occupies the interstitial spaces of the α lattice and dissolves in the α matrix, it distorts the α lattice and increases the strength of the α matrix. This carbon can then contribute to the strength of the α matrix. While the strengthening effect is significant at room temperature, its tendency to dissolve significantly increases at high temperatures, reducing the strengthening effect. Furthermore, carbon interacts strongly with elements such as Nb, V, Ti, and Mo to form carbides, significantly impacting high-temperature strength. Therefore, while ensuring strength, a suitably low carbon content should be designed to increase the bainite transformation temperature and facilitate the formation of granular bainite. Low carbon content also improves the weldability of the steel plate. Therefore, a carbon content of <0.10wt% is recommended.

[0008] Mn: Manganese's primary functions in steel are deoxidation and desulfurization. It also reduces the formation of grain boundary cementite, improving toughness. Furthermore, by reducing carbon diffusion, Mn hinders the coarsening and growth of carbides, thereby refining precipitates. Mn also dissolves in the matrix, contributing to solid solution strengthening. Studies have shown that Mn interacts with nitrogen at around 450°C, improving high-temperature strength but causing temper brittleness and hindering high-temperature ductility. Therefore, a Mn content of 1.20–1.50 wt% is recommended.

[0009] Nb: Niobium has two strengthening effects: one is through network-like precipitation at ferrite grain boundaries and fine precipitation strengthening within the ferrite matrix; the other is through grain refinement caused by the effect of solid solution and precipitation on recrystallization. Nb has a strong affinity with carbon and nitrogen, easily forming finely dispersed Nb (C, N), which hinders grain boundary migration and increases grain growth temperature, thereby achieving grain refinement. Furthermore, by forming fine, dispersed carbonitrides, it exerts precipitation strengthening, ensuring excellent high-temperature properties. Furthermore, its addition promotes induced phase transformation in the steel, raising the critical temperature and lowering the critical deformation for induced transformation. It also inhibits the growth of fine-grained ferrite during induced transformation, favoring the formation of high-strength phases such as bainite, thereby improving the steel's high-temperature strength. Compared with other alloying elements, in addition to significantly improving high-temperature strength, the bainite structure contains a large number of dislocations, resulting in a significant strengthening effect due to the promotion of high-strength phases such as bainite, resulting in a lower yield strength ratio. Therefore, 0.02 to 0.05 wt% of Nb is designed.

[0010] Vanadium (V): Vanadium improves the hardenability of steel and effectively increases the bainite content in the microstructure. Dissolving into ferrite strengthens the steel, forming stable carbides and refining the grains, thereby increasing the steel's high-temperature strength. However, V has no significant inhibitory effect on austenite recrystallization. V has a strong affinity for austenite, and increasing nitrogen content increases the precipitation driving force of V (C, N). If an appropriate amount of V is added to refractory steel while controlling the finish rolling temperature and cooling rate after final rolling, a large number of fine carbon and nitrogen precipitates will form within the steel. These precipitates significantly hinder grain growth at high temperatures, thereby enhancing the steel's refractory properties. Therefore, a V content of 0.05-0.10 wt% is designed.

[0011] Titanium (Ti): Titanium readily forms high-melting-point nitrides, which refine austenite grains and contribute to microstructure refinement. The carbonitrides formed also contribute to precipitation strengthening and provide nucleation sites. As the titanium content increases, strong precipitation strengthening occurs, thereby improving the high-temperature performance of the steel. Therefore, a Ti content of 0.01 to 0.03 wt% is designed.

[0012] Cr: Chromium contributes to high-temperature strength in two main ways: first, through solid solution in the matrix, it strengthens the steel; second, through the formation of precipitates such as Cr7C3 and Cr23C6, it strengthens the steel through precipitation. In Mo-containing steels, the addition of Cr alters the distribution of Mo between carbides and the matrix. When the added Cr is insufficient to form carbides other than carburizers, Cr allows more Mo to dissolve into the matrix. When the added Cr forms carbides other than carburizers, it reduces the number of Mo-containing carbides and similarly allows Mo to dissolve into the matrix. Another disadvantage of Cr is that it dissolves in Mo2C, reducing the lattice constant and stability. Cr effectively improves the high-temperature and creep strength of steel, but its impact is weaker than that of elemental Mo. Therefore, a Cr content of 0.50–0.80 wt% is recommended.

[0013] Molybdenum (Mo) exhibits three forms of high-temperature strengthening in refractory steels: solid solution strengthening, precipitation strengthening, and phase transformation strengthening by promoting bainite transformation. Mo dissolves in ferrite, strengthening the ferrite matrix and increasing the bonding strength of α-Fe atoms. Mo diffuses slowly in ferrite at high temperatures, significantly improving the steel's high-temperature strength and creep strength. Research has also shown that dissolved Mo tends to segregate at grain boundaries, enhancing the steel's high-temperature strength. Mo is the element with the strongest inhibitory effect on pearlite transformation, significantly increasing the stability of supercooled austenite in the pearlite transformation zone. This increases the incubation period for phase transformation and slows the transformation rate, thus facilitating the formation of bainite. The addition of Mo to steel increases the volume fraction of bainite, and the high-strength bainite structure contributes to the excellent high-temperature performance of refractory steel. Similarly, a higher concentration of bainite effectively reduces the steel's yield strength ratio. Mo is an element that shrinks the γ phase. It exists in solid solution or forms carbides in steel, and has a strong affinity for carbon. Therefore, the addition of Mo alloying elements also promotes the formation of more small, stable carbides, Mo2C. These carbides are more stable than Fe3C and are difficult to decompose and grow at high temperatures. Studies have shown that after tempering at 700°C, Mo carbides only spheroidize in situ, with no obvious growth characteristics. These fine alloy carbides hinder grain boundary movement at high temperatures, thereby hindering grain deformation at high temperatures, thereby ensuring the stability of the ferrite structure. The precipitation of Mo in steel to form carbides can improve its high-temperature strength. Therefore, a Mo content of 0.20–0.30 wt% is designed.

[0014] The production of the steel includes clean steel smelting, heating, rolling and heat treatment, as follows: ① Clean steel smelting: LF+VD is used to ensure the cleanliness of molten steel. In the LF refining process, the C, Mn, and Cr contents must not reach the lower limit at the same time. The finished product S is controlled to be ≤0.003%, and Als is controlled to be between 0.020 and 0.035%. Lime is added to each furnace at least 1000 kg, and the white slag holding time is ≥20 minutes. The VD vacuum is ≤67 Pa, the pressure holding time is ≥15 minutes, the constant [H] is ≤1.50 PPm, and the VD off-station superheat is controlled to be 10-20°C. ② Heating: Holding temperature 1200 ~ 1250 ℃, holding time ≥ 2h; adopt low temperature and long holding process to avoid grain coarsening. By refining the grains, more grain boundaries are obtained. These grain boundaries will hinder the movement of dislocations during steel plate deformation, thereby increasing yield strength.

[0015] ③ Rolling: Two-stage temperature-controlled rolling is adopted, with the rolling temperature of the first stage controlled at 1050-1100°C, the rolling temperature of the second stage controlled at 850-900°C, and the final rolling temperature controlled at 800-850°C to obtain finished steel plates with a thickness of 20-60 mm; the steel plates after rolling are cooled by laminar flow with a cooling rate controlled at 5-20°C / s until they are cooled to room temperature; ④ Heat treatment: The steel plate is subjected to quenching and tempering treatment, the quenching heating temperature is controlled at 900-920°C, the holding time is controlled at 1.5 min / mm, the tempering heating temperature is controlled at 600-620°C, the holding time is controlled at 2.0 min / mm, and after tempering, it is air-cooled to room temperature.

[0016] Furthermore, the steel composition also includes 0.10-0.20 wt% of W. Tungsten alloy has good high-temperature stability, which can make the fire-resistant and weathering steel have better high-temperature resistance.

[0017] In the rolling process, the reduction rate in the first stage is controlled at 50-60%, and the reduction rate in the second stage is controlled at 30-40%.

[0018] In the heat treatment step, the quenching medium is a water-soluble quenching liquid with a concentration of 5 to 10%.

[0019] Compared with the prior art, the present invention has the following advantages: The low-carbon and low welding crack sensitivity coefficient design greatly improves the welding performance compared with traditional high-strength steel and solves the problem of cold cracks in thick plate welding. The Cu-Cr-Ni-Mo-Nb-V multi-component synergistic strengthening system is adopted to achieve a balance of high strength and high toughness in thick plates and a comprehensive improvement in mechanical properties. The yield strength at room temperature is stable ≥890MPa, and the impact energy at -40℃ is ≥150J. Among them, the combination of Cu, Cr, and Ni increases the weathering index to I≥7.0, and the micro-alloying of Mo and Nb ensures that the high-temperature strength at 600℃ is ≥2 / 3 of the room temperature strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The technical features of the present invention are further described below in conjunction with the accompanying drawings and embodiments.

[0021] Figure 1 This is a schematic diagram of the metallographic structure of the steel plate obtained in Example 1 of the present invention at 200 times the magnification.

[0022] Figure 2 This is a schematic diagram of the metallographic structure of the steel plate obtained in Example 1 of the present invention at a magnification of 500. DETAILED DESCRIPTION

[0023] The present invention will be further described below with reference to the embodiments. Example

[0024] To produce 20mm thick 890MPa grade fire-resistant and weathering steel, the following chemical composition by mass percentage (unit: wt%) is adopted: C: 0.08, Si: 0.30, Mn: 1.35, P: 0.010, S: 0.003, Cr: 0.60, Mo: 0.20, Ni: 0.55, V: 0.06, Nb: 0.03, Ti: 0.02, Al: 0.03, B: 0.002, Cu: 0.30, and the rest is Fe and unavoidable impurities.

[0025] The main points of the production process are as follows: ① Converter smelting: Before tapping, the slag must be poured and the slag layer thickness must be measured to ensure that the slag layer thickness does not exceed 10mm and that the temperature of the molten steel in the ladle is greater than 1600℃; ②LF refining: The molten steel is transferred to the LF furnace for refining, and slag-forming materials such as lime and fluorite are added. Lime is added to each furnace ≥1000Kg, and the slag basicity is controlled at 3.0. The refining time is 40min, and the refined white slag is kept for ≥20min. After the refining is completed, wire feeding is carried out, and Si-Ca wire is fed for deoxidation and inclusion modification treatment. The Als content of the finished product is controlled at 0.012~0.030%; ③VD vacuum degassing: transfer the refined molten steel into the VD furnace for vacuum degassing treatment, with a vacuum degree of ≤67Pa, a holding time of 20min, a constant [H] of ≤1.50PPm, and soft blowing through the air. It is strictly forbidden to add deoxidizing alloys, and the superheat is controlled at 10-20℃; ④Continuous casting: The vacuum degassed molten steel is continuously cast, the pouring temperature is controlled at 1540℃, the casting speed is 1.0m / min, and a 250mm thick continuous casting billet is obtained; ⑤ Heating: The continuous casting billet is sent into the heating furnace for heating, and the holding temperature is controlled at 1200-1230℃, and the holding time is 2.2h.

[0026] ⑥ Rolling: The heated ingot is rolled using a two-stage controlled rolling process. The rolling temperature in the first stage is controlled at 1050-1100°C, with a reduction rate of 55%. The rolling temperature in the second stage is controlled at 850-900°C, with a reduction rate of 35%. The final rolling temperature is controlled at 800-850°C to obtain a steel plate with a thickness of 20 mm.

[0027] ⑦ Cooling: The rolled steel plate is subjected to laminar cooling with a cooling rate controlled at 5-20°C / s until it cools to room temperature.

[0028] ⑧Heat treatment: The cooled steel plate is subjected to quenching and tempering treatment. The quenching heating temperature is set to 910℃, the holding time is 30min, the quenching medium is a water-soluble quenching liquid with a concentration of 8%; the tempering heating temperature is set to 610℃, the holding time is 40min, and after tempering, it is air-cooled to room temperature. Example

[0029] To produce 60mm thick 890MPa grade fire-resistant and weathering steel, the following chemical composition by mass percentage (unit: wt%) is adopted: C: 0.09, Si: 0.25, Mn: 1.25, P: 0.008, S: 0.002, Cr: 0.65, Mo: 0.25, Ni: 0.65, V: 0.08, Nb: 0.02, Ti: 0.01, Al: 0.02, B: 0.001, Cu: 0.30, W: 0.15, and the rest is Fe and unavoidable impurities.

[0030] The main points of the production process are as follows: ① Converter smelting: Before tapping, the slag must be poured and the slag layer thickness must be measured to ensure that the slag layer thickness does not exceed 10mm and that the temperature of the molten steel in the ladle is greater than 1600℃; ②LF refining: The molten steel is transferred to the LF furnace for refining, and slag-forming materials such as lime and fluorite are added. Lime is added to each furnace ≥1000Kg, and the slag basicity is controlled at 2.8. The refining time is 35min, and the refined white slag is kept for ≥20min. After the refining is completed, wire feeding is carried out, and Si-Ca wire is fed for deoxidation and inclusion modification treatment. The Als content of the finished product is controlled at 0.012~0.030%; ③VD vacuum degassing: transfer the refined molten steel into the VD furnace for vacuum degassing treatment, with a vacuum degree of ≤67Pa, a holding time of 18min, a constant [H] of ≤1.50PPm, soft blowing through the air, and it is strictly forbidden to add deoxidizing alloys, and the superheat is controlled at 10-20℃; ④Continuous casting: The vacuum degassed molten steel is continuously cast, the pouring temperature is controlled at 1535℃, the casting speed is at 0.9m / min, and a 400mm thick continuous casting billet is obtained; ⑤ Heating: The continuous casting billet is sent into the heating furnace for heating, and the holding temperature is controlled at 1200-1230℃ for 2.5h.

[0031] ⑥ Rolling: The heated ingot is rolled using a two-stage controlled rolling process. The rolling temperature in the first stage is controlled at 1050-1100°C, with a reduction rate of 52%. The rolling temperature in the second stage is controlled at 850-900°C, with a reduction rate of 32%. The final rolling temperature is controlled at 800-850°C to obtain a steel plate with a thickness of 60 mm.

[0032] ⑦ Cooling: The rolled steel plate is subjected to laminar cooling with a cooling rate controlled at 5-20°C / s until it cools to room temperature.

[0033] ⑧Heat treatment: The cooled steel plate is subjected to quenching and tempering treatment. The quenching heating temperature is set to 905℃, the holding time is 180min, the quenching medium is a water-soluble quenching liquid with a concentration of 7%; the tempering heating temperature is set to 605℃, the holding time is 240min, and after tempering, it is air-cooled to room temperature.

[0034] The mechanical properties, fire resistance and weather resistance of the steel plates obtained in Example 1 and Example 2 were tested, and the results are shown in Table 1.

[0035]

[0036] As can be seen from the above table, the 20-60 mm thick 890 MPa-grade fire-resistant and weathering steel prepared in Examples 1 and 2 of the present invention has a yield strength of ≥890 MPa at room temperature, an impact energy of ≥150 J at -40°C, a weathering index I ≥7.0, and a high-temperature strength of ≥2 / 3 of the room-temperature strength, which fully meets the application requirements of 890 MPa-grade fire-resistant and weathering steel and can be widely used in construction, bridges, ships and other fields.

[0037] The metallographic examination of the steel plate obtained in Example 1 showed that the main structure was tempered bainite + a small amount of bainite.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limiting. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in this field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for producing 890 MPa grade fire-resistant and weathering steel with a thickness of 20 to 60 mm, characterized in that: The chemical composition by mass percentage is as follows (unit: wt%): C < 0.10, Si: 0.20-0.40, Mn: 1.20-1.50, P ≤ 0.015, S ≤ 0.005, Cr: 0.50-0.80, Mo: 0.20-0.30, Ni: 0.50-0.80, Cu: 0.20-0.40, V: 0.05-0.10, Nb: 0.02-0.05, Ti: 0.01-0.03, Als: 0.020-0.035, B: 0.001-0.003, and the rest are Fe and unavoidable impurities. The crack sensitivity index Pcm is ≤ 0.25%, and the weathering index I is ≥ 7.

0. The production of the steel includes clean steel smelting, heating, rolling and heat treatment, as follows: ① Clean steel smelting: LF+VD is used to ensure the cleanliness of molten steel. In the LF refining process, the C, Mn, and Cr contents must not reach the lower limit at the same time. The finished product S is controlled to be ≤0.003%, and Als is controlled to be between 0.020 and 0.035%. Lime is added to each furnace at least 1000 kg, and the white slag holding time is ≥20 minutes. The VD vacuum is ≤67 Pa, the pressure holding time is ≥15 minutes, the constant [H] is ≤1.50 PPm, and the VD off-station superheat is controlled to be 10-20°C. ② Heating: insulation temperature 1200~1250℃, insulation time ≥2h; ③ Rolling: Two-stage temperature-controlled rolling is adopted, with the rolling temperature of the first stage controlled at 1050-1100°C, the rolling temperature of the second stage controlled at 850-900°C, and the final rolling temperature controlled at 800-850°C to obtain finished steel plates with a thickness of 20-60 mm; the steel plates after rolling are cooled by laminar flow with a cooling rate controlled at 5-20°C / s until they are cooled to room temperature; ④ Heat treatment: The steel plate is subjected to quenching and tempering treatment, the quenching heating temperature is controlled at 900-920°C, the holding time is controlled at 1.5 min / mm, the tempering heating temperature is controlled at 600-620°C, the holding time is controlled at 2.0 min / mm, and after tempering, it is air-cooled to room temperature.

2. The method for producing 890 MPa grade fire-resistant and weathering steel with a thickness of 20 to 60 mm according to claim 1, characterized in that: The steel also contains 0.10 to 0.20% W.

3. The method for producing 890 MPa grade fire-resistant and weathering steel with a thickness of 20 to 60 mm according to claim 1, characterized in that: In the rolling process, the reduction rate in the first stage is controlled at 50-60%, and the reduction rate in the second stage is controlled at 30-40%.

4. The method for producing 890 MPa grade fire-resistant and weathering steel with a thickness of 20 to 60 mm according to claim 1, characterized in that: In the heat treatment step, the quenching medium is a water-soluble quenching liquid with a concentration of 5 to 10%.

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