A method for improving the microstructure and properties of a flat-killed steel

Abstract

This invention discloses a method for improving the microstructure and properties of bulb flat steel, comprising: the chemical composition of the bulb flat steel by weight percentage as follows: C 0.08-0.10%, N 0.017-0.020%, V 0.07-0.09%, Si 0.40-0.55%, Mn 0.85-1.00%, P≤0.010%, S≤0.005%, with the balance being Fe and unavoidable impurities, and the V / N ratio controlled at 3.5-4.0; heating the billet to 1230-1250℃ and holding it at that temperature for at least 60 minutes; starting rolling at 1150-1180℃, and controlling the final rolling temperature at 930-960℃, with a total reduction rate of at least 75%; using a combination of air cooling and thin water mist cooling; and tempering at 680±5℃. The purpose of this invention is to optimize the performance of bulb flat steel.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and particularly relates to a method for improving the microstructure and properties of bulb flat steel. Background Technology

[0002] As a critical structural material for ships, bulb flat steel requires high strength, high toughness, and good uniformity of cross-sectional properties. With the increasing size of ships, the uneven deformation and cooling rate during the rolling process of bulb flat steel can easily lead to inconsistencies in microstructure and properties, especially between the bulb head and the web, becoming a key factor limiting its service reliability. Currently, quenching and tempering (quenching + tempering) is commonly used to improve its overall performance, but this method suffers from high cost, difficulty in control, and long production cycles. Therefore, there is an urgent need to develop a new, low-cost, high-efficiency process method that possesses excellent microstructure uniformity and high toughness. Summary of the Invention

[0003] The purpose of this invention is to provide a method for optimizing the performance of bulb flat steel by controlling the carbon-nitrogen composition ratio, implementing VN microalloying design, and combining optimized hot rolling parameters and tempering heat treatment regime to achieve the synergistic effect of grain refinement, precipitation strengthening and microstructure homogenization.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a method for improving the microstructure and properties of bulb flat steel, comprising:

[0006] The chemical composition of the bulb flat steel by weight percentage is as follows: C 0.08-0.10%, N 0.017-0.020%, V 0.07-0.09%, Si 0.40-0.55%, Mn 0.85-1.00%, P≤0.010%, S≤0.005%, with the balance being Fe and unavoidable impurities, and the V / N ratio is controlled at 3.5-4.0;

[0007] The steel billet is heated to 1230-1250℃ and held at that temperature for no less than 60 minutes;

[0008] Rolling is started at 1150–1180℃, and the final rolling temperature is controlled at 930–960℃, with a total reduction rate of not less than 75%.

[0009] A combination of air cooling and thin water mist cooling is adopted to keep the cooling rate difference between the ball head and the web area within 5℃ / s;

[0010] Temper at 680±5℃ for 2 hours, and then air cool or slowly cool to room temperature.

[0011] The final microstructure is fine polygonal ferrite + spheroidized pearlite, with precipitate grain size ≤10 nm, average grain size ≤5 μm, impact energy at -40℃ not less than 250 J, and ductile-brittle transition temperature less than -55℃.

[0012] Furthermore, the chemical composition of the bulb flat steel by mass percentage is as follows: C 0.09%, N 0.019%, V 0.078%, Si 0.52%, Mn 0.91%, P 0.004%, S 0.003%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, the chemical composition of the bulb flat steel by weight percentage is as follows: C 0.10%, N 0.018%, V 0.072%, Si 0.50%, Mn 0.89%, P 0.005%, S 0.003%, with the balance being Fe and unavoidable impurities.

[0014] Furthermore, the tempering process employs a multi-zone temperature-controlled box-type resistance furnace, with a heating rate controlled at 5–10℃ / min and a temperature control accuracy of ±2℃.

[0015] Furthermore, the cooling system employs a symmetrically arranged spray nozzle air-mist cooling system with a spray particle size of less than 0.3 mm and a cooling time controlled between 90 and 150 seconds.

[0016] Furthermore, the proportion of high-angle grain boundaries in the microstructure obtained by EBSD analysis is not less than 70%, which helps to improve low-temperature impact toughness.

[0017] Furthermore, the yield strength difference between the ball head and web regions of the final produced ball flat steel is no greater than ±5 MPa, and the impact absorption energy difference is no greater than ±10 J.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0019] (1) Significantly improves the yield strength and impact toughness of bulb flat steel, meeting the high strength and toughness matching requirements of large-size marine structural steel; (2) Significantly improves the uniformity of cross-sectional properties, and controls the difference in mechanical properties between the bulb head and the web within ±5 MPa; (3) The process can realize industrial continuous production, and tempering replaces quenching and tempering to significantly reduce energy consumption and heat treatment costs; (4) Grain refinement and precipitation strengthening work together to enhance performance without increasing costs; (5) Excellent low-temperature impact performance, with the ductile-brittle transition temperature reduced to -58℃, meeting the service requirements of extremely cold environments. Detailed Implementation

[0020] A method for optimizing the performance of bulb flat steel by controlling the carbon-nitrogen ratio, implementing VN microalloying design, and combining optimized hot rolling parameters and tempering heat treatment regime to achieve the synergistic effect of grain refinement, precipitation strengthening and microstructure homogenization.

[0021] The specific technical solution of the present invention is as follows:

[0022] A method for improving the microstructure and properties of bulb flat steel, the main production steps of which include:

[0023] Alloy design: Low carbon and high nitrogen composition is selected, C 0.08~0.10%, N 0.017~0.020%, Si 0.40~0.55%, Mn 0.85~1.00%, V 0.07~0.09%, and the V / N mass ratio is controlled between 3.5 and 4.0 to ensure the formation of fine and dispersed V(C,N) precipitates.

[0024] Steel billet smelting and continuous casting: Converter smelting is adopted, and the S and P contents are controlled to be less than 0.005% and 0.010%, respectively; the continuously cast steel billet must ensure low segregation (center segregation level ≤1.0) to avoid subsequent grain coarsening.

[0025] Heating and rolling: The billet heating temperature is controlled at 1230-1250℃ and held for no less than 1 hour to achieve uniform microstructure; the initial rolling temperature is controlled at 1150-1180℃ and the final rolling temperature is controlled at 930-960℃; the total reduction rate is controlled at more than 75%, and controlled cooling is performed immediately after final rolling to minimize the difference in cooling rate between different parts.

[0026] Cooling process: After controlled rolling, a combination of air cooling and thin water mist cooling is adopted to ensure that the cooling rate difference between the ball head and the web is controlled within 5℃ / s, effectively suppressing the formation of granular bainite.

[0027] Tempering heat treatment: Temper at 680℃±5℃ for 2 hours with a temperature control accuracy of ±2℃ to ensure pearlite spheroidization and eliminate granular bainite; cooling methods can be furnace cooling or slow air cooling to avoid stress concentration.

[0028] Microstructure control target: The microstructure should mainly consist of fine polygonal ferrite + spheroidized pearlite, with an average particle size of V(C,N) precipitates not greater than 10 nm and a high-angle grain boundary ratio not less than 70%, in order to balance strength and low-temperature toughness.

[0029] The following is a detailed description of a method for improving the microstructure and properties of bulb flat steel according to the present invention.

[0030] Example: This example is a preferred embodiment of the various embodiments of the present invention.

[0031] Example 1:

[0032] Chemical composition of steel (mass percentage): C 0.09%, N 0.019%, V 0.078%, Si 0.52%, Mn 0.91%, P 0.004%, S 0.003%, balance Fe and impurities.

[0033] Billet preparation: 280 mm × 380 mm continuous casting billets are used, which are refined in two stages (LF+VD) with excellent inclusion control and total oxygen content TO ≤ 0.001%.

[0034] Heating and rolling process: The billet is heated to 1240℃, held for 60 minutes and then taken out of the furnace. The initial rolling temperature is 1170℃, the final rolling temperature is 950℃, and the total reduction rate is 78%. Controlled rolling and controlled cooling process is adopted.

[0035] Cooling process: After finishing rolling, air cooling + fine mist cooling composite cooling is adopted, with a cooling time of 110 seconds and a cooling rate controlled at 12–15℃ / s. The cooling rate difference between the ball head and web area is less than 4℃ / s.

[0036] Tempering process: After holding at 680℃ ± 5℃ in a box furnace for 2 hours, air cool.

[0037] Microstructure: The microstructure consists of fine polygonal ferrite and spheroidized pearlite, with an average grain size of 4.6 μm, a precipitate grain size of 9 nm, and a high-angle grain boundary ratio of 71%.

[0038] Performance parameters (center of ball head): Yield strength: 565 MPa, tensile strength: 682 MPa, elongation: 27%, impact energy at -40℃: 265J, ductile-brittle transition temperature: -58℃.

[0039] Example 2:

[0040] Chemical composition of steel (mass percentage): C 0.10%, N 0.018%, V 0.072%, Si 0.50%, Mn 0.89%, P 0.005%, S 0.003%, balance Fe and impurities.

[0041] Heating and rolling process: The billet is heated to 1235℃, the initial rolling temperature is 1160℃, the final rolling temperature is 945℃, and the total reduction rate is 76%.

[0042] Cooling process: A two-stage air-cooling-mist-cooling control system is adopted, with a cooling time of about 120 seconds and a cooling rate of about 13℃ / s, resulting in uniform cooling.

[0043] Tempering process: Tempering temperature is 680℃, hold for 2 hours, and air cool to room temperature.

[0044] Microstructure characteristics: average grain size is 4.8 μm, precipitate grain size is 10 nm, and high-angle grain boundary ratio is 70%.

[0045] Performance parameters (center of ball head): Yield strength: 558 MPa, tensile strength: 676 MPa, elongation: 25%, impact energy at -40℃: 258J, ductile-brittle transition temperature: -56℃.

[0046] Comparative Example 1 (Unoptimized N / V ratio, conventional cooling):

[0047] Chemical composition of steel (mass percentage): C 0.11%, N 0.010%, V 0.065%, Si 0.44%, Mn 0.86%, P 0.005%, S 0.005%, balance Fe and impurities.

[0048] Heating and rolling process: heating temperature 1240℃, initial rolling temperature 1155℃, final rolling temperature 935℃, reduction rate approximately 75%.

[0049] Cooling process: natural air cooling, cooling time 180 seconds, relatively slow cooling rate (about 7℃ / s), cooling difference between ball head and web plate reaches 8℃ / s.

[0050] Tempering process: Tempering temperature 680℃, holding time 2 hours.

[0051] Microstructure characteristics: It contains granular bainite and coarse pearlite, with a grain size of 6.0 μm and uneven precipitates.

[0052] Performance indicators (center of ball head): Yield strength: 470 MPa, tensile strength: 665 MPa, elongation: 24%, impact energy at -40℃: 190J, ductile-brittle transition temperature: -40℃.

[0053] Comparative Example 2 (high C, low V):

[0054] Chemical composition of steel (mass percentage): C 0.12%, N 0.015%, V 0.070%, Si 0.49%, Mn 0.88%, P 0.005%, S 0.004%, balance Fe and impurities.

[0055] Hot working and cooling: heating to 1230℃, final rolling temperature 930℃, natural air cooling, cooling rate about 6℃ / s.

[0056] Tempering treatment: Temper at 680℃ and hold for 2 hours.

[0057] Microstructure characteristics: Grain size 5.9 μm, bainite mixed with coarse pearlite, and coarse and uneven precipitates.

[0058] Performance indicators (center of ball head): Yield strength: 485 MPa, tensile strength: 673 MPa, elongation: 23%, impact energy at –40℃: 200J, ductile-brittle transition temperature: –42℃.

[0059] Table 1 Chemical composition (wt%) of the examples and comparative examples

[0060] Material Number C N V Si Mn P S Example 1 0.09 0.019 0.078 0.52 0.91 0.004 0.003 Example 2 0.10 0.018 0.072 0.50 0.89 0.005 0.003 Comparative Example 1 0.11 0.010 0.065 0.44 0.86 0.005 0.005 Comparative Example 2 0.12 0.015 0.070 0.49 0.88 0.005 0.004

[0061] Table 2 Comparison of mechanical properties (tempered state, ball head area)

[0062] Material Number Yield strength / MPa Tensile strength / MPa Elongation / % Impact energy at -40℃ / J Brittle-ductile transition temperature / ℃ Example 1 565 682 27 265 -58 Example 2 558 676 25 258 -56 Comparative Example 1 470 665 24 190 -40 Comparative Example 2 485 673 23 200 -42

[0063] As can be seen from the embodiments and comparative examples, the present invention has the following advantages:

[0064] (1) Significant effect of microstructure refinement: By adopting a low carbon and high nitrogen design and optimizing the V content and V / N ratio (controlled at 3.5 to 4.0), Examples 1 and 2 promote the formation of dispersed V(C,N) precipitates, so that the average grain size is controlled at 4.6 to 4.8 μm, which is much smaller than the 5.9 to 6.0 μm of the comparative example; the particle size of the precipitate is also significantly reduced, effectively improving the yield strength and toughness.

[0065] (2) Synergistic improvement of strength and toughness: In the embodiment of the present invention, the yield strength of the ball head region reaches 558-565 MPa, the impact energy at -40℃ exceeds 258 J, and the brittle-ductile transition temperature is reduced to -56--58℃, which is significantly improved compared with the comparative example. In particular, the impact energy of the comparative example 1 is only 190 J, and the transition temperature is as high as -40℃, which is prone to low-temperature brittle fracture, showing the superior reliability of the present invention in extremely cold environments.

[0066] (3) Composition and controlled cooling work together to achieve better uniformity of cross-sectional properties: By precisely controlling the cooling rate and the difference in cooling rate between the ball head and the web (controlled within 4-5℃ / s), and with a reasonable V / N ratio design, the difference in mechanical properties of the cross section is greatly reduced. Actual test data show that the difference in strength and impact toughness between the ball head and the web is controlled within ±5 MPa and ±10J, respectively, and the performance uniformity is better than that of conventional production methods.

[0067] (4) Strong process adaptability and can be promoted in batches: The composition design and controlled rolling-cooling-tempering process adopted in this invention can be realized on existing steel plant equipment. No quenching process is required, which simplifies the heat treatment process, reduces energy consumption and deformation risk, and has good conditions for industrial promotion.

[0068] (5) Adapting to the future demand trend of high-performance marine steel: This method can meet the stringent requirements of large-size spherical flat steel for high strength, high toughness, low temperature resistance to brittle fracture and cross-sectional consistency, and is especially suitable for key structural components such as polar icebreakers, offshore platforms and LNG ships, with significant engineering value and market prospects.

[0069] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for improving the microstructure and properties of bulb flat steel, characterized in that, include: The chemical composition of the bulb flat steel by weight percentage is as follows: C 0.08-0.10%, N 0.017-0.020%, V 0.07-0.09%, Si 0.40-0.55%, Mn 0.85-1.00%, P≤0.010%, S≤0.005%, with the balance being Fe and unavoidable impurities, and the V / N ratio is controlled at 3.5-4.0; The steel billet is heated to 1230-1250℃ and held at that temperature for no less than 60 minutes; Rolling is started at 1150–1180℃, and the final rolling temperature is controlled at 930–960℃, with a total reduction rate of not less than 75%. A combination of air cooling and thin water mist cooling is adopted to keep the cooling rate difference between the ball head and the web area within 5℃ / s; Temper at 680±5℃ for 2 hours, and then air cool or slowly cool to room temperature. The final microstructure is fine polygonal ferrite + spheroidized pearlite, with precipitate grain size ≤10 nm, average grain size ≤5 μm, impact energy at -40℃ not less than 250 J, and ductile-brittle transition temperature less than -55℃.

2. The method for improving the microstructure and properties of bulb flat steel according to claim 1, characterized in that, The chemical composition of the bulb flat steel by weight percentage is as follows: C 0.09%, N 0.019%, V 0.078%, Si 0.52%, Mn 0.91%, P 0.004%, S 0.003%, with the balance being Fe and unavoidable impurities.

3. The method for improving the microstructure and properties of bulb flat steel according to claim 1, characterized in that, The chemical composition of the bulb flat steel by weight percentage is as follows: C 0.10%, N 0.018%, V 0.072%, Si 0.50%, Mn 0.89%, P 0.005%, S 0.003%, with the balance being Fe and unavoidable impurities.

4. The method for improving the microstructure and properties of bulb flat steel according to claim 1, characterized in that, Tempering is performed using a multi-zone temperature-controlled box-type resistance furnace, with a heating rate controlled at 5–10℃ / min and a temperature control accuracy of ±2℃.

5. The method for improving the microstructure and properties of bulb flat steel according to claim 1, characterized in that, Cooling is achieved using a symmetrically arranged air-mist cooling system with spray nozzles. The spray particle size is less than 0.3 mm, and the cooling time is controlled between 90 and 150 seconds.

6. The method for improving the microstructure and properties of bulb flat steel according to claim 1, characterized in that, The proportion of high-angle grain boundaries in the microstructure obtained by EBSD analysis is not less than 70%, which helps to improve low-temperature impact toughness.

7. The method for improving the microstructure and properties of bulb flat steel according to claim 3, characterized in that, The final spherical flat steel has a yield strength difference of no more than ±5 MPa between the spherical head and the web region, and an impact absorption energy difference of no more than ±10 J.