Method for improving the uniformity of the properties of a flat-bulb steel section

Abstract

The application discloses a method for improving the performance uniformity of a bulb flat steel section and belongs to the technical field of ship structure steel manufacturing. The method effectively improves the tissue difference between a bulb head and a web, improves the strength, impact performance and grain consistency of the bulb flat steel, and finally realizes the coordination and unification of the overall performance of the bulb flat steel through micro-alloy design (controlling the V / N ratio), differential rolling reduction, zoned cooling and subsequent normalizing treatment. Experiments show that the yield strength difference between the bulb head and the web is less than or equal to 6 MPa, and the impact energy difference at-20 DEG C is less than or equal to 14 J, so the bulb flat steel has remarkable engineering value and popularization prospect.

Description

Technical Field

[0001] This invention belongs to the field of marine structural steel manufacturing technology, and particularly relates to a method for improving the uniformity of cross-sectional properties of bulb flat steel; it is applicable to the industrial manufacturing and performance consistency control of large-size DH36 and above grade bulb flat steel. Background Technology

[0002] Marine bulb flats are widely used in ship bulkheads, frames, decks, and other structures. Their typical cross-sections include a bulbous head and web with significant differences in size and compression ratio. Due to uneven deformation distribution and unbalanced cooling rates in traditional rolling and cooling processes, the grain size, microstructure, and mechanical properties of the bulbous head and web regions differ considerably, severely impacting the overall service safety and resistance to brittle fracture of the bulb flats. Existing methods often compensate for performance issues through localized adjustments or fine-tuning of alloying elements, but the improvement is limited and lacks industrial applicability. Therefore, there is an urgent need to develop a comprehensive, systematic manufacturing method that coordinates multiple dimensions, including steelmaking, hot deformation, cooling control, and heat treatment, to address the problem of uneven cross-sectional properties in bulb flats.

[0003] The purpose of this invention is to provide a method for improving the uniformity of the cross-sectional properties of bulb flat steel. This method systematically controls the microalloying composition, differential rolling deformation, zoned cooling rate, and subsequent normalizing treatment to achieve refined microstructure and coordinated strength and toughness in the bulb head and web areas, thereby improving the overall performance consistency of the entire bulb flat steel.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: This invention discloses a method for improving the uniformity of cross-sectional properties of bulb flat steel, comprising the following steps: (1) Steel billet smelting and continuous casting control: The converter smelting-LF refining-VD degassing process is adopted to control the total oxygen content of molten steel TO≤0.0008%; the continuous casting uses square or rectangular billets with a cross section of 280 mm×380 mm, the casting speed is 1.1~1.2 m / min, the crystallizer is equipped with a differential cooling device, and the water volume in the secondary cooling zone is automatically adjusted to avoid the corner temperature difference being too cold; (2) Differential rolling deformation control: When rolling the heated steel billet into spherical flat steel, the total reduction rate of the web is controlled at 13-15%, and the reduction rate of the spherical head area is 5-7%; different reduction amounts are set for the roughing and finishing rolling stages, and the final rolling temperature is controlled between 840-860℃. (3) Zoned cooling control: Spray water cooling is used in the web area after rolling, and the cooling rate is controlled at 20-30℃ / s; air cooling or mist cooling is used in the ball head area, and the cooling rate is 10-15℃ / s. The temperature difference between the two areas at the end of cooling is controlled to be less than 80℃. (4) Normalizing heat treatment: After cooling, the bulb flat steel is normally treated at a temperature of 900-920℃ and a holding time of 40-60 minutes, followed by air cooling to further improve the uniformity of the microstructure and impact performance of the bulb head area; The chemical composition of the bulb flat steel is as follows (by mass percentage): C: 0.10–0.13%, Si: 0.20–0.35%, Mn: 1.20–1.40%, P≤0.015%, S≤0.005%, V: 0.05–0.08%, N: 0.005–0.009%, with the balance being Fe and unavoidable impurities, and the V / N mass ratio is controlled between 6 and 9.

[0005] Furthermore, it also includes (5) surface shaping and treatment: high-pressure water shot blasting and laser straightening treatment are carried out before normalizing to control the surface roughness Ra≤2.0 μm and the geometric deviation is not greater than ±1.0 mm.

[0006] Furthermore, specifically: [V] is 0.06%, [N] is 0.007%, and the V / N ratio is controlled at 8.57.

[0007] Furthermore, the final rolling temperature of the bulb flat steel is controlled at 840–850°C in the web region and 850–860°C in the bulb head region.

[0008] Furthermore, the partitioned cooling control method is to automatically switch between air cooling and water mist combined cooling, with the cooling time controlled between 90 and 180 seconds.

[0009] Furthermore, during the normalizing process, the bulb flat steel is heated using a walking beam furnace with atmospheric protection heating, and the heating rate is 5-10℃ / min.

[0010] Furthermore, the billet is heated to 1180–1200℃ before hot rolling, and the holding time is 50–70 minutes.

[0011] Furthermore, the high-pressure water shot blasting has a spray pressure of 10–15 MPa, a nozzle angle of 60–90°, and a spraying time of 15–30 seconds.

[0012] Furthermore, after implementing this method, the difference in impact energy between the web and the ball head region at -20℃ for the bulb flat steel is no greater than 15 J, the difference in yield strength is no greater than 10 MPa, and the difference in grain size is no more than grade 1.

[0013] Compared with the prior art, the beneficial technical effects of the present invention are as follows: (1) Significantly homogenized cross-sectional properties: Through V / N ratio optimization design, differential rolling and zoned cooling control, the differences in grain size, strength and impact toughness between the ball head and web are effectively reduced, and the high consistency of the entire ball flat steel in terms of structure and properties is achieved; (2) Significantly improved low-temperature impact performance: In the example, the impact energy of the ball head at -20℃ increased from 190 J in the comparative example to more than 254 J, an increase of more than 30%, which meets the service requirements of extremely cold environments and enhances the application capability of ball flat steel in the field of high strength and brittle fracture resistance; (3) Synergistic strengthening of microalloying and clean smelting: By adding trace amounts of V, controlling the N content and precisely adjusting the V / N ratio, and with low oxygen control of TO≤0.0008%, the steel precipitates dispersed fine VN particles, which promotes grain refinement and structural stability; (4) Strong industrial adaptability of processing technology: The method of this invention adopts all conventional continuous casting-hot rolling equipment, and is equipped with mature processes such as differential rolling and normalizing treatment. No new expensive equipment is required, and it is easy to promote and apply it directly in the existing production lines of steel plants. Detailed Implementation

[0014] The following is a detailed description of a method for improving the uniformity of cross-sectional properties of spherical flat steel according to the present invention.

[0015] Example: This example is a preferred embodiment of the various embodiments of the present invention. Example

[0016] The following DH36 grade steel billet was selected: C 0.12%, Si 0.28%, Mn 1.32%, P 0.012%, S 0.004%, V 0.06%, N 0.007%, V / N ratio of 8.57, and TO controlled at 0.0007%.

[0017] The steel billet is smelted through a converter-LF refining-VD degassing process and is continuously cast in 280 mm × 380 mm form at a casting speed of 1.2 m / min. The spray in the secondary cooling zone is uniformly controlled, and the crystallizer is equipped with a differential cooling device to suppress uneven cooling of the billet corner.

[0018] The billet was heated to 1190℃ and held at that temperature for 60 minutes before being removed from the furnace. During rolling, the web reduction rate was controlled at 13.5%, and the ball head reduction rate was 6.2%. A differential rolling strategy was adopted, with the final rolling temperatures controlled at 850℃ (web) and 855℃ (ball head). After rolling, the web area was cooled by high-pressure water spray at a rate of approximately 25℃ / s, while the ball head area was cooled by a combination of air cooling and water mist cooling at a rate of approximately 13℃ / s. The temperature difference between the two cooling termination methods was controlled within 72℃.

[0019] After cooling, the entire bulb flat steel bar is subjected to high-pressure shot blasting (blasting pressure 12 MPa) and laser straightening treatment, with the surface roughness controlled within Ra = 2.0 μm and the dimensional deviation within ±1.0 mm. Subsequently, it is normalized at 910℃ for 60 minutes and then air-cooled.

[0020] Performance test results show that the grain size of the ball head and web regions are grade 8.5 and 9.5, respectively, the yield strength is 422 MPa and 428 MPa, respectively, the impact absorption energy at -20℃ is 254 J and 268 J, the strength difference is 6 MPa, the impact energy difference is 14 J, and the cross-sectional properties are uniform. Example

[0021] The selected chemical composition is: C 0.11%, Si 0.30%, Mn 1.30%, P 0.013%, S 0.004%, V 0.05%, N 0.0065%, with a V / N ratio of 7.7 and TO controlled at 0.0008%.

[0022] The billet heating temperature is 1185℃, and the final rolling temperatures for the web and ball end are controlled at 840℃ and 855℃, respectively. The reduction ratio is controlled at 14.0 (web) and 5.8 (ball end). The post-rolling zoned cooling scheme is as follows: the web uses high-speed spray cooling (approximately 24℃ / s), and the ball end uses air cooling + mist cooling (approximately 12℃ / s), with the cooling termination temperature difference controlled within 60℃.

[0023] After shot blasting (Ra = 1.9 μm) and laser straightening, the sample was normalized at 905℃ for 50 minutes and then air-cooled.

[0024] The results showed that the grain size of the web was grade 9.2 and that of the ball head was grade 9.0; the yield strength of the web was 426 MPa and that of the ball head was 421 MPa; the impact energy at -20℃ was 261 J for the web and 251 J for the ball head; the strength difference was 5 MPa and the impact energy difference was 10 J, indicating stable performance.

[0025] Comparative Example 1 (without micro-alloying and controlled deformation): The standard DH36 steel composition is as follows: C 0.12%, Si 0.27%, Mn 1.33%, P 0.014%, S 0.006%, no added V, N 0.008%, TO 0.0011%, and no V / N ratio control.

[0026] The steel billet was heated to 1190℃, held at that temperature, and then directly hot-rolled. The web reduction rate was 13.0%, and the ball reduction rate was only 3.4%. Differential rolling design was not implemented. After rolling, it was naturally air-cooled without a separate cooling zone. Normalizing and shot blasting were not performed.

[0027] Performance tests show that the grain size of the ball head is only grade 6.5, the impact energy is 190 J, the impact energy of the web is 223 J, the difference is as high as 33 J, the strength difference is 16 MPa, and the performance is extremely uneven.

[0028] Comparative Example 2 (Uncontrolled cooling and heat treatment): Chemical composition of steel by mass percentage: C 0.12%, Si 0.29%, Mn 1.31%, P 0.015%, S 0.006%, V not added, N 0.006%, TO 0.0012%.

[0029] The billet was heated and then hot-rolled with a reduction ratio of 13.8 for the web and 5.9 for the ball end, at a final rolling temperature of 850℃. It was then naturally cooled after rolling without zoning control, with a cooling rate of 6℃ / s for the ball end and 23℃ / s for the web, resulting in a final temperature difference of 130℃. No normalizing or surface treatment was performed.

[0030] Test results: The web impact energy was 228 J, the ball head impact energy was 196 J, the difference was 32 J, the strength difference was 11 MPa, and the cross-sectional properties fluctuated greatly.

[0031] Table 1. Comparison of chemical composition between the examples and comparative examples (mass percentage, %) C 0.12 0.11 0.12 0.12 Si 0.28 0.30 0.27 0.29 Mn 1.32 1.30 1.33 1.31 P 0.012 0.013 0.014 0.015 S 0.004 0.004 0.006 0.006 V 0.06 0.05 — (Not added) — (Not added) N 0.007 0.0065 0.008 0.006 V / N 8.57 7.7 Uncontrolled Uncontrolled TO 0.0007 0.0008 0.0011 0.0012 Remark Microalloying + Controlled Cooling and Rolling Microalloying + Partitioned Cooling + Normalizing Standard ingredients Standard ingredients + uncontrolled cooling Table 2 Comparison of Process Performance between Examples and Comparative Examples steel grades DH36 bulb flat steel DH36 bulb flat steel DH36 bulb flat steel DH36 bulb flat steel V / N ratio 8.6 7.7 Uncontrolled Uncontrolled Compression ratio (web / ball head) 13.5 / 6.2 14.0 / 5.8 13.0 / 3.4 13.8 / 5.9 Final rolling temperature (°C) 850 / 855 840 / 855 845 / 870 850 / 850 Cooling rate (°C / s) 25 / 13 24 / 12 28 / 8 23 / 6 Cooling termination temperature difference (°C) 72 60 160 130 Grain size (web / ball head) 9.5 / 8.5 9.2 / 9.0 8.0 / 6.5 8.2 / 6.8 Yield strength (MPa) 428 / 422 426 / 421 422 / 406 423 / 412 Impact energy at -20℃ (J) 268 / 254 261 / 251 223 / 190 228 / 196 Toughness difference (J) 14 10 33 32 Strength difference (MPa) 6 5 16 11 As can be seen from the examples and comparative examples, the present invention has the following advantages: (1) In Examples 1 and 2 of the present invention, by controlling the V / N ratio within a reasonable range, combined with differential rolling and zoned cooling, not only can the grain size of the ball head and web plate reach grade 9 or above, but the difference in strength and toughness is also controlled within 10%, showing good consistency in cross-sectional performance. (2) Comparative Example 1 did not use microalloying elements and differential deformation control, resulting in coarse grains in the ball head, a significant decrease in impact performance, and an impact energy as low as 190J at -20℃. The performance difference was too large, which was not conducive to the overall stability of the hull's stress structure. (3) Although Comparative Example 2 had some deformation control, the cooling was not zoned, the temperature difference was too large, resulting in uneven thermal stress and uncoordinated structure, significant performance fluctuations, and a high proportion of flaw detection defects.

[0032] 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 uniformity of cross-sectional properties of bulb flat steel, characterized in that, Includes the following steps: (1) Steel billet smelting and continuous casting control: The converter smelting-LF refining-VD degassing process is adopted to control the total oxygen content of molten steel TO≤0.0008%; the continuous casting uses square or rectangular billets with a cross section of 280 mm×380 mm, the casting speed is 1.1~1.2 m / min, the crystallizer is equipped with a differential cooling device, and the water volume in the secondary cooling zone is automatically adjusted to avoid the corner temperature difference being too cold; (2) Differential rolling deformation control: When rolling the heated steel billet into spherical flat steel, the total reduction rate of the web is controlled at 13-15%, and the reduction rate of the spherical head area is 5-7%; different reduction amounts are set for the roughing and finishing rolling stages, and the final rolling temperature is controlled between 840-860℃. (3) Zoned cooling control: Spray water cooling is used in the web area after rolling, and the cooling rate is controlled at 20-30℃ / s; air cooling or mist cooling is used in the ball head area, and the cooling rate is 10-15℃ / s. The temperature difference between the two areas at the end of cooling is controlled to be less than 80℃. (4) Normalizing heat treatment: After cooling, the bulb flat steel is normally treated at a temperature of 900-920℃ and a holding time of 40-60 minutes, followed by air cooling to further improve the uniformity of the microstructure and impact performance of the bulb head area; The chemical composition of the bulb flat steel is as follows (by mass percentage): C: 0.10–0.13%, Si: 0.20–0.35%, Mn: 1.20–1.40%, P≤0.015%, S≤0.005%, V: 0.05–0.08%, N: 0.005–0.009%, with the balance being Fe and unavoidable impurities, and the V / N mass ratio is controlled between 6 and 9.

2. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, It also includes (5) surface shaping and treatment: high-pressure water shot blasting and laser straightening treatment are carried out before normalizing to control the surface roughness Ra≤2.0 μm and the geometric deviation is not greater than ±1.0 mm.

3. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, Specifically: [V] is 0.06%, [N] is 0.007%, and the V / N ratio is controlled at 8.

57.

4. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, The final rolling temperature of the bulb flat steel is controlled at 840-850℃ in the web region and 850-860℃ in the bulb head region.

5. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, The partitioned cooling control method is to automatically switch between air cooling and water mist combined cooling, with the cooling time controlled between 90 and 180 seconds.

6. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, During the normalizing process, the bulb flat steel is heated in a walking beam furnace with atmospheric protection heating, and the heating rate is 5-10℃ / min.

7. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, The billet is heated to 1180-1200℃ before hot rolling, and the holding time is 50-70 minutes.

8. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, The high-pressure water shot blasting has a spray pressure of 10–15 MPa, a nozzle angle of 60–90°, and a spraying time of 15–30 seconds.

9. The method for improving the uniformity of cross-sectional properties of bulb flat steel according to claim 1, characterized in that, After implementing this method, the difference in impact energy at -20℃ between the web and the bulb head region of the bulb flat steel shall not exceed 15 J, the difference in yield strength shall not exceed 10 MPa, and the difference in grain size shall not exceed grade 1.