Q345R steel plate for pressure vessel and manufacturing method thereof

Abstract

A kind of Q345R steel plate for pressure vessel and its manufacturing method, the chemical composition of steel is as follows: C=0.14-0.18%, Si=0.15-0.30%, Mn=1.20-1.50%, P≤0.018%, S≤0.005%, Nb=0.015-0.030%, V=0.030-0.060%, Alt=0.020-0.050%, the balance is Fe and inevitable impurities;Process flow includes smelting, continuous casting, heating, rolling, cooling and heat treatment, adopts low compression ratio design with total compression ratio of 1.8-2.2;Rolling process adopts the combination strategy of "high-temperature initial rolling + immediate intensive cooling + accurate pass distribution", the thickness of steel plate is 120-180mm, the metallographic structure of steel plate is ferrite + pearlite, grain size is ≥8.0 level, thickness direction full section center ultrasonic flaw detection reaches I level qualified, 20 ℃ transverse impact energy KV2≥100J, yield strength ≥345MPa, tensile strength ≥510MPa, elongation after fracture ≥21%.

Description

Technical Field

[0001] This invention belongs to the field of low-alloy high-strength steel manufacturing technology, and relates to a Q345R steel plate for pressure vessels that can still ensure excellent core quality and flaw detection performance under low compression ratio conditions, and its manufacturing method. Background Technology

[0002] Q345R steel is a key material for manufacturing pressure-bearing equipment such as boilers and pressure vessels, and its quality directly affects the safe operation of the equipment. Traditional Q345R steel plate production typically employs a high compression ratio, generally >4, and even >5 for thick plates. This large deformation is used to break up the as-cast structure and create a dense core, thereby ensuring mechanical properties and passing ultrasonic testing. However, a high compression ratio means the need for thicker continuously cast billets or greater rolling capacity, which leads to the following problems: (1) Increased raw material costs; (2) Heating energy consumption increased significantly; (3) High load requirements on the rolling mill, resulting in significant equipment wear; (4) Limited production flexibility makes it difficult to economically produce steel plates of different thicknesses using the same specification of billet. It is generally believed in the industry that when the compression ratio is below 3.0, Q345R steel plates, especially in their core area, are prone to problems such as coarse grains and loose microstructure due to insufficient deformation penetration, leading to decreased impact toughness and failure in ultrasonic testing. This severely restricts the development of low-cost, high-efficiency production processes. Therefore, developing an innovative manufacturing method that can stably produce Q345R steel plates with qualified core flaw detection and excellent overall performance under low compression ratios, such as 1.8–2.2, has become a pressing technical challenge in this field. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a Q345R steel plate that can be stably produced at a low compression ratio of 1.8 to 2.2 and has excellent core quality and flaw detection performance, as well as its manufacturing method.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A type of Q345R steel plate for pressure vessels, the chemical composition of the steel by mass percentage is C=0.14~0.18%, Si=0.15~0.30%, Mn=1.20~1.50%, P≤0.018%, S≤0.005%, Nb=0.015~0.030%, V=0.030~0.060%, Alt=0.020~0.050%, with the balance being Fe and unavoidable impurities; the steel plate thickness is 120~180mm, the metallographic structure of the steel plate is ferrite + pearlite, the grain size is ≥8.0 grade, the ultrasonic flaw detection of the core of the entire cross section in the thickness direction reaches Grade I qualified, the transverse impact energy KV2 at 20℃ is ≥100J, the yield strength is ≥345MPa, the tensile strength is ≥510MPa, and the elongation after fracture is ≥21%.

[0005] The manufacturing method of the Q345R steel plate for pressure vessels described above includes smelting, continuous casting, heating, rolling, cooling, and heat treatment. Key process steps include: a. Continuous casting: casting into billets, the ratio of the thickness of the continuously cast billet to the thickness of the final finished steel plate, i.e., the total compression ratio, is 1.8 to 2.2; b. Heating: Heat the continuously cast billet to 1150-1200℃ and hold it at that temperature; c. Preliminary rolling and immediate forced cooling: Descaling is performed on the heated slab, and the initial rolling temperature is controlled at 1050-1100℃; at least 3 passes of preliminary rolling are performed, with a total reduction rate of 30%-40%, and a single pass reduction rate of not less than 14%; within 10 seconds after the completion of preliminary rolling, the upper and lower surfaces of the steel plate are subjected to high-intensity water cooling, with a water cooling intensity of 350-450 m³ / h·m² and a cooling time of 10-30 seconds, so that the surface temperature of the steel plate drops to 930-980℃, while the temperature difference between the core and the surface is ≥180℃; d. Intermediate rolling and finishing rolling: After water cooling in the primary rolling mill, intermediate rolling and finishing rolling are continued, with a total of 6 to 9 passes and the total compression ratio controlled within the range of 1.8 to 2.2; e. Accelerated cooling and heat treatment: The final rolling temperature is 780-820℃, and ACC is used to accelerate cooling to 550-650℃ immediately after rolling, with a cooling rate of 15-25℃ / s; finally, air cooling is performed.

[0006] Further, step c, initial rolling and immediate forced cooling: the initial rolling passes are allocated as follows: the first pass has a reduction rate of 14% to 16%, the second pass has a reduction rate of 14% to 16%, and the third pass has a reduction rate of 12% to 14%.

[0007] Furthermore, in step d, intermediate rolling and finishing rolling: the pass allocation strategy for intermediate rolling and finishing rolling is as follows: the first 2 to 3 passes are the high temperature and high reduction stage, with a single pass reduction rate of 10% to 13%; the last 3 to 4 passes are the low temperature finishing rolling stage, with a single pass reduction rate gradually decreasing to 6% to 9%.

[0008] The innovations and principles of this invention are as follows: This invention employs a synergistic innovation of "low compression ratio design" and "gradient temperature-controlled rolling." Specifically, "gradient temperature-controlled rolling" refers to using a large reduction rate for rapid deformation in the initial rolling stage (primary rolling stage), followed immediately by strong water cooling. This causes the surface temperature of the steel plate to be significantly lower than the core temperature within a short period, creating a large cross-sectional temperature gradient (core-to-surface temperature difference ≥ 180℃). This results in the formation of fine deformation-induced phase transformation structures on the surface. Furthermore, during heat transfer from the high-temperature core to the surface, the "traction" and "constraint" of the low-temperature surface region stimulate the formation of even finer and more uniform austenite recrystallization grains. This mechanism effectively compensates for the insufficient total deformation under low compression ratios, laying a solid foundation for obtaining a refined core microstructure.

[0009] The beneficial effects of this invention include: Revolutionary improvement in cost-effectiveness: Successfully pushing the lower limit of the compression ratio to 2.2 allows for the production of steel plates of the same thickness using thinner continuously cast billets, significantly reducing raw material costs and heating energy consumption (estimated to be reduced by more than 20%). Breakthrough assurance of core quality: Through "gradient temperature-controlled rolling" technology, the industry problem of coarse core microstructure at low compression ratios is solved, with an ultrasonic flaw detection first-pass yield exceeding 99.5%. Excellent and uniform performance: The resulting steel plate has a uniform and fine microstructure across its entire cross-section, with excellent strength and toughness matching. Green and low-carbon: The process flow is shorter and energy consumption is lower, aligning with the current trend of green manufacturing. Attached Figure Description

[0010] Figure 1 This is a process flow diagram of an embodiment of the present invention. Detailed Implementation

[0011] The present invention will be further illustrated by specific embodiments below. Example 1

[0012] The production of 120mm thick Q345R steel plates has the following chemical composition by mass percentage (wt%): C=0.18, Si=0.28, Mn 1.48, P≤0.009, S≤0.002, Nb=0.028, V=0.055, Alt=0.045. The key process steps include: a. Continuous casting: Select a 240mm thick continuous casting billet, with a total compression ratio of 240 / 120 = 2.

[0013] b. Heating: 1180℃, hold for 90 minutes.

[0014] c. Initial rolling and forced cooling: First pass: 240mm → 216mm, reduction rate 10.0%; Second pass: 216mm → 194mm, reduction rate 10.2%; Third pass: 194mm → 174mm, reduction rate 10.3%; 4th pass: 174mm → 156mm, reduction rate 10.3%; total reduction in initial rolling 35%. Immediate water cooling (360 m³ / (h・m²), cooling time 30s), surface temperature drops to 960℃.

[0015] d. Intermediate rolling and finish rolling: After the steel plate temperature is uniform, finish rolling is carried out in 6 passes, with the final rolling temperature controlled at 830℃. The pass distribution is as follows: First pass: 156mm → 143mm, reduction rate 8.3%; Second pass: 143mm → 132mm, reduction rate 7.7%; Third pass: 132mm → 125mm, reduction rate 5.3%; 4th pass: 125mm → 122mm, reduction rate 2.4%; 5th pass: 122mm → 121mm, reduction rate 0.8%; 6th pass: 121mm → 120mm, reduction rate 0.8%.

[0016] e. Cooling and heat treatment: ACC cool to 600℃ (cooling rate 20℃ / s), normalize at 910℃ and hold for 80 minutes. Example 2

[0017] The production process involves manufacturing Q345R steel plates with a thickness of 150mm and 120mm. The chemical composition of the steel plates, by mass percentage (wt%), is C=0.19, Si=0.30, Mn=1.55, P≤0.008, S≤0.001, Nb=0.032, V=0.060, Alt=0.050. The key process steps include: a. Continuous casting: Select a 315mm thick continuous casting billet, with a total compression ratio of 315 / 150 = 2.1.

[0018] b. Heating: 1190℃, hold for 150 minutes.

[0019] c. Initial rolling and forced cooling: Furnace exit temperature ≥ 1190℃, initial rolling temperature 1110℃, 4 passes of initial rolling: First pass: 315mm → 284mm (reduction rate 9.8%) Second pass: 284mm → 256mm (reduction rate 9.9%) Third pass: 256mm → 231mm (reduction rate 9.8%) 4th pass: 231mm → 209mm (reduction rate 9.5%).

[0020] The total reduction rate was 33.7%; the water-cooled strength after initial rolling was 370 m³ / (h・m²), the time was 40s, and the temperature was reduced to 945℃.

[0021] d. Intermediate and finishing rolling: 7 finishing rolling passes, final rolling temperature 825℃: First pass: 209mm → 192mm (8.1%) Second track: 192mm → 178mm (7.3%); Third lane: 178mm → 167mm (6.2%) 4th lane: 167mm → 159mm (4.8%) 5th lane: 159mm → 156mm (1.9%) Lane 6: 156mm → 153mm (1.9%) Lane 7: 153mm → 150mm (2.0%).

[0022] e. Cooling and heat treatment: ACC cool to 580℃ (cooling rate 18℃ / s), normalize at 905℃ and hold for 120 min. Example 3

[0023] The production of Q345R steel plates with a thickness of 170mm has the following chemical composition by weight percentage (wt%): C=0.20, Si=0.32, Mn=1.62, P≤0.007, S≤0.001, Nb=0.035, V=0.065, Alt=0.055. The key process steps include: a. Continuous casting: 374mm thick continuous casting billet is selected, and the total compression ratio = 374 / 170 = 2.2.

[0024] b. Heating: 1200℃, hold for 90 minutes.

[0025] c. Initial rolling and forced cooling: Initial rolling temperature 1120℃, 4 passes of initial rolling: First pass: 374mm → 337mm (9.9%) Second track: 337mm → 304mm (9.8%); Third pass: 304mm → 274mm (9.9%) 4th lane: 274mm → 248mm (9.5%).

[0026] The total reduction rate was 33.7%; the water-cooling strength after initial rolling was 380 m³ / (h・m²), the time was 45 s, and the temperature was reduced to 940℃.

[0027] d. Intermediate and finishing rolling: 8 passes of finishing rolling, final rolling temperature 820℃: First pass: 248mm → 228mm (8.0%) Second pass: 228mm → 211mm (7.5%) Third track: 211mm → 197mm (6.6%). 4th lane: 197mm → 186mm (5.6%) Lane 5: 186mm → 178mm (4.3%) Lane 6: 178mm → 175mm (1.7%) Lane 7: 175mm → 173mm (1.1%) Lane 8: 173mm → 170mm (1.8%).

[0028] e. Cooling and heat treatment: ACC cool to 580℃ (cooling rate 12℃ / s), normalize at 890℃ and hold for 120 min.

[0029] Comparative Example 1: 120mm thick Q345R steel plates were produced using conventional processes. The V content was reduced to 0.005wt% (no V strengthening). The chemical composition of the steel plate, by mass percentage (wt%), was C=0.18, Si=0.28, Mn=1.48, P=0.009, S=0.002, Nb=0.028, V=0.005, Alt=0.045. 240mm thick continuously cast billets (compression ratio 2.0) were used. Conventional two-stage rolling was employed, without immediate strong cooling after initial rolling. The remaining final rolling temperature, cooling, and normalizing procedures were essentially the same as in Example 1.

[0030] Comparative Example 2: A 150mm thick Q345R steel plate was produced using a low compression ratio. A 240mm thick continuously cast billet (compression ratio 1.6) was selected. Conventional two-stage rolling was adopted, with the final rolling temperature, cooling, and normalizing process being basically the same as in Example 2.

[0031] Table 1. Comparison of performance test results between the steel plates produced in the examples and those produced in the comparative examples. .

[0032] As shown in Table 1, the Q345R steel plate produced by the present invention at a low compression ratio of 1.8 to 2.2 exhibits significantly better comprehensive mechanical properties, especially core impact toughness, than those produced using traditional processes. This fully demonstrates the effectiveness and advancement of the method of the present invention.

Claims

1. A Q345R steel plate for pressure vessels, characterized in that: The chemical composition of the steel, by mass percentage, is C=0.14~0.18%, Si=0.15~0.30%, Mn=1.20~1.50%, P≤0.018%, S≤0.005%, Nb=0.015~0.030%, V=0.030~0.060%, Alt=0.020~0.050%, with the balance being Fe and unavoidable impurities; the steel plate thickness is 120~180mm, the metallographic structure of the steel plate is ferrite + pearlite, the grain size is ≥8.0 grade, the ultrasonic flaw detection of the core of the entire cross section in the thickness direction reaches Grade I qualified, the transverse impact energy KV2 at 20℃ is ≥100J, the yield strength is ≥345MPa, the tensile strength is ≥510MPa, and the elongation after fracture is ≥21%.

2. A method for manufacturing Q345R steel plate for pressure vessels, the process flow including smelting, continuous casting, heating, rolling, cooling and heat treatment, characterized in that... Key process steps include: a. Continuous casting: casting into billets, the ratio of the thickness of the continuously cast billet to the thickness of the final finished steel plate, i.e., the total compression ratio, is 1.8 to 2.2; b. Heating: Heat the continuously cast billet to 1150-1200℃ and hold it at that temperature; c. Preliminary rolling and immediate forced cooling: Descaling is performed on the heated slab, and the initial rolling temperature is controlled at 1050-1100℃; at least 3 passes of preliminary rolling are performed, with a total reduction rate of 30%-40%, and a single pass reduction rate of not less than 14%; within 10 seconds after the completion of preliminary rolling, the upper and lower surfaces of the steel plate are subjected to high-intensity water cooling, with a water cooling intensity of 350-450 m³ / h·m² and a cooling time of 10-30 seconds, so that the surface temperature of the steel plate drops to 930-980℃, while the temperature difference between the core and the surface is ≥180℃; d. Intermediate rolling and finishing rolling: After water cooling in the primary rolling mill, intermediate rolling and finishing rolling are continued, with a total of 6 to 9 passes and the total compression ratio controlled within the range of 1.8 to 2.2; e. Accelerated cooling and heat treatment: The final rolling temperature is 780-820℃, and ACC is used to accelerate cooling to 550-650℃ immediately after rolling, with a cooling rate of 15-25℃ / s; finally, air cooling is performed.

3. The method for manufacturing a Q345R steel plate for pressure vessels according to claim 2, characterized in that... Step c: Preliminary rolling and immediate forced cooling: The primary rolling passes are allocated as follows: the first pass has a reduction rate of 14% to 16%, the second pass has a reduction rate of 14% to 16%, and the third pass has a reduction rate of 12% to 14%.

4. A method for manufacturing a Q345R steel plate for pressure vessels according to claim 2 or 3, characterized in that, Step d: Intermediate rolling and finishing rolling: The pass allocation strategy for intermediate rolling and finishing rolling is that the first 2 to 3 passes are high temperature and high reduction stage, with a single pass reduction rate of 10% to 13%; the last 3 to 4 passes are low temperature finishing rolling stage, with a single pass reduction rate gradually decreasing to 6% to 9%.

Images