Method for producing high-strength building structural steel plate with extremely small upper and lower yield strength difference and above 420 Mpa grade by adopting TMCP (Thermal Mechanical Control Processing) process

Abstract

The invention relates to a method for producing a high-strength building structural steel plate with the extremely small upper and lower yield strength difference of more than 420 Mpa by adopting a TMCP (Thermal Mechanical Control Processing) process. The method comprises the following steps: controlling the following smelting chemical components in percentage by weight: 0.10-0.15% of C, 0.15-0.25% of Si, 1.00-1.50% of Mn, less than or equal to 0.018% of P, less than or equal to 0.008% of S, 0.025-0.045% of Nb, 0.015-0.035% of Als, less than or equal to 0.0035% of N and the balance of Fe and impurities, wherein the chemical components further need to meet the formula that C / 12 + Mn / 12 + Nb / 5 + Si / 32 is more than or equal to 0.09% and less than or equal to 0.112%; the finish rolling temperature in the rough rolling stage is controlled to be 1080-1130 DEG C; according to different finished product thicknesses, the finish rolling starting temperature and the machine body descaling system are strictly controlled; in the cooling process, according to different thickness intervals, a head and a tail with the length of 800-2000 mm are adopted for shielding, and opening water in a specific proportion is adopted at the shielding part; the upper yield strength of the finally manufactured finished steel plate is 470-520 MPa, the lower yield strength is 470-510 MPa, the difference between the upper yield strength and the lower yield strength of the same steel plate is smaller than or equal to 10 MPa, the tensile strength is 600-670 MPa, the ductility is larger than or equal to 25%, the yield ratio is smaller than or equal to 0.79, and the straightness of the steel plate is smaller than or equal to 3 mm / 1000 mm.

Description

Technical Field

[0001] The present invention relates to a method for producing high-strength building structural steel plates of 420 MPa or above with minimal upper and lower yield strength differences using a TMCP process. The method belongs to the field of structural steel manufacturing and is suitable for controlling the performance and plate shape of high-strength building structural steel delivered with a thickness of 10 to 100 mm using TMCP. Background Art

[0002] The TMCP process is an effective means of improving steel plate strength. It significantly reduces the addition of precious alloys, simplifies the process, significantly reduces production costs, and improves efficiency, making it highly sought after by the industry. In recent years, with economic and technological development, an increasing number of structural steels for building bridges have been supplied using the TMCP process. Besides cost considerations, the main reason is that TMCP steel often requires a lower carbon equivalent, which significantly improves welding quality and efficiency. However, a low carbon equivalent requires a relatively low carbon content, making it difficult to control the yield ratio of the steel plate, failing to meet the seismic performance requirements of building structural steel. In particular, a significant difference in upper and lower yield strengths can occur, resulting in structural inhomogeneities and high stresses within the steel plate. Furthermore, simply reducing the carbon content requires a large amount of water, which complicates subsequent plate shape control. Therefore, new technologies are urgently needed to produce high-strength building structural steel plates above 420 MPa with minimal upper and lower yield strength differences using the TMCP process. Summary of the Invention

[0003] The object of the present invention is to provide a method for producing high-strength building structural steel plates of 420 MPa or above with extremely small upper and lower yield strength differences using the TMCP process. The finished steel plates produced using the method of the present invention have excellent plate shape, uniform structure, upper and lower yield strength differences of ≤10 MPa on the same plate, elongation ≥25%, yield strength ratio ≤0.79, and steel plate flatness ≤3 mm / 1000 mm.

[0004] The present invention provides a method for producing high-strength building structural steel plates of 420 MPa or higher with minimal upper and lower yield strength differences using a TMCP process, comprising the following steps: (1) Smelting: strictly control N≤0.0035%; (2) Continuous casting: Control the superheat at 12-28°C, the secondary cooling water straightening temperature at 920-935°C, and continuously cast into ingots; (3) Heating: Heat the ingot to 1170-1200°C at a heating rate of 8-9 min / mm; (4) Rolling is divided into two stages: the final rolling temperature of the rough rolling stage is controlled at 1080-1130℃; the starting temperature of the finishing rolling and the start of the machine body descaling system are determined according to the thickness t of the finished steel plate, as follows: When the finished steel plate thickness is 10≤t≤20mm, the rolling temperature is 910~960℃, and the machine body descaling system is turned on for descaling once in the fifth pass from the finishing rolling. When the finished steel plate thickness is 20<t≤40mm, the rolling temperature is 880~900℃, and the machine body descaling system is turned on for descaling once in the third pass from the finishing rolling. When the finished steel plate thickness is 40<t≤60mm, the rolling temperature is 840~870℃, and the machine body descaling system is opened for descaling in the third and fifth passes of finishing rolling respectively; When the finished steel plate thickness is 60<t≤100mm, the rolling temperature is 820~850℃, and the machine body descaling system is opened for descaling in the third, fifth and seventh passes of finishing rolling respectively; (5) Cooling system: starting cooling temperature 740-760℃, final cooling temperature 480-550℃, cooling rate 8-24℃ / s, cooling roller speed and cooling water volume are determined according to the thickness t of the finished steel plate, as follows: When the thickness of the finished steel plate is 10≤t≤30mm, the roller speed is 1.4-1.5m / s, and shielding is performed at 2000mm at the head and tail. The upper water volume is 40-50% of the water volume of the plate body, and the lower water volume is 60-70% of the water volume of the plate body. When the thickness of the finished steel plate is 30<t≤40mm, the roller speed is 1.2~1.3m / s, and shielding is performed at 1500mm at the head and tail. The upper water volume is 45~60% of the water volume of the plate body, and the lower water volume is 65~75% of the water volume of the plate body. When the thickness of the finished steel plate is 40<t≤60mm, the roller speed is 0.9~1.05m / s, and shielding is performed at 1000mm at the head and tail. The upper water volume is 60~70% of the water volume of the plate body, and the lower water volume is 70~85% of the water volume of the plate body. When the finished steel plate thickness is 60<t≤100mm, the roller speed is 0.7~0.8m / s, and shielding is performed at 800mm at the head and tail. The upper water volume is 70~80% of the water volume of the plate body, and the lower water volume is 80~90% of the water volume of the plate body.

[0005] The chemical composition of the ingot in the present invention is controlled according to the following weight percentages: C: 0.10-0.15%, Si: 0.15-0.25%, Mn: 1.00-1.50%, P≤0.018%, S≤0.008%, Nb: 0.025-0.045%, Als: 0.015-0.035%, N≤0.0035%, and the balance is Fe and unavoidable impurities. At the same time, the above chemical composition must also satisfy the formula: 0.09%≤C / 12+Mn / 12+Nb / 5+Si / 32≤0.112%.

[0006] The finished steel plate produced by the method of the present invention has a thickness of 10 to 100 mm, an upper yield strength of 470 to 520 MPa, a lower yield strength of 470 to 510 MPa, a difference between the upper and lower yield strengths of the same plate of ≤10 MPa, a tensile strength of 600 to 670 MPa, an elongation of ≥25%, a yield strength ratio of ≤0.79, and a steel plate flatness of ≤3 mm / 1000 mm.

[0007] The reasons for limiting the amounts of chemical components in the steel sheet of the present invention are described in detail below.

[0008] The method of the present invention uses C, Si, Mn, P, S and a trace amount of Nb alloy and Fe as raw materials, and is a method for preparing a high-strength building structural steel plate of 420 MPa or above with a very small upper and lower yield strength difference without adding equipment such as a warm straightener or a flattening machine, wherein: C: Carbon can significantly increase the hardness and strength of steel. However, when the C content is too high, it will affect the welding performance and is not conducive to the phase transformation control in TMCP. Therefore, the C content is controlled at 0.10-0.15%.

[0009] Si: Silicon is an essential element for steelmaking deoxidation. It can reduce oxide inclusions and also has a certain strengthening effect. However, when it exceeds a certain amount, it will reduce plasticity and toughness and also have an adverse effect on weldability. Therefore, it is controlled at 0.15-0.25%.

[0010] Mn: It can significantly improve the strength and hardness of steel, reduce the Ar3 phase transformation temperature, and promote finer ferrite grains. However, when the Mn content is too high, it increases the risk of center segregation and banded structure. Therefore, the Mn content is controlled within 1.00-1.50%.

[0011] Phosphorus (P) significantly increases the cold brittleness of steel, making it difficult to guarantee low-temperature toughness. However, too low a P content will increase costs and affect production efficiency. Therefore, the designed P content is P ≤ 0.018%.

[0012] S: Sulfur easily forms sulfide inclusions such as MnS, which reduce transverse plasticity and toughness. Large inclusions, in particular, act as stress concentration points and can easily cause localized premature yielding. Therefore, the designed S content is S ≤ 0.008%.

[0013] Nb: It has strong grain refinement and precipitation strengthening effects, ensuring the strength of the steel plate. At the same time, it combines with free C and N atoms to form stable carbonitrides, reducing the harm caused by excess N. Therefore, the design of Nb is: 0.025~0.045% N: Free interstitial nitrogen atoms will form Cottrell gas masses with dislocations, resulting in obvious upper and lower yield platforms. Therefore, the N content should be strictly limited in the present invention, and strictly controlled to be N≤0.0035%.

[0014] Als: Aluminum deoxidizes steel and controls grain size, inhibiting aging in low-carbon steel and improving its oxidation resistance. However, excessive aluminum levels can promote graphite formation and affect its strength and toughness. Therefore, Als is controlled within a range of 0.015-0.035%.

[0015] Furthermore, based on practical production experience and theoretical research, when 0.09% ≤ C / 12 + Mn / 12 + Nb / 5 + Si / 32 ≤ 0.112%, the amount of interstitial atoms (C and N) can be effectively controlled, resulting in sufficient grain refinement while improving steel purity and structural uniformity. Therefore, the present invention further limits the range to 0.09% ≤ C / 12 + Mn / 12 + Nb / 5 + Si / 32 ≤ 0.112%.

[0016] The principle of the production method of the present invention is as follows: The production process of the present invention includes the following steps: 130t converter smelting → argon blowing in an argon station → LF furnace → continuous casting → heating of the ingot → rolling → cooling → finishing. Specifically: 1) Smelting part: Use converter smelting, decarburize as deeply as possible to make the oxygen content reach 500-800ppm, use vacuum degassing to control the nitrogen content below 0.0035%, control the superheat at 12-28℃, and control the secondary cooling water straightening temperature at 920-935℃.

[0017] 2) Rolling section: The finishing temperature of the rough rolling stage is controlled at 1080-1130℃; for finished products with a thickness of 10-20mm, the starting temperature of the finishing rolling is 910-960℃, and the machine body descaling system is turned on for descaling once in the fifth pass from the last pass of the finishing rolling, instead of turning on the machine body descaling; for finished products with a thickness of >20-40mm, the starting temperature of the finishing rolling is 880-900℃, and the machine body descaling system is turned on for descaling once in the third pass from the last pass of the finishing rolling; for finished products with a thickness of >40-60mm, the starting temperature of the finishing rolling is 840-870℃, and the machine body descaling system is turned on for descaling in the fourth and sixth passes from the last pass of the finishing rolling respectively; for finished products with a thickness of >60-100mm, the starting temperature of the finishing rolling is 820-850℃, and the machine body descaling system is turned on for descaling in the second, fourth and sixth passes from the last pass of the finishing rolling respectively; The use of machine body descaling in the finishing rolling stage is due to the fact that during the finishing rolling process, the steel plate is exposed to air at a high temperature, which will regenerate iron oxide scale at a rate of 0.1mm / min and become more dense. If the scale is not removed in time, the hard and brittle iron oxide scale will be pressed into the steel plate matrix during rolling, forming pits; secondly, the surface iron oxide scale will cause fluctuations in the contact friction coefficient between the roller and the steel plate, causing local uneven deformation and the production of mixed crystal structure; thirdly, due to the physical isolation, heat conduction blockage, mechanical interference and other effects of the iron oxide scale, it will cause systemic damage to the uniformity of the steel plate structure and process control; on the other hand, it is started at a specific rolling pass, causing the austenite grains to be violently elongated and flattened, and a large number of deformation bands to form within the grains. The final rolling is completed near the phase transformation point to maximize the promotion of phase deformation nucleation and obtain fine ferrite grains; avoiding the static recovery of austenite caused by continuous machine body descaling and watering in the last few passes, which releases the accumulated deformation energy, weakens the grain refinement strengthening effect, coarsens the grains or forms a martensitic hard layer, affecting the strength and toughness of the steel plate.

[0018] 3) Cooling system: cooling temperature 740 ~ 760 ℃, final cooling temperature 480 ~ 550 ℃, cooling rate 8 ~ 24 ℃ / S, cooling roller speed: for finished steel plates with thickness ≤ 30 mm, roller speed is 1.4 ~ 1.5 m / s, at 2000 mm at the head and tail, the upper water volume is 40 ~ 50% of the plate body water volume, and the lower water volume is 60 ~ 70% of the plate body water volume; for finished steel plates with thickness > 30 ~ 40 mm, roller speed is 1.2 ~ 1.3 m / s, at 1500 mm at the head and tail, the upper water volume is 40 ~ 50% of the plate body water volume, and the lower water volume is 60 ~ 70% of the plate body water volume. The water volume of the upper part is 45-60% of the water volume of the board body, and the water volume of the lower part is 65-75% of the water volume of the board body; for finished products with a thickness of more than 40-60mm, the roller speed is 0.9-1.05m / s, and at 1000mm at the head and tail, the water volume of the upper part is 60-70% of the water volume of the board body, and the water volume of the lower part is 70-85% of the water volume of the board body; for finished products with a thickness of more than 60mm, the roller speed is 0.7-0.8m / s, and at 800mm at the head and tail, the water volume of the upper part is 70-80% of the water volume of the board body, and the water volume of the lower part is 80-90% of the water volume of the board body; Different head and tail shielding lengths and water opening ratios are set according to different finished product thicknesses. This is mainly because when the head of the steel plate first contacts the cooling water, only the front part of the head is exposed to the cooling water, and a stable cooling flow field cannot be formed, which in turn causes uneven actual cooling intensity of the head; at the same time, when the tail is about to leave the cooling zone, the cooling water is suddenly interrupted and lacks continuous cooling. At this time, the residual heat of the tail causes the temperature to return, resulting in abnormal tissue transformations such as pearlite coarsening; in order to compensate for the difference in heat dissipation between the head and tail, and to avoid the harm caused by excessive cooling, the inventor's theoretical research and actual production experience summarizes that different finished product thicknesses use specific shielding lengths and water opening ratios to achieve excellent steel plate shape and uniform control of full-plate performance.

[0019] Compared with the existing technology, the steel of the present invention does not require the production line to be specially equipped with a warm straightening machine at the cooling bed outlet to eliminate the plate shape defects of TMCP high-strength steel caused by uneven cooling temperature and phase change factors. It can be implemented on a large scale in various metallurgical enterprises. The steel plate produced by the method of the present invention has high strength, high plasticity, and low yield strength ratio. In particular, the difference between the upper and lower yield strengths is extremely small, which ensures that the steel plate has more stable and predictable plastic deformation behavior, which is beneficial to subsequent forming processing. At the same time, it has lower stress concentration and potentially higher fatigue life, thereby improving the accuracy and safety of structural design. DETAILED DESCRIPTION

[0020] In order to better explain the technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any form.

[0021] Table 1 is a list of chemical composition values ​​(wt%) of steel plates of various embodiments of the present invention and comparative examples; Table 2 is a list of the values ​​of the main production process parameters of the steel plates of various embodiments of the present invention and comparative examples; Table 3 is a list of the plate shape and mechanical property test results of the steel plates of various embodiments of the present invention and comparative examples.

[0022] According to various embodiments of the present invention, a method for producing high-strength building structural steel plates of 420 MPa or higher with minimal upper and lower yield strength differences using a TMCP process comprises the following steps: (1) Smelting: strictly control N≤0.0035%; (2) Continuous casting: Control the superheat at 12-28°C, the secondary cooling water straightening temperature at 920-935°C, and continuously cast into ingots; (3) Heating: Heat the ingot to 1170-1200°C at a heating rate of 8-9 min / mm; (4) Rolling is divided into two stages: the final rolling temperature of the rough rolling stage is controlled at 1080-1130℃; the starting temperature of the finishing rolling and the start of the machine body descaling system are determined according to the thickness t of the finished steel plate, as follows: When the finished steel plate thickness is 10≤t≤20mm, the rolling temperature is 910~960℃, and the machine body descaling system is turned on for descaling once in the fifth pass from the finishing rolling. When the finished steel plate thickness is 20<t≤40mm, the rolling temperature is 880~900℃, and the machine body descaling system is turned on for descaling once in the third pass from the finishing rolling. When the finished steel plate thickness is 40<t≤60mm, the rolling temperature is 840~870℃, and the machine body descaling system is opened for descaling in the third and fifth passes of finishing rolling respectively; When the finished steel plate thickness is 60<t≤100mm, the rolling temperature is 820~850℃, and the machine body descaling system is opened for descaling in the third, fifth and seventh passes of finishing rolling respectively; (5) Cooling system: starting cooling temperature 740-760℃, final cooling temperature 480-550℃, cooling rate 8-24℃ / s, cooling roller speed and cooling water volume are determined according to the thickness t of the finished steel plate, as follows: When the thickness of the finished steel plate is 10≤t≤30mm, the roller speed is 1.4-1.5m / s, and shielding is performed at 2000mm at the head and tail. The upper water volume is 40-50% of the water volume of the plate body, and the lower water volume is 60-70% of the water volume of the plate body. When the thickness of the finished steel plate is 30<t≤40mm, the roller speed is 1.2~1.3m / s, and shielding is performed at 1500mm at the head and tail. The upper water volume is 45~60% of the water volume of the plate body, and the lower water volume is 65~75% of the water volume of the plate body. When the thickness of the finished steel plate is 40<t≤60mm, the roller speed is 0.9~1.05m / s, and shielding is performed at 1000mm at the head and tail. The upper water volume is 60~70% of the water volume of the plate body, and the lower water volume is 70~85% of the water volume of the plate body. When the finished steel plate thickness is 60<t≤100mm, the roller speed is 0.7~0.8m / s, and shielding is performed at 800mm at the head and tail. The upper water volume is 70~80% of the water volume of the plate body, and the lower water volume is 80~90% of the water volume of the plate body.

[0023] Table 1 Chemical composition values ​​of steel plates of various embodiments of the present invention and comparative examples (wt%) Table 2 List of values ​​of main production process parameters of steel plates in various embodiments of the present invention and comparative examples

[0024] Table 3 Table of plate shape and mechanical properties test results of steel plates in various embodiments of the present invention and comparative examples

[0025] Table 3 above lists the results of flatness measurement and room temperature tensile test of the finished steel plates prepared in the embodiments of the present invention and the comparative examples.

[0026] As can be seen from Table 3 above, the products of the present invention have better comprehensive performance and plate shape. The steel plates produced by the method of the present invention have the characteristics of high strength, high plasticity, and low yield strength ratio. In particular, the finished steel plates have extremely small difference in upper and lower yield strength (the difference in yield strength of the same plate is ≤10MPa), while the yield strength difference of the steel plates produced in the comparative example is 24-42MPa.

[0027] Compared with the existing technology, the steel of the present invention does not require additional warm straightening equipment, has low alloy cost, and has a simple and easy production process. It can be implemented on a large scale in various metallurgical enterprises and can be widely used in various steel structure projects such as buildings, offshore platforms, pipelines, bridges, etc.

[0028] The present invention is not limited to the above-mentioned specific embodiments. Non-substantial changes made by researchers in the technical field of the present invention based on the above-mentioned content and form still fall within the substantial protection scope of the present invention.

Claims

1. A method for producing high-strength building structural steel plates of 420 MPa or above with minimal upper and lower yield strength differences using the TMCP process, characterized in that The steps include: (1) Smelting: strictly control N≤0.0035%; (2) Continuous casting: Control the superheat at 12-28°C, the secondary cooling water straightening temperature at 920-935°C, and continuously cast into ingots; (3) Heating: Heat the ingot to 1170-1200°C at a heating rate of 8-9 min / mm; (4) Rolling is divided into two stages: the final rolling temperature of the rough rolling stage is controlled at 1080-1130℃; the starting temperature of the finishing rolling and the start of the machine body descaling system are determined according to the thickness t of the finished steel plate, as follows: When the finished steel plate thickness is 10≤t≤20mm, the rolling temperature is 910~960℃, and the machine body descaling system is turned on for descaling once in the fifth pass from the finishing rolling. When the finished steel plate thickness is 20<t≤40mm, the rolling temperature is 880~900℃, and the machine body descaling system is turned on for descaling once in the third pass from the finishing rolling. When the finished steel plate thickness is 40<t≤60mm, the rolling temperature is 840~870℃, and the machine body descaling system is opened for descaling in the third and fifth passes of finishing rolling respectively; When the finished steel plate thickness is 60<t≤100mm, the rolling temperature is 820~850℃, and the machine body descaling system is opened for descaling in the third, fifth and seventh passes of finishing rolling respectively; (5) Cooling system: starting cooling temperature 740-760℃, final cooling temperature 480-550℃, cooling rate 8-24℃ / s, cooling roller speed and cooling water volume are determined according to the thickness t of the finished steel plate, as follows: When the thickness of the finished steel plate is 10≤t≤30mm, the roller speed is 1.4-1.5m / s, and shielding is performed at 2000mm at the head and tail. The upper water volume is 40-50% of the water volume of the plate body, and the lower water volume is 60-70% of the water volume of the plate body. When the thickness of the finished steel plate is 30<t≤40mm, the roller speed is 1.2~1.3m / s, and shielding is performed at 1500mm at the head and tail. The upper water volume is 45~60% of the water volume of the plate body, and the lower water volume is 65~75% of the water volume of the plate body. When the thickness of the finished steel plate is 40<t≤60mm, the roller speed is 0.9~1.05m / s, and shielding is performed at 1000mm at the head and tail. The upper water volume is 60~70% of the water volume of the plate body, and the lower water volume is 70~85% of the water volume of the plate body. When the finished steel plate thickness is 60<t≤100mm, the roller speed is 0.7~0.8m / s, and shielding is performed at 800mm at the head and tail. The upper water volume is 70~80% of the water volume of the plate body, and the lower water volume is 80~90% of the water volume of the plate body.

2. The method for producing high-strength building structural steel plates of 420 MPa or higher with minimal upper and lower yield strength differences using the TMCP process according to claim 1, characterized in that: The chemical composition of the ingot is controlled according to the following weight percentages: C: 0.10-0.15%, Si: 0.15-0.25%, Mn: 1.00-1.50%, P≤0.018%, S≤0.008%, Nb: 0.025-0.045%, Als: 0.015-0.035%, N≤0.0035%, and the balance is Fe and unavoidable impurities. At the same time, the above chemical composition must also satisfy the formula: 0.09%≤C / 12+Mn / 12+Nb / 5+Si / 32≤0.112%.

3. The method for producing high-strength building structural steel plates of 420 MPa or higher with minimal upper and lower yield strength differences using the TMCP process according to claim 1, characterized in that: The thickness of the finished steel plate is 10~100mm, the upper yield strength is 470~520MPa, the lower yield strength is 470~510 MPa, the difference between the upper and lower yield strengths of the same plate is ≤10MPa, the tensile strength is 600~670MPa, the elongation is ≥25%, the yield strength ratio is ≤0.79, and the steel plate flatness is ≤3mm / 1000mm.

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