Production method of Q620qE steel plate with low yield ratio

Abstract

The invention discloses a production method of a Q620qE steel plate with a low yield ratio. The steel comprises the following chemical components in percentage by weight: 0.07%-0.08% of C, 0.25%-0.3% of Si, 1.5%-1.55% of Mn, less than or equal to 0.012% of P, less than or equal to 0.002% of S, 0.035%-0.045% of Nb, 0.01%-0.02% of Ti, 0.25%-0.3% of Mo, 0.45%-0.55% of Cr and 0.2%-0.3% of Ni. The process comprises the steps of smelting, continuous casting, heating, controlled rolling and controlled cooling and tempering. A Q620qE steel plate with the thickness reaching 50 mm is produced through a continuous casting billet with the section of 260 mm, the structure of the steel is bainite and ferrite, the yield strength Rp0.2 is larger than or equal to 620 MPa, the tensile strength Rm is larger than or equal to 720 MPa, the ductility A is larger than or equal to 15%, the yield ratio is smaller than or equal to 0.85, and the Charpy impact energy at the temperature of-40 DEG C is larger than or equal to 120 J. A welding evaluation test shows that under the submerged-arc welding heat input of 35 kJ / cm, the impact energy at the temperature of-40 DEG C of a heat affected zone is larger than or equal to 120 J, and the impact energy at the center of a welding seam is larger than or equal to 100 J.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of wide and thick plate production, and relates to a production method for rolling a thick plate of a bridge structure steel with a thickness of ≤50 mm and a low yield ratio Q620qE by using a 260 mm continuous casting blank. BACKGROUND

[0002] Since the 21st century, major engineering construction in China has increased, and projects such as large-scale cross-sea channels and the Sichuan-Tibet railway have accelerated the construction of a traffic power country. In the future, bridge construction will face more complex technologies, larger scales, and more severe service environments, which puts forward higher requirements for bridge steels. At present, the bridge steel with a yield strength of 355-500 MPa widely used in bridge construction cannot fully meet the development requirements of the bridge industry, and the research and application of bridge steels with higher strength levels are imminent.

[0003] Existing steel enterprises have tried to develop 620 MPa grade ultra-high strength bridge steel, but this steel grade needs to be realized through a complex quenching and tempering process and the addition of a large amount of high-cost alloy elements, and the technology is not mature in terms of heat sensitivity and welding stability. There is a great application limitation for a considerable period of time. Chinese patent CN114921711B discloses a production method of a Q620 grade high corrosion resistance high strength offshore structure steel. In order to realize high corrosion resistance, a large amount of Ni, Mo, Cu and other precious alloys are added, which is high in cost. The production method has a yield ratio greater than 0.9, and it is difficult to realize a low yield ratio of less than 0.9. Chinese patent CN112553530B discloses a low yield ratio 700 MPa high strength bridge steel and a manufacturing method thereof. The furnace rolling mill is used for production, the yield strength is 560-570 MPa, the steel grade is low, and the production thickness can only reach 20 mm. The thickness specification and strength grade cannot be expanded to the Q620q steel grade. Chinese patent CN116716538A discloses a high strength bridge steel and a manufacturing method thereof. The yield strength is 410-690 MPa, the tensile strength is 540-770 MPa, the elongation is ≥14%, and the impact energy at -40℃ is ≥47J. The method does not explicitly indicate that the low yield ratio can be guaranteed, and the elongation and impact toughness are low, which cannot meet the requirements of GB / T714-2015 on Q620qE. Chinese patent CN111979481B discloses a thin gauge low yield ratio high strength bridge steel and a production method thereof. The thickness of the steel plate is 8-16 mm; the mechanical properties are as follows: the yield strength is 520-570 MPa, the tensile strength is 670-760 MPa, and the yield ratio is 0.69-0.79. The maximum production thickness can only reach 16 mm, and the steel grade is lower than the requirement of Q620q.

[0004] The Q620qE steel plate has high strength requirement, and the strength and toughness and yield ratio matching condition is harsh, the national standard GB / T714-2015 for bridge structure steel plate, the maximum steel grade of TMCP (hot mechanical rolling) process is only to Q500q, and the Q620q steel grade is only embodied in the quenching and tempering process. Therefore, the production technology of the low yield ratio Q620qE steel plate by TMCP + tempering process is developed, the performance of the steel meets the yield strength R p0.2 ≥620MPa, the tensile strength Rm≥720MPa, the elongation A≥15%, the yield ratio is less than or equal to 0.85, and the Charpy impact energy at-40 DEG C is greater than 120J, which can greatly solve the industry pain point and widen the application of high-grade bridge structure steel. SUMMARY

[0005] The purpose of the application is to provide a production method of the low yield ratio high toughness Q620qE bridge structure steel plate, the 260mm continuous casting billet section rolling is adopted, the quenching and tempering heat treatment after rolling is not needed, the maximum thickness 50mm Q620qE steel plate with excellent performance and low cost can be produced, the structure is bainite + ferrite, the yield strength R p0.2 ≥620MPa, the tensile strength Rm≥720MPa, the elongation A≥15%, the yield ratio is less than or equal to 0.85, and the Charpy impact energy at-40 DEG C is greater than 120J, which can greatly solve the industry pain point and widen the application of high-grade bridge structure steel.

[0006] The technical scheme of the application is as follows: A production method of the low yield ratio Q620qE steel plate, the chemical composition of the steel is as follows: C=0.07%-0.08%, Si=0.25%-0.3%, Mn=1.5%-1.55%, P≤0.012%, S≤0.002%, Nb=0.035%-0.045%, Ti=0.01%-0.02%, Mo=0.25%-0.3%, Cr=0.45%-0.55%, Ni=0.2%-0.3%, the balance is Fe and inevitable impurity elements, CEV≤0.55%, Pcm≤0.25, the 260mm section continuous casting billet is used to produce the maximum thickness 50mm Q620qE steel plate, the structure of the steel is bainite + ferrite, the yield strength R p0.2 ≥620MPa, the tensile strength Rm≥720MPa, the elongation A≥15%, the yield ratio is less than or equal to 0.85, and the Charpy impact energy at-40 DEG C is greater than 120J, which can greatly solve the industry pain point and widen the application of high-grade bridge structure steel. (1) Smelting: the molten steel is refined by LF, treated by RH / VD vacuum, controlled with low C, low P and low S, and the composition of the slab meets P≤0.012%, S≤0.002%.

[0007] (2) Heating: the total heating time of the slab is ≥260 min, the maximum heating temperature of the hearth is ≤1220℃, the soaking temperature is 1180~1200℃, the soaking time is ≥40 min, and the temperature of the core of the slab after tapping is 1180~1200℃.

[0008] (3) Controlled rolling: two-stage controlled rolling is adopted, the reduction rate of at least one pass during the rough rolling process at a rolling temperature ≥1150℃ is ≥18%, and the end temperature of the rough rolling is controlled to be 1130~1180℃; the thickness of the intermediate slab during the finish rolling is set to be ≥2.5 times the thickness of the finished product, the opening rolling temperature during the finish rolling is ≤900℃, and the opening cold temperature is 750~800℃.

[0009] (4) Controlled cooling: after pre-straightening, the slab is cooled to 350~450℃ at a cooling rate of 8~18℃ / s by a water cooling device, and then air-cooled on a cooling bed after hot straightening.

[0010] (5) Tempering: 400℃ tempering heat treatment is adopted, and the furnace time is 48~100 min.

[0011] Further, step (3) controlled rolling: after the rough rolling is completed, the intermediate slab is accelerated water-cooled before the opening rolling of the finish rolling, the cooling rate is 3~8℃ / s, the temperature of the intermediate slab after cooling is ≥930℃, and the waiting time from the rough rolling to the opening rolling of the finish rolling is ≤8 min.

[0012] Further, step (4) controlled cooling: after rolling, the slab is cooled by a water cooling device with a total length of 24 m, and a segmented control cooling mode is adopted, the total cooling water volume of the first 1 / 4 cooling section is 960~1500 L / s, the cooling time is 5~12 s; the total cooling water volume of the last 3 / 4 cooling section is 2250~2700 L / s, the cooling time is 15~36 s; and the cooling rate is 8~15℃ / s to cool to 350~450℃.

[0013] The selection of the main chemical components in the steel plate, the content range and the limitation of the rolling process parameters are obtained through a large number of field tests and performance detection. The invention principle is as follows: The low carbon content of C=0.07%~0.08% is used to improve the toughness of the steel, and the low-carbon bainite + ferrite structure design can also make the thick plate fully cooled in a wider cooling speed range in the thickness direction, avoid the toughness and elongation deterioration caused by the martensitic structure of the medium and high carbon components, and the matching of the soft phase ferrite and the hard phase bainite is also an important means to realize the low yield ratio of the steel plate. Nb and Ti are the main fine-grain strengthening elements, and appropriate addition can not only refine the original austenite grain size during slab heating, but also refine the grain through the second phase particle precipitation and pinning of C and N compounds during controlled cooling, thereby improving the strength and toughness. The atomic size of Ni and Fe is similar, and Ni can be infinitely solid-solved in steel, so the addition of Ni element can improve the low-temperature toughness of the steel material. Since Ni is a precious alloy element, it needs to be added in an appropriate amount for cost control and market factors. Mo and Cr elements have significant solid solution strengthening effect, and Mo element can reduce the austenite phase region while inhibiting the decomposition of austenite, reduce the phase transition temperature, increase the hardenability of the steel plate, improve the strength of the full thickness of the steel plate, and reduce the strength difference between the core and the surface of the thick plate. The addition of 0.25%~0.3% Mo content can fully play the beneficial effect of Mo element on Q620 steel grade thick plate. It should be noted that excessive addition of Mo will sharply deteriorate the toughness and welding performance of the steel plate, and will also greatly increase the alloy cost.

[0014] The requirements for slab heating temperature and time are to solidify the key alloying elements such as Nb, Mo and Cr. At the same time, the highest temperature is limited to 1220℃ to avoid the rapid growth of the original austenite when the composition exceeds 1220℃, which is difficult to refine and uniform in the subsequent controlled rolling and controlled cooling process, resulting in reduced impact toughness and loss of fine-grain strengthening strength.

[0015] The application innovatively adopts the intermediate billet cooling, i.e., the intermediate roller pipe laminar flow water cooling process, which is optimized and quantified through repeated tests. In addition to the advantages of reducing the waiting time and improving the production efficiency of the traditional process, the process performance can be greatly improved. First, the temperature rise caused by deformation heat during rolling can be reduced, the austenite grains are prevented from excessive growth, more deformation energy storage is retained, more nucleation sites are provided for subsequent phase change, and dynamic recrystallization is inhibited. Second, the intermediate billet is rapidly cooled in the high temperature range, which can increase the undercooling degree of austenite, and it is easier to form fine ferrite or bainite structure during subsequent rolling, so as to promote phase change refinement and improve strength and toughness. Third, the abnormal growth of local grains caused by uneven temperature can be prevented, and the uniformity of the structure is ensured. The cooling speed of 3-8℃ / s is proposed to avoid the formation of martensite structure at too fast cooling speed. The temperature after cooling is not lower than 930℃, and the total time from rough rolling to open rolling is ≤8min, so that the intermediate billet can be cooled before the austenite recrystallization zone, and the mixed grains caused by abnormal growth of local grains are prevented, so that the fine grain effect of the non-recrystallization zone during the finishing rolling stage can be greatly improved, and the strength and low temperature impact toughness can be improved at the same time.

[0016] The rolling and cooling system adopted in the application controls the open cooling water inlet temperature to be 750-800℃, the final cooling temperature to be 350-450℃, the cooling speed to be 8-15℃ / s, the cooling time to be 20s-48s, and adopts high and low pressure partition cooling, so as to ensure that the steel plate is transformed in the bainite zone, and the full thickness direction is fully bainite-based, so as to support the strength requirements of the Q620 grade.

[0017] The application has the following beneficial effects: 1) The application does not need complex quenching and tempering heat treatment process, only needs appropriate addition of Mo, Cr, Ni and other alloys, and through the optimized controlled rolling and controlled cooling (TMCP) process and stress relief tempering after rolling, a maximum thickness of 50mm Q620qE steel plate is produced by using a 260mm continuous casting billet, which greatly simplifies the production process path, reduces the cost and shortens the delivery cycle. 2) The application fully utilizes the existing equipment, and innovatively develops the intermediate billet water cooling process, and combines the controlled rolling and controlled cooling process in the finishing rolling stage, so that the fine grain effect is greatly strengthened, and the strength and low temperature impact toughness of the structure are improved. 3) The application develops a partition controlled cooling mode after rolling, through setting different cooling sections, different cooling water quantities and cooling time, sufficient red heat return and heat exchange in the thickness direction are ensured, the hardenability alloy elements are added, the cooling speed and hardenability of the core of the thick plate are improved, and sufficient bainite structure is still present at the core position of the maximum thickness of 50mm, so as to support the strength requirements of the Q620 grade. 4) The Q620GJ of the present application has excellent welding performance, and under the submerged arc welding heat input of 35kJ / cm, the impact energy Akv of the heat affected zone at-40 DEG C is greater than or equal to 120J, and the impact energy Akv of the weld center is greater than or equal to 100J; 5) The Q620qE steel plate with the thickness of 24-50mm is stably produced by the TMCP+T process of the present application, and the yield strength R p0.2 ≥620MPa, the tensile strength Rm≥720MPa, the elongation A≥15%, the yield strength ratio ≤0.85, and the impact energy Akv at-40 DEG C is greater than or equal to 120J. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 It is the metallographic structure diagram of the 1 / 4 thickness of the steel plate of the embodiment 3 of the present application. DETAILED DESCRIPTION

[0019] The content of the present application is further described below in combination with the embodiments.

[0020] Embodiment 1: Production method of Q620qE steel plate with the thickness of 24mm The chemical composition of the smelted steel is that the weight percentage of C is 0.078%, the weight percentage of Si is 0.27%, the weight percentage of Mn is 1.53%, the weight percentage of P is less than or equal to 0.01%, the weight percentage of S is less than or equal to 0.002%, the weight percentage of Nb+Ti is 0.052%, the weight percentage of Mo+Cr+Ni is 1.1%, and the balance is Fe and inevitable impurity elements; and the key process steps include: (1) Smelting: the molten steel is subjected to LF refining and RH / VD vacuum treatment, and low C, low P and low S control, and the composition of the casting blank satisfies P≤0.012% and S≤0.002%.

[0021] (2) Heating: the total heating time of the slab is greater than or equal to 260min, the highest heating temperature of the hearth is 1220 DEG C, the soaking temperature is 1200 DEG C, the soaking time is 59min, and the core temperature of the casting blank after tapping is 1192 DEG C.

[0022] (3) Controlled rolling: two-stage controlled rolling is adopted, the reduction rate of the last pass of rough rolling is greater than or equal to 18.6%, and the rough rolling end temperature is 1142 DEG C; the intermediate blank thickness is set to 80mm, the intermediate water cooling device is used to cool to 950 DEG C at the cooling speed of about 5 DEG C / s, the total time of the rough rolling to the rough rolling start waiting is less than 6min, the rough rolling start temperature is 900 DEG C, and the rough rolling start cooling temperature is 770 DEG C.

[0023] (4) Controlled cooling: After pre-straightening, the steel plate is cooled by a water cooling device with a total length of 24 m, and a segmented controlled cooling mode is adopted. The roller speed is 1.2 m / s, the total cooling water flow of the first 6 m cooling section (1 / 4 of the total length) is 960 L / s, and the cooling time is 5 s; the total cooling water flow of the last 18 m cooling section (3 / 4 of the total length) is 2400 L / s, and the cooling time is 15 s; the cooling rate is 16-18 ℃ / s, and the temperature is cooled to 410-450 ℃, and then the hot straightening is carried out and the steel plate is air-cooled on the cooling bed.

[0024] (5) Tempering: The steel plate is tempered at 400 ℃, and the heat treatment time in the furnace is 48 min.

[0025] The steel plate (1 / 4 thickness) of Example 1 is sampled for tensile and Charpy V-notch impact tests, and the performance is shown in Table 1. The sample of Example 1 is subjected to welding evaluation, and under the heat input of 35 KJ / cm, the impact energy value of the heat-affected zone at -40 ℃ is 153 J, 203 J, and 193 J, the impact energy value of the weld center is 143 J, 128 J, and 128 J, the weld back bending and face bending tests are qualified, and the welding performance is good.

[0026] Example 2: Production method of Q620qE steel plate with a thickness of 40 mm The chemical composition of the steel by weight percentage is C=0.078%, Si=0.27%, Mn=1.53%, P≤0.01%, S≤0.002%, Nb+Ti=0.052%, Mo+Cr+Ni=1.1%, and the balance is Fe and inevitable impurity elements; the key process steps include: (1) Smelting: The molten steel is subjected to LF refining and RH / VD vacuum treatment, and low C, low P and low S control, and the composition of the casting blank satisfies P≤0.012% and S≤0.002%.

[0027] (2) Heating: The total heating time of the slab is ≥260 min, the maximum heating temperature of the hearth is 1220 ℃, the soaking temperature is 1200 ℃, the soaking time is 65 min, and the core temperature of the casting blank after tapping is 1188 ℃.

[0028] (3) Controlled rolling: Two-stage controlled rolling is adopted, the last pass reduction rate of rough rolling is ≥19.3%, and the rough rolling end temperature is 1146 ℃; the intermediate water cooling device is used to cool the intermediate blank to 950 ℃ at a cooling rate of about 5 ℃ / s, the total time from the end of the rough rolling to the start of the rough rolling is <7 min, the rough rolling start temperature is 840 ℃, and the cold start temperature is 780 ℃.

[0029] (4) Controlled cooling: After pre-straightening, the steel plate is cooled by a water cooling device with a total length of 24 m, and a step cooling mode is adopted. The roller speed is 0.6 m / s, the total cooling water flow of the first 6 m cooling section (1 / 4 of the total length) is 1500 L / s, and the cooling time is 10 s. The total cooling water flow of the last 18 m cooling section (3 / 4 of the total length) is 2700 L / s, and the cooling time is 30 s. The cooling rate is 9-10.2 ℃ / s, and the temperature is cooled to 370-420 ℃, and then air-cooled by hot straightening and cooling bed.

[0030] (5) Tempering: 400 ℃ tempering heat treatment is adopted, and the heat treatment time in the furnace is 80 min.

[0031] The steel plate (1 / 4 thickness) of Example 2 is sampled for tensile and Charpy V-notch impact tests, and the performance is shown in Table 1. The sample of Example 1 is subjected to welding evaluation, and under the heat input of 35 KJ / cm, the impact energy value of the heat affected zone at -40 ℃ is 234 J, 178 J, and 186 J, the impact energy value of the weld center is 122 J, 105 J, and 102 J, the weld back bending and face bending tests are qualified, and the welding performance is good.

[0032] Example 3: Production method of Q620qE steel plate with a thickness of 50 mm The chemical composition of the smelted steel is C=0.079%, Si=0.28%, Mn=1.54%, P≤0.01%, S≤0.002%, Nb+Ti=0.053%, Mo+Cr+Ni=1.18%, and the balance is Fe and inevitable impurity elements; the key process steps include: (1) Smelting: The molten steel is subjected to LF refining and RH / VD vacuum treatment, and low C, low P and low S control, and the composition of the cast blank satisfies P≤0.012% and S≤0.002%.

[0033] (2) Heating: The total heating time of the slab is ≥260 min, the maximum heating temperature of the hearth is 1220 ℃, the soaking temperature is 1200 ℃, the soaking time is 672 min, and the core temperature of the cast blank after tapping is 1190 ℃.

[0034] (3) Controlled rolling: Two-stage controlled rolling is adopted, the last pass reduction rate of rough rolling is ≥19.4%, and the rough rolling end temperature is 1144 ℃; the intermediate water cooling device is used to cool the intermediate blank to 940 ℃ at a cooling rate of about 4 ℃ / s, the total time from the end of rough rolling to the start of open rolling is <7 min 40 s, the open rolling temperature is 830 ℃, and the open cooling temperature is 782 ℃.

[0035] (4) Controlled cooling: After pre-straightening, the steel plate is cooled by a water cooling device with a total length of 24 m, and a segmented controlled cooling mode is adopted. The roller speed is 0.45 m / s. The total cooling water flow rate of the first 6 m (1 / 4 of the total length) cooling section is 1500 L / s, and the cooling time is 13.3 s. The total cooling water flow rate of the last 18 m (3 / 4 of the total length) cooling section is 2700 L / s, and the cooling time is 40 s. The cooling rate is 8.8-10 ℃ / s, and the steel plate is cooled to 350-400 ℃, and then air-cooled by hot straightening and a cooling bed.

[0036] (5) Tempering: The steel plate is tempered at 400 ℃, and the heat treatment time in the furnace is 100 min.

[0037] The steel plate (1 / 4 thickness) of Example 3 is sampled for tensile and Charpy V-notch impact tests, and the performance is shown in Table 1. The sample of Example 1 is subjected to welding evaluation. Under the submerged arc welding heat input of 35 KJ / cm, the impact energy value of the heat-affected zone at -40 ℃ is 163 J, 144 J and 115 J, the impact energy value of the weld center is 122 J, 105 J and 102 J, the weld back bending and face bending tests are qualified, and the welding performance is good.

[0038] The physical property test results of the steel plate of Example are shown in Table 1, and the metallographic structure of the steel plate 1 / 4 thickness is shown in Figure 1 .

[0039] Table 1 Physical property test results of the steel plate produced in Example .

Claims

1. A method for producing Q620qE steel plate with low yield strength ratio, characterized in that: The chemical composition of the steel, by weight percentage, is C=0.07%~0.08%, Si=0.25%~0.3%, Mn=1.5%~1.55%, P≤0.012%, S≤0.002%, Nb=0.035%~0.045%, Ti=0.01%~0.02%, Mo=0.25%~0.3%, Cr=0.45%~0.55%, Ni=0.2%~0.3%, with the balance being Fe and unavoidable impurity elements. CEV≤0.55%, Pcm≤0.

25. A maximum thickness of 50mm Q620qE steel plate is produced using 260mm cross-section continuous casting billets. The steel's microstructure is bainite + ferrite, and its yield strength R... p0.2 ≥620MPa, tensile strength Rm≥720MPa, elongation A≥15%, yield strength ratio≤0.85, Charpy impact energy at -40℃≥120J, under submerged arc welding heat input of 35kJ / cm, impact energy of heat-affected zone at -40℃≥120J, and impact energy of weld center≥100J; Its key process steps include: (1) Smelting: Molten steel is refined by LF, treated by RH / VD vacuum, and controlled with low C, low P and low S. The composition of the billet meets the requirements of P≤0.012% and S≤0.002%; (2) Heating: The total heating time of the slab is ≥260min, the maximum heating temperature in the furnace is ≤1220℃, the soaking temperature is 1180℃~1200℃, the soaking time is ≥40min, and the core temperature of the slab after exiting the steel is 1180~1200℃; (3) Controlled rolling: Two-stage controlled rolling is adopted. During the rough rolling process with a rolling temperature ≥1150℃, there is at least one pass with a reduction rate ≥18%. The end temperature of rough rolling is controlled at 1130~1180℃. The thickness of the intermediate billet in the finishing rolling is set to ≥2.5 times the thickness of the finished product. The starting temperature of finishing rolling is ≤900℃, and the starting cooling temperature is 750~800℃. (4) Controlled cooling: After rolling, the rolls are pre-straightened and then cooled to 350-450°C at a cooling rate of 8-18°C / s by a water cooling device, and then cooled to air on a cooling bed after hot straightening. (5) Tempering: Tempering heat treatment at 400℃ for 48~100 min.

2. The method for producing a Q620qE thick plate with a low yield strength ratio according to claim 1, characterized in that... Step (3) Control rolling: After rough rolling is completed, the intermediate billet is accelerated water-cooled before the start of finish rolling. The cooling rate is 3~8℃ / s. After cooling, the temperature of the intermediate billet is ≥930℃. The waiting time from rough rolling to the start of finish rolling is ≤8min.

3. The method for producing a Q620qE thick plate with a low yield strength ratio according to claim 1, characterized in that... Step (4) Controlled cooling: After rolling, the water cooling device with a total length of 24m is used to control the cooling in a segmented manner. The total cooling water volume of the first 1 / 4 cooling section is 960~1500L / s, and the cooling time is 5~12s; the total cooling water volume of the last 3 / 4 cooling section is 2250~2700L / s, and the cooling time is 15~36s; the temperature is cooled to 350~450℃ at a cooling rate of 8~15℃ / s.

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