Ultra-thick Q500qE steel plate for bridges and its manufacturing method

Abstract

The present application provides a steel plate for Q500qE bridges with extremely thick thickness and a manufacturing method thereof. The manufacturing method directly performs a rolling process on a slab after three-stage heating before rolling, and performs a three-stage cooling process after rolling, whereby a Q500qE steel plate with a maximum plate thickness of 150 mm can be obtained. The steel plate satisfies the Z-direction tensile property requirements of grade Z35 and the flaw detection requirements of grade II or above in the GB / T 2970-2016 standard. With such a configuration, the manufacturing process is simple, the manufacturing efficiency is high, and the manufacturing cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of alloy technology, and particularly to a steel plate for extremely thick Q500qE bridges and a manufacturing method thereof.

Background Art

[0002] With the development of the social economy in our country, the construction of extra-large bridges with large spans and heavy loads is increasing. Along with the increase in span and load, the demand for steel materials for bridges with extremely thick specifications is also increasing, and the performance required for steel plates has become more advanced, such as high strength, high toughness, reduction of defects in the central part (core part), and excellent resistance to lamellar tearing. Compared with steel materials for bridges with normal specifications, steel materials for bridges with extremely thick specifications have a smaller compression ratio due to the increase in product thickness, causing insufficient deformation in the central part of the steel plate and problems such as non-welded defects in the central part of the slab and a decrease in resistance to lamellar tearing. Furthermore, due to the increase in steel plate thickness, the cooling penetration ability decreases, and the control of the strength and toughness of the steel plate by online cooling after rolling cannot be effectively carried out.

[0003] The existing manufacturing methods for steel materials for extremely thick bridges are relatively complex, and the thickness is still thin. For example, the patents with patent numbers CN107557690B and CN102041438B manufacture materials by die casting, the patents with patent numbers CN107460278B and CN107287527B obtain materials with large thickness by the composite slab method, and although the patents with patent numbers CN104988435B and CN103540848B manufacture extremely thick plates with continuous casting slabs, the maximum thickness specification of the steel plates that can be manufactured remains at 120 mm. Therefore, a method for manufacturing steel materials for extremely thick bridges with a simpler manufacturing process is an urgent problem to be solved in the industry.

Summary of the Invention

[0004] An object of the present invention is to provide a steel plate for extremely thick Q500qE bridges and a manufacturing method thereof.

[0005] The present invention provides a method for manufacturing a steel plate for extremely thick Q500qE bridges. The chemical composition of the steel plate, in mass percentage, includes C: 0.05 - 0.07%, Si: 0.15 - 0.35%, Mn: 1.5 - 1.7%, Ni: 0.3 - 0.5%, Cr: 0.2 - 0.3%, Mo: 0.2 - 0.3%, Cu: 0.15 - 0.25%, Nb: 0.04 - 0.05%, Ti: 0.01 - 0.02%. The balance consists of Fe and unavoidable impurities, where the impurities are P ≤ 0.01% and S ≤ 0.003%.

[0006] The manufacturing method includes: a step of smelting based on the above chemical composition and obtaining a slab by continuous casting; a step of sequentially performing first-stage heating, second-stage heating, and third-stage heating on the slab, controlling the second-stage heating temperature to 1180 - 1220°C, controlling the temperatures of the first-stage heating and the third-stage heating to be lower than the temperature of the second-stage heating, making the temperature of the slab uniform by the first heating stage and the second heating stage, and forming a temperature gradient that gradually increases from the surface to the center of the slab in the third heating stage; a step of performing one-stage rolling on the heated slab to obtain a rolled steel plate with a maximum thickness of 150 mm; and a step of sequentially performing first-stage cooling, second-stage cooling, and third-stage cooling on the rolled steel plate, controlling the final cooling temperature of the third-stage cooling to 350°C or lower, and including a step of sequentially increasing the cooling rates of the first-stage cooling, the second-stage cooling, and the third-stage cooling.

[0007] As a further improvement in an embodiment of the present invention, the chemical composition of the steel plate satisfies a carbon equivalent CE ≤ 0.485 and a low-temperature cracking susceptibility index Pcm ≤ 0.24. Here, the calculation formula for the carbon equivalent CE is CE = (%C) + (%Mn) / 6 + (%Cr + %Mo + %V) / 5 + (%Ni + %Cu) / 15, The calculation formula for the low-temperature cracking susceptibility index Pcm is Pcm = (%C) + (%Si) / 30 + (%Mn + %Cu + %Cr) / 20 + (%Ni) / 60 + (%Mo) / 15 + (%V) / 10 + 5(%B), wherein the element symbols in parentheses indicate the mass percentages of the corresponding elements, and %element symbol indicates the value obtained by multiplying the mass percentage of the corresponding element by 100.

[0008] As a further improvement of an embodiment of the present invention, the step of smelting based on the chemical composition and continuously casting to obtain a slab includes: Based on the chemical composition, sequentially performing the steps of desulfurization of hot metal preliminary, converter smelting, LF refining, and RH vacuum degassing to obtain molten steel, continuously casting the molten steel to obtain a slab with a thickness of 320 mm, and including the step of stack cooling the slab.

[0009] As a further improvement of an embodiment of the present invention, the step of sequentially performing first-stage heating, second-stage heating, and third-stage heating on the slab includes: Controlling the temperature of the first-stage heating to 900 - 1100 °C and the heating time to 150 min or more, controlling the temperature of the second-stage heating to 1180 - 1220 °C and the heating time to 180 - 240 min, and controlling the temperature of the third-stage heating to 1140 - 1160 °C and the heating time to 30 - 60 min.

[0010] As a further improvement of an embodiment of the present invention, the step of performing single-pass rolling on the slab after heating includes: After removing the scale of the slab after three-stage heating, rolling is performed. In the rolling process, controlling the rolling temperature to 980 - 1080 °C and controlling the reduction amount of rolling passes other than the final pass to 40 mm or more.

[0011] As a further improvement of an embodiment of the present invention, the step of performing single-pass rolling on the slab after heating includes: In the rolling process, further including the step of controlling the bite speed to 0.6 m / s and the rolling speed to 1.2 m / s.

[0012] As a further improvement of an embodiment of the present invention, the step of sequentially performing first-stage cooling, second-stage cooling, and third-stage cooling on the rolled steel plate includes: Control the cooling rate of the first-stage cooling to 0.3 to 1 °C / s and the final cooling temperature to 760 to 780 °C, control the cooling rate of the second-stage cooling to 3 to 5 °C / s and the final cooling temperature to 560 to 600 °C, wait for 20 to 40 seconds after the end of the second-stage cooling, and control the cooling rate of the third-stage cooling to 5 to 8 °C / s and the final cooling temperature to 350 °C or lower.

[0013] The present invention also provides an extra-thick Q500qE bridge steel plate manufactured by the manufacturing method of the above extra-thick Q500qE bridge steel plate.

[0014] As a further improvement of an embodiment of the present invention, the microstructure of the steel plate is a two-phase structure of bainite and martensite, and the amount of the bainite is more than the amount of the martensite.

[0015] As a further improvement of an embodiment of the present invention, the steel plate has the characteristics that the yield strength after descending ≧ 430 MPa, the tensile strength ≧ 540 MPa, the yield ratio ≦ 0.85, the impact value at -40 °C ≧ 180 J, the cross-sectional shrinkage rate in the Z-direction tensile test ≧ 50%, and meets the ultrasonic flaw detection requirements of Class II or above of the GB / T 2970-2016 standard.

[0016] The present invention also provides an extra-thick Q500qE bridge steel plate, and the chemical composition of the steel plate includes the following in mass percentage: C: 0.05 to 0.07%, Si: 0.15 to 0.35%, Mn: 1.5 to 1.7%, Ni: 0.3 to 0.5%, Cr: 0.2 to 0.3%, Mo: 0.2 to 0.3%, Cu: 0.15 to 0.25%, Nb: 0.04 to 0.05%, Ti: 0.01 to 0.02%, the balance consists of Fe and inevitable impurities, and the impurities are P ≦ 0.01% and S ≦ 0.003%.

[0017] Also, the steel plate satisfies the conditions that the carbon equivalent CE ≦ 0.485 and the cold crack susceptibility index Pcm ≦ 0.24. Here, the calculation formula of the carbon equivalent CE is CE = (%C) + (%Mn) / 6 + (%Cr + %Mo + %V) / 5 + (%Ni + %Cu) / 15, The calculation formula of the cold crack susceptibility index Pcm is Pcm = (%C) + (%Si) / 30 + (%Mn + %Cu + %Cr) / 20 + (%Ni) / 60 + (%Mo) / 15 + (%V) / 10 + 5(%B), where the element symbols in parentheses indicate the mass percentages of the corresponding elements, and %element symbol indicates the value obtained by multiplying the mass percentage of the corresponding element by 100.

[0018] As a further improvement of an embodiment of the present invention, the steel plate has the following characteristics: maximum plate thickness of 150 mm, yield strength after descent ≧ 430 MPa, tensile strength ≧ 540 MPa, yield ratio ≦ 0.85, impact value at -40°C ≧ 180 J, cross-sectional shrinkage rate in the Z-direction tensile test ≧ 50%, and meets the ultrasonic flaw detection requirements of Class II or higher according to the GB / T 2970-2016 standard.

[0019] As a further improvement of an embodiment of the present invention, the microstructure of the steel plate is a two-phase structure of bainite and martensite, and the amount of bainite is more than that of martensite.

[0020] The beneficial effects of the present invention will be described. The manufacturing method used in the present invention directly rolls the slab after three-stage heating before rolling and then performs three-stage cooling treatment on the steel plate after rolling, so as to obtain a Q500qE steel plate with a maximum plate thickness of 150 mm. The steel plate can meet the requirements of the Z-direction tensile characteristics of Class Z35 and the ultrasonic flaw detection requirements of Class II or higher according to the GB / T 2970-2016 standard. Its manufacturing process is simple, efficient, and has a low manufacturing cost. First, in the manufacturing process, a steel plate with an extremely thick specification can be manufactured from a normal continuous casting slab, and there is no need for mold casting or the production of composite slabs. Second, in the rolling process, high-temperature single-stage heavy reduction rolling is adopted, and multi-stage low-temperature controlled rolling is not required. The rolling process is simple and the rolling efficiency is high. Third, after rolling, heat treatments such as quenching and tempering, normalizing, and rapid cooling after normalizing are not required. The number of manufacturing processes is small, the manufacturing cycle is short, and the manufacturing cost is low.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0022] To make the objectives, technical solutions and advantages of the present invention clearer, the following will clearly and completely explain the technical solutions of the present invention by using specific embodiments of the present invention and corresponding drawings. Obviously, the described embodiments are some but not all of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative labor are also included in the protection scope of the present invention.

[0023] The following will detail the embodiments of the present invention. Examples of the embodiments are shown in the drawings, where the same or similar reference numerals from beginning to end represent the same or similar elements, or elements having the same or similar functions. The embodiments described below with reference to the drawings are exemplary and for the purpose of explaining the present invention, and should not be understood as limiting the present invention.

[0024] This embodiment provides an extra-thick Q500qE steel plate for bridges and a manufacturing method thereof. This manufacturing method directly rolls a continuously cast slab to obtain a steel plate with a maximum plate thickness of 150 mm. Compared with conventional extra-thick standard steel plates for bridges, it can endow the steel plate with high strength and high toughness under the conditions of a low reduction ratio, no low-temperature controlled rolling, and no heat treatment. The manufacturing method is simple, the performance of the steel plate is excellent, and the steel plate manufactured by this method can be used as a steel plate for bridges of the Q500qE standard.

[0025] The chemical composition of the steel plate provided in this embodiment, by mass percentage, includes C: 0.05 - 0.07%, Si: 0.15 - 0.35%, Mn: 1.5 - 1.7%, Ni: 0.3 - 0.5%, Cr: 0.2 - 0.3%, Mo: 0.2 - 0.3%, Cu: 0.15 - 0.25%, Nb: 0.04 - 0.05%, Ti: 0.01 - 0.02%, and the balance is Fe and inevitable impurities, and the impurities include P ≤ 0.01% and S ≤ 0.003%.

[0026] Specifically, the design principle of the chemical composition of the steel plate is as follows: C: It is a strengthening element. The amount of C content has a significant impact on the microstructure of the steel material, and thereby can effectively guarantee the strength of the steel plate. At the same time, the C content plays an important role in increasing the hardenability and yield ratio of the steel plate. However, excessive C content significantly deteriorates the low-temperature toughness of the steel plate. Therefore, in this embodiment, the C content is controlled to be 0.05 - 0.07%.

[0027] Si: As a deoxidizing element, it improves the strength of the material through solid-solution strengthening. Si dissolves in ferrite and austenite to improve the hardness and strength of the steel. Furthermore, Si has the effect of suppressing pearlite transformation. However, excessive Si content causes an increase in the M / A (martensite / austenite) content and deteriorates the low-temperature toughness of the steel plate. Therefore, in this embodiment, the Si content is controlled to be 0.15 - 0.35%.

[0028] Mn: It is a solid-solution strengthening element, which can improve the hardenability of the steel plate and thereby improve the strength. Also, Mn promotes the formation of acicular ferrite. At the same time, Mn is also an excellent deoxidizer and desulfurizer, which can combine with the harmful element S to reduce the hot brittleness of the steel plate. However, excessive Mn promotes the segregation of elements such as P, Sb, and Sn, deteriorates the low-temperature toughness of the center part of the steel plate, and the spreading during the rolling process causes non-conformance in flaw detection of the steel plate. Therefore, in this invention, the Mn content is controlled to be 1.40 - 1.60%.

[0029] Ni: At the crack tip of the steel material, precipitation behavior mainly based on P and Ni alloys occurs, and chemical precipitation occurs on the crack surface at the rear part of the crack tip. The NiP precipitate induces crack closure, generates residual compressive stress near the crack tip, and effectively reduces the propagation rate of fatigue cracks. By the stress release at the crack tip, the low-temperature toughness of the material is improved. At the same time, Ni effectively improves the hardenability of the steel material. However, excessive addition of Ni increases the alloy cost. Therefore, in this embodiment, the Ni content is controlled to be 0.1 - 0.2%.

[0030] It has a solution strengthening effect similar to that of Cr:Mn and can effectively improve the hardenability of steel. However, the price of Cr alloy is higher than that of Mn, and excessive Cr content affects the stability of supercooled austenite. Therefore, in this embodiment, the Cr content is controlled to be 0.1 - 0.2%.

[0031] Mo: It can significantly increase the hardenability of steel and improve the strength of the steel plate by combining with accelerated cooling after rolling. However, Mo is a precious alloying element, and excessive addition may increase the cost. Therefore, in this embodiment, 0.08 - 0.16% is added.

[0032] Cu: It improves the strength and corrosion resistance of the steel plate, and has a relatively small adverse effect on low-temperature toughness. However, when the Cu content is excessively high, copper brittleness occurs, affecting the surface quality of the steel plate. Therefore, in this embodiment, the Cu content is controlled to be 0.15 - 0.25%.

[0033] Nb: It belongs to micro-alloy strengthening elements, has precipitation strengthening and grain refinement strengthening effects, can improve the strength of steel and improve low-temperature toughness. However, when the Nb content is excessively high, a large amount of large-sized M / A is likely to be formed in the heat-affected zone of welding, deteriorating the welding performance of the steel plate. Therefore, in this embodiment, the Nb content is controlled to be 0.04 - 0.05%.

[0034] Ti: It belongs to micro-alloy strengthening elements and has precipitation strengthening and grain refinement strengthening effects. Nitrides and oxides of Ti can inhibit the growth of austenite grains and refine austenite. In this embodiment, the Ti content is controlled to be 0.010 - 0.020%.

[0035] P, S: They are residual impurity elements and are disadvantageous to the low-temperature toughness of steel. Therefore, the P content ≤ 0.015% and the S content ≤ 0.005% are controlled.

[0036] Furthermore, the chemical composition of the steel plate also meets the following conditions.

[0037] The carbon equivalent CE is set to be ≤ 0.485 to ensure good weldability of the steel plate. Based on the GB / T 1591-2018 standard, the calculation formula for the carbon equivalent CE is as follows.

[0038] CE = (%C) + (%Mn) / 6 + (%Cr + %Mo + %V) / 5 + (%Ni + %Cu) / 15 Here, the element symbols within the parentheses represent the mass percentages of the corresponding elements, and the % element symbols represent the values obtained by multiplying the mass percentages of the corresponding elements by 100.

[0039] The cold cracking susceptibility index Pcm is set to be ≤ 0.24 to reduce the tendency of cracking during welding and ensure weldability. Based on the GB / T 1591-2018 standard, the calculation formula for the cold cracking susceptibility index Pcm is as follows.

[0040] Pcm = (%C) + (%Si) / 30 + (%Mn + %Cu + %Cr) / 20 + (%Ni) / 60 + (%Mo) / 15 + (%V) / 10 + 5(%B), Here, the element symbols within the parentheses represent the mass percentages of the corresponding elements, and the % element symbols represent the values obtained by multiplying the mass percentages of the corresponding elements by 100.

[0041] As shown in Figure 1, the manufacturing method of the ultra-thick Q500qE bridge steel plate includes the following steps.

[0042] S1: According to the above chemical composition ratio, steelmaking and continuous casting are carried out to obtain a slab.

[0043] S2: The slab is sequentially subjected to first-stage heating, second-stage heating, and third-stage heating. Here, the temperature of the second-stage heating is controlled at 1180 - 1220°C, the temperatures of the first-stage heating and the third-stage heating are controlled lower than the temperature of the second-stage heating, the slab is heated until the temperature is uniform in the first heating stage and the second heating stage, and a temperature gradient that gradually increases from the surface to the center of the slab is formed in the third heating stage.

[0044] S3: The heated slab is subjected to one-pass rolling to obtain a rolled steel plate with a maximum plate thickness of 150 mm.

[0045] S4: For the steel sheet in the rolled state, perform first-stage cooling, second-stage cooling, and third-stage cooling sequentially. Here, control the final cooling temperature of the third-stage cooling to ≤350°C, and sequentially increase the cooling rates of the first-stage cooling, second-stage cooling, and third-stage cooling.

[0046] Specifically, step S1 sequentially includes a hot metal preliminary desulfurization step, a converter steelmaking step, an LF refining step, an RH vacuum refining step, a continuous casting step, and a slab stacking and slow cooling step.

[0047] In the hot metal preliminary desulfurization step, perform KR desulfurization treatment on the hot metal to control the sulfur content in the hot metal. After removing the desulfurized slag, charge the hot metal into the converter to perform converter steelmaking.

[0048] In the converter steelmaking step, use the pre-desulfurized hot metal and scrap, etc. as raw materials to perform converter steelmaking to obtain molten steel. During the steelmaking process, sequentially add ferro-silicon alloy, metallic manganese, and carbon powder, etc. to the molten steel to perform deoxidation alloying treatment.

[0049] After refining the molten steel until it meets the predetermined requirements, perform vacuum circulation degassing treatment.

[0050] Subject the refined molten steel to vacuum circulation degassing treatment to remove inclusions and perform further refining.

[0051] After refining, form the molten steel into a slab with a thickness of 320 mm by continuous casting, and stack and slowly cool the obtained slab.

[0052] In this embodiment, control the slab thickness to 320 mm, and in combination with the subsequent rolling process, a steel sheet with a maximum thickness of 150 mm can be obtained. In other embodiments of the present application, according to the desired steel sheet thickness and rolling process parameters, the slab thickness obtained by continuous casting can also be adjusted.

[0053] In Process S2, it includes sequentially performing first-stage heating, second-stage heating, and third-stage heating on the slab. In the first-stage heating, the heating temperature is controlled to be 900 - 1100 °C and the heating time is controlled to be 150 minutes or more (that is, the residence time of the slab in the heating furnace is controlled to be 150 minutes or more). In the second-stage heating, the heating temperature is controlled to be 1180 - 1220 °C and the heating time is controlled to be 180 - 240 minutes (that is, the residence time of the slab in the heating furnace is controlled to be 180 - 240 minutes).

[0054] In the third-stage heating, the heating temperature is controlled to be 1140 - 1160 °C and the heating time is controlled to be 30 - 60 minutes (that is, the residence time of the slab in the heating furnace is controlled to be 30 - 60 minutes).

[0055] By heating the slab to a uniform and appropriate rolling temperature before rolling, the plasticity of the steel can be improved, the deformation resistance can be reduced, and the steel can be made easier to deform, thereby making it possible to use a large reduction ratio in the rolling process of the slab. Also, the internal structure and performance of the slab can be improved by heating, and the non-uniform structure and non-metallic inclusions are homogenized by the diffusion effect of high-temperature heating.

[0056] In the first-stage heating process, by setting a relatively low heating temperature, cracking on the slab surface due to thermal stress caused by an excessively fast temperature rise rate of the slab is prevented. Also, in order to cooperate with the subsequent second-stage heating process, heating is carried out for a long time to ensure that the temperature of the entire slab becomes uniform and reaches the set heating temperature. In the second-stage heating process, a higher heating temperature is set and holding is carried out for a long time to ensure that the inside of the slab is sufficiently heated and the temperatures of the central part and the surface part of the steel plate become uniform. Also, by limiting the maximum heating time, excessive coarsening of crystal grains on the slab surface due to long-time heating under high-temperature conditions, and oxidation or melting of grain boundaries are prevented, thereby preventing problems with the surface quality of the steel plate such as overheating and burning of the surface and adhesion of oxide scale. In the third-stage heating process, compared with the second-stage heating process, by setting a lower heating temperature and a shorter heating time, a temperature gradient that gradually increases from the surface toward the central part in the thickness direction of the slab is formed, thereby making it easier for deformation to penetrate to the central part of the slab in the subsequent rolling process.

[0057] Step S3 specifically includes the following.

[0058] After descaling the slab after three-stage heating, rolling is carried out. During the rolling process, the rolling temperature is controlled at 980 - 1080°C, the biting speed is controlled at 0.6 m / s, the rolling speed is controlled at 1.2 m / s, and the reduction amount in each rolling pass except the finishing pass is controlled to be 40 mm or more to obtain a steel plate with a maximum plate thickness of 150 mm.

[0059] Descaling means removing the primary oxide scale on the slab surface by a descaling device to prepare for the rolling process of the slab.

[0060] In this embodiment, in order to ensure good quality of the central part of the steel plate under the low-pressure ratio condition, hot rolling by a high-temperature, single-pass, large reduction method is adopted. The rolling temperature is controlled at 980 - 1080°C, and large reduction rolling is carried out per pass by utilizing the characteristic that the deformation resistance at high temperature is small. By setting the reduction amount in each pass except the finishing pass to 40 mm or more, it is ensured that the deformation penetrates to the center part of the slab, preventing the occurrence of defects such as densification and microcracks in the center part after rolling, and guaranteeing that the steel plate meets the flaw detection requirements.

[0061] In this embodiment, after heating the continuous casting slab, direct rolling treatment is performed, eliminating the need for block rolling before the rolling process, simplifying the process, reducing the required man-hours, and reducing the energy consumption during manufacturing. During the rolling process, high-temperature single-pass large reduction rolling is adopted, eliminating the need for multi-stage low-temperature controlled rolling. The rolling process is simple and the rolling efficiency is high. Compared with the conventional low-temperature controlled rolling process, the rolling efficiency can be improved by 50% or more.

[0062] Also, in this embodiment, by directly rolling from a continuous casting slab with a thickness of 320 mm to obtain a steel plate with a maximum thickness of 150 mm, the limitation of the reduction ratio in the production of extra-thick plates using conventional continuous casting slabs is broken through.

[0063] Step S4 includes successively performing first-stage cooling, second-stage cooling, and third-stage cooling on the rolled steel plate. In the first-stage cooling, the cooling rate is controlled at 0.3 - 1°C / s, and the final cooling temperature is controlled at 760 - 780°C. In the second-stage cooling, the cooling rate is controlled at 3 - 5°C / s, and the final cooling temperature is controlled at 560 - 600°C. After the second-stage cooling is completed, standby for 20 - 40 seconds. In the third-stage cooling, the cooling rate is controlled at 5 - 8°C / s, and the final cooling temperature is controlled at 350°C or lower.

[0064] The rolled steel plate is subjected to microstructure control by a three-stage cooling method to ensure good strength and toughness. In the first-stage cooling process, slow cooling is adopted to prevent the formation of excessively coarse side lath-like ferrite microstructure near the surface when the steel plate is cooled near 1000 °C, which may affect the low-temperature toughness of the steel plate, and to prepare for microstructure control near the surface in the next stage. In the second-stage cooling process, compared with the first-stage cooling process, the cooling rate is increased to 3 - 5 °C / s to control the microstructure within the range from near the surface of the steel plate to 1 / 4 of the plate thickness, and the steel plate microstructure within this range is phase-transformed in advance to prevent the excessive formation of martensite hard-phase microstructure, which is disadvantageous to toughness, due to the high cooling intensity near the surface during the strong cooling in the next stage. In the third-stage cooling process, compared with the second-stage cooling process, the cooling rate is further increased to control the microstructure within the range from 1 / 4 to 1 / 2 of the plate thickness of the steel plate. Since this range is close to the center of the steel plate, the heat dissipation effect during cooling is significantly weaker compared with the surface of the steel plate. Therefore, it is necessary to further increase the cooling degree to ensure the cooling effect of the center of the steel plate, so the cooling rate is increased to 5 - 8 °C / s. Also, since the surface near the surface has already undergone phase transformation during the second-stage cooling, although the cooling intensity in the third stage is enhanced, it will not have a significant impact on the microstructure near the surface.

[0065] The steel plate is cooled by a three-stage cooling method, a relatively large cooling rate is adopted in the second and third-stage cooling processes, and the final cooling stage temperature is controlled below 350 °C to effectively control the microstructure of the ultra-thick steel plate and obtain a two-phase microstructure mainly composed of bainite and containing a small amount of martensite. Based on the bainite microstructure, the steel plate microstructure is further strengthened by high-strength martensite, thereby controlling the strength and toughness.

[0066] This embodiment also provides an ultra-thick Q500qE steel plate for bridges manufactured by the manufacturing method of the ultra-thick Q500qE steel plate for bridges described above.

[0067] The chemical composition of the steel plate, by mass percentage, contains C: 0.05 - 0.07%, Si: 0.15 - 0.35%, Mn: 1.5 - 1.7%, Ni: 0.3 - 0.5%, Cr: 0.2 - 0.3%, Mo: 0.2 - 0.3%, Cu: 0.15 - 0.25%, Nb: 0.04 - 0.05%, Ti: 0.01 - 0.02%, and the balance consists of Fe and unavoidable impurities, where the impurities are P ≤ 0.01% and S ≤ 0.003%.

[0068] Also, the carbon equivalent CE of the steel plate is 0.485 or less, and the cold cracking susceptibility index Pcm is 0.24 or less.

[0069] The steel plate has a yield strength of 430 MPa or more, a tensile strength of 540 MPa or more, a yield ratio of 0.85 or less, an impact value at -40°C of 180 J or more, a Z-direction tensile reduction of 50% or more, and satisfies the flaw detection requirements of Class II or higher in the GB / T 2970 - 2016 standard.

[0070] As described above, the manufacturing method of the present application directly performs a rolling process on the slab after three-stage heating before rolling, and performs a three-stage cooling process after rolling, thereby obtaining a Q500qE steel plate with a maximum plate thickness of 150 mm. This steel plate satisfies the Z-direction tensile property requirements of Z35 class and the flaw detection requirements of Class II or higher in the GB / T 2970 - 2016 standard. This manufacturing method has a simple manufacturing process, high manufacturing efficiency, and low manufacturing cost. Specifically, first, in the manufacturing process, a steel plate with an extremely thick specification can be manufactured using a normal continuous casting slab, without the need for die casting and the preparation of composite slabs. Second, in the rolling process, high-temperature single-stage heavy reduction rolling is adopted, without the need for multi-stage low-temperature controlled rolling, and the rolling process is simple and the rolling efficiency is high. Third, heat treatments such as tempering, normalizing, and rapid cooling after normalizing after rolling are not required, the number of manufacturing processes is small, the manufacturing cycle is short, and the manufacturing cost is low.

[0071] The specific embodiments of the present invention will be further described by the following examples.

[0072] <Example 1> The chemical composition of the steel plate is C: 0.058%, Si: 0.28%, Mn: 1.63%, P: 0.008%, S: 0.0026%, Ni: 0.36%, Cr: 0.24%, Mo: 0.24%, Cu: 0.20%, Nb: 0.042%, Ti: 0.014%. Ceq = 0.463 and Pcm = 0.193. The balance is Fe and inevitable impurities.

[0073] Through the processes of preliminary desulfurization of hot metal, converter steelmaking, LF refining, RH vacuum refining, continuous casting, and slow cooling of the slab in a stack, a continuous casting slab with a thickness of 320 mm was produced.

[0074] Three-stage heating was carried out on the slab. The heating temperature in the first stage was 988 - 1093 °C, the holding time t1 was 180 minutes, the heating temperature in the second stage was 1198 - 1210 °C, the holding time t2 was 204 minutes, the heating temperature in the third stage was 1143 - 1156 °C, and the holding time t3 was 36 minutes.

[0075] High-temperature one-stage heavy reduction rolling was carried out on the steel plate. After the slab was extracted from the heating furnace, it was immediately rolled after rough scale removal. The rolling temperature was 1006 - 1038 °C, the biting speed was 0.6 m / s, the rolling speed was 1.2 m / s, the reduction per pass was 43 mm, 45 mm, 45 mm, and 37 mm in sequence, and the product plate thickness was 150 mm.

[0076] After rolling, three-stage cooling was carried out on the steel plate. The cooling rate in the first stage was 0.3 °C / s, the cooling stop temperature was 774 °C, the cooling rate in the second stage was 4.5 °C / s, the cooling stop temperature was 572 °C. After waiting for 20 seconds after the end of the second-stage cooling, the cooling rate in the third stage was 7.2 °C / s, and the cooling stop temperature was 328 °C.

[0077] Various properties of the steel plate are shown in Table 1, and a typical microstructure is shown in Figure 2.

[0078]

Table 1

[0079] In the description based on the embodiments of the present invention, it should be understood that each embodiment does not only include independent technical solutions. Such a description method in this specification is only for the sake of clarity. Those skilled in the art can understand this specification as a whole and appropriately combine the technical solutions in each embodiment to constitute other embodiments that can be understood by those skilled in the art.

[0080] The above series of detailed descriptions are only specific descriptions of the feasible embodiments of the present application and do not limit the protection scope of the present application. Equivalent embodiments or modifications within the scope not departing from the technical idea of the present application shall all be included in the protection scope of the present application.

Claims

1. A method for manufacturing a steel plate for ultra-thick Q500qE bridges, comprising: The chemical composition of the steel plate is by mass percentage: C: 0.05 - 0.07%, Si: 0.15 - 0.35%, Mn: 1.5 - 1.7%, Ni: 0.3 - 0.5%, Cr: 0.2 - 0.3%, Mo: 0.2 - 0.3%, Cu: 0.15 - 0.25%, Nb: 0.04 - 0.05%, Ti: 0.01 - 0.02%, the balance consisting of Fe and unavoidable impurities, and the impurities are P≤0.01% and S≤0.003%; A step of smelting based on the above chemical composition and obtaining a slab by continuous casting; A step of sequentially performing first-stage heating, second-stage heating, and third-stage heating on the slab, controlling the temperature of the second-stage heating to 1180 - 1220°C, controlling the temperatures of the first-stage heating and the third-stage heating to be lower than the temperature of the second-stage heating, making the temperature of the slab uniform by the first heating stage and the second heating stage, and forming a temperature gradient that gradually increases from the surface to the center of the slab in the third heating stage; A step of performing single-pass rolling on the heated slab to obtain a rolled steel plate with a maximum thickness of 150 mm; and A method for manufacturing a steel plate for ultra-thick Q500qE bridges, comprising a step of sequentially performing first-stage cooling, second-stage cooling, and third-stage cooling on the rolled steel plate, controlling the final cooling temperature of the third-stage cooling to 350°C or lower, and sequentially increasing the cooling rates of the first-stage cooling, the second-stage cooling, and the third-stage cooling.

2. The chemical composition of the steel plate satisfies a carbon equivalent CE≤0.485 and a cold crack susceptibility index Pcm≤0.24, where the calculation formula for the carbon equivalent CE is CE = (%C) + (%Mn) / 6 + (%Cr + %Mo + %V) / 5 + (%Ni + %Cu) / 15, The calculation formula for the cold crack susceptibility index Pcm is Pcm = (%C) + (%Si) / 30 + (%Mn + %Cu + %Cr) / 20 + (%Ni) / 60 + (%Mo) / 15 + (%V) / 10 + 5(%B), where the element symbols in the parentheses indicate the mass percentages of the corresponding elements, and %element symbol indicates the value obtained by multiplying the mass percentage of the corresponding element by 100. The method for manufacturing a steel plate for ultra-thick Q500qE bridges according to Claim 1.

3. The step of smelting based on the above chemical composition and obtaining a slab by continuous casting is Based on the above chemical composition, the following steps are sequentially carried out: desulfurization of hot metal preliminarily, converter steelmaking, LF refining, and RH vacuum degassing to obtain molten steel, continuous casting of the molten steel to obtain a slab with a thickness of 320 mm, and stack cooling of the slab. The method for manufacturing an extra-thick Q500qE bridge steel plate according to claim 1 or 2.

4. The step of sequentially performing first-stage heating, second-stage heating, and third-stage heating on the slab is as follows: Controlling the temperature of the first-stage heating to 900 - 1100 °C and the heating time to 150 min or more, controlling the temperature of the second-stage heating to 1180 - 1220 °C and the heating time to 180 - 240 min, and controlling the temperature of the third-stage heating to 1140 - 1160 °C and the heating time to 30 - 60 min. The method for manufacturing an extra-thick Q500qE bridge steel plate according to any one of claims 1 - 3.

5. The step of performing single-pass rolling on the heated slab is as follows: After removing the scale of the slab after three-stage heating, rolling is carried out. In the rolling process, the rolling temperature is controlled to 980 - 1080 °C, and the reduction per pass except for the final pass is controlled to 40 mm or more. The method for manufacturing an extra-thick Q500qE bridge steel plate according to any one of claims 1 - 4.

6. The step of performing single-pass rolling on the heated slab is as follows: In the rolling process, the biting speed is controlled to 0.6 m / s, and the rolling speed is controlled to 1.2 m / s. The method for manufacturing an extra-thick Q500qE bridge steel plate according to any one of claims 1 - 5.

7. The step of sequentially performing first-stage cooling, second-stage cooling, and third-stage cooling on the rolled steel plate is as follows: Controlling the cooling rate of the first-stage cooling to 0.3 - 1 °C / s and the final cooling temperature to 760 - 780 °C, controlling the cooling rate of the second-stage cooling to 3 - 5 °C / s and the final cooling temperature to 560 - 600 °C, waiting for 20 - 40 s after the end of the second-stage cooling, and controlling the cooling rate of the third-stage cooling to 5 - 8 °C / s and the final cooling temperature to 350 °C or less. The method for manufacturing an extra-thick Q500qE bridge steel plate according to any one of claims 1 - 6.

8. An extra-thick Q500qE bridge steel plate manufactured by the manufacturing method according to any one of claims 1 - 7.

9. The microstructure of the steel plate is a two-phase structure of bainite and martensite, and the amount of bainite is more than the amount of martensite. The steel plate according to claim 8.

10. The yield strength of the steel plate is 430 MPa or more, the tensile strength is 540 MPa or more, the yield ratio is 0.85 or less, the impact value at -40°C is 180 J or more, the Z-direction tensile sectional shrinkage rate is 50% or more, and it meets the flaw detection requirements of Class II or higher in the GB / T 2970-2016 standard. The steel plate according to claim 8 or 9.

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