Production method of steel medium-thickness plate

Abstract

The invention belongs to the technical field of high-yield structural steel production, and particularly relates to a production method of a steel medium-thickness plate. Comprising the steps of molten iron pretreatment, converter smelting, LF refining, casting, casting blank heating, rolling, cooling after rolling and heat treatment. Rolling comprises rough rolling and finish rolling, and rough rolling is conducted under the conditions that the initial rolling temperature is larger than or equal to 1000 DEG C and the finish rolling temperature is larger than or equal to 920 DEG C; and carrying out finish rolling under the conditions that the initial rolling temperature is less than or equal to 900 DEG C and the finish rolling temperature is 800-840 DEG C to obtain a rolled steel plate. After-rolling cooling is conducted according to the thickness of the rolled steel plate, and when the thickness of the rolled steel plate is 6-12 mm, air cooling is conducted; and when the thickness of the rolled steel plate is larger than 12 mm, water cooling is conducted. And the steel plate subjected to water cooling after rolling is subjected to heat treatment including quenching and tempering, the quenching temperature ranges from 920 DEG C to 940 DEG C, and the tempering temperature ranges from 610 DEG C to 650 DEG C. The prepared steel medium plate is excellent in toughness and high in one-time hit rate, and the production efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-yield structural steel production, and particularly relates to a production method for medium and heavy steel plates. Background Art

[0002] With the rapid development of fields such as heavy construction machinery, offshore platforms, and super high-rise buildings, the demand for medium and heavy steel plates with high strength, high toughness, and excellent welding performance is becoming increasingly urgent. The thickness of the medium and heavy steel plates is ≥ 30 mm. As a typical quenched and tempered high-strength steel, S690QL1 steel has a yield strength ≥ 690 MPa, and due to its excellent comprehensive mechanical properties, it has become the core material for lightweight design of large structures.

[0003] Currently, in the quenching process of existing medium and heavy steel plate S690QL1 steel, a single water cooling method is mostly used, resulting in insufficient cooling rate in the core of the thick plate, and it is easy to generate non-martensite structures such as ferrite, resulting in unqualified mechanical properties such as strength and toughness in the core. Although the performance can be compensated by multiple temperings, tempering cannot change the tissue defects in the core, and it increases energy consumption and production cycle.

[0004] Chinese Patent CN118703890A discloses a steel plate for mobile containers with a yield strength of 690 MPa and its manufacturing method, including smelting, continuous casting, heating, rolling, and heat treatment; two-stage quenching is adopted in the heat treatment process. Although the core hardness is improved to a certain extent, the single water cooling method leads to the risk of microcracks caused by surface supercooling. Summary of the Invention

[0005] In order to solve the technical problem that the single water cooling method in the quenching process of the existing medium and heavy steel plate S690QL1 steel leads to the risk of microcracks caused by surface supercooling. The present invention provides a production method for medium and heavy steel plates. Through a two-stage rolling process, and setting the finishing rolling temperature of the second-stage finish rolling to 800 °C - 840 °C, the grain size of the structure in the core of the steel plate is prevented from becoming larger or the grain size of the steel plate structure from being uneven; at the same time, by adopting a post-rolling cooling process before heat treatment, the grain size during the transformation of ferrite and pearlite is effectively controlled to achieve a state of fine grains and uniform structure; and the quenching temperature during the heat treatment process is controlled at 920 °C - 940 °C to avoid residual ferrite and pearlite; and different tempering temperatures are selected according to the thickness of the steel plate to effectively avoid grain growth and unevenness, and a high-yield strength structural steel medium and heavy plate with good performance, guaranteed flaw detection, and low inclusions is obtained.

[0006] The first object of the present invention is to provide a production method for medium and heavy steel plates. The production method for medium and heavy steel plates includes hot metal pretreatment, converter smelting, LF refining, casting, slab heating, rolling, post-rolling cooling, and heat treatment. The process conditions for the rolling, post-rolling cooling, and heat treatment are as follows:

[0007] The rolling includes rough rolling and finish rolling. The process of the rolling is as follows: rough roll the slab under the conditions that the starting rolling temperature is ≥1000°C and the finishing rolling temperature is ≥920°C to obtain an intermediate billet; then finish roll the intermediate billet under the conditions that the starting rolling temperature is ≤900°C and the finishing rolling temperature is 800°C - 840°C to obtain a rolled steel plate; and the cumulative deformation rate below 900°C during the rolling process is not less than 50%.

[0008] It should be noted that in the rolling process of the present invention, rough rolling and finish rolling are adopted. At the same time, the finishing rolling temperature is set to 800°C - 840°C to avoid the situation that the crystal grains in the core of the steel plate become larger or the crystal grains of the steel plate structure are uneven, resulting in the deterioration of the performance of the steel plate, so as to ensure that the performance of the steel plate can be effectively improved through an appropriate finish rolling temperature.

[0009] Cool the rolled steel plate after rolling according to the thickness of the rolled steel plate. The process conditions for the post-rolling cooling are as follows: when the thickness of the rolled steel plate is 6mm - 12mm, air-cool the rolled steel plate; or when the thickness of the rolled steel plate is 12mm - 50mm, water-cool the rolled steel plate, and the red-return temperature of the rolled steel plate is 700°C - 740°C; or when the thickness of the rolled steel plate is 50mm - 80mm, water-cool the rolled steel plate, and the red-return temperature of the rolled steel plate is 680°C - 720°C.

[0010] It should be noted that usually, heat-treated steel plates do not require water cooling after rolling. Considering the heredity of crystal grains, the present invention pays attention to controlling the grain size throughout the whole process. The post-rolling cooling process can better improve the original grain size and uniformity before heat treatment. When the thickness of the steel plate is relatively low, ferrite-pearlite structure is formed by natural cooling; when the steel plate is medium-thick, by controlling the red-return temperature, bainite + martensite duplex structure is obtained to balance strength and toughness. Through the post-rolling cooling process of the present invention, the grain size during the phase transformation of ferrite and pearlite is effectively controlled to achieve a state of fine grains and uniform structure. The original grain size before this heat treatment has heredity during the subsequent heat treatment process. Compared with the common steel plates that are air-cooled after rolling and then heat-treated, it has finer grains and retains a certain amount of dislocation density, which plays a positive role in the strength and toughness of the steel plate, and the comprehensive performance of the steel plate is better.

[0011] Heat-treat the steel plate cooled after rolling. The heat treatment includes quenching and tempering. The process of the heat treatment is as follows: the quenching temperature is 920°C - 940°C to avoid residual ferrite and pearlite, and the tempering temperature is 610°C - 650°C.

[0012] Adjust the quenching time according to the thickness of the steel plate after post-rolling cooling. When the thickness of the steel plate after post-rolling cooling ≤ 14 mm, the quenching time is 4×H min. When the thickness of the steel plate after post-rolling cooling > 14 mm, the quenching time is 3×H min. Here, H is the thickness of the steel plate after post-rolling cooling, with the unit of mm.

[0013] Adjust the tempering process according to the thickness of the steel plate after post-rolling cooling. When the thickness range of the steel plate after post-rolling cooling is 6 mm ≤ thickness < 20 mm, the tempering temperature of the steel plate after post-rolling cooling is 630°C to 650°C, and the tempering time is 1.5H + 60 min. Or, when the thickness range of the steel plate after post-rolling cooling is 20 mm ≤ thickness ≤ 80 mm, the tempering temperature of the steel plate after post-rolling cooling is 610°C to 630°C, and the tempering time is 1.5H + 60 min. Here, H is the thickness of the steel plate after post-rolling cooling, with the unit of mm. The tempering process selects different temperatures according to the thickness of the steel plate after post-rolling cooling to avoid uneven grain growth and affect the performance uniformity.

[0014] Preferably, the medium-thick steel plate includes the following chemical components by mass percentage: C 0.15% - 0.18%, Si 0.15% - 0.35%, Mn 1.10% - 1.20%, S ≤ 0.005%, P ≤ 0.015%, Nb 0.018% - 0.025%, V ≤ 0.01%, Ti ≤ 0.01%, Als 0.020% - 0.045%, Cr 0.43% - 0.50%, Ni 0.18% - 0.25%, Mo 0.22% - 0.30%, B 0.001% - 0.0025%, and the balance is Fe and unavoidable impurities; the carbon equivalent CEV(%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≤ 0.56%.

[0015] Preferably, the reheat crack sensitivity index PSR = Cr + Cu + 2Mo + 5Ti + 7Nb + 10V - 2 ≤ -0.57. PSR ≤ 0 indicates a low possibility of reheat cracks.

[0016] Compared with the prior art, the present invention has the following technical effects: 1. The present invention involves rough rolling and finish rolling, and sets the finish rolling temperature at 800°C to 840°C to prevent the grain size of the steel plate core from becoming larger or the grain size of the steel plate structure from being uneven, ensuring that an appropriate finish rolling temperature improves the comprehensive performance of the steel plate. At the same time, a post-rolling cooling process is adopted to effectively control the grain size during the ferrite and pearlite phase transformation processes, achieving a state of fine grains and uniform structure. And by controlling the quenching temperature at 920°C to 940°C, retained ferrite and pearlite are avoided; the tempering process selects different temperatures according to the thickness of the steel plate, effectively avoiding uneven grain growth and affecting the performance uniformity. This solves the technical problem that the existing medium and heavy plate S690QL1 steel has a risk of microcracks caused by surface supercooling in the single water cooling method during the quenching process.

[0017] 2. Through the optimization of the rolling, post-rolling cooling, and heat treatment processes, the quenched and tempered S690QL1 steel plate prepared by the present invention has excellent strength and toughness, and the one-time hit rate is ≥96%, improving the production efficiency of the enterprise.

[0018] 3. The requirements for the quenched and tempered S690QL1 steel plate prepared by the present invention are relatively high. For steel grades with more alloying elements, the production of unqualified steel plates is avoided, reducing the manufacturing cost of the enterprise. Through the coordinated and precise control of the entire process, a preparation method with high production efficiency is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a micrograph of a 6-mm-thick medium and heavy steel plate obtained in Example 1.

[0020] Figure 2 It is a micrograph of a 50-mm-thick medium and heavy steel plate obtained in Example 2.

[0021] Figure 3 It is a micrograph of an 80-mm-thick medium and heavy steel plate obtained in Example 3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The following will describe the technical solutions in the present invention clearly and completely in combination with the specific embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0023] In the methods described in the embodiments of the present invention, unless otherwise specified, they are all conventional methods. The materials, reagents, etc. used, unless otherwise specified, can all be obtained from commercial channels.

[0024] It should be noted that the specific production process flow of the present invention is as follows: hot metal pretreatment, converter smelting, argon blowing during converter tapping, LF refining, vacuum degassing treatment, casting, ingot pit cooling, billet heating, rolling, post-rolling cooling, and heat treatment.

[0025] Hot metal pretreatment: During hot metal pretreatment, control the impurity elements As ≤ 0.016% and Sn ≤ 0.012%, and the scrap steel is all self-produced high-quality scrap steel.

[0026] Converter smelting: Adopt double slag operation; the tapping C ≤ 0.04%, P ≤ 0.008%, and S ≤ 0.025%. By discharging the high-phosphorus slag in the early stage in stages and remelting the slag, deep dephosphorization is carried out under the conditions of high alkalinity and oxidability, and the end-point phosphorus content can be controlled below 0.008% to meet the requirements of low-phosphorus steel grades. Improve the purity of molten steel through the converter steelmaking process, reduce and improve the quantity and distribution of inclusions, and improve the crystallization structure of continuous casting billets.

[0027] Argon blowing during converter tapping: The argon blowing time is greater than 2 minutes, and the target end-point C at the argon station is 0.03% - 0.04%, P ≤ 0.010%, and the end-point oxygen is greater than 100 ppm to ensure that the oxygen potential is 30 ppm - 50 ppm when reaching LF refining. Promote the homogenization of molten steel composition and temperature through argon blowing stirring to reduce C and P segregation; at the same time, by precisely controlling the end-point oxygen potential at the argon station to be 30 ppm - 50 ppm, conditions are provided for aluminum deoxidation and inclusion modification in LF refining, avoiding Al burning due to too high oxygen potential or insufficient deoxidation due to too low oxygen potential.

[0028] LF refining: Adopt slightly positive pressure operation, add in the order of ferroniobium - Al wire - special aluminum-clad wire - Ca wire, and the interval time is 3 minutes; ensure that S ≤ 0.003% and Als is 0.040% - 0.050% during tapping. By adding Al wire and aluminum-clad wire step by step, control Als to be 0.040% - 0.050%, which not only ensures the deoxidation effect but also avoids surface defects of the casting billet caused by excessive aluminum. At the same time, form Nb(C,N) precipitation phases through the addition of ferroniobium to refine grains and improve strength and toughness.

[0029] Vacuum degassing treatment: The time of vacuum degassing treatment is greater than 15 minutes, the ultimate vacuum degree ≤ 133 Pa, the ultimate vacuum holding time is greater than 10 minutes, and the tapping requirement is H ≤ 1.5 ppm.

[0030] Casting: The superheat of molten steel casting is ≤ 30°C, the drawing speed is 0.90 m / min, and the specific water volume is 0.5 L / kg.

[0031] Ingot pit cooling: The pit cooling time is greater than 48 hours, and the pit cooling temperature is greater than 750°C.

[0032] Slab heating: The temperature in the preheating section is 700°C to 900°C, the temperature in the first heating section is 1100°C to 1190°C, and the temperature in the second heating section is 1210°C to 1260°C; the temperature in the soaking section is 1250 to 1270°C; Considering the high requirements for the strength and toughness of low-temperature high-strength steel, the heating temperature of the slab is relatively low and the range is narrow. Low-temperature preheating in the preheating section avoids thermal stress cracks in the slab; the temperature is gradually increased from the first heating section to the soaking section to 1250°C to 1270°C to ensure the full solution of microalloying elements such as Nb and V and the homogenization of austenite grains at the same time.

[0033] Rolling: Rolling includes rough rolling and finish rolling. The slab is rough rolled under the conditions of an initial rolling temperature ≥ 1000°C and a final rolling temperature ≥ 920°C to obtain an intermediate billet; then the intermediate billet is finish rolled under the conditions of an initial rolling temperature ≤ 900°C and a final rolling temperature of 800°C to 840°C to obtain the rolled steel plate; and the cumulative deformation rate below 900°C during the rolling process is not less than 50%.

[0034] It should be noted that in the present invention, high-temperature rough rolling promotes the dynamic recrystallization of austenite and refines the grains; low-temperature finish rolling introduces deformation-induced ferrite transformation to form an ultrafine grain structure. During the rolling process, if the finish rolling temperature is too high, the temperature of the thick steel plate will be too high, and the larger the temperature of the steel plate, the larger the grain size of the core structure, which will lead to poor performance of the steel plate. If the finish rolling temperature is too low, rolling is difficult, the grain deformation bands at different positions of the steel plate are reduced, and the number of ferrite nucleation points after phase transformation is small, which is not conducive to refining the grain structure of the steel plate and results in poor performance. Therefore, preferably in the present invention, the final rolling temperature of the finish rolling is set to 800°C to 840°C to avoid the situation that the grain size of the core structure of the steel plate becomes larger or the grain structure of the steel plate is uneven, resulting in poor performance of the steel plate, and further ensuring that the performance of the steel plate can be effectively improved through an appropriate finish rolling temperature.

[0035] Post-rolling cooling: Different water groups are appropriately turned on according to the thickness of the rolled steel plate for post-rolling cooling of the rolled steel plate; when the thickness of the rolled steel plate is 6 mm to 12 mm, the rolled steel plate is air-cooled; or when the thickness of the rolled steel plate is 12 mm to 50 mm, the rolled steel plate is water-cooled, and the return red temperature of the rolled steel plate is 720 ± 20°C; or when the thickness of the rolled steel plate is 50 mm to 80 mm, the rolled steel plate is water-cooled, and the return red temperature of the rolled steel plate is 700 ± 20°C; Heat treatment process: The rolled and cooled steel plate is heat-treated. The heat treatment process is divided into quenching and tempering. The quenching time is adjusted according to the thickness of the rolled and cooled steel plate. The quenching conditions are as follows: The quenching temperature is 920°C to 940°C. When the thickness of the rolled and cooled steel plate ≤ 14 mm, the quenching time is 4×H min. Or, when the thickness of the rolled and cooled steel plate > 14 mm, the quenching time is 3×H min. The tempering process is adjusted according to the thickness of the rolled and cooled steel plate. When the thickness range of the rolled and cooled steel plate is 6 mm ≤ thickness < 20 mm, the tempering temperature is 630°C to 650°C, and the tempering time is 1.5H + 60 min. Or, when the thickness range of the rolled and cooled steel plate is 20 mm ≤ thickness ≤ 80 mm, the tempering temperature is 610°C to 630°C, and the tempering time is 1.5H + 60 min. Wherein, H is the thickness of the rolled and cooled steel plate, and the unit is mm.

[0036] It should be noted that usually, the heat-treated steel plate does not require water cooling after rolling. Considering the heredity of the tissue grains, the present invention pays attention to controlling the grain size throughout the whole process. The original grain size and uniformity before heat treatment can be better improved through the post-rolling cooling process. When the thickness of the steel plate is relatively low, ferrite-pearlite structure is formed by natural cooling. When the steel plate is medium-thick plate, by controlling the recarburization temperature, bainite + martensite duplex structure is obtained to balance strength and toughness.

[0037] The austenitizing final temperature of the steel grade designed by the present invention is 870°C to 890°C. In order to obtain a single lower bainite structure, the preferred quenching temperature is 920°C to 940°C to avoid residual ferrite and pearlite. Different temperatures are selected for the tempering process according to the size effect to avoid uneven grain growth and affect the performance uniformity.

[0038] Example 1 A production method of medium-thick steel plate includes the following steps: Step 1, converter smelting: Double slag operation is adopted to make the C content in the molten steel at the end of the converter be 0.02%; the P content be 0.006%; the S content be 0.018% to obtain molten steel.

[0039] Step 2, LF refining: Micro-alloy addition is carried out on the molten steel in step 1 by using slightly positive pressure operation. When leaving the station, the S content in the molten steel is 0.001%; the Als content is 0.042% to obtain refined molten steel.

[0040] Step 3, RH vacuum degassing: The refined molten steel is treated by RH to ensure that the H content in the refined molten steel when leaving the station is 1.2 ppm.

[0041] Step 4, casting: Pour the molten steel in step 3, with the superheat degree being 25°C, the drawing speed being 0.90 m / min, and the specific water volume being 0.5 L / kg, to obtain a slab with a thickness of 240 mm.

[0042] Step 5, pit cooling of the billet: Perform pit cooling on the slab with a thickness of 240 mm produced in step 4, with the pit cooling time being 48 h and the pit cooling temperature being 770°C, to obtain a billet with a thickness of 240 mm.

[0043] Step 6, heating of the slab: Use a walking beam type reheating furnace to heat the billet with a thickness of 240 mm in step 5. The preheating section of the reheating furnace is at 810°C, the temperature of the first heating section is 1150°C, the temperature of the second heating section is 1240°C, and the soaking section temperature is 1255°C. The residence time of the slab in the furnace is 1.3×240 = 312 min, and the gas ratio is 0.75, to obtain a hot rolled billet suitable for rolling.

[0044] Step 7, rough rolling: Perform rough rolling on the hot rolled billet suitable for rolling obtained in step 6 above at an initial rolling temperature of 1115°C until the thickness of the billet reaches 60 mm to obtain an intermediate billet; among them, the pass distribution for rough rolling is shown in Table 1: Table 1 Pass distribution for rough rolling Step 8, finish rolling: Perform finish rolling on the intermediate billet with a thickness of 60 mm obtained in step 7 above at an initial rolling temperature of 890°C, and the final rolling temperature is 814°C, to obtain a rolled steel plate with a thickness of 6 mm.

[0045] Step 9, air cooling after rolling: Air cool the rolled steel plate obtained in step 8 above, and the steel plate is taken off the production line and stacked for cooling after passing through a hot straightening machine.

[0046] Step 10, quenching of the steel plate: Perform a quenching process on the steel plate that has been stacked and cooled to room temperature in step 9 above through a radiant tube heating roller hearth heat treatment furnace. The quenching temperature is 931°C, the residence time of the steel plate in the furnace is 24 min, and the steel plate is heated and taken out of the furnace and water cooled to room temperature by the slit nozzles and high-density nozzles of a quenching machine.

[0047] Step 11, tempering of the steel plate: Perform high-temperature tempering on the steel plate quenched in step 10 above through an open-flame heating roller hearth heat treatment furnace. The tempering temperature is 642°C, and the residence time in the furnace is 69 minutes, to obtain a medium-thick steel plate with a thickness of 6 mm.

[0048] Example 2 A production method for medium-thick steel plates, comprising the following steps: Step 1, converter smelting: Adopt double slag operation to make the C content in the molten steel at the end of the converter be 0.03%; the P content be 0.005%; the S content be 0.016%, and obtain molten steel.

[0049] Step 2, LF refining: Adopt slightly positive pressure operation to add microalloy to the molten steel in Step 1. When leaving the station, the S content in the molten steel is 0.001%; the Als content is 0.045%, and obtain refined molten steel.

[0050] Step 3, RH vacuum degassing: Treat the refined molten steel through RH to ensure that the H content in the refined molten steel when leaving the station is 1.0 ppm.

[0051] Step 4, casting: Cast the molten steel in Step 3. Among them, the superheat degree is 28 °C, the drawing speed is 0.90 m / min, the specific water volume is 0.5 L / kg, and obtain a slab with a thickness of 240 mm.

[0052] Step 5, ingot pit cooling: Cool the slab with a thickness of 240 mm produced in Step 4 in the pit. Among them, the pit cooling time is 48 h, the pit cooling temperature is 785 °C, and obtain a billet with a thickness of 240 mm.

[0053] Step 6, slab heating: Use a walking beam type heating furnace to heat the billet with a thickness of 240 mm in Step 5. The preheating section of the heating furnace is 860 °C, the temperature of the first heating section is 1140 °C, the temperature of the second heating section is 1240 °C, the temperature of the soaking section is 1260 °C, the residence time of the slab in the furnace is 1.3×240 = 312 min, and the gas ratio is 0.78, and obtain a hot rolled billet that can be rolled.

[0054] Step 7, rough rolling: Rough roll the hot rolled billet that can be rolled obtained in the above Step 6 at an initial rolling temperature of 1120 °C until the thickness of the billet is 110 mm, and obtain an intermediate billet; among them, the rough rolling pass distribution is shown in Table 2: Table 2 Rough rolling pass distribution Step 8, finish rolling: Finish roll the intermediate billet with a thickness of 110 mm obtained in the above Step 7 at an initial rolling temperature of 850 °C, and the final rolling temperature is 835 °C, and obtain a rolled steel plate with a thickness of 50 mm.

[0055] Step 9, water cooling after rolling: The rolled steel plate obtained in the above step 8 is water-cooled, and the return red temperature of the steel plate is 736 °C. After passing through the hot straightening machine, the steel plate is taken off the production line and stacked for air cooling.

[0056] Step 10, quenching of the steel plate: The steel plate that has been water-cooled to room temperature after rolling in the above step 9 is quenched by a walking beam furnace heated by radiant tubes. The quenching temperature is 935 °C, the residence time of the steel plate in the furnace is 150 min, and the steel plate is heated and taken out of the furnace and then water-cooled to room temperature by the slot nozzles and high-density nozzles of the quenching machine.

[0057] Step 11, tempering of the steel plate: The steel plate quenched in the above step 10 is subjected to high-temperature tempering in a walking beam furnace heated by open fire. The tempering temperature is 630 °C, and the residence time in the furnace is 135 min, obtaining a medium and heavy steel plate with a thickness of 50 mm.

[0058] Example 3 A production method of a medium and heavy steel plate, comprising the following steps: Step 1, converter smelting: Adopting double slag operation, so that the C content in the molten steel at the end of the converter is 0.02%; the P content is 0.003%; the S content is 0.020%, obtaining molten steel.

[0059] Step 2, LF refining: Adopting slightly positive pressure operation to add microalloy to the molten steel in step 1. When the molten steel leaves the station, the S content in the molten steel is 0.002%; the Als content is 0.040%, obtaining refined molten steel.

[0060] Step 3, RH vacuum degassing: Refined molten steel is treated by RH to ensure that the H content in the refined molten steel when it leaves the station is 1.3 ppm.

[0061] Step 4, casting: The molten steel in step 3 is cast, wherein the superheat is 26 °C, the drawing speed is 0.90 m / min, and the specific water quantity is 0.5 L / kg, obtaining a slab with a thickness of 240 mm.

[0062] Step 5, ingot pit cooling: The slab with a thickness of 240 mm produced in step 4 is subjected to pit cooling, wherein the pit cooling time is 48 h and the pit cooling temperature is 777 °C, obtaining a billet with a thickness of 240 mm.

[0063] Step 6, billet heating: The billet with a thickness of 240 mm in Step 5 is heated by a walking beam reheating furnace. The preheating section of the reheating furnace is at 840 °C, the temperature of the first heating section is 1170 °C, the temperature of the second heating section is 1255 °C, and the soaking section is at1265 °C. The residence time in the furnace is 1.3×240 = 312 min, and the gas ratio is 0.79 to obtain a hot rolled billet that can be rolled.

[0064] Step 7, rough rolling: The hot rolled billet obtained in the above Step 6 is rough rolled at a starting rolling temperature of 1180 °C until the thickness of the billet is 160 mm to obtain an intermediate billet. Among them, the rough rolling pass distribution is shown in Table 3: Table 3 Rough rolling pass distribution Step 8, finish rolling: The intermediate billet with a thickness of 160 mm obtained in the above Step 7 is finish rolled at a starting rolling temperature of 840 °C, and the final rolling temperature is 830 °C to obtain a rolled steel plate with a thickness of 80 mm.

[0065] Step 9, water cooling after rolling: The rolled steel plate obtained in the above Step 8 is water cooled. The recalescence temperature of the steel plate is 695 °C, and after passing through the hot straightening machine, the steel plate is taken off the production line and stacked for air cooling.

[0066] Step 10, quenching of the steel plate: The steel plate water cooled to room temperature in the above Step 9 is quenched by a radiant tube heated roller hearth heat treatment furnace. The quenching temperature is 930 °C, the residence time of the steel plate in the furnace is 240 min, and the steel plate is heated and taken out of the furnace and water cooled to room temperature by the slot nozzles and high-density nozzles of the quenching machine.

[0067] Step 11, tempering of the steel plate: The steel plate quenched in the above Step 10 is subjected to high-temperature tempering by an open-flame heated roller hearth heat treatment furnace. The tempering temperature is 620 °C, and the residence time in the furnace is 180 min to obtain a medium and heavy steel plate with a thickness of 80 mm.

[0068] (2) Mechanical property testing The present invention conducts mechanical property testing on the medium and heavy steel plates prepared in Examples 1 to 3, and the test results are shown in Table 4. The yield strength, tensile strength, and elongation are all tested according to the test method of "Metallic materials - Tensile testing at ambient temperature" in GB / T 228.1; the impact absorption energy is tested according to the test method in GB / T 229, and each group is tested in parallel three times.

[0069] Table 4 Mechanical property test results of the medium and heavy steel plates prepared in Examples 1 to 3 As can be seen from the test results in Table 4, the quenched and tempered medium and heavy plates of the steel prepared in Examples 1 to 3 of the present invention all have excellent mechanical properties. This may be because in the examples, harmful gases, the size and distribution of inclusions are strictly controlled, and by optimizing the billet heating process, the gas mixing ratio, the post-rolling cooling system and a higher quenching heating temperature, a more uniform and single bainite structure is obtained, thereby obtaining stable and excellent comprehensive properties of high strength and high toughness. The quenched S690QL1 high-yield structural steel obtained through the integrated smelting, continuous casting, rolling and heat treatment processes of the present invention all has excellent and stable mechanical properties.

[0070] As Figures 1 to 3 shown, the medium and heavy plates of the steel prepared in Examples 1 to 3 are all uniform and single bainite structures.

[0071] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present invention.

[0072] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims

1. A production method of medium and heavy steel plates, comprising hot metal pretreatment, converter smelting, LF refining, casting, slab heating, rolling, post-rolling cooling and heat treatment, characterized in that, The process conditions for rolling, post-rolling cooling, and heat treatment are as follows: The rolling includes rough rolling and finish rolling. The rolling process is as follows: The slab is rough-rolled under the conditions that the starting rolling temperature is ≥1000°C and the finishing rolling temperature is ≥920°C to obtain an intermediate billet; then the intermediate billet is finish-rolled under the conditions that the starting rolling temperature is ≤900°C and the finishing rolling temperature is 800°C - 840°C to obtain the rolled steel plate; and the cumulative deformation rate below 900°C during the rolling process is not less than 50%. The post-rolling cooling of the rolled steel plate is carried out according to the thickness of the rolled steel plate. The process conditions for post-rolling cooling are as follows: When the thickness of the rolled steel plate is less than 12 mm, the rolled steel plate is air-cooled; or when the thickness of the rolled steel plate is greater than 12 mm, the rolled steel plate is water-cooled. The heat treatment of the steel plate after post-rolling cooling is carried out. The heat treatment includes quenching and tempering. The process conditions for heat treatment are as follows: The quenching temperature is 920°C - 940°C, and the tempering temperature is 610°C - 650°C.

2. The production method of the medium and heavy steel plate according to claim 1, characterized in that, The rolling process is as follows: The slab is rough-rolled under the conditions that the starting rolling temperature is 1115°C - 1180°C and the finishing rolling temperature is ≥920°C to obtain an intermediate billet; then the intermediate billet is finish-rolled under the conditions that the starting rolling temperature is 840°C - 890°C and the finishing rolling temperature is 800°C - 840°C to obtain the rolled steel plate. And the cumulative deformation rate below 900°C during the rolling process is not less than 50%.

3. The production method of medium and heavy steel plates according to claim 1, characterized in that, The process conditions for post-rolling cooling are as follows: When the thickness of the rolled steel plate is 6 mm - 12 mm, the rolled steel plate is air-cooled; or when the thickness of the rolled steel plate is 12 mm - 50 mm, the rolled steel plate is water-cooled, and the recrystallization temperature of the rolled steel plate is 700°C - 740°C; or when the thickness of the rolled steel plate is 50 mm - 80 mm, the rolled steel plate is water-cooled, and the recrystallization temperature of the rolled steel plate is 680°C - 720°C.

4. The production method of medium and heavy steel plates according to claim 1, characterized in that, The conditions for quenching are as follows: When the thickness of the steel plate after post-rolling cooling is ≤14 mm, the quenching time is 4×H min; or when the thickness of the steel plate after post-rolling cooling is >14 mm, the quenching time is 3×H min; where H is the thickness of the steel plate after post-rolling cooling, and the unit is mm.

5. The production method of the medium and heavy steel plate according to claim 1, characterized in that The conditions for tempering are as follows: When the thickness range of the steel plate after post-rolling cooling is 6 mm ≤ thickness < 20 mm, the tempering temperature of the steel plate after post-rolling cooling is 630°C - 650°C, and the time is 1.5H + 60 min; or when the thickness range of the steel plate after post-rolling cooling is 20 mm ≤ thickness ≤ 80 mm, the tempering temperature of the steel plate after post-rolling cooling is 610°C - 630°C, and the time is 1.5H + 60 min; where H is the thickness of the steel plate after post-rolling cooling, and the unit is mm.

6. The production method of the medium and heavy steel plate according to claim 1, characterized in that, The medium and heavy steel plate comprises chemical components with the following mass percentages: C 0.15% - 0.18%, Si 0.15% - 0.35%, Mn 1.10% - 1.20%, S ≤ 0.005%, P ≤ 0.015%, Nb 0.018% - 0.025%, V ≤ 0.01%, Ti ≤ 0.01%, Als 0.020% - 0.045%, Cr 0.43% - 0.50%, Ni 0.18% - 0.25%, Mo 0.22% - 0.30%, B 0.001% - 0.0025%, the balance being Fe and unavoidable impurities, totaling 100%; the carbon equivalent CEV (%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≤ 0.56%.

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