Low-alloy high-performance super-thick steel plate under low compression ratio condition and preparation method thereof

Abstract

The present application relates to a kind of low alloy high performance extra-thick steel plate under low compression ratio condition and its preparation method, the chemical composition in steel is calculated as follows by weight percentage: C 0.13%~0.15%, Si 0.15%~0.35%, Mn 1.30%~1.50%, P:≤0.015%, S≤0.005%, Ni 0.1%~0.3%, Nb 0.01%~0.02%, Ti 0.015%~0.03%, Als 0.02%~0.03%, the rest is iron and inevitable impurities.The present application produces the high performance extra-thick steel plate of thickness 150~180mm by using continuous casting billet in 1.8~2.3 times compression ratio limit range, the tensile strength of steel plate at room temperature is ≥550MPa, yield strength is ≥370MPa;After breaking elongation is ≥28%, the impact energy of heart part-40 ℃ is ≥150J, Z direction performance is ≥60%, and flaw detection meets the requirement of GB / T2970 first flaw detection.

Description

Technical Field

[0001] This invention belongs to the field of extra-thick steel plate technology, and particularly relates to a low-alloy high-performance extra-thick steel plate under low compression ratio conditions and its preparation method. Background Technology

[0002] With the increasing demand for high-precision products in the market, the demand for extra-thick structural steel plates is growing. As the thickness of the steel plates increases, the requirements for mechanical properties such as strength, elongation, low-temperature impact resistance, and resistance to lamellar tearing become increasingly stringent. Currently, my country uses a compression ratio of more than 3 times for rolling extra-thick plates with a thickness of over 100mm. Therefore, slab production often employs a combination of ingot casting and continuous casting processes. However, this process is cumbersome, has high production costs, long delivery cycles, and many uncontrollable factors, making it unsuitable for large-scale production. While pursuing internal quality, continuous casting is a highly efficient and energy-saving option. However, the slab thickness specifications of each steel mill are fixed. Therefore, there is an urgent need for a process to produce high-performance extra-thick steel plates using existing slab thicknesses (i.e., a smaller compression ratio limit).

[0003] The patent application filed by Baosteel Zhanjiang Iron & Steel Co., Ltd., entitled "A Low Compression Ratio, Thickness, and Lamellar Tear-Resistant Steel Plate and Its Manufacturing Method" (application number: CN202010652883.X), discloses the following steel composition: C: 0.13%–0.20%, Si: 0.25%–0.40%, Mn: 1.5%–1.6%, P: ≤0.015%, S: ≤0.002%, Nb: 0.025–0.04%, Ti: 0.006–0.012%, Alt: 0.015–0.04%. This invention uses a two-stage rolling process followed by normalizing of the continuously cast billet to obtain a steel plate with excellent lamellar tear resistance. However, the steel plate thickness described in this invention is 80–90 mm, which is far lower than the steel plate thickness involved in this invention, and it does not involve the production of extra-thick plates. Furthermore, its composition design is significantly different from that of this invention.

[0004] The patent application submitted by Shandong Iron and Steel Group Rizhao Co., Ltd., entitled "A Method for Producing Low Compression Ratio Resistant Lamellar Tear-Resistant Extra-Thick Plates Based on TMCP" (application number: CN201910986955.1), specifies the following steel composition: C: 0.14%–0.16%, Si: 0.1%–0.4%, Mn: 1.3%–1.6%, P: ≤0.012%, S: ≤0.003%, Nb: 0.015–0.040%, Ti: 0.01–0.025%, Al: 0.01–0.05%. This invention employs a rolling strategy of continuous casting under heavy pressure + TMCP to roll a 270mm continuous casting billet into a 155mm steel plate. The plate exhibits a yield strength of 305–355MPa, a tensile strength of 460–510MPa, an elongation after fracture of 20–25%, a KV2 of 90–150J at -40℃, and a reduction of area of ​​26–45%. The steel plate thickness involved in the previous invention is at the lower limit of the thickness limit of this invention, resulting in lower mechanical properties and a low final rolling temperature with a high mill load. In contrast, this patent uses a one-stage high-reduction differential temperature rolling process, which has a high final rolling temperature, a low mill load, and allows for sufficient grain recrystallization to achieve good mechanical properties. Summary of the Invention

[0005] The purpose of this invention is to provide a low-alloy high-performance extra-thick steel plate and its preparation method under low compression ratio conditions. The invention utilizes continuously cast billets to produce high-performance extra-thick steel plates with a thickness of 150-180 mm within a compression ratio limit of 1.8 to 2.3 times. The steel plate has a room temperature tensile strength ≥550 MPa, a yield strength ≥370 MPa, an elongation after fracture ≥28%, a core impact absorption energy of -40℃ ≥150 J, a Z-axis performance ≥60%, and meets the GB / T2970 Class I flaw detection requirements.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] This invention strictly controls the P and S content in molten steel through converter, LF refining and RH vacuum circulation treatment, reduces the order of magnitude of inclusion size, controls the gas content in the ladle, and obtains a continuous casting billet with excellent internal quality by matching reasonable casting speed, superheat and electromagnetic stirring in the secondary cooling zone under light and heavy pressure during the continuous casting stage. The continuous casting billet is then heated and differentially rolled to refine the grains in the core of the steel plate and improve the recrystallization ratio. After rolling, water cooling and offline heat treatment are performed to obtain an extra-thick steel plate with low compression ratio and high performance produced by continuous casting billet.

[0008] The manufacturing method of this invention includes the following process: molten iron pretreatment—converter—ladle refining LF—RH—continuous casting—slow cooling—heating—differential temperature rolling—slow cooling—heat treatment—flaw detection—performance testing.

[0009] A low-alloy high-performance extra-thick steel plate under low compression ratio conditions, wherein the chemical composition of the steel, calculated by weight percentage, is: C 0.13%–0.15%, Si 0.15%–0.35%, Mn 1.30%–1.50%, P ≤0.015%, S ≤0.005%, Ni 0.1%–0.3%, Nb 0.01%–0.02%, Ti 0.015%–0.03%, Als 0.02%–0.03%, with the remainder being iron and unavoidable impurities.

[0010] The thickness of the steel plate is 150-180 mm.

[0011] The microstructure of the steel plate is ferrite + pearlite.

[0012] The steel plate has a room temperature tensile strength ≥550MPa, a yield strength ≥370MPa, an elongation after fracture ≥28%, a core impact absorption energy of -40℃ ≥150J, and a Z-direction property ≥60%.

[0013] The chemical components of this invention function as follows:

[0014] C element: Although carbon can improve the strength of steel, it also has an adverse effect on the plasticity and toughness of steel plates. The carbon content in this patent is controlled at 0.13% to 0.15%.

[0015] Silicon (Si): Silicon exists in ferrite in steel in the form of solid solution, which improves the strength of steel. However, if the content is too high, it will affect the toughness of steel and have a negative impact on the surface quality of steel plate. The silicon content in this patent is controlled at 0.15% to 0.35%.

[0016] Mn element: Manganese can expand the austenite phase region and improve the strength of steel plates in the form of solid solution. It can also combine with carbon to form alloy carbides. Excessive manganese content will increase segregation and deteriorate performance. The manganese content in this patent is controlled at 1.30% to 1.50%.

[0017] Phosphorus is a harmful element in steel, as it deteriorates the low-temperature performance of steel plates and should be controlled. Therefore, the phosphorus content in the steel of this invention is designed to be ≤0.015%.

[0018] Sulfur (S) in steel combines with manganese to form MnS inclusions, which are detrimental to the mechanical properties of the steel plate and should be controlled. Therefore, the S content in the steel of this invention is designed to be ≤0.005%.

[0019] Al: Aluminum is a good deoxidizer in steel. It combines with nitrogen to form AlN, which can inhibit grain growth and refine the grains. Therefore, the Al content in the steel of this invention is designed to be 0.02% to 0.03%.

[0020] Nitrogen (Nb) can pin austenite grain boundaries during heating, preventing grain boundary migration and refining the initial grains. During rolling, it plays a role in precipitation strengthening through strain-induced precipitation. Therefore, the Nb content in the steel of this invention is designed to be 0.01–0.02%.

[0021] Ti: Ti is a strong carbide precipitating element. It combines with S to form Ti4C2S2, reducing the formation of the hot-brittle phase FeS. In addition, the coarsening rate of TiC is lower than that of NbC, resulting in a more significant pinning effect on austenite grains. Furthermore, the co-precipitation with Nb further enhances the effect of Nb. Therefore, the Ti content in the steel of this invention is designed to be 0.015–0.03%.

[0022] Ni element: Ni can significantly improve the low-temperature performance of steel plates, but due to its high cost, its usage should be controlled. Therefore, the Ni content in the steel of this invention is designed to be 0.1% to 0.3%.

[0023] A method for preparing low-alloy high-performance extra-thick steel plates under low compression ratio conditions includes the following steps:

[0024] 1) Converter smelting: The raw materials are pretreated with hot iron, and the converter smelting adopts top and bottom blowing to ensure that P ≤ 0.01% when tapping steel.

[0025] 2) LF+RH refining: During the LF furnace refining process, white slag is produced, Al deoxidation is used, and silicon-calcium wire is fed in to modify inclusions. The LF treatment time is 30-50 minutes, and the S content at the outlet is ensured to be ≤0.005%. The RH vacuum degassing time is 15-25 minutes, and the argon blowing flow rate and time are reduced to reduce the inclusion content. After RH treatment, the hydrogen content of the molten steel is ≤1.5PPM, the oxygen content is ≤30PPM, and the nitrogen content is ≤50PPM.

[0026] 3) Continuous casting: A combination of light and heavy reduction is used to control the total reduction at 20-30 mm, the superheat is controlled at 10-15℃, and the casting speed is matched with the solidification end at 0.6-0.8 m / min. During the secondary cooling stage, electromagnetic stirring with a current intensity of 600-800 A is used for continuous stirring for 30-60 seconds to ensure that the segregation degree of the continuously cast billet does not exceed C0.5. The continuously cast billets are then slowly cooled in the slow cooling pit for at least 48 hours. The thickness of the continuously cast billet is 330-340 mm.

[0027] 4) Heating: The stacked continuous casting slabs are sent to a walking beam furnace for heating, passing through a preheating section, a heating section, and a soaking section before exiting the furnace. The temperature range of the preheating section is 750-1000℃, and the preheating time is 1.5-2 hours; the temperature range of the heating section is 1150-1240℃, and the heating time is 1.5-2 hours; the temperature range of the soaking section is 1180-1220℃, and the soaking time is 1-2 hours; the total time in the furnace is 4-6 hours to ensure the temperature uniformity of the continuous casting slabs in the furnace.

[0028] 5) Billet tapping: In order to increase the temperature difference between the surface and core of the billet during the initial rolling and to allow deformation to penetrate into the core of the steel plate, the billet is rapidly cooled after tapping. The cooling rate is 10-15℃ / s and the cooling time does not exceed 20s. This ensures that the temperature difference between the surface and core of the steel plate is 80-150℃ after cooling. The billet is then quickly sent to the rolling mill after cooling, ensuring that the interval between cooling and the start of rolling is controlled within 30s.

[0029] 6) Rolling: A differential temperature rolling strategy with high temperature, low speed and high reduction is adopted. Rolling is completed in 3 to 4 passes. The initial rolling surface temperature is 970 to 1000℃. The reduction rate of the first three passes is 18% to 23%. Since the steel plate will have a back-heating phenomenon during the rolling process, it is cooled when the thickness of the intermediate billet is 210 to 230 mm. The cooling rate is 5 to 10℃ / s and the cooling time does not exceed 10s. The cooling is to reduce the surface temperature. A rolling-water cooling coupling process is adopted to allow the deformation during rolling to penetrate into the core of the steel plate and increase the recrystallization ratio in the core. The final rolling temperature is controlled at 900 to 950℃.

[0030] 7) Post-rolling cooling: After rolling, the steel plate enters the cooling system for cooling at a rate of 3-6℃ / s and a reddening temperature of 610-630℃. After the steel plate is removed from the production line, it is stacked for slow cooling. The stacking temperature shall not be lower than 550℃ and the stacking time shall not be lower than 72 hours. This ensures that hydrogen diffuses fully in the steel plate and allows the second phase particles in the steel plate to be fully separated, ensuring that the austenite grain boundaries are pinned during normalizing and the grains are refined.

[0031] 8) Heat treatment: Heating rate 1.4–1.5 min / mm, holding temperature 850–880℃, net holding time 30–40 min; after heat treatment, rapid cooling at 4–8℃ / s to 570–620℃, then stopping rapid cooling, followed by air cooling of the steel plate. Rapid cooling is to refine the grains, while slow cooling during the air cooling stage is equivalent to self-tempering, which is beneficial for the precipitation of second-phase particles, the fragmentation of cementite, and the release of internal stress, thus having a positive effect on mechanical properties.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] This invention provides a method for manufacturing low-compression-ratio, high-performance, extra-thick steel plates using continuously cast billets. The thickness of the continuously cast billets used in production is 330-340 mm, the thickness of the finished steel plates is 150-180 mm, the room temperature tensile strength of the steel plates is ≥550 MPa, the yield strength is ≥370 MPa, the elongation after fracture is ≥28%, the impact absorption energy at -40℃ in the core is ≥150 J, and the Z-axis properties are ≥60%. Compared with the prior art, the beneficial effects are as follows:

[0034] 1) Under conditions of low content of microalloying elements such as Nb and Ti, high-performance extra-thick steel plates were obtained.

[0035] 2) This invention obtains high-quality continuous casting billets through high-purity smelting and light + heavy pressure processes. Through differential temperature rolling and heat treatment, the steel plate is ensured to have a room temperature tensile yield strength ≥370Mpa, tensile strength ≥550Mpa, elongation after fracture ≥28%, Z-direction properties ≥60%, and core impact absorption energy ≥150J at -40℃.

[0036] 3) Through the overall design of smelting + continuous casting + heating + rolling + heat treatment, a production method for low alloy high performance extra-thick steel plates under the condition of low compression ratio (1.8~2.3) was established.

[0037] 4) This invention enables steel plates with a thickness of 150-180mm to meet the requirements of Level 1 flaw detection.

[0038] 5) This invention adopts a production mode of direct heating and rolling of continuous casting billets, which eliminates the need for secondary billet opening or electroslag remelting and forging of continuous casting billets before heating, thereby reducing production processes, shortening delivery cycles, and lowering production costs.

[0039] 6) The steel plate production process is simple, stable, efficient, and highly operable. Attached Figure Description

[0040] Figure 1 The image shows the metallographic structure (ferrite + pearlite) after heat treatment in Example 1. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0042] Table 1 lists the components involved in each embodiment, Table 2 lists the smelting and continuous casting processes of each embodiment, Table 3 lists the heating and cooling processes after tapping of the furnace of each embodiment, Table 4 lists the rolling and cooling processes of each embodiment, Table 5 lists the heat treatment processes of each embodiment, and Table 6 lists the performance of each embodiment after heat treatment.

[0043] Table 1. Chemical composition (mass fraction / %) of the steel in each embodiment.

[0044] element C Si Mn P S Ni Nb Ti Als H O N Example 1 0.14 0.25 1.42 0.012 0.0038 0.23 0.018 0.025 0.027 0.00013 0.00085 0.00018 Example 2 0.13 0.28 1.43 0.013 0.0041 0.18 0.014 0.027 0.025 0.00009 0.0012 0.00026 Example 3 0.13 0.30 1.46 0.012 0.0036 0.28 0.015 0.026 0.026 0.00011 0.0010 0.00017 Example 4 0.15 0.18 1.32 0.010 0.0028 0.21 0.013 0.026 0.026 0.00010 0.00082 0.00019 Example 5 0.14 0.20 1.42 0.011 0.0033 0.11 0.016 0.023 0.022 0.00008 0.0011 0.00022 Example 6 0.15 0.17 1.36 0.014 0.0032 0.13 0.015 0.024 0.025 0.00006 0.00086 0.00015 Example 7 0.13 0.24 1.49 0.011 0.0041 0.26 0.015 0.017 0.028 0.00013 0.0011 0.00013 Example 8 0.15 0.32 1.38 0.012 0.0036 0.19 0.016 0.022 0.021 0.00006 0.0012 0.00016

[0045] Table 2 Smelting and Continuous Casting Processes of Each Example

[0046] Example 1 2 3 4 5 6 7 8 LF processing time / min 46 35 43 40 33 45 48 50 RH vacuum degassing time / min 18 19 21 15 17 18 25 23 Electromagnetic stirring current intensity / A 600 700 700 700 600 800 800 800 Electromagnetic stirring time / s 30 55 40 45 55 50 60 60 Superheat / °C 11 15 12 12 13 15 10 12 Pulling speed (m / min) 0.75 0.69 0.75 0.65 0.72 0.68 0.60 0.66 Total reduction at the end of solidification / mm 22 26 25 29 23 27 28 30 Continuous casting billet thickness / mm 338 334 335 331 337 333 332 330 Segregation of continuously cast billets C0.5 C0.5 C0.5 C0.5 C0.5 C0.5 C0.5 C0.5

[0047] Table 3 Heating and Cooling Processes After Removing Furnace from Each Embodiment

[0048]

[0049] Table 4 Rolling and Cooling Processes of Each Example

[0050] Example 1 2 3 4 5 6 7 8 Steel plate thickness / mm 150 150 160 160 170 170 180 180 Surface temperature before rolling / °C 991 986 993 974 971 985 996 986 First pass reduction / mm 60.84 60.12 60.3 59.58 60.66 63.27 59.76 59.4 Second pass reduction / mm 52.66 54.78 49.45 51.57 63.56 56.64 51.73 51.41 Third pass reduction / mm 44.9 43.82 45.05 39.57 42.56 42.62 39.69 39.45 4th pass reduction / mm 28.74 26.29 19.82 19.83 0 0 0 0 <![CDATA[Cooling rate of intermediate billet / °C·S -1 > 6.5 6.8 7.1 5.6 6.8 9.5 7.8 10 Intermediate billet cooling time / s 7 8 6 8 7 5 9 8 First pass reduction rate / % 0.18 0.18 0.18 0.19 0.18 0.19 0.18 0.18 Second pass reduction rate / % 0.19 0.20 0.18 0.19 0.23 0.21 0.19 0.19 3rd pass reduction rate / % 0.20 0.20 0.20 0.18 0.20 0.20 0.18 0.18 4th pass reduction rate / % 0.16 0.15 0.11 0 0 0 0 0 Final rolling temperature / ℃ 930 925 946 938 950 932 950 946 <![CDATA[Cooling rate after rolling / °C·S -1 > 5 4 6 4 3 4 4 3 Redness temperature / ℃ 618 625 625 617 621 625 628 624

[0051] Table 5 Heat treatment processes for each embodiment

[0052]

[0053]

[0054] Table 6. Performance of each embodiment after heat treatment

[0055]

Claims

1. A low-alloy high-performance extra-thick steel plate under low compression ratio conditions, characterized in that, The chemical composition of the steel, calculated by weight percentage, is as follows: C 0.13%–0.15%, Si 0.15%–0.35%, Mn 1.30%–1.50%, P≤0.015%, S≤0.005%, Ni 0.1%–0.3%, Nb 0.01%–0.02%, Ti 0.015%–0.03%, Als 0.02%–0.03%, with the remainder being iron and unavoidable impurities. The method for preparing low-alloy high-performance extra-thick steel plates under low compression ratio conditions includes the following steps: 1) Continuous casting: The total reduction is controlled at 20-30 mm using a combination of light and heavy reduction, the superheat is controlled at 10-15℃, the casting speed is matched with the solidification end at 0.6-0.8 m / min, and electromagnetic stirring with a current intensity of 600-800 A is used for continuous stirring for 30-60 s during the secondary cooling stage; the thickness of the continuously cast billet is 330-340 mm. 2) Heating: The temperature range of the preheating section is 750-1000℃, and the preheating time is 1.5-2h; the temperature range of the heating section is 1150-1240℃, and the heating time is 1.5-2h; the temperature range of the soaking section is 1180-1220℃, and the soaking time is 1-2h; the total furnace time is 4-6h. 3) Rolling: Rolling is completed in 3 to 4 passes. The initial rolling surface temperature is 970 to 1000℃. The reduction rate of the first three passes is 18% to 23%. When the thickness of the intermediate billet is 210 to 230 mm, it is cooled at a cooling rate of 5 to 10℃ / s and the cooling time does not exceed 10s. The final rolling temperature is controlled at 900 to 950℃. Cooling is performed after rolling. 4) Heat treatment: heating rate 1.4~1.5min / mm, holding temperature 850~880℃, net holding time 30~40min; after heat treatment, rapid cooling is carried out at 4~8℃ / s to 570~620℃ and then rapid cooling is stopped, and the steel plate is then air-cooled.

2. The low-alloy high-performance extra-thick steel plate under low compression ratio conditions according to claim 1, characterized in that, The thickness of the steel plate is 150-180 mm.

3. The low-alloy high-performance extra-thick steel plate under low compression ratio conditions according to claim 1, characterized in that, The steel plate has a room temperature tensile strength ≥550MPa, a yield strength ≥370MPa, an elongation after fracture ≥28%, an impact absorption energy of -40℃ ≥150J, a Z-axis performance ≥60%, and meets the GB / T2970 Class I flaw detection requirements.

4. The low-alloy high-performance extra-thick steel plate under low compression ratio conditions according to claim 1, characterized in that, The microstructure of the steel plate is ferrite + pearlite.

5. A method for preparing low-alloy high-performance extra-thick steel plates under low compression ratio conditions as described in any one of claims 1-4, characterized in that, The methods and steps include the following: 1) Continuous casting: The total reduction is controlled at 20-30 mm using a combination of light and heavy reduction, the superheat is controlled at 10-15℃, the casting speed is matched with the solidification end at 0.6-0.8 m / min, and electromagnetic stirring with a current intensity of 600-800 A is used for continuous stirring for 30-60 s during the secondary cooling stage; the thickness of the continuously cast billet is 330-340 mm. 2) Heating: The temperature range of the preheating section is 750-1000℃, and the preheating time is 1.5-2h; the temperature range of the heating section is 1150-1240℃, and the heating time is 1.5-2h; the temperature range of the soaking section is 1180-1220℃, and the soaking time is 1-2h; the total furnace time is 4-6h. 3) Rolling: Rolling is completed in 3 to 4 passes. The initial rolling surface temperature is 970 to 1000℃. The reduction rate of the first three passes is 18% to 23%. When the thickness of the intermediate billet is 210 to 230 mm, it is cooled at a cooling rate of 5 to 10℃ / s and the cooling time does not exceed 10s. The final rolling temperature is controlled at 900 to 950℃. Cooling is performed after rolling. 4) Heat treatment: heating rate 1.4~1.5min / mm, holding temperature 850~880℃, net holding time 30~40min; after heat treatment, rapid cooling is carried out at 4~8℃ / s to 570~620℃ and then rapid cooling is stopped, and the steel plate is then air-cooled.

6. The method for preparing low-alloy high-performance extra-thick steel plates under low compression ratio conditions according to claim 5, characterized in that, The converter smelting adopts top and bottom blowing to ensure that P ≤ 0.01% when tapping steel.

7. The method for preparing low-alloy high-performance extra-thick steel plates under low compression ratio conditions according to claim 5, characterized in that, LF+RH refining is adopted: LF treatment time is 30-50 min, and the S content is ensured to be ≤0.005% when leaving the station; RH vacuum degassing is 15-25 min, and the hydrogen content, oxygen content, and nitrogen content of molten steel are ≤1.5 PPM, ≤30 PPM, and ≤50 PPM after RH treatment.

8. The method for preparing low-alloy high-performance extra-thick steel plates under low compression ratio conditions according to claim 5, characterized in that, Billet unloading: Cooling rate 10-15℃ / s, cooling time not exceeding 20s, ensuring that the temperature difference between the surface and core of the steel plate after cooling is 80-150℃, and the interval between cooling and starting rolling is controlled within 30s.

9. The method for preparing low-alloy high-performance extra-thick steel plates under low compression ratio conditions according to claim 5, characterized in that, Post-rolling cooling: cooling rate 3-6℃ / s, reddening temperature 610-630℃; after the steel plates are removed from the production line, they are stacked and cooled slowly, the stacking temperature shall not be lower than 550℃, and the stacking time shall not be lower than 72h.

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