Preparation method of an economical steel plate with excellent low-temperature toughness and welding performance

Abstract

The invention discloses a preparation method of an economical steel plate with excellent low-temperature toughness and welding performance. The components of the steel plate include C, Si, Mn, Ti, Als, P, S, and the balance of Fe and impurities. When the thickness of the finished steel plate ≥ 60 mm, a trace amount of Nb is added according to the plate thickness effect; the preparation method of the steel plate includes: heating the continuous casting billet to 1040 - 1080 °C and discharging it from the furnace, adopting two-stage controlled rolling, the rolling speed in the rough rolling stage < 2.2 m / s, and the single-pass reduction amount is controlled at 35 - 40 mm; the thickness of the intermediate billet: when the thickness of the finished steel plate h < 50 mm, the thickness of the intermediate billet is controlled according to 2h + (12 - 20) mm; when the thickness of the finished steel plate h ≥ 50 mm, the thickness of the intermediate billet is controlled according to 2h - (8 - 25) mm, and the finish rolling temperature is controlled at Ar3 + (40 - 50) °C; the cooling rate is controlled at 28 - 32 °C / S; the thickness of the finished steel plate prepared by the invention reaches 100 mm, the impact at -40 °C reaches more than 260 J, the yield strength is 410 - 460 MPa, the tensile strength is 540 - 590 MPa, and the thickness direction tensile performance is more than 52%, and it can be widely applied to steel structures, especially in the wind power field.

Description

Technical Field

[0001] The present invention relates to the technical field of steel manufacturing methods, in particular to a preparation method of an economical steel plate with excellent low-temperature toughness and welding performance. Background Art

[0002] Wind power generation has outstanding advantages in reducing environmental pollution, optimizing the energy structure, etc., and has become the most commercially promising new energy industry. With the upgrading and iteration of wind power industry products and the progress of technology, wind power shows an obvious development trend of large capacity, long blades, and high towers. This also requires wind power steel to have higher strength and toughness and low welding crack sensitivity. However, as the thickness increases, the impact toughness of the steel plate will decrease. To solve this problem, steel mills spare no expense in reducing the C content, increasing precious alloys, or even through heat treatment to ensure the low-temperature toughness of the steel plate. However, adding alloys and heat treatment will increase production costs. In particular, heat treatment requires reheating in the furnace after rolling, with cumbersome processes and long cycles.

[0003] Therefore, it is of great significance to invent an economical ultra-thick steel plate with excellent low-temperature toughness and welding performance. There are many reports on low-temperature toughness steel plates in China, and patents have also been applied for. The Chinese invention patent application with the publication number CN 103014282 discloses a production method of a high-strength and good-toughness wind power steel plate, with a slab thickness of 400 mm, a low-carbon composition design, and tempering treatment after rolling. Generally speaking, the thicker the slab thickness, the greater the reduction ratio, which is more conducive to internal quality and mechanical property control. However, less than 20% of the medium and heavy plate production lines in China can reach a slab thickness of 400 mm, so this process is not widely popularizable. At the same time, tempering treatment after rolling increases costs compared to direct delivery in the as-rolled state and extends the production cycle.

[0004] The Chinese invention patent application with the publication number CN 105821302A discloses a wind power steel without Ni and excellent in low-temperature toughness and its production method. Its chemical composition includes: C: 0.10 - 0.16%, Si: 0.25 - 0.40%, Mn: 1.24 - 1.45%, P ≤ 0.015%, S ≤ 0.008%, Als: 0.030 - 0.045% or 0.015 - 0.028%, N: 0.003 - 0.005%, T: 0.03 - 0.04% or 0.015 - 0.025% or adding Nb: 0.015 - 0.035%. Its tapping temperature is 1220 - 1260 °C. On the one hand, a higher heating temperature is not conducive to energy conservation and environmental protection. On the other hand, a high heating temperature is likely to cause the original austenite grains to grow coarsely, which is not conducive to the internal quality and mechanical properties of the steel plate. In addition, it does not mention the maximum thickness that can be produced, and its C and Ti contents are at most 0.16% and 0.04% respectively, which is not conducive to welding performance. Summary of the Invention

[0005] The object of the present invention is to provide a method for preparing an economical steel plate with excellent low-temperature toughness and welding performance, aiming at the problem that in the current wind power industry, there is an urgent need for thick steel plates with low cost, high toughness and excellent welding performance, while existing steel plates cannot meet the above requirements simultaneously.

[0006] A method for preparing an economical steel plate with excellent low-temperature toughness and welding performance according to the present invention, the steel plate contains the following chemical components in mass percentage: C: 0.09 - 0.14%, Mn: 1.20 - 1.70%, Ti: 0.008 - 0.025%, Si: 0.15 - 0.4%, Als: 0.010 - 0.05%, P ≤ 0.012%, S ≤ 0.006%, the balance being Fe and unavoidable impurities, and at the same time, the residual elements are limited as Cr ≤ 0.05, Mo ≤ 0.007, V ≤ 0.006, Ni ≤ 0.02, Cu ≤ 0.032, and the carbon equivalent is 0.35 - 0.38%, where the carbon equivalent calculation formula is: C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15; when the thickness of the finished steel plate ≥ 60mm, Nb with a mass fraction of 0.012 - 0.025% is added;

[0007] The preparation method of the steel plate includes converter top-bottom combined smelting → LF furnace refining → vacuum treatment → continuous casting → slab heating → rolling mill rolling → cooling → finishing, where:

[0008] (1) Smelting process: The holding time of the white slag in LF refining is controlled at 23 - 30min; the vacuum holding pressure time ≥ 18min; the continuous casting casting speed is controlled at 0.75 - 0.80m / s, and the slab thickness is 200 - 300mm;

[0009] (2) Heating process: The slab heating temperature reaches 1040 - 1080°C, and the heating time is controlled at 250 - 300min;

[0010] (3) Rolling process: The rolling mill rolling adopts two-stage controlled rolling. In the rough rolling stage, the rolling speed < 2.2m / s, and the throwing distance after each pass is controlled at 5 - 6m, the single-pass reduction is controlled at 35 - 40mm. In the third and second passes from the end in the rough rolling stage, water cooling of the rolling mill stand is used to form a temperature gradient difference of 60°C - 80°C between the surface and the core of the steel plate; the thickness of the intermediate slab: when the thickness of the finished steel plate h < 50mm, the thickness of the intermediate slab is controlled according to 2h + (12 - 20)mm; when the thickness of the finished steel plate h ≥ 50mm, the thickness of the intermediate slab is controlled according to 2h - (8 - 25)mm; the finishing rolling temperature in the second stage is controlled at Ar3 + (40 - 50)°C, and the entire finishing rolling process shall not stop;

[0011] (4) Cooling is carried out by water cooling after rolling. The starting cooling temperature is controlled at 750 - 780 °C, the cooling rate is controlled at 28 - 32 °C / S, and the recrystallization temperature is controlled at 560 - 630 °C.

[0012] The finished steel plate prepared by the present invention has a thickness of 10 - 100 mm, the impact energy at -40 °C reaches more than 260 J, the yield strength is 410 - 460 MPa, the tensile strength is 540 - 590 MPa, the thickness direction tensile property is more than 52%, and the impact energy of each area of the welded joint at -40 °C is in the range of 86 - 222 J.

[0013] In the present invention, the thickness of the continuous casting billet is 200 - 300 mm.

[0014] In the present invention, the reasons for the limited amounts of chemical elements are as follows:

[0015] C: Carbon can significantly improve the strength of steel and increase the wear resistance of the steel plate. When the C content is lower than 0.09%, the above strengthening effect of C weakens, resulting in insufficient strength of the steel of the invention; when the C content is higher than 0.14%, it will cause an increase in the cold embrittlement transition temperature of the steel plate. For every 0.1% increase in C, the cold embrittlement transition temperature rises by about 13.9 degrees. Therefore, it is controlled at 0.12 - 0.20%.

[0016] Mn: It has a good solid solution strengthening effect, improves the strength of steel within a certain range, and can make the cementite appearing at the grain boundaries after slow cooling smaller, thereby improving the toughness of the steel. However, the Mn content cannot be too high, otherwise it will affect the decrease of the martensite transformation point, and further increase the amount of retained austenite at room temperature. Therefore, it is controlled at 1.20 - 1.70%.

[0017] Ti: Titanium is a strong deoxidizer in steel, which can make the internal structure of steel dense, refine the grains, reduce the aging sensitivity and cold brittleness, and improve the welding performance. Therefore, the designed Ti content is: 0.008 - 0.025%.

[0018] Nb: Metallic niobium is a strong carbide forming element, which has a strong effect of refining grains and precipitation strengthening. During the rolling process, the fine carbonitride particles formed by Nb can effectively inhibit the growth of austenite grains, and have a strong refining effect on the final structure of the product, thereby improving the strength, low-temperature toughness and elongation performance of the steel plate. Considering the plate thickness effect and cost, for steel plates with a thickness of ≥60 mm, it is controlled at 0.012 - 0.025%.

[0019] Als: Aluminum, as a deoxidizer and nitrogen fixer during steelmaking, refines grains, inhibits the aging of low-carbon steel, improves the toughness of steel at low temperatures, and can also improve the oxidation resistance of steel. However, if the aluminum content is too high, it will promote the graphite tendency of steel and affect the strength and toughness of steel. Therefore, Als is controlled at 0.010 - 0.050%.

[0020] Carbon equivalent: The carbon equivalent directly affects the strength and weldability of the steel plate. The higher the carbon equivalent, the poorer the weldability. When the carbon equivalent is less than 0.35%, the strength of the steel plate will be significantly reduced. Therefore, the carbon equivalent is controlled at 0.35 - 0.38%.

[0021] In the technical solution of the present invention, the principles of the hot rolling and cooling processes and the reasons for controlling the main process parameters are as follows:

[0022] (1) Adopting a relatively low heating temperature can refine the original austenite grains of the slab, and then ensure that the structure after rolling is refined, and the low-temperature impact performance of the steel plate is more excellent;

[0023] (2) The speed in the rough rolling stage is within 2.2 m / s, and the throwing distance after each pass of rolling is controlled at 5 - 6 m, and the reduction per pass is 32 - 40 mm. In the rough rolling stage, the cooling water of the rolling mill is used to form a temperature gradient difference of 60°C - 80°C between the surface and the core of the steel plate, so that the rolling force penetrates to the core, making the grain size of the core reach above grade 9.5, and at the same time improving the healing efficiency of defects such as central porosity and microcracks in the billet;

[0024] (3) Intermediate billet thickness: When the thickness h of the finished steel plate is less than 50 mm, the intermediate billet thickness is controlled according to 2h + (12 - 20) mm; when the thickness h of the finished steel plate is greater than or equal to 50 mm, the intermediate billet thickness is controlled according to 2h - (8 - 25) mm, where h is the thickness of the finished product. At the same time, the reduction rate per pass is taken into account to be not less than 16%, so that the austenite grains can be fully elongated, providing more nucleation sites for the ferrite transformation. The fine ferrite grains can significantly improve the plasticity and toughness of the steel plate;

[0025] (4) The finish rolling temperature is controlled at Ar3 + (40 - 50) °C, ensuring that the entire finish rolling process is completed within austenite. And the finish rolling temperature is relatively close to Ar3, which is easier to control the austenite grain size, providing more nucleation positions for the phase transformation during the post-rolling cooling process, and it is easier to obtain fine and uniform grains;

[0026] (5) The post-rolling cooling rate is controlled at 28 - 32 °C / S. By increasing the cooling rate, the diffusion of saturated carbon atoms is inhibited, thereby reducing the banded structure in the core of the steel plate;

[0027] The technical solution provided by the present invention uses medium and low carbon supplemented with appropriate amounts of Mn and Ti as the main strengthening alloying elements in the steel grade composition, and strictly controls the content of harmful element P; considering the plate thickness effect, only a small amount of Nb is added to ensure the strength for steel plates with a thickness of ≥60 mm, making the composition design more economical and practical.

[0028] In the present invention, the control of heating temperature, rolling speed in the rough rolling stage, casting distance, reduction per pass, and the formation of a temperature gradient difference in the thickness direction by using stand water cooling, intermediate billet thickness, finish rolling temperature, and cooling rate is particularly important. Otherwise, a fine and uniform grain structure cannot be obtained. More importantly, under the above rolling and cooling control means and process parameters, the grain size of the steel plate core can reach above grade 9.5, and there is no obvious banded structure. The thickness of the produced finished steel plate reaches 100 mm, the impact energy at -40 °C reaches above 260 J, the yield strength is between 410 and 460 MPa, the tensile strength is between 540 and 590 MPa, the tensile property in the thickness direction is above 52%, and the impact energy of each area of the welded joint is in the range of 86 - 222 J at -40 °C.

[0029] The method for preparing the ultra-thick steel plate in the present invention has the following beneficial effects:

[0030] (1) The chemical composition of the steel plate is very simple, the process flow is short, the production process is easy to operate, the production cost of the steel plate is much lower compared with that of re-heat treatment, and at the same time, the production efficiency is improved and the manufacturing cycle is shortened;

[0031] (2) Compared with other structural steel plates, the steel plate produced by the method of the present invention has excellent low-temperature toughness and welding performance, and the thickness reaches 100 mm, which can be widely used in the wind power field and belongs to high-value-added products;

[0032] (3) Using a low-cost composition design system, there is no need for major transformation of existing production equipment. Selecting a 200 - 300 mm thick continuous casting billet can be used for production, which is applicable to more than 90% of domestic production lines and has great significance for popularization. Detailed Embodiments

[0033] In order to better explain the technical solution of the present invention, the technical solution of the present invention will be further described below in conjunction with specific embodiments. The following embodiments are only used to exemplarily illustrate the technical solution of the present invention and do not limit the present invention in any form.

[0034] In practical applications, according to different steel plate production specifications and batches, there are different component contents, specific process control conditions, and corresponding mechanical property indexes within the control range. In order to better illustrate and explain the present invention, the components, process conditions, and mechanical properties of the embodiments (steel grades involved in the present invention) and comparative examples (existing steel grades) of the present invention are listed in Tables 1 to 3 for comparison.

[0035] The following Table 1 is a list of the chemical composition (wt%) values of the steel plates of each embodiment and comparative example of the present invention;

[0036] The following Table 2 is a list of the main production process parameter values of the steel plates of each embodiment and comparative example of the present invention;

[0037] Table 3 below lists the main mechanical property test results of the steel plates in the embodiments and comparative examples of the present invention;

[0038] Table 4 below lists the welding property test results of the steel plates in the embodiments and comparative examples of the present invention.

[0039] A preparation method of an economical steel plate with excellent low-temperature toughness and welding properties in the embodiments of the present invention. The steel plate contains the following chemical components in mass percentage: C: 0.09 - 0.14%, Mn: 1.20 - 1.70%, Ti: 0.008 - 0.025%, Si: 0.15 - 0.4%, Als: 0.010 - 0.05%, P ≤ 0.012%, S ≤ 0.006%, and the rest is Fe and inevitable impurities. At the same time, the residual elements are limited as Cr ≤ 0.05, Mo ≤ 0.007, V ≤ 0.006, Ni ≤ 0.02, Cu ≤ 0.032, and the carbon equivalent is 0.35 - 0.38%. The carbon equivalent calculation formula is: C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15; when the thickness of the finished steel plate ≥ 60mm, Nb with a mass fraction of 0.012 - 0.025% is added;

[0040] The preparation method of the steel plate includes combined top and bottom blowing converter smelting → LF furnace refining → vacuum treatment → continuous casting → slab heating → rolling mill rolling → cooling → finishing, where:

[0041] (1) Smelting process: The holding time of the white slag in LF refining is controlled at 23 - 30min; the vacuum holding pressure time ≥ 18min; the continuous casting casting speed is controlled at 0.75 - 0.80m / s, and the slab thickness is 200 - 300mm;

[0042] (2) Heating process: The slab heating temperature reaches 1040 - 1080°C, and the heating time is controlled at 250 - 300min;

[0043] (3) Rolling process: The rolling mill rolling adopts two-stage controlled rolling. The rolling speed in the rough rolling stage < 2.2m / s, and the throwing distance after each pass of rolling is controlled at 5 - 6m, the single-pass reduction is controlled at 35 - 40mm. In the third and second passes from the end in the rough rolling stage, water cooling of the rolling mill stand is used to form a temperature gradient difference of 60°C - 80°C between the surface and the core of the steel plate; Intermediate slab thickness: When the thickness of the finished steel plate h < 50mm, the intermediate slab thickness is controlled according to 2h + (12 - 20)mm; when the thickness of the finished steel plate h ≥ 50mm, the intermediate slab thickness is controlled according to 2h - (8 - 25)mm; The finishing rolling temperature in the second stage is controlled at Ar3 + (40 - 50)°C, and the whole finishing rolling process shall not stop;

[0044] (4) Cooling is carried out by water cooling after rolling. The starting cooling temperature is controlled at 750 - 780 °C, the cooling rate is controlled at 28 - 32 °C / S, and the recrystallization temperature is controlled at 560 - 630 °C.

[0045] Table 1 List of chemical composition (wt%) values of steel plates in each embodiment and comparative example of the present invention

[0046]

[0047] Table 2 List of main production process parameter values of steel plates in each embodiment and comparative example of the present invention

[0048]

[0049] Table 3 List of mechanical property test results of steel plates in each embodiment and comparative example of the present invention

[0050]

[0051] Table 4 List of welding property test results of steel plates in each embodiment and comparative example of the present invention

[0052]

[0053] It can be seen from Table 3 and Table 4 that by using the composition and production process designed in the present invention, the thickness of the produced finished steel plate is 10 - 100 mm, the impact at -40 °C reaches more than 260 J, the yield strength is 410 - 460 MPa, the tensile strength is 540 - 590 MPa, the tensile property in the thickness direction is more than 52%, and the impact energy of each area of the welded joint at -40 °C is in the range of 86 - 222 J, showing high impact toughness and excellent welding performance.

[0054] On the contrary, in Comparative Examples 1 and 2, the yield strength is only 360 - 378 MPa, and the impact toughness is far lower than that of the examples designed by the present invention. Especially for the 100 - mm - thick steel plate with a thick specification, the impact at -40 °C shows a cliff - like drop, and the Z - direction performance is also far lower than the values of the examples of the present invention.

[0055] The above - mentioned embodiments are only specific examples cited by the present invention to explain the present invention, and do not limit the present invention in any form. Any non - substantial change made by anyone based on the above content and form without departing from the protection scope of the claims of the present invention shall be considered to fall within the protection scope of the claims of the present invention.

Claims

1. A preparation method of an economical steel plate with excellent low-temperature toughness and welding performance, characterized in that The steel plate contains chemical components with the following mass percentages: C: 0.09 - 0.14%, Mn: 1.20 - 1.70%, Ti: 0.008 - 0.025%, Si: 0.15 - 0.4%, Als: 0.010 - 0.05%, P ≤ 0.012%, S ≤ 0.006%, and the rest is Fe and inevitable impurities. At the same time, the residual elements are limited as follows: Cr ≤ 0.05, Mo ≤ 0.007, V ≤ 0.006, Ni ≤ 0.02, Cu ≤ 0.032, and the carbon equivalent is 0.35 - 0.38%. The carbon equivalent calculation formula is: C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15; when the thickness of the finished steel plate is ≥ 60 mm, Nb with a mass fraction of 0.012 - 0.025% is added. The preparation method of the steel plate includes the following steps: (1) Smelting process: The holding time of the LF refining white slag is controlled at 23 - 30 min; the vacuum holding pressure time ≥ 18 min; the continuous casting drawing speed is controlled at 0.75 - 0.80 m / s; (2) Heating process: The casting blank time is controlled at 250 - 300 min, and the heating temperature reaches 1040 - 1080 °C; (3) Rolling process: The rolling mill adopts two-stage controlled rolling. In the rough rolling stage, the rolling speed < 2.2 m / s, and the throwing distance after each pass is controlled at 5 - 6 m, the single-pass reduction is controlled at 35 - 40 mm. In the third and second passes from the end of the rough rolling stage, water cooling of the rolling mill frame is used to form a temperature gradient difference of 60 °C - 80 °C between the surface and the core of the steel plate; the thickness of the intermediate billet: when the thickness h of the finished steel plate < 50 mm, the thickness of the intermediate billet is controlled according to 2h + (12 - 20) mm; when the thickness h of the finished steel plate ≥ 50 mm, the thickness of the intermediate billet is controlled according to 2h - (8 - 25) mm; the final rolling temperature in the second stage is controlled at Ar3 + (40 - 50) °C, and the whole finish rolling process shall not stop; (4) Cooling is carried out by water cooling after rolling. The starting cooling temperature is controlled at 750 - 780 °C, the cooling rate is controlled at 28 - 32 °C / S, and the recrystallization temperature is controlled at 560 - 630 °C.

2. The preparation method of an economical steel plate with excellent low-temperature toughness and welding performance according to claim 1, characterized in that: The thickness of the finished steel plate is 10 - 100 mm, the impact at -40 °C reaches more than 260 J, the yield strength is 410 - 460 MPa, the tensile strength is 540 - 590 MPa, the thickness direction tensile property is above 52%, and the impact energy of each area of the welded joint at -40 °C is in the range of 86 - 222 J.

3. The preparation method of an economical steel plate with excellent low-temperature toughness and welding performance according to claim 1, characterized in that: The thickness of the casting blank is 200 - 300 mm.

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