Ultra-low temperature 1000 mpa-grade high-strength hydropower steel plate and production method therefor

Abstract

An ultra-low temperature 1000 MPa-grade high-strength hydropower steel plate, comprising the following chemical components in weight percentage: C: 0.085-0.115%; Mn: 0.90-1.40%; Si: 0.10-0.28%; P: ≤0.015%; S: ≤0.003%; Al: 0.020-0.080%; Nb: ≤0.05%; V: 0.030-0.060%; Ti: 0.008-0.020%; Ni: 1.60-2.50%; Cr: 0.30-0.60%; Mo: 0.30-0.60%; B: 0.001-0.002%; Pcm: ≤0.26%; and the rest being Fe and impurities. The steel plate has improved yield strength and tensile strength, and meets -80°C transverse low-temperature impact requirements, the maximum thickness of the steel plate is 120 mm, and use of the steel plate at a low preheating temperature is achieved.

Description

Ultra-low temperature 1000MPa grade high-strength hydropower steel plate and production method thereof Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and in particular relates to an ultra-low temperature 1000MPa grade high-strength hydropower steel plate and a production method thereof. Background Art

[0002] 800MPa high-strength steel plates are already widely used in large-scale hydropower projects, and demand is also growing for 1000MPa-grade high-strength steel plates for larger pressure pipes. These plates must not only possess high strength but also excellent low-temperature toughness and good weldability. As hydropower projects increasingly move toward high-altitude and frigid regions, 1000MPa-grade hydropower steel plates are required to have a yield strength of 890MPa or higher and a tensile strength of 950-1150MPa. Furthermore, they must withstand -80°C transverse low-temperature impact and a low welding preheat temperature to meet the demanding requirements of hydropower project sites. However, existing 1000MPa-grade hydropower steel plates only meet the -60°C low-temperature impact requirement, failing to meet the lower -80°C low-temperature impact requirement required by project material design.

[0003] CN108385034B discloses an LGB-Q&T method for producing 1000MPa-grade hydropower steel plates no thicker than 100mm. The invention designs the steel plate's composition to be 0.12-0.21% C and 0.30-0.80%. The high C content and the addition of more than 0.30% Si alloy in the designed composition are detrimental to low-temperature impact performance. Using the LGB+Q+T process, the controlled-rolled steel plate needs to be water-cooled to 400-600°C at a cooling rate of 10-25°C / s. The controlled-rolled and controlled-cooled steel plate undergoes an offline reheating quenching and high-temperature tempering heat treatment, placing very high demands on online cooling equipment and making production process control extremely difficult. The maximum thickness produced by this LGB+Q+T process is only 80mm, and the steel plate only meets the -60°C V-type impact energy ≥70J, failing to meet the lower -80°C impact requirement.

[0004] CN108193137B discloses a DQ-Q&T method for 1000MPa-grade hydropower steel plates with a thickness of no more than 80mm. The invention adopts a DQ+Q+T process, that is, the steel plates after controlled rolling are water-cooled to room temperature with a cooling rate of 20-50°C / s. The steel plates after controlled rolling and controlled cooling are subjected to offline reheating quenching + high-temperature tempering heat treatment. The requirements for online cooling equipment are very high, and the production process control is very difficult. The maximum thickness produced by the DQ+Q+T process is only 80mm, and the steel plates only meet the -60°C V-type impact energy ≥70J, and cannot meet the lower -80°C impact requirement.

[0005] In addition, in order to ensure the high strength requirements of the steel plates after Q+T heat treatment, the existing 1000MPa grade quenched and tempered hydropower steel plates generally adopt a C ≥ 0.12% design and a composite addition method of Nb, V, Ti, Ni, Cr, Mo, and Cu alloys. There are many types of alloys and the alloy cost is high. At the same time, they are produced using processes such as LGB+Q+T and DQ+Q+T. That is, the steel plates after controlled rolling need to be quickly cooled in water to a lower temperature or water-cooled to room temperature. The requirements for online cooling equipment are very high, and the production process control is very difficult. Moreover, the maximum thickness of the steel plates produced by the above process does not exceed 100mm, which cannot meet the production needs of extra-thick plates for steel branch pipes in large hydropower projects.

[0006] Summary of the Invention

[0007] The purpose of the present invention is to solve the problems of high impact requirements, great production difficulty and unguaranteed thickness of existing high-strength hydropower steel plates. It provides an ultra-low temperature 1000MPa grade high-strength hydropower steel plate, which can effectively prevent the coarsening of steel plate grains and control carbide precipitation, improve yield strength and tensile strength, meet the lower -80℃ impact requirements while increasing the thickness of the steel plate, and improve the welding quality and welding efficiency at the construction site of hydropower engineering projects.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The invention discloses an ultra-low temperature 1000MPa grade high-strength hydropower steel plate, the chemical composition of which comprises, in weight percentage, 0.085-0.115% of C, 0.90-1.40% of Mn, 0.10-0.28% of Si, ≤0.015% of P, ≤0.003% of S, 0.020-0.080% of Al, ≤0.05% of Nb, 0.030-0.060% of V, 0.008-0.020% of Ti, 1.60-2.50% of Ni, 0.30-0.60% of Cr, 0.30-0.60% of Mo, 0.001-0.002% of B, ≤0.26% of Pcm, and the remainder being Fe and impurities.

[0010] Furthermore, the microstructure of the ultra-low temperature 1000MPa grade high-strength hydropower steel plate is tempered troostite.

[0011] Furthermore, the thickness of the ultra-low temperature 1000MPa grade high-strength hydropower steel plate is 48mm to 120mm.

[0012] The reasons for limiting the chemical composition of the present invention are as follows:

[0013] C is the main element after iron, which directly affects the strength, plasticity, toughness and welding performance of steel. In order to reduce the sensitivity of steel to welding cracks, the C content in steel is generally controlled not to exceed 0.12%. However, in order to ensure sufficient hardenability, the C content must be guaranteed to be no less than 0.08%.

[0014] Si is an important reducing agent and deoxidizer in the steelmaking process. Si can dissolve in ferrite and austenite to increase the hardness and strength of steel. However, a higher Si content will reduce the welding performance of steel. Ultra-low temperature steel has more stringent requirements on Si, and generally requires the Si content not to exceed 0.30%.

[0015] Mn is used to improve the strength of steel. Since Mn is relatively cheap and can be infinitely dissolved in Fe, it has a relatively small effect on the plasticity while improving the strength of steel. Therefore, Mn is widely used as a strengthening element in steel.

[0016] Al is added to steel as a deoxidizer or alloying element. Its primary function is to refine grains and fix nitrogen in the steel. Generally, excessive Al content is insufficient to effectively fix nitrogen and prevent the formation of BN. Increasing Al content can raise the precipitation temperature of aluminum nitride, while excessive Al addition slows down and hinders the nitridation of B. An Al content of ≥0.02% is generally recommended.

[0017] V exists in steel mainly in the form of carbides. In quenched and tempered steel, it mainly refines the grains, improves the strength and yield ratio of the steel, and increases the tempering stability of quenched steel.

[0018] Ti has a strong affinity for nitrogen, oxygen, and carbon. It is a good deoxidizer and degassing agent and an effective element for fixing nitrogen and carbon. Adding trace amounts of Ti can improve the strength of steel, and the alloy cost is very low. However, higher Ti content tends to form large particles of Ti and N precipitation, which will reduce the low-temperature toughness of the steel. Generally, the low-temperature toughness requirement at -60°C and below requires a Ti content of no more than 0.020%.

[0019] The main function of boron in steel is to increase the hardenability of the steel, thereby saving other rare and precious metals. It is used together with nickel, chromium, molybdenum, etc. Its content is generally specified in the range of 0.001% to 0.005%. It is generally necessary to add other nitrogen-fixing elements to the steel to fully utilize the hardenability effect of free boron.

[0020] The lattice constant of nickel is similar to that of γ-iron, so it can form a continuous solid solution. This helps improve the hardenability of steel. Ni can lower the critical point and increase the stability of austenite. On the one hand, it greatly improves the strength of steel, and on the other hand, it always keeps the toughness of iron at an extremely high level. Generally, low-temperature steels at -60℃ require 1.20% or more Ni, and low-temperature steels at -80℃ require 1.60% or more Ni. At the same time, a Ni content greater than 2.50% will increase the carbon equivalent and affect welding performance.

[0021] Cr can increase the hardenability of steel and has a secondary hardening effect. Its main function in the quenched and tempered structure is to improve the hardenability so that the steel has better comprehensive mechanical properties after quenching and tempering.

[0022] Mo in steel can improve hardenability and heat resistance, prevent temper brittleness, improve steel's tempering resistance or tempering stability, and enable parts to be tempered at higher temperatures, thereby more effectively eliminating or reducing residual stress and improving plasticity.

[0023] To further achieve the purpose of the present invention, a method for producing an ultra-low temperature 1000 MPa grade high-strength hydropower steel plate is provided, comprising a steelmaking process, a billet heating process, a rolling process, and a heat treatment process. The specific steps are as follows:

[0024] (1) Steelmaking process: molten steel is smelted according to the designed chemical composition and continuously cast into slabs, wherein the chemical composition by weight is as follows: C: 0.085-0.115%, Mn: 0.90-1.40%, Si: 0.10-0.28%, P≤0.015%, S≤0.003%, Al: 0.020-0.080%, Nb≤0.05%, V: 0.030-0.060%, Ti: 0.008-0.020%, Ni: 1.60-2.50%, Cr: 0.30-0.60%, Mo: 0.30-0.60%, B: 0.001-0.002%, Pcm≤0.26%, and the remainder is Fe and impurities;

[0025] (2) Heating process: The billet is heated in a heating furnace with a heating coefficient of 10.0-14.0 min / cm and a heating temperature of 1140-1160°C. Low-temperature heating is used to prevent grain growth during heating of the billet.

[0026] (3) Rolling process: A two-stage controlled rolling process is adopted. In the first stage, the total reduction ratio is ≥60%, and the final rolling temperature is ≥1000°C. In the second stage, low-temperature rolling technology is adopted, with the starting rolling temperature of 800°C to 880°C and the final rolling temperature of 780°C to 840°C. The steel plate is cooled to room temperature in air after rolling.

[0027] (4) Heat treatment process: The steel plate is put into the furnace for offline quenching + first long tempering + second rapid tempering heat treatment, wherein the quenching temperature is 880-930℃, the furnace time is 1.5-2.0 min / mm, the first tempering temperature is 500-540℃, the furnace time is 3.0-5.0 min / mm, and the second tempering temperature is 600-640℃, the furnace time is 1.5-2.0 min / mm.

[0028] Furthermore, in the step (1), dynamic soft reduction is adopted in the continuous casting, and the center segregation of the continuous casting slab does not exceed the C1.0 level.

[0029] Furthermore, in step (2), the heating coefficient is 10.2 to 12.1 min / cm, and the heating temperature is 1143 to 1156°C.

[0030] Compared with the prior art, the advantages of the technical solution of the present invention are:

[0031] (1) The ultra-low temperature 1000MPa grade high-strength hydropower steel plate of the present invention meets the steel plate yield strength ≥890MPa and tensile strength 950-1150MPa, while the steel plate -80°C transverse low-temperature impact energy ≥100J. The impact performance is better than the existing 1000MPa grade hydropower steel plate that only meets the -60°C low-temperature impact requirement, greatly meeting the lower -80°C low-temperature impact requirement required by the material design of hydropower projects in high-altitude and extremely cold areas;

[0032] (2) The steel plate of the present invention is subjected to controlled rolling after low-temperature heating and air cooling, and is subjected to quenching + first long-term tempering + second rapid tempering heat treatment to obtain an ultra-low-temperature 1000MPa grade high-strength hydropower steel plate. Compared with the existing 1000MPa grade hydropower steel plate produced by LGB+Q+T, DQ+Q+T and other processes, the steel plate of the present invention obtains tempered bainite + martensite lath after the first tempering, and the lath martensite is further transformed into tempered bainite after the second rapid tempering, and effectively prevents grain coarsening and controls carbide precipitation. The low-temperature impact value at -80°C is increased from 40-90J to more than 100J;

[0033] (3) The maximum thickness of the steel plate produced by the present invention is 120 mm. The steel plate after controlled rolling does not need to be quickly cooled to a lower temperature in water or water-cooled to room temperature, which greatly reduces the difficulty of controlling the production process. It is superior to the existing 1000 MPa grade hydropower steel plate with a maximum thickness of only 100 mm, and meets the production demand for extra-thick plates for steel branch pipes in large hydropower projects. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG1 is a metallographic structure diagram at 1 / 4 of the plate thickness in Example 3 of the present invention;

[0035] FIG2 is a table of chemical compositions of steel plates according to embodiments 1 to 4 of the present invention;

[0036] FIG3 is a table showing the mechanical properties of hydropower steel plates according to embodiments 1 to 4 of the present invention. DETAILED DESCRIPTION

[0037] Example 1

[0038] To make the present invention more clear, the following is a further description of an ultra-low temperature 1000MPa grade high-strength hydropower steel plate and its production method of the present invention in conjunction with the accompanying drawings. The specific embodiments described here are only used to explain the present invention and are not used to limit the present invention.

[0039] Taking the ultra-low temperature 1000MPa grade high-strength hydropower steel plate with a thickness of 120mm and the composition content (wt) as shown in Figure 2 as an example, the production method of this steel plate is as follows, which is characterized by:

[0040] (1) Steelmaking process: The steel was smelted according to the composition shown in Figure 2. The continuous casting process adopted dynamic soft reduction technology. The casting was 370 mm continuous casting slab, and the center segregation of the slab was C1.0.

[0041] (2) Heating process: The heating coefficient of the steel billet is 11.8 min / cm and the heating temperature is 1156°C.

[0042] (3) Rolling process: A two-stage controlled rolling process is adopted. The reduction of the last two passes in the first stage is 35 mm and 34 mm, and the final rolling temperature is 1011 °C. The starting rolling temperature in the second stage is 802 °C, and the final rolling temperature is 793 °C. The steel plates after rolling are cooled to room temperature in air.

[0043] (4) Heat treatment process: The steel plate is put into the furnace for offline heat treatment, wherein the quenching temperature is 926℃, the furnace time is 1.8 min / mm; the first tempering temperature is 523℃, the furnace time is 4.3 min / mm; the secondary tempering temperature is 615℃, the furnace time is 1.9 min / mm.

[0044] In this embodiment, the yield strength of the 120 mm thick ultra-high strength hydropower steel plate is 939 MPa, the tensile strength is 978 MPa, and the elongation after fracture is 17.5%. For specific properties, please refer to the table shown in Figure 3.

[0045] Example 2

[0046] Taking the ultra-low temperature 1000MPa grade high-strength hydropower steel plate with a thickness of 80mm and the composition content (wt) as shown in Figure 2 as an example, the production method of this steel plate is as follows, which is characterized by:

[0047] (1) Steelmaking process: The steel was smelted according to the composition shown in Figure 2. The continuous casting process adopted dynamic soft reduction technology. The casting was 370 mm continuous casting slab, and the center segregation of the slab was C1.0.

[0048] (2) Heating process: The heating coefficient of the steel billet is 12.1 min / cm and the heating temperature is 1153 °C.

[0049] (3) Rolling process: A two-stage controlled rolling process is adopted. The last two passes of the first stage have a reduction of 37 mm and 35 mm, and the final rolling temperature is 1013 °C. The second stage has a starting rolling temperature of 815 °C and a final rolling temperature of 797 °C. The steel plates after rolling are cooled to room temperature in air.

[0050] (4) Heat treatment process: The steel plate is put into the furnace for offline heat treatment, wherein the quenching temperature is 914℃, the furnace time is 1.7 min / mm; the first tempering temperature is 508℃, the furnace time is 4.6 min / mm; the secondary tempering temperature is 609℃, the furnace time is 1.8 min / mm.

[0051] In this embodiment, the yield strength of the 80 mm thick ultra-high strength hydropower steel plate is 956 MPa, the tensile strength is 991 MPa, and the elongation after fracture is 19.5%. For specific properties, please refer to the table shown in Figure 3.

[0052] Example 3

[0053] Taking the ultra-low temperature 1000MPa grade high-strength hydropower steel plate with a thickness of 56mm and the composition content (wt) as shown in Figure 2 as an example, the production method of this steel plate is as follows, which is characterized by:

[0054] (1) Steelmaking process: The steel was smelted according to the composition shown in Figure 2. The continuous casting process adopted dynamic soft reduction technology. The casting was 260 mm continuous casting slab, and the center segregation of the slab was C 0.5 level.

[0055] (2) Heating process: The heating coefficient of the steel billet is 10.6 min / cm and the heating temperature is 1151 °C.

[0056] (3) Rolling process: A two-stage controlled rolling process is adopted. The reduction of the last two passes in the first stage is 38mm and 36mm, and the final rolling temperature is 1003℃. The starting rolling temperature in the second stage is 821℃, and the final rolling temperature is 796℃. The steel plates after rolling are cooled to room temperature in air.

[0057] (4) Heat treatment process: The steel plate is put into the furnace for offline heat treatment, wherein the quenching temperature is 903℃, the furnace time is 1.7 min / mm; the first tempering temperature is 511℃, the furnace time is 4.1 min / mm; the first tempering temperature is 627℃, the furnace time is 1.8 min / mm.

[0058] In this embodiment, the yield strength of the 56 mm thick ultra-high strength hydropower steel plate is 969 MPa, the tensile strength is 981 MPa, and the elongation after fracture is 17%. Its metallographic structure is shown in FIG1 , and specific properties are shown in the table shown in FIG3 .

[0059] Example 4

[0060] Taking the ultra-low temperature 1000MPa grade high-strength hydropower steel plate with a thickness of 48mm and the composition content (wt) as shown in Figure 2 as an example, the production method of this steel plate is as follows, which is characterized by:

[0061] (1) Steelmaking process: The steel was smelted according to the composition shown in Figure 2. The continuous casting process adopted dynamic soft reduction technology. The casting was 260 mm continuous casting slab, and the center segregation of the slab was C0.5.

[0062] (2) Heating process: The heating coefficient of the steel billet is 10.2 min / cm and the heating temperature is 1143 °C.

[0063] (3) Rolling process: A two-stage controlled rolling process is adopted. The reduction of the last two passes in the first stage is 40 mm and 38 mm, and the final rolling temperature is 1006 °C. The starting rolling temperature in the second stage is 844 °C, and the final rolling temperature is 803 °C. After rolling, the steel plate is cooled to room temperature in air.

[0064] (4) Heat treatment process: The steel plate is put into the furnace for offline heat treatment, wherein the quenching temperature is 895℃, the furnace time is 1.6 min / mm; the first tempering temperature is 533℃, the furnace time is 3.6 min / mm; the first tempering temperature is 631℃, the furnace time is 1.6 min / mm.

[0065] In this embodiment, the yield strength of the 48 mm thick ultra-high strength hydropower steel plate is 977 MPa, the tensile strength is 1004 MPa, and the elongation after fracture is 16.5%. For specific properties, please refer to the table shown in Figure 3.

[0066] The present invention mainly adopts low carbon steel, Ni+Cr+Mo alloying, and V+Ti+B microalloying design, and controls the steel plate after rolling to undergo quenching + first long-term tempering + secondary rapid tempering heat treatment, effectively preventing the steel plate from coarsening grains and controlling carbide precipitation, thereby obtaining an ultra-low temperature 1000MPa grade high-strength hydropower steel plate with performance reaching yield strength ≥890MPa, tensile strength 950~1150MPa, -80℃ transverse low-temperature impact energy of the steel plate ≥100J, and a maximum thickness of the steel plate of 120mm. Due to the low carbon content and the welding cold crack sensitivity coefficient Pcm≤0.26%, the steel plate can be used at a lower preheating temperature, thereby improving the welding quality and welding efficiency at the construction site of the hydropower project, and realizing efficient production of high-strength, ultra-low temperature hydropower steel plates with good economic benefits. It is expected that the gross profit per ton of steel will be more than RMB 1,000 / ton.

[0067] In addition to the above embodiments, the present invention may also have other implementations. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. An ultra-low temperature 1000MPa grade high-strength hydropower steel plate, characterized by: Its chemical composition includes, by weight percentage, 0.085-0.115% C, 0.90-1.40% Mn, 0.10-0.28% Si, ≤0.015% P, ≤0.003% S, 0.020-0.080% Al, ≤0.05% Nb, 0.030-0.060% V, 0.008-0.020% Ti, 1.60-2.50% Ni, 0.30-0.60% Cr, 0.30-0.60% Mo, 0.001-0.002% B, ≤0.26% Pcm, and the rest is Fe and impurities.

2. The ultra-low temperature 1000MPa high-strength hydropower steel plate according to claim 1, characterized in that: The microstructure of the ultra-low temperature 1000MPa grade high-strength hydropower steel plate is tempered troostite.

3. The ultra-low temperature 1000MPa high-strength hydropower steel plate according to claim 1 or 2, characterized in that: The thickness of the ultra-low temperature 1000MPa grade high-strength hydropower steel plate is 48mm to 120mm.

4. A method for producing an ultra-low temperature 1000MPa grade high-strength hydropower steel plate as claimed in claim 1, comprising a steelmaking process, a billet heating process, a rolling process, and a heat treatment process, wherein the specific steps are as follows, and the method is characterized in that: (1) Steelmaking process: molten steel is smelted according to the designed chemical composition and continuously cast into slabs; (2) Heating process: The ingot is heated in a heating furnace, with a heating coefficient of 10.0 to 14.0 min / cm and a heating temperature of 1140 to 1160°C. Low-temperature heating is used to prevent grain growth during heating of the ingot. (3) Rolling process: A two-stage controlled rolling process is adopted. In the first stage, the total reduction ratio is ≥60%, and the final rolling temperature is ≥1000°C. In the second stage, low-temperature rolling technology is adopted, with the starting rolling temperature of 800°C to 880°C and the final rolling temperature of 780°C to 840°C. The rolled steel plate is cooled to room temperature in air; (4) Heat treatment process: The steel plate is put into the furnace for offline quenching + first long tempering + second rapid tempering heat treatment, wherein the quenching temperature is 880-930°C, the furnace time is 1.5-2.0 min / mm, the first tempering temperature is 500-540°C, the furnace time is 3.0-5.0 min / mm, and the second tempering temperature is 600-640°C, and the furnace time is 1.5-2.0 min / mm.

5. The ultra-low temperature 1000MPa high-strength hydropower steel plate according to claim 4 is characterized in that: In the step (1), dynamic soft reduction is adopted for continuous casting, and the center segregation of the continuous casting slab does not exceed the C1.0 level.

6. The method for producing ultra-high strength steel plate for hydropower according to claim 4, characterized in that: In the step (2), the heating coefficient is 10.2 to 12.1 min / cm, and the heating temperature is 1143 to 1156°C.

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