Low-manganese micro-niobium TMCP-state DH36 thick plate for offshore wind power and manufacturing method of low-manganese micro-niobium TMCP-state DH36 thick plate

Abstract

The invention discloses a low-manganese micro-niobium TMCP state DH36 thick plate for offshore wind power and a manufacturing method of the low-manganese micro-niobium TMCP state DH36 thick plate. The low-manganese micro-niobium TMCP state DH36 thick plate comprises 0.13%-0.17% of C, 0.2%-0.4% of Si, 1.1%-1.4% of Mn, smaller than or equal to 0.018% of P, smaller than or equal to 0.003% of S, smaller than or equal to 0.006% of N, 0.01%-0.03% of Nb, 0.005%-0.02% of Ti, 0.02%-0.04% of Al, 0.001%-0.003% of Ca and the balance Fe and impurity elements. The following relations are satisfied: 0.34% < = Ceq < = 0.38%, 2 < = Ti / N < = 3, Ca treatment, 1.0 < = Ca / S < = 2.5, and (% Ca) * (% S) < = 5 * 10 <-6 >. According to the manufacturing method of the thick plate, the low-manganese micro-niobium component is adopted, fine and uniform ferrite and pearlite structures are obtained in combination with the TMCP technology, and the strength and toughness required by DH36 are obtained by means of the comprehensive strength and toughness effect of solid solution strengthening, precipitation strengthening and fine grain strengthening; the welding performance of the steel plate is improved through carbonitride precipitation and grain refinement, the steel plate can support 50 KJ / cm large heat input welding on the premise that preheating before welding is not conducted and heat treatment is not conducted after welding is conducted, the welding efficiency is greatly improved, the steel plate is particularly suitable for manufacturing a TMCP state DH36 offshore wind power tower barrel, and low-cost stable batch industrial production can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of niobium microalloying and thermomechanical rolling (TMCP) reinforced steel, specifically relating to a low-manganese micro-niobium offshore wind power TMCP-processed DH36 thick plate and its manufacturing method. Background Technology

[0002] Wind power is a rapidly developing new energy industry in recent years. Thick plates are used to support wind turbines in wind turbine towers, and their service stability and cost play a crucial role in the development of the wind power industry. DH36 is currently the main type of steel for offshore wind power, delivered in normalized and TMCP states, and conforms to the standard GB / T 712-2022 "Steel for Shipbuilding and Marine Engineering". Normalized DH36 typically uses a high carbon equivalent and high alloy design, resulting in poorer weldability than TMCP DH36, which is detrimental to the long-term safe and stable service of offshore wind turbine towers. TMCP DH36 is typically produced using a combination of lower C (generally around 0.1% or even lower), medium Mn (generally around 1.5%), and microalloying strengthening elements such as Nb and Ti, combined with the TMCP process. Due to the lower C content and carbon equivalent, higher Nb content is required to improve strength and toughness through precipitation strengthening, resulting in higher overall alloy and manufacturing costs, which limits its application in the wind power field.

[0003] For example, Chinese patent CN 111155028A discloses a normalized DH36 wind turbine steel plate with a thickness of 60-100mm. Its composition by weight percentage is: C: 0.13-0.16%, Si: 0.18-0.26%, Mn: 1.4-1.5%, Al: 0.02-0.05%, Nb: 0.03-0.04%, Ti: 0.01-0.02%, P: 0.015%, S≤0.005%, N≤0.006%, with the remainder being iron and unavoidable impurities. The carbon equivalent Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15≤0.43%, produced using a normalizing process. This technology uses high carbon equivalent and high alloy composition, resulting in poorer weldability and higher alloy costs compared to TMCP process steel plates, which is detrimental to the low-cost commercial application and long-term service safety of wind turbine towers.

[0004] Chinese patent CN 113462958 A discloses a DH36 Haili wind power steel plate with the following composition by weight percentage: C: 0.07-0.11%, Si≤0.25%, Mn<1.60%, Nb: 0.010~0.030%, Ti: 0.01-0.02%, and carbon equivalent≤0.37%. The thickness of the steel plate is less than 36mm, which makes it impossible to produce steel plates with greater thickness. This makes it unsuitable for the development trend of thick wind turbine towers, and the overall alloy cost is relatively high.

[0005] Chinese patent CN 112176248 A discloses a low-carbon equivalent extra-thick DH36 offshore wind power steel plate and its production method. The composition by mass percentage is: C: 0.14-0.15%, Si: 0.43-0.46%, Mn: 1.35-1.45%, Ceq: 0.38-0.40%. It can be used to manufacture 50mm to 100mm extra-thick offshore wind power steel delivered in TMCP condition. This carbon equivalent exceeds the upper limit of the 0.38% requirement of the ≤50mm TMCP condition DH36 standard, making it impossible to produce ≤50mm TMCP condition DH36 wind power steel. Furthermore, the patent does not disclose the weldability of the developed steel plate, and its composition lacks elements such as Nb and Ti to improve weldability, and does not add Ca to improve inclusion morphology. Its weldability and long-term service stability are questionable. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a TMCP-state DH36 thick plate for low-manganese micro-niobium offshore wind power and its manufacturing method.

[0007] This invention ensures the strength and toughness of steel plates through reasonable component matching and appropriate slab heating, rolling and cooling processes, achieving lower Mn content and micro-niobium alloying content than conventional TMCP-state DH36 for offshore wind power. This not only reduces manufacturing costs but also effectively solves the problems of high cost and long manufacturing cycle of DH36 produced by normalizing process. In addition, the weldability of the product is improved through carbonitride precipitation, inclusion modification and grain refinement, making it suitable for manufacturing offshore wind turbine towers.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: On one hand, this invention provides a low-manganese, micro-niobium offshore wind power TMCP-state DH36 thick plate, the weight percentage of which is controlled as follows: C: 0.13-0.17%, Si: 0.2-0.4%, Mn: 1.1-1.4%, P≤0.018%, S≤0.003%, Nb: 0.01-0.03%, Ti: 0.005-0.02%, Al: 0.02-0.04%, Ca: 0.001-0.003%, with the remainder being Fe and impurity elements. Furthermore, it simultaneously satisfies the following conditions: 0.34%≤Ceq≤0.38%, 2≤Ti / N≤3, Ca treatment with 1.0≤Ca / S≤2.5, and (%Ca)×(%S)≤5×10⁻⁶. -6 The proportions.

[0009] This invention's technical solution starts with optimizing the alloy composition design, using medium-C, low-Mn, and micro-Nb / Ti microalloying as a foundation. For extra-thick plates >50mm, it employs ultra-low levels of harmful elements P and S, and optimizes the TMCP process, resulting in a finished steel plate with a microstructure of fine ferrite + pearlite, with an average grain size below 30μm. Specifically, this invention imposes special limitations on the remaining chemical composition of the steel, for the following reasons: 1) 0.01%≤Nb≤0.03%: Micro-niobium alloying hinders austenite grain growth during the slab heating stage and provides grain refinement and precipitation strengthening during rolling and subsequent cooling. Furthermore, micro-niobium alloys can improve weldability by pinning grain boundaries through Nb(C,N) precipitation, thus reducing grain growth in the HAZ region. Excessive niobium alloy addition increases cost and can easily lead to coarse precipitates that degrade impact and weldability; therefore, the addition amount is limited to 0.01-0.03%.

[0010] 2) 1.1%≤Mn≤1.4%: Manganese is a weak deoxidizer during smelting, and an appropriate amount of manganese in steel can effectively improve the strength of the steel. However, manganese is prone to segregation during continuous casting, and the manganese content at segregated areas can increase by more than 60%. High manganese and high carbon equivalent segregated areas are prone to forming hard structures such as lower bainite and martensite during cooling, becoming impact crack sources. In addition, the central segregation of the harmful element sulfur easily forms MnS segregation bands in the core of the steel plate. These segregated elements often form hydrogen traps, causing hydrogen-induced cracks that lead to non-compliance in flaw detection of the base plate or weld. This invention partially replaces the solid solution strengthening effect of manganese by the precipitation strengthening effect of trace niobium alloys, and at the same time relies on the post-rolling water cooling process to refine the grains of the steel plate, thereby improving the segregation of the steel plate and improving its strength and toughness.

[0011] 3) P≤0.018%, S≤0.003%: Phosphorus and sulfur are harmful elements that can significantly reduce the ductile-brittle transition temperature of steel plates and have an adverse effect on the long-term service of wind turbine towers. However, excessively low control of phosphorus and sulfur is not conducive to the reasonable management of steelmaking costs and efficiency improvement, so they are limited to below 0.018% and 0.003% respectively.

[0012] 4) 0.34%≤Ceq≤0.38%: Matching the strength and toughness of the steel plate with an appropriate carbon equivalent. An appropriate carbon equivalent is the basis for the strength of the steel plate. If the carbon equivalent is too low, more microalloying strengthening elements need to be added to obtain the performance of thick-gauge DH36; if the carbon equivalent is too high, it will reduce the toughness of the steel plate while increasing the strength, and cannot meet the impact toughness requirements of DH36.

[0013] 5) 2≤Ti / N≤3: On the one hand, Ti(C,N) particles can pin austenite grain boundaries, reducing grain coarsening in the weld heat-affected zone (HAZ) and thus improving weldability. On the other hand, it can reduce intergranular corrosion sensitivity: Ti fixes carbon and nitrogen elements, reducing carbide precipitation at grain boundaries and mitigating the risk of intergranular corrosion after welding. In this technology, the Ti / N ratio is controlled between 2 and 3 to avoid insufficient TiN precipitation due to low ratio, which would not provide adequate pinning, or excessive precipitation of coarse liquid TiN, which would degrade the impact toughness of the HAZ during welding.

[0014] 6) 1.0 ≤ Ca / S ≤ 2.5 and (% Ca) × (% S) ≤ 5 × 10 -6 Treating molten steel with Ca (Ca) spheroidizes and modulates MnS, reducing the hot brittleness of sulfur and improving the low-temperature toughness of the steel. Simultaneously, good inclusion spheroidization helps reduce the trapping effect of H by MnS segregated in the steel core, mitigating the risk of hydrogen-induced cracking and weld area defects in extra-thick DH36 steel. Too little Ca results in poor spheroidization, while too much leads to increased inclusions, easily causing defects in steel plate inspection and potentially becoming a fracture crack initiation point, reducing the steel's low-temperature toughness and weldability. Furthermore, insufficient CaS inclusion flotation can also lead to defects in steel plate inspection. On the other hand, the present invention provides a method for manufacturing a TMCP-state DH36 thick plate for low-manganese micro-niobium offshore wind power, characterized by the following steps.

[0015] 1) Material preparation: Prepare each component according to the ratio.

[0016] 2) Steelmaking and Continuous Casting: The prepared components are smelted in a converter and continuously cast into slabs. During steelmaking, the molten iron is treated with KR to remove sulfur to below 0.005% before entering the converter for smelting, where phosphorus is removed to below 0.01%. It is then refined in an LF furnace to remove sulfur to below 0.002%, and further sent to an RH furnace for degassing, where O, H, and N are removed to below 0.003%, 0.0002%, and 0.004%, respectively. Finally, it is cast in a continuous casting machine to obtain slabs with a thickness of 230mm. During casting, the superheat is controlled between 18-30℃, electromagnetic stirring is used to homogenize the composition, and dynamic light pressure is used to reduce slab segregation; ensuring that the center segregation of the slab is ≤M3.0 grade.

[0017] 3) Heating: The time the slab is in the furnace is controlled at about 1 min / mm, and the furnace exit temperature is controlled between 1100-1150℃ to ensure that the niobium microalloy is fully dissolved in the austenite, while avoiding excessive coarsening of the original austenite grains and overheating of the slab surface.

[0018] 4) Rolling: The initial rolling temperature in the roughing stage is above 1050℃, and the reduction rate of the last three passes is above 10%. After rolling to the intermediate thickness, the plate is left to warm on the roller table to ensure that the large reduction in the roughing stage can penetrate into the core of the steel plate. In the finishing rolling stage, the austenite recrystallization zone is avoided, and the finishing rolling is completed between 750-900℃, with the reduction in the last pass being >8mm. Avoiding the austenite recrystallization zone during the rolling process can prevent mixed crystals.

[0019] 5) Cooling: The rolled steel plate is brought into the MULPIC equipment at a temperature 10-50°C above the Ar3 temperature for accelerated cooling. The cooling rate is between 10-40°C / s depending on the thickness of the steel plate, and the final cooling temperature is between 400-700°C.

[0020] Ar3 is calculated at the following temperatures: 910-310(%C)-80(%Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%Mo)-0.35(t-8), where t is the final thickness of the steel plate. Depending on the thickness specifications, the final cooling temperature is between 400-700℃, and the post-heat straightening reddening temperature is above 500℃ to avoid producing high-carbon bainitic hard phases that could affect the toughness and weldability of the steel plate. The final microstructure of the steel plate consists of fine and uniform ferrite and pearlite.

[0021] 6) Stacking and slow cooling: After the steel plates with a thickness of ≥30mm are cooled by the MULPIC equipment, they are taken off the line and stacked and slow cooled within 20 minutes. The stacking temperature is above 300℃ and the slow cooling time is more than 24 hours. For steel plates with a thickness of less than 30mm, slow cooling is not required; proceed directly to step 7). 7) Flaw detection: After cooling or slow cooling is completed, the steel plate is inspected for flaws using online or manual flaw detection equipment; 8) Cutting: After the steel plate is cut to length by shearing line or fire cutting, it is put into the finished product warehouse.

[0022] Compared with the prior art, the advantages of the present invention are as follows: This invention achieves the required strength and toughness of DH36 by optimizing the matching of medium carbon, low manganese, micro niobium, titanium alloying, low phosphorus, and sulfur harmful elements, combined with the TMCP process to obtain fine and uniform ferrite and pearlite structures. It relies on the comprehensive strength and toughness effects of solid solution strengthening, precipitation strengthening, and fine grain strengthening to form low manganese micro niobium TMCP state DH36 thick plates covering thicknesses of 12-80mm.

[0023] On the one hand, niobium and titanium microalloying and post-rolling water cooling processes reduce the manganese content and carbon equivalent of the steel plate, which can improve the weldability of the steel. On the other hand, the refined base metal structure, the reduction of inclusions in the steel, the spheroidization effect, and the precipitation of fine and uniform carbonitrides can further improve the weldability of the steel plate. This ensures that after welding with a high heat input of 50 kJ / cm without preheating before welding or heat treatment after welding, the weld and HAZ zone still meet the strength and toughness requirements of DH36. It is particularly suitable for manufacturing TMCP DH36 for offshore wind power and can achieve low-cost, stable mass industrial production. Attached Figure Description

[0024] The present invention will now be described in further detail with reference to the accompanying drawings.

[0025] Figure 1 Metallographic photograph of the finished steel plate produced according to an embodiment of the present invention. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to the embodiments.

[0027] This invention discloses a TMCP-state DH36 thick plate for offshore wind power with low manganese and micro-niobium content, comprising the following components by mass percentage: C: 0.13-0.17%, Si: 0.2-0.4%, Mn: 1.1-1.4%, P≤0.018%, S≤0.003%, Nb: 0.01-0.03%, Ti: 0.005-0.02%, Al: 0.02-0.04%, Ca: 0.001-0.003%, with the remainder being Fe and impurity elements; simultaneously satisfying the following relationships: 0.34%≤Ceq≤0.38%, 2≤Ti / N≤3, 1.0≤Ca / S≤2.5 and (%Ca)×(%S)≤5×10 -6 .

[0028] This invention discloses a method for manufacturing a TMCP-processed DH36 thick plate for offshore wind power with low manganese and micro-niobium content. The method achieves a good balance of strength and toughness by combining the solid solution strengthening effect of appropriate carbon equivalent, the precipitation strengthening effect of trace alloying elements, and the grain refinement effect of the TMCP process. Furthermore, the weldability of the steel plate is improved through grain refinement, sulfide inclusion modification, and the pinning effect of nano-carbonitrides. The specific steps include: The following preferred composition (wt%) was prepared: C: 0.15, Si: 0.28, Mn: 1.31, P: 0.016, S: 0.002, Nb: 0.016, Ti: 0.012, Al: 0.03, Ca: 0.0025, with the remainder being Fe and impurity elements.

[0029] 2) After being treated with KR to remove sulfur to below 0.005%, the molten iron enters the converter for smelting, where phosphorus is removed to below 0.01%. It is then sent to the LF furnace for refining to remove sulfur to below 0.002%, and finally to the RH furnace for further refining and degassing, where O, H, and N are removed to below 0.003%, 0.0002%, and 0.004%, respectively. Vacuum-protected casting is employed, with superheat limited to 18-30℃ during casting. A constant casting speed is maintained, with a pulling speed of approximately 1.0 m / min. Electromagnetic stirring is used in the crystallizer during casting. At the end of the billet solidification process, a light reduction of 4-6 mm is applied to reduce center segregation.

[0030] 3) Slab heating: In order to meet the requirements of sufficient solid solution of microalloying elements and avoid excessive coarsening of austenite grains during heating, the slab heating temperature is controlled at about 1140℃ and the furnace time is 220 minutes.

[0031] 4) Controlled rolling: The finished product thickness is 40mm. Two-stage controlled rolling is used. In the first stage, rough rolling with large reduction is carried out at a temperature above 1050℃ to a thickness of 120mm, and the reduction rate of the last three passes is above 10%. In the second stage, controlled rolling is carried out at around 870℃ in the non-recrystallization region of austenite to avoid mixed crystals, and the final rolling temperature is around 800℃.

[0032] 5) Controlled cooling: The MULPIC equipment is used to accelerate cooling. The inlet water temperature is about 760℃, the target cooling rate is 25℃ / s, and the final cooling target temperature is 580℃.

[0033] 6) Stacking and slow cooling: After the steel plates with a thickness of ≥30mm are cooled by the MULPIC equipment, they are taken off the line and stacked and slow cooled within 20 minutes. The stacking temperature is above 300℃ and the slow cooling time is more than 24 hours. For steel plates with a thickness of less than 30mm, slow cooling is not required; proceed directly to step 7). 7) Flaw detection: After cooling or slow cooling is completed, the steel plate is inspected for flaws using online or manual flaw detection equipment.

[0034] 8) Cutting: After the steel plate is cut to length by shearing line or fire cutting, it is put into the finished product warehouse.

[0035] The heating, rolling, and cooling processes of the steel plate in this embodiment are shown in Table 1. The properties of the steel plate in this embodiment are shown in Tables 2 and 3. The microstructure of the steel plate in this embodiment is as follows: Figure 1 As shown. The weldability evaluation of the steel plate was conducted using submerged arc welding with a 1 / 2 V-groove and a welding heat input of 50±2 kJ / cm.

[0036] Table 1. Heating, rolling, and cooling processes in the examples. .

[0037] Table 2 Rolled properties of the examples .

[0038] Table 3 Post-welding performance of the examples .

Claims

1. A TMCP-state DH36 thick plate for low-manganese, low-niobium offshore wind power, characterized in that, It contains the following components by mass percentage: C: 0.13-0.17%, Si: 0.2-0.4%, Mn: 1.1-1.4%, P≤0.018%, S≤0.003%, N≤0.006%, Nb: 0.01-0.03%, Ti: 0.005-0.02%, Al: 0.02-0.04%, Ca: 0.001-0.003%, with the remainder being Fe and impurity elements; and simultaneously satisfying the following relationships: 0.34%≤Ceq≤0.38%, 2≤Ti / N≤3, Ca treated and 1.0≤Ca / S≤2.5 and (%Ca)×(%S)≤5×10 -6 .

2. The low-manganese, micro-niobium offshore wind power TMCP-state DH36 thick plate according to claim 1, characterized in that, The microstructure of the finished thick plate is fine ferrite + pearlite, with an average grain size of less than 30 μm.

3. The low-manganese, micro-niobium offshore wind power TMCP-state DH36 thick plate according to claim 1, characterized in that, The thick plate can support high heat input welding of 50KJ / cm without preheating before welding or heat preservation after welding.

4. A method for manufacturing a TMCP-state DH36 thick plate for low-manganese, micro-niobium offshore wind power as described in claim 1, characterized in that, Includes the following steps: 1) Material preparation: Prepare each component according to the formula; 2) Steelmaking: The prepared components are smelted in a converter and continuously cast into slabs; during the steelmaking process, the molten iron is treated by KR to remove sulfur to below 0.005% and then enters the converter for smelting, where phosphorus is removed to below 0.01%, and then refined in an LF furnace to remove sulfur to below 0.002%, before being sent to RH for refining and degassing, where O, H, and N are removed to below 0.003%, 0.0002%, and 0.004%, respectively; during the casting process, the superheat is controlled at 18-30℃, electromagnetic stirring is used to homogenize the composition, and dynamic light pressure is used to reduce the degree of segregation in the slabs; 3) Heating: The time the slab is in the furnace is controlled at 0.8-1.20 min / mm, and the furnace exit temperature is controlled between 1100-1150℃; 4) Rolling: The slab is rolled in two stages. The initial rolling temperature of the roughing stage is above 1050℃, and the reduction rate of the last three passes is above 10%. After rolling to the intermediate thickness, it is left to warm on the roller table. The finishing stage is rolled in the austenitic non-recrystallization region between 750-900℃, and the reduction of the last pass is >8mm. 5) Cooling: The rolled steel plate is transported to the MULPIC equipment for accelerated cooling. The cooling rate is between 10-40℃ / s depending on the thickness of the steel plate, and the final cooling temperature is between 400-700℃. 6) Slow cooling: After the steel plates with a thickness of ≥30mm are cooled by the MULPIC equipment, they are taken off the production line and stacked for slow cooling within 20 minutes. The stacking temperature is above 300℃ and the slow cooling time is above 24 hours. For steel plates with a thickness of less than 30mm, slow cooling is not required; proceed directly to step 7). 7) Flaw detection: After cooling or slow cooling is completed, the steel plate is inspected for flaws using online or manual flaw detection equipment; 8) Cutting: After the steel plate is cut to length by shearing line or fire cutting, it is put into the finished product warehouse.

5. The manufacturing method according to claim 4, characterized in that, The thick plate manufactured by the above method can support high heat input welding of 50 KJ / cm without preheating before welding and without heat preservation after welding.

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