A martensitic / austenitic dual-phase ultra-high strength steel plate and its manufacturing method

Abstract

This invention relates to a martensitic / austenitic dual-phase ultra-high strength steel plate and its manufacturing method. A niobium-microalloyed 9Ni steel continuous casting billet with a diameter ≥150mm is selected and its surface is cleaned. The billet is then fed into a walking beam furnace, with the tapping temperature maintained at 1200–1220℃. The initial rolling temperature of the billet is between 1050–1100℃, and high-temperature rolling is employed for finishing, with an initial rolling temperature between 890–920℃ and a single-pass reduction rate of ≤10%. The final rolling temperature is ≥840℃, and the immersion temperature is ≥820℃. After rolling, the steel is cooled to room temperature via ACC+DQ at a rate of 15–25℃ / s. A martensitic / austenitic multiphase ultra-high-strength steel plate was obtained, with an austenitic phase volume fraction of 5-10%, yield strength ≥900MPa, tensile strength ≥1000MPa, yield-to-tensile ratio ≤0.90, elongation ≥12%, and impact toughness ≥90J at -196℃. Ultrasonic testing according to EN 10160 standard showed the plate body area meeting Class S3 requirements and the edge area meeting Class E4 requirements. The steel plate exhibits excellent surface quality and good plate shape, offering advantages for mass production and application.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a martensitic / austenitic dual-phase ultra-high strength steel plate and its manufacturing method. Background Technology

[0002] Low-alloy ultra-high-strength steel plates are mainly used in machinery and equipment manufacturing. To improve the overall level and market competitiveness of China's machinery manufacturing industry and meet the needs of large-scale and lightweight engineering machinery development, the strength and thickness of ultra-high-strength steel plates are constantly being improved. Due to the expanding scope of large-scale applications, steel plates are not only developing towards thicker specifications, but also requiring higher strength and low-temperature resistance. Currently, the low-temperature toughness of delivered 1000MPa yield strength steel only reaches -60℃, and there is no substantial production for even lower temperatures. Furthermore, the hydrogen-induced delayed cracking of ultra-high-strength steel plates themselves and welded joints has not been effectively solved. Most of the low-alloy ultra-high-strength steels developed so far are produced through quenching and tempering or Q+P (Quenched and Partitioned) production, resulting in tempered sorbitic or martensitic steels. Regardless of whether the sorbite crystal structure is body-centered cubic ferrite or body-centered tetragonal martensite, the hydrogen solubility is low, making it easy for hydrogen to accumulate in the cementite film, leading to hydrogen-induced cracking. The steel plate has poor weldability and is prone to cold cracking after welding. Simultaneously, due to its high strength, the preheating temperature cannot be too high, otherwise it will severely reduce the strength of the steel plate. However, the cold cracking sensitivity of ultra-high strength steel requires the preheating temperature to be as high as possible. This contradiction presents a significant challenge to welders. Secondly, the control of welding heat input is extremely strict. Excessive heat input will cause severe softening of the heat-affected zone, thus significantly reducing the joint strength; insufficient heat input will result in poor weld formation, or even make welding impossible.

[0003] In the mid-20th century, researchers recognized that retained austenite in quenched steel could improve its plasticity and toughness. For example, a thin film of retained austenite several nanometers thick exists in strip-shaped martensite, increasing toughness. Utilizing the thermal stabilization phenomenon of austenite, quenching and tempering heat treatment processes for tool steels were proposed. Experiments showed that hydrogen embrittlement cracking is hindered by fcc austenite. Xu Zuyao's calculations proved that carbon diffuses from martensite to retained austenite during the quenching of low-carbon steel. Speer et al. proposed the QP heat treatment process for steel, which involves quenching (Q) to between Ms and Mf, followed by holding at a certain temperature to allow carbon to distribute from martensite (P) to austenite, stabilizing a certain amount of austenite at room temperature to ensure toughness. To inhibit the precipitation of hard cementite, they designed a Q+P steel containing 1-2% Si. Based on the Q+P heat treatment process for steel proposed by Speer et al., Xu Zuyao introduced a precipitation hardening mechanism, which involves adding carbon compound forming elements to the steel. After quenching, carbon is distributed, and dispersed complex carbides precipitate within the martensite, resulting in a combination of high strength and toughness. Examples of Q+P technology include: Chinese patent CN104532156A, which reports an ultra-high-strength steel plate with a yield strength of 1400 MPa produced using QP, characterized by ultrafine lath martensite, nanoscale lath retained austenite, and precipitated carbides; and Chinese patent CN 102925801A, which describes an ultra-high-strength steel plate with a yield strength of 1200 MPa obtained using QP technology. However, because the P process in QP is difficult to achieve in actual industrial production, and the complex phases are difficult to control precisely and stably, mass production is very challenging and industrial-scale production is difficult to realize. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a martensitic / austenitic multiphase ultra-high strength steel plate and its manufacturing method in light of the above-mentioned prior art. The invention selects the composition system of niobium micro-alloyed 9Ni steel and produces the martensitic / austenitic multiphase ultra-high strength steel plate without changing the smelting of Nb micro-alloyed 9Ni steel.

[0005] The technical solution adopted by this invention to solve the above problems is as follows: a martensitic / austenitic multiphase ultra-high strength steel plate, selected from niobium micro-alloyed 9Ni steel. Since the elements C, Mn, Ni, and Nb in steel are one of the key factors affecting the formation of martensitic / austenitic multiphase, in order to achieve impact toughness of -196℃ and obtain ultra-high strength steel, this invention specifies the main element chemical composition of steel C, Mn, Ni, and Nb, wherein the main elements are C: 0.02-0.05%, Mn: 0.60-0.80%, Ni: 8.80-9.20%, Nb: 0.010-0.020%, and the balance is Fe and unavoidable impurities.

[0006] The design principles of the main elements of the steel in this invention are as follows:

[0007] The addition of carbon can increase the hardenability of steel, especially in the production of medium and heavy plates, and can significantly improve the strength. However, excessive carbon content is detrimental to the steel's low-temperature impact resistance, low-temperature strain aging performance, weldability, and corrosion resistance. Therefore, the carbon content in this invention is controlled between 0.02% and 0.05%.

[0008] Manganese (Mn) is an element that improves the hardenability of steel and plays a solid solution strengthening role to compensate for the strength loss caused by the reduction of carbon (C) content in the steel. When the Mn content in the steel is too low, it cannot fully play its role in ensuring strength, but when the Mn content is too high, it will increase its carbon equivalent, thereby impairing weldability. In addition, Mn tends to segregate in the center of the steel plate, reducing the impact toughness of the central part of the steel plate. Therefore, the Mn content in this invention is controlled at 0.60% to 0.80%.

[0009] Ni is an element that improves the hardenability of steel plates and can significantly enhance their low-temperature toughness. It also has a positive impact on impact toughness and the ductile-brittle transition temperature. However, when the Ni content is too high, a highly adhesive iron oxide scale easily forms on the slab surface, which is difficult to remove and affects the surface quality of the steel plate. In addition, Ni is also a precious metal, and excessive content will increase costs. Therefore, this invention controls its content at 8.80%–9.00% while meeting the requirements of classification society specifications, European standards, and American standards, which is beneficial for achieving optimal cost-effectiveness.

[0010] The solute dragging effect of Nb and the pinning effect of Nb(C,N) on austenite grain boundaries both inhibit the recrystallization of deformed austenite, expanding the non-recrystallization range of austenite. This can increase the starting rolling temperature of finishing rolling, ensure the steel plate shape, facilitate high-temperature rolling, and avoid the temperature range where secondary iron oxide scale forms. However, excessive Nb can also lead to the formation of fine grains in the steel plate and increase the yield strength ratio. Therefore, this invention controls its content within a narrow range of 0.010–0.020%.

[0011] This invention provides a method for manufacturing the aforementioned martensitic / austenitic dual-phase ultra-high-strength steel plate. A niobium-microalloyed 9Ni steel continuous casting billet with a thickness of ≥150mm is selected. The upper and lower surfaces of the billet are ground down by 1.5mm using a surface grinding machine. The billet is then placed in a walking beam furnace at an average heating rate of 10–14 cm / min, heated to 1220–1240℃. When the surface temperature reaches half the thickness of the steel plate, a holding time of at least 45 minutes is initiated to ensure sufficient solid solution of the alloying elements in the steel, guaranteeing the uniformity of the composition and properties of the final product. The heating time at 600–900℃ is controlled to be ≥0.35 min / mm, and the tapping temperature is maintained at 1200–1220℃. After tapping, the billet undergoes high-pressure water descaling and is then subjected to a two-stage roughing and finishing rolling process on a medium-thick plate mill. The initial rolling temperature of the roughing rolling is controlled to be between 1050–1100℃, and the reduction rate after the three passes of roughing rolling is ≥15%. The thickness at the initial temperature is ≥1.7H, where H is the finished thickness. Finishing rolling is performed at high temperatures, with an initial rolling temperature between 890 and 920℃ and a single-pass reduction rate ≤10%. The final rolling temperature is ≥840℃, and the water immersion temperature is ≥820℃. After rolling, the steel plate is cooled to room temperature at 15–25℃ / s using an accelerated cooling (ACC) and direct online quenching (DQ) unit.

[0012] Preferably, the rolling compression ratio of the continuous casting billet to the finished steel plate thickness is ≥3.5.

[0013] Compared with the prior art, the advantages of the present invention are as follows:

[0014] This invention selects a niobium microalloyed 9Ni steel composition system. Ni, with its heat-resistant, cold-resistant, and corrosion-resistant properties, plays a significant role in improving the service life of high-strength steel plates. Increasing Ni significantly enhances the low-temperature performance of the steel plate, allowing it to be used in a temperature range of room temperature to -196℃. Niobium microalloying enables high-temperature rolling and effectively reduces segregation problems in high-Ni alloy steels. It also ensures that the grains do not become coarse during high-temperature rolling. Furthermore, the addition of trace amounts of Nb significantly improves the strength of the high-Ni alloy steel plate.

[0015] This invention employs high-temperature rolling for finishing, with an initial rolling temperature between 890 and 920°C and a single-pass reduction rate of ≤10%. This reduces segregation in high-Ni alloy steel while effectively lowering the residual stress in the steel plate before ACC+DQ, which is beneficial for ensuring the flatness of thin and wide steel plates.

[0016] This invention uses ACC+DQ heat treatment to obtain ultra-high strength steel plates, which is different from the previous TMCP+QT treatment of low alloy ultra-high strength steel, and eliminates the heat treatment processes of quenching and tempering.

[0017] This invention, through the selection of the composition system and process design, ultimately obtains a martensitic / austenitic multiphase structure, which reduces the sensitivity to hydrogen embrittlement. In terms of process, it retains the existing smelting process of niobium microalloyed 9Ni steel, achieves low-H treatment, and the steel plate itself has a low H content, which increases the adaptability of the steel plate to unavoidable H-prone environments such as near-shore, offshore, and rainy environments.

[0018] This invention employs a high-temperature controlled rolling process, followed by ACC+DQ (Adjustable Chromatography and Dielectric Quality) to produce martensitic / austenitic multiphase ultra-high-strength steel plates with thicknesses ranging from 15 to 40 mm, yield strength ≥900 MPa, tensile strength ≥1000 MPa, yield-to-tensile ratio ≤0.90, elongation ≥12%, and impact toughness ≥90 J at -196℃. Ultrasonic testing is performed according to EN 10160 standards, achieving Class S3 requirements for the plate body and Class E4 requirements for the edge areas. The steel plates exhibit excellent surface quality and good shape, offering advantages for mass production and application.

[0019] The ultra-high strength steel plate of this invention has a uniform martensite / austenite multiphase structure, with the austenite phase accounting for 5-10% of the volume. This structure is different from the bainitic structure of hot-rolled 9Ni steel and also different from the tempered sorbite structure of finished products. Attached Figure Description

[0020] Figure 1 This is a microstructure image of the metallographic structure at 1 / 4 thickness in Example 3 of the present invention. The original austenite grains are small and uniform in size, with an average grain size of ≤20μm.

[0021] Figure 2 This is a rolling microstructure diagram at 1 / 4 of the thickness of the comparative figure. The microstructure is a complex structure of bainite and martensite with severe segregation.

[0022] Figure 3 The image shows the microstructure after tempering at 1 / 4 of the thickness, which is tempered sorbite. Severe segregation is still visible after quenching.

[0023] Figure 4 In Example 3 of this invention, X-ray diffraction was performed at a thickness of 1 / 4. The test results showed that the volume percentage of martensite phase in the microstructure was 92% and that of austenite phase was 8%.

[0024] Figure 5 This is a diagram showing the steel plate shape after DQ in Embodiment 3 of the present invention. The plate shape is not affected by the DQ process and is in good condition.

[0025] Figure 6 The HV10 Vickers hardness test chart of the steel plate after DQ in Embodiment 3 of the present invention shows that the hardness value along the thickness direction does not change significantly after DQ. Detailed Implementation

[0026] The technical solution of the present invention will be described in more detail with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the invention.

[0027] The production process of a martensitic / austenitic dual-phase ultra-high strength steel plate of the present invention is as follows: converter steelmaking -> RH vacuum degassing -> LF refining -> Ca treatment -> RH high vacuum degassing -> continuous casting -> billet slow cooling treatment -> billet surface cleaning -> heating -> rolling -> ACC -> DQ.

[0028] A method for producing a martensitic / austenitic dual-phase ultra-high strength steel plate according to Embodiments 1-4 of the present invention includes the following steps:

[0029] (1) Five continuous casting billets of niobium microalloyed 9Ni steel were selected from two heat numbers. The thickness of the selected continuous casting billets was 150mm. The composition met the requirements of the classification society and GB / T 37602 9Ni steel. The chemical composition of the martensitic / austenitic dual phase ultra-high strength steel plates corresponding to each embodiment and comparative example is shown in Table 1. The data in the table are the weight percentage (wt,%) of each element. The remainder is Fe and unavoidable impurity elements.

[0030] (2) Grind and clean the billet with a machine, removing 1.5mm from both the upper and lower surfaces. Then, put the continuously cast billet into a walking beam furnace, with an average heating rate of 10-14cm / min, and heat it to 1220-1240℃. When the surface temperature is reached at 1 / 2 of the steel plate thickness, start holding it at that temperature for no less than 1 hour. Control the heating time at 600-900℃ to be ≥0.35min / mm, and ensure the tapping temperature is 1200-1220℃.

[0031] (3) After the billet exits the furnace, it undergoes high-pressure water descaling followed by two-stage controlled rolling: roughing and finishing. The initial rolling temperature for roughing is between 1080-1150℃, and the reduction rate for the three passes after roughing is ≥15%. The thickness after cooling is ≥1.7H, where H is the finished thickness. Finishing is carried out at high temperature, with an initial rolling temperature between 890 and 920℃, and a single-pass reduction rate of ≤10%. The final rolling temperature is ≥840℃, and the water immersion temperature is ≥820℃. After rolling, the steel plate is cooled to room temperature by ACC+DQ at 15-25℃ / s.

[0032] (4) After DQ, the steel plate undergoes transverse tensile testing, transverse impact testing, metallographic inspection, and hardness testing in the thickness direction.

[0033] (5) The diffraction pattern of the sample at 1 / 4 of the thickness of Example 3 was measured on an X-ray diffractometer (XRD). The area of ​​each diffraction peak was calculated by Jade analysis software to obtain the amount of martensite and austenite in the sample.

[0034] The specific ingredients and process parameters are shown in Tables 1 and 2.

[0035] The properties of the steel plates in each embodiment are shown in Tables 3 to 5.

[0036] The XRD test results of each diffraction peak at 1 / 4 thickness in the examples show that the volume ratio of austenite phase is between 5% and 10%. Table 6 gives the diffraction peak information of Example 3.

[0037] Figures 1-3 Typical metallographic morphologies of the embodiments and comparative examples of rolled and finished microstructures are presented. Figure 1 The image shows the microstructure at 1 / 4 of the thickness in Example 3. Based on the XRD test results above, the microstructure of the steel plate is martensite / austenite with no obvious segregation bands. Figure 2 The comparative sample at 1 / 4 of the thickness shows the rolled microstructure of the same composition system, which is a bainite and martensite multiphase structure with severe segregation. Figure 3 The microstructure of the comparative example after tempering is tempered sorbite, and severe segregation is still visible after quenching. It is evident that the microstructure of this example differs significantly from that of the comparative example in the rolled state and after tempering.

[0038] Figure 4 The XRD scan pattern at 1 / 4 thickness in Example 3 is given.

[0039] Figure 5 The appearance morphology after 3DQ in Example 3 is given.

[0040] Figure 6 The test results of HV along the thickness direction of the steel plate after DQ in Example 3 are given.

[0041] Table 1. Chemical composition (wt%) of martensitic / austenitic multiphase ultra-high strength steel plates from the examples.

[0042] Example C Si Mn Ni Nb 1 0.039 0.16 0.69 8.9 0.015 2 0.045 0.17 0.7 8.9 0.012 3 0.039 0.17 0.7 9 0.013 4 0.038 0.16 0.7 8.9 0.011

[0043] Table 2 Rolling and ACC+DQ Process Control

[0044]

[0045] Table 3 Transverse tensile properties of embodiments of the present invention

[0046]

[0047] Table 4 Lateral impact performance of embodiments of the present invention

[0048]

[0049]

[0050] Table 5 Transverse resistance to brittle fracture in embodiments of the present invention

[0051]

[0052] Table 6. XRD test results at 1 / 4 thickness of the steel plate in Example 3 of the present invention (see attached graph). Figure 4 )

[0053]

[0054] This invention employs a niobium microalloyed 9Ni steel composition system, which undergoes high-temperature controlled rolling followed by ACC+DQ processing. By purposefully selecting existing composition systems and controlling aspects such as heating and rolling process, material microstructure, heating and cooling temperatures and times, this invention achieves not only ultra-high strength comparable to conventional ultra-high strength steel plates, but also the ability to be used at temperatures as low as -196℃, resistance to hydrogen embrittlement, and ensures good elongation, surface quality, and plate shape.

[0055] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing a martensitic / austenitic dual-phase ultra-high strength steel plate, wherein the steel plate is a niobium-micro-alloyed 9Ni steel, and the main element chemical composition of the steel, C, Mn, Ni, and Nb, is specified, wherein the main elements, by mass percentage, are C: 0.02-0.05%, Mn: 0.60-0.80%, Ni: 8.80-9.20%, Nb: 0.010-0.020%, with the balance being Fe and unavoidable impurities; the method includes converter steelmaking -> RH vacuum degassing -> LF refining -> Ca treatment -> RH high vacuum degassing -> continuous casting -> billet slow cooling treatment -> billet surface cleaning -> heating -> rolling -> ACC -> DQ, wherein: Select 9Ni steel continuously cast billets with ≥150mm niobium microalloying, put the billets into a walking beam furnace, heat them to 1220-1240℃ at an average heating rate of 10-14cm / min, and ensure the tapping temperature is 1200-1220℃. After the billet exits the furnace, it undergoes high-pressure water descaling and then two-stage controlled rolling of roughing and finishing on a medium-thick plate mill. The initial rolling temperature of roughing is between 1050-1100℃, while finishing is carried out at high temperature with an initial rolling temperature between 890-920℃. The single-pass reduction rate of finishing is ≤10%, the final rolling temperature is ≥840℃, and the water immersion temperature is ≥820℃. After rolling, the steel plate is cooled to room temperature by ACC+DQ at 15-25℃ / s. The reduction rate of the three passes after rough rolling is ≥15%, and the thickness after heating is ≥1.7H, where H is the finished thickness.

2. The method for manufacturing a martensitic / austenitic multiphase ultra-high strength steel plate according to claim 1, characterized in that... The steel plate has a fine and uniform martensite / austenite multiphase structure, with the austenite phase accounting for 5-10% of the volume.

3. The method for manufacturing a martensitic / austenitic multiphase ultra-high strength steel plate according to claim 1, characterized in that... The steel plate has a yield strength ≥900MPa, tensile strength ≥1000MPa, yield-to-tensile ratio ≤0.90, elongation ≥12%, and impact toughness ≥90J at -196℃.

4. The method for manufacturing a martensitic / austenitic multiphase ultra-high strength steel plate according to claim 1, characterized in that: The continuously cast billet is cleaned by surface grinding with a surface grinding machine, removing 1.5 mm from each of the upper and lower surfaces, and then heated.

5. The method for manufacturing a martensitic / austenitic multiphase ultra-high strength steel plate according to claim 1, characterized in that: During the heating stage, heat preservation should begin when the surface temperature reaches 1 / 2 of the steel plate thickness. The heat preservation time should be no less than 45 minutes, and the heating time at 600~900℃ should be controlled to be ≥0.35 minutes / mm.

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