A 980MPa grade ultra-low carbon bainitic steel for marine engineering and its preparation method

Abstract

The present invention relates to the technical field of thick plates of marine engineering structural steel, and in particular to a 980MPa grade ultra-low carbon bainitic steel and a preparation method thereof. A 980MPa grade ultra-low carbon bainitic steel for marine engineering, the chemical composition of which comprises by weight: C≤0.04%, Mn: 1.3%-1.5%, Mo: 1.0%-1.2%, Nb: 0.05%-0.08%, Ni: 6.0%-8.0%, Cr: 0.8%-1.2%, Ti: 0.010%-0.020%, Al: 0.40%-0.60%, and the rest is iron and unavoidable impurities. The steel yield strength of the present invention is ≥980Mpa, the tensile strength is ≥1050MPa, the toughness level reaches F grade, and the impact energy of the coarse grain heat affected zone of the welded joint at 40°C is ≥80J. It can solve the problems of small adaptability of the maximum thickness specification of the existing EH960 steel and relatively low low temperature toughness.

Description

Technical Field

[0001] The invention relates to the technical field of marine engineering structural steel thick plates, and in particular to a 980MPa grade ultra-low carbon bainite steel and a preparation method thereof. Background Art

[0002] Ultra-low carbon bainitic steel (ULCB) is a new type of high-strength, high-toughness, multi-purpose steel developed internationally in recent years. The alloy design concept of ultra-low carbon bainitic steel is different from that of the original high-strength low-alloy steel. By significantly reducing the carbon content, the influence of carbon on the toughness and welding performance of steel is eliminated. At present, ultra-low carbon bainitic steel has been widely used in oil pipelines, transportation buildings and marine facilities.

[0003] At present, the highest strength grade of marine engineering steel in my country is EH960 steel, whose main components are: C≤0.18%, Si≤0.80%, Mn≤1.70%, Ni≤2.00%, Cr≤1.50%, Cu≤0.50%, Mo≤0.70%, Nb≤0.060%, Al≥0.018%, and the steel plate is produced by quenching + tempering process. The maximum thickness of the steel plate prepared by this production process is 50mm, the yield strength is ≥960MPa, the tensile strength is: 980-1150MPa, the transverse impact energy at -40℃ is ≥46J, and the longitudinal impact energy at -40℃ is ≥69J.

[0004] Since the current EH960 steel alloy system has uneven cross-sectional properties when producing steel plates with a thickness of more than 50 mm, the yield strength surplus at the center is small and the toughness level is only E grade (-40°C), which makes EH960 steel unable to be used in marine engineering equipment in extremely cold environments. In addition, the -40°C impact energy performance of the coarse-grained heat-affected zone of the welded joint of EH960 steel is relatively low, which hinders its further expansion of its application range. In order to broaden the thickness specifications of EH960 steel and improve the toughness level of the steel plate to F grade (-60°C) to meet the equipment assessment requirements in extremely cold environments, it is urgent to apply the method of ultra-low carbon bainite structure design to this system steel, carry out corresponding research work, solve the problems of the existing EH960 steel thickness specifications and low-temperature toughness improvement, and meet the new requirements of marine engineering. Summary of the invention

[0005] In view of the above analysis, the embodiments of the present invention aim to provide a 980MPa grade ultra-low carbon bainitic steel for marine engineering and a preparation method thereof, so as to solve the problems of small adaptability of the maximum thickness specification and relatively low low temperature toughness of the existing EH960 steel.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] On the one hand, an embodiment of the present invention provides a 980MPa grade ultra-low carbon bainitic steel for marine engineering, the chemical composition of which comprises by weight: C≤0.04%, Mn: 1.3%~1.5%, Mo: 1.0%~1.2%, Nb: 0.05%~0.08%, Ni: 6.0%~8.0%, Cr: 0.8%~1.2%, Ti: 0.010%~0.020%, Al: 0.40%-0.60%, and the rest is iron and unavoidable impurities.

[0008] Furthermore, the microstructure of the steel is lath bainite, MA component and film-like residual austenite, and the lath bainite has dispersed and precipitated nano-scale Ni3Al precipitation phase.

[0009] Furthermore, the maximum thickness specification of the steel is 80 mm.

[0010] Furthermore, a 980MPa grade ultra-low carbon bainitic steel for marine engineering, the chemical composition of which comprises by weight: C: 0.025%~0.038%, Mn: 1.32%~1.48%, Mo: 1.04%~1.18%, Nb: 0.055%~0.073%, Ni: 6.5%~7.8%, Cr: 0.85%~1.16%, Ti: 0.012%~0.016%, Al: 0.42%-0.58%, and the rest is iron and unavoidable impurities.

[0011] On the other hand, the present invention provides a method for preparing 980MPa grade ultra-low carbon bainitic steel for marine engineering, which is used to prepare the above-mentioned 980MPa grade ultra-low carbon bainitic steel for marine engineering, including molten iron pretreatment-converter smelting-LF refining-RH refining-continuous casting-controlled rolling and controlled cooling-tempering heat treatment process steps, wherein the RH refining controls the impurity content, by weight: P≤0.008%, S≤0.003%, O≤0.0030%, N≤0.0040%, H≤0.0002%.

[0012] Furthermore, the continuous casting process uses strong cooling in the crystallizer and weak cooling in the secondary cooling zone to control the superheat at 15-25°C, the pulling speed ≤1.1m / min, and enters the slow cooling pit for treatment after continuous casting.

[0013] Furthermore, the controlled rolling and controlled cooling heats the continuous casting billet at a heating temperature of 1200 to 1250° C. for a heating time of 1.5 to 2 hours.

[0014] Furthermore, the controlled rolling and controlled cooling adopts two-stage rolling, the first stage rolling is rough rolling, the starting rolling temperature is 1150-1180°C, the final rolling temperature is ≥980°C, and the cumulative deformation is ≥65%.

[0015] Furthermore, in the two-stage rolling, the starting rolling temperature is 900-930° C., the finishing rolling temperature is 800-850° C., and the cumulative deformation is ≥30%.

[0016] Furthermore, the tempering heat treatment process is a heating temperature of 500°C to 600°C and a holding time of 2-3h.

[0017] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0018] 1. The present invention provides a new 980MPa grade marine engineering steel alloy component system. The present invention adopts an ultra-low C design and adds Mn, Mo and Nb to suppress the formation of proeutectoid ferrite, so that the alloy system can obtain a uniform bainite structure within a wide cooling rate range. The addition of a higher content of Ni ensures the excellent low-temperature toughness of the steel. The addition of a high Al content ensures the precipitation of a nano-scale Ni3Al phase during the tempering process, providing a strong precipitation strengthening effect.

[0019] 2. The present invention provides an alloy composition system and preparation method for producing 980MPa marine engineering steel with a maximum thickness of 80mm. The alloy composition system of this steel can obtain ultra-low carbon bainite structure with uniform performance in a wide cooling rate range, ensuring the uniformity of the structure and performance of the material in the cross-sectional thickness direction, and can adapt to the production of steel plates with a maximum thickness of 80mm, exceeding the existing EH960 steel maximum thickness specification of 50mm.

[0020] 3. The present invention provides a 980MPa grade marine engineering steel with high strength and excellent low-temperature toughness. Due to the refinement of the microstructure of ultra-low carbon bainitic steel, the control of high Ni content (6.0-8.0%) and the formation of thin film austenite, the toughness level of the steel is guaranteed to reach F grade, the transverse impact energy at -60℃ is ≥100J, and the longitudinal impact energy at -60℃ is ≥150J, which is higher than the E grade toughness level of the existing EH960 steel.

[0021] 4. The present invention provides a 980MPa grade marine engineering steel with good welding crack resistance and excellent toughness of welded joints. Due to the ultra-low carbon content design, this steel is in the easy welding zone of the Graville diagram, has good crack resistance, and can be welded without preheating at room temperature; at the same time, due to the high Ni content and the control of the microstructure, the impact energy of the coarse-grained heat-affected zone in the welded joint at -40°C is ≥80J, which is higher than the toughness level of the coarse-grained heat-affected zone of the existing EH960 steel welded joint.

[0022] 5. The present invention provides a method for preparing 980MPa-grade marine engineering steel with lower production process cost. It adopts controlled rolling and controlled cooling + tempering process. Compared with the existing EH960 steel production method, it omits the tempering process, saves energy, and reduces the production process cost.

[0023] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0025] Figure 1 This is an optical microscope microstructure diagram of the steel plate of Example 1;

[0026] Figure 2 TEM microstructure diagram of the steel plate of Example 1;

[0027] Figure 3 This is an optical microscope microstructure diagram of the steel plate of Example 2;

[0028] Figure 4 TEM microstructure diagram of the steel plate of Example 2;

[0029] Figure 5 This is an optical microscope microstructure diagram of the steel plate of Example 3;

[0030] Figure 6 This is the transmission electron microscope microstructure diagram of the steel plate in Example 3. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0032] The invention provides a 980MPa grade ultra-low carbon bainite steel for marine engineering, the chemical composition of which comprises by weight: C≤0.04%, Mn: 1.3%-1.5%, Mo: 1.0%-1.2%, Nb: 0.05%-0.08%, Ni: 6.0%-8.0%, Cr: 0.8%-1.2%, Ti: 0.010%-0.020%, Al: 0.40%-0.60%, and the rest is iron and inevitable impurities.

[0033] At present, the highest strength grade of marine engineering steel in my country is EH960 steel. The maximum thickness of the steel plate prepared by this production process is 50mm. When the current EH960 steel alloy system is used to produce steel plates with a thickness of more than 50mm through quenching + tempering process, there is uneven cross-sectional performance, small yield strength surplus at the center, and only E grade (-40℃), which makes EH960 steel unable to be used in marine engineering equipment in extremely cold environments. The maximum thickness of the 980MPa grade ultra-low carbon bainitic steel for marine engineering of the present invention is 80mm, yield strength ≥980Mpa, tensile strength ≥1050MPa, -60℃ transverse impact energy ≥100J, -60℃ longitudinal impact energy ≥150J, and toughness level reaches F grade, which can realize room temperature welding without preheating, no cold cracks after welding, and -40℃ impact energy ≥80J in the coarse grain heat affected zone of the welded joint. It can solve the problems of small adaptability of the maximum thickness specification of the existing EH960 steel and relatively low low temperature toughness.

[0034] The reasons for limiting the composition of the ingot of the 980 MPa grade ultra-low carbon bainite for marine engineering and the preparation method thereof in the present invention are explained below, and only % is used to represent the mass percentage in the composition.

[0035] Carbon (C): C is the most critical factor affecting the structure and performance of bainite. In order to eliminate the damaging effect of C on the weldability and toughness of steel, the C content needs to be reduced to an ultra-low level. According to the current industrial smelting level, the C content is ≤ 0.04%.

[0036] Manganese (Mn): Manganese is an austenite-forming element that can increase the stability of supercooled austenite, which is beneficial to delay the transformation temperature of proeutectoid ferrite and ensure that the material obtains a better low-temperature transformation structure. The addition amount of Mn is 1.3% to 1.5%.

[0037] Molybdenum (Mo): Mo has a significant solute drag effect, which causes a distinct bend in the supercooled austenite transformation curve and significantly delays high-temperature transformation, separating the upper and lower C curves of steel. The addition amount of Mo is 1.0% to 1.2%.

[0038] Niobium (Nb): Nb can inhibit the deformation and recrystallization behavior of high-temperature austenite, increase the recrystallization temperature, expand the non-recrystallization zone, increase the deformation accumulation during rolling in the non-recrystallization zone, introduce high-density dislocations, and promote microstructure refinement. The addition amount of Nb is 0.05% to 0.08%.

[0039] Nickel (Ni): The Ni element can improve the stability of austenite, reduce the ductile-brittle transition temperature of the material, improve toughness, and is also the key to obtaining thin-film austenite. The addition amount of Ni is 6.0% to 8.0%.

[0040] Chromium (Cr): The Cr element can significantly reduce the transformation temperature of bainite, affect the C curve of bainite transformation, and improve the strength of bainitic steel. The addition amount of Cr is 0.8% to 1.2%.

[0041] Titanium (Ti): Ti is a strong nitrogen-fixing element, which combines Ti with N to form TiN, pinning the original austenite in the welding thermal cycle to obtain better toughness of the coarse-grained heat-affected zone. The addition amount of Ti is 0.010% to 0.020%.

[0042] Aluminum (Al): Al plays a deoxidizing role in steel to ensure the purity of molten steel. At the same time, it ensures a certain solid solution aluminum content and forms a dispersed nano-scale Ni3Al phase during the steel tempering process, providing a strong strengthening effect. The addition amount of Al is 0.40%-0.60%.

[0043] Preferably, a 980MPa grade ultra-low carbon bainitic steel for marine engineering, the chemical composition of which comprises by weight: C: 0.025%-0.038%, Mn: 1.32%-1.48%, Mo: 1.04%-1.18%, Nb: 0.055%-0.073%, Ni: 6.5%-7.8%, Cr: 0.85%-1.16%, Ti: 0.012%-0.016%, Al: 0.42%-0.58%, and the rest is iron and unavoidable impurities.

[0044] The present invention also provides a method for preparing the above-mentioned 980MPa grade ultra-low carbon bainite steel for marine engineering, comprising the following steps:

[0045] Step 1: Hot metal pretreatment: Use low P and low S hot metal for treatment, deep desulfurization of the hot metal, and the slag after desulfurization must be thoroughly removed;

[0046] Step 2: converter smelting + LF refining + RH refining: LF furnace makes white slag to refine molten steel, and adjusts the composition of molten steel to reach the target value; RH furnace refining controls the impurity content;

[0047] Step 3: Continuous casting: Continuous casting uses strong cooling in the crystallizer and weak cooling in the secondary cooling zone, controlling the superheat at 15-25°C, the pulling speed ≤1.1m / min, and entering the slow cooling pit for treatment after continuous casting;

[0048] Step 4: controlled rolling and controlled cooling: heating the continuous casting billet, and performing controlled rolling and controlled cooling on the continuous casting billet;

[0049] The continuous casting billet is heated to a temperature of 1200-1250°C and a heating time of 1.5-2h to ensure that the high content of alloy elements is fully dissolved into austenite; the controlled rolling adopts a two-stage rolling method: the first stage is rolling in the recrystallization zone, which is rough rolling, the start rolling temperature is 1150-1180°C, the final rolling temperature is ≥980°C, and the cumulative deformation is ≥65%; the second stage is rolling in the non-recrystallization zone, which is finish rolling, the start rolling temperature is 900-930°C, the final rolling temperature is 800-850°C, and the cumulative deformation is ≥30%; after rolling, it is directly quenched and cooled to room temperature.

[0050] Step 5: Tempering heat treatment: The tempering heating treatment temperature is 500℃~600℃, the holding time is 2-3h, and air cooling is performed after holding.

[0051] It should be noted that the impurity content is controlled by RH furnace refining in step 2, wherein P≤0.008%, S≤0.003%, O≤0.0030%, N≤0.0040%, and H≤0.0002%. The step 2 adjusts the composition of the molten steel to reach the target value, i.e., adjusts the composition of the molten steel by LF refining so that the content of the molten steel meets the content of each element required by the present invention.

[0052] It should be noted that in step 3, the continuous casting adopts the water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone to obtain a continuous casting billet with uniform composition and fewer surface defects; the superheat is controlled at 15-25°C and the pulling speed is ≤1.1m / min to reduce element segregation during the solidification process.

[0053] It should be noted that the rolling in step 4 is divided into rough rolling and finishing rolling, wherein the main purpose of rough rolling is deformation, and rolling is performed in the austenite recrystallization zone. The starting temperature of rough rolling is 1150-1180°C, which can form high-temperature large deformation and penetrate into the core of the billet, which is beneficial to the recrystallization of the core structure; the ending temperature of rough rolling is ≥980°C, 980°C is the termination temperature of austenite recrystallization, and the cumulative deformation is ≥65%, which ensures that the core structure is fully recrystallized, which is beneficial to improve the toughness level of the core structure. Finishing rolling deformation is mainly to regulate the phase transformation process of deformed austenite, so as to obtain fine lath bainite, MA component and film austenite. The starting temperature of finishing rolling is 900-930°C, 900-930°C is in the non-recrystallization zone of austenite, and rolling in the interval is conducive to controlling the final rolling, accumulating the deformation in the non-recrystallization zone, and ensuring the control of the final rolling temperature. The final rolling temperature of fine rolling is 800-850℃, which ensures that the ultra-low carbon bainite steel is rolled in the non-recrystallization zone of austenite. The accumulated deformation provides sufficient driving force for the subsequent phase transformation, thereby increasing the driving force of the austenite to bainite phase transformation and the nucleation rate of ultra-low carbon bainite, thereby obtaining fine ultra-low carbon bainite. The accumulated deformation ≥30% can obtain a good grain refinement effect. After rolling, direct quenching and cooling to room temperature ensures that the organization obtains high dislocation density and fine substructure, and ensures the high strength level of the organization.

[0054] In step 5, a tempering heat treatment process is performed, the tempering heating treatment temperature is 500°C to 600°C, the insulation time is 2-3h, and air cooling is performed after insulation to obtain a dispersed nano-scale Ni3Al precipitation phase, which plays a strong precipitation strengthening role and ensures the high strength source of the material.

[0055] The microstructure of the steel prepared by the preparation method is lath bainite, MA component and film-like residual austenite, and there is a dispersed and precipitated nano-scale Ni3Al precipitation phase in the lath bainite.

[0056] The ultra-low carbon bainitic steel for marine engineering prepared by the present invention has a yield strength of ≥980Mpa (such as 1005-1084MPa) and a tensile strength of ≥1050MPa (such as 1080-1213MPa). At the same time, the weldability of the steel plate is good, the impact energy of the heat-affected zone of the weld joint at -40°C is ≥80J (such as 168-218J), the transverse impact energy at -60°C is ≥100J (such as 244-287J), and the longitudinal impact energy at -60°C is ≥150J (such as 289-324J).

[0057] The advantages of the precise control of the chemical composition, content and preparation process parameters of the present invention will be demonstrated below with specific examples.

[0058] Example 1

[0059] A 980MPa grade ultra-low carbon bainite steel for marine engineering, the chemical composition of which includes by weight: C: 0.025%, Mn: 1.32%, Mo: 1.05%, Nb: 0.062%, Ni: 6.5%, Cr: 0.85%, Ti: 0.012%, Al: 0.45%, and the rest is iron and unavoidable impurities. The preparation method thereof comprises the following steps:

[0060] Step 1: Hot metal pretreatment:

[0061] Use low P and low S molten iron for treatment, deeply desulfurize the molten iron, and thoroughly remove the slag after desulfurization.

[0062] Step 2: Converter smelting + LF refining + RH refining:

[0063] RH refining controls the impurity content: P: 0.0072%, S: 0.0025%, O: 0.0025%, N: 0.0030%, H: 0.00015%.

[0064] Step 3: Continuous Casting:

[0065] A water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone was adopted, with a superheat of 20°C and a pulling speed of 1.0m / min. After continuous casting, the casting entered the slow cooling pit for treatment.

[0066] Step 4: controlled rolling and controlled cooling;

[0067] The continuous casting billet is heated and kept warm at a heating temperature of 1220°C for 2 hours. The rolling is carried out using a controlled rolling and controlled cooling process, which is divided into two stages: rough rolling and finish rolling. The starting temperature of rough rolling is 1150°C, the ending temperature of rough rolling is 985°C, and the cumulative deformation of rough rolling is 70%. The starting temperature of finish rolling is 910°C, the ending temperature of finish rolling is 820°C, and the cumulative deformation of finish rolling is 35%. After finish rolling, the steel plate is directly quenched to room temperature.

[0068] Step 5: Tempering heat treatment:

[0069] The tempering heating treatment temperature is 550°C, the holding time is 2h, and air cooling is performed after holding.

[0070] The 980MPa grade ultra-low carbon bainite steel plate prepared in this embodiment has a thickness of 80mm, and its microstructure is composed of lath bainite, MA component, and film-like retained austenite. Figure 1 shown.

[0071] The mechanical properties are: yield strength is 1005MPa; tensile strength is 1080MPa; transverse impact energy at -60℃ is 254J, longitudinal impact energy at -60℃ is 312J; no cracks occur when welding without preheating at room temperature (25℃), and the impact energy of the coarse-grained heat-affected zone of the weld joint at -40℃ is 218J.

[0072] Example 2

[0073] A 980MPa grade ultra-low carbon bainite steel for marine engineering, the chemical composition of which includes by weight: C: 0.035%, Mn: 1.45%, Mo: 1.15%, Nb: 0.073%, Ni: 7.8%, Cr: 1.15%, Ti: 0.012%, Al: 0.55%, and the rest is iron and unavoidable impurities. The preparation method thereof comprises the following steps:

[0074] Step 1: Hot metal pretreatment:

[0075] Use low P and low S molten iron for treatment, deeply desulfurize the molten iron, and thoroughly remove the slag after desulfurization.

[0076] Step 2: Converter smelting + LF refining + RH refining:

[0077] RH refining controls the impurity content: P: 0.0054%, S: 0.0028%, O: 0.0027%, N: 0.0035%, H: 0.00018%.

[0078] Step 3: Continuous Casting:

[0079] A water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone was adopted, with a superheat of 18°C ​​and a pulling speed of 1.1m / min. After continuous casting, the casting entered the slow cooling pit for treatment.

[0080] Step 4: Controlled rolling and controlled cooling:

[0081] The continuous casting billet is heated and kept warm at 1250℃ for 2h. The rolling adopts controlled rolling and controlled cooling process, which is divided into two stages: rough rolling and finishing rolling. The starting temperature of rough rolling is 1180℃, the ending temperature of rough rolling is 1000℃, and the cumulative deformation of rough rolling is 68%. The starting temperature of finishing rolling is 930℃, the ending temperature of finishing rolling is 850℃, and the cumulative deformation of finishing rolling is 35%. After finishing rolling, the steel plate is directly quenched to room temperature.

[0082] Step 5: Tempering heat treatment:

[0083] The tempering heating treatment temperature is 600°C, the holding time is 3h, and air cooling is performed after holding.

[0084] The 980MPa grade ultra-low carbon bainite steel plate prepared in this embodiment has a thickness of 60mm, and its microstructure is composed of lath bainite, MA component, and film-like retained austenite. Figure 2 shown.

[0085] The mechanical properties are: yield strength is 1084MPa; tensile strength is 1210MPa; transverse impact energy at -60℃ is 282J, longitudinal impact energy at -60℃ is 318J; no cracks appear when welding without preheating at room temperature (25℃), and the impact energy of the coarse-grained heat-affected zone of the weld joint at -40℃ is 196J.

[0086] Example 3

[0087] A 980MPa grade ultra-low carbon bainite steel for marine engineering, the chemical composition of which includes by weight: C: 0.032%, Mn: 1.40%, Mo: 1.12%, Nb: 0.072%, Ni: 7.2%, Cr: 1.05%, Ti: 0.012%, Al: 0.53%, and the rest is iron and unavoidable impurities. The preparation method thereof comprises the following steps:

[0088] Step 1: Hot metal pretreatment:

[0089] Use low P and low S molten iron for treatment, deeply desulfurize the molten iron, and thoroughly remove the slag after desulfurization.

[0090] Step 2: Converter smelting + LF refining + RH refining:

[0091] RH refining controls the impurity content: P: 0.0068%, S: 0.0021%, O: 0.0028%, N: 0.0038%, H: 0.00017%.

[0092] Step 3: Continuous Casting:

[0093] A water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone was adopted, with a superheat of 25°C and a pulling speed of 1.0m / min. After continuous casting, the casting entered the slow cooling pit for treatment.

[0094] Step 4: Controlled rolling and controlled cooling:

[0095] The continuous casting ingot is heated and kept warm at 1210°C for 2h. The rolling adopts controlled rolling and controlled cooling process, which is divided into two stages: rough rolling and finishing rolling. The starting temperature of rough rolling is 1160°C, the ending temperature of rough rolling is 990°C, and the cumulative deformation of rough rolling is 72%. The starting temperature of finishing rolling is 920°C, the ending temperature of finishing rolling is 830°C, and the cumulative deformation of finishing rolling is 35%. After finishing rolling, the steel plate is directly quenched to room temperature.

[0096] Step 5: Tempering heat treatment:

[0097] The tempering heating treatment temperature is 520℃, the holding time is 3h, and air cooling is performed after holding.

[0098] The 980MPa grade ultra-low carbon bainite steel plate prepared in this embodiment has a thickness of 50mm, and its microstructure is composed of lath bainite, MA component, and film-like retained austenite. Figure 3 shown.

[0099] The mechanical properties are: yield strength is 1075MPa; tensile strength is 1164MPa; transverse impact energy at -60℃ is 257J, longitudinal impact energy at -60℃ is 294J; no cracks appear when welding without preheating at room temperature (25℃), and the impact energy of the coarse-grained heat-affected zone of the weld joint at -40℃ is 175J.

[0100] Example 4

[0101] A 980MPa grade ultra-low carbon bainite steel for marine engineering, the chemical composition of which includes by weight: C: 0.025%, Mn: 1.35%, Mo: 1.18%, Nb: 0.068%, Ni: 6.9%, Cr: 0.93%, Ti: 0.015%, Al: 0.42%, and the rest is iron and unavoidable impurities. The preparation method thereof comprises the following steps:

[0102] Step 1: Hot metal pretreatment:

[0103] Use low P and low S molten iron for treatment, deeply desulfurize the molten iron, and thoroughly remove the slag after desulfurization.

[0104] Step 2: Converter smelting + LF refining + RH refining:

[0105] RH refining controls the impurity content: P: 0.0062%, S: 0.0023%, O: 0.0024%, N: 0.0029%, H: 0.00013%.

[0106] Step 3: Continuous Casting:

[0107] A water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone was adopted, with a superheat of 23°C and a pulling speed of 1.0m / min. After continuous casting, the casting entered the slow cooling pit for treatment.

[0108] Step 4: Controlled rolling and controlled cooling:

[0109] The continuous casting billet is heated and kept warm at 1220℃ for 2h. The rolling adopts controlled rolling and controlled cooling process, which is divided into two stages: rough rolling and finishing rolling. The starting temperature of rough rolling is 1150℃, the ending temperature of rough rolling is 985℃, and the cumulative deformation of rough rolling is 72%. The starting temperature of finishing rolling is 910℃, the ending temperature of finishing rolling is 800℃, and the cumulative deformation of finishing rolling is 37%. After finishing rolling, the steel plate is directly quenched to room temperature.

[0110] Step 5: Tempering heat treatment:

[0111] The tempering heating treatment temperature is 525℃, the holding time is 3h, and air cooling is performed after holding.

[0112] The 980 MPa grade ultra-low carbon bainitic steel plate prepared in this embodiment has a thickness of 80 mm, and its microstructure is composed of lath bainite, MA component, and film-like retained austenite.

[0113] The mechanical properties are: yield strength is 1058MPa; tensile strength is 1172MPa; transverse impact energy at -60℃ is 287J, longitudinal impact energy at -60℃ is 324J; no cracks occur when welding without preheating at room temperature (25℃), and the impact energy of the coarse-grained heat-affected zone of the weld joint at -40℃ is 188J.

[0114] Example 5

[0115] A 980MPa grade ultra-low carbon bainite steel for marine engineering, the chemical composition of which includes by weight: C: 0.038%, Mn: 1.48%, Mo: 1.16%, Nb: 0.055%, Ni: 7.5%, Cr: 1.16%, Ti: 0.016%, Al: 0.58%, and the rest is iron and unavoidable impurities. The preparation method thereof comprises the following steps:

[0116] Step 1: Hot metal pretreatment:

[0117] Use low P and low S molten iron for treatment, deeply desulfurize the molten iron, and thoroughly remove the slag after desulfurization.

[0118] Step 2: Converter smelting + LF refining + RH refining:

[0119] RH refining controls the impurity content: P: 0.0052%, S: 0.0019%, O: 0.0018%, N: 0.0030%, H: 0.00014%.

[0120] Step 3: Continuous Casting:

[0121] A water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone was adopted, with a superheat of 20°C and a pulling speed of 1.1m / min. After continuous casting, the casting entered the slow cooling pit for treatment.

[0122] Step 4: Controlled rolling and controlled cooling:

[0123] The continuous casting billet is heated and kept warm at 1220℃ for 2h. The rolling adopts controlled rolling and controlled cooling process, which is divided into two stages: rough rolling and finishing rolling. The starting temperature of rough rolling is 1170℃, the ending temperature of rough rolling is 1000℃, and the cumulative deformation of rough rolling is 70%. The starting temperature of finishing rolling is 920℃, the ending temperature of finishing rolling is 805℃, and the cumulative deformation of finishing rolling is 38%. After finishing rolling, the steel plate is directly quenched to room temperature.

[0124] Step 5: Tempering heat treatment:

[0125] The tempering heating treatment temperature is 520°C, the holding time is 2.5h, and air cooling is performed after holding.

[0126] The 980 MPa grade ultra-low carbon bainitic steel plate prepared in this embodiment has a thickness of 75 mm, and its microstructure is composed of lath bainite, MA component, and film-like retained austenite.

[0127] The mechanical properties are: yield strength is 1028MPa; tensile strength is 1167MPa; transverse impact energy at -60℃ is 244J, longitudinal impact energy at -60℃ is 294J; no cracks occur when welding without preheating at room temperature (25℃), and the impact energy of the coarse-grained heat-affected zone of the weld joint at -40℃ is 168J.

[0128] Example 6

[0129] A 980MPa grade ultra-low carbon bainite steel for marine engineering, the chemical composition of which includes by weight: C: 0.033%, Mn: 1.39%, Mo: 1.16%, Nb: 0.055%, Ni: 7.5%, Cr: 1.16%, Ti: 0.016%, Al: 0.58%, and the rest is iron and unavoidable impurities. The preparation method thereof comprises the following steps:

[0130] Step 1: Hot metal pretreatment:

[0131] Use low P and low S molten iron for treatment, deeply desulfurize the molten iron, and thoroughly remove the slag after desulfurization.

[0132] Step 2: Converter smelting + LF refining + RH refining:

[0133] RH refining controls the impurity content: P: 0.0072%, S: 0.0024%, O: 0.0025%, N: 0.0031%, H: 0.00015%.

[0134] Step 3: Continuous Casting:

[0135] A water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone was adopted, with a superheat of 18°C ​​and a pulling speed of 1.0m / min. After continuous casting, the casting entered the slow cooling pit for treatment.

[0136] Step 4: Controlled rolling and controlled cooling:

[0137] The continuous casting billet is heated and kept warm at 1250℃ for 2h. The rolling adopts controlled rolling and controlled cooling process, which is divided into two stages: rough rolling and finishing rolling. The starting temperature of rough rolling is 1180℃, the ending temperature of rough rolling is 1010℃, and the cumulative deformation of rough rolling is 72%. The starting temperature of finishing rolling is 930℃, the ending temperature of finishing rolling is 840℃, and the cumulative deformation of finishing rolling is 35%. After finishing rolling, the steel plate is directly quenched to room temperature.

[0138] Step 5: Tempering heat treatment:

[0139] The tempering heating treatment temperature is 580°C, the holding time is 2h, and air cooling is performed after holding.

[0140] The 980 MPa grade ultra-low carbon bainitic steel plate prepared in this embodiment has a thickness of 20 mm, and its microstructure is composed of lath bainite, MA component, and film-like retained austenite.

[0141] The mechanical properties are: yield strength is 1084MPa; tensile strength is 1213MPa; transverse impact energy at -60℃ is 255J, longitudinal impact energy at -60℃ is 289J; no cracks appear when welding without preheating at room temperature (25℃), and the impact energy of the coarse-grained heat-affected zone of the weld joint at -40℃ is 182J.

[0142] Table 1 Chemical composition of steel in the example (wt, %)

[0143] serial number C Mn Mo Nb Ni Cr Ti Al Example 1 0.025 1.32 1.05 0.062 6.5 0.85 0.012 0.45 Example 2 0.035 1.45 1.15 0.073 7.8 1.15 0.012 0.55 Example 3 0.032 1.40 1.12 0.072 7.2 1.05 0.012 0.53 Example 4 0.025 1.35 1.18 0.068 6.9 0.93 0.015 0.42 Example 5 0.038 1.48 1.16 0.055 7.5 1.16 0.016 0.58 Example 6 0.033 1.39 1.04 0.074 7.4 1.01 0.014 0.48

[0144] Table 2 Example steel preparation process

[0145]

[0146] Table 3 Mechanical properties of example steel

[0147]

[0148] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A 980MPa grade ultra-low carbon bainitic steel for marine engineering, characterized in that: The chemical composition includes by weight: C: 0.025% to 0.04%, Mn: 1.3% to 1.5%, Mo: 1.0% to 1.2%, Nb: 0.05% to 0.08%, Ni: 6.5% to 8.0%, Cr: 0.8% to 1.2%, Ti: 0.010% to 0.020%, Al: 0.40% to 0.60%, and the rest is iron and unavoidable impurities; The microstructure of the steel is lath bainite, MA component and film-like residual austenite, and the lath bainite has dispersed and precipitated nano-scale Ni3Al precipitation phase; The ultra-low carbon bainite steel for marine engineering is prepared by a method of molten iron pretreatment-converter smelting-LF refining-RH refining-continuous casting-controlled rolling and controlled cooling-tempering heat treatment, wherein the controlled rolling adopts two-stage rolling, the first stage is rough rolling, the rolling start temperature is 1150-1180°C, the second stage is finishing rolling, the rolling start temperature is 900-930°C, and the steel is directly quenched and cooled to room temperature after rolling; The yield strength of the ultra-low carbon bainitic steel for marine engineering is ≥980Mpa.

2. The 980MPa grade ultra-low carbon bainite steel for marine engineering according to claim 1, characterized in that: The maximum thickness specification of the steel is 80 mm.

3. The 980MPa grade ultra-low carbon bainite steel for marine engineering according to claim 1, characterized in that: The chemical composition by weight includes: C: 0.025% ~ 0.038%, Mn: 1.32% ~ 1.48%, Mo: 1.04% ~ 1.18%, Nb: 0.055% ~ 0.073%, Ni: 6.5% ~ 7.8%, Cr: 0.85% ~ 1.16%, Ti: 0.012% ~ 0.016%, Al: 0.42% -0.58%, and the rest is iron and unavoidable impurities.

4. A method for preparing 980MPa grade ultra-low carbon bainitic steel for marine engineering, used for preparing the 980MPa grade ultra-low carbon bainitic steel for marine engineering according to any one of claims 1 to 3, characterized in that: The invention comprises the following process steps: molten iron pretreatment - converter smelting - LF refining - RH refining - continuous casting - controlled rolling and controlled cooling - tempering heat treatment. The RH refining controls the impurity content, which is calculated by weight as follows: P≤0.008%, S≤0.003%, O≤0.0030%, N≤0.0040%, and H≤0.0002%.

5. The method for preparing 980MPa grade ultra-low carbon bainite steel for marine engineering according to claim 4, characterized in that: The continuous casting process adopts strong cooling water in the crystallizer and weak cooling water in the secondary cooling zone, controls the superheat to be 15-25°C, and the pulling speed is ≤1.1m / min. After continuous casting, it enters the slow cooling pit for treatment.

6. The method for preparing 980MPa grade ultra-low carbon bainite steel for marine engineering according to claim 4, characterized in that: The controlled rolling and controlled cooling heats the continuous casting billet at a heating temperature of 1200 to 1250° C. for a heating time of 1.5 to 2 hours.

7. The method for preparing 980MPa grade ultra-low carbon bainite steel for marine engineering according to claim 6, characterized in that: The controlled rolling and controlled cooling adopts two-stage rolling, the first stage rolling is rough rolling, the starting rolling temperature is 1150-1180° C., the final rolling temperature is ≥980° C., and the cumulative deformation is ≥65%.

8. The method for preparing 980MPa grade ultra-low carbon bainite steel for marine engineering according to claim 7, characterized in that: In the two-stage rolling, the starting rolling temperature is 900-930° C., the final rolling temperature is 800-850° C., and the cumulative deformation is ≥30%.

9. The method for preparing 980MPa grade ultra-low carbon bainite steel for marine engineering according to claim 8, characterized in that: The tempering heat treatment process is a heating temperature of 500°C to 600°C and a heat preservation time of 2-3h.

Images