CN116219299A - A 980MPa grade ultra-low carbon bainitic steel for marine engineering and its preparation method - Google Patents

Abstract

The invention relates to the technical field of marine engineering structural steel thick plates, in particular to a 980MPa ultra-low carbon bainite steel and a preparation method thereof. A 980MPa grade ultra-low carbon bainitic steel for marine engineering, the chemical composition includes 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 are iron and unavoidable impurities. The yield strength of the steel of the invention is ≥ 980 MPa, the tensile strength is ≥ 1050 MPa, the toughness level reaches F level, and the impact energy of the welded joint coarse crystal heat-affected zone -40°C is ≥ 80 J. It can solve the problems of small adaptability to the maximum thickness specification of the existing EH960 steel and relatively low low-temperature toughness.

Description

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

technical field

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

Background technique

Ultra-low carbon bainitic steel (ULCB) is a large class of high-strength, high-toughness, multi-purpose steel newly developed in the world in recent years. The alloy design idea of ultra-low carbon bainitic steel is different from the original high-strength low-alloy steel. By greatly reducing the carbon content, the influence of carbon elements on the toughness and welding performance of the steel is eliminated. At present, ultra-low carbon bainitic steel has been widely used in oil pipelines, transportation construction and marine facilities and other fields.

At present, the grade of steel for ocean engineering with the highest strength in my country is EH960 steel, and its 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%, 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°C is ≥46J, and the longitudinal impact energy at -40°C is ≥69J.

Due to the fact that the current EH 960 steel alloy system has uneven cross-sectional properties when producing steel plates with a thickness of more than 50mm, the margin of yield strength at the center is small, and the toughness level is only E (-40°C), so EH960 steel cannot be used in extreme conditions. Marine engineering equipment in cold environment. In addition, the -40°C impact energy performance of the coarse-grained heat-affected zone of EH960 steel is relatively low, which hinders its further expansion of application range. In order to broaden the thickness specification of EH960 steel and increase the toughness level of the steel plate to F level (-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 to solve the problems of the existing EH960 steel thickness specification and low-temperature toughness improvement, and meet the new requirements put forward by ocean engineering.

Contents of the invention

In view of the above analysis, the embodiment of the present invention aims to provide a 980MPa grade ultra-low carbon bainitic steel for marine engineering and its preparation method to solve the problem of the existing EH960 steel with low adaptability to the maximum thickness specification and relatively low low temperature toughness The problem.

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

On the one hand, the embodiment of the present invention provides a 980MPa grade ultra-low carbon bainitic steel for marine engineering, the chemical composition includes 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 are iron and unavoidable impurities .

Further, the microstructure of the steel is lath bainite, MA components and film-like retained austenite, and the lath bainite has nano-scale Ni ₃ Al precipitates dispersed and precipitated.

Further, the maximum thickness specification of the steel is 80mm.

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

On the other hand, the present invention provides a preparation method of 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, the RH refining, control of impurity content, by weight: P≤0.008%, S≤0.003%, O≤0.0030%, N ≤0.0040%, H≤0.0002%.

Further, the continuous casting process adopts the water volume of the strong cooling in the crystallizer and the weak cooling in the secondary cooling zone, controls the degree of superheat to 15-25°C, and the casting speed ≤ 1.1m/min, and enters the slow cooling pit for treatment after continuous casting.

Further, the controlled rolling and controlled cooling heats the continuous casting slab, the heating temperature is 1200-1250° C., and the heating time is 1.5-2 hours.

Further, 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 finishing rolling temperature is ≥980°C, and the cumulative deformation is ≥65%.

Further, 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%.

Further, the tempering heat treatment process is a heating temperature of 500° C. to 600° C. and a holding time of 2-3 hours.

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

1. The present invention provides a new 980MPa-grade steel alloy composition system for marine engineering. The present invention adopts an ultra-low C design, and adds Mn, Mo, and Nb to inhibit the formation of proeutectoid ferrite, so that the alloy system can A uniform bainite structure can be obtained in a wide range of cooling rates. 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 nano-scale Ni ₃ Al phases during tempering, providing strong precipitation strengthening.

2. The present invention provides an alloy composition system and a preparation method capable of producing 980MPa grade marine engineering steel with a maximum thickness of 80mm. The alloy composition system of this steel type can obtain ultra-low carbon bainite structure with uniform performance in a wide cooling rate range, which ensures the uniformity of structure and performance of the material in the thickness direction of the section, and can adapt to the production of steel plates with a thickness of 80mm at most. It exceeds the 50mm maximum thickness specification of the existing EH960 steel.

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

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 design of ultra-low carbon content, this steel type is located in the easy-to-weld area of the Graville diagram and has good crack resistance , 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 is ≥80J at -40°C, which is higher than that of the existing EH960 steel welded joint. Toughness level of crystal heat-affected zone.

5. The present invention provides a preparation method of 980MPa grade marine engineering steel with lower production process cost, which is produced by controlled rolling and controlled cooling + tempering process. Compared with the existing EH960 steel production method, the quenching and tempering process is omitted , saving energy and reducing production process costs.

In the present invention, the above technical solutions can also be combined with each other to realize more preferred combination solutions. Additional features and advantages of the invention will be set forth in the description which follows, and some of the advantages will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the matter particularly pointed out in the written description and appended drawings.

Description of drawings

The drawings are for the purpose of illustrating specific embodiments only and are not to be considered as limitations of the invention, and like reference numerals refer to like parts throughout the drawings.

Fig. 1 is the optical microscope microstructure figure of embodiment 1 steel plate;

Fig. 2 is the transmission electron microscope microstructural figure of embodiment 1 steel plate;

Fig. 3 is the optical microscope microstructure figure of embodiment 2 steel plate;

Fig. 4 is the transmission electron microscope microstructure figure of embodiment 2 steel plate;

Fig. 5 is the optical microscope microstructure figure of embodiment 3 steel plate;

Fig. 6 is a transmission electron microscope microstructure diagram of the steel plate in Example 3.

Detailed ways

Preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the application and together with the embodiments of the present invention are used to explain the principle of the present invention and are not intended to limit the scope of the present invention.

The 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 are iron and unavoidable impurities.

At present, my country's highest-strength steel grade for marine engineering is EH960 steel, and the maximum thickness of the steel plate prepared by this production process is 50 mm. When the current EH 960 steel alloy system is used to produce steel plates with a thickness of more than 50 mm through the quenching and tempering process, there are The properties of the section are not uniform, the margin of yield strength at the center is small, and the toughness level is only E (-40°C), so EH960 steel cannot be used in marine engineering equipment in extremely cold environments. The maximum thickness specification of the 980MPa grade ultra-low carbon bainite steel for marine engineering of the present invention is 80mm, the yield strength is ≥ 980Mpa, the tensile strength is ≥ 1050MPa, the transverse impact energy at -60°C is ≥100J, and the longitudinal impact energy at -60°C is ≥150J. The toughness level reaches F level, which can realize welding at room temperature without preheating, no cold cracks after welding, and the impact energy of the heat-affected zone of the welded joint coarse grain at -40°C is ≥80J. It can solve the problems of small adaptability to the maximum thickness specification of the existing EH960 steel and relatively low low-temperature toughness.

The reasons for limiting the slab composition of the 980MPa grade ultra-low carbon bainite for marine engineering and its preparation method in the present invention will be explained, and % is only used to represent the mass percentage in the composition below.

Carbon (C): C element is the most critical factor affecting the structure and properties of bainite. In order to eliminate the harmful effect of C on the weldability and toughness of steel, it is necessary to reduce the C content to an ultra-low level. According to the existing industrial smelting level, the C content is ≤0.04%.

Manganese (Mn): Manganese is an austenite-forming element, which 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 amount of Mn added is between 1.3% and 1.5%.

Molybdenum (Mo): Mo element has obvious solute dragging effect, which makes the supercooled austenite transformation curve appear obvious river bends, and significantly delays the high temperature transformation, so that the upper and lower C curves of the steel are separated. The amount of Mo added is 1.0% to 1.2%.

Niobium (Nb): Nb element can inhibit the deformation and recrystallization behavior of high-temperature austenite, increase the recrystallization temperature, expand the non-recrystallization zone, increase the deformation accumulation in the non-recrystallization zone during rolling, and introduce high-density dislocations. Promotes tissue refinement. The amount of Nb added is 0.05% to 0.08%.

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

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

Titanium (Ti): Ti, a strong nitrogen-fixing element, makes Ti and N combine to form TiN, pinning the original austenite in the welding heat cycle, and obtaining better toughness of the coarse-grained heat-affected zone. The addition amount of Ti is 0.010% to 0.020% .

Aluminum (Al): On the one hand, Al plays a deoxidizing role in the steel to ensure the purity of the molten steel. At the same time, it ensures a certain solid solution aluminum content and forms a dispersed nano-scale Ni ₃ Al phase during the tempering process of the steel, providing a strong The strengthening effect of Al is 0.40%-0.60%.

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

The present invention also provides a preparation method for the above-mentioned 980MPa grade ultra-low carbon bainitic steel for marine engineering, comprising the following steps:

Step 1: Hot metal pretreatment: choose low P and low S hot metal for treatment, deep desulfurization of hot metal, and thoroughly remove slag after desulfurization;

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;

Step 3: Continuous casting: Continuous casting adopts the water volume of the strong cooling in the crystallizer and the weak cooling in the secondary cooling zone, the superheat is controlled at 15-25°C, the casting speed is ≤1.1m/min, and it enters the slow cooling pit for treatment after continuous casting;

Step 4: Controlled rolling and controlled cooling: heating the continuous casting slab, and performing controlled rolling and controlled cooling of the continuous casting slab;

The heating temperature of the continuous casting slab is 1200-1250 ℃, and the heating time is 1.5-2 hours to ensure that the high-content alloy elements are fully dissolved into the austenite; the controlled rolling adopts a two-stage rolling method: the first stage is rolling in the recrystallization zone, It is rough rolling, the starting rolling temperature is 1150～1180℃, the final rolling temperature is ≥980℃, and the cumulative deformation is ≥65%; the second stage is rolling in the non-recrystallized area, which is finish rolling, and the starting rolling temperature is 900～930℃ , 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.

Step 5: tempering heat treatment: tempering heat treatment temperature is 500°C-600°C, holding time is 2-3h, air cooling after heat preservation.

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

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 slab with uniform composition and less surface defects; ≤1.1m/min, reduce element segregation during solidification.

It should be noted that the rolling in step 4 is divided into rough rolling and finish rolling, wherein the main purpose of rough rolling is deformation and rolling in the austenite recrystallization zone. The rough rolling start temperature is 1150-1180°C, which can form high temperature and large deformation, penetrate into the center of the billet, and is conducive to the recrystallization of the core structure; the rough rolling end temperature is ≥980°C, and 980°C is the termination of austenite recrystallization The temperature, the cumulative deformation ≥ 65%, ensures the full recrystallization of the heart tissue, which is beneficial to improve the toughness level of the heart tissue. The finishing rolling deformation is mainly to control the phase transformation process of deformed austenite, so as to obtain fine lath bainite, M-A components and thin film austenite. The starting temperature of finish rolling is 900-930°C, and 900-930°C is in the austenite non-recrystallization zone. Rolling in the interval is beneficial to control the finish rolling, accumulate the deformation in the non-recrystallization zone, and ensure the control of the finish rolling temperature. The finish rolling temperature is 800-850°C, which ensures that the ultra-low carbon bainite steel is rolled in the austenite non-recrystallized region, and the accumulated deformation provides sufficient driving force for the subsequent phase transformation. Thereby, the driving force of austenite to bainite phase transformation is improved, and the nucleation rate of ultra-low carbon bainite is increased, so as to obtain fine ultra-low carbon bainite, and the accumulated deformation amount ≥ 30% can obtain good grain refinement effect. Quenching and cooling to room temperature directly after rolling ensures that the structure obtains high dislocation density and fine substructure, and ensures the high strength level of the structure.

In step 5, through the tempering heat treatment process, the tempering heat treatment temperature is 500°C-600°C, the holding time is 2-3h, and air cooling after heat preservation is used to obtain dispersed nano-scale Ni ₃ Al precipitates, which play a strong role in the precipitation Reinforcing effect ensures the source of high strength of the material.

The microstructure of the steel prepared by the above preparation method is lath bainite, MA components and film-like retained austenite, and the lath bainite has nano-scale Ni ₃ Al precipitates dispersed and precipitated.

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

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

Example 1

A 980MPa grade ultra-low carbon bainitic steel for marine engineering, its chemical composition 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 are iron and unavoidable impurities. Its preparation method comprises the following steps:

Step 1: Hot metal pretreatment:

Use low-P and low-S molten iron for treatment, deep desulfurization of molten iron, and thoroughly remove slag after desulfurization.

Step 2: Converter smelting + LF refining + RH refining:

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

Step 3: Continuous Casting:

The water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone is adopted. The superheat is 20°C and the casting speed is 1.0m/min. After continuous casting, it enters the slow cooling pit for treatment.

Step 4: controlled rolling and controlled cooling;

The continuous casting slab is heated and kept at a temperature of 1220°C, and the holding time is 2 hours; the rolling adopts a controlled rolling and controlled cooling process, which is divided into two stages: rough rolling and finish rolling. The start temperature of rough rolling is 1150°C, and the end of rough rolling The temperature is 985°C, the cumulative deformation of rough rolling is 70%; the start temperature of finish rolling is 910°C, the end temperature of finish rolling is 820°C, and the cumulative deformation of finish rolling is 35%. After finishing rolling, the steel plate is directly quenched to room temperature.

Step 5: Tempering heat treatment:

Tempering heat treatment temperature is 550°C, holding time is 2h, air cooling after holding.

The thickness specification of the 980MPa grade ultra-low carbon bainite steel plate prepared in this example is 80 mm, and the microstructure is composed of lath bainite, M-A components, and film-like retained austenite, as shown in FIG. 1 .

The mechanical properties are: the yield strength is 1005MPa; the tensile strength is 1080MPa; the transverse impact energy at -60°C is 254J, and the longitudinal impact energy at -60°C is 312J; there is no crack at room temperature (25°C) without preheating welding, and the welded joint The impact energy at -40°C in the heat-affected zone of coarse grains is 218J.

Example 2

A 980MPa grade ultra-low carbon bainitic steel for marine engineering, its chemical composition 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 are iron and unavoidable impurities. Its preparation method comprises the following steps:

Step 1: Hot metal pretreatment:

Use low-P and low-S molten iron for treatment, deep desulfurization of molten iron, and thoroughly remove slag after desulfurization.

Step 2: Converter smelting + LF refining + RH refining:

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

Step 3: Continuous Casting:

The water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone is adopted. The superheat is 18°C and the casting speed is 1.1m/min. After continuous casting, it enters the slow cooling pit for treatment.

Step 4: Controlled rolling and controlled cooling:

The continuous casting slab is heated and kept at a temperature of 1250°C for a holding time of 2 hours; the rolling adopts a controlled rolling and controlled cooling process, which is divided into two stages of rough rolling and finish rolling. The start temperature of rough rolling is 1180°C, and the end temperature of rough rolling is 1000°C, the cumulative deformation of rough rolling is 68%, the start temperature of finish rolling is 930°C, the end temperature of finish rolling is 850°C, and the cumulative deformation of finish rolling is 35%. After finish rolling, the steel plate is directly quenched to room temperature.

Step 5: Tempering heat treatment:

Tempering heat treatment temperature is 600°C, holding time is 3h, air cooling after heat preservation.

The thickness specification of the 980MPa grade ultra-low carbon bainite steel plate prepared in this example is 60 mm, and the microstructure is composed of lath bainite, M-A components, and film-like retained austenite, as shown in FIG. 2 .

The mechanical properties are: the yield strength is 1084MPa; the tensile strength is 1210MPa; the transverse impact energy at -60°C is 282J, and the longitudinal impact energy at -60°C is 318J; there is no crack at room temperature (25°C) without preheating welding, and the welded joint The impact energy in the heat-affected zone of coarse grains at -40°C is 196J.

Example 3

A 980MPa grade ultra-low carbon bainitic steel for marine engineering, its chemical composition 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 are iron and unavoidable impurities. Its preparation method comprises the following steps:

Step 1: Hot metal pretreatment:

Use low-P and low-S molten iron for treatment, deep desulfurization of molten iron, and thoroughly remove slag after desulfurization.

Step 2: Converter smelting + LF refining + RH refining:

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

Step 3: Continuous Casting:

The water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone is adopted. The superheat is 25°C and the casting speed is 1.0m/min. After continuous casting, it enters the slow cooling pit for treatment.

Step 4: Controlled rolling and controlled cooling:

The continuous casting slab is heated and kept at a temperature of 1210°C, and the holding time is 2 hours; the rolling adopts a controlled rolling and controlled cooling process, which is divided into two stages of rough rolling and finish rolling. The start temperature of rough rolling is 1160°C, and the end temperature of rough rolling is 990°C, the cumulative deformation of rough rolling is 72%, the start temperature of finish rolling is 920°C, the end temperature of finish rolling is 830°C, and the cumulative deformation of finish rolling is 35%. After finish rolling, the steel plate is directly quenched to room temperature.

Step 5: Tempering heat treatment:

Tempering heat treatment temperature is 520°C, holding time is 3h, air cooling after holding.

The thickness specification of the 980MPa grade ultra-low carbon bainite steel plate prepared in this example is 50 mm, and the microstructure is composed of lath bainite, M-A components, and film-like retained austenite, as shown in FIG. 3 .

The mechanical properties are: the yield strength is 1075MPa; the tensile strength is 1164MPa; the transverse impact energy at -60°C is 257J, and the longitudinal impact energy at -60°C is 294J; there is no crack at room temperature (25°C) without preheating welding, and the welded joint The impact energy in the heat-affected zone of coarse grains at -40°C is 175J.

Example 4

A 980MPa grade ultra-low carbon bainitic steel for marine engineering, its chemical composition 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 are iron and unavoidable impurities. Its preparation method comprises the following steps:

Step 1: Hot metal pretreatment:

Use low-P and low-S molten iron for treatment, deep desulfurization of molten iron, and thoroughly remove slag after desulfurization.

Step 2: Converter smelting + LF refining + RH refining:

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

Step 3: Continuous Casting:

The water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone is adopted. The superheat is 23°C and the casting speed is 1.0m/min. After continuous casting, it enters the slow cooling pit for treatment.

Step 4: Controlled rolling and controlled cooling:

The continuous casting slab is heated and kept at a temperature of 1220°C for a holding time of 2 hours; the rolling adopts a controlled rolling and controlled cooling process, which is divided into two stages of rough rolling and finish rolling. The start temperature of rough rolling is 1150°C, and the end temperature of rough rolling is 985°C, the cumulative deformation of rough rolling is 72%, the start temperature of finish rolling is 910°C, the end temperature of finish rolling is 800°C, and the cumulative deformation of finish rolling is 37%. After finish rolling, the steel plate is directly quenched to room temperature.

Step 5: Tempering heat treatment:

Tempering heat treatment temperature is 525°C, holding time is 3h, air cooling after holding.

The thickness specification of the 980MPa grade ultra-low carbon bainite steel plate prepared in this embodiment is 80mm, and the microstructure is composed of lath bainite, M-A components, and film-like retained austenite.

The mechanical properties are: the yield strength is 1058MPa; the tensile strength is 1172MPa; the transverse impact energy at -60°C is 287J, and the longitudinal impact energy at -60°C is 324J; there is no crack at room temperature (25°C) without preheating welding, and the welded joint The impact energy in the heat-affected zone of coarse grains at -40°C is 188J.

Example 5

A 980MPa grade ultra-low carbon bainitic steel for marine engineering, its chemical composition 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 are iron and unavoidable impurities. Its preparation method comprises the following steps:

Step 1: Hot metal pretreatment:

Use low-P and low-S molten iron for treatment, deep desulfurization of molten iron, and thoroughly remove slag after desulfurization.

Step 2: Converter smelting + LF refining + RH refining:

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

Step 3: Continuous Casting:

The water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone is adopted. The superheat is 20°C and the casting speed is 1.1m/min. After continuous casting, it enters the slow cooling pit for treatment.

Step 4: Controlled rolling and controlled cooling:

The continuous casting slab is heated and kept at a temperature of 1220°C, and the holding time is 2 hours; the rolling adopts a controlled rolling and controlled cooling process, which is divided into two stages of rough rolling and finish rolling. The start temperature of rough rolling is 1170°C, and the end temperature of rough rolling is 1000°C, the cumulative deformation of rough rolling is 70%, the start temperature of finish rolling is 920°C, the end temperature of finish rolling is 805°C, and the cumulative deformation of finish rolling is 38%. After finish rolling, the steel plate is directly quenched to room temperature.

Step 5: Tempering heat treatment:

Tempering heat treatment temperature is 520°C, holding time is 2.5h, air cooling after holding.

The thickness specification of the 980MPa grade ultra-low carbon bainite steel plate prepared in this embodiment is 75 mm, and the microstructure is composed of lath bainite, M-A components, and film-like retained austenite.

The mechanical properties are: the yield strength is 1028MPa; the tensile strength is 1167MPa; the transverse impact energy at -60°C is 244J, and the longitudinal impact energy at -60°C is 294J; there is no crack at room temperature (25°C) without preheating welding, and the welded joint The impact energy in the heat-affected zone of coarse grains at -40°C is 168J.

Example 6

A 980MPa grade ultra-low carbon bainitic steel for marine engineering, its chemical composition 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 are iron and unavoidable impurities. Its preparation method comprises the following steps:

Step 1: Hot metal pretreatment:

Use low-P and low-S molten iron for treatment, deep desulfurization of molten iron, and thoroughly remove slag after desulfurization.

Step 2: Converter smelting + LF refining + RH refining:

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

Step 3: Continuous Casting:

The water volume scheme of strong cooling in the crystallizer and weak cooling in the secondary cooling zone is adopted, the superheat is 18°C, and the casting speed is 1.0m/min. After continuous casting, it enters the slow cooling pit for treatment.

Step 4: Controlled rolling and controlled cooling:

The continuous casting slab is heated and kept at a temperature of 1250°C for a holding time of 2 hours; the rolling adopts a controlled rolling and controlled cooling process, which is divided into two stages of rough rolling and finish rolling. The start temperature of rough rolling is 1180°C, and the end temperature of rough rolling is 1010°C, the cumulative deformation of rough rolling is 72%, the start temperature of finish rolling is 930°C, the end temperature of finish rolling is 840°C, and the cumulative deformation of finish rolling is 35%. After finish rolling, the steel plate is directly quenched to room temperature.

Step 5: Tempering heat treatment:

Tempering heat treatment temperature is 580°C, holding time is 2h, air cooling after heat preservation.

The thickness specification of the 980MPa grade ultra-low carbon bainite steel plate prepared in this embodiment is 20 mm, and the microstructure is composed of lath bainite, M-A components, and film-like retained austenite.

The mechanical properties are: the yield strength is 1084MPa; the tensile strength is 1213MPa; the transverse impact energy at -60°C is 255J, and the longitudinal impact energy at -60°C is 289J; there is no crack at room temperature (25°C) without preheating welding, and the welded joint The impact energy in the heat-affected zone of coarse grains at -40°C is 182J.

Table 1 embodiment steel chemical composition (wt, %)

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

Table 2 Example steel preparation process

Table 3 Example steel mechanical properties

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed in the present invention can easily think of changes or Replacement should be covered within the protection scope of the present invention.

Claims (10)

1. A 980MPa grade ultra-low carbon bainitic steel for marine engineering, characterized in that the chemical composition comprises by weight: C≤0.04%, Mn: 1.3% to 1.5%, Mo: 1.0% to 1.2%, Nb Ni: 0.05% to 0.08%, Ni: 6.0% to 8.0%, Cr: 0.8% to 1.2%, Ti: 0.010% to 0.020%, Al: 0.40% to 0.60%, and the rest are iron and unavoidable impurities. 2. according to the described 980MPa level marine engineering ultra-low carbon bainitic steel of claim 1, it is characterized in that, the microstructure of described steel is lath bainite, MA constituent element and thin-film retained austenite, plate There are nanoscale Ni ₃ Al precipitates dispersed in the striped bainite. 3. The 980MPa grade ultra-low carbon bainitic steel for marine engineering according to claim 1, wherein the maximum thickness of the steel is 80mm. 4. The 980MPa grade ultra-low carbon bainitic steel for marine engineering according to claim 1, wherein the chemical composition includes 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%, the rest is iron and unavoidable impurities. 5. A preparation method for ultra-low carbon bainitic steel for 980MPa grade marine engineering, for preparing the ultra-low carbon bainitic steel for 980MPa grade marine engineering according to any one of claims 1 to 4, characterized in that, Including hot metal pretreatment-converter smelting-LF refining-RH refining-continuous casting-controlled rolling and controlled cooling-tempering heat treatment process steps, the RH refining, control of impurity content, by weight: P≤0.008%, S≤0.003% , O≤0.0030%, N≤0.0040%, H≤0.0002%. 6. according to the preparation method of the described 980MPa level marine engineering of claim 5, ultra-low carbon bainitic steel, it is characterized in that, described continuous casting process adopts the water yield of strong cooling in crystallizer and weak cooling of secondary cooling zone, controls over The temperature is 15-25°C, the casting speed is ≤1.1m/min, and it enters the slow cooling pit after continuous casting. 7. The method for preparing 980MPa grade ultra-low carbon bainitic steel for marine engineering according to claim 5, characterized in that the controlled rolling and controlled cooling heats the continuous casting slab, the heating temperature is 1200-1250 ° C, and the heating time 1.5～2h. 8. according to the preparation method of the described 980MPa level marine engineering ultra-low carbon bainitic steel of claim 7, it is characterized in that, described controlled rolling and controlled cooling adopts two-stage rolling, and the first-order stage rolling is rough rolling, The rolling start temperature is 1150-1180°C, the finish rolling temperature is ≥980°C, and the cumulative deformation is ≥65%. 9. The preparation method of 980MPa grade ultra-low carbon bainitic steel for marine engineering according to claim 8, characterized in that, in the two-stage rolling, the starting rolling temperature is 900-930° C., and the finishing rolling temperature is 800° C. ~850℃, accumulative deformation ≥30%. 10. The method for preparing 980MPa grade ultra-low carbon bainitic steel for marine engineering according to claim 9, characterized in that the tempering heat treatment process is a heating temperature of 500°C-600°C and a holding time of 2-3h.

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