Ultrahigh-strength steel for ocean engineering and manufacturing method thereof

Abstract

The invention relates to ultrahigh-strength steel for ocean engineering and a manufacturing method thereof, belongs to the technical field of steel for ocean engineering, and solves the problems of poor low-temperature toughness, poorer weldability, insufficient corrosion resistance, higher manufacturing and welding cost and the like of ultrahigh-strength steel in the prior art. The invention provides ultrahigh-strength steel for ocean engineering. The steel for ocean engineering comprises the following components in percentage by mass: 0.020%-0.050% of C, 6.00%-10.00% of Ni, 1.50%-3.50% of Cu, 0.60%-1.0% of Mo, 0.80%-1.50% of Cr, 0.60%-1.0% of Mn, 0.10%-0.30% of Si, 0.040%-0.060% of V, 0.010%-0.020% of Ti, 0 < N < = 0.04%, < = 0.005% of P, < = 0.003% of S and the balance of Fe and inevitable impurities. The steel for ocean engineering is excellent in comprehensive performance, good in obdurability matching degree, excellent in welding performance and capable of achieving low-temperature preheating welding.

Description

Technical Field

[0001] The present invention relates to the technical field of marine engineering steel, and in particular to an ultra-high strength marine engineering steel and a manufacturing method thereof. Background Art

[0002] In recent years, as countries around the world attach importance to the development and utilization of marine resources and trade, the marine equipment industry such as ships, offshore platforms, and submersibles has risen rapidly, and is also developing in the direction of lightweight, high-end, and low-cost. This has put forward new requirements for the comprehensive performance of structural steel used in equipment construction, such as the need for higher-strength steel to replace low-strength steel to achieve the purpose of equipment weight reduction, the need for better low-temperature toughness to ensure the safety of equipment under impact loads, and good weldability and corrosion resistance, etc., to achieve low cost and reliability during equipment construction and service.

[0003] At present, the existing 1000MPa grade ultra-high strength steel is generally used for engineering machinery manufacturing. The design idea is to use Cr-Mo-V / Nb alloying technology on the basis of low carbon, and obtain nano-scale micro-alloy carbides and tempered martensite structure through quenching and tempering heat treatment process to achieve high strength. However, the weldability of this type of high-strength steel is poor. In order to avoid cold cracks at the welded joints after welding, the steel plate needs to be preheated at 200℃ or above before welding, resulting in high welding costs; and this type of high-strength steel generally has insufficient low-temperature toughness reserves, and has great safety risks in service.

[0004] In recent years, in order to meet the construction needs of high-end marine equipment and further improve the comprehensive performance of ultra-high strength steel such as low-temperature toughness and welding, researchers have developed 1000MPa ultra-high strength steel through alloy design and process improvement. For example, the invention patent with publication number CN 112779472 A describes a 1GPa-grade marine engineering steel plate with excellent low-temperature toughness and its preparation method. It has a good match between strength and toughness, with a yield strength of 1000MPa and a low-temperature toughness of 150J at -80°C. However, the welding performance of the steel is poor, and in order to obtain higher low-temperature toughness, it must be quenched twice, and the process cost is relatively high.

[0005] In summary, the existing steel grades focus on the strength of steel during design and development, and do not take into account the steel's low-temperature toughness, weldability, corrosion resistance and other properties in alloy design and process manufacturing. They cannot meet the demand for ultra-high-strength steel for high-end marine equipment manufacturing, and generally have high manufacturing and welding costs. Summary of the invention

[0006] In view of the above analysis, an embodiment of the present invention aims to provide an ultra-high strength marine engineering steel and a manufacturing method thereof, so as to solve at least one of the problems of ultra-high strength steel in the prior art, such as low strength-toughness matching, poor low-temperature toughness, poor weldability, and high manufacturing and welding costs.

[0007] The present invention provides an ultra-high strength marine engineering steel, wherein the components in the marine engineering steel are calculated by mass percentage as follows: C: 0.020% to 0.050%, Ni: 6.00% to 10.00%, Cu: 1.50% to 3.50%, Mo: 0.60% to 1.0%, Cr: 0.80% to 1.50%, Mn: 0.60% to 1.0%, Si: 0.10% to 0.30%, V: 0.040% to 0.060%, Ti: 0.010% to 0.020%, 0<N≤0.0 4%, P≤0.005%, S≤0.003%, the balance being Fe and unavoidable impurities; and Ceq=[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.63%; Pcm=[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.57%, wherein [X] is the mass percentage of the corresponding element X.

[0008] Preferably, the components in the marine engineering steel are as follows by mass percentage: C: 0.023% to 0.042%, Ni: 6.56% to 7.01%, Cu: 1.55% to 2.89%, Mo: 0.65% to 0.76%, Cr: 0.86% to 1.26%, Mn: 0.62% to 0.75%, Si: 0.11% to 0.23%, V: 0.041% to 0.048%, Ti: 0.011% to 0.015%, N: 0.02% to 0.035%, P ≤0.005%, S≤0.003%, the balance being Fe and unavoidable impurities; and Ceq=[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.28%; Pcm=[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.47%, wherein [X] is the mass percentage of the corresponding element X.

[0009] Preferably, the components in the marine engineering steel are calculated by mass percentage as follows: C: 0.043% to 0.048%, Ni: 7.15% to 9.6%, Cu: 2.9% to 3.49%, Mo: 0.78% to 0.97%, Cr: 0.88% to 1.47%, Mn: 0.76% to 1.0%, Si: 0.24% to 0.30%, V: 0.049% to 0.058%, Ti: 0.016% to 0.019%, N: 0.036% to 0.038%, P≤ 0.005%, S≤0.003%, the balance being Fe and unavoidable impurities; and Ceq=[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.52%; Pcm=[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.54%, wherein [X] is the mass percentage of the corresponding element X.

[0010] Specifically, the microstructure of the steel is composed of tempered martensite and nano-scale precipitation phase, wherein the volume fraction of martensite is 97.0% to 99.0%, the volume fraction of nano-scale precipitation phase is 1.0% to 3.0%, and the sum of the two is 100%.

[0011] Specifically, the nanoscale precipitated phase consists of a Cu-rich phase and a (V, Mo)C phase, and the particle size of the nanoscale precipitated phase ranges from 5 to 20 nm; the precipitation strengthening increment accounts for more than 20% of the yield strength.

[0012] Specifically, the yield strength of the marine engineering steel is ≥1000MPa, the elongation is ≥16%, the -80℃ Charpy V-notch impact energy is ≥200J, the -80℃ section fiber rate is ≥80%, the ductile-brittle transition temperature is lower than -100℃, the annual corrosion rate is ≤0.14mm / a, and low-temperature preheating welding can be achieved.

[0013] The present invention also provides a method for manufacturing the marine engineering steel, which is characterized in that it specifically includes the following steps: S1 converter smelting, S2 slab continuous casting, S3 low-temperature heating, S4 segmented rolling, S5 solution heat treatment and quenching, S6 tempering heat treatment; wherein the S4 segmented rolling includes recrystallization zone rolling and non-recrystallization zone rolling.

[0014] Specifically, the specific process parameters of the S3 low-temperature heating step are: heating temperature is 1150-1180° C., and insulation time is 90-120 min.

[0015] Specifically, the specific process parameters of the recrystallization zone rolling are: a total of 2 to 5 passes of large reduction rolling within the recrystallization zone temperature range, the starting rolling temperature is 1050 to 1150° C., the final rolling temperature is 980 to 1000° C., the rolling speed is ≤1.5 m / s, and the reduction of 2 to 3 passes is ≥20%;

[0016] The specific process parameters of the non-recrystallization zone rolling are: a total of 3 to 10 low-temperature and large-reduction rolling passes within the non-recrystallization zone temperature range, starting rolling temperature: 920 to 980°C, final rolling temperature: 820 to 880°C, laminar cooling to room temperature after rolling, wherein the reduction in 3 to 6 passes is ≥18%.

[0017] Specifically, the specific process parameters of the S5 solution heat treatment and quenching step are: the rolled steel plate is subjected to solution heat treatment, the solution temperature is 800-880°C, the solution time is 1.5-2h, and then quenching is performed, the water quenching speed is controlled at 5-10m / min, the cooling rate is ≥20°C / s, and the final cooling temperature of the steel plate surface is ≤200°C.

[0018] Specifically, the specific process parameters of the S6 aging heat treatment step are: subjecting the quenched steel plate to aging heat treatment in a heat treatment furnace at a heating temperature of 550-580° C. for an aging time of 100-120 min, and then air cooling to room temperature.

[0019] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0020] 1. The present invention achieves matching of strength and toughness while ensuring high strength of marine engineering steel through reasonable composition design, and has excellent low-temperature toughness. Each element plays its own indispensable role and achieves the final comprehensive effect through synergy. The elements / element groups with more prominent effects are designed as follows:

[0021] The marine engineering steel adopts an ultra-low carbon design, which significantly improves the low-temperature toughness of the steel and improves the weldability; Cu is added as a key precipitation strengthening element, which precipitates from the steel in the form of nanophase during the aging process, thereby producing a significant precipitation strengthening effect, which can make up for the strength loss caused by reducing the C content. At the same time, Cu can also improve the seawater corrosion resistance of the steel; the large amount of Ni added can refine the austenite grain size, improve the hardenability, and refine the martensite substructure size, thereby improving the low-temperature toughness of the steel, reducing the ductile-brittle transition temperature, and making the steel have excellent low-temperature toughness. At the same time, the above-mentioned role of Ni can provide a favorable nucleation position for the precipitation of Cu, promote the precipitation of Cu, and enhance the precipitation strengthening effect of Cu. In addition, the addition of Ni can also prevent the hot brittleness effect of Cu, significantly improve the surface hot crack tendency of Cu-containing steel, and improve the corrosion resistance of steel; solid solution Mo can significantly improve the hardenability of steel, and also has a certain solid solution strengthening effect, and promotes the precipitation of V during tempering. At the same time, Mo can improve the tempering stability and significantly reduce the tempering brittleness; V is a strong carbide forming element, which can combine with elements such as C and Mo during the aging process to form nano-scale (V, Mo) C particles, and precipitate complementary with Cu, which can play a good precipitation strengthening role; Ti is a strong carbonitride forming element, and a trace amount of Ti can combine with N in steel to form TiN, which prevents the growth of austenite grains during soaking, and can also prevent the growth of austenite grains in the welding heat affected zone, thereby improving weldability.

[0022] The marine engineering steel provided by the present invention has excellent comprehensive performance. While meeting the 1000MPa grade ultra-high strength, the steel of the present invention has excellent low temperature toughness, -80℃ Charpy V-notch impact energy KV 2(-80℃) ≥200J (single value), and the impact fracture fiber rate (FA -80℃ )≥80% (single value), the ductile-brittle transition temperature is lower than -100°C. At the same time, the elongation of the steel is ≥16%, and the annual corrosion rate of the steel is ≤0.14mm / a.

[0023] 2. The carbon content and carbon equivalent of the marine engineering steel provided by the present invention are controlled at a low level, the welding performance is excellent, and low-temperature (≤100°C) preheating welding can be achieved, which saves energy consumption, reduces welding costs, and has low application costs.

[0024] 3. The manufacturing method of the marine engineering steel provided by the present invention is simple, and excellent comprehensive performance can be achieved by quenching and tempering heat treatment after hot rolling. The solid solution treatment allows all alloy elements to fully dissolve and reach a solid solution equilibrium state, and then roller quenching is performed to allow the steel plate to undergo a martensitic phase transformation under rapid cooling conditions, and then aging heat treatment (i.e. tempering) is performed to allow the dissolved Cu, V and other alloy elements to precipitate, forming nano-scale Cu-rich phases, C (V, Mo) phases, etc., which play a strengthening role; and the preparation process cost is relatively low.

[0025] 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

[0026] 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.

[0027] Figure 1 is the metallographic diagram of the ultra-high strength marine engineering steel in Example 1;

[0028] Figure 2 The electron microscope photograph and energy spectrum diagram (lower left corner) of the nanoscale precipitated phase particle morphology of the ultra-high strength marine engineering steel in Example 1. DETAILED DESCRIPTION

[0029] 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.

[0030] The present invention provides an ultra-high strength marine engineering steel, wherein the components in the marine engineering steel are as follows by mass percentage: C: 0.020% to 0.050%, Ni: 6.00% to 10.00%, Cu: 1.50% to 3.50%, Mo: 0.60% to 1.0%, Cr: 0.80% to 1.50%, Mn: 0.60% to 1.0%, Si: 0.10% to 0.30%, V: 0.040% to 0.060%, Ti: 0.010% to 0.020%, N≤0.04 %, P≤0.005%, S≤0.003%, the balance being Fe and unavoidable impurities; and Ceq=[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.63%; Pcm=[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.57%, wherein [X] is the mass percentage of the corresponding element X.

[0031] The effects (including synergistic effects) and proportions of the elements of the present invention are as follows:

[0032] C: has a significant solid solution strengthening effect and can improve the hardenability of steel, but the increase in C content in steel is very unfavorable to the low temperature toughness and weldability of steel. Therefore, the present invention adopts an ultra-low carbon design, and the C content is controlled at 0.02-0.05wt.%.

[0033] Ni: The addition of Ni can refine the austenite grain size, improve hardenability, and refine the martensite substructure size, thereby improving the low-temperature toughness of the steel, reducing the ductile-brittle transition temperature, and making the steel have excellent low-temperature toughness. At the same time, the above-mentioned role of Ni can provide a favorable nucleation position for the precipitation of the Cu precipitation phase, promote Cu precipitation, and enhance the Cu precipitation strengthening effect. In addition, the addition of Ni can also prevent the hot brittleness effect of Cu, significantly improve the surface hot crack tendency of Cu-containing steel, and improve the corrosion resistance of the steel. Considering the toughness and corrosion resistance of the steel of the present invention, the Ni content in the present invention is controlled at 6.00-10.00wt.%.

[0034] Cu: The key precipitation strengthening element of the steel of the present invention. Cu can be precipitated in the steel in the form of nanoclusters during the aging process without relying on C and N elements, thereby producing a significant precipitation strengthening effect, which can make up for the strength loss caused by reducing C. At the same time, Cu can also improve the seawater corrosion resistance of the steel. In order to ensure that the steel has a certain strength, the content of Cu in the present invention is controlled at 1.50-3.50wt.%.

[0035] Mo: Solid solution Mo can significantly improve the hardenability of steel, and also has a certain solid solution strengthening effect, and promotes the precipitation of V during the aging process. At the same time, Mo can improve the tempering stability and significantly reduce the tempering brittleness. In the present invention, the Mo content is controlled at 0.60-1.0wt.%.

[0036] Cr: Solid solution Cr can significantly improve the hardenability of steel (it has the effect of reducing Ac3 temperature and improving the stability of supercooled austenite). At the same time, Cr is easy to form a dense oxide film, which can significantly improve the corrosion resistance of steel. It is one of the more critical alloying elements in the steel of the present invention. The Cr content in the present invention is controlled at 0.80-1.50wt.%.

[0037] Mn: It can significantly improve the hardenability of steel, and has a certain solid solution strengthening effect. It can also reduce the Ac3 temperature, making the temperature window of rolling in the non-recrystallization zone wider. However, when the Mn content is too high, the corrosion resistance of the steel is reduced, and coarse M / A islands are easily formed in the welding heat affected zone, which seriously affects the low-temperature toughness of the welding heat affected zone. In the present invention, the Mn content is controlled at 0.60-1.00wt.%.

[0038] Si: One of the deoxidizing elements in steel. Silicon is also a non-carbide forming element. It exists in the steel matrix in the form of solid solution and has a certain solid solution strengthening effect. However, excessive silicon is not good for the low-temperature toughness of steel. The sensitivity of steel to welding cold cracks and hot cracks will increase. The Si content of the present invention is controlled to 0.10-0.30wt.%.

[0039] V: is a strong carbide-forming element, which can combine with elements such as C and Mo during the aging process to form nano-scale (V, Mo)C particles, which precipitate complementary to Cu precipitation and can play a good precipitation strengthening role. Considering the strength of the present invention, the V content in the present invention is controlled at 0.04-0.06%.

[0040] Ti: A strong carbonitride-forming element. A trace amount of Ti can combine with N in steel to form TiN, which prevents the growth of austenite grains during soaking and can also prevent the growth of austenite grains in the heat-affected zone of welding, thereby improving weldability. In the present invention, the Ti content is controlled at 0.01-0.02wt.%.

[0041] N: A small amount of N element can combine with Ti element to form TiN particles. The N content is generally determined according to the Ti content. If too much is added, it will exist in the form of solid solution to increase the carbon equivalent. If too little is added, Ti cannot be completely fixed. Therefore, the N content is controlled at 0<N≤0.04%.

[0042] P and S: impurity elements in steel, which are very detrimental to the performance of steel, especially low-temperature toughness. The content of P and S in the steel of the present invention should be strictly controlled. The two impurity elements are controlled at P≤0.005% and S≤0.003%.

[0043] Preferably, the components in the marine engineering steel are as follows by mass percentage: C: 0.023% to 0.042%, Ni: 6.56% to 7.01%, Cu: 1.55% to 2.89%, Mo: 0.65% to 0.76%, Cr: 0.86% to 1.26%, Mn: 0.62% to 0.75%, Si: 0.11% to 0.23%, V: 0.041% to 0.048%, Ti: 0.011% to 0.015%, N: 0.02% to 0.035%, P ≤0.005%, S≤0.003%, the balance being Fe and unavoidable impurities; and Ceq=[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.28%; Pcm=[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.47%, wherein [X] is the mass percentage of the corresponding element X.

[0044] Preferably, the components in the marine engineering steel are calculated by mass percentage as follows: C: 0.043% to 0.048%, Ni: 7.15% to 9.6%, Cu: 2.9% to 3.49%, Mo: 0.78% to 0.97%, Cr: 0.88% to 1.47%, Mn: 0.76% to 1.0%, Si: 0.24% to 0.30%, V: 0.049% to 0.058%, Ti: 0.016% to 0.019%, N: 0.036% to 0.038%, P≤ 0.005%, S≤0.003%, the balance being Fe and unavoidable impurities; and Ceq=[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.52%; Pcm=[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.54%, wherein [X] is the mass percentage of the corresponding element X.

[0045] Specifically, the microstructure of the marine engineering steel is composed of tempered martensite and nano-scale precipitation phase, wherein the volume fraction of martensite is 97.0% to 99.0%, the volume fraction of nano-scale precipitation phase is 1.0% to 3.0%, and the sum of the two is 100%. Although the steel of the present invention has a low C content, it contains a high content of alloy elements such as Ni that improve the hardenability of the steel. After quenching, the steel can obtain full martensitic steel, and the volume fraction of martensite can reach more than 97%. Since Ni can refine the organization, the size of the martensitic substructure lath block is very small, so the strength and toughness of the steel can be guaranteed. In addition to martensite, the composition of steel also has a certain volume fraction (1.0% to 3.0%) of nano-scale precipitates, such as Cu-rich phase, (V, Mo)C phase, etc., which play a role in precipitation strengthening. If the volume fraction of the precipitate phase is too low (<1.0%), the precipitation strengthening effect is poor. If it is too high (>3.0%), more Cu, V, Mo, C and other elements need to be added, and the carbon equivalent of the steel will increase, which will affect the weldability.

[0046] Specifically, the nanoscale precipitated phase is composed of a Cu-rich phase and a (V, Mo) C phase, and the particle size of the nanoscale precipitated phase ranges from 5 to 20 nm; the precipitation strengthening increment accounts for more than 20% of the yield strength. The size of nanoscale precipitated phases such as the Cu-rich phase and the (V, Mo) C phase is controlled at 5 to 20 nm, and the strengthening effect is optimal. If the size is less than 5 nm, the strengthening effect is more significant, but the toughness level of the steel will be greatly reduced at this time. If the size exceeds 20 nm, the strengthening effect will be reduced, and the strengthening effect of the steel of the present invention will not be achieved.

[0047] Specifically, the yield strength of the marine engineering steel is ≥1000MPa, the elongation is ≥16%, the -80℃ Charpy V-notch impact energy is ≥200J, the -80℃ section fiber rate is ≥80%, the ductile-brittle transition temperature is lower than -100℃, the annual corrosion rate is ≤0.14mm / a, and low-temperature preheating welding can be achieved.

[0048] The present invention also provides a method for manufacturing the marine engineering steel, which is characterized in that it specifically includes the following steps: S1 converter smelting, S2 slab continuous casting, S3 low-temperature heating, S4 segmented rolling, S5 solution heat treatment and quenching, S6 aging heat treatment; wherein the S4 segmented rolling includes recrystallization zone rolling and non-recrystallization zone rolling.

[0049] Specifically, the function of the S1 converter smelting step is mainly to remove impurities in the molten iron and make the content of each component reach a preset value. The existing process and equipment can meet the implementation needs.

[0050] Specifically, the S2 slab continuous casting step adopts low superheat pouring, the pouring superheat is no more than 20°C, and light pressure reduction and electromagnetic stirring technology are adopted to reduce the degree of segregation in the core of the steel plate.

[0051] Specifically, the specific process parameters of the S3 low-temperature heating step are: heating temperature is 1150-1180° C., and heat preservation is 90-120 minutes, so that the alloy elements in the steel are fully dissolved, and the steel plate is controlled to have a finer austenite grain size.

[0052] Specifically, in step S4, the specific process parameters of the recrystallization zone rolling are: a total of 2 to 5 passes of large reduction rate rolling within the temperature range of the recrystallization zone, a start rolling temperature of 1050 to 1150°C, a final rolling temperature of 980 to 1000°C, a rolling speed of ≤1.5 m / s, wherein the single pass reduction of 2 to 3 passes is ≥20%; during rolling, the single pass reduction is large and the rolling speed is slow, so that the steel plate has a large rolling penetration force, so that the recrystallization occurs to the maximum extent on its surface and core, and the segregation of elements is eliminated to a certain extent;

[0053] The rough rolling stage (recrystallization zone rolling) mainly adopts the technical idea of ​​high temperature and large reduction. Within the temperature range of the recrystallization zone of steel, at least 2 to 3 large deformation rolling passes with a single pass reduction of not less than 20% are carried out to eliminate coarse austenite grains, allow the austenite grains to fully recrystallize, and refine the austenite grain size, especially the grain size in the core of the steel plate, to about level 8. While refining the austenite grains, its uniformity is ensured, thereby improving the uniformity of the strength and toughness of the steel plate.

[0054] Specifically, in step S4, the specific process parameters of the non-recrystallization zone rolling are: a total of 3 to 10 passes of low-temperature and large reduction rolling within the temperature range of the non-recrystallization zone, starting rolling temperature: 920 to 980°C, final rolling temperature: 820 to 880°C, laminar cooling to room temperature after rolling, wherein the single pass reduction of 3 to 6 passes is ≥18%.

[0055] The finishing rolling stage (rolling in the non-recrystallization zone) mainly uses non-recrystallization rolling to make the austenite non-recrystallized, flatten the austenite grains, produce more substructures, dislocations and other defects inside, increase the free energy, and when phase transformation occurs during the subsequent quenching process, the martensite has more nucleation sites, which promotes the refinement of the martensite substructure, thereby improving the strength and toughness of the steel.

[0056] Specifically, the specific process parameters of the S5 solid solution heat treatment and quenching step are: the rolled steel plate is subjected to solid solution heat treatment, and the solid solution temperature is A C3 The temperature is 30-50℃ above the normal temperature, i.e. 800-880℃, the solution time is 1.5-2h, and then quenching is carried out. The water quenching speed is controlled at 5-10m / min, the cooling rate is ≥20℃ / s, and the final cooling temperature of the steel plate surface is ≤200℃.

[0057] Specifically, solution quenching heat treatment is an important process for obtaining excellent strength and toughness of the steel of the present invention. If the temperature is too low, the alloy elements cannot be fully dissolved, and if the temperature is too high, the austenite grains will grow and coarsen, thereby affecting the low-temperature toughness. Therefore, the temperature is controlled within the range of A C3 The temperature should be above 30-50℃, i.e. 800-880℃, and the solution time should be controlled within 1.5-2h.

[0058] Specifically, the specific process parameters of the S6 aging heat treatment step are: the quenched steel plate is subjected to aging heat treatment in a heat treatment furnace, the heating temperature is 10 to 50° C. below the AC1 temperature of the steel, i.e., 550 to 580° C., the aging time is 100 to 120 min, and then air-cooled to room temperature.

[0059] Aging heat treatment is a key process to obtain excellent strength and toughness of the steel of the present invention. If the aging temperature is low, the size of the nanophase is too small, or even a cluster structure. At this time, the strength is very high, but the low-temperature toughness is very poor. If the aging temperature is too high, the toughness will be improved, but the nanophase will grow and coarsen, the matrix will recover, and even recrystallize, and the strength will be reduced. Therefore, the aging temperature is controlled at 10 to 50°C below the AC1 temperature of the steel, 550 to 580°C, and the aging time is preferably controlled at 100 to 120 minutes.

[0060] Examples and Comparative Examples

[0061] Table 1 shows the chemical composition of the examples and comparative steels in the present invention. A total of 8 furnaces were smelted, among which G1 to G5 steels are the example group, and D1 to D3 are the comparative example group. See Table 1 for details.

[0062] Table 1 Chemical composition of steel in the embodiments and comparative examples of the present invention (wt.%)

[0063]

[0064] The manufacturing process adopts: S1 converter smelting, S2 slab continuous casting, S3 low temperature heating, S4 segmented rolling, S5 solution heat treatment and quenching, S6 aging heat treatment; wherein, the S4 segmented rolling includes recrystallization zone rolling and non-recrystallization zone rolling.

[0065] The specific process parameters of steps S3 and S4 are shown in Table 2.

[0066] Table 2 Specific rolling process parameters of steel in the embodiments and comparative examples

[0067]

[0068] The specific process parameters of steps S5 and S6 are shown in Table 3.

[0069] Table 3 Heat treatment process parameters of steel in the examples and comparative examples

[0070]

[0071] The mechanical properties of the steel plates prepared in the embodiments and comparative examples were tested, and the specific results are shown in Table 4.

[0072] Table 4 Mechanical properties of the steels of the embodiments and comparative examples

[0073]

[0074] The marine engineering steel / steel plate Rp prepared by the present invention 0.2 ≥1050MPa, R m ≥1080MPa, elongation / extension ≥18.5%, section shrinkage ≥72%, KV 2(-80℃) ≥208J (average of parallel samples ≥210J), FA (-80℃)≥96% (average of parallel samples ≥98.67%), ductile-brittle transition temperature ≤-110°C. In comparative example D1, the Ni content is low, resulting in a low low-temperature toughness level, with an impact energy of only 71J at -80°C and a ductile-brittle transition temperature of -75°C; in comparative example D2, the carbon content is high, reaching 0.12%, resulting in poor low-temperature toughness and welding performance, with an impact energy of only 79J at -80°C and a ductile-brittle transition temperature of -60°C, and preheating to 160°C is required before welding; in comparative example D3, the Cu content is low, only 1.1%, resulting in a low precipitated Cu content, a weak precipitation strengthening effect, and a yield strength of less than 1000MPa.

[0075] The cold crack sensitivity test of the inclined Y-shaped groove was carried out on the steel plates prepared in the embodiment and the comparative example. The specific results are shown in Table 5, and low-temperature preheating welding can be achieved.

[0076] Table 5 Experimental results of cold crack sensitivity of inclined Y-shaped groove of the embodiment and comparative example steel in the present invention

[0077]

[0078]

[0079] The steel plates prepared in the examples and comparative examples were subjected to full immersion corrosion tests (the experiments were conducted in accordance with the metal material laboratory uniform corrosion full immersion test method GB 10124-88). The specific results are shown in Table 6.

[0080] Table 6 Annual corrosion rate of the steel in the embodiments of the present invention and the comparative example (mm / a)

[0081] Steel Grade G1 G2 G3 G4 G5 D1 D2 D3 Annual corrosion rate 0.138 0.120 0.124 0.130 0.124 0.155 0.125 0.146

[0082] The annual corrosion rate of the marine engineering steel / steel plate prepared by the present invention is ≤0.138mm / a. In comparative example D1, due to the low Ni content, the corrosion resistance and annual corrosion rate reach 0.155mm / a. In comparative example D3, due to the low alloy content of Cu, Cr, etc., the corrosion resistance and annual corrosion rate reach 0.146mm / a. Although comparative example D2 has relatively good corrosion resistance, which is roughly at the same level as the embodiment, the low temperature toughness and welding performance are significantly insufficient, and the comprehensive performance cannot meet the application requirements.

[0083] In summary, the steel of the present invention adopts the alloy design concept of ultra-low carbon, high Ni toughening, and Cu aging strengthening, combined with two-stage rolling and quenching and tempering heat treatment process, and designs and invents an ultra-high strength steel for marine engineering with a yield strength of not less than 1000MPa, excellent low-temperature toughness, easy weldability and corrosion resistance. The steel has excellent comprehensive performance, R p0.2 ≥1050MPa, R m≥1080MPa, elongation / extension ≥18.5%, section shrinkage ≥72%, K V2 (-80℃)≥208J (average of parallel samples ≥210J), FA(-80℃)≥96% (average of parallel samples ≥98.67%), ductile-brittle transition temperature≤-110℃, low-temperature (≤100℃) preheating welding can be achieved, annual corrosion rate≤0.138mm / a, which is significantly improved compared with the existing technology.

[0084] 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. An ultra-high strength marine engineering steel, characterized in that: The components of the marine engineering steel are as follows by mass percentage: C: 0.020% to 0.050%, Ni: 6.00% to 10.00%, Cu: 1.50% to 3.50%, Mo: 0.60% to 1.0%, Cr: 0.80% to 1.50%, Mn: 0.60% to 1.0%, Si: 0.10% to 0.30%, V: 0.040% to 0.060%, Ti: 0.010% to 0.020%, 0<N≤0.04%, P≤0.005%, S≤0.003%, and the balance is Fe and unavoidable impurities; and C eq =[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.63%; P cm =[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.57%, wherein [X] is the mass percentage of the corresponding element X.

2. The marine engineering steel according to claim 1, characterized in that: The components of the marine engineering steel are as follows by mass percentage: C: 0.023% to 0.042%, Ni: 6.56% to 7.01%, Cu: 1.55% to 2.89%, Mo: 0.65% to 0.76%, Cr: 0.86% to 1.26%, Mn: 0.62% to 0.75%, Si: 0.11% to 0.23%, V: 0.041% to 0.048%, Ti: 0.011% to 0.015%, N: 0.02% to 0.035%, P≤0.005%, S≤0.003%, and the balance is Fe and unavoidable impurities; and C eq =[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.28%; P cm =[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.47%, wherein [X] is the mass percentage of the corresponding element X.

3. The marine engineering steel according to claim 1, characterized in that: The components of the marine engineering steel are calculated by mass percentage as follows: C: 0.043% to 0.048%, Ni: 7.15% to 9.6%, Cu: 2.9% to 3.49%, Mo: 0.78% to 0.97%, Cr: 0.88% to 1.47%, Mn: 0.76% to 1.0%, Si: 0.24% to 0.30%, V: 0.049% to 0.058%, Ti: 0.016% to 0.019%, N: 0.036% to 0.038%, P≤0.005%, S≤0.003%, and the balance is Fe and unavoidable impurities; and C eq =[C]+([Mn]+[Si]) / 6+([Ni]+[Cu]) / 15+([Cr]+[Mo]+[V]) / 5≤1.52%; P cm =[C]+[Si] / 30+[Ni] / 60+([Mn]+[Cr]+[Cu]) / 20+[Mo] / 15+[V] / 10+5[B]≤0.54%, wherein [X] is the mass percentage of the corresponding element X.

4. The marine engineering steel according to claim 1, characterized in that: The microstructure of the steel consists of tempered martensite and nano-scale precipitation phase, wherein the volume fraction of martensite is 97.0% to 99.0%, the volume fraction of the nano-scale precipitation phase is 1.0% to 3.0%, and the sum of the two is 100%.

5. The marine engineering steel according to claim 4, characterized in that: The nano-scale precipitation phase consists of a Cu-rich phase and a (V, Mo)C phase, and the particle size of the nano-scale precipitation phase ranges from 5 to 20 nm. The precipitation strengthening increment accounts for more than 20% of the yield strength.

6. The marine engineering steel according to claim 1, characterized in that: The marine engineering steel has a yield strength of ≥1000MPa, an elongation of ≥16%, a Charpy V-notch impact energy of ≥200J at -80°C, a fiber rate of a -80°C section of ≥80%, a ductile-brittle transition temperature of less than -100°C, an annual corrosion rate of ≤0.14mm / a, and can achieve low-temperature preheating welding.

7. A method for manufacturing marine engineering steel according to any one of claims 1 to 6, characterized in that: The specific steps include: S1 converter smelting, S2 slab continuous casting, S3 low temperature heating, S4 segmented rolling, S5 solution heat treatment and quenching, S6 aging heat treatment; wherein, the S4 segmented rolling includes recrystallization zone rolling and non-recrystallization zone rolling.

8. The manufacturing method according to claim 7, characterized in that: The specific process parameters of the S3 low-temperature heating step are: heating temperature is 1150-1180° C., and heat preservation is 90-120 minutes.

9. The manufacturing method according to claim 7, characterized in that: The specific process parameters of the recrystallization zone rolling are: 2 to 5 passes of high reduction rate rolling within the recrystallization zone temperature range, starting rolling temperature: 1050 to 1150°C, final rolling temperature: 980 to 1000°C, rolling speed ≤ 1.5 m / s, wherein the reduction of 2 to 3 passes is ≥ 20%; The specific process parameters of the non-recrystallization zone rolling are: 3 to 10 low-temperature and large-reduction rolling passes within the non-recrystallization zone temperature range, starting rolling temperature: 920 to 980°C, final rolling temperature: 820 to 880°C, laminar cooling to room temperature after rolling, wherein the reduction in 3 to 6 passes is ≥18%.

10. The manufacturing method according to claim 7, characterized in that: The specific process parameters of the S5 solid solution heat treatment and quenching step are: the rolled steel plate is subjected to solid solution heat treatment at a solid solution temperature of 800-880°C and a solid solution time of 1.5-2h, and then quenched, the water quenching speed is controlled at 5-10m / min, the cooling rate is ≥20°C / s, and the final cooling temperature of the steel plate surface is ≤200°C; The specific process parameters of the S6 aging heat treatment step are: subjecting the quenched steel plate to aging heat treatment in a heat treatment furnace at a heating temperature of 550-580° C. for an aging time of 100-120 min, and then air cooling to room temperature.

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