CN114086051B - High-strength high-toughness easily-welded nano steel with thickness of 60-120 mm and thickness of 850MPa and preparation method thereof - Google Patents

Abstract

The invention discloses a high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and 850MPa, and a preparation method thereof, wherein the composition is as follows: and C:0.05 to 0.08, si:0.25 to 0.5, mn: 0.8-1.5, P is less than or equal to 0.01, S is less than or equal to 0.0015, cu:1.0 to 1.5, ni:3.5 to 5.0, cr:0.7 to 1.5, mo:0.5 to 1.0, nb:0.02 to 0.1, ti:0.01 to 0.05, al: 0.005-0.05, and the balance of Fe and unavoidable impurities, comprising the following steps: smelting and refining-casting-rolling-heat treatment; the nano steel has the yield strength of more than or equal to 850MPa, the Charpy V notch impact energy of more than or equal to 150J at-84 ℃, the elongation rate of more than or equal to 15 percent, and the characteristics of high strength, high toughness, high plasticity and easy welding.

Description

A 60-120mm thick 850MPa-grade high-strength, high-toughness and easy-to-weld nano-steel and its preparation method

technical field

The invention belongs to the field of alloy steel, and in particular relates to a 60-120mm thick 850MPa-grade high-strength, high-toughness easy-to-weld steel and a manufacturing method thereof, which can be used in the fields of ships, marine engineering equipment, pipelines, heavy machinery equipment, and the like.

Background technique

In China, steel plates with a thickness greater than 60mm are generally called 60-120mm extra-thick plates. Extra-thick plates can be widely used in marine engineering equipment, high-rise buildings, pressure vessels, chemical reaction synthesis towers, bridges and armor. The strengthening mechanism of traditional ultra-thick high-strength steel is mainly martensite or lower bainite structure with high carbon content. In order to ensure sufficient hardenability, it is usually necessary to add higher content of alloying elements such as C, Ni, Cr, Mo, etc., resulting in poor plasticity and toughness of the material, and the increase of carbon equivalent also makes the welding performance poor.

Nano-phase strengthening is an effective strengthening method to increase strength without losing plasticity and toughness. By using Cu nano-precipitated phase strengthening instead of traditional carbon strengthening, a good strengthening effect can also be achieved in the center of extra-thick plates. Lower carbon content also results in better weldability. At present, most high-strength steels above 800MPa are still designed with low-carbon and micro-alloying elements. The high carbon content leads to unsatisfactory plasticity, toughness and welding performance.

In the patent document with the publication number CN112143958A, a 1000MPa-grade steel plate with ultra-thickness, ultra-high toughness and excellent weldability and its manufacturing method are disclosed. The yield strength is greater than 890MPa. At the same time, the TMCP + off-line modulation process is used, the nickel element content is high, and vanadium and boron need to be added to improve hardenability, and the carbon content is high, greater than 0.08%. In addition, this steel only shows the impact performance at -60°C. In contrast, a 60-120mm thick 850MPa-grade high-strength, high-toughness easy-weldable steel of the present invention has better welding performance and low-temperature toughness, and can be used in more severe conditions. Serve under harsh conditions.

In the patent document with the publication number CN106544590A, a 1000MPa grade high toughness, high performance, uniformity and easy welding extra-thick steel plate and its manufacturing method are disclosed. The content is higher than 0.1%, and the impact energy at -40°C is less than 100J.

In the patent document with the publication number CN106636961A, a Cu nano-phase strengthened easy-welding steel and its preparation method are disclosed. Its aluminum content is relatively high, and at the same time only the preparation of steel below 15mm is carried out. There is no explanation for steel plates with a thickness above 15mm. The invention controls the aluminum element at a lower level, adopts secondary refining and post-rolling slow cooling process at the same time, and can produce steel plates with greater thickness.

To sum up, there is currently no large-thickness steel material that can satisfy high strength, high toughness and good welding performance at the same time, which is a great challenge to traditional microstructure design ideas and heat treatment processes.

Contents of the invention

Purpose of the invention: the present invention aims to provide a 60-120mm thick 850MPa grade high-strength, high-toughness easy-to-weld steel and its preparation method, which can meet the requirements for high strength, high toughness and good welding performance of the steel plate. Aging treatment Two-step heat treatment process, the yield strength of the steel plate is ≥850MPa, the elongation is ≥15%, and the Charpy V-notch impact energy at -84°C is ≥150J.

Technical scheme of the present invention is:

A method for preparing 850MPa-grade high-strength, high-toughness and easy-to-weld nano-steel with a thickness of 60-120 mm, comprising the following steps:

(1) Smelting and refining: use the molten iron smelted in an electric furnace, blow oxygen to dephosphorize and decarburize, and deoxidize aluminum, then transfer it to a ladle furnace for refining, and add alloy materials at the same time. In terms of mass fraction, the mass percentage of each element in the alloy is C: 0.05～0.08, Si: 0.25～0.5, Mn: 0.8～1.5, P≤0.01, S≤0.0015, Cu: 1.0～1.5, Ni: 3.5～5.0, Cr: 0.7～1.5, Mo: 0.5～1.0, Nb: 0.02～0.1, Ti: 0.01～0.05, Al: 0.005～0.05, the balance is Fe and unavoidable impurities, adjust the composition to the target composition and then carry out dehydrogenation and deoxidation in VD vacuum furnace;

(2) Pouring into steel ingots: pouring the smelted molten iron into steel ingots by die casting, and stacking the ingots for slow cooling for more than 24 hours;

(3) Billet opening: heat the steel ingot at 1150-1200°C for 18-24 hours, and then use hot rolling to open the billet after heating. In order to ensure the strength of the extra-thick plate, it is rolled under high temperature and high pressure, and the pressing amount is greater than 45mm in at least 3 passes , the final rolling temperature is higher than 900°C; rolling: heat the billet to 1150-1200°C, keep it warm for 2-6 hours, use high-pressure water to remove scale before rolling, use high-pressure water to remove phosphorus during rolling, rolling includes Rough rolling and finishing rolling, the rough rolling temperature is controlled at 1000-1150°C; the finishing rolling start temperature is 950-1050°C, the final rolling temperature is higher than 900°C, and rolled into 60-120mm thick steel plate;

(4) Heat treatment: After the steel plate is held at 800°C-950°C for 60-300 minutes, it is ultra-fast quenched to room temperature, and the quenching retention time is t quenching retention=30+(H-10)×1.5, the unit is min, H is the thickness of the finished steel plate, the unit is mm; then temper at 550-700°C for 120-540 minutes, air-cool to room temperature, and the tempering holding time is ttempering holding=60+(H-10)×2.5, the unit is min , H is the thickness of the finished steel plate, in mm.

A 60-120mm thick 850MPa-grade high-strength, high-toughness and easy-to-weld nano-steel has a microstructure composed of ultra-low carbon lath martensite and flaky inverted austenite.

The yield strength of the nano-steel is ≥850MPa, the Charpy V-notch impact energy at -84°C is ≥150J, and the elongation is ≥15%.

Compared with the prior art, the present invention has the following beneficial effects:

1. The 60-120mm thick 850MPa-grade high-strength, high-toughness, easy-to-weld steel of the present invention replaces traditional carbon strengthening by using copper-rich nano-precipitation phase strengthening, which has low carbon content and good welding performance; at the same time, the content of alloy elements is low and the cost is low. lower. High strength and toughness improve the safety and stability of large and heavy steel structures, and good weldability saves the cost of component manufacturing, especially for ultra-high-strength steel plates, the sensitivity of welding cold cracks is greatly reduced, and the welding preheating and postheating temperatures Lower, wider range of heat in and out, greatly reducing costs.

2. The 60-120mm thick 850MPa-grade high-strength, high-toughness, easy-to-weld steel of the present invention fully exerts the potential of hardenability and hardenability of alloying elements through the regulation and control of rolling and heat treatment processes, and effectively refines the original austenite and martensite The size of the lath bundles ensures a higher density of large-angle grain boundaries, and obtains excellent properties such as yield strength ≥ 850 MPa, elongation ≥ 15%, and Charpy V-notch impact energy ≥ 150J at -84°C.

Description of drawings

The optical microscope photograph of Fig. 1 embodiment 1;

The engineering stress-strain curve of Fig. 2 embodiment 1.

Detailed ways

Below in conjunction with accompanying drawing and embodiment the application of the present invention will be further described:

A 60-120mm thick 1000MPa-grade high-strength, high-toughness easy-weldable steel and its preparation method described in the present invention, the composition of the high-strength steel is as follows: by mass percentage, C: 0.05-0.08, Si: 0.25-0.5, Mn : 0.8～1.5, P≤0.01, S≤0.0015, Cu: 1.0～1.5, Ni: 3.5～5.0, Cr: 0.7～1.5, Mo: 0.5～1.0, Nb: 0.02～0.1, Ti: 0.01～0.05, Al : 0.005 ~ 0.05, the balance is Fe and unavoidable impurities.

The invention principle and composition design basis of the 850MPa grade high-strength, high-toughness and easy-to-weld steel are as follows:

Invention principle: the microstructure of the 60-120mm thick 850MPa grade high-strength and high-toughness easy-weldable steel of the present invention is lath martensite, copper-rich nickel-aluminum composite nano-precipitated phase and reversed austenite. The high strength of the steel of the present invention mainly comes from four aspects: one is precipitation strengthening of copper-rich nano phase, the other is solid solution strengthening of alloy elements, the third is fine grain strengthening of lath martensite, and the fourth is dislocation strengthening. Precipitation strengthening mainly comes from the precipitation of copper, nickel and manganese elements added in the alloy during the aging process, which are evenly distributed in the matrix phase, hindering the movement of dislocations and playing a strengthening role. In the present invention, it can reach more than 200 MPa in the core and the surface at the same time Enhanced effect. The LF ladle refining can ensure the uniform distribution of alloy components, thereby ensuring the effect of precipitation strengthening and solid solution strengthening, and ensuring that these two strengthening methods can play the same role in the core and surface of the medium and thick plate. Fine-grain strengthening is due to the pinning of grain boundaries by compounds of Nb and other elements in the recrystallization rough rolling stage, and the subsequent refinement of austenite grain size in the non-crystallization zone finish rolling stage, and the second is due to lath martensite The effective grain size is the lath bundle size, and the lath bundle size is only a fraction of the original austenite size, so fine-grain strengthening brings a great strength contribution. Dislocation strengthening mainly comes from the high density of dislocations in the lath martensite.

In order to ensure that enough lath martensite or lower bainite can be obtained after quenching, the present invention uses the ideal critical diameter DI to simulate and calculate the hardenability of different alloy compositions, and adjust the alloy composition to make DI greater than 1.5 times the thickness of the steel plate To ensure that the center of the plate also has a sufficient proportion of martensite to provide fine grain strengthening and dislocation strengthening, ensuring the strength of the material.

The parameters of the ideal critical diameter DI formula used in the present invention are obtained from the experiments of this series of high-strength steels, which are more in line with this type of steel, and the ideal critical diameter DI is calculated according to carbon and alloy element factors.

D _I ＝25.4 f _C f _Mn f _Si f _Ni f _Cr f _Mo ψf _Cu (mm)

The toughness of the steel of the present invention mainly comes from the hindering effect of lath martensite or lower bainite on crack propagation and the inhibition of crack initiation and propagation by the passivation of crack tip by reverse transformed austenite. control of things. By adjusting the ideal critical diameter DI of the alloy to control the number density of lath martensite in the core of the medium-thick plate steel, and by VD vacuum degassing to control the content of impurity elements and inclusions, it ensures that the core of the medium-thick plate steel Resilience can reach high levels.

Composition Design Basis:

C: Carbon is a solid solution strengthening element, which plays an important role in improving the strength. Traditional steel materials mainly increase strength through solid solution strengthening of carbon, but excessive carbon will form large pieces of brittle cementite during tempering, seriously affecting toughness, and the increase of high carbon content will affect weldability . The present invention replaces traditional carbon strengthening by using nano phase strengthening, so the carbon content is controlled at 0.05-0.08%.

Cu: Copper is the most important forming element of the precipitated phase. It can increase the strength without losing the plasticity and toughness by forming the nano-scale precipitated phase. At the same time, copper also has the effect of refining the grain. If the copper content is too low, it will affect the strengthening effect, if it is too high, it will easily cause hot embrittlement, which will affect welding and thermal processing. Therefore, the copper content of the present invention is controlled at 1.0-1.5%.

Ni: Nickel is one of the main elements for the formation of nano-precipitated phases. It forms a B2 ordered structure wrapped on the surface of the precipitated phase formed by copper elements, which can increase the thermal stability of the precipitated phase; at the same time, nickel, as an austenite stabilizing element, can fine The size of martensitic lath bundles can be reduced, which can significantly improve the low-temperature toughness; nickel in copper-containing steel can also eliminate copper embrittlement and reduce intergranular cracking during hot rolling; for extra-thick plates, there must be sufficient nickel content to improve quenching. permeability. Therefore, the nickel content of the present invention is controlled at 3.5-5.0%.

Mn: Manganese is one of the main constituent elements of the nano-precipitated phase. At the same time, it can also refine the grain, improve the strength and low-temperature toughness of the steel, but if the content is too high, it will easily cause slab segregation, large structural stress, and decreased welding performance. The manganese content of the invention is controlled at 0.8-1.5%.

Al: Aluminum is a strong deoxidizing element in the steelmaking process, and it can also have the effect of refining grains, but when the content is too high, it will promote the graphitization tendency of carbon in steel and reduce the effect of refining grains , the aluminum content of the present invention is controlled at 0.005-0.05%.

Cr: Chromium can increase the corrosion resistance of steel, while improving hardenability and improving the tempering stability of steel. Since the steel plate of the present invention is relatively thick, the chromium content is controlled at 0.7-1.5%.

Mo: Molybdenum can increase the hardenability of steel, refine grains, and can also form carbides to improve strength, and at the same time, it can promote the nucleation of nano-precipitated phases. Since the steel plate of the present invention is relatively thick, the molybdenum content of the present invention is controlled at 0.5-1.0.

Nb: Niobium can form carbonitrides to pin austenite grain boundaries, prevent grain growth, and also play a role in precipitation strengthening to improve strength. The content of niobium in the present invention is controlled at 0.02-0.1%.

Ti: Titanium can form carbonitrides to pin grain boundaries and refine grains. The niobium content of the present invention is controlled at 0.01-0.05%

The 850MPa grade high-strength high-toughness easy-to-weld steel of the present invention and the preparation method thereof comprise the following steps:

Smelting into molten iron in an electric furnace → oxygen blowing dephosphorization and decarburization → LF ladle refining → VD vacuum furnace treatment → casting → blank opening → rolling → quenching → tempering → flaw detection → performance inspection;

The specific operation of the main process is as follows:

1) Smelting and refining: use the molten iron smelted in an electric furnace, blow oxygen to dephosphorize and decarburize, and deoxidize aluminum, then transfer to a ladle furnace for refining, and add alloy materials at the same time, adjust the composition to the target composition, and then carry out dehydrogenation in a VD vacuum furnace deoxidation;

2) Pouring into steel ingots: pouring the smelted molten iron into steel ingots by die casting, and stacking the ingots for slow cooling for more than 24 hours;

3) Blanking: heat the steel ingot at 1150-1200°C for 18-24 hours, and then use hot rolling to open the billet after heating. In order to ensure the strength of the extra-thick plate, it is rolled under high temperature and high pressure. The finishing temperature is higher than 900°C.

3) Rolling: heat the steel billet to 1150-1200°C, keep it warm for 2-6 hours, use high-pressure water to remove scale before rolling, and use high-pressure water to remove phosphorus during rolling.

Rolling includes rough rolling and finishing rolling. The temperature of rough rolling is controlled at 1000-1150°C; the starting temperature of finish rolling is 950-1050°C, the final rolling temperature is higher than 900°C, and rolled into 60-120mm thick steel plate.

Heat treatment: After the steel plate is held at 800°C-950°C for 60-300 minutes, it is ultra-fast cooled and quenched to room temperature, and the quenching retention time is t quenching retention=30+(H-10)×1.5, the unit is min, and H is the finished product Steel plate thickness, in mm; then tempered at 550-700°C for 120-540 minutes, air-cooled to room temperature, tempering retention time is ttempering retention=60+(H-10)×2.5, the unit is min, H is The thickness of the finished steel plate, in mm.

The chemical composition of the embodiment of the present invention is shown in Table 1 (mass percentage), and the balance is Fe and unavoidable impurities.

Table 1

C Si mn P S Cu Ni Cr Mo Nb Ti Al Example 1 0.055 0.32 1.0 0.006 0.0008 1.5 4.0 1.05 0.98 0.045 0.017 0.037

Embodiment Smelting in an electric furnace, blowing oxygen for dephosphorization and decarburization, and aluminum deoxidation, then transfer to a ladle furnace for refining, carry out deep desulfurization, temperature rise, refining treatment and adjust the composition to the target composition, and blow argon into the molten steel from the breathable brick at the bottom of the ladle The gas is stirred to ensure uniform composition, and then refined in the VD vacuum furnace for degassing and inclusion removal to fully remove the gas and inclusions to ensure the purity of molten steel. Finally, it is cast into ingots and stacked and cooled slowly for 48 hours;

Heat the steel ingot at 1150-1200°C for 18-24 hours. After heating, hot rolling is used to open the billet. In order to ensure the strength of the extra-thick plate, it is rolled under high temperature and high pressure. 900°C.

Subsequently, the billet is reheated to 1160-1200°C, kept at a temperature of 2-6 hours, and then rolled. Use high-pressure water to remove scale before rolling, and use high-pressure water to remove phosphorus during rolling. The rolling includes two steps of rough rolling and finish rolling. The temperature of rough rolling is controlled at 1000-1150°C;

After the steel plate is held at 800-950°C for 60-300 minutes, it is ultra-fast water-cooled and quenched to room temperature; then tempered at 550-700°C for 120-540 minutes, and air-cooled to room temperature. Table 2 is the main rolling process parameters of the embodiment.

Table 2

Table 3 is the heat treatment process parameters.

table 3

After heat treatment, the steel plate was sampled transversely and processed into tensile and impact samples for mechanical performance testing. The results are shown in Table 4.

Table 4

FIG. 1 is an optical microscope photo of the steel plate in Example 1, and the structure is lath martensite. This organization not only ensures that the steel has better strength and toughness, but also ensures better elongation.

FIG. 2 shows the tensile curve of the steel plate in Example 1.

The invention has a wide range of uses and can be applied to key structures such as ships, marine engineering, and aerospace engineering.

The invention discloses a 60-120mm thick 850MPa-grade high-strength, high-toughness, easy-weldable steel and a preparation method thereof. The high-strength, high-toughness, easy-weldable steel is composed as follows: in terms of mass percentage of alloy elements, C: 0.05-0.08, Si: 0.25 ～0.5, Mn: 0.8～1.5, P≤0.01, S≤0.0015, Cu: 1.0～1.5, Ni: 3.5～5.0, Cr: 0.7～1.5, Mo: 0.5～1.0, Nb: 0.02～0.1, Ti: 0.01 ～0.05, Al: 0.005～0.05, the balance is Fe and unavoidable impurities. The preparation method of the high-strength and high-toughness easy-weld steel comprises the following steps: smelting and refining-casting-rolling-heat treatment. In the case of ultra-low carbon content, the extra-thick plate steel of the present invention precipitates a large amount of nano-precipitated phases to improve the strength by adjusting the content of nano-precipitated phase-forming elements and thermomechanical treatment process, and at the same time controls the morphology of reversed austenite, Distribution and volume fraction optimize plasticity and low-temperature toughness, which can achieve yield strength ≥ 850MPa, Charpy V-notch impact energy ≥ 150J at -84°C, and elongation ≥ 15%. It has the characteristics of high strength, high toughness, high plasticity and easy welding . The high-strength and high-toughness easily weldable steel of the invention can be widely used in key structures such as ships, marine engineering, engineering machinery, bridges, oil pipelines, and aerospace engineering.

It should be pointed out that the above examples are only examples of the present invention, and obviously the present invention is not limited to the above examples, but there are many similar changes as required. If a person skilled in the art derives or associates all deformations directly from the content disclosed in the present invention, it shall belong to the protection scope of the present invention.

Claims (3)

1. The preparation method of the high-strength high-toughness easily-welded nano steel with the thickness of 60-120 mm and the thickness of 850MPa is characterized by comprising the following steps:

(1) Smelting and refining: molten iron smelted by adopting an electric furnace, oxygen blowing dephosphorization decarburization, aluminum deoxidation, refining in a ladle furnace, and adding alloy materials at the same time, wherein the mass percentages of elements in the alloy are as follows: 0.055 to 0.08, si:0.25 to 0.5, mn: 0.8-1.5, P is less than or equal to 0.01, S is less than or equal to 0.0015, cu:1.0 to 1.5, ni:3.5 to 5.0, cr:1.05 to 1.5, mo:0.5 to 1.0, nb:0.02 to 0.1, ti:0.01 to 0.05, al: 0.005-0.037, the balance being Fe and unavoidable impurities, adjusting the components to target components, and then performing dehydrogenation and deoxidation in a VD vacuum furnace;

(2) Casting into steel ingots: casting molten iron after smelting into steel ingots by adopting a die casting method, stacking and slowly cooling the ingot for more than 24 hours;

(3) Cogging: heating the steel ingot at 1150-1200 ℃ for 18-24 h, hot rolling and cogging, and rolling at high temperature and high pressure for ensuring the strength of the super-thick plate, wherein the reduction of at least 3 passes is more than 45mm, and the final rolling temperature is higher than 900 ℃; rolling: heating the billet to 1150-1200 ℃, preserving heat for 2-6 hours, removing iron scales by using high-pressure water before rolling, removing phosphorus by using high-pressure water in the rolling process, wherein the rolling comprises rough rolling and finish rolling, and the rough rolling temperature is controlled to be 1000-1150 ℃; the initial rolling temperature of the finish rolling is 950-1050 ℃, the final rolling temperature is higher than 900 ℃, and the finish rolling is performed to obtain a 60-120 mm thick steel plate;

(4) And (3) heat treatment: after the steel plate is subjected to heat preservation for 60-300 minutes at 800-950 ℃, carrying out ultra-fast quenching to room temperature, wherein the quenching maintenance time is tquenching maintenance=30+ (H-10) multiplied by 1.5, the unit is min, and the H is the thickness of the finished steel plate, and the unit is mm; and tempering at 550-700 ℃ for 120-540 minutes, and air cooling to room temperature, wherein the tempering maintaining time is t tempering maintaining=60+ (H-10) ×2.5, the unit is min, and the unit is mm of the thickness of the finished steel plate.

2. A nanosteel prepared by the method of claim 1 wherein the microstructure consists of ultra-low carbon lath martensitic structure and lamellar inverse austenite.

3. The nanosteel of claim 2, wherein the yield strength is not less than 850MPa, the charpy V-notch impact energy at-84 ℃ is not less than 150J, and the elongation is not less than 15%.

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