CN113549828B - Low-yield-ratio ultrahigh-strength marine steel and manufacturing method thereof - Google Patents

Abstract

A low-yield-ratio ultra-high-strength marine steel and a manufacturing method thereof, wherein the chemical composition in the steel is calculated by weight percentage: C0.06%-0.10%, Si0.2%-0.40%, Mn1.40%-1.65% , P≤0.012%, S≤0.002%, Cu0.15%～0.35%, Ni0.20%～0.45%, Cr0.25%～0.50%, Nb0.020%～0.045%, Mo0.15%～0.30% , Ti0.01% to 0.025%, Alt0.02% to 0.05%, and the balance is Fe and unavoidable impurities. The yield strength of the steel plate of the invention is ≥500MPa, the tensile strength is 610-770MPa, the elongation after fracture is ≥16%, the yield strength ratio is ≤0.85, excellent low-temperature toughness, the impact energy at 40°C is ≥100J, and the structure and performance are uniform.

Description

A low-yield-ratio ultra-high-strength marine steel and its manufacturing method

technical field

The invention relates to the field of steel material preparation, in particular to a low-yield-ratio ultra-high-strength marine steel and a manufacturing method thereof.

Background technique

As a key structural material for offshore engineering equipment, steel is widely used in offshore wind power, production platforms, and submarine pipelines. The service period of marine engineering equipment is generally 30 years, which is 50% longer than that of traditional ships. The service environment of marine engineering equipment is very harsh. coming impact. This requires that special sea conditions must be considered in the design structure and material selection of marine steel. Therefore, steel for offshore platforms not only requires high strength, but also requires good low yield ratio, low temperature toughness and post-weld performance to ensure sufficient ductility before plastic failure to prevent catastrophic brittle fracture. In order to meet the requirements of marine engineering for high performance and high safety service, it is urgent to develop ultra-high strength marine steel with low yield ratio and excellent welding performance.

The patent document of publication number CN102433507B "Low Yield Ratio Easy Welding High-Strength Steel Plate and Its Preparation Process" through reasonable design of alloy composition, controlled rolling and controlled cooling process, the thickness of the finished steel plate is ≥ 15mm, and the structure is ferrite + Bainite, yield strength 460-560MPa, tensile strength 700-790MPa, elongation after fracture ≥ 14%, low yield ratio < 0.7, steel plate with good weldability. However, the maximum thickness of the embodiment is only 30mm, and the elongation after fracture cannot be guaranteed to be ≥ 16%, which cannot meet the current service requirements of marine engineering steel.

The invention patent of publication number CN102644024B "a low-alloy low-yield-ratio marine engineering structural steel and its production method" provides a low-alloy low-yield-ratio marine engineering structural steel and its production method, with low alloy composition (only added Trace amounts of Nb and Ti alloys), reasonable and convenient for industrial production, rolling and water cooling, to produce low yield ratio, high strength, high toughness marine engineering structural steel. However, its yield strength cannot be guaranteed to be greater than or equal to 500 MPa, and its thickness in the examples is only 40 mm, which cannot meet the requirements of large-thickness marine steel and limits its application range.

The patent document of publication number CN109868412A "a large-thickness low-carbon equivalent 500MPa grade high-strength steel without preheating before welding and its manufacturing method" realizes full control of nanoscale precipitates by adopting reasonable chemical composition and process design, so that the product The carbon equivalent is not more than 0.40%, the yield strength reaches more than 500MPa, the tensile strength reaches more than 600MPa, the yield strength ratio is not more than 0.85, and has the characteristics of high strength toughness and no preheating before welding. However, it cannot guarantee the impact toughness at -40°C, and it requires the Alt content to be ≤0.008%. The current international standards and relevant classification society regulations for steel for marine engineering all require the lower limit of the Al element content, which limits its application range.

The patent document of publication number CN111057965B "a steel for marine engineering with low yield ratio and its preparation method" discloses a method for preparing steel for marine engineering with low yield ratio. The preparation method adopts two times of quenching + three times of tempering heat treatment process. Its Ni content is 3.0%-3.2%, and its economy is poor, which is far beyond the scope of use of steel alloy content for ships and marine engineering.

It can be seen from the above existing technologies that the currently available marine steel with low yield ratio and excellent mechanical properties has the following disadvantages:

1. The thickness of the product is small and the scope of application is narrow;

2. The cost of the alloy is high, and on-site control is difficult, which is not conducive to large-scale implementation and production promotion.

In view of the above deficiencies, the present invention adopts the low-precious metal alloy content design, adopts the coupling design of alloy composition design-smelting-controlled rolling-controlled cooling-tempering process, and finally obtains a low yield ratio ultra-high strength seawater with a maximum thickness of 80mm. Industrial steel and its manufacturing method.

Contents of the invention

The object of the present invention is to provide a low-yield-ratio ultra-high-strength marine steel and its manufacturing method. The steel plate of the present invention has ultra-high strength, yield strength ≥ 500 MPa, tensile strength 610-770 MPa, and elongation after fracture ≥ 16%; Yield strength ratio ≤0.85, excellent low temperature toughness, -40°C impact energy ≥100J, uniform structure and properties.

In order to achieve the above object, the present invention adopts the following technical solutions to realize:

A low-yield-ratio ultra-high-strength marine steel, the chemical composition of which is calculated by weight percentage: C 0.06%-0.10%, Si0.2%-0.40%, Mn 1.40%-1.65%, P≤0.012%, S ≤0.002%, Cu 0.15%～0.35%, Ni 0.20%～0.45%, Cr 0.25%～0.50%, Nb 0.020%～0.045%, Mo 0.15%～0.30%, Ti 0.01%～0.025%, Alt 0.02%～ 0.05%, the balance is Fe and unavoidable impurities.

The action mechanism of each alloy composition in the steel of the present invention is as follows:

C: It is a necessary element to ensure the strength. Solid solution strengthening and precipitation strengthening have a significant effect on improving the strength of steel, but the increase of carbon content seriously affects the welding performance and low temperature toughness of steel. From the perspective of product performance, the preferred C content is controlled at 0.06-0.10%.

Si: It is the main deoxidizing component in the steelmaking process. In order to obtain sufficient deoxidizing effect, it must contain more than 0.10%.

Mn: As the most important alloying element in the steel, in addition to increasing the strength of the steel plate, it also has the effect of expanding the austenite phase region, reducing the Ar3 point temperature, refining the ferrite grains and improving the low temperature toughness of the steel plate, but when Mn When the element quality is too high, the segregation of Mn will make the low-temperature toughness of the thick plate core poor, and the performance of the welding heat-affected zone will decrease. Therefore, the preferred Mn content range is 1.40% to 1.65%.

P: It is an element that adversely affects the impact value. It can segregate at the center of the slab and aggregate at the grain boundary to damage the low-temperature toughness. The material of the present invention is controlled at no more than 0.012%.

S: It is an element that adversely affects the impact value, and can form sulfide inclusions and become the source of cracks. The material of the present invention is controlled at no more than 0.002%.

Cu: Adding an appropriate amount of Cu to the steel can improve the corrosion resistance, strength, weldability, formability and machinability of the steel. When used together with Ni, hot embrittlement can also be avoided. The Cu content ranges from 0.15% to 0.35%.

Ni: Nickel dissolves in austenite, inhibits austenite recrystallization, refines austenite grains, and improves low-temperature toughness of the steel plate. , but as the nickel content increases, the production cost will increase significantly. Therefore, considering the performance of the steel plate and the production cost, the Ni content of the present invention is controlled at 0.20% to 0.45%.

Cr: It can improve the hardenability and strength of the steel plate, and Cr can also inhibit the transformation of proeutectoid ferrite and pearlite, which is beneficial to obtain acicular ferrite structure. However, if the content of Cr is too high, it will increase the tendency of temper brittleness and increase the difficulty of welding, while if the content of Cr is too low, it will not be able to effectively exert its strengthening effect. The content of Cr in the present invention is controlled to be 0.25-0.50%.

Nb: The addition of niobium is to promote the grain refinement of the steel rolling microstructure, which can improve the strength and toughness at the same time. Niobium can effectively refine the microstructure by inhibiting austenite recrystallization during the controlled rolling process. And the matrix is strengthened by precipitation. During the welding process, the segregation and precipitation of niobium atoms can hinder the coarsening of austenite grains during heating, and ensure a relatively fine heat-affected zone structure after welding to improve welding performance. The Nb content is preferably controlled at 0.020-0.045%.

Mo: Mo element can improve the hardenability of steel, inhibit the generation of polygonal ferrite and pearlite, and promote the formation of ferrite or bainite with a large number of dislocations in the crystal within a large cooling range, resulting in phase transformation strengthening And dislocation strengthening effect, significantly improve the strength and structure uniformity of steel. When the Mo content is lower than 0.10%, the strength and uniformity of the steel are not significantly improved; but if the Mo content is too high, the cost will be increased on the one hand, and the toughness and weldability of the steel will be reduced on the other hand. Therefore, in the present invention The Mo content is controlled at 0.15-0.30%.

Ti: When Ti is contained in a small amount, it forms nitrides, carbides, or carbonitrides, and has the effect of making crystal grains finer and improving the toughness of the base material. However, if the content exceeds 0.025%, the toughness of the base metal and the welded heat-affected zone will be reduced. Therefore, the content is preferably controlled at 0.01-0.025%.

Alt: As the deoxidizing and grain-refining elements that must be added in the present invention, the added content is above 0.01%, but when it exceeds 0.08%, it is easy to generate slab hot cracks, and at the same time, the toughness of the steel decreases. The preferred content of Alt is controlled at 0.02%-0.04%.

The yield strength of the marine engineering steel plate is ≥500MPa, the yield strength ratio is ≤0.85, and the impact toughness at -40°C is ≥100J.

The maximum thickness of the finished marine steel plate is 80 mm.

A method for manufacturing ultra-high-strength marine steel with a low yield ratio, comprising the steps of:

1) Smelting, continuous casting and slow cooling of billet: production is carried out by deep desulfurization of molten iron, converter smelting, refining outside the furnace, vacuum treatment and continuous casting. Reduction process, light reduction 6-9mm, continuous casting slabs obtained after continuous casting, continuous casting slab thickness 250-360mm, continuous casting slabs stacking and slow cooling after off-line, stacking temperature ≥ 650 ℃, stacking time ≥ 60h;

2) Rolling process: three-stage controlled rolling is adopted, the temperature in the soaking section of the cast slab is 1130-1180°C, the first stage is high-temperature controlled rolling, the starting temperature is 950-1000°C, and the rolling pass is single-passed except for the stretching pass. The reduction rate is ≥15%, which improves the as-cast structure of the slab, reduces the thickness of the slab to be heated, and shortens the time for the steel plate to be heated. The second and third stages adopt controlled rolling. The rolling temperature of the second stage is 880-920°C, the reduction rate of a single pass is ≥12%, and the cumulative reduction rate is ≥25%. The starting temperature of the third stage is 800-840°C. Sub-reduction rate ≥ 10%, cumulative reduction rate ≥ 50%, final rolling temperature 740-790°C; the purpose of the above three-stage rolling is to fully deform austenite grains and provide energy storage and location for phase deformation nuclei , improve the phase deformation nucleation rate, obtain a uniform and fine final structure, and ensure the low temperature toughness of the steel plate.

3) Cooling process: Controlled cooling after steel plate straightening, cooling adopts DQ+ACC rapid cooling system with average cooling rate ≥ 3°C/s, starting cooling temperature is 660-720°C, red return temperature is 200-300°C;

4) Slow cooling process: Immediately put into the slow cooling pit after the controlled cooling is completed, adopt the alternate stacking method with the hot steel plates above 300 ℃, and carry out the slow cooling pit insulation and cooling, and the stacking time is ≥ 24 hours;

5) Tempering process: After slow cooling, the steel plate is tempered. The tempering temperature is 350-450°C. The time in the furnace is 2-4min/mm*steel plate thickness. Air-cool after being out of the furnace to obtain the finished steel plate.

The evaluation of the continuous casting slab in the above step 1) after off-line is carried out by hot acid etching (the corrosion solution is 1:1 hydrochloric acid aqueous solution, the temperature is 75±5°C, and the corrosion time is 40 minutes), and it is rated against the Mannesmann standard atlas, and the rating result is ≤ 2 grades.

Compared with prior art, the beneficial effect of the present invention is:

1) The product steel plate produced by the process of the present invention is purified and smelted, combined with a specific continuous casting process and controlled rolling and controlled cooling process to control the segregation of the cast slab and the grain size of the rolled steel plate, and through fine control of tempering, to achieve 500MPa grade marine steel bending. Strength ratio ≤0.85, impact toughness at -40°C ≥100J.

2) The invention takes full advantage of the technical equipment advantages of wide and thick plate rolling mills, combined with continuous casting slabs with a thickness of 250-360mm, and develops ultra-high-strength marine steel thick steel plates with a low yield ratio. The maximum thickness of the finished product is 80mm.

3) Adopt reasonable straightening process and controlled cooling process to ensure the flatness of the plate shape, and can realize the unevenness of the steel plate within 2 meters ≤ 6mm.

4) The microstructure of the steel plate is a mixed structure of ferrite + bainite.

Description of drawings

Fig. 1 is the metallographic structure photo (500 times) of embodiment 2.

Detailed ways

The following examples are used to specifically illustrate the contents of the present invention, and these examples are only general descriptions of the contents of the present invention, and do not limit the contents of the present invention.

The chemical composition of the steel of the embodiment of the present invention is shown in Table 1, the smelting of the steel of the embodiment of the present invention, the process parameters of continuous casting and the evaluation results of the slab are shown in Table 2, and the rolling and cooling process of the steel plate of the embodiment of the present invention are shown in Table 3. The tempering heat treatment process of the steel plate of the example is shown in Table 4, the mechanical properties of the steel plate of the embodiment of the present invention are shown in Table 5, and the unevenness of the steel plate of the embodiment of the present invention is shown in Table 6.

Table 1 steel chemical composition wt% of the embodiment of the present invention

Example C Si mn P S Cu Ni Cr Nb Mo Ti Alt 1 0.091 0.27 1.42 0.009 0.001 0.33 0.44 0.38 0.041 0.27 0.014 0.027 2 0.075 0.24 1.65 0.01 0.002 0.35 0.39 0.37 0.39 0.25 0.013 0.034 3 0.082 0.3 1.63 0.007 0.001 0.2 0.28 0.35 0.044 0.23 0.015 0.03 4 0.071 0.25 1.57 0.011 0.002 0.26 0.33 0.41 0.038 0.22 0.012 0.021 5 0.078 0.33 1.62 0.008 0.001 0.24 0.29 0.31 0.035 0.26 0.022 0.029 6 0.068 0.28 1.59 0.012 0.002 0.23 0.31 0.29 0.031 0.24 0.011 0.031 7 0.088 0.37 1.49 0.009 0.002 0.17 0.27 0.33 0.034 0.21 0.017 0.036 8 0.062 0.34 1.64 0.01 0.001 0.21 0.22 0.48 0.024 0.17 0.024 0.033

Table 2 The smelting and stacking process parameters and slab evaluation results of the steel of the embodiment of the present invention

Table 3 Steel plate rolling and cooling process of the embodiment of the present invention

Table 4 Steel sheet tempering heat treatment process of the embodiment of the present invention

Table 5 Mechanical properties of the steel plate of the embodiment of the present invention

Table 6 The unevenness of the steel plate of the embodiment of the present invention

Example Steel plate thickness/mm Measuring length 2000mm steel plate roughness/mm 1 80 3 2 75 3 3 70 2 4 70 3 5 65 3 6 60 4 7 60 3 8 50 5

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Claims (2)

1. The low-yield-ratio ultrahigh-strength marine steel is characterized in that the steel comprises the following chemical components in percentage by weight: 0.06 to 0.10 percent of C, 0.2 to 0.40 percent of Si, 1.62 to 1.65 percent of Mn, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S, 0.15 to 0.35 percent of Cu, 0.20 to 0.29 percent of Ni, 0.31 to 0.50 percent of Cr, 0.020 to 0.045 percent of Nb, 0.15 to 0.30 percent of Mo, 0.01 to 0.025 percent of Ti, 0.02 to 0.05 percent of Alt, and the balance of Fe and inevitable impurities;

the maximum thickness of the finished marine steel plate is 80mm;

the manufacturing method of the low yield ratio ultrahigh strength marine steel comprises the following steps:

1) The rolling process comprises the following steps: three-stage controlled rolling is adopted, the temperature of a soaking section of a casting blank is 1130-1180 ℃, the first stage is high-temperature controlled rolling, the initial rolling temperature is 950-1000 ℃, the single-pass reduction rate except the widening pass is more than or equal to 15%, the second and third stages are controlled rolling, the initial rolling temperature of the second stage is 880-920 ℃, the single-pass reduction rate is more than or equal to 12%, the accumulated reduction rate is more than or equal to 25%, the initial rolling temperature of the third stage is 800-840 ℃, the single-pass reduction rate is more than or equal to 10%, the accumulated reduction rate is more than or equal to 50%, and the final rolling temperature is 740-768 ℃;

2) And (3) a cooling process: after the steel plate is straightened, controlled cooling is carried out, wherein a DQ + ACC rapid cooling system with the average cooling speed of more than or equal to 3 ℃/s is adopted for cooling, the starting cooling temperature is 660-699 ℃, and the temperature of red returning is 200-269 ℃;

3) The slow cooling process comprises the following steps: immediately placing the steel plate into a slow cooling pit after the controlled cooling is finished, and carrying out heat preservation and cooling on the slow cooling pit in an alternative stacking mode with hot steel plates at the temperature of more than 300 ℃, wherein the stacking time is more than or equal to 24 hours;

4) And (3) tempering: tempering temperature is 350-450 ℃, furnace time is 2-4 min/mm x the thickness of the steel plate, and the steel plate is air-cooled after being discharged to obtain a steel plate finished product;

further comprises smelting, continuous casting and casting blank slow cooling: the superheat degree of the tundish molten steel is 25-30 ℃, the whole process is protected and cast, a soft reduction process is matched, the soft reduction is 6-9 mm, the thickness of a continuous casting billet is 250-360 mm, the continuous casting billet is stacked and slowly cooled after being taken off the line, the stacking temperature is more than or equal to 650 ℃, and the stacking time is more than or equal to 60 hours; after the continuous casting billet is off-line, the hot acid etching is adopted for evaluation, the evaluation is carried out by referring to a Mannesmann standard map, and the rating result is less than or equal to level 2.

2. The low yield ratio and ultrahigh strength marine steel as claimed in claim 1, wherein the yield strength of the steel plate of the marine steel is not less than 500MPa, the yield ratio is not more than 0.85, and the impact toughness at-40 ℃ is not less than 100J.

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