CN116607076A - Thin-specification 690 MPa-level low-yield-ratio steel for ocean engineering and manufacturing method - Google Patents
Thin-specification 690 MPa-level low-yield-ratio steel for ocean engineering and manufacturing method Download PDFInfo
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- 229910000734 martensite Inorganic materials 0.000 claims abstract description 16
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- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
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- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
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Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
- B21B1/026—Rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/04—Making ferrous alloys by melting
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
- B21B2001/028—Slabs
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Abstract
A thin-specification low-yield-ratio 690 MPa-grade steel for ocean engineering and a manufacturing method thereof. The steel comprises the following chemical components, by mass, C=0.06-0.15%, si=0.10-0.80%, mn=0.50-1.60%, P is less than or equal to 0.015%, S is less than or equal to 0.006%, nb=0.015-0.05%, V=0.02-0.10%, ti=0.01-0.03%, ni=0.30-1.00%, cu=0.20-1.50%, cr=0.30-1.20%, mo=0.20-0.70%, al=0.015-0.045%, ca=0.001-0.015%, and the balance Fe and other unavoidable impurities. The low yield ratio full-flow regulation and control production process of smelting, continuous casting, controlled rolling, controlled cooling direct quenching and tempering is adopted to design the hardenability chemical components with matched thin specification and thickness and the controlled quenching process of relaxation after rolling, and the two-phase proportion of soft ferrite and hard martensite is regulated and controlled, so that the 690 MPa-level ultrahigh-strength ocean engineering steel plate with the yield ratio lower than 0.94 and other mechanical properties meeting the requirements of the specifications of the class society is produced.
Description
Technical Field
The invention belongs to the technical field of low-alloy high-strength steel production, relates to a thin steel plate and a manufacturing method thereof, and is particularly suitable for producing steel with low yield ratio for ocean engineering structures.
Background
With the large-scale development of engineering structures such as marine equipment, ships, bridges, machinery and the like, the requirements on the strength level of structural steel are higher and higher, so that the design targets of light weight of the structure and gravity center balance of the whole structure are realized. For example, ultra-high strength grades in international social specifications have increased yield strength from a highest grade of 690MPa to 960MPa. However, the higher the steel sheet strength grade, the higher the yield strength to tensile strength ratio (yield ratio) is. The European sponsored structural integrity evaluation project (SINTAP) performs statistical analysis on a large amount of data of steel yield strength and yield ratio, and regression shows that the upper limit relationship between the steel yield strength and the yield ratio is Y/T=1/[ 1+2 (150/YS) 2.5]. The SINTAP project concludes that too high a yield ratio is detrimental to the safety of the engineering structure as a whole, and should be included as an important parameter for the safety design of the engineering structure. The higher the steel plate yield ratio is, the smaller the deformation fracture allowance of the manufactured structural member is, and the safety risk of the marine engineering structure in the service process is increased. Therefore, the specifications of the class-Cooperation materials such as Norway class-Cooperation (DNV) and UK Law class-Cooperation (LR) specify that the yield ratio of the steel sheet having a yield strength exceeding 420MPa is not more than 0.94.
Due to the characteristic that the yield ratio increases with the increase of the strength level, the application of the ultra-high strength steel in marine engineering equipment is limited, and EH690 is the highest level of the current mass application. The yield ratio lower than 0.94 index is a recognized technical problem for producing steel plates with the level of 690MPa and above by a quenching and tempering process, and particularly for producing steel plates with the thin specification with the thickness of 6-20 mm. This is because the thin gauge steel sheet is quenched to obtain a full-thickness-direction lath martensitic structure, and complex changes such as internal stress release, carbide precipitation, lath softening occur during tempering, resulting in insufficient work hardening ability from yielding to the failure stage of necking.
Chinese patent CN114231714A discloses a heat treatment method of 890 MPa-grade ultra-high strength low yield ratio ocean engineering steel, which comprises the steps of performing online cooling quenching, sub-temperature quenching and tempering water cooling after rolling, wherein the finishing temperature of a steel plate with the thickness of 10-40 mm is 820-860 ℃, cooling to be less than or equal to 200 ℃, then heating the steel plate to the temperature of a two-phase region of 820-850 ℃, preserving the temperature for 0.3-0.5 min/mm, performing rapid cooling quenching, finally heating the steel plate subjected to sub-temperature quenching to 560-620 ℃ for tempering, discharging water cooling to be less than or equal to 80 ℃, and obtaining the ultra-high strength low yield ratio ocean engineering steel plate with the yield ratio not higher than 0.93 and the yield strength not lower than 890 MPa. However, the temperature of the two-phase region is limited to a narrow temperature range of 820-850 ℃, so that the control requirement on the chemical components of steel is high, and the process of tempering and water cooling is required, so that the method is not suitable for the existing industrial production equipment conditions.
Chinese patent CN114134414A discloses a low yield ratio high toughness steel and a preparation method thereof, wherein the steel is subjected to two-stage rolling process of continuously cast steel billets, after transverse and longitudinal rolling in the first stage, an intermediate billet is accelerated to be cooled to 730 ℃, the second stage is started to be lower Wen Zhongga, water cooling and stacking slow cooling are carried out on the rolled steel plate, the thickness of the obtained low yield ratio high toughness steel plate is less than or equal to 80mm, the yield strength is more than or equal to 500MPa, the tensile strength is more than or equal to 640MPa, the elongation after breaking is more than or equal to 20.0%, the yield ratio is less than or equal to 0.80 and the Charpy impact power is more than 200J. The patent aims at the bridge and building structural steel with the strength level of 500MPa, adopts a low-temperature final rolling and cooling control process, and cannot meet the requirement of 890MPa on the ultra-high strength level.
Chinese patent CN114134416A discloses a low yield ratio high strength medium-thickness steel plate and a short process manufacturing method thereof, wherein a small amount of microalloying elements are added on the basis of low carbon component design to reduce cost, meanwhile, a bainite structure with a soft and hard substructure is formed through rolling and water cooling process development, quenching and tempering heat treatment are not needed, water cooling is directly carried out after rolling, the thickness of the steel plate is 15-60 mm, the yield strength is 580-680 MPa, the tensile strength is 700-820 MPa, the elongation after breaking is 16-23%, the longitudinal Charpy impact absorption power is 80-260J at the test temperature of minus 40 ℃, and the yield ratio is less than or equal to 0.82. But the manufacturing method is suitable for producing the steel with the yield strength of 580-680 MPa.
Chinese patent CN114058793A discloses a heat treatment method for reducing yield ratio of ultra-high strength marine steel EH890, which adopts quenching, sub-temperature quenching and tempering processes, wherein the primary quenching process is 910-940 ℃, the heat preservation time is PLC+ (30-40) min, water cooling is carried out to room temperature after discharging, the quenching temperature of the sub-temperature quenching process is 810-830 ℃, the heat preservation time is PLC+ (10-30) min, water cooling is carried out to room temperature after discharging, the tempering temperature is 630-660 ℃, the total heating time is 4.0-5.0min/mm, and the total heating time is carried out for air cooling to room temperature after discharging, so that steel plates with thickness specification of 15-50mm can be produced, the yield strength is more than or equal to 890MPa, the tensile strength is 940-1100MPa, the yield ratio is less than or equal to 0.94, and the V-shaped transverse-40 ℃ impact power is more than or equal to 50J. However, the patent adopts a plurality of heat treatment processes of quenching, sub-temperature quenching and tempering, the production period is long, the process cost is high, and meanwhile, the surface quality of the steel plate is deteriorated after the plurality of heat treatments, so that the coating requirement of ocean engineering equipment is difficult to meet.
ChinesepatentCN114032459Adisclosesapreparationmethodofamedium-thicknesssteelplatewithhighstrengthandtoughnessandlowyieldratioandhighyieldstrength,whichcanbeusedforproducingasteelplatewiththethicknessof10-50mm,whereinahot-rolledsteelplateisheatedto300-650℃forpre-heatpreservation,theheatpreservationtimeisnotlessthan60min,M-Aisfullydecomposedtoobtainuniformandfinecementite,thentheuniformandfinecementiteisheatedtoacertaintemperatureinan(alpha+gamma)two-phaseregionataheatingrateofnotlessthan1℃persecond,waterquenchingiscarriedoutafterheatpreservationiscarriedoutfor30-120min,thenthemedium-lowtemperaturetemperingiscarriedout,andtheheatpreservationiscarriedoutfor30-120min,sothataprecipitation-strengthenedferrite(softphase)andtemperedmartensite(hardphase)structureisobtained. However, the patent adopts a very complex heat treatment process route, needs to heat the hot rolled steel plate to 300-650 ℃ for pre-heat preservation, then heat the hot rolled steel plate to a two-phase region for heat preservation, then quench the hot rolled steel plate, and then heat the hot rolled steel plate to 200-450 ℃ for medium-low temperature tempering, so that the hot rolled steel plate cannot be suitable for industrial production equipment of medium-thickness steel plates.
Chinese patent CN111705268B discloses a steel for ultra-high strength and high toughness pressure-resistant housing with low yield ratio and a preparation method thereof, which adopts a secondary quenching heat treatment, the primary quenching is completely austenitized, and then the secondary quenching and tempering are carried out, finally, a complex phase structure of tempered martensite, critical ferrite, residual austenite and the like is obtained, and the steel for ultra-high strength and high toughness pressure-resistant housing with low yield ratio of not more than 0.90 is realized. Aiming at the steel for the pressure-resistant shell, 5.00% -10.00% of Ni is added to obtain enough residual austenite to reduce the yield ratio, and meanwhile, the long-period heat treatment process of two times of quenching is used, so that the alloy and the process cost are extremely high.
Disclosure of Invention
The invention aims to provide steel with low yield ratio for ocean engineering structures and a manufacturing method thereof, and the steel is used for producing thin-specification steel plates with the thickness of 6-20 mm, the yield strength of not lower than 690MPa and the ratio of the yield strength to the tensile strength of not higher than 0.94.
The technical scheme of the invention is as follows:
the steel for the marine engineering with the thin specification and the low yield ratio of 690MPa comprises the following chemical components, by mass, C=0.06-0.15%, si=0.10-0.80%, mn=0.50-1.60%, P is less than or equal to 0.015%, S is less than or equal to 0.006%, nb=0.015-0.05%, V=0.02-0.10%, ti=0.01-0.03%, ni=0.30-1.00%, cu=0.20-1.50%, cr=0.30-1.20%, mo=0.20-0.70%, al=0.015-0.045%, ca=0.001-0.015%, and the balance Fe and other unavoidable impurities; the thickness of the steel plate is 6-20 mm, the yield strength is not lower than 690MPa, and the ratio of the yield strength to the tensile strength is not higher than 0.94.
A manufacturing method of thin-specification low-yield-ratio 690 MPa-grade ocean engineering steel comprises the following steps:
1) Smelting in a converter: after carrying out pre-desulfurization treatment on molten iron produced by a blast furnace, pouring into a 100-300 ton converter for smelting, carrying out top-bottom re-blowing oxygen treatment, completing oxidation decarburization, dephosphorization and desulfurization of the molten iron, then adding Mn and Ni ferroalloy raw materials, continuously blowing oxygen, adjusting the temperature of the molten steel to 1620-1660 ℃, adding aluminum ingots for sedative deoxidation according to the detected oxygen content in the molten steel, and carrying out bottom blowing argon before tapping;
2) Secondary refining: pouring molten steel into a ladle, conveying the ladle to an LF refining furnace, and continuously stirring the molten steel by bottom blowing argon; sampling and detecting molten steel components in the refining process, and finely adjusting the alloy content to enable all the components to meet the requirements, wherein the treatment time is 20-30 minutes; feeding 200m pure calcium wires before LF treatment is finished; then the ladle is sent to an RH vacuum refining furnace for vacuum degassing treatment, the time is more than or equal to 15 minutes, and the gas content of molten steel is removed until the gas content of [ H ] < 2 ppm, [ O ] < 20 ppm, [ N ] < 60 ppm;
3) Continuous slab casting: continuously casting a slab with the thickness of 150-320 mm and the width of 1650-2650 mm under the coverage of protective atmosphere and protective slag by using an intermediate ladle;
4) And (3) heating a plate blank: after flame cutting, stacking and slowly cooling the continuous casting high-temperature plate blanks in a blank field for 45-50 hours, and then conveying the plate blanks to a heating furnace to heat the plate blanks to 1150-1220 ℃ for 4-8 h, wherein the austenite grain size is more than 6 grades;
5) Rolling a steel plate: after removing iron oxide scales on the surface of a billet by using high-pressure water, carrying out stretching rolling for 3-7 times along the width direction, wherein the rolling temperature is 1100-1150 ℃, rolling to the required width, then turning the steel for 90 DEG, and then rolling to the required thickness of the steel plate at the temperature of 900-1050 ℃ above an austenite non-recrystallization region, wherein austenite grains are kept to be in a recrystallized equiaxial shape, and the grain size is above 8 grades;
6) And (3) cooling the steel plate: spraying water to the rolled steel plate by using a laminar flow header to accelerate cooling speed, wherein the cooling rate is 10-30 ℃ per second, the final cooling temperature is 200-450 ℃, and the lath martensite proportion in the microstructure of the steel plate is more than 60%;
7) And (3) heat treatment of the steel plate: and carrying out primary tempering heat treatment on the steel plate.
In the step 3), nonmetallic inclusion in the plate blank meets the following grade requirements: the A-class inclusion is less than or equal to 0.5, the B-class inclusion is less than or equal to 0.5, the C-class inclusion (silicate) is less than or equal to 0.5, and the D-class inclusion is less than or equal to 0.5.
The principle of the invention:
in the technical scheme, the microstructure of the 6-20 mm low yield ratio 690 MPa-grade ocean engineering steel is a complex phase structure such as tempered martensite, proeutectoid ferrite, residual austenite and the like, and a large amount of nano-sized Nb-V-Ti composite precipitation strengthening phases exist in the matrix, so that the performance index requirements of low yield ratio and over 690MPa yield strength are met.
In the technical scheme, the low yield ratio 690 MPa-grade steel for ocean engineering has the yield strength more than or equal to 690MPa and can practically reach 710-850 MPa; the tensile strength is more than or equal to 760 MPa; the elongation after breaking is more than or equal to 15 percent; the yield ratio is less than or equal to 0.92; the impact energy at the temperature of minus 40 ℃ is more than or equal to 100J, and the actual impact energy can reach 120-230J.
In the technical scheme, when controlled rolling is performed, the final rolling temperature is 750-850 ℃, the initial cooling temperature is 680-750 ℃, and the final cooling temperature is 200-350 ℃. In the tempering treatment, the tempering temperature is 550-620 ℃ and the tempering time is 30-60 min. The actions of controlled rolling, controlled cooling on-line quenching and tempering enable the steel plate to form martensite not less than 60% and bainitic ferrite matrix structure not more than 40%, meanwhile, (Nb, ti) C+VC composite nano particles are separated out, the yield ratio is reduced by the double-phase matrix structure, and the yield strength is ensured not to be lower than 690MPa by high-density composite nano phase precipitation strengthening.
The yield strength of the steel plate for ocean engineering manufactured by the method is not lower than 690MPa, the yield ratio is not higher than 0.94, and the mass percentage content of each alloy component in the steel plate for ocean engineering with the thickness specification of 6-20 mm is set based on the following mechanism:
the C element is an ultra-high strength steel basic strengthening element, and can obviously improve the forming capacity of a martensitic phase in the quenching process, thereby improving the hardenability. However, the high C content is unfavorable for weldability and low-temperature toughness of the quenched and tempered ultra-high strength steel, and the C content is controlled to be 0.06% -0.15% in order to ensure balance of the ultra-high strength, the low-temperature toughness and the welding performance of the steel.
Si is one of main deoxidizing elements in the steelmaking process and is also a solid solution strengthening element, but the Si content is too high, so that ferrous silicate Fe2SiO4 which is difficult to remove is easily formed in the heating process of a steel billet and a steel plate, iron scales on the surface of the steel plate are difficult to peel off, and the surface quality of the steel plate is affected, so that the Si content is controlled to be 0.10% -0.80%.
Mn element is the most main strengthening element and also is an austenite stabilizing element, improves the hardenability of steel in the quenching process, and plays roles of solid solution strengthening and fine grain strengthening. However, mn is extremely easy to form a banded structure in the center of a continuous casting billet, forms MnS inclusions which are easy to deform with S, and is easy to react with hydrogen to cause hydrogen induced cracking in a marine environment, so that the Mn content is controlled to be 0.50% -1.60%.
Nb is a strong carbon-nitrogen compound forming element, nb (C, N) nano-scale particles are dispersed and separated out in steel, and austenite grain growth in the slab heating process is effectively refined. Meanwhile, nb can effectively reduce the non-recrystallization temperature of austenite, inhibit the recrystallization process of austenite, refine austenite grains in a hot rolling high-temperature zone, and can effectively refine the final ferrite structure of the steel plate by combining with controlling the rolling process, thereby improving the strength and toughness of the steel plate. Part of Nb is kept in a solid solution state in the quenching process, and is precipitated in the tempering process, so that the precipitation strengthening contribution is improved. To exert the above effects, the Nb content is controlled to be 0.015-0.050%.
The V element is carbide forming element in steel, exists in a solid solution state in the hot rolling and quenching process, and precipitates nano-scale VC or (MoV) C in the tempering process, so that the strength of the steel is improved. The V content is controlled to be 0.02% -0.10%.
The Ti element is a strong oxide and nitride forming element, O and N gases in molten steel are effectively removed in the steel refining process, tiO and TiN are formed, and the growth of austenite grains in the subsequent heating process is inhibited. However, excessive Ti is added to precipitate large-sized square TiCN particles at the austenitic grain boundary in the solidification process of the steel billet, so that cracks on the surface of the steel billet are caused. Therefore, the Ti content is controlled to be 0.01% -0.03%.
The Ni element is a strong austenite stabilizing element, improves the hardenability and martensite forming capability of the steel, and forms high-heat-stability residual austenite in a Ni micro-segregation zone in the quenching and tempering processes, thereby obviously improving the low-temperature toughness of the high-strength steel produced by the quenching and tempering process. The Ni element has the additional function of inhibiting Cu embrittlement caused by low-melting-point Cu melting in the high-temperature heating process of the Cu-containing steel, and improving the surface quality of the steel. However, ni content is controlled to be 0.30% -1.00% due to high Ni cost.
The Cu element can improve the corrosion resistance of the marine environment service steel, and meanwhile, cu-rich phase nano-scale particles are separated out in the quenching and tempering process, so that a separation strengthening effect is realized. However, cu has a low melting point, cracks are easily caused by melting at austenite grain boundaries in the heating process of the steel billet, and Ni needs to be added for inhibition, so that the Cu content is controlled to be 0.20% -0.50%.
Cr and Mo are strong hardenability elements of steel, promote the formation of martensite in the quenching process, and are also strong carbide forming elements, promote the formation of M23C6 and MC type carbides, effectively inhibit the tempering brittleness phenomenon, and reduce the pearlite band structure of the steel plate, so that the Cr and Mo contents are respectively controlled to be 0.30% -1.20% and 0.30% -0.70%.
Al is a strong deoxidizing and fine-grain element in the steelmaking process, effectively removes the O gas content in molten steel, forms a stable AlN precipitated phase with N, plays a role in growing austenite grains in the pinning heating process, but excessive Al is added to keep excessive Al2O3 inclusion in the molten steel, and is agglomerated at a casting nozzle to cause accidents, so that the Al content is controlled to be 0.015% -0.045%.
Ca is a strong sulfide forming element to form Ca-Mn-S composite inclusions, so that the shape of MnS inclusions and the forming performance of a thermal deformation process are changed, particularly in an Mn segregation zone at the core part of the steel plate, sulfide inclusions are kept spherical, layering phenomena of tensile and impact fracture are inhibited, and the uniformity of the transverse and longitudinal performances of the steel plate is improved. The Ca content should be controlled to be 0.001% -0.015% by keeping Ca/S ratio.
P and S are harmful elements and should be removed by steelmaking process as much as possible. In the solidification process of the continuous casting blank, P is extremely easy to generate segregation, and particularly, a high P content enrichment region is formed at the solidification tail end of the casting blank core; in the steelmaking process, S and Mn form manganese sulfide inclusions, particularly a large number of MnS inclusions are formed at the center Mn segregation position of a casting blank, and the subsequent rolling process is deformed into a strip shape, so that the manganese sulfide inclusions are particularly obvious for thin-specification steel plates. Therefore, the P and S contents should be controlled at lower level as much as possible, and the treatment difficulty of the steelmaking process is balanced with the consideration of the method for improving the inclusion by Ca treatment according to the invention, so that the P is less than or equal to 0.015 percent and the S is less than or equal to 0.006 percent in the steel is controlled.
The outstanding characteristics and remarkable effects of the invention are mainly shown in:
(1) According to the invention, through a low-carbon +MnNiCrMo alloying +Nb-V-Ti composite microalloying component system, controlled rolling +controlled cooling direct quenching +tempering production process, the low-yield-ratio thin steel plate for ocean engineering, which has the thickness of 6-20 mm, the thin specification, the yield strength higher than 690MPa, the elongation after fracture higher than 17%, the yield ratio lower than 0.92 and the impact energy higher than 100J at-40 ℃, is produced.
(2) The invention is a short-flow process which is suitable for the existing technical route of the industrialized production of the steel plate, and avoids the long-period high-energy consumption process of the current twice quenching technology with low yield ratio. The method has the advantages of high cooling rate and uniformity of the thin-specification steel plate, adopts the design of low-carbon low-alloy components with matched hardenability, regulates and controls the appearance and state of austenite by controlling the finishing temperature after rolling, controls the cooling temperature and cooling rate by controlling the roller way transmission time, ensures that deformed austenite of the steel plate can be properly relaxed, firstly separates out part of ferrite soft phases of grain boundaries, then enables the rest relaxed austenite to be converted into martensite hard phases by forced water cooling, reduces the yield ratio of the thin-specification steel plate to be below 0.92, and simultaneously maintains the solid solution state of Nb, ti and V microalloy elements.
(3) The high-density nano-scale composite microalloy carbide (Nb, ti) C+VC is obtained through optimized heat treatment of tempering temperature and time, the yield strength is improved to more than 690MPa, and meanwhile, the tempering recovers quenched martensite with high dislocation density and high C.
Drawings
In fig. 1, (a), (b) and (c) correspond to the optical microstructure of the steel sheet after controlled rolling and controlled cooling direct quenching in example 1, example 2 and example 3, respectively.
Fig. 2 (a), (b) and (c) correspond to the core average orientation error maps obtained from the electron back scattering diffraction data of the steel sheet after the direct quenching by controlled rolling and controlled cooling in example 1, example 2 and example 3, respectively. The local distribution of dislocation density is reflected in the figure, and blue represents that the dislocation density is extremely low, corresponding to the ferrite region in fig. 1; yellow represents regions of high dislocation density, corresponding to the martensitic regions in fig. 1.
In fig. 3, (a), (b) and (C) are corresponding to the transmission electron micrographs of the tempered steel sheet of examples 1, 2 and 3, respectively, it is seen that nano-scale VC- (Nb, ti) C complex carbide is precipitated at high density.
Detailed Description
The technical scheme of the invention is further described below with reference to specific embodiments. The steel products produced in the examples were subjected to performance tests using the following relevant criteria: (1) yield strength, tensile strength, elongation after break: GB/T228.1-2021 Metal materials tensile test part 1: room temperature test method; (2) -40 ℃ Charpy impact energy: GB/T229-2020 metal material Charpy pendulum impact test method; (3) The yield ratio is the ratio of the yield strength to the tensile strength and is obtained according to the GB/T228.1-2021 test method.
Example 1
Manufacturing an EH690 grade ocean engineering steel plate with the thickness of 6mm and the yield ratio of less than or equal to 0.92, smelting molten steel according to set components, and casting into a slab with the thickness of 200mm, wherein the chemical components in percentage by weight are as follows: 0.06% C, 0.25% Si, 0.80% Mn, 0.25% Mo, 0.012% P, 0.005% S, 0.030% Nb, 0.03% V, 0.015% Ti, 0.30% Ni, 0.25% Cu, 0.35% Cr, 0.25% Mo, 0.03% Al, 0.008% Ca, the balance being Fe and unavoidable impurities.
The process comprises the following steps: heating a casting blank with the thickness of 200mm to 1200 ℃, preserving heat for 3 hours, pushing out of a heating furnace, removing iron scales on the upper surface and the lower surface of a steel blank by using 19MPa high-pressure water, and then performing two-stage controlled rolling: the rolling temperature in the first stage is 1150-950 ℃, the accumulated reduction rate is 80% to the thickness of the intermediate billet of 40mm, and the austenite is ensured to be fully recrystallized and refined; the second stage rolling temperature is 920-850 ℃, the rolling reduction is 80%, and finally the steel plate is hot rolled to 6mm at 850 ℃. After hot rolling, the steel sheet was air cooled to 750℃by a 0.5. 0.5 mm/s speed transfer roll to fully recover the deformed work-hardened austenite. The steel plate passes through 12 groups of header laminar flow sections at the speed of 1.5 mm/s on an accelerated cooling roller way, and the final cooling temperature is 260 ℃ to obtain 80% martensite. And (3) rolling under control and cooling under control, directly quenching, conveying the steel plate to a tempering furnace, soaking at 600 ℃ and preserving heat for 30min. The yield strength of the obtained steel is 795MPa, the tensile strength is 873MPa, the elongation after break is 20%, the yield ratio is 0.91, and the half-size sample impact energy is 122J at minus 40 ℃.
Example 2
Manufacturing an EH690 grade ocean engineering steel plate with the thickness of 15mm and the yield ratio of less than or equal to 0.92, smelting molten steel according to set components, and casting into a slab with the thickness of 200mm, wherein the components in percentage by weight are as follows: 0.06% C, 0.25% Si, 1.10% Mn, 0.35% Mo, 0.012% P, 0.004% S, 0.035% Nb, 0.04% V, 0.015% Ti, 0.45% Ni, 0.30% Cu, 0.42% Cr, 0.30% Mo, 0.03% Al, 0.009% Ca, the balance being Fe and unavoidable impurities.
The process comprises the following steps: heating a casting blank with the thickness of 200mm to 1200 ℃, preserving heat for 3 hours, pushing out of a heating furnace, removing iron scales on the upper surface and the lower surface of a steel blank by using 19MPa high-pressure water, and then performing two-stage controlled rolling: the rolling temperature in the first stage is 1150-950 ℃, the accumulated rolling reduction rate is 70% to the thickness of the intermediate billet of 60mm, and the austenite is ensured to be fully recrystallized and refined; the rolling temperature of the second stage is 900-820 ℃, the rolling reduction is 75 percent, and finally the steel plate is hot rolled to 15mm at 790 ℃. The hot rolled steel plate is cooled to 720 ℃ by a conveying roller with the speed of 0.3 mm/s, so that the deformed work hardening austenite fully recovers and relaxes. The steel plate passes through 15 groups of collector tube laminar flow sections at the speed of 1.0 mm/s on an accelerated cooling roller way, and the final cooling temperature is 300 ℃ to obtain 75% martensite. And (3) rolling under control and cooling under control, directly quenching, conveying the steel plate to a tempering furnace, soaking at 600 ℃ and preserving heat for 45min. The yield strength of the obtained steel is 757MPa, the tensile strength is 842MPa, the elongation after breaking is 22%, the yield ratio is 0.90, and the impact energy of a full-size sample at-40 ℃ is 172J.
Example 3
Manufacturing an EH690 grade ocean engineering steel plate with the thickness of 20mm and the yield ratio of less than or equal to 0.92, smelting molten steel according to set components, and casting into a slab with the thickness of 200mm, wherein the components are as follows in percentage by weight: 0.06% C, 0.25% Si, 1.20% Mn, 0.45% Mo, 0.012% P, 0.004% S, 0.035% Nb, 0.06% V, 0.015% Ti, 0.70% Ni, 0.35% Cu, 0.55% Cr, 0.45% Mo, 0.03% Al, 0.009% Ca, the balance being Fe and unavoidable impurities.
The process comprises the following steps: heating a casting blank with the thickness of 200mm to 1200 ℃, preserving heat for 3 hours, pushing out of a heating furnace, removing iron scales on the upper surface and the lower surface of a steel blank by using 19MPa high-pressure water, and then performing two-stage controlled rolling: the rolling temperature in the first stage is 1150-950 ℃, the accumulated reduction rate is 60% to the thickness of the intermediate billet of 80mm, and austenite recrystallization refinement is ensured; the second stage rolling temperature is 880-810 ℃, the rolling reduction is 75%, and finally the steel plate is hot-rolled to 20mm at 760 ℃. The hot rolled steel sheet was cooled to 700 c by air cooling with a 0.25 mm/s speed transfer roll to fully relax the deformed work hardened austenite. The steel plate passes through 21 groups of header laminar flow sections at the speed of 0.80 mm/s on an accelerated cooling roller way, and the final cooling temperature is 320 ℃ to obtain 70% martensite. And (3) rolling under control and cooling under control, directly quenching, conveying the steel plate to a tempering furnace, soaking at 600 ℃ and preserving heat for 60min. The obtained steel has the yield strength of 731MPa, the tensile strength of 821MPa, the elongation after break of 24%, the yield ratio of 0.89 and the full-size sample impact energy of 212J at minus 40 ℃.
Claims (3)
1. A thin-specification low-yield-ratio 690 MPa-grade steel for ocean engineering is characterized in that: the steel comprises the following chemical components, by mass, C=0.06-0.15%, si=0.10-0.80%, mn=0.50-1.60%, P is less than or equal to 0.015%, S is less than or equal to 0.006%, nb=0.015-0.05%, V=0.02-0.10%, ti=0.01-0.03%, ni=0.30-1.00%, cu=0.20-1.50%, cr=0.30-1.20%, mo=0.20-0.70%, al=0.015-0.045%, ca=0.001-0.015%, and the balance Fe and other unavoidable impurities; the thickness of the steel plate is 6-20 mm, the yield strength is not lower than 690MPa, and the ratio of the yield strength to the tensile strength is not higher than 0.94.
2. A manufacturing method of thin-specification low-yield-ratio 690 MPa-grade ocean engineering steel is characterized by comprising the following steps:
1) Smelting in a converter: after carrying out pre-desulfurization treatment on molten iron produced by a blast furnace, pouring into a 100-300 ton converter for smelting, carrying out top-bottom re-blowing oxygen treatment, completing oxidation decarburization, dephosphorization and desulfurization of the molten iron, then adding Mn and Ni ferroalloy raw materials, continuously blowing oxygen, adjusting the temperature of the molten steel to 1620-1660 ℃, adding aluminum ingots for sedative deoxidation according to the detected oxygen content in the molten steel, and carrying out bottom blowing argon before tapping;
2) Secondary refining: pouring molten steel into a ladle, conveying the ladle to an LF refining furnace, and continuously stirring the molten steel by bottom blowing argon; sampling and detecting molten steel components in the refining process, and finely adjusting the alloy content to enable all the components to meet the requirements, wherein the treatment time is 20-30 minutes; feeding 200m pure calcium wires before LF treatment is finished; then the ladle is sent to an RH vacuum refining furnace for vacuum degassing treatment, the time is more than or equal to 15 minutes, and the gas content of molten steel is removed until the gas content of [ H ] < 2 ppm, [ O ] < 20 ppm, [ N ] < 60 ppm;
3) Continuous slab casting: continuously casting a slab with the thickness of 150-320 mm and the width of 1650-2650 mm under the coverage of protective atmosphere and protective slag by using an intermediate ladle;
4) And (3) heating a plate blank: after flame cutting, stacking and slowly cooling the continuous casting high-temperature plate blanks in a blank field for 45-50 hours, and then conveying the plate blanks to a heating furnace to heat the plate blanks to 1150-1220 ℃ for 4-8 h, wherein the austenite grain size is more than 6 grades;
5) Rolling a steel plate: after removing iron oxide scales on the surface of a billet by using high-pressure water, carrying out stretching rolling for 3-7 times along the width direction, wherein the rolling temperature is 1100-1150 ℃, rolling to the required width, then turning the steel for 90 DEG, and then rolling to the required thickness of the steel plate at the temperature of 900-1050 ℃ above an austenite non-recrystallization region, wherein austenite grains are kept to be in a recrystallized equiaxial shape, and the grain size is above 8 grades;
6) And (3) cooling the steel plate: spraying water to the rolled steel plate by using a laminar flow header to accelerate cooling speed, wherein the cooling rate is 10-30 ℃ per second, the final cooling temperature is 200-450 ℃, and the lath martensite proportion in the microstructure of the steel plate is more than 60%;
7) And (3) heat treatment of the steel plate: and carrying out primary tempering heat treatment on the steel plate.
3. The method for manufacturing the thin gauge 690 MPa-grade steel for ocean engineering according to claim 2, wherein the method comprises the steps of: in the step 3), nonmetallic inclusion in the plate blank meets the following grade requirements: the A-class inclusion is less than or equal to 0.5, the B-class inclusion is less than or equal to 0.5, the C-class inclusion (silicate) is less than or equal to 0.5, and the D-class inclusion is less than or equal to 0.5.
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| CN112831717A (en) * | 2020-12-03 | 2021-05-25 | 南京钢铁股份有限公司 | 690 MPa-grade low-yield-ratio thin-specification weather-resistant bridge steel and manufacturing method thereof |
| CN115216610A (en) * | 2022-07-28 | 2022-10-21 | 湖南华菱湘潭钢铁有限公司 | Production method of Q690 high-corrosion-resistance high-strength quenched and tempered steel plate for offshore structure |
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