CN113832413B - Ultra-thick 800 MPa-grade quenched and tempered steel plate with excellent core low-temperature impact toughness and weldability and manufacturing method thereof - Google Patents
Ultra-thick 800 MPa-grade quenched and tempered steel plate with excellent core low-temperature impact toughness and weldability and manufacturing method thereof Download PDFInfo
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Abstract
The component of the super-thick 800 MPa-grade quenched and tempered steel plate is based on a low C-ultralow Si-low Mn- (Cu + Ni + high Cr + low Mo) alloying- (Ti + V + B) microalloying component system, the acid-soluble Als content in the steel is properly increased, (% Als)/[ (% N) -0.292 (% Ti)]Not less than 33, control [1.57 (% C) 0.5 +7.86(％Si)+23.23(％P)+8.17(％S)]×[1.21(％Mn)+2.65(％Mo)]Less than or equal to 4.2; ca treatment, a physical metallurgy design control means of alloy components with Ca/S ratio of 1.00-3.00; the manufacturing method adopts an optimized CR + offline gradient quenching and tempering heat treatment process, so that the microstructure of a finished steel plate (particularly a steel plate core) is fine low-carbon tempered martensite and tempered lower bainite, the average grain size of the steel plate is below 20 mu m, 800 MPa-grade high strength of steel plates with different thicknesses can be ensured, and simultaneously, the low-temperature impact toughness of the steel plate core, and the weldability and crack resistance of the steel plate are also excellent.
Description
Technical Field
The invention relates to the technical field of quenched and tempered steel plates and manufacturing thereof, in particular to an ultra-thick 800MPa quenched and tempered steel plate with excellent core low-temperature impact toughness and weldability and a manufacturing method thereof.
Background
As is well known, low carbon (high strength) low alloy steel is one of the most important engineering structural materials, and is widely used in petroleum and gas pipelines, offshore platforms, shipbuilding, bridge structures, boiler vessels, building structures, automobile industry, railway transportation and machinery manufacturing. The properties of low carbon (high strength) low alloy steels depend on their chemical composition and manufacturing process, where strength, toughness, plasticity, weldability and the match between them are the most important properties of low carbon (high strength) low alloy steels, which ultimately depend on the microstructure state of the finished steel. With the continuous forward development of science and technology, people put forward higher requirements on the toughness, plasticity and weldability of high-strength reinforced steel plates, namely that the steel plates have the brittle fracture resistance and plasticity unstability fracture resistance under the condition of low temperature (40 ℃ below zero), and simultaneously the fracture elongation and the uniform elongation reach the level of steel plates with the tensile strength of 600 MPa. Under the conditions of relatively low alloy content, particularly precious metal content and relatively low manufacturing cost, the comprehensive mechanical property and the service performance of the steel plate are greatly improved, the alloy consumption of steel is reduced, the cost is saved, the self weight, the stability and the safety of the steel member are reduced due to high strength and light weight, and more importantly, the cold and hot workability and the safety and the reliability in the service process of the steel member are further improved.
At present, the research of developing a new generation of high-performance steel materials is highly advanced within the scope of Japanese and Korean European Union, better microstructure matching is obtained through alloy combination design optimization design, control of a fine structure of a sub-microstructure and innovation manufacturing technology, and ultra-fine metallographic microstructure, fine structure of the sub-structure (dislocation configuration, pack, block and variant) and phase angle control are attempted to ensure that the ultra-thick high-strength steel plate obtains more excellent matching of strong plasticity and ductility, particularly core impact toughness, weldability and environmental brittleness resistance (hydrogen induced delayed fracture resistance).
The traditional high-strength steel plate with the tensile strength of more than or equal to 780MPa is mainly produced by an off-line hardening and tempering process (RQ + T), which requires that the steel plate has enough high hardenability and hardenability, namely, the hardenability index DI is more than or equal to 2.0 multiplied by the thickness of the finished steel plate (DI = 0.311C) 1/2 (1 + 0.64Si) x (1 + 4.10Mn) x (1 + 0.27Cu) x (1 + 0.52Ni) x (1 + 2.33Cr) x (1 + 3.14Mo) x 25.4 (mm) to ensure that the steel plate has high enough strength, excellent low-temperature toughness and uniformity of microstructure and performance in the plate thickness direction; therefore, it is inevitable to add a large amount of alloy elements such as Cr, mo, ni, cu, etc., particularly a large amount of Ni, to steel (CAMP-ISIJ, vol.4, 1991, 1949; CAMP-ISIJ, vol.4, 1991, 1950; CAMP-ISIJ, vol.7, 1994, 836; CAMP-ISIJ, vol.7, 1994, 837; japanese patent No. 59-129724;1-219121; "New Ri iron and Steel research" No. 314-1984; japanese Steel pipe technical report No.107-1985; xinri Fei technical newspaper 348 (1993); vol.4 (No. 3) -1972 of Kawasaki iron making technical paper; chuKawasaki iron technical report Vol.7 (No. 2) -1975).
More importantly, the traditional quenched and tempered steel component system and the manufacturing process are adopted, so that the steel plate is long in manufacturing period, high in manufacturing cost and difficult to process the scrap steel containing high Cu and Ni, and the recycling efficiency of the scrap steel is limited; and for the 80 kg-grade quenched and tempered steel produced by the traditional quenching and tempering process, the steel plate has the characteristics of low elongation, high yield ratio, poor weldability (high welding cold crack sensitivity, high welding heat affected zone embrittlement degree, high reheating crack sensitivity and the like), high uniformity control difficulty of performance in the plate thickness direction and the like due to high alloy content of the steel plate; the lower elongation rate is not only unfavorable for the cold and hot processing performance of the steel plate, but also has great influence on the fatigue resistance, stress concentration resistance sensitivity and structural stability of the steel plate, and has great potential safety hazard when used in large-scale engineering construction and large-scale equipment such as penstocks and steel branch pipes, thermal power turbogenerators, ocean platform structures, marine floating cranes, giant excavators and the like in hydropower engineering; therefore, when the large-scale fatigue heavy-load steel structure adopts high-strength steel, the 80 kg-grade high-strength steel is generally expected to have excellent obdurability and strong plasticity matching, and particularly the tensile elongation delta 5 Above 18%.
A large number of Patent documents are available which only describe how to achieve the strength and low temperature toughness of a base steel sheet, thereby improving the welding performance of the steel sheet and obtaining excellent HAZ low temperature toughness in a weld heat affected zone, but which are less described, and which do not relate to how to improve the tensile strength of the steel sheet and the uniformity of the tensile elongation and the mechanical properties in the thickness direction of the steel sheet (Japanese Patent No. Sho 63-93845, sho 63-79921, sho 60-258410, japanese Patent No. 4-285119, japanese Patent No. 4-308035, hei 3-264614, hei 2-250917, hei 4-143246, US Patent No. US 5798004, european Patent No. EP 0288054A2, west Mount memorial technology lecture No. 159-160, and P79-P80).
Chinese patents ZL200710042357.6, ZL200810036416.3, ZL200910055353.0, ZL2018101636154, etc. disclose the preparation of steel plates by different processes, although the comprehensive mechanical properties of steel plates produced by these patent technologies also reach very high levels: the tensile strength is more than or equal to 780MPa, the yield strength is more than or equal to 690MPa, the temperature Charpy transverse impact energy (single value) at-40 ℃ and below is more than or equal to 47J, and the weldability of the steel plate is excellent; however, the steel plate inevitably contains a certain amount of Cu and Ni alloy elements, and particularly, a large amount of Ni element.
The Chinese patent 2009100482874 discloses a low-cost 80 kg-level super-thick quenched and tempered steel plate and a manufacturing method thereof, the chemical components of the steel plate do not contain Cu and Ni elements, but the impact toughness can only meet the temperature requirements of-20 ℃ and above. In addition, a controlled rolling, offline quenching and tempering process is adopted; the manufacturing process is multiple, long in manufacturing period and high in manufacturing cost, energy consumption in the manufacturing process is relatively high (the steel plate is rolled and naturally cooled to room temperature, and then is heated to the quenching temperature again after shot blasting treatment), and energy conservation and environmental protection are not facilitated; moreover, the potential of full hardening and hardenability of alloy elements cannot be fully exerted by adopting the off-line quenching and tempering process, and the full hardening and hardenability of the elements cannot be exerted to the maximum extent; therefore, in order to obtain the same strength and toughness level, more alloy elements (especially Ni, mo, cr and the like) must be added, which not only further increases the manufacturing cost, but also impairs the weldability of the steel plate, especially for ultrahigh-strength steel plates, the welding cold cracking sensitivity is greatly improved, the welding preheating and postheating (namely PWHT) at higher temperature are required, the proper welding heat input range is narrower, and the processing and manufacturing cost is greatly improved correspondingly.
The 'nickel-free high-toughness 80 kilogram grade high-strength steel and the manufacturing method thereof' disclosed by Chinese patent ZL201210209649.5 adopt 800MPa grade quenched and tempered steel plates which are successfully developed by adopting a DQ (data analysis) process and have excellent performance and do not contain noble alloy elements such as Cu and Ni, but the low-temperature toughness cannot meet the low-temperature requirement of-50 ℃ and the steel plates cannot adopt preheating-free welding and welding heat input and cannot be higher than 50kJ/cm; however, all of the inventive steel sheets do not relate to impact toughness at a position of 1/2 of the thickness of the steel sheet (i.e., a steel sheet core), let alone the steel sheet core has excellent low-temperature impact toughness.
Disclosure of Invention
The invention aims to provide an ultra-thick 800 MPa-grade quenched and tempered steel plate with excellent core low-temperature impact toughness and weldability and a manufacturing method thereof, which ensure that steel plates with different thicknesses can obtain 800 MPa-grade high strength, and simultaneously ensure that the low-temperature impact toughness of the core part of the steel plate, the weldability of the steel plate and the crack resistance and crack arrest characteristics of the steel plate are also excellent; the tensile strength is more than or equal to 780MPa, the yield strength is more than or equal to 690MPa, and the elongation at break is delta 5 More than or equal to 15 percent, and the steel plate core part has-40 ℃ transverse impact energy (single value) KV 2 Not less than 69J, and has excellent weldability (preheating temperature not more than 120 ℃, welding heat input of 30-50 kJ/cm, no need of postweld heat treatment); the method is particularly suitable for manufacturing pressure steel pipes, steel branch pipes, spiral cases, ocean platforms, deep-sea submersibles, engineering machinery (comprising marine machinery such as a marine floating crane), coal mine machinery, heavy trucks, special loading trucks and other heavy equipment of hydroelectric engineering (particularly high-water-head and large-HD value pumped storage power stations).
In order to achieve the purpose, the technical scheme of the invention is as follows:
the invention takes a low C-ultralow Si-low Mn- (Cu + Ni + high Cr + low Mo) alloying- (Ti + V + B) microalloying component system as a basis, properly improves the acid-soluble Als content in steel, and (% Als)/[ (% N) -0.292 (% Ti)]Not less than 33, control [1.57 (% C) 0.5 +7.86(％Si)+23.23(％P) +8.17(％S)]×[1.21(％Mn)+2.65(％Mo)]Not more than 4.2, ca treatment, ca/S ratio of 1.00-3.00, etc., so that the microstructure of the finished steel plate (especially the core part of the steel plate) is fine low-carbon tempered martensite plus tempered lower bainite, and the average grain size of the steel plate is below 20 microns.
Specifically, the ultra-thick 800 MPa-grade quenched and tempered steel plate with excellent core low-temperature impact toughness and weldability comprises the following components in percentage by weight:
C：0.07％～0.11％，
Si：≤0.20％，
Mn：0.60％～1.00％，
P：≤0.014％，
S：≤0.0030％，
Cu：0.15％～0.40％，
Ni：0.70％～1.70％，
Cr：0.55％～0.95％，
Mo：0.25％～0.50％，
Nb：0.010％～0.030％，
V：0.025％～0.055％，
Ti：0.006％～0.013％，
B：0.0010％～0.0018％，
Als：0.040％～0.070％，
N：≤0.0050％，
Ca：0.0010％～0.0035％，
the balance of Fe and inevitable impurities; and the content of the elements must satisfy the following relation at the same time:
(％Als)/[(％N)-0.292(％Ti)]≥33；
[1.57(％C) 0.5 +7.86(％Si)+23.23(％P)+8.17(％S)]×[1.21(％Mn)+2.65 (％Mo)]≤4.2；
ca treatment, the ratio of Ca/S is between 1.00 and 3.00, and ((% Ca) × (% S) 0.18 )≤2.5×10 －3 ；
130≤[(％C)×DI×(T Quenching 1 )×(T Quenching 2 )]/[H×(T Tempering )]190, wherein:
DI is the index of hardenability in mm,
DI＝0.367C 0.5 (1+0.7Si)(1+3.33Mn)(1+0.35Cu)(1+0.36Ni)(1+2.16Cr)(1+3 Mo)(1+1.75V)(1+1.77Al)×25.4；
T quenching 1 Is the first quenching temperature and has the unit of;
T quenching 2 The second quenching temperature is expressed in unit of;
T tempering Is the tempering temperature, and the unit is;
h is the thickness of the steel plate and is in mm.
In the composition design of the steel plate of the invention:
c is the most effective hardening element, can effectively improve the hardness of the quenched steel plate and is the most effective alloy element for improving the quenched and tempered steel plate, so that C is of great importance for controlling the content range of C in the high-strength quenched and tempered steel plate; the proper increase of C content in steel can not only increase the hardenability and strength of steel plate, but also reduce the consumption of other alloy elements to reduce cost. As is well known, C has a great influence on the strength, low-temperature toughness, elongation and weldability of a wrought steel plate, and from the viewpoint of improving the intrinsic ductility and weldability of a high-strength steel plate, it is desirable that the C content in the steel is controlled to be appropriately low; but the C content is not easy to control to be too low from the aspects of hardenability of the steel plate (particularly the hardenability of the steel plate core part), obdurability/strong plasticity matching, weldability, microstructure control and alloy cost control; when the content of C is too high, C not only reduces the ductility and toughness of the quenched and tempered steel plate, but also more importantly, the high content of C easily causes serious segregation of the steel plate core, causes the generation of a high-carbon martensite structure and seriously deteriorates the low-temperature impact toughness of the steel plate core; therefore, the reasonable range of the C content is 0.07 percent to 0.11 percent.
Si promotes molten steel deoxidation and can improve the strength of a steel plate, but the molten steel deoxidized by Al has small deoxidation effect, si can improve the strength of the steel plate, but Si inhibits carbide precipitation in ferrite, stabilizes undercooled austenite, reduces the martensite transformation critical cooling speed, promotes coarse martensite transformation, inhibits the division effect of lower bainite transformation on prior austenite crystal grains, coarsens the packet crystal cluster size and small angle of block lath grain boundaries, seriously damages the low-temperature toughness, crack arrest characteristics, elongation and weldability of a quenched and tempered steel plate (particularly a high-strength quenched and tempered steel plate), si not only reduces the intrinsic toughness of the quenched and tempered steel plate and promotes the formation of M-A islands, but also forms the M-A islands with larger size and uneven distribution, and seriously damages the toughness of a welding Heat Affected Zone (HAZ); si promotes the solidification segregation of molten steel, so that a high-carbon martensite structure of a steel plate core is easily generated, and the low-temperature impact toughness of the steel plate core is seriously deteriorated; therefore, the Si content in the steel should be controlled as low as possible, and the Si content should be controlled to be less than 0.20% in view of economy and operability in the steel making process.
Mn, the most important alloying element, improves the strength of the steel sheet, expands the austenite phase region, and reduces Ar in the steel 3 Point temperature, refined bainite/martensite packing structure size, and orientation difference of bainite lath/martensite lath grain boundaryThe low-temperature toughness of the quenched and tempered steel plate is improved, and the low-temperature transformation structure bainite/martensite is promoted to form, so that the quenched and tempered steel plate has excellent toughness/strong plasticity matching; however, mn is easy to segregate in the molten steel solidification process, and particularly when the Mn content is high, not only is casting operation difficult, but also conjugate segregation phenomenon is easy to occur with elements such as C, P, S and the like, so that segregation and porosity of the central part of a casting blank are increased, and serious central region segregation of the casting blank is easy to form abnormal structures in the subsequent controlled rolling, quenching and tempering heat treatment and welding processes, so that low-temperature toughness severe cracking (particularly when long-strip-shaped MnS inclusions occur) of a high-strength steel plate core and cracks occur in a welding joint are caused; therefore, according to the strength grade and the content range of C in steel, the selection of the proper Mn content range is necessary for the 800 MPa-grade quenched and tempered steel plate which needs the low-temperature impact toughness of the core part, and the Mn content of the quenched and tempered steel plate is proper to be 0.60-1.00%.
P serving as harmful impurities in steel not only promotes the solidification and segregation of molten steel, but also has great damage effect on the low-temperature impact toughness (particularly the low-temperature impact toughness of a core part), the elongation, the weldability and the SR performance of a welded joint of a quality-improved steel plate, and the lower the requirement is, the better the requirement is theoretically; however, in consideration of steel-making workability and steel-making cost, the P content of an 800 MPa-grade quenched and tempered steel plate which requires excellent weldability, core impact toughness at-40 ℃ and ductility and toughness matching needs to be controlled to be less than or equal to 0.014%.
S is used as harmful impurities in steel, not only promotes the solidification and segregation of molten steel, but also has great damage effect on the low-temperature impact toughness (particularly the low-temperature impact toughness of a core part), the elongation, the weldability and the SR performance of a welding structure of a steel plate; more importantly, S is combined with Mn in steel to form MnS inclusions, the plasticity of MnS enables the MnS to extend along the rolling direction in the hot rolling process to form MnS inclusion strips along the rolling direction, the low-temperature impact toughness, the steel plate elongation, the Z-direction performance and the weldability of a steel plate core are seriously damaged, and meanwhile, S is also a main element generating hot brittleness in the hot rolling process, and the lower the S is required to be, the better the S is theoretically required to be; however, considering the steel-making operability, steel-making cost and the principle of smooth material flow, the S content needs to be controlled to be less than or equal to 0.0030 percent.
Cu is an austenite stabilizing element, and Cu may be addedReduction of Ar 3 The point temperature improves the hardenability of the steel plate and the atmospheric corrosion resistance of the steel plate; however, the addition of Cu is too much and is higher than 0.40%, so that the problems of copper brittleness, casting blank surface cracking and internal cracking and especially the SR performance degradation of the high-strength quenched and tempered steel plate welding joint are easily caused; the addition amount of Cu is too small, less than 0.15%, and the effect is small; therefore, the Cu content is preferably controlled between 0.15 percent and 0.40 percent; besides, the composite addition of Cu and Ni can reduce the copper brittleness of copper-containing steel and reduce intergranular cracking in the hot rolling process, and more importantly, both Cu and Ni are austenite stabilizing elements, and the composite addition of Cu and Ni can greatly reduce Ar 3 The driving force of austenite to ferrite phase transformation is improved, the structure size of martensite/bainite packets is refined, the growth of martensite/bainite laths in each orientation is promoted, the orientation difference between martensite/bainite blocks and the orientation difference between phase transformation variants become larger, the resistance of cracks passing through the martensite/bainite blocks and the martensite/bainite laths is increased, and the low-temperature toughness of the high-strength steel plate is improved.
The addition of Ni has the following effects:
1) The dislocation low-temperature P-N force (crystal lattice friction force) in the BCC (body centered cubic) crystal is reduced, the dislocation cross slip is promoted, and the low-temperature intrinsic toughness of a martensite/bainite structure of the BCC crystal structure is improved;
2) Ni increases the orientation difference between the martensite/bainite block and the phase change variant, and improves the resistance of the crack to pass through the martensite/bainite block and the martensite/bainite lath;
3) Ni is used as an austenite stabilizing element, reduces the temperature of an Ar3 point and refines the structure size of martensite/bainite packets, so that Ni has the function of simultaneously improving the strength, the elongation and the low-temperature toughness of a quenched and tempered steel plate, is an indispensable toughening element for a high-strength toughened steel plate, is segregated in a steel plate core to cause the increase of the local Ni content, and has great significance for improving the low-temperature impact toughness of the steel plate core;
4) The addition of Ni in the steel can also reduce the copper brittleness of the copper-containing steel, reduce intergranular cracking in the hot rolling process and improve the atmospheric corrosion resistance of the steel plate.
Therefore, theoretically, the higher the Ni content in the steel is in a certain range, the better the Ni content is, but Ni is a precious alloy element, and the manufacturing cost is increased sharply due to the fact that a large amount of Ni is added; for the 800 MPa-grade quenched and tempered steel plate, considering the balance of performance and manufacturing cost, the steel must have certain Ni content so as to ensure that the extremely high-strength quenched and tempered steel plate, particularly a steel plate core has enough hardenability, and simultaneously ensure that the toughness/strong plasticity of the steel plate is matched and the low-temperature toughness of the core is excellent; therefore, the reasonable range of the Ni content is 0.70-1.70%.
Cr is used as a weak carbide forming element, the added Cr not only improves the hardenability of the steel plate and promotes the formation of martensite/bainite, but also has the functions of strongly increasing the meta-position difference of martensite/bainite laths (namely phase transformation variants) and increasing the resistance of cracks passing through a martensite/bainite block structure, and has the functions of strongly improving the ductility and toughness and the crack resistance and crack resistance of the steel plate while improving the strength of the steel plate, and in addition, the Cr can inhibit the segregation of C and Mn in the process of solidifying molten steel, thereby having great significance for improving the low-temperature impact toughness of a steel plate core; however, when the amount of Cr added is too large, weldability of the steel sheet, particularly toughness after weld joint annealing treatment (i.e., after SR), is seriously impaired; however, for 800 MPa-grade quenched and tempered steel plates, a certain Cr content is required to ensure that the steel plates have sufficient hardenability and the low-temperature impact toughness of the steel plate core is improved; therefore, the reasonable range of the Cr content is 0.55 to 0.95 percent.
Adding Mo greatly improves the hardenability of the steel plate, promotes the formation of martensite, improves the tempering characteristic and tempering process window of the steel plate, improves the toughness and the strong plastic matching of the tempered steel plate, but the Mo is used as a strong carbide forming element, increases the size of a martensite/bainite packet and promotes the formation of small-angle grain boundaries among martensite/bainite block laths while promoting the formation of the martensite/bainite, reduces the resistance of cracks passing through the martensite/bainite block structure, and in addition, the Mo promotes the over-quenching of the surface (sub) layer of the high-strength steel; therefore, mo greatly improves the strength of the steel plate, reduces the low-temperature toughness and the elongation of the high-strength quenched and tempered steel plate, and induces the over-quenching of the surface (sub) surface layer of the steel plate; when Mo is excessively added (particularly when the contents of C and Mn are high), segregation of the slab core is promoted, so that the low-temperature impact toughness of the steel plate core, the elongation of the steel plate, the weldability and the SR performance of a welded joint are seriously damaged, and the SR brittleness and the production cost of the steel plate are increased; however, for 800 MPa-grade quenched and tempered steel sheets, a certain Mo content is required to ensure that the steel sheets have sufficient hardenability and temper softening resistance. Therefore, the Mo content is controlled to be between 0.25% and 0.50% by comprehensively considering the phase transformation strengthening effect of Mo and the influence on the low-temperature toughness, elongation and weldability of the base steel plate (especially the steel plate core).
The purpose of adding trace Nb element into the steel is to perform non-recrystallization controlled rolling, refine the microstructure of a base steel plate and improve the strength and the temper softening resistance of the steel plate, an optimal matching interval exists between the Nb content range and the C content range, and in the interval, nb has less damage to the weldability of the steel plate while exerting the optimal non-recrystallization controlled rolling and strengthening and toughening effects; therefore, when the low-carbon component design is adopted, the Nb content range can be properly increased so as to ensure the strength, low-temperature toughness and SR softening resistance of the high-strength quenched and tempered steel plate; when the addition amount of Nb is less than 0.010%, the above-described effects of Nb in the ultra-low carbon content steel sheet cannot be effectively exerted; when the addition amount of Nb exceeds 0.030%, the formation of upper bainite (Bu) and the secondary precipitation embrittlement of Nb (C, N) are easily induced in the welding process, and the low-temperature toughness of a welding Heat Affected Zone (HAZ) is seriously damaged; nb is used as a strong carbide forming element, so that the phase difference of bainite/martensite lath grain boundaries is reduced, the segregation of the slab core is increased, and the low-temperature toughness of the steel plate core and the welding HAZ is greatly damaged; therefore, the Nb content is controlled between 0.010 percent and 0.030 percent.
The content of V is between 0.025% and 0.055%, and the upper limit value of the content of V can be properly selected along with the increase of the thickness of the steel plate. The purpose of V addition is to improve the strength of the quenched and tempered steel sheet by dispersing and precipitating V (C, N) in bainite/martensite laths. Too little V is added, so that the separated V (C, N) is too little, and the strength of the 800 MPa-grade quenched and tempered steel plate cannot be effectively improved; if the amount of V added is too large, it is higher than 0.055%, and the low-temperature toughness, elongation, weldability, and low-temperature toughness of the weld HAZ are impaired.
The Ti content is controlled between 0.006% and 0.013%, excessive growth of austenite grains in the slab heating and rolling process is inhibited, the low-temperature toughness of the steel plate is improved, more importantly, the growth of HAZ grains in the welding process is inhibited, and the low-temperature toughness of the HAZ is improved; in addition, ti has the function of fixing N, eliminates free N in steel, ensures that B element exists in a solid solution B form, and stabilizes the hardenability of the solid solution B to a steel plate; however, when the Ti content exceeds 0.013%, under the condition of high acid-soluble aluminum content, excessive Ti precipitates as TiC coherent on martensite/bainite laths and at the grain boundaries, severely embrittling the high-strength steel microstructure (the martensite/bainite structure is more susceptible to coherent precipitation embrittlement than the ferrite + pearlite structure).
The content of B is controlled between 0.0010 percent and 0.0018 percent, so that the hardenability of the steel plate is ensured, and meanwhile, the weldability, HAZ toughness and the surface quality of the plate blank of the steel plate are not damaged.
Als in steel can fix free [ N ] in steel]Removal of weld Heat Affected Zone (HAZ) free [ N ]]Besides improving the low-temperature toughness of the welded HAZ, more importantly, ensuring that the steel has certain solid solution B and the hardenability of the steel plate is stable, and controlling the lower limit of Als to be 0.040%; however, excessive addition of Als to the steel not only causes casting difficulties, but also forms a large amount of dispersed acicular Al in the steel 2 O 3 Inclusions damage the integrity, low-temperature toughness and weldability of the steel plate, and the upper limit of Als is controlled to be 0.070%, so that the reasonable range of the content of Als is 0.040% -0.070%.
In order to ensure the existence of solid solution B in the steel plate and prevent a large amount of coarse AlN from precipitating along the prior austenite grain boundary and damaging the transverse impact toughness and plasticity of the steel plate, the content of N in the steel is less than or equal to 0.0050 percent.
Performing Ca treatment on the steel, on one hand, purifying the molten steel, and on the other hand, performing modification treatment on sulfides in the steel to convert the sulfides into non-deformable, stable and fine spherical sulfides, inhibiting precipitation of long-strip MnS, improving slab core segregation (high-melting Ca (O, S) particles promote the formation of slab center equiaxial crystals, improving slab core segregation), inhibiting S hot brittleness, improving low-temperature toughness, elongation and Z-direction performance of the steel plate, improving low-temperature impact toughness and anisotropy and weldability of the steel plate core, and improving pouring of high-acid-soluble aluminum molten steel by adopting Ca treatment; the addition amount of Ca depends on the content of S in steel, the addition amount of Ca is too low, and the treatment effect is not great; the Ca is added in an excessive amount, the size of formed Ca (O, S) is too large, the brittleness is increased, the Ca can be used as a starting point of fracture and crack, the low-temperature toughness and the elongation of the steel and the weldability of a steel plate are reduced, and meanwhile, the purity of the steel is reduced and the molten steel is polluted. The Ca content is generally controlled as ESSP = (wt% Ca) [1-1.24 (wt% O) ]/1.25 (wt% S), where ESSP is a sulfide inclusion shape control index, and thus a suitable range of Ca content is 0.0010% to 0.0035%.
The following are specially controlled in the component design of the invention:
(% Als)/[ (% N) -0.292 (% Ti) ] > 33, eliminating steel and weld heat affected zone free [ N ]:
1) The steel contains enough solid solution [ B ], the steel plate has enough hardenability, especially the hardenability of the steel plate core has enough hardenability and the hardenability is stable, and the strength and the low-temperature impact toughness of the 800 MPa-grade quenched and tempered steel plate, especially the low-temperature impact toughness, the obdurability/strong plasticity matching of the steel plate core are realized;
2) The content of free [ N ] in a welding heat affected zone is reduced, and the low-temperature impact toughness and the crack resistance and crack arrest characteristics of the welding heat affected zone are improved (martensite and bainite tissues are extremely sensitive to the free [ N ], the embrittlement effect of the free [ N ] on the martensite tissues is more than the bainite tissues is more than ferrite and pearlite tissues);
3) Ensuring that a welding Heat Affected Zone (HAZ) has enough solid solution [ B ] in the welding process, ensuring that the HAZ has enough hardenability, ensuring that the HAZ obtains a martensite/bainite structure, and realizing the matching of the obdurability/obdurability of a welding joint and excellent low-temperature impact toughness.
[1.57(％C) 0.5 +7.86(％Si)+23.23(％P)+8.17(％S)]×[1.21(％Mn)+2.65 (％Mo)]≤4.2：
1) The method has the advantages that the degree of conjugate segregation (the phenomenon that segregation among elements is mutually strengthened) of elements such as C, mn, P, S, mo and the like in the process of solidifying the molten steel is reduced, the segregation of a slab core is reduced, the generation of high-carbon martensite of a steel plate core is inhibited, and the sensitivity of low-temperature impact toughness and environmental brittleness of the steel plate core is improved;
2) The martensite phase transformation critical cooling speed is increased, the formation of a low-carbon martensite + lower bainite (M + BL) structure is promoted, the steel plate obtains excellent low-temperature impact toughness, and particularly, the steel plate core has excellent low-temperature impact toughness and crack resistance and crack arrest characteristics;
3) The weldability of the steel plate is improved, the number of M-A islands for welding HAZ is reduced, the size of the M-A islands is reduced, the shapes (from blocks to strips) and the distribution of the M-A islands are improved, and the low-temperature impact toughness and the crack resistance and crack arrest characteristics of the welding HAZ are improved;
this is one of the key core technologies of the present invention.
Ca treatment, ca/S ratio is between 1.00 and 3.00, and (% Ca) × (% S) 0.18 ≤2.5×10 －3 : the equiaxed crystal formation of the slab core is promoted, the transgranular structure of a high segregation zone of the slab core is reduced, the segregation degree of the steel plate core is greatly reduced, and the low-temperature impact toughness of the 800 MPa-grade quenched and tempered steel plate core and the plasticity and toughness of the steel plate are improved; fine Ca (O, S) particles are used for pinning HAZ austenite grains in a welding heat affected zone to grow large, so that the low-temperature weldability of the HAZ is improved, and the environmental brittleness of 800 MPa-grade quenched and tempered steel plates is inhibited (the segregation of the steel plate core is improved, and the delayed hydrogen-induced cracking in the central part of the plate thickness is inhibited).
130≤[(％C)×DI×(T Quenching 1 )×(T Quenching 2 )]/[H×(T Tempering )]Less than or equal to 190: the steel plate hardenability (chemical component ratio), the steel plate thickness and the gradient hardening and tempering process are matched; when ensuring that the steel plates with different thicknesses can obtain high strength, the 800MPa high-strength quenched and tempered steel plate is obtained at reasonable cost: the low-temperature impact toughness of the steel plate core, the weldability of the steel plate and the crack resistance and crack arrest characteristics are also excellent; and successfully solves the problems of contradiction and difficult blending between the strength, the ductility and the toughness of the 800 MPa-grade quenched and tempered steel plate, the low-temperature toughness of the steel plate core (namely, the steel plate core is required to have enough hardenability to cause higher carbon equivalent of the steel plate) and the excellent weldability.
The invention relates to a method for manufacturing an ultra-thick 800 MPa-grade quenched and tempered steel plate with excellent core low-temperature impact toughness and weldability, which comprises the following steps:
1) Smelting and casting
Smelting and casting the components into a plate blank, wherein the pouring superheat degree of a tundish is 7-26 ℃, the pulling speed is 0.6-1.0 m/min, the fluctuation of the liquid level of a crystallizer is less than or equal to 5mm, and the light pressure rate of the solidification tail end is 2-5%;
2) Rolling of
The first stage is common rolling, and the heating temperature of the plate blank is controlled between 1100 ℃ and 1160 ℃; the rolling pass reduction rate is more than or equal to 7 percent, and the accumulated reduction rate is more than or equal to 50 percent;
rolling is controlled by adopting non-recrystallization in the second stage, the rolling start temperature is controlled to be 780-860 ℃, the rolling reduction rate of the average rolling pass is more than or equal to 10%, the accumulated rolling reduction rate of a non-recrystallization area is more than or equal to 40%, and the final rolling temperature is 770-850 ℃;
3) Cooling and heat preservation
After rolling is finished, entering a slow cooling pit for heat preservation and slow cooling; the heat preservation slow cooling process is to preserve heat for at least 24 hours under the condition that the temperature surface of the steel plate is more than 300 ℃;
4) Thermal treatment, quenching and tempering process, namely quenching and tempering process
Quenching according to the hardenability index, ac of the steel sheet 3 Adjusting the quenching temperature of the steel plate, wherein the first quenching temperature is 900-930 ℃, the quenching holding time is more than or equal to 15min, the second quenching temperature is 870-900 ℃, the quenching holding time is more than or equal to 10min, and the quenching holding time is the heat preservation time for starting timing when the central temperature of the steel plate reaches the quenching target temperature;
tempering, wherein the tempering temperature of the steel plate is 585-625 ℃, the tempering retention time is more than or equal to 30min, and the tempering retention time is the heat preservation time when the central temperature of the steel plate reaches the tempering target temperature; and naturally cooling the steel plate to room temperature after tempering.
In the manufacturing method of the present invention:
according to the requirements of the steel of the invention such as component system, mechanical property, weldability, internal soundness (namely UT flaw detection) and the like, the design scheme of the manufacturing process is as follows: continuous casting is adopted, the superheat degree of pouring in a tundish is controlled to be 7-26 ℃, the pulling speed is controlled to be 0.6-1.0 m/min, the fluctuation of the liquid level of a crystallizer is less than or equal to 5mm, and the soft reduction rate of the solidification tail end is 2-5%.
The first stage is common rolling, and the heating temperature of the plate blank is controlled between 1100 ℃ and 1160 ℃; the rolling is carried out under a large reduction, the reduction rate of rolling passes is more than or equal to 7 percent, the cumulative reduction rate is more than or equal to 50 percent, and the uniform and fine microstructure of the intermediate billet is ensured.
And the second stage adopts non-recrystallization controlled rolling, the rolling start temperature is controlled to be 780-860 ℃, the rolling reduction rate of the average rolling pass is more than or equal to 10%, the accumulated rolling reduction rate is more than or equal to 40%, and the final rolling temperature is 770-850 ℃.
And (3) after the steel plate is rolled, the steel plate enters a heating type slow cooling pit for heat preservation and slow cooling, and the heat preservation process is characterized in that the steel plate is subjected to heat preservation for at least 24 hours under the condition that the temperature surface of the steel plate is more than 300 ℃, so that the steel plate is fully dehydrogenated, and hydrogen-induced cracks are prevented.
The heat treatment process adopts an off-line special (quenching and tempering) process, namely a gradient quenching and tempering process to produce the steel plate according to the hardenability index (namely the chemical component ratio) of the steel plate and Ac 3 The point temperature is adjusted, the quenching temperature of the steel plate is adjusted, the first quenching temperature is 900-930 ℃, and the quenching holding time is more than or equal to 15min; the secondary quenching temperature is 870-900 ℃, the quenching holding time is more than or equal to 10min, and the quenching holding time is the heat preservation time for starting timing when the central temperature of the steel plate reaches the quenching target temperature, so as to realize the matching between the hardenability and the quenching temperature of the steel plate, obtain fine and uniform low-carbon martensite and a small amount of low-carbon bainite tissues, and further obtain the performance requirements of the steel plate.
The tempering temperature (plate temperature) of the steel plate is 585-625 ℃, the tempering retention time is more than or equal to 30min, and the tempering retention time is the heat preservation time when the central temperature of the steel plate reaches the tempering target temperature; and naturally cooling the steel plate to room temperature after tempering.
The invention has the beneficial effects that:
the invention adopts reasonable cost alloy combination design, and the manufacturing method adopts controlled rolling and gradient hardening and tempering processes, thereby not only fully playing the potential of hardenability of alloy elements, but also effectively refining steel plate packets (i.e. crystal clusters), increasing the large-angle grain boundary density between martensite/bainite laths (i.e. variants), effectively refining martensite/bainite substructure, and being capable of reducing the content of precious alloys (particularly Mo element and the like); the method not only further reduces the manufacturing cost, but also improves the weldability of the steel plate, and particularly for 800MPa quenched and tempered steel plates, the welding cold crack sensitivity is greatly reduced, the welding preheating, the post-heating temperature are reduced, the proper welding heat input range is wider, and the processing and manufacturing cost of a user is correspondingly reduced.
Through reasonable components and process design, the steel plates with different thicknesses can obtain high strength, and meanwhile, the low-temperature impact toughness of the steel plate core part, the weldability of the steel plate and the crack resistance and crack arrest characteristics are also excellent; the method successfully solves the problems of contradiction and difficult blending between the strength, the ductility and the toughness of the 800 MPa-grade quenched and tempered steel plate, the low-temperature toughness of the core part of the steel plate (namely the carbon equivalent of the steel plate is higher due to the requirement of sufficient hardenability of the core part of the steel plate) and the excellent weldability, and improves the safety stability and the fatigue resistance of a large heavy steel structure.
Drawings
FIG. 1 is a photograph showing the microstructure (1/4 thickness and 1/2 thickness) of the steel of example 5 of the present invention.
Detailed Description
The invention is further illustrated by the following examples and figures.
The steel composition examples of the present invention are shown in Table 1, the manufacturing processes of the steel of the present invention are shown in tables 2 to 4, and the properties of the steel sheet are shown in tables 5 to 6.
As can be seen from the microstructure in FIG. 1, the microstructure of the steel plate is fine low-carbon tempered martensite and tempered lower bainite, the average grain size of the steel plate is below 20 μm, and no obvious segregation zone exists in the core of the steel plate.
The invention adopts reasonable cost alloy combination design and is matched with a controlled rolling and gradient hardening and tempering process, thereby not only fully playing the potential of hardenability of alloy elements, but also effectively refining steel plate packets (crystal clusters), increasing the large-angle grain boundary density between martensite/bainite laths (varients), effectively refining martensite/bainite substructure and being capable of reducing the content of precious alloys (particularly Mo element and the like); the manufacturing cost is further reduced, the weldability of the steel plate is improved, the welding cold crack sensitivity is greatly reduced particularly for 800MPa quenched and tempered steel plates, the welding preheating and post-heating temperature are reduced, the range of proper welding heat input is wider, and the processing and manufacturing cost of a user is correspondingly reduced.
As a strategic base material, the ultra-thick 800 MPa-grade quenched and tempered steel plate with excellent low-temperature impact toughness and weldability of the core has wide market prospect. The method is mainly used for manufacturing heavy equipment such as pressure steel pipes, steel branch pipes, spiral cases, ocean platforms, deep-sea submersibles, engineering machinery (including marine machinery such as marine floating cranes) and the like, coal mine machinery, heavy trucks, special loading trucks and the like in hydroelectric engineering (particularly high-water-head and large-HD-value pumped storage power stations).
Claims (4)
1. The super-thick 800 MPa-grade quenched and tempered steel plate with excellent low-temperature impact toughness and weldability of the core part comprises the following components in percentage by weight:
C：0.07％～0.11％，
Si：≤0.20％，
Mn：0.60％～1.00％，
P：≤0.014％，
S：≤0.0030％，
Cu：0.15％～0.40％，
Ni：0.70％～1.70％，
Cr：0.55％～0.95％，
Mo：0.25％～0.50％，
Nb：0.010％～0.030％，
V：0.025％～0.055％，
Ti：0.006％～0.013％，
B：0.0010％～0.0018％，
Als：0.040％～0.070％，
N：≤0.0050％，
Ca：0.0010％～0.0035％，
the balance of Fe and inevitable impurities; and the contents of the elements simultaneously satisfy the following relationship:
(％Als)/[(％N)-0.292(％Ti)]≥33；
[1.57(％C) 0.5 +7.86(％Si)+23.23(％P)+8.17(％S)]×[1.21(％Mn)+2.65(％Mo)]≤4.2；
ca treatment, the Ca/S ratio is 1.00-3.00, (% Ca) × (% S) 0.18 )≤2.5×10 －3 ；
130≤[(％C)×DI×(T Quenching 1 )×(T Quenching 2 )]/[H×(T Tempering )]190, wherein:
DI is the index of hardenability in mm,
DI＝0.367C 0.5 (1+0.7Si)(1+3.33Mn)(1+0.35Cu)(1+0.36Ni)(1+2.16Cr)(1+3Mo)(1+1.75V)(1+1.77Al)×25.4；
T quenching 1 Is the first quenching temperature and has the unit of;
T quenching 2 The second quenching temperature is expressed in unit of;
T tempering Is the tempering temperature, and the unit is;
h is the thickness of the steel plate, and the unit is mm;
the tensile strength of the quenched and tempered steel plate is more than or equal to 780MPa, the yield strength is more than or equal to 690MPa, and the fracture elongation delta 5 Not less than 15 percent and single value KV of transverse impact energy of steel plate core at-40 DEG C 2 ≥69J。
2. The extra thick 800 MPa-grade quenched and tempered steel sheet with excellent core low-temperature impact toughness and weldability according to claim 1, wherein the microstructure of the quenched and tempered steel sheet is fine low-carbon tempered martensite + tempered lower bainite, and the average grain size of the steel sheet is 20 μm or less.
3. The method for manufacturing the 800 MPa-grade quenched and tempered steel sheet having excellent low-temperature impact toughness and weldability as claimed in claim 1 or 2, comprising the steps of:
1) Smelting and casting
Smelting and casting the components into a plate blank according to the claim 1, wherein the superheat degree of pouring in a tundish is 7-26 ℃, the drawing speed is 0.6-1.0 m/min, the fluctuation of the liquid level of a crystallizer is less than or equal to 5mm, and the soft reduction rate at the solidification tail end is 2-5%;
2) Rolling of
The first stage is common rolling, and the heating temperature of the plate blank is controlled between 1100 ℃ and 1160 ℃; the rolling pass reduction rate is more than or equal to 7 percent, and the accumulated reduction rate is more than or equal to 50 percent;
rolling is controlled in the second stage by adopting non-recrystallization, the rolling start temperature is controlled to be 780-860 ℃, the rolling reduction rate of the average rolling pass is more than or equal to 10%, the cumulative rolling reduction rate of a non-recrystallization area is more than or equal to 40%, and the final rolling temperature is 770-850 ℃;
3) Cooling and heat preservation
After rolling is finished, entering a slow cooling pit for heat preservation and slow cooling; the heat preservation slow cooling process is to preserve heat for at least 24 hours under the condition that the temperature surface of the steel plate is more than 300 ℃;
4) Thermal treatment, quenching and tempering process, namely quenching and tempering process
Quenching according to hardenability index and Ac of steel plate 3 The steel plate quenching temperature is adjusted, the first quenching temperature is 900-930 ℃, the quenching holding time is more than or equal to 15min, the second quenching temperature is 870-900 ℃, the quenching holding time is more than or equal to 10min, and the quenching holding time is the heat preservation time for starting timing when the central temperature of the steel plate reaches the quenching target temperature;
tempering, wherein the tempering temperature of the steel plate is 585-625 ℃, the tempering retention time is more than or equal to 30min, and the tempering retention time is the heat preservation time when the central temperature of the steel plate reaches the tempering target temperature; and naturally cooling the steel plate to room temperature after tempering.
4. The method of manufacturing a quenched and tempered steel sheet of 800MPa grade having excellent low-temperature impact toughness and weldability as claimed in claim 3, wherein the microstructure of the quenched and tempered steel sheet is fine low-carbon tempered martensite + tempered lower bainite, and the average grain size of the steel sheet is 20 μm or less.
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| CN101845597B (en) * | 2009-03-26 | 2011-12-21 | 宝山钢铁股份有限公司 | Low-cost 80-kilo grade super-thick quenched and tempered steel plate and production method thereof |
| CN102041459B (en) * | 2009-10-23 | 2012-09-19 | 宝山钢铁股份有限公司 | Steel plate HT690 capable of being subjected to high heat input welding and manufacturing method thereof |
| CN102337478B (en) * | 2010-07-15 | 2012-11-14 | 宝山钢铁股份有限公司 | Excellent 100 kg-grade quenched and tempered steel plate with strong toughness and plasticity, and manufacturing method thereof |
| CN102691007B (en) * | 2011-03-23 | 2013-09-04 | 宝山钢铁股份有限公司 | High tempering parameter PWHT embrittlement resistant, extra thick cryogenic steel plate and manufacture method thereof |
| CN102719757B (en) * | 2012-06-25 | 2014-03-19 | 宝山钢铁股份有限公司 | Nickel-free high-toughness 80kg-grade high-strength steel and manufacturing method thereof |
| KR20150075541A (en) * | 2013-12-26 | 2015-07-06 | 주식회사 포스코 | High strength hot rolled steel sheet have excellent formability and fatigue resistance And Method of manufacturing the same |
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| CN110760765B (en) * | 2018-07-27 | 2021-03-12 | 宝山钢铁股份有限公司 | 600 MPa-grade quenched and tempered steel plate with ultralow cost, high elongation and strain aging embrittlement resistance and manufacturing method thereof |
| CN110157987B (en) * | 2019-06-18 | 2020-10-09 | 贝斯山钢(山东)钢板有限公司 | NQT-process-based large-thickness wear-resistant steel plate with good-40-DEG C low-temperature toughness and preparation method thereof |
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2020
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