CN110195193B - 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability and manufacturing method thereof - Google Patents
800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability and manufacturing method thereof Download PDFInfo
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- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
Abstract
800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability and a manufacturing method thereof are characterized in that an ultralow C-ultralow Si-high Mn- (Cu + Ni + Mo + high Cr) alloying- (Ti + V + B) microalloying component system is adopted as a basis, the acid-soluble Als content in the steel is properly increased, and the percent Als is more than or equal to 10 x [ (% N) (% Als)t)－0.292(％Ti)]Controlling the large angle bainite lath/martensite lath grain boundary forming index chi ≥ 2.1, (% C) × [ (% Si) +0.83 (% Mo) +1.12 (% Nb) +0.89 (% Als) +0.45 (% V)]Less than or equal to 0.031, DI index × ζ × TOpen cold×[(TOpen cold‑TStopping cooling)/(TOpen cold+TStopping cooling)]the/T is more than or equal to 1535, the Ca treatment and the Ca/S ratio is between 1.00 and 3.00, and the like, and the CR + DQ + off-line tempering process T is optimized, so that the microstructure of the finished steel plate is uniform and fine lower bainite + lath martensite, the average crystal group size is below 20 mu m, and excellent strong plasticity and toughness are obtained.
Description
Technical Field
The invention relates to a high-strength steel plate, in particular to a 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability and a manufacturing method thereof, wherein the quenched and tempered steel plate has the tensile strength of more than or equal to 780MPa, the yield strength of more than or equal to 690MPa, the-50 ℃ Charpy transverse impact energy (single value) of more than or equal to 47J and the fracture elongation delta5Not less than 15 percent, and has excellent weldability.
Background
As is well known, low-carbon (high-strength) low-alloy steel is one of the most important engineering structural materials, and is widely applied to petroleum and natural gas pipelines, ocean platforms, shipbuilding, bridge structures, boiler containers, building structures, automobile industry, railway transportation and mechanical manufacturing; the properties of low carbon (high strength) low alloy steel depend on its chemical composition and manufacturing process, where strength, toughness, plasticity and weldability are the most important properties of low carbon (high strength) low alloy steel, which ultimately depend on the microstructure state of the finished steel; with the continuous forward development of metallurgical technology, people put forward higher requirements on the toughness, plasticity and weldability of high-strength steel, namely that the steel plate has high strength, high elongation, brittle fracture resistance and plastic unstable fracture resistance and has excellent welding performance at a low temperature; under the condition of lower manufacturing cost, the comprehensive mechanical property and the use performance of the steel plate are greatly improved, the cost is saved by reducing the using amount of steel, the self weight, the stability and the safety of a steel member are reduced, and more importantly, the cold and hot workability of the strong quenched and tempered steel and the safety and the reliability in the service process are further improved; at present, the research of developing a new generation of high-performance steel materials is high in the scope of the Japanese and Korean European Union, and better microscopic structure matching and ultrafine structure and structure matching of high-strength steel are obtained through alloy combination design optimization and innovative manufacturing technology.
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); this requires that the steel sheet have a sufficiently high hardenability and hardenability, i.e. a hardenability index DI ≥ 2 × the thickness of the finished steel sheet [ DI ═ 0.311C ]1/2(1+0.64Si) × (1+4.10Mn) × (1+0.27Cu) × (1+0.52Ni) × (1+2.33Cr) × (1+3.14Mo) × 25.4 (mm)) to ensure sufficiently high strength, excellent low-temperature toughness and uniformity of microstructure and properties in the plate thickness direction, and therefore, it is inevitable to add large amounts of alloy elements such as Cr, Mo, Ni, Cu, etc., particularly large amounts of Ni element, to the steel (see 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. Sho 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 traditional manufacturing process are adopted, so that the steel plate has long manufacturing period and high manufacturing cost, and the scrap steel containing high Cu and Ni is difficult to treat, 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 low elongation rate is not only adverse to 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 a penstock and a steel branch pipe in hydroelectric engineering, a thermal power turbine generator and an ocean platform structure, a marine floating crane, a giant excavator and the like; 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 delta5Above 18%.
A large number of prior Patent documents only describe how to achieve the strength and low temperature toughness of a base steel plate, and improve the welding performance of the steel plate, and to obtain an excellent weld heat affected zone HAZ low temperature toughness, and do not relate to how to improve the tensile strength of the steel plate and the uniformity of the tensile elongation and the mechanical properties in the thickness direction of the steel plate (Japanese Patent No. Sho 63-93845, No. Sho 63-79921, No. Sho 60-258410, No. Hei 4-285119, No. Hei 4-308035, No. Hei 3-264614, No. Hei 2-250917, No. Hei 4-143246, U.S. Pat. No. US Patent5798004, European Patent No. EP0288054A2, West mountain commemorative technical lecture No. 159-160, and P79-P80).
Chinese patent No. CN200710042357.6 discloses a HT780 steel plate with excellent weldability and low yield ratio and a manufacturing method thereof; chinese patent No. CN200810036416.3 discloses a steel plate with strong toughness and strong plasticity and a manufacturing method thereof; the Chinese patent No. CN200910048287 discloses a low-cost 80 kg-grade extra-thick quenched and tempered steel plate and a manufacturing method thereof, although the comprehensive mechanical properties of the steel plates produced by the patent technologies also reach a very high level: 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 chemical components of the steel plate do not contain Cu and Ni elements, but the impact toughness can only meet the temperature requirement of minus 20 ℃ and above (for example, the Chinese patent No. CN200910048287 is a super-thick quenched and tempered steel plate with low cost and 80 kg grade and the manufacturing method thereof). In addition, a controlled rolling, offline quenching and tempering process is adopted; the method has the advantages of multiple manufacturing procedures, long manufacturing period, high manufacturing cost, and relatively high energy consumption in the manufacturing process (the steel plate is naturally cooled to room temperature after being rolled and then is heated to the quenching temperature again after being shot-blasted), so that the method is not beneficial to energy conservation and environmental protection; 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 kg-grade high-strength steel and the manufacturing method thereof' disclosed by Chinese patent No. CN201210209649.5 are developed successfully by adopting a DQ (data transfer) process without adding noble alloy elements such as Cu, Ni and the like and 800 MPa-grade quenched and tempered steel plates with excellent performance, but the low-temperature toughness cannot meet the low-temperature requirement of minus 50 ℃ and the steel plates cannot adopt non-preheating welding and welding heat input and cannot be higher than 50 kJ/cm.
Disclosure of Invention
The invention aims to provide an 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability and a manufacturing method thereof, wherein on the basis of adding a small amount of Cu and Ni alloy elements, high strength is obtained at low cost, the ultralow temperature toughness and the weldability of the steel plate are also excellent, the steel plate can bear high heat input welding, and the problem of mutual contradiction among the strength, the ultralow temperature toughness and the weldability of the 800 MPa-grade quenched and tempered steel plate is successfully solved; the method is particularly suitable for large-scale steel structures and equipment such as hydroelectric pressure water pipes, steel branch pipes, volute casings, ocean platforms, large-scale engineering machinery, coal mine machinery and the like.
In order to achieve the purpose, the technical scheme of the invention is as follows:
the invention adopts a component system of ultralow C-ultralow Si-high Mn- (Cu + Ni + Mo + high Cr) alloying- (Ti + V + B) microalloying as a basis, properly improves the content of acid-soluble Als in steel and ensures that the content of (% Als) is more than or equal to 10 x [ (% N) ]t)－0.292(％Ti)]Controlling the large angle bainite lath/martensite lath grain boundary forming index chi ≥ 2.1, (% C) × [ (% Si) +0.83 (% Mo) +1.12 (% Nb) +0.89 (% Als) +0.45 (% V)]Less than or equal to 0.031, DI index × ζ × TOpen cold×[(TOpen cold-TStopping cooling)/(TOpen cold+TStopping cooling)]the/T is more than or equal to 1535, the Ca treatment and the Ca/S ratio is between 1.00 and 3.00, and the like, and the combination of the non-recrystallization controlled rolling CR, the online quenching DQ and the offline tempering process T is optimized, so that the microstructure of the finished steel plate is fine low-carbon tempered martensite and tempered lower bainite, the average crystal group size is below 20 mu m, and the 800MPa grade high-strength steel plate with excellent strong plasticity and toughness is obtained.
Specifically, the 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability comprises the following components in percentage by weight:
C：0.05％～0.08％
Si：≤0.10％
Mn：1.55％～1.85％
P：≤0.013％
S：≤0.0030％
Cu：0.10％～0.40％
Ni：0.10％～0.40％
Cr：0.50％～0.90％
Mo：0.10％～0.30％
Nb：0.010％～0.035％
V：0.025％～0.055％
Ti：0.007％～0.014％
B：0.0010％～0.0018％
Als：0.025％～0.060％
N：≤0.0055％
Ca：0.001％～0.0035％
the balance of Fe and inevitable impurities; and the content of the elements must satisfy the following relation at the same time:
the forming index chi of the large-angle bainite lath/martensite lath grain boundary is controlled to be more than or equal to 2.1, the bainite/martensite lath grain boundary is ensured to be the large-angle grain boundary in the quenching and subsequent steel plate welding heat cycle process of the steel plate, the resistance of the crack penetrating through the bainite lath/martensite lath grain boundary is increased, and the low-temperature toughness of the high-strength quenched and tempered steel plate and the welding HAZ is improved; wherein χ ═ 1.33 (% Mn) + (% Ni) +0.55 (% Cu) +0.21 (% Cr) -0.93 (% Mo) -2.55 (% Nb) -1.91 (% Ti) -1.42 (% V); this is one of the key core technologies of the present invention.
(％C)×[(％Si)+0.83(％Mo)+1.12(％Nb)+0.89(％Als)+0.45(％V)]Less than or equal to 0.031, and can increase the critical cooling rate of martensite phase transformation and promote lower bainite BLForming, effectively dividing prior austenite grains, refining the size of a crystal group of a 800 MPa-grade quenched and tempered steel plate, and improving the low-temperature toughness of a base steel plate; promote carbide precipitation, inhibit M/A island precipitation in HAZ, reduce the number and size of M/A islands, improve the form of M/A islands, and improve the low-temperature toughness of the HAZ in high-heat-input welding of the steel plate; this is one of the key core technologies of the present invention.
DI index × ζ × TOpen cold×[(TOpen cold-TStopping cooling)/(TOpen cold+TStopping cooling)]The/t is more than or equal to 1535, the microstructure of the steel plate is uniform and fine lower bainite and lath martensite after on-line quenching (namely DQ), and the steel plate is ensured to have excellent obdurability, strong plastic matching, excellent low-temperature toughness and edgeThe mechanical property in the plate thickness direction is uniform; wherein the content of the first and second substances,
zeta is online quenching DQ hardenability factor, and its value is 1.4;
t is the thickness (mm) of the finished steel plate;
DI＝0.367(％C)0.5[1+0.7(％Si)][(1+3.33(％Mn)][(1+0.35(％Cu)][(1+0.36(％Ni)][(1+2.16(％Cr)][(1+3(％Mo)][(1+1.75(％V)][(1+1.77(％Al)][(1+200(％B)]×25.4(mm)；
Topen coldQuenching temperature (i.e., start temperature of accelerated cooling), T, for in-line quenching of DQStopping coolingThe quenching end temperature (i.e., the stop temperature for accelerated cooling) for on-line quenching of DQ is given in units of; this is one of the key core technologies of the present invention.
An Als content of ≧ 10 [ (% N)t)－0.292(％Ti)]So as to ensure solid solution of B in the steel and precipitation of AlN in a fine dispersion state, improve the hardenability of the steel plate, refine the grain extraction of the quenched and tempered steel plate and improve the low-temperature toughness of the quenched and tempered steel plate.
Ca treatment is carried out, and the Ca/S ratio is between 1.00 and 3.00; so as to improve the low-temperature toughness, the matching of the obdurability and the strong plasticity, the weldability and the lamellar tearing resistance of the steel plate.
The component data in the above relational expression is calculated according to percentage, for example, the carbon content is 0.10%, and when the relational expression is calculated, the calculation is carried out by substituting 0.10.
In the composition design of the steel of the invention:
c has great influence on the strength, low-temperature toughness, elongation and weldability of the high-strength steel, and the content of C in the steel is expected to be controlled to be lower from the viewpoint of improving the intrinsic ductility and weldability of the high-strength steel; but from the aspects of hardenability, strong plasticity and obdurability matching of high-strength tempered steel, micro-structure control in rolling control and DQ processes and manufacturing cost, the content of C is not easy to be controlled to be too low, particularly 800MPa grade on-line quenched and tempered steel plates; the reasonable range of the factor C content is 0.05-0.08%.
Mn, the most important alloying element, improves the strength of the heat-treated steel, and also has the effects of enlarging the austenite phase region and greatly reducing Ar in the steel3Point temperature, refined bainite/martensite packing structure size, BehcetThe orientation difference of the lath/martensite lath crystal boundary improves the toughness of the quenched and tempered steel plate and the low temperature, promotes the formation of bainite/martensite of a low temperature transformation structure, and ensures that the quenched and tempered steel plate has excellent toughness, strong plasticity and matching; however, Mn is easy to segregate in the molten steel solidification process, particularly when the Mn content is high, not only can casting operation be difficult, but also conjugate segregation with elements such as C, P, S and the like can be easily caused, particularly when the C content in steel is high, segregation and porosity of the central part of a casting blank are aggravated, and the serious central part segregation of the casting blank is easy to form abnormal structures in subsequent controlled rolling, on-line quenching DQ and welding processes, so that low ultralow-temperature toughness of high-strength high-; 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 800 MPa-grade quenched and tempered steel plates, and the proper Mn content of the quenched and tempered steel plate is 1.55-1.85%.
Si promotes molten steel deoxidation and can improve the strength of a quenched and tempered steel plate at 800MPa, but Si deoxidized by Al has small deoxidation effect, Si can improve the strength of the quenched and tempered steel plate, but Si reduces the critical cooling speed of martensite formation, inhibits the formation of lower bainite, seriously damages the low-temperature toughness, elongation and weldability of a high-strength quenched steel plate, particularly under the condition of high linear energy welding, Si not only promotes the formation of M-A islands, but also forms thicker and unevenly distributed M-A islands, and seriously damages the toughness of a welding Heat Affected Zone (HAZ) and the SR performance of a welding joint, so that the Si content in the steel is controlled to be extremely low as possible, and therefore, the Si content in the steel is controlled to be less than 0.10%.
P has great damage effect on the mechanical properties of the quality-improved steel plate, particularly on the low-temperature impact toughness, the elongation, the weldability and the SR performance of a welding joint as harmful impurities in steel, and the lower the requirement is, the better the theoretical requirement is; however, considering steel-making operability and steel-making cost, the P content of the quenched and tempered steel plate which requires excellent weldability, toughness at-50 ℃ and excellent toughness matching needs to be controlled to be less than or equal to 0.013 percent.
S has a great damage effect on the low-temperature toughness of a tempered steel plate as harmful inclusions in steel, more importantly, S is combined with Mn in the steel to form MnS inclusions, the plasticity of MnS enables MnS to extend along the rolling direction in the hot rolling process to form MnS inclusion bands along the rolling direction, the low-temperature impact toughness, the elongation, the Z-direction performance, the weldability and the SR performance of a welded joint of the steel plate are seriously damaged, and simultaneously S is also a main element generating hot brittleness in the hot rolling process, and the lower the S is theoretically required to be, the better the S is; but considering the steel-making operability, the steel-making cost and the principle of smooth logistics, the S content of the quenched and tempered steel plate which requires excellent weldability, toughness at-50 ℃ and excellent toughness matching needs to be controlled to be less than or equal to 0.0030 percent.
Cu austenite stabilizing element, and Ar can be reduced by adding Cu3The point temperature improves the through hardenability of the quenched and tempered steel plate and the atmospheric corrosion resistance of the steel plate; however, the addition of Cu is too much and higher than 0.40%, so that the problems of copper brittleness, surface cracking and internal cracking of a casting blank and the like are easily caused; for 800 MPa-grade quenched and tempered steel plates, the addition amount of Cu is too small and is lower than 0.10%, and any effect is small; therefore, the Cu content is controlled between 0.10 percent and 0.40 percent. In addition, the Cu and Ni composite addition not only reduces the copper brittleness of the copper-containing steel and reduces the original austenite intergranular cracking in the hot rolling process, but also more importantly, both Cu and Ni are austenite stabilizing elements, and the composite addition of Cu and Ni can greatly reduce Ar3The driving force of the austenite to ferrite phase transformation is improved, so that the martensite/bainite laths can grow in each orientation, the orientation difference of the martensite/bainite lath grain boundary is enlarged, the resistance of cracks passing through the martensite/bainite lath grain boundary is increased, and the low-temperature toughness of the quenched and tempered steel plate is improved.
The addition of Ni can not only improve the dislocation mobility in a ferrite phase, promote the slippage and the cross slippage of the dislocation under the low-temperature condition, and improve the intrinsic toughness of bainite/martensite laths (essentially all BCC ferrite phase); ni as austenite stabilizing element to reduce Ar3Point temperature, increasing phase transformation driving force, refining the size of martensite/bainite crystal groups, and increasing the orientation difference of martensite/bainite lath grain boundaries; therefore, Ni has the function of simultaneously improving the strength, the elongation and the low-temperature toughness of the quenched and tempered steel plate; 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 Ni content in the steel isThe higher the Ni content is, the better the Ni content is in a certain range, but the excessively high Ni content can not only harden the welding heat affected zone and cause adverse effects on the weldability of the steel plate, but also cause high manufacturing cost of the steel plate and seriously affect the market competitiveness of the steel plate; therefore, the appropriate Ni content is controlled to be between 0.10% and 0.30% to ensure through hardenability of the steel sheet, the matching of the toughness and the plasticity of the steel sheet and excellent low-temperature toughness without greatly increasing the manufacturing cost of the steel sheet.
Cr is used as a weak carbide forming element, and the addition of Cr not only improves the hardenability of the steel plate and promotes the formation of martensite/bainite, but also increases the orientation difference of martensite/bainite lath grain boundaries and increases the resistance of cracks passing through the martensite/bainite grain boundaries, thereby improving the strength of the steel plate and having a certain function of improving the toughness of the steel plate; however, when the amount of Cr added is too large, chromium-containing carbide precipitates and aggregates at the prior austenite grain boundary in the processes of tempering and welding heat cycle, and the low-temperature toughness and weldability of the steel plate are seriously damaged, but for a DQ type quenched and tempered steel plate of 800MPa grade, a certain Cr content is required to ensure that the steel plate has sufficient hardenability; therefore, the proper Cr content is controlled between 0.50 percent and 0.90 percent.
The addition of Mo improves the hardenability of the steel plate and promotes the formation of martensite/bainite, but Mo is used as a strong carbide forming element, so that the formation of martensite/bainite is promoted, the size of a martensite/bainite packet is increased, the meta-position difference of the formed martensite/bainite block is small (less than or equal to 5 degrees), and the resistance of cracks passing through a martensite/bainite grain boundary is reduced, so that the strength of the steel plate is greatly improved, and the low-temperature toughness and the elongation of high-strength steel are reduced; and when Mo is added excessively, not only elongation and weldability of the steel sheet, especially large heat input weldability, are seriously impaired, but also the manufacturing cost of the steel sheet is increased; however, in the 800MPa class DQ quenched and tempered steel sheet, a certain Mo content is required to ensure that the steel sheet has sufficient hardenability and temper softening resistance. Therefore, the phase transformation strengthening effect of Mo and the influence on the low-temperature toughness, the elongation and the weldability of the base steel plate are comprehensively considered, and the content of Mo is properly controlled to be between 0.10 and 0.30 percent.
The purpose of adding trace Nb element in the steel is to perform non-recrystallization controlled rolling, increase the hardenability of the rolled steel plate on-line DQ, refine the microstructure of the 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 ultra-low carbon composition 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-mentioned effects of Nb in the ultra-low carbon content steel sheet cannot be effectively exerted; when the addition amount of Nb exceeds 0.035%, the formation of upper bainite (Bu) and the secondary precipitation embrittlement of Nb (C, N) are induced under the condition of high heat input welding, and the low-temperature toughness of a high heat input 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, and the low-temperature toughness of the welding HAZ is greatly damaged; therefore, the content of Nb is properly controlled between 0.010 percent and 0.035 percent.
The Ti content is between 0.007 and 0.014 percent, the excessive growth of austenite grains in the processes of heating, rolling and DQ of a plate blank is inhibited, the low-temperature toughness of the steel plate is improved, and more importantly, the growth of HAZ grains in the process of large heat input welding is inhibited, and the low-temperature toughness of HAZ is improved; in addition, Ti has the function of fixing N, eliminates free N in steel and ensures that B element exists in a solid solution B form; however, when the Ti content exceeds 0.014%, excess Ti precipitates as TiC in a coherent manner on martensite/bainite laths and at the grain boundaries under the condition of a high acid-soluble aluminum content, and severely embrittles the microstructure of the high-strength steel.
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 improving the hardenability of the steel plate; therefore, the lower limit of Als is controlled to be 0.025%; however, excessive addition of Als to steel not only causes casting difficulties, but also causes problems in the steelForm a large amount of dispersed needle-shaped Al2O3Inclusions impair the soundness of the steel sheet inner quality, low-temperature toughness and weldability, so the upper limit of Als is controlled to 0.060%.
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 adding V is to improve the strength of the DQ 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 800 MPa-grade DQ quenched and tempered steel cannot be effectively improved; when 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 welded HAZ of the steel sheet are impaired.
In order to ensure the presence of solid solution [ B ] in the steel sheet and to prevent a large amount of coarse AlN from precipitating along prior austenite grain boundaries to impair the transverse impact toughness and plasticity of the steel sheet, the N content in the steel should not exceed 0.0055%.
Performing Ca treatment on the steel, on one hand, purifying the molten steel, and on the other hand, performing denaturation treatment on sulfides in the steel to convert the sulfides into non-deformable, stable and fine spherical sulfides, inhibiting the hot brittleness of S, improving the low-temperature toughness, the elongation and the Z-direction performance of the steel plate, and improving the anisotropy and the weldability of the toughness of the steel plate; 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 formed Ca (O, S) size is too large, the brittleness is increased, the Ca can be used as a fracture crack starting point, 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 and the pollution to molten steel are reduced. The Ca content is generally controlled as ESSP (wt% Ca) [1-1.24 (wt% O) ]/1.25 (wt% S), which is a sulfide inclusion shape control index, and thus a suitable range of the Ca content is 0.0010% to 0.0035%.
The invention relates to a method for manufacturing 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability, which comprises the following steps:
1) smelting and casting
Smelting and continuously casting the components into a plate blank;
2) rolling, wherein the total compression ratio of the steel plate, namely the thickness of the plate blank/the thickness of the finished steel plate, is more than or equal to 3.5;
the first stage is common rolling, and the heating temperature of the plate blank is controlled between 1100 ℃ and 1180 ℃; rolling under high reduction, wherein the reduction rate of rolling passes is more than or equal to 8 percent, and the cumulative reduction rate of passes is more than or equal to 45 percent;
rolling in the second stage by using unrecrystallized control, wherein the rolling start temperature is controlled to be 830-880 ℃, the rolling pass reduction rate is more than or equal to 7 percent, the cumulative reduction rate of an unrecrystallized area is more than or equal to 50 percent, and the final rolling temperature is 820-850 ℃;
3) cooling down
Immediately conveying the steel plate to cooling equipment after the recrystallization and rolling control are finished, and then quickly cooling the steel plate, wherein the starting cooling temperature of the steel plate is 800-830 ℃, the cooling speed is more than or equal to 7 ℃/s, and the stopping cooling temperature is controlled to be less than or equal to 300 ℃; then sending the steel plate into a slow cooling pit for heat preservation, wherein the interval time between the end of cold stop and the time when the steel plate enters a heating type slow cooling pit for heat preservation is not more than 30min, and the heat preservation technology 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 ℃;
4) tempering process
The tempering temperature of the steel plate is 550-600 ℃, the tempering retention time T is more than or equal to (0.60-1.00) multiplied by the thickness T of the finished steel plate, the tempering retention time is the heat preservation time started to time when the central temperature of the steel plate reaches the tempering target temperature, the time T unit is min, and the thickness T unit is mm; and naturally cooling the steel plate to room temperature after tempering.
According to the requirements of the steel plate of the invention on component system, mechanical property, weldability, internal soundness (UT flaw detection) and the like, continuous casting is adopted, the superheat degree of pouring in a tundish is controlled at 8-30 ℃, the pulling speed is controlled at 0.6-1.0 m/min, the fluctuation of the liquid level of a crystallizer is controlled at less than or equal to 5mm, and the process of solidification tail end soft reduction is 2-5%.
And 2) controlling rolling, wherein in order to ensure that the microstructure of the ultrahigh-strength quenched and tempered steel plate is uniform and fine, the total compression ratio (the thickness of the plate blank/the thickness of a finished steel plate) of the steel plate is more than or equal to 3.5.
The first stage is common rolling, in order to ensure that [ Al ] + BN → AlN + [ B ] occurs in the heating and rolling processes and ensure that enough solid solution B exists in steel, the heating temperature of a plate blank is controlled between 1100 ℃ and 1180 ℃; the rolling is carried out under high reduction, the rolling pass reduction rate is more than or equal to 8 percent, and the accumulated pass reduction rate is more than or equal to 45 percent, so as to ensure that the microstructure of the intermediate billet is uniform, fine and sufficient solid solution B, and lay the foundation for the subsequent non-recrystallization controlled rolling and on-line quenching.
And the second stage adopts non-recrystallization controlled rolling, the rolling starting temperature is controlled to be 830-880 ℃, the rolling pass reduction rate is more than or equal to 7%, the cumulative reduction rate of a non-recrystallization area is more than or equal to 50%, and the final rolling temperature is 820-850 ℃.
Step 3), immediately conveying the steel plate to DQ + ACC equipment after the recrystallization controlled rolling is finished, and then carrying out accelerated cooling on the steel plate; the steel plate is cooled at 800-830 deg.c, cooling speed not lower than 7 deg.c/s and cooling stopping temperature not higher than 300 deg.c.
The interval time between the end of cooling stop and the time when a steel plate with the thickness of more than or equal to 40mm enters the heating type slow cooling pit for heat preservation is not more than 30min, the heat preservation process is that the steel plate is preserved for at least 24 hours under the condition that the temperature surface of the steel plate is more than 300 ℃, the steel plate is ensured to be fully dehydrogenated, and hydrogen-induced cracks are prevented from being generated.
In the tempering process in the step 4), the tempering temperature (plate temperature) of a steel plate is 550-600 ℃, the tempering temperature is higher than the upper limit when the steel plate is relatively thin, the tempering temperature is lower than the lower limit when the steel plate is relatively thick, the tempering retention time is not less than (0.60-1.00) multiplied by the thickness T of a finished steel plate, the tempering retention time is the heat preservation time started to time when the central temperature of the steel plate reaches the tempering target temperature, the time T unit is min, and the thickness T unit is mm; and naturally cooling the steel plate to room temperature after tempering.
The invention has the beneficial effects that:
the invention obtains the excellent 800MPa grade high strength quenched and tempered steel plate, simultaneously, the strength and toughness, the plasticity and toughness and the welding manufacturability of the steel plate are also excellent, the mutual contradiction among the strength, the plasticity, the low temperature toughness and the weldability of the high strength quenched and tempered steel plate is successfully solved, and the safety stability and the fatigue resistance of the large heavy steel structure are improved; the on-line quenching not only reduces the manufacturing procedures, shortens the manufacturing period and reduces the manufacturing cost of the steel plate, but also more importantly, the steel plate of the invention adopts the controlled rolling and on-line DQ + off-line tempering process to fully exert the potential of hardenability of the alloy element, the hardenability and the hardenability of the alloy element are exerted to the maximum extent, the high strength, excellent strong plasticity and plastic toughness matching and excellent weldability can be obtained under the conditions of less precious alloy content (such as Ni and Mo) and very low carbon equivalent and Pcm, thereby not only further reducing the manufacturing cost, but also improving the weldability of the steel plate, and particularly for the high strength quenched and tempered steel plate, the welding cold crack sensitivity is greatly reduced, the welding preheating temperature is greatly reduced, the heat treatment (namely SR) is not needed after welding, the range of the proper welding heat input quantity is wider, and correspondingly reducing the cost and procedures of the processing and manufacturing of users, the manufacturing time of the user steel member is shortened, and great value is created for users, so that the steel plate is not only a product with high added value and environmental protection.
Drawings
FIG. 1 is a photograph of the microstructure (1/4 thickness) of the steel of example 5 of the present invention.
Detailed Description
The invention is further illustrated with reference to the following figures and examples.
The steel composition of the steel of the embodiment of the invention is shown in table 1, tables 2 to 5 are process parameters of the embodiment of the invention, and table 6 is performance parameters of the steel of the embodiment of the invention.
As can be seen from figure 1, the microstructure of the steel plate is uniform and fine lower bainite + lath martensite through combination of component optimization design, TMCP and subsequent tempering process, and the average crystal cluster size is below 20 μm.
According to the invention, through the combination design of steel plate alloy elements and the combination of the processes of controlling rolling CR, online quenching DQ and offline tempering T, the strength and toughness, the plastic toughness and the welding manufacturability of the steel plate are also excellent while obtaining the excellent 800MPa grade high-strength quenched and tempered steel plate, the mutual contradiction among the strength, the plasticity, the low-temperature toughness and the weldability of the high-strength quenched and tempered steel plate is successfully solved, and the safety stability and the fatigue resistance of a large heavy steel structure are improved; the on-line quenching not only reduces the manufacturing procedures, shortens the manufacturing period and reduces the manufacturing cost of the steel plate, but also more importantly, the steel plate of the invention adopts the controlled rolling and on-line quenching DQ + off-line tempering process to fully exert the potential of hardenability of the alloy element, the hardenability and the hardenability of the alloy element are exerted to the maximum extent, the high strength, excellent strong plasticity and plastic toughness matching and excellent weldability can be obtained under the conditions of less precious alloy content (such as Ni and Mo) and very low carbon equivalent and Pcm, thereby not only further reducing the manufacturing cost, but also improving the weldability of the steel plate, and particularly for the high strength quenched and tempered steel plate, the welding cold crack sensitivity is greatly reduced, the welding preheating temperature is greatly reduced, the heat treatment (namely SR) is not needed after welding, the range of the proper welding heat input quantity is wider, and correspondingly reducing the cost and procedures of the processing and manufacturing of users, the manufacturing time of the user steel member is shortened, and great value is created for users, so that the steel plate is not only a product with high added value and environmental protection, but also conforms to the green energy-saving emission-reduction production policy advocated by the state.
The 800MPa grade high strength, high toughness and excellent weldability quenched and tempered steel plate is mainly used for manufacturing a pressure water pipe, a volute casing and a steel branch pipe of hydropower engineering, a large-scale engineering mechanical structure and an offshore oil platform, and is a key material for important national economic construction. With the continuous development of national economy of China, the requirement of an economical harmonious society is built; manufacturing and development of large equipment required by national foundation engineering construction, clean energy engineering construction (such as hydroelectric engineering and wind power), ocean development construction and national economic construction are put to the daily agenda, and the quenched and tempered steel plate with high strength, high toughness and excellent weldability has wide market prospect as a strategic base material, namely 800MPa grade quenched and tempered steel plate.
Claims (8)
1. The 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability comprises the following components in percentage by weight:
C：0.05％～0.08％
Si：≤0.10％
Mn：1.55％～1.85％
P：≤0.013％
S：≤0.0030％
Cu：0.10％～0.40％
Ni：0.10％～0.40％
Cr：0.50％～0.90％
Mo：0.10％～0.30％
Nb：0.010％～0.035％
V：0.025％～0.055％
Ti：0.007％～0.014％
B：0.0010％～0.0018％
Als：0.025％～0.060％
N：≤0.0055％
Ca：0.001％～0.0035％，
the balance of Fe and inevitable impurities;
and the content of the elements must satisfy the following relation at the same time:
controlling a large-angle bainite lath/martensite lath grain boundary formation index χ ≧ 2.1, χ ═ 1.33 (% Mn) + (% Ni) +0.55 (% Cu) +0.21 (% Cr) -0.93 (% Mo) -2.55 (% Nb) -1.91 (% Ti) -1.42 (% V);
(％C)×[(％Si)+0.83(％Mo)+1.12(％Nb)+0.89(％Als)+0.45(％V)]≤0.031；
DI index × ζ × TOpen cold×[(TOpen cold-TStopping cooling)/(TOpen cold+TStopping cooling)]The ratio/t is more than or equal to 1535, wherein,
zeta is online quenching DQ hardenability factor, and its value is 1.4;
t is the thickness of the finished steel plate in unit mm;
DI＝0.367(％C)0.5[1+0.7(％Si)][(1+3.33(％Mn)][(1+0.35(％Cu)][(1+0.36(％Ni)][(1+2.16(％Cr)][(1+3(％Mo)][(1+1.75(％V)][(1+1.77(％Al)][(1+200(％B)]x 25.4, unit mm;
Topen coldQuenching temperature for in-line quenching of DQ, i.e. start temperature for accelerated cooling, TStopping coolingThe quenching end temperature of on-line quenching DQ, namely the stop temperature of accelerated cooling, and the temperature unit is;
an Als content of ≧ 10 [ (% N)t)－0.292(％Ti)]；
Ca treatment and the ratio of Ca/S is between 1.00 and 3.00.
2. The low-cost, high-toughness, and good-weldability 800 MPa-grade quenched and tempered steel sheet as claimed in claim 1, wherein the microstructure of said steel sheet is uniform and fine lower bainite + lath martensite.
3. The low-cost, high-toughness, and good-weldability 800 MPa-grade quenched and tempered steel plate as claimed in claim 1 or 2, characterized in that the steel plate has a tensile strength of 780MPa or more, a yield strength of 690MPa or more, a Charpy transverse impact energy of-50 ℃ of 47J or more, and a fracture elongation of δ5≥15％。
4. The method for manufacturing the 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability according to claim 1, which comprises the steps of:
1) smelting and casting
Smelting and continuously casting the components according to the claim 1 into a slab;
2) rolling, wherein the total compression ratio of the steel plate, namely the thickness of the plate blank/the thickness of the finished steel plate, is more than or equal to 3.5;
the first stage is common rolling, and the heating temperature of the plate blank is controlled between 1100 ℃ and 1180 ℃; rolling under high reduction, wherein the reduction rate of rolling passes is more than or equal to 8 percent, and the cumulative reduction rate of passes is more than or equal to 45 percent;
rolling in the second stage by using unrecrystallized control, wherein the rolling start temperature is controlled to be 830-880 ℃, the rolling pass reduction rate is more than or equal to 7 percent, the cumulative reduction rate of an unrecrystallized area is more than or equal to 50 percent, and the final rolling temperature is 820-850 ℃;
3) cooling down
Immediately conveying the steel plate to cooling equipment after the recrystallization and rolling control are finished, and then quickly cooling the steel plate, wherein the starting cooling temperature of the steel plate is 800-830 ℃, the cooling speed is more than or equal to 7 ℃/s, and the stopping cooling temperature is controlled to be less than or equal to 300 ℃; then sending the steel plate into a slow cooling pit for heat preservation, wherein the interval time between the end of cold stop and the time when the steel plate enters a heating type slow cooling pit for heat preservation is not more than 30min, and the heat preservation technology 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 ℃;
4) tempering process
The tempering temperature of the steel plate is 550-600 ℃, the tempering retention time T is more than or equal to (0.60-1.00) multiplied by the thickness T of the finished steel plate, the tempering retention time is the heat preservation time started to time when the central temperature of the steel plate reaches the tempering target temperature, the time T unit is min, and the thickness T unit is mm; and naturally cooling the steel plate to room temperature after tempering.
5. The method for manufacturing 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability according to claim 4, characterized in that continuous casting is adopted in the step 1), the superheat degree of pouring in a tundish is controlled at 8-30 ℃, the drawing speed is controlled at 0.6-1.0 m/min, the fluctuation of the liquid level of a crystallizer is controlled at less than or equal to 5mm, and the soft reduction process at the solidification tail end is 2-5%.
6. The method for manufacturing a quenched and tempered steel sheet of 800MPa grade having low cost, high toughness and excellent weldability according to claim 4 or 5, wherein the microstructure of the steel sheet is uniform and fine lower bainite + lath martensite, and the average grain size is 20 μm or less.
7. The method for manufacturing 800 MPa-grade quenched and tempered steel plate with low cost, high toughness and excellent weldability according to claim 4 or 5, characterized in that the steel plate has tensile strength of 780MPa or more, yield strength of 690MPa or more, charpy transverse impact energy of-50 ℃ or more of 47J or more, and elongation at break δ5≥15％。
8. The low cost of claim 6,The manufacturing method of the 800 MPa-grade quenched and tempered steel plate with high toughness and excellent weldability is characterized in that the tensile strength of the steel plate is more than or equal to 780MPa, the yield strength is more than or equal to 690MPa, the charpy transverse impact energy single value at-50 ℃ is more than or equal to 47J, and the fracture elongation delta5≥15％。
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