CN115537668A - Low-temperature steel bar and production method thereof - Google Patents
Low-temperature steel bar and production method thereof Download PDFInfo
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- B21B1/16—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling wire rods, bars, merchant bars, rounds wire or material of like small cross-section
- B21B1/18—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling wire rods, bars, merchant bars, rounds wire or material of like small cross-section in a continuous process
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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Abstract
The invention discloses a low-temperature steel bar and a production method thereof. The steel bar comprises the following chemical components in percentage by mass: 0.03 to 0.06 percent of C, 0.12 to 0.25 percent of Si, 1.65 to 1.85 percent of Mn, 0.92 to 1.25 percent of Ni, 0.25 to 0.48 percent of Cu, 0.045 to 0.06 percent of Al, 0.02 to 0.06 percent of Ti, 0.008 to 0.015 percent of N, and the balance of Fe and impurities; [ Ni ] +0.5[ Cu ] +1.5[ Al ] +1.2[ Ti ] +5[ N ] is 1.30 to 1.65%, carbon equivalent Ceq is not more than 0.46%. The steel bar disclosed by the invention has the advantages of low alloy cost, low production difficulty, excellent normal-temperature mechanical property, welding property, low-temperature mechanical property and plastic toughness, and a welding joint also has excellent normal-temperature property and low-temperature property.
Description
Technical Field
The invention belongs to the technical field of steel materials, and particularly relates to a low-temperature steel bar and a production method thereof.
Background
Natural gas is used as a clean energy source, and accounts for a larger and larger proportion of the energy systems used at present. The storage and transportation quantity of natural gas is greatly improved by compressing and liquefying the natural gas, but the temperature of the liquefied natural gas is reduced to-165 ℃, and the conventional steel-concrete structure storage tank cannot meet the service requirement. In view of the severe low temperature environment, various low temperature reinforcing steel bars for liquefied natural gas storage tanks have been developed successively at home and abroad.
The low-temperature steel bar needs to be welded in the actual construction process, so the low-temperature steel bar needs to have excellent welding performance and low-temperature mechanical property besides excellent normal-temperature mechanical property; furthermore, the ductility and toughness directly influence the difficulty of the low-temperature steel bar in production, for example, the low-temperature steel bar with poor ductility and toughness is easy to have microcracks in rolling, so that the rolling difficulty is increased.
Disclosure of Invention
In order to solve the above technical problems, an object of the present invention is to provide a low temperature steel bar and a method for producing the same, which have excellent room temperature mechanical properties, welding properties, low temperature mechanical properties, and ductility and toughness at the same time of low alloy cost.
In order to achieve the above object, an embodiment of the present invention provides a low temperature steel bar, which comprises the following chemical components by mass: 0.03 to 0.06 percent of C, 0.12 to 0.25 percent of Si, 1.65 to 1.85 percent of Mn, 0.92 to 1.25 percent of Ni, 0.25 to 0.48 percent of Cu, 0.045 to 0.06 percent of Al, 0.02 to 0.06 percent of Ti, less than or equal to 0.010 percent of P, less than or equal to 0.012 percent of S, 0.008 to 0.015 percent of N, less than or equal to 20ppm of O, less than or equal to 2ppm of H, and the balance of Fe and inevitable impurities; and [ Ni ] +0.5, [ Cu ] +1.5, [ Al ] +1.2, [ Ti ] + 5] + N ] is 1.30-1.65%, carbon equivalent Ceq = [ C ] + [ Mn ]/6+ ([ Cr ] + [ Mo ] + [ V ])/5 + ([ Ni ] + [ Cu ])/15 ≦ 0.46%, wherein [ C ], [ Mn ], [ Cr ], [ Mo ], [ V ], [ Ni ], [ Cu ], [ Al ], [ Ti ], [ N ] respectively represent the mass percent of the corresponding elements in the low-temperature steel bar.
Preferably, the chemical components of the low-temperature steel bar comprise, by mass: 0.03 to 0.06 percent of C, 0.12 to 0.18 percent of Si, 1.65 to 1.85 percent of Mn, 0.92 to 1.20 percent of Ni, 0.27 to 0.48 percent of Cu, 0.045 to 0.06 percent of Al, 0.03 to 0.06 percent of Ti, less than or equal to 0.010 percent of P, less than or equal to 0.012 percent of S, 0.012 to 0.015 percent of N, less than or equal to 20ppm of O, less than or equal to 2ppm of H, and the balance of Fe and inevitable impurities; and [ Ni ] +0.5[ Cu ] +1.5[ Al ] +1.2[ Ti ] +5[ N ] is 1.30 to 1.60%, the carbon equivalent Ceq is not more than 0.46%.
Furthermore, the structure of the low-temperature steel bar is polygonal ferrite, bainite and a very small amount of pearlite, wherein the bainite content is more than or equal to 75%, and the pearlite content is less than 1%.
Further, at normal temperature, the yield strength R of the steel bar p0.2 450 to 485MPa, tensile strength R m 650-685 MPa, elongation after break A of 23-26%, and maximum force total elongation A gt 13 to 15 percent of the total weight of the alloy, and the yield ratio R m /R p0.2 Is 1.35 to 1.50.
Further, after welding the low-temperature steel bar as a base material, a fracture point in a room-temperature tensile test is formed at the steel bar base material, and the welded joint is free of cracks by 180 DEG cold bending at room temperature, D =4D dw Wherein D is the core diameter of the bend, D dw Is the diameter of the rebar.
Furthermore, after the low-temperature steel bar is welded as a base material, the structure of the welded joint is polygonal ferrite, acicular ferrite and granular bainite, wherein the proportion of the polygonal ferrite is less than or equal to 8 percent, and the proportion of the granular bainite is greater than or equal to 65 percent.
Further, the yield strength R of the steel bar of the low-temperature steel bar under the low-temperature condition of-165 ℃ in a non-notch form p0.2 Greater than or equal to 600MPa, tensile strength R m Not less than 720MPa, maximum force total elongation A gt More than or equal to 5.5 percent; yield strength R in notched form p0.2 More than or equal to 610MPa, tensile strength R m Not less than 700MPa, maximum force total elongation A gt Not less than 3.5 percent; the notch sensitivity index is more than or equal to 1.18, and the tensile strength R is in a notched form m Yield strength R in unnotched form p0.2 。
In order to achieve the above object, an embodiment of the present invention provides a method for producing a low-temperature steel bar, which includes a molten iron pretreatment process, a converter smelting process, an LF refining process, an RH refining process, a continuous casting process, and a controlled rolling and controlled cooling process, which are sequentially performed;
in the LF refining process, the components of the slagging agent adopted in slagging comprise the following components in percentage by mass: 45 to 55 percent of Ca, 30 to 40 percent of Al and the balance of O, and controlling the alkalinity of the refining slag to be 0.8 to 1.2; and bottom blowing argon gas for soft stirring in the whole slagging process, wherein the flow of the argon gas is 120-160L/min, and the soft stirring time is 7-10min;
in the RH refining process, deoxidation and dehydrogenation are carried out, then alloy core-spun yarns are fed by a wire feeder according to the wire feeding amount of 0.25-0.45 kg and the wire feeding speed of 0.8-1.2 m/s of molten steel per ton, and the core-spun yarns comprise the following components in percentage by mass: 50-55% of Al, 15-20% of Si, 5-10% of N and the balance of Fe;
in the continuous casting process, full-protection casting is carried out, dynamic soft reduction is adopted in a solidification secondary cooling zone, the reduction amount is 2-4.5 mm, the reduction rate is 0.4-0.55 mm/min, and the continuous casting drawing speed is 0.18-0.25 m/min;
in the controlled rolling and controlled cooling process, after a continuous casting slab leaves a continuous casting machine, the continuous casting slab directly enters a heating furnace for heating at the temperature of 550-600 ℃, and the soaking temperature is 1070-1120 ℃; then, the heated continuous casting billet is sequentially subjected to a rough rolling stage, a medium rolling stage, a first finish rolling stage and a second finish rolling stage on a continuous rolling mill, and the inlet temperature of each stage is 960-985 ℃, 930-955 ℃, 850-875 ℃ and 800-825 ℃ in sequence; finally, the steel bar rolled by the continuous mill is cooled to room temperature in an air cooling bed, and the temperature of the upper cooling bed is 500-550 ℃; and before the upper cooling bed, the reinforcing steel bars leave the continuous rolling mill and are cooled by a water penetrating device.
Preferably, the temperature is controlled between the rough rolling stage and the intermediate rolling stage in an aerosol cooling mode, and the temperature is controlled between the intermediate rolling stage and the first finish rolling stage and between the first finish rolling stage and the second finish rolling stage by water penetrating devices respectively.
Preferably, in the controlled rolling and controlled cooling process, the maintaining time of the continuous casting billet in the heating furnace is more than 1100 ℃ and less than or equal to 5min.
Compared with the prior art, the invention has the beneficial effects that: by adjusting the respective contents of Ni, cu, al, ti and N, establishing the correlation of Ni, cu, al, ti and N [ Ni ] +0.5[ Cu ] +1.5[ Al ] +1.2[ Ti ] +5[ N ], limiting the value of the relation and reversely constraining the content of the corresponding elements, the embodiment ensures the excellent low-temperature performance of the low-temperature steel bar without adding Cr which is a low-temperature performance element and reducing Ni which is a low-temperature performance element, and does not cause the problems of plasticity deterioration, inclusion increase and the like of the low-temperature steel bar; compared with other existing low-temperature reinforcing steel bars, the low-temperature reinforcing steel bar disclosed by the invention has the advantages of more excellent normal-temperature performance, toughness, welding performance and low-temperature performance, low alloy cost and low production difficulty.
Detailed Description
The technical solution of the present invention will be further described with reference to the following specific embodiments.
The invention provides a low-temperature steel bar, which comprises the following chemical components in percentage by mass: 0.03 to 0.06 percent of C, 0.12 to 0.25 percent of Si, 1.65 to 1.85 percent of Mn, 0.92 to 1.25 percent of Ni, 0.25 to 0.48 percent of Cu, 0.045 to 0.06 percent of Al, 0.02 to 0.06 percent of Ti, less than or equal to 0.010 percent of P, less than or equal to 0.012 percent of S, 0.008 to 0.015 percent of N, less than or equal to 20ppm of O, less than or equal to 2ppm of H, and the balance of Fe and inevitable impurities; and [ Ni ] +0.5, [ Cu ] +1.5, [ Al ] +1.2, [ Ti ] + 5] + N ] is defined as 1.30 to 1.65%, and the carbon equivalent Ceq = [ C ] + [ Mn ]/6+ ([ Cr ] + [ Mo ] + [ V ])/5 + ([ Ni ] + [ Cu ])/15 ≦ 0.46%, wherein [ C ], [ Mn ], [ Cr ], [ Mo ], [ V ], [ Ni ], [ Cu ], [ Al ], [ Ti ], [ N ] respectively represent the mass percentages of the respective elements in the low-temperature steel bar.
Further preferably, the chemical components of the low-temperature steel bar comprise, by mass: 0.03 to 0.06 percent of C, 0.12 to 0.18 percent of Si, 1.65 to 1.85 percent of Mn, 0.92 to 1.20 percent of Ni, 0.27 to 0.48 percent of Cu, 0.045 to 0.06 percent of Al, 0.03 to 0.06 percent of Ti, less than or equal to 0.010 percent of P, less than or equal to 0.012 percent of S, 0.012 to 0.015 percent of N, less than or equal to 20ppm of O, less than or equal to 2ppm of H, and the balance of Fe and inevitable impurities; and [ Ni ] +0.5, [ Cu ] +1.5, [ Al ] +1.2, [ Ti ] + 5], [ N ] is defined as 1.30 to 1.60%, and the carbon equivalent Ceq = [ C ] + [ Mn ]/6+ ([ Cr ] + [ Mo ] + [ V ])/5 + ([ Ni ] + [ Cu ])/15. Ltoreq.0.46%.
The preferred chemical composition of the cryogenic steel reinforcement is described below.
C: c is a cheap and effective strength-enhancing element. But the C content is too high, so that the cold brittleness and aging sensitivity of the steel can be improved, the ductile-brittle transition temperature is improved, and the low-temperature performance of the steel bar is reduced; meanwhile, too high C content increases carbon equivalent and deteriorates weldability. In the present embodiment, C is 0.03 to 0.06%.
Si: si has a solid solution strengthening effect, increases the elastic limit and the yield limit, and improves the strength and the wear resistance of the steel. However, too high a Si content reduces the plasticity of the steel and affects the low-temperature toughness. In the present embodiment, si is 0.12 to 0.25%, and more preferably 0.12 to 0.18%.
Mn: mn is also an effective solid solution strengthening element, and can enhance the hardenability of steel, reduce the brittleness index of the steel and obviously improve the strength. The increase of the Mn/C ratio is beneficial to reducing the ductile-brittle transition temperature and improving the low-temperature performance. However, the addition of Mn increases the carbon equivalent and directly affects the weldability. In the present embodiment, mn is 1.65 to 1.85%.
Ni: ni is an element for effectively improving the low-temperature toughness of steel, can expand the austenite phase region of the steel, strengthen stable austenite, reduce the critical quenching speed, refine crystal grains, obviously reduce the ductile-brittle transition temperature of the steel and synchronously improve the ductility and toughness. In the present embodiment, ni is 0.92 to 1.25%, and more preferably 0.92 to 1.20%.
Cu: cu acts similarly to Ni, and by dissolving in steel in a solid solution, enlarges the stable austenite phase region, improves hardenability, lowers the ductile-brittle transition temperature, and can replace a part of Ni, but excessive addition thereof easily causes segregation, and affects plasticity. Cu also forms high melting point compounds with high Ni content, reducing the tendency to hot shortness. In the present embodiment, cu is 0.25 to 0.48%, and more preferably 0.27 to 0.48%.
Al: al is an effective deoxidizing element, and effectively reduces the oxygen content in steel. Meanwhile, al and a proper amount of N are combined to form AlN, so that the grain structure is refined, and the low-temperature toughness is improved. In the present embodiment, al is 0.045 to 0.06%.
Ti: ti is similar to Al, and can be combined with a proper amount of N to precipitate fine dispersed TiN and refine grains so as to obtain better low-temperature strength and toughness, but excessive addition can easily generate large-size inclusions. In the present embodiment, the Ti content is 0.02 to 0.06%, and more preferably 0.03 to 0.06%.
N: the proper amount of N can effectively exert the fine grain strengthening effect of Al and Ti and improve the low-temperature performance. An excessive amount may produce large-sized brittle inclusions and deteriorate the ductility and toughness. In the present embodiment, N is 0.008 to 0.015%, and more preferably 0.012 to 0.015%.
P, S: s and Mn form long-strip MnS inclusions to influence the plasticity and toughness of the steel plate; p is segregated in the grain boundary, which lowers the grain boundary strength and deteriorates the low-temperature toughness. In the embodiment, P is less than or equal to 0.010 percent and S is less than or equal to 0.010 percent.
O, H: o is easy to generate large-size oxide inclusions to influence the ductility and toughness and lead a welding area to generate cracks, and the O is limited to be less than or equal to 20ppm in the embodiment; h can generate hydrogen embrittlement, and particularly, the higher the strength of the steel, the lower the service temperature and the higher the hydrogen embrittlement sensitivity, H is limited to be less than or equal to 2ppm in the embodiment.
In summary, in terms of chemical composition, the present embodiment, on one hand, by adjusting respective contents of Ni, cu, al, ti and N, and on the other hand, by establishing an association of Ni, cu, al, ti and N [ Ni ] +0.5[ Cu ] +1.5[ Al ] +1.2[ Ti ] + 5] + N ], and defining a value of the relationship, and reversely constraining contents of corresponding elements, ensures excellent low temperature performance of low temperature steel bars without adding low temperature performance element Cr and reducing low temperature performance element Ni, and does not cause problems of plastic degradation of low temperature steel bars, increase of inclusions, and the like; compared with other existing low-temperature reinforcing steel bars, the low-temperature reinforcing steel bar has the advantages of excellent normal-temperature performance, plasticity and toughness, excellent welding performance, excellent low-temperature performance, low alloy cost and low production difficulty.
It should be noted that the relationship [ Ni ] +0.5[ Cu ] +1.5[ Al ] +1.2[ Ti ] +5[ N ] between the elements Ni, cu, al, ti and N has an important effect on the performance of the low-temperature steel bar (especially low-temperature performance), which is the first creative achievement and proposal of the inventor, and is specifically defined as the low-temperature performance index LTE of the low-temperature steel bar, namely LTE = [ Ni ] +0.5[ Cu ] +1.5[ Al ] +1.2[ Ti ] +5[ N ]. Moreover, the present invention preferably defines 1.30% ≦ [ Ni ] +0.5 ], [ Cu ] +1.5 ], [ Al ] +1.2 ], [ Ti ] + 5], [ N ], [ 1.65%, and further may preferably be 1.30% to 1.60%, based on which the obtained low temperature steel bar can have unexpected low temperature performance, plasticity, welding performance and normal temperature performance while having low alloy cost, and the welded joint obtained after welding also has excellent low temperature performance, which can meet the use demand of the low temperature steel bar.
Specifically, the diameter of the low-temperature steel bar is 6-40 mm, the structure of the low-temperature steel bar is polygonal ferrite, bainite and a very small amount of pearlite, wherein the bainite accounts for more than or equal to 75%, and the pearlite accounts for less than 1%; yield strength R of steel bar at normal temperature p0.2 450 to 485MPa, tensile strength R m 650-685 MPa, elongation after break A of 23-26%, and maximum force total elongation A gt 13 to 15 percent of the total weight of the alloy, and the yield ratio R m /R p0.2 1.35 to 1.50; yield strength R in unnotched form at-165 deg.C p0.2 Not less than 600MPa, tensile strength R m Not less than 720MPa, maximum force total elongation A gt Not less than 5.5 percent; yield strength R in notched form p0.2 Not less than 610MPa, tensile strength R m Not less than 700MPa, maximum force total elongation A gt Not less than 3.5 percent; the notch sensitivity index is more than or equal to 1.18, and the tensile strength R is in a notched form m Yield strength R in unnotched form p0.2 。
When the low-temperature steel bar is used as a base material for welding, the obtained welded joint structure is excellent and comprises polygonal ferrite, acicular ferrite and granular bainite, wherein the proportion of the polygonal ferrite is less than or equal to 8%, and the proportion of the granular bainite is more than or equal to 65%; and, the obtained welded joint is cooled to room temperature and then subjected to a performance test, and a fracture point in a room temperature tensile test is formed in the steel barAt the base material, the welding joint is free of cracks through 180 DEG cold bending at room temperature, and D =4D dw Wherein D is the radius of curvature, D dw Is the diameter of the rebar.
Furthermore, the present embodiment also provides a preferred production method of the low-temperature steel bar, and it should be noted that the low-temperature steel bar may be prepared by other existing processes besides the production method, and the production method has advantages compared with the existing production processes.
In this embodiment, the production method includes a molten iron pretreatment process, a converter smelting process, an LF refining process, an RH refining process, a continuous casting process, and a controlled rolling and controlled cooling process, which are sequentially performed, and each process is described in detail below.
(1) Molten iron pretreatment process
And (3) carrying out desulfurization pretreatment on the blast furnace molten iron, wherein the slagging rate of the desulfurized slag of the pretreated molten iron is more than or equal to 98%. Before pretreatment, the chemical composition content of molten iron is calculated by mass percent: si is less than or equal to 0.10 percent, and P is less than or equal to 0.12 percent. The temperature of the pretreated molten iron is more than or equal to 1385 ℃, and the content of S in the molten iron is less than or equal to 0.005 percent by mass percent.
(2) Converter smelting process
The pretreated molten iron enters a converter for oxygen blowing smelting, and the smelting end point C is less than or equal to 0.03%, P is less than or equal to 0.08%, and Si is less than or equal to 0.05%. The tapping temperature of the converter is 1610 to 1625 ℃, and nitrogen or argon purging is carried out on the ladle before tapping, so that oxygen absorption of molten steel is reduced. Adding ferromanganese alloy, a nickel plate and a copper block when tapping 1/4, and adding ferrotitanium alloy when tapping 1/2.
(3) LF refining Process
After molten steel discharged from the converter enters an LF (ladle furnace) station, slagging is carried out in a manner that 5.5-8.5 kg of alkaline slagging agent is added to each ton of molten steel. Wherein the slag former comprises the following components in percentage by mass: 45 to 55 percent of Ca, 30 to 40 percent of Al and the balance of O. Controlling the alkalinity of the refining slag to be 0.8 to 1.2, wherein the refining slag mainly comprises CaO and Al 2 O 3 Therefore, slag pouring can be facilitated, and refractory materials can be protected. Argon is blown at the bottom for soft stirring in the whole slagging process, the flow of the argon is 120-160L/min, and the soft stirring time is 7-10min, so that impurities float upwards and are extracted to the maximum extentAnd (5) increasing the cleanliness of the molten steel.
Electrifying to heat up after soft stirring, sampling, detecting and replenishing alloy, controlling the LF refining tapping to be 1565-1590 ℃, and controlling the Si element in the tapping water to be less than or equal to 0.08 percent by mass percent.
(4) RH refining step
And the molten steel obtained by LF refining enters an RH furnace for refining.
Specifically, argon circulation deoxidation and dehydrogenation are carried out, the vacuum degree is controlled to be less than or equal to 1.5mbar, the static circulation time is controlled to be more than or equal to 15min, H is controlled to be less than or equal to 0.00016%, and O is controlled to be less than or equal to 0.0015%.
Then, feeding alloy core-spun yarn through a wire feeder according to the wire feeding amount of 0.25-0.45 kg and the wire feeding speed of 0.8-1.2 m/s per ton of molten steel, wherein the diameter of the core-spun yarn is 6-8mm, the core-spun yarn is prepared from aluminum particles and ferrosilicon nitride, and the core-spun yarn comprises the following components in percentage by mass: 50-55% of Al, 15-20% of Si, 5-10% of N and the balance of Fe. Thus, after RH deoxidation and dehydrogenation, al can be greatly reduced by adding Al in a core-spun yarn feeding mode and regulating and controlling the content of Si and N 2 O 3 -SiO 2 Impurities are contained, the yield of Al is improved, and the content of N is accurately controlled.
Finally, vacuum breaking tapping is carried out, and the RH tapping temperature is 1555-1580 ℃.
(5) Continuous casting process
And (3) preparing the molten steel obtained by RH refining into a continuous casting billet with the cross section size of 150 x 150mm by adopting the continuous casting billet. The full-protection casting is carried out in the continuous casting process, and specifically, for example, a ladle long nozzle and argon seal, an alkaline tundish covering agent, an immersion nozzle, low-carbon casting powder and other protection modes are adopted. The solidification secondary cooling area adopts dynamic soft reduction, the reduction is 2-4.5 mm, and the reduction rate is 0.4-0.55 mm/min. The continuous casting drawing speed is 0.18-0.25 m/min, so that the Cu, al and Ti elements can be prevented from segregating at the core and 1/4 of the core to form micro-cracks, and further cracking in the subsequent controlled rolling process is avoided.
(6) Controlled rolling and controlled cooling process
Firstly, after leaving the continuous casting machine, the continuous casting slab directly enters a heating furnace for heating at the temperature of 550-600 ℃ (namely when the temperature of the continuous casting slab is not reduced to be lower than 550 ℃). The soaking temperature is controlled to be 1070 to 1120 ℃, the total heating time is controlled to be 40 to 55min, and the maintaining time above 1100 ℃ is less than or equal to 5min in the heating process. Therefore, on one hand, the continuous casting billet enters the heating furnace with the temperature (for example, 550-600 ℃), so that the cold billet continuous casting cracks can be reduced, the surface quality of a steel bar finished product is improved, and the operation rate is improved; on the other hand, the total heating time is short, the soaking temperature is low, the copper-rich phase can be prevented from being partially melted in the steel billet, and further the cracking is reduced.
Subsequently, the continuous casting slab enters a continuous rolling mill for controlled rolling after leaving the heating furnace. The controlled rolling process comprises a rough rolling stage, a middle rolling stage, a first finish rolling stage and a second finish rolling stage, wherein the initial rolling temperature of the rolling process is controlled to be 960-985 ℃ (namely the inlet temperature of the rough rolling stage is 960-985 ℃), the inlet temperature of the middle rolling stage is 930-955 ℃, the inlet temperature of the first finish rolling stage is 850-875 ℃, and the inlet temperature of the second finish rolling stage is 800-825 ℃. In this way, the controlled rolling process of the present embodiment adopts a gradient cooling rolling manner, which can increase the rolling force and enhance the work hardening, and the finish rolling process (including the first stage and the second stage of the finish rolling) is in the non-recrystallization region, so that the fine grain strengthening effect of AlN and TiN can be fully exerted through the deformation induction effect while the ferrite is fully refined.
Preferably, the rough rolling stage comprises 8-pass rough rolling defined by a 1-8 # rolling mill, the medium rolling stage comprises 4-pass medium rolling defined by a 9-12 # rolling mill, the finish rolling first stage comprises 2-pass finish rolling defined by a 13-14 # rolling mill, and the finish rolling second stage comprises 4-pass finish rolling defined by a 15-18 # rolling mill; of course, it will be appreciated that the number of passes in each stage is not limited to this preferred design. In addition, the temperature is controlled by adopting an air mist cooling mode between the rough rolling stage and the intermediate rolling stage, for example, an air mist cooling device is arranged between an 8# rolling mill and a 9# rolling mill, and the temperature is controlled by the air mist cooling device before a rolled piece leaves the 8# rolling mill and enters the 9# rolling mill; furthermore, the temperature of the rolled piece is controlled by water passing devices between the middle rolling stage and the first finishing rolling stage, and between the first finishing rolling stage and the second finishing rolling stage, for example, a first water passing device is arranged between the 12# rolling mill and the 13# rolling mill, the temperature of the rolled piece is controlled by the first water passing device before the rolled piece leaves the 12# rolling mill and enters the 13# rolling mill, a second water passing device is arranged between the 14# rolling mill and the 15# rolling mill, and the temperature of the rolled piece is controlled by the second water passing device before the rolled piece leaves the 14# rolling mill and enters the 15# rolling mill.
In one embodiment, the diameter of the steel bar rolled by the continuous rolling mill is 6-40 mm.
Finally, cooling the steel bar rolled by the continuous rolling mill on a cooling bed to room temperature, wherein the temperature of the cooling bed is 500-550 ℃; and before the upper cooling bed, the reinforcing steel bars leave the continuous rolling mill and are cooled by a water penetrating device. Specifically, for example, a third water passing device is arranged after the continuous rolling mill (such as 18# rolling mill), and the steel bar is rapidly cooled to 500-550 ℃ after leaving the continuous rolling mill (such as 18# rolling mill) by using the third water passing device and then is loaded on a cooling bed. Therefore, after finish rolling, the steel bar rapidly passes through the pearlite transformation area by strong water penetration to enter the bainite transformation area, deformed austenite is fully transformed into bainite to obtain an ideal structure, so that the ideal steel bar structure can be obtained on the premise of low carbon and low silicon, and the strength of the obtained low-temperature steel bar is excellent.
Still further, an embodiment of the present invention further provides a flash butt welding method for the low temperature steel bar. The flash butt welding method specifically comprises a pretreatment process, a clamping process, a preheating process, a welding process and a cooling process which are sequentially carried out. The respective steps will be described in detail below.
(1) Pretreatment step
And polishing the surface of the end part to be welded of the low-temperature steel bar. Thus, the oxide skin and the rust on the surface of the end part to be welded are removed by polishing, so that the oxygen caused by the oxide is prevented from exceeding the standard, and the influence on the low-temperature performance caused by the microcrack of the finally obtained welding joint is avoided.
Wherein the surface comprises an end face of the end to be welded and a circumferential face of a certain length starting from the end face. Preferably, the certain length is controlled to be 25-40 mm, that is, the length of the polished end to be welded is 25-40 mm. Further preferably, the certain length is not less than a length L of the end to be welded within a heating range of the induction coil, which will be described later, so that all regions of the end to be welded within the heating range of the induction coil are ground in the preprocessing step.
(2) Clamping procedure
The polished end part to be welded passes through an induction coil of a flash butt welding machine and is clamped on an electrode; it will be appreciated that a flash butt welder has electrodes arranged in pairs, with the two ends to be welded each being clamped to one of the pair of electrodes.
After clamping is finished, the gap d between the end faces of the two end portions to be welded is controlled to be 2-5 mm, the length L of each end portion to be welded in the heating range of the induction coil is controlled to be 20-30 mm, and the distance between the two electrodes is larger than 2L + d.
Preferably, the distance between the two electrodes is controlled between 50 and 80mm.
(3) Preheating step
The end part to be welded is preheated to 800-880 ℃ in 15-30 s by using an induction coil heating mode. That is, the induction coil of the flash butt welding machine is started to heat the end part to be welded, so that the temperature of the end part to be welded is rapidly increased to 800-880 ℃ from the normal temperature within 15-30 s, and then the induction coil is controlled to finish heating (namely, the preheating process is finished and the next welding process is started). Therefore, on one hand, complete solid solution of alloy elements can be ensured, and on the other hand, the preheating time (such as 15-30 s) is short, so that the internal structure of the end part to be welded can be controlled, and the performance of the final welded joint is further facilitated.
During preheating, the argon atmosphere is maintained at the end part to be welded at the flow rate of 5-10 mL/s, so that the probability of oxidation of the end part to be welded after being heated is reduced, and the performance of the final welded joint is further ensured.
(4) Welding process
As mentioned above, after the preheating is finished (i.e. the end to be welded reaches 800-880 ℃), the induction coil is controlled to finish heating, and the flash butt welder is started to enter the welding process. The welding process comprises two stages of a flash stage and a pressure upset stage which are sequentially performed.
Firstly, in a flashing stage, a flash butt welding machine is carried out according to preset flashing parameters, so that the end part to be welded reaches a semi-molten state from 800-880 ℃. The flash parameters comprise flash heat of 0.2-0.5 kJ/mm 2 ×S dw +Q f The flash distance is (0.8-1.2) x d dw + d, flash time 8-15S, S in flash heat dw Cross-sectional area of end to be welded, Q f For floating heat, d dw Is the diameter of the rebar.
Wherein, with respect to Q f There are a number of options for the setting of (c). In a preferred embodiment, Q f Is associated with the carbon equivalent Ceq, in particular, for example, as mentioned above, the carbon equivalent Ceq of the low-temperature steel bar is less than or equal to 0.46%, then: when Ceq is less than or equal to 0.40 percent, Q f =1 to 5kJ; when Ceq is more than 0.40% and less than or equal to 0.44%, Q f = 5-15 kJ; when Ceq is more than 0.44% and less than or equal to 0.46%, Q f =15 to 40kJ, of course, Q f Is not limited to this preferred embodiment.
As mentioned above, d dw The diameter of the steel bar is, for example, 6 to 40mm.
Moreover, the flash parameters set above can realize that the end part to be welded can reach a semi-molten state in a short time (for example, the flash time is 8-15 s), so as to meet the requirement of subsequent fusion, avoid oxidation and improve the performance of the finally obtained welding joint.
Next, in the pressure upsetting stage, the flash butt welder is performed according to preset pressure upsetting parameters, so that the two ends to be welded are fused together, i.e. welded together. The pressure upsetting parameters comprise upsetting stress of 105-150 MPa and upsetting time of less than or equal to 0.8s.
In addition, in the whole welding process, the argon atmosphere is maintained at the flow rate of 12-18 mL/s in the whole process, so that the probability of oxidation of the end part to be welded is reduced, and the performance of the final welding joint is further ensured.
(5) Cooling Process
After the pressure upsetting is finished, the cooling process is performed, and in summary, the whole cooling process is divided into three stages: in the first stage, firstly, the argon atmosphere is maintained at the flow rate of 45-60 mL/s (namely, the flow rate of the argon is increased to 45-60 mL/s from 12-18 mL/s in the welding process), the obtained welding joint is controlled to be cooled to 600-650 ℃ at the cooling rate of 10-15 ℃/s, and the first stage is a rapid cooling stage, so that the effects of reducing ferrite phase change and surface oxidation can be realized; in the second stage, the argon atmosphere is maintained at the flow rate of 5-10 mL/s (namely, the argon flow rate is reduced from 45-60 mL/s in the first stage to 5-10 mL/s), and the welding joint is controlled to maintain 530-560 ℃ and 300-480 s in a heating mode of an induction coil during the second stage, namely, in the cooling process of the second stage, when the welding joint is cooled to 560 ℃, the induction coil is used for controlling the welding joint to maintain 300-480 s in a temperature range of 530-560 ℃, so that the welding joint is fully phase-changed to obtain an ideal granular bainite structure, and the performance of the final welding joint is further ensured; and in the third stage, when the temperature of the welding joint is reduced to below 530 ℃, the welding joint is controlled to be cooled to room temperature at a cooling rate of 0.2-0.5 ℃/s, so that the internal stress of upset forging fusion can be fully released through the heat preservation stage.
In the third stage of this process, there are various ways to control the temperature of the welded joint to be slowly reduced to room temperature, and in a preferred embodiment, the induction coil is specifically heated, argon gas is stopped, and a heat-insulating cover is added, so that the temperature of the welded joint is controlled to be reduced to room temperature at a cooling rate of 0.2-0.5 ℃/s. Of course, the specific implementation is not limited thereto.
Compared with the prior art, the flash butt welding method has the advantages that on one hand, the traditional welding technology of the low-temperature steel bar is arc welding or gas shielded welding, the flash butt welding technology is disclosed for the first time in the welding processing of the low-temperature steel bar due to the requirement of the low-temperature steel bar on the low-temperature performance of a welding joint, the flash butt welding technology can ensure the excellent low-temperature performance of the welding joint of the low-temperature steel bar, for example, the welding joint obtained by adopting the flash butt welding technology and taking the low-temperature steel bar as a base material is excellent in structure and comprises polygonal ferrite, acicular ferrite and granular bainite, wherein the ratio of polygonal ferrite is less than or equal to 8%, and the ratio of granular bainite is more than or equal to 65%, so that the flash butt welding method is ensured to have the advantages of arc welding or gas shielded weldingExcellent normal temperature and low temperature properties, for example, fracture points of two welded low temperature steel bars in a room temperature tensile test are formed at a steel bar base material, and D =4D at 180 ° cold bending property of a welded joint at room temperature dw The appearance has no cracks, wherein D is the diameter of a bending core; on the other hand, the flash butt welding technology of the invention has greater advantages compared with arc welding or gas shielded welding, and particularly, the method can automatically and continuously work in the whole process after the low-temperature steel bar is clamped on the flash butt welding machine until the welding is finished and the steel bar is cooled to the room temperature, and can ensure the excellent low-temperature performance and the room-temperature performance of the welding joint without carrying out extra off-line heat treatment (namely, without dismounting the steel bar from the flash butt welding machine) before clamping or after the welding is finished as in the prior art, and the whole process has fewer steps, short time and high efficiency.
The detailed description set forth above is merely a specific description of possible embodiments of the present invention and is not intended to limit the scope of the invention, which is intended to include within the scope of the invention equivalent embodiments or modifications that do not depart from the technical spirit of the present invention.
Several embodiments of the present invention are provided below to further illustrate the technical solutions of the present invention. Of course, these embodiments are preferably only some, but not all, of the many variations encompassed by the present invention.
Examples A1 to G1 of the low-temperature reinforcing bars, which are manufactured according to the technical solutions of the foregoing embodiments of the present invention, for example, the chemical compositions of which are consistent with the design of one embodiment of the present invention, and which are manufactured by the manufacturing method of one embodiment of the present invention (of course, the manufacturing method of these reinforcing bars is not limited to the present invention). Referring to tables 1 to 3 below, table 1 shows the chemical compositions of the low-temperature steel bars of examples A1 to G1, table 2 shows the specifications (i.e., diameters), microstructures, and normal-temperature mechanical properties of the low-temperature steel bars of examples A1 to G1, and table 3 shows the low-temperature mechanical properties of the low-temperature steel bars of examples A1 to G1.
[ Table 1]
[ Table 2]
[ Table 3]
The embodiment shows that the low-temperature steel bar has excellent structure, low-temperature performance and normal-temperature performance, and also has excellent ductility and toughness, and the low-temperature steel bar is low in alloy cost, low in production cost, high in production efficiency and low in difficulty.
Further, the welding tests were carried out using the low-temperature steel bars of the above-described examples A1 to G1 as base materials, and the obtained test examples A2 to G2 and A3, A4 were respectively shown in table 4, specifically, the base materials used in the respective test examples (i.e., the examples in table 1 are referenced), the welding techniques used, the microstructures of the obtained welded joints, and the results of performance tests at room temperature are shown in table 4; in addition, referring to table 5, the low temperature mechanical properties of the welded joints obtained in test examples A2 to G2 are also shown.
[ Table 4]
[ Table 5]
As can be seen from tables 4 and 5, the test examples A2-G2 adopt the flash butt welding method provided by the invention for welding, while the test examples A3 and A4 adopt the existing known arc welding and gas shielded welding for welding, the low-temperature steel bar of the invention has excellent welding performance, and the welding joint meets the application requirements of the low-temperature steel bar after welding; in addition, the flash butt welding method disclosed by the invention is used as a flash butt welding technology which is disclosed for the first time in the industry and is suitable for low-temperature steel bars, the obtained welding joint has an excellent microstructure, the normal-temperature tensile property, the cold bending property and the low-temperature mechanical property are all excellent, the application requirement of the low-temperature steel bars after welding is also met, and the flash butt welding method further has the advantages of full-automatic continuous operation, less steps of the whole process, short time, high efficiency and the like.
Claims (10)
1. The low-temperature steel bar is characterized by comprising the following chemical components in percentage by mass: 0.03 to 0.06 percent of C, 0.12 to 0.25 percent of Si, 1.65 to 1.85 percent of Mn, 0.92 to 1.25 percent of Ni, 0.25 to 0.48 percent of Cu, 0.045 to 0.06 percent of Al, 0.02 to 0.06 percent of Ti, less than or equal to 0.010 percent of P, less than or equal to 0.012 percent of S, 0.008 to 0.015 percent of N, less than or equal to 20ppm of O, less than or equal to 2ppm of H, and the balance of Fe and inevitable impurities; and [ Ni ] +0.5, [ Cu ] +1.5, [ Al ] +1.2, [ Ti ] + 5] + N ] is 1.30-1.65%, carbon equivalent Ceq = [ C ] + [ Mn ]/6+ ([ Cr ] + [ Mo ] + [ V ])/5 + ([ Ni ] + [ Cu ])/15 ≦ 0.46%, wherein [ C ], [ Mn ], [ Cr ], [ Mo ], [ V ], [ Ni ], [ Cu ], [ Al ], [ Ti ], [ N ] respectively represent the mass percent of the corresponding elements in the low-temperature steel bar.
2. The cryogenic steel bar of claim 1, wherein the chemical composition comprises, in mass percent: 0.03 to 0.06 percent of C, 0.12 to 0.18 percent of Si, 1.65 to 1.85 percent of Mn, 0.92 to 1.20 percent of Ni, 0.27 to 0.48 percent of Cu0.045 to 0.06 percent of Al, 0.03 to 0.06 percent of Ti, less than or equal to 0.010 percent of P, less than or equal to 0.012 percent of S, 0.012 to 0.015 percent of N, less than or equal to 20ppm of O, less than or equal to 2ppm of H, and the balance of Fe and inevitable impurities; and [ Ni ] +0.5[ Cu ] +1.5[ Al ] +1.2[ Ti ] +5[ N ] is 1.30 to 1.60%, the carbon equivalent Ceq is not more than 0.46%.
3. The cryogenic steel bar according to claim 1, wherein the structure is polygonal ferrite + bainite + a very small amount of pearlite, wherein bainite is 75% or more and pearlite is < 1%.
4. The cryogenic steel of claim 1, wherein the yield strength R of the steel at room temperature p0.2 450 to 485MPa, tensile strength R m 650-685 MPa, elongation A after fracture of 23-26%, and maximum force total elongation A gt 13 to 15 percent of the total weight of the alloy, and the yield ratio R m /R p0.2 Is 1.35 to 1.50.
5. The low-temperature steel bar as claimed in claim 1, wherein a fracture point in a room-temperature tensile test is formed at the base material of the steel bar after the steel bar is welded as the base material, and the welded joint is free from cracks by 180 ° cold bending at room temperature, and D =4D dw Wherein D is the core diameter of the bend, D dw Is the diameter of the rebar.
6. The cryogenic steel according to claim 1, wherein the welded joint has a structure of polygonal ferrite + acicular ferrite + granular bainite, wherein a ratio of polygonal ferrite is not more than 8% and a ratio of granular bainite is not less than 65% after the steel bar is welded as a base material.
7. The cryogenic steel bar of claim 1, wherein the steel bar has a yield strength R in an unnotched state at-165 ℃ cryogenic conditions p0.2 Greater than or equal to 600MPa, tensile strength R m Not less than 720MPa, maximum force total elongation A gt More than or equal to 5.5 percent; yield strength R in notched form p0.2 Not less than 610MPa, tensile strength R m Not less than 700MPa, maximum force total elongation A gt Not less than 3.5 percent; the notch sensitivity index is more than or equal to 1.18, and the tensile strength R is in a notched form m Yield strength R in unnotched form p0.2 。
8. A method for producing low-temperature steel bars as claimed in any one of claims 1 to 7, comprising a molten iron pretreatment process, a converter smelting process, an LF refining process, an RH refining process, a continuous casting process and a controlled rolling and controlled cooling process, which are sequentially performed;
in the LF refining process, the components of the slagging agent adopted in slagging comprise the following components in percentage by mass: 45 to 55 percent of Ca, 30 to 40 percent of Al and the balance of O, and controlling the alkalinity of the refining slag to be 0.8 to 1.2; and bottom blowing argon gas for soft stirring in the whole slagging process, wherein the flow of the argon gas is 120-160L/min, and the soft stirring time is 7-10min;
in the RH refining process, deoxidation and dehydrogenation are carried out, then alloy core-spun yarns are fed by a wire feeder according to the wire feeding amount of 0.25-0.45 kg and the wire feeding speed of 0.8-1.2 m/s of molten steel per ton, and the core-spun yarns comprise the following components in percentage by mass: 50-55% of Al, 15-20% of Si, 5-10% of N and the balance of Fe;
in the continuous casting process, full-protection casting is carried out, dynamic soft reduction is adopted in a solidification secondary cooling area, the reduction amount is 2-4.5 mm, the reduction rate is 0.4-0.55 mm/min, and the continuous casting drawing speed is 0.18-0.25 m/min;
in the controlled rolling and controlled cooling process, after leaving a continuous casting machine, the continuous casting blank directly enters a heating furnace for heating at the temperature of 550-600 ℃, and the soaking temperature is 1070-1120 ℃; then, the heated continuous casting slab is sequentially subjected to a rough rolling stage, a medium rolling stage, a first finish rolling stage and a second finish rolling stage on a continuous rolling mill, and the inlet temperature of each stage is 960-985 ℃, 930-955 ℃, 850-875 ℃ and 800-825 ℃ in sequence; finally, the steel bar rolled by the continuous mill is cooled to room temperature in an air cooling bed, and the temperature of the upper cooling bed is 500-550 ℃; and before the upper cooling bed, the reinforcing steel bars leave the continuous rolling mill and are cooled by a water penetrating device.
9. The method for producing low-temperature steel bars according to claim 8, wherein the temperature between the rough rolling stage and the intermediate rolling stage is controlled by means of gas mist cooling, and the temperature between the intermediate rolling stage and the first finish rolling stage and the temperature between the first finish rolling stage and the second finish rolling stage are controlled by means of water passing devices respectively.
10. The method for producing low-temperature steel bars according to claim 8, wherein in the controlled rolling and controlled cooling process, the continuous casting billet is maintained in the heating furnace for more than 1100 ℃ for less than or equal to 5min.
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