WO2021052434A1 - 一种Nb微合金化高强高扩孔钢及其生产方法 - Google Patents

一种Nb微合金化高强高扩孔钢及其生产方法 Download PDF

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WO2021052434A1
WO2021052434A1 PCT/CN2020/115964 CN2020115964W WO2021052434A1 WO 2021052434 A1 WO2021052434 A1 WO 2021052434A1 CN 2020115964 W CN2020115964 W CN 2020115964W WO 2021052434 A1 WO2021052434 A1 WO 2021052434A1
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steel
strip
strength
microalloyed
rolling
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PCT/CN2020/115964
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English (en)
French (fr)
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吴建春
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宝山钢铁股份有限公司
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Priority claimed from CN201910888756.7A external-priority patent/CN112522629B/zh
Priority claimed from CN201910889397.7A external-priority patent/CN112522596B/zh
Application filed by 宝山钢铁股份有限公司 filed Critical 宝山钢铁股份有限公司
Priority to DE112020004399.1T priority Critical patent/DE112020004399T5/de
Priority to US17/761,719 priority patent/US20220389534A1/en
Publication of WO2021052434A1 publication Critical patent/WO2021052434A1/zh

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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
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Definitions

  • the invention relates to a high-reamed steel manufacturing technology, in particular to a Nb microalloyed high-strength and high-reamed steel and a production method thereof.
  • tin (Sn) and copper (Cu) are typical residual elements or harmful elements in steel. It is very difficult and very expensive to fully remove Sn and Cu during the steelmaking process. Once the steel contains Sn , Cu, basically can not be completely eliminated, only by diluting molten steel to reduce the content of Sn, Cu, these all cause the increase in the smelting cost of steel products.
  • Traditional thin strip steel is mostly produced by continuous rolling of cast billets with a thickness of 70-200mm through multiple passes.
  • the traditional hot rolling process is: continuous casting + billet reheating and heat preservation + rough rolling + finishing rolling + cooling + Coiling, that is, firstly obtain a cast slab with a thickness of about 200mm through continuous casting. After reheating and holding the cast slab, rough rolling and finishing rolling are performed to obtain a steel strip with a thickness generally greater than 2mm, and finally the steel strip is processed Laminar cooling and coiling complete the entire hot rolling production process. If you want to produce steel strips with a thickness of less than 1.5mm (inclusive), the difficulty is relatively large, and the hot-rolled steel strips are usually subjected to subsequent cold rolling and annealing to complete. In addition, the long process flow, high energy consumption, many units and equipment, and high capital construction costs result in high production costs.
  • the thin slab continuous casting and rolling process is: continuous casting + slab heat preservation and soaking + hot continuous rolling + cooling + coiling.
  • the main difference between this process and the traditional process is that the thickness of the cast slab in the thin slab process is greatly reduced to 50-90mm. Because the cast slab is thin, the cast slab only needs to undergo 1 to 2 passes of rough rolling (the thickness of the cast slab is 70-90mm When the thickness of the cast slab is 50mm, the thickness of the continuous casting slab in the traditional process must be repeatedly rolled to reduce the thickness to the required specifications before the finish rolling; and the thin slab casting process is not necessary to go through rough rolling (when the thickness of the cast slab is 50mm). Without cooling, the billet directly enters the soaking furnace for soaking and heat preservation, or a small amount of temperature compensation.
  • the thin slab process greatly shortens the process flow, reduces energy consumption, reduces investment, and reduces production costs.
  • the strength of the steel will increase and the yield ratio will increase, thereby increasing the rolling load, so that the thickness specification of the hot-rolled product can be economically produced and the thickness specification is not too thin, generally ⁇ 1.5mm, see Chinese patents CN 2 00610123458.1, CN 2 00610035800.2 and CN 2 00710031548.2, and these patents do not involve the elements Sn and Cu.
  • ESP all-headless thin slab continuous casting and rolling process
  • slab flame cutting and heating furnace for heat preservation and soaking and slab transition are eliminated, and the length of the entire production line is greatly shortened to about 190 meters.
  • the slab casted by the continuous casting machine The thickness is 90-110mm, and the width is 1100-1600mm.
  • the continuously cast slab passes through a section of induction heating roller table to keep and heat the slab, and then enters rough rolling, finishing rolling, layer cooling, and coiling in turn.
  • the hot-rolled sheet is obtained through the process.
  • the thinnest hot-rolled sheet of 0.8mm thickness can be obtained by this process, which expands the specification range of the hot-rolled sheet, and its single production line output can reach 2.2 million t/year scale.
  • the process has been rapidly developed and promoted.
  • the shorter process flow than thin slab continuous casting and rolling is the thin strip continuous casting and rolling process.
  • the thin strip continuous casting technology is a cutting-edge technology in the field of metallurgy and materials research. Its appearance has brought a revolution to the steel industry. It changes the production process of steel strip in the traditional metallurgical industry, integrating continuous casting, rolling, and even heat treatment into a whole, so that the produced thin strip will be formed into thin steel strip at one time after one online hot rolling.
  • the production process is simplified and the production cycle is shortened.
  • the length of the process line is only about 50m; the equipment investment is also reduced, and the product cost is significantly reduced. It is a low-carbon and environmentally friendly hot-rolled thin strip production process.
  • the twin-roll thin strip continuous casting process is a main form of the thin strip continuous casting process, and it is also the world's only industrialized thin strip continuous casting process.
  • the typical process flow of twin-roll thin strip continuous casting is shown in Figure 1.
  • the molten steel in the ladle 1 is directly poured into one by two through the ladle nozzle 2, the tundish 3, the immersion nozzle 4 and the distributor 5
  • the molten steel solidifies on the rotating circumferential surfaces of the crystallizing rollers 8a, 8b to form a solidified shell and gradually grow, and then the two
  • the minimum gap (nip point) between the crystallizing rolls 8a and 8b forms a steel strip 11 with a thickness of 1-5 mm.
  • the steel strip is guided to the pinch roll 12 via the guide plate 9 and sent to the rolling mill 13 to be rolled into a thin strip of 0.7-2.5 mm. After being cooled by the cooling device 14, after being cut by the flying shear device 16, it is finally sent to the coiler 19 to be wound into a roll.
  • High-reamed steel is an important steel grade of advanced high-strength steel (AHSS). It has high strength, elongation, excellent formability and flanging performance, and can meet the requirements of complex shape automotive zero
  • the component requirements such as the swing arm of the rear axle suspension of the car chassis, can also be used for other components that require flange flanging.
  • the flanging capacity is expressed by the hole expansion rate.
  • the hole expansion performance is a formability index of steel, which reflects the resistance of the material in the direction perpendicular to the hole edge due to the excessive local elongation and deformation of the hole edge during the hole expansion process. The ability to locally crack.
  • the hole reaming performance of the steel plate is related to the composition, strength and uniformity of the steel plate. Because it contains more precious alloying elements Cr, Nb, Ti, V and Mo, etc., although the cooling rate is lower Under the conditions, a ferrite/bainite dual phase structure can be obtained, but its cost is relatively high.
  • the most commonly used high-reamed steel strength grades are mainly concentrated in the 440MPa and 590MPa grades, and their microstructures are mainly ferrite and bainite, and sometimes contain a small amount of martensite.
  • the hole reaming performance of the steel plate is related to many factors, these factors mainly include: the level of inclusions, the performance difference of each phase in the structure, the uniformity of the structure, the yield ratio and the type of organization.
  • ferrite and bainite structures have relatively high hole expansion performance, but their strength is relatively low, and it is difficult to reach the level of 780MPa and above. This is also the current high hole expansion steel mainly concentrated in 440MPa and 590MPa. The main reason for each intensity level. High-reamed steel has become one of the important varieties of automotive steel plates.
  • thin strip continuous casting Due to the advantages of the natural process flow of thin strip continuous casting, compared with the traditional hot rolling process, thin strip continuous casting is easy to generate bainite type microstructure during the cooling process after rolling, and it is easy to produce products with excellent expansion. Hole performance. Therefore, the use of thin strip continuous casting to produce high-expansion steel has natural advantages.
  • the use of thin strip continuous casting to produce high-reamed steel is mainly aimed at the hot-rolled thin-gauge automotive steel market with a thickness of less than 1.8mm (inclusive). Due to the thinner thickness, the thin strip continuous casting process has strong manufacturing and cost advantages.
  • the product specifications of the high-reamed steel strip directly supplied in the hot-rolled/pickled state are 1.2mm, 1.25mm, 1.4mm, 1.5mm, 1.6mm, 1.7mm, 1.8mm, etc. Due to the thinner product thickness, traditional Many manufacturers often cannot supply thin-gauge high-expansion steel due to the capacity limitations of traditional hot-rolling lines; or use the hot-rolling process first and then cold-rolled. This production process adds thin-gauge, high-expansion steel. The production cost of Kong Gang.
  • the high temperature strip is in contact with the cooling water.
  • the thickness of the oxide scale on the surface of the cast strip increases quickly.
  • the contact of high-temperature strip steel with cooling water will cause many problems: first, water spots (rust spots) will be formed on the surface of the strip steel, which will affect the surface quality; second, cooling water for laminar cooling or spray cooling It is easy to cause uneven local cooling on the surface of the strip, resulting in uneven microstructure inside the strip, resulting in uneven performance of the strip, and affecting product quality; third, uneven local cooling on the surface of the strip will cause the deterioration of the plate shape , Affect the shape quality.
  • thin strip continuous casting due to its own rapid solidification process characteristics, thin strip continuous casting generally has the problems of uneven structure, low elongation, high yield ratio, and poor formability.
  • the cast strip has austenite crystals. The grains have obvious non-uniformity, which will cause the final product structure obtained after austenite transformation to be non-uniform, resulting in unstable product performance. Therefore, it is difficult and challenging to use thin strip continuous casting production lines to produce some products with high hole expansion performance required by the automotive industry and petrochemical industry. Therefore, when using thin strip continuous casting to produce high hole expansion steel, copy the traditional The composition process of the product cannot be produced, and a breakthrough in the composition and process is required.
  • Chinese patent CN103602890 discloses a high-expanded steel plate with a tensile strength of 540MPa and a manufacturing method thereof. This patent adopts traditional continuous casting + traditional hot rolling process to produce, and adopts one-stage conventional laminar cooling method.
  • Chinese patent CN103602890 discloses a high-expanded steel plate with a tensile strength of 440MPa and a manufacturing method thereof. This patent adopts traditional continuous casting + traditional hot rolling process to produce, and adopts one-stage conventional laminar cooling method.
  • Chinese patents CN105154769 and CN106119702 respectively disclose a 780MPa and 980MPa grade hot-rolled high-strength and high-reamed steel and its manufacturing method, both of which belong to the category of high-strength steel. They are compared by adding Ti, Mo and Ti, Nb, Cr, V, etc. Multiple microalloying elements are used to strengthen the steel grade, and the alloy cost is relatively high; at the same time, its production method adopts traditional continuous casting + traditional hot rolling process to produce.
  • the purpose of the present invention is to provide a Nb microalloyed high-strength and high-expansion steel and a production method thereof, which can fully utilize the short process advantage of thin strip continuous casting, further reduce the production process cost and improve product performance.
  • the present invention makes full use of scrap steel as a raw material to reduce the cost of molten steel.
  • the present invention selectively adds Nb and other microalloying elements to steel (including scrap steel containing Cu and/or Sn); during the smelting process, the basicity of the slag, the type and melting point of the inclusions in the steel, and the free oxygen content in the molten steel are controlled during the smelting process.
  • Acid-soluble aluminum Als content then double-roll thin strip continuous casting is carried out to cast a 1.5-3mm thick casting strip. After the casting strip comes out of the crystallizing roller, it directly enters a lower closed chamber with a non-oxidizing atmosphere, and is closed In this case, it enters the online rolling mill for hot rolling; after rolling, the strip is cooled by gas atomization cooling method.
  • the gas atomization cooling method can effectively reduce the thickness of the surface oxide scale of the strip, improve the temperature uniformity of the strip, and increase the strip
  • the surface quality of the steel; the final produced steel coil can be used directly as a hot-rolled sheet, or it can be used after pickling and leveling.
  • the Nb microalloyed high-strength and high-expansion steel of the present invention has the following chemical composition weight percentages: C: 0.01-0.05%, Si: 0.2-0.6%, Mn: 0.8-1.5%, P ⁇ 0.02%, S ⁇ 0.005%, N ⁇ 0.008%, Als: ⁇ 0.001%, Ca ⁇ 0.0050%, Nb: 0.01-0.08%, optional one or two of Cu 0.1-0.6% and Sn 0.005-0.04%, Among them, Mn/S>250, total oxygen [O] T : 0.007 to 0.020%, and the balance is Fe and unavoidable impurities.
  • the weight percentage of the chemical composition of the Nb microalloyed high-strength and high-expansion steel is: C: 0.01-0.05%, Si: 0.2-0.6%, Mn: 0.8-1.5%, P ⁇ 0.02%, S ⁇ 0.005%, N ⁇ 0.008%, Als: ⁇ 0.001%, Ca ⁇ 0.0050%, Nb: 0.01-0.08%, Mn/S>250, total oxygen [O] T : 0.007 ⁇ 0.020%, the balance is Fe And unavoidable impurities, and meet: contain one or two of Cu 0.1-0.6% and Sn 0.005-0.04%.
  • these Nb microalloyed high-strength high-strength hole-expanded steels are high-strength hole-expanded steels based on scrap steel.
  • the weight percentage of the chemical composition of the Nb microalloyed high-strength and high-expansion steel is: C: 0.01-0.05%, Si: 0.2-0.6%, Mn: 0.8-1.5%, P ⁇ 0.02%, S ⁇ 0.005%, N ⁇ 0.008%, Als: ⁇ 0.001%, Ca ⁇ 0.0050%, Nb: 0.01-0.08%, Mn/S>250, total oxygen [O] T : 0.007 ⁇ 0.020%, the balance is Fe And unavoidable impurities.
  • the microstructure of the high-expansion steel of the present invention is ferrite (F) + bainite (B), and the proportion of bainite (B) phase is ⁇ 15%.
  • the Nb microalloyed high-strength and high-expansion steel of the present invention has a yield strength ⁇ 440MPa, a tensile strength ⁇ 590MPa, an elongation ⁇ 19%, and a hole expansion rate ⁇ 100%.
  • C is the most economical and basic strengthening element in steel. It improves the strength of steel through solid solution strengthening and precipitation strengthening. C is an essential element for the precipitation of cementite during austenite transformation. Therefore, the level of C content determines the strength level of steel to a large extent, that is, a higher C content corresponds to a higher strength level.
  • the interstitial solid solution and precipitation of C have great harm to the plasticity and toughness of steel, and the excessively high C content is detrimental to the welding performance, so the C content cannot be too high, and the strength of the steel can be compensated by adding alloy elements appropriately .
  • casting in the peritectic reaction zone is prone to slab surface cracks, and breakout accidents may occur in severe cases.
  • the casting strip in the peritectic reaction zone is prone to surface cracks, and in severe cases, the strip will break. Therefore, the continuous casting of Fe-C alloy thin strip also needs to avoid the peritectic reaction zone. Therefore, the C content range used in the present invention is 0.01-0.05%.
  • Si plays a role of solid solution strengthening in steel.
  • adding Si also plays a role of deoxidation, which can improve the purity of steel; at the same time, Si can expand the range of ferrite formation and avoid the appearance of pearlite phase.
  • excessive Si content is likely to form "red iron scale" defects on the surface of the steel sheet after rolling. Therefore, the range of Si content used in the present invention is 0.2-0.6%.
  • Mn is one of the cheapest alloying elements. It can improve the hardenability of steel and has a considerable solid solubility in steel. It can improve the strength of steel through solid solution strengthening, and at the same time, it can basically improve the plasticity and toughness of steel. No damage, it is the most important strengthening element to increase the strength of steel, and it can also play a role in deoxidizing steel. However, excessive Mn content will cause deterioration of weldability and toughness of the welding heat-affected zone. Therefore, the range of Mn content used in the present invention is 0.8-1.5%.
  • P High content of P is easy to segregate in grain boundaries, increase the cold brittleness of steel, deteriorate welding performance, reduce plasticity, and deteriorate cold bending performance.
  • the solidification and cooling rate of the cast strip is extremely fast, which can effectively inhibit the segregation of P, thereby effectively avoiding the disadvantages of P and giving full play to the advantages of P. Therefore, in the present invention, a higher P content than the traditional production process is adopted, the content of P element is appropriately relaxed, and the dephosphorization process is eliminated in the steelmaking process. In actual operation, there is no need to deliberately perform the dephosphorization process, and no additional process is required. Add phosphorus, the range of P content is less than 0.02%.
  • S Under normal circumstances, S is a harmful element in steel, causing steel to produce hot brittleness, reducing the ductility and toughness of steel, and causing cracks during rolling. S is easy to form MnS in steel.
  • the amount and shape of sulfide in steel directly affect the hole expansion rate of steel plate. S must be less than 0.005%.
  • the number and morphology of inclusion elements have a great influence on the hole expansion performance of the steel plate, especially the strip-shaped sulfide inclusions are likely to cause cracks during deformation. Therefore, in the present invention, S is controlled as an impurity element, and its content range is ⁇ 0.005%. And, Mn/S>250.
  • N Similar to the C element, the N element can improve the strength of steel through interstitial solid solution. However, the interstitial solid solution of N has great harm to the plasticity and toughness of steel. The existence of free N will increase the yield ratio of steel, so the N content cannot be too high.
  • the range of N content used in the present invention is ⁇ 0.008%. In some embodiments, the content of N is 0.004-0.008%.
  • Nb In the thin strip continuous casting process, due to its unique rapid solidification and rapid cooling characteristics, the added alloying element Nb can mainly exist in the steel strip in a solid solution state. Even if the steel strip is cooled to room temperature, it is almost impossible to observe To the precipitation of Nb.
  • the Nb element dissolved in steel can play a role of solid solution strengthening.
  • the content of Nb designed in the present invention ranges from 0.01% to 0.08%.
  • Ca It can change the form of sulfides in steel, transform long MnS inclusions into spherical CaS inclusions, improve the plasticity and toughness of the steel plate, and help increase the hole expansion rate of the steel plate.
  • the present invention controls Ca ⁇ 0.0050%. In some embodiments, the content of Ca is 0.001-0.005%.
  • Cu It mainly plays a role of solid solution and precipitation strengthening in steel. Because Cu is an element that is easy to segregate, the Cu content is generally strictly controlled in the traditional process. Using the rapid solidification effect of thin strip continuous casting, the present invention increases the upper limit of Cu to 0.60%.
  • Figure 2 shows the effect of copper on the interface heat flow. Copper elements of different compositions are added to steel. It can be seen from the experimental results that as the copper content increases, the peak heat flow of the interface heat transfer of the steel decreases, and the average heat flow also decreases. When the Cu content reaches 0.80%, there is still a high peak heat flow and average heat flow. When the Cu content is greater than 2.5%, the peak heat flow and average heat flow are significantly reduced.
  • the present invention controls the content of Cu to be between 0.1% and 0.6%, and the peak heat flow and average heat flow caused by the Cu element have little effect.
  • the increase of Cu content can, to a certain extent, realize the effective use of copper in scrap steel or inferior mineral resources (high copper ore), promote the recycling of steel, reduce production costs, and achieve the purpose of sustainable development. It is worth noting that, in the present invention, the Cu element in the scrap steel is fully utilized, and no additional metal Cu is required, which will increase the cost of steelmaking.
  • Sn element is also one of the main participating elements in scrap steel. It is recognized as a harmful element in steel. Because Sn is an element that is easy to segregate, a small amount of Sn will be enriched at the grain boundary, resulting in cracks and other defects. In the traditional process, the content of Sn element is strictly controlled. Due to the characteristics of rapid solidification of thin strip continuous casting, the segregation of elements in the dendrites is greatly reduced, which can greatly increase the amount of solid solution of the elements. Therefore, under the conditions of the thin strip continuous casting process, the range of Sn elements can be expanded, so it can greatly Reduce steelmaking costs.
  • Figure 3 is the relationship between the Sn element and the average heat flux. It can be seen from Fig.
  • Figure 4 shows the relationship between Sn content and surface roughness. Because the cracks on the surface of the cast strip are usually produced at the uneven wrinkles on the surface of the cast strip, the surface roughness is used to characterize the occurrence of surface cracks. If the roughness is large, the probability of occurrence of cracks is high. It can be seen from Fig. 4 that the increase in Sn content does not adversely affect the surface quality of the cast strip under the conditions of rapid solidification. From the results of Fig. 3 and Fig. 4, it can be seen that Sn has no adverse effect on the solidification and surface quality of the cast strip.
  • the requirements for Sn content can be further relaxed, and the design Sn content range is 0.005-0.04%. It is worth noting that, in the present invention, the Sn element in the scrap steel is fully utilized, and there is no need to add metal Sn, which will increase the steelmaking cost.
  • the production method of Nb microalloyed high-strength and high-expansion steel according to the present invention includes the following steps:
  • the continuous casting adopts double-roll thin strip continuous casting to form a 1.5-3mm thick cast strip at the smallest gap between the two crystallizing rolls; the diameter of the crystallizing roll is between 500-1500mm, preferably 800mm; the inside of the crystallizing roll is cooled by water.
  • the casting speed of the machine is 60-150m/min; the continuous casting flow adopts a two-stage molten steel distribution flow system, that is, the tundish + flow distributor;
  • the temperature of the casting belt is 1420-1480°C, and it enters the lower closed chamber directly.
  • the lower closed chamber is filled with non-oxidizing gas.
  • the oxygen concentration (volume) in the lower closed chamber is controlled to be less than 5%.
  • the lower closed chamber The temperature of the exit casting belt is between 1150-1300°C;
  • the cast strip is sent to the rolling mill via pinch rolls in the lower enclosed chamber and rolled into strip steel.
  • the rolling temperature is 1100-1250°C, and the hot rolling reduction rate is controlled to 10-50%, preferably 30-50%. ;
  • the thickness of the steel strip after hot rolling is 0.8-2.5mm, preferably 1.0-1.8mm;
  • the cooling adopts gas atomization cooling method, and the cooling rate is ⁇ 50°C/s;
  • the cooled hot-rolled steel strip is directly coiled into coils at a coiling temperature of 470-570°C.
  • step 1) the smelting adopts electric furnace steelmaking or converter steelmaking, and then enters LF furnace, VD/VOD furnace or RH furnace for refining.
  • step 1) of some embodiments 100% scrap steel is selected as raw material for smelting, without pre-screening, and molten steel smelting adopts electric furnace steelmaking; or, smelting adopts converter steelmaking, and scrap accounts for more than 20% of smelting raw materials.
  • the non-oxidizing gas is CO 2 gas obtained by sublimation of N 2 , Ar or dry ice.
  • the gas-water ratio of the gas atomization cooling is 15:1-10:1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.
  • air-water ratio refers to the flow ratio of compressed air and water, and the unit of flow is m 3 /h.
  • the cooling rate is 50-75°C/s.
  • the coiling adopts a double-coiler form, or adopts a Carrousel winding form.
  • the cooled hot-rolled steel strip is directly coiled into a coil after being cut by a cutting head to remove the poor quality head, and the coiling temperature is 470-570°C.
  • MnO-SiO 2 -Al 2 O 3 ternary inclusions As shown in the shaded area in Figure 2, MnO-SiO 2 -Al 2 O 3 ternary inclusions
  • the MnO/SiO 2 in the composition is controlled within 0.5-2, preferably 1-1.8.
  • O in steel is an essential element for the formation of oxidized inclusions.
  • the present invention requires the formation of low-melting ternary inclusions of MnO-SiO 2 -Al 2 O 3 , which requires steel
  • the free oxygen [O] Free range in water is: 0.0005 to 0.005%.
  • molten steel smelting can be made through electric furnace or converter, and then enter the refining process, such as LF furnace, VD/VOD furnace, RH furnace, etc.
  • the strip steel after online hot rolling is cooled after rolling, and the cooling adopts the gas atomization cooling method.
  • the gas atomization cooling method can effectively reduce the thickness of the surface oxide scale of the strip steel, improve the temperature uniformity of the strip steel, and improve the surface quality of the strip steel.
  • the gas-water ratio of the gas atomization cooling is 15:1-10:1, the air pressure is 0.5-0.8MPa, and the water pressure is 1.0-1.5MPa.
  • high-pressure water mist is sprayed on the surface of the steel strip. On the one hand, it can reduce the temperature of the steel strip. On the other hand, the water mist will form a dense gas film covering the surface of the steel strip, which can prevent the strip from being oxidized.
  • This cooling method can avoid the problems caused by traditional spraying or laminar cooling, make the surface temperature of the strip drop uniformly, improve the uniformity of the strip temperature, and achieve the effect of homogenizing the internal microstructure; at the same time, the uniform cooling can improve the strip The shape quality and performance stability of the steel; effectively reduce the thickness of the oxide scale on the surface of the strip.
  • the cooling rate range of gas atomization cooling is ⁇ 50°C/s, and the strip is cooled to 470-570°C, so that the high-temperature austenite after rolling is transformed into a mixed microstructure of ferrite + a small amount of bainite, as shown in Figure 6. Shown.
  • the cooled hot-rolled steel strip is cut by the cutting head to remove the poor quality head, and then directly coiled into a coil with a coiling temperature of 470-570°C.
  • the coiling machine adopts double coiling form, and can also adopt Carrosel coiling form to ensure the continuous production of strip steel.
  • a carrousel winding form is adopted.
  • converter steelmaking to provide molten steel requires manufacturers to have the conditions to provide molten iron. Under normal circumstances, blast furnace ironmaking or non-blast furnace ironmaking equipment is required, which belongs to the current long-process steel production mode. However, today with increasingly rich scrap steel resources, the country is advocating to increase the converter scrap ratio, so as to achieve the purpose of energy saving, consumption reduction and cost reduction.
  • the average level of converter scrap ratio in the past was about 8%. Now and in the future, the target of converter scrap ratio It is 15-25%.
  • Scrap steel As the main raw material.
  • Traditional processes such as die casting or thick plate continuous casting have a solidification and cooling rate of only 10 -1 -10°C/s. These residual elements in the scrap will solidify Grain boundary segregation occurs during the process, which deteriorates the performance and quality of the steel. In severe cases, cracks and fractures occur directly. Therefore, in the traditional process, these harmful elements must be strictly controlled.
  • some pre-screening is required.
  • some special treatments are required in the steelmaking process, such as adding some concentrates for dilution, etc., which undoubtedly increase the cost of production operations. Due to the need to control the composition of steel, there are certain quality requirements for the used scrap raw materials.
  • the scrap steel needs to be pre-screened and classified.
  • some domestic electric furnace steel plants will choose to add refined materials such as purchased sponge iron and iron carbide to the raw material composition to dilute the harmful elements that are difficult to remove in the scrap and improve the quality of molten steel.
  • Some domestic steel mills that have both blast furnaces and electric furnaces use self-produced molten iron into the electric furnace as raw materials to improve the production efficiency of the electric furnace, thereby greatly shortening the tapping time of the electric furnace, and the hot metal blending ratio in the electric furnace can reach 30-50% .
  • twin-roll thin strip continuous casting technology is a typical sub-rapid solidification process.
  • the solidification cooling rate is as high as 10 2 -10 4 °C/s.
  • Some harmful residual elements in the scrap such as Cu, Sn, P, etc., can be maximized
  • the ground is dissolved in the steel matrix without grain boundary segregation. Therefore, 100% scrap steel smelting can be realized during electric furnace steelmaking without pre-screening, which greatly reduces the cost of raw materials; the smelting adopts converter steel smelting, and scrap smelting is based on More than 20% of the raw materials are added to the converter without pre-screening, which maximizes the scrap ratio of the converter and greatly reduces the production cost.
  • Cast Ability of Strip Casting refers to the feasibility of double-roll casting for a steel grade.
  • Good castability means that there will be no restrictive problems that affect the inability of the casting process or the quality of the cast product cannot meet the requirements during the casting process; poor castability means that there are often problems such as molten steel during the casting process. Poor fluidity, agglomeration and bridging of the molten pool, severe belt breaks, surface cracks and surface slag inclusions, etc., have caused the production to be unable to proceed normally and stably or the product quality does not meet the requirements.
  • the steel grades of the present invention control from carbon content (avoid the peritectic zone to solve uneven solidification shrinkage); basicity control, Als control, free oxygen total oxygen control, and low melting point MnO-SiO 2 -Al 2 O 3 Ternary inclusion control (improving the uniformity of interface heat transfer to solve the uniformity of solidification); Mn/S control (avoiding hot brittleness) and other aspects, strictly meet the castability of thin strip continuous casting.
  • Traditional continuous casting also uses spray cooling, but the area and temperature are different.
  • the casting slab is spray-cooled at the exit fan-shaped area of the casting mold. At this time, the temperature of the casting slab is higher, which is shown in the phase diagram. Look at the high-temperature austenite single-phase region.
  • the main purpose of spray cooling in this area is to control the position of the solidification end, accelerate the surface cooling of the cast slab, refine the surface austenite grain structure, increase the surface strength of the cast slab, improve the surface quality of the cast slab, and avoid the occurrence of cracks.
  • the present invention sprays and cools the ultra-thin steel strip after the on-line hot rolling of the cast strip. The temperature is relatively low.
  • the phase diagram From the phase diagram, it is in the solid phase transformation zone where high-temperature austenite transforms to ferrite.
  • spray cooling by adjusting the spray cooling intensity, the type of microstructure after solid phase transformation can be effectively controlled, and the performance requirements of the final product can be achieved.
  • the present invention preferably adopts the Carrousel coiling method, which can realize the co-coiling of the hot-rolled steel coil, ensure the uniformity of the coiling temperature, and greatly improve the stability of the product performance of the steel coil.
  • the Carrosel coiler is widely used in the cold rolling field. Its main advantage is that it can realize thinner strip coiling, and it has a small footprint, which can greatly shorten the length of the production line. Due to the low temperature of the strip steel, it is relatively easy to realize.
  • the present invention proposes to adopt the Carrousel coiling method in the field of ultra-thin hot-rolled steel strip coiling, taking into account the high temperature resistance of the equipment, and realizes the coiling of ultra-thin hot-rolled steel strip.
  • the coiling method is more advanced than the coiling method of the Casttrip thin strip continuous casting production line of Nucor in the United States.
  • the most obvious feature of the present invention which is different from the existing thin strip continuous casting technology is the roll diameter of the crystallizing roll and the corresponding flow distribution method.
  • the technical feature of EUROSTRIP is ⁇ 1500mm large diameter crystallizing roller, large crystallizing roller, large molten steel capacity in molten pool, easy flow distribution, high cost of crystallizing roller manufacturing and operation and maintenance.
  • the technical feature of CASTRIP is the ⁇ 500mm small diameter crystallizing roller, the crystallizing roller is small, the molten steel volume is small, and the flow distribution is very difficult, but the cost of manufacturing and operation and maintenance of the casting machine is low.
  • CASTRIP adopts a three-stage molten steel distribution flow system (tundish + transition bag + flow distributor).
  • the use of a three-stage flow distribution system directly leads to an increase in the cost of refractory materials; more importantly, the three-stage flow distribution system makes the path of molten steel flow longer and the temperature drop of molten steel is also greater.
  • the tapping temperature needs to be greatly increased. The increase in the tapping temperature will cause problems such as increased steelmaking costs, increased energy consumption, and shortened life of refractory materials.
  • the molten steel flowing out of the distributor forms different distribution patterns along the roller surface and the two end surfaces, and flows in two paths without interfering with each other. Due to the use of a two-stage flow distribution system, compared with a three-stage flow system, the cost of refractory materials is greatly reduced; the shortening of the molten steel flow path reduces the temperature drop of the molten steel, which can lower the tapping temperature, compared with the three-stage flow system , The tapping temperature can be reduced by 30-50°C.
  • the lowering of the tapping temperature can effectively reduce the cost of steelmaking, save energy and extend the life of refractory materials.
  • the invention is matched with a crystallizing roller with a roller diameter of ⁇ 800mm and adopts a two-stage molten steel distribution system, which not only realizes the requirement of stable molten steel distribution, but also realizes the goals of simple structure, convenient operation and low processing cost.
  • Chinese patent CN101353757 uses low-carbon microalloy components to produce 440MPa tensile strength reamed steel, with a small amount of Nb:0-0.25% and Ti:0-0.03% added to the composition. Due to the 600°C coiling temperature, this patent adopts the traditional Continuous casting + traditional hot rolling process. For carbon-manganese steel hot-rolled sheets, there is often a band structure, which leads to a decrease in the plate hole expansion rate. At the same time, a variety of microalloys are added, which increases the cost of steelmaking.
  • the invention is obviously different from the patent in the production process.
  • the invention adopts the thin strip continuous casting process to produce, which can greatly shorten the production process, and can avoid the band structure. At the same time, it can save the amount of microalloys and only need to add a small amount of microalloys. The same or even better performance can be achieved.
  • Chinese patent CN101928881 discloses a hot-rolled high-expanded steel sheet with a tensile strength of 590MPa and its manufacturing process.
  • the patent composition adds a small amount of Nb:0-0.10% and Ti:0-0.04%, using traditional continuous casting + traditional heat It is produced by rolling process.
  • the finished steel plate is cooled to 600-750°C at a cooling rate of 50°C/s ⁇ 100°C/s, and then cooled in air at a cooling rate of 5°C/s ⁇ 15°C/s for 3 ⁇ After 10 seconds, the steel plate is again cooled to 350-500°C at a cooling rate of 70°C/s-150°C/s and coiled, and then air-cooled to room temperature.
  • the subsequent cooling adopts complicated three-stage cooling, the coiling temperature fluctuates greatly, the performance fluctuation of the coil head, middle and tail will be relatively large, and the hole expansion rate fluctuates also.
  • the invention adopts the thin strip continuous casting process to produce, greatly simplifies the production process, and does not need to use complicated three-stage cooling, and has obvious advancement.
  • Japanese patent JP2006063394 discloses a hot-rolled high-expansion steel with a carbon content of 0.20 ⁇ 0.48%, its tensile strength ⁇ 440MPa, and its addition of Cr alloy elements, but its hole expansion rate is only ⁇ 70%, and after hot rolling An annealing treatment at 640°C is also required.
  • the carbon content design of the invention has reached the range of medium and high carbon steel, which is obviously higher than the low carbon design of the invention.
  • the hot-rolled high-strength steel sheet disclosed in Japanese patent JP2006305700 adopts the composition design of C-Si-Mn+Ti to obtain a tensile strength of more than 780MPa and a hole expansion rate of only 68% or more.
  • Japanese patent JP2003/016614 discloses a hot-rolled high-expansion steel with carbon content of 0.02 ⁇ 0.10%, Si ⁇ 0.5%, and its tensile strength ⁇ 590MPa, but it contains a lot of alloys such as Nb, Ti, V, Cr, RE, etc. Element, steelmaking costs are relatively high, and good surface paintability is the main goal.
  • the present invention adopts a simple alloy composition system and a thin strip continuous casting process to realize the performance of high-expansion steel.
  • the present invention has the characteristics of simplicity and high efficiency.
  • the hot-rolled steel sheet disclosed in US Patent US2006096678 has a strength of 780MPa or more, an elongation rate of 22% or more, and a hole expansion rate of 60% or more.
  • the hot-rolled steel sheet disclosed in US Patent No. 4,415,376 has a yield strength of 80 ksi (550 MPa) or more, a hole expansion rate of 58% or more, and is strengthened by Nb and V.
  • the production processes used in these patents are all traditional continuous casting + traditional hot rolling production processes, which are different from the production process of the present invention, and the product hole expansion rate is low.
  • the present invention eliminates the need for complex processes such as slab heating and multi-pass repeated hot rolling. Through double-roll thin strip continuous casting + one online hot rolling process, the production process is shorter, the efficiency is higher, and the investment cost of the production line is reduced. The production cost is greatly reduced.
  • the present invention eliminates many complicated intermediate steps in the production of traditional high-expansion steel. Compared with traditional high-expansion steel, the energy consumption and CO 2 emission of production are greatly reduced, and it is a green and environmentally friendly product.
  • thin strip continuous casting Due to the advantages of the natural process flow of thin strip continuous casting, compared with the traditional hot rolling process, thin strip continuous casting is easy to generate bainite type microstructure during the cooling process after rolling, and it is easy to produce excellent products. The reaming performance.
  • the present invention adopts the thin strip continuous casting process to produce high-reamed steel.
  • the thickness of the cast strip itself is relatively thin.
  • the purpose of supplying hot-rolled sheets of specifications can significantly improve the cost-effectiveness of sheets and strips.
  • the smelting of the present invention uses electric furnace steel smelting, and the raw materials smelted can realize 100% all-scrap smelting in the true sense without pre-screening, which greatly reduces the cost of raw materials; the smelting is through converter steel smelting, and the scrap accounts for more than 20% of the smelting raw materials. It is added to the converter in a proportion and does not require pre-screening, which maximizes the scrap ratio of the converter and greatly reduces smelting costs and energy consumption.
  • the present invention uses scrap steel containing Cu and Sn to "turn harm into benefits" for Cu and Sn in the steel, and realize the full use of existing scrap steel or low-quality low-quality mineral resources (high tin ore, high copper ore) Utilize, promote the recycling of scrap steel, reduce production costs, and realize the sustainable development of the steel industry.
  • the present invention adopts the strip steel gas atomization cooling method after rolling, which can avoid the problems caused by traditional spraying or laminar cooling, make the surface temperature of the strip uniformly drop, improve the uniformity of the strip temperature, and achieve uniform internal microcosmic The effect of the structure; at the same time, uniform cooling can improve the shape quality and performance stability of the strip steel; effectively reduce the thickness of the oxide scale on the surface of the strip steel.
  • the precipitation of alloying elements occurs during the cooling process of the traditional slab.
  • the utilization of alloying elements is often reduced due to insufficient re-dissolution of the alloy elements.
  • the high-temperature cast strip is directly hot-rolled, and the added alloy elements mainly exist in a solid solution state, which can improve the alloy utilization rate.
  • the carrousel coiler is selected for the coiling of the present invention, which effectively shortens the length of the production line; at the same time, the co-position coiling can greatly improve the control accuracy of the coiling temperature and improve the stability of the product performance.
  • Figure 1 is a schematic diagram of the process layout of the twin-roll thin strip continuous casting process
  • Figure 2 is a schematic diagram of the influence of Cu on the interface heat flow
  • Figure 3 is a schematic diagram of the relationship between Sn content and average heat flux
  • Figure 4 is a schematic diagram of the relationship between the Sn content and the surface roughness of the cast strip
  • Figure 5 is a ternary phase diagram of MnO-SiO 2 -Al 2 O 3 (shaded area: low melting point area);
  • Figure 6 is a photo of the microstructure of the steel in the example of the invention.
  • the molten steel designed in accordance with the chemical composition of the present invention is directly poured in a large ladle 1, through a ladle nozzle 2, a tundish 3, an immersion nozzle 4, and a distributor 5, which can rotate relatively quickly and quickly
  • the molten steel solidifies on the rotating circumferential surface of the crystallizing rollers 8a, 8b, and then forms a solidified shell and gradually grows on the two crystallizing rollers.
  • a 1.5-3mm thick cast belt 11 is formed.
  • the diameter of the crystal roll described in this embodiment is between 500-1500mm, and the inside is cooled by water; according to the thickness of the cast belt, the casting speed range of the casting machine Between 60-150m/min.
  • the temperature of the casting belt is 1420-1480°C, and it directly enters the lower airtight chamber 10.
  • the lower airtight chamber 10 protects the casting belt with non-oxidizing gas to realize the oxidation protection of the casting belt
  • the atmosphere for the anti-oxidation protection can be N 2 , Ar, or other non-oxidizing gases, such as CO 2 gas obtained by sublimation of dry ice, etc.
  • the oxygen concentration in the lower closed chamber 10 is controlled to be less than 5%.
  • the airtight chamber 10 protects the cast strip 11 from oxidation to the entrance of the rolling mill 13; the temperature of the cast strip at the exit of the lower airtight chamber 10 is between 1150-1300°C; then the cast strip is sent to it through the swing guide 9, the pinch roller 12 and the roller table 15.
  • the rolling mill 13 forms a 0.8-2.5mm hot-rolled steel strip after hot rolling, and the rolled steel strip is cooled by the gas atomization rapid cooling device 14 to improve the temperature uniformity of the strip steel.
  • the flying shear device 16 After cutting the head by the flying shear device 16, the cutting head falls into the flying shear pit 18 along the flying shear guide plate 17, and the hot-rolled steel strip after the cutting head enters the coiler 19 for coiling.
  • the steel coil is removed from the coiler, it is naturally cooled to room temperature.
  • the final produced steel coil can be used directly as a hot-rolled sheet, or it can be used after pickling and leveling.
  • the chemical composition of the embodiment of the present invention is shown in Table 1, and the balance of the composition is Fe and other unavoidable impurities.
  • the process parameters are shown in Table 2, and the properties of the final product obtained are shown in Table 3.
  • the method for measuring the hole expansion rate is implemented in accordance with the international standard ISO16630:2009.
  • the present invention uses thin strip continuous casting technology to manufacture high-expansion steel according to the designed steel composition, with yield strength ⁇ 440MPa, tensile strength ⁇ 590MPa, elongation ⁇ 19%, and hole expansion rate ⁇ 100 %.
  • Table 1 The chemical composition of the example steel (wt.%)
  • Example 1 0.04 0.27 1.35 0.008 0.004 0.0064 0.0093 0.0009 0.06 0.003
  • Example 2 0.05 0.20 0.90 0.013 0.003 0.0068 0.0110 0.0006 0.05 0.004
  • Example 3 0.02 0.38 1.28 0.015 0.004 0.0048 0.0150 0.0004 0.03 0.005
  • Example 4 0.01 0.22 1.26 0.013 0.005 0.0067 0.0130 0.0008 0.03 0.004
  • Example 5 0.03 0.41 0.85 0.009 0.002 0.0062 0.0120 0.0007 0.01 0.003
  • Example 6 0.04 0.45 0.80 0.012 0.002 0.0046 0.0070 0.0008 0.04 0.001
  • Example 7 0.02 0.28 0.95 0.015 0.003 0.0040 0.0100 0.0005 0.06 0.002
  • Example 8 0.05 0.37 1.30 0.014 0.005 0.0080 0.0085 0.0006 0.05 0.004
  • Example 9 0.04 0.36 0.84 0.018 0.003
  • Example 13 0.05 0.45 1.37 0.018 0.004 0.0045 0.0132 0.0006 0.08 0.002
  • Example 14 0.02 0.28 1.40 0.017 0.003 0.0064 0.0075 0.0005 0.03 0.003
  • the chemical composition of the examples of the present invention based on scrap steel is shown in Table 4, and the composition balance is Fe and other unavoidable impurities.
  • the process parameters are shown in Table 5, and the properties of the final product obtained are shown in Table 6.
  • the high-expansion steel produced by the thin strip continuous casting process technology according to the steel grade composition design range provided by the present invention has a yield strength ⁇ 440MPa, tensile strength ⁇ 590MPa, elongation ⁇ 19%, and hole expansion rate ⁇ 100%.
  • Example 15 0.03 0.26 1.37 0.008 0.004 0.0054 0.0093 0.0009 0.03 0.35 0.022 0.005
  • Example 16 0.01 0.20 0.92 0.013 0.003 0.0071 0.0110 0.0006 0.04 0.16 0.005 0.004
  • Example 17 0.04 0.35 1.28 0.015 0.004 0.0068 0.0150 0.0004 0.06 0.10
  • Example 18 0.05 0.28 1.26 0.013 0.005 0.0067 0.0130 0.0008 0.04 0.56 0.040 0.003
  • Example 19 0.04 0.45 0.85 0.009 0.002 0.0052 0.0120 0.0007 0.01 0.44 0.014 0.005
  • Example 20 0.05 0.41 0.80 0.012 0.002 0.0046 0.0070 0.0008 0.03
  • Example 21 0.03 0.29 0.95 0.015 0.003 0.0040 0.0100 0.0005 0.07 0.38 0.035 0.002
  • Example 22 0.02 0.38 1.30 0.014
  • Example 26 0.05 0.57 1.50 0.012 0.005 0.0055 0.0118 0.0003 0.08 0.35 0.013 0.001
  • Example 27 0.03 0.46 1.38 0.018 0.004 0.0045 0.0132 0.0006 0.07 To 0.036 0.004
  • Example 28 0.02 0.27 1.40 0.017 0.003 0.0074 0.0075 0.0005 0.04 0.27 0.027 0.003

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Abstract

一种Nb微合金化高强高扩孔钢及其生产方法，其化学成分重量百分比为：C 0.01-0.05％，Si 0.2-0.6％，Mn：0.8-1.5％，P≤0.02％，S≤0.005％，N≤0.008％，Als＜0.001％，Ca≤0.0050％，Nb 0.01-0.08％，任选的Cu 0.1-0.6％和Sn 0.005-0.04％中的一种或两种，其中，Mn/S＞250，总氧[O] T：0.007～0.020％，余量Fe和不可避免杂质。通过有选择地添加Nb等微合金元素；冶炼过程中通过控制渣的碱度、钢中夹杂物类型及熔点、钢水中的游离氧含量、酸溶铝Als含量；然后双辊薄带连铸浇铸出铸带，铸带直接进入一个有非氧化性气氛的下密闭室中，并在密闭情况下进入在线轧机热轧；轧后采用气雾化冷却方式对带钢进行冷却，最后生产的钢卷可以作为热轧板直接使用，也可以经过酸洗-平整后使用。

Description

一种Nb微合金化高强高扩孔钢及其生产方法 技术领域
本发明涉及高扩孔钢制造技术，特别涉及一种Nb微合金化高强高扩孔钢及其生产方法。
背景技术
在传统钢铁生产流程中，锡(Sn)、铜(Cu)是钢中典型的残余元素或有害元素，炼钢过程中要充分地去除Sn、Cu非常困难而且也是非常昂贵，一旦钢中含有Sn、Cu，基本是无法彻底消除的，只能通过稀释钢水来降低Sn、Cu的含量，这些都造成钢铁产品冶炼成本的升高。
近年来，由于废钢的连续循环利用，废钢资源越来越多，电价也持续降低，国内基于废钢的短流程电炉炼钢日益兴起，导致钢中的Sn、Cu等残余元素的含量逐渐升高，钢中的Sn、Cu是易偏析元素，容易富集在晶界导致裂纹等缺陷发生，因此在传统的工艺中Sn、Cu元素的含量是被严格控制的，在普通结构用钢中，对Sn、Cu的含量均有明确的要求：Sn(wt％)≤0.005％；Cu(wt％)≤0.2％。
因此，如果能对钢(特别是废钢)中Sn、Cu等残余元素做到合理利用，“化害为利”，将对整个冶金界产生积极的影响；可以实现对现有废钢或低品质劣质矿资源(高锡矿、高铜矿)的有效利用，促进钢的循环利用，降低生产成本，实现钢铁业可持续发展。
传统的薄带钢大都是由厚达70-200mm的铸坯经过多道次连续轧制生产出来的，传统热轧工艺流程是：连铸+铸坯再加热保温+粗轧+精轧+冷却+卷取，即首先通过连铸得到厚度为200mm左右的铸坯，对铸坯进行再加热并保温后，再进行粗轧和精轧，得到厚度一般大于2mm的钢带，最后对钢带进行层流冷却和卷取，完成整个热轧生产过程。如果要生产厚度小于1.5mm(含)的钢带，则难度相对较大，通常要对热轧钢带进行后续冷轧以及退火来完成。且工艺流程长、能耗高、机组设备多、基建成本高，导致生产成本较高。
薄板坯连铸连轧工艺流程是：连铸+铸坯保温均热+热连轧+冷却+卷取。该工艺与传统工艺的主要区别是：薄板坯工艺的铸坯厚度大大减薄，为50-90mm，由于铸坯薄，铸坯只要经过1～2道次粗轧(铸坯厚度为70-90mm时)或者不需要经过粗轧(铸坯厚度为50mm时)，而传统工艺的连铸坯要经过反复多道次轧制，才能减薄到精轧前所需规格；而且薄板坯工艺的铸坯不经冷却，直接进入均热炉进行均热保温，或者少量补温，因此薄板坯工艺大大缩短了工艺流程，降低了能耗，减少了投资，从而降低了生产成本。但薄板坯连铸连轧由于较快的冷速会导致钢材强度提高，屈强比提高，从而增加轧制载荷，使得可经济地生产热轧产品的厚度规格也不可能太薄，一般为≥1.5mm，见中国专利CN 200610123458.1，CN 200610035800.2以及CN 200710031548.2，且这些专利均未涉及元素Sn和Cu。
近年来兴起的一种全无头薄板坯连铸连轧工艺(简称：ESP)，是在上述半无头薄板坯连铸连轧工艺的基础上发展起来的一种改进工艺，ESP实现了板坯连铸的无头轧制，取消了板坯火焰切割和起保温均热、板坯过渡作用的加热炉，整条产线长度大大缩短到190米左右，连铸机连铸出来的板坯厚度在90-110mm，宽度在1100-1600mm，连铸出来的板坯通过一段感应加热辊道对板坯起到保温均热的作用，然后再依次进入粗轧、精轧、层冷、卷取工序得到热轧板，这种工艺由于实现了无头轧制，可以得到最薄0.8mm厚度的热轧板，拓展了热轧板的规格范围，再加上其单条产线产量可达220万t/年规模。目前该工艺得到了快速发展和推广，目前世界上已有多条ESP产线在运营生产。
比薄板坯连铸连轧更短的工艺流程是薄带连铸连轧工艺，薄带连铸技术是冶金及材料研究领域内的一项前沿技术，它的出现为钢铁工业带来一场革命，它改变了传统治金工业中钢带的生产过程，将连续铸造、轧制、甚至热处理等整合为一体，使生产的薄带坯经过一道次在线热轧就一次性形成薄钢带，大大简化了生产工序，缩短了生产周期，其工艺线长度仅50m左右；设备投资也相应减少，产品成本显著降低，是一种低碳环保的热轧薄带生产工艺。双辊薄带连铸工艺是薄带连铸工艺的一种主要形式，也是世界上唯一实现产业化的一种薄带连铸工艺。
双辊薄带连铸典型的工艺流程如图1所示，大包1中的熔融钢水通过大包长水口2、中间包3、浸入式水口4以及布流器5直接浇注在一个由两个相对转动并能够快速冷却的结晶辊8a、8b和侧封装置6a、6b围成的熔池7中，钢水在结晶辊8a、 8b旋转的周向表面凝固形成凝固壳并逐渐生长，进而在两结晶辊8a、8b辊缝隙最小处(nip点)形成1-5mm厚的钢带11，钢带经由导板9导向夹送辊12送入轧机13中轧制成0.7-2.5mm的薄带，随后经过冷却装置14冷却，经飞剪装置16切头后，最后送入卷取机19卷取成卷。
高扩孔钢，是先进高强钢(AHSS)的一个重要钢种，它具有较高的强度、延伸率，优良的成形性和翻边性能，能满足成形性能要求很高的复杂形状的汽车零部件的要求，比如汽车底盘后桥悬架摆臂，也可用于其它需要凸缘翻边的零部件上。其翻边能力以扩孔率表示，扩孔性能作为钢材的一项成形性能指标，反映的是在扩孔过程中材料抵抗因孔缘局部伸长变形过大而在垂直于孔缘方向上抵抗局部开裂的能力。
随着汽车设计对底盘结构的要求日益提高，零件成形更加复杂，对钢板的翻边和扩孔性能要求进一步提高，通过汽车零件翻边和局部扩孔形状设计可提高零件的强度和刚性，可达到汽车钢板减薄和轻量化的目的，传统碳锰固溶强化钢和低合金析出强化钢结构钢板难易满足汽车底盘、悬臂零件成形要求，比如传统440MPa钢板碳锰固溶强化钢和低合金析出强化钢的扩孔率仅在50～70％之间，由此诞生了高扩孔钢。20世纪90年代，美国、日本等相继开发出440～780MPa级高扩孔性能的热轧钢板，其扩孔率为70％～131％，主要应用于成形性，尤其是翻边性能要求良好的汽车底盘、车轮等部件，钢板的扩孔性能与钢板的成分、强度和组织均匀性有关，由于其含有较多贵重合金元素Cr、Nb、Ti、V和Mo等，虽然在冷速较低的条件下可以获得铁素体/贝氏体双相组织，但其成本较高。
过去，为满足钢板在轿车底盘上的使用条件，一般有两种选择，一种是使用强度降低的钢板(≤300MPa)，以获得较高的扩孔性能；另一种是在零件设计中减少翻边量，以降低对钢板扩孔性能的要求。随着汽车用钢强度的不断提高，传统汽车用钢的扩孔率随之降低，已难以满足轿车底盘对钢板扩孔率的要求。而随着汽车设计对底盘结构的要求日益提高，其零件形状日趋复杂，强度要求不断提高，对钢板扩孔率也有所增加，高扩孔钢已成为一个重要的汽车用钢品种。
目前最常用的高扩孔钢强度级别主要集中在440MPa和590MPa级别上，其显微组织主要是铁素体和贝氏体，有时含少量的马氏体组织。钢板的扩孔性能与多种因素有关，这些因素主要包括：夹杂物水平、组织中各相的性能差异、组织均匀性、 屈强比以及组织类型等。从组织类型看，铁素体型和贝氏体型的组织具有比较高的扩孔性能，但其强度相对较低，难以达到780MPa及以上级别，这也是目前高扩孔钢主要集中在440MPa和590MPa两个强度级别上的主要原因。高扩孔钢已成为汽车钢板的一个重要品种之一。
由于薄带连铸天然的工艺流程优势，相比传统热轧流程，薄带连铸在轧后冷却过程中很容易生成贝氏体类型的微观组织，很容易使生产出来的产品具备优良的扩孔性能。因此，采用薄带连铸来生产高扩孔钢也就具备天然的优势了。
采用薄带连铸来生产高扩孔钢，主要瞄准厚度小于1.8mm(含)的热轧薄规格汽车用钢市场。由于厚度较薄，薄带连铸工艺具有较强的制造和成本优势。高扩孔钢带以热轧/酸洗状态直接供货的产品规格特征厚度为1.2mm、1.25mm、1.4mm、1.5mm、1.6mm、1.7mm和1.8mm等，由于产品厚度较薄，传统薄规格的高扩孔钢，很多厂家由于传统热连轧线的能力限制，往往无法全规格供应；或者采用先热连轧工艺生产，然后进行冷轧，这样的生产流程增加了薄规格高扩孔钢的生产成本。
热轧带钢作为薄规格热轧产品使用时，对带钢表面质量要求不是最高。一般要求带钢表面氧化皮的厚度越薄越好，这就需要在铸带后续的各个阶段控制氧化铁皮的生成，在结晶辊直至轧机入口均采用密闭室防止铸带氧化，在密闭室内如美国专利US6920912添加氢气以及在美国专利US20060182989中控制氧气含量小于5％，均可以控制铸带表面的氧化皮厚度。但是在轧机至卷取这段输送过程如何控制氧化皮的厚度很少有关专利涉及，尤其是在采用层流冷却或喷淋冷却对带钢进行冷却的过程中，高温的带钢与冷却水接触，铸带表面的氧化皮厚度增长很快。同时，高温的带钢与冷却水接触还会带来很多问题：其一，会在带钢表面形成水斑(锈斑)，影响表面质量；其二，层流冷却或喷淋冷却用的冷却水容易造成带钢表面局部冷却不均匀，造成带钢内部微观组织的不均匀，从而造成带钢性能的不均匀，影响产品质量；其三，带钢表面局部冷却不均匀，会造成板形的恶化，影响板形质量。
但是，薄带连铸由于其本身的快速凝固工艺特性，生产的钢种普遍存在组织不均匀、延伸率偏低、屈强比偏高、成型性不好的问题；同时铸带奥氏体晶粒具有明显不均匀性，会导致奥氏体相变后所获得的最终产品组织也不均匀，从而导致产品的性能不稳定。因此采用薄带连铸生产线来生产一些汽车行业、石化行业需要的具有高扩孔性能的产品，具有一定难度，具有一定的挑战，因此，采用薄带连铸生产 高扩孔钢时，照搬传统的成分工艺是无法生产的，需要在成分和工艺上有突破。
中国专利CN103602890公开了一种抗拉强度540MPa级高扩孔钢板及其制造方法。该专利采用传统连铸+传统热轧工艺来生产，并采用一段式常规层流冷却方式。
中国专利CN103602890公开了一种抗拉强度440MPa级高扩孔钢板及其制造方法。该专利采用传统连铸+传统热轧工艺来生产，并采用一段式常规层流冷却方式。
中国专利CN105154769、CN106119702分别公开了一种780MPa、980MPa级热轧高强度高扩孔钢及其制造方法，均为高强钢的范畴，分别通过添加Ti、Mo和Ti、Nb、Cr、V等较多微合金元素来实现钢种的强化，合金成本较高；同时其生产方式采用传统连铸+传统热轧工艺来生产。
国际专利WO200928515采用C、Si、Mn添加少量Nb、Ti合金元素，可生产抗拉强度在490MPa以上扩孔钢，热轧必须采用两段式层流冷却方式，在实验室可较精确模拟两段式冷却控制，得到很好的试验结果，但在热轧生产中，热轧轧制中带钢速度变化大，又无法测量空冷段的钢板温度，采用两段式冷却模型控制层流冷却，钢板实际温度波动大，易导致钢卷头中尾性能波动很大。
发明内容
本发明的目的在于提供一种Nb微合金化高强高扩孔钢及其生产方法，充分利薄带连铸的短流程优势，进一步降低生产工序成本以及提高产品性能。在一些实施方案中，本发明充分利用废钢作为原料降低钢水成本，通过薄带连铸的短流程优势，可以进一步降低生产工序成本以及提高产品性能。
为达到上述目的，本发明的技术方案是：
本发明在钢(包括含有Cu和/或Sn的废钢)中有选择地添加Nb等微合金元素；冶炼过程中通过控制渣的碱度、钢中夹杂物类型及熔点、钢水中的游离氧含量、酸溶铝Als含量；然后进行双辊薄带连铸浇铸出1.5-3mm厚的铸带，铸带出结晶辊后，直接进入到一个有非氧化性气氛的下密闭室中，并在密闭情况下进入到在线轧机进行热轧；轧制后采用气雾化冷却方式对带钢进行冷却，气雾化冷却方式可以有效减小带钢表面氧化皮厚度，改善带钢温度均匀性，提高带钢表面质量；最后生产的钢卷可以作为热轧板直接使用，也可以经过酸洗-平整后使用。
具体的，本发明所述Nb微合金化高强高扩孔钢，其化学成分重量百分比为： C：0.01-0.05％，Si：0.2-0.6％，Mn：0.8-1.5％，P≤0.02％，S≤0.005％，N≤0.008％，Als：＜0.001％，Ca≤0.0050％，Nb：0.01-0.08％，任选的Cu 0.1-0.6％和Sn 0.005-0.04％中的一种或两种，其中，Mn/S＞250，总氧[O] T：0.007～0.020％，余量为Fe和不可避免杂质。
在一些实施方案中，所述Nb微合金化高强高扩孔钢的化学成分重量百分比为：C：0.01-0.05％，Si：0.2-0.6％，Mn：0.8-1.5％，P≤0.02％，S≤0.005％，N≤0.008％，Als：＜0.001％，Ca≤0.0050％，Nb：0.01-0.08％，Mn/S＞250，总氧[O] T：0.007～0.020％，余量为Fe和不可避免杂质，且满足：含有Cu 0.1-0.6％和Sn 0.005-0.04％中的一种或两种。优选地，这些Nb微合金化高强高扩孔钢是基于废钢的高强扩孔钢。
在一些实施方案中，所述Nb微合金化高强高扩孔钢的化学成分重量百分比为：C：0.01-0.05％，Si：0.2-0.6％，Mn：0.8-1.5％，P≤0.02％，S≤0.005％，N≤0.008％，Als：＜0.001％，Ca≤0.0050％，Nb：0.01-0.08％，Mn/S＞250，总氧[O] T：0.007～0.020％，余量为Fe和不可避免杂质。
本发明所述高扩孔钢的微观组织为铁素体(F)+贝氏体(B)，其中贝氏体(B)相的比例≥15％。
本发明所述的Nb微合金化高强高扩孔钢的屈服强度≥440MPa，抗拉强度≥590MPa，延伸率≥19％，扩孔率≥100％。
在本发明所述Nb微合金化高强高扩孔钢化学成分设计中：
C：C是钢中最经济、最基本的强化元素，通过固溶强化和析出强化来提高钢的强度。C是奥氏体转变过程中析出渗碳体必不可少的元素，因此C含量的高低在很大程度上决定钢的强度级别，即较高的C含量对应较高的强度级别。但是，由于C的间隙固溶和析出对钢的塑性和韧性有较大危害，而且，过高的C含量对焊接性能不利，因此C含量不能过高，钢的强度通过适当添加合金元素来弥补。同时，对常规板坯连铸来说，在包晶反应区浇铸易产生铸坯表面裂纹，严重时会发生漏钢事故。对薄带连铸来说也同样如此，在包晶反应区浇铸铸带坯易发生表面裂纹，严重时会发生断带。因此，Fe-C合金的薄带连铸同样需要避开包晶反应区。故本发明采用的C含量范围是0.01-0.05％。
Si：Si在钢中起固溶强化作用，在本发明中加Si还起到脱氧的作用，能提高钢质纯净度；同时Si可以扩大铁素体形成范围，避免珠光体相的出现。但Si含量过 高容易在轧制后钢板表面形成“红铁皮”缺陷。故本发明采用的Si含量范围是0.2-0.6％。
Mn：Mn是价格最便宜的合金元素之一，它能提高钢的淬透性，在钢中具有相当大的固溶度，通过固溶强化提高钢的强度，同时对钢的塑性和韧性基本无损害，是提高钢的强度最主要的强化元素，还可以在钢中起到脱氧的作用。但Mn含量过高会导致可焊性和焊接热影响区韧性恶化。故本发明采用的Mn含量范围是0.8-1.5％。
P：高含量的P容易在晶界偏析，增加钢的冷脆性，使焊接性能变坏，降低塑性，使冷弯性能变坏。在薄带连铸工艺中，铸带的凝固和冷却速率极快，可有效抑制P的偏析，从而可有效避免P的劣势，充分发挥P的优势。故在本发明中，采用较传统工艺生产时高的P含量，适当放宽P元素的含量，炼钢工序中取消脱磷工序，在实际操作中，不需要刻意进行脱磷工序，也不需要额外添加磷，P含量的范围≤0.02％。
S：在通常情况下S是钢中有害元素，使钢产生热脆性，降低钢的延展性和韧性，在轧制时造成裂纹。S在钢中易形成MnS，钢中硫化物数量和形态直接影响钢板的扩孔率，S必须低于0.005％。夹杂元素数量和形态对钢板的扩孔性能有很大的影响，特别是条状硫化物夹杂在变形中容易导致裂纹发生。故在本发明中，S作为杂质元素来控制，其含量范围是≤0.005％。且，Mn/S＞250。
Als：为控制钢中的夹杂物，本发明要求不能用Al脱氧，耐材的使用中，也应尽量避免Al的额外引入，严格控制酸溶铝Als的含量：＜0.001％。
N：与C元素类似，N元素可通过间隙固溶提高钢的强度。但是，N的间隙固溶对钢的塑性和韧性有较大危害，自由N的存在会提高钢的屈强比，因此N含量也不能过高。本发明采用的N含量范围是≤0.008％。在一些实施方案中，N的含量为0.004-0.008％。
Nb：在薄带连铸工艺中，由于其独特的快速凝固和快速冷却特性，可以使添加的合金元素Nb主要以固溶态存在于钢带中，即使钢带冷却到室温，也几乎观察不到Nb的析出。固溶于钢中的Nb元素，可以起到固溶强化的作用。本发明设计Nb的含量范围是0.01-0.08％。
Ca：可改变钢中硫化物的形态，使长条MnS夹杂转化球状CaS夹杂，提高钢 板的塑性和韧性，有助于提高钢板的扩孔率。本发明控制Ca＜0.0050％。在一些实施方案中，Ca的含量为0.001-0.005％。
Cu：在钢中主要起固溶和沉淀强化作用，由于Cu是易偏析元素，传统工艺流程中一般对Cu含量有较严格的控制。运用薄带连铸的快速凝固效应，本发明将Cu的上限提高到0.60％。图2为铜对界面热流的影响，在钢中添加不同成分的铜元素，由实验结果可见，随着铜含量的增加，钢的界面传热的峰值热流减小，平均热流也减小。Cu的含量达到0.80％时，仍然有较高的峰值热流和平均热流，Cu含量大于2.5％时，峰值热流和平均热流都明显减小。本发明将Cu的含量控制在0.1-0.6％之间，由Cu元素造成的峰值热流和平均热流影响不大。Cu含量的提高，在一定意义上可以实现废钢或劣质矿资源(高铜矿)中铜的有效利用，促进钢的循环利用，降低生产成本，实现可持续发展的目的。值得说明的是，本发明中，是充分利用废钢中的Cu元素，无需额外添加金属Cu，会增加炼钢成本。
Sn：Sn元素也是废钢中的主要参与元素之一，它被公认为钢中的有害元素，因为Sn是易偏析元素，少量的Sn就会在晶界富集，导致是裂纹等缺陷发生，因此在传统的工艺中Sn元素的含量是被严格控制的。薄带连铸由于快速凝固的特点，元素在枝晶间的偏析大大减小，可以大大提高元素的固溶量，因此在薄带连铸工艺条件下，Sn元素的范围可以扩大，因此可以大大降低炼钢成本。图3是Sn元素与平均热流密度的关系。由图3可见，当Sn加入量小于0.04％时，对热流密度的影响不大，即对薄带凝固过程没有影响。图4是Sn含量与表面粗糙度的关系。因为铸带表面的裂纹通常都是在铸带表面凹凸不平的皱褶处产生，用表面粗糙度来表征表面裂纹发生情况。如果粗糙度大，则裂纹发生的概率高。由图4可知，Sn含量的增加，在快速凝固条件下并没有对铸带的表面质量产生不良的影响。由图3和图4的结果可知，Sn没有对铸带的凝固和表面质量产生不良影响。故在本发明中，对Sn含量的要求可进一步放宽，设计Sn的含量范围在0.005-0.04％。值得说明的是，本发明中，是充分利用废钢中的Sn元素，无需额外添加金属Sn，会增加炼钢成本。
本发明所述的Nb微合金化高强高扩孔钢的生产方法，其包括如下步骤：
1)冶炼
按上述化学成分进行冶炼，冶炼过程中，造渣的碱度a＝CaO/SiO 2(质量比)控制在a＜1.5，优选a＜1.2，或a＝0.7～1.0；需要获得低熔点MnO-SiO 2-Al 2O 3三元夹杂 物，MnO-SiO 2-Al 2O 3三元夹杂物中的MnO/SiO 2(质量比)控制在0.5～2，优选在1～1.8；钢水中的自由氧[O] Free范围为：0.0005-0.005％，钢水的成分中Mn/S＞250；
2)连铸
连铸采用双辊薄带连铸，在两结晶辊辊缝隙最小处形成1.5-3mm厚的铸带；结晶辊直径在500-1500mm之间，优选直径为800mm；结晶辊内部通水冷却，铸机的浇铸速度为60-150m/min；连铸布流采用两级钢水分配布流***，即中间包+布流器；
3)下密闭室保护
铸带出结晶辊后，铸带温度在1420-1480℃，直接进入到下密闭室内，下密闭室内通非氧化性气体，下密闭室内的氧浓度(体积)控制在＜5％，下密闭室出口铸带的温度在1150-1300℃；
4)在线热轧
铸带在下密闭室内经夹送辊送至轧机，轧制成带钢，轧制温度为1100-1250℃，控制热轧压下率为10-50％，优选热轧压下率30-50％；热轧后钢带的厚度是0.8-2.5mm，优选厚度为1.0-1.8mm；
5)轧后冷却
对在线热轧后的带钢进行轧后冷却，冷却采用气雾化冷却方式，冷却速率≥50℃/s；
6)带钢卷取
冷却后的热轧带钢直接进行卷取成卷，卷取温度470-570℃。
优选的，步骤1)中，冶炼采用电炉炼钢或者转炉炼钢，再进入LF炉、VD/VOD炉或RH炉精炼。
优选的，在一些实施方案的步骤1)中，冶炼原料选用100％全废钢，无需预筛选，钢水冶炼采用电炉炼钢；或者，冶炼采用转炉炼钢，废钢按占冶炼原料20％以上的比例加入转炉，且无需预筛选；然后再进入LF炉、VD/VOD炉或RH炉精炼。
优选的，步骤3)中，所述非氧化性气体为N 2、Ar或干冰升华得到的CO 2气体。
优选的，步骤5)中，气雾化冷却的气水比为15∶1～10∶1，气压0.5～0.8MPa，水压1.0～1.5MPa。本文中，气水比指压缩空气和水的流量比，流量的单位为m 3/h。
优选的，步骤5)中，冷却速率为50-75℃/s。
优选的，步骤6)中，卷取采用双卷取机形式，或采用卡罗塞尔卷取形式。
优选的，步骤6)中，冷却后的热轧带钢经切头剪切除质量较差的头部后，直接进行卷取成卷，卷取温度470-570℃。
在本发明所述的Nb微合金化高强高扩孔钢的生产方法中：
为提高薄带连铸钢水的可浇铸性，炼钢过程造渣的碱度a＝CaO/SiO 2控制在a＜1.5，优选a＜1.2，或a＝0.7-1.0。
为提高薄带连铸钢水的可浇铸性，需要获得低熔点MnO-SiO 2-Al 2O 3三元夹杂物，如图2的阴影区域，MnO-SiO 2-Al 2O 3三元夹杂物中的MnO/SiO 2控制在0.5～2，优选1～1.8。
为提高薄带连铸钢水的可浇铸性，钢中的O是形成氧化夹杂物的必要元素，本发明需要形成低熔点的MnO-SiO 2-Al 2O 3的三元夹杂物，要求钢水中的自由氧[O] Free范围为：0.0005-0.005％。
为提高薄带连铸钢水的可浇铸性，上述成分中，Mn和S的控制：Mn/S＞250。
按所设计化学成分冶炼，钢水冶炼可以通过电炉炼钢，也可以通过转炉炼钢，再进入精炼工序，比如LF炉、VD/VOD炉、RH炉等。
对在线热轧后的带钢进行轧后冷却，冷却采用气雾化冷却方式，气雾化冷却方式可以有效减小带钢表面氧化皮厚度，改善带钢温度均匀性，提高带钢表面质量。气雾化冷却的气水比为15∶1～10∶1，气压0.5～0.8MPa，水压1.0～1.5MPa。气雾化后形成高压水雾喷射在钢带表面，一方面起到了降低钢带温度的作用，另一方面水雾会形成致密的气膜包覆在带钢表面，起到带钢防氧化的作用，从而有效控制了热轧带钢表面氧化皮的生长。该种冷却方式可以避免传统喷淋或者层流冷却带来的问题，使带钢表面温度均匀下降，提高带钢温度均匀性，从而达到均匀化内部微观组织的效果；同时冷却均匀，可以提高带钢的板形质量和性能稳定性；有效减少带钢表面的氧化皮厚度。气雾化冷却的冷却速率范围为≥50℃/s，将带钢冷却到470-570℃，使轧后的高温奥氏体转变为铁素体+少量贝氏体混合微观组织，如图6所示。
冷却后的热轧带钢经切头剪切除质量较差的头部后，直接进行卷取成卷，卷取温度470-570℃。卷取机采用双卷取形式，也可以采用卡罗塞尔卷取形式，保证带钢的连续生产。优选地，采用卡罗塞尔卷取形式。
关于可选用100％全废钢无需预筛选作为原料的说明：
现代钢铁生产企业为节省投资成本与生产成本，积极对现有生产工艺流程进行技术革新。针对现有热带钢生产工艺流程长、设备多且复杂的问题，许多生产厂家将连铸连轧技术与传统工艺紧密结合，以满足连铸连轧工艺的需求。
采用转炉炼钢提供钢水，则需要生产厂家具备有提供铁水的条件，一般情况下，需要有高炉炼铁或者非高炉炼铁设备，属于目前长流程钢铁生产模式。但是，在废钢资源日益丰富的今天，国家正在倡导提高转炉废钢比，从而可以达到节能降耗、降成本的目的，以前转炉废钢比的平均水平在8％左右，现在和今后转炉废钢比的目标是15-25％。
采用电炉炼钢提供钢水，则以废钢为主要原料，传统工艺比如模铸或者厚板连铸，其凝固冷却速度仅有10 -1-10℃/s，废钢中的这些残余元素，会在凝固过程中发生晶界偏析，恶化钢的性能和质量，严重时直接发生开裂、断裂现象，所以在传统工艺中，要对这些有害元素严加控制，在废钢原料的选择上，需要进行一些预筛选，以及在炼钢过程中需要进行一些特殊处理，比如添加一些精料进行稀释等等，这无疑增加了生产作业成本。由于需要控制钢的成分，对采用的废钢原料有一定的质量要求，一般情况下，需要对废钢进行预筛选分类。有些国内电炉钢厂为了提高生产效率，会在原料组成里选择添加精料如外购海绵铁、碳化铁等来稀释废钢中难以脱除的有害元素，提高钢水的质量。有些国内同时拥有高炉和电炉的钢厂利用自产铁水兑入到电炉中作为电炉原料来提高电炉的生产效率，从而大大缩短电炉的出钢时间，电炉中的铁水勾兑比可达到30～50％。
而采用双辊薄带连铸技术，是典型的亚快速凝固过程，凝固冷却速度高达10 2-10 4℃/s，废钢中的一些有害残余元素，比如Cu、Sn、P等，可以最大限度地固溶到钢的基体中而不产生晶界偏析，因此在电炉炼钢时，可以实现100％全废钢冶炼，无需进行预筛选，大大降低原料成本；冶炼采用转炉炼钢，废钢按占冶炼原料20％以上的比例加入转炉，且无需预筛选，最大限度提高转炉废钢比，大大降低生产成本。这些残余元素还可以起到固溶强化的作用，从而生产出性能优良的超薄热轧带钢。实现劣质废钢资源的生产综合利用，对废钢中的这些有害残余元素起到“化害为利”，“废物利用”的效果。
本发明所述的关于薄带连铸可浇铸性的理由说明：
对于可浇铸性，目前尚无确切定义，从传统意义上讲，它是一个与钢水流动性、激冷倾向、收缩特性以及产品品质密切相关、频繁使用的概念，它是相对于金属种类及其工艺因素来说的。定义“薄带连铸可浇铸性(Cast Ability of Strip Casting，CASC)”，是指一个钢种进行双辊铸造的可行性。可浇铸性好，意味着在浇铸过程中不会出现影响浇铸过程不能进行的或浇铸的产品品质达不到要求的限制性问题；可浇铸性不好，意味着在浇铸过程中经常出现诸如钢水流动性差、熔池结块搭桥、严重断带、表面裂纹和表面夹渣等问题，导致生产无法正常稳定进行或产品品质达不到要求。
通过对薄带连铸可浇铸性的研究分析，判断一个钢种的薄带连铸可浇铸性，简要归纳起来，可从以下几个方面考虑：一是能否避开不均匀的凝固收缩；二是能否改善界面传热的均匀性，从而改善凝固的均匀性；三是能否改善或控制凝固过程中的热脆性。当一个钢种的薄带连铸可浇铸性很差，就意味着生产过程稳定性很差，生产的产品质量稳定性也很差，最终会导致产能不能发挥，而且产品的合格率会很低，这样的产品是不适合采用薄带连铸工艺生产的。
本发明所述的钢种，从含碳量控制(避开包晶区解决不均匀凝固收缩)；碱度控制、Als的控制、自由氧总氧控制以及低熔点MnO-SiO 2-Al 2O 3三元夹杂物控制(改善界面传热均匀性解决凝固均匀性)；Mn/S控制(避免热脆性)等方面，严格地满足了薄带连铸的可浇铸性。
本发明所述的薄带连铸热轧钢卷轧后优选采用喷雾冷却的理由：
传统连铸也采用喷雾冷却，但作用的区域和温度不同，传统连铸在铸坯出结晶器的出口扇形段区域对铸坯进行喷雾冷却，此时铸坯的温度较高，在相图上看处在高温奥氏体单相区域。在此区域进行喷雾冷却主要目的是，控制凝固末端位置，加速铸坯表面冷却，细化表面奥氏体晶粒组织，提高铸坯表面强度，改善铸坯表面质量，避免裂纹的发生。本发明在铸带经在线热轧后对超薄带钢进行喷雾冷却，温度较低，在相图上看处于高温奥氏体向铁素体转变的固态相变区域，在此区域对带钢进行喷雾冷却，通过调节喷雾冷却强度，可以有效控制固态相变后的微观组织类型，进而实现最终产品的性能要求。
本发明涉及的薄带连铸热轧钢卷优选采用卡罗塞尔同位卷取机的理由：
目前绝大多数超薄热轧钢卷的生产线都采用地下双卷取或地下三卷取方式，主 要原因是这些产线还兼顾了厚规格热轧板的生产，比如阿维迪(Avedi)公司的ESP产线的卷取采用地下三卷取方式，唐钢引进达涅利(Danieli)的FTSC产线的卷取采用地下双卷取方式。而美国纽柯(Nucor)的Castrip薄带连铸产线选用了传统的做法，也采用了地下双卷取方式。地下卷取机与卷取机之间的距离间隔一般在8-10米(典型值9.4m)，薄带连铸生产超薄热轧带钢时，带钢在空气中的冷却速度也非常快，这段间隔足以影响到卷取温度的差异，两个卷取机之间的温度偏差可达49℃，这会严重影响钢卷的性能偏差。
而本发明优选采用卡罗塞尔卷取方式，可以实现热轧钢卷的同位卷取，确保了卷取温度的同一性，进而大大提高钢卷产品性能的稳定性。目前市面上，卡罗塞尔卷取机被广泛使用在冷轧领域，其主要优点是可以实现较薄的带钢卷取，而且占地面积小，可大大缩短产线长度，但在冷轧领域由于带钢温度较低，比较容易实现。本发明提出在超薄热轧带钢卷取领域采用卡罗塞尔卷取方式，考虑了设备的耐高温事宜，实现了超薄热轧带钢的卷取。该卷取方式更先进于美国纽柯(Nucor)的Castrip薄带连铸产线的卷取方式。
本发明与已有技术的区别和改进之处：
现有的薄带连铸生产薄带材产品和工艺的专利有很多，但采用薄带连铸生产本发明涉及的高扩孔钢，还未见直接报导。
本发明区别于现有薄带连铸技术最明显的一个特征就是结晶辊的辊径及其相对应的布流方式。EUROSTRIP技术特征就是Φ1500mm大辊径结晶辊，结晶辊大、熔池钢水容量大，布流容易，结晶辊制造及运维成本高。CASTRIP技术特征就是Φ500mm小辊径结晶辊，结晶辊小、熔池钢水容量小，布流非常困难，但铸机设备制造与运维成本低。CASTRIP为解决小熔池的均匀布流问题，采用三级钢水分配布流***(中间包+过渡包+布流器)。由于采用了三级布流***，会直接导致耐材成本增加；更为主要的是，三级布流***使钢水流动的路径变长，钢水的温降也较大，为了满足熔池钢液的温度，出钢温度需要大大提高。出钢温度的提高，会导致炼钢成本增加、能耗增加以及耐材寿命缩短等问题。
本发明结晶辊直径在500-1500mm优选Φ800mm辊径的结晶辊，采用两级钢水分配布流***(中间包+布流器)。从布流器流出的钢水，沿辊面和两个端面形成不同的布流模式，且分两路流动，互不干扰。由于采用了两级布流***，相比三级 布流***，耐材成本大幅度降低；钢水流动路径的缩短，使钢水温降减小，可以降低出钢温度，相比三级布流***，出钢温度可降低30-50℃。出钢温度的降低，可有效降低炼钢成本、节约能耗以及延长耐材寿命。本发明配合优选Φ800mm辊径的结晶辊，采用两级钢水分配布流***，既实现了钢水稳定布流的要求，又实现了结构简单、操作方便、加工成本低的目标。
中国专利CN101353757采用低碳微合金成分生产抗拉强度440MPa的扩孔钢，成分中添加微量的Nb：0-0.25％和Ti：0-0.03％，由于采用600℃卷取温度，该专利采用传统连铸+传统热轧工艺来生产，对碳锰钢热轧板来讲，常存在带状组织，导致钢板扩孔率下降，同时添加多种微合金，增加炼钢成本。本发明与该专利在生产工艺上明显不同，本发明采用薄带连铸工艺来生产，可以大大缩短生产流程，而且可以避免带状组织，同时可以节约微合金用量，仅需添加少量的微合金就可以达到相同甚至更加优异的性能。
中国专利CN101928881公开了抗拉强度为590MPa级热轧高扩孔钢板及其制造工艺，该专利成分中添加微量的Nb：0-0.10％和Ti：0-0.04％，采用传统连铸+传统热轧工艺来生产，终轧后的钢板以50℃/s～100℃/s的冷却速度冷却到600～750℃，随后以5℃/s～15℃/s的冷却速度在空气中冷却3～10秒，随后钢板再次以70℃/s～150℃/s的冷却速度冷却至350～500℃并卷取，然后空冷至室温。后续冷却采用复杂的三段式冷却，卷取温度波动大，钢卷头中尾性能波动会比较大，扩孔率波动也会比较大。本发明采用薄带连铸工艺来生产，大大简化生产工艺流程，也无需采用复杂的三段式冷却，具有明显的先进性。
日本专利JP2006063394公开了一种热轧高扩孔钢，碳含量0.20～0.48％，其抗拉强度≥440MPa，其加入了Cr合金元素，但其扩孔率仅为≥70％，且热轧后还要进行640℃的退火处理。该发明碳含量设计已经到达中高碳钢的范围，较本发明的低碳设计明显偏高。日本专利JP2006305700公开的热轧高强钢板采用C-Si-Mn+Ti的成分设计，获得抗拉强度780MPa以上，扩孔率仅为68％以上。日本专利JP2003/016614公开的热轧高扩孔钢，碳含量0.02～0.10％，Si≤0.5％，其抗拉强度≥590MPa，但其加入了不少Nb、Ti、V、Cr、RE等合金元素，炼钢成本较高，以良好的表面可涂装性为主要目标。相比该专利，本发明采用简单的合金成分体系，采用薄带连铸工艺来实现高扩孔钢的性能，本发明具有简约高效的特点。
美国专利US2006096678公开的热轧钢板强度为780MPa以上，延伸率为22％以上，扩孔率为60％以上。美国专利US 4415376公开的热轧钢板屈服强度为80ksi(550MPa)以上，扩孔率为58％以上，并采用Nb、V强化。这些专利采用的生产工艺都为传统连铸+传统热轧工艺生产，与本发明生产工艺不同，且产品扩孔率较低。
本发明的主要优点：
利用薄带连铸技术生产Nb微合金化高强高扩孔钢，尤其是利用薄带连铸技术生产Nb微合金化的含锡(Sn)、铜(Cu)/含锡(Sn)、铜(Cu)的高强高扩孔钢及其方法，迄今为止尚未见报导，归纳优点如下：
1.本发明省去了板坯加热、多道次反复热轧等复杂过程，通过双辊薄带连铸+一道次在线热轧工序，生产流程更短、效率更高，产线投资成本和生产成本大幅降低。
2.本发明省去了传统高扩孔钢生产中诸多复杂的中间步骤，与传统高扩孔钢相比，生产的能耗和CO 2排放大幅度降低，是一种绿色环保的产品。
3.由于薄带连铸天然的工艺流程优势，相比传统热轧流程，薄带连铸在轧后冷却过程中很容易生成贝氏体类型的微观组织，很容易使生产出来的产品具备优良的扩孔性能。
4.本发明采用薄带连铸工艺生产高扩孔钢，铸带厚度本身较薄，通过在线热轧至期望产品厚度，薄规格产品的生产不需要经过冷轧，直接供给市场使用，达到薄规格热轧板供货的目的，可以显著提高板带材的性价比。
5.本发明冶炼通过电炉炼钢，冶炼的原料可以实现真正意义上的100％全废钢冶炼，无需进行预筛选，大大降低原料成本；冶炼通过转炉炼钢，废钢按占冶炼原料20％以上的比例加入转炉，且无需预筛选，最大限度提高转炉废钢比，大大降低冶炼成本和能耗。
6.本发明利用含Cu、Sn的废钢，对钢中的Cu、Sn做到“化害为利”，实现对现有废钢或低品质劣质矿资源(高锡矿、高铜矿)的充分利用，促进废钢的循环利用，降低生产成本，实现钢铁业可持续发展。
7.本发明采用轧后带钢气雾化冷却方式，可以避免传统喷淋或者层流冷却带来的问题，使带钢表面温度均匀下降，提高带钢温度均匀性，从而达到均匀化内部微 观组织的效果；同时冷却均匀，可以提高带钢的板形质量和性能稳定性；有效减少带钢表面的氧化皮厚度。
8.传统工艺板坯冷却过程中发生合金元素析出，板坯再加热时往往会由于合金元素回溶不充分而降低合金元素利用率。本发明薄带连铸工艺中，高温铸带直接热轧，所添加的合金元素主要以固溶态存在，可提高合金利用率。
9.本发明卷取选用卡罗塞尔卷取机，有效缩短产线长度；同时同位卷取可以大大提高卷取温度的控制精度，提高产品性能的稳定性。
附图说明
图1为双辊薄带连铸工艺的工艺布置示意图；
图2为Cu对界面热流的影响示意图；
图3为Sn含量与平均热流密度的关系示意图；
图4为Sn含量与铸带表面粗糙度的关系示意图；
图5为MnO-SiO 2-Al 2O 3三元相图(阴影区域：低熔点区)；
图6为本发明实施例钢的微观组织照片。
具体实施方式
下面用实施例和附图对本发明作进一步阐述，但这些实施例绝非对本发明有任何限制。本领域技术人员在本说明书的启示下对本发明实施中所作的任何变动都将落在本发明权利要求保护范围内。
参见图1，将符合本发明化学成分设计的钢水经大包1，通过大包长水口2、中间包3、浸入式水口4以及布流器5直接浇注在一个由两个相对转动并能够快速冷却的结晶辊8a、8b和侧封板装置6a、6b围成的熔池7中，钢水在结晶辊8a、8b旋转的周向表面凝固，进而形成凝固壳并逐渐生长随后在两结晶辊辊缝隙最小处(nip点)形成1.5-3mm厚的铸带11，本实施例所述的结晶辊直径在500-1500mm之间，内部通水冷却；根据铸带厚度不同，铸机的浇铸速度范围介于60-150m/min。
铸带11出结晶辊8a、8b后，铸带温度在1420-1480℃，直接进入到下密闭室10内，下密闭室10通非氧化性气体保护铸带，实现对铸带的防氧化保护，防氧化保护的气氛可以是N 2，也可以是Ar，也可以是其他非氧化性气体，比如干冰升华 得到的CO 2气体等，下密闭室10内的氧浓度控制在＜5％，下密闭室10对铸带11的防氧化保护到轧机13入口；下密闭室10出口铸带的温度在1150-1300℃；然后通过摆动导板9、夹送辊12、辊道15将铸带送至轧机13，热轧后形成0.8-2.5mm的热轧带钢，轧制后的带钢使用气雾化快速冷却装置14以气雾化冷却方式进行冷却，改善带钢温度均匀性。经飞剪装置16切头之后，切头沿着飞剪导板17掉入飞剪坑18中，切头后的热轧带钢进入卷取机19进行卷取。将钢卷从卷取机上取下后，自然冷却至室温。最后生产的钢卷可以作为热轧板直接使用，也可以经过酸洗-平整后使用。
本发明实施例化学成分如表1所示，其成分余量为Fe和其他不可避免杂质。经本发明的制造方法，工艺参数见表2，最终获得产品的性能见表3。扩孔率的测定方法按照国际标准ISO16630：2009执行。
综上所述，本发明利用薄带连铸工艺技术，按设计的钢种成分制造的高扩孔钢，屈服强度≥440MPa，抗拉强度≥590MPa，延伸率≥19％，扩孔率≥100％。
表1：实施例钢的化学成分(wt.％)
  C Si Mn P S N O Als Nb Ca
实施例1 0.04 0.27 1.35 0.008 0.004 0.0064 0.0093 0.0009 0.06 0.003
实施例2 0.05 0.20 0.90 0.013 0.003 0.0068 0.0110 0.0006 0.05 0.004
实施例3 0.02 0.38 1.28 0.015 0.004 0.0048 0.0150 0.0004 0.03 0.005
实施例4 0.01 0.22 1.26 0.013 0.005 0.0067 0.0130 0.0008 0.03 0.004
实施例5 0.03 0.41 0.85 0.009 0.002 0.0062 0.0120 0.0007 0.01 0.003
实施例6 0.04 0.45 0.80 0.012 0.002 0.0046 0.0070 0.0008 0.04 0.001
实施例7 0.02 0.28 0.95 0.015 0.003 0.0040 0.0100 0.0005 0.06 0.002
实施例8 0.05 0.37 1.30 0.014 0.005 0.0080 0.0085 0.0006 0.05 0.004
实施例9 0.04 0.36 0.84 0.018 0.003 0.0078 0.0200 0.0003 0.04 0.004
实施例10 0.02 0.45 0.90 0.020 0.001 0.0065 0.0125 0.0004 0.07 0.004
实施例11 0.02 0.60 0.85 0.010 0.002 0.0080 0.0090 0.0009 0.04 0.003
实施例12 0.03 0.59 1.50 0.012 0.005 0.0075 0.0118 0.0003 0.06 0.001
实施例13 0.05 0.45 1.37 0.018 0.004 0.0045 0.0132 0.0006 0.08 0.002
实施例14 0.02 0.28 1.40 0.017 0.003 0.0064 0.0075 0.0005 0.03 0.003
表2：实施例的工艺参数
Figure PCTCN2020115964-appb-000001
表3：实施例钢的产品性能
Figure PCTCN2020115964-appb-000002
Figure PCTCN2020115964-appb-000003
本发明基于废钢的实施例化学成分如表4所示，其成分余量为Fe和其他不可避免杂质。经本发明的制造方法，工艺参数见表5，最终获得产品的性能见表6。
综上所述，利用薄带连铸工艺技术按本发明提供的钢种成分设计范围制造的高扩孔钢，屈服强度≥440MPa，抗拉强度≥590MPa，延伸率≥19％，扩孔率≥100％。
表4：实施例钢的化学成分(wt.％)
  C Si Mn P S N O Als Nb Cu Sn Ca
实施例15 0.03 0.26 1.37 0.008 0.004 0.0054 0.0093 0.0009 0.03 0.35 0.022 0.005
实施例16 0.01 0.20 0.92 0.013 0.003 0.0071 0.0110 0.0006 0.04 0.16 0.005 0.004
实施例17 0.04 0.35 1.28 0.015 0.004 0.0068 0.0150 0.0004 0.06 0.10   0.003
实施例18 0.05 0.28 1.26 0.013 0.005 0.0067 0.0130 0.0008 0.04 0.56 0.040 0.003
实施例19 0.04 0.45 0.85 0.009 0.002 0.0052 0.0120 0.0007 0.01 0.44 0.014 0.005
实施例20 0.05 0.41 0.80 0.012 0.002 0.0046 0.0070 0.0008 0.03   0.023 0.001
实施例21 0.03 0.29 0.95 0.015 0.003 0.0040 0.0100 0.0005 0.07 0.38 0.035 0.002
实施例22 0.02 0.38 1.30 0.014 0.005 0.0080 0.0085 0.0006 0.05 0.60 0.015 0.005
实施例23 0.04 0.33 0.85 0.018 0.003 0.0078 0.0200 0.0003 0.03 0.37   0.002
实施例24 0.05 0.43 0.90 0.020 0.001 0.0065 0.0125 0.0004 0.06 0.53 0.016 0.004
实施例25 0.02 0.60 0.85 0.010 0.002 0.0080 0.0090 0.0009 0.03   0.038 0.005
实施例26 0.05 0.57 1.50 0.012 0.005 0.0055 0.0118 0.0003 0.08 0.35 0.013 0.001
实施例27 0.03 0.46 1.38 0.018 0.004 0.0045 0.0132 0.0006 0.07   0.036 0.004
实施例28 0.02 0.27 1.40 0.017 0.003 0.0074 0.0075 0.0005 0.04 0.27 0.027 0.003
表5：实施例钢的工艺参数
Figure PCTCN2020115964-appb-000004
表6：实施例钢的产品性能
Figure PCTCN2020115964-appb-000005
Figure PCTCN2020115964-appb-000006

Claims (13)

  1. 一种Nb微合金化高强高扩孔钢，其化学成分重量百分比为：C：0.01-0.05％，Si：0.2-0.6％，Mn：0.8-1.5％，P≤0.02％，S≤0.005％，N≤0.008％，Als：＜0.001％，Ca≤0.0050％，Nb：0.01-0.08％，任选的Cu 0.1-0.6％和Sn 0.005-0.04％中的一种或两种，其中，Mn/S＞250，总氧[O] T：0.007～0.020％，余量为Fe和不可避免杂质。
  2. 如权利要求1所述的Nb微合金化高强高扩孔钢，其特征在于，所述Nb微合金化高强高扩孔钢的化学成分重量百分比为：C：0.01-0.05％，Si：0.2-0.6％，Mn：0.8-1.5％，P≤0.02％，S≤0.005％，N≤0.008％，Als：＜0.001％，Ca≤0.0050％，Nb：0.01-0.08％，Mn/S＞250，总氧[O] T：0.007～0.020％，余量为Fe和不可避免杂质，且满足：含有Cu 0.1-0.6％和Sn 0.005-0.04％中的一种或两种。
  3. 如权利要求1或2所述的Nb微合金化高强高扩孔钢，其特征在于，所述高扩孔钢的微观组织为铁素体+贝氏体，其中贝氏体相的比例≥15％。
  4. 如权利要求1-3中任一项所述的Nb微合金化高强高扩孔钢，其特征在于，所述高扩孔钢的≥440MPa，抗拉强度≥590MPa，延伸率≥19％，扩孔率≥100％。
  5. 如权利要求1所述的Nb微合金化高强高扩孔钢，其特征在于，所述Nb微合金化高强高扩孔钢的厚度是0.8-2.5mm，优选为1.0-1.8mm。
  6. 如权利要求1-5中任一项所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，包括如下步骤：
    1)冶炼
    按权利要求1所述化学成分进行冶炼，冶炼过程中，造渣的碱度a＝CaO/SiO 2控制在a＜1.5，优选a＜1.2，或a＝0.7～1.0；需要获得低熔点MnO-SiO 2-Al 2O 3三元夹杂物，MnO-SiO 2-Al 2O 3三元夹杂物中的MnO/SiO 2控制在0.5～2，优选在1～1.8；钢水中的自由氧[O] Free范围为：0.0005-0.005％，钢水的成分中Mn/S＞250；
    2)连铸
    连铸采用双辊薄带连铸，在两结晶辊辊缝隙最小处形成1.5-3mm厚的铸带；结晶辊直径在500-1500mm之间，优选直径为800mm；结晶辊内部通水冷却，铸机的浇铸速度为60-150m/min；连铸布流采用两级钢水分配布流***，即中间包+布流器；
    3)下密闭室保护
    铸带出结晶辊后，铸带温度在1420-1480℃，直接进入到下密闭室内，下密闭室内通非氧化性气体，下密闭室内的氧浓度控制在＜5％，下密闭室出口铸带的温度在1150-1300℃；
    4)在线热轧
    铸带在下密闭室内经夹送辊送至轧机，轧制成带钢，轧制温度为1100-1250℃，控制热轧压下率为10-50％，优选热轧压下率30-50％；热轧后钢带的厚度是0.8-2.5mm，优选厚度为1.0-1.8mm；
    5)轧后冷却
    对在线热轧后的带钢进行轧后冷却，冷却采用气雾化冷却方式，冷却速率≥50℃/s；
    6)带钢卷取
    冷却后的热轧带钢直接进行卷取成卷，卷取温度470-570℃。
  7. 如权利要求6所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，步骤1)中，冶炼采用电炉炼钢或者转炉炼钢，再进入LF炉、VD/VOD炉或RH炉精炼。
  8. 如权利要求6所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，步骤1)中，冶炼采用电炉炼钢，冶炼原料为100％全废钢，无需预筛选；或者，冶炼采用转炉炼钢，废钢按占冶炼原料20wt％以上的比例加入转炉，且无需预筛选；然后再进入LF炉、VD/VOD炉或RH炉精炼。
  9. 如权利要求6所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，步骤3)中，所述非氧化性气体为N 2、Ar或干冰升华得到的CO 2气体。
  10. 如权利要求6所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，步骤5)中，气雾化冷却的气水比为15∶1～10∶1，气压0.5～0.8MPa，水压1.0～1.5MPa。
  11. 如权利要求6所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，步骤6)中，卷取采用双卷取机形式或采用卡罗塞尔卷取形式。
  12. 如权利要求6所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，步骤5)中，冷却速率为50-75℃/s。
  13. 如权利要求6所述的Nb微合金化高强高扩孔钢的生产方法，其特征是，步骤6)中，冷却后的热轧带钢经切头剪切除质量较差的头部后，直接进行卷取成卷。
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