WO2023241665A1 - 一种具有高耐候性能的高强度高塑性热轧带钢及其制造方法 - Google Patents

一种具有高耐候性能的高强度高塑性热轧带钢及其制造方法 Download PDF

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WO2023241665A1
WO2023241665A1 PCT/CN2023/100513 CN2023100513W WO2023241665A1 WO 2023241665 A1 WO2023241665 A1 WO 2023241665A1 CN 2023100513 W CN2023100513 W CN 2023100513W WO 2023241665 A1 WO2023241665 A1 WO 2023241665A1
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hot
rolled strip
steel
temperature
rolling
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PCT/CN2023/100513
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French (fr)
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柏明卓
李自刚
杨阿娜
李丰滨
温东辉
周庆军
宋凤明
胡晓萍
刘海亭
刘生
张华�
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宝山钢铁股份有限公司
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the invention belongs to the technical field of low alloy steel production, and particularly relates to a high-strength and high-plasticity hot-rolled strip steel with high weather resistance and a manufacturing method thereof.
  • Atmospheric corrosion-resistant steel is widely used in the production of outdoor steel structures with atmospheric corrosion resistance requirements such as containers, railway vehicles, bridges, etc. With the increasing requirements for green, low-carbon and environmental protection, the application scenarios of atmospheric corrosion-resistant steel are also expanding. People use the weather resistance of atmospheric corrosion-resistant steel to make some structural profiles that serve in the atmospheric environment, such as guardrails, mast towers, support brackets, photovoltaic brackets, etc. These steel structures can be used directly bare or with a light coating on the surface, which can achieve very high weather resistance and thus can replace some traditional steel surface anti-corrosion processes, such as pre-galvanized, pre-galvanized aluminum-magnesium coating , post galvanizing, etc. The use of atmospheric corrosion-resistant steel can not only reduce energy consumption and pollution problems caused by the metal coating process, but also improve the service life of steel structures and reduce later anti-corrosion maintenance costs.
  • Chinese patent CN202011384068.6 discloses a low-alloy, high-strength and high-weather-resistant structural steel for highway guardrails, with a yield strength of about 500MPa.
  • the main design idea is to increase the P element content to 0.07 ⁇ 0.12% and the Cr element to 0.30 ⁇ 1.25% to form a P-rich and Cr-rich layer on the surface of the rust layer, thereby making the rust layer stable and dense, so as to improve the weather resistance of structural steel. performance.
  • the structure of the steel is ferrite and pearlite, with the volume content of pearlite ranging from 5 to 25%.
  • P is an impurity element in the steel. Excessive content will lead to central segregation of P and segregation at the grain boundaries, affecting the formability and toughness of the steel, and is not conducive to the processing performance and service safety of the steel. .
  • Chinese patent CN202010116991.5 discloses a high-strength weather-resistant steel, which is mainly used in railway vehicles. Its purpose is also to provide a high-strength weather-resistant steel to solve the problem that steel materials in the existing technology cannot achieve high strength and high weather resistance at the same time. question.
  • the steel mainly contains the following chemical composition: C 0.06 ⁇ 0.07%, Si 0.23 ⁇ 0.26%, Mn 1.40 ⁇ 1.50%, Ni 0.0 ⁇ 0.19%, Cr 0.0 ⁇ 0.51%, Cu 0.31 ⁇ 0.33%, Ti 0.110 ⁇ 0.12%, Nb 0.030 ⁇ 0.036%, Sb 0.0 ⁇ 0.09%. This kind of steel adopts a nearly complete ferrite structure, and the pearlite content in the steel is only less than 2%.
  • the yield strength of steel is between 636MPa and 710MPa, and the tensile strength is between 698MPa and 775MPa.
  • This invention obtains higher strength through Nb and Ti composite precipitation strengthening.
  • the disadvantage of this solution is that the Cr content in the steel is low, below 0.51%, so its weather resistance is still at the level of ordinary weathering steel.
  • the invention also mentions the use of Sb to improve corrosion resistance, but Sb is a harmful element in steel and will deteriorate the performance of steel, especially the low-temperature toughness. At the same time, due to the low melting point of Sb, the yield after smelting is difficult to control. Therefore, this solution is difficult to apply in practice.
  • Chinese patent CN201810154871.7 patent discloses a high-strength weathering steel with a yield strength of 550MPa, which is strengthened by 0.05 to 0.09wt% Ti, and its Cr: 0.30 to 0.60wt%.
  • the invention purpose of these patents is to obtain high-strength weather-resistant steel, and the weather resistance of the product is still based on general design.
  • the object of the present invention is to provide a high-strength and high-plasticity hot-rolled strip steel with high weather resistance and a manufacturing method thereof.
  • the hot-rolled strip steel of the present invention significantly improves the atmospheric corrosion resistance of the material on the basis of high-strength weather-resistant steel. Its corrosion rate is less than 30% of that of Q355B ordinary structural steel, and is three times its weather resistance. Compared with ordinary weather-resistant steel The weather resistance of steel (such as Q450NQR1 steel) has also been more than doubled.
  • the hot-rolled strip steel of the present invention can replace the post-galvanizing anti-corrosion protection, and can be used bare for guardrails, mast towers, photovoltaics and other supporting bracket structural parts without surface coating.
  • the present invention achieves the above technical objectives by optimizing the chemical composition of hot-rolled strip steel.
  • the high Cr content of 1.5 to 4.5% is used to promote the formation of a uniform and dense rust layer on the surface of hot-rolled strip steel during use, and Cr can be rapidly enriched in the thinner rust layer, at the interface between the rust layer and the matrix. Cr concentration can be above 12% to significantly increase corrosion potential and electrochemical impedance, medium Corrosion continues to occur, thereby achieving ultra-high atmospheric corrosion resistance.
  • the hot-rolled strip steel of the present invention contains the following components in mass percentage: C: 0.04 ⁇ 0.09%, Si ⁇ 0.50%, Mn: 0.10 ⁇ 1.50%, P ⁇ 0.03%, S ⁇ 0.01%, Al ⁇ 0.60%, Cr: 1.5 ⁇ 4.5%, Cu: 0.10 ⁇ 0.60%, Ti: 0.05 ⁇ 0.18%, Ni ⁇ 0.30%, Nb ⁇ 0.06%, N ⁇ 0.008%, the balance is Fe and other unavoidable impurities , and at the same time satisfy: 2Mn+Cr ⁇ 6%, the mass percentage of the corresponding element in the hot-rolled strip is entered into the calculation. For example, when the Mn content in steel is 0.10%, substitute the value 0.10% for calculation.
  • the composition of the hot-rolled strip is Ti-3N ⁇ 0.04%. In this case, the strengthening effect of Ti can be fully exerted.
  • the mass percentage of the corresponding element in the hot-rolled strip is used for calculation.
  • the composition of the hot-rolled strip is Si+2Ni ⁇ 0.10%.
  • the influence of copper brittleness can be reduced.
  • the mass percentage of the corresponding element in the hot-rolled strip is used for calculation.
  • the hot-rolled strip steel of the present invention also contains at least one selected from the following: Ca ⁇ 0.015%, Mg ⁇ 0.015%, B ⁇ 0.003%, Mo ⁇ 0.30%, V ⁇ 0.15%, Re ⁇ 0.015% .
  • the microstructure of the hot-rolled strip steel of the present invention is polygonal ferrite + a small amount of pearlite + bainite mainly in the form of MA.
  • the grain size of the ferrite is above level 8, preferably above level 9. .
  • the polygonal ferrite content is 70 to 90%, and the pearlite content is 5 to 30%.
  • the pearlite content in the steel of the present invention is more preferably 15% or more and 25% or less.
  • the bainite content is 5-15%, of which the MA structure accounts for approximately 20-70% of the bainite, preferably 30-70%.
  • the content of the steel microstructure in the present invention refers to the volume fraction.
  • the hot-rolled strip steel of the present invention has very high weather resistance.
  • the corrosion rate of the steel is less than 30% of the corrosion rate of ordinary structural steel Q355B, that is, the weather resistance is more than 3 times that of Q355B steel, and it is the same as ordinary weather-resistant steel (such as Q450NQR1 Steel) more than 2 times.
  • the corrosion rate of the hot-rolled strip steel of the present invention decreases rapidly as the corrosion depth increases, and the simulated corrosion test results show that its corrosion depth in 25 years is ⁇ 0.1mm.
  • the C is an effective strengthening element in steel. In addition to solid solution strengthening, it can also form nanoscale second phase precipitation particles with micro-alloying elements such as Ti and Nb to play the role of precipitation strengthening and refinement of the structure.
  • the C content in the present invention is ⁇ 0.04%; however, too much C will form too many carbides or bainitic hard phase structures in the steel, which will not only reduce the toughness and formability of the material, but also It will form a galvanic cell, which will reduce the corrosion resistance of the steel and also reduce the welding performance of the steel. Therefore, the C content is ⁇ 0.09%.
  • the content of elements in the hot-rolled strip steel of the present invention refers to the mass fraction.
  • Si is a commonly used deoxidizing element in steel and also has a solid solution strengthening effect on steel. It can also improve the corrosion resistance of the material and play a certain role in reducing copper brittleness. However, higher Si content will cause red iron scale defects on the surface of hot-rolled strip steel, seriously affecting the surface quality of the strip steel. It will also reduce the welding performance of the material and cause the toughness of the welding heat-affected zone to deteriorate. Therefore, the Si content in the present invention is ⁇ 0.50%, preferably 0.06% ⁇ Si ⁇ 0.50%.
  • Mn is an important strengthening and toughening element in steel. It has the effect of solid solution strengthening. It can also reduce the supercooled austenite transformation temperature and ferrite phase transformation temperature, which is beneficial to the refinement of the structure, thereby improving the strength and toughness of the material. However, excessive Mn content will significantly inhibit the transformation of ferrite into bainite, resulting in a decrease in the plasticity and cold forming properties of the material. Therefore, the Mn content in the present invention is 0.1-1.5%, preferably 0.3-1.2%, and more preferably 0.5-1.0%.
  • Cr is an important element to improve the weather resistance of steel plates.
  • the main mechanism to improve the weather resistance of weathering steel is as follows.
  • the corrosion potential of the matrix is increased by adding corrosion-resistant elements to reduce the corrosion rate by increasing the electrochemical impedance.
  • Cr physically blocks the corrosive medium by promoting the formation of a dense rust layer on the surface, changing the corrosion environment at the substrate position, and gradually slows down the corrosion as the corrosion depth increases.
  • the Cr content in steel exceeds 1.5%, a uniform and dense rust layer can be formed on the surface of the substrate under the combined action of Cr, Cu and other elements.
  • electrochemical corrosive media such as water and improving electrochemical impedance.
  • Cr due to the high concentration of Cr, with the rust As the layer thickness increases, Cr is enriched at the front edge of the rust layer, causing the Cr concentration in the rust layer to increase rapidly. Before the thickness of the rust layer reaches 0.1mm, the Cr concentration at the interface between the rust layer and the substrate can reach more than 12%.
  • the hot-rolled strip steel of the present invention will produce a stainless steel-like effect, that is, the corrosion potential at the interface between the rust layer and the substrate at the corrosion front is very high, coupled with the barrier of the dense rust layer and low corrosive medium As a result, the electrochemical impedance on the strip surface is very high, and the corrosion reaction is basically interrupted.
  • the Cr content in the matrix should not be too high. As the Cr content in the matrix increases, the corrosion potential of the matrix will increase. For example, when the Cr content exceeds 4.5%, it will lead to enhanced selective corrosion in the early stages of the formation of the rust layer, and the uniformity of the thickness of the rust layer.
  • the corrosion resistance deteriorates, that is, the Cr concentration at the front of the rust layer and the corrosive environment cause the chemical resistance to become uneven, increasing the corrosion potential difference at the corrosion front, exacerbating the galvanic cell effect, thereby worsening the corrosion inhibition effect.
  • the Cr content in the present invention is between 1.5% and 4.5%, preferably between 2.27% and 3.68%.
  • Figure 1 shows the effect of the Cr content in the steel on the relative corrosion rate of the hot-rolled strip in the weekly immersion test
  • Figure 2 shows the long-term corrosion depth of the hot-rolled strip with the Cr content in the steel. Impact.
  • the corrosion rate of the hot-rolled strip steel of the present invention decays rapidly with time, and the simulated corrosion depth value in 25 years can be below 0.1mm, for example, when the Cr content in the steel is 2%.
  • the Cr content in steel is 5%
  • the simulated corrosion depth in 25 years is estimated to be about 0.12mm, and the corrosion resistance decreases.
  • Cr is also an element that increases the hardenability of steel. A higher Cr content can cause the steel to form air-cooled bainite or air-cooled martensite at a lower air-cooling rate, thereby significantly improving the tensile strength of the material and reducing the material's hardenability.
  • the yield-to-strength ratio is beneficial to reducing forming springback and improving the stability of forming dimensions.
  • the present invention combines the design with higher Cr content, fully utilizes this function of Cr, and cooperates with the strengthening effect of C, Mn and other elements to further improve the strength of the steel.
  • Cu is also one of the important corrosion resistance elements, and the effect is more obvious when added together with Cr.
  • Cu can promote the formation of a dense rust layer on the steel surface. Adding more than 0.10% Cu can significantly improve the atmospheric corrosion resistance of steel.
  • Cu is a metal with a low melting point. Strip steel containing more Cu will easily form copper brittle cracks and warping defects on the surface during the hot rolling process, deteriorating the surface quality of the steel.
  • Cu is also a precious element.
  • the Cu content in the present invention is 0.10-0.60%, preferably 0.2-0.5%, and more preferably 0.25-0.38%.
  • P is often added as a corrosion-resistant element in traditional atmospheric corrosion-resistant steel, which can promote the formation of a protective rust layer on the surface, thereby effectively improving the atmospheric corrosion resistance of steel.
  • P is also a harmful impurity element in steel and is prone to segregation in the thickness center during continuous casting of steel billets.
  • P is prone to segregation at grain boundaries, reducing the grain boundary binding energy, thereby reducing the toughness and plasticity of steel.
  • P is also very detrimental to the welding performance of steel. Therefore, the present invention does not use atmospheric corrosion-resistant steel with high P content, but reduces the P content in the steel as much as possible, requiring P content ⁇ 0.03%.
  • S is a common harmful impurity element in steel, which has adverse effects on low-temperature toughness, welding performance, cold forming performance, etc.
  • the S content in the steel of the present invention is ⁇ 0.01%.
  • Al is a very effective deoxidizing element. At the same time, Al is conducive to refining grains and improving the strength and toughness of steel. At the same time, Al can also promote the generation of ferrite, inhibit the transformation of pearlite, and is conducive to the transformation of the dual-phase structure of ferrite and bainite. However, a higher Al content is not conducive to smooth pouring and can easily block the nozzle. Therefore, the present invention requires that the Al content in the steel is ⁇ 0.60%, preferably 0.011-0.56%, and more preferably 0.02-0.30%.
  • Ti is a strong carbonitride-forming element that can precipitate in the form of extremely fine TiC or Ti(C,N) second phase particles, thereby significantly improving the strength of the material and is a very effective strengthening element. Only when Ti is above 0.05% will it exert a strong precipitation strengthening effect. At the same time, the precipitation of TiC significantly reduces the formation of larger-sized carbides or pearlite from free C, thereby reducing the galvanic cell effect of heterogeneous phases in the corrosion process, improving the material's ability to resist intergranular corrosion, thereby improving strength while At the same time, it is also conducive to improving the corrosion resistance of the material. However, if the content is too high, the precipitation strengthening effect of Ti will gradually weaken and begin to affect the low-temperature toughness of the steel. Therefore, the Ti content in the present invention is 0.05-0.18%, preferably 0.072-0.160%.
  • Ni can improve the corrosion resistance of steel and also improve the surface copper brittleness problem caused by Cu.
  • Ni is very expensive, and excessive addition will significantly increase the alloy cost of the material. Therefore, in the present invention, the Ni content ⁇ 0.30% is preferably above 0.1% and below 0.25%, and more preferably below 0.20%.
  • Nb is also a strong nitrogen carbide-forming element and can also form NbC and Nb(CN) carbide particles to precipitate as the second phase, resulting in precipitation strengthening.
  • the cost of Nb is much higher than that of Ti, so it is uneconomical to add Nb to improve strength compared with Ti.
  • excessive Nb content will also affect the quality of the slab during the cooling process of strip casting, causing defects such as surface cracks and corner cracks. Therefore, the Nb content in the present invention is ⁇ 0.06%.
  • N is an impurity element in steel.
  • N is easily combined with Ti during the smelting process. Precipitation forms coarse TiN inclusions.
  • TiN inclusions will damage the toughness of steel, and on the other hand, they also reduce the effective Ti content in the steel. Therefore, the N content in the present invention is ⁇ 0.008%.
  • composition design of the hot-rolled strip steel of the present invention also needs to satisfy: 2Mn+Cr ⁇ 6%. Both Mn and Cr will shift the C curve of ferrite transformation to the right, significantly inhibiting the transformation of ferrite and making the transformation time longer. According to thermal simulation and CCT calculation, when 2Mn+Cr>6%, it is difficult for the strip steel to undergo sufficient ferrite transformation during laminar cooling and post-cooling cooling, and sufficient ferrite transformation has an adverse effect on the steel's properties. Performance is very important. First of all, the amount of ferrite transformation will directly affect the elongation performance of the steel of the present invention, that is, the plasticity of the material.
  • the composition of the hot-rolled strip of the present invention satisfies Ti-3N ⁇ 0.04%, and the mass percentage of the corresponding element in the hot-rolled strip is included in the calculation. If N in steel preferentially combines with Ti to form TiN inclusions, the contribution of Ti to the strength of the steel will be weakened.
  • Ti-3N is defined as the effective Ti content. When Ti-3N ⁇ 0.04%, it can be ensured that sufficient Ti is combined with C to form TiC precipitated particles to play a precipitation strengthening role.
  • the composition of the hot-rolled strip steel of the present invention satisfies Si+2Ni ⁇ 0.10%, and the mass percentage of the corresponding element in the hot-rolled strip steel is taken into account when calculating.
  • Cu easily forms copper brittle defects on the surface of the strip, while Si and Ni both have the effect of improving copper brittle defects, and they can complement each other. Compared with Si, Ni has a better effect on improving copper embrittlement, but the cost is also higher.
  • the hot-rolled strip steel of the present invention may contain one or both of Si and Ni elements.
  • Si ⁇ 0.50%, Ni ⁇ 0.30% and Si+2Ni ⁇ At 0.10% the complementary relationship between the two can be used to adjust the balance between material design economy, material surface quality and copper brittleness problem, so that the copper brittleness problem can be economically controlled.
  • Si+2Ni ⁇ 0.30% Preferably, Si+2Ni ⁇ 0.30%.
  • Another aspect of the present invention provides a method for manufacturing the above-mentioned hot-rolled strip, which includes the following steps:
  • the slab comes out of the heating furnace and is set to width, it is rough rolled.
  • high-pressure water above 15MPa preferably above 20MPa, is used for descaling.
  • the outlet temperature of rough rolling is below 1080°C; the rough-rolled strip is subjected to finish rolling.
  • the multi-stand continuous rolling process is adopted to control the final rolling temperature at 820 ⁇ 880°C;
  • Laminar flow cooling is used for cooling, and the coiling temperature of the strip is 630-680°C; after laminar flow cooling, the steel coils are slowly cooled through hot coil stacking, slow cooling walls or insulation covers, and the steel coils are cooled at 530°C
  • the above cooling rate is ⁇ 1°C/min.
  • the rough rolling outlet temperature is 1040-1080°C.
  • the final rolling temperature is 860-880°C; if the thickness of the finished hot-rolled strip is 3-5 mm, the final rolling temperature is 840°C. ⁇ 860°C; if the thickness of the finished hot-rolled strip is more than 5mm, the final rolling temperature is 820 ⁇ 840°C.
  • the manufacturing process of the present invention optimizes the temperature rise curve of the steel billet during the heating process.
  • the surface temperature of the slab quickly crosses the sensitive temperature range of 1050 to 1150°C that produces copper brittleness, and the heating time is controlled within 15 minutes to allow melting.
  • the Cu can be absorbed by the newly formed oxide scale on the surface of the substrate, preventing Cu from penetrating into the steel billet substrate, thereby inhibiting the occurrence of copper brittleness.
  • the temperature is maintained at 1230-1290°C, and the soaking time is controlled at 30-90 minutes to ensure full solid solution of Ti.
  • the temperature of coming out of the oven is also 1230 ⁇ 1290°C.
  • the technical solution of the present invention overcomes the contradiction between Cu-containing steel that needs low-temperature heating to suppress copper embrittlement and high-Ti steel that needs high-temperature heating to fully dissolve into solid solution. It can not only ensure that Ti is fully solid-solubilized, but also provide conditions for the subsequent precipitation and strengthening of Ti; it can also reduce copper The occurrence of brittleness improves the surface quality of hot-rolled strip steel.
  • the slab comes out of the heating furnace and has a fixed width, it enters rough rolling.
  • a high enough descaling pressure should be ensured to obtain a good descaling effect.
  • high-pressure water above 15MPa preferably above 20MPa
  • the removal of oxide scale has a significant improvement effect in reducing the copper embrittlement problem caused by the enriched Cu in the surface layer. Therefore, in the manufacturing process of the present invention, the pressure of the descaling high-pressure water is required to be above 15 MPa.
  • the rough rolling exit temperature should be below 1080°C, preferably 1040 to 1080°C.
  • the strip after rough rolling enters finishing rolling, which adopts a multi-stand continuous rolling process.
  • the final rolling temperature is controlled between 820 and 880°C.
  • the steel of the present invention needs to adopt a lower final rolling temperature.
  • the final rolling temperature is 860 ⁇ 880°C; when the thickness of the finished hot-rolled strip is 3 ⁇ 5mm
  • the final rolling temperature is 820-840°C.
  • the purpose of controlling the final rolling temperature is to increase the rolling deformation energy and promote the transformation of fine-grained ferrite after laminar cooling of the strip.
  • the precipitation of ferrite will also promote the precipitation of TiC, which can significantly improve the precipitation strengthening effect. Therefore, the use of low-temperature final rolling is very important to improve the plasticity and strength of the hot-banded steel strip of the present invention.
  • the steel of the present invention Since the steel of the present invention has high Mn and Cr contents, it will inhibit the ferrite phase transformation, and the ferrite phase transformation plays an important role in improving the plasticity and formability of the material, and promoting the precipitation of TiC to improve the strength. Therefore, in addition to obtaining more phase changes by lowering the final rolling temperature during the hot rolling stage In addition to the force, after laminar cooling, the coiling temperature must be increased to give the ferrite a higher phase transformation temperature and a more sufficient phase transformation time. Therefore, in the present invention, the strip coiling temperature is 630-680°C.
  • the strip steel is also required to ensure the slow cooling of the steel coils through hot coil stacking, slow cooling walls or thermal insulation covers, so that the cooling rate of the steel coils above 530°C is ⁇ 1°C/min to ensure that the ferrite
  • the full occurrence of phase change and the full precipitation of TiC particles improve the plasticity and strength of the material.
  • the higher coiling temperature during the strip manufacturing process is also beneficial to the strip shape control.
  • the hot-rolled strip steel of the present invention has a structure mainly composed of polygonal ferrite, in which the grain size of the ferrite is above grade 8, or even up to grade 11.
  • the polygonal ferrite content in the microstructure of hot-rolled strip steel is 70 to 90%.
  • a large number of TiC precipitates with a diameter of less than 10 nm are dispersed in the ferrite, which significantly contributes to the high strength of the material.
  • hot-rolled strip steel also contains 5-30%, preferably 15-30%, more preferably 15-25% pearlite, and a small amount of bainite mainly in MA form of 5-15%, which can further improve the strength of the steel. .
  • the bainite contains 30-70% MA structure.
  • the large amount of TiC nano-precipitated phase in steel gives the material high strength.
  • the yield strength is ⁇ 600MPa and the tensile strength is ⁇ 700MPa.
  • the hot-rolled strip steel of the present invention has high plasticity due to its mainly ferrite structure.
  • the present invention utilizes high Cr addition of 1.5 to 4.5%, and simultaneously adds Cu and other components to promote the formation of a uniform and dense rust layer on the surface of the hot-rolled strip steel of the present invention when used in the atmosphere, and can achieve a thin layer of ⁇ 0.1mm.
  • the interface between the rust layer and the strip steel matrix quickly enriches the Cr concentration to more than 12%.
  • the hot-rolled strip steel has ultra-high atmospheric corrosion resistance (the service environment of the hot-rolled strip steel of the present invention is the C1-C3 corrosion level specified in GB/T 19292.1-2018). Weekly immersion tests were conducted on Q355B steel and the hot-rolled strip steel of the present invention respectively. The results show that the corrosion rate of the hot-rolled strip steel of the present invention is less than 30% of that of Q355B steel, that is, its weather resistance is 3 times that of ordinary structural steel Q355B.
  • the hot-rolled strip steel of the present invention utilizes the relatively economical precipitation strengthening of Ti, controls the content of Mn and Cr in the steel and requires 2Mn+Cr ⁇ 6%, so that the strip steel can obtain good corrosion resistance. Furthermore, with a specific high-temperature coiling and slow cooling process after coiling, the hot-rolled strip steel has a ferrite volume content of 70-90%, preferably 75-90%, and a large amount of nano-TiC distributed in the ferrite. The precipitated phase gives the material high strength and high plasticity.
  • the yield strength is ⁇ 600MPa
  • the tensile strength is ⁇ 700MPa
  • the elongation at break is ⁇ 24%
  • the manufacturing method of the present invention overcomes the contradiction between Cu-containing steel that requires low-temperature heating to suppress copper brittleness and high-Ti steel that requires high-temperature heating to fully dissolve the solid solution.
  • high-temperature rapid firing is used to make the slab
  • the surface temperature can quickly cross the copper brittleness sensitive range of 1050 to 1150°C, and then by controlling the heating time and temperature of the soaking section, the sufficient high-temperature solid solution of Ti can be ensured, providing conditions for the subsequent precipitation and strengthening of Ti; on the other hand, combined with rough rolling High-pressure water descaling with a section above 15MPa reduces the occurrence of steel and copper brittleness and ensures the surface quality of the strip.
  • Figure 1 shows the effect of Cr content in the experimental hot-rolled strip on the relative corrosion rate in the weekly immersion test
  • Figure 2 shows the corrosion depth of experimental hot-rolled strips with different Cr contents in long-term atmospheric exposure
  • Figure 3 is a microstructure photograph of the hot-rolled strip steel in Example 2 of the present invention.
  • Figure 4 is a microstructure photograph of the hot-rolled strip steel in Example 6 of the present invention.
  • Figure 5 is a thin film TEM dark field photograph of the hot-banded steel strip in Example 2 of the present invention, in which the bright parts are precipitated phases with a diameter of less than 10 nm.
  • the main processes for manufacturing the hot-rolled strip steel of Examples 1-8 of the present invention are as follows:
  • the steel billet is heated by the heating furnace.
  • the heating process of the steel billet is controlled so that the heating time when the temperature of the slab crosses the 1050-1150°C range is within 15 minutes; the furnace temperature in the soaking section is maintained at 1230-1290°C, the soaking time is controlled at 30-90 minutes, and the furnace temperature is It is 1230 ⁇ 1290°C.
  • the steel billet is subjected to rough rolling and descaling, side pressing to determine width, rough rolling, flying shearing, finishing rolling and descaling, and finish rolling to obtain strip steel.
  • rough rolling stage high-pressure water above 15MPa is used for descaling, and the rough rolling outlet temperature is below 1080°C; the rough rolled strip is subjected to finish rolling.
  • the multi-stand continuous rolling process is used for the finish rolling, and the final rolling temperature is controlled between 820°C and 820°C. 880°C.
  • Example 2 The specific process parameters used in the production process of Examples 1-8 are listed in Table 2. After obtaining the hot-rolled strip according to the above steps 1-4, perform performance inspection on the hot-rolled strip. The specific strip properties are shown in Table 3. Table 3 also records the microstructure of Examples 1-8.
  • the corrosion resistance of the hot-rolled strip steel of Examples 1-8 and the steel of Comparative Examples 1-2 was tested in accordance with TB/T2375 "Cyclic Wetting Corrosion Test Method of Weathering Steel for Railways".
  • the "relative corrosion rate” in Table 3 is the corresponding Corrosion rates of hot-rolled strip steel and Q450NQR1 steel relative to Q345B steel in Examples 1-8.
  • the ferrite grain size in Examples 1-8 was measured in accordance with GB/T 6394-2017 "Metal Average Grain Size Determination Method".
  • Figures 3-4 respectively show the metallographic structures of hot-rolled strip steel in Examples 2 and 6 of the present invention. From Figure 3-4 It can be seen that the hot-rolled strip steel in Examples 2 and 6 of the present invention has a microstructure dominated by ferrite, in which the proportion of ferrite is more than 80%, and there is also a small amount of atypical pearlite structure in the steel. There is actually some bainite mixed in the pearlite structure, and the MA structure can be seen in the bainite.
  • Figure 5 is the dark field morphology of the hot-rolled strip steel in Example 2 observed by transmission electron microscopy (TEM).
  • TEM transmission electron microscopy
  • the existing Q355B steel and Q450NQR1 steel are used as comparative examples. It can be seen from the results in Table 3 that the hot-rolled strip steel of the present invention shows excellent corrosion resistance.

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Abstract

本发明公开了一种热轧带钢及其制造方法。其中，热轧带钢的成分以质量百分比计为：C 0.04～0.09％，Si≤0.50％，Mn 0.10～1.50％，P≤0.03％，S≤0.01％，Al≤0.60％，Cr 1.5～4.5％，Cu 0.10～0.60％，Ti 0.05～0.18％，Ni≤0.30％，Nb≤0.06％，N≤0.008％，余量为Fe和不可避免杂质，且，2Mn+Cr≤6％，计算时代入相应元素在所述热轧带钢中的质量百分比。本发明显示了优异的耐大气腐蚀性能和力学性能。

Description

一种具有高耐候性能的高强度高塑性热轧带钢及其制造方法 技术领域
本发明属低合金钢的生产技术领域，特别涉及一种高耐候性能的高强度高塑性热轧带钢及其制造方法。
背景技术
耐大气腐蚀钢广泛应用于集装箱、铁道车辆、桥梁等户外有耐大气腐蚀要求的钢结构生产中。随着对绿色、低碳、环保的要求的提高，耐大气腐蚀钢的应用场景也在拓展。人们利用耐大气腐蚀钢的耐候性能，制作一些在大气环境中服役的结构型材件，如护栏、桅杆塔架、支撑支架、光伏支架等等。这些钢结构可以直接裸用，或者对其表面进行轻涂装后使用，可以实现非常高的耐候性能，从而能够替代一些传统钢的表面防腐工艺，如预镀锌、预镀锌铝镁涂层、后镀锌等。采用耐大气腐蚀钢不但能减少金属涂镀工艺带来的能耗、污染问题，也能提高钢结构的服役寿命、减少后期防腐维护成本。
现有技术中有涉及高强度耐候钢的方案，例如：
中国专利CN202011384068.6公开了一种公路护栏用低合金高强高耐候结构钢，其屈服强度约500MPa。主要设计思路是：通过增加P元素含量至0.07～0.12％以及提高Cr元素至0.30～1.25％，使锈层表面形成富P和富Cr层，从而使锈层稳定致密，以提高结构钢的耐候性能。该钢的组织为铁素体和珠光体，其中珠光体的体积含量在5～25％之间。但是对于结构钢来说，P是钢中的杂质元素，含量过高会导致P的中心偏析及在晶界的偏聚，影响钢的成形性能和韧性，不利于钢的加工性能和服役安全性。
中国专利CN202010116991.5公开了一种高强度耐候钢，主要应用于铁路车辆，其目的也是提供一种高强度耐候钢，以解决现有技术中存在的钢材无法同时达到高强度与高耐候性的问题。该钢主要含有以下化学组成：C 0.06～0.07％，Si 0.23～0.26％，Mn 1.40～1.50％，Ni 0.0～0.19％， Cr 0.0～0.51％，Cu 0.31～0.33％，Ti 0.110～0.12％，Nb 0.030～0.036％，Sb 0.0～0.09％。这种钢采用近全铁素体组织，钢中珠光体的含量仅在2％以下。钢的屈服强度在636MPa～710MPa之间，抗拉强度在698MPa～775MPa之间。该发明通过Nb、Ti复合析出强化获得了较高的强度。但该方案的缺点在于钢中Cr的含量较低，在0.51％以下，因此其耐候性能仍是普通耐候钢的水平。该发明也提到采用Sb来提高耐蚀性能，但Sb是钢中有害元素，会劣化钢的性能，尤其是低温韧性；同时由于Sb的熔点较低，冶炼后的收得率很难控制，因此该方案难以实际应用。
中国专利CN201810154871.7专利公开了一种屈服强度550MPa级高强度耐候钢利用了0.05～0.09wt％的Ti强化，其Cr：0.30～0.60wt％。CN202110398903.X则公开了一种耐大气腐蚀的700MPa级高强耐候钢板，也是采用Ti：0.100％～0.140％进行强化，Cr：0.60％～1.00％。但这些专利的发明目的都是获得高强度的耐候钢，产品的耐候性还是基于一般的设计。
发明内容
本发明的目的在于提供一种高耐候性能的高强度高塑性热轧带钢及其制造方法。本发明的热轧带钢在高强度耐候钢的基础上显著提高了材料的耐大气腐蚀性能，其腐蚀速率为Q355B普通结构钢的30％以下，是其耐候性的3倍，相对普通耐候钢(例如Q450NQR1钢)的耐候性能也提高一倍以上。本发明的热轧带钢能够取代后镀锌防腐保护，可在表面无涂装的情况下裸用于护栏、桅杆塔架、光伏等支撑支架结构件。且，本发明钢具有较高的强度，屈服强度≥600MPa，抗拉强度≥700MPa，同时保留了很高的塑性，延伸率≥24％，180°D＝0.5t冷弯合格，优选180°D＝0t冷弯合格，具有极高的冷加工成型性能，可以完全对折而不开裂，能满足复杂截面型材构件的辊压成型。
本发明通过优化热轧带钢的化学成分以实现上述技术目的。
具体地，利用1.5～4.5％的高Cr含量，促使热轧带钢在使用时表面形成均匀致密锈层，且Cr能在较薄的锈层内快速富集，在锈层与基体的界面位置Cr浓度可以在12％以上，以显著提高腐蚀电位和电化学阻抗，中 断腐蚀继续发生，从而获得超高的耐大气腐蚀性能。另一方面在高Cr含量下，能获得70％体积以上的细小铁素体为主的显微组织，并在铁素体内有大量直径在10nm以下的TiC纳米析出相，从而较经济地获得具有高强度、高塑性、高耐候性以及极好的加工成形性能的热轧带钢。
具体地，本发明的热轧带钢，其包含以质量百分比计的如下成分：C：0.04～0.09％，Si≤0.50％，Mn：0.10～1.50％，P≤0.03％，S≤0.01％，Al≤0.60％，Cr：1.5～4.5％，Cu：0.10～0.60％，Ti：0.05～0.18％，Ni≤0.30％，Nb≤0.06％，N≤0.008％，余量为Fe和其它不可避免杂质，且同时满足：2Mn+Cr≤6％，计算时代入相应元素在所述热轧带钢中的质量百分比。例如，当钢中Mn的含量为0.10％时，代入数值0.10％进行计算。优选地，所述热轧带钢的成分中，Ti-3N≥0.04％，在该情况下可充分发挥Ti的强化作用，计算时代入相应元素在所述热轧带钢中的质量百分比。
优选地，所述热轧带钢的成分中，Si+2Ni≥0.10％，在该情况下可减少铜脆的影响，计算时代入相应元素在所述热轧带钢中的质量百分比。
优选地，本发明的热轧带钢还包含选自如下中的至少一种：Ca≤0.015％，Mg≤0.015％，B≤0.003％，Mo≤0.30％，V≤0.15％，Re≤0.015％。
优选地，本发明的热轧带钢的显微组织为多边形铁素体+少量珠光体+以MA形态为主的贝氏体，铁素体的晶粒度在8级以上，优选9级以上。其中，多边形铁素体含量为70～90％，珠光体含量为5～30％，本发明的钢中珠光体含量的更优选在15％以上，25％以下。贝氏体含量为5～15％，其中MA组织约占贝氏体的20-70％，优选30-70％。铁素体组织内有大量直径10nm以下的TiC纳米析出相，TiC析出相在铁素体中的含量约为0.005％-0.03％。如无特殊说明，本发明中钢显微组织的含量均指体积分数。
本发明的热轧带钢具有非常高的耐候性能，该钢的腐蚀速率为普通结构钢Q355B腐蚀速率的30％以下，即耐候性是Q355B钢的3倍以上，以及是普通耐候钢(例如Q450NQR1钢)的2倍以上。此外，本发明的热轧带钢的腐蚀速率随腐蚀深度增加而快速减小，模拟腐蚀试验结果显示其25年的腐蚀深度≤0.1mm。
本发明所述的热轧带钢的屈服强度≥600MPa，优选650-800MPa；抗 拉强度≥700MPa，优选750-850MPa；断裂延伸率≥20％，优选在24％以上，更优选在26％以上；冷弯性能满足180°D＝0.5t冷弯合格，优选180°D＝0t冷弯合格(D为弯曲直径，t为钢板厚度)；具有极好的成形性能，可以完全对折而不开裂，能够满足复杂截面型材构件的辊压成型加工。
在本发明所述的热轧带钢的成分中，各元素的作用如下：
C是钢中有效的强化元素，除固溶强化外，还能与Ti、Nb等微合金元素形成纳米级的第二相析出粒子，发挥析出强化和细化组织的作用。作为最经济的强化元素，本发明中C含量≥0.04％；但过多的C会在钢中形成过多的碳化物或者贝氏体硬相组织，不仅会降低材料的韧性和成形性能，还会形成原电池作用降低钢材的耐腐蚀性能，同时也降低钢材的焊接性能，因此，C含量≤0.09％。如无特殊说明，本发明的热轧带钢中元素的含量均指质量分数。
Si是钢中常用的脱氧元素，对钢也具有固溶强化作用。也可以提高材料的耐蚀性能，对于减轻铜脆也有一定的作用。但较高的Si含量会导致热轧带钢表面产生红铁皮缺陷，严重影响带钢的表面质量，同时会降低材料的焊接性能，造成焊接热影响区韧性的恶化。因而本发明中Si含量≤0.50％，优选0.06％≤Si≤0.50％。
Mn是钢中重要的强韧化元素，有固溶强化的作用，也能降低过冷奥氏体转变温度，降低铁素体相变温度，利于组织细化，从而提高材料的强度和韧性。但是Mn含量过多会显著抑制铁素体转变为贝氏体，导致材料的塑性和冷成形性能下降。所以本发明中Mn含量为0.1～1.5％，优选为0.3-1.2％，更优选为0.5-1.0％。
Cr是提高钢板耐候性能的重要元素。提高耐候钢的耐候性的主要机理如下，一方面，通过添加耐蚀性元素提高基体的腐蚀电位，以通过提高电化学阻抗来降低腐蚀速率。另一方面，Cr通过促使表面形成致密锈层对腐蚀介质实现了物理阻隔，改变基体位置的腐蚀环境，随着腐蚀深度增加腐蚀逐渐减缓。当钢中Cr含量超过1.5％时，在Cr与Cu等元素的综合作用下，可在基体表面形成均匀致密的锈层，较高的Cr含量配合Cu等元素作用，使锈层中的α-FeOOH非常细小，有利于阻隔水等电化学腐蚀介质的进一步渗透，提高电化学阻抗。同时由于Cr的浓度较高，随着锈 层厚度增加，Cr在锈层前沿富集，使锈层中Cr的浓度迅速增加，在锈层厚度达到0.1mm之前，在锈层与基体界面位置Cr浓度可以达到12％以上。当Cr富集浓度达到12％以上时，本发明的热轧带钢会产生类似不锈钢的效应，即腐蚀前沿锈层与基体界面处的腐蚀电位非常高，加上致密锈层低腐蚀介质的阻隔作用，使带钢表面的电化学阻抗十分高，腐蚀反应基本中断。但基体中Cr含量不宜过高，随着基体中Cr含量增加，基体的腐蚀电位会提高，如Cr含量超过4.5％时，会导致锈层生成初期的选择性腐蚀增强，锈层的厚度的均匀性变差，也即锈层前沿的Cr浓度以及腐蚀环境造成化学阻抗变得不均匀，提高了腐蚀前沿的腐蚀电位差，加剧了原电池效应，从而使抑制腐蚀的效果变差。在这种情况下，尽管在有限次的试验条件的相对腐蚀速率会降低，但是长周期来看其腐蚀的深度却并不减少，不能达到在长期使用时提升耐候性的效果。因此，本发明中Cr含量在1.5％～4.5％，优选为2.27～3.68％。
参见图1、图2，图1显示了在周浸试验中，钢中Cr含量对热轧带钢的相对腐蚀率的影响；图2显示了钢中Cr含量热轧带钢的长周期腐蚀深度的影响。
由图2可见，本发明的热轧带钢的腐蚀速率随时间快速衰减，25年的腐蚀深度模拟推测值可以在0.1mm以下，例如当钢中Cr含量为2％时。而当钢中Cr的含量为5％时，其25年的腐蚀深度模拟推测值约0.12mm，耐腐蚀性反而下降。另外，Cr也是增加钢淬透性的元素，较高的Cr含量，可以使钢材在较低的空冷速率下形成空冷贝氏体或者空冷马氏体，从而显著提高材料的抗拉强度，降低材料的屈强比，利于减少成形回弹、提高成形尺寸的稳定性。本发明结合较高的Cr含量设计，充分利用了Cr的这一作用，配合C、Mn等元素的强化作用，进一步提高了钢强度。
Cu也是重要耐蚀性元素之一，和Cr一起加入时效果更加明显。Cu能促进钢表面致密性锈层的形成。加入0.10％以上的Cu可显著改善钢的耐大气腐蚀性能。但Cu是一种熔点较低的金属，含Cu较多的带钢在热轧过程中容易在表面形成铜脆网裂和翘皮缺陷，恶化钢的表面质量，同时Cu也是贵重元素，出于上述原因，本发明中Cu含量为0.10～0.60％，优选为0.2-0.5％，更优选为0.25-0.38％。
P在传统耐大气腐蚀钢中常作为耐蚀元素添加，其能够促进表面保护性锈层的形成，从而有效提高钢的耐大气腐蚀性能。但P也是钢中的有害杂质元素，易在钢坯连铸时在厚度中心偏析。同时，P易在晶界处产生偏聚，降低晶界结合能，从而降低钢的韧性及塑性。基于同样的原理，P对钢的焊接性能也十分不利。因此，本发明不采用高P含量的耐大气腐蚀钢，而要尽量降低钢中P的含量，要求P含量≤0.03％。
S是钢中常见的有害杂质元素，对低温韧性、焊接性能、冷成形性能等都有不利影响，在本发明的钢中S含量≤0.01％。
Al是十分有效的脱氧元素，同时Al有利于细化晶粒，改善钢材的强韧性能。同时Al也能促进铁素体的产生，抑制珠光体转变，有利于铁素体贝氏体双相组织的转变。但较高的Al含量不利于顺利浇注，容易堵塞水口，所以本发明要求钢中Al含量≤0.60％，优选0.011～0.56％，更优选0.02-0.30％。
Ti是一种强烈的碳氮化物形成元素，可以以极细小的TiC或Ti(C,N)第二相粒子的形式析出，从而显著提高材料的强度，是一种十分有效的强化元素。Ti在0.05％以上时才会发挥较强的析出强化作用。同时TiC的析出显著减少了自由C形成尺寸较大的碳化物或者珠光体，从而减少了异质相在腐蚀过程中的原电池效应，提高了材料抗晶间腐蚀的能力，从而在提高强度的同时，还利于材料耐蚀性能的提高。但是过高含量添加，Ti的析出强化作用会逐渐减弱，并开始影响钢的低温韧性。因此本发明中Ti含量为0.05～0.18％，优选0.072～0.160％。
Ni可提高钢的耐腐蚀性能，也能改善Cu引起的表面铜脆问题。但Ni的价格十分昂贵，过多添加会大幅提高材料的合金成本。因此本发明中Ni含量≤0.30％优选在0.1％以上，0.25％以下，更优选在0.20％以下。
Nb也是一种强的氮碳化物形成元素，也可形成NbC、Nb(CN)碳化物粒子作为第二相析出，产生析出强化作用。但Nb的成本远高于Ti，因此，与Ti相比，通过添加Nb以提高强度不经济。同时过高含量的Nb，还会影响带钢连铸冷却过程中的铸坯质量，产生表面裂纹、角裂等缺陷。因此本发明中Nb含量≤0.06％。
N是钢中的杂质元素。在含Ti钢中，N容易与Ti结合在冶炼过程中 析出形成粗大的TiN夹杂。一方面TiN夹杂会损害钢的韧性，另一方面也减少了钢中的有效Ti含量。因此本发明中N含量≤0.008％。
另外，在本发明的热轧带钢的成分设计还需要满足：2Mn+Cr≤6％。Mn、Cr都会使铁素体转变的C曲线右移，显著抑制铁素体的转变，使转变时间变长。经热模拟和CCT计算，在2Mn+Cr＞6％时，带钢在层流冷却以及卷后冷却过程中都很难有足量铁素体转变，而足量的铁素体转变对钢的性能十分重要。首先，铁素体的转变量会直接影响到本发明钢的延伸率性能，即材料的塑性。延伸率不足，材料就难以满足复杂截面的成型要求。其次，铁素体的转变对于TiC纳米相的析出也有重要影响，Ti在α铁素体相中的扩散系数较高，同时C在α铁素体相中溶解度很低，所以随着γ→α相变发生，TiC可以以相间沉淀或者弥散析出的形式快速析出，形成纳米级别的析出相，从而显著提高强度。如果Mn、Cr含量过高，铁素体相不发生转变，TiC就难以快速析出，随着温度降低，大量组织转变为贝氏体，钢中的自由C元素会以碳化物的形式析出，从而不再有机会与Ti结合形成TiC，也就不能产生足够的强化效果。因此铁素体充分转变对于钢的强度和塑性有重要影响。因此，在本发明中，要求2Mn+Cr≤6％，计算时代入相应元素在所述热轧带钢中的质量百分比。优选地，3％≤2Mn+Cr≤5％。
优选地，本发明的热轧带钢的成分满足Ti-3N≥0.04％，计算时代入相应元素在所述热轧带钢中的质量百分比。若钢中N优先与Ti结合形成TiN夹杂，则会削弱Ti对钢的强度的贡献。在本发明中，将Ti-3N定义为有效Ti含量，当Ti-3N≥0.04％，可以保证有足够的Ti与C结合形成TiC沉淀粒子起到析出强化作用。优选地，Ti-3N≥0.06％，更优选地，Ti-3N≥0.09％。
优选地，本发明的热轧带钢的成分满足Si+2Ni≥0.10％，计算时代入相应元素在所述热轧带钢中的质量百分比。Cu易在带钢表面形成铜脆缺陷，而Si、Ni都具有改善铜脆缺陷的作用，它们可以相互补充。与Si相比，Ni对铜脆改善的效果更好，但成本也更高。当Si+2Ni含量在0.10％以上时即可起到改善铜脆缺陷的效果。因此，本发明的热轧带钢可以含有Si、Ni元素中的一种或两种，当钢中Si≤0.50％，Ni≤0.30％且Si+2Ni≥ 0.10％时，利用二者可以互补的关系，可以调整材料设计经济性、材料表面质量与铜脆问题之间的平衡，使铜脆问题能较经济地处于可控状态。优选地，Si+2Ni≥0.30％。
本发明的另一方面提供了一种上述热轧带钢的制造方法，其包括如下步骤：
1)冶炼、铸造
按上述成分冶炼钢水，然后铸造成板坯；
2)板坯加热
对板坯进行快速升温，使板坯的温度跨越1050～1150℃温度区间的加热时间在15min以内；将均热段炉温保持在1230～1290℃，均热时间控制在30～90min，出炉温度为1230～1290℃；
3)轧制
板坯出加热炉并定宽后进行粗轧，粗轧阶段采用15MPa以上优选20MPa以上的高压水除鳞，粗轧出口温度为1080℃以下；对粗轧后的带钢进行精轧，精轧采用多机架连轧工艺，控制终轧温度在820～880℃；
4)冷却、卷取
冷却采用层流冷却，所述带钢的卷取温度为630～680℃；层流冷却结束后，通过热卷堆垛、缓冷墙或者保温罩方式使钢卷缓慢冷却，钢卷在530℃以上的冷却速率≤1℃/min。
优选地，粗轧出口温度为1040～1080℃。
优选地，在步骤3)精轧阶段，若成品热轧带钢厚度为3mm以下，则终轧温度为860～880℃；若成品热轧带钢厚度为3～5mm，则终轧温度为840～860℃；若成品热轧带钢厚度为5mm以上，则终轧温度为820～840℃。
在本发明所述热轧带钢的制造方法中：
对于含Cu钢来说，通常采用低温加热的方法来避免铜脆发生。而对于含Ti钢来说，通常又需要高温加热来使Ti能充分固溶，为后面TiC的析出强化提供条件。由于本发明的热轧带钢既含Cu又含Ti，在加热制度 上存在矛盾。
本发明的制造工艺优化了加热过程中钢坯的升温曲线，通过高温快烧使板坯的表面温度快速跨越产生铜脆的敏感温度区间1050～1150℃，并将加热时间控制在15min以内，让熔融的Cu能够被基体表层新形成的氧化皮吸留，避免Cu向钢坯基体渗透，从而抑制铜脆的产生。
在均热段，温度保持在1230～1290℃，均热时间控制在30～90min，保证Ti的充分固溶。出炉温度也在1230～1290℃。本发明的技术方案克服了含Cu钢需要低温加热抑制铜脆和高Ti钢需要高温加热充分固溶的矛盾，既可以保证Ti充分固溶，为后面Ti的析出强化提供条件；又能减少铜脆的发生，提高热轧带钢的表面质量。
板坯出加热炉并定宽后进入粗轧，粗轧阶段应该保证足够高的除鳞压力以获得好的除鳞效果。由实际生产证实，15MPa以上优选20MPa以上的高压水对Cr含量较高的钢坯表面较致密的一次氧化皮具有较好的破碎和去除作用。而氧化皮的去除对于减少因表层富集的Cu而产生的铜脆问题有显著的改善效果。因此，在本发明的制造工艺中，要求除鳞高压水的压力在15MPa以上。
此外，为减少表面铜脆的发生，同时兼顾Ti的析出强化效果，粗轧出口温度应为1080℃以下，优选为1040～1080℃。
粗轧后的带钢进入精轧，精轧采用多机架连轧工艺，根据轧制带钢的厚度不同，控制终轧温度在820～880℃。相对一般轧钢工艺，本发明钢需要采用更低的终轧温度，如当成品热轧带钢厚度为3mm以下时，终轧温度采用860～880℃；当成品热轧带钢厚度在3～5mm之间时，终轧温度为840～860℃；当成品热轧带钢厚度为5mm以上时，终轧温度为820～840℃。控制终轧温度的目的是为了增加轧制形变能，促进带钢层流冷却后的细晶铁素体转变。铁素体的析出同时会促进TiC的析出，能显著提高析出强化效果。因此采用低温终轧对于提高本发明的热扎带钢的塑性和强度十分重要。
由于本发明的钢的Mn、Cr含量较高，会抑制铁素体的相变，而铁素体相变对于提高材料的塑性和成形性能，以及促进TiC的析出以提高强度都有重要作用。因此，除在热轧阶段通过降低终轧温度获得更多相变动 力之外，还要在层流冷却之后，通过提高卷取温度给与铁素体相变更高的相变温度和更充分的相变时间。因此，在本发明中，带钢卷取温度在630-680℃。层冷结束后，带钢还要求通过热卷堆垛、缓冷墙或者保温罩等方式保证钢卷缓慢冷却，使钢卷在530℃以上的冷却速率≤1℃/min，以保证铁素体相变的充分发生以及TiC粒子的充分析出，从而提高材料的塑性和强度。带钢制造过程中较高的卷取温度，也有利于带钢的板形控制。
通过上述热轧工艺的控制，使本发明的热轧带钢具有以多边形铁素体为主的组织，其中铁素体的晶粒度在8级以上，甚至可达11级。热轧带钢的显微组织中多边形铁素体含量为70～90％，在铁素体内大量弥散分布着直径在10nm以下TiC析出相，对材料的高强度有着显著的贡献。另有热轧带钢还含有5～30％、优选15-30％、更优选15-25％的珠光体，以及5～15％的少量MA形态为主的贝氏体，可以进一步提高钢强度。优选地，贝氏体中含30-70％的MA组织。钢中大量存在的TiC纳米析出相，使材料具有较高的强度，具体地，屈强强度≥600MPa，抗拉强度≥700MPa。本发明的热轧带钢由于具有以铁素体为主的组织而具有很高的塑性，具体地，断裂延伸率≥24％，180°D＝0.5t冷弯合格，优选180°D＝0t冷弯合格。即本发明的热扎带钢具有高强度和高塑性。
与现有技术相比，本发明的的技术方案具有以下优势：
首先，本发明利用1.5～4.5％的高Cr添加，同时添加Cu等成分，促使本发明的热轧带钢在大气中使用时，表面形成均匀致密锈层，并能在≤0.1mm的较薄的锈层与带钢基体的界面位置，使Cr浓度快速富集至12％以上。通过在带钢表面形成均匀致密的锈层以及锈层中Cr富集带来的高Cr浓度，实现带钢基体表面腐蚀电位和电化学阻抗的显著提高，阻碍表面腐蚀继续发生，从而使本发明的热轧带钢具有超高的耐大气腐蚀性能(本发明的热轧带钢服役环境是GB/T 19292.1-2018中规定的C1-C3腐蚀等级)。分别对Q355B钢和本发明的热轧带钢进行周浸试验，结果显示，本发明的热轧带钢的腐蚀速率为Q355B钢的30％以下，即其耐候性是普通结构钢Q355B的3倍以上，是普通耐候钢(Q450NQR1)的2倍以上；且其在大气中的腐蚀速率随带钢表面腐蚀深度增加而快速衰减，本 发明的带钢模拟25年腐蚀的结果显示其在大气中25年的腐蚀深度≤0.1mm。
其次，本发明的热轧带钢利用比较经济的Ti的析出强化，控制钢中Mn、Cr的含量并要求2Mn+Cr≤6％，能使带钢获得良好的耐蚀性。进一步地，配合特定的高温卷取及卷后缓冷工艺，使热轧带钢具有70-90％、优选75～90％体积含量的铁素体，以及在铁素体中分布的大量纳米TiC析出相，使材料具有高强度以及高塑性。具体地，屈服强度≥600MPa，抗拉强度≥700MPa，断裂延伸率≥24％，冷弯性能可以满足180°D＝0.5t冷弯合格，优选180°D＝0t冷弯合格(冷弯性能采用GB/T 232-2010“金属材料弯曲试验方法”标准进行评价，其中，D为弯曲直径，t为钢板厚度)。具有极好成形性能，可以实现完全对折不开裂，能够满足复杂截面型材构件的辊压成型加工。
另外，本发明中优选满足Si+2Ni≥0.10％的成分设计，由此获得了材料设计经济性、材料表面质量与铜脆问题之间的平衡。
同时本发明的制造方法在加热制度上，克服了含Cu钢需要低温加热抑制铜脆和高Ti钢需要高温加热充分固溶的矛盾，通过控制铸坯升温速度，采用高温快烧使其板坯表面温度能快速跨越铜脆敏感区间1050～1150℃，然后通过控制均热段加热时间和加热温度，保证Ti的充分高温固溶，为后面Ti的析出强化提供条件；另一方面，结合粗轧段在15MPa以上的高压水除鳞，减少了钢铜脆问题的发生，保证了带钢表面质量。
附图说明
图1显示了试验热轧带钢中Cr含量对周浸试验相对腐蚀速率的影响；
图2显示了具有不同Cr含量的试验热轧带钢在长周期大气曝露中的腐蚀深度；
图3为本发明实施例2的热轧带钢的显微组织照片；
图4为本发明实施例6的热轧带钢的显微组织照片；
图5为本发明实施例2的热扎带钢的薄膜TEM暗场照片，其中亮的部分为直径10nm以下的析出相。
具体实施方式
下面结合实施例和附图对本发明的技术方案做进一步的说明。
制造本发明实施例1-8的热轧带钢的主要工艺如下：
1)根据表1所示的化学成分进行转炉冶炼、精炼，然后进行连铸，获得钢坯。
2)钢坯经加热炉加热。加热过程中控制钢坯升温进程，使板坯的温度跨越1050～1150℃区间的加热时间在15min以内；将均热段炉温保持在1230～1290℃，均热时间控制在30～90min，出炉温度为1230～1290℃。
3)对钢坯进行粗轧除鳞、侧压定宽、粗轧、飞剪、精轧除鳞、精轧，获得带钢。其中粗轧阶段采用15MPa以上的高压水除鳞，粗轧出口温度为1080℃以下；对粗轧后的带钢进行精轧，精轧采用多机架连轧工艺，控制终轧温度在820～880℃。
4)带钢精轧后进行层流冷却和卷取，获得本发明的热轧带钢。其中带钢的卷取温度为630～680℃，层流冷却结束后，通过热卷堆垛、缓冷墙或者保温罩方式使钢卷缓慢冷却，钢卷在530℃以上的冷却速率≤1℃/min。
实施例1-8的生产工艺采用的具体工艺参数列在表2中。按上述步骤1-4获得热轧带钢后，对热轧带钢进行性能检验，具体的带钢性能见表3。表3中还记载了实施例1-8的显微组织。
按照TB/T2375“铁路用耐候钢周期浸润腐蚀试验方法”检测实施例1-8的热轧带钢以及对比例1-2的钢的耐腐蚀性能，表3中的“相对腐蚀速率”为相应实施例1-8中的热轧带钢以及Q450NQR1钢相对Q345B钢的腐蚀速率。
实施例1-8的热轧带钢的屈服强度、抗拉强度、断裂延伸率按照GB/T 228.1-2021“金属材料拉伸试验第1部分：室温试验方法”进行测试，冷弯性能按照GB/T 232-2010“金属材料弯曲试验方法”进行测试。
实施例1-8中铁素体晶粒度按照GB/T 6394-2017“金属平均晶粒度测定方法”进行测量。
图3-4分别示出本发明实施例2和6的热轧带钢金相组织。由图3-4 可见，本发明实施例2和6的热轧带钢具有以铁素体为主的显微组织，其中铁素体的占比在80％以上，钢中还有少量非典型的珠光体组织，混杂在珠光体组织中实际还有一些贝氏体，贝氏体中可见MA组织。
图5是实施例2的热轧带钢经透射电镜(TEM)观察到的暗场形貌，在钢中可以发现大量的10nm以下TiC析出分布在铁素体晶界和晶内，这些极其细小的TiC粒子对材料的强度有强烈的贡献。
在本发明中，以现有的Q355B钢和Q450NQR1钢作为对比例。从表3的结果可以看出，本发明的热轧带钢显示出优良的耐蚀性能。


Claims (10)

  1. 一种热轧带钢，其以质量百分比计包含如下成分：C：0.04～0.09％，Si≤0.50％，Mn：0.10～1.50％，P≤0.03％，S≤0.01％，Al≤0.60％，Cr：1.5～4.5％，Cu：0.10～0.60％，Ti：0.05～0.18％，Ni≤0.30％，Nb≤0.06％，N≤0.008％，余量为Fe和其它不可避免杂质，且同时满足：2Mn+Cr≤6％，计算时代入相应元素在所述热轧带钢中的质量百分比。
  2. 如权利要求1所述的热轧带钢，其特征在于，Ti-3N≥0.04％，计算时代入相应元素在所述热轧带钢中的质量百分比。
  3. 如权利要求1或2所述的热轧带钢，其特征在于，Si+2Ni≥0.10％，计算时代入相应元素在所述热轧带钢中的质量百分比。
  4. 如权利要求1-3中任一项所述的热轧带钢，其特征在于，所述热轧带钢还包含选自如下中的至少一种：Ca≤0.015％，Mg≤0.015％，B≤0.003％，Mo≤0.30％，V≤0.15％，Re≤0.015％。
  5. 如权利要求1-4中任一项所述的热轧带钢，其特征在于，所述热轧带钢的显微组织为多边形铁素体+少量珠光体+以MA形态为主的贝氏体，铁素体的晶粒度在8级以上，优选9级以上，其中，以体积百分比计，多边形铁素体含量为70～90％，珠光体含量为5～30％，贝氏体含量为5～15％；所述铁素体内有0.005％-0.03％体积的直径在10nm以下的TiC纳米析出相。
  6. 如权利要求1-5中任一项所述的热轧带钢，其特征在于，所述热轧带钢的腐蚀速率为Q355B钢的30％以下；所述热轧带钢模拟25年服役周期内的腐蚀深度≤0.1mm；和/或，所述热轧带钢的屈服强度≥600MPa，抗拉强度≥700MPa，断裂延伸率≥20％，冷弯性能满足180°D＝0.5t弯曲合格，优选180°D＝0t弯曲合格。
  7. 如权利要求1～6中任一项所述的热轧带钢的制造方法，其特征在于，所述方法包括如下步骤：
    1)冶炼、铸造
    按权利要求1-4中任一项所述的成分冶炼钢水，然后铸造成板坯；
    2)板坯加热
    对板坯进行升温，使板坯表面温度跨越1050～1150℃温度区间的加热时间在15min以内；将均热段炉温保持在1230～1290℃，均热时间控制在30～90min，出炉温度为1230～1290℃；
    3)轧制
    板坯出加热炉并定宽后进行粗轧，粗轧阶段采用15MPa以上优选20MPa以上的高压水除鳞，粗轧出口温度为1080℃以下；对粗轧后的带钢进行精轧，精轧采用多机架连轧工艺，控制终轧温度在820～880℃；
    4)冷却、卷取
    冷却采用层流冷却，所述带钢的卷取温度为630～680℃；
    层流冷却结束后，通过热卷堆垛、缓冷墙或者保温罩方式使钢卷缓慢冷却，钢卷在530℃以上的冷却速率≤1℃/min。
  8. 如权利要求7所述的热轧带钢的制造方法，其特征在于，粗轧出口温度为1040～1080℃。
  9. 如权利要求7所述的热轧带钢的制造方法，其特征在于，在步骤3)精轧阶段，若成品热轧带钢厚度在3mm以下，则终轧温度为860～880℃；若成品热轧带钢厚度为3～5mm，则终轧温度为840～860℃；若成品热轧带钢厚度在5mm以上，则终轧温度为820～840℃。
  10. 如权利要求7所述的热轧带钢的制造方法，其特征在于，步骤2)板坯加热采用加热炉或者均热炉均热。
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