WO2018001019A1 - 一种磷化性能优异的冷轧低密度钢板及其制造方法 - Google Patents

一种磷化性能优异的冷轧低密度钢板及其制造方法 Download PDF

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WO2018001019A1
WO2018001019A1 PCT/CN2017/086174 CN2017086174W WO2018001019A1 WO 2018001019 A1 WO2018001019 A1 WO 2018001019A1 CN 2017086174 W CN2017086174 W CN 2017086174W WO 2018001019 A1 WO2018001019 A1 WO 2018001019A1
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steel sheet
cold
rolled low
density steel
sheet according
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PCT/CN2017/086174
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English (en)
French (fr)
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金鑫焱
杨旗
赵艳亮
王利
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宝山钢铁股份有限公司
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Priority to JP2019514171A priority Critical patent/JP6824393B2/ja
Priority to US16/304,947 priority patent/US11371112B2/en
Priority to KR1020187031774A priority patent/KR102163179B1/ko
Priority to EP17819016.1A priority patent/EP3476967B1/en
Publication of WO2018001019A1 publication Critical patent/WO2018001019A1/zh

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    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
    • 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
    • C21D8/0236—Cold rolling
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • C21D8/0247—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
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • 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
    • 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
    • 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
    • C21D2211/001—Austenite
    • 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

Definitions

  • the present invention relates to a cold rolled steel sheet and a method for producing the same, and more particularly to a cold rolled steel sheet excellent in phosphating performance and a method for producing the same.
  • the path to achieve lightweight vehicles from the material point of view is: use light alloys such as aluminum and magnesium instead of steel; use higher-strength steel instead of traditional low-strength steel to achieve material thinning; increase the specific strength of steel by reducing the density of steel, ie Develop low density steel.
  • the publication number is CN104220609A, and the publication date is December 17, 2014.
  • the Chinese patent document entitled "High-strength non-clearral low-density steel and the preparation method of the steel” discloses a high-strength non-clear gap. Density steel and its preparation method, in order to reduce the density, the steel has an Al content of 6-9%.
  • the Chinese Patent Publication No. CN101755057A published on June 23, 2010, entitled “Low Density Steel with Good Compressibility", discloses a hot rolled ferritic steel sheet in which Al is The content is 6% ⁇ Al ⁇ 10%.
  • the materials used in auto parts are usually phosphatized to form a phosphate film on the metal surface.
  • the normal phosphating film is porous, uniform and fine, and the coating penetrates into the pores. It increases the adhesion of the coating, and at the same time has the corrosion resistance during the electrophoresis of the electrophoretic paint and enhances the electrophoresis effect. Therefore, the phosphating film becomes a good bottom layer which is indispensable for the coating, and the quality of the coating cannot be ensured without the phosphating or phosphating effect.
  • the alloying elements will enrich the surface of the annealed steel sheet to form an oxide film, which is not conducive to the uniform reaction of the phosphating process, and is easy to cause low phosphating coverage and coarse phosphating crystals. Defects such as looseness do not meet the requirements of automobile manufacturing. Poor phosphating performance is also high strength steel plate Common problems in car manufacturing.
  • the controlled annealing process is another way to improve the phosphating performance of high-strength steels.
  • drawbacks in the prior art for example, the control of the annealing process cannot be applied to most continuous annealing lines, or the process parameters for annealing production are difficult, for example, the atmosphere dew point is controlled below -45 ° C, or the annealing process step is increased. Increased production costs.
  • the prior art mainly solves the adverse effect on the phosphating performance of the surface enrichment of the Si and Mn elements in the improvement of the phosphating performance of the high-strength steel, and the mass percentage of the Al element in the steel sheet is usually ⁇ 1%.
  • One of the objects of the present invention is to provide a cold-rolled low-density steel sheet excellent in phosphating performance, which has a low density by controlling the mass percentage of Al element, and forms an iron particle layer by controlling oxidation of the surface of the steel sheet. It has high strength and excellent phosphating performance, thereby solving the technical problem that the prior art has high Al element content and excellent phosphating performance.
  • the present invention provides a cold rolled low density steel sheet excellent in phosphating performance, the surface of the cold rolled low density steel sheet having an iron particle layer having dispersed iron particles therein;
  • the cold rolled low density steel sheet contains an Al element having a mass percentage of 3.0 to 7.0%.
  • the design principle of the Al element is that the Al element is a ferrite forming element.
  • the addition of the Al element can significantly lower the density of the steel sheet, and therefore, in the present invention, the mass percentage of the Al element is not less than 3.0%.
  • the mass percentage of the Al element exceeds 7.0%, which inhibits the formation of austenite.
  • the Al element can significantly increase the stacking fault energy of austenite in the steel, and thus the mass percentage of the Al element exceeds 7.0%. It is suppressed that the retained austenite in the steel is induced to undergo martensite transformation during deformation, so that it is difficult to obtain good strength and plasticity matching of the steel sheet.
  • the present invention defines the mass percentage of the Al element to be 3.0 to 7.0%. Further, the surface of the cold-rolled low-density steel sheet according to the present invention has an iron particle layer which can solve the problem of phosphating of low-density steel when the Al content is high.
  • the inner side of the iron particle layer has an inner oxide layer adjacent to the iron particle layer, and the inner oxide layer contains an oxide of Al.
  • the formation of external oxidation of Al 2 O 3 is suppressed by controlling the dew point of the annealing atmosphere, and is converted into internal oxidation of the inner oxide layer while being on the surface of the steel sheet. Iron particles are formed, thereby solving the problem of phosphating of cold-rolled high-strength low-density steel.
  • the inner oxide layer further contains an oxide of Mn.
  • the inner oxide layer has a thickness of 0.2 to 10 ⁇ m.
  • the thickness of the inner oxide layer is less than 0.2 ⁇ m, the external oxidation of the Al element cannot be more effectively suppressed, and when the thickness of the inner oxide layer is >10 ⁇ m, The forming property of the secondary surface layer of the steel sheet is affected, and therefore, preferably, the thickness of the inner oxide layer is controlled to be 0.2 to 10 ⁇ m.
  • the oxide in the inner oxide layer exists in the grain boundary and the crystal.
  • the oxide in the inner oxide layer is mainly an oxide of Al and an oxide of Mn, and the oxide is simultaneously distributed inside the grain and at the grain boundary position in the inner oxide layer.
  • the thickness of the iron particle layer is smaller than the thickness of the inner oxide layer.
  • the iron particle layer has a thickness of 0.1 to 5 ⁇ m.
  • the present invention defines the iron particle layer to have a thickness of 0.1 to 5 ⁇ m.
  • the iron particle layer of the cold-rolled low-density steel sheet according to the present invention has a thickness of 0.3 to 3 ⁇ m.
  • the iron particles have a particle diameter of 0.1 to 5 ⁇ m.
  • the present invention defines that the iron particles have a particle diameter of 0.1 to 5 ⁇ m.
  • the iron particles cover the surface of the steel sheet More than 30% of the area.
  • the present invention defines that the iron particles cover an area of 30% or more of the surface of the steel sheet.
  • the spacing between adjacent iron particles is not more than 10 times the average particle diameter of the iron particles.
  • the phosphating may cause non-phosphating at intervals between the iron particles. Accordingly, preferably, the present invention defines that the spacing between adjacent iron particles does not exceed a maximum of 10 times the average particle size of the iron particles.
  • the microstructure of the steel sheet is ferrite and retained austenite.
  • the ratio of the retained austenite is 6 to 30%.
  • the mass percentage of the C element in the retained austenite is not less than 0.8%.
  • C is an important solid solution strengthening element to promote austenite formation, and in low-density steel rich in Al element, if retained in austenite
  • mass percentage of C is less than 0.8%, the content and mechanical stability of retained austenite are relatively low, and thus the strength and ductility of the steel sheet are low. Therefore, the C content in the retained austenite of the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention is not less than 0.8%.
  • the density is less than 7,500 kg/m3, so that the cold-rolled low-density steel has a low density and a light weight, and is suitable for use in the manufacture of automobile structural members.
  • the chemical element mass percentage is: C: 0.25 to 0.50%, Mn: 0.25 to 4.0%, Al: 3.0 to 7.0%, and the balance is Fe. And other inevitable impurities.
  • the inevitable impurities are mainly S, P and N elements, and can control P ⁇ 0.02%, S ⁇ 0.01%, N ⁇ 0.01%.
  • C is an important solid solution strengthening element that promotes austenite formation.
  • the C mass percentage is less than 0.25%, the retained austenite content and mechanical stability are relatively low, so the strength and ductility of the steel sheet are low; when the C mass percentage is higher than 0.5%, the lamellar layer Carbides and carbide particles distributed at the grain boundaries of the ferrite are coarse, thereby reducing the rolling deformation ability of the steel sheet. Therefore, the present invention controls the C mass percentage to be 0.25 to 0.50%.
  • Mn can increase the stability of austenite, reduce the critical cooling rate of steel during quenching and improve the hardenability of steel. Mn also improves the work hardening properties of steel, thereby increasing the strength of the steel sheet. However, too high Mn content will cause Mn segregation in the slab and obvious band-like structure distribution in the hot-rolled sheet, thereby reducing the ductility and bending properties of the steel sheet; and too high Mn content tends to cause the hot-rolled sheet to be cold. Cracks occur during rolling deformation. Therefore, the present invention controls the mass percentage of Mn to be 0.25 to 4.0%.
  • the Al element is a ferrite forming element.
  • the addition of the Al element can significantly lower the density of the steel sheet, and therefore, in the present invention, the mass percentage of the Al element is not less than 3.0%. However, the mass percentage of the Al element exceeds 7.0%, which inhibits the formation of austenite.
  • the Al element can significantly increase the stacking fault energy of austenite in the steel, and thus the mass percentage of the Al element exceeds 7.0%. It is suppressed that the retained austenite in the steel is induced to undergo martensite transformation during deformation, so that it is difficult to obtain good strength and plasticity matching of the steel sheet. Therefore, the present invention defines the mass percentage of the Al element to be 3.0 to 7.0%.
  • P is a solid solution strengthening element; however, P increases the cold brittleness of the steel, reduces the plasticity of the steel, and deteriorates the cold bending property and the welding property. Therefore, the present invention defines a P mass percentage ⁇ 0.02%.
  • S:S causes the steel to be hot brittle, reduces the ductility and toughness of the steel, deteriorates the weldability, and reduces the corrosion resistance of the steel. Accordingly, the present invention defines an S mass percentage of ⁇ 0.01%.
  • N:N forms AlN with Al, and can refine columnar dendrites during solidification.
  • the N content is too high, the formed coarse AlN particles affect the ductility of the steel sheet.
  • excess AlN reduces the thermoplasticity of the steel. Accordingly, the invention defines N mass percent ⁇ 0.01%.
  • At least one of Si, Ti, Nb, V, Cr, Mo, Ni, Cu, B, Zr, and Ca may be selectively added.
  • the elongation is >25% and the tensile strength is >800 MPa.
  • Another object of the present invention is to provide a method for producing a cold-rolled low-density steel sheet according to the above aspect, which is capable of producing a cold-rolled low-density steel sheet having any of the above-described phosphating properties.
  • the present invention provides a method of manufacturing a cold rolled low density steel sheet according to any of the above, comprising the steps of:
  • the soaking temperature and the holding time of the continuous annealing in the step (5) are mainly selected to form an iron particle layer on the surface of the steel sheet after continuous annealing.
  • the soaking temperature is controlled at 750°C-950°C, and the holding time is limited to 30-600s, because: if the soaking temperature is lower than 750°C and the holding time is lower than 30s, Markov in the cold-rolled low-density steel plate matrix
  • the austenite reverse phase transformation has not fully formed austenite grains, and the carbides in the cold rolled low density steel plate steel matrix have not been completely dissolved to form austenite grains, and the strip of high temperature iron cannot fully perform dynamic recrystallization.
  • the iron particle layer on the surface of the annealed steel sheet is not sufficiently formed, and the phosphating performance is inferior.
  • the soaking temperature is higher than 950 ° C and the holding time is higher than 600 s
  • the austenite grains in the matrix structure of the steel sheet are coarsened after the soaking treatment, and the austenite stability in the steel is reduced, thereby causing the residual in the steel sheet matrix after annealing.
  • the solid content is reduced and the retained austenite stability is also reduced.
  • the mechanical properties of the steel sheet after annealing deteriorate.
  • the iron particle size on the surface of the steel sheet after annealing is too large, and the thickness of the inner oxide layer is too thick. It is not conducive to the surface forming properties of the steel sheet.
  • the formation of the iron particle layer in the present technical solution is also closely related to the dew point of the annealing atmosphere defined in the technical solution, and the formation of the external oxidation of Al 2 O 3 is suppressed by controlling the dew point of the annealing atmosphere of the continuous annealing, and is converted into Internal oxidation of the inner oxide layer causes iron particles to form on the surface of the steel sheet.
  • the annealing atmosphere is all reductive to Fe, and thus the iron oxide is reduced.
  • the dew point of the annealing atmosphere is lower than -15 ° C, the above annealing atmosphere is still oxidized to the Al element in the steel matrix, and Al in the steel matrix forms a continuous dense Al 2 O 3 film on the surface of the steel sheet, thereby affecting the phosphating performance. . If the dew point of the annealing atmosphere is higher than +20 ° C, the oxygen potential in the annealing atmosphere is too high, and the ability of O atoms to diffuse into the steel matrix is increased, and the inner oxide layer is too thick on the surface of the steel sheet and alloy elements such as Al and Mn, affecting the steel sheet. Formability of the surface.
  • the holding time in the step (5) is 30-200 s.
  • the soaking time is limited to 30-200 s.
  • the heating temperature is 1000-1250 ° C
  • the holding time is 0.5-3 h
  • the finishing temperature is 800-900 ° C.
  • the hot rolled sheet is taken up at a temperature of 500 to 750 °C.
  • the heating temperature in the step (2) is limited to 1000 to 1250 ° C because the heating temperature exceeds 1250 ° C, which may result in
  • the slab of the steel sheet is over-fired, and the coarse grain structure in the slab leads to a decrease in hot workability, and the ultra-high temperature causes severe decarburization on the surface of the slab;
  • the heating temperature is lower than 1000 ° C, the slab is descaled by high-pressure water and initially After rolling, if the finishing temperature is too low, the deformation resistance of the slab is too large, and it is difficult to manufacture a steel sheet having neither surface defects nor a predetermined thickness.
  • the holding time is limited to 0.5-3 h in the step (2) because the holding time exceeds 3 h, which causes the sheet of the steel sheet.
  • the grain structure in the billet is large and the surface of the slab is decarburized seriously.
  • the holding time is less than 0.5 h, and the inside of the slab is not even.
  • the finishing rolling temperature is defined in the step (2) at 800-900 ° C in order to complete hot rolling of the slab, and final rolling If the temperature is too low, the deformation resistance of the slab is too high, so that it is difficult to manufacture hot-rolled steel sheets and cold-rolled steel sheets having the required thickness specifications and no surface and edge defects; in addition, for the present invention, when the finish rolling temperature is lower than At 800 ° C, the hot-rolled strip-shaped high-temperature ferrite inside the slab cannot be sufficiently recovered and recrystallized to be refined. Since the plate temperature will naturally drop during the hot rolling process after the slab is released, it is difficult to control the finish rolling temperature to be higher than 900 °C.
  • the hot-rolled sheet is wound at a temperature of 500 to 750 ° C, and if the coiling temperature is higher than 750 ° C, It is easy to cause flat rolls, and the microstructure non-uniformity of the head, middle, and tail materials of the hot rolled coil is increased; if the coiling temperature is lower than 500 ° C, the high tensile strength of the hot rolled coil may cause difficulty in cold rolling.
  • the cold rolling reduction is 30 to 90%.
  • the cold rolling reduction is defined in the step (4) in order to perform cold rolling deformation on the hot-rolled steel sheet after pickling.
  • the cold rolling reduction of >30% can increase the austenite formation rate during the subsequent annealing process, help to form the deformed high-temperature ferrite and improve the uniformity of the annealed steel sheet, thereby improving the ductility of the annealed steel sheet.
  • the cold rolling reduction is >90%, and the deformation resistance of the material is very high due to work hardening. It is extremely difficult to prepare a cold-rolled steel sheet having a predetermined thickness and a good plate shape. Therefore, the cold rolling reduction of the cold rolled low density steel sheet according to the present invention is controlled to be 30 to 90%.
  • the cold rolling reduction is 50-80%.
  • the atmosphere of the heating section and the holding section is a mixed gas of N 2 and H 2 , wherein the volume content of H 2 is 0.5-20%.
  • the volume content of H 2 is 1-5%.
  • the annealing atmosphere has a dew point of -10 to 0 °C.
  • the heating rate is 1-20 ° C / s
  • the cooling rate after soaking is 1-150 ° C / s.
  • the cooling rate after soaking is selected to be 1-150 ° C / s, preferably the cooling rate is 10-50. °C / s, the selection of the cooling rate needs to avoid the decomposition of austenite during the cooling process.
  • the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention has excellent phosphating performance because it has an iron particle layer;
  • the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention is excellent in mechanical properties, and has an elongation of >25% and a tensile strength of >800 MPa.
  • Fig. 1 is a schematic view showing the structure of a cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention.
  • Fig. 2 is a cross-sectional metallographic structure of a cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention.
  • Fig. 3 is a surface scanning electron microscope secondary electron image of Example A2 of the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention.
  • Figure 4 is a view showing the surface of Example A7 of the cold rolled low density steel sheet excellent in phosphating performance according to the present invention; Scanning electron microscope secondary electron image.
  • Fig. 5 is a surface scanning electron microscope secondary electron image of Comparative Example B1 of the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention.
  • Fig. 6 is a surface SEM low-power backscattered electron image of Example A2 of the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention after phosphating.
  • Fig. 7 is a high-order secondary electron image of a surface scanning electron microscope after phosphating of Example A2 of the cold-rolled low-density steel sheet having excellent phosphating performance according to the present invention.
  • Fig. 8 is a surface-scanning electron microscope low-power backscattered electron image of a comparatively-prepared B1 of a cold-rolled low-density steel sheet having excellent phosphating performance according to the present invention after phosphating.
  • Fig. 9 is a high-order secondary electron image of a surface scanning electron microscope after phosphating of Comparative Example B1 of a cold-rolled low-density steel sheet having excellent phosphating performance according to the present invention.
  • Fig. 1 shows the structure of a cold rolled low-density steel sheet excellent in phosphating performance according to the present invention.
  • the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention comprises a steel substrate 1, an iron particle layer 3 on the surface of the steel sheet, and an inner layer of the iron particle layer having an inner oxide layer 2 in close proximity to the iron particle layer.
  • Fig. 2 is a view showing a cross-section metallographic structure of a cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention.
  • the formation of external oxidation of the iron particle layer 3 on the surface of Al 2 O 3 is suppressed by controlling the dew point of the annealing atmosphere, and It is converted into internal oxidation of the inner oxide layer 2, and at the same time, iron particles are formed on the surface of the steel sheet, and after phosphating, a surface having uniform appearance and complete coverage of the phosphate film can be obtained.
  • the thickness of the inner oxide layer 2 is 0.2-10 ⁇ m, and the oxide of the inner oxide layer 2 exists in the grain boundary and the crystal, the thickness of the iron particle layer 3 is smaller than the thickness of the inner oxide layer, and the thickness of the iron particle layer 3 is 0.1. -5 ⁇ m.
  • Table 1 lists the mass percentages of the chemical elements of the cold-rolled low-density steel sheets excellent in the phosphating performance of Examples A1 to A16 and the conventional steel sheets of B1-B6.
  • Table 2 lists the cold-rolled low-density steel sheets excellent in phosphating performance in Examples A1 to A16 and
  • Figure 3 shows a surface scanning electron microscope secondary electron image of Example A2.
  • Figure 4 shows a surface scanning electron microscope secondary electron image of Example A7.
  • Figure 5 shows a surface SEM secondary electron image of Comparative Example B1.
  • FIG. 3 and Fig. 4 iron particles were present on the surfaces of Examples A2 and A7, except that the iron particles of Example A2 were sufficiently grown, and the gap between the iron particles was small, while the iron particles of Example A7 were not. When fully grown, the gap between the iron particles is large.
  • the soaking time of the embodiment A2 is longer than the soaking time of the embodiment A7. Therefore, the soaking time of the present invention is preferably 30-200 s. .
  • Figure 5 shows the surface electron microscope secondary electron image of Comparative Example B1. A layer of Al 2 O 3 film was observed on the surface, but no iron particles were observed, and the surface topography was completely different from that shown in Figure 3 and Figure 4. In the examples, the cross-section metallographic pattern revealed that no iron particle layer and an inner oxide layer were formed.
  • Table 3 lists the respective performance parameters of the cold rolled low density steel sheets excellent in the phosphating performance and the conventional steel sheets rolled in the B1-B6 in Examples A1 to A16.
  • the phosphating performance is tested by randomly selecting 10 500-fold fields of view to observe the phosphating film on the surface of the phosphating steel sheet under scanning electron microscopy, and using the image software to measure the coverage of the phosphating film. If the average coverage of the phosphating film in the field of view is ⁇ 75%, it is judged that the phosphating performance is good (indicated by ⁇ ), and if the average coverage of the phosphating film in 10 fields of view is ⁇ 75%, it is judged that the phosphating performance is poor. (with X table Show).
  • Fig. 6 is a surface SEM low-power backscattered electron image of Example A2 of the cold-rolled low-density steel sheet excellent in phosphating performance according to the present invention after phosphating.
  • Fig. 7 is a high-order secondary electron image of a surface scanning electron microscope after phosphating of Example A2 of the cold-rolled low-density steel sheet having excellent phosphating performance according to the present invention.
  • Fig. 8 is a surface-scanning electron microscope low-power backscattered electron image of a comparatively-prepared B1 of a cold-rolled low-density steel sheet having excellent phosphating performance according to the present invention after phosphating.
  • Fig. 9 is a high-order secondary electron image of a surface scanning electron microscope after phosphating of Comparative Example B1 of a cold-rolled low-density steel sheet having excellent phosphating performance according to the present invention.
  • the phosphating of Example A2 was observed to be uniform at a low magnification of the scanning electron microscope, and further, As can be seen from the high power microscope shown in Fig. 7, the phosphate film in Example A2 completely covered the surface of the steel sheet, and the phosphating crystal was uniform. As shown in Fig. 8, the phosphating unevenness of Comparative Example B1 can be seen under the low magnification observation of the scanning electron microscope, in which the black region is a position having phosphating crystals, the white region is a position where no phosphating crystal is formed, and surface phosphating is performed. The coverage is low, and the image after further enlargement is shown in Fig. 9. As can be seen from Fig. 9, the surface of Comparative Example B1 has only partial phosphating crystals.
  • the dew point of the annealing atmosphere of the embodiment is -15 ° C to +20 ° C, which can promote the conversion of Al element from external oxidation to internal oxidation within the above dew point range, avoiding the formation of continuous dense Al 2 O on the surface of the steel sheet of the example.
  • Effects of phosphate film 3, and a thickness of 0.2-10 ⁇ m is formed such that the Al oxide layer is an element in the steel. Since the iron particle layer was present in the surface layer of the steel sheet of the example, when the steel sheet of the example was phosphatized, it was equivalent to phosphating on the surface of ordinary mild steel.

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Abstract

一种磷化性能优异的冷轧低密度钢板，所述冷轧低密度钢板的表面具有铁颗粒层，所述铁颗粒层中具有分散的铁颗粒；所述冷轧低密度钢板含有质量百分含量为3.0～7.0％的Al元素。

Description

一种磷化性能优异的冷轧低密度钢板及其制造方法 技术领域
本发明涉及一种冷轧钢板及其制造方法，尤其涉及一种磷化性能优异的冷轧钢板及其制造方法。
背景技术
随着环保法规以及节能降耗的要求越来越高，轻量化是汽车发展的方向之一。从材料角度实现汽车轻量化的路径有：使用铝、镁等轻质合金替代钢材；使用更高强度的钢替代传统的低强度钢实现材料减薄；通过降低钢材密度提高钢材的比强度，即开发低密度钢。
由于铝的密度比钢低的多，因此现有技术通过在钢中添加一定含量的铝来实现降低材料密度。
例如,公开号为CN104220609A，公开日为2014年12月17日，名称为“高强度无晶隙低密度钢及所述钢的制备方法”的中国专利文献公开了一种高强度无晶隙低密度钢及其制备方法，为了降低密度，该钢的Al含量为6-9％。
又例如，公开号为CN101755057A，公开日为2010年6月23日，名称为“具有良好可压延性的低密度钢”的中国专利文献公开了一种热轧铁素体钢片材，其中Al的含量为6％≤Al≤10％。
然而对于Al含量较高的钢进行磷化处理时，会出现磷化结晶覆盖率低的情况，无法满足汽车用户涂装的要求。用于汽车零部件的材料在除油、除锈后，通常要进行磷化处理，使金属表面生成一层磷化膜，正常的磷化膜具有多孔性，均匀细致，涂料渗入到孔隙中，增加涂层的附着力的作用，同时具有耐电泳漆电泳过程中的腐蚀性，增强电泳效果的作用。所以，磷化膜成为涂层不可缺少的良好底层，不磷化或磷化效果不佳都无法保证涂装质量。
由于高强钢添加了较多的合金元素，合金元素会在退火后的钢板表面富集而形成氧化膜，从而不利于磷化过程的均匀反应，易造成磷化覆盖率低、磷化结晶粗大、疏松等缺陷，不符合汽车制造的要求。磷化性能差也是高强度钢板 在汽车制造方面的常见问题。
为了获得高强钢优异的磷化处理性能，控制合金成分含量是一种方法，但对合金成分含量进行限制，会影响材料性能的发挥。
除控制成分之外，控制退火工艺是另一种改善高强钢磷化性能的方法。但现有技术中存在缺陷：例如对退火工艺的控制无法适用于大多数连续退火生产线，或对于退火生产时工艺参数难度较大例如气氛露点控制在-45℃以下，或增加了退火工艺步骤因而增加了生产成本。
现有技术在改善高强钢磷化性能时主要解决Si和Mn两个元素表面富集后对磷化性能的不利影响，而这种钢板中，Al元素的质量百分比通常≤1％。
发明内容
本发明的目的之一在于提供一种磷化性能优异的冷轧低密度钢板，该冷轧低密度钢板通过控制Al元素的质量百分比使其具有低密度，并通过控制钢板表面氧化形成铁颗粒层使其具有高强度且磷化性能优异，从而解决现有技术中Al元素含量高和磷化性能优异不可兼得的技术问题。
为了实现上述目的，本发明提出了一种磷化性能优异的冷轧低密度钢板，所述冷轧低密度钢板的表面具有铁颗粒层，所述铁颗粒层中具有分散的铁颗粒；所述冷轧低密度钢板含有质量百分含量为3.0～7.0％的Al元素。
在本发明所述的磷化性能优异的冷轧低密度钢板中，Al元素的设计原理为：Al元素是铁素体形成元素。添加Al元素可显著降低钢板的密度，因此，在本发明中，Al元素的质量百分比含量不低于3.0％。然而，Al元素的质量百分比含量超过7.0％，会抑制奥氏体形成，另外，Al元素可显著增加钢中奥氏体的堆垛层错能，因而Al元素的质量百分比含量超过7.0％，会抑制钢中残余奥氏体在形变时被诱导发生马氏体相变，从而使钢板难以获得良好的强度和塑性匹配。因此，本发明限定Al元素的质量百分比含量为3.0～7.0％。此外，本发明所述的冷轧低密度钢板的表面具有铁颗粒层，该铁颗粒层可以解决Al含量较高时低密度钢的磷化问题。
进一步地，在本发明所述的冷轧低密度钢板中，所述铁颗粒层内侧具有紧邻铁颗粒层的内氧化层，所述内氧化层中含有Al的氧化物。
在本发明所述的磷化性能优异的冷轧低密度钢板中，通过控制退火气氛的 露点抑制Al2O3外氧化的形成，并将其转化成内氧化层的内氧化，同时在钢板表面形成铁颗粒，从而解决了冷轧高强低密度钢的磷化问题。
进一步地，在本发明所述的冷轧低密度钢板中，所述内氧化层中还含有Mn的氧化物。
进一步地，在本发明所述的冷轧低密度钢板中，所述内氧化层的厚度为0.2-10μm。
在本发明所述的磷化性能优异的冷轧低密度钢板中，当内氧化层的厚度<0.2μm时，无法更有效地抑制Al元素的外氧化，当内氧化层厚度＞10μm时可能会影响钢板次表层的成形性能，因此，优选地，所述的内氧化层的厚度控制为0.2-10μm。
进一步地，在本发明所述的冷轧低密度钢板中，所述内氧化层中的氧化物存在于晶界和晶内。所述内氧化层中的氧化物主要是Al的氧化物和Mn的氧化物，氧化物同时分布在内氧化层中的晶粒内部和晶界位置。
进一步地，在本发明所述的冷轧低密度钢板中，所述铁颗粒层的厚度小于内氧化层的厚度。
进一步地，在本发明所述的冷轧低密度钢板中，所述铁颗粒层的厚度为0.1-5μm。
在本发明所述的磷化性能优异的冷轧低密度钢板中，当铁颗粒层的厚度<0.1μm时，磷化性能相对来说不太好；而当铁颗粒层厚度>5μm时，则需要更长的退火保温时间形成铁颗粒层，因此，优选地，本发明限定了铁颗粒层的厚度为0.1-5μm。
进一步优选地，本发明所述的冷轧低密度钢板的铁颗粒层的厚度为0.3-3μm。
进一步地，在本发明所述的冷轧低密度钢板中，所述铁颗粒的粒径为0.1-5μm。
在本发明所述的磷化性能优异的冷轧低密度钢板中，当铁颗粒的粒径<0.1μm时，铁颗粒的厚度及覆盖面积较少，磷化性能相对较差；当铁颗粒的粒径>5μm时，会导致铁颗粒层过厚，因此，优选地，本发明限定了铁颗粒的粒径为0.1-5μm。
进一步地，在本发明所述的冷轧低密度钢板中，所述铁颗粒覆盖钢板表面 30％以上的面积。
在本发明所述的磷化性能优异的冷轧低密度钢板中，当铁颗粒覆盖钢板表面小于30％的面积时，没有铁颗粒覆盖的钢板表面面积过大，可能会导致这些位置磷化性能较差。因此，优选地，本发明限定了所述铁颗粒覆盖钢板表面30％以上的面积。
进一步地，在本发明所述的冷轧低密度钢板中，相邻铁颗粒之间的间距最大不超过铁颗粒平均粒径的10倍。
上述方案中，若相邻铁颗粒之间的间隔最大超过铁颗粒平均粒径的10倍，则磷化时，铁颗粒之间的间隔位置可能会出现未磷化。因此，优选地，本发明限定了相邻铁颗粒之间的间距最大不超过铁颗粒平均粒径的10倍。
进一步地，在本发明所述的冷轧低密度钢板中，所述钢板的微观组织为铁素体和残余奥氏体。
进一步地，在本发明所述的冷轧低密度钢板，所述残余奥氏体的相比例为6-30％。
进一步地，在本发明所述的冷轧低密度钢板中，所述残余奥氏体中C元素的质量百分含量不低于0.8％。
在本发明所述的磷化性能优异的冷轧低密度钢板中，C是重要的固溶强化元素，促进奥氏体生成，在富含Al元素的低密度钢中，若残余奥氏体中的C的质量百分比低于0.8％时，残余奥氏体的含量和力学稳定性相对较低，因而钢板的强度和延展性均较低。因此，本发明所述的磷化性能优异的冷轧低密度钢板残余奥氏体中的C含量不低于0.8％。
进一步地，在本发明所述的冷轧低密度钢板中，其密度＜7500kg/m3，使所述的冷轧低密度钢密度较低、轻量化，从而适用于制造汽车结构件。
进一步地，在本发明所述的冷轧低密度钢板中，其化学元素质量百分含量为：C：0.25～0.50％，Mn：0.25～4.0％，Al：3.0～7.0％，余量为Fe和其他不可避免的杂质。
其中，不可避免的杂质主要是S、P和N元素，可以控制P≤0.02％，S≤0.01％，N≤0.01％。
上述冷轧低密度钢板中的各化学元素的设计原理为：
C：C是重要的固溶强化元素，促进奥氏体生成，在富含Al的低密度钢中， C质量百分含量低于0.25％时，残余奥氏体的含量和力学稳定性相对较低，因而钢板的强度和延展性均较低；C质量百分含量高于0.5％时，片层状碳化物和分布在铁素体晶界处的碳化物颗粒粗大，从而降低钢板的轧制变形能力。因此，本发明控制C质量百分比为0.25～0.50％。
Mn：Mn能增加奥氏体的稳定性，降低钢淬火时临界冷却速度以及提高钢的淬透性。Mn还能够提高钢的加工硬化性能，从而提高钢板的强度。但是，过高的Mn含量会引起板坯中Mn偏析以及热轧板中明显的带状组织分布，从而降低钢板的延展性和弯曲性能；并且，过高的Mn含量容易造成热轧板在冷轧变形时产生裂纹。因此，本发明控制Mn质量百分含量为0.25～4.0％。
Al元素是铁素体形成元素。添加Al元素可显著降低钢板的密度，因此，在本发明中，Al元素的质量百分比含量不低于3.0％。然而，Al元素的质量百分比含量超过7.0％，会抑制奥氏体形成，另外，Al元素可显著增加钢中奥氏体的堆垛层错能，因而Al元素的质量百分比含量超过7.0％，会抑制钢中残余奥氏体在形变时被诱导发生马氏体相变，从而使钢板难以获得良好的强度和塑性匹配。因此，本发明限定Al元素的质量百分比含量为3.0～7.0％。
P：P是固溶强化元素；但是P会增加钢的冷脆性，降低钢的塑性，使冷弯性能和焊接性能变坏。因此，本发明限定P质量百分含量≤0.02％。
S：S使钢产生热脆性，降低钢的延展性和韧性，使焊接性能变坏，降低钢的耐蚀性。因此，本发明限定S质量百分含量≤0.01％。
N：N与Al形成AlN，凝固过程中可细化柱状枝晶，但N含量过高时，形成的粗大AlN颗粒影响钢板的延展性。另外，过量AlN会降低钢的热塑性。因此，本发明限定N质量百分含量≤0.01％。
此外，在本发明所述的冷轧低密度钢板中还可以选择添加Si、Ti、Nb、V、Cr、Mo、Ni、Cu、B、Zr、Ca元素的至少其中之一。
进一步地，在本发明所述的冷轧低密度钢板，其延伸率＞25％，抗拉强度＞800MPa。
本发明的另一目的在于提供一种所述的冷轧低密度钢板的制造方法，通过该方法可以制造得到上述任意一项磷化性能优异的冷轧低密度钢板。
为了实现上述目的，本发明提出了如上述任意一项的冷轧低密度钢板的制造方法，包括步骤：
(1)冶炼和铸造；
(2)热轧；
(3)酸洗；
(4)冷轧；
(5)连续退火：加热至均热温度750-950℃后保温30-600s，退火气氛的露点为-15℃～20℃，然后将经过均热的带钢冷却后卷取。
在本技术方案中，步骤(5)连续退火的均热温度及保温时间的选取主要是为了在连续退火后在钢板表面形成铁颗粒层。将均热温度控制在750℃-950℃，保温时间限定在30-600s，是因为：若均热温度低于750℃，保温时间低于30s，则冷轧低密度钢板钢基体中的马氏体尚未充分发生奥氏体逆相变生成奥氏体颗粒，冷轧低密度钢板钢基体中的碳化物尚未完全溶解生成奥氏体颗粒，并且条状的高温铁素无法充分进行动态再结晶而细化，并且会使得退火后的钢板表面的铁颗粒层未充分形成，磷化性能较差。而当均热温度高于950℃，保温时间高于600s时，均热处理后钢板基体组织中奥氏体晶粒粗化，钢中奥氏体稳定性降低，从而引起退火后钢板基体中残余奥氏体含量减少，并且残余奥氏体稳定性也会降低。相应的，退火后钢板的力学性能恶化，在均热温度高于950℃，保温时间高于600s时，会使得退火后钢板表面的铁颗粒粒径过大，内氧化层厚度过厚，而这都是不利于钢板的表面成形性能的。
此外，本技术方案中铁颗粒层的形成还与本技术方案中限定的退火气氛的露点密切相关，通过控制连续退火的退火气氛的露点抑制Al2O3的外氧化的形成，并将其转化为内氧化层的内氧化，使其在钢板表面形成铁颗粒。在上述露点范围内，退火气氛对Fe都是还原性的，因此会把氧化铁还原。若退火气氛露点低于-15℃，则上述退火气氛对钢基体中的Al元素依然是氧化的，钢基体中的Al会在钢板表面形成连续致密的Al2O3薄膜，从而影响磷化性能。若退火气氛露点高于+20℃，则退火气氛中的氧势过高，O原子向钢基体扩散的能力加大，在钢板表面与Al、Mn等合金元素形成内氧化层过厚，影响钢板表面的成形性能。
优选地，步骤(5)中保温时间为30-200s。
优选地，对于本技术方案来说，为了实现更好的实施效果，均热保温时间限定为30-200s。
进一步地，在本发明所述的冷轧低密度钢板的制造方法中，在所述步骤(2)中，加热温度为1000-1250℃，保温时间为0.5-3h，终轧温度800-900℃，然后在500-750℃的温度下卷取热轧板。
在本发明所述的磷化性能优异的冷轧低密度钢板的制造方法中，在所述步骤(2)中限定加热温度为1000-1250℃，是因为：加热温度超过1250℃时，会造成钢板的板坯过烧，板坯内晶粒组织粗大导致其热加工性能降低，并且超高温会引起板坯表面严重脱碳；加热温度低于1000℃时，板坯经高压水除鳞和初轧后，精轧温度过低，会造成板坯变形抗力过大，从而难以制造出既无表面缺陷又具有规定厚度的钢板。
在本发明所述的磷化性能优异的冷轧低密度钢板的制造方法中，在所述步骤(2)中限定保温时间为0.5-3h，是因为：保温时间超过3h，会造成钢板的板坯内晶粒组织初大和板坯表面脱碳严重，保温时间低于0.5h，板坯内部尚未均匀。
在本发明所述的磷化性能优异的冷轧低密度钢板的制造方法中，在所述步骤(2)中限定终轧温度800-900℃，是为了完成对铸坯的热轧，终轧温度过低会造成板坯变形抗力过高，从而难以制造出所需厚度规格并且无表面和边部缺陷的热轧钢板和冷轧钢板；另外，对于本发明来说，当终轧温度低于800℃时，板坯内部热轧条状高温铁素体无法获得充分回复和再结晶而细化。由于板坯在出炉后的热轧过程中板温会自然下降，要控制终轧温度高于900℃难度较大。
在本发明所述的磷化性能优异的冷轧低密度钢板的制造方法中，在所述步骤(2)中限定500-750℃下卷取热轧板，如果卷取温度高于750℃，容易引起扁卷，并且热轧卷的头部、中部、尾部材料显微组织不均匀性增加；如果卷取温度低于500℃，热轧卷抗拉强度过高会导致冷轧轧制困难。
进一步地，在本发明所述的冷轧低密度钢板的制造方法中，在所述步骤(4)中，冷轧压下量为30-90％。
在本发明所述的磷化性能优异的冷轧低密度钢板的制造方法中，在所述步骤(4)中限定冷轧压下量是为了：对酸洗后的热轧钢板实施冷轧变形至规定厚度，冷轧压下量＞30％可以在后续退火过程中提高奥氏体形成速率，有助于形成形变高温铁素体以及提高退火钢板的组织均匀性，从而提高退火钢板的延展性。但冷轧压下量＞90％，因加工硬化导致材料的变形抗力非常高，使得制 备规定厚度和良好板型的冷轧钢板变得异常困难。因此，本发明所述的冷轧低密度钢板的冷轧压下量控制在30-90％。
优选地，对于本技术方案来说，为了实现更好的实施效果，冷轧压下量为50-80％。
进一步地，在本发明所述的冷轧低密度钢板的制造方法，在所述步骤(5)中，加热段和保温段的气氛采用N2和H2混合气体，其中H2的体积含量为0.5-20％。
优选地，对于本技术方案来说，为了实现更好的实施效果，H2的体积含量为1-5％。
优选地，对于本技术方案来说，为了实现更好的实施效果，退火气氛露点为-10到0℃。
进一步地，在本发明所述的冷轧低密度钢板的制造方法，在所述步骤(5)中，加热的速率为1-20℃/s，均热之后的冷却速度为1-150℃/s。
在本发明所述的磷化性能优异的冷轧低密度钢板的制造方法中，在所述步骤(5)中，均热后的冷速选择1-150℃/s，优选冷速10-50℃/s，该冷速的选取需要避免钢板在冷却过程中奥氏体发生分解。
本发明所述的磷化性能优异的冷轧低密度钢板具有以下优点和有益效果：
(1)因为Al元素含量高，因此其密度低，本发明所述的冷轧低密度钢板的密度低于7500kg/m3，从而可以实现轻量化；
(2)本发明所述的磷化性能优异的冷轧低密度钢板因为具有铁颗粒层，因此其具有优异的磷化性能；
(3)本发明所述的磷化性能优异的冷轧低密度钢板的力学性能优异，其延伸率＞25％，抗拉强度＞800MPa。
附图说明
图1为本发明所述的磷化性能优异的冷轧低密度钢板的结构示意图。
图2为本发明所述的磷化性能优异的冷轧低密度钢板的截面金相组织。
图3为本发明所述的磷化性能优异的冷轧低密度钢板的实施例A2的表面扫描电镜二次电子像。
图4为本发明所述的磷化性能优异的冷轧低密度钢板的实施例A7的表面 扫描电镜二次电子像。
图5为本发明所述的磷化性能优异的冷轧低密度钢板的对比例B1的表面扫描电镜二次电子像。
图6为本发明所述的磷化性能优异的冷轧低密度钢板的实施例A2经磷化处理后的表面扫描电镜低倍背散射电子像。
图7为本发明所述的磷化性能优异的冷轧低密度钢板的实施例A2经磷化处理后的表面扫描电镜高倍二次电子像。
图8为本发明所述的磷化性能优异的冷轧低密度钢板的对比例B1经磷化处理后的表面扫描电镜低倍背散射电子像。
图9为本发明所述的磷化性能优异的冷轧低密度钢板的对比例B1经磷化处理后的表面扫描电镜高倍二次电子像。
具体实施方式
下面将结合附图说明和具体的实施例对本发明所述的磷化性能优异的冷轧低密度钢板及其制造方法做进一步的解释和说明，然而该解释和说明并不对本发明的技术方案构成不当限定。
图1显示了本发明所述的磷化性能优异的冷轧低密度钢板的结构。如图1所示，本发明所述的磷化性能优异的冷轧低密度钢板包括钢基板1、钢板表面的铁颗粒层3以及铁颗粒层内层具有紧邻铁颗粒层的内氧化层2。
图2显示本发明所述的磷化性能优异的冷轧低密度钢板的截面金相组织。如图2所示，在本发明所述的磷化性能优异的冷轧低密度钢板中，通过控制退火气氛的露点抑制Al2O3表面的铁颗粒层3的外氧化的形成，并将其转化成内氧化层2的内氧化，同时在钢板表面形成铁颗粒，进行磷化后，均能获得外观均匀、磷化膜完全覆盖的表面。其中，内氧化层2的厚度为0.2-10μm，且内氧化层2的氧化物存在于晶界和晶内，铁颗粒层3的厚度小于内氧化层的厚度，铁颗粒层3的厚度为0.1-5μm。
实施例A1-A16以及对比例B1-B6
表1列出了实施例A1-A16的磷化性能优异的冷轧低密度钢板以及B1-B6的常规钢板的化学元素的成分的质量百分比
表1.(wt％，余量为Fe)
  C Mn Al Si N S P
成分I 0.37 1.1 4.1 0.31 0.0025 0.002 0.004
成分II 0.45 2 6.1 - 0.0040 0.003 0.007
成分III 0.34 2.8 5.2 - 0.0027 0.003 0.007
从表1中可以看出，成分I、II、III中化学元素质量百分含量范围控制在：C：0.25～0.50％，Mn：0.25～4.0％，Al：3.0～7.0％，P≤0.02％，S≤0.01％，N≤0.01％，且成分I中还添加了Si。
实施例A1-A16中的磷化性能优异的冷轧低密度钢板以及B1-B6的常规钢板采用以下步骤制得：
(1)按照表1相应成分的化学元素质量百分含量冶炼和铸造；
(2)热轧，控制加热温度为1000-1250℃后，保温时间为0.5-3h，终轧温度≥800℃，然后在低于750℃的温度下卷取热轧板；
(3)酸洗；
(4)冷轧，冷轧压下量控制在30-90％；
(5)连续退火：加热至均热温度750-950℃后保温30-600s，然后将经过均热的带钢冷却后卷取，其中加热段和加热段和保温段的气氛采用N2和H2混合气体，其中H2的体积含量为0.5-20％，退火气氛的露点为-15℃～20℃，其中加热速率为1-20℃/s，均热之后的冷却速度为1-150℃/s。
表2列出了实施例A1-A16中的磷化性能优异的冷轧低密度钢板以及
B1-B6的常规钢板的具体工艺参数。
表2.
Figure PCTCN2017086174-appb-000001
Figure PCTCN2017086174-appb-000002
图3显示了实施例A2的表面扫描电镜二次电子像。图4显示了实施例A7的表面扫描电镜二次电子像。图5显示了对比例B1的表面扫描电镜二次电子像。
如图3、图4所示，实施例A2、A7表面均出现了铁颗粒，不同的是实施例A2的铁颗粒充分长大，铁颗粒间的间隙较小，而实施例A7的铁颗粒未充分长大，铁颗粒间的间隙较大，结合表2可知，实施例A2的均热保温时间比实施例A7的均热保温时间长，因此，本发明的均热保温时间优选在30-200s。图5显示了对比例B1的表面扫描电镜二次电子像，表面观察到一层Al2O3薄膜，但观察不到铁颗粒，其表面形貌特征完全不同于图3、图4所示的实施例，截面金相图可发现没有形成铁颗粒层以及内氧化层。
表3列出了实施例A1-A16中的磷化性能优异的冷轧低密度钢板以及B1-B6的常规钢板轧的各性能参数。
其中，磷化性能采用的测试方法为：在扫描电镜下随机选择10个500倍的视场观察磷化后的钢板表面磷化膜，并用图像软件对磷化膜的覆盖率进行统计，若10个视场的磷化膜平均覆盖率≥75％，则判断为磷化性能良好(用○表示)，若10个视场的磷化膜平均覆盖率<75％，则判断为磷化性能差(用X表 示)。
表3.
Figure PCTCN2017086174-appb-000003
由表3可以看出，实施例A1-A16的密度均＜7500kg/m3，延伸率均其延伸率＞25％，抗拉强度＞800MPa，并且磷化性能均优于对比例B1-B6。
图6为本发明所述的磷化性能优异的冷轧低密度钢板的实施例A2经磷化处理后的表面扫描电镜低倍背散射电子像。图7为本发明所述的磷化性能优异的冷轧低密度钢板的实施例A2经磷化处理后的表面扫描电镜高倍二次电子像。图8为本发明所述的磷化性能优异的冷轧低密度钢板的对比例B1经磷化处理后的表面扫描电镜低倍背散射电子像。图9为本发明所述的磷化性能优异的冷轧低密度钢板的对比例B1经磷化处理后的表面扫描电镜高倍二次电子像。
如图6所示，在扫描电镜低倍观察到实施例A2磷化均匀，进一步地，在 图7所示的高倍镜观察下可以看出，实施例A2中的磷化膜完整覆盖钢板表面，磷化结晶均匀。如图8所示，在扫描电镜的低倍观察下可以看到对比例B1磷化不均匀，其中黑色区域为有磷化结晶的位置，白色区域为未形成磷化结晶的位置，表面磷化覆盖率低，进一步放大后图像如图9所示，从图9中可以看出，对比例B1表面仅局部有磷化结晶。
这是由于：实施例的退火气氛露点为-15℃到+20℃，在上述露点范围内，能促使Al元素由外氧化转变为内氧化，避免在实施例钢板表面形成连续致密的Al2O3薄膜影响磷化，并且使得Al元素在钢板内氧化层形成厚度0.2-10μm。由于实施例钢板表层存在铁颗粒层，因此当实施例钢板进行磷化时，相当于在普通的软钢表面进行磷化。而对照例由于钢基板表面未形成有效的铁颗粒层，而是连续致密的Al2O3氧化膜，阻碍了磷化液与铁的反应，因此未形成有效的磷化膜。
需要注意的是，以上列举的仅为本发明的具体实施例，显然本发明不限于以上实施例，随之有着许多的类似变化。本领域的技术人员如果从本发明公开的内容直接导出或联想到的所有变形，均应属于本发明的保护范围。

Claims (21)

  1. 一种磷化性能优异的冷轧低密度钢板，其特征在于：
    所述冷轧低密度钢板的表面具有铁颗粒层，所述铁颗粒层中具有分散的铁颗粒；
    所述冷轧低密度钢板含有质量百分含量为3.0～7.0％的Al元素。
  2. 如权利要求1所述的冷轧低密度钢板，其特征在于，所述铁颗粒层内侧具有紧邻铁颗粒层的内氧化层，所述内氧化层中含有Al的氧化物。
  3. 如权利要求2所述的冷轧低密度钢板，其特征在于，所述内氧化层中还含有Mn的氧化物。
  4. 如权利要求2或3所述的冷轧低密度钢板，其特征在于，所述内氧化层的厚度为0.2-10μm。
  5. 如权利要求2或3所述的冷轧低密度钢板，其特征在于，所述内氧化层中的氧化物存在于晶界和晶内。
  6. 如权利要求2或3所述的冷轧低密度钢板，其特征在于，所述铁颗粒层的厚度小于内氧化层的厚度。
  7. 如权利要求1所述的冷轧低密度钢板，其特征在于，所述铁颗粒层的厚度为0.1-5μm。
  8. 如权利要求1所述的冷轧低密度钢板，其特征在于，所述铁颗粒的粒径为0.1-5μm
  9. 如权利要求1所述的冷轧低密度钢板，其特征在于，所述铁颗粒覆盖钢板表面30％以上的面积。
  10. 如权利要求1所述的冷轧低密度钢板，其特征在于，相邻铁颗粒之间的间距最大不超过铁颗粒平均粒径的10倍。
  11. 如权利要求1所述的冷轧低密度钢板，其特征在于，所述钢板的微观组织为铁素体和残余奥氏体。
  12. 如权利要求11所述的冷轧低密度钢板，其特征在于，所述残余奥氏体的相比例为6-30％。
  13. 如权利要求11或12所述的冷轧低密度钢板，其特征在于，所述残余奥氏体中C元素的质量百分含量不低于0.8％。
  14. 如权利要求1所述的冷轧低密度钢板，其特征在于，其密度＜7500kg/m3。
  15. 如权利要求1或14所述的冷轧低密度钢板，其特征在于，其化学元素质量百分含量为：C：0.25～0.50％，Mn：0.25～4.0％，Al：3.0～7.0％，余量为Fe和其他不可避免的杂质。
  16. 如权利要求15所述的冷轧低密度钢板，其特征在于，其延伸率＞25％，抗拉强度＞800MPa。
  17. 如权利要求1-16中任意一项所述的冷轧低密度钢板的制造方法，其特征在于，包括步骤：
    (1)冶炼和铸造；
    (2)热轧；
    (3)酸洗；
    (4)冷轧；
    (5)连续退火：加热至均热温度750-950℃后保温30-600s，退火气氛的露点为-15℃～20℃，然后将经过均热的带钢冷却后卷取。
  18. 如权利要求17所述的冷轧低密度钢板的制造方法，其特征在于，在所述步骤(2)中，加热温度为1000-1250℃，保温时间为0.5-3h，终轧温度800-900℃，然后在500-750℃的温度下卷取热轧板。
  19. 如权利要求17所述的冷轧低密度钢板的制造方法，其特征在于，在所述步骤(4)中，冷轧压下量为30-90％。
  20. 如权利要求17所述的冷轧低密度钢板的制造方法，其特征在于，在所述步骤(5)中，加热段和保温段的气氛采用N2和H2混合气体，其中H2的体积含量为0.5-20％。
  21. 如权利要求17所述的冷轧低密度钢板的制造方法，其特征在于，在所述步骤(5)中，加热的速率为1-20℃/s，均热之后的冷却速度为1-150℃/s。
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