WO2021136352A1 - 低碳低成本超高强复相钢板/钢带及其制造方法 - Google Patents

低碳低成本超高强复相钢板/钢带及其制造方法 Download PDF

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WO2021136352A1
WO2021136352A1 PCT/CN2020/141301 CN2020141301W WO2021136352A1 WO 2021136352 A1 WO2021136352 A1 WO 2021136352A1 CN 2020141301 W CN2020141301 W CN 2020141301W WO 2021136352 A1 WO2021136352 A1 WO 2021136352A1
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steel
strength
steel strip
ultra
steel sheet
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PCT/CN2020/141301
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English (en)
French (fr)
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张瀚龙
孙宜强
毛新平
张玉龙
王成
金鑫焱
王利
汪水泽
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宝山钢铁股份有限公司
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Priority to AU2020416427A priority Critical patent/AU2020416427A1/en
Priority to KR1020227021167A priority patent/KR20220115575A/ko
Priority to BR112022010497A priority patent/BR112022010497A2/pt
Priority to JP2022538801A priority patent/JP7482231B2/ja
Priority to US17/789,089 priority patent/US20230052592A1/en
Priority to EP20911271.3A priority patent/EP4086363A4/en
Publication of WO2021136352A1 publication Critical patent/WO2021136352A1/zh

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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • B21C47/02—Winding-up or coiling
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B32B15/00—Layered products comprising a layer of metal
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21—METALLURGY OF IRON
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the invention belongs to the field of metal materials, and specifically relates to a low-carbon, low-cost, ultra-high-strength, complex-phase steel plate/steel strip and a manufacturing method thereof, and is mainly applied to manufacturing automobile chassis and suspension system components.
  • the "lightweight” of automobiles can directly reduce emissions and reduce fuel consumption, which is the goal of today's automobile manufacturing industry.
  • An important measure for the "lightweight” of automobiles is to use high-strength and ultra-high-strength steel plates to replace low-strength steel plates.
  • the concept of "lightweight” is further applied to automobile chassis and suspension systems. Increasingly stringent environmental requirements and market demands also require the use of high-strength steel for automobile chassis materials to achieve "lightweight”.
  • chassis and suspension systems also require steel plates to have a higher yield tensile strength ratio (that is, the ratio of yield strength to tensile strength, hereafter referred to as yield ratio).
  • yield ratio the ratio of yield strength to tensile strength
  • chassis and suspension systems often require the material to be free from any plastic deformation, so the yield ratio of the material is extremely demanding. high.
  • the fatigue limit of a material is often proportional to its yield strength.
  • the higher the yield strength of the material means that the material has a higher fatigue limit. Therefore, for the chassis and For suspension system components, having a higher yield strength or being used for a higher yield ratio at the same tensile strength level is one of its goals.
  • the parts of automobile chassis and suspension systems often have complex structures (as shown in Figure 1), and processes such as flanging, reaming, bending, and drawing often coexist. Therefore, they have extremely high requirements on the forming performance of the material.
  • the steel needs to have a higher elongation and a larger hole expansion rate, and the steel needs to have excellent bending properties.
  • the three performance indexes of steel plate's strength and yield ratio, elongation and hole expansion rate mutually restrict each other, because this requires that the steel has phase components with higher strength and hardness (such as martensite).
  • a softer phase component (such as ferrite) is needed to ensure plasticity and elongation, and the organization needs to be uniform enough to improve bending performance, and there is also a need for a soft phase and a hard phase.
  • the difference in hardness is as small as possible to ensure high yield ratio and high hole expansion performance.
  • Chinese Patent Publication No. CN109055657A discloses an ultra-low-carbon high-strength composite steel plate. Through the large addition of noble metal elements such as niobium, molybdenum and nickel, the yield strength is ⁇ 690MPa and the yield ratio is 0.89 ⁇ 0.92, but the steel plate is not considered The reaming performance.
  • Chinese Patent Publication No. CN101906567A discloses a high-strength hot-rolled steel sheet. By adding a large amount of expensive alloy elements such as niobium, molybdenum, and nickel, the tensile strength is greater than 780MPa, and the hole expansion rate (the original hole is punched) is More than 40% performance.
  • one of the main methods is not to add expensive alloys such as molybdenum, vanadium, and niobium, but to choose a large amount of relatively low-cost alloying elements such as manganese, chromium, and titanium, through phase transformation Strengthening, solid solution strengthening and second-phase strengthening methods enhance the strength of the complex phase steel.
  • Chinese Patent Publication No. CN102732790A discloses an ultra-low carbon bainite steel sheet and a manufacturing method thereof. The steel sheet has a tensile strength greater than 770 MPa, although it does not contain precious metal microalloying elements such as niobium, molybdenum, and vanadium.
  • Chinese Patent Publication No. CNl01285156A discloses a bainite steel produced by thin slab continuous casting and rolling technology.
  • the patent uses thin slab continuous casting and rolling technology to design and manufacture a kind of bainite steel that does not contain niobium, molybdenum, High-strength bainitic steel with precious metal microalloying elements such as vanadium can combine high strength, high yield ratio, high elongation and certain bending properties.
  • Another low-cost high-strength composite steel preparation method is to increase the strength of the composite steel through solid solution strengthening by adding a large amount of silicon.
  • Chinese Patent Publication No. CNl756853A discloses a high-strength hot-rolled duplex steel composed of ferrite, bainite ⁇ martensite and a second phase and its manufacturing method. This patent guarantees the ultra-high strength of the complex phase steel by adding a large amount of silicon (up to 1.5%), thereby reducing the use of other alloying elements.
  • the automotive steel with a silicon content of 0.5% It is found that there are evenly spaced strip-shaped iron sheets on the surface of the strip steel, and the defects of the red iron sheet account for about 30% of the surface of the strip steel. Therefore, the use of high-silicon solutions to design and manufacture ultra-high-strength complex phase steel is not suitable for application in the automotive industry.
  • low-carbon high-strength composite steel with a tensile strength of 800 MPa used in the manufacture of automobile chassis and suspension system components
  • the existing technology cannot solve its low-cost design.
  • the purpose of the present invention is to provide a low-cost ultra-high-strength composite steel plate/steel strip and a manufacturing method thereof.
  • the tensile strength of the steel plate is ⁇ 780MPa, the yield strength is ⁇ 680MPa, the elongation is ⁇ 15%, and the yield ratio is ⁇ 0.9.
  • the porosity meets: if the original hole is a punched hole: the hole reaming rate is ⁇ 85%; if the original hole is a reamed hole: the hole reaming rate is ⁇ 115%, and the bending performance meets the 180° bending test.
  • Low-cost ultra-high-strength composite steel sheet/steel its composition weight percentage is: C: 0.03 ⁇ 0.07%, Si: 0.1 ⁇ 0.5%, Mn: 1.3 ⁇ 1.9%, P ⁇ 0.02%, S ⁇ 0.01%, Al: 0.01 ⁇ 0.05%, Cr: 0.2 ⁇ 0.5%, also containing Ti: 0.07 ⁇ 0.14%, (Ni+Nb+Mo+V) ⁇ 0.03%, the rest is Fe and inevitable impurities; and at the same time, it must meet:
  • C 0.04-0.06%, calculated by weight percentage.
  • Si 0.1-0.27%, calculated by weight percentage.
  • Mn 1.45 to 1.75%, calculated by weight percentage.
  • Cr 0.35 to 0.50%, calculated by weight percentage.
  • the steel sheet/steel strip does not contain V, Mo and Ni, and the Nb content is ⁇ 0.03%.
  • the microstructure of the steel sheet/steel strip of the present invention contains ferrite, lower bainite, carbide precipitation phase, inclusion phase and/or trace martensite phase, in which the content of ferrite is ⁇ 70%, and iron The content of element body + lower bainite is ⁇ 90%.
  • the ratio of columnar crystals is less than or equal to 10%, or the thickness of the columnar crystal region is less than 40 mm.
  • the average ferrite grain diameter is less than 6 ⁇ m, or the grain size ASTM rating is greater than 11.8.
  • the microstructure of the finished steel sheet/steel strip contains TiN particles, and the longest side length of a single particle is less than 10 ⁇ m or the area is less than 50 ⁇ m 2 .
  • the longest side length of a single particle of the TiN particles is less than 8 ⁇ m.
  • the tensile strength of the steel plate/strip through the above solution is ⁇ 800MPa
  • the yield strength is ⁇ 710MPa
  • the yield ratio of the steel plate/steel strip in the above solution is ⁇ 0.9.
  • Carbon directly affects the strength, weldability, formability, and manufacturability of thin slab continuous casting of steel plates/strips. The higher the carbon content, the more beneficial it is to increase the strength of the steel plate. If the carbon content is less than 0.03%, the strength of the steel plate/steel strip will not meet the target requirements; if the carbon content is higher than 0.07%, the strength of the steel plate/steel strip is likely to be too high. As a result, the hole expansion rate does not meet the requirements. Therefore, the present invention controls the carbon content in a range of 0.03 to 0.07%.
  • Silicon has a certain solid solution strengthening effect. The higher the Si content, the more beneficial it is to increase the yield strength of the steel plate/strip. At the same time, silicon also has the effect of inhibiting the precipitation of carbides. The addition of silicon can form a bainite structure without carbide precipitation; but when the silicon content is higher than 0.5%, the surface of the hot-rolled steel sheet/steel strip is prone to serious formation The red iron oxide scale not only deteriorates the surface quality of the steel sheet/steel strip, but also damages the plateability of the steel sheet/steel strip, which is not conducive to the production of hot-dip galvanized steel sheet/steel strip. Therefore, the present invention limits the silicon content to the range of 0.1% to 0.5%.
  • Manganese can effectively increase the strength of the steel plate/steel strip, and the cost is relatively low compared to other alloying elements. Therefore, the present invention uses manganese as the main additive element. However, when the manganese content is higher than 1.90%, not only the ratio or thickness of columnar crystals in the as-cast structure of the slab will be greatly increased, which will have a serious adverse effect on the control of the uniformity of the subsequent finished product, but the increase in Mn content will also lead to the final product. The increase of the martensite content in the structure impairs the hole expansion performance; when the manganese content is less than 1.40%, the strength of the steel plate/steel strip is insufficient. Therefore, the present invention limits the manganese content to 1.30% to 1.90%.
  • Aluminum is added as the main deoxidizer in the steelmaking process, but when the aluminum content is less than 0.01%, the deoxidizing effect is insufficient: when the aluminum content exceeds 0.05%, it affects the viscosity of molten steel and may cause nozzle nodules and damage Welding performance of steel plate/steel strip. Therefore, the present invention limits the aluminum content to 0.01 to 0.05%.
  • Chromium is conducive to expanding the bainite phase region, ensuring that the steel sheet/steel can obtain the bainite structure during cooling after rolling, and is conducive to improving the strength and hole expansion rate.
  • the addition amount exceeds 0.5%, the increase in strength is no longer significant, but it will not be conducive to the weldability of the steel sheet/steel strip, resulting in a substantial increase in the proportion or thickness of columnar crystals in the as-cast structure of the slab, and also causing the surface of the steel sheet to appear Chromium is enriched and formed, which affects the uniformity of the final product.
  • the present invention limits the chromium content to 0.2% to 0.5%.
  • Titanium is the main alloying element in the multi-phase steel of the present invention. It improves the strength of the multi-phase steel through solid solution strengthening and second phase strengthening after the formation of fine carbides. When the content of microalloy is low and 0.07%, the strength of the steel plate/steel is insufficient, and when the content of microalloy is higher than 0.14%, on the one hand, it will increase the cost, on the other hand, it is easy to form carbide center segregation, which is not conducive to the hole expansion performance. .
  • Niobium, vanadium, molybdenum and nickel (Nb, V, Mo, Ni): adding niobium, vanadium, molybdenum and nickel can also produce solid solution strengthening and second phase strengthening, thereby enhancing the strength of the composite steel.
  • the inclusion of these noble metal microalloying elements can also achieve the effect of grain refinement, which is beneficial to the hole expansion rate of the complex phase steel.
  • alloys of niobium, vanadium, molybdenum and nickel are extremely expensive.
  • niobium, vanadium, molybdenum and nickel in the present invention, that is, Nb+V+Mo+Ni ⁇ 0.03%.
  • the upper limit of impurity elements in steel is controlled at P: ⁇ 0.02%, S: ⁇ 0.01%. The purer the steel, the better the effect.
  • the microstructure of the steel plate/strip of the present invention is a microstructure of ferrite + lower bainite, and the ferrite content is less than or equal to 70%.
  • the content of ferrite + lower bainite is ⁇ 90%. If the ferrite structure is higher than 70%, the steel plate/strip will not reach the required strength; if the ferrite + lower bainite content is less than 90%, the hole expansion performance of the steel plate/strip will not meet the requirements .
  • the microstructure of the steel plate/steel strip of the present invention may also contain carbide precipitation phase, inclusion phase and/or trace martensite phase.
  • the content of carbide precipitation phase is usually below 5%, and the content of inclusion phase is usually below 5%. Below 0.01% (occasionally found in the field of view), the content of the martensite phase is usually below 0.5%.
  • the inclusions may be common inclusions in steel such as MnS, TiN, and AlN.
  • the average ferrite grain diameter is less than 6 ⁇ m, or the grain size ASTM rating is greater than 12.3. If the average grain diameter is not less than 6 ⁇ m or the grain size rating is not more than 12.3, the steel plate/strip will not be able to achieve the required strength.
  • the microstructure of the steel sheet/steel strip of the present invention contains TiN particles, and the longest side length of a single particle is less than 10 ⁇ m. If the longest side length of a single particle is not less than 10 ⁇ m, the steel plate/steel strip will not be able to achieve the required hole expansion performance.
  • the manufacturing method of the ultra-low-carbon, low-cost, ultra-high-strength composite steel sheet/steel strip of the present invention includes the following steps:
  • the slab enters the heating furnace at a temperature not lower than 700°C to heat the cast slab at a heating temperature of 1100 ⁇ 1250°C; the reduction rate of each pass of the first and second hot rolling passes is ⁇ 55%, and the finishing rolling is finished
  • the rolling temperature is 850 ⁇ 950°C;
  • step 3 pickling, it also includes a hot-dip galvanizing annealing process to obtain a finished hot-rolled hot-dip galvanized steel sheet.
  • the slab cooling rate during continuous casting will affect the grain size in the final structure of the steel sheet/steel strip, the size of inclusions formed in the liquid phase, and the ratio of columnar crystals in the slab structure. If the cooling rate is lower than 5°C/s, on the one hand, the thickness or proportion of the columnar crystals of the slab will be higher than the design requirements, which will easily form a band-like structure in the subsequent finished product structure, which will affect the bending performance of the steel plate/strip; on the other hand, On the other hand, the decrease in the cooling rate of the slab during continuous casting will cause the grain size in the final structure to be out of design requirements, and will cause the size of the inclusions (typically TiN) generated in the liquid phase in the steel to be coarse, which is very important for hole expansion and bending. Performance is adversely affected.
  • the cooling rate is lower than 5°C/s, on the one hand, the thickness or proportion of the columnar crystals of the slab will be higher than the design requirements, which will easily form a band-like structure in
  • the lowest temperature before the slab enters the heating furnace will affect the final performance of the product.
  • the minimum temperature of the slab before entering the heating furnace is less than 700°C
  • titanium carbide will precipitate in the slab in large quantities, and in the subsequent reheating process, the titanium carbide that has been precipitated in the slab cannot be completely re-dissolved into the slab.
  • the solid solution titanium and titanium carbide in the matrix after hot rolling are both less, resulting in insufficient product strength.
  • the finishing temperature of finishing rolling is less than 850°C, ferrite will precipitate before finishing rolling, resulting in low bainite content in the final structure and making the steel plate/strip unable to reach the set strength.
  • the final rolling temperature of the finishing rolling should not exceed 950°C.
  • the reduction rate per pass of the first and second hot rolling passes is ⁇ 55%; when the reduction rate is insufficient, the fineness cannot be obtained.
  • the uniform structure of the structure results in insufficient strength of the steel plate/steel strip, and the bending performance cannot meet the design requirements.
  • the high pressure reduction rate in the above step 2) must match the high cooling rate of the slab during continuous casting in step 1).
  • the continuous casting cooling rate cannot reach 5°C/s or more, it will cause the liquid phase in the slab
  • the size of the generated inclusions (mainly TiN) is too large.
  • a large reduction ratio of ⁇ 55% is used in step 2), it will cause the coarse TiN to crack, as shown in Figure 1, and become a steel plate/
  • the crack source inside the steel strip causes the deterioration of the hole expansion and bending performance of the steel plate/steel strip; and if the continuous casting cooling rate can reach more than 5°C/s, the inclusions (mainly TiN) formed in the liquid phase in the slab
  • the small size, as shown in Figure 2 will not break during the large hot rolling reduction in step 2), so that it will not adversely affect the hole reaming and bending properties of the steel plate/steel strip.
  • the coiling temperature is one of the most critical process parameters for obtaining high strength and high hole expansion rate.
  • the coiling temperature is greater than 640°C, the alloy carbides are strongly precipitated and coarsened, which will have a negative effect on the hole expansion rate of the steel plate.
  • the coiling temperature is less than 550°C, the precipitation of carbides will be severely inhibited, resulting in The strength of the steel plate cannot meet the set requirements, therefore.
  • the present invention limits the coiling temperature to 550 to 630°C.
  • the performance of the ultra-high-strength hot-rolled steel sheet/steel strip meets the following indicators:
  • the tensile strength is ⁇ 780MPa, preferably ⁇ 800MPa; the yield strength is ⁇ 680MPa, preferably ⁇ 710MPa; the yield ratio is ⁇ 0.9. In some embodiments, the tensile strength is 780-900 MPa, and the yield strength is 680-830 MPa.
  • the hole expansion rate is greater than 85%, preferably ⁇ 95%, and the highest can reach more than 100%; in some embodiments, the hole expansion rate is 86% to 110%;
  • the hole expansion rate is greater than 115%, preferably ⁇ 120%, which can reach up to 130% or more; in some embodiments, the hole expansion rate is 117% to 140%.
  • the present invention adopts low-cost composition design to reduce the addition of alloy elements, especially precious metal alloy elements, and requires [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20( Nb+V)] ⁇ 3.0, and (Nb+V+Mo+Ni) ⁇ 0.03, to ensure that the complex phase steel has extremely low alloy cost.
  • alloy elements especially precious metal alloy elements
  • niobium and vanadium alloys have the highest cost.
  • the unit price can reach 20 times the unit cost of manganese; followed by molybdenum and nickel.
  • the unit price can be It reaches 10 times the unit cost of manganese. Therefore, this type of element is basically not used in the present invention.
  • the optimal composition design is selected to minimize the amount of addition to achieve the lowest overall cost.
  • the present invention basically does not add expensive alloying elements, which tends to cause insufficient material strength, such as CN103667948A.
  • the present invention mainly adopts fine-grain strengthening combined with Ti element strengthening, by optimizing the ratio of precipitation strengthening and solid solution strengthening, and achieving high uniformity of the microstructure and small enough inclusions to achieve high strength and high elongation The combination of high rate, high hole expansion rate and excellent bending performance.
  • a certain amount of manganese, chromium, titanium, especially chromium and titanium is still added.
  • the strengthening is achieved by solid dissolving in the matrix, that is, solid solution strengthening; on the other hand, strengthening is achieved by the precipitation of carbides. That is, the second phase is strengthened.
  • solid solution strengthening improves the strength weaker than the second phase strengthening, but the precipitation of carbides will damage the hole expansion performance of the duplex steel. Therefore, it is necessary to find a balance between the two strengthening mechanisms, that is, the following relationship between the addition of alloying elements and the content of carbon elements is required: (Mn+2Cr+4Ti+4Nb+4V+4Mo-Si/3+2C) ⁇ 3.0.
  • the formula of the present invention expresses the contribution of each alloy element in the material to the precipitation effect of the second phase.
  • the ability of Mn, Cr, Ti (or Nb, Mo, V) three types of elements to form carbide precipitation phases is gradually improved, so a gradient is adopted
  • the Si element has the effect of inhibiting the precipitation of carbides, so it is designed as a negative coefficient in the formula. Therefore, the larger the value of the above formula, the higher the overall contribution of the alloy elements added to the material to the precipitation strengthening.
  • the present invention finds that when the value of the above formula is higher than 3.0, the hole expansion performance of the material will be greatly reduced.
  • the first and second passes of hot rolling are designed with a large reduction rolling process to further destroy the columnar crystals while obtaining a fine structure, achieving both high strength, high bending, and high elongation.
  • the ultra-high-strength hot-rolled steel sheet products, steel strip products, and hot-dip galvanized steel sheet products manufactured by the present invention can be used to manufacture automobile chassis and suspension system parts, and at the same time have low cost, high strength, high hole expansion and good bending. Performance, thus making up for the gap of low-cost and high-quality chassis steel products that the market urgently needs in the automotive industry chain.
  • Figure 1 shows the size of TiN particles when the continuous casting cooling rate reaches more than 5°C/s and their morphology after hot rolling (hot-rolled microstructure photo).
  • Figure 2 shows the size of TiN particles when the continuous casting cooling rate is less than 5°C/s and their morphology after hot rolling (hot-rolled microstructure photo).
  • Figure 3 shows that the microstructure of the steel sheet/steel strip of the present invention contains ferrite and lower bainite, wherein the content of ferrite + lower bainite is ⁇ 90%.
  • punching and reaming are used to prepare the original hole in the center of the test piece, and the subsequent tests and test methods are in accordance with the ISO/DIS 16630 standard.
  • the reaming rate test method is executed.
  • the 180° bending test is performed using the bending performance measurement method in the GB/T232-2010 standard.
  • Examples A to I are steels of the present invention, and the content of carbon or manganese or other alloying elements in Comparative Examples J to N is beyond the range of the present invention.
  • M refers to the composition of [Mn+1.5Cr+5( Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)] item calculated value
  • R refers to the (Mn+2Cr+4Ti+4V+4Nb+4Mo-Si/3+2C) item in the composition Calculated value.
  • Comparative Example O and Comparative Example P are the disclosed examples in CN101906567A and CN101285156A respectively.
  • Table 2 shows the different manufacturing processes of each steel grade in Table 1, which are also divided into two categories: Examples and Comparative Examples.
  • the processes of Comparative Example O and Comparative Example P are the manufacturing processes disclosed in the corresponding patent applications.
  • Table 3 shows the measured values of the mechanical properties of the above-mentioned examples and comparative examples. Among them, the performance of Comparative Example O and Comparative Example P is the performance disclosed in the corresponding patent application. As can be seen from the table, the performance of Comparative Example O and Comparative Example P are both inferior to the embodiments of the present invention.
  • the strength will not meet the design standard of the present invention; if the coiling temperature is too low, such as the comparative example D-2, the precipitation of carbides in the steel will be inhibited , Resulting in too low strength of the steel plate.
  • the present invention controls a reasonable composition range, limits the content of alloying elements, optimizes the ratio of each element, and greatly reduces alloy costs.
  • the conventional automobile steel production line it is further connected Casting cooling rate, hot rolling reduction rate, coiling temperature, low-cost ultra-high-strength hot-rolled steel sheet/steel strip with high strength, high hole expansion performance and excellent bending performance are produced, and its yield strength is not less than 680MPa, and its tensile strength is not less than 680MPa.

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Abstract

低成本超高强复相钢板/钢带及其制造方法，其成分重量百分比为：C 0.03～0.07％，Si 0.1～0.5％，Mn 1.3～1.9％，P≤0.02％，S≤0.01％，Al 0.01～0.05％，Cr 0.2～0.5％，Ti 0.07～0.14％、(Ni+Nb+Mo+V)＜0.03％，其余为Fe和其他不可避免的杂质；且Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)＜3.0；Mn+2Cr+4Ti+4Nb+4V+4Mo-Si/3+2C≤3.0。所述钢板的抗拉强度≥780MPa，屈服强度≥680MPa，延伸率≥15％，屈强比≥0.9，扩孔率满足：若原始孔为冲压孔：扩孔率≥85％；若原始孔为铰孔：扩孔率≥115％，弯曲性能满足180°弯曲实验弯角直径等于0.5板厚(d＝0.5a)时不裂；主要用于汽车底盘、悬挂***零部件的制备。

Description

低碳低成本超高强复相钢板/钢带及其制造方法 技术领域
本发明属于金属材料领域，具体涉及一种低碳低成本超高强复相钢板/钢带及其制造方法，主要应用于制造汽车底盘及悬挂***的零部件产品。
背景技术
汽车“轻量化”可直接减少排放，降低油耗，是当今汽车制造业发展的目标。汽车“轻量化”的一个重要的措施就是采用高强度和超高强度的钢板来代替低强度钢板。目前，“轻量化”概念进一步沿用至汽车底盘及悬挂***，日益严苛的环保要求和市场需求也要求汽车底盘材料采用高强钢实现“轻量化”。
然而，除了要求钢板具有更高的强度，汽车底盘及悬挂***的结构件还要求钢板具有较高的屈服抗拉强度比(即屈服强度与抗拉强度的比值，本文以下简称屈强比)，良好的延伸率、良好的扩孔性能和良好的弯曲性能。不同于一些车身零部件需要在碰撞时通过一定的塑性变形来吸收碰撞能量或避免对行人造成刚性碰撞伤害，底盘及悬挂***往往要求材料不能出现任何塑性变形，因此对材料的屈强比要求极高。此外，根据公知，材料的疲劳极限往往与其屈服强度成正比，因此在同一抗拉强度级别的情况下，材料具有越高的屈服强度就意味着材料具有更高的疲劳极限，因此，对于底盘及悬挂***零部件来说，具有更高的屈服强度或在同一抗拉强度级别用于更高的屈强比是其所追求的目标之一。
此外，汽车底盘及悬挂***的零部件，往往具有复杂的结构(如图1)，翻边、扩孔、弯曲、拉延等工序往往并存，因而对材料的成形性能具有极高的要求，不仅需要钢材具有较高的延伸率、较大的扩孔率，也需要钢板具有优异的弯曲性能。但是，根据公知，钢板的强度和屈强比、延伸率与扩孔率三个性能指标彼此之间相互制约，因为这既需要钢中有强度和硬度较高的相组分(如马氏体、贝氏体)来保证强度，需要有较软的相组分(如铁素体)来保证塑性和延伸率，还需要组织足够均匀来提高弯曲性能，同时还需要软相和硬相之间的硬度差异尽量小来保证高屈强比 和高扩孔性能。
除了上述苛刻的性能要求以外，汽车底盘及悬挂***的零部件制备还需要材料具有良好的表面质量、可涂装性和正常均匀的色泽，以及较低的碳当量水平以保证零部件的可焊性能，因此常见的成分设计是低碳成分(碳含量小于0.1％)。而在这种成分体系下，为获得兼具高强度高屈强比，高延伸率、高扩孔性和高弯曲性能的钢材，以满足形状复杂、高品质高服役性能的汽车底盘及悬挂***的零部件的制造，通常会选用以贝氏体为主，铁素体为辅的复相钢，并通过添加大量合金元素，尤其是铌、钼、钒、镍、铝等昂贵的合金元素来实现析出强化，从而提升铁素体强度并保证铁素体塑性，以此来提升屈强比并保证钢材具有较高的延伸率和扩孔率。
如中国专利公开号CN109055657A公开了一种超低碳高强度的复相钢板，通过大量添加铌、钼、镍等贵金属元素，实现获得屈服强度≥690MPa，屈强比0.89～0.92，但未考虑钢板的扩孔性能。同样，中国专利公开号CN101906567A公开了一种高强度热轧钢板，通过大量添加铌、钼、镍等昂贵的合金元素，获得了抗拉强度大于780MPa，扩孔率(原始孔为冲孔)为40％以上的性能。
但是，随着钢铁行业和汽车行业内部竞争日益激烈，这种添加大量合金元素、尤其是钼、钒、铌等昂贵合金的钢材，极大地增加了整个汽车行业产业链的成本压力，因此，市场上急需一种不添加或几乎不添加昂贵的合金元素(即低成本)，又兼具高强度及高屈强比、高延伸率、高扩孔与高弯曲性能的具有优异服役和综合成形性能的钢板。
但是一旦不添加钼、钒、铌等昂贵合金，低成本复相钢很难达到高强度(如抗拉强度800MPa)级别，如中国专利公开号CN103667948A公开了一种不含铌、钒、镍、钼等贵金属微合金元素的复相钢及其制造方法，该专利设计的复相钢虽然具有较低的合金成本，且复相钢的扩孔性能较好，但是强度较低，实施例中的屈服强度只有不高于520MPa的屈服强度和不高于725MPa的抗拉强度，因此只能设计用来制造汽车车轮，而无法满足汽车底盘零部件对复相钢产品零部件强度的要求。
为解决低成本复相钢强度不足的问题，主要的方法之一是不添加钼、钒、铌等昂贵合金，而选择大量加入锰、铬、钛等相对价格较低的合金元素，通过相变强化、固溶强化和第二相强化的方式提升复相钢强度。如中国专利公开号CN102732790A公开了一种超低碳贝氏体钢板及其制造方法，该钢板抗拉强度大于770MPa，虽然 不包含铌、钼、钒等贵金属微合金元素，但是。却含有3.0％～4.5％的锰，导致其成本依旧较高，而该专利并未考虑复相钢的扩孔性能与弯曲性能。再比如中国专利公开号CNl01285156A公开了一种利用薄板坯连铸连轧技术生产制造的贝氏体钢，虽然该专利通过薄板坯连铸连轧技术设计、制造出一种不含铌、钼、钒等贵金属微合金元素的高强贝氏体钢，可以兼具高强度、高屈强比、高延伸率和一定的弯曲性能，但是由于其大量添加铬、锰、钛等元素，其产品的生产制造成本仍然较高，且该产品弯曲性能一般，且依旧未考虑其扩孔性能。实际上，锰、铬元素极易在钢板中形成偏聚，造成局部成分和相组分差异，从而导致钢板的扩孔率和弯曲性能恶化，而锰元素在连铸时容易造成板坯内柱状晶粗大，粗大的柱状晶会对后续钢板热轧和冷轧组织产生遗传影响，进而影响钢板组织均匀性、对扩孔性能和弯曲性能产生不利影响。
另一种低成本高强度复相钢的制备方法，是通过大量添加硅元素，通过固溶强化提升复相钢强度。如中国专利公开号CNl756853A公开了一种含有铁素体、贝氏体\马氏体和第二相组成的高强度热轧复相钢及其制造方法。该专利通过添加大量的硅元素(最高1.5％)来保证复相钢具备超高强度，从而降低其他合金元素的使用量。但是，硅元素含量过高会导致热轧钢板/钢带表面出现严重的红色氧化铁皮，进而直接造成钢板/钢带成品的表面出现严重色差，直接影响其作为汽车用结构件的表面质量和美观。根据公知：于洋，王畅，王林等，基于高温氧化特性的含Si钢红铁皮缺陷研究[J]，轧钢，2016，33(2)：10—15；于洋，王畅，王林等，基于高温氧化特性的含Si钢红铁皮缺陷研究[J]，轧钢，2016，33(2)：10—15；王畅，于洋，王林等，硅元素对炉生铁皮界面微观结构的影响研究[J]，轧钢，2016，33(5)：6—10；当钢中硅含量较高时，可形成红鳞等缺陷而导致钢材表面质量降低，其中含0.5％的硅含量的汽车用钢中发现带钢表面存在等间距的条带状铁皮，红铁皮缺陷占带钢表面比例约30％。因此，采用高硅元素的方案来设计并制造超高强复相钢并不适合应用于汽车工业。
因此，对于这类应用于汽车底盘和悬挂***零部件制造用的抗拉强度达到800MPa级别的低碳复相钢(以下简称低碳高强复相钢)，现有的技术无法解决其低成本设计与高综合服役性能之间的矛盾，即便采用现在已公开的低成本设计，材料的强度、屈强比、扩孔性能和弯曲性能也无法兼得。因此，如何获得兼具低成本 与高强度高扩孔特性的复相钢板/钢带，以满足汽车零部件的生产制造需要，一直是钢铁工业界的难题。
发明内容
本发明的目的在于提供一种低成本超高强复相钢板/钢带及其制造方法，该钢板的抗拉强度≥780MPa、屈服强度≥680MPa，延伸率≥15％，屈强比≥0.9，扩孔率满足：若原始孔为冲压孔：扩孔率≥85％；若原始孔为铰孔：扩孔率≥115％，弯曲性能满足180°弯曲实验弯角直径等于0.5板厚(d＝0.5a)时不裂；主要用于汽车底盘、悬挂***零部件的制备。
为达到上述目的，本发明的技术方案是：
低成本超高强复相钢板/钢带，其成分重量百分比为：C：0.03～0.07％，Si：0.1～0.5％，Mn：1.3～1.9％，P≤0.02％，S≤0.01％，Al：0.01～0.05％，Cr：0.2～0.5％，还含有Ti：0.07～0.14％、(Ni+Nb+Mo+V)＜0.03％，其余为Fe和不可避免的杂质；且同时需满足：
[Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)]＜3.0；
(Mn+2Cr+4Ti+4Nb+4V+4Mo-Si/3+2C)≤3.0。
优选的，所述的钢板/钢带的化学成分中C：0.04～0.06％，以重量百分比计。
优选的，所述的钢板/钢带的化学成分中Si：0.1～0.27％，以重量百分比计。
优选的，所述的钢板/钢带的化学成分中Mn：1.45～1.75％，以重量百分比计。
优选的，所述的钢板/钢带的化学成分中Cr：0.35～0.50％，以重量百分比计。
优选的，所述的钢板/钢带不含有V、Mo和Ni，且Nb含量≤0.03％。
本发明所述钢板/钢带的微观组织含有铁素体、下贝氏体，还含有碳化物析出相、夹杂物相和/或微量马氏体相，其中铁素体含量≤70％，铁素体+下贝氏体含量≥90％。
优选的，所述的钢板/钢带的板坯铸态组织中，柱状晶的比例≤10％，或柱状晶区的厚度＜40mm。
优选的，所述的钢板/钢带成品态的微观组织中，铁素体晶粒平均直径＜6μm，或晶粒度ASTM评级＞11.8。
优选的，所述的钢板/钢带的成品态的微观组织中含有TiN颗粒，且单个颗粒 的最长边长＜10μm或面积＜50μm 2。优选地，所述TiN颗粒单个颗粒的最长边长＜8μm。
通过上述方案所述钢板/钢带的抗拉强度≥780MPa、屈服强度≥680MPa，扩孔率性能指标：若原始孔为冲压孔：扩孔率≥85％；若原始孔为铰孔：扩孔率≥115％；180°弯曲d＝0.5a合格。
优选地，通过上述方案所述钢板/钢带的抗拉强度≥800MPa、屈服强度≥710MPa，扩孔率性能指标：若原始孔为冲压孔：扩孔率≥95％；若原始孔为铰孔：扩孔率≥120％；180°弯曲d＝0.5a合格。
优选地，上述方案所述钢板/钢带的屈强比≥0.9。
在本发明钢的成分设计中：
碳(C)：碳直接影响钢板/钢带的强度、焊接性、成形性和薄板坯连铸的可制造性。碳含量越高，越有利于提高钢板的强度，若碳含量低于0.03％，钢板/钢带的强度达不到目标要求；碳含量高于0.07％，容易造成钢板/钢带强度过高，从而导致扩孔率不满足要求。因此，本发明控制碳含量的范围为0.03～0.07％。
硅(Si)：硅具有一定的固溶强化作用，Si含量越高越有利于提升钢板/钢带的屈服强度。同时，硅还有抑制碳化物析出的效果，可以通过添加硅元素来形成无碳化物析出的贝氏体组织；但当硅含量高于0.5％时，热轧钢板/钢带表面易生成严重的红色氧化铁皮，不仅恶化钢板/钢带的表面质量，同时损害钢板/钢带的可镀性，不利于生产热镀锌钢板/钢带。因此，本发明将硅含量限定在0.1～0.5％的范围内。
锰(Mn)：锰可以有效地提升钢板/钢带的强度，而且成本相对其他合金元素较低，因此本发明将锰作为主要添加元素。但是当锰含量高于1.90％时，不仅板坯铸态组织中柱状晶的比例或厚度会大幅增加，从而对后续成品组织均匀性的控制产生严重不良影响以外，Mn含量增加还会导致最终成品组织中马氏体含量的增加，损害扩孔性能；当锰含量低于1.40％时，钢板/钢带的强度不足。因此本发明将锰含量限定在1.30～1.90％。
铝(Al)：铝是作为炼钢过程的主要脱氧剂而加入，但铝含量小于0.01％时，脱氧效果不足：铝含量超过0.05％时，影响钢水粘度，可能会造成水口结瘤，并损害钢板/钢带的焊接性能。因此，本发明将铝含量限定在0.01～0.05％。
铬(Cr)：铬有利于扩大贝氏体相区，保证钢板/钢带在轧后冷却中可以得到贝 氏体组织，有利于提高强度和扩孔率。但添加量超过0.5％时，强度提升不再显著，反而会不利于钢板/钢带的可焊性，导致板坯铸态组织中柱状晶的比例或厚度大幅增加，且还会导致钢板表面出现铬元素富集而形成，从而影响最终产品的组织均匀性。但当含量小于0.2％时，对贝氏体相区的扩大并不显著。因此，本发明将铬含量限定在0.2～0.5％。
钛(Ti)：钛是本发明复相钢中的主要合金元素，通过固溶强化，和形成细小的碳化物之后的第二相强化来提升复相钢的强度。当微合金含量低与0.07％时，钢板/钢带的强度不足，而当微合金含量高于0.14％时，一方面会增加成本，另一方面容易形成碳化物中心偏析，不利于扩孔性能。
铌、钒、钼和镍(Nb、V、Mo、Ni)：加入铌、钒、钼和镍也可以产生固溶强化和第二相强化，从而提升复相钢强度。此外，这些贵金属微合金元素的计入还可以实现细化晶粒的效果，从而有利于复相钢的扩孔率。但，相比钛元素，铌、钒、钼和镍的合金成本极高。虽可作为可选元素加入钢中，但从低成本角度考虑，本发明中不建议添加铌、钒、钼和镍，即Nb+V+Mo+Ni＜0.03％。
钢中的杂质元素的上限控制在P：≤0.02％，S：≤0.01％，钢质越纯净效果更佳。
本发明所述钢板/钢带的微观组织为为铁素体+下贝氏体的微观组织，铁素体含量≤70％。铁素体+下贝氏体含量≥90％。若铁素体组织高于70％，钢板/钢带将无法达到所要求的强度；若铁素体+下贝氏体含量低于90％，则钢板/钢带的扩孔性能达不到要求。本发明所述钢板/钢带的微观组织还可含有碳化物析出相、夹杂物相和/或微量马氏体相，碳化物析出相的含量通常在5％以下，夹杂物相的含量通常在0.01％以下(偶尔可在视场中发现)，马氏体相的含量通常在0.5％以下。所述夹杂物可以是MnS、TiN和AlN等钢中常见夹杂物。
本发明所述的钢板/钢带的微观组织中，铁素体晶粒平均直径＜6μm，或晶粒度ASTM评级＞12.3。若晶粒平均直径不低于6μm或晶粒度评级不大于12.3，则钢板/钢带将无法达到所要求的强度。
本发明所述的钢板/钢带的微观组织中含有TiN颗粒，且单个颗粒的最长边长＜10μm。若单个颗粒的最长边长不小于10μm，将导致钢板/钢带无法达到所要求的扩孔性能。
此外，以各元素的含量百分比计，上述合金元素与碳元素的计量关系还应满足如下公式：
①[Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)]＜3.0，以保证钢板/钢带具有低合金成本；
②(Mn+2Cr+4Ti+4Nb+4V+4Mo-0.5Si+5C)≤3.0，以保证钢板/钢带微观组织中能够析出一定量碳化物颗粒，而不发生碳化物析出过多、过粗大或过于聚集而造成的扩孔率降低。从而可以保证钢板/钢带兼具较高的强度和扩孔性能。
本发明的超低碳低成本超高强复相钢板/钢带的制造方法，包括如下步骤：
1)冶炼、连铸
按上述化学成分冶炼并通过连铸铸造成铸坯，连铸时板坯冷却速率≥5℃/s；
2)热轧轧制、轧后冷却
板坯在不低于700℃的温度下进加热炉，对铸坯加热，加热温度为1100～1250℃；热轧第一、第二道次每道次压下率≥55％，精轧终轧温度为850～950℃；
3)轧后冷却、卷取
轧后采用水冷，卷取温度为550～630℃；
4)酸洗。
进一步地，步骤3)酸洗后，还包括热镀锌退火工艺，获得热轧热镀锌钢板成品。
上述步骤1)中，连铸时的板坯冷却速率将影响钢板/钢带最终组织中的晶粒尺寸、液相中形成的夹杂物尺寸和板坯组织中柱状晶的比例。若冷却速度低于5℃/s时，一方面板坯柱状晶的厚度或比例会高于设计要求，从而容易在后续成品组织中形成带状组织，影响钢板/钢带的弯曲性能；另一方面，连铸时板坯冷速的下降会导致最终组织中的晶粒尺寸将无法设计要求，而且会导致钢中液相生成的夹杂物(典型如TiN)的尺寸粗大，对扩孔和弯曲性能产生不利影响。
上述步骤2)中，板坯进加热炉之前的最低温度将影响产品最终的性能。当板坯进加热炉之前的最低温度小于700℃时，碳化钛会在板坯中大量析出，而在后续的再加热过程中，板坯中已析出的碳化钛无法再完全重溶进板坯中，造成热轧后基体中的固溶钛和碳化钛均较少，导致产品强度不足。而当精轧终轧温度小于850℃时，在精轧前就会有铁素体析出，造成最终组织中贝氏体含量偏低，使钢板/钢带无 法达到设定强度。但考虑到板坯加热温度，精轧终轧温度不超过950℃。此外，上述步骤2)中，为保证钢板/钢带具有细小和高度均匀的组织，热轧第一、第二道次每道次压下率≥55％；压下率不足时，无法获得细小的组织均匀的组织，导致钢板/钢带强度不够，弯曲性能不能达到设计要求。不仅如此，上述步骤2)中的高压下率必须与步骤1)中连铸时板坯的高冷速相配合，若连铸冷速无法达到5℃/s以上，会导致板坯中液相生成的夹杂物(以TiN为主)尺寸过大，此时若在步骤2)中采用≥55％的大压下率，会导致粗大的TiN开裂，如附图1所示，从而成为钢板/钢带内部的裂纹源，造成钢板/钢带扩孔和弯曲性能的恶化；而若连铸冷速可以达到5℃/s以上时，板坯中液相生成的夹杂物(以TiN为主)尺寸细小，如附图2所示，不会在步骤2)中的大热轧压下时破裂，从而不会对钢板/钢带的扩孔和弯曲性能产生不利影响。
上述步骤3)中，卷取温度是获得高强度、高扩孔率的最为关键的工艺参数之一。当卷取温度大于640℃时，由于合金碳化物的强烈析出和粗化，对钢板扩孔率有负面作用，另一方面，当卷取温度小于550℃时会严重抑制碳化物的析出，造成钢板强度无法达到设定要求，因此。本发明将卷取温度限定为550～630℃。
经检测，本发明提供的超高强热轧钢板/钢带性能满足如下指标：
1、常温力学性能：
抗拉强度≥780MPa，优选≥800MPa；屈服强度≥680MPa，优选≥710MPa；屈强比≥0.9。在一些实施方案中，抗拉强度为780-900MPa，屈服强度为680-830MPa。
2、断裂延伸率：
A50≥15％或A5≥19％。
3、扩孔率性能：
若原始孔为冲压孔：则扩孔率大于85％，优选≥95％，最高可达到100％以上；在一些实施方案中，扩孔率为86％到110％；
若原始孔为铰孔：则扩孔率大于115％，优选≥120％最高可达到130％以上；在一些实施方案中，扩孔率为117％到140％。
4、弯曲性能：
180°冷弯，d＝0.5a合格。
本发明的有益效果：
1.本发明采用低成本的成分设计，减少合金元素，尤其是贵金属合金元素的添加量，并要求[Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)]≤3.0，和(Nb+V+Mo+Ni)＜0.03，以确保复相钢具备极低的合金成本。在复相钢的常见合金元素中，铌和钒合金成本最高，在考虑品位的情况下，单价可以达到锰元素成本单价的20倍；其次是钼和镍，在考虑品位的情况下，单价可以达到锰元素成本单价的10倍，因此，这类元素在本发明中基本不予采用。而其他的合金元素，如钛、铝、锰、铬等，则选取最优化的成分设计，尽量减小添加量，实现综合成本最低。
2.出于低成本的市场需求，本发明基本不添加昂贵的合金元素，从而容易造成材料强度不足，如CN103667948A。为提升材料强度，本发明主要采用细晶强化结合Ti元素强化，通过优化析出强化和固溶强化的配比，并实现微观组织的高度均匀与夹杂物的足够细小，来实现高强度、高延伸率、高扩孔率与优良弯曲性能的兼得。在本发明中，仍然添加了一定量的锰、铬、钛，尤其是铬和钛，一方面通过固溶于基体中实现强化，即固溶强化；另一方面通过形成碳化物析出实现强化，即第二相强化。这两种强化机制，固溶强化对强度的提升弱于第二相强化，但是碳化物的析出会损害复相钢的扩孔性能。因此需要在两种强化机制中寻找平衡，即要求合金元素添加量与碳元素含量之间满足如下关系：(Mn+2Cr+4Ti+4Nb+4V+4Mo-Si/3+2C)≤3.0。本发明公式表示材料中各合金元素对第二相析出效应的贡献，其中，Mn、Cr、Ti(或Nb、Mo、V)三类元素形成碳化物析出相的能力逐渐提升，故采用了梯度的系数设计，而Si元素有抑制碳化物析出的作用，故在公式中设计为负系数，因此，上式值越大，代表材料中添加的合金元素整体对析出强化的贡献会更高。本发明发现，当上式值高于3.0时，材料的扩孔性能会大幅下降。
3.通过成分和工艺的设计，实现细小的高均匀的组织及尺寸细小的夹杂物，从而获得优良的弯曲性能。在设计中首先采用较低的Mn和Cr的设计，以避免连铸后钢板板坯中出现大尺寸的柱状晶组织，以尽量降低柱状晶对后续生产获得高均匀度组织的不利影响；其次，在连铸中采用高冷速设计，一方面继续降低板坯中柱状晶比例提升细小等轴晶的比例，另一方面降低液相中生成的夹杂物的尺寸(以TiN为代表)；最后在热轧的第一、二道次，采用大压下的轧制工艺设计，在进一步破 坏柱状晶的同时，获得细小的组织，实现高强度、高弯曲、和高延伸率的兼得。
本发明制造的超高强热轧钢板产品和钢带产品及热镀锌钢板成品可用于制备汽车底盘、悬挂***零部件的制造，并同时兼备低成本、高强度、高扩孔性与良好的弯曲性能，从而弥补了汽车产业链中市场亟需的低成本高质量的底盘用钢产品的空白。
附图说明
图1为连铸冷速达到5℃/s以上时TiN颗粒的尺寸及其在热轧大压下之后的形貌(热轧态组织照片)。
图2为连铸冷速不足5℃/s时TiN颗粒的尺寸及其在热轧大压下之后的形貌(热轧态组织照片)。
图3显示本发明所述钢板/钢带的微观组织含有铁素体和下贝氏体，其中，铁素体+下贝氏体含量≥90％。
具体实施方式
下面结合实施例对本发明做进一步说明。
将表1中所示的不同成分的钢经冶炼后按表2所示加热+热轧工艺后得到厚度小于4mm的钢板。取沿纵向50mm标距和5mm标距的拉伸试样测定屈服、抗拉强度及延伸率，取钢板中部区域测定扩孔率和180°弯曲性能；试验数据如表2所示。其中，扩孔率采用扩孔试验测定，用凸模把中心带孔的试件压入凹模，使试件中心孔扩大，直到板孔边缘出现颈缩或贯穿裂纹为止。由于试件中心原始孔的制备方式对扩孔率测试结果存在较大影响，因此，分别采用冲孔和铰孔制备试件中心原始孔，后续试验及测试方法按ISO/DIS 16630标准中规定的扩孔率测试方法执行。180°弯曲实验采用GB/T232-2010标准中弯曲性能的测定方法执行。
表1中，实施例A～I为本发明的钢，对比例J～N中碳或锰或其他合金元素含量超出本发明成分的范围，表中M指成分中[Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)]项的计算值，R指成分中(Mn+2Cr+4Ti+4V+4Nb+4Mo-Si/3+2C)项的计算值。此外，比较例O和比较例P分别是CN101906567A和CN101285156A中的公开的实施例，从对比中可以看出，比 较例O和比较例P的M和R值都超过本发明设计范围，表示这两个比较例的合金成本都高于本发明案例，且未采用本发明设计的公式优化合金配比。
表2为表1中各钢种的不同制造工艺，同样分为实施例和比较例两大类，其中比较例O和比较例P的工艺为对应专利申请中公开的制造工艺。表3则为上述实施例和比较例的力学性能检测值。其中，比较例O和比较例P的性能为对应专利申请中公开的性能，从表中可以看出，比较例O和比较例P的性能均逊于本发明中的实施例。
可见，当C、Mn、Ti等合金成分偏离本发明范围时，如Mn、Ti含量较低时，如对比例K和M，会导致钢板的强度均小于设计要求；而当C、Ti含量或R值高出本发明的成分范围时，如对比例J、L和N，其中C、Mn含量超标会导致组织中生产大量的马氏体，恶化材料的扩孔性能和弯曲性能，而Ti含量和R值过高也会导致组织中碳化物粗化，恶化材料的扩孔性能，均不符合本发明的目的。
当板坯进炉温度过低时，如对比钢A-2，会导致强度不满足本发明设计标准；若卷取温度过低，如对比例D-2，会导致钢中碳化物析出受到抑制，导致钢板强度过低。当热轧前两道次压下率不够时，无法彻底消除钢板的带状组织，且不能充分细化晶粒，实现组织均匀性，会导致钢板延伸率的弯曲性能变差，如对比例B-2；而当连铸冷却速度不够，但热轧却追求大压下率时，会导致钢中粗大的TiN颗粒破碎，形成潜在裂纹源，大幅恶化材料的延伸率、扩孔性能和弯曲性能，如对比例C-2。
综上所述，本发明在碳锰钢的基础上，控制合理的成分范围，限制合金元素的含量，优化各元素配比，大幅降低合金成本，在常规的汽车用钢生产线基础上，进一步连铸冷却速度，热轧压下率，卷取温度，生产出兼具高强度、高扩孔性能与优良弯曲性能的低成本超高强热轧钢板/钢带，其屈服强度不小于680MPa，抗拉强度不小于780MPa，扩孔率不小于85％(原始孔为冲孔)或不小于115％(原始孔为铰孔)，180°弯曲d＝0.5a，以弥补汽车行业市场对兼具低成本和高强度高成性能的底盘、悬挂件用材的迫切需求。
Figure PCTCN2020141301-appb-000001
Figure PCTCN2020141301-appb-000002
Figure PCTCN2020141301-appb-000003

Claims (15)

  1. 超高强复相钢板/钢带，其成分质量百分比为：C：0.03～0.07％，Si：0.1～0.5％，Mn：1.3～1.9％，P≤0.02％，S≤0.01％，Al：0.01～0.05％，Cr：0.2～0.5％，Ti：0.07～0.14％，(Ni+Nb+Mo+V)＜0.03％，其余为Fe和其他不可避免的杂质；且同时需满足：
    [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)]≤3.0；
    (Mn+2Cr+4Ti+4Nb+4V+4Mo-Si/3+2C)≤3.0。
  2. 如权利要求1所述的超高强复相钢板/钢带，其特征在于，所述C含量为：0.04～0.06％，以重量百分比计。
  3. 如权利要求1所述的超高强复相钢板/钢带，其特征在于，所述Si含量为：0.1～0.27％，以重量百分比计。
  4. 如权利要求1所述的超高强复相钢板/钢带，其特征在于，所述Mn含量为：1.45～1.75％，以重量百分比计。
  5. 如权利要求1所述的超高强复相钢板/钢带，其特征在于，所述Cr含量为：0.35～0.50％，以重量百分比计。
  6. 如权利要求1所述的超高强复相钢板/钢带，其特征在于，所述化学成分中Nb+Mo+V＜0.03％，以重量百分比计。
  7. 如权利要求1-6任一项所述的超高强复相钢板/钢带，其特征在于，钢中组织含有铁素体、下贝氏体，还含有碳化物析出相、夹杂物相和/或微量马氏体相，其中铁素体含量≤70％，铁素体+下贝氏体含量≥90％。
  8. 如权利要求7所述的超高强复相钢板/钢带，其特征在于，所述钢板/钢带的微观组织中还含有TiN颗粒，且单个颗粒的最长边长＜10μm或面积＜50μm 2。
  9. 如权利要求7所述的超高强复相钢板/钢带，其特征在于，所述铁素体晶粒平均直径＜6μm，或铁素体晶粒度ASTM评级＞11.8。
  10. 如权利要求1-9任一项所述的超高强复相钢板/钢带，其特征在于，所述钢板/钢带的抗拉强度≥780MPa、屈服强度≥680MPa，扩孔率满足：若原始孔为冲压孔：扩孔率≥85％；若原始孔为铰孔：扩孔率≥115％，弯曲性能满足180°弯曲d＝0.5a合格。
  11. 如权利要求1所述的超高强复相钢板/钢带，其特征在于，所述钢板/钢带的屈强比≥0.9，延伸率≥15％。
  12. 如权利要求1-11任一项所述的超高强复相钢板/钢带的制造方法，包括如下步骤：
    1)冶炼、连铸
    按权利要求1-6任一项所述的化学成分冶炼并通过连铸铸造成板坯，连铸时冷速≥5℃/s；
    2)板坯热送、轧制、轧后冷却、卷取
    板坯在不低于700℃的温度下进加热炉，对板坯加热，加热温度为1100～1250℃；板坯热轧时前两道次压下率均≥55％；精轧终轧温度为850～950℃，卷取温度为550～630℃；
    3)酸洗。
  13. 如权利要求12所述的超高强复相钢板/钢带的制造方法，其特征在于，步骤3)酸洗后，还包括热镀锌退火工艺，获得热轧热镀锌钢板成品。
  14. 如权利要求12所述的超高强复相钢板/钢带的制造方法，其特征在于，步骤1)中，所述板坯铸态组织中柱状晶的比例≤10％，或柱状晶区的厚度＜40mm。
  15. 如权利要求12所述的超高强复相钢板/钢带的制造方法，其特征在于，所述钢板/钢带的厚度为0.7～4.0mm。
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