WO2022042733A1 - 一种780MPa级高表面超高扩孔钢及其制造方法 - Google Patents

一种780MPa级高表面超高扩孔钢及其制造方法 Download PDF

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WO2022042733A1
WO2022042733A1 PCT/CN2021/115435 CN2021115435W WO2022042733A1 WO 2022042733 A1 WO2022042733 A1 WO 2022042733A1 CN 2021115435 W CN2021115435 W CN 2021115435W WO 2022042733 A1 WO2022042733 A1 WO 2022042733A1
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steel
ultra
content
high hole
steel according
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French (fr)
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王焕荣
杨峰
张晨
华骏山
刘运华
倪亚平
王明
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宝山钢铁股份有限公司
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Definitions

  • the invention belongs to the field of high-strength steel, and particularly relates to a 780MPa grade high-surface ultra-high hole-expanding steel and a manufacturing method thereof.
  • Pickling products are commonly used for many components in passenger cars, especially chassis and body parts.
  • the lightweight of passenger cars is not only the development trend of the automobile industry, but also the requirements of laws and regulations.
  • the fuel consumption is stipulated in the laws and regulations, in fact, it is a disguised requirement to reduce the weight of the body, which is reflected in the requirement of high strength, thinning and light weight.
  • High-strength weight reduction is an inevitable requirement for subsequent new models, which will inevitably lead to higher steel grades, and at the same time, the chassis structure will inevitably bring changes: if the parts are more complex, the material properties, surfaces and forming technologies such as hydroforming, hot stamping, Laser welding has put forward higher requirements, which are then transformed into high strength, stamping, flanging, springback and fatigue properties of materials.
  • the development of domestic high-strength and high-hole-expanding steel not only has a relatively low strength level, but also has poor performance stability.
  • the high-hole-expansion steel used by domestic auto parts companies is basically high-strength steel with a tensile strength below 600MPa, and the competition for high-hole-expansion steel below 440MPa is fierce.
  • the high hole-expanding steel with a tensile strength of 780MPa is gradually being used in batches in China, but higher requirements are also put forward for the two important indicators in the forming process, elongation and hole expansion rate.
  • the automobile industry has reached an inflection point and competition has become more intense.
  • the hole expansion rate index should be further increased from the current ⁇ 50% to ⁇ 70%.
  • 780MPa grade high hole-expanding steel mostly adopts the design idea of high silicon composition system, and the structure is mainly bainite, and there is also a certain amount of precipitation strengthening.
  • the surface of the strip steel after pickling not only has obvious red iron scale, but also the hole expansion rate is basically between 50-65%, and the elongation of bainite structure is low, which cannot meet the higher hole expansion rate proposed by users. performance requirements.
  • Cikona grade pickling high hole expansion steel has been involved in many patent applications.
  • Chinese patent application CN103602895A relates to a low-carbon Nb-Ti micro-alloyed high-hole-enlarging steel.
  • Its composition design features are low-carbon and high-silicon Nb-Ti micro-alloying, the guaranteed hole expansion rate is ⁇ 50%, and the high silicon composition
  • the design usually brings red iron scale on the surface of the steel plate, and the coiling temperature range required to form bainite is around 500 °C. It is difficult to control the temperature of the entire length of the steel coil, which is easy to cause large fluctuations in the performance of the entire length.
  • Patent application CN105821301A involves A 800MPa grade hot-rolled high-strength and high-hole-expanding steel, its composition design is also characterized by low-carbon and high-silicon Nb-Ti microalloying, and its Ti content reaches a very high level, ranging from 0.15 to 0.18%.
  • This kind of composition design idea not only has defects such as red iron skin on the surface of the strip, but also easily forms coarse TiN with ultra-high Ti content, which is very unfavorable for the stability of hole expansion rate;
  • Chinese patent application CN108570604A relates to a 780MPa grade hot-rolled acid-washed high-hole-expansion steel, whose composition design features are low-carbon, high-aluminum, and high-chromium, and a three-stage cooling process is adopted in the process design.
  • the high-aluminum design is likely to cause the casting nozzle to block in the actual production process, and the process is complicated, the three-stage cooling process is difficult to control, and the hole expansion rate is not high.
  • the above patents all have the problems of red iron sheet, difficulty in making steel, and difficulty in controlling the temperature uniformity of the entire length of the strip.
  • the elongation of the material is inversely proportional to the hole expansion ratio, that is, the higher the elongation, the lower the hole expansion ratio; conversely, the lower the elongation, the higher the hole expansion ratio. Tata Steel's technicians have also found this pattern in practice. Then it is very difficult to obtain high-strength high-enlarging steel with high elongation and high-enlargement at the same time.
  • the object of the present invention is to provide a 780MPa grade high surface ultra-high hole expanding steel and a manufacturing method thereof, the obtained steel plate/coil has good surface quality, avoids red iron scale on the surface of the strip steel, and improves the surface quality of the pickling high strength steel; Yield strength ⁇ 700MPa, tensile strength ⁇ 780MPa, elongation A50 ⁇ 17%, hole expansion rate ⁇ 80%, to achieve a good match of high surface quality, high strength, high plasticity, ultra-high hole expansion rate, and can be used in passenger cars Chassis parts such as control arms and subframes require high strength and thinning.
  • the technical scheme of the present invention is:
  • a 780MPa grade high-surface ultra-high hole-expanding steel, its chemical composition weight percentage is:
  • the rest is Fe and other inevitable impurities.
  • Nb ⁇ 0.06%, V ⁇ 0.05%, Cu ⁇ 0.5%, Ni ⁇ 0.5%, Cr ⁇ 0.5%, B ⁇ 0.001%, Ca ⁇ 0.005% can be added, wherein, The Nb and V contents are preferably ⁇ 0.03%, the Cu, Ni, and Cr contents are preferably ⁇ 0.3%, the B content is preferably ⁇ 0.0005%, and the Ca content is preferably ⁇ 0.002%.
  • the chemical composition weight percentage of the ultra-high hole-expanding steel of the present invention is: C: 0.03-0.08%, Si: ⁇ 0.2%, Mn: 0.5-2.0%, P: ⁇ 0.02%, S: ⁇ 0.003%, Al: 0.01 ⁇ 0.08%, N: ⁇ 0.004%, Ti: 0.05 ⁇ 0.20%, Mo: 0.1 ⁇ 0.5%, Mg: ⁇ 0.005%, O: ⁇ 0.0030%, Nb ⁇ 0.06%, V ⁇ 0.05%, Cu ⁇ 0.5%, Ni ⁇ 0.5%, Cr ⁇ 0.5%, B ⁇ 0.001%, Ca ⁇ 0.005%, the rest is Fe and other inevitable impurities, and it contains Nb, V, Cu, Ni, Cr At least one or two or more of , B and Ca.
  • the microstructure of the ultra-high hole-expanding steel of the present invention is irregular ferrite, nano-sized carbides in ferrite grains and a very small amount of pearlite.
  • the volume percentage of pearlite is less than or equal to 0.5% by volume ratio.
  • the ultra-high hole-expanding steel of the present invention has a yield strength ⁇ 700MPa, preferably ⁇ 730MPa, more preferably ⁇ 750MPa, tensile strength ⁇ 780MPa, preferably ⁇ 800MPa, more preferably ⁇ 810MPa, elongation A50 ⁇ 17%, preferably ⁇ 19% , the hole expansion ratio is ⁇ 80%, preferably ⁇ 85%, more preferably ⁇ 90%.
  • the ultra-high hole-expanding steel of the present invention has a yield strength ⁇ 750MPa, a tensile strength ⁇ 810MPa, an elongation A50 ⁇ 19%, and a hole expansion rate ⁇ 90%.
  • the ultra-high hole-expanding steel of the present invention has a thickness of 1.8-6.0 mm.
  • Carbon the basic element in steel, is also one of the important elements in the present invention. Carbon expands the austenite phase region and stabilizes the austenite. As an interstitial atom in steel, carbon plays a very important role in improving the strength of steel, and has the greatest impact on the yield strength and tensile strength of steel.
  • the structure to be obtained is full ferritic, in order to obtain a high-strength steel with a tensile strength of 780 MPa, it is necessary to ensure that the carbon content is above 0.03%. When the carbon content is below 0.03%, the tensile strength of the ferrite structure is difficult to reach 780MPa; but the carbon content cannot be higher than 0.08%. If the carbon content is too high, it is easy to form pearlite structure during hot rolling and coiling, which is not good for hole expansion performance. Therefore, the carbon content should be controlled between 0.03-0.08%, and the preferred range is between 0.04-0.06%.
  • Silicon is the basic element in steel. As mentioned above, in order to meet the requirements of high strength, high plasticity and ultra-high hole expansion ratio proposed by users, more silicon is usually added in the composition design, but the composition design of high silicon brings about the reduction of the surface quality of the steel plate. More red tin defects. In the present invention, in order to ensure good surface quality, the content of silicon should be strictly controlled in the composition design. In other words, silicon is an impurity element in the present invention. Considering that silicon manganese needs to be used for deoxidation in actual steelmaking, it is difficult to completely avoid the addition of silicon.
  • the silicon content when the silicon content is below 0.2%, surface red iron sheet defects can be avoided during hot rolling, and usually below 0.15% can guarantee no red iron sheet. Therefore, the content of silicon in the steel should be controlled within 0.2%, preferably within 0.15%. In some embodiments, the silicon content is 0.05-0.2%. In some embodiments, the silicon content is 0.05-0.15%.
  • Mn is an important element to expand the austenite phase region, which can stabilize austenite, refine grains and delay the transformation of austenite to pearlite.
  • the Mn content is usually controlled at more than 0.5%; at the same time, the Mn content should generally not exceed 2.0%, otherwise, Mn segregation is likely to occur during steelmaking, and the slab will be continuously Hot cracking is also prone to occur during casting. Therefore, the Mn content in the steel is generally controlled at 0.5-2.0%, preferably in the range of 1.0-1.6%.
  • Phosphorus is an impurity element in steel. P is easy to segregate on the grain boundary. When the content of P in the steel is high ( ⁇ 0.1%), Fe 2 P is formed to precipitate around the grain, reducing the plasticity and toughness of the steel, so the lower the content, the better. It is better to control within 0.02% without increasing the cost of steelmaking.
  • Sulfur is an impurity element in steel.
  • S in steel usually combines with Mn to form MnS inclusions, especially when the content of S and Mn is high, more MnS will be formed in the steel, and MnS itself has a certain plasticity.
  • the deformation in the rolling direction not only reduces the transverse plasticity of the steel, but also increases the anisotropy of the structure, which is unfavorable for the hole expansion performance. Therefore, the lower the S content in the steel, the better.
  • the S content needs to be strictly controlled, and the S content is required to be controlled within 0.003%, and the preferred range is below 0.0015%.
  • the role of aluminum in steel is mainly deoxidation and nitrogen fixation.
  • the main role of Al is to deoxidize and refine grains.
  • the content of Al is usually controlled at 0.01-0.08%; if the content of Al is less than 0.01%, it cannot achieve the effect of refining grains; similarly, When the Al content is higher than 0.08%, the grain refinement effect is saturated. Therefore, the content of Al in the steel can be controlled between 0.01-0.08%, and the preferred range is between 0.02-0.05%.
  • Nitrogen is an impurity element in the present invention, and the lower the content, the better. But nitrogen is an inevitable element in the steelmaking process. Although its content is small, when combined with strong carbide forming elements such as Ti, the formed TiN particles have a very adverse effect on the properties of the steel, especially on the hole expansion performance. Due to the square shape of TiN, there is a large stress concentration between its sharp corners and the matrix. During the process of hole expansion and deformation, the stress concentration between TiN and the matrix is likely to form a crack initiation source, which greatly reduces the hole expansion performance of the material. Since the present invention adopts a high-titanium design in the composition system, in order to minimize the adverse effect on hole expansion caused by TiN. Therefore, the nitrogen content should be controlled below 0.004%, preferably below 0.003%.
  • Titanium is one of the important elements in the present invention. Ti mainly plays two roles in the invention: one is to combine with the impurity element N in the steel to form TiN, which plays a part of "fixing nitrogen"; the other is to form a uniform dispersion from the ferrite during the coiling phase transformation Fine nano-scale carbides improve strength, plasticity and hole expansion.
  • the content of titanium is lower than 0.05%, there is no obvious precipitation strengthening effect; when the content of titanium is higher than 0.20%, the coarse TiN easily leads to poor impact toughness of the steel plate. Therefore, the content of titanium in the steel should be controlled between 0.05-0.20%, and the preferred range is 0.07-0.11%;
  • Molybdenum is one of the important elements in the present invention.
  • the addition of molybdenum to steel can greatly delay the transformation of ferrite and pearlite, which is beneficial to obtain irregular ferrite structure.
  • the formed nano-scale titanium carbide and molybdenum precipitation phase has the effect of resisting high temperature roughening, which can ensure that no roughening occurs for a long time after coiling and avoid strength reduction.
  • molybdenum has strong resistance to welding softening properties. Since the main purpose of the present invention is to obtain ferrite and nano-precipitated structure, adding a certain amount of molybdenum can effectively reduce the degree of welding softening. Therefore, the content of molybdenum should be controlled in the range of 0.1-0.5%, preferably in the range of 0.15-0.45%.
  • Magnesium is one of the important elements in the present invention.
  • the addition of magnesium to steel can preferentially form finely dispersed MgO in the steelmaking stage. These fine MgO can be used as the nucleation particles of TiN.
  • the nucleation points of TiN can be effectively increased and the size of TiN can be reduced. Since TiN has a great influence on the hole expansion rate of the final steel plate, it is easy to cause the hole expansion rate to be unstable. Therefore, the Mg content in the steel can be controlled within 0.005%.
  • Oxygen is an unavoidable element in the steelmaking process.
  • the oxygen content in the steel can generally reach below 30 ppm after deoxidation, which will not cause obvious adverse effects on the performance of the steel plate. Therefore, it is sufficient to control the O content in the steel within 30 ppm.
  • Copper is an additive element in the present invention. Adding copper to steel can improve the corrosion resistance of steel. When it is added together with P element, the corrosion resistance effect is better; to a strong precipitation strengthening effect. However, the addition of Cu is easy to form the phenomenon of "Cu embrittlement" during the rolling process. In order to make full use of the effect of improving the corrosion resistance of Cu in some applications, and at the same time not cause significant "Cu embrittlement" phenomenon, usually Cu The content of the element is controlled within 0.5%, preferably within 0.3%.
  • Nickel is an additive element in the present invention.
  • the addition of nickel to steel has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper.
  • the addition of nickel to steel has little effect on the tensile properties of steel, but it can refine the structure and precipitation of steel, and greatly improve the low temperature toughness of steel. ; At the same time, adding a small amount of nickel can inhibit the occurrence of "Cu brittleness" in the steel added with copper.
  • the addition of higher nickel has no obvious adverse effect on the properties of the steel itself. If copper and nickel are added at the same time, it can not only improve the corrosion resistance, but also refine the structure and precipitation of the steel, greatly improving the low temperature toughness. But since copper and nickel are both relatively precious alloying elements. Therefore, in order to minimize the cost of alloy design, the addition amount of nickel is usually ⁇ 0.5%, and the preferred range is ⁇ 0.3%.
  • Chromium is an additive element in the present invention. Chromium is added to steel to improve the strength of steel mainly through solid solution strengthening or microstructure refinement. Since the structure in the present invention is fine bainitic ferrite plus nano-precipitated carbides, since a large number of dispersed fine nano-scale carbides strongly pin the dislocations, the ratio of the yield strength to the tensile strength of the steel is the yield strength. The strength ratio is high, usually above 0.90. Adding a small amount of chromium element can appropriately reduce the yield strength of steel, thereby reducing the yield strength ratio. In addition, the addition of a small amount of chromium can also play a role in improving corrosion resistance, usually the addition amount of chromium is ⁇ 0.5%, and the preferred range is ⁇ 0.3%.
  • Niobium is one of the elements that can be added in the present invention. Similar to titanium, niobium is a strong carbide element in steel. The addition of niobium to steel can greatly increase the non-recrystallization temperature of steel. In the finishing rolling stage, deformed austenite with higher dislocation density can be obtained. The final tissue can be refined. However, the addition amount of niobium should not be too much. On the one hand, the addition amount of niobium exceeds 0.06%, which is easy to form relatively coarse carbonitrides of niobium in the structure, which consumes some carbon atoms and reduces the precipitation strengthening effect of carbides.
  • the high content of niobium also easily causes the anisotropy of the hot-rolled austenite structure, which is inherited to the final structure in the subsequent cooling transformation process, which is detrimental to the hole expansion performance. Therefore, the niobium content in the steel is usually controlled to be ⁇ 0.06%, and the preferred range is ⁇ 0.03%.
  • Vanadium is an addable element in the present invention. Like titanium and niobium, vanadium is also a strong carbide former. However, the carbides of vanadium have a low solid solution or precipitation temperature, and are usually all dissolved in austenite in the finishing rolling stage. Vanadium begins to form in ferrite only when the temperature is lowered to start the phase transformation. Since the solid solubility of vanadium carbides in ferrite is greater than that of niobium and titanium, the size of vanadium carbides formed in ferrite is relatively large, which is not conducive to precipitation strengthening and contributes far to the strength of steel.
  • the addition amount of vanadium in steel is usually ⁇ 0.05%, and the preferred range is ⁇ 0.03%.
  • Boron is an addable element in the present invention. Boron can greatly improve the hardenability of steel, which is beneficial to obtain martensite structure. Considering that the structure expected to be obtained in the hot rolling stage of the present invention is ferrite instead of martensite, the content of boron element in the steel needs to be strictly controlled to prevent the formation of martensite due to excessive addition of boron element. In addition, the addition of boron to steel can form irregular ferrite structure or even Mao component, which is detrimental to the low temperature impact toughness of steel. Therefore, the addition amount of boron in steel is usually controlled at ⁇ 0.001%, and the preferred range is ⁇ 0.0005%.
  • Calcium is an addable element in the present invention.
  • Calcium can improve the morphology of sulfides such as MnS, so that elongated sulfides such as MnS become spherical CaS, which is beneficial to improve the morphology of inclusions, thereby reducing the adverse effect of elongated sulfides on hole expansion performance.
  • the addition of calcium oxide will increase the amount of calcium oxide, which is detrimental to the hole expansion performance. Therefore, the addition amount of steel grade calcium is usually ⁇ 0.005%, and the preferred range is ⁇ 0.002%.
  • the manufacturing method of the 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention comprises the following steps:
  • the rolling temperature is 1050-1150°C, and the rough rolling is carried out at a high pressure of 3-5 passes above 1050°C and the accumulated deformation is ⁇ 50%, and then the intermediate billet is air-cooled or water-cooled to 950-1000°C, and then 3-7 passes of refining are carried out. Rolling and the accumulated deformation ⁇ 70%, the final rolling temperature is 850-950 °C, preferably 850-930 °C;
  • the strip pickling speed is adjusted within the range of 30-140m/min, the pickling temperature is controlled between 75-85°C, the pull-correction rate is controlled within ⁇ 3%, and the rinsing is carried out at the temperature range of 35-50°C to ensure The surface quality of strip steel is dried and oiled at 120-140°C.
  • Casting billet (ingot) heating temperature: ⁇ 1230°C, holding time: 1-2 hours, rolling temperature: 1050-1150°C, under 3-5 passes of rough rolling and high pressure above 1050°C and the accumulated deformation ⁇ 50% , the main purpose is to refine the austenite grains while retaining more solid solution titanium.
  • the cumulative deformation of rough rolling is > 80%.
  • high-temperature coiling forms ferrite and nano-precipitates within the ferrite with uniform and fine structure.
  • the rolling rhythm should be completed as quickly as possible to ensure that more solid solution titanium is in the austenite.
  • the strip is cooled online to 550-650°C at a cooling rate of ⁇ 10°C/s to obtain ferrite and nano-precipitated structures.
  • the accumulated deformation in finish rolling is > 80%.
  • the cooling rate is 15-70°C/s.
  • the thermal stress and microstructure stress inhomogeneity formed inside the steel coil during the phase transformation of high temperature coiling are fully released during pickling and straightening, which further improves the microstructure uniformity.
  • the coiling temperature is 550-650°C.
  • the present invention adopts the design idea that does not contain Si or contains a small amount of Si.
  • the formation of red iron scale on the surface of the strip can be avoided, which is beneficial to the stable control of key process parameters in the actual production process; on the other hand, high temperature coiling is adopted.
  • This conventional process can obtain ferrite and nano-precipitation structure with uniform performance, which is beneficial to the stable performance of the full length of the strip.
  • Mg deoxidation is used to preferentially form finely dispersed MgO in molten steel, creating more nucleation particles for the formation of TiN in the subsequent continuous casting process, which can effectively refine TiN particles and improve hole expansion. rate stability.
  • the addition of relatively high Ti and a small amount of Mo can ensure that the nano-precipitated carbides will not be significantly coarsened after high temperature coiling and reduce the strength of the steel.
  • the high-temperature coiled strip steel has a low phase transformation stress as much as possible, but there is also a phenomenon of uneven internal stress. After several times of pickling and bending, the internal stress is reduced and homogenized. In the case of small plastic loss, there are Conducive to the improvement of hole expansion rate.
  • the invention adopts the design idea of low-cost components, and adopts innovative controlled rolling and pickling processes at the same time, and firstly obtains ferritic high-strength steel with uniform structure and performance on the existing hot continuous rolling production line, and then goes through the pickling process. , so that the internal stress in the ferrite structure is more uniformly distributed.
  • the uniform and finely dispersed nano-scale carbides in the ferrite on the one hand, endow the steel plate with high strength and high plasticity, and at the same time, the good structure and uniform distribution of internal stress endow the steel plate with an ultra-high hole expansion ratio.
  • the yield strength of the steel plate/coil is ⁇ 700MPa, the tensile strength is ⁇ 780MPa, and the thickness is 1.8-6.0mm. rate ⁇ 80%), showing excellent matching of strength, plasticity and hole expansion performance, with excellent strength, plasticity and ultra-high hole expansion rate, can be used in automobile chassis, sub-frames, etc. that require high-strength thinning and hole expansion
  • the manufacture of complex parts such as edges has a very broad application prospect.
  • the process of the invention is simple and suitable for mass production.
  • FIG. 1 is a process flow diagram of the method for manufacturing 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention.
  • FIG. 2 is a schematic diagram of the rolling process in the manufacturing method of the 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention.
  • FIG. 3 is a schematic diagram of the cooling process in the manufacturing method of the 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention.
  • FIG. 4 is a typical metallographic photograph of Example 1 of the 780MPa-grade high-surface ultra-high hole-expanding steel according to the present invention.
  • FIG. 5 is a typical metallographic photograph of Example 3 of the 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention.
  • FIG. 6 is a typical metallographic photograph of Example 7 of the 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention.
  • the manufacturing method of 780MPa grade high-surface ultra-high hole-expanding steel according to the present invention includes the following steps:
  • the rolling temperature is 1050-1150°C, and the rough rolling is carried out at a high pressure of 3-5 passes above 1050°C and the accumulated deformation is ⁇ 50%, and then the intermediate billet is air-cooled or water-cooled to 950-1000°C, and then 3-7 passes of refining are carried out.
  • Rolled and the cumulative deformation is ⁇ 70%, and the final rolling temperature is 850-950 °C;
  • the operating speed of strip pickling is adjusted within the range of 30-140m/min, the pickling temperature is controlled between 75-85°C, the pull-straightening rate is controlled within ⁇ 3%, the rinsing is carried out in the temperature range of 35-50°C, and the Dry the surface between 120-140°C and apply oil.
  • Tables 2 and 3 are the production process parameters of the steel embodiments of the present invention, wherein the thickness of the billet in the rolling process is 230 mm; Table 4 is the embodiment of the present invention.
  • Mechanical properties of steel plates Tensile properties (yield strength, tensile strength, elongation) are tested according to ISO6892-2-2018 international standard; hole expansion rate is tested according to ISO16630-2017 international standard.
  • Figure 4 Figure 5 show typical metallographic photos of Examples 1, 3, and 7 when coiling at different temperatures. It can be clearly seen from the figures that the composition system designed by the present invention is The structure obtained during coiling is uniform and fine all-ferrite. In the subsequent pickling process, the ferrite microstructure stress and internal stress are further improved, and the strength, plasticity and hole expansion performance are improved.
  • the yield strength of the steel coil is ⁇ 700MPa
  • the tensile strength is ⁇ 780MPa
  • the elongation A50 is ⁇ 17%
  • the hole expansion ratio is ⁇ 80%.
  • the 780MPa high-strength steel involved in the present invention has high strength, high plasticity and a good match for ultra-high hole expansion rate, and is especially suitable for parts such as automobile chassis structures that require high-strength thinning and hole-enlarged flanging.
  • the manufacture of control arms, etc. can also be used for complex parts such as wheels that need to be turned, and has broad application prospects.

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Abstract

一种780MPa级高表面超高扩孔钢及其制造方法，其化学成分重量百分比为：C 0.03～0.08％，Si≤0.2％，Mn 0.5～2.0％，P≤0.02％，S≤0.003％，Al 0.01～0.08％，N≤0.004％，Ti 0.05～0.20％，Mo 0.1～0.5％，Mg≤0.005％，O≤0.0030％，其余为Fe以及其它不可避免的杂质。本发明获得的钢板/卷表面质量良好，避免了带钢表面出现红铁皮，提高酸洗高强钢的表面质量；其屈服强度≥700MPa，抗拉强度≥780MPa，延伸率A50≥17％，扩孔率≥80％，实现高表面质量、高强度、高塑性、超高扩孔率良好匹配，可应用在乘用车底盘零件如控制臂以及副车架等需要高强减薄的部位。

Description

一种780MPa级高表面超高扩孔钢及其制造方法 技术领域
本发明属于高强钢领域，特别涉及一种780MPa级高表面超高扩孔钢及其制造方法。
背景技术
乘用车中许多零部件尤其是底盘和车身零件通常使用酸洗产品。乘用车的轻量化不仅是汽车行业的发展趋势，而且还是法律法规的要求。法律法规中规定了油耗，实际上是变相地要求降低车身重量，反映到材料上的要求是高强减薄轻量化。高强减重是后续新车型的必然要求，这势必造成用钢级别更高，同时底盘结构上也必然带来变化：如零件更复杂，对材料性能、表面以及成型技术如液压成形、热冲压、激光焊接等提出了更高要求，进而转化为材料的高强、冲压、翻边、回弹以及疲劳等性能上。
国内高强度高扩孔钢的开发与国外相比不仅强度级别相对较低，而且性能稳定性也不好。如国内汽车零部件企业使用的高扩孔钢基本是抗拉强度600MPa以下的高强钢，440MPa以下级别的高扩孔钢竞争白热化。抗拉强度780MPa级别的高扩孔钢目前国内正在逐渐开始批量使用，但是对延伸率和扩孔率这两个成形过程中的重要指标也提出了更高要求。随着国内乘用车销量的逐渐下滑，汽车行业已经出现拐点，竞争愈加激烈。乘用车企业为了进一步降低工序成本，对材料的性能要求进一步提高。如在生产汽车底盘控制臂这一结构件时，为了减少冲压工序和降低成本，要求在高强度高塑性的同时，扩孔率指标进一步提高。如要求在现有780MPa级高扩孔钢基础上，将扩孔率指标从目前≥50％的基础上进一步提高至≥70％以上。目前780MPa级高扩孔钢多采用高硅成分体系的设计思路，组织主要以贝氏体为主，同时还有一定的析出强化。酸洗后的带钢表面不仅有明显的红铁皮，而且扩孔率基本在50-65％之间，且贝氏体组织的延伸率偏低，这些均不能满足用户提出的更高扩孔率的性能要求。
780MPa级酸洗高扩孔钢已有较多专利申请涉及。如中国专利申请 CN103602895A涉及了一种低碳Nb-Ti微合金化高扩孔钢，其成分设计特点为低碳高硅Nb-Ti微合金化，扩孔率保证值≥50％，高硅成分设计通常带来钢板表面红铁皮，再加上形成贝氏体所需要的卷取温度区间在500℃左右，钢卷全长温度控制难度大，易造成全长性能波动大；专利申请CN105821301A涉及了一种800MPa级热轧高强度高扩孔钢，其成分设计特点也是低碳高硅Nb-Ti微合金化，其Ti含量达到很高的程度，为0.15～0.18％，在实际生产过程中，此种成分设计思路不仅带钢表面有红铁皮等缺陷，而且超高Ti含量容易形成粗大的TiN，对扩孔率的稳定性非常不利；专利
中国专利申请CN108570604A涉及了一种780MPa级热轧酸洗高扩孔钢，其成分设计特点为低碳高铝高铬，在工艺设计上采用了三段式冷却工艺。尽管其带钢表面无红铁皮，但高铝的设计容易在实际生产过程中造成浇铸水口堵塞，且工艺过程复杂，三段式冷却工艺控制难度大，扩孔率不高。与本申请专利相比，上述专利均存在红铁皮、炼钢困难以及带钢全长温度均匀性控制难度大等问题。
为了满足用户对更高表面质量、更好的性能稳定性、更优的强度、塑性和超高扩孔性匹配等需求，需要对现有的酸洗780MPa级高扩孔钢作出颠覆性的改变。
众所周知，在通常情况下，材料的延伸率与扩孔率呈反比关系，即延伸率越高，扩孔率越低；反之，延伸率越低，扩孔率越高。塔塔钢铁的技术人员在实践中也发现有此规律。那么要获得高延伸高扩孔，同时又具有高强度的高扩孔钢就显得非常困难。
此外，在相同或相似的强化机制下，材料的强度越高，扩孔率越低。为了获得具有良好的塑性和扩孔翻边性能的钢材，需要更好的平衡两者之间的关系。另一方面，为获得强度、塑性和扩孔性的良好匹配，较多硅元素的加入似乎是高强高塑高扩孔钢不可或缺的，但是高硅的成分设计带来较差的钢板表面，即在热轧环节形成的红铁皮缺陷在后面的酸洗工序很难清除彻底，使得酸洗高强钢的表面出现条纹状红铁皮，严重影响表面质量。
发明内容
本发明的目的在于提供一种780MPa级高表面超高扩孔钢及其制造方法，获得的钢板/卷表面质量良好，避免了带钢表面出现红铁皮，改善酸洗高强钢的表面质量；其屈服强度≥700MPa，抗拉强度≥780MPa，延伸率A50≥17％，扩孔率≥80％， 实现高表面质量、高强度、高塑性、超高扩孔率良好匹配，可应用在乘用车底盘零件如控制臂以及副车架等需要高强减薄的部位。
为达到上述目的，本发明的技术方案是：
一种780MPa级高表面超高扩孔钢，其化学成分重量百分比为：
C：0.03～0.08％，
Si：≤0.2％，
Mn：0.5～2.0％，
P：≤0.02％，
S：≤0.003％，
Al：0.01～0.08％，
N：≤0.004％，
Ti：0.05～0.20％，
Mo：0.1～0.5％，
Mg：≤0.005％，
O：≤0.0030％，
其余为Fe以及其它不可避免的杂质。
进一步，还可添加Nb≤0.06％，V≤0.05％，Cu≤0.5％，Ni≤0.5％，Cr≤0.5％，B≤0.001％，Ca≤0.005％中的一种或一种以上，其中，Nb、V含量分别优选为≤0.03％，Cu、Ni、Cr含量分别优选为≤0.3％，B含量优选为≤0.0005％，Ca含量优选为≤0.002％。
在一些实施方案中，本发明所述超高扩孔钢的化学成分重量百分比为：C：0.03～0.08％，Si：≤0.2％，Mn：0.5～2.0％，P：≤0.02％，S：≤0.003％，Al：0.01～0.08％，N：≤0.004％，Ti：0.05～0.20％，Mo：0.1～0.5％，Mg：≤0.005％，O：≤0.0030％，Nb≤0.06％，V≤0.05％，Cu≤0.5％，Ni≤0.5％，Cr≤0.5％，B≤0.001％，Ca≤0.005％，其余为Fe以及其它不可避免的杂质，且其含有Nb、V、Cu、Ni、Cr、B和Ca中的至少一种或两种以上。
本发明所述超高扩孔钢的显微组织为不规则铁素体、铁素体晶粒内纳米碳化物以及极少量珠光体。在一些实施方案中，以体积比计，本发明所述超高扩孔钢的显微组织中，珠光体体积百分比≤0.5％。
本发明所述超高扩孔钢的屈服强度≥700MPa、优选≥730MPa、更优选≥750MPa，抗拉强度≥780MPa、优选≥800MPa、更优选≥810MPa，延伸率A50≥17％、 优选≥19％，扩孔率≥80％、优选≥85％、更优选≥90％。在优选的实施方案中，本发明所述超高扩孔钢的屈服强度≥750MPa，抗拉强度≥810MPa，延伸率A50≥19％，扩孔率≥90％。
在一些实施方案中，本发明所述超高扩孔钢的厚度为1.8-6.0mm。
在本发明所述超高扩孔钢的成分设计中：
碳，是钢中的基本元素，同时也是本发明中的重要元素之一。碳扩大奥氏体相区，稳定奥氏体。碳作为钢中的间隙原子，对提高钢的强度起着非常重要的作用，对钢的屈服强度和抗拉强度影响最大。在本发明中，由于要获得的组织为全铁素，为获得抗拉强度达780MPa级的高强钢，必须保证碳含量在0.03％以上。碳含量在0.03％以下，铁素体型组织抗拉强度难以达到780MPa；但碳含量也不能高于0.08％。碳含量太高，热轧卷取时容易形成珠光体组织，对扩孔性能不利。因此，碳含量应控制在0.03-0.08％之间，优选范围在0.04-0.06％之间。
硅，是钢中的基本元素。前已述及，为了满足用户提出的高强、高塑和超高扩孔率的要求，通常在成分设计时添加较多的硅，但是高硅的成分设计带来的是钢板表面质量降低，有较多的红铁皮缺陷。在本发明中，为了保证获得良好的表面质量，在成分设计时应严格控制硅的含量。换言之，硅在本发明中属于杂质元素，考虑到实际炼钢时需要用到硅锰进行脱氧，完全避免硅的添加比较困难。根据实际生产大量统计数据显示，硅含量在0.2％以下时，热轧时可避免出现表面红铁皮缺陷，通常在0.15％以下可保证不出现红铁皮。因此，钢中硅的含量应控制在0.2％以内，优选范围在0.15％以内。在一些实施方案中，硅的含量为0.05～0.2％。在一些实施方案中，硅的含量为0.05～0.15％。
锰，是钢中最基本的元素，同时也是本发明中最重要的元素之一。Mn是扩大奥氏体相区的重要元素，可稳定奥氏体，细化晶粒并推迟奥氏体向珠光体转变。在本发明中，为保证钢板的强度和晶粒细化效果，Mn含量通常控制在0.5％以上；同时，Mn的含量一般也不宜超过2.0％，否则炼钢时容易发生Mn偏析，板坯连铸时也容易发生热裂。因此，钢中Mn含量一般控制在0.5-2.0％，优选范围在1.0-1.6％。
磷，是钢中的杂质元素。P极易偏聚到晶界上，钢中P的含量较高(≥0.1％)时，形成Fe 2P在晶粒周围析出，降低钢的塑性和韧性，故其含量越低越好，一般控制在0.02％以内较好且不提高炼钢成本。
硫，是钢中的杂质元素。钢中的S通常与Mn结合形成MnS夹杂，尤其是当 S和Mn的含量均较高时，钢中将形成较多的MnS，而MnS本身具有一定的塑性，在后续轧制过程中MnS沿轧向发生变形，不仅降低了钢的横向塑性，而且增加了组织的各项异性，对扩孔性能不利。故钢中S含量越低越好，为了减少MnS的含量，需对S含量严格控制，要求S含量控制在0.003％以内，优选范围在0.0015％以下。
铝，在钢中的作用主要是脱氧和固氮。在有强碳化物形成元素如Ti等存在的前提下，Al的主要作用是脱氧和细化晶粒。在本发明中，Al作为常见的脱氧元素和细化晶粒的元素，其含量通常控制在0.01-0.08％即可；Al含量低于0.01％，起不到细化晶粒的作用；同样，Al含量高于0.08％时，其细化晶粒效果达到饱和。因此，钢中Al的含量控制在0.01-0.08％之间即可，优选范围在0.02-0.05％之间。
氮，在本发明中属于杂质元素，其含量越低越好。但是氮在炼钢过程中是不可避免的元素。虽然其含量较少，但是与强碳化物形成元素如Ti等结合，形成的TiN颗粒对钢的性能带来非常不利的影响，尤其对扩孔性能非常不利。由于TiN呈方形，其尖角与基体之间存在很大的应力集中，在扩孔变形的过程中，TiN与基体之间的应力集中容易形成起裂源，从而大大降低材料的扩孔性能。由于本发明在成分体系上采用了高钛设计，为了尽量减少TiN带来的对扩孔不利影响。因此，氮含量应控制在0.004％以下，优选范围在0.003％以下。
钛，是本发明中的重要元素之一。Ti在本发明中主要起两个作用：一是与钢中的杂质元素N结合形成TiN，起到一部分“固氮”的作用；二是在卷取相变过程中从铁素体中形成弥散均匀细小的纳米级碳化物，提高强度、塑性和扩孔率。钛的含量低于0.05％时，起不到明显的析出强化效果；当钛的含量高于0.20％时，粗大的TiN容易导致钢板的冲击韧性很差。因此，钢中钛的含量应控制在0.05-0.20％之间，优选范围在0.07～0.11％；
钼，是本发明中的重要元素之一。钼加入钢中可以大大推迟铁素体和珠光体相变，有利于获得不规则铁素体组织。钼和钛同时加入钢中，所形成的纳米级碳化钛钼析出相具有抗高温粗化的作用，可确保在卷取后长时间不发生粗化，避免强度降低。同时，钼具有很强的抗焊接软化特性。由于本发明的主要目的是获得铁素体加纳米析出组织，加入一定量的钼可以有效减小焊接软化程度。因此，钼的含量应控制在0.1-0.5％之间，优选范围在0.15-0.45％。
镁，是本发明中的重要元素之一。镁加入钢中可在炼钢阶段优先形成弥散细小 的MgO，这些细小的MgO可作为TiN的形核质点，在后续连铸过程中，可有效增加TiN的形核点并减小TiN的尺寸。由于TiN对最终钢板的扩孔率有较大影响，易造成扩孔率不稳定。因此，钢中Mg含量控制在0.005％以内即可。
氧，是炼钢过程中不可避免的元素，对本发明而言，钢中氧的含量通过脱氧之后一般都可以达到30ppm以下，对钢板性能不会造成明显不利影响。因此，将钢中的O含量控制在30ppm以内即可。
铜，是本发明中的一种可添加元素。铜加入钢中可提高钢的耐蚀性，当其与P元素共同加入时，耐蚀效果更佳；当Cu加入量超过1％时，在一定条件下，可形成ε-Cu析出相，起到较强的析出强化效果。但Cu的加入容易在轧制过程中形成“Cu脆”现象，为了在某些应用场合下充分利用Cu的改善耐蚀性效果，同时又不至于引起显著的“Cu脆”现象，通常将Cu元素的含量控制在0.5％以内，优选范围在0.3％以内。
镍，是本发明中的一种可添加元素。镍加入钢中具有一定的耐蚀性，但耐蚀效果较铜弱，镍加入钢中对钢的拉伸性能影响不大，但可以细化钢的组织和析出相，大大提高钢的低温韧性；同时在添加铜元素的钢中，添加少量的镍可以抑制“Cu脆”的发生。添加较高的镍对钢本身的性能无明显不利影响。若铜和镍同时添加，不仅可以提高耐蚀性，而且对钢的组织和析出相进行细化，大大提高低温韧性。但由于铜和镍均属于比较贵重的合金元素。因此，为了尽量降低合金设计的成本，镍的添加量通常≤0.5％，优选范围≤0.3％。
铬，是本发明中的可添加元素。铬加入钢中主要通过固溶强化或细化组织等方式提高钢的强度。由于本发明中的组织为细小贝氏体铁素体加纳米析出碳化物，由于大量弥散细小的纳米级碳化物强烈地钉扎位错，使得钢的屈服强度和抗拉强度之比即屈强比较高，通常达到0.90以上。加入少量的铬元素，可适当降低钢的屈服强度，从而降低屈强比。此外，少量铬的加入还可以起到提高耐蚀性的作用，通常铬的加入量≤0.5％，优选范围≤0.3％。
铌，是本发明的可添加元素之一。铌与钛相似，是钢中的强碳化物元素，铌加入钢中可大大提高钢的未再结晶温度，在精轧阶段可获得位错密度更高的形变奥氏体，在后续相变时可细化最终组织。但铌的加入量不可太多，一方面铌的加入量超过0.06％，易在组织中形成比较粗大的铌的碳氮化物，消耗了部分碳原子，降低了碳化物的析出强化效果。同时，铌的含量较多，还容易造成热轧态奥氏体组织的各 向异性，在后续的冷却相变过程中遗传给最终的组织，对扩孔性能不利。因此，钢中铌含量通常控制在≤0.06％，优选范围在≤0.03％。
钒，是本发明中的可添加元素。钒与钛、铌类似，也是一种强碳化物形成元素。但钒的碳化物固溶或析出温度低，在精轧阶段通常全部固溶在奥氏体中。只有当温度降低开始相变时，钒才开始在铁素体中形成。由于钒的碳化物在铁素体中的固溶度大于铌和钛的固溶度，故钒的碳化物在铁素体中形成的尺寸较大，不利于析出强化，对钢的强度贡献远小于碳化钛或碳化钛钼，但由于钒的碳化物形成也消耗了一定的碳原子，对钢的强度提高不利。因此，钢中钒的添加量通常≤0.05％，优选范围≤0.03％。
硼，是本发明中的可添加元素。硼能够大大提高钢的淬透性，有利于获得马氏体组织。考虑到本发明在热轧阶段期望获得的组织为铁素体而非马氏体，因此，钢中需要严格控制硼元素的含量，防止由于硼元素的过量添加导致形成马氏体。此外，硼元素加入钢中可形成不规则的铁素体组织甚至马奥组元，对钢的低温冲击韧性不利。故钢中硼的添加量通常控制在≤0.001％，优选范围在≤0.0005％。
钙，是本发明中的可添加元素。钙能够改善硫化物如MnS形态，使长条形的MnS等硫化物变为球形CaS，有利于改善夹杂物形态，进而减小长条形硫化物对扩孔性能的不利影响，但过多钙的加入会增加氧化钙的数量，对扩孔性能不利。因此，钢种钙的添加量通常≤0.005％，优选范围在≤0.002％。
本发明所述的780MPa级高表面超高扩孔钢的制造方法，其包括如下步骤：
1)冶炼、浇铸
按上述成分采用转炉或电炉冶炼、真空炉二次精炼后浇铸成铸坯或铸锭；
2)铸坯或铸锭再加热，加热温度≥1230℃，保温时间1～2小时；
3)热轧
开轧温度1050～1150℃，在1050℃以上3-5道次粗轧大压下且累计变形量≥50％，随后中间坯空冷或水冷至950-1000℃，然后进行3-7道次精轧且累计变形量≥70％，终轧温度850-950℃、优选850-930℃；
4)控制冷却
终轧结束后，先经过0-10秒空冷以进行动态回复和动态再结晶，然后以≥10℃/s的冷速将钢板水冷至550-650℃，卷取后缓慢冷却(冷速≤20℃/h)至室温；
5)酸洗
带钢酸洗运行速度在30～140m/min的区间内调整，酸洗温度控制在75～85℃之间，拉矫率控制在≤3％，在35-50℃温度区间进行漂洗，以保证带钢表面质量，并在120-140℃之间进行表面烘干，涂油。
在本发明所述超高扩孔钢的制造方法中：
铸坯(锭)加热温度：≥1230℃，保温时间：1～2小时，开轧温度：1050～1150℃，在1050℃以上3-5道次粗轧大压下且累计变形量≥50％，主要目的是细化奥氏体晶粒，同时保留更多的固溶钛。在一些实施方案中，粗轧累计变形量≥80％。
在热轧阶段，高温卷取形成组织均匀细小的铁素体和铁素体内纳米析出相。
在粗轧和精轧阶段，轧制节奏应尽量快速完成，保证更多的固溶钛在奥氏体中。高温终轧结束后，先经过0-10秒空冷以进行动态回复和动态再结晶后，带钢以≥10℃/s的冷却速度在线冷却至550-650℃以获得铁素体和纳米析出组织。在一些实施方案中，精轧累计变形量≥80％。在一些实施方案中，冷却速度为15-70℃/s。
在酸洗过程中，高温卷取相变过程中钢卷内部形成的热应力和组织应力不均匀性，在酸洗拉矫时得到充分释放，进一步改善了组织均匀性。在一些实施方案中，卷取温度为550-650℃。
本发明在成分设计上，采用了不含Si或含有少量Si的设计思路，一方面可以避免带钢表面红铁皮形成，有利于实际生产过程关键工艺参数的稳定控制；另一方面采用高温卷取这种常规工艺即可获得性能均匀的铁素体和纳米析出组织，有利于带钢全长性能稳定。在炼钢工艺上，采用Mg脱氧的方式，在钢水中优先形成弥散细小的MgO，为后续连铸过程中TiN的形成创造更多的形核质点，可有效地细化TiN颗粒，提高扩孔率稳定性。添加相对较高的Ti和少量Mo，可以保证在高温卷取之后纳米析出碳化物不致于明显粗化，降低钢的强度。高温卷取的带钢尽量相变应力不高，但也存在内应力不均匀现象，经过酸洗多次拉弯工序后内应力得到的降低和均匀化，在塑性损失较小的情况下，有利于扩孔率的提高。
本发明的有益效果：
采用独特的成分设计思路，同时与相应的工艺匹配，
本发明采用低成本成分设计思路，同时采用创新性的控轧和酸洗工艺，在现有的热连轧产线上首先获得组织和性能均匀的铁素体型高强钢，然后再经过酸洗工 序，使得铁素体组织中的内应力得到更均匀分布。铁素体中均匀细小弥散分布的纳米级碳化物一方面赋予钢板高强度和高塑性，同时良好的组织和内应力均匀分布赋予钢板超高的扩孔率。
钢板/卷的屈服强度≥700MPa，抗拉强度≥780MPa，且厚度在1.8-6.0mm，表面质量高，同时且具有良好的延伸率(横向A50≥17％)和超高扩孔性能(扩孔率≥80％)，表现出优异的强度、塑性和扩孔性能匹配，具有优异的强度、塑性和超高扩孔率，可应用于汽车底盘、副车架等需要高强减薄和扩孔翻边等复杂零部件的制造，具有非常广阔的应用前景。
本发明工艺过程简单，适合大批量生产。
附图说明
图1为本发明所述780MPa级高表面超高扩孔钢制造方法的工艺流程图。
图2为本发明所述780MPa级高表面超高扩孔钢制造方法中轧制工艺示意图。
图3为本发明所述780MPa级高表面超高扩孔钢制造方法中冷却工艺示意图。
图4为本发明所述780MPa级高表面超高扩孔钢实施例1的典型金相照片。
图5为本发明所述780MPa级高表面超高扩孔钢实施例3的典型金相照片。
图6为本发明所述780MPa级高表面超高扩孔钢实施例7的典型金相照片。
具体实施方式
下面结合实施例和附图对本发明做进一步说明。
参见图1～图3，本发明所述的780MPa级高表面超高扩孔钢的制造方法，其包括如下步骤：
1)冶炼、浇铸
按上述成分采用转炉或电炉冶炼、真空炉二次精炼后浇铸成铸坯或铸锭；
2)铸坯或铸锭再加热，加热温度≥1230℃，保温时间1～2小时；
3)热轧
开轧温度1050～1150℃，在1050℃以上3-5道次粗轧大压下且累计变形量≥50％，随后中间坯空冷或水冷至950-1000℃，然后进行3-7道次精轧且累计变形量≥70％，终轧温度850-950℃；
4)控制冷却
终轧结束后，先经过0-10秒空冷以进行动态回复和动态再结晶，然后以≥10℃/s的冷速将钢板水冷至550-650℃，卷取后缓慢冷却(冷速≤20℃/h)至室温；
5)酸洗
带钢酸洗运行速度在30～140m/min的区间内调整，酸洗温度控制在75～85℃之间，拉矫率控制在≤3％，在35-50℃温度区间进行漂洗，并在120-140℃之间进行表面烘干，涂油。
本发明所述超高扩孔钢实施例的成分参见表1，表2、表3为本发明钢实施例的生产工艺参数，其中，轧制工艺中钢坯厚度230mm；表4为本发明实施例钢板的力学性能。拉伸性能(屈服强度、抗拉强度、延伸率)按照ISO6892-2-2018国际标准进行检测；扩孔率按照ISO16630-2017国际标准进行检测。
图4、图5、图6分别给出了实施例1、3、7在不同温度卷取时的典型金相照片，从图中可以清楚地看出，采用本发明所设计的成分体系在高温卷取时得到的组织为均匀细小的全铁素体。在后续的酸洗过程中，铁素体组织应力和内应力得到进一步改善，提高了强度、塑性和扩孔性能。
从表4可以看出，钢卷的屈服强度≥700MPa，抗拉强度≥780MPa，延伸率A50≥17％，扩孔率≥80％。从上述实施例可以看出，本发明所涉及的780MPa高强钢具有高强度、高塑性和超高扩孔率良好匹配，特别适合汽车底盘结构等需要高强减薄和扩孔翻边成形的零件如控制臂等的制造，也可用于车轮等需要翻孔的复杂零部件，具有广阔的应用前景。
表1(单位：重量百分比)
Figure PCTCN2021115435-appb-000001
Figure PCTCN2021115435-appb-000002
表2
Figure PCTCN2021115435-appb-000003
表3
Figure PCTCN2021115435-appb-000004
Figure PCTCN2021115435-appb-000005
表4：钢板的力学性能
Figure PCTCN2021115435-appb-000006

Claims (15)

  1. 一种780MPa级高表面超高扩孔钢，其化学成分重量百分比为：
    C：0.03～0.08％，
    Si：≤0.2％，
    Mn：0.5～2.0％，
    P：≤0.02％，
    S：≤0.003％，
    Al：0.01～0.08％，
    N：≤0.004％，
    Ti：0.05～0.20％，
    Mo：0.1～0.5％，
    Mg：≤0.005％，
    O：≤0.0030％，
    其余为Fe以及其它不可避免的杂质。
  2. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述超高扩孔钢还含有Nb≤0.06％，V≤0.05％，Cu≤0.5％，Ni≤0.5％，Cr≤0.5％，B≤0.001％和Ca≤0.005％中的一种或一种以上，其中，Nb、V含量分别优选为≤0.03％，Cu、Ni、Cr含量分别优选为≤0.3％，B含量优选为≤0.0005％，Ca含量优选为≤0.002％。
  3. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述C含量在0.04-0.06％之间。
  4. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述Si含量在0.15％以内，和/或所述S含量控制在0.0015％，和/或所述N含量控制在0.003％以下。
  5. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述Mn含量在1.0-1.6％。
  6. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述Al含量控制在0.02-0.05％。
  7. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述Ti含量 控制在0.07～0.11％。
  8. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述Mo含量在0.15-0.45％。
  9. 如权利要求1所述的780MPa级高表面超高扩孔钢，其特征在于，所述超高扩孔钢的显微组织为不规则铁素体、铁素体晶粒内纳米碳化物以及极少量珠光体。
  10. 如权利要求1或9所述的780MPa级高表面超高扩孔钢，其特征在于，所述超高扩孔钢的屈服强度≥700MPa，抗拉强度≥780MPa，延伸率A50≥17％，扩孔率≥80％。
  11. 如权利要求1或9所述的780MPa级高表面超高扩孔钢，其特征在于，所述超高扩孔钢的屈服强度≥750MPa，抗拉强度≥810MPa，延伸率A50≥19％，扩孔率≥90％。
  12. 如权利要求1～11中任何一项所述的780MPa级高表面超高扩孔钢的制造方法，其特征是，包括如下步骤：
    1)冶炼、浇铸
    按权利要求1～8中任一项所述成分采用转炉或电炉冶炼、真空炉二次精炼后浇铸成铸坯或铸锭；
    2)铸坯或铸锭再加热，加热温度≥1230℃，保温时间1～2小时；
    3)热轧
    开轧温度1050～1150℃，在1050℃以上3-5道次粗轧大压下且累计变形量≥50％，随后中间坯空冷或水冷至950-1000℃，然后进行3-7道次精轧且累计变形量≥70％，终轧温度850-950℃；
    4)控制冷却
    终轧结束后，先经过0-10秒空冷，然后以≥10℃/s的冷速将钢板水冷至550-650℃，卷取后缓慢冷却至室温；
    5)酸洗
    带钢酸洗运行速度在30～140m/min的区间内调整，酸洗温度控制在75～85℃之间，拉矫率控制在≤3％，然后漂洗、带钢表面烘干，涂油。
  13. 如权利要求12所述的780MPa级高表面超高扩孔钢的制造方法，其特征在于，步骤5)酸洗后，在35-50℃温度区间进行漂洗，并在120-140℃之间进行表面烘干，涂油。
  14. 如权利要求12所述的780MPa级高表面超高扩孔钢的制造方法，其特征在于，步骤3)中，粗轧累计变形量≥80％，精轧累计变形量≥80％。
  15. 如权利要求12所述的780MPa级高表面超高扩孔钢的制造方法，其特征在于，步骤4)中，冷速为15-70℃/s。
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