WO2023246899A1 - 高扩孔钢及其制造方法 - Google Patents

高扩孔钢及其制造方法 Download PDF

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Publication number
WO2023246899A1
WO2023246899A1 PCT/CN2023/101836 CN2023101836W WO2023246899A1 WO 2023246899 A1 WO2023246899 A1 WO 2023246899A1 CN 2023101836 W CN2023101836 W CN 2023101836W WO 2023246899 A1 WO2023246899 A1 WO 2023246899A1
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Prior art keywords
steel
rolling
cooling
temperature
bainite
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PCT/CN2023/101836
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English (en)
French (fr)
Inventor
王焕荣
张晨
杨阿娜
庞厚君
范佳杰
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宝山钢铁股份有限公司
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Priority claimed from CN202210714666.8A external-priority patent/CN117305731A/zh
Priority claimed from CN202210714629.7A external-priority patent/CN117305730A/zh
Application filed by 宝山钢铁股份有限公司 filed Critical 宝山钢铁股份有限公司
Publication of WO2023246899A1 publication Critical patent/WO2023246899A1/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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the invention belongs to the field of steel and its manufacturing method, and particularly relates to a highly expanded steel and its manufacturing method.
  • the high-expansion steel used by domestic auto parts companies is basically high-strength steel with a tensile strength of less than 600MPa.
  • high-expansion steel with a tensile strength of less than 540MPa is considerable competition for high-expansion steel with a tensile strength of less than 540MPa.
  • high-expansion steel with a tensile strength of 780MPa is gradually being used in batches in China.
  • higher requirements are also put forward for elongation and hole expansion rate, two important indicators in the forming process.
  • performance stability is also required. The requirements are becoming more stringent.
  • the material is required to have high strength and high plasticity as well as a high hole expansion rate.
  • the hole expansion rate of 780MPa grade high-hole expansion steel is required to be guaranteed to be ⁇ 50%, and it is best to further increase it to 70% or even more than 80%.
  • Existing high-expansion steels, especially 780MPa grade high-expansion steels mostly have a hot-rolled bainite structure and are strengthened with precipitated phases. Most of the process paths used are medium-temperature coiling, but the temperature control accuracy is not high and the finished product structure is uniform. Poor performance results in uneven hole expansion rate and other properties of the hot-rolled steel obtained, and stamping cracking is prone to occur during subsequent processing.
  • Chinese patent CN103602895A discloses a low-carbon Nb-Ti micro-alloyed high hole expansion steel, which adopts a composition design of low carbon, high silicon and Nb-Ti micro-alloying, which can ensure a hole expansion rate of ⁇ 50%.
  • the design of high silicon content usually brings red iron scale on the surface of the steel plate.
  • the coiling temperature required to form bainite is usually around 500°C, which makes it difficult to control the temperature of the entire length of the steel coil, resulting in fluctuations in the performance of the entire length of the steel coil. big.
  • Chinese patent CN105821301A discloses an 800MPa grade hot-rolled high-strength and high-pore-expansion steel, which also adopts a composition design of low carbon and high silicon combined with Nb-Ti microalloying.
  • the Ti content in the steel is very high, ranging from 0.15 to 0.18%. In the actual production process, this composition design idea will lead to defects such as red iron sheets on the surface of the strip. On the other hand, due to the high Ti content, coarse TiN is easily formed in the steel, which is detrimental to the stability of the hole expansion rate.
  • Chinese patent CN108570604A discloses a 780MPa grade hot-rolled pickled high-hole expansion steel, which adopts a low-carbon, high-aluminum and high-chromium composition design, and adopts a three-stage cooling process in the process design.
  • the high-aluminum composition design can easily cause clogging of the casting nozzle during the actual production process.
  • the production process of this steel is complex, especially the three-stage cooling process is difficult to control, resulting in a low hole expansion rate of the steel.
  • Chinese patent CN114107792A discloses a 780MPa grade hot-rolled pickled high-hole expansion steel, which is designed with a low-carbon, high-titanium composition and adds an appropriate amount of molybdenum element to the steel. Since the phase transformation process of molybdenum-containing steel is relatively slow, the phase transformation process mainly occurs after coiling. Therefore, in the actual production process, there are problems such as low strength of the inner and outer rings of steel coils.
  • the object of the present invention is to provide a steel and a manufacturing method thereof.
  • the steel of the present invention has high strength, high plasticity and high hole expansion rate, and these properties are well matched, and can be used in passenger car chassis parts such as control arms and subframes that require high strength and thinning.
  • the elongation of a material is inversely proportional to the hole expansion rate, that is, the higher the elongation, the lower the hole expansion rate; conversely, the lower the elongation, the higher the hole expansion rate.
  • the higher the strength of the material the lower the hole expansion rate.
  • the addition of more silicon elements seems to be indispensable for high-strength, high-plasticity, and high-pore-expanding steels.
  • high-silicon composition design usually leads to poor surface quality of steel plates. Specifically, the red iron scale defects formed during the hot rolling process are difficult to completely remove in the subsequent pickling process, causing striped red iron scale to appear on the surface of the pickled high-strength steel, seriously affecting the surface quality.
  • the invention optimizes the chemical composition of existing hot-rolled steel, adopts low-carbon and high-vanadium element design, does not deliberately add silicon elements to the steel, and by adding V elements, uses the nano-scale vanadium carbide formed by it to improve the strength and strength of the steel.
  • Plasticity Bainitic precipitation-strengthened high-strength steel with uniform structure and properties can be obtained without changing the existing hot continuous rolling production line.
  • the steel according to the present invention has the following compositions in mass percentage: C 0.01 ⁇ 0.10%, Si ⁇ 0.2%, Mn 0.5 ⁇ 2.0%, P ⁇ 0.02%, S ⁇ 0.003%, Al 0.01 ⁇ 0.08% , N ⁇ 0.004%, V 0.10 ⁇ 0.50%, O ⁇ 0.003%, the balance includes Fe and other inevitable impurities.
  • the above-mentioned steel also contains Ti.
  • the upper limit of the Ti content is 0.2%, preferably 0.18%, more preferably 0.015%, and the lower limit of the Ti content is 0.05%, preferably 0.08%.
  • the steel also contains Mo.
  • the content of Mo is preferably 0.1-0.5%, more preferably 0.20-0.40%, and even more preferably 0.2-0.3%.
  • the above-mentioned steel also contains one or more components selected from the group consisting of Nb ⁇ 0.1%, Cu ⁇ 0.5%, Ni ⁇ 0.5%, Cr ⁇ 0.5%, and B ⁇ 0.002%, among which Cu is more preferably 0.3% or less; Ni More preferably, it is 0.3% or less; Cr is more preferably 0.3% or less; Nb is more preferably 0.06% or less; B is more preferably 0.002% or less, still more preferably 0.001% or less.
  • the composition of the above-mentioned steel satisfies at least one of the following: C 0.03 ⁇ 0.07%; Si ⁇ 0.10%; Mn 0.8 ⁇ 1.6%; S ⁇ 0.0018%; Al 0.02 ⁇ 0.05%; N ⁇ 0.003%; O ⁇ 0.002 %.
  • Carbon is a basic element in steel and one of the important elements in the present invention. Carbon can expand the austenite phase area and stabilize austenite. As an interstitial atom in steel, carbon plays a very important role in improving the strength of steel, among which it has the greatest impact on the yield strength and tensile strength of steel.
  • the carbon content since the structure to be obtained during the hot rolling stage is low carbon bainite, in order to obtain high-expansion steel with final tensile strength reaching different strength levels, the carbon content must be above 0.01%; at the same time, the carbon content must also be It cannot be higher than 0.10%. The carbon content is too high, and low-carbon martensite is easily formed during low-temperature coiling. Therefore, this The invention controls the carbon content to 0.01-0.10%, preferably 0.03-0.07%.
  • Silicon is the basic element in steel. As mentioned before, in order to meet the user's requirements for high strength, high plasticity and high hole expansion rate, more silicon is usually added in the composition design. However, the composition design of high silicon brings about a reduction in the surface quality of the steel plate, which has a greater impact. Many red iron sheet defects. In the present invention, in order to ensure good surface quality, the silicon content should be strictly controlled during component design. In other words, silicon is an impurity element in the present invention. Considering that silicon-manganese is needed for deoxidation in actual steelmaking, it seems difficult to completely avoid the addition of silicon. According to a large amount of statistical data from actual production, when the silicon content is below 0.2%, surface red iron scale defects can be avoided during the hot rolling process. Usually, when the silicon content is below 0.10%, it is guaranteed that no red scale will appear. Therefore, the silicon content in the steel of the present invention is controlled within 0.2%, preferably within 0.10%.
  • Manganese is also the most basic element in steel and one of the most important elements in the present invention.
  • Mn is an important element in expanding the austenite phase area. It can reduce the critical quenching speed of steel, stabilize austenite, refine grains, and delay the transformation of austenite to pearlite.
  • the Mn content is usually controlled above 0.5%; at the same time, the Mn content is generally not suitable to exceed 2.0%, otherwise Mn segregation will easily occur during steelmaking, and the slab will Hot cracking is also prone to occur during continuous casting. Therefore, the Mn content in the steel of the present invention is controlled at 0.5-2.0%, preferably 0.8-1.6%.
  • Phosphorus is an impurity element in steel. P is easily segregated to the grain boundaries. When the content of P in the steel is high ( ⁇ 0.1%), Fe 2 P will form and precipitate around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower the content, the better.
  • the present invention controls the phosphorus content within 0.02%, obtains steel with better mechanical properties and does not increase steelmaking costs.
  • Sulfur is an impurity element in steel.
  • S in steel usually combines with Mn to form MnS inclusions. Especially when the contents of S and Mn are both high, more MnS will be formed in the steel. MnS itself has a certain plasticity. During the subsequent rolling process, MnS will form along 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 detrimental to the hole expansion performance.
  • the S content needs to be strictly controlled. The lower the S content in the steel, the better. In the present invention, the S content is controlled within 0.003%, preferably below 0.0018%.
  • Aluminum's main function in steel is to deoxidize and fix nitrogen.
  • the main function of Al is to deoxidize and refine grains.
  • Al is a common deoxidizing element and an element that refines grains, and its content is usually controlled at 0.01-0.08%, that is, Yes; if the Al content is less than 0.01%, it will not have the effect of refining the grains; similarly, when the Al content is higher than 0.08%, its effect of refining the grains will reach saturation. Therefore, the Al content in the steel of the present invention is controlled between 0.01-0.08%, preferably between 0.02-0.05%.
  • Nitrogen is an impurity element in the present invention, and the lower its content, the better. But nitrogen is an inevitable element in the steelmaking process. Although its content is small, it is combined with strong carbide-forming elements such as V to form VN particles that have an adverse impact on the properties of steel, especially the hole expansion performance. Due to the square shape of VN, there is a large stress concentration between its sharp corners and the matrix. During the process of hole expansion deformation, the stress concentration between VN and the matrix can easily form crack sources, thus greatly reducing the hole expansion performance of the material. Since the present invention adopts a high vanadium design in the component system, in order to minimize the adverse impact of VN on hole expansion. Therefore, the present invention controls the nitrogen content below 0.004%, preferably below 0.003%.
  • Vanadium is an important element in the present invention. Similar to titanium and niobium, vanadium is also a strong carbide-forming element. However, the solid solution or precipitation temperature of vanadium carbide is low, and it is usually completely dissolved in austenite during the finishing rolling stage. Only when the temperature is lowered and phase transformation begins, vanadium begins to form in the ferrite. In order to make full use of the precipitation strengthening effect of vanadium, the addition amount of vanadium in steel should be at least 0.10% to have obvious precipitation strengthening effect; as the vanadium content increases, the precipitation strengthening effect of vanadium gradually increases.
  • the added amount of vanadium in the steel of the present invention is controlled to be ⁇ 0.50%.
  • the V content is between 0.10-0.20%, 590MPa grade high hole expansion steel can be obtained; when the V content is between 0.20-0.35%, 780MPa grade high hole expansion steel can be obtained; when the V content is between 0.35 -0.50%, 980MPa grade high-expansion steel can be obtained.
  • Molybdenum is one of the important elements in the present invention.
  • the addition of molybdenum to steel can greatly delay the phase transformation of ferrite and pearlite, which is beneficial to obtaining a bainite structure.
  • molybdenum has strong resistance to solder softening. Since the main purpose of the present invention is to obtain a low-carbon bainite structure, and low-carbon bainite is prone to softening after welding, adding a certain amount of molybdenum can effectively reduce the degree of welding softening. Therefore, in the present invention, the molybdenum content is controlled at 0.10-0.50%, preferably 0.20-0.40%, and more preferably 0.2-0.3%. When combined with the segmented cooling process, molybdenum plays a certain inhibitory role in the formation of ferrite during the segmented cooling process. When the molybdenum content is within the above range, its effect can be fully exerted.
  • Niobium is one of the additive elements in the present invention.
  • Niobium like titanium, is a strong carbide element in steel. Adding niobium to steel can greatly increase the non-recrystallization temperature of the steel, and dislocations can be obtained during the finishing rolling stage. The higher density deformed austenite can refine the final phase transformation structure during the subsequent transformation process. However, the amount of niobium added should not be too much. On the one hand, if the amount of niobium added exceeds 0.10%, it is easy to form relatively coarse niobium carbonitrides in the structure, consuming part of the carbon atoms and reducing the precipitation strengthening effect of carbides.
  • the high content of niobium can easily cause anisotropy of the hot-rolled austenite structure, which is inherited to the final structure during the subsequent cooling phase transformation process, which is detrimental to the hole expansion performance. Therefore, the niobium content in the steel of the present invention is controlled at ⁇ 0.10%, preferably ⁇ 0.06%.
  • Titanium is an additive element in the present invention. Adding a small amount of Ti to steel can, on the one hand, combine with N to form TiN during the high-temperature stage, which can fix nitrogen and help reduce the subsequent formation of VN. On the other hand, the excess Ti after combining with N can combine with carbon in the subsequent process. Nano-TiC is formed, and together with nano-VC, it improves the performance of steel. When the titanium content is higher than 0.20%, more coarse TiN is easily formed during the high temperature stage and deteriorates the impact toughness of the steel. Therefore, the content of the additive element titanium in the steel of the present invention is within 0.20%, preferably within 0.18%, more preferably within 0.015%, and most preferably within 0.10%. On the other hand, the Ti content is preferably 0.05% or more, more preferably 0.08% or more, thereby providing an excellent precipitation strengthening effect.
  • 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 the P element, the corrosion resistance effect is better; when the amount of Cu added exceeds 1%, under certain conditions, an ⁇ -Cu precipitation phase can be formed, causing A strong precipitation strengthening effect is achieved. However, the addition of Cu can easily cause the "Cu brittleness" phenomenon during the rolling process. In order to make full use of Cu's corrosion resistance improvement effect in certain applications without causing significant "Cu brittleness” phenomenon, the present invention will The Cu content is controlled within 0.5%, preferably within 0.3%.
  • Nickel is an additive element in the present invention. Nickel added to steel has certain corrosion resistance, but the corrosion resistance effect is weaker than that of copper. Nickel added to steel has little effect on the tensile properties of the steel, but it can refine the structure and precipitated phases of the steel, greatly improving the low-temperature toughness of the steel. ;At the same time, in steel with added copper element, adding a small amount of nickel can inhibit the occurrence of "Cu embrittlement". Adding higher nickel has no significant 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 precipitated phases of the steel, greatly improving the low-temperature toughness. But both copper and nickel are relatively expensive alloy elements. Therefore, in order to reduce the cost of alloy design as much as possible, the added amount of nickel in the steel of the present invention is ⁇ 0.5%, preferably ⁇ 0.3%.
  • Chromium is an additive element in the present invention. Chromium is added to steel mainly to improve the strength of steel through solid solution strengthening or structure refinement. Since the structure of the steel in the present invention is fine bainite ferrite plus Nano-precipitated carbides make the ratio of yield strength and tensile strength of steel, that is, the yield-strength ratio, higher, usually reaching more than 0.90. Adding a small amount of chromium 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%, preferably ⁇ 0.3%.
  • Boron is an additive element in the present invention. Boron can greatly improve the hardenability of steel, promote bainite transformation, and promote lath bainite transformation during medium-temperature bainite transformation. Therefore, adding a trace amount of boron to steel is beneficial to obtaining a fine lath bainite structure, but the boron content should not be too much. Adding too much boron will lead to the formation of martensite and more Maho islands, which will affect the plasticity and expansion. The hole is unfavorable. Therefore, the added amount of boron in the steel of the present invention is controlled at ⁇ 0.002%, preferably ⁇ 0.001%.
  • Oxygen is an impurity element in the present invention.
  • the lower the oxygen content in the steel the better.
  • a lower oxidation amount will increase the cost of steelmaking.
  • the oxygen content in the invention steel is controlled within 0.003%, preferably within 0.002%.
  • Existing high-expansion steels are usually designed with high Ti components, in which the main purpose of adding micro-alloying element titanium is to refine the grains, and the amount added is generally within 0.1%.
  • the present invention adopts a high vanadium composition design, and Ti exists as an optional element in the steel of the present invention.
  • the main purpose of adding vanadium in the present invention is to combine it with carbon to form dispersed nano-vanadium carbide, which plays a precipitation strengthening role.
  • the steel of the present invention obtains steel with both high tensile strength and hole expansion rate by containing V with a content as high as 0.1-0.5%.
  • V content in the steel is 0.10-0.20%
  • the tensile strength of the steel is 590MPa. grade
  • the hole expansion rate is ⁇ 70%, preferably ⁇ 80%
  • the V content in the steel is 0.20 ⁇ 0.35%
  • the tensile strength of the steel is 780MPa
  • the hole expansion rate is ⁇ 50%
  • the hole expansion rate is ⁇ 30%, preferably ⁇ 40%.
  • the microstructure of existing high-expansion steels is mostly ferrite or ferrite plus bainite.
  • nano-titanium carbide is used for strengthening.
  • the high hole expansion steel of the present invention does not contain Ti
  • its microstructure is bainite and nanometer vanadium carbide in bainite, and high hole expansion steel of different strength levels can be obtained according to the difference in vanadium content to meet the needs of downstream users for different strengths.
  • the microstructure is ferrite and bainite, wherein the ferrite contains nano-TiC and the bainite contains nano-VC.
  • the present invention also provides a method for manufacturing the steel of the present invention, which includes the following steps:
  • composition it is smelted in a converter or electric furnace, secondary refined in a vacuum furnace, and cast into a slab or ingot.
  • the rolling temperature is 1000-1100°C
  • 3-5 passes of rough rolling are performed at a high pressure above 950°C and the cumulative deformation is ⁇ 50%
  • the intermediate billet is air-cooled or water-cooled to 900 ⁇ 950°C, and then subjected to 7 passes of finish rolling with a cumulative deformation of ⁇ 70%.
  • the final rolling is completed between 800 ⁇ 900°C to obtain a steel strip, and then rolled at ⁇ 10°C/s
  • the above-mentioned steel strip is water-cooled to 400-550°C for coiling at a cooling rate of ⁇ 20°C/s, and then slowly cooled to room temperature at a cooling rate of ⁇ 20°C/s to obtain hot-rolled strip steel.
  • the opening rolling temperature of the hot rolling is 1050-1150°C, and the rolling is performed at a high pressure of 3-5 passes above 1050°C and the cumulative deformation is The amount is ⁇ 50%, then the intermediate billet is heated to 950 ⁇ 1000°C, and then the final 3 ⁇ 7 passes of rolling are performed with the cumulative deformation ⁇ 70%, and the final rolling temperature is 800 ⁇ 950°C to obtain the steel strip;
  • Cooling adopts segmented cooling. After final rolling, the above steel strip is water-cooled to between 600 and 750°C at a cooling rate of ⁇ 30°C/s. After air cooling for 1 to 10 seconds, it is then cooled to a cooling rate of ⁇ 10°C/s. Coil between 400 and 550°C, and then cool to room temperature at a cooling rate of ⁇ 20°C/h to obtain hot-rolled strip steel.
  • the steel of the present invention contains Ti
  • it is carried out using the above-mentioned method including segmented cooling.
  • the hot rolling rolling temperature is 1050 ⁇ 1150°C
  • the main purpose is to refine the austenite grains, and at the same time Retain more solid solution titanium.
  • the rolling should be completed as quickly as possible to ensure that there is more solid solution titanium and vanadium in the austenite.
  • the strip is first cooled to 600-750°C at a cooling rate of ⁇ 30°C/s. Ferrite and intra-ferrite nano-TiC are formed in the air cooling stage, and then cooled at a cooling rate of ⁇ 10°C/s. Rapidly water cool to 400-550°C to obtain bainite and nano-precipitated VC, and finally obtain a microstructure dominated by ferrite and bainite as well as nano-precipitated TiC and VC inside ferrite and bainite.
  • the main purpose of adding more V is to make it bond with C.
  • the main purpose of adding more V is to make it bond with C.
  • nano-VC which plays a precipitation strengthening role.
  • the high Ti and high V composition design is combined with the segmented cooling process to form nano-TiC inside the ferrite grains in the ferrite formation interval, and nano-VC is formed inside the bainite in the bainite formation interval.
  • the excess titanium combined with N can be combined with carbon to form nano-TiC in the air cooling stage after the first stage of water cooling, which can strengthen ferrite.
  • the method of the present invention also includes step 4) pickling, wherein the pickling operating speed of the hot-rolled strip is 30-120m/min, the pickling temperature is controlled at 75-85°C, and the tensile straightening rate is controlled at ⁇ 3 %, rinse in the temperature range of 35 to 50°C, and dry and oil the surface between 120 to 140°C to obtain pickled highly expanded steel.
  • the invention adopts a medium-temperature coiling process and combines it with an innovative low-carbon, high-vanadium and low-silicon composition design to obtain a high-strength and high-pore-expanding steel with excellent performance stability.
  • Nano TiC and VC Gives ferrite and bainite respectively higher strength and more balanced properties.
  • the thermal stress and structural stress inhomogeneity formed inside the steel coil during the medium-temperature coiling phase transformation process are fully released during pickling and straightening, which can further improve the structural uniformity. It is precisely based on this innovative composition and process design idea that the present invention can obtain a series of high-surface hot-rolled pickled high-pore expansion steel products with different strength levels, plasticity, hole expansion, and performance stability.
  • the present invention adopts a silicon-free and high-vanadium composition design to avoid the appearance of red iron scale on the surface of the strip steel and improve the surface quality of pickling high-strength steel.
  • the present invention further adopts innovative low-carbon, high-titanium and high-vanadium composition design, combined with innovative segmented cooling and medium-temperature coiling processes, to obtain hot-rolled pickling of different strength levels with excellent surface, strength, plasticity and hole expansion properties. Highly expanded steel with good performance stability.
  • the steel of the present invention has a tensile strength of ⁇ 590 ⁇ 980MPa, a thickness of 1.5-6.0mm, and good elongation (transverse A50 ⁇ 13% ⁇ 18%) and high hole expansion performance (hole expansion rate ⁇ 30% ⁇ 80%), showing excellent surface, strength, plasticity and hole expansion performance matching, can be used in the manufacture of complex parts such as automobile chassis, subframes, etc. that require high strength thinning and hole expansion flanging, and has a very broad application application prospects.
  • Figure 1 is a schematic diagram of the rolling and cooling process of steel in Examples 1-9 of the present invention.
  • Figure 2 is a schematic diagram of the rolling and cooling process of steel in Examples 10-18 of the present invention.
  • Figure 3 is a schematic diagram of the post-rolling cooling process of the steel in Examples 10-18 of the present invention.
  • composition of the steel in Examples 1-18 of the present invention is shown in Table 1, and the balance of the composition is Fe and inevitable impurities.
  • the process path of the steel in Examples 1-18 of the present invention is:
  • converter smelting or electric furnace smelting is used, and then secondary refining is performed in a vacuum furnace, and then cast into a slab or ingot.
  • the heating temperature is ⁇ 1100°C, between 1100-1300°C; in Examples 10-18, the heating temperature is above 1200°C, between 1200-1300°C.
  • the holding time is 1-2 hours.
  • Example 1-9 primary cooling was used. Specifically, the rolling temperature is 1000 to 1100°C, and 3 to 5 passes of rough rolling are carried out under high pressure above 950°C with a cumulative deformation of ⁇ 50%. Then the intermediate billet is air-cooled or water-cooled to 900 to 950°C, and then 7 passes are performed. Finish rolling and the cumulative deformation is ⁇ 70%, the final rolling is completed between 800 and 900°C, and then the steel plate is water-cooled to 400 ⁇ 550°C at a cooling speed of ⁇ 10°C/s, and then coiled at a cooling speed of ⁇ 20°C/s. Cool slowly to room temperature.
  • the strip steel pickling operating speed is 30 ⁇ 140m/min, preferably 30 ⁇ 120m/min, the pickling temperature is controlled at 75 ⁇ 85°C, the tensile straightening rate is controlled at ⁇ 3%, and rinsing is carried out in the temperature range of 35 ⁇ 50°C.
  • the surface is dried and oiled between 120 and 140°C.
  • Examples 10-18 used secondary cooling in step 3 above.
  • the opening rolling temperature of hot rolling is 1050 ⁇ 1150°C
  • rolling is performed under high pressure in 3 ⁇ 5 passes above 1050°C and the cumulative deformation is ⁇ 50%, and then the intermediate billet is heated to 950 ⁇ 1000°C, and then the final 3 to 7 passes of rolling with cumulative deformation ⁇ 70%, final rolling temperature at 800 to 950°C; cooling adopts segmented cooling, and after final rolling, the steel plate is water-cooled to 600 to 750°C at a cooling rate of ⁇ 30°C/s After cooling in air for 1 to 10 seconds, it is then cooled to a temperature between 400 and 550°C at a cooling rate of ⁇ 10°C/s, and then cooled to room temperature at a cooling rate of ⁇ 20°C/h.
  • Figure 1 shows the flow of the manufacturing method of the present invention.
  • Table 2 shows the production process parameters of the steel in Examples 1-18 of the present invention.
  • Table 3 shows the performance parameters of the pickled steel in Examples 1-18 of the present invention.
  • the steel in Comparative Examples 1-3 is selected from CN103602895A, and the steel in Comparative Example 4 is selected from CN114107792A.
  • Comparative Examples 1-4 do not contain vanadium element, while Comparative Examples 1-3
  • the ingredients also contain high silicon. Therefore, from the perspective of the surface quality of the steel plates, the surfaces of the steel plates of Comparative Examples 1-3 inevitably contain red iron scale, while the embodiments of the present invention are all designed with silicon-free components and have excellent surface quality.
  • Comparative Examples 1 to 4 only adopt high-titanium composition designs, and no V is added to the steel, indicating that nano-titanium carbide precipitation strengthening is used; while the embodiments of the present invention are high-vanadium composition designs, and nano-vanadium carbide is used for strengthening.
  • the present invention can obtain yield strength ⁇ 500- ⁇ 800MPa, tensile strength ⁇ 590- ⁇ 980MPa, and elongation (transverse A50) High hole expansion steel with three different strength levels: ⁇ 13- ⁇ 18%, hole expansion rate ⁇ 30- ⁇ 80%.
  • the steel structure of Examples 1-9 is bainite, and the bainite contains nano-VC, wherein the volume fraction of nano-VC in the bainite is about 0.005-0.05%, preferably 0.005-0.03%.
  • the steel structures of Examples 10-18 are ferrite and bainite, wherein the ferrite contains nano-TiC and the bainite contains nano-VC.
  • the volume fraction of ferrite in steel is 10-40%
  • the volume fraction of nano-TiC in ferrite is about 0.005-0.02%
  • the volume fraction of bainite in steel is 60-90%
  • nano-VC is in
  • the volume fraction in bainite is approximately 0.005 to 0.03%.
  • the highly expanded steel involved in the present invention has high strength, high plasticity and high hole expansion rate, which are well matched, and is particularly suitable for manufacturing parts such as automobile chassis structures that require high strength thinning and expansion and flanging forming. ,have a broad vision of application.

Abstract

本发明提供了一种钢及其制造方法。所述钢的成分以质量百分比计为：C 0.01～0.10％，Si≤0.2％，Mn 0.5～2.0％，P≤0.02％，S≤0.003％，Al 0.01～0.08％，N≤0.004％，V 0.10～0.50％，O≤0.003％，余量为Fe及其它不可避免的杂质。本发明的钢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

高扩孔钢及其制造方法 技术领域
本发明属于钢及其制造方法领域，特别涉及一种高扩孔钢及其制造方法。
背景技术
汽车在国民经济发展中占据非常重要的地位。乘用车中许多零部件尤其是底盘和车身的部分零件经常需要用到热轧酸洗产品。乘用车的轻量化是汽车行业的发展趋势。汽车高强减重需要用钢级别更高，也需要改进底盘结构，造成底盘零件更复杂，对材料性能、表面状况以及成型技术，如液压成形、热冲压、激光焊接等有更高要求，进而转化为对材料的高强、冲压、翻边、回弹以及疲劳等性能上的要求。
目前，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，540MPa以下级别的高扩孔钢竞争相当充分。同时，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较高要求，同时对性能稳定性的要求也愈加严格。
乘用车企业为了降低工序成本，对材料的性能要求进一步提高。如在生产汽车底盘零件时，为了减少冲压工序，要求材料在高强高塑的同时，还具有高扩孔率。如780MPa级高扩孔钢的扩孔率要求保证≥50％，最好进一步提高至70％甚至80％以上。现有高扩孔钢特别是780MPa级高扩孔钢多具有热轧贝氏体组织同时配合析出相进行强化，采用的工艺路径多为中温卷取，但温度控制精度不高且成品组织均匀性欠佳，导致获得的热轧钢扩孔率等性能不均匀，后续加工过程中易出现冲压开裂。
现有技术中对780MPa级酸洗高扩孔钢研究列举如下：
中国专利CN103602895A公开了一种低碳Nb-Ti微合金化高扩孔钢，其采用低碳高硅配合Nb-Ti微合金化的成分设计，可以保证扩孔率≥50％。 但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通常在500℃左右，导致钢卷全长温度控制难度大，从而使钢卷全长性能波动大。
中国专利CN105821301A公开了一种800MPa级热轧高强度高扩孔钢，其也采用低碳高硅配合Nb-Ti微合金化的成分设计，钢中Ti含量很高，为0.15～0.18％。在实际生产过程中，此种成分设计思路会导致带钢表面有红铁皮等缺陷。另一方面，由于高Ti含量，钢中易形成粗大TiN，对扩孔率稳定性不利。
中国专利CN108570604A公开了一种780MPa级热轧酸洗高扩孔钢，其采用低碳高铝高铬的成分设计，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出现，但高铝的成分设计容易在实际生产过程中造成浇铸水口堵塞。另一方面，该钢的生产工艺复杂，尤其是三段式冷却工艺控制难度大，导致钢的扩孔率不高。
中国专利CN114107792A公开了一种780MPa级热轧酸洗高扩孔钢，其采用低碳高钛的成分设计，并在钢中添加了适量的钼元素。由于含钼钢的相变过程较为缓慢，相变过程主要发生在卷取之后。因此，实际生产过程中存在钢卷内外圈强度偏低等问题。
发明内容
鉴于上述现有技术中存在的问题，本发明的目的在于提供一种钢及其制造方法。本发明的钢具有高强度、高塑性和高扩孔率，且这些性能之间匹配良好，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：
为满足用户对更高表面质量、更好的性能稳定性、更优的强度、塑性和扩孔性匹配等需求，需要对传统高扩孔钢进行改进。
在通常情况下，材料的延伸率与扩孔率呈反比，即延伸率越高，扩孔率越低；反之，延伸率越低，扩孔率越高。在相同或相似的强化机制下，材料的强度越高，扩孔率越低。为了获得同时具有良好的塑性和扩孔翻边性能的钢材，需要更好地平衡两者之间的关系。为实现强度、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通常导致较差的钢板表面质量。具体地，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。
本发明优化了现有热轧钢的化学成分，采用低碳高钒的元素设计，不刻意在钢中添加硅元素，并且通过添加V元素，利用其形成的纳米级碳化钒提升钢的强度和塑性。在不改变现有的热连轧产线的情况下即可获得组织和性能均匀的贝氏体型析出强化高强钢。
具体地，本发明所述的钢，其成分以质量百分比计为：C 0.01～0.10％，Si≤0.2％，Mn 0.5～2.0％，P≤0.02％，S≤0.003％，Al 0.01～0.08％，N≤0.004％，V 0.10～0.50％，O≤0.003％，余量包含Fe及其它不可避免的杂质。
优选地，上述钢中还含有Ti，以质量百分比计，Ti含量的上限为0.2％，优选为0.18％，更优选为0.015％，Ti含量的下限为0.05％，优选为0.08％。
优选地，所述钢还含有Mo，以质量百分比计，Mo的含量优选为0.1～0.5％，更优选0.20～0.40％，还更优选0.2-0.3％。
优选地，上述钢还含有选自Nb≤0.1％，Cu≤0.5％，Ni≤0.5％，Cr≤0.5％，B≤0.002％中的一种以上成分，其中Cu更优选为0.3％以下；Ni更优选为0.3％以下；Cr更优选为0.3％以下；Nb更优选为0.06％以下；B更优选为0.002％以下，还更优选为0.001％以下。
优选地，上述钢的成分满足如下中的一个以上：C 0.03～0.07％；Si≤0.10％；Mn 0.8～1.6％；S≤0.0018％；Al 0.02～0.05％；N≤0.003％；O≤0.002％。
在本发明所述钢的成分设计中：
碳，是钢中的基本元素，同时也是本发明中的重要元素之一。碳能扩大奥氏体相区，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，其中对钢的屈服强度和抗拉强度影响最大。在本发明中，由于在热轧阶段所要获得的组织为低碳贝氏体，为获得最终抗拉强度达不同强度级别的高扩孔钢，碳含量必须在0.01％以上；同时，碳含量也不能高于0.10％。碳含量太高，低温卷取时易形成低碳马氏体。因此，本 发明将碳含量控制在0.01-0.10％，优选为0.03-0.07％。
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，完全避免硅的添加似乎是比较困难的。根据实际生产的大量统计数据显示，硅含量在0.2％以下时，热轧过程中可避免出现表面红铁皮缺陷，通常在0.10％以下可保证不出现红铁皮。因此，本发明钢中硅的含量控制在0.2％以内，优选为0.10％以内。
锰，也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。众所周知，Mn是扩大奥氏体相区的重要元素，可以降低钢的临界淬火速度，稳定奥氏体，细化晶粒，推迟奥氏体向珠光体的转变。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，同时板坯连铸时也容易发生热裂。因此，本发明钢中Mn含量控制在0.5-2.0％，优选为0.8-1.6％。
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，会形成Fe2P在晶粒周围析出，降低钢的塑性和韧性，故其含量越低越好。本发明将磷含量控制在0.02％以内，获得的钢力学性能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。为了减少MnS的含量，需对S含量严格控制，钢中S含量越低越好。本发明将S含量控制在0.003％以内，优选为0.0018％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，本发明钢中Al的含量控制在0.01-0.08％之间，优选在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V等结合，形成的VN颗粒对钢的性能带来不利的影响，尤其对扩孔性能非常不利。由于VN呈方形，其尖角与基体之间存在很大应力集中，在扩孔变形的过程中，VN与基体之间的应力集中容易形成裂纹源，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钒设计，为了尽量减少VN带来的对扩孔不利影响。因此，本发明将氮含量控制在0.004％以下，优选在0.003％以下。
钒，是本发明中的重要元素。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。为了充分利用钒的析出强化效果，钢中钒的加入量至少应达到0.10％以上才有明显的析出强化作用；随着钒含量的增加，钒的析出强化效果逐渐增强，当钒的含量超过0.50％时，钒的析出强化效果饱和且形成的碳化钒尺寸较大，对钢强度的贡献反而降低。因此，本发明的钢中钒的添加量控制为≤0.50％。具体地，当V含量在0.10-0.20％之间时，可获得590MPa级高扩孔钢；当V含量在0.20-0.35％之间时，可获得780MPa级高扩孔钢；当V含量在0.35-0.50％之间时，可获得980MPa级高扩孔钢。
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，有利于获得贝氏体组织。另外，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低碳贝氏体组织，而低碳贝氏体在焊接之后易发生软化现象，加入一定量的钼可以有效减小焊接软化程度。因此，在本发明中，钼的含量控制在0.10-0.50％，优选为0.20-0.40％，更优选为0.2～0.3％。当配合分段冷却工艺时，钼对分段冷却过程中铁素体形成起到一定的抑制作用。当钼含量在上述范围内时，可充分发挥其效果。
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。铌加入钢中可以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，在后续转变过程中可细化最终的相变组织。但铌的加入量不可太多，一方面铌的加入量超过0.10％，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，本发明的钢中铌含量控制在≤0.10％，优选为≤0.06％。
钛，是本发明中的一种可添加元素。少量Ti加入钢中，一方面可以在高温阶段与N结合形成TiN，起到固氮的作用，有利于减少后续VN形成；另一方面，与N结合之后多余的Ti可在后续过程中与碳结合形成纳米TiC，并与纳米VC一起共同提高钢的性能。钛的含量高于0.20％时，高温阶段易形成较多粗大TiN并恶化钢的冲击韧性。因此，本发明钢中可添加元素钛的含量为0.20％以内，优选为0.18％以内，更优选为0.015％以内，最优选为0.10％以内。另一方面，Ti的含量优选在0.05％以上，更优选为0.08％以上，由此可提供出色的析出强化效果。
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，本发明将Cu含量控制在0.5％以内，优选为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，本发明的钢中镍的添加量≤0.5％，优选为≤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。由于本发明中钢的组织为细小贝氏体铁素体加 纳米析出碳化物，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≤0.3％。
硼，是本发明中的可添加元素。硼能够大大提高钢的淬透性，促进贝氏体转变，在中温贝氏体相变时，促进板条贝氏体的转变。因此，钢中加入微量的硼有利于获得细小的板条贝氏体组织，但硼含量不宜过多，过多硼的加入会导致形成马氏体和较多的马奥岛，对塑性和扩孔不利。故本发明钢中硼的添加量控制在≤0.002％，优选为≤0.001％。
氧，是本发明中的杂质元素，为了获得性能更优异的钢，钢中氧的含量越低越好，但更低的氧化量会提高炼钢成本，在保证带钢性能的条件下，本发明钢中氧含量控制在0.003％以内，优选为0.002％以内。
现有高扩孔钢通常采用高Ti成分设计，其中添加微合金元素钛的主要目的是细化晶粒，添加量一般在0.1％以内。而本发明采用了高钒成分设计，Ti在本发明的钢中以可选元素存在。本发明中添加钒的主要目的是使其与碳结合，形成弥散分布的纳米碳化钒，起到析出强化作用。
本发明的钢通过包含含量高达0.1～0.5％的V获得了同时具有高抗拉强度和扩孔率的钢，其中，当钢中V含量为0.10～0.20％时，钢的抗拉强度为590MPa级，扩孔率≥70％、优选≥80％；当钢中V含量为0.20～0.35％时，钢的抗拉强度为780MPa级，扩孔率≥50％；当钢中V的含量为0.35～0.50％时，钢的抗拉强度为980MPa级，扩孔率≥30％、优选≥40％。
现有高扩孔钢的微观组织大多为铁素体或铁素体加贝氏体，为了获得更高的强度，采用纳米碳化钛进行强化。本发明高扩孔钢不含Ti时其微观组织为贝氏体和贝氏体中纳米碳化钒，并且可根据钒含量的不同，获得不同强度级别的高扩孔钢，满足下游用户对不同强度级别高扩孔钢的需求。本发明的钢在包含Ti时的微观组织为铁素体和贝氏体，其中，铁素体内含有纳米TiC，贝氏体内含有纳米VC。
本发明还提供一种制造本发明的钢的方法，包括如下步骤：
1)冶炼、铸造：
按上述成分采用转炉或电炉冶炼，真空炉二次精炼，铸造成铸坯或铸锭。
2)铸坯或铸锭再加热：
加热温度≥1100℃，优选在1200℃以上；保温时间：1～2小时。
3)热轧、冷却。
作为本发明方法的一个实施方式，优选地，本发明方法的步骤3)中，开轧温度1000～1100℃，在950℃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900～950℃，然后进行7道次精轧且累计变形量≥70％，在800～900℃之间终轧结束，获得钢带，再以≥10℃/s的冷速将上述钢带水冷至400～550℃卷取，然后以≤20℃/s的冷速缓慢冷却至室温，获得热轧带钢。
作为本发明方法的另一实施方式，优选地，本发明方法的步骤3)中，热轧的开轧温度1050～1150℃，在1050℃以上3～5道次大压下轧制且累计变形量≥50％，随后中间坯待温至950～1000℃，然后进行最后3～7道次轧制且累计变形量≥70％，终轧温度在800～950℃，获得钢带；
冷却采用分段冷却，终轧后以≥30℃/s的冷速将上述钢带水冷至600～750℃之间，空冷1～10秒后，再以≥10℃/s的冷速冷却至400～550℃之间卷取，然后以≤20℃/h的冷速冷却至室温，获得热轧带钢。
优选地，本发明的钢包含Ti时，其采用上述包括分段冷却方式的方法进行。
上述步骤3)中，采用热轧开轧温度为1050～1150℃，在1050℃以上3-5道次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。
在步骤3)的粗轧和精轧阶段，轧制应尽量快速完成，以保证奥氏体中具有更多的固溶钛和钒。高温终轧结束后，带钢以≥30℃/s的冷却速度首先冷却到600-750℃，在空冷阶段形成铁素体和铁素体晶内纳米TiC，随后以≥10℃/s的冷速水冷至400-550℃以获得贝氏体和纳米析出VC，最终获得以铁素体和贝氏体以及铁素体和贝氏体内部纳米析出TiC和VC为主的微观组织。
当钢中同时含高Ti和高V时，添加较多V的主要目的是使其与C结 合，形成弥散分布的纳米VC，起到析出强化作用。高Ti、高V成分设计与分段冷却工艺相结合，在铁素体形成区间，在铁素体晶粒内部形成纳米TiC，在贝氏体形成区间，在贝氏体内部形成纳米VC。通过成分和工艺的配合，与N结合之后多余的钛可在第一阶段水冷之后的空冷阶段与碳结合形成纳米TiC，起到强化铁素体的作用。在铁素体中引入纳米TiC可减小铁素体与贝氏体之间性能差异，有利于扩孔率提高；通过V含量的不同，控制贝氏体的强度，可获得不同强度级别的高扩孔钢，满足下游用户对不同强度级别高扩孔钢的需求。
优选地，本发明的方法还包括步骤4)酸洗，其中，热轧带钢的酸洗运行速度在30～120m/min，酸洗温度控制在75～85℃，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，获得经酸洗的高扩孔钢。
在制造本发明的钢的方法中：
本发明采用中温卷取工艺，并与创新性的低碳高钒以及低硅成分设计相结合，可获得一种性能稳定性优异的高强度高扩孔钢。
在中低温贝氏体转变区间，通过对卷取温度的精确控制，保证了带钢全长性能均匀性。中温卷取过程中发生贝氏体相变，同时伴随着钒的纳米析出。通过对关键元素碳含量和钒含量进行定量化设计，同时与特定的卷取相配合，可获得不同强度级别不同扩孔率的高扩孔钢系列产品。
传统高钛型高扩孔钢大多采用高温卷取工艺，本发明则采用中温卷取工艺。因此，在粗轧和精轧阶段，轧制节奏应尽量快速完成，保证更多的钒固溶在奥氏体中。高温终轧结束后，带钢以≥10℃/s的冷却速度在线冷却至400-550℃中温区以获得贝氏体和纳米析出组织。
当钢中含Ti且制造工艺中采用分段冷却时，在轧后第一次水冷后的空冷阶段，形成所需数量的铁素体并在铁素体晶粒内部析出纳米碳化钛，提高铁素体的性能；在第二次水冷后的中温卷取阶段，利用钒的析出作用，在贝氏体中形成纳米碳化钒，提高贝氏体的强度，通过成分和工艺的创新，获得了纳米碳化钛强化的铁素体和纳米碳化钒强化的贝氏体组织。通过对钒含量的不同设计，可获得不同强度级别的高扩孔钢系列产品，表现出成分和工艺的创新性以及由此带来的组织和性能的独特性。纳米TiC和VC 分别赋予铁素体和贝氏体更高的强度和更平衡的性能。
在随后的酸洗过程中，中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，可进一步改善组织均匀性。正是基于这种创新性的成分和工艺设计思路，本发明可获得不同强度等级、塑性、扩孔以及性能稳定性等优异的高表面热轧酸洗高扩孔钢系列产品。
与现有技术相比，本发明的优点或有益效果在于：
与中国专利CN103602895A和CN105821301A采用高硅成分设计相比，本发明采用不含硅和高钒的成分设计，避免带钢表面出现红铁皮，提高酸洗高强钢的表面质量。
与中国专利CN108570604A相比，尽管其硅含量在0.05～0.5％，但仍无法保证带钢表面完全消除红铁皮缺陷，且三段冷却工艺控制难度大，性能稳定性难以保证，而本发明在采用了多路径冷却精确控制技术，可保证带钢的性能均匀性。
本发明进一步采用创新性的低碳高钛高钒成分设计，结合创新性的分段冷却和中温卷取工艺，可获得表面、强度、塑性和扩孔性能优异的不同强度级别的热轧酸洗高扩孔钢且性能稳定性良好。
本发明的钢的抗拉强度≥590～≥980MPa，且厚度在1.5-6.0mm的，具有良好延伸率(横向A50≥13％～≥18％)和高扩孔性能(扩孔率≥30％～≥80％)，表现出优异的表面、强度、塑性和扩孔性能匹配，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。
附图说明
图1为本发明实施例1-9的钢的轧制和冷却工艺示意图；
图2为本发明实施例10-18的钢的轧制和冷却工艺示意图；
图3为本发明实施例10-18的钢的轧后冷却工艺示意图
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。
本发明实施例1-18的钢的成分参见表1，成分余量为Fe和不可避免的杂质。
本发明实施例1-18的钢的工艺路径为：
1)冶炼、铸造
按照表1所示的成分采用转炉冶炼或电炉冶炼，然后在真空炉中进行二次精炼，再铸造成铸坯或铸锭。
2)对铸坯或铸锭再加热
实施例1-9中加热温度≥1100℃，在1100-1300℃之间；实施例10-18中加热温度在1200℃以上，在1200-1300℃之间。保温时间为1-2小时。
3)热轧、冷却
实施例1-9中采用一次冷却。具体地，开轧温度1000～1100℃，在950℃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900～950℃，然后进行7道次精轧且累计变形量≥70％，在800～900℃之间终轧结束，再以≥10℃/s的冷速将钢板水冷至400～550℃卷取，然后以≤20℃/s的冷速缓慢冷却至室温。
4)酸洗
带钢酸洗运行速度在30～140m/min，优选在30～120m/min，酸洗温度控制在75～85℃，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。
实施例10-18在上述步骤3中采用二次冷却。具体地，热轧的开轧温度1050～1150℃，在1050℃以上3～5道次大压下轧制且累计变形量≥50％，随后中间坯待温至950～1000℃，然后进行最后3～7道次轧制且累计变形量≥70％，终轧温度在800～950℃；冷却采用分段冷却，终轧后以≥30℃/s的冷速将钢板水冷至600～750℃之间，空冷1～10秒后，再以≥10℃/s的冷速冷却至400～550℃之间卷取，然后以≤20℃/h的冷速冷却至室温。
图1示出本发明制造方法的流程。表2为本发明实施例1-18的钢的生产工艺参数。表3为本发明实施例1-18的酸洗钢的性能参数。
对比例1-3的钢选自CN103602895A，对比例4的钢选自CN114107792A。
从表1可以看出，对比例1-4成分中均不含钒元素，同时对比例1-3 的成分中还含有高硅。因此，从钢板表面质量看，对比例1-3的钢板表面不可避免含有红铁皮，而本发明实施例均为不含硅的成分设计，表面质量优异。
另外，对比例1-4均仅采用了高钛成分设计，钢中未添加V，表明其采用了纳米碳化钛析出强化；而本发明实施例为高钒成分设计，采用纳米碳化钒进行强化。
从表3可以看出，本发明通过对成分与关键工艺过程参数进行定量化设计和精确控制，可获得屈服强度≥500-≥800MPa，抗拉强度≥590-≥980MPa，延伸率(横向A50)≥13-≥18％，扩孔率≥30-≥80％的三种不同强度级别的高扩孔钢。
表3中钢的力学性能按如下测量：
钢的屈服强度、抗拉强度和延伸率(横向A50)按照GB/T 228.1-2021“金属材料拉伸试验第1部分：室温试验方法”进行测试。
钢的扩孔率按照GB/T 24524-2021“金属材料薄板和薄带扩孔实验方法”进行测试。
实施例1-9的钢组织为贝氏体，贝氏体内含有纳米VC，其中纳米VC在贝氏体中的体积分数约为0.005～0.05％，优选0.005-0.03％。
实施例10-18的钢组织为铁素体和贝氏体，其中铁素体内含有纳米TiC，贝氏体内含有纳米VC。具体地，钢中铁素体的体积分数为10～40％，纳米TiC在铁素体中的体积分数约为0.005～0.02％，钢中贝氏体的体积分数为60～90％，纳米VC在贝氏体中的体积分数约为0.005～0.03％。
从上述实施例可以看出，本发明所涉及的高扩孔钢具有高强度、高塑性和高扩孔率良好匹配，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制造，具有广阔的应用前景。


Claims (13)

  1. 一种钢，其成分以质量百分比计为：C 0.01～0.10％，Si≤0.2％，Mn 0.5～2.0％，P≤0.02％，S≤0.003％，Al 0.01～0.08％，N≤0.004％，V 0.10～0.50％，O≤0.003％，余量为Fe及其它不可避免的杂质。
  2. 如权利要求1所述的钢，其特征在于，以质量百分比计，所述钢还含有0.05～0.2％、优选0.08％～0.15％、更优选0.08％～0.10％的Ti。
  3. 如权利要求1或2所述的钢，其特征在于，以质量百分比计，所述钢还含有选自Nb≤0.1％，Cu≤0.5％，Ni≤0.5％，Cr≤0.5％，B≤0.002％中的一种以上成分。
  4. 如权利要求3所述的钢，其特征在于，以质量百分比计，所述钢还包含0.1～0.5％、优选0.20～0.40％、更优选0.2-0.3％的Mo。
  5. 如权利要求1或2所述的钢，其特征在于，所述钢的成分满足如下中的一个以上：C 0.03～0.07％；Si≤0.10％；Mn 0.8～1.6％；S≤0.0018％；Al 0.02～0.05％；N≤0.003％；O≤0.002％。
  6. 如权利要求3所述的钢，其特征在于，所述钢的成分满足如下中的一个以上：Nb≤0.06％、Cu≤0.3％、Ni≤0.3％、Cr≤0.3％和B≤0.001％。
  7. 如权利要求1所述的钢，其特征在于，所述钢的组织为贝氏体和贝氏体内的纳米析出VC。
  8. 如权利要求2所述的钢，其特征在于，所述钢的组织为铁素体和贝氏体，其中，铁素体内含有纳米TiC，贝氏体内含有纳米VC。
  9. 如权利要求1-8中任一项所述的钢，其特征在于，所述钢的屈服强度为500MPa以上，抗拉强度为590MPa以上、优选780MPa以上，延伸率横向A50为14％以上、30％以下。
  10. 一种制造权利要求1～9中任一项所述的钢的方法，其特征是，所述方法包括如下步骤：
    1)冶炼、铸造
    按权利要求1～9中任一项所述的成分采用转炉或电炉冶炼，真空炉二次精炼，铸造成铸坯或铸锭；
    2)铸坯或铸锭再加热
    加热温度≥1100℃，保温时间：1～2小时；
    3)热轧、冷却。
  11. 根据权利要求10所述的方法，其中，在步骤3)中，开轧温度1000～1100℃，在950℃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900～950℃，然后进行7道次精轧且累计变形量≥70％，在800～900℃之间终轧结束，获得钢带，再以≥10℃/s的冷速将所述钢带水冷至400～550℃卷取，然后以≤20℃/s的冷速缓慢冷却至室温，获得热轧带钢。
  12. 根据权利要求10所述的方法，其中，在步骤3)中，
    热轧的开轧温度1050～1150℃，在1050℃以上3～5道次大压下轧制且累计变形量≥50％，随后中间坯待温至950～1000℃，然后进行最后3～7道次轧制且累计变形量≥70％，终轧温度在800～950℃，获得钢带；
    冷却采用分段冷却，终轧后以≥30℃/s的冷速将上述钢带水冷至600～750℃之间，空冷1～10秒后，再以≥10℃/s的冷速冷却至400～550℃之间卷取，然后以≤20℃/h的冷速冷却至室温，获得热轧带钢。
  13. 如权利要求11或12所述的方法，其特征在于，所述方法还包括步骤4)酸洗，其中，所述热轧带钢的酸洗运行速度在30～140m/min，优选在30～120m/min，酸洗温度控制在75～85℃，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。
PCT/CN2023/101836 2022-06-22 2023-06-21 高扩孔钢及其制造方法 WO2023246899A1 (zh)

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