WO2023246898A1 - 一种高塑性钢及其制造方法 - Google Patents

一种高塑性钢及其制造方法 Download PDF

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Publication number
WO2023246898A1
WO2023246898A1 PCT/CN2023/101832 CN2023101832W WO2023246898A1 WO 2023246898 A1 WO2023246898 A1 WO 2023246898A1 CN 2023101832 W CN2023101832 W CN 2023101832W WO 2023246898 A1 WO2023246898 A1 WO 2023246898A1
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Prior art keywords
steel
content
present
strength
cooling
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PCT/CN2023/101832
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English (en)
French (fr)
Inventor
王焕荣
张晨
杨阿娜
庞厚君
范佳杰
陆敏
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Baoshan Iron and Steel Co Ltd
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Baoshan Iron and Steel Co Ltd
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Priority claimed from CN202210714634.8A external-priority patent/CN117305690A/zh
Priority claimed from CN202210713392.0A external-priority patent/CN117305685A/zh
Application filed by Baoshan Iron and Steel Co Ltd filed Critical Baoshan Iron and Steel Co Ltd
Priority to EP23826543.3A priority Critical patent/EP4527963A4/en
Priority to JP2024574813A priority patent/JP2025521326A/ja
Priority to KR1020257001157A priority patent/KR20250022191A/ko
Priority to US18/876,887 priority patent/US20250369066A1/en
Publication of WO2023246898A1 publication Critical patent/WO2023246898A1/zh
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • 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
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    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
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Definitions

  • the invention belongs to the field of steel and its manufacturing method, and particularly relates to a high plasticity steel and its manufacturing method.
  • Chinese patent CN104233092A discloses a 780MPa grade ultra-high plasticity steel. Its composition is designed to be low carbon and high silicon. At the same time, a certain amount of precious alloy elements such as Cr, Mo, and Nb are added, so the alloy cost is relatively high.
  • Chinese patent CN107815593A discloses a low-silicon, high-aluminum ultra-high plasticity steel, which is composed of low-silicon, high-aluminum, and a certain amount of precious element Cu.
  • the process path is mainly to perform heat treatment and heat preservation in the two-phase zone for 1 to 3 minutes, and then perform phase transformation in the bainite zone to obtain 780MPa grade heat-treated ultra-high plasticity steel.
  • the heat treatment process path cannot be expanded to the existing hot-rolled steel. Production line.
  • the purpose of the present invention is to provide a high plasticity steel and a manufacturing method thereof, which have good mechanical properties and can achieve a good match between low yield strength, low yield ratio, high tensile strength and ultra-high elongation.
  • the above-mentioned steel can be widely used in parts with complex shape requirements such as commercial vehicles or passenger cars, or other parts that require high strength and thinning.
  • the first aspect of the present invention provides a kind of steel.
  • the composition of the steel is calculated as mass percentage: C 0.10 ⁇ 0.35%, Si 0.8 ⁇ 2.0%, Mn 1.0 ⁇ 3.0%, P ⁇ 0.02% , S ⁇ 0.005%, Al 0.1 ⁇ 2.0%, N ⁇ 0.005%, the balance includes A and other unavoidable impurities.
  • the above-mentioned steel also contains Ti, and the content of Ti is less than 0.2% in terms of mass percentage, preferably 0.05-0.2%, and more preferably 0.05-0.1%.
  • the above-mentioned steel also contains one or more selected from the group consisting of Mo, Nb, V, Cu, Ni, Cr, and B, wherein the content of Mo is less than 0.5%, preferably less than 0.3% in terms of mass percentage; Nb
  • the content of V is below 0.1%, preferably below 0.06%; the content of V is below 0.1%, preferably below 0.06%; the content of Cu is below 0.5%, preferably below 0.3%; the content of Ni is below 0.5%, preferably below The content of Cr is below 0.5%, preferably below 0.3%; the content of B is below 0.001%, preferably below 0.0005%.
  • O ⁇ 0.003%
  • S S ⁇ 0.003%
  • N N ⁇ 0.004%
  • the composition of the above-mentioned steel satisfies at least one of the following in terms of mass percentage: C 0.15 ⁇ 0.25%, Si 1.0 ⁇ 1.6%, Mn 1.5 ⁇ 2.5%, Al 0.3 ⁇ 1.0%.
  • Carbon is a basic element in steel and one of the important elements in the present invention. Carbon expands the austenite phase area and stabilizes 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. At the same time, carbon is an effective element for stabilizing retained austenite, and its content in steel is usually high. In the present invention, in order to obtain high-strength steel with tensile strength reaching different strength levels, it is also necessary to have more stable retained austenite in the steel structure, and the carbon content must be ensured to be above 0.10%; however, the carbon content is also It cannot be higher than 0.35%. If the carbon content is too high, it will easily lead to high strength, reduced elongation and poor weldability. Therefore, the carbon content is between 0.10-0.35%.
  • Silicon is a basic element in steel and one of the important elements in the present invention. Adding silicon to steel can reduce the non-recrystallization temperature of austenite and expand the austenite rolling process window, so that the steel can complete dynamic recrystallization during the finishing rolling stage, which is beneficial to improving the transverse and longitudinal performance differences of the steel. Another function of silicon added to steel is to inhibit the formation of cementite. In the present invention, in order to have more retained austenite in the steel structure, higher silicon needs to be added to suppress the formation of cementite.
  • the silicon content in steel is between 0.8-2.0%, preferably 1.0-1.6%.
  • Manganese is the most basic element in steel and one of the most important elements in the present invention. Mn expands the austenite phase area, reduces the critical quenching speed of steel, stabilizes austenite, refines grains, and delays the transformation of austenite to pearlite. At the same time, manganese can be partitioned during the heat treatment process. Manganese diffuses from bainite to retained austenite, further stabilizing the retained austenite and obtaining more retained austenite. The manganese content must be at least 1.0% to achieve the above effect, but the manganese content should not be too high. If the manganese content exceeds 3.0%, the continuous casting billet is prone to segregation and more MnS inclusions will be formed. Therefore, the manganese content in steel is 1.0-3.0%, preferably 1.5-2.5%.
  • 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 is formed and precipitates around the grains, reducing the plasticity and toughness of the steel. Therefore, its content The lower the amount, the better. Generally, it is better to control it 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 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. Therefore, the lower the S content in steel, the better. In order to reduce the MnS content, the S content needs to be strictly controlled. The S content is required to be within 0.005%, preferably within 0.003%.
  • Aluminum is one of the important elements in the present invention. In addition to the basic functions of deoxidation and nitrogen fixation, aluminum also has two other important functions in the present invention. Since in the present invention, the content of austenite stabilizing elements such as carbon and manganese is relatively high and the austenite stability is strong, it is difficult to form the required content of ferrite during the short air cooling stage of segmented cooling after rolling, and a proportion of the ferrite needs to be added. Ordinary high-strength steel contains more aluminum, which accelerates the ferrite phase transformation in order to obtain a sufficient amount of ferrite; on the other hand, in order to obtain super stable retained austenite, more aluminum also needs to be added.
  • the aluminum content in the steel of the present invention is between 0.1-2.0%, preferably between 0.3-1.0%.
  • 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 Ti to form TiN particles that have an adverse impact on the properties of steel. Therefore, the nitrogen content in the present invention is 0.005% or less, preferably 0.004% or less.
  • Titanium is one of the elements that can be added in the present invention. Because ultra-high plasticity high-strength steel contains a lot of retained austenite, the retained austenite is a soft phase and has a low yield strength. In order to improve the yield strength of steel, under certain conditions, micro-alloying elements such as titanium can be added to increase the yield strength through the precipitation strengthening effect of titanium in proeutectoid ferrite. As the titanium content increases, the precipitation strengthening effect gradually increases. When the titanium content increases to 0.20%, the precipitation strengthening effect of titanium is saturated. Therefore, the amount of titanium added can be adjusted according to actual needs.
  • the titanium content in the steel of the present invention is within 0.20%, preferably 0.05-0.2%, more preferably 0.05-0.1%.
  • Molybdenum is one of the elements that can be added 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 structure dominated by ferrite, bainite and retained austenite, and ferrite and bainite are prone to softening after welding, adding a certain amount of molybdenum can effectively reduce welding stress. degree of softening. Considering that molybdenum is a precious metal, adding more molybdenum increases the cost of the alloy. Therefore, the content of molybdenum in the present invention is within 0.5%, preferably within 0.3%.
  • Oxygen is an inevitable element in the steelmaking process.
  • the oxygen content in the steel can generally reach less than 0.003% after deoxidation, which will not cause obvious adverse effects on the performance of the steel plate. Therefore, in the present invention, the O content in the steel is 0.003% or less, preferably 0.002% or less.
  • Copper is one of the addable elements 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 the corrosion resistance improvement effect of Cu in certain applications without causing significant "Cu brittleness” phenomenon, in the present invention The Cu content is within 0.5%, preferably within 0.3%.
  • Nickel is one of the addable elements 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 amount of nickel added in the present invention is below 0.5%, preferably below 0.3%.
  • Chromium is one of the elements that can be added in the present invention. Chromium is added to steel mainly to improve the strength of steel through solid solution strengthening or structure refinement. Chromium easily dissolves into ferrite and strengthens ferrite; at the same time, adding a small amount of chromium can also improve corrosion resistance. Therefore, the amount of chromium added in the present invention is below 0.5%, preferably below 0.3%.
  • Niobium is one of the additive elements in the present invention. Niobium is similar to titanium and is a strong carbide element in steel. The addition of niobium to steel can greatly increase the non-recrystallization temperature of the steel. Deformed austenite with higher dislocation density can be obtained during the finishing rolling stage, and the final phase transformation structure can be refined 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%, relatively coarse niobium carbonitrides are easily formed in the structure, which is detrimental to the low-temperature impact toughness of the steel. At the same time, the high content of niobium can easily cause anisotropy of the hot-rolled austenite structure. Therefore, the niobium content in the steel of the present invention is 0.10% or less, preferably 0.06% or less.
  • Vanadium is one of the addable elements in the present invention. Vanadium, similar to titanium and niobium, is 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. Since the solid solubility of vanadium carbide in ferrite is greater than the solid solubility of niobium and titanium, the size of vanadium carbide formed in ferrite is larger and it is easy to form on the grain boundaries, which has a negative impact on the steel. Resilience is bad. Therefore, the added amount of vanadium in the steel of the present invention is 0.10% or less, preferably 0.06% or less.
  • Boron is one of the addable elements in the present invention. Boron is an element that easily segregates. When rolling in the austenite zone, the boron element can segregate to the austenite grain boundaries, reducing the interface energy at the austenite grain boundaries, which is not conducive to the subsequent cooling phase transformation of ferrite. body formation. Since the desired structures of the present invention are ferrite, bainite and stable retained austenite, the content of boron in the steel needs to be strictly controlled to prevent excessive addition of boron from inhibiting the formation of ferrite. Therefore, the added amount of boron in the steel of the present invention is 0.001% or less, preferably 0.0005% or less.
  • the content of elements in the steel of the present invention refers to mass fraction.
  • the structure of the steel of the present invention is ferrite, bainite and retained austenite with a content of ⁇ 5%.
  • the volume fraction of ferrite in the steel is between 30-50%, preferably between 35-45%, and the volume fraction of bainite is between 40-60%, preferably between 45-55% , the volume fraction of retained austenite is between 5-15%, preferably between 10-15%.
  • the yield strength of the above-mentioned steel is above 500MPa, preferably above 600MPa, and more preferably above 700MPa; the tensile strength is above 780MPa, preferably above 980MPa; and the elongation is above 25%, preferably above 30%.
  • the hole expansion rate of the steel is above 30%, preferably above 50%.
  • the yield strength of the steel is ⁇ 600MPa
  • the tensile strength is ⁇ 780MPa
  • the elongation is ⁇ 30%
  • the hole expansion rate is ⁇ 50%
  • the Ti content is 0.05-0.2% and the C content is 0.25-0.35%
  • the steel The yield strength is ⁇ 700MPa
  • the tensile strength is ⁇ 980MPa
  • the elongation is ⁇ 25%
  • the hole expansion rate is ⁇ 30%.
  • the existing 780MPa ultra-high plasticity steel uses C-Si-Mn as the main element. If necessary, micro-alloying elements such as Nb and Ti are also added to refine the grains.
  • the Al content is less than 0.1%, and the deoxidation of Al element is mainly used. Nitrogen fixation.
  • the present invention adopts a high Al component design, and the Al content is above 0.1%.
  • the main purpose of adding high Al is to promote ferrite phase transformation and to further improve the stability of retained austenite.
  • the existing 780MPa ultra-high plasticity steel has low yield strength or low yield-to-strength ratio, and the retained austenite is not stable enough. Under deformation, the retained austenite in the structure can easily transform into martensite.
  • the ultra-high plasticity steel involved in the present invention can obtain tensile properties with different yield strengths and different yield ratios.
  • the retained austenite in the structure is more stable, and the content of retained austenite is more than 5%.
  • the yield strength levels are different, the tensile strength and elongation remain at a very high level, which is more conducive to processing and use by downstream users.
  • the steel of the present invention can also have a higher hole expansion rate, which is particularly beneficial to the stamping processing of parts that require higher drawing and flanging forming requirements.
  • the method for manufacturing the aforementioned steel includes the following steps:
  • composition it is smelted in a converter or electric furnace, then refined in a vacuum furnace, and then cast into a slab or ingot;
  • the opening temperature of hot rolling is ⁇ 1000°C, rolling at high pressure for 5 to 7 passes above 1000°C and the cumulative deformation is ⁇ 50%, then the intermediate billet is heated to ⁇ 950°C, and then the final 3 to 7 passes are performed. Roll and the cumulative deformation is ⁇ 70%, and the final rolling temperature is between 800 and 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 350 and 550°C, and then cool to room temperature at a cooling rate of ⁇ 50°C/h to obtain hot-rolled strip steel.
  • the above method also includes step 4) pickling, wherein the pickling 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 ⁇ 2%. It is then rinsed in the temperature range of 35 to 50°C, and the surface is dried and oiled between 120 to 140°C to obtain pickled high-strength and ultra-high plasticity steel.
  • the opening temperature of hot rolling is ⁇ 1000°C, and the main purpose is to refine the austenite grains under 5-7 passes of high pressure above 1000°C and with a cumulative deformation of ⁇ 50%.
  • the contents of ferrite, bainite and retained austenite in the steel are controlled through a segmented cooling process.
  • the cooling water cooling stop temperature and air cooling time in the first stage after rolling determine the ferrite content
  • the coiling temperature after the second stage cooling determines the bainite and retained austenite content.
  • the present invention can obtain low yield strength hot-rolled or pickled ultra-high plasticity and high-strength steel through the design of medium-low carbon, high-silicon and high-aluminum components, combined with innovative hot-rolling segmented cooling, medium-temperature coiling and pickling processes.
  • the relatively high carbon content is conducive to obtaining high strength, and at the same time, more available carbon atoms are diffused into the retained austenite, thereby obtaining a very stable retained austenite.
  • the main purpose of adding a higher silicon content is to suppress the formation of carbides and expand the ferrite formation temperature range; adding a higher aluminum content can promote the diffusion of carbon atoms from bainitic ferrite to retained austenite, continuing Improve the stability of retained austenite.
  • the present invention adds high Ti to steel, and combines the segmented cooling process to form nano-TiC inside the ferrite grains during the ferrite phase transformation process, thereby improving the strength of the ferrite and reducing the ferrite strength.
  • the performance difference between bainite and bainite increases the hole expansion rate of steel.
  • the higher manganese content in the steel of the invention further improves the stability of retained austenite.
  • the structural types of the highly expanded ultra-high plasticity steel involved in the present invention are ferrite, bainite and residual Remaining austenite.
  • Bainite gives the steel plate high tensile strength
  • ferrite and a higher content of metastable retained austenite give the steel plate ultra-high elongation through the TRIP effect, in which the retained austenite content is ⁇ 5%; containing nano-precipitated phases
  • Ferrite increases the yield strength through precipitation strengthening, reduces the hardness difference between ferrite and bainite, and greatly increases the hole expansion rate while obtaining ultra-high plasticity.
  • Coiling at 350-550°C after hot rolling is mainly to obtain bainite and super-stable retained austenite.
  • the microstructure types of the ultra-high plasticity steel involved in the present invention are mainly ferrite, bainite and stable retained austenite, and the retained austenite content is ⁇ 5%. Ferrite gives the steel plate a lower yield strength, bainite gives the steel plate high tensile strength, and stable retained austenite gives the steel plate ultra-high elongation.
  • the present invention can obtain low yield strength hot-rolled or pickled ultra-high plasticity and high-strength steel, with a yield strength of above 500MPa, preferably above 600MPa, and more preferably above 700MPa.
  • the tensile strength is above 780MPa, preferably above 980MPa; the elongation of hot-rolled or pickled steel coils is above 25%, preferably above 30%.
  • the present invention adopts a medium-low carbon, high-silicon and high-aluminum design in the composition design, which is different from the traditional hot-rolled ultra-high plasticity steel with low carbon, high silicon and low silicon and high aluminum composition design. completely different.
  • Chinese patent CN104233092A also adds precious metal elements such as Cr, Mo, Nb, etc., and the alloy cost is relatively high;
  • Chinese patent CN107815593A has a composition design of low silicon and high aluminum, as well as a certain amount of Cu, but its process path is mainly It is through heat treatment in the two-phase zone and then phase transformation in the bainite zone. The process path cannot be applied to the hot rolling production line.
  • the above-mentioned patent is not only different from the present invention in terms of composition design, but also has problems such as high alloy cost and the inability to transplant the process path to the hot rolling production line.
  • the invention adopts innovative design ideas of medium-low carbon and high aluminum components, and matches the innovative segmented cooling and medium-temperature coiling processes. It can obtain high tensile strength and ultra-high tensile strength on the existing hot continuous rolling production line. Hot-rolled pickled high-expansion ultra-high plasticity steel with high elongation and high hole expansion rate.
  • the high-strength and ultra-high plasticity steel produced using the technology provided by the present invention has a yield strength of ⁇ 500MPa, a tensile strength of ⁇ 780MPa, a low yield-strength ratio and ultra-high elongation (A can to reach more than 30%), showing excellent low yield strength, low yield ratio, high tensile strength, ultra-high plasticity, and high hole expansion rate, and can be used in the manufacturing of various complex parts for passenger cars or commercial vehicles. , has good application prospects.
  • Figure 1 is a schematic diagram of the steel rolling process of the present invention
  • Figure 2 is a schematic diagram of the cooling process of steel according to the present invention.
  • Figure 3 is a metallographic photograph of Example 1 of the present invention.
  • Figure 4 is a metallographic photograph of Example 6 of the present invention.
  • Figure 5 is a metallographic photograph of Example 10 of the present invention.
  • Figure 6 is a metallographic photograph of Example 14 of the present invention.
  • Figure 7 is a metallographic photograph of Example 17 of the present invention.
  • Figure 8 is a metallographic photograph of Example 19 of the present invention.
  • Figure 9 is a metallographic photograph of Example 21 of the present invention.
  • Figure 10 is a metallographic photograph of Example 23 of the present invention.
  • the components of the steels in the examples and comparative examples of the present invention are shown in Table 1.
  • the balance of the components in the table is Fe and other unavoidable impurities.
  • the molten steel is smelted in a converter or electric furnace, then refining in a vacuum furnace, and then cast into a slab or ingot.
  • the opening temperature of hot rolling is ⁇ 1000°C, rolling at high pressure for 5 to 7 passes above 1000°C and the cumulative deformation is ⁇ 50%, then the intermediate billet is heated to ⁇ 950°C, and then the final 3 to 7 passes are performed. Roll and the cumulative deformation is ⁇ 70%, and the final rolling temperature is between 800 and 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 350 and 550°C, and then cool to room temperature at a cooling rate of ⁇ 50°C/h to obtain hot-rolled strip steel.
  • Examples 1-8 and 17-24 did not undergo a pickling step to obtain hot-rolled steel.
  • the steel of Examples 9-16 underwent a pickling step to obtain pickled steel.
  • the steel of Comparative Examples 1-3 was selected from CN104233092A.
  • Table 2 shows the specific production process parameters of the steel in the embodiment of the present invention. The specific process parameters of pickling are not shown.
  • Table 3 shows the performance parameters of the hot-rolled steel in Examples 1-8 and 17-24 of the present invention, and Table 4 shows the performance parameters of the pickled steel in Examples 9-16 of the present invention.
  • composition of the comparative example is low carbon, high silicon and low aluminum, while the embodiment of the present invention is medium low carbon, high silicon and high aluminum.
  • the two are completely different in terms of composition design of carbon content and aluminum content.
  • the ferrite content in the structure of the comparative example is below 15%, while the ferrite content in the steel structure of the example is between 25% and 45%; the bainite content in the structure of the comparative example is 70%.
  • the bainite content in the structure of the embodiment is 44 to 53%, and there are obvious differences in the structure design between the two.
  • the yield strength of plastic steel coils or steel plates is above 500MPa, up to above 600MPa, or even above 700MPa, the tensile strength is above 780MPa, up to above 980MPa, and the elongation is above 25, up to above 30%.
  • the hole expansion rate is above 30%, and can even reach above 50%. It has good yield strength, tensile strength, ultra-high plasticity and high hole expansion rate. It is especially suitable for parts such as automobile chassis structures that require complex forming and cold drawing, and has broad application prospects.
  • Figures 3 to 6 show the metallographic photos of Examples 1, 6, 10 and 14 respectively. It can be seen from this that by using the composition and process path designed in the present invention, a structure mainly composed of carbide-free lath-shaped bainite and residual austenite between bainite laths can be obtained. Figures 7 to 10 show typical metallographic photos of Examples 17, 19, 21 and 23 respectively. It can be clearly seen from the figure that by using the composition and process path designed in the present invention, a microstructure dominated by intragranular nano-precipitated ferrite, bainite and retained austenite can be obtained, showing lower It has a good match of yield strength, high tensile strength, ultra-high plasticity and high hole expansion rate, and has excellent comprehensive performance.
  • the steel of the present invention has good strength, ultra-high plasticity and high hole expansion characteristics, is particularly suitable for automobile chassis structures and other parts that require complex forming, and has broad application prospects.

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Abstract

本发明提供了一种高塑性钢及其制造方法。其中，钢的成分以质量百分比计为：C 0.10～0.35％，Si 0.8～2.0％，Mn 1.0～3.0％，P≤0.02％，S≤0.005％，Al 0.1～2.0％，N≤0.005％，余量为Fe及其它不可避免的杂质。本发明的钢能实现低屈服强度、低屈强比、高抗拉强度和超高延伸率的良好匹配，可广泛应用于商用车或乘用车等具有复杂形状要求的零部件或其他需要高强减薄的部位。

Description

一种高塑性钢及其制造方法 技术领域
本发明属于钢及其制造方法领域，特别涉及一种高塑性钢及其制造方法。
背景技术
目前，汽车行业作为国民经济重要支柱产业之一，对先进高强钢的需求越来越迫切，对低排放甚至零碳产品的需求也迅速增长。高强减薄不仅是乘用车行业的发展趋势，在商用车领域，高强减薄和节能减排的进展也逐步加快，这不仅是行业发展的需要，更是汽车行业转型升级的必然要求，特别是在商用车领域，传统的大吨位车辆已不能适应日益严格的法规要求。因此，汽车行业的设计人员和生产厂家正在重新审视传统设计和生产理念，从底盘、车身、座椅等多个方面进行高强减重，甚至采用其他新材料如铝合金、碳纤维等。而商用车行业则从底盘、车厢、上装等加快轻量化进程，随着商用车政策法规执行力度不断加强，未来商用车领域轻量化潜力巨大。
许多乘用车和商用车零件车身、防撞梁、油箱托架、电池托架、气瓶托架、弯管等大多采用低强度级别厚规格普通钢如Q235或Q345进行生产，而且工艺过程较为复杂，有的需要螺栓连接，有的需要焊接。随着轻量化的发展，已有许多用户希望将商用车的这些零件采用冷冲压的方式进行一体成形，既减少了工序，又实现了轻量化。这就对热轧高强钢的性能提出了更高的要求，在确保高强度的同时使钢具有更高的延伸率和更好的成形性等。如油箱托架等零件在采用传统的高强钢进行冲压时，容易在零件的大圆弧处发生开裂，无法实现顺利冲压。这就需要开发出具有高抗拉强度和高成形性能的新型高强钢。同时考虑到用户的模具寿命，新型高强钢的屈服强度不能太高，否则在实际冲压时零件反弹严重，成形难度大。
基于上述多种原因，需要开发具有低屈服强度、高抗拉强度、超高延伸率的热轧或酸洗高强钢，适用于对冷拉延成形要求特别高的复杂零件冲 压成型，具有良好的可制造性和广阔的应用前景。
超高塑性钢已有较多专利涉及，大多集中在冷轧高强钢领域，在热轧领域也有部分超高塑性钢专利文献。如：
中国专利CN104233092A公开了一种780MPa级超高塑性钢，其成分设计为低碳高硅，同时添加了一定量Cr，Mo，Nb等贵重合金元素，故其合金成本较高。
中国专利CN107815593A公开了一种低硅高铝超高塑性钢，其成分为低硅高铝，同时还有一定量贵重元素Cu。其工艺路径主要是通过在两相区进行热处理保温1～3min，然后在贝氏体区进行相变，获得780MPa级热处理型超高塑性钢，但其热处理工艺路径无法拓展至现有的热轧产线。
发明内容
本发明的目的在于提供一种高塑性钢及其制造方法，该具有良好的力学性能，可以实现低屈服强度、低屈强比、高抗拉强度和超高延伸率之间的良好匹配。上述钢能广泛应用于商用车或乘用车等具有复杂形状要求的零部件或其他需要高强减薄的部位。
为达到上述目的，本发明的第一方面提供了一种钢，所述钢的成分以质量百分比计为：C 0.10～0.35％，Si 0.8～2.0％，Mn 1.0～3.0％，P≤0.02％，S≤0.005％，Al 0.1～2.0％，N≤0.005％，余量包含为及其它不可避免的杂质。
优选地，上述钢还含有Ti，以质量百分比计，Ti的含量在0.2％以下，优选为0.05-0.2％，更优选为0.05-0.1％。
优选地，上述钢还含有选自Mo、Nb、V、Cu、Ni、Cr、B中的一种以上，其中，以质量百分比计，Mo的含量在0.5％以下，优选在0.3％以下；Nb的含量在0.1％以下，优选在0.06％以下；V的含量在0.1％以下，优选在0.06％以下；Cu的含量在0.5％以下，优选在0.3％以下；Ni的含量在0.5％以下，优选在0.3％以下；Cr的含量在0.5％以下，优选在0.3％以下；B的含量在0.001％以下，优选在0.0005％以下。
优选地，在上述钢不可避免的杂质中，以质量百分比计，O≤0.003％、 优选≤0.002％；S≤0.003％；和/或N≤0.004％。
优选地，上述钢的成分以质量百分比计满足如下中的一个以上：C 0.15～0.25％，Si 1.0～1.6％，Mn 1.5～2.5％，Al 0.3～1.0％。
在本发明的钢中，各元素的设计思路如下：
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。同时，碳作为稳定残余奥氏体的有效元素，其含在钢中的含量通常较高。在本发明中，为获得抗拉强度达不同强度级别的高强钢，同时还需要钢组织中有较多的稳定的残余奥氏体，必须保证碳的含量在0.10％以上；但是碳的含量也不能高于0.35％。碳的含量太高，容易导致强度偏高，延伸率降低，焊接性变差。因此，碳的含量在0.10-0.35％之间。
硅，是钢中的基本元素，同时也是本发明中的重要元素之一。硅加入钢中可降低奥氏体的未再结晶温度，扩大奥氏体轧制工艺窗口，使得钢在精轧阶段即可完成动态再结晶，有利于改善钢的横纵向性能差异。硅加入钢中的另一个作用是抑制渗碳体的形成。在本发明中，为了使钢组织中具有较多残余奥氏体，需要添加较高的硅来抑制渗碳体的形成。硅的这种抑制碳化物形成的作用必须在其含量达到0.8％以上时才表现明显；但硅的含量不宜太高，否则实际轧制过程中轧制力负荷过大，钢板表面存在较多的红铁皮，不利于轧制稳定生产。因此，钢中硅的含量在0.8-2.0％之间，优选在1.0～1.6％。
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扩大奥氏体相区，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。同时，锰在热处理过程可发生配分，锰从贝氏体向残余奥氏体中扩散，进一步稳定残余奥氏体，获得更多含量的残余奥氏体。锰含量至少在1.0％以上才能起到上述效果，但锰含量也不宜太高，锰含量超过3.0％，连铸坯容易发生偏析，而且会形成较多的MnS夹杂。因此，钢中锰的含量在1.0-3.0％，优选在1.5-2.5％。
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，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的含量在0.005％以内，优选在0.003％以内。
铝，是本发明中的重要元素之一。铝在本发明中除脱氧和固氮基本作用外，还有其他两个重要作用。由于本发明中，奥氏体稳定化元素如碳和锰等含量较高，奥氏体稳定性强，在轧后分段冷却短时空冷阶段难以形成所需含量的铁素体，需要加入比普通高强钢更多的铝，起到加快铁素体相变的作用，以便获得足够数量的铁素体；另一方面，为了获得超级稳定的残余奥氏体，也需要加入较多的铝，铝加入钢中除了加快铁素体相变外，在贝氏体相变过程中，铝既可以起到抑制渗碳体形成的作用，还可以促进碳原子从贝氏体铁素体向残余奥氏体中扩散，从而加快碳原子在残余奥氏体中的扩散，提高残余奥氏体中的碳浓度，获得超级稳定的残余奥氏体。当铝含量≥0.1％时才能起到上述提到的各种有利作用，但当铝含量超过2.0％时，其促进碳扩散和富集的效果达到饱和，且钢水粘度增大，容易堵塞浇注水口。因此，本发明钢中铝的含量在0.1-2.0％之间，优选在0.3～1.0％。
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不利的影响。因此，本发明中氮的含量在0.005％以下，优选在0.004％以下。
钛，是本发明中可添加元素之一。由于超高塑性高强钢中含有较多残余奥氏体，残余奥氏体属于软相，其屈服强度较低。为了提高钢的屈服强度，在一定条件下，可添加微合金元素如钛等，通过钛在先共析铁素体中的析出强化效应提高屈服强度。随着钛含量的增加，析出强化效果逐渐增强。当钛的含量增加到0.20％时，钛的析出强化效果饱和。因此，可根据实际需要，调整钛的添加量，本发明的钢中钛的含量在0.20％以内，优选 为0.05-0.2％，更优选为0.05-0.1％。
钼，是本发明中可添加元素之一。钼加入钢中可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铁素体、贝氏体和残余奥氏体为主的组织，而铁素体和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轻焊接软化程度。考虑到钼属于贵重金属，添加较多的钼增加合金成本。因此，本发明中钼的含量在0.5％以内，优选在0.3％以内。
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0.003％以下，对钢板的性能不会造成明显不利影响。因此，本发明将钢中的O含量在0.003％以下，优选在0.002％以下。
铜，是本发明中的可添加元素之一。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，本发明中Cu的含量在0.5％以内，优选在0.3％以内。
镍，是本发明中的一种可添加元素之一。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，本发明中镍的添加量在0.5％以下，优选在0.3％以下。
铬，是本发明中的可添加元素之一。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。铬易固溶至铁素体中，起到强化铁素体的作用；同时，加入少量铬元素还可以起到提高耐蚀性的作用。因此，本发明中铬的加入量在0.5％以下，优选在0.3％以下。
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碳氮化物，对钢的低温冲击韧性不利。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。因此，本发明的钢中铌的含量在0.10％以下，优选在0.06％以下。
钒，是本发明中的可添加元素之一。钒与钛、铌类似，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，且容易在晶界上形成，对钢的韧性不利。因此，本发明的钢中钒的添加量在0.10％以下，优选在0.06％以下。
硼，是本发明中的可添加元素之一。硼是易偏聚元素，在奥氏体区轧制时，硼元素能够偏聚到奥氏体晶界，降低了奥氏体晶界处的界面能，不利于后续冷却相变时的铁素体形成。由于本发明期望的组织为铁素体、贝氏体和稳定的残余奥氏体，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抑制了铁素体的形成。故本发明的钢中硼的添加量在0.001％以下，优选在0.0005％以下。
如无特殊说明，本发明的钢中元素的含量均指质量分数。
优选地，本发明的钢的组织为铁素体、贝氏体和含量≥5％的残余奥氏体。具体地，钢中铁素体的体积分数在30-50％之间，优选在35-45％之间，贝氏体的体积份数在40-60％之间，优选在45-55％之间，残余奥氏体的体积分数在5-15％之间，优选在10-15％之间。
优选地，上述钢的屈服强度在500MPa以上，优选在600MPa以上，更优选在700MPa以上；抗拉强度在780MPa以上，优选在980MPa以上；延伸率在25％以上，优选在30％以上。
优选地，上述钢的扩孔率在30％以上，优选在50％以上。
优选地，在钢中，当Ti的含量为0.05-0.2％、C的含量为0.10～0.25％时，钢的屈服强度≥600MPa，抗拉强度≥780MPa，延伸率≥30％，扩孔率≥50％；当Ti的含量为0.05-0.2％、C的含量在0.25～0.35％时，所述钢 的屈服强度≥700MPa，抗拉强度≥980MPa，延伸率≥25％，扩孔率≥30％。
现有780MPa级超高塑性钢以C-Si-Mn为主要元素，必要时也添加Nb、Ti等微合金元素以细化晶粒，其中Al的含量在0.1％以下，主要利用Al元素的脱氧固氮作用。
而本发明则采用了高Al成分设计，Al含量在0.1％以上。添加高Al的主要目的一是为了促进铁素体相变，二是为了进一步提高残余奥氏体的稳定性。
从性能上看，现有780MPa级超高塑性钢屈服强度低或屈强比低，残余奥氏体不够稳定，在变形的情况下，组织中的残余奥氏体很容易转变为马氏体。
而本发明所涉及的超高塑性钢则可获得不同屈服强度不同屈强比的拉伸性能，同时组织中的残余奥氏体更为稳定，且残余奥氏体的含量在5％以上。尽管屈服强度级别不同，但抗拉强度和延伸率仍保持在很高的水平上，更有利于下游用户的加工使用。并且，本发明的钢还可以具有较高的扩孔率，特别有益于对拉延和翻边成形要求更高的零件冲压加工。
制造前述的钢的方法，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，然后在真空炉中进行二次精炼，再铸造成铸坯或铸锭；
2)铸坯或铸锭再加热
加热温度≥1100℃，保温时间：1～2小时；
3)铸坯或铸锭热轧、冷却
热轧的开轧温度≥1000℃，在1000℃以上5～7道次大压下轧制且累计变形量≥50％，随后中间坯待温至≥950℃，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350～550℃之间卷取，然后以≤50℃/h的冷速冷却至室温，获得热轧带钢。
进一步地，上述方法还包括步骤4)酸洗，其中，上述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≤2％，随后在35～50℃温度区间进行漂洗，并在120～140℃之间进行表面烘干，涂油，获得酸洗高强度超高塑性钢。
在制造本发明所述的钢的方法中：
热轧的开轧温度≥1000℃，在1000℃以上5-7道次大压下且累计变形量≥50％，主要目的是细化奥氏体晶粒。
在800-950℃温度区间终轧后，通过分段冷却工艺控制钢中铁素体、贝氏体和残余奥氏体的含量。其中，轧后第一阶段冷却水冷停止温度和空冷时间决定了铁素体的含量，第二阶段冷却后的卷取温度决定了贝氏体和残余奥氏体的含量。
通过分段冷却工艺与创新性的成分设计，可定量控制铁素体、贝氏体和残余奥氏体三相的含量，而这种创新性的成分设计和工艺的结合可获得残余奥氏体非常稳定的超高塑性钢。
本发明的创新点在于：
本发明通过中低碳高硅高铝成分设计，配合创新性的热轧分段冷却和中温卷取以及酸洗工艺可获得低屈服强度热轧或酸洗超高塑性高强钢。相对较高的碳含量有利于获得高强度，同时有较多可利用的碳原子扩散至残余奥氏体中，从而获得非常稳定的残余奥氏体。添加较高的硅含量主要目的是为了抑制碳化物形成，扩大铁素体形成温度范围；添加较高的铝，可促进碳原子从贝氏体铁素体向残余奥氏体中的扩散，继续提高残余奥氏体的稳定性。
优选地，本发明在钢中添加高Ti，通过结合分段冷却工艺，在铁素体相变过程中，在铁素体晶粒内部形成纳米TiC，提高铁素体的强度，减小铁素体和贝氏体之间的性能差异，提高钢的扩孔率。
此外，本发明的钢中较高的锰含量可进一步提高残余奥氏体的稳定性。
本发明所涉及的高扩孔超高塑性钢的组织类型为铁素体、贝氏体和残 余奥氏体。贝氏体赋予钢板高抗拉强度、铁素体和较高含量的亚稳态残余奥氏体通过TRIP效应赋予钢板超高延伸率，其中残余奥氏体含量≥5％；含有纳米析出相的铁素体通过析出强化提高了屈服强度，缩小了铁素体与贝氏体之间的硬度差，在获得超高塑性的同时，大大提高了扩孔率，通过上述成分和工艺的精确配合，实现了高抗拉强度、超高延伸率和高扩孔率的良好匹配。
热轧之后在350-550℃卷取主要是为了获得贝氏体和超级稳定的残余奥氏体。本发明所涉及的超高塑性钢组织类型主要为铁素体、贝氏体和稳定的残余奥氏体且残余奥氏体含量≥5％。铁素体赋予钢板较低的屈服强度、贝氏体赋予钢板高抗拉强度、稳定的残余奥氏体赋予钢板超高的延伸率。
正是基于这种创新性的成分和工艺设计思路，本发明可获得低屈服强度热轧或酸洗超高塑性高强钢，其屈服强度在500MPa以上，优选在600MPa以上，更优选在700MPa以上，抗拉强度在780MPa以上，优选在980MPa以上；热轧或酸洗钢卷的延伸率在25％以上，优选在30％以上。
与现有技术相比，本发明的优点在于：
与现有的超高塑性钢专利相比，本发明在成分设计上采用了中低碳高硅高铝的设计，与传统热轧超高塑性钢低碳高硅和低硅高铝等成分设计完全不同。
中国专利CN104233092A除低碳高硅外，还添加了Cr,Mo,Nb等贵重金属元素，合金成本较高；中国专利CN107815593A成分设计为低硅高铝，同时还有一定量Cu，但其工艺路径主要是通过在两相区进行热处理，然后在贝氏体区进行相变，其工艺路径无法应用至热轧产线上。
因此，上述专利不仅在成分设计上与本发明不同，而且存在合金成本高以及工艺路径无法移植到热轧产线等问题。
本发明采用创新性的中低碳和高铝成分设计思路，与创新性的分段冷却和中温卷取工艺匹配，在现有的热连轧产线上即可获得具有高抗拉强度、超高延伸率和高扩孔率的热轧酸洗高扩孔超高塑性钢。
利用本发明所提供的技术制造获得的高强超高塑性钢的屈服强度≥500MPa，抗拉强度≥780MPa，具有低屈强比同时具有超高延伸率(A可 以达到30％以上)，表现出优异的低屈服强度、低屈强比、高抗拉强度和超高塑性、高扩孔率匹配，可应用于乘用车或商用车各种复杂零件的制造，具有良好的应用前景。
附图说明
图1为本发明的钢的轧制工艺示意图；
图2为本发明的钢的冷却工艺示意图；
图3为本发明的实施例1的金相照片；
图4为本发明的实施例6的金相照片；
图5为本发明的实施例10的金相照片；
图6为本发明的实施例14的金相照片；
图7为本发明的实施例17的金相照片；
图8为本发明的实施例19的金相照片；
图9为本发明的实施例21的金相照片；
图10为本发明的实施例23的金相照片。
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。
本发明实施例和对比例的钢的成分参见表1，表中成分的余量为Fe和其他不可避免的杂质。
本发明实施例的工艺路径为：
1)冶炼、铸造。
按表1的成分采用转炉或电炉冶炼钢水，然后在真空炉中进行二次精炼，再铸造成铸坯或铸锭。
2)铸坯或铸锭再加热。
加热温度≥1100℃，保温时间：1～2小时。
3)铸坯或铸锭的热轧、冷却。
热轧的开轧温度≥1000℃，在1000℃以上5～7道次大压下轧制且累计变形量≥50％，随后中间坯待温至≥950℃，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350～550℃之间卷取，然后以≤50℃/h的冷速冷却至室温，获得热轧带钢。
实施例1-8、17-24未进行酸洗步骤，获得热轧钢，实施例9-16的钢进行了酸洗步骤，获得酸洗钢。
酸洗步骤的具体工艺如下：
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≤2％，随后在35～50℃温度区间进行漂洗，并在120～140℃之间进行表面烘干，涂油。
上述步骤3的具体工艺如图1、图2所示。
对比例1-3的钢选自CN104233092A。
表2为本发明实施例的钢的具体生产工艺参数，酸洗的具体工艺参数未示出。表3为本发明实施例1-8、17-24的热轧钢的性能参数，表4为本发明实施例9-16的酸洗钢的性能参数。
表3-4中钢的性能按如下测量：
钢的屈服强度、抗拉强度和延伸率按照GB/T 228.1-2021“金属材料拉伸试验第1部分：室温试验方法”进行测试；
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。
从表1可以看出，对比例成分设计为低碳高硅低铝，而本发明实施例为中低碳高硅高铝，两者在碳含量和铝含量的成分设计方面完全不同。
从表3的性能对比可以看出，对比例的延伸率在20％左右，而本发明实施例延伸率可达到30％左右，表明本发明所涉及的超高塑性钢具有更优异的强度和超高塑性匹配。
从表3和表4可以看出，对比例的组织中铁素体含量在15％以下，而实施例的钢组织中铁素体含量在25～45％；对比例组织中贝氏体含量在70％以上，而实施例的组织中贝氏体含量在44～53％，两者在组织设计方面存在明显不同。
从表3、表4还可以看出，本发明所涉及的热轧或酸洗高强度超高塑 性钢钢卷或钢板的屈服强度在500MPa以上，可达600MPa以上，甚至可达700MPa以上，抗拉强度在780MPa以上，可达980MPa以上，延伸率在25上，可达30％以上。扩孔率在30％以上，甚至可达50％以上。具有良好的屈服强度、抗拉强度、超高塑性以及高扩孔率的匹配，特别适合汽车底盘结构等需要复杂成形和冷拉延等零件，具有广阔的应用前景。
图3～图6分别给出了实施例1、6、10和14的金相照片。从中可以看出，采用本发明所设计的成分和工艺路径，可获得具有无碳化物板条状贝氏体和贝氏体板条之间残余奥氏体为主的组织。图7～图10分别给出了实施例17、19、21和23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和工艺路径，可获得具有晶内纳米析出的铁素体、贝氏体和残余奥氏体为主的组织，表现出较低的屈服强度、高抗拉强度、超高塑性和高扩孔率的良好匹配，具有优异的综合性能表现。
本发明的钢具有良好的强度、超高塑性和高扩孔特性匹配，特别适合汽车底盘结构等需要复杂成形的零件等，具有广阔的应用前景。



Claims (11)

  1. 一种钢，其成分以质量百分比计为：C 0.10～0.35％，Si 0.8～2.0％，Mn 1.0～3.0％，P≤0.02％，S≤0.005％，Al 0.1～2.0％，N≤0.005％，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，所述钢还含有Ti，以质量百分比计，Ti的含量在0.2％以下，优选为0.05-0.2％，更优选为0.05-0.1％。
  3. 如权利要求1或2所述的钢，其特征在于，所述钢还含有选自Mo、Nb、V、Cu、Ni、Cr、B中的一种以上，其中，以质量百分比计，
    Mo的含量在0.5％以下，优选在0.3％以下；
    Nb的含量在0.1％以下，优选在0.06％以下；
    V的含量在0.1％以下，优选在0.06％以下；
    Cu的含量在0.5％以下，优选在0.3％以下；
    Ni的含量在0.5％以下，优选在0.3％以下；
    Cr的含量在0.5％以下，优选在0.3％以下；
    B的含量在0.001％以下，优选在0.0005％以下。
  4. 如权利要求1或2所述的钢，其特征在于，在不可避免的杂质中，以质量百分比计，O≤0.003％、优选≤0.002％；S≤0.003％；和/或N≤0.004％。
  5. 如权利要求1或2所述的钢，其特征在于，所述钢的成分以质量百分比计满足如下中的一个以上：C 0.15～0.25％，Si 1.0～1.6％，Mn 1.5～2.5％，Al 0.3～1.0％。
  6. 如权利要求1～5中任一项所述的钢，其特征在于，所述钢的组织为铁素体、贝氏体和含量≥5％的残余奥氏体。
  7. 如权利要求1～6中任一项所述的钢，其特征在于，所述钢的屈服强度在500MPa以上，优选在600MPa以上，更优选在700MPa以上；抗拉强度在780MPa以上，优选在980MPa以上；延伸率在25％以上，优选在30％以上。
  8. 如权利要求2所述的钢，其特征在于，以质量百分比计，C的含量为0.10～0.25％，Ti的含量为0.05-0.2％，钢的屈服强度≥600MPa，抗拉强度≥780MPa，延伸率≥30％，扩孔率≥50％；或者，以质量百分比 计，C的含量为0.25～0.35％，Ti的含量为0.05-0.2％，钢的屈服强度≥700MPa，抗拉强度≥980MPa，延伸率≥25％，扩孔率≥30％。
  9. 如权利要求1～8中任一项所述的钢，其特征在于，所述钢的扩孔率在30％以上，优选在50％以上。
  10. 一种制造权利要求1～9中任一项所述的钢的方法，其特征在于，所述方法包括如下步骤：
    1)冶炼、铸造
    按权利要求1～5中任一项所述的成分采用转炉或电炉冶炼，然后在真空炉中进行二次精炼，再铸造成铸坯或铸锭；
    2)铸坯或铸锭再加热
    加热温度≥1100℃，保温时间：1～2小时；
    3)铸坯或铸锭热轧、冷却
    热轧的开轧温度≥1000℃，在1000℃以上5～7道次大压下轧制且累计变形量≥50％，随后中间坯待温至≥950℃，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的冷速将所述钢带水冷至600～750℃之间，空冷1～10秒后，再以≥10℃/s的冷速冷却至350～550℃之间卷取，然后以≤50℃/h的冷速冷却至室温，获得热轧带钢。
  11. 如权利要求10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30～120m/min，酸洗温度控制在75～85℃，拉矫率控制≤2％，随后在35～50℃温度区间进行漂洗，并在120～140℃之间进行表面烘干，涂油。
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