WO2021249446A1 - 一种塑性优异的超高强度钢及其制造方法 - Google Patents

一种塑性优异的超高强度钢及其制造方法 Download PDF

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WO2021249446A1
WO2021249446A1 PCT/CN2021/099258 CN2021099258W WO2021249446A1 WO 2021249446 A1 WO2021249446 A1 WO 2021249446A1 CN 2021099258 W CN2021099258 W CN 2021099258W WO 2021249446 A1 WO2021249446 A1 WO 2021249446A1
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ultra
strength steel
steel
excellent plasticity
strength
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PCT/CN2021/099258
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English (en)
French (fr)
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陈濛潇
钟勇
陈孟
王利
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宝山钢铁股份有限公司
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Priority to US18/009,049 priority Critical patent/US20230272511A1/en
Priority to KR1020227044159A priority patent/KR20230024905A/ko
Priority to EP21822958.1A priority patent/EP4166685A4/en
Priority to JP2022575846A priority patent/JP2023528673A/ja
Publication of WO2021249446A1 publication Critical patent/WO2021249446A1/zh

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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
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Definitions

  • the invention relates to a kind of steel and a manufacturing method thereof, in particular to a high-strength steel and a manufacturing method thereof.
  • Advanced high-strength steel reduces the thickness of the steel plate by increasing the strength of the steel plate while maintaining excellent formability. It is currently the most comprehensively competitive lightweight material for the car body.
  • TRIP steel is composed of ferrite, bainite and retained austenite. This phase structure limits the further improvement of its strength. Using martensite instead of bainite as the main strengthening phase can continue to increase Strength of TRIP steel.
  • the main factors that determine its ductility are the shape, volume fraction and stability of retained austenite in the steel. The stability of retained austenite is closely related to its size and carbon content.
  • the existing advanced high-strength steels are mostly based on carbon-manganese steel composition, adding more alloy elements such as Cr, Mo, Nb, Ti, B, etc., which not only increases the material cost, but also improves the steelmaking
  • the manufacturability of hot rolling and cold rolling brings difficulties.
  • the publication number is CN104245971A
  • the publication date is December 24, 2014
  • the Chinese patent document entitled "High-strength cold-rolled steel sheet and method for producing the same” discloses a high-strength steel sheet.
  • its composition is: C: 0.1% to 0.3%, Si: 0.4% to 1.0%, Mn: 2.0% to 3.0%, Cr ⁇ 0.6%, Si+0.8Al+Cr : 1.0% ⁇ 1.8%, Al: 0.2% ⁇ 0.8%, Nb ⁇ 0.1%, Mo ⁇ 0.3%, Ti ⁇ 0.2%, V ⁇ 0.1%, Cu ⁇ 0.5%, Ni ⁇ 0.5%, S ⁇ 0.01%, P ⁇ 0.02%, N ⁇ 0.02%, B ⁇ 0.005%, Ca ⁇ 0.005%, Mg ⁇ 0.005%, REM ⁇ 0.005%, and the balance is Fe and unavoidable impurities.
  • the microstructure (vol%) is: retained austenite 5%-20%, bainite + bainite ferrite + tempered martensite ⁇ 80%, polygonal ferrite ⁇ 10%, martensite -Austenite composition ⁇ 20%. It should be pointed out that because the steel components involved in this technical solution need to add a certain amount of Cr and Mo, although its tensile strength is ⁇ 980MPa, the elongation is only about 14%, which is very important for the cost and formability of steel for auto parts. The advantage is not obvious. .
  • the manufacturing method is as follows: heating a steel slab meeting the compositional conditions to 1100-1300°C, finishing-rolling exit side temperature 800-1000°C, average coiling temperature 450-700°C, after pickling Keep the steel plate at 450°C ⁇ Ac1 temperature for 900 ⁇ 36000s, perform cold rolling at a reduction rate of 30% or more, heat the steel plate to 820 ⁇ 950°C for the first annealing, and then the average cooling rate to 500°C is 15°C/ Under the above conditions, cool to below Ms temperature, and then heat to 740 ⁇ 840°C for the second annealing, cool to 150 ⁇ 350°C at a cooling rate of 1 ⁇ 15°C/s, and then heat to 350 ⁇ 550°C for more than 10s .
  • warm rolling is required, and two annealing treatments are adopted, the production process is cumbersome, and the manufacturing cost increases. Therefore, its application in the automotive field is greatly restricted.
  • One of the objectives of the present invention is to provide an ultra-high-strength steel with excellent plasticity.
  • the ultra-high-strength steel with excellent plasticity adopts a simple composition design and makes full use of the influence law of C, Si and Mn on the phase transformation of the material.
  • its yield strength is 850-1000MPa
  • tensile strength is 1180-1300MPa
  • uniform elongation is ⁇ 11%
  • elongation at break is 15%-20%. It has good application prospects and value.
  • the present invention proposes an ultra-high-strength steel with excellent plasticity, the mass percentage of chemical elements is:
  • the balance is Fe and other unavoidable impurities.
  • C In the ultra-high-strength steel with excellent plasticity according to the present invention, C is the most important solid solution strengthening element, which can effectively ensure the strength of the steel.
  • too high C content in steel will reduce the weldability of steel. If the mass percentage of C in steel exceeds 0.30%, more twins are prone to appear after quenching, which increases crack sensitivity. Based on this, in the ultra-high-strength steel with excellent plasticity according to the present invention, the mass percentage of C is controlled between 0.26 and 0.30 wt%.
  • the mass percentage of element C can be controlled between 0.26 and 0.28 wt%.
  • the Si element can strongly inhibit the formation of cementite during the partitioning process, thereby promoting the enrichment of carbon into the retained austenite and increasing the content of the retained austenite. stability.
  • the content of Si element required to effectively suppress cementite is at least 0.8%. It should be noted that if the content of Si element in the steel is too high, the high temperature plasticity of the steel will be reduced and the incidence of hot rolling defects will increase. At the same time, when the Si content is too high, stable oxides will be formed on the surface of the steel sheet, which will adversely affect the subsequent pickling process. Based on this, the mass percentage of Si in the ultra-high-strength steel with excellent plasticity according to the present invention is controlled to be between 0.8 and 1.00 wt%.
  • the mass percentage of Si element can be controlled between 0.9 and 1.00 wt%.
  • Mn In the ultra-high-strength steel with excellent plasticity described in the present invention, Mn can expand the austenite phase region, reduce Ms and Mf points, effectively improve the stability of austenite and the hardenability of steel, and reduce critical transformation The rate is conducive to the preservation of retained austenite to room temperature. At the same time, Mn is also a more important solid solution strengthening element, but it should be noted that excessive Mn content in steel will deteriorate corrosion resistance and welding performance, and at the same time, it will aggravate the tendency of grain coarsening and reduce the plasticity and toughness of steel. Based on this, the mass percentage of Mn in the ultra-high-strength steel with excellent plasticity according to the present invention is controlled between 2.80 and 3.30 wt%.
  • the mass percentage of Mn element can be controlled between 2.9-3.1 wt%.
  • Al In the ultra-high-strength steel with excellent plasticity of the present invention, when Al element exists in a solid solution state, it can not only increase the stacking fault energy, but also inhibit the precipitation of cementite and the transformation of ⁇ to martensite. Improve the stability of austenite.
  • the Al element can also form finely dispersed insoluble points with C and N, which can refine the grains, but the strengthening effect of Al is weaker than that of Si, and the ability to stabilize austenite is also weaker than that of Si. If the mass percentage of the Al element in the steel is too high, a large number of oxide inclusions are easily formed, which is not conducive to the cleanliness of the molten steel. Therefore, the mass percentage of Al in the ultra-high-strength steel with excellent plasticity according to the present invention is controlled to be between 0.04 and 0.08 wt%.
  • the mass percentage of each chemical element satisfies at least one of the following items:
  • the ultra-high-strength steel with excellent plasticity according to the present invention also contains at least one of the following elements:
  • the aforementioned Cr, Mo, Nb, Ti, V and B elements can further improve the performance of the high-strength steel according to the present invention.
  • Cr and Mo elements can improve the hardenability of steel and adjust the strength of steel, but Cr will be enriched on the surface of the steel plate, which will affect the welding performance, and the higher mass percentage of Mo will increase the resistance to cold rolling deformation of the steel.
  • Nb, Ti, V elements can form fine carbides with C to promote structure refinement, but the formation of such fine carbides is not conducive to the enrichment of C into retained austenite and the stabilization of retained austenite .
  • the main function of the B element is to improve the hardenability of the steel.
  • B tends to segregate at the austenite grain boundary and delay the transformation of austenite to ferrite. A lower content will have a significant effect. If the mass percentage of B is too high, it will Causes the strength of steel to increase, which is not conducive to good shaping. Therefore, the mass percentage of B can be controlled as 0 ⁇ B ⁇ 0.001%.
  • the addition of the above-mentioned elements will increase the cost of the material. Taking into account the performance and cost control, in the technical solution of the present invention, at least one of the above-mentioned elements can be preferably added.
  • each chemical element satisfies at least one of the following items:
  • the elements P, S and N are all impurity elements.
  • P can play a solid solution strengthening effect, inhibit the formation of carbides, and help improve the stability of retained austenite
  • the mass percentage of P is too high It will weaken the grain boundary, increase the brittleness of the material, and deteriorate the welding performance, that is, the positive effect of the P element is weaker than its negative effect. Therefore, the mass percentage of P is preferably controlled to P ⁇ 0.01wt%.
  • N too high mass percentage of N will bring difficulties to steelmaking and continuous casting, and is not conducive to inclusion control. Therefore, it is preferable to control the mass percentage of N to N ⁇ 0.006wt%.
  • too high a mass percentage of S in the steel will significantly deteriorate the plasticity of the material, so the mass percentage of S is controlled to be S ⁇ 0.01wt%.
  • the mass percentage of chemical elements of the ultra-high-strength steel with excellent plasticity according to the present invention is:
  • Si 0.8-1.00wt%, preferably 0.9-1.00wt%;
  • Mn 2.80 ⁇ 3.30wt%, preferably 2.9 ⁇ 3.1wt%
  • Nb ⁇ 0.03wt%, preferably 0.01wt%
  • V ⁇ 0.03wt%, preferably 0.01wt%
  • the balance is Fe and other unavoidable impurities.
  • the microstructure (volume ratio) is 20%-40% ferrite + 50%-70% martensite + retained austenite .
  • the ultra-high-strength steel with excellent plasticity in the ferrite, grains of 10 ⁇ m or less account for 90% or more, and grains of 5 ⁇ m or less account for 60% or more of the ferrite.
  • the average grain size of the retained austenite is less than or equal to 2 ⁇ m; and/or the average C content in the retained austenite is more than or equal to 1.1 wt%. In some embodiments, in the ultra-high-strength steel with excellent plasticity according to the present invention, the average grain size of the retained austenite is between 0.6-1.6 ⁇ m. In some embodiments, in the ultra-high-strength steel with excellent plasticity according to the present invention, the average C content in the retained austenite is between 1.1-1.35 wt%.
  • the yield strength is 850-1000MPa
  • the tensile strength is 1180-1300MPa
  • the uniform elongation is ⁇ 11%
  • the elongation at break is 15%-20%.
  • another object of the present invention is to provide the above-mentioned manufacturing method of ultra-high-strength steel with excellent plasticity.
  • the manufacturing method adopts a thin slab continuous casting process combined with a pickling or pickling process to obtain a plastic with excellent plasticity after continuous annealing.
  • Ultra-high-strength steel is simple to produce, and the obtained high-strength steel can further improve the elongation of the steel under the same strength condition.
  • the present invention proposes the above-mentioned manufacturing method of ultra-high-strength steel with excellent plasticity, which includes the following steps:
  • Continuous annealing annealing at 800 ⁇ 920°C, slowly cooling to 690 ⁇ 760°C at a cooling rate of 3 ⁇ 10°C/s to obtain a certain proportion of ferrite; then quickly cooling to 250 ⁇ 350°C, cooling The speed is 50-100°C/s to partially transform austenite into martensite; then it is heated to 360-460°C for 100-400s, and finally cooled to room temperature.
  • step (1) adopts thin slab continuous casting, the rough rolling process can be omitted, and the hot rolling deformation can be reduced, thereby ensuring that the subsequent steps (4) and step ( 5) The performance of the steel plate.
  • step (1) adopts thin slab continuous casting, it can reduce the energy consumption required for heating by making full use of the heat of the slab, thereby obtaining a more uniform ferrite or ferrite + pearlite structure, which is beneficial to
  • step (5) a certain amount of fine-grained ferrite is maintained in the microstructure of the finished product, which improves the uniformity of the structure and is beneficial to increase the plasticity.
  • step (3) controlling the thickness of the surface oxide scale of the steel strip after hot rolling to be ⁇ 6 ⁇ m, and (FeO+Fe 3 O 4 ) ⁇ 40wt% in the surface oxide scale of the steel strip after the hot rolling can be beneficial to the subsequent steps
  • the progress of (4) has an important influence on the performance of the steel sheet obtained after continuous annealing.
  • a uniform austenite structure or austenite + ferrite structure can be formed by controlling the annealing temperature of 800 to 920°C; then, it is slowly cooled to a temperature of 3 to 10°C/s. 690 ⁇ 760°C, in order to further adjust the content of ferrite in the structure and improve the shaping of the material; afterwards, it is cooled to 250 ⁇ 350°C (between the temperature of Ms and Mf) at a rate of 50 ⁇ 100°C/s.
  • the critical cooling rate needs to be no less than 50°C/s, and if the cooling rate exceeds 100°C/s, the production cost will increase significantly.
  • the austenite partly transforms into martensite to ensure that the steel has high strength; then heat to 360 ⁇ 460°C and keep for 100 ⁇ 400s, so that carbon will be distributed in martensite and austenite to form a certain
  • the amount of carbon-rich retained austenite is kept stable to room temperature. Due to the TRIP effect, the work hardening ability and formability of the steel can be significantly improved, and an ultra-high-strength steel plate with excellent plasticity can be obtained.
  • the above distribution process is set like this because: when the reheating temperature is lower than 360°C or the reheating time is less than 100s, the stabilization process of retained austenite in the steel is insufficient, and the final retained austenite content obtained at room temperature will be insufficient; When the reheating temperature is higher than 460°C or the reheating time is longer than 400s, the steel will undergo obvious tempering softening, which will result in a significant decrease in the final material strength.
  • the ultra-high-strength steel in this case adopts high-carbon, high-manganese composition design and ferrite grain refining mechanism, during the continuous annealing process, the nucleation point of the reverse phase transformation of austenite increases, and the size is further increased.
  • the average grain size of the retained austenite that is refined and stably maintained to room temperature can be ⁇ 2 ⁇ m; the average C content in the retained austenite is ⁇ 1.1%.
  • the martensite formed by rapid cooling basically does not decompose during the partitioning process, so as to ensure the content of martensite in the structure and ensure the strength of the steel.
  • step (1) the continuous casting drawing speed is controlled to be 2-5 m/min.
  • step (2) the slab is heated to 1200-1300°C.
  • step (3) the finishing rolling temperature is controlled to be 860 to 930°C, and the coiling temperature is to be 450 to 600°C.
  • step (4) when pickling + cold rolling is adopted, the deformation is controlled to be 40% to 70%.
  • step (5) the continuous annealing process is controlled to satisfy at least one of the following items:
  • Annealing temperature is 820 ⁇ 870°C
  • the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace is controlled to 10-15%.
  • the ultra-high-strength steel with excellent plasticity and its manufacturing method according to the present invention have the following advantages and beneficial effects:
  • the high-strength steel of the present invention is based on carbon-silicon-manganese steel without adding any expensive alloy elements, and by optimizing the ratio of carbon-silicon-manganese, a high-strength cold-rolled steel sheet with excellent ductility is obtained.
  • the manufacturing method of the present invention has a unique production process, and due to the adoption of thin slab continuous casting technology, it has inherent advantages in terms of organization uniformity and segregation control.
  • the obtained ultra-high-strength steel can significantly increase its elongation under the same strength conditions, and will have a good application prospect in automotive safety structural parts. It is especially suitable for manufacturing vehicle structural parts with complex shapes and high formability requirements.
  • Safety parts such as A/B pillars, door anti-collision bars, longitudinal beams, bumpers, etc.
  • Figure 1 is a photo of the microstructure of the ultra-high-strength steel of Example 4.
  • Fig. 2 is an EBSD photograph of the phase composition of the ultra-high-strength steel of Example 4.
  • the ultra-high-strength steel with excellent plasticity of Examples 1-24 was prepared by the following steps:
  • Hot rolling The thickness of the oxide scale on the surface of the steel strip after hot rolling is ⁇ 6 ⁇ m, and the (FeO+Fe 3 O 4 ) in the oxide scale on the surface of the strip after hot rolling is ⁇ 40wt%, and the final rolling temperature is controlled at 860 ⁇ 930°C , The coiling temperature is 450 ⁇ 600°C.
  • Continuous annealing annealing at 800 ⁇ 920°C, slowly cooling to 690 ⁇ 760°C at a cooling rate of 3 ⁇ 10°C/s to obtain a certain proportion of ferrite; then quickly cooling to 250 ⁇ 350°C, cooling The speed is 50 ⁇ 100°C/s, so that the austenite is partially transformed into martensite; then it is heated to 360 ⁇ 460°C, kept for 100 ⁇ 400s, and finally cooled to room temperature.
  • control parameters can be further controlled to satisfy at least one of the following items:
  • Annealing temperature is 820 ⁇ 870°C
  • the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace is controlled to 10-15%.
  • the comparative examples 1-3 were manufactured using conventional processes.
  • Table 1 lists the mass percentage ratios of the chemical elements of the ultra-high-strength steels with excellent plasticity of Examples 1-24 and the comparative steels of Comparative Examples 1-3.
  • Table 2-1 and Table 2-2 list the specific process parameters of the ultra-high-strength steel with excellent plasticity in Examples 1-24 and the comparative steel in Comparative Examples 1-3.
  • Table 3 lists the mechanical performance test results of the ultra-high-strength steels with excellent plasticity in Examples 1-24 and the comparative steels in Comparative Examples 1-3.
  • the ultra-high-strength steel with excellent plasticity in Examples 1-24 of this case has excellent ductility while ensuring its strength.
  • Its yield strength YS is 850 ⁇ 1000MPa
  • its tensile strength TS is 1180 ⁇ 1300MPa.
  • the uniform elongation UEL is ⁇ 11%
  • the breaking elongation TEL is 15%-20%.
  • the microstructure of the ultra-high-strength steel with excellent plasticity in Examples 1-24 of this case is 20%-40% ferrite + 50%-70% martensite + retained austenite
  • grains below 10 ⁇ m account for more than 90%
  • grains below 5 ⁇ m account for more than 60%
  • the average grain size of retained austenite is ⁇ 2 ⁇ m
  • retained austenite The average C content in the body is ⁇ 1.1wt%.
  • Figure 1 is a photo of the microstructure of the ultra-high-strength steel of Example 4.
  • Fig. 2 is an EBSD photograph of the phase composition of the ultra-high-strength steel of Example 4.
  • the microstructure of the ultra-high-strength steel of Example 4 is 20% to 40% ferrite + 50% to 70% martensite + retained austenite.
  • crystal grains below 10 ⁇ m account for more than 90%, and crystal grains below 5 ⁇ m account for more than 60%.
  • the average grain size of retained austenite is ⁇ 2 ⁇ m; and/or the average C content in retained austenite is ⁇ 1.1wt%.
  • the ultra-high-strength steel of the present invention is based on carbon-silicon-manganese steel without adding any expensive alloying elements.
  • the manufacturing method of the present invention has simple production process, and the obtained ultra-high-strength steel can significantly increase its elongation under the same strength condition, and will have a good application prospect in automobile safety structural parts, and is particularly suitable for manufacturing complex shapes.
  • Vehicle structural parts and safety parts that require high formability such as A/B pillars, door anti-collision bars, longitudinal beams, bumpers, etc.

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Abstract

本发明公开了一种塑性优异的超高强度钢，其化学元素质量百分比为：C：0.26～0.30wt％；Si：0.8～1.00wt％；Mn：2.80～3.30wt％；Al：0.04～0.08wt％；余量为Fe和其他不可避免的杂质。此外，本发明还公开了上述的塑性优异的超高强度钢的制造方法，其包括步骤：(1)冶炼和薄板坯连铸：其中连铸出口端的板坯厚度控制为55～60mm；(2)加热；(3)热轧：热轧后钢带表面氧化皮厚度≤6μm，并且热轧后带钢表面氧化皮中的(FeO+Fe3O4)≤40wt％；(4)酸洗或者酸洗+冷轧；(5)连续退火：在800～920℃退火，以3～10℃/s的冷速缓冷至690～760℃，以获得一定比例的铁素体；再快冷至250～350℃，冷却速度为50～100℃/s，使奥氏体部分转变为马氏体；然后再加热至360～460℃，保温100～400s，最后冷却至室温。

Description

一种塑性优异的超高强度钢及其制造方法 技术领域
本发明涉及一种钢种及其制造方法，尤其涉及一种高强度钢及其制造方法。
背景技术
近年来，为实现车身减重，达到节能减排、提高碰撞安全性和降低制造成本的目的，汽车用先进高强钢已在汽车制造行业中得到广泛使用。先进高强钢通过提高钢板强度以减薄钢板厚度，同时保持优秀的成形性能，其是目前最具综合竞争力的车身轻量化用材。
基于相变诱导塑性(TRIP)效应的先进高强钢在保持高强度的同时，还具有较好的延展性。从微观组织看，TRIP钢由铁素体，贝氏体和残余奥氏体组成，这种相结构限制了其强度的进一步提高，以马氏体代替贝氏体作为主要的强化相可以继续提高TRIP钢的强度。对基于TRIP效应的先进高强钢而言，决定其延展性的主要因素是钢中残余奥氏体的形态、体积分数及稳定性，残余奥氏体的稳定性又与其尺寸、碳含量密切相关。
为保证钢板的强度和延展性能，现有先进高强钢多以碳锰钢成分为基础，添加较多的Cr、Mo、Nb、Ti、B等合金元素，不仅提高了材料成本，而且对炼钢、热轧、冷轧的可制造性带来难度。
例如：公开号为CN104245971A，公开日为2014年12月24日，名称为“高强度冷轧钢板和生产该钢板的方法”的中国专利文献公开了一种高强度钢板。在该专利文献所公开的技术方案中，其成分为：C:0.1％～0.3％、Si:0.4％～1.0％、Mn:2.0％～3.0％、Cr≤0.6％、Si+0.8Al+Cr:1.0％～1.8％、Al:0.2％～0.8％、Nb＜0.1％、Mo＜0.3％、Ti＜0.2％、V＜0.1％、Cu＜0.5％、Ni＜0.5％、S≤0.01％、P≤0.02％、N≤0.02％、B＜0.005％、Ca＜0.005％、Mg＜0.005％、REM＜0.005％，余量为Fe和不可避免的杂质。显微组织(体积％)为：残余奥氏体5％～20％、贝氏体+贝氏体铁素体+回火马氏体≥80％、多边形铁素体≤10％、马氏体-奥氏体组分≤20％。需要指出的是，由于该技术方案所涉及的钢种成分需要加入一定量的Cr、Mo，虽然其抗拉强度≥980MPa但 延伸率仅14％左右，对汽车零部件用钢的成本和成形性优势不明显。。
又例如：公开号为CN106574342A，公开日为2017年4月19日，名称为“高强度钢板及其制造方法、以及高强度镀锌钢板的制造方法”的中国专利文献公开了一种高强度钢板。在该专利文献所公开的技术方案中，其制造方法为：满足成分条件的钢坯加热至1100～1300℃，精轧出口侧温度800～1000℃，平均卷取温度450～700℃，酸洗后将钢板在450℃～Ac1温度保持900～36000s，以30％以上压下率进行冷轧，钢板加热至820～950℃进行第一次退火，然后在至500℃为止的平均冷却速度15℃/s以上条件下冷却至Ms温度以下，再加热至740～840℃进行第二次退火，以1～15℃/s的冷却速度冷至150～350℃，再加热至350～550℃保温10s以上。在该专利文献所公开的技术方案中需要加温轧制，而且采用了两次退火处理，生产工序繁琐，制造成本增加，因此，使得其在汽车领域的应用受到较大限制。
再例如：公开号为WO2018/116155，公开日为2018年6月28日，名称为“HIGH-STRENGTH COLD ROLLED STEEL SHEET HAVING HIGH FORMABILITY AND A METHOD OF MANUFACTURING THEREOF”的国际专利文献公开了一种具有高成形性的高强度冷轧钢板。在该专利文献所公开的技术方案中，其成分为：C:0.19％～0.24％、Mn:1.9％～2.2％、Si:1.4％～1.6％、Al:0.01％～0.06％、Cr:0.2％～0.5％、P≤0.02％、S≤0.003％，任选的一种或几种：Nb:0.0010％～0.06％、Ti:0.001％～0.08％、V:0.001％～0.1％、Ca:0.001％～0.005％，余量为Fe和不可避免杂质。需要指出的是，该技术方案所涉及的钢种的抗拉强度≥1150MPa，延伸率≥13％、扩孔率≥30％，虽然其抗拉强度较高，但是加入了较多的Cr及Nb、Ti元素，因而，不适合作为成本控制要求非常严格的汽车用钢。
发明内容
本发明的目的之一在于提供一种塑性优异的超高强度钢，该塑性优异的超高强度钢采用了简单的成分设计，其充分利用C、Si以及Mn元素对材料相变的影响规律，以保证钢板的强度以及延展性能，其屈服强度为850～1000MPa，抗拉强度为1180～1300MPa，均匀延伸率≥11％，断裂延伸率为15％～20％，具有良好的使用前景和价值。
为了实现上述目的，本发明提出了一种塑性优异的超高强度钢，其化学元素质量百分比为：
C：0.26～0.30wt％；
Si：0.8～1.00wt％；
Mn：2.80～3.30wt％；
Al：0.04～0.08wt％；
余量为Fe和其他不可避免的杂质。
在本发明所述的技术方案中，采用了普通碳硅锰钢成分设计，充分利用C、Si、Mn元素对材料相变的影响规律，从而实现了本发明所述的高强度钢在高强度与高延展性的统一，并最终得到性能优异的钢板产品。各化学元素的设计原理具体如下所述：
C：在本发明所述的塑性优异的超高强度钢中，C是最重要的固溶强化元素，对于可以有效保证钢的强度。钢中C元素的质量百分比越高，残余奥氏体的分数越多，配分时C在残余奥氏体中的富集程度越高，有利于增强残余奥氏体稳定性，产生TRIP效应，提高材料延展性。但需要注意的是，钢中C含量过高会降低钢的焊接性，若钢中的C的质量百分比超过0.30％，淬火后容易出现较多孪晶，增加裂纹敏感性。基于此，在本发明所述的塑性优异的超高强度钢中，将C的质量百分比控制在0.26～0.30wt％之间。
在一些优选的实施方式中，C元素的质量百分比可以控制在0.26～0.28wt％之间。
Si：在本发明所述的塑性优异的超高强度钢中，Si元素在配分处理过程中可以强烈抑制渗碳体形成，从而促进碳向残余奥氏体中富集，提高残余奥氏体的稳定性。有效抑制渗碳体所需Si元素的含量至少为0.8％，需要注意的是，若钢中Si元素含量过高，则会降低钢的高温塑性，增大热轧缺陷发生率。同时，Si含量过高时会在钢板表面形成稳定氧化物，对后续的酸洗工序产生不利影响。基于此，在本发明所述的塑性优异的超高强度钢中将Si的质量百分比控制在0.8～1.00wt％之间。
在一些优选的实施方式中，Si元素的质量百分比可以控制在0.9～1.00wt％之间。
Mn：在本发明所述的塑性优异的超高强度钢中，Mn元素能够扩大奥氏体相区，降低Ms和Mf点，有效提高奥氏体稳定性和钢的淬透性，降低临界转变速率，有利于残余奥氏体保存至室温。同时Mn也是较为重要的固溶强化元素，但需要注意的是，钢中Mn元素含量过高，会恶化耐腐蚀性能和焊接性能，同时也加剧晶粒粗化趋势，降低钢的塑性和韧性。基于此，在本发明所述的塑性优异的超高强度钢中将Mn的质量百分比控制在2.80～3.30wt％之间。
在一些优选的实施方式中，Mn元素的质量百分比可以控制在2.9～3.1wt％之间。
Al：在本发明所述的塑性优异的超高强度钢中，Al元素以固溶态存在时，其不仅能够增加层错能，同时也可以抑制渗碳体析出和γ到马氏体转变，提高奥氏体稳定性。此外，Al元素还可以与C、N形成细小弥散分布的难溶质点，可以细化晶粒，但是Al的强化效果弱于Si，稳定奥氏体的能力也较Si弱。若钢中Al元素的质量百分比过高，则容易形成大量氧化物夹杂，不利于钢水的洁净度。因此，在本发明所述的塑性优异的超高强度钢中控制Al的质量百分比在0.04～0.08wt％之间。
进一步地，在本发明所述的塑性优异的超高强度钢中，其各化学元素的质量百分比满足下列各项的至少其中之一：
C：0.26～0.28wt％；
Si：0.9～1.00wt％；
Mn：2.9～3.1wt％。
进一步地，在本发明所述的塑性优异的超高强度钢中，还含有下列各元素的至少其中之一：
0＜Cr≤0.05wt％；
0＜Mo≤0.05wt％；
0＜Nb≤0.03wt％；
0＜Ti≤0.05wt％；
0＜V≤0.03wt％；
0＜B≤0.001wt％。
上述的Cr、Mo、Nb、Ti、V以及B元素均可以进一步提高本发明所述的高强度钢的性能。例如：Cr、Mo元素可以提高钢的淬透性，调节钢的强度，但是Cr会在钢板表面发生富集，影响焊接性能，而Mo的质量百分比较高会导致钢的冷轧变形抗力增大。又例如：Nb、Ti、V元素可以与C形成细小的碳化物，促进组织细化，但是此类细小碳化物的形成不利于C向残余奥氏体中富集与残余奥氏体的稳定化。而B元素的主要作用是提高钢的淬透性，B容易在奥氏体晶界偏聚，延缓奥氏体向铁素体转变，含量较低就有明显效果，B的质量百分比过高会引起钢强度升高，不利于得到良好塑形。因此，可以将B的质量百分比控制为0＜B≤0.001％。
此外，上述元素的加入会增加材料的成本，综合考虑到性能与成本控制，在本发明所述的技术方案中，可以优选地添加上述元素的至少其中之一。
更进一步地，在本发明所述的塑性优异的超高强度钢中，其中各化学元素满足下列各项的至少其中之一：
0＜Cr≤0.03wt％；
0＜Mo≤0.03wt％；
0＜Nb≤0.01wt％；
0＜Ti≤0.03wt％；
0＜V≤0.01wt％。
进一步地，在本发明所述的塑性优异的超高强度钢中，在其他不可避免的杂质中：P≤0.01wt％，S≤0.01wt％，N≤0.006wt％。
上述方案中，P、S和N元素均为杂质元素，其中，P虽然能起到固溶强化作用，抑制碳化物形成，有利于提高残余奥氏体的稳定性，但是P的质量百分比过高会弱化晶界，增大材料脆性，恶化焊接性能，也就是说P元素的正面作用弱于其负面作用，因此，优选地将P的质量百分比控制为P≤0.01wt％。而至于N，由于N的质量百分比过高会给炼钢、连铸带来困难，不利于夹杂物控制，因此，优选地将N的质量百分比控制为N≤0.006wt％。相应地，钢中S元素的质量百分比过高将会明显恶化材料的塑性，因此将S的质量百分比控制为S≤0.01wt％。
进一步地，本发明所述的塑性优异的超高强度钢的化学元素质量百分比为：
C：0.26～0.30wt％，优选0.26～0.28wt％；
Si：0.8～1.00wt％，优选0.9～1.00wt％；
Mn：2.80～3.30wt％，优选2.9～3.1wt％；
Al：0.04～0.08wt％；
Cr：≤0.05wt％，优选0.03wt％；
Mo：≤0.05wt％，优选0.03wt％；
Nb：≤0.03wt％，优选0.01wt％；
Ti：≤0.05wt％，优选0.03wt％；
V：≤0.03wt％，优选0.01wt％；
B：≤0.001wt％；
P：≤0.01wt％；
S：≤0.01wt％；
N≤0.006wt％；
余量为Fe和其他不可避免的杂质。
进一步地，在本发明所述的塑性优异的超高强度钢中，其微观组织(体积比)为20％～40％的铁素体+50％～70％的马氏体+残余奥氏体。
进一步地，在本发明所述的塑性优异的超高强度钢中，其中在铁素体中，10μm以下的晶粒占比90％以上，5μm以下的晶粒占比60％以上。
进一步地，在本发明所述的塑性优异的超高强度钢中，其中残余奥氏体的平均晶粒尺寸≤2μm；并且/或者残余奥氏体中平均C含量≥1.1wt％。在一些实施方案中，在本发明所述的塑性优异的超高强度钢中，其中残余奥氏体的平均晶粒尺寸在0.6-1.6μm之间。在一些实施方案中，在本发明所述的塑性优异的超高强度钢中，残余奥氏体中平均C含量在1.1-1.35wt％之间。
进一步地，在本发明所述的塑性优异的超高强度钢中，其屈服强度为850～1000MPa，抗拉强度为1180～1300MPa，均匀延伸率≥11％，断裂延伸率为15％～20％。
相应地，本发明的另一目的在于提供上述的塑性优异的超高强度钢的制造方法，该制造方法通过采用薄板坯连铸工艺配合酸洗或酸轧工艺，在连续退火后获得塑性优异的超高强度钢。该制造方法生产简单，所得到的高强度钢在同等强度条件下，进一步提高钢的延伸率。
为了实现上述目的，本发明提出了上述的塑性优异的超高强度钢的制造方法，其包括步骤：
(1)冶炼和薄板坯连铸：其中连铸出口端的板坯厚度控制为55～60mm；
(2)加热；
(3)热轧：热轧后钢带表面氧化皮厚度≤6μm，并且热轧后带钢表面氧化皮中的(FeO+Fe 3O 4)≤40wt％；
(4)酸洗或者酸洗+冷轧；
(5)连续退火：在800～920℃退火，以3～10℃/s的冷速缓冷至690～760℃，以获得一定比例的铁素体；再快冷至250～350℃，冷却速度为50～100℃/s，使奥氏体部分转变为马氏体；然后再加热至360～460℃，保温100～400s，最后冷却至室温。
在本发明所述的技术方案中，由于步骤(1)采用的是薄板坯连铸，因此，可以省却粗轧工序，减小热轧变形量，从而保证在后续的步骤(4)以及步骤(5)的钢板性能。此外，由于步骤(1)采用的是薄板坯连铸，其可以通过充分利用板坯热量， 降低加热所需能耗，从而获得更加均匀的铁素体或铁素体+珠光体组织，有利于后续步骤(5)中的成品微观组织中保持一定量的细晶粒铁素体，提高组织均匀性，有利于提高塑性。
而在步骤(3)中，控制热轧后钢带表面氧化皮厚度≤6μm，并且热轧后带钢表面氧化皮中的(FeO+Fe 3O 4)≤40wt％，可以有利于后续在步骤(4)的进行，并对连续退火后所获得的钢板性能有着重要影响，这是因为：在本发明所述的技术方案中，FeO、Fe 3O 4比Fe 2O 3更加难酸洗，而控制本案的热轧后钢带表面氧化皮厚度以及热轧后带钢表面氧化皮中的(FeO+Fe 3O 4)≤40wt％可以提高酸洗效果，获得可用于直接连续退火的酸洗板表面，而由于酸洗板可以直接进行连续退火，使得热轧组织变形量小，钢板组织以铁素体与珠光体或贝氏体为主，因而，在相同连续退火条件下可以降低材料强度，使得组织更加均匀，从而获得优异的延展性。
而在步骤(5)中，通过控制800～920℃的退火温度可以形成均匀化的奥氏体组织或奥氏体+铁素体组织；然后以3～10℃/s的冷速缓冷至690～760℃，以进一步调整组织中铁素体的含量，提高材料的塑形；之后以50～100℃/s的速度冷却至250～350℃(即介于Ms与Mf温度之间)，这是因为：为保证冷却过程仅发生马氏体相变临界冷却速度需不小于50℃/s，而若冷速超过100℃/s后生产成本将大幅上升。此时，奥氏体部分转变为马氏体，保证钢具有较高的强度；再加热至360～460℃并保温100～400s，使碳在马氏体和奥氏体中发生配分，形成一定量富碳的残余奥氏体，稳定保持到室温，由于TRIP效应，可以显著提高钢的加工硬化能力和成形性，得到塑性优异的超高强度钢板。上述配分工艺设定如此是因为：当再加热温度低于360℃或再加热时长少于100s时钢中残余奥氏体稳定化过程不充分，最终室温下获得的残余奥氏体含量将不足；当再加热温度高于460℃或再加热时长多于400s时钢发生明显的回火软化，会导致最终的材料强度明显下降。
由于本案的超高强度钢是采用高碳、高锰的成分设计及铁素体晶粒细化机理，因而在连续退火过程中，奥氏体逆相变的形核点增多的同时，尺寸进一步细化，稳定保持到室温的残余奥氏体的平均晶粒尺寸可以≤2μm；残余奥氏体中平均C含量≥1.1％。另外，由于采用高Si设计，因而，快速冷却形成的马氏体在配分过程中基本不发生分解，以保证组织中马氏体的含量，保证钢的强度。
进一步地，在本发明所述的制造方法中，在步骤(1)中，控制连铸拉速为2～5m/min。
进一步地，在本发明所述的制造方法中，在步骤(2)中，将板坯加热到1200～1300℃。
进一步地，在本发明所述的制造方法中，在步骤(3)中，控制终轧温度为860～930℃，卷取温度为450～600℃。
进一步地，在本发明所述的制造方法中，在步骤(4)中，当采用酸洗+冷轧步骤时，控制其变形量为40％～70％。
进一步地，在本发明所述的制造方法，在步骤(5)中，控制连续退火工艺满足下述各项的至少其中之一：
退火温度820～870℃；
以3～10℃/s的冷速缓冷至700～730℃；
快冷至270～330℃；
快冷后再加热至400～430℃，保温150～300s；
控制连续退火炉内还原性气氛中氢气的体积含量10～15％。
本发明所述的塑性优异的超高强度钢及其制造方法相较于现有技术具有如下所述的优点以及有益效果：
本发明所述的高强度钢以碳硅锰钢为基础，不添加任何昂贵的合金元素，通过优化碳硅锰配比，获得具有优异延展性的高强度冷轧钢板。
本发明所述的制造方法生产工艺独特，由于采用了薄板坯连铸技术，在组织均匀性及偏析控制方面具有先天优势。所获得的超高强度钢在同等强度条件下，延伸率可显著提高，在汽车安全结构件中将具有较好的应用前景，特别适合于制造形状复杂、对成形性能要求高的车辆结构件和安全件，如A/B柱、车门防撞杆、纵梁、保险杠等。
附图说明
图1为实施例4的超高强度钢的微观组织照片。
图2为实施例4的超高强度钢的相组成EBSD照片。
具体实施方式
下面将结合说明书附图以及具体的实施例对本发明所述的塑性优异的超高强度钢及其制造方法做进一步的解释和说明，然而该解释和说明并不对本发明的技术方 案构成不当限定。
实施例1-24以及对比例1-3
实施例1-24的塑性优异的超高强度钢采用以下步骤制得：
(1)按照表1所示的化学成分进行冶炼和薄板坯连铸：其中连铸出口端的板坯厚度控制为55～60mm，并且控制连铸坯拉速为2～5m/min。
(2)加热：将板坯加热到1200～1300℃。
(3)热轧：热轧后钢带表面氧化皮厚度≤6μm，并且热轧后带钢表面氧化皮中的(FeO+Fe 3O 4)≤40wt％，控制终轧温度为860～930℃，卷取温度为450～600℃。
(4)酸洗或者酸洗+冷轧：当采用酸洗+冷轧步骤时，控制其变形量为40％～70％。
(5)连续退火：在800～920℃退火，以3～10℃/s的冷速缓冷至690～760℃，以获得一定比例的铁素体；再快冷至250～350℃，冷却速度为50～100℃/s，使奥氏体部分转变为马氏体；然后再加热至360～460℃，保温100～400s，最后冷却至室温。
需要说明的是，在一些优选的实施方式中，在步骤(5)中，可以进一步控制参数满足下述各项的至少其中之一：
退火温度820～870℃；
以3～10℃/s的冷速缓冷至700～730℃；
快冷至270～330℃；
快冷后再加热至400～430℃，保温150～300s；
控制连续退火炉内还原性气氛中氢气的体积含量10～15％。
而对比例1-3则采用常规工艺制造获得。
表1列出了实施例1-24的塑性优异的超高强度钢以及对比例1-3的对比钢的各化学元素的质量百分配比。
表1 (wt％，余量为Fe和除了P、S以及N以外的其他不可避免的杂质)
Figure PCTCN2021099258-appb-000001
Figure PCTCN2021099258-appb-000002
表2-1以及表2-2列出了实施例1-24的塑性优异的超高强度钢以及对比例1-3的对比钢的具体工艺参数。
表2-1
Figure PCTCN2021099258-appb-000003
Figure PCTCN2021099258-appb-000004
表2-2
Figure PCTCN2021099258-appb-000005
Figure PCTCN2021099258-appb-000006
表3列出了实施例1-24的塑性优异的超高强度钢以及对比例1-3的对比钢的力学性能测试结果。采用ISO 6892：1998(金属材料室温拉伸试验方法)、P14(A 50)拉伸试样标准进行测试。
表3
Figure PCTCN2021099258-appb-000007
Figure PCTCN2021099258-appb-000008
由表3可以看出，本案实施例1-24的塑性优异的超高强度钢在保证强度的同时，延展性能同样表现优异，其屈服强度YS为850～1000MPa，抗拉强度TS为1180～1300MPa，均匀延伸率UEL均≥11％，断裂延伸率TEL为15％～20％。
表4实施例1-24的塑性优异的超高强度钢的微观组织观察结果。
表4
Figure PCTCN2021099258-appb-000009
结合表3和表4可以看出，本案实施例1-24的塑性优异的超高强度钢的微观组织为20％～40％的铁素体+50％～70％的马氏体+残余奥氏体，其中在铁素体中，10μm 以下的晶粒占比90％以上，5μm以下的晶粒占比60％以上，残余奥氏体的平均晶粒尺寸≤2μm；并且/或者残余奥氏体中平均C含量≥1.1wt％。由此，说明了本案各实施例的塑性优异的超高强度钢由于具有一定量的细晶粒铁素体，并组织均匀性好，因而，使得各个实施例的高强度钢可以在强度高的同时具有极好的塑性。
图1为实施例4的超高强度钢的微观组织照片。
图2为实施例4的超高强度钢的相组成EBSD照片。
结合图1和图2可以看出，实施例4的超高强度钢的微观组织为20％～40％的铁素体+50％～70％的马氏体+残余奥氏体，其中在铁素体中，10μm以下的晶粒占比90％以上，5μm以下的晶粒占比60％以上，残余奥氏体的平均晶粒尺寸≤2μm；并且/或者残余奥氏体中平均C含量≥1.1wt％。
综上所述可以看出，本发明所述的超高强度钢以碳硅锰钢为基础，不添加任何昂贵的合金元素，通过优化碳硅锰配比，且采用薄板坯连铸技术，在组织均匀性及偏析控制方面具有先天优势。
本发明所述的制造方法生产工艺简单，所获得的超高强度钢在同等强度条件下，延伸率可显著提高，在汽车安全结构件中将具有较好的应用前景，特别适合于制造形状复杂、对成形性能要求高的车辆结构件和安全件，如A/B柱、车门防撞杆、纵梁、保险杠等。
需要说明的是，本发明的保护范围中现有技术部分并不局限于本申请文件所给出的实施例，所有不与本发明的方案相矛盾的现有技术，包括但不局限于在先专利文献、在先公开出版物，在先公开使用等等，都可纳入本发明的保护范围。
此外，本案中各技术特征的组合方式并不限本案权利要求中所记载的组合方式或是具体实施例所记载的组合方式，本案记载的所有技术特征可以以任何方式进行自由组合或结合，除非相互之间产生矛盾。
还需要注意的是，以上所列举的实施例仅为本发明的具体实施例。显然本发明不局限于以上实施例，随之做出的类似变化或变形是本领域技术人员能从本发明公开的内容直接得出或者很容易便联想到的，均应属于本发明的保护范围。

Claims (15)

  1. 一种塑性优异的超高强度钢，其特征在于，其化学元素质量百分比为：
    C：0.26～0.30wt％；
    Si：0.8～1.00wt％；
    Mn：2.80～3.30wt％；
    Al：0.04～0.08wt％；
    余量为Fe和其他不可避免的杂质。
  2. 如权利要求1所述的塑性优异的超高强度钢，其特征在于，其各化学元素的质量百分比满足下列各项的至少其中之一：
    C：0.26～0.28wt％；
    Si：0.9～1.00wt％；
    Mn：2.9～3.1wt％。
  3. 如权利要求1或2所述的塑性优异的超高强度钢，其特征在于，还含有下列各元素的至少其中之一：
    0＜Cr≤0.05wt％；
    0＜Mo≤0.05wt％；
    0＜Nb≤0.03wt％；
    0＜Ti≤0.05wt％；
    0＜V≤0.03wt％；
    0＜B≤0.001wt％。
  4. 如权利要求3所述的塑性优异的超高强度钢，其特征在于，其中各化学元素满足下列各项的至少其中之一：
    0＜Cr≤0.03wt％；
    0＜Mo≤0.03wt％；
    0＜Nb≤0.01wt％；
    0＜Ti≤0.03wt％；
    0＜V≤0.01wt％。
  5. 如权利要求1所述的塑性优异的超高强度钢，其特征在于，在其他不可避免的杂质中：P≤0.01wt％，S≤0.01wt％，N≤0.006wt％。
  6. 如权利要求1所述的塑性优异的超高强度钢，其特征在于，其微观组织为20％～40％的铁素体+50％～70％的马氏体+残余奥氏体。
  7. 如权利要求6所述的塑性优异的超高强度钢，其特征在于，其中在铁素体中，10μm以下的晶粒占比90％以上，5μm以下的晶粒占比60％以上。
  8. 如权利要求6所述的塑性优异的超高强度钢，其特征在于，其中残余奥氏体的平均晶粒尺寸≤2μm；并且/或者残余奥氏体中平均C含量≥1.1wt％。
  9. 如权利要求1所述的塑性优异的超高强度钢，其特征在于，其屈服强度为850～1000MPa，抗拉强度为1180～1300MPa，均匀延伸率≥11％，断裂延伸率为15％～20％。
  10. 如权利要求1-9中任意一项所述的塑性优异的超高强度钢的制造方法，其特征在于，其包括步骤：
    (1)冶炼和薄板坯连铸：其中连铸出口端的板坯厚度控制为55～60mm；
    (2)加热；
    (3)热轧：热轧后钢带表面氧化皮厚度≤6μm，并且热轧后带钢表面氧化皮中的(FeO+Fe 3O 4)≤40wt％；
    (4)酸洗或者酸洗+冷轧；
    (5)连续退火：在800～920℃退火，以3～10℃/s的冷速缓冷至690～760℃，以获得一定比例的铁素体；再快冷至250～350℃，冷却速度为50～100℃/s，使奥氏体部分转变为马氏体；然后再加热至360～460℃，保温100～400s，最后冷却至室温。
  11. 如权利要求10所述的制造方法，其特征在于，在步骤(1)中，控制连铸拉速为2～5m/min。
  12. 如权利要求10所述的制造方法，其特征在于，在步骤(2)中，将板坯加热到1200～1300℃。
  13. 如权利要求10所述的制造方法，其特征在于，在步骤(3)中，控制终轧温度为860～930℃，卷取温度为450～600℃。
  14. 如权利要求10所述的制造方法，其特征在于，在步骤(4)中，当采用酸洗+冷轧步骤时，控制其变形量为40％～70％。
  15. 如权利要求10所述的制造方法，其特征在于，在步骤(5)中，控制连续退火工艺满足下述各项的至少其中之一：
    退火温度820～870℃；
    以3～10℃/s的冷速缓冷至700～730℃；
    快冷至270～330℃；
    快冷后再加热至400～430℃，保温150～300s；
    控制连续退火炉内还原性气氛中氢气的体积含量10～15％。
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