WO2018019220A1 - 一种1500MPa级高强塑积汽车用钢及其制造方法 - Google Patents

一种1500MPa级高强塑积汽车用钢及其制造方法 Download PDF

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WO2018019220A1
WO2018019220A1 PCT/CN2017/094247 CN2017094247W WO2018019220A1 WO 2018019220 A1 WO2018019220 A1 WO 2018019220A1 CN 2017094247 W CN2017094247 W CN 2017094247W WO 2018019220 A1 WO2018019220 A1 WO 2018019220A1
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steel
plastic
annealing
strength
austenite
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PCT/CN2017/094247
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English (en)
French (fr)
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张玉龙
韩启航
王利
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宝山钢铁股份有限公司
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Priority to US16/320,235 priority Critical patent/US11047027B2/en
Priority to JP2019503712A priority patent/JP6808811B2/ja
Priority to KR1020197004638A priority patent/KR102251635B1/ko
Priority to EP17833523.8A priority patent/EP3492618B1/en
Publication of WO2018019220A1 publication Critical patent/WO2018019220A1/zh

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Definitions

  • the invention relates to a steel grade, a manufacturing method thereof and a use thereof, in particular to a steel for automobile and a manufacturing method thereof.
  • the “lightweight” of automobiles makes the application of ultra-high-strength steel plates more and more in automotive structural parts.
  • the most used steel sheets such as duplex steel, martensitic steel, phase change induced plastic steel (TRIP steel), and duplex steel have a strong plastic product of about 10 GPa%.
  • the tensile strength of ultra-high-strength martensitic steel is 1500 MPa
  • the elongation is about 5%, which cannot meet the double requirements of automotive safety performance and forming performance in the manufacturing process.
  • the publication number is CN101638749, and the publication date is February 3, 2010.
  • the Chinese patent document entitled "A low-cost high-strength plastic-plastic automobile steel and a preparation method thereof" discloses a low-cost high-strength plastic-shaped automobile steel.
  • the manufacturing method includes a cold-rolled steel sheet having a strong plastic product of 35 to 55 GPa% by a process route of smelting, hot rolling, hood furnace annealing, cold rolling, and hood furnace annealing.
  • a reverse austenite transformation a sufficient austenite volume fraction is obtained, and after a cold rolling, a hood furnace is used, and the annealing time is 1 to 10 hours.
  • the technical strength of the automotive steel is 700-1300 MPa, which cannot meet the 1500 MPa grade.
  • the publication number is CN102758133A, and the publication date is October 31, 2012.
  • the Chinese patent document entitled "A 1000MPa class high-strength plastic-plastic automobile steel and its manufacture” discloses a 1000MPa grade high-strength plastic-plastic automobile steel. And a manufacturing method thereof, which relates to a steel sheet having a strong plastic product of more than 30 GPa% by a continuous annealing method, and is suitable for an industrial production line of various steel plants at present.
  • the automotive grade of this technical solution is 1000 MPa, which cannot meet the 1500 MPa grade.
  • the company expects to obtain a steel material for automobiles, which has high strength and good strength and plastic product, and can be used for manufacturing automobile parts to meet the demand for automotive steel.
  • the company also expects to obtain the manufacturing method of the automobile steel, which has a simple process and strong applicability, and can be used in various actual production lines.
  • One of the objects of the present invention is to provide a steel for 1500 MPa high-strength plastic-plastic automobile, which can reach a grade of 1500 MPa and a strong plastic product of not less than 30 GPa%.
  • the present invention provides a steel of 1500 MPa high-strength plastic-plastic automobile, and the chemical element mass percentage thereof is:
  • the microstructure of the 1500 MPa high-strength plastic-plastic automobile steel is austenite + martensite + ferrite or austenite + martensite.
  • Carbon acts as a solid solution strengthening and is also a major element in stabilizing austenite. It has a great influence on the strength, formability and weldability of steel. When the mass percentage of carbon is less than 0.1%, the strength of martensite in the structure is low, resulting in lower strength of the steel, and also results in poor austenite stability and lower elongation; however, when carbon When the mass percentage is more than 0.3%, the forming and welding properties of the steel are deteriorated, so that the carbon mass percentage in the steel of the 1500 MPa class high-strength plastic-plastic automobile according to the present invention is controlled to be 0.1% to 0.3%.
  • Silicon is an essential element for deoxidation of steelmaking, and it also has a certain solid solution strengthening effect, while silicon has the effect of inhibiting the precipitation of carbides. Therefore, when the mass percentage of silicon is less than 0.1%, it is difficult to obtain a sufficient deoxidation effect; at the same time, silicon has a tendency to prevent precipitation of cementite, and promotes the occurrence of inverse martensitic transformation. Thus, when the mass percentage of silicon is higher than 2.0%, the effect of continuing to increase silicon is not significant. In view of this, the silicon mass percentage in the steel for 1500 MPa grade high-strength plastic-plastic automobile according to the present invention is controlled to be 0.1% to 2.0%.
  • Manganese is an element that enlarges the austenite phase region, and the stability of austenite and austenite can be improved by the diffusion of heat-treated manganese.
  • manganese is controlled by inverse martensite transformation. The main elements of size, distribution and stability. When the mass percentage of manganese is less than 7.5%, it is difficult to obtain a sufficient amount of austenite at room temperature. However, when the mass percentage of manganese is more than 12%, partial ⁇ martensite is obtained at room temperature, which adversely affects the properties of steel. . In order to ensure the strength and toughness of the steel, the mass percentage of manganese in the steel for 1500 MPa class high-strength plastic-plastic automobile according to the present invention is controlled to be 7.5-12%.
  • Aluminum has a deoxidizing effect in the steel making process and is an element added to improve the purity of molten steel. At the same time, aluminum can also fix nitrogen in the steel to form a stable compound, effectively refining the grains. Moreover, the addition of aluminum to the steel has the effect of preventing the precipitation of cementite and promoting the reverse martensitic transformation. When the mass percentage of aluminum is less than 0.01%, the effect of adding aluminum is not significant. For this reason, the mass percentage of aluminum of the 1500 MPa class high-strength plastic-plastic automobile steel according to the present invention is limited to 0.01% to 2.0%.
  • the 1500 MPa high-strength plastic-plastic automobile steel of the present invention defines the microstructure as austenite + martensite + ferrite or Austenite + martensite.
  • the chemical element further has Nb: 0.01 to 0.07%, Ti: 0.02 to 0.15%, V: 0.05 to 0.20%, and Cr: 0.15 to 0.50. %, Mo: at least one of 0.10 to 0.50%.
  • the alloying element is added to further improve the performance of the 1500MPa grade high-strength plastic-plastic automobile steel according to the present invention, and the design principle is:
  • ⁇ can effectively delay the recrystallization of deformed austenite, prevent the growth of austenite grains, increase the austenite recrystallization temperature, refine grains, and improve strength and elongation.
  • the mass percentage of bismuth is less than 0.01%, it does not have the effect, but when the mass percentage of bismuth is higher than 0.07%, the production cost is increased, and the improvement effect on steel performance is no longer significant. Therefore, in the technical solution of the present invention, the mass percentage of ruthenium is controlled to be 0.01 to 0.07%.
  • Titanium forms fine composite carbides, prevents austenite grains from growing, refines grains, and also acts as a precipitation strengthening agent. The strength of the steel is increased without lowering the elongation and the hole expansion ratio. When the mass percentage of titanium is less than 0.02%, there is no grain refinement and precipitation strengthening effect. However, when the mass percentage of titanium is more than 0.15%, the content thereof is further increased, and the effect on improving steel is not remarkable. In view of this, the mass percentage of titanium of the 1500 MPa high strength plastic-plastic automobile steel according to the present invention is limited to 0.02 to 0.15%.
  • Vanadium The role of vanadium is to form carbides and increase the strength of steel. When the mass percentage of vanadium is less than 0.05%, the precipitation strengthening effect is not remarkable. However, when the mass percentage of vanadium is more than 0.20%, the content is further increased, and the improvement effect is not remarkable. For this reason, in the steel for 1500 MPa class high-strength plastic-plastic automobile according to the present invention, the mass percentage of vanadium is limited to 0.05 to 0.20%.
  • Chromium contributes to the refinement of austenite grains and the formation of fine bainite during rolling, and increases the strength of steel. When the mass percentage of chromium is less than 0.15%, the effect is not significant. However, when the mass percentage of chromium exceeds 0.5%, the cost is increased and the weldability is remarkably lowered. Therefore, in the steel for 1500 MPa class high-strength plastic-plastic automobile according to the present invention, the mass percentage of chromium is limited to 0.15 to 0.50%.
  • Molybdenum contributes to the refinement of austenite grains and the formation of fine bainite during rolling, and increases the strength of steel. When the mass percentage of molybdenum is less than 0.15%, the effect is not remarkable. However, when the mass percentage of molybdenum exceeds 0.5%, the cost is increased and the weldability is remarkably lowered. Therefore, in the steel for 1500 MPa class high-strength plastic-plastic automobile according to the present invention, the mass percentage of molybdenum is limited to 0.15 to 0.50%.
  • the austenite ratio is 20% to 40%.
  • the ratio of martensite is 50% to 70%.
  • the ratio of austenite is 20% to 50%.
  • the strong plastic product is not less than 30 GPa%.
  • the steel for 1500MPa grade high-strength plastic-plastic automobile according to the present invention can have a tensile strength of more than 1500 MPa and a strong plastic product of not less than 30 GPa%, so that the steel for automobiles meets the demand for lightweight and high strength of modern automobile steel.
  • Another object of the present invention is to provide a method for manufacturing a steel of 1500 MPa grade high-strength plastic automobile according to the present invention, which comprises the steps of:
  • annealing temperature is 600 ⁇ 700 ° C, annealing time is 1 ⁇ 48h;
  • the first annealing after cold rolling the annealing temperature is between Ac1 and Ac3, and the annealing time is greater than 5 min;
  • the annealing temperature is 750-850 ° C, and the annealing time is 1-10 min;
  • Tempering The tempering temperature is 200-300 ° C, and the tempering time is not less than 3 min.
  • ART austenite reverse phase transformation
  • plastic accumulation By controlling the chemical composition design and process parameters of the steel sheet, the steel is subjected to full martensite structure after hot rolling and cold rolling, and in the subsequent annealing process (annealing temperature is between Ac1 and Ac3 temperatures)
  • annealing temperature is between Ac1 and Ac3 temperatures
  • the reverse martensite transformation is promoted to form part of austenite. Due to the partitioning of carbon and manganese elements and enrichment in austenite, austenite can be stably present at room temperature.
  • austenite structure can be obtained at room temperature. Under stress, austenite undergoes stress/strain induced martensitic transformation, forming so-called phase transformation induced plasticity (TRIP), and improves the performance of the steel sheet.
  • TRIP phase transformation induced plasticity
  • the conventional ART annealing temperature is usually only slightly higher than the Ac1 temperature, and the microstructure of austenite + ferrite is obtained after annealing.
  • the steel strength of this microstructure is less than 1500 MPa, which cannot meet the requirements of the technical solution. . If the annealing temperature is increased, the microstructure of ferrite + martensite + austenite can be obtained, however, the austenite stability in these microstructures is poor. If the stress is small, a phase change occurs, and the TRIP effect cannot be produced, so that the elongation of the steel sheet is deteriorated, and a high strong plastic product cannot be obtained.
  • the inventors have found through research that to obtain a 1500 MPa high-strength steel plate, the microstructure must contain a large amount of martensite, and at the same time, it must contain more austenite with higher stability.
  • the inventors have creatively proposed an annealing process based on the composition of the present invention, so that the microstructure of the steel has a large amount of martensite and austenite having a high stability.
  • the microstructure after hot rolling is martensite, and the martensite strength is high, but is brittle, so the step (3) is passed.
  • the hood furnace is annealed to soften the steel to perform the cold rolling of step (4).
  • the austenite is transformed into martensite, and the microstructure in the steel is further adjusted by the step (5), the step (6) and the step (7), thereby obtaining the high strength of 1500 MPa.
  • Plastic steel for automobiles is further adjusted by the step (5), the step (6) and the step (7), thereby obtaining the high strength of 1500 MPa.
  • the hood furnace annealing of step (3) and the first annealing after cold rolling of step (5) are both ART annealing, and the annealing temperature is between Ac1 and Ac3 temperatures.
  • the first annealing after cold rolling in step (5) is to convert the microstructure of the cold rolled steel sheet from martensite to austenite plus ferrite by ART annealing, in preparation for the subsequent process.
  • the annealing temperature of the second annealing after cold rolling in the step (6) in the technical solution is higher (close to the two-phase region of the Ac3 temperature or the single-phase austenite region), and the annealing time is higher. short.
  • the principle and principle are as follows: the microstructure of the steel plate obtained after the first annealing after the cold rolling in step (5) is ferrite + austenite, and the Mn content in the austenite structure is high and the stability is good. At this time, when the steel sheet is heated to a higher temperature, the ferrite structure in the original steel sheet is turned into a new austenite phase.
  • the newly formed austenite phase contains relatively low Mn content, and due to the slow diffusion rate of Mn, Mn cannot be sufficiently diffused during short-time annealing, so two parts of austenite are formed in the microstructure at high temperature.
  • Body ie Mn-rich and Mn-depleted austenite.
  • the Mn-depleted austenite is transformed into martensite, and the Mn-rich austenite is stably present. In this way, a large amount of martensite and austenite with high stability can be obtained.
  • the microstructure of martensite + austenite + a small amount of ferrite can be obtained by controlling the annealing temperature and the annealing time; (6)
  • the microstructure obtained by controlling the annealing temperature and the annealing time is martensite + austenite.
  • the annealing temperature of the step (6) is limited to 750 to 850 ° C, and the annealing time is controlled to be 1 to 10 min.
  • the annealing temperature is greater than 850 ° C or the annealing time is more than 10 min, the austenite stability will be deteriorated.
  • the austenite ratio at room temperature is lower, resulting in a steel with a strong plastic product of less than 30 GPa%; however, when the annealing temperature is less than 750 °C or annealing time less than 1min will result in less ferrite transformation to austenite during annealing, and there will still be a large amount of ferrite after cooling to room temperature, in this case, despite the elongation and strong plastic product of steel. Can be higher, but the strength of steel can not meet 1500MPa.
  • the purpose of the tempering in the step (7) is to remove the internal stress generated when the martensite is formed, and the tempering without causing the obtained steel sheet to be brittle and the elongation is low.
  • the slab is heated to 1,100 to 1,260 ° C, and the rolling is controlled, and the rolling temperature is 950 ⁇ .
  • the finishing temperature is 750-900 ° C
  • the coiling temperature is 500-850 ° C, and after coiling, it is cooled to room temperature to obtain a full martensite structure.
  • the cold rolling reduction is not less than 40%.
  • the tensile strength of the 1500MPa grade high-strength plastic-plastic automobile according to the present invention can reach 1500MPa or more, and the strong plastic product can reach 30GPa% or more.
  • the manufacturing method of the steel for 1500 MPa grade high-strength plastic-plastic automobile according to the present invention also has the above advantages and advantageous effects.
  • the manufacturing method optimizes the process flow and improves the performance of the steel through reasonable chemical composition design and controlled annealing process, thereby obtaining high-strength and high-strength plastic steel for the demand, and saving manufacturing cost.
  • FIG. 1 is a schematic view showing a process curve of a method for manufacturing a 1500 MPa high-strength plastic-plastic automobile according to the present invention.
  • Hot rolling After the slab is heated to 1,100 to 1,260 ° C, the rolling is controlled, the rolling temperature is 950 to 1150 ° C, the finishing temperature is 750 to 900 ° C, and the coiling temperature is 500 to 850 ° C. Cool to room temperature to obtain a full martensite structure.
  • the annealing temperature is 600-700 ° C, and the annealing time is 1 to 48 h.
  • annealing temperature is between Ac1 and Ac3 temperatures, annealing time is greater than 5 min.
  • Second annealing after cold rolling annealing temperature is 750-850 ° C, annealing time is 1-10 min. It should be noted that, in order to show the influence of the second annealing process parameters after cold rolling in this case on the implementation effect of the present invention, the annealing temperature used in Comparative Examples 1-3 is not within the scope of the present case, and the comparison The second annealing temperature of cold rolling in Example 1 was 720 ° C, the second annealing time of cold rolling in Comparative Example 2 was 15 min, and the second annealing temperature in cold rolling of Comparative Example 3 was 760 ° C.
  • Tempering The tempering temperature is 200-300 ° C, and the tempering time is not less than 3 min.
  • the thickness of the hot rolled steel sheet in the step (2) is not more than 8 mm.
  • the thickness of the cold rolled steel sheet in the step (4) is not more than 2.5 mm.
  • the electric furnace or the induction furnace may be used for smelting.
  • Table 1 lists the mass ratios of the respective chemical elements in Examples 1-8 and Comparative Examples 1-4.
  • Table 2 lists the specific process parameters in the manufacturing methods of Examples 1-8 and Comparative Examples 1-4.
  • Table 3 lists the performance parameters of the steel of 1500 MPa grade high-strength plastic automobile of Examples 1-8 and the steel sheets of Comparative Examples 1-4.
  • the strong plastic product is the product of tensile strength and elongation.
  • the 1500 MPa high-strength plastic-plastic automobile steel of each embodiment of the present invention has a tensile strength of >1500 Mpa and a strong plastic product of >30 GPa%, indicating that the automotive steel of each embodiment has high strength and Good tensile ductility.
  • the mass percentage of manganese in Comparative Example 4 was less than 7.5%, and the strong plastic product did not reach 30 GPa%, and the elongation was low. This is because the mass percentage of manganese in Comparative Example 4 is low, so the ratio and stability of austenite produced during the second annealing of cold rolling are insufficient, resulting in lower elongation and lower strong plastic product. .
  • the second annealing temperature of the cold rolling in Comparative Example 1 is lower than 750 ° C. Therefore, the amount of ferrite transformation to austenite is less during the second annealing of cold rolling, and is cooled to room temperature. There is still a large amount of ferrite present.
  • the steel sheet of Comparative Example 1 had an elongation of more than 30%, a strong plastic product of more than 30 GPa%, but a tensile strength of less than 1,500 MPa.
  • the second annealing time of cold rolling in Comparative Example 2 is greater than 10 min, and the second annealing temperature of cold rolling in Comparative Example 3 is higher than 850 ° C, thus causing austenite stability to change. Poor, the ratio of austenite was lower at room temperature, and the strength of the steel sheets of Comparative Example 2 and Comparative Example 3 was less than 30 GPa%.
  • Fig. 1 is a view showing the process curve of a method for producing a 1500 MPa class high-strength plastic-plastic automobile according to Embodiment 1 of the present invention.
  • the manufacturing process involved in the technical solution performs one annealing after hot rolling 1, that is, a hood furnace annealing 2, and then cold rolling 3, and after cold rolling, secondary annealing, that is, cold rolling is performed.
  • hot rolling that is, a hood furnace annealing 2, and then cold rolling 3, and after cold rolling, secondary annealing, that is, cold rolling is performed.
  • secondary annealing that is, cold rolling is performed.
  • the horizontal axis represents time and the vertical axis represents temperature, and therefore the graph of Fig. 1 schematically shows the case where the temperature changes with time. It can be seen from Fig.
  • the annealing of the cover furnace 2 and the first annealing after cold rolling 4 are performed by ordinary ART annealing
  • the second annealing after cold rolling 5 is performed by annealing higher than ordinary ART.
  • the annealing temperature and the shorter annealing time thereby obtaining the microstructure desired in the present technical solution, that is, a large amount of martensite structure and more austenite structure.

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Abstract

一种1500MPa级高强塑积汽车用钢及其制造方法，其化学元素质量百分比为：C:0.1%~0.3%，Si：0.1%~2.0%，Mn：7.5%~12%，Al：0.01%~2.0%；余量为铁和其他不可避免的杂质；该高强塑积汽车用钢的微观组织为奥氏体+马氏体+铁素体或奥氏体+马氏体。该汽车用钢能够达到1500MPa级，且其强塑积不小于30GPa%。

Description

一种1500MPa级高强塑积汽车用钢及其制造方法 技术领域
本发明涉及一种钢种及其制造方法和用途，尤其涉及一种汽车用钢及其制造方法。
背景技术
汽车“轻量化”使得超高强度钢板在汽车结构件中的应用越来越多。目前用量最大的钢板如双相钢、马氏体钢、相变诱导塑性钢(TRIP钢)、复相钢等，其强塑积最大约为10GPa％左右。例如超高强马氏体钢的抗拉强度在1500MPa级别时，其延伸率约为5％左右，已不能满足汽车领域对汽车安全性能和制造过程中成形性能的双重要求。上世纪末，人们相继开发了高强塑积的奥氏体钢和孪生诱发塑性钢(TWIP钢)，其抗拉强度为800～1000MPa，延伸率高达60％，强塑积达到60GPa％级别，被称为第二代汽车用钢。第二代汽车用钢中加入了大量的合金元素，成本较高，同时其可制造性较差，使得其推广过程受到极大的限制。因此，强塑积大于30GPa％，兼具高强度和高延伸率，成本较低的第三代汽车用钢受到的广泛的关注。
公开号为CN101638749，公开日为2010年2月3日，名称为“一种低成本高强塑积汽车用钢及其制备方法”的中国专利文献公开了一种低成本高强塑积的汽车用钢的制造方法，其中涉及通过冶炼、热轧、罩式炉退火、冷轧和罩式炉退火的工艺路线得到了强塑积为35～55GPa％的冷轧钢板。为了实现逆奥氏体相变，得到足够的奥氏体体积分数，冷轧后采用罩式炉，其退火时间为1～10小时。然而，该技术方案的汽车用钢强度为700-1300MPa，无法满足1500MPa级别。
公开号为CN102758133A，公开日为2012年10月31日，名称为“一种1000MPa级别的高强塑积汽车用钢及其制造”的中国专利文献公开了一种1000MPa级别的高强塑积汽车用钢及其制造方法，其中涉及通过连续退火的方法生产强塑积大于30GPa％的钢板，适用于目前各钢铁厂的工业生产线。然而， 该技术方案的汽车用钢级别为1000MPa，无法满足1500MPa级别。
鉴于此，企业期望获得一种汽车用钢材料，其具有较高的强度且较好的强塑积，可以用于制造汽车零部件，满足汽车用钢需求。与此同时，企业还期望获得该种汽车用钢的制造方法，工艺流程简单，适用性强，可用于多种实际生产线上。
发明内容
本发明的目的之一在于提供一种1500MPa级高强塑积汽车用钢，该汽车用钢能够达到1500MPa级，且其强塑积不小于30GPa％。
基于上述发明目的，本发明提供了一种1500MPa级高强塑积汽车用钢，其化学元素质量百分比为：
C：0.1％～0.3％，Si：0.1％～2.0％，Mn：7.5％～12％，Al：0.01％～2.0％；余量为铁和其他不可避免的杂质；
所述1500MPa级高强塑积汽车用钢的微观组织为奥氏体+马氏体+铁素体或奥氏体+马氏体。
本发明所述1500MPa级高强塑积汽车用钢的各化学元素的设计原理为：
碳：碳起固溶强化作用，同时也是稳定奥氏体的主要元素，对钢的强度、成形性能和焊接性能影响很大。当碳的质量百分比低于0.1％时，组织中的马氏体强度较低，从而导致钢的强度较低，同时也导致奥氏体的稳定性较差，延伸率较低；然而，当碳的质量百分比高于0.3％时会使钢的成形和焊接性能恶化，因此在本发明所述的1500MPa级高强塑积汽车用钢中的碳质量百分比控制在0.1％～0.3％。
硅：硅是炼钢脱氧的必要元素，也具有一定的固溶强化作用，同时硅有抑制碳化物的析出的作用。因此，当硅的质量百分比低于0.1％时，难以获得充分的脱氧效果；同时硅具有阻止渗碳体的析出的左右，促进逆马氏体相变的发生。因而，当硅的质量百分比高于2.0％时，继续增加硅的作用不明显。鉴于此，本发明所述的1500MPa级高强塑积汽车用钢中的硅质量百分比控制在0.1％～2.0％。
锰：锰是扩大奥氏体相区的元素，通过热处理的锰的扩散，可提高奥氏体的相比例和奥氏体的稳定性。在本发明技术方案中，锰是控制逆马氏体转变的 尺寸、分布和稳定性的主要元素。当锰的质量百分比小于7.5％时，在室温下难以得到足够含量的奥氏体，然而，当锰质量百分比大于12％时，室温下回得到部分ε马氏体，对钢的性能产生不利影响。为了保证钢的强度和韧性，本发明所述的1500MPa级高强塑积汽车用钢中的锰的质量百分比控制在7.5-12％。
铝:铝在炼钢过程中具有脱氧的作用，是为了提高钢水纯净度而添加的元素。同时，铝还能固定钢中的氮使之形成稳定的化合物，有效的细化晶粒。而且，钢中加入铝具有阻止渗碳体的析出，促进逆马氏体相变的作用。铝的质量百分比低于0.01％时，铝的添加效果不明显。为此，本发明所述的1500MPa级高强塑积汽车用钢的铝的质量百分比限定在0.01％～2.0％。
此外，为了使汽车用钢达到1500MPa级的同时具有强塑积不小于30GPa％,本发明所述的1500MPa级高强塑积汽车用钢限定微观组织为奥氏体+马氏体+铁素体或奥氏体+马氏体。
需要说明的是，基于上述技术方案，在本发明所述的1500MPa级高强塑积汽车用钢中，其他不可避免的杂质主要是指磷、硫和氮，这些杂质元素可以控制为：P≤0.02％，S≤0.02％，N≤0.02％。
进一步地，在本发明所述的1500MPa级高强塑积汽车用钢中，其化学元素还具有Nb：0.01～0.07％，Ti：0.02～0.15％，V：0.05～0.20％，Cr：0.15～0.50％，Mo：0.10～0.50％的至少其中之一。
添加合金元素是为了进一步改善本发明所述的1500MPa级高强塑积汽车用钢的性能，其设计原理是：
铌：铌能够有效的延迟变形奥氏体的再结晶，阻止奥氏体晶粒的长大，提高奥氏体再结晶温度，细化晶粒，同时提高强度和延伸率。当铌的质量百分比小于0.01％时，不能起到应有的效果，但当铌的质量百分比高于0.07％时，会使生产成本增加，且对于钢性能的改善效果不再显著。因此，在本发明技术方案中，铌的质量百分比控制在0.01～0.07％。
钛：钛形成细小的复合碳化物，阻止奥氏体晶粒长大，细化晶粒，也能起到沉淀强化的作用。在提高钢的强度的同时并不降低延伸率和扩孔率。当钛的质量百分比低于0.02％时，无细化晶粒和沉淀强化效应。但当钛的质量百分比大于0.15％时，再增加其含量，对于改善钢的效果并不显著。鉴于此，本发明所述的1500MPa级高强塑积汽车用钢的钛的质量百分比限定在0.02～0.15％。
钒：钒的作用是形成碳化物，提高钢的强度。当钒的质量百分比小于0.05％时，沉淀强化效果不显著。然而，当钒的质量百分比大于0.20％时，再增加其含量，改善效果并不显著。为此，在本发明所述的1500MPa级高强塑积汽车用钢中，钒的质量百分比限定在0.05～0.20％。
铬：铬有助于轧制时奥氏体晶粒的细化和细小的贝氏体的生成，提高钢的强度。当铬的质量百分比小于0.15％时，效果不显著。然而，当铬的质量百分比超过0.5％时，成本提高，可焊性明显降低。因此，在本发明所述的1500MPa级高强塑积汽车用钢中，铬的质量百分比限定在0.15～0.50％。
钼：钼有助于轧制时奥氏体晶粒的细化和细小的贝氏体的生成，提高钢的强度。当钼的质量百分比小于0.15％时，效果不显著。然而，当钼的质量百分比超过0.5％时，成本提高，可焊性明显降低。因此，在本发明所述的1500MPa级高强塑积汽车用钢中，钼的质量百分比限定在0.15～0.50％。
进一步地，在本发明所述的1500MPa级高强塑积汽车用钢中，当其微观组织为奥氏体+马氏体+铁素体时，奥氏体的相比例为20％～40％，马氏体的相比例为50％～70％。
进一步地，在本发明所述的1500MPa级高强塑积汽车用钢中，当其微观组织为奥氏体+马氏体时，奥氏体的相比例为20％～50％。
进一步地，在本发明所述的1500MPa级高强塑积汽车用钢中，其强塑积不小于30GPa％。
本发明所述的1500MPa级高强塑积汽车用钢能够抗拉强度大于1500MPa，且其强塑积不小于30GPa％，因而该汽车用钢符合现代汽车用钢对于轻量化和高强度的需求。
本发明的另一目的在于提供本发明所述的1500MPa级高强塑积汽车用钢的制造方法，其依次包括步骤：
(1)冶炼和铸造；
(2)热轧；
(3)罩式炉退火，退火温度为600～700℃，退火时间为1～48h；
(4)冷轧；
(5)冷轧后第一次退火：退火温度在Ac1和Ac3温度之间，退火时间大于5min；
(6)冷轧后第二次退火：退火温度为750～850℃，退火时间为1～10min；
(7)回火：回火温度为200～300℃，回火时间不小于3min。
在本发明所述的1500MPa级高强塑积汽车用钢的制造方法中，由于Mn的质量百分比为7.5～12％，因此发明人希望采用奥氏体逆相变(ART)退火工艺得到高的强塑积。ART退火的原理如下：通过控制钢板化学成分设计和工艺参数，使得钢在热轧和冷轧后得到全马氏体组织，在随后的退火过程中(退火温度介于Ac1和Ac3温度之间)促使逆马氏体相变，形成部分奥氏体，由于碳和锰元素的配分，并在奥氏体中富集，使得奥氏体可以在室温下稳定存在。通过ART退火，可以在室温下得到奥氏体组织，应力作用下奥氏体会发生应力/应变诱发马氏体相变，形成所谓的相变诱发塑性(TRIP)，提高钢板的性能。
然而，常规的ART退火温度通常仅略高于Ac1温度，且退火后得到奥氏体+铁素体的微观组织，这种微观组织的钢强度根本达不到1500MPa，无法满足本技术方案的要求。若将退火温度升高，则可得到铁素体+马氏体+奥氏体的微观组织，然而，这些微观组织中的奥氏体稳定性较差。若应力较小时，就会发生相变，不能产生TRIP效应，使得钢板的延伸率变差，得不到高的强塑积。
发明人经过研究发现，要得到1500MPa高强塑积的钢板，微观组织中必须含有大量的马氏体，同时还要含有较多的稳定性较高的奥氏体。基于此目的，发明人创造性地提出了基于本案成分设计的退火工艺，从而使得钢的微观组织在还有大量马氏体的同时还具有较多稳定性高的奥氏体。
在本发明所述的1500MPa级高强塑积汽车用钢的制造方法的步骤(2)中，热轧后微观组织为马氏体，马氏体强度高，但较脆，因此通过步骤(3)的罩式炉退火，使钢软化才能进行步骤(4)的冷轧。步骤(4)的冷轧过程中，奥氏体转变为马氏体，通过步骤(5)、步骤(6)以及步骤(7)进一步调整钢中的微观组织，从而获得所述的1500MPa级高强塑积汽车用钢。
其中，步骤(3)的罩式炉退火及步骤(5)的冷轧后第一次退火均为ART退火，退火温度在Ac1和Ac3温度之间。步骤(5)的冷轧后第一次退火是为了在通过ART退火，将冷轧后的钢板的微观组织由马氏体转变为奥氏体加铁素体，为后续工艺做准备。
尤其是，本技术方案中的步骤(6)中的冷轧后第二次退火的退火温度较高(接近Ac3温度的两相区内或单相奥氏体区内)，而退火时间则较短。其目 的和原理如下：步骤(5)的冷轧后第一次退火后所获得的钢板微观组织为铁素体+奥氏体，且奥氏体组织内Mn含量高，稳定性好。此时，将钢板加热到较高温度时，原始钢板内的铁素体组织会转别为新的奥氏体相。这部分新生成的奥氏体相含Mn量相对较低，且由于Mn的扩散速度较慢，在短时间的退火过程中Mn不能充分扩散，因此高温下组织内会形成两种成分的奥氏体，即富Mn和贫Mn的奥氏体。冷却到室温后，贫Mn的奥氏体会转变为马氏体，富Mn的奥氏体稳定存在。通过这种方式就可以得到大量的马氏体和稳定性高的奥氏体。
因此，当步骤(6)的冷轧后第二次退火的退火温度位于两相区时，控制退火温度和退火时间可得到马氏体+奥氏体+少量铁素体的微观组织；当步骤(6)的冷轧后第二次退火的退火温度在单相奥氏体区时，控制退火温度和退火时间可获得的微观组织为马氏体+奥氏体。
鉴于此，在本发明所述的技术方案中，步骤(6)的退火温度限定为750～850℃，退火时间控制在1～10min。当退火温度大于850℃或退火时间大于10min，会导致奥氏体稳定性变差，室温下奥氏体的相比例较低，导致钢的强塑积小于30GPa％；然而，当退火温度小于750℃或退火时间小于1min，会导致退火过程中，铁素体向奥氏体转变量较少，冷却至室温后仍有大量的铁素体存在，此时，尽管钢的延伸率和强塑积可以较高，但是钢的强度无法满足1500MPa。
步骤(7)的回火的目的是为了去除马氏体形成时产生的内应力，不进行回火会使得所获得的钢板较脆，延伸率较低。
进一步地，本发明所述的1500MPa级高强塑积汽车用钢的制造方法中，在所述步骤(2)中，将铸坯加热到1100～1260℃后控制轧制，开轧温度为950～1150℃，终轧温度为750～900℃，卷取温度为500～850℃，卷取后冷却到室温，获得全马氏体组织。
进一步地，在本发明所述的1500MPa级高强塑积汽车用钢的制造方法中，所述步骤(4)中，冷轧压下量不小于40％。
进一步地，在本发明所述的1500MPa级高强塑积汽车用钢的制造方法中，在所述步骤(3)和(4)之间还具有酸洗步骤。用以去除热轧过程中产生的氧化铁皮。
本发明所述的1500MPa级高强塑积汽车用钢的抗拉强度可达到1500MPa以上，强塑积可达到30GPa％以上。
本发明所述的1500MPa级高强塑积汽车用钢的制造方法同样具有上述优点和有益效果。除此以外，所述的制造方法通过合理的化学成分设计及控制退火工艺，从而优化了工艺流程，改善了钢的性能，从而获得符合需求的高强度高强塑积的汽车用钢，此外节省了制造成本。
附图说明
图1为本发明所述的1500MPa级高强塑积汽车用钢的制造方法的工艺曲线示意图。
具体实施方式
下面将结合附图说明和具体的实施例对本发明所述的1500MPa级高强塑积汽车用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1-8和对比例1-4
上述实施例1-8的1500MPa级高强塑积汽车用钢和对比例1-4的钢板采用下述步骤制得：
(1)冶炼和铸造：采用转炉冶炼，控制各化学元素的质量百分比如表1所示。
(2)热轧：将铸坯加热到1100～1260℃后控制轧制，开轧温度为950～1150℃，终轧温度为750～900℃，卷取温度为500～850℃，卷取后冷却到室温，获得全马氏体组织。
(3)罩式炉退火，退火温度为600～700℃，退火时间为1～48h。
(4)冷轧：冷轧压下量不小于40％。
(5)冷轧后第一次退火：退火温度在Ac1和Ac3温度之间，退火时间大于5min。
(6)冷轧后第二次退火：退火温度为750～850℃，退火时间为1～10min。需要说明的是，为了显示本案限定的冷轧后第二次退火工艺参数对本案实施效果的影响，对比例1-3采用的退火温度是不在本案限定的范围内的，其中对比 例1的冷轧第二次退火温度为720℃，对比例2的冷轧第二次退火时间为15min，对比例3的冷轧第二次退火温度为760℃。
(7)回火：回火温度为200～300℃，回火时间不小于3min。
此外，需要说明的是，步骤(2)中热轧钢板的厚度不大于8mm。步骤(4)中冷轧钢板的厚度不大于2.5mm。
另外，需要说明的是，在其他实施方式中，步骤(1)中，还可以采用电炉或感应炉进行冶炼。
此外，需要说明的是，在其他实施方式中，优选地，所述步骤(3)和(4)之间还具有酸洗步骤。
表1列出了实施例1-8和对比例1-4中各化学元素的质量百分配比。
表1.(wt％，余量为Fe和除了杂质元素S、P和N之外的其他杂质元素)
Figure PCTCN2017094247-appb-000001
表2列出了实施例1-8和对比例1-4的制造方法中的具体工艺参数。
表2
Figure PCTCN2017094247-appb-000002
Figure PCTCN2017094247-appb-000003
需要说明的是，表2中的组分序号是指各实施例与对比例采用表1所对应的组分序号。
对上述实施例1-8的1500MPa级高强塑积汽车用钢和对比例1-4的钢板取样，进行各项性能测试，将试验测得到的相关性能参数列于表3中。
表3列出了实施例1-8的1500MPa级高强塑积汽车用钢和对比例1-4的钢板的性能参数。强塑积为抗拉强度与延伸率的乘积。
表3
Figure PCTCN2017094247-appb-000004
Figure PCTCN2017094247-appb-000005
从表3中可以看出，本案各实施例的1500MPa级高强塑积汽车用钢，其抗拉强度＞1500Mpa，强塑积＞30GPa％，说明各实施例的汽车用钢具备较高的强度和良好的拉伸延展性。
结合表1和表3可知，对比例4中锰的质量百分比低于7.5％，其强塑积未达到30GPa％，延伸率较低。这是由于对比例4中锰的质量百分比较低，因此在冷轧第二次退火过程中产生的奥氏体的相比例和稳定性不足，致使其延伸率较低，强塑积也较低。
结合表2和表3可知，对比例1中的冷轧第二次退火温度低于750℃，因此，在冷轧第二次退火过程中铁素体向奥氏体转变量较少，冷却至室温后仍有大量的铁素体存在。由此，对比例1的钢板的延伸率大于30％，强塑积大于30GPa％，但其抗拉强度低于1500MPa。
继续结合表2和表3可知，对比例2中的冷轧第二次退火时间大于10min，对比例3中的冷轧第二次退火温度高于850℃，因此，导致奥氏体稳定性变差，室温下奥氏体的相比例较低，对比例2和对比例3的钢板的强塑积均小于30GPa％。
图1显示了本发明实施例1的1500MPa级高强塑积汽车用钢的制造方法的工艺曲线示意。
从图1中可以看出，本技术方案涉及的制造工艺在热轧1后进行一次退火，即罩式炉退火2，然后进行冷轧3，在冷轧后，进行二次退火，即冷轧后的第一次退火4，然后又进行三次退火，即冷轧后的第二次退火5，最后进行回火6。图1中的横轴表示时间，纵轴表示温度，因此图1曲线示意性地表示了温度随时间变化的情况。从图1中可看出，罩式炉退火2和冷轧后第一次退火4采用的是普通ART退火，而冷轧后第二次退火5则采用了相较于普通ART退火更高的退火温度和更短的退火时间，从而获得了本技术方案想要获得的微观组织，即大量的马氏体组织和较多的奥氏体组织。
需要注意的是，以上列举的仅为本发明的具体实施例，显然本发明不限于以上实施例，随之有着许多的类似变化。本领域的技术人员如果从本发明公开的内容直接导出或联想到的所有变形，均应属于本发明的保护范围。

Claims (9)

  1. 一种1500MPa级高强塑积汽车用钢，其特征在于，其化学元素质量百分比为：
    C：0.1％～0.3％，Si：0.1％～2.0％，Mn：7.5％～12％，Al：0.01％～2.0％，；余量为铁和其他不可避免的杂质；
    所述1500MPa级高强塑积汽车用钢的微观组织为奥氏体+马氏体+铁素体或奥氏体+马氏体。
  2. 如权利要求1所述的1500MPa级高强塑积汽车用钢，其特征在于，其化学元素还具有Nb：0.01～0.07％，Ti：0.02～0.15％，V：0.05～0.20％，Cr：0.15～0.50％，Mo：0.10～0.50％的至少其中之一。
  3. 如权利要求1或2所述的1500MPa级高强塑积汽车用钢，其特征在于，当其微观组织为奥氏体+马氏体+铁素体时，奥氏体的相比例为20％～40％，马氏体的相比例为50％～70％。
  4. 如权利要求1或2所述的1500MPa级高强塑积汽车用钢，其特征在于，当其微观组织为奥氏体+马氏体时，奥氏体的相比例为20％～50％。
  5. 如权利要求1所述的1500MPa级高强塑积汽车用钢，其特征在于，其强塑积不小于30GPa％。
  6. 如权利要求1-5中任意一项所述的1500MPa级高强塑积汽车用钢的制造方法，其依次包括步骤：
    (1)冶炼和铸造；
    (2)热轧；
    (3)罩式炉退火，退火温度为600～700℃，退火时间为1～48h；
    (4)冷轧；
    (5)冷轧后第一次退火：退火温度在Ac1和Ac3温度之间，退火时间大于5min；
    (6)冷轧后第二次退火：退火温度为750～850oC，退火时间为1～10min；
    (7)回火：回火温度为200～300oC，回火时间不小于3min。
  7. 如权利要求6所述的1500MPa级高强塑积汽车用钢的制造方法，其特征在 于，在所述步骤(2)中，将铸坯加热到1100～1260℃后控制轧制，开轧温度为950～1150℃，终轧温度为750～900℃，卷取温度为500～850℃，卷取后冷却到室温，获得全马氏体组织。
  8. 如权利要求6所述的1500MPa级高强塑积汽车用钢的制造方法，其特征在于，所述步骤(4)中，冷轧压下量不小于40％。
  9. 如权利要求6所述的1500MPa级高强塑积汽车用钢的制造方法，其特征在于，在所述步骤(3)和(4)之间还具有酸洗步骤。
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