硼钢B1500HS的热冲压关键参数测试及其淬火性能研究
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摘要
节能、环保、安全、舒适,是当今汽车技术发展的总趋势,汽车轻量化技术是各大汽车公司实现汽车低油耗、少废气排放的主要措施之一。为了使汽车轻量化后仍满足其碰撞安全要求,各大汽车公司在优化汽车框架结构的同时,把工作重点转向了新材料及新工艺的应用方面,尤其在车身结构方面,通过对先进高强度钢和超高强度钢的研究和使用,既提高了汽车的碰撞安全性能,同时也实现了汽车轻量化的要求。但是,随着钢材强度的提高,其成形性能却大大降低,在成形过程中容易产生破裂、起皱、尺寸难以控制和形状不良等问题。
高强度硼钢热冲压工艺是近几年发展起来的一种新型的板料成形技术。在热冲压工艺中,硼钢板料在加热炉中被加热到奥氏体化温度,并保温5分钟左右至板料完全奥氏体化,然后转移到配有冷却系统的冲压模具中,进行热成形并淬火,使成形件获得完全的马氏体组织。与传统的冷冲压成形相比,板料的热冲压成形具有以下优点：变形抗力小、塑性好、成形极限高；能够生产具有复杂几何形状的工件,成形件具有良好的尺寸精度及较高的强度；配以合适的热处理方式,可使板料发挥其最佳的性能。
高强度硼钢热冲压工艺是硼钢板料在高温下进行的复杂成形工艺,也是形变强化和相变强化相结合的一种综合强化工艺。本文针对热冲压工艺及热冲压材料特点,对硼钢材料的TTT曲线、热物性参数、界面换热系数、本构关系、淬火性能及热冲压有限元数值模拟技术进行了系统研究。
TTT曲线能比较直观地反映过冷奥氏体在等温转变过程中各相转变量与时间的关系,是热处理过程中判断组织转变、计算组织转变量的主要理论依据,也可为热冲压工艺参数的制订提供理论指导。针对硼钢B1500HS,设计了TTT曲线的测试方案,用DIL805A膨胀相变仪测试了B1500HS的相变点Ac1、Ac3、Ar3、Arl、Ms和Mf。在不同保温温度下,测试得到了系列相变膨胀曲线,并得到了各温度下的相变开始点和终止点,利用光学显微镜观察试样的组织状态,结合显微硬度判定在相应温度下得到的组织类型。根据测试和计算结果,绘制了B1500HS的TTT曲线。
热冲压过程的数值模拟需要准确可靠的材料热物性参数,硼钢处于奥氏体、铁素体+珠光体、贝氏体和马氏体各组织状态时的热物性参数更是热冲压数值模拟时急需的数据。目前,关于硼钢处于以上各组织状态时热物性参数还不完善。针对热冲压硼钢B1500HS,根据热膨胀系数、密度、等压比热容、导热系数等热物性参数的特点,设计了系列实验方案,分别制备了硼钢处于奥氏体、铁素体+珠光体、贝氏体和马氏体各组织状态时的试样并进行了实验。根据实验的测试结果,对热物性参数的数据进行了回归分析,建立了热膨胀系数、密度、等压比热容、导热系数、弹性模量等热物性参数与温度之间的函数关系式。
利用数值模拟方法研究硼钢热冲压工艺时,除了需要准确的材料热物性参数之外,还需要硼钢与模具之间的界面换热系数。界面换热系数是一个极为关键的边界参数,其准确程度直接影响到热冲压硼钢的温度场、应力／应变场、组织场(相变量)及力学性能的求解精度。根据硼钢热冲压工艺的特点,设计了相应的测试工装,建立了基于USB接口的温度高速采集系统,对课题组开发的淬火过程换热系数反传热求解系统进行了改进,并验证了反传热求解系统的精度。根据测试的温度曲线,利用有限元数值模拟方法和反传热求解系统,研究了冷却水与模具之间的界面换热系数,结果显示界面换热系数随模具表面温度的升高而降低,而且呈非线性变化；研究了硼钢与模具之间的界面温度及界面压力对界面换热系数的影响,结果显示试样及工装接触界面处的温度对界面换热系数基本无影响,而界面压力对界面换热系数影响较大；研究了界面压力与界面换热系数的关系,通过对冷却工装与硼钢试样界面施加1MPa、10MPa、20MPa及40MPa的压力,采集了温度冷却曲线,计算结果显示界面换热系数与界面压力之间近似成线性关系,随着界面压力的升高,界面换热系数也逐渐增大。
硼钢的高温本构方程是热冲压数值模拟中不可缺少的热力学模型,它反映了流动应力、应变、应变速率以及温度之间的关系。在硼钢热冲压过程中,热成形阶段的坯料组织为奥氏体组织；在冷却阶段,由于成形件的各部位与模具之间的传热条件不同,多数奥氏体转变为马氏体,另外,还可能有部分奥氏体转变为铁素体+珠光体或贝氏体。为了研究热冲压硼钢B1500HS高温时奥氏体组织的流变力学行为,采用Gleeble1500D热模拟试验机制备奥氏体试样,在600℃-900℃温度区间,分别以0.01s-1、0.1s-1、1.0s-1、10s-1的应变速率对硼钢B1500HS奥氏体组织试样进行等温单向拉伸试验,计算得到了各个测试条件下的真实应力-应变曲线。采用包含变形激活能和变形温度的双曲正弦形式修正的Arrhenius关系来描述硼钢奥氏体组织的热激活变形行为。通过对实验数据进行拟合回归分析,获得了以应变量、应变速率与变形温度表示的奥氏体组织的流变应力关系式。同理,根据测试的硼钢TTT曲线,设计了铁素体+珠光体、贝氏体和马氏体组织的应力-应变曲线测试方案,利用Gleeble1500D试验机进行了试样的制备及各条件下应力应变数据的测试,研究了应变量、加热温度、应变速率等因素对材料流动应力的影响规律。根据铁素体+珠光体、贝氏体和马氏体组织在室温和中高温时的应力-应变曲线,构建了一种新型的本构关系式,得到了铁素体+珠光体、贝氏体和马氏体组织的本构关系式。利用得到的本构关系式分别对奥氏体、铁素体+珠光体、贝氏体和马氏体组织的流变应力进行了计算,并将计算数据与实验数据进行了对比,结果表明：计算数据与实验数据吻合得较好,所构建的奥氏体、铁素体+珠光体、贝氏体和马氏体组织的本构方程可较好地反映硼钢热冲压过程的宏观力学行为。
针对热冲压工艺的特点,研究了淬火介质、淬火工艺参数及形变历史等热冲压关键参数对硼钢B1500HS性能的影响。利用四种冷却介质(水、空气、内置冷却水道的45钢块和铬锆铜块)研究了硼钢的淬火性能,研究结果表明：除空气冷却外,其余淬火方式对硼钢试样的拉伸强度和硬度的影响不大；B1500HS钢板具有良好的可淬性,在钢模中淬火可达到较高的硬度和抗拉强度。研究了加热温度和保温时间对试样微观组织、硬度以及抗拉强度的影响规律,实验结果表明：在850℃-910℃区间保温5min后进行钢模淬火,试样获得板条状马氏体组织,具有较高的淬火硬度、抗拉强度和延伸率,其中,奥氏体温度为910℃时淬火性能最佳。研究了硼钢板形变历史(应变速率、应变量、冷却速度)对硼钢板微观组织、显微硬度及连续冷却曲线的影响,研究结果表明：马氏体相变开始点对应变速率不敏感,但对真实应变较敏感,真实应变越大,马氏体相变开始点越低；工件冷却速度在临界值以上时,随着冷却速度的提高,试样硬度相应增大；在同等冷却速度下,随着应变量的增加,试样硬度值降低,CCT曲线有向左(高冷却速度方向)偏移的现象。
为了研究奥氏体化温度和保温时间对热冲压硼钢B1500HS淬火硬度、抗拉强度和延伸率的影响规律,以奥氏体化温度和保温时间为设计因子进行了二因子五水平的实验设计,根据实验设计的方案进行了B1500HS试样的淬火实验,测试了淬火后试样的硬度、抗拉强度和延伸率。利用三次响应曲面对实验结果进行了回归分析,得到了淬火硬度、抗拉强度和延伸率的响应曲面模型。根据响应曲面模型,对奥氏体化温度和保温时间进行了优化,得到了最佳淬火工艺参数。
对课题组开发的淬火工艺有限元模拟软件进行了改善,将其作为子程序加入到ABAQUS软件包,实现了温度-组织-应力/应变的耦合分析。通过用户子程序将本文测试得到的TTT曲线、热物性参数、界面换热系数等数据加入到软件包中。选择典型形状零件,利用实验和数值模拟方法研究热冲压温度对硼钢热冲压件性能和组织状态的影响。将测试的实验数据和数值模拟数据进行了对比,结果表明：坯料的加热及保温温度对硼钢热冲压件的抗拉强度、微观组织状态影响较大；随着坯料加热及保温温度的升高,板条状马氏体变得越来越粗大；同一热冲压件,其凸缘、侧壁和底部的微观组织状态差别不明显；坯料的加热及保温温度对硼钢表面硬度的影响不明显。热成形实验的结果与有限元数值模拟的结果基本吻合,表明了本文中测试的材料TTT曲线、热物性参数、界面换热系数等数据可以较好地保证有限元数值模拟结果的准确性和可靠性。本文测试的热冲压参数可以用于硼钢热冲压的有限元数值模拟,并可为热冲压工艺的编制提供技术指导。
Energy saving, emission reduction, safety and comfort are the main aims of automotive industry. In order to satisfy the requirement of crash safety and the reduction of car weight, the automotive industry has been putting emphasis upon the application of new materials besides optimizing the frameworks of cars. By using advanced high-strength steels (AHSS) and ultra high-strength steels (UHSS), the car's safety can be improved, and its weight will be reduced remarkably. Whereas, with tensile strength enhancement of AHSS and UHSS, the formability of steel reduces remarkably, and some defects (such as cracking, corrugation, spring-back) easily appear in sheet cold forming.
In order to improve the forming performance of UHSS sheets, the hot stamping for the quenchable boron steel sheet was presented and developed in recent years. At the beginning of the process, the blank is put into a furnace, heated up to an austenitization temperature, austenitized for about five minutes to get a homogeneous austenitic microstructure, then transferred quickly to a cooled die for forming and quenching simultaneously, and finally receives a full martensitic structure with an increase in hardness and strength. During the process, the material undergoes three physical changes in microstructure, ferrite+pearlite in blank, austenite in high temperature, and martensite after quenching. By comparison with cold forming proess, the hot stamping process has advantage in the forming of AHSS and UHSS, such as less deformation resistance, good ductility for the blank in high temperature, good formability in complex shapes, good dimensions, little spring-back and high strength.
Hot stamping of quenchenable boron steels is a new and complex forming technology combining metal hot forming and quenching process, the key technology is to control the hot plastic forming and quenching with pressure, essentially, the thermal and thermo-mechanical properties like the thermal physical parameters, heat transfer coefficient, the flow curves etc. and the quenching properties of the material are the main factors. So the study of the relative parameters is important for hot stamping technology, especially for a reliable numerical process design by FEM. At present, the research of hot stamping technology is still in primary stage, the thermal property data of materials are needed to be perfect. Thereby it is necessary and important to study the thermal property parameters of boron steel. Then1.6mm sheet of UHSS B1500HS, which is commonly used and quenchenable, is used for research in this paper.
Time Temperature Transformation (TTT) curve and thermo-physical parameters of boron steel B1500HS are obtained using the expansion principle and relative models of thermo-physical parameters. TTT curve could obviously describe the phase transformation history of undercooled austenite at isothermal temperatures. The time-temperature characteristics are useful to the practical production and FEM of hot stamping. In order to gain TTT curves, the phase-transformation temperatures of boron steel B1500HS are measured by DIL805A dilatometer. According to the expansion curves of phase-transformation at different isothermal temperatures, the start and finish temperatures of phase-transformation are calculated by using the expansion method and the tangential method. The types of metallographic phases after phase-transformation are judged by the optical microscope and micro hardness, TTT curves are drawn in terms of the results of testing and calculating. For the next, the models of expansion coefficient, density, specific heat, conductivity and modulus of elasticity are built, and the relative specimens of austenitic, ferriticpearlitic, bainitic and martensitic structures are made and measured. As a result of the regression analysis to the relative testing data, the functions based on temperature are respectively obtained for the relative thermo-physical parameters of each phase specimen.
A method to research the relative surface heat transfer coefficient (SHTC) in hot stamping process of B1500HS is presented. Due to the characteristics of hot stamping process, the test device to get the temperature data of specimens is designed, the temperature recording system based on the USB port is used, and the inverse heat conduction calculation software developed by research team is improved according to the TTT curves and thermo-physical parameters of B1500HS measured, and used for the relevant SHTC calculation according to the recorded temperature data. The obtained results show, the SHTC between cooling water and cooled die is nonlinear, and it decreases with the die temperature increases, the contact surface temperatures of the specimen and cooled die have little effect but the contact pressure has much effect on the SHTC between the specimen and cooled die. With pressures of1MPa,10MPa,20MPa and40MPa on the cooling system, the result shows that SHTC is approximately linear with relevant interfacial pressure. For advantageous application in FEM, a simplified mathematical model is built using the linear regression method.
Flow behaviors of B1500HS are investigated and relative thermodynamics constitutive equations are built based on phase structures, and a new model is presented to describe respectively the flow behaviors of the ferriticpearlitic, bainitic and martensitic structures at lower and middle temperatures. Constitutive equations of material describe the relationships among strain, strain rate and deformation temperature, which are important for building a correct FEM model to improve strain-stress calculation accuracy. In practical manufacture, the phases of austenite, ferrite and pearlite, bainite, martensite of the boron steel could appear due to different cooling rate during hot stamping process. Depending on the TTT curves of B1500HS, the relative phase specimens are made and isothermal tests are performed with strain rate0.01s-1,0.1s-1,1.0s-1and10s-1by Gleeble1500D thermo-mechanical simulator, and the stress-strain curves at the relative deformation temperatures are gained. The modified Arrhenius model, which is a hyperbolic sine function including the deformation activation energy and deformation temperature, is used to describe the hot deformation activation character in austenitic micro structure in the higher temperature range, and a new model is built to describe the ferriticpearlitic, bainitic and martensitic structures in the lower and middle temperature ranges. Simultaneously, the constitutive relationships are confirmed by the comparison of the computational data and experimental data being consistent, the results show the constitutive equations are reasonable.
The quenching property of boron steel B1500HS is researched by combining the effects of quenchants, austenitizing temperature, holding time, strain, strain rate or cooling rate on the Ms and Mf microstructure, hardness, tensile strength behaviours of boron steel, and the response surface methodology is used to optimize the key process parameters design. In the experiment, the quenchants are water in room temperature, room air,45steel die and CuCrZr die both with water cooled in channels, The results show that B1500HS has a good quenching performance, the tensile strength and hardness of the samples quenched in a water-cooled steel die are similar to those of the samples queneched with water involved in. The research about the effect of austenitizing temperature and holding time on the mechanical properties of boron steel B1500HS is performed using the water-cooled steel die. The experimental results show that, while austenitizing temperatures is in the range of850-910℃and holding time is5min, the quenched specimens have better lath martensitic structures and quenching properties of higher tensile strength, hardness and ductility. The research is performed about the effects of deformation history (strain, strain rate, cooling rate) on the microstructures, hardness and CCT curves of the austenized specimens. The results show that Ms is insensitive to the strain rate, and sensitive to the true strain, Ms decreases with the strain increasing; the hardness of quenched specimen increases with the cooling rate (above the critical) increasing; at the same cooling rate, when strain increases the hardness decreases due to soft microstuctures transformed, thereby CCT curves may be shifted to the left side with the strain increasing..
In order to research the effect of austenitizing temperature and holding time on the hardness, tensile strength and ductility of boron steel B1500HS, the experimental project of two factors and five levels is designed using the austenitizing temperature and holding time as the design factors by design Expert7.0. The experiments are performed according to the design result. The cubic response surface models of hardness, tensile strength and ductility are obtained using the regression analysis prediction method and the results of experiment. The austenitizing temperature and holding time are optimized according to the cubic response surface models, and the optimum quenching parameters are gained.
The quenching process FEM simulation system developed by subject team is improved according to TTT curves, the thermal-physical parameters, surface heat transfer coefficient etc. measured in this paper, then embedded in Abaqus software to implement the coupling calculation of temperature-strain/stress-phase transformation during hot stamping process of B1500HS. The hot stamping of typical part is performed to research the effect of blank temperature on the property and microstructure of the final part by the both ways of experiment and FEM simulation, and the die is designed without a blank holder. The experimental and computational results show that austenizing temperature has an obviously effect on the tensile strength and microstructure of quenched part, lath martensites are getting larger with blank austenizing temperature increasing. The microstructures in the flange, side and bottom regions of the quenched part are almost the same for one part. The austenizing temperature has a little effect on the hardness of quenched part. The computational results are almost the same with the relevant experimental results, which shows the TTT curves, thermal-physical parameters, surface heat transfer coefficient etc. obtained in this paper can make the FEM model correct and reliable, the obtained parameters in this paper are useful in the hot stamping process of FEM simulation or practical manufacture for B1500HS sheet.
高强度硼钢热冲压工艺是近几年发展起来的一种新型的板料成形技术。在热冲压工艺中,硼钢板料在加热炉中被加热到奥氏体化温度,并保温5分钟左右至板料完全奥氏体化,然后转移到配有冷却系统的冲压模具中,进行热成形并淬火,使成形件获得完全的马氏体组织。与传统的冷冲压成形相比,板料的热冲压成形具有以下优点：变形抗力小、塑性好、成形极限高；能够生产具有复杂几何形状的工件,成形件具有良好的尺寸精度及较高的强度；配以合适的热处理方式,可使板料发挥其最佳的性能。
高强度硼钢热冲压工艺是硼钢板料在高温下进行的复杂成形工艺,也是形变强化和相变强化相结合的一种综合强化工艺。本文针对热冲压工艺及热冲压材料特点,对硼钢材料的TTT曲线、热物性参数、界面换热系数、本构关系、淬火性能及热冲压有限元数值模拟技术进行了系统研究。
TTT曲线能比较直观地反映过冷奥氏体在等温转变过程中各相转变量与时间的关系,是热处理过程中判断组织转变、计算组织转变量的主要理论依据,也可为热冲压工艺参数的制订提供理论指导。针对硼钢B1500HS,设计了TTT曲线的测试方案,用DIL805A膨胀相变仪测试了B1500HS的相变点Ac1、Ac3、Ar3、Arl、Ms和Mf。在不同保温温度下,测试得到了系列相变膨胀曲线,并得到了各温度下的相变开始点和终止点,利用光学显微镜观察试样的组织状态,结合显微硬度判定在相应温度下得到的组织类型。根据测试和计算结果,绘制了B1500HS的TTT曲线。
热冲压过程的数值模拟需要准确可靠的材料热物性参数,硼钢处于奥氏体、铁素体+珠光体、贝氏体和马氏体各组织状态时的热物性参数更是热冲压数值模拟时急需的数据。目前,关于硼钢处于以上各组织状态时热物性参数还不完善。针对热冲压硼钢B1500HS,根据热膨胀系数、密度、等压比热容、导热系数等热物性参数的特点,设计了系列实验方案,分别制备了硼钢处于奥氏体、铁素体+珠光体、贝氏体和马氏体各组织状态时的试样并进行了实验。根据实验的测试结果,对热物性参数的数据进行了回归分析,建立了热膨胀系数、密度、等压比热容、导热系数、弹性模量等热物性参数与温度之间的函数关系式。
利用数值模拟方法研究硼钢热冲压工艺时,除了需要准确的材料热物性参数之外,还需要硼钢与模具之间的界面换热系数。界面换热系数是一个极为关键的边界参数,其准确程度直接影响到热冲压硼钢的温度场、应力／应变场、组织场(相变量)及力学性能的求解精度。根据硼钢热冲压工艺的特点,设计了相应的测试工装,建立了基于USB接口的温度高速采集系统,对课题组开发的淬火过程换热系数反传热求解系统进行了改进,并验证了反传热求解系统的精度。根据测试的温度曲线,利用有限元数值模拟方法和反传热求解系统,研究了冷却水与模具之间的界面换热系数,结果显示界面换热系数随模具表面温度的升高而降低,而且呈非线性变化；研究了硼钢与模具之间的界面温度及界面压力对界面换热系数的影响,结果显示试样及工装接触界面处的温度对界面换热系数基本无影响,而界面压力对界面换热系数影响较大；研究了界面压力与界面换热系数的关系,通过对冷却工装与硼钢试样界面施加1MPa、10MPa、20MPa及40MPa的压力,采集了温度冷却曲线,计算结果显示界面换热系数与界面压力之间近似成线性关系,随着界面压力的升高,界面换热系数也逐渐增大。
硼钢的高温本构方程是热冲压数值模拟中不可缺少的热力学模型,它反映了流动应力、应变、应变速率以及温度之间的关系。在硼钢热冲压过程中,热成形阶段的坯料组织为奥氏体组织；在冷却阶段,由于成形件的各部位与模具之间的传热条件不同,多数奥氏体转变为马氏体,另外,还可能有部分奥氏体转变为铁素体+珠光体或贝氏体。为了研究热冲压硼钢B1500HS高温时奥氏体组织的流变力学行为,采用Gleeble1500D热模拟试验机制备奥氏体试样,在600℃-900℃温度区间,分别以0.01s-1、0.1s-1、1.0s-1、10s-1的应变速率对硼钢B1500HS奥氏体组织试样进行等温单向拉伸试验,计算得到了各个测试条件下的真实应力-应变曲线。采用包含变形激活能和变形温度的双曲正弦形式修正的Arrhenius关系来描述硼钢奥氏体组织的热激活变形行为。通过对实验数据进行拟合回归分析,获得了以应变量、应变速率与变形温度表示的奥氏体组织的流变应力关系式。同理,根据测试的硼钢TTT曲线,设计了铁素体+珠光体、贝氏体和马氏体组织的应力-应变曲线测试方案,利用Gleeble1500D试验机进行了试样的制备及各条件下应力应变数据的测试,研究了应变量、加热温度、应变速率等因素对材料流动应力的影响规律。根据铁素体+珠光体、贝氏体和马氏体组织在室温和中高温时的应力-应变曲线,构建了一种新型的本构关系式,得到了铁素体+珠光体、贝氏体和马氏体组织的本构关系式。利用得到的本构关系式分别对奥氏体、铁素体+珠光体、贝氏体和马氏体组织的流变应力进行了计算,并将计算数据与实验数据进行了对比,结果表明：计算数据与实验数据吻合得较好,所构建的奥氏体、铁素体+珠光体、贝氏体和马氏体组织的本构方程可较好地反映硼钢热冲压过程的宏观力学行为。
针对热冲压工艺的特点,研究了淬火介质、淬火工艺参数及形变历史等热冲压关键参数对硼钢B1500HS性能的影响。利用四种冷却介质(水、空气、内置冷却水道的45钢块和铬锆铜块)研究了硼钢的淬火性能,研究结果表明：除空气冷却外,其余淬火方式对硼钢试样的拉伸强度和硬度的影响不大；B1500HS钢板具有良好的可淬性,在钢模中淬火可达到较高的硬度和抗拉强度。研究了加热温度和保温时间对试样微观组织、硬度以及抗拉强度的影响规律,实验结果表明：在850℃-910℃区间保温5min后进行钢模淬火,试样获得板条状马氏体组织,具有较高的淬火硬度、抗拉强度和延伸率,其中,奥氏体温度为910℃时淬火性能最佳。研究了硼钢板形变历史(应变速率、应变量、冷却速度)对硼钢板微观组织、显微硬度及连续冷却曲线的影响,研究结果表明：马氏体相变开始点对应变速率不敏感,但对真实应变较敏感,真实应变越大,马氏体相变开始点越低；工件冷却速度在临界值以上时,随着冷却速度的提高,试样硬度相应增大；在同等冷却速度下,随着应变量的增加,试样硬度值降低,CCT曲线有向左(高冷却速度方向)偏移的现象。
为了研究奥氏体化温度和保温时间对热冲压硼钢B1500HS淬火硬度、抗拉强度和延伸率的影响规律,以奥氏体化温度和保温时间为设计因子进行了二因子五水平的实验设计,根据实验设计的方案进行了B1500HS试样的淬火实验,测试了淬火后试样的硬度、抗拉强度和延伸率。利用三次响应曲面对实验结果进行了回归分析,得到了淬火硬度、抗拉强度和延伸率的响应曲面模型。根据响应曲面模型,对奥氏体化温度和保温时间进行了优化,得到了最佳淬火工艺参数。
对课题组开发的淬火工艺有限元模拟软件进行了改善,将其作为子程序加入到ABAQUS软件包,实现了温度-组织-应力/应变的耦合分析。通过用户子程序将本文测试得到的TTT曲线、热物性参数、界面换热系数等数据加入到软件包中。选择典型形状零件,利用实验和数值模拟方法研究热冲压温度对硼钢热冲压件性能和组织状态的影响。将测试的实验数据和数值模拟数据进行了对比,结果表明：坯料的加热及保温温度对硼钢热冲压件的抗拉强度、微观组织状态影响较大；随着坯料加热及保温温度的升高,板条状马氏体变得越来越粗大；同一热冲压件,其凸缘、侧壁和底部的微观组织状态差别不明显；坯料的加热及保温温度对硼钢表面硬度的影响不明显。热成形实验的结果与有限元数值模拟的结果基本吻合,表明了本文中测试的材料TTT曲线、热物性参数、界面换热系数等数据可以较好地保证有限元数值模拟结果的准确性和可靠性。本文测试的热冲压参数可以用于硼钢热冲压的有限元数值模拟,并可为热冲压工艺的编制提供技术指导。
Energy saving, emission reduction, safety and comfort are the main aims of automotive industry. In order to satisfy the requirement of crash safety and the reduction of car weight, the automotive industry has been putting emphasis upon the application of new materials besides optimizing the frameworks of cars. By using advanced high-strength steels (AHSS) and ultra high-strength steels (UHSS), the car's safety can be improved, and its weight will be reduced remarkably. Whereas, with tensile strength enhancement of AHSS and UHSS, the formability of steel reduces remarkably, and some defects (such as cracking, corrugation, spring-back) easily appear in sheet cold forming.
In order to improve the forming performance of UHSS sheets, the hot stamping for the quenchable boron steel sheet was presented and developed in recent years. At the beginning of the process, the blank is put into a furnace, heated up to an austenitization temperature, austenitized for about five minutes to get a homogeneous austenitic microstructure, then transferred quickly to a cooled die for forming and quenching simultaneously, and finally receives a full martensitic structure with an increase in hardness and strength. During the process, the material undergoes three physical changes in microstructure, ferrite+pearlite in blank, austenite in high temperature, and martensite after quenching. By comparison with cold forming proess, the hot stamping process has advantage in the forming of AHSS and UHSS, such as less deformation resistance, good ductility for the blank in high temperature, good formability in complex shapes, good dimensions, little spring-back and high strength.
Hot stamping of quenchenable boron steels is a new and complex forming technology combining metal hot forming and quenching process, the key technology is to control the hot plastic forming and quenching with pressure, essentially, the thermal and thermo-mechanical properties like the thermal physical parameters, heat transfer coefficient, the flow curves etc. and the quenching properties of the material are the main factors. So the study of the relative parameters is important for hot stamping technology, especially for a reliable numerical process design by FEM. At present, the research of hot stamping technology is still in primary stage, the thermal property data of materials are needed to be perfect. Thereby it is necessary and important to study the thermal property parameters of boron steel. Then1.6mm sheet of UHSS B1500HS, which is commonly used and quenchenable, is used for research in this paper.
Time Temperature Transformation (TTT) curve and thermo-physical parameters of boron steel B1500HS are obtained using the expansion principle and relative models of thermo-physical parameters. TTT curve could obviously describe the phase transformation history of undercooled austenite at isothermal temperatures. The time-temperature characteristics are useful to the practical production and FEM of hot stamping. In order to gain TTT curves, the phase-transformation temperatures of boron steel B1500HS are measured by DIL805A dilatometer. According to the expansion curves of phase-transformation at different isothermal temperatures, the start and finish temperatures of phase-transformation are calculated by using the expansion method and the tangential method. The types of metallographic phases after phase-transformation are judged by the optical microscope and micro hardness, TTT curves are drawn in terms of the results of testing and calculating. For the next, the models of expansion coefficient, density, specific heat, conductivity and modulus of elasticity are built, and the relative specimens of austenitic, ferriticpearlitic, bainitic and martensitic structures are made and measured. As a result of the regression analysis to the relative testing data, the functions based on temperature are respectively obtained for the relative thermo-physical parameters of each phase specimen.
A method to research the relative surface heat transfer coefficient (SHTC) in hot stamping process of B1500HS is presented. Due to the characteristics of hot stamping process, the test device to get the temperature data of specimens is designed, the temperature recording system based on the USB port is used, and the inverse heat conduction calculation software developed by research team is improved according to the TTT curves and thermo-physical parameters of B1500HS measured, and used for the relevant SHTC calculation according to the recorded temperature data. The obtained results show, the SHTC between cooling water and cooled die is nonlinear, and it decreases with the die temperature increases, the contact surface temperatures of the specimen and cooled die have little effect but the contact pressure has much effect on the SHTC between the specimen and cooled die. With pressures of1MPa,10MPa,20MPa and40MPa on the cooling system, the result shows that SHTC is approximately linear with relevant interfacial pressure. For advantageous application in FEM, a simplified mathematical model is built using the linear regression method.
Flow behaviors of B1500HS are investigated and relative thermodynamics constitutive equations are built based on phase structures, and a new model is presented to describe respectively the flow behaviors of the ferriticpearlitic, bainitic and martensitic structures at lower and middle temperatures. Constitutive equations of material describe the relationships among strain, strain rate and deformation temperature, which are important for building a correct FEM model to improve strain-stress calculation accuracy. In practical manufacture, the phases of austenite, ferrite and pearlite, bainite, martensite of the boron steel could appear due to different cooling rate during hot stamping process. Depending on the TTT curves of B1500HS, the relative phase specimens are made and isothermal tests are performed with strain rate0.01s-1,0.1s-1,1.0s-1and10s-1by Gleeble1500D thermo-mechanical simulator, and the stress-strain curves at the relative deformation temperatures are gained. The modified Arrhenius model, which is a hyperbolic sine function including the deformation activation energy and deformation temperature, is used to describe the hot deformation activation character in austenitic micro structure in the higher temperature range, and a new model is built to describe the ferriticpearlitic, bainitic and martensitic structures in the lower and middle temperature ranges. Simultaneously, the constitutive relationships are confirmed by the comparison of the computational data and experimental data being consistent, the results show the constitutive equations are reasonable.
The quenching property of boron steel B1500HS is researched by combining the effects of quenchants, austenitizing temperature, holding time, strain, strain rate or cooling rate on the Ms and Mf microstructure, hardness, tensile strength behaviours of boron steel, and the response surface methodology is used to optimize the key process parameters design. In the experiment, the quenchants are water in room temperature, room air,45steel die and CuCrZr die both with water cooled in channels, The results show that B1500HS has a good quenching performance, the tensile strength and hardness of the samples quenched in a water-cooled steel die are similar to those of the samples queneched with water involved in. The research about the effect of austenitizing temperature and holding time on the mechanical properties of boron steel B1500HS is performed using the water-cooled steel die. The experimental results show that, while austenitizing temperatures is in the range of850-910℃and holding time is5min, the quenched specimens have better lath martensitic structures and quenching properties of higher tensile strength, hardness and ductility. The research is performed about the effects of deformation history (strain, strain rate, cooling rate) on the microstructures, hardness and CCT curves of the austenized specimens. The results show that Ms is insensitive to the strain rate, and sensitive to the true strain, Ms decreases with the strain increasing; the hardness of quenched specimen increases with the cooling rate (above the critical) increasing; at the same cooling rate, when strain increases the hardness decreases due to soft microstuctures transformed, thereby CCT curves may be shifted to the left side with the strain increasing..
In order to research the effect of austenitizing temperature and holding time on the hardness, tensile strength and ductility of boron steel B1500HS, the experimental project of two factors and five levels is designed using the austenitizing temperature and holding time as the design factors by design Expert7.0. The experiments are performed according to the design result. The cubic response surface models of hardness, tensile strength and ductility are obtained using the regression analysis prediction method and the results of experiment. The austenitizing temperature and holding time are optimized according to the cubic response surface models, and the optimum quenching parameters are gained.
The quenching process FEM simulation system developed by subject team is improved according to TTT curves, the thermal-physical parameters, surface heat transfer coefficient etc. measured in this paper, then embedded in Abaqus software to implement the coupling calculation of temperature-strain/stress-phase transformation during hot stamping process of B1500HS. The hot stamping of typical part is performed to research the effect of blank temperature on the property and microstructure of the final part by the both ways of experiment and FEM simulation, and the die is designed without a blank holder. The experimental and computational results show that austenizing temperature has an obviously effect on the tensile strength and microstructure of quenched part, lath martensites are getting larger with blank austenizing temperature increasing. The microstructures in the flange, side and bottom regions of the quenched part are almost the same for one part. The austenizing temperature has a little effect on the hardness of quenched part. The computational results are almost the same with the relevant experimental results, which shows the TTT curves, thermal-physical parameters, surface heat transfer coefficient etc. obtained in this paper can make the FEM model correct and reliable, the obtained parameters in this paper are useful in the hot stamping process of FEM simulation or practical manufacture for B1500HS sheet.
引文
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