Delayed fracture of tensile strength 980 MPa grade steels for automotive applications

Three different types of tensile strength( TS) 980 MPa grade advanced high-strength steels used in automotive applications,namely,980 MS( martensite steel),980DP( dual phase) and 980QP( quenching and partitioning) steels were examined. The delayed fracture resistance of the steels was evaluated using a U-bend test,slowstrain rate test( SSRT) and a constant load tensile test. The results indicated that all the steels could pass the300h HCl solution immersion test and none of the U-bend specimens was fractured in the test. However,the steels exhibited different susceptibilities to delayed fracture under SSRT and the constant load tensile tests. 980 DP exhibited the highest resistance to delayed fracture among all the samples,while 980 MS was found to be the most susceptible to delayed fracture.     

Investigation on the Failure Mechanism of DP590-22MnB5 (Quenched) and DP590-DP590 Spot-Welding Joints

The failure mechanism of DP590-22MnB5(quenched) and DP590-DP590 spot-welding joints is studied through lap-shearing experiments,metallographic observation and three-dimensional finite element simulation.Both joints cracked on the DP590 steel,but the tensile shear strength of the DP590-22MnB5(quenched) joint is greater than that of the DP590-DP590 joint.A finite element model for the lap-shearing experiment is established according to the mechanical properties of DP590 and 22MnB5(quenched) steels and the metallographic analysis of welding spots.The simulation results show that the difference in the axis rotation of the two welding spots causes different distributions of stress and strain,which shifts shear loading condition to opening loading condition.Due to larger axis rotation angle of the DP590-DP590 joint,the stress concentration occurs at the middle of the nugget circumference,and it results in lower tensile shear strength of the DP590-DP590 joint.     

Comparative study of resistance spot welding performance between cold-rolled DP980 and Q&P980 steels

Advanced high strength steel (AHSS) has been widely used in the automobile industry.The resistance spot welding performance of DP980 and Q&P980 steels was studied through comparing the two steels’welding current range,tensile shear strength (TSS),cross tension strength (CTS),weld spots’microhardness,etc.The following conclusions were achieved:It is easy for both DP980 and Q&P980 steels to get a nugget size bigger than 4 mm,they all have welding current ranges exceeding 2 kA and high weld strength.     

Characteristics of Welding Crack Defects and Failure Mode in Resistance Spot Welding of DP780 Steel

The mechanical properties of welded joints in resistance spot welding of DP780 steel were tested,and three dif-ferent types of welding cracks in welded joints were investigated by optical microscopy,scanning electron microscopy and electron back-scattered diffraction.Finally,the failure mode of the welded joints in shear tensile test was dis-cussed.It is found the shear tensile strength of welded joints can be greatly improved by adding preheating current or tempering current.The surface crack in welded joint is intergranular fracture,while the inner crack in welded joint is transgranular fracture,and the surface crack on the edge of the electrode imprint can be improved by adding prehea-ting current or tempering current.The traditional failure mode criterion advised by American Welding Society is no longer suitable for DP780 spot welds and the critical nugget size suggested by Pouranvari is overestimated.     

硫含量和夹杂物控制对冷轧马氏体钢板冷弯性能的影响

冷轧马氏体钢主要用于汽车安全件,成形以冷弯或辊压为主,因此,马氏体钢的冷弯性能至关重要。以宝钢工业产线轧制的980 MPa级冷轧马氏体钢为研究对象,研究了不同S含量水平和Ca处理对冷轧马氏体钢弯曲性能的影响,分析了不同S含量水平下钢中的夹杂物数量和级别。通过控制S含量和采用Ca处理,可以明显降低马氏体钢中的夹杂物数量、尺寸,最终明显提高冷弯性能。宝钢开发的980 MPa级冷轧马氏体钢已经成功应用于乘用车安全部件的制造。     

Formability of High Strength Dual‐phase Steels

The application of ferritic‐martensitic dual‐phase (DP) steels has become an increasing trend in the automotive industry due to the possibility to achieve significant weight reduction and fuel efficiency with improved crash performance while keeping the manufacturing costs at affordable levels. In order to meet the different design requirements of individual auto‐body components, a wide variety of DP grades exhibiting different strength and ductility levels is currently industrially produced. Despite the numerous studies on the relationship between the mechanical properties and the microstructural characteristics of DP steels over the last decades, it is still a challenge to increase their formability at a constant strength level (or equivalently increasing the strength while maintaining a high ductility). One of the possibilities to increase strength is grain refinement. Ultrafine‐grained ferritic‐martensitic microstructures were produced by intercritical annealing of a cold‐rolled, pre‐processed dual‐phase steel. Ferrite mean grain sizes in the order of ~ 1.5 μm were obtained. The mechanical properties of these steels are studied, revealing the beneficial effect of grain refinement. Ultimate tensile strength above 800 MPa is achievable, while reaching remarkable high uniform and total elongations, which are only slightly affected by the martensite volume fraction. Moreover, the yield to tensile strength ratio can be adjusted between 0.4 and 0.5. Light and electron microscopy investigations, fracture profile and fracture surface analyses, hole expansion tests and additional ultramicrohardness measurements are used for the interpretation of the results and for the correlation of the mechanical properties and the formability characteristics with the microstructure of the steel.     

Fracture morphologies of a hot stamped steel and comparisons with several sheet metals

Hot stamping has been widely used in car industry to produce safety components.Most existing researches focused on the stamping and quenching process,but less on the mechanical properties of stamped parts.The fracture behaviors of hot stamped boron steel B1500 HS have been studied,and other four commonly used sheet metals with different strengths,including Q235,TRIP780,QP980 and MS1300,were also introduced for comparison.Both uniaxial tests and mechanical trimming tests were performed,and the fracture surfaces under different stress states were observed and discussed.The SEM observations showed that the fracture models are closely related to the stress states,i.e.,the tensile surfaces have ductile rupture characters while the trimming surfaces have brittle rupture characters.Compared with other steels,the quenched boron steel has smaller dimple size accompanied by shear planes in the tensile surface,and has smaller burnish zone in the trimming surface,and its cutting surface with‘S'like shape is also very different with others.Furthermore,two fitted empirical models were derived to describe the quantitative correlations between the average dimple diameter and the steel strength and between the percentage of burnish zone and the steel strength.     

Influence of strain rate on tensile properties and fracture behaviour of DP600 and DP780 dual-phase steels

The influences of the strain rate on the tensile properties and fracture behaviour of DP600 and DP780 advanced high-strength sheet steels have been studied. The variation of their mechanical properties depending on the strain rate have been researched by applying uniaxial tensile tests at three different strain rates (0.001, 0.01, 0.06?s^?1). The influences of strain rate on fracture behaviour have been investigated by displaying the fracture surfaces of the material. Strain rate increase has been determined to increase the yield strength, tensile strength, total elongation and hardening rate. The strain hardening coefficient has been found not to be significantly affected by the strain rate. It has been determined that, the fracture has occurred faster during necking while load-carrying capacity has increased with strain rate increase.     

Prediction of roll force in skin pass rolling using numerical and artificial neural network methods

A combination of finite element method and neural network methods was used for rapid prediction of the roll force during skin pass rolling of 980DP and 1180CP high strength steels. The FE based commercial package DEFOEM-2D was used to develop a mathematical model of the skin pass rolling operation. Numerical experiments were designed with different process parameters to produce training data for a neural network algorithm. The friction coefficient was considered as an input parameter in the neural network but it was optimised using an iterative method employing an equation that relates the friction coefficient to the rolling force. The load prediction method described in this paper is sufficiently rapid that it can be used in real-time as an adjustment tool for skin pass rolling mills with error within 10% (based on plant data from POSCO).     

DP590/DP780高强钢管液压成形的性能

为对生产进行指导,研究了DP590/DP780高强钢焊管在液压成形过程中的变形行为;使用场发射扫描电镜观察管材周向的横截面以确定基体的组织,通过VMHT30M显微硬度计确定管材的焊缝及热影响区的大小,以便研究液压成形破裂行为;采用液压成形试验机对两种管件进行液压成形研究。实验结果表明:管材在胀形过程中的破裂压力比理论计算公式得到的破裂压力大,破裂位置全部位于靠近焊缝及热影响区的母材区域;随着管径的增大和长径比的增大,管材的极限膨胀率呈现下降趋势;在自由胀形过程中,管材的焊缝区域基本上不发生减薄,最小壁厚位于管材的热影响区和基体的过渡区域,并且壁厚的减薄率在胀形最高点所在截面最大,越靠近管材夹持区,壁厚的减薄率越小。最终得到以下结论:管材液压成形实验是准确获得管材力学性能参数的途径;提高焊接质量有助于控制失效破裂位置;合理选择管材的长径比有利于管材性能的充分发挥;通过合理控制各处的减薄有利于降低液压成形件的破裂风险。      

Cold-rolling technologies of advanced high strength steels in Ansteel

Low - carbon becomes a high - frequency and fashionable word which gets the greatest concern in the world.Low - carbon refers to a minimal output of greenhouse gas emissions into the biosphere, specifically refers to the greenhouse gas carbon dioxide.To reduce energy consumption of automobile,more and more high strength steels are used by vehicle companies.To meet the request of vehicle companies, various high strength steels are developed in steel companies all over the world.Ansteel can provide,ultra-low carbon bake hardening(BH) steels,dual phase(DP) steels and transformation - induced plasticity (TRIP) steels with the grades of under 780 MPa,up to now.AHSS steels have much different composition, microstructure and strenght than conventional vehicle steels,so there are some trouble in producing in cold rolling mills,for example,difficult to join,poor thickness and flatness,accurate temperature and velocity control and so on.To reduce the opportunity of strip breakage,we have done many welding experiments and special research.Now,DP and TRIP steels can be continuously produced in Ansteel.To assure thickness and flatness of strips,we optimized the hot rolling parameter to get low deformation resistance,optimized rolling oil to get fit frictional coefficient and fix on the targat rolling curve.To get more accurate and repetitive results over the production shifts,the Mathematical Model(MM) is used in Ansteel.The MM analyses information transmitted by various sensors and transmitters,compares the collected data with the chosen parameters and adjusts the settings of the various pieces of equipment to hone on the parameter setpoints.At present,the key production technologies of AHSS were grasped by cold rolling mill Ansteel,and Ansteel is the first company to apply the TRIP with the grade of 780 MPa.     

双相钢激光焊管的焊缝组织演变及扩口性能研究

介绍了马氏体含量不同的双相钢(DP590和DP780)激光焊管的焊缝组织演变,研究了焊缝硬度分布特征及其对扩口性能的影响。结果表明:激光焊缝的熔合区存在明显硬化,组织为单一的马氏体,DP780钢热影响区出现了显著的软化,DP590钢无软化区存在,造成这一差别的原因在于两者马氏体含量不同;软化区的存在导致DP780钢激光焊管的扩口率急剧下降,仅为DP590钢的10%左右;建立了焊管扩口的数值模型,模拟结果与试验结果相吻合。     

Enhanced mechanical properties of dual-phase steel by repetitive intercritical annealing

The effect of repetitive intercritical annealing on the mechanical properties and work-hardening response of a typical C–Mn dual-phase (DP) steel was studied by consideration of different pre-intercritical annealing microstructures and martensite volume fractions. It was revealed that the DP steels produced from the initial martensitic microstructure have much better mechanical properties compared with those originated from the ferritic–pearlitic banded microstructure. The repetitive intercritical annealing of the initial martensitic microstructure was effective for the enhancement of strength–ductility balance, where at a comparable ductility; a twofold increase in the tensile strength was obtained. Finally, these results were discussed based on the Crussard–Jaoul work-hardening rate analysis.     

Microstructure based modelling of stress–strain relationship on dual phase steels

The deformation in microstructures of DP600, DP800 and DP1000 commercial advanced high strength steels have been researched through using the representative volume element method. For this purpose, deformation analyses have been carried out by transferring geometrical models and mechanical properties of phases of each material to the finite element software. Deformation relation between ferrite and martensite phases was determined. According to the loading direction, Shear bands have been observed to occur in the range of approximately 40°–45°. It has been understood that the failure mode is shear for DP600, DP800 and DP1000 steels. Crack propagation has been observed to occur in the ferrite phase trapped at the martensite grain boundary.     

热轧双相钢微观应力-应变模型

利用扫描电子显微镜和透射电子显微镜对热轧后双相钢的微观组织进行分析,用Image-Pro Plus软件测定双相钢微观组织中各独立相的体积分数.根据多相材料中间混合法则和Swift方程,建立热轧双相钢微观应力-应变模型,并用DP590和DP780钢单向拉伸曲线进行验证.结果表明,该应力-应变关系微观模型基本阐明热轧双相钢微观组织参数与宏观力学性能的内在联系,能够准确地描述材料的变形行为,同时很好地预测热轧双相钢宏观的拉伸曲线.      

鞍钢先进高强汽车用钢的研制开发

介绍了鞍钢先进高强汽车用钢开发研制情况,包括以980 MPa级DP钢、TRIP钢、TWIP钢和QP钢为代表的热轧、冷轧和热镀锌先进高强钢系列产品,以及鞍钢开发的先进高强钢热镀锌生产技术,并对鞍钢先进高强汽车钢未来的发展方向进行了展望。     

Influence of constant and variable restraining force on springback of TRIP700 and stainless steel grades

Newly developed high strength steels (HSS) like dual phase (DP) and transformation induced plasticity (TRIP) grades as well as austenitic grades like AISI 304 and AISI 301 show superior strength compared to the microalloyed grades with the same formability (e.g. HSLA340). However, due to the multiphase microstructure and the austenite martensite transformation during forming, a higher springback will appear. In this paper the influence of different process parameters of six high strength steels on springback after stamping is investigated. The material properties were determined with uniaxial tensile tests. A modified Duncan Shabel test was used to draw U‐profiles with a controlled restraining force. To investigate different process parameters, constant restraining forces of 1 and 5kN and die radii of 3, 6, 9, and 12 mm were applied. In a second step, a shape set process was used. A constant restraining force of 1kN was used until a draw depth of 80 % was reached. Then the restraining was increased to 5.5 kN. The springback and the sidewall force were measured and analysed. An increased restraining force and a reduced die radius increases the sidewall force and reduces the springback. This resulted in a significant decrease in springback. The tests with variable restraining forces, also known as shape set process have shown that it combines a good formability with a reduced springback.     

Spot weldability comparison of high strength steel DP590 with different chemical composition

This paper introduces the cold-rolled DP590 high strength automotive steel sheets produced by Sougang Steel,which involve two different composition systems,one with high Al content and the other with C-Si-Mn.These two materials are pot-welded and the optimized welding current range and the best welding current are obtained.Both the two kinds of materials welding current’s scope is 1 800 A,but the current of C-Si-Mn system DP590 is 1 400 A higher then the high Al content one’ s;when these two kind of materials are welded with the optimal current,the nugget can be get by no defect.There have some difference in these two base materials.High Al DP590 has a bandy metallurgical structure compose with ferrite and martensite,the volume fraction of martensite is 8%,the grain size is 10.5.C-Si-Mn system DP590 compose with ferrite and martensite also while the volume fraction of martensite is 9%,the grain size is 9.5.Weld structure of high Al DP590 are bainite and lath martensite when C-Si-Mn system DP590’s is lath martensite only.The variation of HV is same for these two materials nugget,the length of the are both 10 mm,there have no soft zone in the weld scope.The HV of the both materials are the same of 210 -220.The HV of high Al DP590 weld scope is 280,when C-Si-Mn system DP590 is much more then it with 425.After test these two welded sample get the same failure modes,the maximum shearing resistance and maximum positive tension of high Al DP590 are both less than the C-Si-Mn system DP590.     

Economically alloyed high-strength steel for use in mine equipment

The development of low-temperature, high-strength weldable steels for very heavy-duty mine vehicles and pit props is considered. Steels of strength classes S70/60 (σ_y ≥ 590 N/mm²) and S80/70 (σ_y ≥ 690 N/mm²) are based on alloying with Cr, Mn, and Si, without added Ni and Mo; wear-resistant steels of strength class S100 (σ_y ≥ 950 N/mm²) are based on alloying with Cr, Mn, and B, with a small content of Ni and Mo. The bainitic–martensitic or martensitic structure with small lath size obtained after quenching and tempering ensures strength, plasticity, and low-temperature strength of the steels. Microalloying reduces the austenite grain size in recrystallizing rolling (microalloying with V) and in heating of the steel before quenching (microalloying with V and Nb) and also ensures precipitation hardening on account of the precipitation of vanadium- and niobium-carbonitride nanoparticles. The new steels may be used to replace imports of high-strength weldable steels of the same strength classes.