A lot of research has been focused on the necking process during the plastic deformation of sheet metals, but the localized necking is rarely distinguished form diffused necking by experiments, due to the limit of mea...A lot of research has been focused on the necking process during the plastic deformation of sheet metals, but the localized necking is rarely distinguished form diffused necking by experiments, due to the limit of measurement equipment and method. Quenching and Partitioning (Q&P) steel is a 3rd generation advanced high strength steel (AHSS). Its good combination of high strength and ductility ensures potential application in automobile industry. Uniaxial tensile tests of QP980 steel sheet at five strain rates are performed to investigate the necking process and the effect of strain rate on necking behavior of Q&P steel. Digital image correlation (DIC) method is applied during tensile tests, and evolutions of major strain, minor strain and normal strain distributions along gauge section of the tensile specimens are obtained. The diffused and localized necking strains are determined according to SWIFT necking theory and HILL necking theory respectively. The test results indicate that with the increasing of strain rate in the investigated range, the diffused necking strain decreases from 0.152 to 0.120 and localized necking strain decreases from 0.245 to 0.137. Meanwhile, the difference of the two strains decreases form 0.096 to 0.017. Thus it can be concluded that strain rate has an influence on both necking strains during the deformation of QP980 steel sheet. Diffused and localized necking strains are determined by uniaxial tensile tests with the aid of DIC technique and the effect of strain rate on necking strains is evaluated.展开更多
The success of obtaining both high strength and high formability in low-carbon quenched and partitioned( Q & P) steels depends on their microstructural constituents. In this regard,the effect of annealing temperat...The success of obtaining both high strength and high formability in low-carbon quenched and partitioned( Q & P) steels depends on their microstructural constituents. In this regard,the effect of annealing temperature on the volume fraction and distribution of carbon in retained austenite in a low-carbon Q & P steel was studied. The microstructures of Q & P steels subjected to different annealing temperatures were studied in detail by electron microscopy,electron microprobe,and X-ray diffraction techniques. The results suggest that intercritical annealing is beneficial in increasing the volume fraction of retained austenite,which is a consequence of the distribution of alloying elements during intercritical annealing. Moreover,the mechanical properties of intercritically annealed Q & P steel,especially its ductility,are significantly enhanced.展开更多
Since it has excellent tensile strength, the quenching and partitioning （Q ＆ P） treated steel has been widely acclaimed. Some researched is currently being conducted on Q ＆ P steel, but Q ＆ P techniques and relat...Since it has excellent tensile strength, the quenching and partitioning （Q ＆ P） treated steel has been widely acclaimed. Some researched is currently being conducted on Q ＆ P steel, but Q ＆ P techniques and related mechanisms have been mostly research based on conventional transformation-induced plasticity （TRIP） steel. This study researches the effect of Mn on the properties of Q ＆P sheet steel. Two kinds of steel sheet with different levels of Mn content （1.46% Mn and 2.75% Mn） are investigated. After being fully annealed, the steel sheets were quenched to a temperature between Ms and Mr. Allocation treatment then caused the carbon to be partitioned into the remaining austenite.The microstructure and tensile properties were analyzed. The results show that the increment of Mn content can increase the stability of the steel and decrease the sensitivity of the quenching temperature （QT）. The results also indicate that increasing the level of Mn can effectively suppress the formation of bainite.展开更多
Ballistic impact response of resistance-spot-welded(RSW)double-layered(2×1.6 mm)plates(190 mm×150 mm)for Q&P980 steel impacted by a round-nosed steel bullet(12 mm diameter and 30 mm length)was investigat...Ballistic impact response of resistance-spot-welded(RSW)double-layered(2×1.6 mm)plates(190 mm×150 mm)for Q&P980 steel impacted by a round-nosed steel bullet(12 mm diameter and 30 mm length)was investigated by using gas gun and high-speed camera system.The RSW specimens were spot welded using a 6 mm diameter electrode face producing a 7.2 mm diameter fusion zone of the spot weld.The ballistic curve and energy balance for the tests of the spot weld of the RSW specimens at different velocity were analyzed to characterize the ballistic behavior of the RSW specimens under bullet impact.The fracture mechanisms of the RSW specimens under bullet impact were presented.For the tests below the ballistic limit,the cracks initiated from the notch-tip and propagated along the faying surface or obliquely through the thickness depending on the impact velocity.For the tests above the ballistic limit,the plug fracture in the front plate of the RSW specimen could be caused by the thinning-induced necking in the BM near the HAZ,while the plug fracture in the rear plate of the RSW specimens may be consist of the circumferential cracking from the rear surface and the bending fracture of the hinged part of material.The effects of the electrode indentation and the weld interfaces on deformation and fracture of the RSW specimens under bullet impact were revealed.For the tests above the ballistic limit,the circumferential fracture from the rear surface of the RSW specimens was always initiated along the interior periphery of the electrode indentation and the crack paths were along the FZ/CGHAZ or CGHAZ/FGHAZ interface.When the circumferential crack also formed outside the electrode indentation,the fracture on the BM/HAZ interface could be found.On the front plate of the RSW specimens,the shear/bending induced cracking from the notch-tip were observed and the crack paths were along the FZ/CGHAZ or CGHAZ/FGHAZ interface.展开更多
The resistance spot weldability of galvanized ultra-high-strength steels is not satisfed,the joints are prone to interfacial fracture and the weldable current range is narrow.To solve the problems,a novel method calle...The resistance spot weldability of galvanized ultra-high-strength steels is not satisfed,the joints are prone to interfacial fracture and the weldable current range is narrow.To solve the problems,a novel method called resistance spot welding with double-sided cover sheets was introduced to weld a galvanized Q&P980 steel with the thickness of 1.2 mm.Two thin SPCC mild steel sheets were chosen as cover sheets and were placed symmetrically at both sides between the Q&P980 steels and the electrodes,then the RSW process was carried out.Compared with the traditional RSW method,the joints obtained by using the novel method achieved larger tensile shear strength and energy absorption,which increased by 26.9%and 52.6%,respectively.With increasing the welding current,the failure mode transferred from interfacial fracture to nugget pull-out fracture or base metal tearing fracture.By contrast,the joints always showed interfacial fracture without cover sheets.The improvement of the joint performance was mainly attributed to the enlargement of the nugget.With the help of fnite element simulation,it was found that the cover sheets helped increase the contact area and reduced the current density during welding,which postponed the expulsion,and a larger area could be evenly heated.The application of the novel method can be easily extended to the resistance spot welding of other ultra-high-strength steels with various thicknesses.展开更多
在GLEEBLE3800热模拟机上模拟了低碳Si．Mn系Q＆P（Quenching and Partitioning）钢的控冷热处理工艺。结果表明，Q＆P钢具有高的抗拉强度（σb=1050MPa）和良好的塑性（δ=25％）。采用扫描和透射电镜进行了组织观察与分析，发现其组...在GLEEBLE3800热模拟机上模拟了低碳Si．Mn系Q＆P（Quenching and Partitioning）钢的控冷热处理工艺。结果表明，Q＆P钢具有高的抗拉强度（σb=1050MPa）和良好的塑性（δ=25％）。采用扫描和透射电镜进行了组织观察与分析，发现其组织主要由板条马氏体和8％～15％的残留奥氏体组成，残留奥氏体呈膜状。利用XRD技术对残留奥氏体进行了定量分析并用扫描电镜进行了断口观察。结果表明，拉伸试样纤维区为比较典型的韧窝状形貌，体现出良好的韧性断裂特征，残留奥氏体在组织中起到了相变诱发塑性的作用。展开更多
文摘A lot of research has been focused on the necking process during the plastic deformation of sheet metals, but the localized necking is rarely distinguished form diffused necking by experiments, due to the limit of measurement equipment and method. Quenching and Partitioning (Q&P) steel is a 3rd generation advanced high strength steel (AHSS). Its good combination of high strength and ductility ensures potential application in automobile industry. Uniaxial tensile tests of QP980 steel sheet at five strain rates are performed to investigate the necking process and the effect of strain rate on necking behavior of Q&P steel. Digital image correlation (DIC) method is applied during tensile tests, and evolutions of major strain, minor strain and normal strain distributions along gauge section of the tensile specimens are obtained. The diffused and localized necking strains are determined according to SWIFT necking theory and HILL necking theory respectively. The test results indicate that with the increasing of strain rate in the investigated range, the diffused necking strain decreases from 0.152 to 0.120 and localized necking strain decreases from 0.245 to 0.137. Meanwhile, the difference of the two strains decreases form 0.096 to 0.017. Thus it can be concluded that strain rate has an influence on both necking strains during the deformation of QP980 steel sheet. Diffused and localized necking strains are determined by uniaxial tensile tests with the aid of DIC technique and the effect of strain rate on necking strains is evaluated.
文摘The success of obtaining both high strength and high formability in low-carbon quenched and partitioned( Q & P) steels depends on their microstructural constituents. In this regard,the effect of annealing temperature on the volume fraction and distribution of carbon in retained austenite in a low-carbon Q & P steel was studied. The microstructures of Q & P steels subjected to different annealing temperatures were studied in detail by electron microscopy,electron microprobe,and X-ray diffraction techniques. The results suggest that intercritical annealing is beneficial in increasing the volume fraction of retained austenite,which is a consequence of the distribution of alloying elements during intercritical annealing. Moreover,the mechanical properties of intercritically annealed Q & P steel,especially its ductility,are significantly enhanced.
文摘Since it has excellent tensile strength, the quenching and partitioning （Q ＆ P） treated steel has been widely acclaimed. Some researched is currently being conducted on Q ＆ P steel, but Q ＆ P techniques and related mechanisms have been mostly research based on conventional transformation-induced plasticity （TRIP） steel. This study researches the effect of Mn on the properties of Q ＆P sheet steel. Two kinds of steel sheet with different levels of Mn content （1.46% Mn and 2.75% Mn） are investigated. After being fully annealed, the steel sheets were quenched to a temperature between Ms and Mr. Allocation treatment then caused the carbon to be partitioned into the remaining austenite.The microstructure and tensile properties were analyzed. The results show that the increment of Mn content can increase the stability of the steel and decrease the sensitivity of the quenching temperature （QT）. The results also indicate that increasing the level of Mn can effectively suppress the formation of bainite.
基金supported by the National Natural Science Foundation of China,China under the grant No.11372149K.C.Wong Magna Fund in Ningbo University。
文摘Ballistic impact response of resistance-spot-welded(RSW)double-layered(2×1.6 mm)plates(190 mm×150 mm)for Q&P980 steel impacted by a round-nosed steel bullet(12 mm diameter and 30 mm length)was investigated by using gas gun and high-speed camera system.The RSW specimens were spot welded using a 6 mm diameter electrode face producing a 7.2 mm diameter fusion zone of the spot weld.The ballistic curve and energy balance for the tests of the spot weld of the RSW specimens at different velocity were analyzed to characterize the ballistic behavior of the RSW specimens under bullet impact.The fracture mechanisms of the RSW specimens under bullet impact were presented.For the tests below the ballistic limit,the cracks initiated from the notch-tip and propagated along the faying surface or obliquely through the thickness depending on the impact velocity.For the tests above the ballistic limit,the plug fracture in the front plate of the RSW specimen could be caused by the thinning-induced necking in the BM near the HAZ,while the plug fracture in the rear plate of the RSW specimens may be consist of the circumferential cracking from the rear surface and the bending fracture of the hinged part of material.The effects of the electrode indentation and the weld interfaces on deformation and fracture of the RSW specimens under bullet impact were revealed.For the tests above the ballistic limit,the circumferential fracture from the rear surface of the RSW specimens was always initiated along the interior periphery of the electrode indentation and the crack paths were along the FZ/CGHAZ or CGHAZ/FGHAZ interface.When the circumferential crack also formed outside the electrode indentation,the fracture on the BM/HAZ interface could be found.On the front plate of the RSW specimens,the shear/bending induced cracking from the notch-tip were observed and the crack paths were along the FZ/CGHAZ or CGHAZ/FGHAZ interface.
基金Supported by National Key Research and Development Program of China(Grant No.2017YFB0304403)National Natural Science Foundation of China(Grant No.51871154).
文摘The resistance spot weldability of galvanized ultra-high-strength steels is not satisfed,the joints are prone to interfacial fracture and the weldable current range is narrow.To solve the problems,a novel method called resistance spot welding with double-sided cover sheets was introduced to weld a galvanized Q&P980 steel with the thickness of 1.2 mm.Two thin SPCC mild steel sheets were chosen as cover sheets and were placed symmetrically at both sides between the Q&P980 steels and the electrodes,then the RSW process was carried out.Compared with the traditional RSW method,the joints obtained by using the novel method achieved larger tensile shear strength and energy absorption,which increased by 26.9%and 52.6%,respectively.With increasing the welding current,the failure mode transferred from interfacial fracture to nugget pull-out fracture or base metal tearing fracture.By contrast,the joints always showed interfacial fracture without cover sheets.The improvement of the joint performance was mainly attributed to the enlargement of the nugget.With the help of fnite element simulation,it was found that the cover sheets helped increase the contact area and reduced the current density during welding,which postponed the expulsion,and a larger area could be evenly heated.The application of the novel method can be easily extended to the resistance spot welding of other ultra-high-strength steels with various thicknesses.
文摘在GLEEBLE3800热模拟机上模拟了低碳Si．Mn系Q＆P（Quenching and Partitioning）钢的控冷热处理工艺。结果表明，Q＆P钢具有高的抗拉强度（σb=1050MPa）和良好的塑性（δ=25％）。采用扫描和透射电镜进行了组织观察与分析，发现其组织主要由板条马氏体和8％～15％的残留奥氏体组成，残留奥氏体呈膜状。利用XRD技术对残留奥氏体进行了定量分析并用扫描电镜进行了断口观察。结果表明，拉伸试样纤维区为比较典型的韧窝状形貌，体现出良好的韧性断裂特征，残留奥氏体在组织中起到了相变诱发塑性的作用。
