Microstructure and mechanical properties of spray-deposited ultra-high carbon steel after hot rolling

The tensile mechanical properties of as-cast ingot metal (IM), spray-formed (SF), and as-hot-rolled (HR) ultra-high carbon steels (UHCS) containing silicon were investigated in this paper. The relationship between microstructure and tensile properties was described for these steels. The carbide networks, the pearlitic interlamellar spacing, the size of carbide particles, and the volume ratio between lamellar and spheroidized structure are all microstructure factors influencing the tensile properties in UHCS.     

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10?nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.     

Enhancement of mechanical properties of low carbon steel joints via graphene addition

Steel base metal laps or welding electrode surfaces were coated using graphene suspensions with various concentrations, and then the steel plates were welded using the shielded metal arc welding process. Microstructural observations showed that the addition of graphene to the weldment significantly refines the microstructure and promotes the formation of fine acicular ferrite. The results of mechanical testing indicated that with lower concentrations of graphene in the weldment, both the strength and ductility improve, but the hardness remains unchanged in comparison to the unreinforced weld metal. However, reinforcing with a higher concentration of graphene gives rise to the significant enhancement of the hardness and strength without deterioration of the ductility.     

Influence of austempering temperature on the mechanical properties of a low carbon low alloy steel

In this investigation, a new low alloy and low carbon steel with exceptionally high strength and high fracture toughness has been developed. The effect of austempering temperature on the microstructure and mechanical properties of this new steel was examined. The influence of the microstructure on the mechanical properties and the fracture toughness of this steel was also studied.Test results show that the austempering produces a unique microstructure consisting of bainitic ferrite and austenite in this steel. There were significant improvement in mechanical properties and fracture toughness as a result of austempering heat treatments. The mechanical properties as well as the fracture toughness were found to decrease as the austempering temperature increases. On the other hand, the strain hardening rate of steel increases at higher austempering temperature. A linear relationship was observed between strain hardening exponent and the austenitic carbon content.     

Hot rolling and annealing effects on the microstructure and mechanical properties of ODS austenitic steel fabricated by electron beam selective melting

The grain morphology, nano-oxide particles and mechanical properties of oxide dispersion strengthened (ODS)-316L austenitic steel synthesized by electron beam selective melting (EBSM) technique with different post-working processes, were explored in this study. The ODS-316L austenitic steel with superfine nano-sized oxide particles of 30–40 nm exhibits good tensile strength (412 MPa) and large total elongation (about 51%) due to the pinning effect of uniform distributed oxide particles on dislocations. After hot rolling, the specimen exhibits a higher tensile strength of 482 MPa, but the elongation decreases to 31.8% owing to the introduction of high-density dislocations. The subsequent heat treatment eliminates the grain defects induced by hot rolling and increases the randomly orientated grains, which further improves the strength and ductility of EBSM ODS-316L steel.     

Effect of Ni content on the tensile properties and strain-induced martensite transformation for 304 stainless steel

The effect of Ni content (8.3-12 wt.%) on the tensile properties and strain hardening behavior was studied on type 304 stainless steels (STS) used for the membrane of LNG storage tanks. The tensile test temperature was varied from 25 °C to −196 °C. At room temperature, the hardening and ductility indices (tensile strength, strain hardening exponent and elongation) increased with decreasing Ni content. For the 8.3-9.0 wt.% Ni STS, a lower yield point was observed at temperatures below −60 °C. It was due to the dynamic strain softening and/or transformation-induced plasticity (TRIP) that accompanied the rapid increase in the amount of strain-induced martensite (α′) at low strains. Neither dynamic strain softening nor TRIP was observed for the 12 wt.% Ni STS because only the ?-martensite transformation was produced at the low strains.     

Mn含量对低碳中锰TRIP钢组织性能的影响

为研究连续退火工艺生产中锰TRIP钢汽车板的可行性,采用CCT-AY-Ⅱ型钢板连续退火机模拟分析了不同锰含量对中锰TRIP钢组织性能的影响规律.采用SEM、TEM和EBSD等微观分析方法观察不同锰含量中锰TRIP的微观组织,利用XRD法测量了残留奥氏体量,实验测量其力学性能.结果表明:试验钢在650℃保温3 min时,随着锰质量分数(4.8%≤w(Mn)≤8%)的增加,屈服强度先增加后降低,抗拉强度持续升高,断后延伸率则基本不变,维持在20%左右,残余奥氏体含量也随着锰含量的增加而增加;当锰质量分数超过6%(含6%)时,真实应力-应变曲线由于动态应变时效而呈锯齿状,且加工硬化指数远大于5Mn钢.试验钢的高塑性由亚稳奥氏体的TRIP效应和超细晶铁素体或马氏体共同提供.     

变形制度对低碳锰钢组织性能的影响

为了获得细晶铁素体/贝氏体的复相组织,通过控轧控冷工艺研究了低碳锰钢在奥氏体区变形时变形量、终轧温度和卷取温度对组织演变和力学性能的影响规律.研究表明,增加变形量(对应道次间隔时间缩短)可以细化铁素体晶粒,但当终轧温度降低到800℃时,变形量的增加以及开冷温度的降低不利于贝氏体组织的获得.通过调整变形量、终轧温度、可开冷温度并适当降低卷取温度,可使实验钢获得晶粒尺寸约为5μm的铁素体和10%~20%的贝氏体组织,低碳锰钢强塑性能良好.     

Copper precipitation and its impact on mechanical properties in a low carbon microalloyed steel processed by a three-step heat treatment

The structure–mechanical property relationship, with particular focus on effect of tempering process on the microstructural evolution and mechanical properties was investigated in a low carbon Cu-bearing steel that was processed in three-steps, namely, intercritical annealing, intercritical tempering, and tempering heat treatment. The objective of adopting three steps was to elucidate the nature and evolution of microstructural constituents that contributed to high strength–ductility combination in the studied steel. The three-step processing led to a microstructure primarily comprising of ferrite, retained austenite, and small amount of bainite/martensite. The mechanical properties obtained were: yield strength > 720 MPa, tensile strength > 920 MPa, uniform elongation > 20%, total elongation > 30%, and low yield ratio of 0.78. The tempering step led to a significant increase in both yield and tensile strength and decrease in yield ratio, without reducing ductility, a behavior attributed to the precipitation of copper in retained austenite and ferrite. The precipitation of copper enhanced the stability of retained austenite and work hardening rate, leading to a high volume fraction of retained austenite (∼29%), with consequent increase in elongation and significant increase in yield and tensile strength during tempering.     

Microstructural and mechanical characterisation of laser-welded lap joints with linear and circular beads in thin low carbon steel sheets

This paper presents a microstructural and mechanical characterisation of laser-welded lap joints in low carbon steel thin sheets. Different combinations of steel types (DC05, S355MC) and thickness values are used to assemble welded specimens with linear and circular weld bead. Metallurgical observations and micro-hardness tests are used to characterise the weld microstructure. Mechanical response in tensile test is then used to evaluate the static strength, rotation angle of weld bead and failure mode of welded specimens. Lap-joints with circular weld showed a lower rotation angle compared to linear welds. The fracture in all tested specimens occurred at the base metal, far away from the weld. A simplified mechanical model is finally proposed to derive theoretical formulae for estimating the tensile strength of welded joints as a function of material properties and weld geometry. The analytical results are in good agreement with experimental findings and they estimate an increased strength for circular welds, compared to linear weld with same lateral width. A design chart is also derived to allow a design of laser-welded joints with virtually equal strength of base metal and weld zone.     

Effect of the loading frequency on fatigue properties of JIS S15C low carbon steel and some discussions based on micro-plasticity behavior

Ultrasonic testing method has been often used to investigate fatigue properties of various metallic materials. Since ultrasonic fatigue tests are conducted at a very high loading frequency, they are particularly convenient for fatigue tests in the very high cycle regime. Indeed, ultrasonic fatigue method allows us to conduct fatigue tests up to 10⁹–10¹⁰ cycles in a definite period. However, due to the huge gap of loading frequency between ultrasonic testing method (around 20 kHz) and usual testing method (most of the cases in the range 1–100 Hz), the frequency effect on the fatigue property is still unclear. Low carbon steel is one of typical metallic materials to provide a significant discrepancy between fatigue strengths obtained under ultrasonic testing frequency and under usual testing frequency range.Thus, by preparing a lot of specimens of JIS S15C low carbon steel (0.15% C), fatigue tests were carried out in a wide range of the loading frequency. The frequency effect on the S–N property was first examined and a useful procedure was proposed to obtain a common S–N property normalized by the lower yield stress. In addition, micro-plasticity behavior such as the stress–strain hysteresis loop and the local misorientation were also measured and the frequency effect on the fatigue property was discussed.     

"针状"先共析铁素体形成及对钢丝性能的影响

运用膨胀法及显微组织观察分析了35#钢连续加热和冷却过程中相变特点,讨论了经正火处理的中碳钢组织中出现一定量的"针状"先共析铁素体形成原因.通过对正火处理的35#钢丝室温拉伸变形行为测试与分析,发现其呈现两个阶段屈服变形行为,两次屈服分别对应于先共析铁素体与珠光体塑性屈服.     

800MPa级冷轧耐候双相钢连续冷却相变及组织性能研究

为了探索一种800 MPa级冷轧耐候双相钢的连续冷却转变规律及退火后组织性能变化,利用For-master-FⅡ全自动相变仪及连续退火模拟实验机,进行了连续冷却转变(CCT)曲线的测定及连续退火实验.结果表明:实验钢的过冷奥氏体在很低的冷却速度(0.5℃/s)下即可发生马氏体转变,而珠光体转变较少.当冷速为80℃/s时,仅发生马氏体转变;退火后实验钢显微组织中的马氏体呈带状分布,经最优工艺退火后实验钢的显微组织为多边形铁素体(79%)+块状马氏体(16%)+细小的残余奥氏体(5%),残余奥氏体主要分布于马氏体晶粒内部或铁素体的晶界处;实验钢屈服强度为387 MPa,抗拉强度为863 MPa,延伸率为18%,强塑积达到15534.     

类金刚石薄膜对轴承钢表面机械性能和滚动接触疲劳寿命的影响

使用等离子体浸没离子注入与沉积(PIII&D)技术在轴承钢基体表面合成类金刚石(DLC)薄膜,研究了薄膜的结构和性能,结果表明,所制备的DLC薄膜主要是由金刚石键(sp3)和石墨键(sp2)组成的混合无定形碳,且sp3键含量大于10%,DLC膜层致密均匀,与基体结合良好,DLC膜具有很高的硬度和杨氏模量,分别达到40 GPa和430 GPa;其最低摩擦系数由基体的0.87下降到0.2,被处理薄膜试件在90%置信区间下的L10、L50、La和平均寿命L较基体分别延长了10.1倍、4.2倍、3.5倍和3.4倍,PIII&D轴承钢滚动接触疲劳寿命的分散性得到了显著改善.     

Effect of pre‐processing annealing and ram speed on mechanical properties for low carbon steel processed by constrained groove pressing

Constrained groove pressing (CGP) has emerged for producing ultra‐fine‐grained materials with distinguished properties. Low carbon steel sheets were subjected to severe plastic deformation by constrained groove pressing process. The effect of pre‐processing annealing temperature, ram speed and number of passes on microstructure, mechanical properties and wear behaviour of the sheets were investigated. The 3 mm thick sheets were deformed by a constrained groove pressing die at ram speeds: 5 mm/min, 10 mm min^?1 and 20 mm min^?1. Furthermore, the as received sheets were annealed at 600 °C and 900 °C, then deformed at ram speed 20 mm min^?1. The annealing temperature 900 °C led to slightly coarser grains, lower strength and larger ductility compared to those obtained after annealing at 600 °C. With lowering the ram speed to 5 mm min^?1, the number of passes could be increased to 10 passes while increasing ram speed from 5 mm min^?1 to 20 mm min^?1 improved the mechanical properties; after 3 constrained groove pressing passes, the ultimate tensile strength increased from 420 MPa to 490 MPa, the hardness from 174 HV 1 to 190 HV 1 and the elongation from 7.6 % to 9.5 %. Finer grains were also obtained by increasing ram speed. Wear resistance was greatly enhanced by constrained groove pressing and by the increase in ram speed.