Effect of Isothermal Bainite Treatment on Microstructure and Mechanical Properties of Low‐Carbon TRIP Seamless Steel Tube

In this work, a low‐carbon transformation‐induced‐plasticity (TRIP) seamless steel tube (Fe–0.15C–1.34Si–1.45Mn–0.029Nb–0.024Ti, in wt%), having potential in application of hydroforming process, has been successfully manufactured by using piercing, cold‐drawing, and two‐stage heat‐treatment process. The optimal heat‐treatment conditions, inter‐critical annealing (IA), and isothermal bainite treatment (IBT) were firstly obtained to maximize the volume fraction and stability of the retained austenite (RA). The effects of temperature and holding time IBT on the microstructures of the TRIP steel tube were studied via optical microscopy (OM), scanning microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffractometer (XRD). The mechanical properties in the axial direction and hydroformability were also evaluated by conventional tensile test and flaring test, respectively. Two‐stage heat‐treatment was finally performed to achieve the required mechanical properties for the hydroformed tube. The results shows that the RA volume fraction increased at first and then decreased with the increase of IBT holding time and IBT temperature for a particular set of IA temperature and IA holding time. It was also demonstrated that high tensile strength of 618 MPa, total elongation of 35.5%, n‐value of 0.23, and better hydroformability could be successfully produced in this TRIP steel tube at IA temperature of 800°C, holding for 10 min, and IBT of 410°C for 4 min holding time.     

残余奥氏体对贝氏体钢轨性能的影响

 无碳化物贝氏体组织中的残余奥氏体对提高贝氏体钢轨的韧塑性作出了突出贡献,为了在铁路运营时使钢轨仍保持较高的韧塑性,需要控制好贝氏体钢轨残余奥氏体的稳定性。通过对热轧空冷、热轧空冷+低温回火贝氏体钢轨在不同环境温度下残余奥氏体稳定性的分析,回火贝氏体钢轨在不同试验温度（包括低温）条件下拉伸性能的分析,在模拟钢轨运营的试验条件下疲劳性能的分析及相应条件下残余奥氏体含量的测定,说明低温回火处理提高了贝氏体钢轨中残余奥氏体的稳定性,模拟钢轨运营的试验条件下,贝氏体钢轨中的残余奥氏体基本是稳定的。     

Bainite Transformation Under Continuous Cooling of Nb-Microalloyed Low Carbon Steel

Utilizing Gleeble-1500 thermomechanical simulator, the influences of hot deformation parameters on continuous cooling bainite transformation in Nb-microalloyed low carbon steel were investigated. The results indicate that bainite starting temperature decreases with raising cooling rate and increases with increasing deformation temperature. Deformation has an accelerative effect on the bainite transformation when the specimens are deformed at 950 ℃. When the deformation temperature increases, the effect of deformation on bainite starting temperature is weakened. The amount of bainite is influenced by strain, cooling rate, and deformation temperature. When the specimens are deformed below 900 ℃, equiaxed ferrites are promoted and the bainite transformation is suppressed.     

Comparison of Microstructure and Mechanical Properties of a High-Carbon Bainitic Steel Treated by Long-Time Bainitic Austempering and Short-Time Austempering Plus Tempering Processes

Herein, the microstructure and mechanical properties of a high-carbon bainitic steel treated by long-time bainitic austempering and short-time austempering plus tempering processes are compared. The multiphase microstructures are characterized by dilatometry, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy to correlate with mechanical properties. Results show that although long-time austempering treatment can reduce the volume fraction of brittle martensite, no significant improvement is observed in fracture damage resistance. Besides, the cementite is prone to precipitation from the austenite at the later period of the long-time austempering process. The cementite precipitation in austenite decreases the carbon content in retained austenite (RA) and consequently reduces the mechanical stability of RA. In contrast, the cementite has not been able to precipitate from austenite after short-time austempering treatment, whereas the martensite is softened and the stability of RA is improved during subsequent tempering. Therefore, excellent mechanical properties are obtained in the samples treated by short-time austempering plus tempering process: ultimate tensile strength, 1489 MPa, yield strength, 1014 MPa, total elongation, 33.2%, and the product of strength and elongation (PSE) of 48.4 GPa%, where PSE is increased by 27% compared with the sample after long-time bainitic austempering.     

Factors Affecting the Mechanical Properties of Ultra-high-strength Bainitic Steel Containing W and 0.33 mass％ C

A novel ultra-high-strength bainitic steel was designed.The analysis of its mechanical properties by quasistatic testing showed that upper bainitic steel exhibited an ultimate tensile strength of 2 260 MPa(engineering stress)and an ultimate compressive strength of more than 2 700MPa(true stress).The ultra-high strength of upper bainitic steel was mainly attributed to untempered martensite and upper bainite with a feather-like microstructure.Moreover,lower bainitic steel demonstrated an ultimate tensile strength of 1 922 MPa(engineering stress)and an ultimate compressive strength of 2 500MPa(true stress).The ultra-high strength of lower bainitic steel was primarily due to untempered martensite and lower bainite with an acicular microstructure.The untempered martensite in the two kinds of bainitic steels was produced in different ways.The dynamic test results showed that the ultimate compressive strengths of the two bainitic steels were maintained at 1 600MPa(true stress)under high strain rates(1 100and2 200s-1)at 600℃,because of the added tungsten,confirming the satisfactory hot hardness property of the steel.Furthermore,lower bainitic steel showed better comprehensive mechanical properties than upper bainitic steel.     

Effect of Cold Deformation and Multiphase Treatment Conditions on Low‐Carbon,Low‐Silicon Multiphase Steel

This paper presents multiphase (MP) treatments of a low‐C, low‐Si cold rolled steel. Despite the much lower content of Si compared to a typical TRIP steel, up to about 8 pct of retained austenite (γ_r) with 1.2 % carbon content can be obtained. Increasing prior cold deformation (i.e. decrease of parent austenite grain size) accelerates the transformation to bainite resulting in a decrease of the volume fraction of residual austenite (γ_r + martensite). Tensile strength of MP steel intercritically annealed at high temperature increases with higher cold reduction degree due to the smaller grain size of the present phases. On the contrary, the ductility and strength‐ductility balance deteriorate because the banded structure becomes more pronounced and the γ_r volume fraction diminishes. Decreasing intercritical annealing temperature results in an increasing γ_r fraction and a uniform distribution of second phases. Hence, the ductility and strength‐ductility balance are improved. Crystallographic preferred orientation is evident in the ferrite and martensite and its extent increases with higher cold deformation.     

Influence of Holding Time After Deformation on Bainite Transformation in Niobium Microalloyed Steel

  Using Gleeble 1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. The results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite (RA) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co existed with RA. The formation mechanism of two kinds of carbides was analyzed by combining TEM micrographs with the model.     

不同锰铬含量对贝氏体钢组织转变的影响

文章利用全自动相变仪对不同锰铬含量的贝氏体钢组织转变进行了试验研究,对比分析了不同锰铬元素含量对贝氏体钢过冷奥氏体相变点的影响,并研究了不同转变温度对贝氏体钢组织转变的影响,试验结果表明贝氏体钢组织转变的最佳温度为330℃,为贝氏体钢生产工艺开发提供了理论依据。     

淬火温度对Q-P钢组织和性能的影响

 采用盐浴的方法模拟Q-P钢的热处理工艺,对钢的组织和性能进行分析和研究。其结果表明,淬火配分组织主要为板条马氏体和残留奥氏体,并随淬火温度升高马氏体含量降低,奥氏体含量升高,从而降低钢的强度,提高塑性,拉伸断口呈现典型的韧窝状形貌,当淬火温度为250℃时,得到最高的强塑积为26224MPa·%。     

微量硼对低碳贝氏体钢过冷奥氏体转变的影响

采用Gleeble-3800热模拟实验机对加入微量硼的低碳钢进行了动态CCT曲线的测定和组织分析,研究了钢中微量硼在奥氏体晶界上的偏聚作用。结果表明,微量硼的加入降低了碳原子在晶界上的吸附趋势,导致了晶界处珠光体形核率的下降,改变了动态CCT曲线的形状,抑制了珠光体的转变,能明显提高钢的淬透性和强韧性。