Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

Research and development of hot-rolled ultra-high strength steel at Baosteel

The effects of the composition and cooling process on the microstructures and properties of hot-rolled ultra-high strength low alloy （HSLA） steel, complex phase steel and martensite steel were studied in the laboratory. And S700MC and MP1200 ultra-high strength steels were trial produced at the 1 880 mm hot-rolling line of Baosteel. Compared with conventional hot-rolled high strength products,the idea that water is alloy was applied in the newly developed hot-rolled ultra-high strength steel. By the use of the economical composition design and controlled cooling after hot-rolling effectively,ultra-high strength steel of different steel grades can be obtained.

Influence of refined hierarchical martensitic microstructures on yield strength and impact toughness of ultra-high strength stainless steel

The hierarchical martensitic features in ultra-high strength stainless steel(UHSSS),including the prior austenite grains,martensite packets,blocks and laths with the descending size,were refined to various extents by employing different thermomechanical processes and then carefully characterized.Their relation to yield strength and impact toughness was analyzed.We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength,which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries(HABs).Impact toughness of UHSSS depends on the frequency and capability for retained austenite(RA)grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation,in which the film-like RA grains at lath boundaries appear to make the greater contribution.

Effect of Martensite Fine Structure on Mechanical Properties of an 1100 MPa Grade Ultra-high Strength Steel

An 1100 MPa grade ultra-high strength steel with different martensite fine structures, characterized by prior austenite grain size, martensite packet size, block width and lath width, was studied by various heat treatment processes. The result shows that with decreasing prior austenite grain size, both the packet size and block width decrease, while the lath width has virtually no change. Accordingly, both strength and toughness increase, while total elongation decreases. The yield strength has a Hall Petch type relationship with the prior austenite grain size, packet size and block width, and the block width may be regarded as a key factor influencing strength. On the other hand, the ductile to brittle transition temperature （DBTT） is found to be more related lo the packet size, which may be considered as a dominant factor influencing toughness.

Effect of Thermomechanical Treatment Temperature on Structure and Properties of CFB/M Ultra-High Strength Steel

Modified CCT diagram of carbide-flee bainite-martensite （CFB/M） ultra-high strength steel was established by applying controlled cooling of small samples. In addition, the influence of thermomechanical treatment tem- perature on the structure and properties was discussed. The experimental results showed that when deformed at 860℃ and below, ferrite transformation occurred due to strain. With the decrease of ausforming temperature, the quantity of ferrite increased and strength and toughness were deteriorated. Therefore, certain information was provided for optimizing technical parameter of ausforming process., firstly, the thermomechanical treatment temperature should not be lower than 860 ℃ in order to avoid ferrite formation induced by deformation; secondly, rapid cooling rate is also significant after deformation in order to avoid ferrite precipitation during subsequent cooling stage.

车用高强钢厚板模内淬火组织演变研究

目的揭示高强钢厚板在热冲压模内淬火过程中的马氏体组织演变特征和规律。方法建立了6mm厚硼钢板热冲压模内淬火过程马氏体相变有限元预测模型,分析了马氏体分布情况及演变特征,阐明了保压力等关键淬火参数对组织演变的影响规律。结果沿板面方向,马氏体从角部开始转化,逐渐由边缘向内部扩展,呈现出“边缘高、中部低”的分布特征。沿板厚方向,马氏体呈现出“表层高、心部低”的分布特征;随淬火过程的进行,这种厚向梯度分布特征逐渐弱化。与薄板相比,厚板的马氏体相变启动和结束时刻均较晚,且马氏体转化率明显较低,马氏体沿厚向分布的不均匀性也更明显。随保压力的增大,马氏体转化结束时刻前移,马氏体转化率增大且沿厚向分布更均匀,抗拉强度增大,延伸率减小;当保压力超过40 MPa时,上述影响逐渐减弱。结论与薄板相比,厚板组织沿板面和板厚方向均呈现更明显的梯度分布特征。保压力对厚板组织演变和力学性能有较大影响,但这种影响随保压力的增大而逐渐减弱。

马氏体/下贝氏体复相热处理对GD钢强韧性的影响

本文研究了高强韧低合金冷模具钢6CrNiMnSiMoV(GD 钢)马氏体/下贝氏体复相热处理的组织和强韧性,以及下贝氏体和残余奥氏体量对 GD 钢强韧性的影响。结果表明,GD 钢复相热处理的强韧性明显优于常规热处理的强韧性,其工艺为900℃奥氏体化,250℃等温55min,空冷,180℃×2h 回火,此时组织中含有约27%的下贝氏体,具有最佳的强韧性配合,并进行了生产验证。

激光选区熔化成形2 GPa级超高强钢及其热处理

采用激光选区熔化(SLM)技术制备了均匀致密的MS1超高强马氏体模具钢,并对其进行了850℃固溶1 h+550℃时效4 h的热处理,采用扫描电镜、显微硬度仪和万能试验机等研究了打印态和热处理态试样的微观组织和力学性能。结果表明:MS1超高强马氏体模具钢SLM打印态的组织主要由板条状马氏体和奥氏体组成,组织均匀细小,但存在少量微小气孔;热处理后,MS1超高强马氏体模具钢的组织为细小的全马氏体板条,微小气孔缺陷消失;与打印态试样相比,热处理态试样的强度显著增加,塑性有所降低,屈服强度提升超过1倍,抗拉强度超过2 GPa,提升70%以上,硬度提升82%以上,伸长率由10.7%降至5.6%。

差厚板热成形B柱仿真分析及试验研究

针对差厚板保压淬火过程中温度降速不均匀问题,通过AutoForm软件对成形过程进行仿真分析,优化了工艺参数及模具零件间隙,解决了马氏体转化率不均匀导致的制件抗拉强度和硬度不足的问题,并且应用在某车型B柱上。

模具间隙对高强度钢热冲压过程的影响

利用DEFORM软件,建立了不等模具间隙下高强度钢板热冲压的有限元模型,研究了模具间隙对22Mn B5钢板热冲压成形中温度场以及马氏体转变的影响规律。数值模拟结果表明:模具间隙是影响板料温度变化的主要因素,板料的最大温差随着模具间隙先增大后减小;模具间隙为0.95t^1.00t时,板料各部位的马氏体分布均匀,转变率高。在此基础上,进行了热冲压试验,测试了板料侧壁位置点和底部圆角位置点的温度变化,并观察了板料成形后的微观组织,通过试验结果与数值模拟结果一致性对比,验证了数值模拟结果的可靠性与准确性。