Morphology of bainite and widmanstätten ferrite

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowB _s and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.     

低碳贝氏体钢低温CTOD试验断口分离分析

通过对低碳贝氏体钢在-20℃CTOD试验中出现的分离断口进行微观分析,研究了断口分离和失稳断裂的原因.结果表明,断口分离面均为脆性断裂,至主断面为韧性扩展,分离裂纹产生的主要因素不是夹杂物,而是材料内部的带状组织和成带状的硬相组织受三维应力作用的结果.拉伸材料的分离裂纹是在达到一定抗拉强度之后开始萌生和扩展,不影响材料的使用性能.在断裂韧度试验中,一方面分离裂纹能降低材料裂纹尖端的三维应力约束,提高材料的韧性;另一方面较大程度的分离裂纹,减小了裂纹形核功和扩展功,诱发主裂纹的失稳扩展.     

两相区位错增殖对低碳贝氏体/铁素体复相钢组织和性能的影响

利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、电子探针(EPMA)、X射线衍射仪(XRD)、室温拉伸等手段,通过两相区保温-淬火(IQ)、两相区形变后保温-淬火(DIQ)、两相区保温-淬火-配分-贝氏体区等温(IQ&PB)及两相区形变后保温-淬火-配分-贝氏体区等温(DIQ&PB)热处理工艺,研究高温形变对室温组织、性能、残余奥氏体稳定性的综合影响作用.结果表明,经15%的压缩形变后铁素体中位错密度由0. 290×1014增加至1. 286×1014m-2,马氏体(原奥氏体)中C、Cu元素富集浓度提高,高温形变产生位错增殖对元素配分有明显促进作用. DIQ&PB工艺下,形变后贝氏体板条尺寸变短且宽度增加0. 1μm左右,贝氏体转变量较未变形时增加14%,多边形铁素体尺寸明显减小.力学性能方面,两相区形变热处理后抗拉强度增加132. 85 MPa,断后伸长率增加7%,强塑积可达25435 MPa·%.形变后残余奥氏体体积分数由7. 8%提高到8. 99%,残余奥氏体中碳质量分数由1. 05%提高到1. 31%.     

两相区位错增殖对低碳贝氏体/铁素体复相钢组织和性能的影响

利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、电子探针(EPMA)、X射线衍射仪(XRD)、室温拉伸等手段, 通过两相区保温-淬火(IQ)、两相区形变后保温-淬火(DIQ)、两相区保温-淬火-配分-贝氏体区等温(IQ&PB)及两相区形变后保温-淬火-配分-贝氏体区等温(DIQ&PB)热处理工艺, 研究高温形变对室温组织、性能、残余奥氏体稳定性的综合影响作用.结果表明, 经15%的压缩形变后铁素体中位错密度由0.290×1014增加至1.286×1014 m-2, 马氏体(原奥氏体)中C、Cu元素富集浓度提高, 高温形变产生位错增殖对元素配分有明显促进作用.DIQ&PB工艺下, 形变后贝氏体板条尺寸变短且宽度增加0.1 μm左右, 贝氏体转变量较未变形时增加14%, 多边形铁素体尺寸明显减小.力学性能方面, 两相区形变热处理后抗拉强度增加132.85 MPa, 断后伸长率增加7%, 强塑积可达25435 MPa·%.形变后残余奥氏体体积分数由7.8%提高到8.99%, 残余奥氏体中碳质量分数由1.05%提高到1.31%.      

低碳贝氏体工程机械用Q460级厚钢板的研制

在工业试制条件下,通过合理的化学成分设计和TMCP工艺设计,采用细晶强化、析出强化和贝氏体组织强化等手段,研制了厚度为65~80 mm的低碳贝氏体工程机械用Q460级厚钢板。试制钢板具有良好组织及力学性能,同时降低了生产成本。     

低碳钢中贝氏体组织在A1温度以下重加热过程中的演化与热稳定性

通过原位追踪金相观察、维氏硬度测试、透射电子显微术、电子背散射衍射等实验手段研究了低碳钢中贝氏体组织在550~675℃范围内重加热过程中的演化与热稳定性.实验结果表明:贝氏体组织通过回复与再结晶方式演化为多边形铁素体,在该过程中粒状贝氏体首先演化为多边形铁素体,然后多边形铁素体再吞噬贝氏体铁素体,贝氏体铁素体表现出了高于粒状贝氏体的热稳定性;在回复过程中,贝氏体铁素体中相邻铁素体板条之间的小角度晶界部分撤除,铁素体板条发生倾转与合并;贝氏体组织在重加热过程中的演化存在一个稳定阶段,处于回复与再结晶之间,其持续时间随温度的降低而显著延长.     

贝氏体形态对细晶铁素体/贝氏体双相钢室温力学性能的影响

基于过冷奥氏体动态相变的思想,通过两道次压缩变形结合控制冷却的热模拟轧制工艺,获得不同贝氏体含量及形态的细晶铁素体贝氏体双相钢。通过显微组织观察及力学性能测试,考察了第二相贝氏体特征对双相钢室温拉伸变形行为的影响。研究结果表明,形变后快速冷却可获得无碳板条状贝氏体,较慢的冷速或在贝氏体转变区保温处理可获得粒状贝氏体。贝氏体体积分数大于20%左右的细晶铁素体/贝氏体双相钢具有低的屈服强度,高的抗拉强度,高的伸长率,低屈强比以及连续屈服特性。屈服强度既与铁素体晶粒尺寸相关,也与贝氏体形态和数量相关。板条贝氏体引起的屈服强度提高大于粒状贝氏体,粒状贝氏体具有比板条贝氏体更好的塑性。     

Effect of bainite on the mechanical properties of dual-phase steels

The bainite-martensite-ferrite steels (tri-phase steels) were made in the laboratory by intercritical annealing, bainite transformation and oil quenching in sequence. With bainite inclusions, ductility was improved substantially without significant reduction of tensile strength. The ductility increase was found to be due to large deformation after necking and increased work-hardening.     

Structure and mechanical properties of hot-deformed low-carbon martensitic steel

The structural changes in low-carbon martensitic 15Kh2G2NMFBA steel induced by its hot forging in the temperature range 1150–850°C have been studied. The calculated cracking resistance parameter I_c is in agreement with its experimental value. A relation is found between the lath sizes in the martensite structure and the change in the impact toughness characteristics. A combined regime of hot deformation and hot treatment of the low-carbon martensitic steel is proposed to form submicrometer-sized structural elements and high strength and impact toughness characteristics.     

Retained austenite and mechanical properties in bainite transformed low alloy steels

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austenite. In this work, intercritical annealing followed by bainite transformation resulted in the retention of austenite with sufficient stability for transformation plasticity interactions. The effect of retained austenite on mechanical properties was studied in two low-alloy steels. Bainite transformation was carried out in the range of 400 to 500°C. The strength properties (yield strength and ultimate tensile strength) were more sensitive to bainite isothermal transformation temperature than holding time. Maximum strength properties were obtained for the lower transformation temperatures. On the other hand, high uniform and total elongation values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation time. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount of retained austenite. The same was true for total elongation and ultimate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processing with respect to austenite stabilisation may lead to a new class of triple-phase high-strength high-formability low-alloy steels.     

钼微合金化低碳钢的显微组织与力学性能

借助物理模拟系统采用四种不同的多道次变形及控制冷却工艺,研究了成分为0.12C-0.78Si-1.42Mn-0.74Al-0.32Mo钢的显微组织和力学性能.结果显示：使用物理模拟系统进行高温区的多道次热连轧,并结合控制冷却处理,能够得到不同的复相组织（铁素体/贝氏体组织,贝氏体/马氏体组织）.依贝氏体含量和形态的不同,铁素体/贝氏体复相组织钢的屈服强度为388~558 MPa,抗拉强度为681~838 MPa,总延伸率为15%~27%;贝氏体/马氏体复相组织钢的屈服强度为746 MPa,抗拉强度为960 MPa,总延伸率为19%.     

低碳多相钢的组织调控与力学性能

采用优化后的临界区再加热-淬火中温等温(T1、T2)热处理工艺,对具有不同前躯体组织的(0.22/0.17)C-(1.91/1.85)Mn-(1.32/0.94)Si两类热轧6 mm钢板分别进行处理,获得了具有铁素体、贝氏体、马氏体以及弥散分布于原奥氏体晶界、相界等处的残余奥氏体所构成的多相组织.利用扫描电镜、X射线衍射以及电子背散射衍射分析技术等对不同热处理阶段钢的微观组织进行了表征.结果证实,采用不同的前躯体组织设计可以很好地调控临界区再加热逆转变奥氏体的组织形貌、比例以及碳含量,进而通过后续处理来实现对钢中多相组织的调控.前躯体为马氏体的0.22C钢,经T1工艺后获得了以针状铁素体为基体的多相组织,其强塑积超过了30 GPa·%;前躯体为铁素体+马氏体的0.17C钢经T2工艺后获得了以块状铁素体为基体的多相组织,其强塑积超过了27 GPa·%.     

Characteristic properties of continuous annealed low-carbon low-aluminium steels

Continuous annealed low-carbon low-aluminium steel grades with good aging resistance and bake-hardenability were investigated. Taking into account the amounts of solute carbon and nitrogen in supersaturated ferrite it was tried to optimize the steel composition. Aging sensitivity depends mainly on solute nitrogen though the contribution to aging of solute carbon cannot be neglected. A reduced nitrogen aging requires a low nitrogen content (20 ppm) as well as Al/N ratios not lower than 5 and carbon contents higher than 0,025%. Both, low amounts of solute carbon and decrease of ferrite grain sizes of steels coiled at 650°C increase the effect of bake hardening. In comparison with aged steels the yield strength of overaged steels is conservative after baking. Bake-hardenable steels with good aging resistance are obtained in the carbon range 0,025 to 0,030%. Optimization according to the r?-value is not achieved in this range.     

Nonuniform structure and properties in hot-rolled low-carbon steel coils

The austenite phase transitions in the industrial production of hot-rolled steel coils are studied magnetometrically. Nonuniformity of the structure and mechanical properties of low-carbon steel strip over its length and width may be attributed to incomplete phase transitions in the strip on the output roller conveyer and nonuniformity of the subsequent coil cooling in regular production conditions. Inheritance of the structural inhomogeneity formed after hot rolling may be responsible for the thickness variation observed after cold rolling of the strip. Recommendations are made for reducing or eliminating the structural inhomogeneity so as to improve product quality.     

On the structure and properties of high-nitrogen low-carbon non-austenitic steels

Phase transformations, when cooling and heating non-austenitic high-nitrogen low-carbon steels containing chromium and other alloying elements, as well as structure and mechanical properties of these steels were analyzed. It was confirmed that these steels have high temperature chromium diffusion controlled pearlitic type transformation and martensitic type transformation. Experimental high nitrogen steels after quenching and tempering provide mechanical properties of about the same level as high strength commercial alloyed steels. Features of nitrogen as an alloying element in steels discussed allow the supposition of a possible reduction of the consumption of nickel, manganese, molybdenum or tungsten in high strength alloyed steels.     

Structure and properties of low-carbon pipe steel after pneumatic testing

The structure and properties of low-carbon pipe steel with ferrite-bainite structure subjected to pneumatic tests are investigated. The relation between the properties determined in static and dynamic loading and the results of the pneumatic tests is considered. It is found that the structure of steel not subjected to pneumatic tests includes large bainite sections and coarse martensite-austenite inclusions.     

调质低碳贝氏体钢的组织和性能

研究了C-Mn-Mo-Cu-Nb-Ti-B系低碳微合金钢915℃淬火和490~640℃回火的调质工艺对钢的组织及力学性能的影响.用扫描电镜和透射电镜对实验钢的组织、析出物形态和分布以及断口形貌进行观察,采用X射线衍射仪分析钢中残余奥氏体的体积分数.结果表明:调质后,实验钢获得贝氏体、少量马氏体及残余奥氏体复相组织,贝氏体板条宽度只有250 nm,残余奥氏体的体积分数随着回火温度的升高而降低,经淬火与520℃回火后残余奥氏体的体积分数为2.1%.调质后析出物的数量激增,6~15 nm的析出物占70%以上.实验钢经过915℃淬火与520℃回火后,其屈服强度达到915 MPa,抗拉强度990 MPa,-40℃冲击功为95 J.细小的析出物及窄的板条提高了钢的强度.板条间有残余奥氏体存在,改善了实验钢的韧性.