中碳V-N微合金钢连续冷却奥氏体分解及晶内铁素体的形成

采用Gleeble-1500热模拟实验机,对一种中碳V-N微合金钢生产过程的中间冷却、再加热、张力减径及其后冷却过程进行了物理模拟.在张力减径变形后分别在800、730、650、630、615、600、585和550℃采用水冷至室温.研究了张力减径变形后控冷过程中奥氏体的分解规律,重点分析了诱导晶内铁素体形成的因素.结果表明:奥氏体的分解转变依次为晶界铁素体、晶内铁素体和珠光体;在745～639℃范围内,奥氏体分解主要为晶界铁素体形核、长大阶段;冷却到639℃后,大量的晶内铁素体和珠光体开始形核析出.除已广泛接受的依附夹杂物形核外,析出相及先形成铁素体晶界同样是晶内铁素体的有效形核位置.能谱分析表明,诱导晶内铁素体形核的夹杂物附近没有出现贫化区,钢中夹杂物诱导晶内铁素体的形核应该是由惰性界面能和应力-应变能的共同作用.     

低碳钢中晶内形核铁素体三维形貌的唯象研究

采用光学显微镜和扫描电镜观察了低碳钢中晶内针状铁素体的二维金相组织形貌,运用计算机软件对铁素体的三维形貌进行了重构。试验发现钢中的MnS夹杂物对晶内铁素体形核有明显的促进作用,晶内有大量铁素体组织形成,晶内形核铁素体的三维形貌呈扁长片状,许多铁素体在空间中相互连接,并有相当数量的铁素体与夹杂物相连。由此推测铁素体的晶内形核有以下3种形核方式：在原有铁素体上激发生核;在夹杂物表面上直接生核;在已经存在的铁素体和夹杂物的交汇处形核。     

二次热循环对晶内铁素体的影响

采用物理模拟的方法研究了二次热循环对晶内铁素体的影响。用自动图像分析仪测量了夹杂物的大小、分布和密度，用透射电镜观察了晶内铁素体的形貌，并用能谱对诱导晶内铁素体形核的夹杂物进行了成分分析。研究结果表明，二次热循环对夹杂物的直径大小、分布和密度几乎没有影响。经过二次热循环过程后，诱导晶内铁素体形核、长大的夹杂物为MnO、TiO、SiO2、Al2O3和MnS、CuS、(Mn，Cu)S的复合物。800-500℃的时间为10a时，出现了晶内铁素体的感生形核。晶内铁素体的感生形核是由于一次品内铁素体有较高的界面能和高密度的位错，两者共同促进晶内铁素体的感生形核。     

夹杂物对微合金钢熔敷金属针状铁素体形核的影响

研究了微合金钢熔敷金属中夹杂物大小、尺寸分布和化学成分对针状铁素体形核的影响。结果表明，作为针状铁素体形核核心的夹杂物，其尺寸大多数位于0．2．0．6μm之间，并且是含有多种元素的复合夹杂物，具有化学成分不均匀的性质。夹杂物作为一种高能的惰性表面，降低了形核能垒，促进了针状铁素体的形核。当一个针状铁素体以夹杂物为初始核心先期形核后，又可诱发大量的相互交错的针状铁素体感应形核。     

含Ti氧化物对低碳钢高温热轧组织及力学性能的影响

采用扫描电镜、光学显微镜和万能力学试验机等研究了含Al和Ti的低碳钢高温热轧后的夹杂物特征、微观组织及力学性能.结果 表明:含Al低碳钢中夹杂物以Al2O3为主,组织主要由粗大的贝氏体铁索体(BF)和准多边形铁索体(QPF)组成;含Ti低碳钢中夹杂物主要类型是Ti2O3和Ti2O3-MnS,组织主要由尺寸细化的BF+QPF及针状铁素体(AF)组成.与含Al低碳钢相比,含Ti低碳钢中夹杂物的数量显著增多,尺寸明显细化,由于夹杂物对原奥氏体晶界迁移的钉扎效果明显且Ti2O3-MnS型夹杂物可有效诱导针状铁素体(AF)形核,含Ti低碳钢组织细化明显.含Ti低碳钢的低温冲击韧性良好,而含Al低碳钢韧性在-20～0℃内急剧恶化,夹杂物的钉扎作用和诱导AF形核是含Ti低碳钢低温冲击韧性良好的主要原因.     

含Zr低碳钢中针状铁素体等温转变行为

文章研究了含Zr低碳钢等温处理过程中针状铁素体的转变特征;分析了夹杂物,尤其是Zr的氧化物对针状铁素体形核长大的作用。结果表明:实验钢中针状铁素体转变开始温度在600~650℃之间;在550℃等温180 s后得到大量的针状铁素体组织,针状铁素体分割奥氏体晶粒,显著细化组织。较高温度下大尺寸夹杂物易于形核,一般形成较粗的单个铁素体板条,较低温度下小尺寸夹杂物也易于形核,可形核多个细小板条而呈放射状。Zr处理能明显促进低碳钢中针状铁素体转变,与Zr氧化物上Mn S的复合析出有关。     

晶内铁素体在夹杂物上形核机制的讨论

从夹杂物与金属之间的界面能和热膨胀系数不同导致的应变能,夹杂物周围基体局域溶质贫乏等角度对晶内铁素体在夹杂物上形核机制进行讨论．并试图分析解释不同夹杂物对铁素体形核作用的差别。     

Ti-B微合金化焊缝金属的韧化机制

针对 5 0 0～ 6 0 0MPa低合金高强度钢的韧化问题 ,研究了夹杂物对Ti-B微合金化焊缝金属针状铁素体形成的影响。钢中的非金属夹杂物 ,对促进晶内针状铁素体的形成 ,提高焊缝金属韧性有显著的作用。通过电镜分析 ,探讨了夹杂物的类型、成分对针状铁素体形成的作用。结果表明 ,Ti、Mn、Al、Si的复合氧化物夹杂可有效地促进针状铁素体形核 ,而MnO·SiO2 夹杂和MnS夹杂对针状铁素体形核作用不大。夹杂物对针状铁素体形核的主要用是 ,在夹杂物周围能形成一个塑性畸变区 ,这个由夹杂物与母相热膨胀系数差引起的塑性区 ,对针状铁素体的形成有一定作用     

氧化物冶金技术及其发展展望

氧化物冶金技术是利用钢中细小非金属夹杂物诱导晶内铁素体形核来细化晶粒的一种技术,利用该技术可以获得高强度、高韧性以及焊接性能良好匹配的高性能钢材。然而当前研究结果对晶内铁素体形核机理的解释并不统一,每种形核机理均存在无法解释的反例。文中介绍了氧化物冶金技术的中心思想,描述了晶内铁素体的显微组织特征,概括总结了能够作为钢中形核质点的夹杂物种类及性状,分析了晶内铁素体的晶体学特征,并指出了氧化物冶金技术亟待解决的问题。     

夹杂物对高强钢焊缝金属组织和冷裂纹影响分析

采用自行研制的高强钢药芯焊丝对Q960钢进行了钨极氩弧焊焊接,在研究焊缝金属的组织和抗拉强度,以及25、0、-20和-60℃焊缝金属冲击功的基础上,分析了夹杂物的尺寸、形状和分布对焊缝金属组织中针状铁素体组织形成的影响,以及对焊缝金属冷裂纹形成和扩展的影响。结果表明:当焊缝金属中夹杂物尺寸在0.4～1.0μm之间时有利于针状铁素体形核,形核夹杂物为核壳结构,其核心为Mn、Ti和Al的氧化物,外壳为Mn的硫化物;焊缝金属中有形核夹杂物和非形核夹杂物两种;冷裂纹从非形核夹杂物处形成并向远处扩展,裂纹扩展中遇到形核夹杂物,其周围针状铁素体可阻碍其扩展,若遇非形核夹杂物则会产生二次裂纹。     

Inclusions nucleating intragranular polygonal ferrite and acicular ferrite in low alloyed carbon manganese steel welds

The inclusion-assisted formation of two types of intragranular ferrite in low alloyed C-Mn steel welds, intragranular polygonal ferrite (IPF) and acicular ferrite (IAF), was investigated in relation to the inclusion characteristics (mainly size and chemistry) and the welding heat input. For this analysis, inclusions engulfed by one ferrite grain and those shared by two ferrite grains were considered as the IPF nucleants, and the rest of them (in contact with more than three ferrite grains) were considered as the IAF nucleants. All inclusions were multi-component oxides having Ti oxides as the nucleoid. The inclusion size exhibited a log-normal distribution and the average size increased as the heat input increased. Inclusions larger than ∼0.4 μm were found to be effective on IAF nucleation, and the effectiveness increased as the inclusion size increased. Accordingly, the overall IAF nucleant fraction increased as the heat input increased due to the corresponding inclusion size increase. However, the probability of the IAF nucleant in the same inclusion size range was insensitive to the heat input. There was no difference in the elemental distribution between the IAF and IPF nucleants except for Si. The Si distribution was uniform in the IPF nucleants while its content was relatively high at the inclusion periphery in the case of the IAF nucleants regardless of the heat input and the inclusion size.     

In situ observation of the formation of intragranular acicular ferrite at non-metallic inclusions in C–Mn steel

Intragranular acicular ferrite is regarded as a most desirable microstructure feature, in view of its strength and toughness, both in weld metals and in the heat-affected zone. This paper systematically investigated the effect of Ti addition on the evolution of intragranular acicular ferrite in the heat-affected zone of C–Mn steel. We also systematically studied the effects of austenite grain size, alloy content and the characteristic of inclusions on the formation of intragranular acicular ferrite. The nucleation and growth of intragranular acicular ferrite was directly observed by laser scanning confocal microscopy. Subsequently, microscopy analysis was used to quantitatively determine and distinguish the potent and inactive inclusions with respect to the nucleation of intragranular acicular ferrite. Finally, some possible reasons are given to explain the formation of intragranular acicular ferrite in the C–Mn steel.     

Ti脱氧钢含Ti复合夹杂物诱导晶内针状铁素体的原位观察

采用高温共聚焦激光显微镜(CSLM)对含Ti复合夹杂物诱导晶内针状铁素体(IAF)进行动态原位观察,并用OM,FE-SEM,EDS和EPMA研究奥氏体晶粒尺寸和冷却速率对含Ti复合夹杂物诱导生成IAF的影响.结果表明,IAF的开始生成温度随奥氏体晶粒尺寸的增加,先升高后降低；生成IAF体积分数随冷却速率的提高显著增加,...     

Ti处理改善船体钢焊接粗晶区的低温韧性研究

采用Gleeble 1500D热模拟试验机对Ti和Al处理船体钢进行不同热输入焊接热模拟实验, 并利用OM和SEM研究了母材和热模拟粗晶区氧化物夹杂及显微组织. 结果表明: Ti处理钢中弥散分布的Ti氧化物具有良好的高温稳定性, 75 kJ/cm的焊接热输入对其形貌、成分及尺寸无影响, 能有效促进晶内针状铁素体(AF)形核长大. Al处理钢中以Al₂O₃为核心的复合夹杂高温易分解, 不能促进晶内AF形核. 线能量大于50 kJ/cm的大热输入条件下, Ti处理钢模拟粗晶区的低温韧性明显高于Al处理钢. t_8/5>40 s时, Ti处理钢中较多的晶内AF组织抑制了M-A岛形成, 细化了基体铁素体组织, Al处理钢中的TiN和Nb(C, N)第二相粒子粗化, 粗晶区晶粒异常长大, 大于Ti处理钢中的奥氏体晶粒尺寸.     

Crystallographic analysis for acicular ferrite formation in low carbon steel weld metals

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steelweld metals. The samples from electro slag welding (ESW) and submerged arc welding (SAW) deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were of multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and energy-dispersive X-ray spectrometer analyses that the Ti-enriched layer was TiO. The acicular ferrite had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, themorphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas the growth of ferrites nucleated from TiO was enough in ESW, the size of nucleated ferrite in SAW was a few hundred millimetres in size. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov–Sachs orientation relationship (K–S relationship) in both ESW and SAW. However, there was a difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to the K–S relationship in ESW. On the other hand, the nucleated ferrite in SAW stopped growing and the newly nucleated ferrite which had K–S relationship prior to austenite was formed adjacently because of large super cooling due to small heat input.     

New insights into intragranular ferrite in a low-carbon low-alloy steel

Single and multiple nucleation events on non-metallic inclusions have been observed, leading to the intragranular formation of ferrite as a function of transformation temperature. Three-dimensional shapes have also been characterized. When the ferrite forms at elevated temperatures it is in the form of isolated idiomorphs, whereas larger undercoolings are associated with the multiple nucleation of plates emanating from the inclusions. The plates grow at a fixed orientation to the parent austenite. Nanohardness tests indicate that the idiomorphs are softer when compared with the plates. The formation of intragranular ferrite laths or plates can facilitate the attainment of fine-grained microstructures.     

锰质量分数和压缩比对中碳钢相变组织的影响

采用光学显微镜和电子探针观察了中碳钢显微组织形貌,分析了不同锰质量分数和轧制压缩比对相变组织的影响。研究表明：试验钢显微组织为形态不同的铁素体+珠光体;增加Mn质量分数抑制晶界铁素体形核和长大,同时细化铁素体晶粒,促进退化珠光体的形成;提高轧制压缩比有利于原奥氏体晶内蜂窝状铁素体的形成,该铁素体均匀的分割原奥氏体晶粒,与晶界铁素体具有相同的方向性;MnS或其复合夹杂物是铁素体在原奥氏体晶粒内部形核的有效位置。     

Ferrite nucleation potency of non-metallic inclusions in medium carbon steels

In recent years, there has been renewed interest in the role of non-metallic inclusions in controlling the microstructure of steels. The potency of various inclusions and precipitates such as SiO₂, MnO·SiO₂, MnS, Al₂O₃, Ti₂O₃ and VN for the nucleation of intragranular ferrite has been examined in the present study. Among them, single SiO₂, MnO·SiO₂, Al₂O₃, TiN and MnS particles seem to be inert to the nucleation of intragranular ferrite under the present experimental condition. Ti₂O₃ particles in a Mn-containing steel are very effective for the nucleation of intragranular ferrite, being (Ti,Mn)₂O₃ particles by absorbing Mn atoms from a steel matrix. On the other hand, Ti₂O₃ particles in a Mn-free steel are not effective. MnS and Al₂O₃ particles in high nitrogen steels containing vanadium also appear to be potent for the nucleation of intragranular ferrite. The decrease in transformation temperature causes a change in the morphology of intragranular ferrite from idiomorphic ferrite to acicular ferrite.     

INTRAGRANULAR FERRITE FORMED IN ASSOCIATION WITH INCLUSIONS IN A VANADIUM MICROALLOYED STEEL

lntragranular ferrite was formed at inclusions in a vanadium microalloyed steel with excess amount of sulfur. The chemical composition of inclusions in the steel was analyzed by SEM-EDS. The inclusions were mainly composed of MnS and aluminum oxides. The precipitation of MnS at aluminum oxides might result in Mn depletion, which, in turn, promotes the formation of intragranular ferrite. Optical and SEM observations and threedimensional (3D) reconstruction demonstrated that intragranular ferrite was formed at inclusions. The morphology of intragranular ferrite changed with undercooling. At higher temperatures intragranular ferrite was nearly equiaxed whereas it was plate-like or lath-like at lower temperatures.