超低碳贝氏体钢焊接接头组织的微观力学性能与EBSD分析
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摘要
通过组织超细化可以使钢铁材料同时获得高的强度和优良的韧性,但超细组织在焊接条件下的粗化可能降低超细组织钢的性能。本文对采用驰豫-析出-控制相变工艺TMCP(Thermomechanical Control Process)-RPC (Relaxation-Precipitation-Controlled Transformation)生产的新型超细组织钢进行了焊接接头微观组织的表征及Electron Back Scattered Diffraction (EBSD)分析和微观力学性能的研究。
利用光学显微镜,扫描电子显微镜对RPC超细组织钢的焊接接头进行观察与分析。焊接接头主要分为:焊缝,熔合区,粗晶区,细晶区和不完全重结晶区。焊缝组织为针状铁素体,熔合区主要为贝氏体组织,粗晶区主要为板条贝氏体束,由相互平行的贝氏体板条构成。细晶区主要为多边形铁素体和贝氏体组织。不完全重结晶区主要为重结晶的细小铁素体晶粒与原母材的微细贝氏体板条混合组织。
对焊接接头组织进行扫描电子显微镜观察和能谱分析的结果表明,焊缝区域针状铁素体以夹杂物为核心多维形核呈放射状生长。研究认为,该夹杂物主要是以Al2O3为核心形成的钛氧化物,以钛复合氧化物为核心形成铁素体。可能由于夹杂物中存在阳离子空位,使Mn通过阳离子空位扩散进入Ti2O3夹杂,或者由于溶质元素在夹杂物和铁素体中溶解度有差异,形成Mn溶质贫乏区,促进晶内铁素体形核。
利用EBSD对焊接接头组织进行晶体学取向研究的结果发现,焊缝针状铁素体晶粒取向并不完全随机分布,在某些晶体学方向上存在取向择优。各针状铁素体之间呈大角度晶界。从同一夹杂物上长出的针状铁素体,观察到沿同一方向背向生长的针状铁素体具有相同取向。粗晶区原奥氏体晶粒内从晶界向晶内有多个方向生长的板条贝氏体束,沿某一方向生长的板条贝氏体束具有明显的优势。生长方向不同的贝氏体束之间呈大角度晶界。粗晶区针状铁素体能有效分割原奥氏体晶粒,从而促进晶粒细化。观察到部分贝氏体与其邻近的针状铁素体板条取向相同或相近,本文认为,这些贝氏体可能以铁素体板条为基体在铁素体板条界面形核。
利用维氏硬度仪和纳米硬度测定仪对焊接接头组织进行了微观力学性能的测定。整体看来,维氏硬度值与纳米硬度随组织变化具有一致性。焊缝针状铁素体硬度最高,焊接热影响区粗晶区硬度最低。对于焊接热影响区粗晶区,由于晶粒粗化以及母材的一些强化途径在焊接热过程中受到削弱或消失,导致硬度降低。细晶区由细小的多边形铁素体组成,硬度较低。
The steel with high strength and good toughness can be realized by the refinement of its microstructures. However, the properties of weld joint of the steel are tended to be deteriorated due to the coarsening of microstructures during welding of the steel. In this thesis, the characterization, Electron Back Scattered Diffraction (EBSD) analysis and micro mechanical properties of weld joint microstructures of a ultra fine-grained high strength steel were studied, which was processed by a TMCP (Thermomechanical Control Process)-RPC (Relaxation-Precipitation-Controlled transformation) technique.
Microstructural observations of weld joint of the steel were conducted by means of optical microscopy and scanning electron microscopy. The weld joint consisted of weld metal, fusion region, coarse-grained region, fine-grained region and incomplete recrystallization region. The weld metal was mainly made up of fine acicular ferrite. The fusion region was mainly made up of bainitic ferrite. The coarse-grained region was mainly made up of bainitic ferrite packets which consisted of parallel bainitic ferrite laths or plates. The fine-grained region consisted of fine polygonal ferrite grains. The incomplete recrystallization region was made up of the mixture of fine recrystallized polygonal ferrite grains and fine lath-like or plate-like bainitic ferrite matrix.
SEM-EDS (Scanning Electron Microscopy-Electron Dispersed Spectrum) observations and analysis of weld metal was conducted on a field emission scanning electron microscope. Results showed that several ferrite laths or plates were nucleated on one inclusion. The inclusion consisted of an Al2O3 core and an outer layer of titanium oxide. The nucleation of intragranular ferrite was promoted by the formation of Mn depleted zone, probably due to Mn diffusion into titanium oxide by means of cation vacancies, or to the solubility difference of Mn in ferrite matrix and in inclusion.
EBSD analysis was conducted with a FE-SEM to investigate the crystallographic orientation of weld joint microstructures. Results indicated that the crystallographic orientation of acicular ferrite was not completely random. A certain crystallographic orientation was dominant for acicular ferrite. Acicular ferrite was separated by high angle boundaries. Some acicular ferrite laths or plates which were formed on one inclusion and grew in opposite directions had the same crystallographic orientation, presumably because it had some crystallographic orientation relations with the prior austenite. In coarse-grained region, some bainitic ferrite packets grew in a certain crystallographic orientation were dominant among the packets in a prior austenite. Bainitic ferrite packets grew in different directions were separated by high angle boundaries. The acicular ferrite laths or plates in coarse-grained region which was formed prior to bainitic ferrite packets sectioned the prior austenite into many small zones so that fine-grained mixed microstructures could thus be obtained. It was found that some banitic ferrite laths or plates which were adjacent to acicular ferrite laths or plates had the same or similar crystallographic orientation with them. These banitic ferrite laths or plates might be formed by sympathetic nucleation on the pre-formed acicular ferrite laths or plates.
The Vickers hardness and nano hardness measurements were carried out on BUEHLER MICROMET 5101 and Nano Indenter II (MTS), respectively. Generally speaking, the value of Vickers hardness and nano hardness were both in agreement with microstructure change in the weld joint. The hardness of weld metal was the highest and coarse-grained region lowest among the different regions in weld joint. The hardness of coarse-grained region was reduced by grain coarsening and the weakening or disappearing of matrix strengthening approaches during welding. The hardness of fine-grained region was lower, because the microstructure in this region was made up of fine polygonal ferrite grains.
利用光学显微镜,扫描电子显微镜对RPC超细组织钢的焊接接头进行观察与分析。焊接接头主要分为:焊缝,熔合区,粗晶区,细晶区和不完全重结晶区。焊缝组织为针状铁素体,熔合区主要为贝氏体组织,粗晶区主要为板条贝氏体束,由相互平行的贝氏体板条构成。细晶区主要为多边形铁素体和贝氏体组织。不完全重结晶区主要为重结晶的细小铁素体晶粒与原母材的微细贝氏体板条混合组织。
对焊接接头组织进行扫描电子显微镜观察和能谱分析的结果表明,焊缝区域针状铁素体以夹杂物为核心多维形核呈放射状生长。研究认为,该夹杂物主要是以Al2O3为核心形成的钛氧化物,以钛复合氧化物为核心形成铁素体。可能由于夹杂物中存在阳离子空位,使Mn通过阳离子空位扩散进入Ti2O3夹杂,或者由于溶质元素在夹杂物和铁素体中溶解度有差异,形成Mn溶质贫乏区,促进晶内铁素体形核。
利用EBSD对焊接接头组织进行晶体学取向研究的结果发现,焊缝针状铁素体晶粒取向并不完全随机分布,在某些晶体学方向上存在取向择优。各针状铁素体之间呈大角度晶界。从同一夹杂物上长出的针状铁素体,观察到沿同一方向背向生长的针状铁素体具有相同取向。粗晶区原奥氏体晶粒内从晶界向晶内有多个方向生长的板条贝氏体束,沿某一方向生长的板条贝氏体束具有明显的优势。生长方向不同的贝氏体束之间呈大角度晶界。粗晶区针状铁素体能有效分割原奥氏体晶粒,从而促进晶粒细化。观察到部分贝氏体与其邻近的针状铁素体板条取向相同或相近,本文认为,这些贝氏体可能以铁素体板条为基体在铁素体板条界面形核。
利用维氏硬度仪和纳米硬度测定仪对焊接接头组织进行了微观力学性能的测定。整体看来,维氏硬度值与纳米硬度随组织变化具有一致性。焊缝针状铁素体硬度最高,焊接热影响区粗晶区硬度最低。对于焊接热影响区粗晶区,由于晶粒粗化以及母材的一些强化途径在焊接热过程中受到削弱或消失,导致硬度降低。细晶区由细小的多边形铁素体组成,硬度较低。
The steel with high strength and good toughness can be realized by the refinement of its microstructures. However, the properties of weld joint of the steel are tended to be deteriorated due to the coarsening of microstructures during welding of the steel. In this thesis, the characterization, Electron Back Scattered Diffraction (EBSD) analysis and micro mechanical properties of weld joint microstructures of a ultra fine-grained high strength steel were studied, which was processed by a TMCP (Thermomechanical Control Process)-RPC (Relaxation-Precipitation-Controlled transformation) technique.
Microstructural observations of weld joint of the steel were conducted by means of optical microscopy and scanning electron microscopy. The weld joint consisted of weld metal, fusion region, coarse-grained region, fine-grained region and incomplete recrystallization region. The weld metal was mainly made up of fine acicular ferrite. The fusion region was mainly made up of bainitic ferrite. The coarse-grained region was mainly made up of bainitic ferrite packets which consisted of parallel bainitic ferrite laths or plates. The fine-grained region consisted of fine polygonal ferrite grains. The incomplete recrystallization region was made up of the mixture of fine recrystallized polygonal ferrite grains and fine lath-like or plate-like bainitic ferrite matrix.
SEM-EDS (Scanning Electron Microscopy-Electron Dispersed Spectrum) observations and analysis of weld metal was conducted on a field emission scanning electron microscope. Results showed that several ferrite laths or plates were nucleated on one inclusion. The inclusion consisted of an Al2O3 core and an outer layer of titanium oxide. The nucleation of intragranular ferrite was promoted by the formation of Mn depleted zone, probably due to Mn diffusion into titanium oxide by means of cation vacancies, or to the solubility difference of Mn in ferrite matrix and in inclusion.
EBSD analysis was conducted with a FE-SEM to investigate the crystallographic orientation of weld joint microstructures. Results indicated that the crystallographic orientation of acicular ferrite was not completely random. A certain crystallographic orientation was dominant for acicular ferrite. Acicular ferrite was separated by high angle boundaries. Some acicular ferrite laths or plates which were formed on one inclusion and grew in opposite directions had the same crystallographic orientation, presumably because it had some crystallographic orientation relations with the prior austenite. In coarse-grained region, some bainitic ferrite packets grew in a certain crystallographic orientation were dominant among the packets in a prior austenite. Bainitic ferrite packets grew in different directions were separated by high angle boundaries. The acicular ferrite laths or plates in coarse-grained region which was formed prior to bainitic ferrite packets sectioned the prior austenite into many small zones so that fine-grained mixed microstructures could thus be obtained. It was found that some banitic ferrite laths or plates which were adjacent to acicular ferrite laths or plates had the same or similar crystallographic orientation with them. These banitic ferrite laths or plates might be formed by sympathetic nucleation on the pre-formed acicular ferrite laths or plates.
The Vickers hardness and nano hardness measurements were carried out on BUEHLER MICROMET 5101 and Nano Indenter II (MTS), respectively. Generally speaking, the value of Vickers hardness and nano hardness were both in agreement with microstructure change in the weld joint. The hardness of weld metal was the highest and coarse-grained region lowest among the different regions in weld joint. The hardness of coarse-grained region was reduced by grain coarsening and the weakening or disappearing of matrix strengthening approaches during welding. The hardness of fine-grained region was lower, because the microstructure in this region was made up of fine polygonal ferrite grains.
引文
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