热循环对低碳贝氏体焊缝金属组织和性能的影响研究
发布时间：2018-11-14 16:59
【摘要】：近些年来钢铁材料的发展非常迅速,其力学性能和焊接性能都大幅度提高,且用途也多元化,超低碳贝氏体钢就是其中的一种。但是对于性能如此优良的钢种而言,如何提升焊缝性能使其与母材性能相匹配是非常重要的。因此研究焊缝金属组织与性能之间的关系,对这种性能优良的贝氏体钢的轧制过程与焊接工艺都是非常有意义的。焊缝金属的组织和性能不仅受焊缝中合金元素成分的影响,同时也受焊接工艺参数的影响。通常在多层多道焊焊接过程中,焊缝金属再热区会发生晶粒粗化进而导致冲击韧性变差,通过焊缝金属的连续冷却转变(CCT)曲线能够预测焊缝金属的组织和性能。本课题通过Gleeble-3800热/力模拟系统对两种Ni含量存在差别的低碳贝氏体焊缝金属进行了不同连续冷却速率下的热循环模拟试验,测得热循环过程中膨胀量随温度的变化曲线。对经历不同热循环下的焊缝进行了微观组织观察和显微硬度测定,绘制了该焊缝金属的连续冷却转变曲线(CCT曲线)和组织转变比例变化图;再将上述用于测定CCT曲线的热模拟工艺分别用于这两种贝氏体焊缝金属的冲击试样,对经过热模拟的冲击试样进行室温和低温(-50℃)的夏比冲击试验,测得不同连续冷却速度下的冲击韧性,绘制出贝氏体焊缝金属在不同冷却速度下冲击韧性的转变规律图,并研究不同Ni含量对冷却后组织相变以及奥氏体晶粒大小的影响。贝氏体焊缝金属的热模拟试验、热模拟粗晶区组织观察、显微硬度测试和热模拟夏比冲击试验结果表明:冷却速率不断增大,含Ni为0%贝氏体焊缝金属热模拟粗晶区组织变化为:准多边形铁素体→针状铁素体(少量)+粒状贝氏体,硬度值总体呈上升趋势,但由于组织类型和组织比例的变化,硬度曲线上升的快慢也有所不同,室温冲击韧性和-50℃冲击韧性变化规律一致,在低冷却速度范围,冲击韧性较差且变化不大,中等冷却速度范围,冲击韧性迅速上升,高冷却速度范围,冲击韧性达到最高值后缓慢下降;Ni含量为4%的贝氏体焊缝金属热模拟粗晶区组织转变为:粒状贝氏体+块状铁素体(少量)→板条贝氏体+针状铁素体(少量)+马氏体,硬度值呈先下降后上升趋势,在粒状贝氏体中粗大块状M-A组元含量最少时达到最小值,室温冲击韧性呈现上升后下降趋势,在中等偏慢冷却速度即粗大块状M-A组元含量最少时达到最高值,-50℃冲击韧性也呈现上升后下降趋势,在中等偏高冷却速度即板条贝氏体含量最多时达到最高值。Ni含量对贝氏体焊缝金属影响的研究表明:在贝氏体焊缝金属中加入合金元素Ni,可以强烈推迟铁素体的转变,降低贝氏体的相变温度区间,扩大形成贝氏体的冷速范围,有利于形成更多的板条贝氏体,在高冷却速度下还可形成马氏体。4%Ni含量的焊缝金属热模拟粗晶区原始奥氏体晶粒尺寸要小于0%Ni含量的焊缝金属,说明添加适量的合金元素Ni可以起到细化晶粒的作用。Ni元素有利于形成更多板条贝氏体和细化晶粒的作用都可提高焊缝金属的低温韧性。而Ni含量过高的副作用在于:在较慢冷却速度下容易造成元素偏析形成硫化物和磷化物,导致韧性断裂的断口上出现缺陷,致使冲击韧性变差,在较大冷却速度下硬化相马氏体含量增加,导致塑性变形较大的断口上出现解理面和缺陷,使得韧性变差。
[Abstract]:In recent years, the development of steel materials is very rapid, the mechanical property and the welding performance are greatly improved, and the application is also diversified, and the ultra-low carbon bainite steel is one of them. However, it is very important to improve the performance of the weld to match the properties of the parent material for steel grades that are such excellent in performance. Therefore, the relationship between the microstructure and the properties of the weld is studied, and the rolling process and the welding process of the bainitic steel with excellent performance are very important. The microstructure and properties of the weld metal are not only affected by the composition of the alloy elements in the weld, but also the influence of the parameters of the welding process. In the process of multi-layer multi-track welding, the grain coarsening occurs in the weld metal and the impact toughness is poor, and the microstructure and performance of the weld metal can be predicted by the continuous cooling transformation (CCT) curve of the weld metal. The thermal cycling simulation of the low-carbon bainite weld metal with the difference of the two Ni contents is carried out by the Gleeble-3800 thermal/ force simulation system, and the variation curve of the expansion quantity with the temperature during the thermal cycle is measured. The microstructure and microhardness of the weld under different thermal cycling were measured, and the continuous cooling transition curve (CCT curve) of the weld metal and the change of the tissue transformation ratio were drawn. the thermal simulation process used for measuring the CCT curve is used for the impact test samples of the two bainitic weld metal, and the impact toughness at different continuous cooling rates is measured for the summer specific impact test of the heat-simulated impact test sample at room temperature and low temperature (-50 DEG C), The effect of different Ni content on the microstructure and grain size after cooling was studied. The results of thermal simulation of the bainitic weld metal, the microstructure observation, the micro-hardness test and the thermal simulation of the thermal simulation of the crude crystal area show that the cooling rate is increasing, and the microstructure of the metal heat-simulated coarse-crystal zone containing Ni in the range of 0% is as follows: In the quasi-polygonal ferrite, the acicular ferrite (small amount) + granular bainite and the hardness value of the ferrite are generally on the rise, but due to the change of the tissue type and the proportion of the tissue, the speed of the increase of the hardness curve is also different, and the impact toughness of the room temperature and the impact toughness of the-50 DEG C are consistent, in the low cooling speed range, the impact toughness is poor and the change is not large, the medium cooling speed range, the impact toughness is rapidly increased, the high cooling speed range is high, the impact toughness reaches the highest value, and the low-cooling speed range is slowly reduced; and the microstructure of the bainite weld metal thermal simulation coarse-crystal region with the Ni content of 4 percent is converted into the following: The granular bainite + block ferrite (small amount) is a lath bainite + needle-like ferrite (small amount) + martensite, the hardness value of the granular bainite + acicular ferrite (small amount) and the martensite and the hardness value of the granular bainite + block ferrite reaches a minimum when the content of the coarse block M-A group in the granular bainite reaches a minimum, and the impact toughness of the room temperature exhibits a downward trend after the increase of the temperature of the room temperature. The maximum value is reached at the medium partial slow cooling rate, that is, the content of the coarse block M-A group is the least, and the impact toughness of-50 DEG C also shows a downward trend after the rise, and the maximum value is reached when the medium-bias high cooling speed, that is, the lath bainite content is the most. The study of the effect of Ni content on the metal of the bainitic weld shows that the transformation of the ferrite can be strongly retarded by the addition of the alloy element Ni to the bainitic weld metal, the phase transition temperature range of the bainite is reduced, the cold-speed range of the bainite is expanded, and more lath bainite is formed, At the high cooling rate, the martensite is also formed. The weld metal with the content of 4% Ni can be used to simulate the weld metal with the original austenite grain size of less than 0% Ni in the rough crystal region, and the addition of the appropriate amount of the alloy element Ni can play a role in refining the crystal grains. The Ni element is beneficial to the formation of more lath bainite and refined grain, and the low-temperature toughness of the weld metal can be improved. and the side effect of high Ni content is that a sulfide and a phosphide are easily caused to form a sulfide and a phosphide at a slow cooling speed, so that defects on the fracture of the ductile fracture are caused, so that the impact toughness is poor, and the martensite content of the hardened phase is increased at the higher cooling speed, The fracture surface and the defect are formed on the fracture with large plastic deformation, so that the toughness is poor.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG406
本文编号：2331764
[Abstract]:In recent years, the development of steel materials is very rapid, the mechanical property and the welding performance are greatly improved, and the application is also diversified, and the ultra-low carbon bainite steel is one of them. However, it is very important to improve the performance of the weld to match the properties of the parent material for steel grades that are such excellent in performance. Therefore, the relationship between the microstructure and the properties of the weld is studied, and the rolling process and the welding process of the bainitic steel with excellent performance are very important. The microstructure and properties of the weld metal are not only affected by the composition of the alloy elements in the weld, but also the influence of the parameters of the welding process. In the process of multi-layer multi-track welding, the grain coarsening occurs in the weld metal and the impact toughness is poor, and the microstructure and performance of the weld metal can be predicted by the continuous cooling transformation (CCT) curve of the weld metal. The thermal cycling simulation of the low-carbon bainite weld metal with the difference of the two Ni contents is carried out by the Gleeble-3800 thermal/ force simulation system, and the variation curve of the expansion quantity with the temperature during the thermal cycle is measured. The microstructure and microhardness of the weld under different thermal cycling were measured, and the continuous cooling transition curve (CCT curve) of the weld metal and the change of the tissue transformation ratio were drawn. the thermal simulation process used for measuring the CCT curve is used for the impact test samples of the two bainitic weld metal, and the impact toughness at different continuous cooling rates is measured for the summer specific impact test of the heat-simulated impact test sample at room temperature and low temperature (-50 DEG C), The effect of different Ni content on the microstructure and grain size after cooling was studied. The results of thermal simulation of the bainitic weld metal, the microstructure observation, the micro-hardness test and the thermal simulation of the thermal simulation of the crude crystal area show that the cooling rate is increasing, and the microstructure of the metal heat-simulated coarse-crystal zone containing Ni in the range of 0% is as follows: In the quasi-polygonal ferrite, the acicular ferrite (small amount) + granular bainite and the hardness value of the ferrite are generally on the rise, but due to the change of the tissue type and the proportion of the tissue, the speed of the increase of the hardness curve is also different, and the impact toughness of the room temperature and the impact toughness of the-50 DEG C are consistent, in the low cooling speed range, the impact toughness is poor and the change is not large, the medium cooling speed range, the impact toughness is rapidly increased, the high cooling speed range is high, the impact toughness reaches the highest value, and the low-cooling speed range is slowly reduced; and the microstructure of the bainite weld metal thermal simulation coarse-crystal region with the Ni content of 4 percent is converted into the following: The granular bainite + block ferrite (small amount) is a lath bainite + needle-like ferrite (small amount) + martensite, the hardness value of the granular bainite + acicular ferrite (small amount) and the martensite and the hardness value of the granular bainite + block ferrite reaches a minimum when the content of the coarse block M-A group in the granular bainite reaches a minimum, and the impact toughness of the room temperature exhibits a downward trend after the increase of the temperature of the room temperature. The maximum value is reached at the medium partial slow cooling rate, that is, the content of the coarse block M-A group is the least, and the impact toughness of-50 DEG C also shows a downward trend after the rise, and the maximum value is reached when the medium-bias high cooling speed, that is, the lath bainite content is the most. The study of the effect of Ni content on the metal of the bainitic weld shows that the transformation of the ferrite can be strongly retarded by the addition of the alloy element Ni to the bainitic weld metal, the phase transition temperature range of the bainite is reduced, the cold-speed range of the bainite is expanded, and more lath bainite is formed, At the high cooling rate, the martensite is also formed. The weld metal with the content of 4% Ni can be used to simulate the weld metal with the original austenite grain size of less than 0% Ni in the rough crystal region, and the addition of the appropriate amount of the alloy element Ni can play a role in refining the crystal grains. The Ni element is beneficial to the formation of more lath bainite and refined grain, and the low-temperature toughness of the weld metal can be improved. and the side effect of high Ni content is that a sulfide and a phosphide are easily caused to form a sulfide and a phosphide at a slow cooling speed, so that defects on the fracture of the ductile fracture are caused, so that the impact toughness is poor, and the martensite content of the hardened phase is increased at the higher cooling speed, The fracture surface and the defect are formed on the fracture with large plastic deformation, so that the toughness is poor.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG406
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