熱循環(huán)對低碳貝氏體焊縫金屬組織和性能的影響研究
發(fā)布時間：2018-11-14 16:59
【摘要】：近些年來鋼鐵材料的發(fā)展非常迅速,其力學性能和焊接性能都大幅度提高,且用途也多元化,超低碳貝氏體鋼就是其中的一種。但是對于性能如此優(yōu)良的鋼種而言,如何提升焊縫性能使其與母材性能相匹配是非常重要的。因此研究焊縫金屬組織與性能之間的關系,對這種性能優(yōu)良的貝氏體鋼的軋制過程與焊接工藝都是非常有意義的。焊縫金屬的組織和性能不僅受焊縫中合金元素成分的影響,同時也受焊接工藝參數(shù)的影響。通常在多層多道焊焊接過程中,焊縫金屬再熱區(qū)會發(fā)生晶粒粗化進而導致沖擊韌性變差,通過焊縫金屬的連續(xù)冷卻轉(zhuǎn)變(CCT)曲線能夠預測焊縫金屬的組織和性能。本課題通過Gleeble-3800熱/力模擬系統(tǒng)對兩種Ni含量存在差別的低碳貝氏體焊縫金屬進行了不同連續(xù)冷卻速率下的熱循環(huán)模擬試驗,測得熱循環(huán)過程中膨脹量隨溫度的變化曲線。對經(jīng)歷不同熱循環(huán)下的焊縫進行了微觀組織觀察和顯微硬度測定,繪制了該焊縫金屬的連續(xù)冷卻轉(zhuǎn)變曲線(CCT曲線)和組織轉(zhuǎn)變比例變化圖;再將上述用于測定CCT曲線的熱模擬工藝分別用于這兩種貝氏體焊縫金屬的沖擊試樣,對經(jīng)過熱模擬的沖擊試樣進行室溫和低溫(-50℃)的夏比沖擊試驗,測得不同連續(xù)冷卻速度下的沖擊韌性,繪制出貝氏體焊縫金屬在不同冷卻速度下沖擊韌性的轉(zhuǎn)變規(guī)律圖,并研究不同Ni含量對冷卻后組織相變以及奧氏體晶粒大小的影響。貝氏體焊縫金屬的熱模擬試驗、熱模擬粗晶區(qū)組織觀察、顯微硬度測試和熱模擬夏比沖擊試驗結(jié)果表明:冷卻速率不斷增大,含Ni為0%貝氏體焊縫金屬熱模擬粗晶區(qū)組織變化為:準多邊形鐵素體→針狀鐵素體(少量)+粒狀貝氏體,硬度值總體呈上升趨勢,但由于組織類型和組織比例的變化,硬度曲線上升的快慢也有所不同,室溫沖擊韌性和-50℃沖擊韌性變化規(guī)律一致,在低冷卻速度范圍,沖擊韌性較差且變化不大,中等冷卻速度范圍,沖擊韌性迅速上升,高冷卻速度范圍,沖擊韌性達到最高值后緩慢下降;Ni含量為4%的貝氏體焊縫金屬熱模擬粗晶區(qū)組織轉(zhuǎn)變?yōu)?粒狀貝氏體+塊狀鐵素體(少量)→板條貝氏體+針狀鐵素體(少量)+馬氏體,硬度值呈先下降后上升趨勢,在粒狀貝氏體中粗大塊狀M-A組元含量最少時達到最小值,室溫沖擊韌性呈現(xiàn)上升后下降趨勢,在中等偏慢冷卻速度即粗大塊狀M-A組元含量最少時達到最高值,-50℃沖擊韌性也呈現(xiàn)上升后下降趨勢,在中等偏高冷卻速度即板條貝氏體含量最多時達到最高值。Ni含量對貝氏體焊縫金屬影響的研究表明:在貝氏體焊縫金屬中加入合金元素Ni,可以強烈推遲鐵素體的轉(zhuǎn)變,降低貝氏體的相變溫度區(qū)間,擴大形成貝氏體的冷速范圍,有利于形成更多的板條貝氏體,在高冷卻速度下還可形成馬氏體。4%Ni含量的焊縫金屬熱模擬粗晶區(qū)原始奧氏體晶粒尺寸要小于0%Ni含量的焊縫金屬,說明添加適量的合金元素Ni可以起到細化晶粒的作用。Ni元素有利于形成更多板條貝氏體和細化晶粒的作用都可提高焊縫金屬的低溫韌性。而Ni含量過高的副作用在于:在較慢冷卻速度下容易造成元素偏析形成硫化物和磷化物,導致韌性斷裂的斷口上出現(xiàn)缺陷,致使沖擊韌性變差,在較大冷卻速度下硬化相馬氏體含量增加,導致塑性變形較大的斷口上出現(xiàn)解理面和缺陷,使得韌性變差。
[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.
【學位授予單位】：蘭州理工大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG406
本文編號：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.
【學位授予單位】：蘭州理工大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG406
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