Ti处理改善船体钢焊接HAZ组织与性能研究

【作者】 杨银辉；

【导师】 代建清； 苏航；

【作者基本信息】 昆明理工大学 ， 材料物理与化学， 2008， 硕士

【摘要】 在大热输入焊接的条件下,传统Al处理船体钢中易出现焊接HAZ低温韧性下降,焊接裂纹敏感性增加的问题。在炼钢过程中产生弥散分布的Ti氧化物能促进晶内针状铁素体的形核,有效改善HAZ的组织和性能,能促进针状铁素体形核的第二相夹杂物中,Ti₂O₃在高温下最稳定,在焊接过程中促进针状铁素体形核能力最强。本文先对Ti氧化物促进晶内针状铁素体形核机理进行分析。对于不含Nb和其他强化元素的Ti脱氧钢采用Gleeble3500D进行连续冷却淬火试验。相变温度为725℃时,晶界铁素体和侧板条铁素体在晶界开始形成,同时晶内针状铁素体开始形核长大。到650℃时晶界铁素体已基本完全形成,晶内针状铁素体迅速长大,抑制了侧板条铁素体由晶界向晶内生长。575℃时针状铁素体组织已在奥氏体晶内形成大量交错密排的组织。利用Bonding扩散试验从宏观角度探讨了Ti₂O₃周围贫锰区（MDZ）形成的机理,以及高温保温时间对其形核能力的影响。结果表明,在Ti₂O₃粉末与基体界面处有铁素体带形成,而Al₂O₃粉末与基体界面处无铁素体带形成,说明Ti₂O₃有较强的促进铁素体形核能力。随着奥氏体化温度的降低,界面处铁素体层的宽度显著下降,当奥氏体化温度为950℃时,铁素体带消失。电子探针分析发现钢的基体材料与氧化物接触的过渡层中形成了约10μm贫锰区。Ti₂O₃吸附了周围的Mn原子,促进了贫锰区形成。随着高温保温时间的增加,Ti氧化物促进针状铁素体形核能力下降。利用焊接物理模拟技术,在Gleeble1500D热验机上对不同热输入下的焊接粗晶区组织进行模拟。-20℃焊接热模拟冲击试验表明,当焊接线能量大于50KJ/cm时,Ti处理船体钢的焊接粗晶区的冲击韧性明显高于Al处理船体钢,在75KJ/cm焊接热输入时-20℃冲击功达60J。气体保护焊试验中,Al处理钢在大热输入焊接（50KJ/cm）时,其焊接粗晶区.20℃冲击功小于40J,而Ti处理钢在50KJ/cm和20KJ/cm的焊接热输入条件下,焊接粗晶区-20℃冲击功均大于140J,因此Ti处理较大地改善了船体钢焊接粗晶区低温韧性。Ti处理船体钢母材夹杂主要是以Ti氧化物为核心的圆形夹杂,少量MnS在其外层析出,其粒径主要分布在2～3.2μm,t_8/5=100s（75KJ/cm）的焊接热输入对其形貌、成分及尺寸无影响。Al处理船体钢母材中夹杂主要是以Al₂O₃为核心,外层是较多的富Ti的TiN+Mn-Ti-O析出,边缘有少量MnS的复合夹杂,其粒径主要分布在2.5～3.5μm。在t_8/5=100s的焊接热循环后,氧化物外层富钛的析出物发生了分解,粒径减小,得到复合夹杂以Al₂O₃为核心,外层是少量富Ti的（Ti,Nb）（C,N）析出。对Ti处理船体钢采用50KJ/cm焊接热循环,当温度连续冷却到600℃、650℃淬火,得到了较多的Ti氧化物促进针状铁素体形核长大组织。采用SEM和金相显微镜在t_8/5=100s、60s的热循环组织中观察到了Ti氧化物促进针状铁素体形核长大组织,晶内针状铁素板条减少了粒状贝氏体的数量,有效分割晶粒,细化了组织。在900～1100℃的奥氏体化温度区间,Al处理船体钢奥氏体晶粒长大速度慢于Ti处理船体钢,当奥氏体化温度超过1100℃时,Al处理钢晶粒异常长大,Ti处理钢晶粒尺寸仍正常长大。Al处理钢在1300℃奥氏体晶粒尺寸达到180μm,明显高于Ti处理钢。焊接峰值温度Tm=1350℃时,当t_8/5小于40s时,Ti处理钢的晶粒长大速度要高于Al处理钢,当t8/5＞40s时,Al处理钢奥氏体晶粒长大速度远高于Ti处理钢,t8/5=100s,Al处理钢奥氏体晶粒尺寸为160μm,明显大于Ti处理钢。更多还原

【Abstract】 The low temperature toughness declined and cold cracking susceptibility increased in traditional Al treated ship hull steel when the high welding heat input was adopt.The disperse distribution of Ti oxides inclusions promoted nucleation of intragranular accicular ferrite in steel making process and it was effective to improve the microstructures and properities of HAZ（Heat Affected Zone）in high strength low alloy ship hull steel.Ti₂O₃ was the most stable inclusions at high temperature and showed the strongest ability to promote the formation of acicular ferrite during welding among the second phase inclusions.In this paper,first the mechanism about how the intragranular ferrite nucleation facilitated on the Ti oxides was studied.The continuous quenching test of Ti deoxidized steel was performed using Gleeble3500D without Nb and other strengthing elements.With a decrease in temperature to 725℃,the GBF（grain boundary ferrite）and FSP（ferrite side plate）begin to nucleate along the austenite grain boundaries,at the same time the acicular ferrite mucleate at Ti₂O₃ within austenite grains.The phase transformation of GBF basically completed and the acicular ferrite intragranularly growed rapidly which suppressed the growth of FSB at 650℃,the interlocking microstructure of acicular ferrite formed in austenite grains when temperature continued to 575℃.Bonding diffusion experiment has been performed to analyze the mechanism of MDZ（Mn-depleted zone）around Ti₂O₃ from macro perspective and the effect of high temperature holding time on ferrite nucleation ability has also been discussed.The results showed that the ferrite layers formed along the interfaces between the steels and the Ti₂O₃ powders and the ferrite layers hasn’t been found along the interfaces between the steels and the Al₂O₃ powders.It confirmed that Ti₂O₃ particle has high ability to promote acicular ferrite nucleation.The width of the MDZ decreased with decreasing austenitizing temperature and the ferrite layers disappeared the austenitizing temperature was at 950℃.The 10μm width of MDZ formed near steel- Ti₂O₃ interfaces in the bonded apecimens was analyzed by electron probe microanalysis（EPMA）.The MDZ developed in the vicility of steel- Ti₂O₃ powder interfaces because Ti₂O₃ itself absorb neighboring Mn within an austenite matrix.The nucleation ability decreased at Ti₂O₃ with increasing of high temperature holding time. The thermal cycle of CGHAZ for the welding heat input with different t_8/5was simulated using Gleeble1500D.The charpy v-notch impact toughness at -20℃showed that the toughness of CGHAZ in Ti treated steel was higher than Al treated steel when the welding heat input was higher than 50KJ/cm,and the toughness of Ti treated hull ship steel attained 60J when the welding heat input was 75KJ/cm.It is concluded from Gas Metal Arc welding experiment at -20℃that the toughness of CGHAZ in Al treated steel was lower than 40J with the welding heat input of 50KJ/cm,but the CGHAZ in Ti treated steel was higher than 140J with the welding heat input of 50KJ/cm and 20KJ/cm.So the Ti treated steel improved low temperature toughness of CGHAZ greatly in ship hull steel.It is found that most spherical inclusions in Ti treated steel had Ti rich oxide core with a little MnS layer on the surface of particles,ranging from 2 to 3.2μn,the morphology、size and chemical composition hadn’t been affected after thermal cycle with welding heat input of 50KJ/cm.It is also found that most complex irregular inclusions in Al treated steel had Al rich oxides core with outer layer of TiN+Mn-Ti-O and a little MnS on the edge of particles,ranging from 2.5 to 3.5μm.The precipitates of outer layer in complex inclusions dissolved and the size of particles decreased after thermal cycle with welding heat input of 75KJ/cm.The complex inclusions had mostly Al₂O₃ core with outer layer of （Ti,Nb）（C,N）.The thermal cycle for the welding heat input of 50KJ/cm was simulated,once the samples continued to 600℃and 650℃,they were quenched in water and a number of acicular ferrites nucleated at Ti oxides were got within austenite grain.The microstructures of acicular ferrites were observed with welding heat input of t_8/5=100s and t_8/5=60s using Metallurgical Microscope and SEM,The acicular ferrite decreased the amount of granular bainite and it split austenite grain and the the microstructures were refined.The austenite grain size growed more quickly in Ti treated hull ship steel than that in Al treated hull ship steel when austenite temperature range from 900℃to 1100℃,The Al treated steel started abnormal grain growth when the austenite temperature above 1300℃,but the Ti treated steel still continued to normal growth.The austenite grain size（180μm）was much more larger in Al treated steel than that in Ti treated steel when the temperature reached 1300℃.As the thermal cycle peak temperature was at 1350℃,The austenite grain size growed more quickly in Ti treated hull ship steel than that in Al treated hull ship steel when t_8/5＜40s,the growth of anstenite was much more quickly in Al treated steel than that in Ti treated steel when t_8/5＞40s,and the size of austenite grain was 160μm in Al treated steel which was much more larger than Ti treated steel.更多还原