【作者】 柴锋；

【导师】 徐洲； 杨才福；

【作者基本信息】 上海交通大学 ， 材料学， 2008， 博士

【摘要】 为改善低合金高强度船体钢焊接性和提高船体钢大热输入焊接的适应性,本文研究了超低碳合金设计以及钢中第二相粒子（包括碳氮化物、氧化物）对低合金高强度船体钢焊接热影响区（Heat Affected Zone,以下简称HAZ）组织和性能的影响,探讨了HAZ的韧化机理。结果表明,采用超低碳成分设计,大幅度地降低钢中的碳含量及碳当量,利用细晶强化或Cu的时效沉淀强化作用可以弥补降碳带来的强度损失。采用上述技术思路设计的超低碳440MPa和550MPa试验钢较传统船体钢焊接性显著提高;不同类型的第二相粒子促进铁素体形核的能力各不相同,在本论文研究的各种第二相粒子（Ti、Zr、Ce、Mg的氧化物、Ti的氮化物）中,以Ti2O3在高温条件下最为稳定,焊接过程中促进铁素体形核的能力最强;研究发现了钢中Ti氧化物的获得方法,利用Ti脱氧取代传统Al脱氧的方法可以在钢中得到大量含Ti氧化物,钢中的含Ti氧化物促进了晶内针状铁素体的形核,抑制了晶界铁素体和侧板条铁素体的向晶内的生长,间接的细化了粗大的奥氏体晶粒尺寸;在Ti处理钢中,同时添加微量Mg（约0.0010%）或Zr（约0.0043%）可进一步细化第二相粒子的尺寸,增大第二相粒子的表面积,从而进一步增强第二相粒子促进晶内针状铁素体形核的能力,大幅度的提高了HAZ的低温韧性。在传统440MPa和550MPa级较高碳含量的船体钢基础上,分别开展了超低碳合金成分设计。为保证设计强度,440MPa级船体钢主要采用Nb微合金化+TMCP工艺弥补降碳带来的强度损失,550MPa级船体钢则主要利用Cu在时效过程中的弥散沉淀强化提高钢的强度。结果表明:采用超低碳合金设计后,两种强度级别的试验钢母材均获得了良好的强韧性配合,HAZ低温韧性和抗冷裂纹敏感性显著提高,实现了试验钢0℃不预热焊接的目标。研究发现,采用超低碳+细晶强化或超低碳+时效沉淀强化两种技术思路,研制高强度、高韧性和良好焊接性的新型船体钢是可行的。为选择最佳的第二相粒子,首次利用加压Bonding试验,系统的研究了Ti₂O₃、TiO₂、ZrO₂、TiN、CeO₂、MgO以及Ti₂O₃+MgO混合粉末对铁素体相变的促进作用。研究结果首次发现,上述各种第二相粒子促进铁素体相变的机理是不同的。其大致可分为三类:第一类为界面上和钢发生了化学反应,造成界面附近形成贫Mn区、贫Si区,从而促进了铁素体相变。这类粉末包括TiO₂、ZrO₂、CeO₂、MgO。第二类为在界面上和钢不发生反应,也能促进铁素体相变,Ti₂O₃粉末属于此类。这类夹杂主要通过吸收界面附近区域的Mn形成贫Mn区（MDZ）,促进了铁素体相变。第三类为在界面上和钢不发生反应,也不能促进铁素体相变,TiN属于此类。奥氏体化温度对Ti₂O₃诱导铁素体形核存在显著影响,研究首次发现,Ti₂O₃诱导晶内铁素体相变取决于Mn在奥氏体中的扩散速度。奥氏体化温度较高（>950℃）时,Mn在奥氏体中扩散较快,易形成贫Mn区,显著促进铁素体相变。反之,Ti₂O₃促进铁素体相变能力明显减弱。在本文研究的第二相粒子粉末中,Ti₂O₃高温稳定性最好,促进铁素体形核的能力最强。研究了Ti及Ti复合氧化物对HAZ组织性能的影响,同时对Al、Ti处理钢焊接粗晶区连续冷却相变规律进行了深入探讨。结果表明,Ti及Ti的复合氧化物具有促进晶内针状铁素体形核的能力,大热输入焊接时HAZ的低温韧性显著提高。Ti处理钢焊接粗晶区连续冷却过程中,晶界铁素体、侧板条铁素体以及晶内铁素体的相变开始温度基本相同,但各自长大的动力学条件不同。晶界铁素体在725～650℃温度区间快速长大,而侧板条铁素体和晶内铁素体则在650～500℃温度区间快速长大。针状铁素体在晶内第二相夹杂物上形核长大后抑制了侧板条铁素体沿着奥氏体晶界向晶内的生长。试验结果还表明,在钢中获得大量的、细小的、弥散分布的含Ti氧化物是提高船体钢大热输入焊接性的技术关键。在Ti氧化物显著促进晶内铁素体相变、大幅度提高HAZ韧性的基础上,为进一步细化含Ti氧化物夹杂,提高钢HAZ韧性,首次系统的研究了微量Mg、Zr对Ti处理钢中第二相粒子尺寸及其大热输入焊接时HAZ组织和韧性的影响。研究首次发现,Mg含量对钢中第二相粒子的类型、尺寸存在显著影响。当钢中添加12ppmMg时,钢中形成等摩尔数的Ti₂O₃和Mg₂TiO₄氧化物夹杂,此时钢中含Ti氧化物夹杂的粒度最小,数量最多,大热输入焊接时HAZ的低温韧性最高。微量Mg加入到钢中能降低含Ti氧化物聚集长大的能力,有效的细化了第二相氧化物夹杂的尺寸,提高了氧化物夹杂促进针状铁素体形核的能力。Zr含量对钢中第二相粒子的类型和尺寸同样存在显著影响。当钢中添加43ppmZr时,钢中形成等摩尔数的Ti₂O₃和ZrO₂,此时含Ti第二相粒子的粒度最为细小,数量最多,大热输入焊接时HAZ的低温韧性最高。这主要是由于Zr氧化物夹杂比重较大,不容易在钢液中上浮,细小的ZrO₂夹杂能够为随后形成的含Ti氧化物夹杂提供大量形核核心,避免含Ti氧化物的聚集长大,增加了晶内针状铁素体的形核核心的密度。更多还原

【Abstract】 To improve the weldability and high heat input welding adaptability of high strength low alloy ship hull steel, effect of ultra-low carbon alloy designing and the second phase particles （including carbonitride and oxide） on the microstructures and properties of Heat Affected Zone （HAZ） in high strength low alloy ship hull steel were studied in this dissertation. The results showed that carbon content and carbon equivalent （Ceq） were greatly decreased when adopting ultra-low carbon alloy designing. Yield strength of the steel significantly increased through grain refinement and Cu precipitation. Weldability of ultra-low carbon content 440MPa and 550MPa ship hull steel significantly increased after adopting the upper alloy designing idea. Different inclusions have different ability to promote the nucleation of ferrite. For all the inclusions focused in this dissertation, Ti₂O₃ was the most stable inclusions at high temperature and showed the strongest ability to promote the formation of acicular ferrite during welding. Large amounts of Ti oxides gained through Ti treated, not traditional Al treated, and the Ti-bearing oxide inclusions promote the formation of acicular ferrite which suppressed the growth of grain boundary ferrite （GBF） and ferrite side plate （FSP）, it refined the austenite grain size indirectly. Microcontent Mg （⁰.0010%Mg） or Zr （⁰.0043%Zr） added to the Ti steel could further refine the size of inclusions and increased the area fraction of inclusions, which increased the ability of Ti-bearing inclusions promoting the formation of acicular ferrite and improve the toughness of HAZ significantly.Ultra-low carbon alloy designing was carried out based on traditional 440MPa and 550MPa high carbon content ship hull steel. Nb microalloying+TMCP technique were used to improve the yield strength of 440MPa steel and Cu age hardening precipitation was employed to improve the yield strength of 550MPa steel. The results showed that the two strength grade steel both gained the high strength and toughness after ultra-low carbon alloy designing, and low temperature toughness and cold cracking resistance were significantly increased, the experimental steel could be welded at 0℃with no preheating. It was feasible to gain high strength, high toughness and good weldability for the ship hull steel through ultra-low carbon content alloy designing with grain refining strengthening or with age hardening precipitation.The role of Ti₂O₃, TiO₂, ZrO₂, TiN, CeO₂, MgO and Ti₂O₃+MgO compound inclusions promoting the nucleation of ferrite were systematic investigated through force bonding experiment. The results showed the mechanism of inclusions promoting the nucleation of ferrite is different for different inclusions. It including the following three types: chemical reaction occurred in the boundary, and Mn/Si depletion region formed which promote the nucleation of ferrite. These inclusions including TiO₂, ZrO₂, CeO₂, MgO; no chemical reaction occurred in the boundary and the inclusions also promote the nucleation of ferrite, the inclusions including Ti₂O₃; no chemical reaction occurred and no ferrite formed in the boundary, the inclusions including TiN. The austenite temperature showed great effect on the ability of Ti₂O₃ promoting the nucleation of ferrite. Mn diffused much faster in the austenite at higher austenite temperature, which was favorable for the inclusions promoting the nucleation of ferrite. Ti₂O₃ was the most stable inclusions at high temperature and showed the strongest ability to promote nucleation of ferrite in the present study.Effect of Ti and Ti compound oxides on the microstructures and properties of HAZ and the phase transformation regularity of coarse-grain HAZ in Ti treated steel were investigated. The results showed that Ti and Ti compound oxides could promote the formation of acicular ferrite, the low temperature toughness of high heat input welding HAZ significantly improved. The phase transformation starting temperature of GBF, FSP and Intragranular ferrite were almost the same, but kinetics of microstructures growth was different. The growing rate of GBF was the fastest at the temperature of 725⁶50℃, but FSP and Intragranular ferrite showed the fasted growing rate at the temperature of 650⁵00℃. Acicular ferrite formed in the austenite suppressed the growth of GBF and FSP. The results also showed the key points to improve the high heat input welding HAZ toughness was how to get large amounts, more fine and dispersed Ti-bearing inclusions in the steel.To get large amounts of dispersed Ti-bearing inclusions in the ship hull steel, effect of small amounts of Mg, Zr on the inclusions and microstructure and HAZ toughness during large heat input welding in Ti treated steel were systematic studied. It was first time to find, Mg content greatly affected the types, size, distribution of Ti-bearing inclusions. The high input welding HAZ showed the highest low temperature toughness and the size of inclusions was much fine when 12ppm Mg added to the Ti treated steel, and the same amounts of Ti₂O₃ and Mg₂TiO₄ formed at this time. Microcontent Mg added to the Ti treated steel decreased the attractive force of inclusions significantly, effectively refined the size of Ti-bearing oxides. Zr content also affected the types and size of inclusions in Ti treated steel greatly. The same amounts of Ti₂O₃ and ZrO₂ formed and HAZ toughness was the highest during high input welding when 43ppm Zr added to the Ti treated steel. The density of Zr oxide was relatively higher, and it was not easy for the oxides rising in the liquid steel. Large amounts of fine Zr oxide could provide the nucleation core for the Ti oxide formed later, which avoided the coarsening of Ti oxides and increase the density of nucleation core of Intragranular acicular ferrite.更多还原