【作者】 夏文勇；

【导师】 雍岐龙； 杨才福；

【作者基本信息】 钢铁研究总院 ， 材料学， 2012， 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线能量焊接的适应性,本文对高强度船体钢中第二相氧化物粒子形成的热力学进行了深入分析,研究了Ti处理改善高强度船板钢焊接热影响区的(Heat Affected Zone,以下简称HAZ)组织和性能,并探讨了Ti氧化物促进HAZ晶内铁素体形核的机理。深入研究了大线能量焊接船板钢的关键合金设计与关键冶金工艺,并对研制的大线能量焊接船板钢进行了焊接性能分析,提出了可大线能量焊接工艺范围。本文首先对大线能量焊接船板钢中氧化物的形成进行了热力学研究,提出了Si-Mn复合脱氧+Ti脱氧的基本技术思路。结果表明：采用Si-Mn复合脱氧,钢液平衡的[O]含量降低。[Si]含量为0.30%时,采用Si单独脱氧相平衡的[O]含量为0.0084%；采用Si-Mn复合脱氧,钢液平衡[O]则可以降低到0.0062%。在船板钢常规Ti含量(0.01-0.02%)、常规氧含量(0.0006%-0.0030%)范围内,钢中的钛优先与氧生成Ti2O3。钢中的A1对Ti2O3的形成有显著,二者在钢液中发生氧化还原反应。因此,当钢液经过Si. Mn弱脱氧后,为了促进Ti203的形成,需将钢液中的氧含量控制小于30ppm,[A]含量控制小于94ppm。对Ti氧化物促进晶内针状铁素体形核机理进行分析。采用Gleeble3500D对Ti脱氧钢进行连续冷却淬火实验。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℃时晶界铁素体己基本完全形成,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的生长。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扩散实验从宏观角度探讨了Ti203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A12O3粉末与基体界面处却无铁素体带形成,说明Ti2O3有较强的促进铁素体形核能力。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的贫锰区。Ti2O3吸附了周围的Mn原子,促进了贫锰区的形成,提高了相变平衡温度Ae3,有效地促进了铁素体形成。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,这主要因为保温时间越长,Mn越容易扩散到Ti203中并形成相对饱和,造成MDZ的减少,从而导致晶内铁素体形核能力减弱。为获得大量、细小、弥散分布的Ti氧化物夹杂,显著促进针状铁素体形核、提高HAZ的韧性,系统研究了微量Mg. Zr对Ti脱氧钢中夹杂物及其大线能量焊接时焊接热影响区组织和韧性的影响。结果表明：当钢中添加12ppm Mg时,钢中形成等摩尔数的Ti2O3和Mg2TiO4氧化物颗粒,此时钢中含Ti氧化物颗粒的粒度最小,数量最多,大线能量焊接时焊接热影响区的低温韧性最高。微量Mg加入到钢中能降低含Ti氧化物聚集长大的能力,有效的细化了氧化物的尺寸,提高了氧化物夹杂促进针状铁素体形核的能力。当钢中添加43ppm Zr时,钢中形成等摩尔数的Ti2O3和ZrO2,此时含Ti氧化物的粒度最为细小,数量最多,大线能量焊接时焊接热影响区的低温韧性最高。这主要是由于Zr氧化物的密度较大,不容易在钢液中上浮,细小的ZrO2颗粒能够为随后形成的含Ti氧化物提供大量的形核核心,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。首次开发了大线能量焊接船板钢的关键冶金工艺技术。通过Ti合金丝喂线的方法,可以在钢中形成大量弥散分布的细小含Ti氧化物颗粒,其平均粒度尺寸约为1～3μm,氧化物类型主要为Mg-Al-Ti-O复合氧化物。脱氧时间对钢中Ti氧化物形成存在显著影响。随着脱氧时间的增长,钢中氧化物尺寸增大,数量减少。采用Mg-Ti复合处理的E36大线能量焊接船板钢,能够适应最大240KJ/cm的线能量焊接要求。更多还原

【Abstract】 To improve the weldability and high heat input welding adaptability of high strength low alloy ship hull steel, analysis of formation thermodynamics on the second phase oxide particles in high strength ship steel, microstructures and properties of Heat Affected Zone (HAZ) in Ti-treated high strength ship hull steel, and mechanism of Ti oxide promoting the formation of acicular ferrite in HAZ were studied in this dissertation. Then key alloy design, metallurgical technology and weldability of high heat input welding ship steel were investigated, proposing the technology range of approvingly high heat input welding.Firstly analysis of formation thermodynamics on the oxide particles in high heat input ship steel was studied in this paper, proposing the basic technical thought of Si-Mn complex deoxidization. The result shows that, oxygen content of melt equilibration decrease by using Si-Mn complex deoxidization technique. When Si content was0.3%, oxygen content of melt equilibration was0.0084%by using Si deoxidization, while it lowed to0.0062%by using Si-Mn complex deoxidization. Within the convention scope of Ti (0.01～0.02%) and O(0.0006%-0.0030%) content in ship steel, titanium and oxygen in the liquid steel forms the reaction of Ti2O3pirorly. Also the formation of Ti2O3was effect by aluminium, they can cause oxidation-reduction reaction. So, to promote the formation of Ti2O3, oxygen content of liquid must less than30ppm and aluminium content less than94ppm before Si-Mn complex deoxidization.Nucleation mechanism of acicular ferrite promoted by Ti oxides was investigated. The continuous quenching test of Ti deoxidized steel was performed using Gleeble3500D. With a decrease in temperature to725℃, 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 Ti2O3within austenite grains.The phase transformation of GBF basically completed and the acicular ferrite intragranularly growed rapidly which suppressed the growth of FSB at650℃,the interlocking microstructure of acicular ferrite formed in austenite grains when temperature continued to575℃.Bonding diffusion experiment had been performed to analyze the mechanism of MDZ(Mn-depleted zone) around Ti2O3from macro perspective and the effect of high temperature holding time on ferrite nucleation ability had also been discussed.The results showed that the ferrite layers formed along the interfaces between the steels and the Ti2O3powders and the ferrite layers hadn’t been found along the interfaces between the steels and the Al2O3powders.It confirmed that Ti2O3particle had 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 at950℃.The lOμm width of MDZ formed near steel-Ti2O3interfaces in the bonded apecimens was analyzed by electron probe microanalysis(EPMA).The MDZ developed in the vicility of steel-Ti2O3powder interfaces because Ti2O3itself absorb neighboring Mn within an austenite matrix, then ferrite formed remarkably because of phase equilibrium temperature increased. The nucleation ability decreased at Ti2O3with increasing of high temperature holding time. It was becaused that with increasing of high temperature holding time, manganese diffused into Ti2O3easily and formed into relative saturation, which reduced the MDZ and nucleation ability of intragranular ferrite.To gain abundant, fine, dispersive Ti oxides which promote acicular ferrite nucleation notably and enhance toughness of HAZ, effct 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. The result showed that the high input welding HAZ showed the highest low temperature toughness and the size of inclusions was much fine when12ppm Mg added to the Ti treated steel, and the same amounts of Ti2O3and Mg2TiO4formed 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. The same amounts of Ti2O3and ZrO2formed and HAZ toughness was the highest during high input welding when43ppm 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 o f Ti oxides and increase the density of nucleation core of intragranular acicular ferrite. Key metallurgical technology of high heat input welding ship steel were empoldered firstly. Large amounts of dispersed Ti-bearing inclusions which average granularity was1～3μm and mainly were Mg-Al-Ti-O complex oxides formed in steel by using Ti wire feeding treatment. Deoxidation time affected the formation of inclusions in Ti treated steel greatly, size of inclusions increased and quantity decreased with increasing deoxidation time. High heat input welding ship steel for E36can adapt240KJ/cm welding energy by using Mg-Ti complex treatment.更多还原