稀土对高强耐蚀钢中夹杂物的影响

Effect of rare earth on inclusions in a high-strength corrosion resistant steel

随着油气田开采条件的不断恶化,深层含H2S油气层的开发日益增多,油气管的腐蚀失效问题逐渐突出。非金属夹杂物是导致油气管抗氢性能和抗硫化物应力腐蚀性能下降的主要因素之一,而稀土具有调控钢中夹杂物特性的能力。为此基于工业生产试验,通过系统取样、SEM-EDS检测、热力学计算等手段研究了Ce-La稀土合金对铝脱氧钙处理工艺下的110级石油套管钢中夹杂物的影响规律,并与不加稀土的工艺进行了对比。研究结果表明,不加稀土的生产工艺钢中夹杂物主要为Ca-Al(-Mg)-O、Ca-Al(-Mg)-O+CaS、CaS和TiN类型,而加Ce-La合金(La、Ce添加量(质量分数)分别为0.0139%、0.0278%)后的夹杂物主要为Ce-La-O(-S)、Ce-La-O(-S)+CaS、Ce-La-P-As和TiN类型,其中前两类既可以在当前钢水条件下直接生成,也可对钢水中钙铝酸盐夹杂物改性获得;Ce-La-P-As和TiN类型的夹杂物主要在钢水凝固过程中产生。热力学计算表明,加稀土后钢中夹杂物的演变顺序为Inc_(liq)(Ca-Al-O)→CaS+Inc_(liq)→CaS+CaO+Inc_(liq)→CaS+CaO+ReAlO_(3)+Inc_(liq)→CaS+CaO+ReAlO_(3)→CaS+CaO+ReAlO_(3)+Re_(2)O_(3)→CaS+CaO+Re_(2)O_(3)→CaS+Re_(2)O_(3)。抗硫化物应力腐蚀A法检测结果表明,在110级套管钢中加入Ce-La合金可显著提升钢的抗硫性能。对相关机理进行了分析。

Corrosion failure is becoming a serious issue for OCTG(oil country tubular goods)due to the deteriorative exploitation conditions and the increasing demand for deep oil fields operation with H2S medium.Non-metallic inclusions in steel are one of the key factors to worsen both the hydrogen resistance and sulfide stress corrosion resistance of OCTG.It is noted that rare earth elements have the potential to control the properties of non-metallic inclusions in steels.Based on the industrial production experiments,the effect of Ce-La alloy on inclusions in 110 grade oil casing steel produced by an Al-killed and Ca treatment process was studied by employing systematic sampling,SEM-EDS detection and thermodynamic calculation.The comparison was carried out between the cases with and without rare earth addition.The results showed that the inclusions in steel without rare earth addition were mainly of Ca-Al(-Mg)-O,Ca-Al(-Mg)-O+CaS,CaS and TiN.The inclusions after adding Ce-La alloy(the mass percent of La and Ce was 0.0139%and 0.0278%,respectively),however,became mainly of Ce-La-O(-S),Ce-La-O(-S)+CaS,Ce-La-P-As,and TiN.The Ce-La-O(-S)and Ce-La-O(-S)+CaS inclusions were possibly produced directly under the current condition of molten steel or indirectly modified by rare earth on the Ca-Al-O inclusions.The Ce-La-P-As and TiN inclusions were mainly generated in the solidification process of molten steel.The thermodynamic calculation showed that the evolution process of inclusions in steel after adding rare earth was as follows:Inc_(liq)(Ca-Al-O)→CaS+Inc_(liq)→CaS+CaO+Inc_(liq)→CaS+CaO+ReAlO_(3)+Inc_(liq)→CaS+CaO+ReAlO_(3)→CaS+CaO+ReAlO_(3)+Re_(2)O_(3)→CaS+CaO+Re_(2)O_(3)→CaS+Re_(2)O_(3).The test results of anti-sulfide stress corrosion by Method A showed that the sulfide resistance of 110 grade casing steel could be significantly improved by adding Ce-La alloy,and the relevant mechanism was analyzed as well.