火花发射光谱法测定IF钢中超低碳的质量控制

采用铣削和砂带研磨两种不同制样方式,对火花发射光谱仪测定IF钢中超低碳的结果进行分析,发现铣削方式测试数据精度优于砂带研磨方式,但由于冶炼生产采用圆柱形试样,试样经砂轮切割机切断后,样品表面会发生一定程度倾斜,放入研削机制样,需多次铣削才能将试样铣平,耗时过长,因此在快节奏的大生产中并未应用。新砂带与旧砂带研磨方式测试同一生产试样的结果表明,新砂带研磨的数据波动较大且系统略高于旧砂带。对影响火花发射光谱分析超低碳的因素进行了全面分析,除试样制备因素外,氩气纯度、激发室的清洁程度、类型标准化等因素对火花发射光谱测定超低碳也有较大影响。针对砂带研磨制样方式,选取两块不同含量的超低碳生产样品进行精度试验,标准偏差分别为0.000 09%、0.000 14%,按3倍标准偏差(置信度99.7%)计算,重复性限为0.000 27%、0.000 42%,能满足生产需要。采用自制样品类型标准化后,火花发射光谱测试IF生产试样结果与红外碳硫测试结果比对,碳成分差值均不大于0.000 5%,一致性较好。通过对大量生产实际数据的统计分析,按假设目标差异为0,上下限设定为±0.000 6%,通过MINITAB软件计算出该条件下的过程能力指数(Cpk)为1.16,介于1.0~1.33之间。按Cpk等级评定及处理原则,等级为B级,即过程控制能力较好。

Discussion on the quality control of ultra-low carbon determination in IF steel by spark source optical emission spectrometry

The sample was prepared by two methods, i.e., milling and abrasive belt grinding. The analysis results of ultra-low carbon in IF steel by spark source optical emission spectrometry indicated that the precision of determination data of milling method were better than those of abrasive belt grinding method. However, the samples were cylindrical in smelting production, and the sample surface was slant to a certain extent after cutting with grinding wheel cutting machine. Many times of milling were required for sample preparation. The time-consuming operation limited its application in fast-paced production. The analysis results of one production sample after grinding with new abrasive belt and old abrasive belt showed that the data fluctuation was large and the results were higher for new abrasive belt than those of old abrasive belt. The factors influencing the determination of ultra-low carbon by spark source optical emission spectrometry were comprehensively analyzed. Apart from sample preparation, the determination of ultra-low carbon was also greatly affected by the argon purity, the cleanliness degree of excitation chamber and type standardization. For the sample preparation mode by abrasive belt grinding, the precision test was conducted using two production samples with different contents of ultra-low carbon. The standard deviation was 0.000 09% and 0.000 14%, respectively. According to three times of standard deviation (confidence coefficient of 99.7%), the repeatability limit was 0.000 27% and 0.000 42%, respectively, which could meet the production requirements. After type standardization, the determination results of ultra-low carbon in IF steel production samples by spark source optical emission spectrometry and infrared carbon-sulfur analyzer were compared, and the difference was not higher than 0.000 5% for carbon, indicating the good consistence. The statistical analysis of many production data was performed. The target difference and bounds was set as 0 and ±0.000 6%, respectively. The process capability index (Cpk) under this condition was calculated by MINITAB software. The value was 1.16, which was between 1.0 and 1.33. According to the grade evaluation and management principle, the grade was level B. In other words, the process control capability was good.