风电法兰用钢S355NL探伤缺陷的分析和工艺改进

风电法兰用钢S355NL（/%:0.14~0.48C，0.15~0.40Si，1.30~1.60Mn，≤0.013P，≤0.005S，0.04~0.12V，0.03~0.05Nb，≤0.009Ti，≤0.30Cr，≤0.50Ni，≤0.20Cu，≤0.10Mo，≥0.020Alt）的冶金流程为100t UHP EAF—LF—VD—Φ650 mm,Φ800 mm坯CC工艺。该钢1~2 mm裂纹探伤不合格的分析结果表明,其主要原因为氧的铝类夹杂和铸坯疏松缺陷所致。通过在LF终点喂钙线0.45 kg/t,VD处理时间由原25 min增至27 min,降低钢中N]至78×10^-6~82×10^-6,H]为1.1×10^-6~1.3×10^-6,O]为12×10^-6~15×10^-6,软吹氩气流量由2×25L/min降至2×20 L/min,时间≥12 min,降低钢水过热度5℃等工艺措施,使铸坯锻造后的探伤合格率超过99%。

Analysis on Nondestructive Examination Defects of Steel S355NL for Wind Power Flange and Process Improvement

The metallurgical flowsheet for steel S355NL for wind power flange (/% : 0.14-0.48C, 0.15-0.40Si, 1.30-1.60Mn, ≤0.013P, ≤0.005S, 0.04-0.12V, 0.03-0.05Nb, ≤0.009Ti, ≤0.30Cr, ≤0.50Ni, ≤0.20Cu, ≤0.10Mo, ≥0.020Alt) is 100t UHP EAF-LF-VD-Φ650 mm, Φ800 mm bloom CC process. The results of analysis on unqualified steel by nondestructive examination due to 1-2 mm cracks show that cracks are mainly caused by alumina inclusions and casting bloom porosity defects. By the process measures including feeding calcium wire 0.45 kg/t at end LF, increasing VD treading time to 27 min from original 25 min, decreasing the N] in steel to 78 x 10^-6-82 x 10^-6 , the H]to 1.1 x 10^-6-1.3 x 10^-6 and the O] to 12 x 10^-6- 15 x 10^-6 , decreasing the soft argon blowing rate to 2 x 20 L/min from 2 x 25 L/min, time≥12 min and decreasing the liquid overheating extent by 5℃ , the qualified ratio of steel forged from casting bloom is more than 99%.