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Effect of impurity level and inclusions on the ductility and toughness of an ultra-high-strength steel

机译：杂质含量和夹杂物对超高强度钢延展性和韧性的影响

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摘要

The effect of composition, size and number density of inclusions on the ductility of an experimental direct quenched ultra-high-strength low-alloy steel with a minimum specified tensile strength of 1300 MPa has been investigated. Two steels with impurity levels, i.e. total O + N + S contents, of 55 and 91 mass ppm have been studied. Both steels were Al-killed and Ca-treated, and had a martensitic microstructure after hot rolling and direct quenching. Ductility was characterized using tensile and Charpy V-notch testing. The number, size and composition of the inclusions were characterized using field emission scanning electron microscope with energy dispersive spectrometer (FESEM-EDS). At the higher impurity level of 91 ppm, the inclusion structure consisted of titanium nitrides, spherical calcium aluminates and elongated manganese sulphides, whereas at the lower impurity level of 55 ppm, the inclusion structure consisted of mainly fine spherical calcium aluminates with sulphide shells. The impurity level did not have a significant effect on the number density of inclusions, as with higher and lower impurity level the number of inclusions mm~(-2) was 80.5 and 73.8, respectively. However, the impurity level affected somewhat more the size distribution of the inclusions, as well as the composition of coarse inclusions with their longest length more than 8 μm. The number density of coarse inclusions mm~(-2) rose from 0.7 to 1.2 with increasing impurity level, and with the higher impurity level of 91 ppm, 72% of the coarse inclusions were titanium nitrides or manganese sulphides, whereas with the lower impurity level of 55 ppm, only 14% of the coarse inclusions were titanium nitrides while no manganese sulphides were detected. Coarse titanium nitrides were especially detrimental to the impact toughness. The number density of them should be below 0.4 mm~(-2) in order to guarantee the best possible toughness in the steel in question.
机译：研究了夹杂物的成分，尺寸和数量密度对实验性直接淬火的最低规定抗拉强度为1300 MPa的超高强度低合金钢的延展性的影响。研究了两种杂质水平即O + N + S总含量分别为55和91质量ppm的钢。两种钢均经过Al镇静和Ca处理，并且在热轧和直接淬火后均具有马氏体显微组织。使用拉伸和夏比V型缺口测试对延性进行了表征。夹杂物的数量，大小和组成使用具有能量色散光谱仪的场发射扫描电子显微镜（FESEM-EDS）进行表征。在较高的91 ppm杂质含量下，夹杂物结构由氮化钛，球形铝酸钙和细长的硫化锰组成，而在较低的55 ppm杂质含量下，夹杂物结构主要由带有硫化物壳的细球形铝酸钙组成。杂质水平对夹杂物的数量密度没有显着影响，因为杂质水平较高和较低时，夹杂物的数量mm〜（-2）分别为80.5和73.8。但是，杂质含量对夹杂物的尺寸分布以及最长长度大于8μm的粗夹杂物的组成的影响更大。随着杂质含量的增加，粗夹杂物的数量密度mm〜（-2）从0.7增加到1.2，杂质含量较高时为91 ppm，其中72％的粗夹杂物为氮化钛或硫化锰，而杂质含量较低含量为55 ppm时，只有14％的粗夹杂物是氮化钛，而未检测到硫化锰。粗氮化钛特别不利于冲击韧性。它们的数量密度应低于0.4 mm〜（-2），以确保所讨论的钢具有最佳的韧性。

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