Inclusion Control And Its Effect On Mechanical Properties In High Strength Low Alloy Steel

High-strength Low-alloy steel is a common structural material in the field of marine and offshore engineering,with high strength,high toughness and low cost.The harsh service environment puts more demanding requirements on the low-temperature toughness and isotropy of the material,while non-metallic inclusions in steel have adverse effects on both plasticity and toughness.The implementation of an effective inclusions control strategy is essential to improve the mechanical properties of high-strength low-alloy steels.In this paper,the effects of inclusions on the mechanical properties of 600 MPa high-strength low-alloy steel and their regulation strategies were studied,and the effects of different smelting methods on inclusions and mechanical properties were investigated.A comparative analysis of the pilot steel plates of three compositions of high-strength low-alloy steel with different impurity element contents was carried out.The results of metallographic and EBSD analyses showed that the three compositions of the pilot steel plates have the same tissue type,grain size and grain orientation.The distribution of inclusions was investigated by metallographic analysis.Randomly distributed spherical inclusions were the common feature of the three steel plates,and strip sulfides were the main difference.EDS results showed that the main types of inclusions were Al₂O₃,MnS and nitrides containing Nb and Ti,and some MnS were attached to the periphery of oxides or nitrides to form composite inclusions.The results of the dimensional study showed that the inclusions are small in size,with the average diameter of Al₂O₃being about 2μm.The sulfides are biased and unevenly distributed in the steel plate,and the size and number of MnS in the center area are much higher than those in the sides.The distribution,morphology and size of inclusions have an important influence on the mechanical properties and fracture morphology of the steel plate.Al₂O₃has no obvious influence on the mechanical properties due to its small size and good morphology,and MnS is the main factor affecting the mechanical properties of high-strength low-alloy steel.The influence of MnS on tensile properties is reflected in the local deformation stage.When the tensile direction is perpendicular to the MnS direction,the section shrinkage is reduced by ovewr 10%.When the MnS direction is perpendicular to the impact direction,it has no effect on the impact properties of the steel plate;when the MnS direction is parallel to the impact direction,the weak interface and gap between MnS and the matrix will promote the expansion of cracks and form long dimples at room temperature,which will lead to decoupling fracture at low temperature.The length of MnS is the most important factor affecting the impact properties of the material,and the aspect ratio and number are relatively minor factors.The critical size of MnS affecting the impact properties of high-strength low-alloy steels at-80°C is 6μm.Heat treatment temperature and time affect the number,size and morphology of MnS.850°C-1100°C solid solution for 0.5h-5h does not change the distribution of inclusions.With the increase of solid solution temperature or the extension of holding time,MnS inclusions and compound inclusions tips and angles are obviously blunted.Large MnS fused and split out small MnS and small MnS gradually spheroidized,and overall large MnS reduced.Aging at 500℃-700℃for 1h has no obvious effect on inclusions.When aging at 650℃for 1h-10h,no significant changed in inclusions at 1h-5h but significant growth occurred after 10h.Cross rolling can change the morphology of MnS,so that the dimensions of MnS in longitudinal and cross sections tend to be the same,and reduce the anisotropy of impact properties of steel plates.However,cross rolling does not significantly improve the number and size of large MnS,which is large in both directions,resulting in relatively low low-temperature impact properties in both longitudinal and transverse directions.The calcium and magnesium elements introduced in the smelting and refining process of the electric furnace+LF+VD process changed the composition and morphology of oxides and sulfides and refined their sizes,resulting in more than 95%of inclusions have a size less han 6μm.At the same time,the inclusions in both longitudinal and transverse sections tend to be of the same size,and the transverse properties of the steel plates are significantly improved.The number of nitrides in low-alloy high-strength steels melted by the electric furnace process can account for up to 15%,but due to their small size,they have no effect on the impact properties at-80°C.