以抗拉强度700 MPa级Nb-Ti微合金化C-Mn钢为研究对象，采用高功率光纤激光器对4.6 mm 厚钢板进行了拼焊实验，研究了热输入(162、175、187、226 J/mm2)对全熔透焊接接头显微组织、显微硬度、拉伸性能及冲击韧性的影响。研究表明，4种热输入条件下，焊缝区和粗晶区组织均以板条马氏体为主，存在不同含量的贝氏体和铁素体；随着热输入的增加，粗晶区原始奥氏体晶粒尺寸逐渐增大；热输入对细晶区和混晶区的组织影响不明显。焊缝区和热影响区平均硬度均高于母材；随着热输入的提高，焊缝区硬度和焊接接头的峰值硬度逐渐降低，主要原因是热输入增大形成贝氏体和铁素体。4 种热输入条件下焊接接头的抗拉强度高于母材。4 种热输入条件下焊接接头冲击吸收功均高于母材，且均为韧性断裂，研究范围内的热输入对焊接接头冲击性能影响不明显。

Tensile strength 700 MPa Nb-Ti microalloyed C-Mn steel is welded using high power fiber laser. The effects of heat input (162、175、187、226 J/mm2) on microstructure, tensile properties, microhardness, impact toughness are studied. The results show that the microstructure of welded seam (WS) and coarse grain heat affected zone (CG-HAZ) are mainly lath martensite, and a little of bainite and ferrite. The original austenite grain size of CGHAZincreases with heat input increasing. The effects of heat input on microstructure of fine grain heat affected zone (FG-HAZ) and mixed grain heat affected zone (MG-HAZ) are not obvious. The average hardness of WS and HAZ are greater than that of base material (BM), and the hardness of WS and peak hardness of weld joint decrease with the heat input increase because of the formation of ferrite and bainite with the increase of heat input. The tensile strength of welded joint are higher than that of BM at four different heat inputs. The impact toughness of weld joint is better than that of BM at four different heat inputs, and both weld joint and BM fracture mode are ductile fracture. The effect of heat input on weld joint impact energy is not obvious.