The　hot　roll　bending　patchwork　components　of　B1500HS　uncoated　ultra-high　strength　steel　processed　at　industrial　scale　were　welded　with　filler　wire.　The　microstructure,　grain　orientation,　mechanical　properties　and　fracture　behavior　of　the　joint　were　investigated.　The　results　showed　that　the　weld　metal　(WM)　was　mainly　composed　of　lath　martensite　and　lower　bainite　(LB).　Various　microstructure　appeared　in　different　positions　of　the　HAZ,　among　which　martensite-austenite　(M-A)　constituents　in　the　incompletely　quenched　zone　and　tempered　martensite　(TM)　in　the　tempered　zone　were　the　key　factors　determining　the　performance　of　the　joint.　The　welded　joint　exhibited　complex　structure　and　anisotropic　mechanical　response　due　to　the　decarburization　and　rolling　texture　of　the　original　base　material　(BM).　In　the　tempered　zone,　lath　merge　and　boundaries　disappear　lead　to　the　annihilation　of　dislocations,　carbide　precipitation　leads　to　reduction　of　mobile　dislocations,　and　recrystallization　and　recovery　eliminate　work　hardening.　The　reduction　of　the　effect　of　dislocation　strengthening　results　in　the　formation　of　a　softening　belt　at　6　mm　from　the　center　of　the　weld,　with　a　minimum　hardness　of　266　HV.　The　tensile　strength　of　the　joint　in　the　rolling　direction　(RD)　was　983　MPa,　and　the　fracture　occurred　in　the　softening　belt.　Both　brittle　and　ductile　fracture　features　were　observed.　The　reason　why　the　fracture　occurs　in　the　high　temperature　tempered　zone　instead　of　the　incompletely　quenched　zone　was　the　difference　in　the　effect　of　＂constraint　strengthening＂.　Fracture　occurred　through　delamination　cracking　along　the　＂weak　interface＂　owing　to　precipitated　high-hardness　cementite　particles　and　the　unfavorably　oriented　ferrite　cleavage　planes.