Welded high-strength steel structures and method of manufacturing the same

A welded high-strength steel structure comprising a base metal and a weld metal, wherein the base metal is a steel whose microstructure is substantially formed of a mixed structure of martensite and lower bainite and which has a tensile strength of not less than 900 MPa;

• and the weld metal is a steel which comprises the following alloy elements based on % by weight:
• C:
• 0.01% to 0.15%;
• Si:
• 0.02% to 0.6%;
• Mn:
• 0.6% to 3%;
• Al:
• 0.004% to 0.08%;
• Ti:
• 0.003% to 0.03%;
• O (oxygen):
• not greater than 0.06%
• B:
• 0.0002% to 0.005%;
• Cu:
• 0% to 1.2%;
• Ni:
• 0% to 3%;
• Cr:
• 0% to 1.2%;
• Mo:
• 0% to 1%;
• V:
• 0% to 0.05%;
• and
• Nb:
• 0% to 0.05%,
• and which satisfies the equations 1) and 2) below:
• 0.25≦Pcm ≦0.32
• Pcm =C+(Si/30)+(Mn/20)+(Ni/60)+(Cu/20) +(Cr/20)+(Mo/15)+(V/10)+5B 1)
• 0.6≦Al/O (oxygen) ≦1.4 2)
• wherein each atomic symbol in equations 1) and 2) represents its content (wt. %) within the steel.