High-tensile steel plate, welded steel pipe or tube, and methods of manufacturing thereof

A high-tensile steel plate comprising 0.02% to 0.1% C, at most 0.6% Si, 1.6% to 2.5% Mn, 0.1% to 0.7% Ni, 0.01% to 0.1% Nb, 0.005% to 0.03% Ti, at most 0.1% sol. Al, 0.001% to 0.006% N, 0% to 0.0025% B, 0% to 0.6% Cu, 0% to 0.8% Cr, 0% to 0.6% Mo, 0% to 0.1% V, 0% to 0.006% Ca, 0% to 0.006% Mg, 0% to 0.03% a rare earth element, at most 0.015% P, and at most 0.003% S, the balance consisting of Fe and impurities, said high-tensile steel plate having:

• a carbon equivalent Pcm in Expression (1) in the range from 0.180% to 0.220%;
• a surface hardness of at most Vickers hardness of 285;
• a ratio of a martensite austenite constituent in the surface layer of at most 10%;
• a ratio of a mixed structure of ferrite and bainite on the inner side beyond said surface layer of at least 90%;
• a ratio of the bainite in the mixed structure of at least 10%, a length of the major axis of cementite precipitate grains in a lath of said bainite of at most 0.5 μm;
• and
• a segregation ratio as the ratio of the Mn concentration of the center segregation part to a Mn concentration of a part in a depth equal to ¼ of the thickness of the plate from the surface of at most 1.3.
• Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B (1)
• where the element symbols represent the % by mass of the respective elements.

A welded steel pipe or tube produced using a high-tensile steel plate, said high-tensile steel plate comprising 0.02% to 0.1% C, at most 0.6% Si, 1.6% to 2.5% Mn, 0.1% to 0.7% Ni, 0.01% to 0.1% Nb, 0.005% to 0.03% Ti, at most 0.1% sol. Al, 0.001% to 0.006% N, 0% to 0.0025% B, 0% to 0.6% Cu, 0% to 0.8% Cr, 0% to 0.6% Mo, 0% to 0.1% V, 0% to 0.006% Ca, 0% to 0.006% Mg, 0% to 0.03% a rare earth element, at most 0.015% P, and at most 0.003% S, the balance consisting of Fe and impurities, said high-tensile steel plate having:

• a carbon equivalent Pcm in Expression (1) in the range from 0.180% to 0.220%;
• a surface hardness of at most Vickers hardness of 285;
• a ratio of a martensite austenite constituent in the surface layer of at most 10%;
• a ratio of a mixed structure of ferrite and bainite on the inner side beyond the surface layer of at least 90%;
• a ratio of the bainite in the mixed structure of at least 10%, a lath of the bainite having a thickness of at most 1 μm and a length of at most 20 μm;
• and
• a segregation ratio as the ratio of the Mn concentration of a center segregation part to the Mn concentration of a part in a depth equal to ¼ of the thickness of the plate from the surface of at most 1.3.
• Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B (1)
• where the element symbols represent the % by mass of the respective elements.

A welded steel pipe or tube produced using a high-tensile steel plate, said high-tensile steel plate comprising 0.02% to 0.1% C, at most 0.6% Si, 1.6% to 2.5% Mn, 0.1% to 0.7% Ni, 0.01% to 0.1% Nb, 0.005% to 0.03% Ti, at most 0.1% sol. Al, 0.001% to 0.006% N, 0% to 0.0025% B, 0% to 0.6% Cu, 0% to 0.8% Cr, 0% to 0.6% Mo, 0% to 0.1% V, 0% to 0.006% Ca, 0% to 0.006% Mg, 0% to 0.03% a rare earth element, at most 0.015% P, and at most 0. 003% S, the balance consisting of Fe and impurities, said high-tensile steel plate having:

• a carbon equivalent Pcm in Expression (1) in the range from 0.180% to 0.220%;
• a surface hardness of at most Vickers hardness of 285;
• a ratio of a martensite austenite constituent in the surface layer of at most 10%;
• a ratio of a mixed structure of ferrite and bainite on the inner side beyond said surface layer of at least 90%;
• a ratio of the bainite in the mixed structure of at least 10%, a length of the major axis of cementite precipitate grains in a lath of said bainite of at most 0.5 μm;
• and
• a segregation ratio as the ratio of the Mn concentration of the center segregation part to a Mn concentration of a part in a depth equal to ¼ of the thickness of the plate from the surface of at most 1.3.
• Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B (1)
• where the element symbols represent the % by mass of the respective elements.

A method of manufacturing a high-tensile steel plate, comprising the steps of:

• continuously casting molten steel into a slab, said molten steel comprising:
• 0. 02% to 0.1% C, at most 0.6% Si, 1.6% to 2.5% Mn, 0.1% to 0.7% Ni, 0.01% to 0.1% Nb, 0.005% to 0.03% Ti, at most 0.1% sol. Al, 0.001% to 0.006% N, 0% to 0.0025% B, 0% to 0.6% Cu, 0% to 0.8% Cr, 0% to 0.6% Mo, 0% to 0.1% V, 0% to 0.006% Ca, 0% to 0.006% Mg, 0% to 0.03% a rare earth element, at most 0.015% P, and at most 0.003% S, the balance consisting of Fe and impurities, said molten steel having a carbon equivalent Pcm in Expression (1) in the range from 0.180% to 0.220%;
• and
• rolling said slab into said high-tensile steel plate,
• said step of casting including the steps of:
• injecting said molten steel into a cooled cast and forming said slab having a solidified shell on the surface and unsolidified molten steel inside,
• drawing said slab downwardly from said cast;
• reducing said slab by at least 30 mm in the thickness-wise direction in a position upstream of the final solidifying position of said slab where the center solid phase ratio of said slab is more than 0 and less than 0.2;
• and
• carrying out electromagnetic stirring to said slab so that said unsolidified molten steel is let to flow in the width-wise direction of said slab in a position at least 2 m upstream of said reducing position,
• said step of rolling including the steps of:
• heating said slab in the range from 900° C. to 1200° C.;
• rolling said heated slab into said steel plate so that the cumulative rolling reduction in an austenite no-recrystallization temperature range is in the range from 50% to 90%;
• and
• cooling said steel plate at a cooling rate in the range from 10° C./sec to 45° C./sec from a temperature of at least Ar3-50° C.
• Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B (1)
• where the element symbols represent the % by mass of the respective elements.

A method of producing a slab for a high-tensile steel plate using a continuous casting device, comprising the steps of:

• injecting molten steel into a cooled cast and forming a slab having a solidified shell on the surface and unsolidified molten steel inside, said molten steel comprising 0.02% to 0.1% C, at most 0.6% Si, 1.6% to 2.5% Mn, 0.1% to 0.7% Ni, 0.01% to 0.1% Nb, 0.005% to 0.03% Ti, at most 0.1% sol. Al, 0.001% to 0.006% N, 0% to 0.0025% B, 0% to 0.6% Cu, 0% to 0.8% Cr, 0% to 0.6% Mo, 0% to 0.1% V, 0% to 0.006% Ca, 0% to 0.006% Mg, 0% to 0.03% a rare earth element, at most 0.015% P, and at most 0.003% S, the balance consisting of Fe and impurities, the carbon equivalent Pcm in Expression (1) being from 0.180% to 0.220%;
• drawing said slab downwardly from said cast;
• reducing said slab by at least 30 mm in the thickness-wise direction in a position upstream of the final solidifying position of said slab where the center solid phase ratio of said slab is more than 0 and less than 0.2;
• and
• carrying out electromagnetic stirring to said slab so that said unsolidified molten steel is let to flow in the width-wise direction of said slab in a position at least 2 m upstream of said reducing position,
• Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B (1)
• where the element symbols represent the % by mass of the respective elements.