An apparatus line for manufacturing seamless steel pipes and tubes includes: a heating apparatus for heating a steel raw material; a piercing apparatus for piercing the heated steel raw material thus forming a hollow material; and a rolling apparatus for applying working to the hollow material to form a seamless steel pipe having a predetermined shape. A cooling apparatus is arranged on an exit side of the rolling apparatus. A heated steel raw material is worked by the rolling apparatus after being pierced by the piercing apparatus, and thereafter, using a surface temperature of a hollow piece before being cooled by the cooling apparatus as a cooling start temperature, the hollow piece is cooled to a cooling stop temperature differing by 50° C. or more from the cooling start temperature and being equal to or above 600° C. at an average cooling speed of 1.0° C./s or more in terms of an outer surface temperature.

An apparatus line for manufacturing seamless steel pipes and tubes includes: a heating apparatus for heating a steel raw material; a piercing apparatus for piercing the heated steel raw material thus forming a hollow material; and a rolling apparatus for applying working to the hollow material to form a seamless steel pipe having a predetermined shape. A cooling apparatus is arranged on an exit side of the rolling apparatus. A heated steel raw material is worked by the rolling apparatus after being pierced by the piercing apparatus, and thereafter, using a surface temperature of a hollow piece before being cooled by the cooling apparatus as a cooling start temperature, the hollow piece is cooled to a cooling stop temperature differing by 50° C. or more from the cooling start temperature and being equal to or above 600° C. at an average cooling speed of 1.0° C./s or more in terms of an outer surface temperature.

Provided is a hot press-formed member having excellent crack propagation resistance and ductility. The hot press-formed member includes: a base steel sheet and a zinc or zinc alloy plating layer on at least one surface of the base steel sheet. The base steel sheet contains, by wt %, carbon (C): 0.08-0.30%, silicon (Si): 0.01-2.0%, manganese (Mn): 3.1-8.0%, aluminum (Al): 0.001-0.5%, phosphorus (P): 0.001-0.05%, sulfur (S): 0.0001-0.02%, nitrogen (N): 0.02% or less, and a balance of iron (Fe) and other impurities. The hot press-formed member comprises 1-30 area % of retained austenite as a microstructure, and a Mn(wt %)/Zn(wt %) content ratio in an oxide layer of 0.5-1.2 μm in a thickness direction from a surface layer of the plating layer is 0.1 or more.

Provided is a hot press-formed member having excellent crack propagation resistance and ductility. The hot press-formed member includes: a base steel sheet and a zinc or zinc alloy plating layer on at least one surface of the base steel sheet. The base steel sheet contains, by wt %, carbon (C): 0.08-0.30%, silicon (Si): 0.01-2.0%, manganese (Mn): 3.1-8.0%, aluminum (Al): 0.001-0.5%, phosphorus (P): 0.001-0.05%, sulfur (S): 0.0001-0.02%, nitrogen (N): 0.02% or less, and a balance of iron (Fe) and other impurities. The hot press-formed member comprises 1-30 area % of retained austenite as a microstructure, and a Mn(wt %)/Zn(wt %) content ratio in an oxide layer of 0.5-1.2 μm in a thickness direction from a surface layer of the plating layer is 0.1 or more.

Provided is a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4≤0.4*Cr+Si≤3.2 (wherein element symbols denote measurements of respective element contents by weight %).

Provided is a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4≤0.4*Cr+Si≤3.2 (wherein element symbols denote measurements of respective element contents by weight %).

A first steel strip and a second steel strip (7) are rolled in succession in at least one roll stand (1) of a cold rolling mill. A rolling pause, in which no steel strip is rolled, is provided between the rolling of the first and the second steel strip (7). The first steel strip is fed over a first path starting from a first pay-off reel (2), and the second steel strip (7) is fed over a second path starting from the first pay-off reel (2), or from a second pay-off reel different from the first pay-off reel (2). The first steel strip is not heated as it is fed to the rolling mill (1), whereas, by contrast, the second steel strip (7) is heated. The second path is longer than the first path.

A first steel strip and a second steel strip (7) are rolled in succession in at least one roll stand (1) of a cold rolling mill. A rolling pause, in which no steel strip is rolled, is provided between the rolling of the first and the second steel strip (7). The first steel strip is fed over a first path starting from a first pay-off reel (2), and the second steel strip (7) is fed over a second path starting from the first pay-off reel (2), or from a second pay-off reel different from the first pay-off reel (2). The first steel strip is not heated as it is fed to the rolling mill (1), whereas, by contrast, the second steel strip (7) is heated. The second path is longer than the first path.

A hot-rolled steel sheet includes a specific chemical composition, and includes a microstructure represented by, in vol %: retained austenite: 2% to 30%; ferrite: 20% to 85%; bainite: 10% to 60%; pearlite: 5% or less; and martensite: 10% or less. A proportion of grains having an intragranular misorientation of 5° to 14° in all grains is 5% to 50% by area ratio, the grain being defined as an area which is surrounded by a boundary having a misorientation of 15° or more and has a circle-equivalent diameter of 0.3 μm or more.

A hot-rolled steel sheet includes a specific chemical composition, and includes a microstructure represented by, in vol %: retained austenite: 2% to 30%; ferrite: 20% to 85%; bainite: 10% to 60%; pearlite: 5% or less; and martensite: 10% or less. A proportion of grains having an intragranular misorientation of 5° to 14° in all grains is 5% to 50% by area ratio, the grain being defined as an area which is surrounded by a boundary having a misorientation of 15° or more and has a circle-equivalent diameter of 0.3 μm or more.