The present disclosure provides a cold-rolled steel sheet having excellent high-temperature properties and room-temperature workability, including, by weight: carbon (C): 0.0005 to 0.003%, manganese (Mn): 0.20 to 0.50%, aluminum (Al): 0.01 to 0.10%, phosphorus (P): 0.003 to 0.020%, nitrogen (N): 0.0005 to 0.004%, sulfur (S): 0.015% or less, niobium (Nb): 0.005 to 0.040%, chromium (Cr): 0.10 to 0.50%, tungsten (W): 0.02 to 0.07%, and a balance of iron (Fe) and other inevitable impurities, wherein C, Nb, and W satisfy the following relationship 1, a microstructure comprises 95 area % or more of polygonal ferrite and 5 area % or less of acicular ferrite, and the cold-rolled steel sheet comprises (Nb,W)C-based precipitates having an average size of 0.005 to 0.10 μm and a method for manufacturing the same:
0.00025≤(2×Nb/93)×(W/184)/(C/12)≤0.0015  [Relationship 1]
where, C, Nb, and W are in weight %.

A high-strength hot-rolled steel sheet that has excellent punching workability and hole expandability, and a method for manufacturing the same. The hot-rolled steel sheet has a tensile strength of 980 MPa or more. The hot-rolled steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, N, Ti, Cr, and B, and has a microstructure including a bainite phase having an area ratio of 85% or more as a main phase, and a martensite phase or martensite-austenite constituent having an area ratio of 15% or less as a second phase, the balance being a ferrite phase. The second phase has an average grain diameter of 3.0 μm or less, prior-austenite grains have an average aspect ratio of 1.3 or more and 5.0 or less, and recrystallized prior-austenite grains have an area ratio of 15% or less relative to non-recrystallized prior-austenite grains.

A high-strength hot-rolled steel sheet that has excellent punching workability and hole expandability, and a method for manufacturing the same. The hot-rolled steel sheet has a tensile strength of 980 MPa or more. The hot-rolled steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, N, Ti, Cr, and B, and has a microstructure including a bainite phase having an area ratio of 85% or more as a main phase, and a martensite phase or martensite-austenite constituent having an area ratio of 15% or less as a second phase, the balance being a ferrite phase. The second phase has an average grain diameter of 3.0 μm or less, prior-austenite grains have an average aspect ratio of 1.3 or more and 5.0 or less, and recrystallized prior-austenite grains have an area ratio of 15% or less relative to non-recrystallized prior-austenite grains.

An ultra-high-strength hot-rolled steel plate and steel strip having good fatigue and reaming properties and a manufacturing method therefor. The weight percentages of the components of the steel plate and the steel strip are: C: 0.07-0.14%, Si: 0.1-0.4%, Mn: 1.55-2.00%, P≤0.015%, S≤0.004%, Al: 0.01-0.05%, N≤0.005%, Cr: 0.15-0.50%, V: 0.1-0.35%, Nb: 0.01%-0.06%, Mo: 0.15-0.50%, Ti≤0.02%, and the balance of Fe and unavoidable impurities. Such components need to meet: 1.0≤[(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)]≤1.6. The tensile strength of the ultrahigh-strength hot-rolled steel plate and steel strip is ≥780 MPa, the yield strength thereof is ≥660 MPa, the tensile fatigue limit (10 million cycles) FL thereof is ≥570 MPa, or the fatigue limit to tensile strength FL/Rm thereof is ≥0.72. The reaming rate meets: if an original hole is a punched hole, the reaming rate thereof is >85%; and if the original hole is a reamed hole, the reaming rate thereof is >120%.

An ultra-high-strength hot-rolled steel plate and steel strip having good fatigue and reaming properties and a manufacturing method therefor. The weight percentages of the components of the steel plate and the steel strip are: C: 0.07-0.14%, Si: 0.1-0.4%, Mn: 1.55-2.00%, P≤0.015%, S≤0.004%, Al: 0.01-0.05%, N≤0.005%, Cr: 0.15-0.50%, V: 0.1-0.35%, Nb: 0.01%-0.06%, Mo: 0.15-0.50%, Ti≤0.02%, and the balance of Fe and unavoidable impurities. Such components need to meet: 1.0≤[(Cr/52)/(C/4)+(Nb/93+Ti/48+V/51+Mo/96)/(C/12)]≤1.6. The tensile strength of the ultrahigh-strength hot-rolled steel plate and steel strip is ≥780 MPa, the yield strength thereof is ≥660 MPa, the tensile fatigue limit (10 million cycles) FL thereof is ≥570 MPa, or the fatigue limit to tensile strength FL/Rm thereof is ≥0.72. The reaming rate meets: if an original hole is a punched hole, the reaming rate thereof is >85%; and if the original hole is a reamed hole, the reaming rate thereof is >120%.

A steel sheet has a composition comprising 0.060%≤C≤0.085%, 1.8%≤Mn≤2.0%, 0.4%≤Cr≤0.6%, 0.1%≤Si≤0.5%, 0.010%≤Nb≤0.025%, 3.42N≤Ti≤0.035%, 0≤Mo≤0.030%, 0.020%≤Al≤0.060%, 0.0012%≤B≤0.0030%, S≤0.005%, P≤0.050%, 0.002%≤N≤0.007% and optionally 0.0005%≤Ca≤0.005%, the remainder of the composition being iron and unavoidable impurities. The microstructure consists of 34% to 80% bainite, 10% to 16% martensite, and 10% to 50% of ferrite. The surface fraction of unrecrystallized ferrite, with respect to the whole structure, is of less than 30%. The martensite consists of self-tempered martensite and fresh martensite, the surface fraction of self-tempered martensite being comprised between 4% and 10%.

A steel sheet has a composition comprising 0.060%≤C≤0.085%, 1.8%≤Mn≤2.0%, 0.4%≤Cr≤0.6%, 0.1%≤Si≤0.5%, 0.010%≤Nb≤0.025%, 3.42N≤Ti≤0.035%, 0≤Mo≤0.030%, 0.020%≤Al≤0.060%, 0.0012%≤B≤0.0030%, S≤0.005%, P≤0.050%, 0.002%≤N≤0.007% and optionally 0.0005%≤Ca≤0.005%, the remainder of the composition being iron and unavoidable impurities. The microstructure consists of 34% to 80% bainite, 10% to 16% martensite, and 10% to 50% of ferrite. The surface fraction of unrecrystallized ferrite, with respect to the whole structure, is of less than 30%. The martensite consists of self-tempered martensite and fresh martensite, the surface fraction of self-tempered martensite being comprised between 4% and 10%.

Disclosed is a stainless steel having excellent surface electrical conductivity for a fuel cell separator. According to an embodiment of the disclosed stainless steel having excellent surface electrical conductivity for a fuel cell separator, a value of the following surface oxide atomic ratio (1) may be 0.5 or less, as measured on the surface of a stainless steel containing 15 wt % or more of Cr by X-ray angle-resolved photoemission spectroscopy using an Al-Kα X-ray source under the condition where a take-off angle of photoelectrons is from 12° to 85°.

[00001]$\begin{array}{cc}\frac{\begin{array}{c}\mathrm{sum}\mathrm{of}\mathrm{atomic}\mathrm{concentrations}\left(\mathrm{at}\%\right)\\ \mathrm{of}\mathrm{metal}\mathrm{elements}\mathrm{in}\mathrm{metal}\mathrm{oxide}\left(\mathrm{MO}\right)\end{array}}{\begin{array}{c}\mathrm{sum}\mathrm{of}\mathrm{atomic}\mathrm{concentrations}\left(\mathrm{at}\%\right)\\ \mathrm{of}\mathrm{metal}\mathrm{elements}\mathrm{in}\mathrm{total}\mathrm{oxides}\mathrm{and}\mathrm{hydroxides}\end{array}}& \left(1\right)\end{array}$

The metal oxide (MO) includes a mixed oxide: M represents an alloying element other than Cr and Fe or a combination thereof in the matrix; and O represents oxygen. The total oxides and hydroxides include a Cr oxide, a Cr hydroxide, an Fe oxide, an Fe hydroxide, and the metal oxide (MO).

Disclosed is, as a high-strength steel plate of API X80 grade or higher with a thickness of 38 mm or more, a thick steel plate for structural pipes or tubes that exhibits high strength in the rolling direction and excellent Charpy properties at its mid-thickness part without addition of large amounts of alloying elements. The thick steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure at its mid-thickness part that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50%, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80% or more with respect to the whole area of the ferrite; a tensile strength of 620 MPa or more; and a Charpy absorption energy vE.sub.−20+ C. at −20° C. at the mid-thickness part of 100 J or more.

Disclosed is, as a high-strength steel plate of API X80 grade or higher with a thickness of 38 mm or more, a thick steel plate for structural pipes or tubes that exhibits high strength in the rolling direction and excellent Charpy properties at its mid-thickness part without addition of large amounts of alloying elements. The thick steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure at its mid-thickness part that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50%, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80% or more with respect to the whole area of the ferrite; a tensile strength of 620 MPa or more; and a Charpy absorption energy vE.sub.−20+ C. at −20° C. at the mid-thickness part of 100 J or more.