A cold-rolled and heat-treated steel sheet having a microstructure of, in surface fraction: between 10% and 30% of retained austenite, said retained austenite being present as films having an aspect ratio of at least 3 and as Martensite Austenite islands, less than 8% of such Martensite Austenite islands having a size above 0.5 μm, at most 10% of fresh martensite and
recovered martensite containing precipitates of at least one element chosen among niobium, titanium and vanadium. A manufacturing method thereof is also provided.

A high-strength galvanized steel sheet includes a steel sheet containing a predetermined component element, a mass ratio of a content amount of Si to a content amount of Mn in the steel (Si/Mn) being 0.2 or more, and a steel structure in which an average grain size of inclusions existing in an area extending from a surface to a position of ⅓ of a sheet thickness is 50 μm or less, and an average nearest distance between ones of the inclusions is 20 μm or more; and a galvanized layer provided on a surface of the steel sheet, in which an amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm, oxides containing predetermined elements in an outer layer portion of the steel sheet account for 0.010 g/m.sup.2 or more per one surface, and a tensile strength is 1100 MPa or more.

Disclosed are a Nb microalloyed high strength high hole expansion steel and a production method therefor. The chemical ingredients of the steel in percentages by weight are as follows: 0.01-0.05% of C, 0.2-0.6% of Si, 0.8-1.5% of Mn, ≤0.02% of P, ≤0.005% of S, ≤0.008% of N, <0.001% of Als, ≤0.0050% of Ca, 0.01-0.08% of Nb, and optionally one or both of 0.1-0.6% of Cu and 0.005-0.04% of Sn, wherein Mn/S>250, total oxygen [O].sub.T is 0.007-0.020%, and the balance is Fe and inevitable impurities. In the present invention, microalloy elements such as Nb are selectively added, and the basicity of slag, the type and melting point of the inclusion in steel, the content of free oxygen in molten steel, and the content of acid-soluble aluminum Als during the smelting process are controlled, and then, a strip is cast by means of twin-roll thin strip continuous casting, and the strip directly enters a lower closed chamber in a non-oxidizing atmosphere and enters an online rolling mill for hot rolling in closed conditions, and after rolling, the strip steel is cooled by air atomization cooling, and finally, the produced steel coil can be used directly as a hot rolled plate or can be used after acid pickling and leveling.

Provided is a hot rolled steel sheet having excellent formability and fatigue properties comprising, in percentage by weight: 0.3-0.8% of C; 13-25% of Mn; 0.1-1.0% of V; 0.005-2.0% of Si; 0.01-2.5% of Al; 0.03% or less of P; 0.03% or less of S; 0.04% or less (excluding 0%) of N; and the balance being Fe and inevitable impurities, wherein, when viewed in a cross section in the thickness direction, the hot rolled steel sheet comprises, by area fraction, 20-70% of a non-recrystallized structure and 30-80% of a recrystallized structure.

A hot-rolled steel plate/strip for sulfuric acid dew point corrosion resistance and manufacturing method therefor. In said method, elements such as Sn and Cu remaining in steel scrap are fully utilized to smelt molten steel, and micro-alloy elements such as Cr, Ti, and Sb are selectively added; in a smelting process, basicity of slag, types and melting points of inclusions in steel, and a free oxygen content and an acid-soluble aluminum (Als) content in molten steel are controlled, a cast strip (11) is casted by means of twin-roll strip continuous casting, the cast strip (11) exits from crystallization rolls (8a, 8b) and directly enters a lower closed chamer (10) having a non-oxidizing atmosphere, then enters, in a closed condition, an on-line rolling mill (13) for hot rolling, after rolling, strip steel is cooled by means of gas atomization cooling, and finally the strip steel is wound up. The steel can be widely applied to the fields of products, such as tobacco baking apparatuses, air preheater heat exchange elements in industries such as petroleum, chemical industry, electric power, and metallurgy, delivery pipe, flue, and stack manufacturing structural parts, and boiler preheater and economizer equipment, of which the use environments have requirements for sulfuric acid dew point corrosion resistance performance.

A high-strength thin-gauge checkered steel plate/strip and a manufacturing method therefor, wherein residual elements such as Sn and Cu in steel scrap are fully utilized as alloy elements in the smelting of molten steel, and the steel has selectively added micro-alloy elements such as B; during the smelting process, the alkalinity of the slag, the types of inclusion in the steel and the melting point thereof, the content of free oxygen and the content of soluble aluminum (Als) in the molten steel are controlled; and twin-roll thin-strip continuous casting is performed to cast a cast strip (11); after exiting crystallization rollers (8a, 8b), the cast strip (11) directly enters a lower sealed chamber (10) containing a non-oxidizing atmosphere, and enters an online rolling machine (13) in a sealed manner so as to undergo hot rolling, then after rolling, the strip steel is cooled by means of air atomization. The resultant steel roll can be used directly as hot-rolled checkered plate/strip, or as a finished checkered plate/strip after being cut and finished, and is widely applicable to the fields of architecture, mechanical production, automobile, bridges, transportation, ship building, etc.

This application discloses thin metal strips and methods of making thin metal strip. Particular embodiments of such methods include cooling the thin metal strip to a temperature equal to or less than a bainite or a martensite start transformation temperature B.sub.S or M.sub.S to thereby form bainite and/or martensite, respectively, within the thin metal strip, reheating the thin metal strip to a reheat temperature equal to or greater than transformation temperature Ac.sub.3 and holding the thin metal strip at the reheat temperature for at least 2 seconds and thereby forming austenite within the thin metal strip with at least 75% of austenite grains having a grain size equal to or less than 15 μm, and rapidly recooling the thin metal strip to a temperature equal to or less than the martensite start transformation temperature M.sub.S and thereby providing finer martensite within the thin metal strip from a finer prior austenite.

Provided is a method of manufacturing a hot rolled steel sheet having excellent formability and fatigue properties, including: preparing a slab including, by weight%, 0.3 to 0.8% of carbon C, 13 to 25% of manganese (Mn), 0.1 to 1.0% of vanadium (V), 0.005 to 2.0% of silicon (Si), 0.01 to 2.5% of aluminum (Al), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), 0.04% or less (excluding 0%) of nitrogen (N), and a remainder of iron (Fe) and inevitable impurities; heating the slab to 1050 to 1250° C.; finish rolling, the slab heated in the heating, at a temperature of not lower than a recrystallization temperature of a region having an average V concentration and not higher than a recrystallization temperature of a region having twice the average V concentration, to obtain a hot rolled steel sheet; and coiling the hot-rolled steel sheet at 50 to 700° C.

A welding additive for electric arc welding and laser beam welding of mixed joins composed of austenitic and high-manganese-containing and ferritic steel, where the high-manganese-containing steel has a manganese content of at least 7-30% by weight includes the following alloy elements in % by weight: C 0.04-1.0; Mn 7-30; Si≦6; Al≦4; Mo≦2; Ti≦0.5; Zr 0.01-01; B 0.001-0.01; P<0.005; S<0.002; N<0.008; balance iron and unavoidable steel accompanying elements.

A manufacturing method for strip casting 700 MPa-grade high strength atmospheric corrosion-resistant steel, comprising the following steps: 1) smelting, where the chemical composition of a molten steel in terms of weight percentage is that C is between 0.03-0.1%, Si≦0.4%, Mn is between 0.75-2.0%, P is between 0.07-0.22%, S≦0.01%, N≦0.012%, Cu is between 0.25-0.8%, Cr is between 0.3-0.8%, and Ni is between 0.12-0.4%, additionally, also comprised is at least one micro-alloying element among Nb, V, Ti, and Mo, where Nb is between 0.01-0.1%, V is between 0.01-0.1%, Ti is between 0.01-0.1%, and Mo is between 0.1-0.5%, and where the remainder is Fe and unavoidable impurities; 2) strip casting, where a 1-5 mm-thick cast strip is casted directly; 3) cooling the cast strip, where the cooling rate is greater than 20° C./s; 4) online hot rolling the cast strip, where the hot rolling temperature is between 1050-1250° C., where the reduction rate is between 20-50%, and where the deformation rate is >20s.sup.−1; austenite online recrystallizing after hot rolling, where the thickness of the hot rolled strip is between 0.5-3.0 mm; and, 5) cooling and winding, where the cooling rate is between 10-80° C./s, and where the winding temperature is between 520-670° C. The microscopic structure of a steel strip acquired is primarily constituted by evenly distributed bainite and acicular ferrite.