摘要:
Disclosed is a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, fatigue properties, stretch flangeability, surface characteristics, and sheet passage ability that can be obtained by providing a predetermined chemical composition and a steel microstructure that contains, by area, 20-50 % of ferrite, 5-25 % of bainitic ferrite, 1-10 % of martensite, and 5-15 % of tempered martensite, and that contains, by volume, 10 % or more of retained austenite, in which the retained austenite has a mean grain size of 2 µm or less, a mean Mn content in the retained austenite in mass% is at least 1.2 times the Mn content in the steel sheet in mass%, the retained austenite has a mean free path of 1.2 µm or less, and the tempered martensite has a mean free path of 1.2 µm or less.
摘要:
A high-strength steel sheet with excellent formability and high yield ratio that has TS of 980 MPa or more and YR of 68 % or more is obtained by providing a predetermined chemical composition and a steel microstructure that contains, in area ratio, 15 to 55 % of polygonal ferrite, 8% or more of non-recrystallized ferrite, and 15 to 30 % of martensite, and that contains, in volume fraction, 12 % or more of retained austenite, in which the polygonal ferrite has a mean grain size of 4 µm or less, the martensite has a mean grain size of 2 µm or less, the retained austenite has a mean grain size of 2 µm or less, and a value obtained by dividing an Mn content in the retained austenite (in mass%) by an Mn content in the polygonal ferrite (in mass%) equals 2.0 or more.
摘要:
Provided are a high-strength hot-pressed part and a method for manufacturing the part. A raw material is a steel sheet obtained by heating a cold-rolled steel sheet having a chemical composition containing, by mass%, C: 0.090% or more and less than 0.30% and Mn: 3.5% or more and less than 11.0% at a temperature in a temperature range equal to or higher than the Ac1 transformation temperature and 850°C or lower, by holding the heated steel sheet at the temperature for 100 seconds or more and 48 hours or less, and by cooling the steel sheet held at the temperature. The raw material is subjected to a heating process in which the raw material is held at a temperature in a temperature range of 800°C to 1000°C for 600 seconds or less and to a hot press forming process in which the heated material is subjected to press forming and quenching at the same time by using a forming tool in order to obtain a hot-pressed part having a specified shape. With this, it is possible to obtain a high-strength hot-pressed part having a microstructure including 3.0% or more of a retained austenite phase and the balance being a martensite phase and tensile properties represented by a tensile strength TS of 1500 MPa or more and a uniform elongation uEl of 6.0% or more. Also, in the case where the cold-rolled steel sheet to be used is a steel sheet having a Zn-based coating layer or an Al-based coating layer on the surface thereof, since it is possible to inhibit the generation of scale on the surface of the part, there is an increase in the productivity of a hot-pressed part.
摘要:
A cold-rolled steel sheet having excellent spot weldability suitable for use in vehicles, electric appliances, etc. is provided. The cold-rolled steel sheet has a steel composition containing, in mass%: C: 0.05% to 0.13%; Si: 0.05% to 2.0%; Mn: 1.5% to 4.0%; P: 0.05% or less; S: 0.005% or less; Al: 0.01% to 0.10%; Cr: 0.05% to 1.0%; Nb: 0.010% to 0.070%; Ti: 0.005% to 0.040%; and N: 0.0005% to 0.0065%, with a mass ratio Ti/N of Ti and N being 2.5 or more and 7.5 or less, and a balance being Fe and incidental impurities, wherein 70 mass% or more ofTi in steel exists as a precipitate, and 15 mass% or more of Nb in the steel exists as solute Nb, and a tensile strength is 980 MPa or more.
摘要:
A steel sheet has a microstructure that contains ferrite in an area ratio of 20 % or more, martensite in an area ratio of 5 % or more, and tempered martensite in an area ratio of 5 % or more. The ferrite has a mean grain size of 20.0 µm or less. An inverse intensity ratio of ³-fiber to ±-fiber in the ferrite is 1.00 or more and an inverse intensity ratio of ³-fiber to ±-fiber in the martensite and the tempered martensite is 1.00 or more.
摘要:
Provided is a high-carbon cold-rolled steel sheet having a thickness of less than 1.0 mm and capable of having good impact and hardness characteristics after a short solution treatment, and thereafter quenching and low-temperature tempering. A high-carbon cold-rolled steel sheet having a steel sheet chemical composition containing, in terms of mass%, C: 0.85-1.10%, Mn: 0.50-1.0%, Si: 0.10-0.35%, P: 0.030% or less, S: 0.030% or less, and Cr: 0.35-0.45%, and furthermore containing Nb: 0.005-0.020 mass% with the remainder being Fe and unavoidable impurities, having a steel sheet structure in which the average particle diameter (d av ) of carbide dispersed in the steel sheet is 0.2-0.7 (µm) and the spheroidization ratio is 90% or higher, and having a thickness of less than 1.0 mm. Mechanical characteristics having an excellent impact characteristic in which the impact value is 5 j/cm 2 or higher and a sufficient hardness characteristic within the range of 600-750 HV can thereby be manifested by a short solution treatment of 3-15 minutes, and thereafter quenching and low-temperature tempering.
摘要:
Disclosed is a method comprising: preparing a steel slab with a predetermined chemical composition; subjecting the steel slab to hot rolling by heating it to a temperature of 1100-1300 °C, hot rolling it with a finisher delivery temperature of 800-1000 °C to form a hot-rolled steel sheet, and coiling the steel sheet at a mean coiling temperature of 200-500 °C; subjecting the steel sheet to pickling treatment; and subjecting the steel sheet to annealing by retaining the steel sheet at a temperature of 740-840 °C for 10-900 s, then cooling the steel sheet at a mean cooling rate of 5-50 °C/s to a cooling stop temperature of higher than 350 °C and 550 °C or lower, and retaining the steel sheet in a temperature range of higher than 350 °C to 550 °C for 10 s or more.