Disclosed is a high-strength steel sheet having excellent processability and a tensile strength of 980 MPa or more. The high-strength steel sheet has the following composition (in mass%): C: 0.1 to 0.3% inclusive, Si: 2.0% or less, Mn: 0.5 to 3.0% inclusive, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less, with the remainder being Fe and unavoidable impurities. The steel structure of the high-strength steel sheet comprises 50% or more of martensite, 50% or less (including 0%) of ferrite, 10% or less (including 0%) of bainite, and 10% or less (including 0%) of residual austenite. In the high-strength steel sheet, the half width in the degree distribution of the nano-hardness obtained by the measurement of the hardness distribution of the martensite is 2.0 GPa or more, and the steel sheet has a tensile strength of 980 MPa or more.

High strength steel sheet and method for manufacturing the same

Fe-6mass%Ni-(0.0008∼0.29)mass%C alloys were hot-deformed in torsion at 600-720°C (above the cooling transformation start temperatures A r3 ) after austenitization. An in-situ X-ray diffraction study revealed that γ→α transformation occurred during deformation in a wide range of condition, even above A p 3 (paraequilibrium γ→α transformation temperature). Corresponding to this transformation, apparent decrease in deformation stress from that expected for austenite was observed. Microstructural study of the specimens quenched after the deformation showed that a large amount of fine-grained ferrite was formed due to the deformation. The analysis of deformation stress and chemical driving-force of the transformation indicated that the transformation occurred in order to reduce the total energy of deformed material since the deformation of energy of α was revealed to be considerably smaller than that of γ and the amount of deformation energy saved by the transformation was shown to be much greater than the chemical energy consumed by the transformation at the tested temperatures.

https://www.jstage.jst.go.jp/article/isijinternational1989/40/2/40_2_200/_pdf

Dynamic γ→α Transformation during Hot Deformation in Iron–Nickel–Carbon Alloys

Large deformation for a common C-Mn steel at the temperatures around Ar3 results in an ultrafine-grained ferrite structure This is due to dynamic transformation and recrystallization of ferrite during deformation To apply the new finding to an actual plant rolling, multi-pass deformations which substitute for a single pass heavy deformation have been studiedIt was realized that when the interpass time during successive deformations is shorter than 2s, the effect of accumulative strain can make ferrite grains fineA trial of strip rolling in plant provided a mean ferrite grain size of ASTM No 135

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Evolution of Ultrafine-grained Ferrite in Hot Successive Deformation

A high strength galvanized steel sheet having a TS of 780 MPa or more and exhibiting excellent stretch frangeability and bendability and a method for manufacturing the same are provided. The component composition contains C: 0.05% to 0.15%, Si: 0.8% to 2.5%, Mn: 1.5% to 3.0%, P: 0.001% to 0.05%, S: 0.0001% to 0.01%, Al: 0.001% to 0.1%, N: 0.0005% to 0.01%, Cr: 0.1% to 1.0%, Ti: 0.0005% to 0.1%, B: 0.0003% to 0.003%, and the remainder composed of iron and incidental impurities, on a percent by mass basis. The microstructure includes 30% or more of ferrite phase and 30% or more, and 70% or less of martensite phase on an areal fraction basis, wherein regarding the above-described martensite phase, the proportion of a tempered martensite phase is 20% or more relative to the whole martensite phase and the proportion of a martensite phase having a grain diameter of 1 µm or less is 10% or less relative to the whole martensite phase.

High-strength hot dip zinc plated steel sheet having excellent moldability, and method for production thereof

Mathematical models for predicting microstructural evolution and mechanical properties of hot strips have been reviewed. The metallurgical features of the hot strip rolling are discussed and the fundamental idea of their modelling is introduced.As applications of the mathematical models, the on-line prediction of the microstructure and strength, the resistance to hot deformation and the cooling curves affected by the heat evolution due to transformation are given.Finally, the future work and prospective of the mathematical model in hot strip rolling is also presented.

Mathematical Models for Predicting Microstructural Evolution and Mechanical Properties of Hot Strips

https://www.jstage.jst.go.jp/article/isijinternational1966/23/2/23_2_100/_pdf

Strength and Structural Changes under High Strain-rate Hot Deformation of C Steels

PURPOSE: To obtain a high strength hot rolled steel plate excellent in stretch- flange formability and ductility by successively subjecting a slab of steel with a specific composition to hot rolling, cooling, and coiling under respectively specified conditions and providing a specific structure. CONSTITUTION: A slab of a steel having a composition consisting of, by weight, 0.07-0.18% C, 0.5-1.0& Si, 0.7-1.5% Mn, ≤0.02% P, ≤0.005% S, 0.0005-0.0050% Ca, 0.01-0.10% Al, and the balance Fe with inevitable impurities is heated up to 1000-1200°C and hot-rolled, and finish rolling is completed at a temp. between (Ar 3 transformation point + 60°C and 950°C. Subsequently, cooling is exerted at ≥50°C/sec cooling rate within 3sec from the completion of finish rolling and rapid cooling is finished at a temp. between (T°C), computed from equation, and (T-70)°C, and, after air cooling, ceiling is carried out at >350-500°C. By this method, the hot rolled steel plate in which the structure ratio of cementite of ≥0.1μm circle-equivalent radius is regulated to ≤0.1% and/or the structure ratio of martensite is regulated to ≤5% and which has ≥50kgf/mm 2 tensile strength and stretch-flange formability of ≥1.8 blanking hole widening ratio and also has excellent ductility is obtained. COPYRIGHT: (C)1992,JPO&Japio

Production of high strength hot rolled steel plate excellent in stretch-flange formability and ductility

PURPOSE:To manufacture with low cost a steel sheet of fine grain structure superior in ductility by heating, rolling and cooling a usual carbon steel under the specified condition so as to generate fine ferrite grains. CONSTITUTION:A rolling of the steel consisting basically of usual carbon steel constituents is started under the total reduction ratio higher than 80% in the temp. region at just above usual Ar3 transformation point (temp. at which ferrite tansformation is started on the way of slow cooling of the steel sheet after the rolling at austenite region) and below (Ar3+100 deg.C), and the rolling is finished at the temp. region at just below Ar3 point and above (Ar3-30 deg.C). This steel is air cooled after the rolling or cooled to = 20 deg.C/sec so as to obtain the finer grains. Thereby, the steel sheet consisting of main structure of ferrite and having fine grains having <=4.5mu mean grain diameter is obtained.

Manufacture of high strength hot rolled steel sheet of super fine grain

PURPOSE: To economically prepare a high strength hot rolled steel plate excellent in an elongation flange property, by continuously casting steel with a specific composition which contains C, Si, Mn and Al and reduced in the content of S, P and Ca, and rolling and cooling the cast steel under a specific condition. CONSTITUTION: Steel containing, on a wt. basis, 0.12W0.30% C, Si<1.5%, 1.0W 2.0% Mn, 0.01W0.10% Al, S<0.005% and, if necessary one or more of P<0.008% and Ca<0.005% and comprising the remainder of Fe and inevitable impurities is continuously cast and heated at 1,200°C or less to be rolled under such a condition that a total draft at 1,000°C or less is set to 90% or more and finish rolling is completed in a temp. region of Ar 3 +50°CWAr 3 +150°C. After the completion of finish rolling, the cooling of the steel plate is started within 2sec and the cooling speed in this case is set so as to satisfy formula logCR(°C/sec)≥31/10(1- Ceq) (wherein Ceq=C+Mn/5). Subsequently, the steel plate is wound up at 500°C or less and a super 70kg grade high strength hot rolled steel plate having 50% or more of a bainite structure and excellent in an elongation flange property is obtained. COPYRIGHT: (C)1986,JPO&Japio

Preparation of super 70kg grade high strength hot rolled steel plate excellent in elongation flange property

A building low yield ratio hot-dip galvanized cold rolled steel sheet having an improved refractory property, consists essentially of, by weight, 0.1% or less of C, 0.1 to 1.5% of Mn, 0.1% of less of P, 0.06% or less of Al, 0.6 to 2.0% of Cu, and a remainder of Fe and unavoidable impurities and having a 0.6 times or more yield point strength at 600° C. compared to that at normal temperatures. The method of forming the steel sheet comprises the steps of: forming a slab from a steel consisting essentially of, by weight, 0.1% or less of C, 0.1 to 1.5% of Mn, 0.1% or less of P, 0.06% or less of Al, 0.6 to 2.0% of Cu, and a remainder of Fe and unavoidable impurities, immediately or after heating the slab at a temperature of 1200° C. or less, hot rolling the slab at a temperature of 750° C. or more and cold rolling, effecting a continuous hot-dip galvanizing process, heating the rolled slab at a temperature of 700° to 900° C. in a reducing atmosphere, cooling the rolled slab at an average cooling rate of 3° C./sec or more and dipping the rolled slab in a hot-dip galvanizing bath.

Nobuhiko Matsuzu

Papers

Strength and Structural Changes under High Strain-rate Hot Deformation of C Steels

Production of high strength hot rolled steel plate excellent in stretch-flange formability and ductility

Manufacture of high strength hot rolled steel sheet of super fine grain

Preparation of super 70kg grade high strength hot rolled steel plate excellent in elongation flange property

Building low yield ratio hot-dip galvanized cold rolled steel sheet having improved refractory property