Kazutoshi Kunishige

High Speed Deformation of Ultrafine Grained TWIP Steel

Tensile properties of twinning induced plasticity (TWIP) steels (31%Mn-3%Al-3%Si-Fe) with various mean grain sizes ranging from ultrafine grain size (1.1μm) to conventional one (35.5μm) at a wide range of strain rates from 10-3sec-1 to 103sec-1 were studied. The ultrafine grained TWIP steel exhibits a large work hardening and keeps an adequate elongation at any strain rate. The strength held to the Hall-Petch relationship at each strain rate and the Hall-Petch slopes do not change largely.

SOFTENING IN WELD HEAT AFFECTED ZONE OF DUAL PHASE STEEL SHEET FOR AUTOMOTIVE WHEEL RIM

Dual phase steel offers potential for wheel weight reduction, but has not been completely succeeded, in part, because of a localized necking or a fracture during rim fabrication. This failure should be related to a softening in the weld heat affected zone (HAZ). This paper investigates mechanical properties through microstructural change produced in the HAZ of as-rolled dual phase steel and finally suggests a practical solution. The softening, caused by heating the steel up to Ac1 in the HAZ, is found to be affected not only by tempering of martensite, but also by the variation of hardness in ferrite matrix. Moreover strain aging as well as work hardening due to up-setting stage during the flash welding also concerns this problem. Coiling temperature in manufacturing as-rolled dual phase steel has a large effect on the conditions of ferrite and martensite. Control of the temperature is of great importance to avoid such a localized necking or a fracture in the HAZ.

Surface Hot-Shortness of Steels Induced by a Small Amount of Copper and Tin from Scrap Steels and its Suppression Methods

The recycling of scrap steels can be difficult due to the tramp elements that they can contain. During the steelmaking process, tramp elements such as Cu and Sn are difficult to be removed; and it is these elements that cause surface cracking of steels during hot rolling process (i.e. Cu and Sn liquid embrittlement).The paper consists of three different experiments into the suppression of surface cracking during the hot rolling process. For the oxidation in air, the surface cracking most severely occurred in the specimens which were oxidized around 1100°C in the tested range of 950-1200°C after a 1250°C heating. For the change in oxidation atmosphere from air to water vapor, the surface cracking occurred more severely although the mass gains were smaller in water vapor than in air oxidation. For the addition of Si and Ni in the water vapor conditions of 0%-30%H2O, the surface cracking was found to be suppressed effectively when the mass gain increased. The Cu and Sn enriched alloys at the scale/steel interface were closely observed by optical microscopy and scanning electron microscopy. The mechanism for suppression of the surface cracking was explained in terms of back diffusion of Cu and Sn into the steel and/or occlusion of Cu and Sn into the scale through the development of a rugged scale/steel interface.

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar 3, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar 3, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar 3 temperature, decreased the time of transformation, and increased the hardness.

Strain Rate Sensitivity of 31Mn-3Al-3Si TWIP Steel with Partially Recrystallized Fine Grained Structure

Strain rate sensitivity of the strength of TWIP (Twinning Induced Plasticity) steel with the mixture of recrystallized fine grains and rolling-deformation microstructures was studied. The 31mass%Mn-3%Al-3%Si TWIP steel sheet was severely cold-rolled to a reduction of 92% and subsequently annealed at various temperatures ranging from 600oC to 700oC in order to obtain the partial recrystallized microstructure with various fraction of recrystallized microstructure. The 600oC annealed specimen keeps similar morphologies as observed in the as-rolled structure consisting of both the fine lamellar dislocation cell structure and the twin/matrix lamellar structure; whereas, in the specimen annealed at 625oC or 675oC , the partially recrystallized fine grains (d~1µm) with a few dislocations evolve. The volume fraction of recrystallized fine grains increases with increasing of the annealing temperature while the mean diameter of the recrystallized grains is not changed largely. The tensile deformation behaviors were measured at various strain rates ranging from 10-3sec-1 to 102sec-1. The strength and elongation become smaller and larger, respectively, with increasing the fraction of the recrystallized microstructure. The activation volume of dislocations becomes larger with increasing the fraction of recrystallized microstructure.

DEVELOPMENT OF A METALLURGICAL MODEL FOR THE PREDICTION OF PEARLITE TRANSFORMATION AND ITS APPLICATION TO HIGH-CARBON SHEET STEEL PRODUCTION

A computer simulation model based on metallurgical theories have been developed in order to predict pearlite transformation during hot-run cooling in a hot strip production line. The model has been constructed on the basis of available theories; the regular solution sublattice model of Fe-C-Mn-Si systems for the estimation of thermodynamic parameters, thermodynamics and kinetics of phase transformation for nucleation and growth rate equations in isothermal condition. The model is able to predict kinetic information of pearlite transformation during continuous cooling and able to estimate the effect of latent heat evolution on the cooling behaviour of the materials. As a consequence the accurate prediction of transformation behaviour on a run-out table is accomplished for high-carbon steels where the effect of latent heat evolution is not negligible. It is demonstrated that the predicted cooling behaviours of steel sheets are in good agreement with the observed temperature profile of steel sheets on a run-out table in a hot strip mill. The optimisation procedure of cooling condition is discussed in order to improve the uniformity in mechanical properties within a high-carbon steel sheet.

Formable ferrite-degenerated pearlite steel (FDP-55) for automotive use

In order to help the gauge reduction of wheels and chassis parts of automobiles, a formable and weldable hot rolled steel of 550 MPa grade, named FDP-55, has been developed. FDP-55 is an 0.14% C, 0.1% Si, 1.1% Mn and Nb free Alkilled steel obtained by controlled-cooling to a low coiling temperature on a runout table, and it is featured by ferrite-degenerated pearlite microstructure. Results of co-operative works with automotive makers showed that FDP-55 was successful in the application to wheels and chassis parts attaining the large weight reduction. This paper reports the metallurgical features and characteristics of the steel.