The behavior for injection of magnesium vapor into molten steel and the potential of magnesium vapor for miniaturization of TiN size were evaluated by the experiment in tammann furnace and the experiment in vacuum furnace. From the results of these experiments, the partial pressure of magnesium reacting with molten steel could be increased, and the both high concentration in molten steel and high additive yield of magnesium into molten steel could be done together. It was confirmed that Spinel MgO·Al2O3 was formed as a heterogeneous nucleus of TiN. The dissolved magnesium concentration governed the heterogeneous nucleus for TiN. The injection of magnesium vapor into molten steel could control the generation of heterogeneous nucleus during solidification.
Recently requests for high added value steel products have been increased to produce vehicles. Since strip temperature in hot strip mills is one of the most important factors to determine the material properties, temperature control on Run Out Table (ROT) is crucial. In conventionally used temperature model, running steel strip is divided into segments of the same length, and then strip temperature is calculated for each segment. In this paper, we propose a new temperature model in which area of ROT is divided into sections fixed in space and strip temperature is calculated in each section. The proposed model is considered to be useful for model-based design of the temperature control since positional relation between cooling valves, i.e. control inputs and sections can be invariant. The validity of the proposed model is confirmed by numerical simulations.
In this study, hydrogen absorption behavior of steels on a vehicle during driving and parking in deicing-salt-spraying areas was investigated by using temperature-compensating hydrogen absorption monitoring system. During the driving of the vehicle in the areas, hydrogen permeation current of steel was increased due to picking up of salt water from the road when the road was wet. On the other hand, during the parking of the vehicle, the magnitude of the hydrogen permeation current were indicated that the hydrogen permeation current in the automobile driving environments is related to both driving states of a vehicle and environmental conditions such as temperature and relative humidity.
In order to clarify the mechanism of abnormal grain growth in steel for cold forging and carburizing, the effect of spheroidized annealing on the behavior of abnormal grain growth during carburizing was investigated. Abnormal grain growth was observed in annealed steel, whereas it was suppressed in normalized steel. However, both the normalized steel and annealed steel have almost the same size distribution of Nb (C,N) nano-precipitates. The effect of annealing on abnormal grain growth was not explained through the conventional theory based on pinning force by nano-precipitates.
Spheroidized cementite was remained in annealed steel subsequently quenched from quasi-carburizing temperature 1203 K. Dissolution of spheroidized cementite and occurrence of abnormal grain growth took place simultaneously. Spheroidized cementite was thermally stabilized by Cr concentration. DICTRA simulation was carried out to discuss kinetics of cementite dissolution. Cr concentration affects dissolution rate of spheroidized cementite through the relationship between Cr concentration and cementite size. Cr concentration in cementite has wide distribution. Therefore, dissolution time is different in each cementite. This dissolution time lag among each cementite causes local and ununiform decreasing of pinning force by cementite. As a consequence, abnormal grain growth is likely to occur in annealed steel.
Dual-phase (DP) steel sheets composed of soft ferrite and hard martensite phases are typical advanced high strength steel sheets applicable to a variety of automobile parts. The crystallite texture of steel sheets is an important factor which influences the press formability. However, the texture of the martensite itself in DP steel sheets has not been discussed, since the texture was generally measured by the X-ray diffraction method, which does not distinguish the texture of martensite from that of ferrite. The objective of this study is to investigate the texture evolution behavior of each compromising phase; especially the martensite phase, in DP steel sheets by a newly-developed analysis method using Electron Back-Scatter Diffraction (EBSD). The chemical composition of the steel used was 0.088%C -1.23%Si -2.29%Mn -0.093%Ti (mass%). The two sequent annealing was conducted, changing the second annealing temperature, both in the intercrtical region and in the γ single-phase region. The obtained DP microstructures were controlled to have the same volume fraction of martensite of approximately 40%. The overall texture including martensite after the intercritical annealing was similar to the texture before 2nd-annealing, while the texture after the γ single-phase annealing became weak. The new analysis technique using OIM clearly revealed that the discriminate textures from only martensite were close to, but slightly weaker than those of ferrite under the two annealing conditions.
This paper introduces the microstructure, retained austenite characteristics, strain-induced transformation-deformation mechanism and mechanical properties of transformation-induced plasticity (TRIP)-aided martensitic (TM) steels for the automotive applications. Because the microstructure consists of a wide lath-martensite structured matrix and a mixture of narrow lath-martensite and metastable retained austenite (MA-like phase), the TM steel produced a good combination of tensile strength and cold formability. If Cr and/or Mo were added into 0.2%C-1.5%Si-1.5%Mn steel to enhance its hardenability, the resultant TM steel achieved superior notch fatigue strength and impact and fracture toughness to conventional structural steel such as SCM420. These enhanced mechanical properties were found to be mainly caused by: (1) plastic relaxation of the stress concentration, which results from expansion strain on the strain-induced transformation of the metastable retained austenite; and (2) the presence of a large quantity of finely dispersed MA-like phase, which suppresses crack or void initiation and subsequent connection.
To investigate an effect of internal hydrogen on mechanical properties of a new austenitic stainless steel “STH2” having a nominal composition of Fe-15%Cr-9%Mn-6%Ni-2.5%Cu-0.15~0.2%N, tensile tests and fatigue crack growth tests were conducted for the specimens containing around 80 ppm hydrogen charged by the exposure in 45 MPa hydrogen gas at 300˚C for 200 h and the ones heat treated in air with the same heat pattern.
At room temperature and –40˚C, no significant ductility drop by hydrogen charging was observed and about 80% of relative reduction of area was obtained, which is the same as that of JIS SUS316L with the same amount of internal hydrogen. In the specimens tensile tested at –40˚C, a small quantity of quasicleavage fracture surfaces were observed. In the banded areas in which Mn, Ni and Cu were negatively segregated, some relatively coarse voids coalesced with cracks extended along the maximum shear stress plane, which is similar to what is called void-sheet type of fracture. It is quite different from the case for tensile tests in 90 MPa hydrogen gas, in which coarse longitudinal cracks form, suggesting that the concentration of hydrogen at crack tips in 90 MPa hydrogen gas is higher than 80 ppm. It was also confirmed that fatigue crack growth rates were not accelerated by 80 ppm internal hydrogen although some faceted fracture surfaces composed of (111) γ formed.
It is confirmed that STH2 has excellent properties not only in high pressure hydrogen gas but also in the circumstance of internal hydrogen of around 80 ppm.
New ferritic heat resistant steels with high nitrogen content were developed and these microstructure and the mechanical properties at high temperature were evaluated. 0.3 mass% N could be added into ferritic steels without blow holes by applying pressurized melting methods with pressurized atmosphere up to 4.0 MPa. The high nitrogen ferritic heat resistant steels contained several kind of nitrides within the lath martensitic structure. V-rich coarse particles were identified as crystallized VN. Fine VN or Cr2N particles were precipitated on the martensitic grain boundaries depending on the amount of V content. The martensitic structure in the high nitrogen steels contained a hierarchical structure of martensitic lath, block, packet and prior austenitic grain. These martensitic structure satisfied the K-S relationship as with the conventional carbon steel. The creep strength of the developed steels were comparable to Gr.91 steel though weaker than Gr.92. It is required additional precipitates other than nitrides for further strengthening of the developed steels.