Benefit of intensive water cooling before controlled rolling was introduced and laboratory tests were conducted with water flow rate of 0.10-0.17m3/m2s. Cooling performance was quantified in conditions of relatively higher surface temperature of 850-1100ºC. Temperature drop in specimen increases with higher water flow rate. Upper limit of cooling performance was not found in the range of experimental conditions. At higher surface temperature, where is more than 1000ºC, cooling performance at upper surface is much larger than at lower surface, whereas difference of cooling performance between upper surface and lower surface is small at lower temperature. Effect of transfer speed was not seen in cooling time of 0.2-0.3s. Based on experimental data and thermal analysis, equivalent heat transfer coefficient was calculated to be 10-29MW/m2 K, which is extremely higher than in literature data. As a result, it was clarified that repeat of short time intensive cooling inter pass is possible to realize effective production in controlled rolling.
In our previous paper, we reported that the surface carbon concentration (Cs) in carburized Nb-bearing steel decreases substantially with increasing the machining speed before carburizing. In the present study, a systematic study was made to clarify the effects of Cr,Nb addition and machining speed on the Cs. Cr oxide film was observed on the surface of specimens exhibited a large decrease in Cs. The coverage of specimen surface by Cr oxide film was increased with increasing Cr,Nb content and machining speed. Since Cs decreased with the increase in the coverage of Cr oxide film, it was considered that the decrease in Cs is due to Cr oxide film formed during carburizing. The carbon concentration profile was calculated assuming that carbon absorption was ceased after the complete coverage of specimen surface by Cr oxide film. It is confirmed that the present calculation fits well with the measured carbon concentration profile of low Cs. Moreover, it is shown that Cs becomes smaller as time period for Cr oxide film formation becomes shorter. The role of Nb is considered to be the retardation of recovery, recrystallization and grain growth through the pinning effect of Nb(CN) during carburizing. The high density of lattice defects introduced by machining is maintained by Nb(CN) and promotes faster Cr diffusion. In the present study, it has been clarified that in machined specimens of Nb-bearing steel, entire coverage of specimen surface by Cr oxide film during carburizing occurs in steels containing even a small amount of Cr like 1%.
Electrochemical hydrogen permeation tests of pure Fe sheets rusted by cyclic corrosion test (CCT) and atmospheric exposure were carried out under controlled temperature and humidity to investigate the influence of atmospheric corrosion on the hydrogen entry behavior. The hydrogen entry into the Fe specimens rusted by CCT increased under wet condition, and the hydrogen entry was increased with the CCT cycle number. During drying process after the wetting, hydrogen entry was further enhanced and a peak of hydrogen current was observed. The peak hydrogen permeation current tended to increase with the growth of rust layer, and the peak value of the hydrogen permeation current became remarkably higher than that at the highest humidity when the rust layer was relatively thick. Similar enhancement of hydrogen entry into an outdoor-exposed specimen was also observed during drying. Drying process after CCT resulted in an increase in hydrogen content of 5 mm-thick steel specimens measured by means of thermal desorption analysis, indicating the enhancement of hydrogen entry during drying process and showing a good agreement with the electrochemical hydrogen permeation test results. It is required to take into consideration the enhanced hydrogen entry to estimate concentration of hydrogen from the environment.
The effects of Cr, Mo and Ni additions on the microstructure and stretch-flangeability of 0.2%C-1.5%Si-1.5%Mn-0.05%Nb, in mass%,TRIP-aided martensitic sheet steel produced by isothermal transformation process at a temperature below martensite-finish temperatures were investigated to develop the 3rd-generation sheet steel for automobile requiring high hardenability. When 0.5% or 1.0% Cr was added to the base steel, the combination of tensile strength of 1.5 GPa and hole-expanding ratio of 40% was attained. On the other hand, the Cr-Mo or Cr-Mo-Ni addition hardly influenced the stretch-flangeability and stretch-formability, although it increased the yield and tensile strengths, comparing to the base steel. The good balance of Cr-bearing steel was mainly caused by suitable combination of (1) volume fraction and (2) interparticle path of finely dispersed martensite-austenite complex phase (M-A phase), which suppressed a void initiation at the matrix/M-A phase interface on hole-punching and void coalescence or crack extension on hole-expanding.
In this study, the precipitation hardening behavior of middle carbon steels with single and multiple additions of 0.3mass%V and 2mass%Cu has been investigated. The precipitation treatment was carried out isothermally at 550ºC, 600ºC and 650ºC.
In the 0.3mass%V bearing steel, precipitation hardening was observed at 600ºC and 650ºC while no significant increase in hardness was observed at 550ºC. When 2mass% Cu was added to the 0.3mass%V bearing steel, the hardness attributed to the precipitation hardening of Cu was added to that attributed to the precipitation hardening of VC in the cases of 600ºC and 650ºC. In the case of 550ºC, the V addition into the Cu bearing steel markedly increased the hardness although the precipitation hardening of VC was hardly observed in the 0.3mass%V bearing steel.
In the paper, the different precipitation hardening behavior of the three steels is discussed in detail in consideration of the observed precipitation behavior.
Kinetic study was carried out in order to clarify effects of slag properties and operation factors on alkali elution of steelmaking slag component into artificial seawater. The seawater was rotated by an impeller in a cylindrical vessel and time change in pH was measured by pH meter. Operating factors such as vessel diameter, seawater volume, slag volumes, rotating speed were varied for various slag diameter and composition.
[OH–] increased lineally in the early stages of the experiment. From a theoretical study on pH buffering ability of carbonate in the seawater, it was found that the double-digit [Ca2+] increase was achieved compared with [OH–] and that [Ca2+]/[OH–] was kept almost constant. As alkali increase in the seawater resulted from dissolved free CaO of the steelmaking slag, its rate proceeded at the constant rate. When slag diameter was too small, alkali elution rate decreased because the penetration of the seawater in the lower part of slag layer was insufficient, whereas when slag diameter was too large, it also decreased due to the decrease in interfacial area of reaction. Alkali elution rate increased according to 1/3 power of liquid circumferential velocity, increased with free CaO content in steelmaking slag within about 2% and decreased with an increase in ratio of dredged soil.