| In order to meet the tendency towards large-scale, high-speed and multiple-sea-area working, the hull ship steel with good combination properties of high strength, low-temperature toughness, better weldability and good marine corrosion resistance has rapidly developed. Most present research of copper-bearing steel with yield strength more than 690 MPa are concentrated on the properties, microstructure and welding processing of laboratory trial steel, but little study has been done on the production process and little performance data of industrial trail plate has been issued. From the published data studied in the field of HSLA100 steel, some shortages such as higher yield ratio, narrower window of tempering process and lower allowance in low-temperature toughness are founded. In the present study, an effort has been made to study the effect on the microstructures and mechanical properties of heat treatment processing parameter to the laboratory rolled HSLA-100 steel plates. At the same time, the comprehensive performances of industrial steel treated by optimizing heat treatment process with 10~30mm thickness plate made in Wuhan Steel Plate Plant, which should fill the domestic blank in the field of high strength & toughness construction steel of ship hull with good comprehensive properties.For plotting the CCT curve of this steel, a Formast-F automatic dilatometer was used. The results indicated that the full lath martensite zone was abstained at a cooling rate more than 17°C/s and a mixed zone of typical lath bainite and granular bainite at a cooling rate less than 17°C/s, owing to ultra-low carbon(≤0.06%) design which reduces the hardenability greatly.The effect on the mechanical properties such as tempering stability, yield ratio and low temperature toughness of different heat treatment with the conventional quenching and tempering heat treatment process(QT) and the secondary hardening heat treatment process in two-phase region(QLT) was studied by the laboratory HSLA100 steel. There was an obvious advantages treated by QLT that it could significantly reduce the ductile-brittle transition temperature, increase tempering stability and lower yield ratio.The microstructure was characterized and studied in different heat treatment status of HSLA100 steel such as Q(a single phase quenching), QL(an intercritical quenching at two-phase region) and QLT(tempering following by QL) by using SEM(scan electron microscope), TEM(transmission electron microscopy), OM(optical microscopy), EBSD(electron backscatter diffraction) and XRD(X-ray diffraction). The results showed that there was typical lath bainitic structure in the Q condition and a mixed miscrostructure of lath bainite ferrite(BF) & martensite islands(M) in the QL conditions. With the increasing of temperature during the two-phase region, the amount of martensite islands distributing along lath ferrite became from particulate to long-thick, but also the ferrite became from single lath-shaped to polygonal shape and in which the dislocation density was reduced. When quenched at 820℃, it had returned to the lath bainitic microstructure in which the dislocation density reduced significantly and little M islands along the grain boundaries was observed compared with the first quenching structure. Lath bainite and particulate ε-Cu precipitated phases were obtained at QLT condition. There was no obvious reverted austenite.Partial austenite transformation occurred during heating in the two-phase region of HSLA100 steel. The research results showed that the reheating temperature at the two-phase region had a near-linear relationship with the fraction of austenite transformation which determines the number of newborn martensite or bainite, and the carbon content in newborn austenite determines the stability of the austenite and morphology of miscrostructure evolution, which ultimately determines the mechanical properties of steel. When tested steels were reheated at 700℃, 720℃, 740℃, 760℃ and 820℃, the transformation quantity of austenite was 10%, 24%, 38%, 60% and 100% respectively, and the carbon content in austenite was 0.345%, 0.293%, 0.243%, 0.193% and 0.045% respectively.Mechanism study results showed that the secondary quenching process at two-phase region of HSLA100 steel could effectively refine the effective grain size and increase the number of high angle boundary orientation difference greater than 15° who effectively prevent the crack growth, which was the main reason for substantial increasing low-temperature toughness of QLT heat treatment. The QLT microstructures had characteristics of dual phase steel with bainite ferrite and ε-Cu, in which the soft ferrite and the relatively hard ε-Cu phase reduced the yield ratio improve the low temperature toughness of steel.According to the results of laboratory studies, industrial production of HSLA100 steel was carried out. The effects on HSLA100 steel comprehensive mechanical property of first quenching temperature, second quenching temperature, tempering temperature and tempering time were investigated by orthogonal test method. The best heat treatment process optimized by range analysis and variance analysis was that the first quenching temperature,the second quenching temperature,tempering temperature and tempering time was 860 ℃ ~880 ℃,750 ℃ ~780 ℃, 530 ℃ ~560 ℃ and 80 min respectively.The industrialization plates of HSLA100 steel with 10~30mm thickness had been conducted and treated by optimized QLT heat treatment process. At the same time, the comprehensive properties had been analyzed by QT and QLT processes comparatively. The results showed that the comprehensive properties of QLT steel met the technology request and had a big prosperous. Compared with QT steel, QLT steel had lower yield ratio(below 0.90), lower toughness-brittleness transformation temperature(-100℃), better anti-crack initiation properties(the maximum-40℃ CTOD feature value δu was 0.72mm), better anti-crack propagation properties(DT upper mounting plate energy was 2140~2360J and-40℃ DT energy was 1080J)。In addition, Industrialization plate produced by QLT process had good welding properties. On the condition of heat inputting about 17KJ/cm and preheating at 80℃, the surface flaw ratio of HSLA100 steel was 0. Both the welding joints’ tensile strength and impact value of three regions all met the welding technology qualification of HSLA100 steel which was done by gas protection welding with WER80 and manual electric arc welding with CJ807 respectively. |
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