Study on the Precipitation Behavior of Nano-sized Carbide in the Novel HSLA Steel
Date Issued
2007
Date
2007
Author(s)
Chen, Chih-Yuan
DOI
en-US
Abstract
Abstract
High strength low alloy (HSLA) steel usually possess many advantages: such as good strength, better stretch flange, good weld-ability and easy formability. Therefore, this type of steel is commonly used in the automobile and pipeline industry. From the various strengthening mechanisms; grain size minimization and generation large amounts of precipitates in the matrix can strengthen steel effectively. However, it is very difficult to obtain grain size less than 2~3μm in the thermomechanical process (TMCP). Nano-sized carbides in the ferrite matrix can lead to very greatly strengthening effect; nevertheless, it is very difficult to obtain these nano-sized carbides in the steel due to complex manufacturing process. In fact, most of microalloy elements exhaust in the soaking and hot deformation process, which result in less microalloy carbide precipitation in the coiling. Therefore, studying the various microalloy additions and thermomechanical process parameters is the main objective of present study, which can generate large amounts of precipitates in the steel.
In this work, isothermal aging experiments at 750℃ for 5min, 10min, and 60min were carried out for six different steels. The isothermal aging used in the present study corresponds to coiling process in the hot rolled steel strip. The corresponding metallographs and transmission micrographs for the different aging times of six steel have been investigated. Interface precipitation carbides during γ→α transformation can generate large amounts of carbides in the steel. The result also indicates that the [(TiMo)C] has slower coarsening rate compared with other carbides during isothermal aging experiment, which is beneficial for strengthening steel effectively. This is because the larger difference in diffusivity in Ti/Mo compared to Ti/Nb indicates that it takes more time to grow [(TiMo)C] than to grow [(TiNb)C].
In continuous and interrupted cooling experiments, three different composition steels were conducted for studying the precipitation behavior of various carbides. Single component carbide (TiC) has larger coarsening rate compared to other carbides; hence, it can be seen that the interface precipitation of titanium carbide can occur in the continuous experiment. However, Complex carbide, such as [(TiMo)C] and [(TiNb)C], cannot happen in the continuous cooling. On the other hand, all steels can generate interface precipitates in the interrupted cooling experiment, and more quickly cooling rates lead to smaller carbides in the ferrite matrix. Therefore, this result suggests that it can generate nano-sized carbides in the HSLA steel if it avoids staying at high temperature for longer times.
From isothermal aging at two phase region, it finds out that not only interface precipitation but also supersaturated precipitation occurring in the ferrite region, which can harden ferrite matrix effectively. The spacing of intersheet for interface precipitation mainly determines by the characteristic of austenite and ferrite boundary. For low isothermal forming temperature, the large driving force induces more incoherent parts (ledge) which can get smaller intersheet. The supersaturated precipitates form when the migration rate of ledge becomes fast, which remain some microalloy elements in the ferrite matrix and precipitate in the subsequently treatment. It reveals that much of nano-sized carbides of interface precipitation and supersaturated precipitation generating in the ferrite matrix when the isothermal temperature is low.
Comparing the effect of different categories of microalloy addition in the same isothermal aging experiment, it finds that complex carbide has a low coarsening rate due to both thermodynamic and dynamic reasons. The spinodal decomposition of complex carbide and discrepancy of diffusivity for various microalloy elements occurring in the two phase region aging makes the excellent thermal stability for complex carbide. Niobium carbide will form in the high temperature region, which will absorb surrounding microalloy elements and reduce the amount of precipitation carbide in the low temperature, which reduces the precipitation strengthening effect for the Nb-bearing steel.
The effect of high temperature deformation also discusses in the present thesis. The acceleration of austenite decomposition occurring in the high temperature deformation process can results higher migration rate of ledge, which inhibits the interface precipitation occurring. It is thus more supersaturated precipitation carbides can happen in mostly of ferrite grains. However the high temperature deformation cannot alter the precipitation status such as size and quantity, which also be determined by the isothermal aging temperature.
The effect of soaking temperature on the precipitation behavior in the HSLA steel also discusses in the present thesis. It should be emphasized that higher soaking temperature can dissolve all microalloy elements in the austenite and bring their strengthening effect into full play. Large amounts of deformation can lead to smaller ferrite grains which can balance the high temperature soaking effect.
In the following continuous cooling and interrupted cooling experiment, it finds that accelerated cooling can remain most of microalloy elements in the ferrite which would be benefit for getting high hardness. In the interrupted cooling experiment, the suitable interrupted cooling temperature is benefit for getting much of nano-sized carbides in the ferrite matrix. Therefore to discovery a proper interrupted cooling temperature is very important parameter for hot rolled strip.
Isothermal aging in the bainite phase, the microalloy elements have two effects:
1.Carbide forming in the high temperature region can promote partition of carbon atoms which can promote the bainite phase transformation further and reduce the existence of blocky austenite.
2. The nano-sized carbide nucleating at dislocation can sustain their tiny size and get
Subjects
界面析出物
過飽和析出物
單一成份碳化物
複合碳化物
奈米級析出物
熱穩定性
汽車鋼板
HSLA steel
Interface precipitation
Supersaturated precipitation
Single component carbide
Complex carbide
Thermal stability
Automobile steel
Nano-sized carbide
and HRTEM
Type
thesis
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