| Engineering construction efficiency can be significantly improved and the production cost can also be saved by applying high heat input welding technology during large-scale steel structures fabrication in the fields of shipbuilding,architecture and so on.Conventional steels cannot meet the requirement because the HAZ toughness deteriorates severely under high heat input welding condition.Thus,developing steels with good weldability under high heat input is urgently needed.Japan has started research on high heat input welding steels for a long period and keeps an internationally leading position.In recent years,obvious progress has been achieved in China.However,the product grade is not high enough and the property stability is insufficient so that extensive domestic demands cannot be fulfilled yet.Oxide metallurgy is an effective technology for developing high heat input welding steel.However,this technology has not been maturely applied in domestic steel industry because of its complication and technical secrecy.With this background,research on high heat input welding steel production process based on oxide metallurgy has been carried out in this study.Main contents are as follows.In order to clarify inclusion precipitation rules under oxide metallurgy technology to provide guidance for experimental and industrial research,deoxidization thermodynamics of oxide metallurgy was analyzed.Calculation results showed that Ti2O3 is the stablest among different Ti oxide types in common microalloy steel.In order to avoid Al2O3 precipitation,Al content should be controlled below 0.004%when Ti content is 0.01%.Zr and Mg are very strong deoxidizer such that a minute amount of Zr can reduce Al2O3 and Ti2O3 and a trace of Mg can change Al2O3 to MgAl2O4 spinel.Deoxidation reaction will proceed during solidification because of elements segregation.MnS and TiN will also precipitate at the end of solidification.The increase in solidification cooling rate could weaken segregation and is in favor of disperse distribution of third inclusions.Influence of different microstructures on HAZ thoughness was analyzed as well as the HAZ embrittlement and toughening mechanisms in order to specify microstructural design route of high heat input welding steel.It showed that intragranular acicular ferrite fraction and grain boundary allotriomorphic ferrite size are the main factors influencing toughness.According to the experiment results,microstructure control of high toughness HAZ includes the following characteristics.Controlling the effective particles for ferrite nucleation distributing finely and abundantly so as to sufficiently promote intragranular ferrite transformation;inhibiting prior austenite grain coarsening in order to decrease allotriomorphic ferrite size;restraining the formation of grain boundary ferrite and side plates through enhancing intragranular transformation and increasing hardenability properly,or promoting polygonal ferrite formation by raising nucleation rate at grain boundary.Systemic experimental research on TiO and MgO oxide metallurgy was carried out.Inclusion distribution for different oxide metallurgy types and high heat input HAZ characteristics were studied.The key points of process control were made clear.In TiO steels,it was favorable for fine inclusion dispersion to control oxygen about 0.005%before Ti addition,shorten killing time before casting and increase solidification cooling rate,TiOx-MnS inclusions in TiO steel induced intragranular ferrite formation.Complex deoxidization with Ti and strong deoxidizer M(Zr,Mg,Ca,REM)could further refine inclusion size.The resultant TiOx-MOy(M(O,S))-MnS-TiN complex inclusions were effective for inducing acicular ferrite nucleation.In MgO steel,the main inclusion type was submicron-sized MgO-TiN-MnS.Prior austenite grain was significantly pinned.However,under the condition of increase in grain boundary area and lack of effective intragranular nucleation,grain boundary ferrite and lath packet structure tended to increase,which will not benefit remarkable improvement of HAZ toughness.With regard to the deficiency of MgO steel,further development was made based on the 3rd generation oxide metallurgy,and HAZ microstructure and properties were further improved.Fraction of inclusions containing Ti oxide was increased by adopting Ti-REM/Zr?Mg deoxidation process,and acicular ferrite transformation was further improved.V microalloying of MgO steel could promote the complex precipitation of MgO,TiN and V(C,N),which would increase the ferrite nucleation potency by coherent interface mechanism,such that the inclusions obtained both effects in pinning austenite grains and promoting intragranular transformation.Combining the two treatment measures,HAZ microstructure received the best refining effect,coarse ferrite side plates and allotriomorphs disappeared and the entire micro structure was fine and uniform instead.The impact toughness reached above 200J at-20? under 500kJ/cm heat input,which got further improved on the basis of normal MgO treated HTUFF steel(?100J at 0?).CGHAZ microstructure evolution behavior and inclusion-induced intragranular ferrite transformation mechanism were analyzed by performing austenite continuous cooling and isothermal transformation experiment.In the continuous cooling transformation of coarse-grained austenite of Ti-Zr deoxidized steel,grain boundary ferrite and acicular ferrite was formed at low cooling rates.At higher cooling rates,prior austenite was segmented by acicular ferrite resulting in great refinement of bainite or martensite packet size.With the decrease of isothermal temperature,intragranular idiomorphic ferrite,coarse acicular ferrite,finer acicular ferrite and intragranular bainite were obtained respectively.With the increase in chemical driving force,the size of effective inclusions became smaller,and several fine plates could form at one inclusion simultaneously.Transformation mechanism of acicular ferrite is similar to that of bainite,which exhibits incomplete reaction phenomenon.Mn depletion zone mechanism is predominant for inclusion-induced nucleation in the present Ti-Zr steel.Investigation of Mn-free and Ni-bearing steel also indicated the significance of Mn element in intragranular ferrite transformation.Influence of commonly used alloying elements on high heat input HAZ toughness was researched.Results showed low C and high Mn is in favor of toughness improvement,butthere exists a proper range of Ceq and C content.Grain boundary ferrite needs to be inhibited when C content is ultra low.Relatively high HAZ toughness could still be obtained with 0.01%Nb.However,excessive Nb will cause bainite formation and toughness deterioration.A percentage of 0.05%of V could increase matrix strength and keep good HAZ toughness,while a larger amount will impair toughness because of precipitation strengthening.Ni and Cu can improve combined properties of strength and toughness.Cr and Mo are detrimental to toughness when C content is relatively high.With optimized compositions,model high heat input welding steels of Q345,EH460 and X80 grade based on different oxide metallurgy processes were produced in laboratory scale.The steels fulfilled high heat input welding property of 100-800kJ/cm respectively.Combining experimental research results,industrial production technology of oxide metallurgical steel for high heat input welding was developed.Normal steelmaking process was improved and new oxide metallurgy technology based on supplying oxygen and deoxidization in RH was developed,which would further promote uniform distribution of fine oxide and the increase in property stability of steel product.Industrially trial-produced steel plates exhibited CGHAZ toughness of above 200J at-60? under 200kJ/cm EGW condition.In comparison with the best domestic product level of?170J at-20? under 180kJ/cm,low temperature HAZ toughness for high heat input of the new developed steel improved significantly.In order to realize the improving effect of oxide metallurgy on steel matrix microstructure and properties,a new process route of“Oxide Metallurgy + NG-TMCP" was proposed.Experimental research was carried out based on the process.It was indicated that inclusion could still promote acicular ferrite nucleation within deformed and recrystallized austenite grains.The increase in deformation temperature and cooling rate contributed to promoting acicular ferrite transformation.The experimental steel obtained fine-grained acicular ferrite type microstructure under NG-TMCP process of high temperature rolling and ultra-fast cooling,which resulted in notable increase in strength and toughness in comparison with conventional steel.The application of this process will have particular significance in improving as-rolled properties remarkably for steel products which are not proper for heavy reduction at low temperature such as heave plate,tube steel and shape steel. |
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