Papers by Keyword: Acicular Ferrite

Abstract: Steel with low yield ratio (YR0.75) and high heat input welding was made by combining Zr, Ti microalloying. Its microstructure investigated by optical and electron microscopes (SEM, TEM), consists of ferrite with the fine pearlite which not only ensures the high strength and toughness but also decreases the yield ratio. The steel was simulated in 100 kJ/cm heat input welding. The microstructures for heat affected zone (HAZ) of weldments mainly consisted of acicular ferrites which nucleate on fine Zr and Ti contained inclusions. The acicular ferrites decrease detrimental influence of grain coursing in the HAZ and make the steel good toughness in low temperature.

Abstract: Acicular ferrite (AF) can significantly improve the performance of steels and can be promoted through special inclusions. The present experimental steel was a low carbon Ti-Zr deoxidized and microalloyed steel. The welding heat affected zone (HAZ) simulation indicated that under high heat input (100 kJ/cm) and peak temperature (1400°C) conditions, the HAZ exhibited high impact toughness (150 J, -20°C) and desired microstructure. Isothermal heat treatment data suggested that grain boundary ferrite formed at ~600°C and AF formed at ~500°C. Intragranular plates sheaves formed at ~450°C. MnS coherent precipitation on ZrO₂TiO_x inclusions promoted AF nucleation.

Abstract: The rapid development of the large steel structure brings a great opportunity for steelmaking industry. The steel plates not only have the strength and toughness but also can withstand the high energy input welding. Using the calcium oxide of high melting-point and high stability to pin the grain boundaries is an effective method to improve the welding performance of the structure steel. This experiment takes the HSLA steel as the research object, the second phase particles which would not be dissolved or aggregated at high temperature will be expected by means of adding calcium into the steel in the form of Ca-Si alloy. The effect of calcium addition on the cast microstructure of HSLA steel was analysed. The results show that the cast microstructure is mainly consist of lamellar and acicular ferrite, a small amount of pearlite and bainite. Compared with the original steel, there are acicular ferrites presenting in the experimental steel after adding 5 wt% Ca, which are the microstructure that we hope to get. The acicular ferrite will have a positive impact on the mechanical properties of the subsequent rolled steel.

Abstract: A new technology to obtain a fine-structured and high-toughness HAZ of HSLA steel for high heat input welding is developed using metallurgical thermodynamics, physical chemistry of metallurgy and material processing methods synthetically in this study. A kind of HSLA steel is designed in this experiment. The thermal stability second phase particles which would not be dissolved or aggregated at high temperature will be expected by means of adding magnesium into the steel in the form of Mg-Zr alloy. The effect of magnesium addition on the cast microstructure of HSLA steel was analysed. The results show that The cast microstructure is mainly consist of lamellar and acicular ferrite, a small amount of pearlite and bainite. Compared with the original steel, there are acicular ferrites presenting in the experimental steel after adding 3 wt% Mg and 5 wt% Mg, which are the microstructure that we hope to get. The acicular ferrite will have a positive impact on the mechanical properties of the subsequent rolled steel.

Abstract: This experiment takes the X80 pipeline steel as the research object, the thermal stability second phase particles which would not be dissolved or aggregated at high temperature will be expected by means of adding nanocalcium oxide into the steel with the method of carrier dispersion addition. The effect of nanocalcium oxide addition on the cast microstructure of X80 pipeline steel was analysed. The results show that the cast microstructure is consist of the ferrite and a small amount bainite. And the bainite is distributed at the boundary of the ferrite grains. When adding 0.02 wt% nanocalcium oxides, there is more bainite in the microstructure by contrast. The number of the acicular ferrite significantly increases in the cast microstructure, and the grains become more fine and uniform. Simultaneously, the granular bainite is uniformly distributed at the grain boundary and edge of the ferrite.

Abstract: Microstructure of high strength pipeline steels containing different Nb contents was investigated using optical microscopy and transmission electron microscopy. Second phase particles were analyzed by extraction replica method. The results show that microstructure of the low Nb steel comprises granule bainite and the low bainite with a few of martensite-austenite (M-A) constituents. However, microstructure is consisted of acicular ferrite with M-A constituents in high Nb content steel. Moreover, M-A constituent is consisted of twinning martensite, lath martensite and retained austenite. In the low Nb steel precipitates are a large of square TiN particles. Second particles of high Nb content are mainly the large size duplex type (Nb,Ti)C and small NbC precipitates.

Abstract: The Ti addition effect on the characteristics of weld metal, such as impact energy, microstructure and nonmetallic inclusions, was investigated to develop a suitable gas metal arc welding wire for the high strength of TMCP (Thermo Mechanical Control Process)-600 steel. The fraction of acicular ferrite which was known to be a favorable weld metal microstructure for toughness was increased with Ti content from 0.002% to 0.025%, The impact energy of weld metal was increased whereas the ductile to brittle transition temperature was decreased with increasing Ti content. The size of nonmetallic inclusion was decreased while the density of inclusions was decreased with increasing Ti content. It was found that Ti content on the weld metal toughness had a plus effect by increasing the fraction of acicular ferrite in the weld metal microstructure.

Abstract: The continuous cooling transformation behaviors were researched on X70 pipeline steel through two pass deformation and non-deformed austenite using Gleeble-3500 thermal mechanical simulator, and static continuous cooling transformation curve and dynamic continuous cooling transformation curve were measured through thermal dilation method and metallographic method. The influence of cooling rate and deformation parameters on microstructure was analyzed. The results show that the hot deformation accelerates the acicular ferrite and polygonal ferrite phase transformation, increases the starting transformation temperature and the finishing transformation temperature significantly, and shifts the CCT curve moving upward to the left side corner. Acicular ferrite is obtained in practice using accelerated cooling rate after deformation Acicular ferrite can be obtained in wider range of cooling rates, and microstructure and island structure is finer through hot deformation.

Abstract: A new hot-rolled ship plate with high strength and high toughness is successfully developed through chemical composition design and TMCP process. Experimental methods, such as OM, TEM and X-EDS, were used to study the microstructure and precipitates of steel. The primary microstructural constituent is acicular ferrite, quasi-polygonal ferrite with second constituents along grain boundaries. Lath width of acicular ferrite is about 1μm. Cubic particles about several hundreds nanometers and nanometer particles exist in experimental steel. It can be concluded that acicular ferrite is the main reason for high strength and super toughness. precipitation hardening due to dispersed precipitations of carbonitrides can not be overlooked.

Abstract: The application of high strength low alloy (HSLA) steels has been limited by unavailability of suitable joining and filler metals in submerged arc welding (SAW) processes. The present work aims at the design and development of flux for Submerged Arc Welding of HSLA steel. In the work L8 array of Taguchi Design is used to formulate eight types of fluxes to vary basicity index (BI) from 1.26 to 2.81 and to study the effect of flux constituents and basicity index on tensile strength, microhardness and microstructure of the weld metal. Empirical models for ultimate tensile strength and microhardness at the centre of weld versus flux constituents and basicity index have been developed. From the experiments it is found that ultimate tensile strength increase with increase of basicity index with minimum at 1.26 increases upto 2.33 and then further decreases whereas opposite in case of microhardness which is highest at 1.26 and minimum at 1.9. Increase of CaO in the flux increases ultimate tensile strength but microhardness remains unaffected whereas increase of SiO₂ decreases ultimate tensile strength but microhardness remains constant. Microhardness decreases critically with increase of CaF₂.