Development in oxide metallurgy for improving the weldability of high -strength low-alloy steel-Combined deoxidizers and microalloying elements

The mechanisms of oxide metallurgy include inducing the formation of intragranular acicular ferrite(IAF)using micron-sized inclusions and restricting the growth of prior austenite grains(PAGs)by nanosized particles during welding.The chaotically oriented IAF and refined PAGs inhibit crack initiation and propagation in the steel,resulting in high impact toughness.This work summarizes the com-bined effect of deoxidizers and alloying elements,with the aim to provide a new perspective for the research and practice related to im-proving the impact toughness of the heat affected zone(HAZ)during the high heat input welding.Ti complex deoxidation with other strong deoxidants,such as Mg,Ca,Zr,and rare earth metals(REMs),can improve the toughness of the heat-affected zone(HAZ)by re-fining PAGs or increasing IAF contents.However,it is difficult to identify the specific phase responsible for IAF nucleation because ef-fective inclusions formed by complex deoxidation are usually multiphase.Increasing alloying elements,such as C,Si,Al,Nb,or Cr,con-tents can impair HAZ toughness.A high C content typically increases the number of coarse carbides and decreases the potency of IAF formation.Si,Cr,or Al addition leads to the formation of undesirable microstructures.Nb reduces the high-temperature stability of the precipitates.Mo,V,and B can enhance HAZ toughness.Mo-containing precipitates present good thermal stability.VN or V(C,N)is ef-fective in promoting IAF nucleation due to its good coherent crystallographic relationship with ferrite.The formation of the B-depleted zone around the inclusion promotes IAF formation.The interactions between alloying elements are complex,and the effect of adding dif-ferent alloying elements remains to be evaluated.In the future,the interactions between various alloying elements and their effects on ox-ide metallurgy,as well as the calculation of the nucleation effects of effective inclusions using first principles calculations will become the focus of oxide metallurgy.

Influences of microstructure and texture on crack propagation path of X70 acicular ferrite pipeline steel

The aspects of two pipeline steels with different technologies were investigated by using transmission electron microscopy （TEM） and electron back-scattered diffraction （EBSD）. The microstructure presents a typical acicular ferrite characteristic with fine particles of martensite/austenite （M/A） constituent, which distributes in grains and at grain boundaries. The bulk textures of the pipeline steel plate are {112}〈110〉 and 〈111〉 fibers, respectively, and the {112}〈110〉 component is the favorable texture benefiting for drop weight tear test. Moreover, low angle boundaries and low coincidence site lattice boundaries are inactive and more resistant to fracture than high energy random boundaries.

Toughening mechanisms of a high-strength acicular ferrite steel heavy plate

An ultra-low carbon acicular ferrite steel heavy plate was obtained with an advanced thermo-mechanical control process-relaxed precipitation controlled transformation （TMCP-RPC） at Xiangtan Steel, Valin Group. The heavy plate has a tensile strength of approximately 600 MPa with a lower yield ratio. The impact toughness of the heavy plate achieves 280 J at ？40°C. The fine-grained mixed microstructures of the heavy plate mainly consist of acicular ferrite, granular bainite, and polygonal ferrite. The high strength and excellent toughness of the heavy plate are attributed to the formation of acicular ferrite microstructure. The prevention of blocks of martensite/retained austenite （M/A） and the higher cleanness are also responsible for the superior toughness.

Precipitation behaviors of X70 acicular ferrite pipeline steel

The morphology, structure, and chemical composition of precipitates in the final microstructure of Nb-V-Ti microalloyed X70 acicular ferrite pipeline steel were investigated using transmission electron microscopy （TEM） and energy dispersive X-ray spectroscopy （EDS）. Precipitates observed by TEM can be classified into two groups. The large precipitates are complex compounds that comprise square-shaped TiN precipitate as core with fine Nb-containing precipitate nucleated on pre-existing TiN precipitate as caps on one or more faces at high temperature. In contrast, the fine and spherical Nb carbides and/or carbonitrides precipitate heterogeneously on dislocations and sub-boundaries at low temperature. From the analysis in terms of thermodynamics, EDS and chemical cornposition of the steel, NbC precipitation is considered to be the predominant precipitation behavior in the tested steel under the processing conditions of this research.

Effect of inclusions on the formation of acicular ferrite in Ti-bearing non quenched-and-tempered steel

Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, Al2O3, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as ferrite nucleation nuclei. The acicular ferrite percentage changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an increase in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular ferrite.

Processing of Ultralow Carbon Pipeline Steels with Acicular Ferrite

Acicular ferrite microstructure was achieved for an ultralow carbon pipeline steel through the improved thermome chanical control process (TMCP), which was based on the transformation process of deformed austenite of steel. Compared with commercial pipeline steels, the experimental ultralow carbon pipeline steel possessed the satisfied strength and toughness behaviors under the current improved TMCP, although it contained only approximately 0.025% C, vvhich should mainly be attributed to the microstructural characteristics of acicular ferrite.

Effect of acicular ferrite on banded structures in low-carbon microalloyed steel

The effect of acicular ferrite （AF） on banded structures in low-carbon microalloyed steel with Mn segregation during both iso- thermal transformation and continuous cooling processes was studied by dilatometry and microscopic observation. With respect to the iso- thermal transformation process, the specimen isothermed at 550℃ consisted of AF in Mn-poor bands and martensite in Mn-rich bands, whereas the specimen isothermed at 450℃ exhibited two different morphologies of AF that appeared as bands. At a continuous cooling rate in the range of 4 to 50℃/s, a mixture of AF and martensite formed in both segregated bands, and the volume fraction of martensite in Mn-rich bands was always higher than that in Mn-poor bands. An increased cooling rate resulted in a decrease in the difference of martensite volume fraction between Mn-rich and Mn-poor bands and thereby leaded to less distinct microstrucmral banding. The results show that Mn segregation and cooling rate strongly affect the formation of AF-containing banded structures. The formation mechanism of microstructural banding was also discussed.

Mechanical properties and grain refinement by acicular ferrite in high strength high toughness weld metal

The volume fraction and morphology of acicular ferrite evolution in a high strength high toughness weld metal were studied and the mechanical properties of weld metal under heat input of 21 kJ/cm with and without fast cooling were tested. The results show the weld metal can obtain a large proportion of acicular ferrite during a wide range of cooling rate and the sizes of acicular ferrite in length and thickness decrease with cooling rate increasing. The weld metal exhibited high tensile strength （895 MPa and 870 MPa） and good low temperature toughness （average AKv-30℃ 104 J and 79. 2 J）. The higher tensile strength and better low temperature toughness of the weld metal under fast cooling are due to the more refined grain of acicular ferrite.

Kinetics of Acicular Ferrite Transformation in Weld Metal

Acicular Ferrite(AF) is beneficial in welds because it provides a tough and strong microstructure.Therefore,much works have been done in the study of the effect of alloying elements and welding conditions aimed at obtaining more AF in the weld metal.However,the knowledge about the nature of AF is limited due to the lack of kinetics researches.In this paper,the kinetic curves of AF transformation in an X65 weld metal were established by means of welding thermal simulation test.The results showed that AF transformation is a typical diffusion process of nucleation and growth with incubation period.AF transformation in weld metal obeys the relationship described by JMA Equation.The effect of welding heat input was also studied.

MICROSTRUCTURE AND INCLUSION CHARACTERIZATION IN THE SIMULATED COARSE-GRAIN HEAT AFFECTED ZONE WITH LARGE HEAT INPUT OF A Ti-Zr-MICROALLOYED HSLA STEEL

The microstructure and the characteristics of the inclusions embedded in ferrite matrix in simulated coarse-grain heat affected zone (CGHAZ) of a Ti-Zr-treated high strength low alloy (HSLA) steel have been investigated. The microstructure of the simulated CGHAZ dominantly consisted of intragranular acicular ferrite (IAF) combining with a small amount of polygonal ferrite (PF), widmanst tten ferrite (WF), bainite ferrite (BF), pearlite and martensite-austenite (M-A) islands. The PF, WF and BF were generally observed at the prior austenite grain boundaries and the interlocking acicular ferrite was usually found intragranularly. It was found that the inclusions were composed of Ti2O3, ZrO2, Al2O3 locating at the center of the particles and MnS lying on the surface layer of the inclusions. The intragranular complex inclusions promoted the acicular ferrite formation and the refinement of microstructure whilst those at prior austenite grain boundaries caused PF formation on the inclusions. The simulated CGHAZ consisting of such complicated microstructure exhibited desired mechanical properties.

EFFECT OF Zr ADDITION TO Ti-KILLED STEEL ON INCLUSION FORMATION AND MICROSTRUCTURAL EVOLUTION IN WELDING INDUCED COARSE-GRAINED HEAT AFFECTED ZONE

Effects of Zirconium on the chemical component and size distribution of Ti-bearing inclusions, favored the grain refinement of the welding reduced, coarse-grained heat affected zone （CGHAZ） with enhanced impact toughness in Ti-killed steels, which were examined based on experimental observations and thermodynamic calculations. It indicated that the chemical constituents of inclusions gradually varied from the TiO oxide to the Ti-O＋Zr-O compound oxide and a single phase of the ZrO2 oxide, as the Zr content increased from zero to 0.0100%. A trace of Zr （0.0030%-0.0080%, depending on the oxygen content in liquid steel） provided a large amount of nucleating core for Ti oxide because of the larger specific density of ZrO2 oxide, and produced a small size distribution of the inclusions favorable for acicular ferrite transformation with a high nucleation rate in the CGHAZ, and a high volume fraction of acicular ferrite was obtained in the CGHAZ, with enhanced impact toughness. Otherwise, a high content of Zr （-0.0100%） produced a single phase Zr02, which was impotent to nucleate acicular ferrite, and a microstructure composed of ferrite side plate and grain boundary ferrite developed in the CGHAZ. The experimental results were confirmed by thermodynamic calculations.

Grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels

The microstructural features and grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels were investigated using optical microscopy, scanning electron microscopy, and electron backscattering dif- fraction. The coarse-grained region of the heat-affected zone consists of predominantly bainite and a small proportion of acicular ferrite. Bainite packets are separated by high angle boundaries. Acicular ferrite laths or plates in the coarse-grained region of the heat-affected zone formed prior to bainite packets partition austenite grains into many smaller and separate areas, resulting in fine-grained mixed microstruc- tures. Electron backscattefing diffraction analysis indicates that the average crystallographic grain size of the coarse-grained region of the heat-affected zone reaches 6-9 μm, much smaller than that of anstanite grains.

Effects of welding wire composition and welding process on the weld metal toughness of submerged arc welded pipeline steel

The effects of alloying elements in welding wires and submerged arc welding process on the microstructures and low-temperature impact toughness of weld metals have been investigated. The results indicate that the optimal contents of alloying elements in welding wires can improve the low-temperature impact toughness of weld metals because the proeutectoid ferrite and bainite formations can be suppressed, and the fraction of acicular ferrite increases. However, the contents of alloying elements need to vary along with the welding heat input. With the increase in welding heat input, the contents of alloying elements in welding wires need to be increased accordingly. The microstructures mainly consisting of acicular ferrite can be obtained in weld metals after four-wire submerged arc welding using the wires with a low carbon content and appropriate contents of Mn, Mo, Ti-B, Cu, Ni, and RE, resulting in the high low-temperature impact toughness of weld metals.

Effect of deformation and cooling rate on the transformation behavior and microstructure of X70 steels

The effects of the deformation in the non-recrystallization region of austenite and the cooling rate on the transformation behavior and microstructure of low-carbon low-alloy steel for pipeline application were studied on the thermal-mechanical simulator Gleeble-1500. It was shown that an increase in deformation amount can greatly increase the nucleation site of ferrite when deformed in the non-recrystallization region of austenite, and an increase in nucleation ratio can greatly refine grains. When the cooling rate is accelerated, the driving force of nucleation is increased and the nucleation rate also improves. Ultra-refine grains can be obtained by controlled rolling. The high density of ferrite nucleus, which forms along the austenite grain boundary, twin interface, and deformation band are introduced in the matrix of austenite by the control of hot rolling, after which the microstructure can be refined. It was found that the acicular ferrite has a very fine sub-structure, high dislocation density, and a thin slab with ultra-fine grains. Small M/A islands and cementite are precipitated on the matrix of the slabs by the analysis technique of TEM and SEM.

Weldability and SAW welding wire of X80 pipeline steel

Weldability test was carried out on the newly developed fine grain, low sulphur, high strength and high toughness pipeline steel of X80 and its matching SAW wire. Test of maximum hardness in welding heat-affected zone and test of Y groove cracking show that X80 steel features low hardenability and good cracking resistance. The submerged arc welding joint made with the newly developed low carbon and multi-alloyed SAW wire of WGX2 exhibits a little higher strength than the base metal, qualified bending performance, under maximum limitation hardness and good impact toughness, which can completely meet the technical requirement of X80 steel. Despite somewhat coarsening the grain size in welding heat-affected zone is still much finer than that of traditional steels, and the microstructure in weld metal is almost full acicular ferrite. The results show that X80 steel and WGX2 wire are of great weldability.

Investigation of the impact toughness of self- shielded flux- cored wire girth welds for X80 pipelines

Self-shielded flux-cored wire is a convenient and efficient consumable for pipeline field girth welding because of its self-protection characteristic and high deposition rate, especially for remote construction sites in rugged terrain. From the perspective of pipeline safety, the impact toughness of the girth welds is an important factor in pipeline integrity ,which determines the crack arrest behavior in the girth welds. Therefore, improving the girth weld impact toughness is of primary importance in the field of pipeline girth welding. Three self-shielded flux-cored wires comprising different chemical composition systems have been applied to large diameter X80 UOE （U-ing-O- ing-Expanding） pipeline semi-automatic girth welding,and the impact toughness of the welds has been evaluated by girth weld chemical composition analysis and microstructural analysis using scanning electron microscopy （SEM） and energy dispersive spectrometry （EDS） to investigate pipeline girth weld impact toughness and find ways to improve it. This helps in determining the main factors that influence girth weld impact toughness. Pipeline girth weld impact toughness is mainly determined by the final microstructure produced in the solid-state phase transition. In the as-weld state,acicular ferrite （AF） and fine bainite （FB） are a benefit to the impact toughness. For multilayer semiautomatic self-shielded flux-cored wire welding, the normalizing and tempering function of the latter beads to the initial beads plays an important role in the transition of girth weld microstructure, which affects the impact toughness. The original AF and FB and the corresponding heat treatment microstructure of the fine and uniform block ferrite and pearlite result in very good impact toughness. The following two mechanisms are found to promote the production of AF and FB in the girth weld. First, elements promoting the broadening of the austenitic region, such as Ni, C, Cu, and Mn, induce low temperature phase transitions and restrain the opposing function of Al, which is a benefit to the production of AF and FB. Second, dispersed high-melting-point inclusions, especially Al2O3 ,induce the nucleated production of AF. The advantageous function of inclusions is determined by their shape, distribution, and dimension. Dispersed spherical inclusions of small dimension are a benefit to the production of AF, and result in good impact toughness.

STUDY OF ELECTROMAGNETIC STIRRING REFINING MICRO-STRUCTURES OF PIPE-LINE STEEL SAW DEPOSITS

The effects of electromagnetic stirring on the microstructures of pipe-linesteel SAW deposited metal were investigated. The results showed that electromagnetic stirringincreased the number density of inclusions with 0.2-0.6 mum in diameter and promoted the formationand refining of acicular ferrite within austenite grains. The low temperature toughness of depositedmetal was improved.

Effects of B and Ti on the Toughness of HSLA Steel Weld Metals

Effects of microalloying Ti and B on the microstructures and low temperature toughness of manual metal arc (MMA) deposits were investi- gated.Weld metals containing 200-300 ppm Ti and 29-60 ppm B deposited by manual coated elec- trodes provided an optimum low temperature toughness.The addition of B in weld metals low- ered the γ→α transformation temperature which promoted the acicular ferrite (AF) transformation. Solid solutioned B suppressed grain boundary ferrite as well as side plate ferrite formation and benefited the acicular ferrite formation.Titanium protected B from oxidizing as well as nitriding and formed Ti-Mn silicate inclusions.Ultra-high volt- age electron microscope analyses showed that TiO structure in the Ti-Mn silicate inclusions was the favorable nucleation site for acicular ferrite forma- tion.

Weldability of Copper-bearing Aging Steel

The weldability of copper-bearing aging steel is evaluated using calculated cracking susceptibility index Pcm,oblique Y-groove cracking test,heat-affected zone （HAZ） maximum hardness measurement,submerged arc welding （SAW） test and gas metal arc welding （GMAW） test.The results show that this copper-bearing aging steel has low hardenability and cold cracking susceptibility.SAW test of 40 mm thick plate with WS03 wire matched by CHF101 flux reveals that the welded joints obtain high strength and good impact toughness at low temperature.The HAZ has no hardening but there exists a slightly softening phenomenon.Thus,line energy should be limited or controlled strictly to avoid softening behavior in HAZ during SAW.GMAW tests of 12mm and 24mm thick plates using WER70NH wire show that the tensile strength of joints reaches 720MPa,higher than the stipulated strength requirement of base metal.The average impact energy at-40℃ in the welded joints is more than 140J exceeding minimum stipulated requirement by a wide margin.There are no hardening and softening behaviors in the heat-affected zones of GMAW.All weld metals exhibit acicular ferrite （AF） plus small amount of proeutectoid ferrite （PF） structure,of which the former can significantly improve impact toughness of weld metal.The predominant microstructure in coarse grain HAZ is bainite.

A CAD System for Tube Drawing Based on Finite Element Simulation of Drawing Process

Acicular Ferrite (AF) is beneficial in welds because it provides a tough and strong microstructure. Therefore, much works have been done in the study of the effect of alloying elements and welding conditions aimed at obtaining more AF in the weld metal. However, the knowledge about the nature of AF is limited due to the lack of kinetics researches. In this paper, the kinetic curves of AF transformation in an X65 weld metal were established by means of welding thermal simulation test. The results showed that AF transformation is a typical diffusion process of nucleation and growth with incubation period. AF transformation in weld metal obeys the relationship described by JMA Equation. The effect of welding heat input was also studied.