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The distribution of microstructure and mechanical properties in welding heat affected zone (HAZ) of a high-Nb high strength pipeline steel was studied by simulating two type welding heat inputs on Gleeble-3500 thermal simulator. The results show that the micro-hardness of HAZ is higher then base metal without obvious softening. However, the toughness for intercritical heat affected zone and coarse grain zone of heat affected zone deteriorates. M/A islands with large size distributed in chain structure along prior austenite grain boundary cause the decrease of toughness in intercritical heat affected zone. Coarsening and mixed crystal of prior austenite grain, coarse bainite lath cluster and M/A island with large size distributed at boundary of bainite lath cluster result in deterioration of toughness in coarse grain. High alloy content and carbon equivalent are main reason to result in the decrease of the toughness.     

Effect of Nb on Austenite Formation and Decomposition in an X80 Linepipe Steel

Advanced high strength steels for pipeline applications,e.g.X80 grades,have complex microstructures and are frequently microalloyed with Nb.In the hot rolled product it is sought to have Nb precipitated as Nb(CN).However,when processing these steels Nb may be in solution and critically affects the microstructure evolution,e.g.austenite decomposition on the run-out table of a hot mill.Further,microstructure changes in the heat affected zone (HAZ) during girth welding of these linepipe steels may occur with Nb precipitated or in solution.In the HAZ,depending on welding procedures,the material undergoes a number of austenite formation and decomposition cycles and the amount of Nb in solution varies along these stages.In selected positions of the HAZ,thermal cycles peak at the intercritical region and the partial formation of austenite and subsequent decomposition constitutes additional complexity.Developing reliable process models for run-out table cooling and the HAZ hinges on an accurate tracking of microstructure evolution,which is strongly influenced by the amount of Nb in solution.The present study provides more insight into the effect of Nb on austenite formation and decomposition.Firstly,a novel experimental methodology is presented to measure quantitatively the effect of Nb on transformation temperatures pertinent to austenite decomposition,notably ferrite.A model for ferrite formation that accounts for solute drag of Nb is proposed to describe the experimental observations.Secondly,an experimental study will be presented to quantify the effect of Nb in and out of solution on austenite formation in the intercritical region.It is found that the morphology of intercritical austenite,as well as the kinetics of its formation is strongly affected by the starting microstructure and the state of Nb.     

700 MPa微合金高强钢焊接软化机理及解决方案

700MPa级Ti Nb成分体系控轧控冷高强钢以其生产成本低、高强韧性以及优良的可焊性,近年来在专用车轻量化领域得到广泛应用。本文采用80％Ar+20％CO2(体积分数)混合气体保护焊,对高Ti、Nb元素析出强化高强钢进行了焊接强度试验研究。结果表明,随着焊接热输入增大,焊接接头强度有降低趋势,焊接热影响区较母材硬度降低,存在软化行为,其软化机理表现在细晶强化、变形强化和析出强化效果的丧失。通过母材的B微合金化、控制焊接热输入等措施可有效缓解软化倾向,可为此种高强钢进一步推广应用提供技术参考。     

Effect of Dissolution and Precipitation of Nb on Phase Transformation,Microstructure, and Microhardness of Two High-Nb Pipeline Steels

Niobium (Nb) as an important microalloying element is widely applied in high strength pipeline steels. In this work, the continuous cooling transformation diagrams of two high-Nb steels with and without hot deformation were studied using a Gleeble 3500 thermal simulator. The amounts of dissolved Nb, undissolved Nb, and precipitated Nb were determined by inductively coupled plasma-atomic emission spectrometry. Results show that the increasing of Nb content in the high-Nb steels can restrain the prior austenite grain growth, dynamic, and/or static recrystallization; moreover, it can suppress polygonal ferrite transformation and promote acicular ferrite and bainite transformation, refining the microstructure and increasing the microhardness as a consequence. Nevertheless, the amplified Nb content in steels escalates trends of strain-induced Nb(C,N) precipitation. The increase in the amount of Nb(C,N) precipitates promote the polygonal ferrite and acicular ferrite transformation, while also decrease the microhardness. The results from this work show that the higher Nb content of 0.13% in the tested steel is unnecessary.     

In-situ Corrosion Characterization of API X80 Steel and Its Corresponding HAZ Microstructures in an Acidic Environment

During heat treatment processing,microstructures of heat affected zone(HAZ)were formed in X80 pipeline steel.After observation by optical microscopy,scanning electron microscopy and transmission electron microscopy,microstructure of the as-received X80 steel was confirmed to be acicular ferrite,while the microstructures of quenched,normalized and annealed X80 steels were lath bainite,granular ferrite and quasi-polygonal ferrite,respectively.After immersion in the simulated acidic soil solution for 48 h,corrosion rates of these four steels were determined by mass loss measurements and corrosion products were examined by Raman spectroscopy and X-ray photoelectron spectroscopy.Scanning vibrating electrode technique was used to characterize the micro-galvanic corrosion behaviors of the synthetic bimetallic electrodes which were formed by coupling each of the simulated HAZ microstructures with the as-received steel in direct physical and electric contact.It is demonstrated that the as-received steel acts as cathode in the as-received/quenched and as-received/normalized couples,while the annealed steel acts as cathode when coupling with the as-received steel.The distinction of current density between the galvanic couples reduces with prolonging the immersion time.     

超低碳贝氏体钢及其焊接特性

超低碳贝氏体（ULCB）钢采用极低的碳含量，充分利用Mn,Mo,Nb,Ni,Ti,B等元素的合金化作用，通过ULCB组织最高的强韧性及优良的低温韧性。由于ULCB钢碳含量极低，焊接性优良，焊接热影响区（HAZ）韧性明显,裂纹敏感性显著降低。ULCB钢焊接时，焊缝金属是焊接接头的薄弱环节，研制开发超低碳贝氏体焊接材料是实现ULCB钢焊接的关键环节。     

Instabilities in stabilized austenitic stainless steels

The effect of aging on the precipitation of grain boundary phases in three austenitic stainless steels (AISI 347, 347AP, and an experimental steel stabilized with hafnium) was investigated. Aging was performed both on bulk steels as well as on samples which were subjected to a thermal treatment to simulate the coarse grain region of the heat affected zone (HAZ) during welding. Aging of the bulk steels at 866 K for 8000 hours resulted in the precipitation of Cr₂₃C₆ carbides, σ, and Fe₂Nb phases; the propensity for precipitation was least for the hafnium-stabilized steel. Weld simulation of the HAZ resulted in dissolution of the phases present in the as-received 347 and 347AP steels, leading to grain coarsening. Subsequent aging caused extensive grain boundary Cr₂₃C₆ carbides and inhomogeneous matrix precipitation. In addition, steel 347AP formed a precipitate free zone (PFZ) along the grain boundaries. The steel containing hafnium showed the best microstructural stability to aging and welding. Formerly with Exxon Research and Engineering Company.     

The Effect of Nb on Mechanical Properties of Quenching and Partitioning Steels

In order to study the effect of alloy elements on mechanical properties of quenching and partitioning steels,the Q and P heat treatments on different chemical composition steels were carried on in lab.The tensile test results indicated the strength of Nb+Ti-bearing steel was not increasing as expected,but lower than that of the Nb+Ti-free steel,and the elongation was raised to 26% from 9%.The Nb+Ti-bearing steel microstructures after tensile test were detected by TEM and found a certain amount of twins in the deformed microstructure while the deformed microstructure mainly was lath martensite in Nb+Ti-free steel,which means the addition of Nb and Ti elements could cause the twinning induced plasticity by inhibiting the phase transformation from austenite to martensite.Based on above analysis,adding trace Nb element could greatly increase the stacking fault energy of the retained austenite,which is beneficial to the formation of twins,and the formation of twins would lower the strength slightly and raise the elongation drastically.     

低合金钢焊接热影响区的静载断裂特性

 低应力破坏是焊接结构常见的断裂形式之一,为确定其影响因素,采用插销法焊接试验和扫描电镜断口观察,研究了低合金钢焊接热影响区的静载断裂特性。试验结果表明,为防止焊接热影响区产生低应力断裂,采用预热措施是十分有效的;预热80℃以上时,断裂强度与母材强度相接近。断口观察表明,不预热时绝大部分为结晶状断口,微观上呈现氢致沿晶断裂和氢致准解理断裂;预热时绝大部分是纤维状断口,微观上呈韧窝状。     

氧化物冶金工艺对EH36钢HAZ组织性能的影响

设计了Ti- Ca和Ti- Mg两种氧化物冶金脱氧工艺的EH36实验钢来考察粗晶热影响区的组织性能和冲击韧性。结果表明,两种处理工艺的实验钢热模拟后的焊接热影响区内都有大量细小的晶内针状铁素体产生;与Ti- Ca脱氧工艺相比,采用Ti- Mg脱氧工艺的实验钢,焊接热影响区中针状组织更加明显,夹杂物的类型也更加复杂,同时Ti- Mg复合脱氧工艺在焊接热循环中能够更好地钉扎奥氏体晶界。-40℃的冲击数据表明,Ti- Mg脱氧工艺处理后的实验钢HAZ冲击性能优于Ti- Ca处理工艺。     

Modeling of γ→α Transformation Temperature Ar3 for High-Nb Pipeline Steel During Cooling Process

 According to the research on the deformation resistance and the ferrite transformation behavior of X80 pipeline steel by using Gleeble-3500 thermal simulator, a mathematical model of the α-phase start transformation temperature for high-Nb pipeline steel was established, based on the transformation kinetics and thermodynamics. The influence of deformation and cooling rate as well as Nb content on the α-phase starting temperature was thoroughly investigated. The results given by the model were in good agreement with the experimental results, which showed that the model could predict the α-phase starting temperature for high-Nb pipeline steel during cooling process.     

Softening Kinetics in the Subcritical Heat-Affected Zone of Dual-Phase Steel Welds

Welds in dual-phase (DP) steels exhibit heat-affected zone (HAZ) softening in which the tempered or subcritical HAZ exhibits a lower hardness vs that of the parent material. The rate of this softening reaction with respect to welding heat input was determined for four DP steels by making several bead-on-plate laser welds using a variety of heat inputs and measuring the resulting minimum hardness. The reduction in hardness was then fit to the Avrami equation, enabling a comparison of the relative heat needed to soften each steel. It was found that the heat input required for HAZ softening decreased as the C content of the martensite within the DP structure increased. However, the presence of carbide forming alloying elements such as Cr and Mo was able to increase resistance to softening.     

Effects of Niobium on the Mechanical Properties and Corrosion Behavior of Simulated Weld HAZ of HSLA Steel

Simulated microstructures of the TZ, ICHAZ, FGHAZ, and CGHAZ of weld joints made from two kinds of HSLA steels with 0 or 0.079 wt pct Nb were prepared by means of heat treatment. Optical microscopy and transmission electron microscopy were used to observe microstructures and the distribution of nanosized precipitates in the simulated weld heat-affected zone (HAZ). Mechanical properties of the simulated HAZ were measured by tensile tests, and the corrosion behavior in simulated seawater was studied using electrochemical and immersion tests. It was shown that the ICHAZ and CGHAZ possess the worst overall mechanical properties in both kinds of HSLA steels, and the corrosion resistance in the descending order was the BM, TZ, FGHAZ, ICHAZ, and CGHAZ. Contrasting Nb-bearing and Nb-free steel demonstrated that the strength and corrosion resistance of the simulated HAZ were enhanced by Nb microalloying, which resulted in precipitation, homogeneous microstructures, and other relative sequences. Moreover, the surface of the Nb-bearing steel formed compact corrosion product films with higher resistance to ion migration; thus, the initiation and propagation of pitting holes were effectively inhibited.     

Microstructual Evolution of a Nb‐Microalloyed Advanced High Strength Steel Treated by Quenching‐Partitioning‐Tempering Process

The recently developed “quenching and partitioning” heat treatment and “quenching‐partitioning‐tempering” heat treatment are novel processing technologies, which are designed for achieving advanced high strength steels (AHSS) with combination of high strength and adequate ductility. Containing adequate amount of austenite phase is an important characteristic of the above steel, and the partitioning treatment is a key step in Q&P or Q‐P‐T process during which the austenite phase is enriched with carbon and achieves thermal stability. However, the microstructural evolution of the steel during the partitioning process is rather complicated. In present study, evolution of complex microstructure in a low carbon steel containing Nb during the Q‐P‐T process has been studied in detail. The microstructural evolution of the steel was investigated in terms of X‐ray diffraction, scanning electron microscope and transmission electron microscope. The experimental results show that the Nb‐microalloyed steel demonstrates a complex multiphase microstructure which consists of lath martensite with high dislocation density, retained austenite, alloy carbide, transition carbide, and a few twin martensite after the Q‐P‐T process. The experimental results can be helpful for the design of Q‐P‐T heat treatment and for the control of mechanical properties of Q‐P‐T steel.     

大线能量焊接用船体结构钢的研究进展

介绍了大线能量焊接在船舶建造中的应用,对比了船体结构钢与其它钢种对大线能量焊接适应性的不同要求。针对大线能量焊接热影响区韧性下降问题,提出了目前提高热影响区韧性的主要措施。指出降低碳当量、细化热影响区奥氏体晶粒尺寸、利用有益氧化物诱导晶内铁素体析出是提高船体结构钢大线能量焊接适应性的有效途径。介绍了鞍钢在热影响区组织调控技术和氧化物诱导机理研究等方面取得的成果。     

Weldability and toughness assessment of Ti-microalloyed offshore steel

The present study has been carried out to investigate the coarse-grained heat-affected zone (CGHAZ) microstructure and crack tip opening displacement (CTOD) toughness of grade StE 355 Ti-microalloyed offshore steels. Three parent plates (40-mm thick) were studied, two of which had Ti microalloying with either Nb + V or Nb also present. As a third steel, conventional StE 355 steel without Ti addition was welded for comparison purposes. Multipass tandem submerged arc weld (SAW) and manual metal arc weld (SMAW) welds were produced. Different heat-affected zone (HAZ) microstructures were simulated to ascertain the detrimental effect of welding on toughness. All HAZ microstructures were examined using optical and electron microscopy. It can be concluded that Ti addition with appropriate steel processing, which disperses fine TiN precipitates uniformly, with a fine balance of other microalloying elements and with a Ti/N weight ratio of about 2.2, is beneficial for HAZ properties of StE 355 grade steel.     

Influence of post-weld heat treatment on the microstructure and hardness of laser weld seams on hot-rolled TRIP 800 steel

The transformation induced plasticity (TRIP) steels effect occurs because of the martensitic transformation of retained austenite during plastic deformation,and it provides the steel with excellent strength and ductility.While welding remains a vital part of auto body manufacturing,the weldability of TRIP steels is problematic,and this prevents its adoption for many applications in the automotive industry.This present work studies the effects of welding and post-weld heat treatment on the microstructure of TRIP steels.It is found that the microstructures of the fusion zone and the heat affected zone (HAZ) are changed after high-temperature heat treatment.Hardness tests revealed that fusion zone hardness decreased with increasing of temperatures in the post-weld heat treatment on the laser weld seam.The rolling performance of the welding seam and the seam of post-weld heat treatment were also studied.     

钒对高强钢轨钢可焊性的影响

通过焊接热模拟试验,研究了不同钒含量对高强钢轨钢可焊性的影响。结果表明,中０．０８％左右的钒对钢轨钢焊接热影响区的组织、性能没有不利的影响。     

Effect of hot deformation and Nb precipitation on continuous cooling transformation of a high-Nb steel

In this work, the effects of hot deformation on continuous cooling transformation of a high-Nb steel were investigated on a Gleeble 3500 thermal simulator. The amounts of dissolved Nb were determined by inductively coupled plasma-atomic emission spectrometry. Furthermore, the effects of hot deformation and Nb precipitation on phase transformation were discussed. Results showed that high-Nb steel is suitable for acicular ferrite pipeline steels because the acicular ferrite microstructure can be obtained in a wide cooling rate range. Hot deformation strongly accelerates the polygonal ferrite transformation and increases the critical cooling rate to obtain a full acicular ferrite microstructure. Moreover, hot deformation markedly refines the final microstructure and improves the mechanical properties of acicular ferrite obtained at a high cooling rate. However, hot deformation can also promote Nb precipitation during holding and even cooling at low rates after hot deformation. Nb precipitation dramatically promotes the polygonal ferrite, weakens the effect of Nb in solution on phase transformation and strengthening, and decreases the microhardness.     

厚钢板焊接热影响区失效机理分析及改善方法研究

分析了厚钢板大线能量焊接后热影响区(HAZ)的失效机理,介绍了利用微细夹杂物改善HAZ性能的研究情况.粗晶热影响区脆化是由于晶粒粗大及不良组织而引起,粗大的奥氏体晶粒是焊接热影响区韧性恶化的主要原因.抑制焊接HAZ晶粒长大是改善厚钢板可焊性的关键因素.用真空感应炉分别冶炼了不同成分的钢,研究Mg对低碳钢HAZ性能的影响.结果表明含Mg钢HAZ的低温韧性较比不含Mg钢有较大幅度提高.通过激光高温显微镜原位观察发现,含Mg钢在1400℃保温300 s后奥氏体晶粒仍然保持着细小的结构,这主要归功于Mg添加后生成的细小粒子所产生的钉扎作用,该发现为改善厚板HAZ韧性提供了一种方法.