Influence of Ti on Weld Microstructure and Mechanical Properties in Large Heat Input Welding of High Strength Low Alloy Steels

The influence of Ti on weld microstructure and mechanical properties in large heat input welding of high strength low alloy steels is investigated.The results indicate that a moderate amount of Ti is still effective for grain refinement even under larger heat input and a large amount of acicular ferrite(AF)is formed in the weld metal when Ti content is within 0.028%-0.038%.With increasing Ti content,proeutectoid ferrite in the weld metal decreases,whereas bainite and M-A constituent increase.The type of inclusion in the welds varies from Mn-Si-Al-O to TiMn-Al-O and finally to Ti-Al-O as Ti content increases from 0up to 0.064%.As for adding 0.028%-0.038% Ti,high weld toughness could be attained since most inclusions less than 2μm which contain Ti2O3 provide the effective nuclei for acicular ferrite formation.However,the toughness of the weld metals severely reduces when Ti content is over the optimum range of 0.028%-0.038%.     

Effect of Aluminum and Titanium Treatment on Non-Metallic Inclusions and Microstructures of CGHAZ in HSLA Steel

The Effect of A1 and Ti treatment on non-metallic inclusions and microstructures of coarse-grain HAZ in HSLA stee1 was investigated in this paper based on experiments and thermodynamic calculations.The results showed that the inclusions in A1 treated steel were mainly aluminum oxides and titanium nitrides which could not promote the formation of acicular ferrite microstructures.Microstructure of coarse-grain HAZ in A1 treated steels consists of heavy grain boundary ferrite and ferrite side plate.The inclusions in Ti treated steel were A1,Ti,Mg,Ca composite oxides with size in the range of 0.5-3μm and titanium nitrides with size less than 0.3μm.Ti composite oxide could promote the formation of acicular ferrite and microstructures of coarse-grain HAZ in Ti treated steel consists of grain boundary ferrite,small amounts of ferrite side plate and large amounts of intragranular acicular ferrite.The size of grain boundaries ferrite was increased and the amount of ferrite side plate was decreased with the increase of soaking time at the peak temperature.The amount of grain boundary ferrite and the size of acicular ferrite were also increased with the increase of cooling rate during ferrite phase formation.     

Charpy V-notch properties and microstructures of narrow gap ferritic welds of a quenched and tempered steel plate

Multipass welds of quenched and tempered 50-mm-thick steel plate have been deposited by a single wire narrow gap process using both gas metal arc welding (GMAW) and submerged arc welding (SAW). Of the five welds, two reported much lower Charpy V-notch (CVN) values when tested at −20 °C. The CVN toughness did not correlate with either the welding process or whether the power source was pulsed or nonpulsed. The only difference in the ferritic microstructure between the two welds of low Charpy values and the three of high values was the percentage of acicular ferrite. There was no effect of the percentage of as-deposited reheated zones intersected by the Charpy notch or the microhardness of the intercellular-dendritic regions. In all welds, austenite was the microconstituent between the ferrite laths. The percentage of acicular ferrite correlated with the presence of MnO, TiO₂, γ Al₂O₃, or MnO. Al₂O₃ as the predominant crystalline compound in the oxide inclusions. In turn, the crystalline compound depended on the aluminum-to-titanium ratio in both the weld deposits and the oxide inclusions. In addition to the presence of less acicular ferrite, the two welds that showed lower Charpy values also reported more oxide inclusions greater than 1 μm in diameter. The combination of more oxide inclusions greater than 1 μm and less acicular ferrite is considered to be the explanation for the lower Charpy values.     

Development of oxide metallurgy technology at Baosteel

Oxide metallurgy technology has been developed at Baosteel for improving the heat affected zone toughness of steel plates for high heat input welding. After deoxidation with strong deoxidizers of Mg alloy,the complex inclusions containing parts of compounds of MgO,Ti2O3,MnO,Al2O3,MnS,CaS and TiN are formed.These kinds of inclusions are beneficial for promoting the formation of intragranular ferrite.After two-electrode vibratory electrogas arc welding with the V-type groove and a high heat input of 395 kJ/cm in a single pass,the former austenite grain is very fine in size,with an average grain size of only 85 μm.Excellent heat affected zone toughness is obtained for the developed steel plates with a thickness up to 68 mm.     

Effect of Magnesium on Evolution of Oxide and Sulphide in Liquid Iron at 1873K

To reveal the effects of magnesium on the evolution of oxide and sulphide inclusions in liquid iron, both thermodynamic calculations and deoxidization experiments were carried out. The samples extracted from the liquid iron were polished and analyzed by optical microscopy and scanning electron microscopy. The results showed that magne sium could modify oxide and sulphide inclusions simultaneously. Spherical MgO and irregular spinel inclusions were observed in the samples. The elongated MnS inclusions were replaced by small MgO · MgS or MgO · MgS · MnS complex inclusions, and the sulphides were distributed dispersively. The evolution mechanisms of inclusions were discussed comprehensively, and a proposed model for the formation of oxysulphide was set up.     

Nucleation Behavior Analysis of Intragranular Acicu-lar Ferrite in a Ti-killed C-Mn Steel

By using a Gleeble 3500Dthermo-mechanical simulator,the nucleation behavior of intragranular acicular ferrites(IAF)was studied in a Ti-killed C-Mn steel.During continuous cooling transformation,the allotriomorphic ferrite(AF)and ferrite side plate(FSP)microstructures grew more rapidly with the temperature decreasing from800to 650℃,and the IAF microstructure was dominant within austenite grain with further cooling to 600℃.The diffusion bonding experiment and the effect of C,Mn and Si concentrations on the Ae3temperature by thermodynamic calculation confirm that Ti2O3itself absorbs neighboring Mn atoms to form Mn-depleted zone(MDZ),which promotes the nucleation of IAF microstructure effectively.High temperature holding tests indicate that the nucleation potential of IAF microstructure was lowered in the Ti-killed C-Mn steel when it was treated at high temperature(1 250℃)for a longer time,which is attributed to the saturated absorption degree of Mn atoms by titanium oxide.     

Effect of Boron on Morphology of Inclusions in Tire Cord Steel

 Abstract： B2O3 was added to tire cord steel during refining in a carbon tube furnace. The influence of boron on the deformability of oxide inclusions was studied by metallographic and scanning electron microscope (SEM) observations. The melting points of boron-bearing compound oxide inclusions were calculated using Factsage software. The results showed that the main inclusion was a boron-bearing compound oxide and the deformation properties of the inclusions were clearly improved by adding B2O3. As the boron content was increased from 00046% to 0039%, the proportion of long strip type inclusions changed slightly and the number of inclusions decreased. The low-melting point areas of the MnO-SiO2-Al2O3 and CaO-SiO2-Al2O3 ternary system increased with the addition of B2O3. Moreover, the area increased with the increase of B2O3 content. Clear improvements in the deformation ability of the inclusions occurred when the B2O3 mass percent rose to 5% in the CaO-SiO2-Al2O3 inclusion system and to 10% in the MnO-SiO2-Al2O3 inclusion system.     

Weldability of 610MPa Grade High Strength Plate Steel 12MnNiVR for Oil Storage Tanks

The weldability of 12MnNiVR was examined in terms of the simulated HAZ continuous cooling transformation (SH-CCT) diagram,microstructure and mechanical properties of the simulated coarse grain heat-affected zone (CGHAZ).When t 8/5 is shorter,the microstructure mainly consists of lath bainite.When t 8/5 is 60 s,the microstructure becomes coarser bainite.Some acicular ferrite appears beside lath bainite when t 8/5 =100s.Finally,a microstructure composed of polygonal ferrite,acicular ferrite,and small amount pearlite is obtained with a small amount of bainite at t 8/5 >100s.With the increase of t 8/5,the hardness of CGHAZ decreases considerably.The minimum impact toughness of CGHAZ appears at t 8/5 =100s.The hardness and the toughness of CGHAZ remain above the specified values for steel 12MnNiVR.     

Metallurgical Interpretation on Grain Refinement and Synergistic Effect of Mn and Ti in Ti-microalloyed Strip Produced by TSCR

The grain refinement mechanism and synergistic effect of Mn and Ti involved in the Ti-microalloying technology of thin slab casting and direct rolling （TSCR） were elucidated. Because the inevitable precipitation of TiN in high Ti-containing liquid steel decreases the volume fraction of TiN precipitated from austenite and the rapid coarse- ning rate leads to a large size of TiN particles, a relatively weak inhibition effect on the recrystallized grain growth was obtained compared with that in the low Ti-containing steel. However, the ferrite grain size in high Ti-containing steel can be refined by the so-called non-recrystallization rolling. The complex addition of Mn and Ti can improve the strength and toughness of strip remarkably, and the mechanisms are that Mn decreases the transformation tempera- ture, refines the ferrite grains, and enhances the formation of bainite and TiC precipitation in ferrite.     

Microstructural Evolution and Mechanical Properties of Nb-Ti Microalloyed Pipeline Steel

 The correlation between microstructures and mechanical properties of a Nb-Ti microalloyed pipeline steel was investigated. The results revealed that with decreasing the finish rolling temperature and the cooling stop temperature, the matrix microstructure was changed from quasi-polygonal ferrite to acicular ferrite, as a result of improvement of both strength and low temperature toughness. By means of electron backscattered diffraction observation, an effective acicular ferrite packet contained several low angle boundaries or subboundaries plates which made important contributions to improvement of strength. It was found that many fine quasi-polygonal ferrite grains with high angle boundaries as the toughening structure were introduced into the acicular ferrite matrix to refine effective grain size and improve the toughness.     

Effect of Magnesium on Inclusion Formation in Ti-Killed Steels and Microstructurai Evolution in Welding Induced Coarse-Grained Heat Affected Zone

Effects of Mg on the chemical component and size distribution of Ti-bearing inclusions favored grain refinement of the welding induced coarse-grained heat affected zone (CGHAZ),with enhanced impact toughness in Ti-killed steels,which were examined based on experimental observations and thermodynamic calculations.The results indicated that the chemical constituents of the inclusions gradually varied from the Ti-O+Ti-Mg-O compound oxide to the Ti-Mg-O+ MgO compound oxide and the single-phase MgO,as the Mg content increased from 0.002 3% to 0.006%.A trace addition of Mg (approximately 0.002 %) led to the refinement of Ti-bearing inclusions by creating the Ti-Mg-O compound oxide and provided favorable size distribution of the inclusions 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 Mg (approximately 0.006%) produced a single-phase MgO,which was impotent to nucleate an acicular ferrite,and a microstructure comprised of a ferrite side plate and a grain boundary ferrite developed in the CGHAZ.The experimental results were confirmed by thermodynamic calculations.     

Effect of magnesium addition in low carbon steel part 2: toughness and microstructure of the simulated coarse-grained heat-affected zone

A critical investigation into the role of Mg on the toughness and microstructure of coarse grain heat-affected zone (CGHAZ) in low carbon steel has been investigated. In this research, the specimens (Mg-free and Mg-added) underwent weld thermal cycle with heat input of 54, 80, and 100?kJ?cm^?1 at 1350°C peak temperature using a thermal simulator. The typical inclusions characteristics were characterised by means of scanning electron microscopy and equilibrium calculations. The precipitates were characterised by transmission electron microscopy and energy-dispersive spectroscopy. It is revealed that the occurrence of Mg in steel mostly exists in the form of Mg-Al-O oxide inclusions, but a few in the form of solid solution state and (Nb,Ti)(C,N)+MgO precipitates when the concentration of Mg is 0.0026%. The improvement of CGHAZ toughness is obtained when the heat input is 80 and 100?kJ?cm^?1. The possible reasons about the effects of Mg on the toughness of CGHAZ, including Mg-Al-O inclusions, precipitates, and soluble Mg, are discussed in detail.     

Toughness improvement by Zr addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effect of Zr addition on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels subjected to 100?kJ?cm^-1 heat input was investigated. The second- phase particles were mainly Al–Ti complex oxides and (Ti,Nb)N precipitates in Zr-free steel, whereas lots of finer Zr–Al–Ti complex oxides and (Al,Ti,Nb)N precipitates were formed in Zr-bearing steel because of Zr addition. These finer oxides and precipitates effectively restricted the austenite grain growth by pinning effect during welding thermal cycle, and smaller and more uniform prior austenite grains were obtained in CGHAZ of Zr-bearing steel. Furthermore, more acicular ferrite grains nucleated on Zr–Al–Ti complex oxides, inducing formation of fine-grained microstructure in CGHAZ of Zr-bearing steel. The toughness improvement in CGHAZ of Zr-bearing steel with dimple fracture surface was attributed to the grain refinement by pinning effect and acicular ferrite formation.     

The significant impact of Ti content on microstructure–toughness relationship in the simulated coarse-grained heated-affected zone of high-strength low-alloy steels

This study aims to investigate the influence of Ti addition on microstructure and toughness in the simulated coarse-grained heated-affected zone (CGHAZ) of high-strength low-alloy steels. The steels with low and high Ti content respectively were subjected to 100?kJ/cm heat input welding thermal cycle. The results indicated that the second-phase particles were mainly oxide covered with MnS and fine (Ti,Nb)N precipitate in low-Ti steel, which were modified to the oxide surrounded by TiN and coarse (Ti,Nb)N precipitate in high-Ti steel. Compared with low-Ti steels, the coarser precipitates induced larger austenite grain in CGHAZ of high-Ti steel. Moreover, the wrapping of TiN decreases the ability of inclusion to promote the nucleation of acicular ferrite, resulting in lower fraction of acicular ferrite in CGHAZ of high-Ti steel. Content of martensite-austenite constituent increased in CGHAZ of high-Ti steel. They were all responsible for the degeneration in toughness in CGHAZ of high-Ti steel.     

Addition of Dispersoid Titanium Oxide Inclusions in Steel and Their Influence on Grain Refinement

In this article, the addition of dispersoid titanium oxide inclusions into liquid steel, the effect of additions on the inclusions found in the steel and on grain refinement, and acicular ferrite formation were studied. Different TiO₂-containing materials and addition procedures into liquid steel were tested in experimental heats to obtain inclusions that promote grain refinement and acicular ferrite formation in C-Mn-Cr steel. Different additives with metallic Ti and TiO₂ were added into the steel melt just before casting or into the mold during casting to create Ti-containing inclusions. The aluminum content in steel was lowered by an addition of iron oxide. The samples taken from steel melts and ingots were studied with a scanning electron microscope to find inclusions and to analyze them. Thermodynamic calculations showed that the Al content should be low (<50 ppm) to obtain Ti oxide dominating inclusions, whereas Al₂O₃ were formed at higher Al contents. When TiO₂ was added late before casting, the oxide inclusions were Ti oxides and were mixed with Ti, Al, and Mn oxides. Small inclusions around 1 μm were detected in the samples with TiO _x or TiN as the main component. It could be concluded that the additions resulted in a clearly higher number and in a smaller size of TiO _x inclusions than just by adding metallic Ti. Selected samples were brought for subsequent hot rolling and heat-treatment experiments to find out the grain-refining effect and the eventual formation of acicular ferrite. Grain refinement was observed clearly, but the presence of acicular ferrite could not be confirmed definitely.     

Effect of inclusions on toughness and microstructures in simulated coarse-grain heat-affected zones of Al/Ti deoxidised and Nb/V microalloyed steels

To explore the effect of inclusions in the coarse-grain heat-affected zone (CGHAZ), four groups were designed using the ‘Al/Ti deoxidised?+?Nb/V microalloyed’ technical route. The toughness of the CGHAZ of Al–Ti–V–N steel was greatly promoted by the presence of polygonal ferrite (PF) after high heat input rather than the traditionally used acicular ferrite (AF). Furthermore, micron-sized inclusions in AF or PF were analysed, and the results indicated that the TiN and V (CN) particles attached on the Ti/Al oxide inclusions induced the nucleation of AF and PF. That can be concluded that the nucleation mechanism of PF is similar to that of AF. In addition, smaller micron- and nanoscale-size inclusions observed by transmission electron microscopy could not induce the nucleation of AF and PF because of the low heterogeneous nucleation energy. Nevertheless, these inclusions can have a drag effect on grain boundary migration.     

锆对含钛F40级船板钢粗晶热影响区低温韧性的影响

在Gleeble3500热模拟试验机上,针对添加质量分数0.01%的锆与未添加锆的两种成分的F40级船板钢,分别进行了不同相变冷却时间T_8/5下的焊接热模拟试验.结果表明,微量锆的加入,使含钛F40钢耐大热输入焊接性能大幅提高,T_8/5提高至100s时,-60℃冲击功可达238J.利用热力学计算并结合扫描电镜观察含锆的F40钢板粗晶热影响区组织发现,随着T_8/5的增大,伴随着原有含锆复合夹杂物尺寸形态的变化,有利于诱导针状铁素体形核的尺寸为1~3μm的锆-钛复合夹杂物数量呈非单调减少趋势,导致粗晶热影响区冲击韧性随T_8/5增大呈现一定的规律性.      

钛处理对EH36船板钢中夹杂物及组织特征影响

 为了研究结晶器喂钛线对EH36船板钢中夹杂物的影响,采用无水有机溶液电解分离提取钢中夹杂物,结合扫描电镜和能谱仪分析其三维形貌,尺寸和成分。试验结果表明,在结晶器喂钛线后,钢中硅铝酸钙夹杂物+外包裹MnS转变为硅铝酸钙钛+MnS夹杂物,三维表面从光滑转变为粗糙多孔的形貌。在焊接热模拟后的试样中,组织形貌从未加钛试样中的晶界铁素体和侧板条铁素体转变为钛处理试样中的针状铁素体,且夹杂物周围铁素体从块状转变为针状,韧性提高了70 J。通过热力学理论计算,分析船板钢中含钛氧化物夹杂物形成条件。计算结果表明,钛、铝与氧反应生成氧化物的过程存在竞争关系,当钢中钛质量分数为0.02%时,钢液中应严格控制铝质量分数不高于0.003 5%,才能保证钢液中大量生成含钛氧化物粒子。     

低合金钢焊接粗晶区连续冷却铁素体相变规律

 利用焊接粗晶区连续冷却淬火方法,对比分析了钛处理钢和普通C Mn钢焊接粗晶区连续冷却不同阶段的相变组织,研究了铁素体相变规律。结果表明,C Mn钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体;钛处理钢焊接粗晶区主要为晶界铁素体+魏氏组织铁素体+晶内铁素体组织。在钛处理钢中,晶界铁素体、魏氏组织铁素体和晶内铁素体的相变开始温度相同,但各自长大的动力学条件不同。当晶内铁素体和魏氏组织铁素体竞争发生相变时,晶内铁素体在晶内弥散分布氧化物夹杂上的非均质形核抑制了魏氏组织铁素体向晶内的长大。