Thermodynamic calculations and experiments on inclusions to be nucleation sites for intragranular ferrite in Si-Mn-Ti deoxidized steel

Microstructures and inclusions in the Si-Mn-Ti deoxidized steels after cooling in the furnace were investigated. The composition and morphology of the inclusions were analyzed using a field emission scanning electron microscope （FE-SEM） with energy dispersive X-ray spectrometry （EDS）. The kind and composition of the inclusions calculated from the thermodynamic database were in good agreement with the experimental results. There were two main kinds of inclusions formed in the Si-Mn-Ti deoxidized steels. One kind of inclusion was the manganese titanium oxide （Mn-Ti oxide）. Another kind of inclusion was the MnS inclusion with segregation points containing Ti and N. According to the thermodynamic calculation, those segregation points were TiN precipitates. The formation of intragranular ferrite （IGF） microstructures refined the grain size during the austenite-ferrite transformation. The mechanisms of IGF formation were discussed. Mn-Ti oxide inclusions with Mn-depleted zone （MDZ） were effective to be nucleation sites for IGF formation, because the MDZ increased the austenite-ferrite transformation temperature. TiN had the low misfit ratio with IGF, so the TiN precipitated on the MnS surface also promoted the formation of IGF because of decreasing interfacial energies.

INTRAGRANULAR FERRITE FORMED IN ASSOCIATION WITH INCLUSIONS IN A VANADIUM MICROALLOYED STEEL

Intragranular ferrite was formed at inclusions in a vanadium microalloyed steel with excess amount of sulfur. The chemical composition of inclusions in the steel was analyzed by SEM-EDS. The inclusions were mainly composed of MnS and aluminum oxides. The precipitation of MnS at aluminum oxides might result in Mn depletion, which, in turn, pro- motes the formation of intragranular ferrite. Optical and SEM observations and three- dimensional (3D) reconstruction demonstrated that intragranular ferrite was formed at inclusions. The morphology of intragranular ferrite changed with undercooling. At higher temperatures intragranular ferrite was nearly equiaxed whereas it was plate-like or lath-like at lower temperatures.

Effect of Nanophase on the Nucleation of Intragranular Ferrite in Microalloyed Steel

MnS, MnS＋V（C, N） complex precipitates in micro-alloyed ultra-fine grained steels were precisely analyzed to investigate the grain refining mechanism. The experimental results shows that MnS, MnS＋V（C, N） precipitates provide nucleation center for Intra-granular ferrite （IGF）, so that refined grain remarkably. Moreover, substructures such as grain boundary, sub-boundary, distortion band, dislocation and dislocation cell in austenite increased as the deformation energy led by heavy deformation at low temperature （deformation temperature≤800 ℃, deformation quantity≥50%）. As a result, V（C, N） nanophase precipitated at these substructures, which pinned and stabilized substructures. The substructures rotated and transformed into ultra-fine ferrite. 20 nm-50 nm were the best grain size range of V（C, N） as it provided nucleating center for intragranular ferrite. The grain size of V（C, N） were less than 30 nm in the microalloyed steels that with volume ratio of ultra-fine ferrite more than 80% and grain size less than 4 μm.

A new step-cooling process for strength and toughness matching control of vanadium-containing railway wheels:effect of intragranular ferrite

To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite(γ)on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was studied using an optical microscope(OM),a scanning electron microscope(SEM),a transmission electron microscope(TEM),and mechanical property tests.The results show that when the wheel steel is slightly cooled to an appropriate temperature above A c3 point for a short time after it has been austenitized at an elevated temperature,the solid-solved vanadium is pre-precipitated in the form of V(C,N)second phase semicoherent with the matrix in the originalγgrain.This phase hardly participates in matrix strengthening.Due to the small mismatch between V(C,N)and ferrite(α),during the subsequent-cooling phase transformation stage,the pre-precipitated second phase becomes theαnucleation point,causing granular and ellipsoidal intragranular ferrite(IGF,with an average size of 4-6μm)to nucleate in the originalγ.The IGF production and strength loss increases with the increasing undercooling degree.Based on this,Masteel Co.,Ltd.has developed a new heat-treatment step-cooling process that can promote the formation of IGF,considerably improving the level and uniformity of fracture toughness on the premise that the strength and hardness of the wheel are almost unchanged.

稀土Ce对高强工程机械用钢夹杂物和高温塑性的影响

通过向高强工程机械用钢中加入稀土合金,研究了Ce对夹杂物和高温塑性的影响。通过扫描电镜、能谱分析、夹杂物自动分析系统和热力学计算对夹杂物的类型、数量和尺寸进行分析。利用Gleeble1500热模拟机测定了实验钢的高温塑性,并使用光学金相显微镜和扫描电镜观察断口形貌及组织。结果表明,加入Ce后,钢中夹杂物由不规则的Al_(2)O_(3)、MnS和Al_(2)O_(3)+MnS复合夹杂物转变为球状或椭球状的Ce_(2)O_(2)S和CeS,数量增多,尺寸减小了37.64%。650~1050℃范围内含Ce钢的断面收缩率均高于无Ce钢,具有更好的高温塑性。这是由于Ce处理生成的小球状稀土夹杂物减轻了应力集中,与基体能够协调变形,同时它们能够诱导晶内铁素体形核,最终高温塑性得到改善。

夹杂物诱发晶内针状铁素体及形核聚合的基础研究进展

采用氧化物冶金技术提高钢板的焊接热影响区韧性是当前钢铁冶金领域研究的热点。本文从夹杂物诱导晶内针状铁素体(Intragranular Acicular Ferrite,IAF)形核和复合夹杂物有效形核二方面,综述了容易诱发IAF的夹杂物、夹杂物间的晶体学关系、钢液凝固过程中IAF的形核机理以及复合夹杂物形核聚合的研究。这能为进一步明确氧化物冶金中有益夹杂物的形核和生长演变过程提供思路。

Effect of Mg addition on formation of intragranular acicular ferrite in heat-affected zone of steel plate after high-heat-input welding

The effects of Mg content, inclusion size, and austenite grain size on the intragranular acicular ferrite （IAF） nucleation in heat-affected zone of steel plate after high-heat-input welding of 400 kJ/cm were investigated by welding simulation and observation using a scanning electron microscope equipped with an energy dispersive spectrometer and an optical microscope. The IAFs are observed in steel with Mg addition, and the volume fraction of IAF is as high as 55.4% in the steel containing 0.0027 mass% Mg. The MgO-Al2O3-Ti2O3-MnS inclusions with size around 2 μm are effective nucleation sites for IAF, whereas Al2O3-MnS inclusions are impotent to nucleate the acicular ferrite. The prior-austenite grain （PAG） size distribution in low Mg steel is similar to that in steel without Mg addition. The austenite grain with size about 200 μm is favorable for the IAF formation. In the steel with high Mg content of 0.0099%, the growth of PAG is greatly inhibited, and PAG sizes are smaller than 100 μm. Therefore, the nucleation of IAF can hardly be observed.

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

By using a Gleeble 350013 thermo-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 from 800 to 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 Ao3 temperature by thermodynam- ic calculation confirm that Ti2O3 itself absorbs neighboring Mn atoms to form Mn-depleted zone （MDZ）, which pro- motes 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 （1250 ℃ ） for a longer time, which is attributed to the saturated absorption degree of Mn atoms by titanium oxide.

Ti-Deoxidized Products and Formation Mechanism of Intragranular Ferrite in High Grade Pipeline Steels

High grade pipeline steels were prepared using vacuum carbon deoxidization process combined with a final Ti-deoxidation process.The microstructure of the as-cast steels was investigated by using scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.SEM observation shows that the formation of intragranular ferrite（IGF）structure is induced by fine inclusions.TEM selected area diffraction（SAD）patterns and elemental distribution analysis indicate that these inclusions are mainly Ti2O3 and MnS.It is also found that Ti2O3 may act as nucleus in the formation of MnS during solidification process.Raman spectroscopic analysis demonstrates the presence of another phase,MnTiO3,which could be formed through entrapment of Mn by Ti2O3.It is believed that the formation of Mn-depleted region in the inclusions and thus the formation of MnTiO3 phase will increase the Mn pickup from matrix and promote the formation of IGF during solidification of molten steel.

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.

Formation of Intragranular Ferrite During Isothermal/Thermomechanical Processes in a Medium Carbon V-Ti-N Microalloyed Steel

For a V-Ti-N microalloyed steel with 0.34%C-1.54%Mn,intragranular ferrite (IGF) was obtained in both isothermal austenite decomposition processes and thermomechanical processes simulating the industrial seamless tubing manufacture process.Results show that with decrease of the isothermal temperature in range of 600℃ down to 450℃,not only the morphology of IGF changed from equiaxed to acicular,but also the equiaxed IGF and the acicular IGF were refined.More importantly,it is found that the amount of equiaxed ferrite increased significantly in the thermomechanical process sample water quenched from 550℃ after 800℃ deformation than that in the isothermally treated sample at 550℃ sample without hot deformation.It implies that appropriate controlled deformation with controlled cooling can significantly promote equiaxed IGF formation,and not solely rely on nucleation mechanisms related with inclusions.Hot deformation of austenite without dynamic and complete static recrystallization causes high energy regions,therefore further promotes the nucleation potency of IGF.

氧化物冶金技术及其应用

氧化物冶金是利用钢中细小非金属夹杂物诱导晶内铁素体形核细化晶粒的新技术,应用氧化物冶金技术已成功开发出了高强度高韧性的非调质钢和低碳钢。讨论了氧化物冶金型钢的显微组织特征,分析了氧化物冶金型钢中非金属夹杂物的性质和晶内铁素体的形核机理,简述了氧化物冶金技术的应用。

晶内铁素体的形核机理

系统地研究了MnS、CuS及其复合物附近的微区在晶内铁素体形核过程中的成分变化,未发现诱导晶内铁素体形核的MnS、CuS及其复合物附近的微区锰含量有明显的变化。夹杂物诱导晶内铁素体形核的机理是:夹杂物作为一种惰性介质具有较高的惰性界面能,它对诱导晶内铁素体形核起主要作用。夹杂物造成其附近有较高的应力 应变能,夹杂物与铁素体的错配度较小均促进晶内铁素体的形核、长大。

二次热循环对晶内铁素体的影响

采用物理模拟的方法研究了二次热循环对晶内铁素体的影响。用自动图像分析仪测量了夹杂物的大小、分布和密度 ,用透射电镜观察了晶内铁素体的形貌 ,并用能谱对诱导晶内铁素体形核的夹杂物进行了成分分析。研究结果表明 ,二次热循环对夹杂物的直径大小、分布和密度几乎没有影响。经过二次热循环过程后 ,诱导晶内铁素体形核、长大的夹杂物为MnO、TiO、SiO2 、Al2 O3 和MnS、CuS、(Mn ,Cu)S的复合物。 80 0～ 5 0 0℃的时间为 10s时 ,出现了晶内铁素体的感生形核。晶内铁素体的感生形核是由于一次晶内铁素体有较高的界面能和高密度的位错 ,两者共同促进晶内铁素体的感生形核。

冷却速度对含Ti非调质钢中晶内铁素体形成的影响

观察了不同冷速下获得的含Ti非调质钢的显微组织,并绘制了静态连续冷却转变曲线（CCT曲线）。结果表明,在0·5-2·5℃/s的冷速范围内,会有晶内铁素体析出,其中在2℃/s冷却时可以获得大量针状铁素体。空冷条件下获得细小无序分布的晶内铁素体,明显提高试验用含Ti非调质钢的韧性。

锻后控冷对38MnVS非调质钢晶内铁素体形成及韧性的影响

研究了锻后控冷工艺对中碳含钒微合金38MnVS非调质钢锻件晶内铁素体形成的影响，对锻后不同控冷工艺条件下微合金非调质钢锻件的显微组织、拉伸性能、冲击韧性和冲击断口进行了试验分析。结果表明，在正常的锻造加热温度（1180～1200℃）和终锻温度（1000～1050℃）条件下，采用锻后快冷并在550℃左右保温缓冷的工艺条件，可抑制先共析铁素体沿原奥氏体晶界的析出，获得较多细小条、块状的晶内铁素体，微合金钢锻件的冲击韧性提高了近4倍，同时仍具有较高的强度，且塑性略有提高。

钒氮钢中的晶粒细化研究

采用Gleeble-3 5 0 0热模拟试验机,在相同工艺条件下,对比研究了钒氮钢、钒钢以及碳锰钢的晶粒细化效果。结果表明,钒氮钢的晶粒细化效果最显著,铁素体的晶粒尺寸可达6.1μm,这主要与在奥氏体区中析出的V(C,N)有关。奥氏体区析出的V(C,N),不但可抑制奥氏体晶粒长大,同时还可以作为铁素体的形核核心,诱导晶内铁素体形成,大大增加铁素体形核率,从而提高了相变细化的比率。

X80管线钢埋弧焊用烧结焊剂的研制

从高强度低合金钢(HSLA)焊缝熔敷金属中的针状铁素体形核机理出发,确定以CaF2-MgO-Al2O3-MnO-TiO2-B2O3氟碱性渣系进行高强韧性埋弧焊烧结焊剂设计。结果表明,焊剂中添加MnO有利于焊缝中Mn元素的过渡,降低γ→α温度,从而抑制高温铁素体的生成而对焊缝中针状铁素体的形成有利。由于焊缝金属裂纹敏感指数(Pcm)较高,焊后空冷奥氏体晶粒较细,过量添加MnO,将会导致焊缝中Mn元素过高而细化奥氏体晶粒,使得奥氏体晶界增加。进一步冷却,贝氏体形核质点增加,因而对晶内形核的针状铁素体不利。稀土元素对提高焊缝金属中针状铁素体含量有一定的促进作用。

晶内铁素体型高强韧性微合金非调质钢的进展

晶内铁素体型微合金非调质钢成分一般为(%):0.2～0.4C,0.2～0.8Si,1.0～1.7Mn,0.05～0.15V,采用再结晶锻造或轧制、控制相变区间冷却速率以促进形成细小弥散的晶内铁素体组织是提高铁素体珠光体型微合金非调质钢强韧性的重要手段。选择合适的脱氧工艺,获得一定组成的细小、弥散的氧化物,使之成为MnS、VN和V4 C3等的析出核心,利于晶内铁素体析出的氧化物冶金是近来提高该钢强韧性的重要进展。介绍了国内外晶内铁素体型微合金非调质钢的化学成分和生产工艺,并分析了晶内铁素体形成机理。

夹杂物诱导晶内铁素体形核物理模拟研究

用物理模拟法研究了夹杂物MnS、Al2O3和Al2O3·Mns在晶内铁素体形核过程中的作用;用数字式金相显微镜观察了夹杂物界面附近区域的金相显微组织变化;用电子探针分析仪对夹杂物界面附近微区的化学成分进行了分析.研究结果表明:MnS、Al2O3和Al2O3·MnS不能使夹杂物与金属界面附近微区的化学成分发生变化,但MnS与Al2O3·MnS具有诱导晶内铁素体形核的能力,Al2O3不具有诱导晶内铁素体形核的能力;在氧、硫复合物Al2O3·MnS中,夹杂物表面的物质Mns在晶内铁素体形核过程中起着重要作用.