Effects of aluminum content on cracking phenomenon occurring during cold rolling of three ferrite-based lightweight steel

The effects of Al content on cracking phenomena occurring during cold rolling of ferritic lightweight steels were investigated in relation to microstructural modification including κ-carbides. Three steels were fabricated, varying the Al content between 4 and 6 wt.%, and the center and edge areas of steel sheets containing 6 wt.% Al were cracked during cold rolling. The three steels were composed of ferrite grains and κ-carbides in a banded shape, and the overall volume fraction of κ-carbide increased with increasing Al content. The shape of lamellar κ-carbides inside κ-carbide bands was changed from short and thin to long and thick. Microstructural observation of the deformed region of fractured tensile specimens revealed that deformation bands were homogeneously formed in wide areas of ferrite matrix in the steels containing 4–5 wt.% Al, and κ-carbide bands and boundary κ-carbides were hardly cracked. In the steel containing 6 wt.% Al, however, microcracks were initiated at grain boundary κ-carbides and long lamellar κ-carbides inside κ-carbide bands. They led to the center or edge cracking during cold rolling. To prevent or minimize cracking, it was necessary to avoid the lengthening or thickening of lamellar κ-carbides. Therefore, it was recommended that the steels should be rapidly cooled from the finish rolling temperature to the coiling temperature through the formation temperature of κ-carbide.     

Microstructural Developments and Tensile Properties of Lean Fe-Mn-Al-C Lightweight Steels

Concepts of Fe-Al-Mn-C-based lightweight steels are fairly simple, but primary metallurgical issues are complicated. In this study, recent studies on lean-composition lightweight steels were reviewed, summarized, and emphasized by their microstructural development and mechanical properties. The lightweight steels containing a low-density element of Al were designed by thermodynamic calculation and were manufactured by conventional industrial processes. Their microstructures consisted of various secondary phases as κ-carbide, martensite, and austenite in the ferrite matrix according to manufacturing and annealing procedures. The solidification microstructure containing segregations of C, Mn, and Al produced a banded structure during the hot rolling. The (ferrite + austenite) duplex microstructure was formed after the annealing, and the austenite was retained at room temperature. It was because the thermal stability of austenite nucleated from fine κ-carbide was quite high due to fine grain size of austenite. Because these lightweight steels have outstanding properties of strength and ductility as well as reduced density, they give a promise for automotive applications requiring excellent properties.     

基于FactSage的Fe-15Mn-8Al-0.25C低密度钢的组织及力学性能

借助FactSage数值模拟软件对Fe-(10~20)Mn-(5~10)Al-(0~0.5)C系低密度钢的凝固及冷却路径、相变及析出相进行了研究,利用FactSage软件中的FSstel数据库对该体系的垂直截面图进行计算,分析了Mn、Al及C元素对凝固及冷却过程中相变及析出相的影响,并得到了Fe-15Mn-8Al-0.25C低密度钢的平衡凝固相变路径图。结果表明,Fe-15Mn-8Al-0.25C低密度钢中热力学计算出的平衡相有液相、铁素体、奥氏体和κ-碳化物, 由1600 ℃冷却至600 ℃完整的平衡相变路径为:液相→液相+铁素体→液相+铁素体+奥氏体→铁素体+奥氏体→铁素体+奥氏体+κ-碳化物。C和Mn含量的增加可扩大Fe-15Mn-8Al-0.25C低密度钢奥氏体相区,Al元素增加缩小奥氏体相区。κ-碳化物的析出温度随着Al与C含量的增加而升高,Al与C元素均可促进κ-碳化物析出。Fe-15Mn-8Al-0.25C低密度钢800 ℃时效3 h后的抗拉强度为602 MPa,屈服强度为520 MPa,断后伸长率为28.6%,时效5 h后的抗拉强度为729 MPa,屈服强度为685 MPa,断后伸长率为22.4%,随着时效时间增加,试验钢的强度增加,断后伸长率降低。Fe-15Mn-8Al-0.25C低密度钢的密度为6.99 g/cm³,相比普通钢材减重效果达10.4%。     

砷及镧对E36船板钢连续冷却转变曲线的影响

采用相变膨胀仪、金相显微镜以及显微硬度计测定了不同含As量及加入稀土元素La后的E36船板钢的连续冷却转变(CCT)曲线.结果表明,As的加入能扩大船板钢的先共析铁素体相区、降低珠光体转变开始点,并且能促进晶内铁素体转变.含As的钢中加入稀土元素La后,船板钢的先共析铁素体相区缩小,贝氏体组织变得细小.     

Simulation of κ-Carbide Precipitation Kinetics in Aged Low-Density Fe–Mn–Al–C Steels and Its Effects on Strengthening

Fe–Al–Mn–C alloy systems are low-density austenite-based steels that show excellent mechanical properties. After aging such steels at adequate temperatures for adequate time, nano-scale precipitates such as κ-carbide form, which have profound effects on the mechanical properties. Therefore, it is important to predict the amount and size of the generated κ-carbide precipitates in order to control the mechanical properties of low-density steels. In this study, the microstructure and mechanical properties of aged low-density austenitic steel were characterized. Thermo-kinetic simulations of the aging process were used to predict the size and phase fraction of κ-carbide after different aging periods, and these results were validated by comparison with experimental data derived from dark-field transmission electron microscopy images. Based on these results, models for precipitation strengthening based on different mechanisms were assessed. The measured increase in the strength of aged specimens was compared with that calculated from the models to determine the exact precipitation strengthening mechanism.     

Effect of rolling in the intercritical region on the tensile properties of dual-phase steel

A steel containing 0.088 wt% C, 1.2 wt% Mn, and 0.78 wt% Cr was rolled at intercritical temperature (790 °C) and quenched to produce dual-phase microstructure. Rolling caused anisotropic increase in tensile strength and little change in ductility. The results suggest that rolling increased strength by a combination of strengthening of the ferrite and an increase in the stress transferred to the martensite. Up to 20% rolling reduction strengthened the ferrite by work hardening, larger reductions then reduced the strength of ferrite, anisotropically, due to increased recovery. Subgrains in ferrite were observed after rolling in the intercritical region which can contribute to the ultimate strength of the rolled material.     

Effects of Carbon Content and Microstructure on Corrosion Property of New Developed Steels in Acidic Salt Solutions

New steels with different carbon contents were self-developed by thermo-mechanical controlled processing.The effects of the carbon content and the microstructure on the corrosion properties of new steels were investigated by immersion test and SEM.The results indicated that the ferrite phase（both the proeutectoid and eutectoid ferrite） dissolved preferentially.Cementite reserved and accumulated on the surface.As carbon content increased,the content of ferrite decreased and cathode/anode area ratio increased.Therefore,the corrosion rate of new steels increased from 0.30 to0.90 mm/years when the carbon content rose from 0.05 to 0.13 wt%.The corrosion process of new steels was studied using electrochemical impedance spectroscopy experiments during 72 h.It indicated that the impedance modulus |Z|_0.01 Hz of the new steels reduces with the increase of the immersion time.While the corrosion process of the new steel with 0.11 wt%C developed faster than that with 0.07 wt%C,although their |Z|_0.01 Hz was similar at the initial stage.     

Effect of Second Phase on the Deformation and Fracture Behavior of Multiphase Low-Density Steels

The effects of the second-phase characteristics on the deformation and fracture behavior in low-density steel are investigated in Fe-0.2C-(4, 8)Mn-5Al-1Si alloys, where the second phase can be retained austenite or a mixture of κ-carbide and ferrite depending on the processing condition. With austenite as second phase, the mechanical stability of the austenite is critical regarding ductility because the fracture is likely to occur before the onset of necking when the austenite transforms into martensite in the early stage of deformation. In the case of the mixture of ferrite and κ-carbide, the round shape of κ-carbide is more favorable than the dense lamellar structure. The prominent influence of the second phase on the deformation and fracture behavior is attributable to the propensity of matrix ferrite for cleavage fracture due to its large aluminum concentration.     

RESEARCH ON TRANSFORMATION OF GRANULAR STRUCTURE IN LOW CARBON Mn–Si STEEL

对低碳Mn--Si钢中的粒状组织进行了实验研究, 考察了粒状组织中小岛的形状及其分布. 结果表明, 不含Cr时, 空冷后形成了由先共析铁素体基体+岛状物所组成的粒状组织,其中的岛状物形态及分布可以是无规则的, 也可以是长条状的小岛沿一定方向分布于基体上. 对于这2种形态的粒状组织, 尤其是对后者形态的形成和铁素体基体的分析表明, 小岛的形状和分布是先共析铁素体形态的反映, 条形规则排列的小岛可能受先共析铁素体基体台阶长大机制控制, 粒状组织小岛在一定条件下也可能具有方向性. 加适量Cr后, 连续冷却后的组织中不出现粒状组织.     

药芯焊丝焊缝中先共析铁素体的数量预测

合理控制低合金高强钢药芯焊缝金属中的先共析铁素体含量,对获得理想的焊缝性能具有重要意义。将焊缝凝固过程中的偏析效应引入基于扩散控制增长的先共析铁素体生长机制,指出成份偏析效应改变了先共析铁素体生长温度和时间区间,从而影响偏析微区内先共析铁素体的体积比例,建立了预测和分析药芯焊丝焊缝中先共析铁素体体积比例的模型,与实际试验结果有很好的吻合。     

Influence of cooling rate and boron content on the microstructure and mechanical properties of hot-rolled high strength interstitial-free steels

A pilot hot strip rolling and cooling test that simulates an actual hot strip rolling and continuous cooling process was performed. We then examined the effect of cooling rates ranging from 0.1 °Cs^?1 to 100 °Cs^?1 on the microstructure and mechanical properties of high strength interstitial-free (IF) steels containing 0.003 wt% of boron, 0.0005 wt% of boron and no boron. The mechanical properties and microstructures of the boron-added high strength IF steels were analyzed using uni-axial tensile test and electron back-scattered diffraction (EBSD) following the pilot hot strip rolling and cooling test. Results show that the microstructure of high strength IF steel with no boron is influenced significantly by cooling rates. There is a critical cooling rate for building up polygonal ferrite (PF) grains. PF grains do not occur when high strength IF steels with a boron content of 0.0005 wt% and 0.003 wt% undergo a cooling rate of 5.0 °Cs^?1, however widmanst?tten ferrite (WF), granular ferrite (GF) and quasi-polygonal ferrite (QF) grains are present. Under the same hot rolling and slow cooling conditions, high strength IF steel with no boron has recrystallized PF grains. On the contrary, high strength IF steel with boron cooled at above 3 °Cs^?1 doesn??t have GF or QF grains, and subsequently generates the unrecrystallized ferritic grains and WF grains, which increase the yield and tensile strengths. It is deduced that we need to control both the cooling rate and coiling temperature when boron-added high strength IF steel sheet is manufactured in an actual hot strip rolling mill.     

Cold cracking susceptibility of boron added high-strength bainitic steels

This study evaluated the cold cracking susceptibility of high-strength bainitic steels having a strength level of over 800 MPa and 1.1 GPa according to their respective chemical compositions. For evaluation of cold cracking susceptibility, a modified implant test was employed. Two different shielding gases (100% CO₂ gas and 98% CO₂ mixed with 2% H₂ gas) were used to estimate the effect of diffusible hydrogen on the cold cracking. The diffusible hydrogen contents measuring 100% CO₂ and 98% CO₂+2% H₂ gas were recorded as 0.58 and 9.01 ml/100 g, respectively. In the case of the lower hydrogen content, cold cracking susceptibility was very low in spite of the very high strength of the base steel regardless of chemical composition and rolling condition. However, when the mixed shielding gas was used, cold cracking susceptibility increased due to an increase in the hydrogen content. Most notably, the cold cracking resistance of steels containing higher alloy content deteriorated significantly due to the higher hardness of the coarsegrained heat-affected zone. However, the effect of the rolling condition on the cold cracking susceptibility turned out to be negligible compared to that of the chemical composition.     

Effects of C and Si on strain aging of strain-based API X60 pipeline steels

Four types of strain-based API X60 pipeline steels were fabricated by varying the C and Si contents, and the effects of C and Si on strain aging were investigated. The 0.05 wt% C steels consisted mainly of polygonal ferrite (PF), whereas the 0.08 wt% C steels consisted of acicular ferrite (AF). The volume fraction of AF increased with increasing C content because C is an austenite stabilizer element. The volume fractions of bainitic ferrite (BF) of the 0.15 wt% Si steels were higher than those of the 0.25 wt% Si steels, whereas the volume fractions of the secondary phases were lower. From the tensile properties before and after the aging process of the strainbased API X60 pipeline steels, the yield strength increased and the uniform and total elongation decreased, which is the strain aging effect. The strain aging effect in the strain-based API X60 pipeline steels was minimized when the volume fraction of AF was increased and secondary phases were distributed uniformly. On the other hand, an excessively high C content formed fine precipitates, and the strain aging effect occurred because of the interactions among dislocations and fine precipitates.     

Modified Law of Mixture to Describe the Tensile Deformation Behavior of Thermomechanically Processed Dual-Phase Steel

Low alloy steel containing 0.09 wt.% C was thermomechanically processed with various rolling reductions at intercritical temperature of 790 °C, followed by quenching in the iced brine solution. The flow of the material due to this plastic deformation increased the aspect ratio of the microstructure (α + γ) in the rolling than in the transverse directions. The strengths, both in the longitudinal and transverse directions of rolling were increased because of the development of substructure in ferrite, observed previously. The fibrous microstructure formed after rolling increased the surface area of contact of ferrite and martensite. During tensile deformation the fibrous dual-phase composite had positioned itself for better stress transfer from soft ferrite to hard martensite particles. These microstructural changes associated with hot deformation of the material were accommodated in current modifications in the law of mixture applied to dual-phase steel. A computer simulation was developed to present the deformation behavior of ferrite, martensite, and composite from the experimental tensile data (loads and strain). Different variables were introduced in the simulation for allowing the composite curves to pass through the experimental data points to demonstrate the tensile deformation behavior of ferrite and martensite. The systematic changes in these variables with degree of hot rolling in the intercritical region clearly described the deformation behaviour of ferrite and martensite individually.     

Influence of cooling rate on secondary phase precipitation and proeutectoid phase transformation of micro-alloyed steel containing vanadium

During continuous casting process of low carbon micro-alloyed steel containing vanadium, the evolution of strand surface microstructure and the precipitation of secondary phase particles (mainly V(C, N)) are significantly influenced by cooling rate. In this paper, influence of cooling rate on the precipitation behavior of proeutectoid α-ferrite at the γ-austenite grain boundary and in the steel matrix are in situ observed and analyzed through high temperature confocal laser scanning microscopy. The relationship between cooling rate and precipitation of V(C, N) from steel continuous casting bloom surface microstructure is further studied by scanning electron microscopy and electron dispersive spectrometer. Relative results have shown the effect of V(C, N) precipitation on α-ferrite phase transformation is mainly revealed in two aspects: (i) Precipitated V(C, N) particles act as inoculant particles to promote proeutectoid ferrite nucleation. (ii) Local carbon concentration along the γ-austenite grain boundaries is decreased with the precipitation of V(C, N), which in turn promotes α-ferrite precipitation.     

Effects of drawing strain and post-annealing conditions on microstructural evolution and tensile properties of medium- and high-carbon steels

Variation in the microstructure and tensile properties with the warm drawing strain and subsequent annealing of 0.45 wt% C (45C) medium-carbon steel and 0.82 wt% C (82C) high-carbon steel was investigated. The morphology and size of ferrite and pearlite changed considerably with applied strain during drawing and with annealing temperature and time, which made the tensile properties of the drawn steels vary considerably. With increasing drawing strain to ~2.5, the yield strength increased significantly from 393 to 1332 MPa for the 45C steel and from 673 to 1876 MPa for the 82C steel; this was attributed mainly to the strain hardening caused by severely deformed ferrite and/or enhanced particle hardening caused by cementite segmentation. During annealing of heavily drawn steels, ferrite grain growth and coarsening of broken cementite particles occurred simultaneously, which caused effective recovery of the ductility of severely drawn steels. These results demonstrate that the tensile strength and elongation of medium- and high-carbon steel wires can be tuned considerably by controlling the extents of drawing and annealing, thereby widening their applicability and facilitating their manufacture to match their mechanical properties to each application.     

Effect of Ni and Mn on the Mechanical Properties of 22Cr Microduplex Stainless Steel

The tensile properties of 22Cr–2Ni–4Mn–0.2N micro-duplex stainless steels with different Ni and Mn contents were investigated. Duplex stainless steels were vacuum induction melted and hot rolled, then annealed at 1,000–1,100 °C, at which temperature both the austenite and ferrite phases were stable. The volume fraction of the ferrite phase was markedly affected by the alloying elements of Mn and Ni; 1 wt% of Mn was equivalent to 0.4 wt% of Ni. All of the steels tested at room temperature showed the common strain-hardening behavior, while the steels tested at lower temperatures(-30 or-50 °C)showed a distinct inflection point in their stress–strain curves, which resulted from the transformation of the austenite to straininduced martensite. The onset strain(e0) of the inflection point in the stress–strain curve depended on the Md30 value of the steel. Testing at lower temperatures resulted in smaller e0 and consequently higher strengths and fracture strains(ef). The tensile behavior was examined from the perspective of austenite stability of the micro-duplex stainless steels with the different Ni and Mn contents.     

电工钢热压缩时动、静态再结晶组织及取向分析

采用热模拟方法及EBSD技术,研究Fe-3 wt%Si电工钢在不同温度下组织的动、静态再结晶及晶粒取向特征,特别是少量奥氏体对铁素体动、静态再结晶组织及取向的影响.结果表明,不同温度形变的组织主要分三类:形变长条铁素体、珠光体和等轴细小铁素体.长条形变铁素体内发生动态回复或连续式的动态再结晶,奥氏体周同的铁素体动态再结晶加速,部分以传统的不连续方式动态再结晶.铁素体、奥氏体都可发生静态再结晶.奥氏体的静态再结晶在1050℃以上明显,铁素体的静态再结晶随温度的升高逐渐进行,最显著的再结晶发生在1050℃.不同温度形变的样品,其形变晶粒取向主要以<111>和<100>为主,小等轴铁素体晶粒除与大形变铁素体取向相近外,出现了<110>取向及其它取向.     

硅对中碳低温贝氏体钢组织与性能的影响

对Si含量分别为0.3%和1.5%(质量分数)的中碳试验钢进行低温贝氏体热处理,研究了Si对贝氏体钢组织和性能的影响。结果表明:不同Si含量的试验钢的微观组织有较大的差别,其中0.3%Si试样的显微组织主要为贝氏体铁素体束,M/A岛以及大量的渗碳体析出,1.5%Si试样的显微组织主要为贝氏体铁素体束和M/A岛。1.5%Si试样的硬度和冲击性能较0.3%Si试样高,高的硬度主要是固溶强化和细晶强化的作用;高的冲击韧性主要是添加的适量Si可以抑制渗碳体的析出,从而提高残留奥氏体的体积分数及其碳含量,进而产生较为明显的TRIP效应。     

Alloy Design,Combinatorial Synthesis,and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels

We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and <4–5 wt.% Al or even <8 wt.% Al when naturally aged. The second one is a class of κ-carbide strengthened austenitic steels with even higher Al content. Here, κ-carbides form either at 500–600°C or even during quenching for >10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement.