Microstructural evolution of submicron sized ferrite in bimodal structural ultrafine grained ferrite/cementite steels by annealing below austenized temperature

The microstructural evolution of submicron sized ferrite in bimodal structural ultrafine grained ferrite/cementite steels with 0.15 pct carbon content and 0.45 pct carbon content upon annealing below the austenized temperature was investigated. The average grain sizes of the ferrites with a normal density and with a high density of cementite particles were plotted, respectively, as a function of the annealed temperature and time, and exhibited different coarsening behaviors. The average grain sizes of the ferrites with a normal density of cementite particles gradually coarsened by increasing the annealing temperature or time, while those with a high density of cementite particles hardly changed at first, and then coarsened after reaching a certain annealing condition. The coarsening of the ferrite grain size in the steel with 0.15 pct carbon content occurred much more readily than that in the steel with 0.45 pct carbon content upon annealing. The spacing and the critical spacing of cementite particle were measured and hypothetically calculated, respectively. The size and the distribution of cementite particles was one of the critical factors affecting the microstructural evolution in this type of cementite particle spherodized steels. Most of the coarsening of the ferrite grain size occurred after the cementite particle spacing reached the required critical value.     

细小弥散碳化物与回火马氏体脆性

本文利用透射电子显微镜及物理化学相分析技术研究了中碳硅-锰钢中回火马氏体脆性(TME)的微观机制,着重探讨了回火过程中析出的碳化物对TME的作用。结果发现,对应于40Si2Mn2钢中TME产生的回火温度区间,存在有由碳化物向渗碳体的转变,析出了细小弥散的颗粒状渗碳体。这种对应现象在40Si2Mn2Mo钢中也得到证实。40Si2Mn2Mo钢的TME产生的回火温度和弥散渗碳体的析出温度都要高于40Si2Mn2钢。由此得出:在回火过程中大量细小弥散渗碳体的析出是中碳硅-锰钢中TME产生的一个重要原因。     

Model for Predicting the Lankford Coefficient of Industrial Sheet of Automotive Steels

A model is proposed for predicting the Lankford coefficient of steel sheet produced for the auto industry at PAO Severstal. The set of empirical model parameters is determined using an experimental database on the mean Lankford coefficient (for 138 strips of 13 steels differing greatly in composition) and additional databases on calculated steel microstructure parameters after hot rolling, and after heat treatment of cold-rolled strips. The mean error in calculations of the Lankford coefficient using the model is 7.1%. It is found that the Lankford coefficient is determined not only by the carbon content in the solid solution before cold rolling but also by the amount of carbon deposited at the ferrite grain boundaries in the form of tertiary cementite. The additional free carbon appearing in the solution when the tertiary cementite particles dissolve on heating of the cold-rolled sheet significantly affects the recrystallization and the texture formation.     

Development of Fine‐Grained High‐Carbon Steel in High Reduction Low Temperature Rolling

After blanking and bending to form parts with the desired shape, high‐carbon steels are quenched and tempered to produce various machine parts. Thus, the spheroidization, formability and hardenability are very important properties for high‐carbon steels. Thermo‐Mechanical control Process of rolling has been widely used in the steel industry. However, it is difficult to apply this process to high‐carbon steels because of the heavy rolling load. Thus, fine‐grained high‐carbon hot strips were developed through high‐reduction and low‐temperature rolling by using single roll rolling mills with different diameters and laminar flow cooling devices in the finishing train, the grain size of these steels was about 3 microns. Also developed annealed strips with fine homogeneously dispersed spheroidal cementite had many excellent characteristics. For example, burring formability investigated by the hole‐expanding and surface hardness evaluated by laser hardening of the developed high‐carbon annealed steels, were excellent.     

The Microstructure and Mechanical Properties of Ultrafine Grained Plain C‐Mn Steels

An ultrafine microstructure was produced in plain C‐Mn steels with different carbon contents (0.15 ‐ 0.3 mass% C) by heavy warm deformation. The rolling was simulated by the plane strain compression test with a simulated post rolling coiling. The final microstructure consists of an ultrafine grained ferrite matrix with the average grain size of 1.1 ‐ 1.4 μm and spheroidized cementite particles of two different size groups. The fraction of high‐angle grain boundaries maintained in the range of 60% to 65%. With the increase of C content from 0.15 mass% to 0.3 mass% the strength increases by about 100 MPa, while the total elongation of 23% hardly changes. The (specific) upper shelf energy decreases from 320 J/cm² to 236 J/cm² but a rather low ductile‐to‐brittle transition temperature (DBTT) of about 206 K does not rise with increasing C content. The ultrafine steel with higher C content (0.3 mass%) exhibits a superior strength‐toughness combination.     

Spheroidizing Kinetics and Optimization of Heat Treatment Parameters in CK60 Steel Using Taguchi Robust Design

For processing parts made from medium carbon steel,toughness and flexibility are of importance.Therefore,to achieve these properties,the cementite in the steels is spheroidized through heat treatment.Different parameters such as the time and temperature of spheroidizing and the initial microstructure of the steel affect the amount of spheroidized cementite.In the present work,the percent of contribution of two parameters,i.e.initial microstructure and spheroidizing time,to the percent of spheroidization in CK60 steel was investigated using Taguchi robust design.The initial microstructures consisted of martensite,coarse pearlite,fine pearlite and bainite and the chosen spheroidization times were 4,8,12,and 16h.Spheroidizing was done at the constant temperature 700℃.After spheroidizing was completed,the samples were prepared in order to observe their microstructure under an optical microscope and to determine the spheroidized percent using MIPTM(metallographic image processing)software.It was found that the spheroidizing time had the most influence(58.5%)on spheroidized percent and the initial microstructure only had a 31.1% contribution.Finally,the instantaneous growth rate of the carbide was also deduced.     

Influence of processing parameters on the recrystallized microstructure of extra-low-carbon steels

This article deals with the influence of processing parameters of a new procedure for ferritic rolling on the recrystallized microstructure of extra-low-carbon (ELC) steels. Parameters such as coil transfer temperature and degree of reduction during ferritic rolling are shown to control the morphology of cementite particles and the precipitation of AlN process. The recrystallized grain morphology and the percentage of recrystallization after annealing cycles simulating the industrial coiling process are shown to be strongly influenced by processing parameters.     

Laboratory Investigations on Copper‐alloyed Interstitial Free Steel – Part II: Effect of Coiling Temperature

The effect of coiling temperature on the annealing behaviour of copper‐alloyed interstitial free steel has been studied during batch and continuous annealing. The batch annealing kinetics undergoes a severe sluggishness in the so‐called industrial low temperature coiling condition, while retardation is less with high temperature coiling. The mechanism is believed to be the particle pinning effect exerted by peak‐aged or over‐aged copper precipitates. Room and high temperature coiled materials show similar strength and ductility after batch annealing. However, the texture development is different in the two cases, which leads to a variation in deep drawability. Copper precipitation has been observed to give rise to an unusual trend in strain hardening with the progress of batch annealing. The strength and formability parameters of the continuous annealed copper‐alloyed interstitial free steel do not depend on the coiling temperature due to dissolution of copper precipitates of the hot rolled material during continuous annealing. Irrespective of the coiling temperature continuous annealed copper‐alloyed interstitial free steels are as good as conventional interstitial free steels particularly in formability parameters.     

Improvement of Hole-Expansion Property for Medium Carbo Steels by Ultra Fast Cooling After Hot Strip Rolling

 The improvement of hole-expansion properties for medium carbon steels by ultra fast cooling (UFC) after hot strip rolling was investigated. It was found that finely dispersed spherical cementite could be formed after ultra fast cooling, coiling and annealing treatment. Tensile strength of the steel after annealing was measured to be about 440 MPa. During hole-expansion test, cracks were observed in the edge region around the punched hole because necking or cracking took place when tangential elongation exceeded the forming limit. Cracks were mainly formed by the coalescence of micro-voids. Fine and homogeneous microstructure comprised of ferrite and spheroidized cementite could increase elongation values of the tested sheets by suppressing the combination of the adjacent micro-voids, resulting in the improved hole-expansion property.     

低碳锰(铌)钢控轧控冷实验研究

通过TMCP研究了低碳锰钢和添加微量Nb的低碳锰铌钢的组织和力学性能的影响因素.研究了Nb对实验钢显微组织和力学性能的影响.Nb的添加能够细化铁素体晶粒,促进铁素体转变,对贝氏体形成有一定的抑制作用.相对低碳锰钢来说,铌的加入减少了贝氏体的体积分数,贝氏体强化效果减弱,钢的屈强比升高.通过增加冷速和降低卷取温度,可以使低碳锰铌钢获得一定量的贝氏体,综合性能较佳.低碳锰铌钢的主要强化机制有细晶强化、贝氏体相变强化和析出强化.     

Analysis of Fishscaling Resistance of Low Carbon Heavy Plate Steels

The precipitates and hydrogen permeation behavior in three kinds of hot rolled low carbon heavy plate steels for enameling were analyzed;then,both sides of the steels were enameled.The experimental results show that a large amount of coarse Ti4C2S2 and fine Ti(C,N) particles exist in the optimized Ti-bearing steel,quite a lot of fine Ti(C,N)particles exist in the optimized carbon steel,but only a little bit fine Ti(C,N) particles exist in the carbon steel.The fishscaling resistance of the steels can be correlated to the effective hydrogen diffusion coefficient,and a model of correlation between the effective hydrogen diffusion coefficient and the volume fraction of the precipitates was established and verified.The effective hydrogen diffusion coefficient should be lower than 3.96×10-6 cm2/s to avoid fishscaling in heavy plate steels.     

Neutron Scattering Study on Precipitation and Recrystallization Behaviours in P‐Free and P‐Alloyed IF‐Steel Sheets

In order to investigate the effect of coiling temperature and alloying of phosphorus (P) on the formation of precipitates and recrystallization behaviour, initial samples of P‐free and P‐alloyed IF‐steels were prepared by coiling at 580 and 720°C. Variations of precipitates and textures after coiling, cold rolling and recrystallization annealing were studied by means of small angle neutron scattering (SANS), neutron texture measurements and TEM observations. Hardness measurements and microstructure observations disclose that both coiling temperature and alloying of P affect softening of cold rolled samples. Coiling at a lower temperature increases the number of fine precipitates smaller than 10 nm in the hot band. Upon recrystallization annealing, precipitates ranging in size from 10 to 50 nm increase at the expense of fine particles smaller than 10 nm. Recrystallization behaviours in cold rolled IF‐steel samples were discussed by tackling the distribution of precipitates and the amount of solute P atoms in solution.     

轧制工艺对CSP线热轧汽车用高强钢板组织特征的影响

研究了薄板坯连铸连轧(CSP)工艺生产高强度汽车用大梁板的工艺控制参数与力学性能和显微组织间的关系.根据柔性工艺控制的指导思想,在珠钢电炉CSP流程下实现了生产不同级别高强度钢板的柔性轧制工艺.利用扫描电镜和透射电镜研究了其组织和强度差异产生的原因.研究表明,钢板最终组织为多边形铁素体和少量珠光体组成,平均铁素体晶粒尺寸约为3.7~5.6μm;当降低卷取温度,部分渗碳体已破碎成细小的碳化物粒子分布于铁素体基体上,钢板中有少量贝氏体出现.     

Microstructures and Mechanical Properties of a New As-Hot-Rolled High-Strength DP Steel Subjected to Different Cooling Schedules

Controlled rolling followed by accelerated cooling was carried out in-house to study the microstructure and mechanical properties of a low carbon dual-phase steel. The objective of the study described here was to explore the effect of cooling schedule, such as air cooling temperature and coiling temperature, on the final microstructure and mechanical properties of dual-phase steels. Furthermore, the precipitation behavior and yield ratio are discussed. The study demonstrates that it is possible to obtain tensile strength and elongation of 780 MPa and 22 pct, respectively, at the two cooling schedules investigated. The microstructure consists of 90 pct ferrite and 10 pct martensite when subjected to moderate air cooling and low temperature coiling, such that the yield ratio is a low 0.69. The microstructure consists of 75 pct ferrite and 25 pct granular bainite with a high yield ratio of 0.84 when the steel is directly cooled to the coiling temperature. Compared to the conventional dual-phase steels, the high yield strength is attributed to precipitation hardening induced by nanoscale TiC particles and solid solution strengthening by high Si content. The interphase precipitates form at a suitable ledge mobility, and the row spacing changes with the rate of ferrite transformation. There are different orientations of the rows in the same grain because of the different growth directions of the ferrite grain boundaries, and the interface of the two colonies is devoid of precipitates because of the competitive mechanisms of the two orientations.     

Cleanliness of Low Carbon Aluminum‐Killed Steels during Secondary Refining Processes

Efficient secondary refining process is necessary for massive and stable production of low carbon aluminum‐killed (LCAK) steels. Plant trials were performed to investigate the cleanliness of steels. Characteristics of composition, cleanliness, and inclusions of LCAK steel during different secondary refining processes, including LF, CAS, RH‐LIT, and RH, were studied and compared. The results showed that CAS, RH‐LIT, and RH processes had better control of low carbon, silicon, and nitrogen than LF process. High cleanliness of LCAK steels could be achieved by all the mentioned refining processes. It was concluded that the total oxygen (T.O.) should be <35 ppm to reduce the amount of inclusions and reach the level of clean steels. The removal rate of inclusions during RH‐LIT and RH processes was much higher than that of LF and CAS. The T.O. content and the amounts of inclusions during CAS, RH‐LIT, and RH could be quickly decreased to a low value within 10 min. The results showed that CAS and RH‐LIT as well as RH refining processes can produce LCAK steels that meet the requirements of high efficient, low cost, clean, and stable production, while LF is more suitable for the heats with poor control of end point of BOF, and for the process with calcium treatment to control sulfur content and lower the clogging of Submerged Entry Nozzle during thin slab continuous casting.     

碳含量和组织类型对低合金钢耐蚀性的影响

研究了不同碳含量和显微组织的低合金钢的耐腐蚀性能和腐蚀行为,并与商业耐候钢09CuPCrNi进行了相应的比较.通过金相显微镜、扫描电镜观察,轧后水冷钢的主体组织为板条状贝氏体,轧后空冷钢为针状铁素体、粒状贝氏体、M/A小岛和少量渗碳体(珠光体)的混合物.用干湿循环加速腐蚀实验对耐蚀性测定结果表明:低碳钢(0.03%C)和轧后水冷的较高碳含量钢(0.1%C)的耐蚀性均明显优于09CuPCrNi;低碳含量钢的组织类型对其耐蚀性影响不大;较高碳含量情况下,单相贝氏体钢的耐蚀性优于由铁素体、渗碳体(珠光体)等构成的复相组织钢;轧后水冷时,不同碳含量的钢耐蚀性差别不大;轧后空冷时,低碳含量钢的耐蚀性优于较高碳含量钢.用扫描电镜对锈层进行观察,可以看出耐蚀性较好的样品在腐蚀后期形成了致密的内锈层.     

Tempered martensite embrittlement in Fe-Mo-C and Fe-W-C steel

A study on the phenomenon of tempered martensite embrittlement (TME) has been made in experimental Fe-Mo-C and Fe-W-C steel. Charpy impact testing was conducted to evaluate the impact toughness, sensitive to TME. Retained austenite was observed by an analytical transmission electron microscopy in both steels. Both steels represented TME. TME was correlated with the formation of the interlath cementite, resulting from the decomposition of interlath retained austenite. TME occurred in a limited range of test temperatures where the interlath cementite could act as a source of embrittling cracks. Therefore, both the interlath cementite resulting from the decomposition of the interlath retained austenite, and the level of matrix toughness, enabling the interlath cementite to act as an effective embrittler, are necessary to produce TME.     

卷取温度对Ti-IF钢第二相粒子及晶粒尺寸的影响

采用热模拟技术，研究了卷取温度对Ti-IF钢第二相粒子析出行为及晶粒尺寸的影响，结果表明，卷取温度的变化，对TiN,TiS和Ti4C2S2粒子的影响不大，但对TiC粒子的影响较大，高温卷取有利于TiC粒子的析出，长大，高温卷取有利于铁素体晶料珠长大，碳含量较高，会析出较多的TiC粒子，对退火织构的发展不利。     

Formation of the cottrell atmosphere during strain aging of bake-hardenable steels

A model has been developed to describe the formation of the carbon atmosphere around dislocations during the aging of a bake-hardenable steel, by taking into account the concurrent segregation of carbon atoms to dislocations, grain boundaries, and pre-existing cementite particles. The effects of the segregation of carbon to grain boundaries and to pre-existing cementite particles on the formation of the carbon atmosphere have been discussed theoretically. Strain-aging experiments have been carried out with a vacuum-degassed ultra-low-carbon bake-hardening steel, and the experimental results been compared with the theoretical predictions. It is shown that the model developed can describe the formation of the carbon atmosphere around dislocations in both low-carbon and ultra-low-carbon bake-hardenable steels in industrial processing conditions.     

Pearlite phase transformation in Si and V steel

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pct C). The products comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of vanadium carbide. As the carbon content is increased further (up to 1.05 wt pct), the amount of grain boundary ferrite gradually decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1 to 25:1 by the addition of the alloy elements. A compre-hensive model has been suggested to explain these effects. Other effects of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions, an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the rate of interphase precipitation of vanadium carbides. Formerly Graduate Student, Department of Materials, Oxford University Formerly University Lecturer, Department of Materials, Oxford University