A Study about the Influence of Carbon Content in the Steel on the Casting Behavior

In the casting process of steels with a C‐content ranging from 0.09 to 0.53 mass%, austenite is formed as secondary crystal phase by peritectic reaction between crystal of δ ferrite and residual melt. For unalloyed or micro‐alloyed steels the C‐content or C‐equivalent influences the casting behavior of steel in the mould, such as strand shell growth, crack formation, heat transfer, temperature fluctuation in the copper plate, mould level fluctuation and oscillation marks formation. The negative casting behavior like the uneven strand shell growth, the deep oscillation mark formation, the high mould level fluctuation, the crack formation on the strand surface were found mostly for steel with C‐content or Cp between 0.10–0.13 mass%. The strand shell structure (strand shell growth, mushy zone, δ + γ phase transformation) and shrinkage of the strand shell were simulated depending on the C‐content by means of mathematical simulation. On the basis of the simulation results and of the measured high temperature strength of steel the dependence of stiffness and the irregularity of the shrinkage of strand shell on the C‐content was investigated. It was found that the stiffness and irregularity of the shrinkage of the strand shell reach the maximum value at a C‐content of about 0.12 mass%.     

Modelling of steel shrinkage and optimisation of mould taper for high speed continuous casting

Abstract

In high speed continuous casting, optimisation of mould taper is key for intensifying heat transfer and for improving the quality of the cast products. Mathematical modelling has been carried out by combining heat transfer, steel shrinkage and parabolic continuous taper model in order to optimise the mould taper profile. These models have been assembled to a set of software, the inputs of which include the steel grade, casting speed, casting temperature, length and the cross-section of mould tube, while the outputs consist of surface temperature of the strand, thickness of solidified shell, thermal linear expansion coefficient, steel shrinkage, distortion of the mould tube wall, the actual air gap, total taper and the continuous taper profile. Optimum mould taper has a parabolic profile which is tapered inwards that changes continuously along the length of the strand in order to achieve reduction in air gap while avoiding distortion of the mould tube.     

High productivity and technological developments at Corus DSP thin slab caster

Abstract

The Corus IJmuiden Direct Sheet Plant focuses on production of thin gauged material through a single casting strand and an (semi-) endless rolling process. The caster has a 90 mm thick funnel shaped mould and is equipped with liquid core reduction, allowing a slab thickness of 70 mm. To reach casting speeds up to 5·4 mm min^?1, steel is fed into the mould using a specially designed submerged entry nozzle and an adjustable multi-pole electromagnetic braking system is installed. Although still increasing, productivity is now at the design level of 1·3 Mt/year. Average casting speed (including start and stop procedures) is >5·0 m min^?1 with sequence lengths up to nine 300 t ladles, representing a maximum planned casting time of 12 h. Significant growth has occurred in the production of (thin gauged) high strength steels. Breakouts have been a constant threat but stabilisation of the casting process has resulted in significant reduction in these events, allowing steady growth to productivity targets. Developments to improve mould fluid flow, mould level control, mould heat transfer and strand lubrication are described.     

品种钢连铸生产实践

邢钢引进奥钢联(VAI)技术建成的品种钢连铸机于2002年12投产。该铸机采用了结晶器电磁搅拌、液面自动控制、DYNAFLEX非正弦液压振动、DIAMOLD钻石结晶铜管、连续矫直等先进技术。在试生产阶段成功生产出冷镦钢、钢绞线、焊丝钢，取得了良好的生产和质量指标。     

连铸保护渣性能与钢种、工艺参数关系的初探

从低、中、高碳钢等钢种本身性能及凝固收缩特点出发研究了不同钢种用保护渣性能特点,并分析了拉速、铸坯断面形状、尺寸及结晶器振动特性等连铸工艺参数对保护渣性能的要求,找出了不同钢种及工艺条件对保护渣性能设计的特点。     

Leaner alloys for the PM industry

Abstract

Numerous lean alloy systems are now available to the PM parts manufacturer as potential lower cost solutions without sacrificing part performance. More recently, steels containing 0·3 and 0·5% prealloyed molybdenum have been introduced which can be tailored with reduced Ni and/or Cu additions to meet specific properties for use in press and sinter applications or as quench and temper grades. These reduced Mo prealloys complement the already familiar 0·85 and 1·5%Mo grades to provide a full range of prealloyed molybdenum steels. Another method to reduce susceptibility to market pricing has been through incorporating manganese, which is historically inexpensive yet highly beneficial to steel properties. Combining Mn with moderate levels of Mo in specially designed alloy grades provides a lean alternative to Ni and Cu containing hybrid alloy steels. This study will examine these lean alloys and explore where there may be opportunity to use them in place of traditional PM grades.     

Three-dimensional analysis and design of petal-like mould for continuous casting of steels

Abstract

Understanding the shrinkage behaviour of a steel billet is very important for designing the continuous casting mould, and conversely, a well designed mould is beneficial when matching the shrinkage behaviour of the billet. The shrinkage behaviour of a billet is hard to measure in situ during continuous casting but can be calculated by numerical simulation. A three-dimensional finite element model has been built to simulate the thermal and stress fields of the billet in the mould. The dynamic thermal boundary condition, the effect of ferrostatic pressure and the temperature dependent thermophysical parameters have been considered in the model. The shrinkage of billet when considering ferrostatic pressure is on average 0·08 mm smaller than when not considering ferrostatic pressure. The temperature and stress distributions are analysed in the present paper, and based on this analysis, a novel petal-like mould was designed and its taper determined. The designed mould has been tested in industrial practice showing better lifetime and billet quality.     

Development of mould fluxes based on lime–alumina slag system for casting high aluminium TRIP steel

Abstract

With excellent strength and ductility properties, transformation induced plasticity (TRIP) steels are superior to ordinary commercial steels and have great application potential in the automobile industry. However, the continuous casting of TRIP steels is associated with challenges owing to its high aluminium content, approximately 0?5–1?8%. The reaction of aluminium near the steel/slag interface continuously changes the basicity of conventional lime–silica based mould fluxes causing variation in the flux physical properties between the solidifying shell and mould wall. Therefore, the heat transfer process is unstable and the surface quality of the as-cast slab is not guaranteed.

To reduce the reactivity of aluminium in TRIP steel during casting, it is suggested to replace the lime–silica based fluxes with a lime–alumina based mould flux, based on laboratory measurements and industrial trials.

The trials showed great improvement in the slab surface quality and no depression defect was noticed so far.     

Design of corners of mould in square billet casting

Abstract

In continuous casting, heat flow optimisation in the mould is key to improving the quality of the product and production savings. The heat flow influences, and is influenced by, several phenomena of a mechanical or metallurgical nature, so its optimisation should include these. In particular, shrinkage of the strand and solid phase formation are among the most influencing factors affecting the cooling of the solidifying product. The present paper describes a model implemented by a software tool that can carry out simulation of shell formation of carbon steel within the mould, for rectangular shapes. The first aim of the software is to simulate formation of the solid shell in the strand and the deformations to which this solid is subjected. Deformations are a result of both thermal shrinkage, related to phase changes, and stresses caused by metallostatic pressure or the mould-shell interaction. The output of the model consists of temperature maps of the strand, maps of formation of the shell and the ideal mould contour.     

包晶钢结晶器液面周期性波动原因及控制

针对鞍钢股份有限公司鲅鱼圈钢铁分公司板坯连铸在生产包晶系列钢种时．连铸结晶器钢液面周期性剧烈波动的原因进行了分析。包晶钢液在凝固过程的包晶相变造成初生坯壳的不均匀生长是导致结晶器出现液面周期性剧烈波动的主要原因。通过调整结晶器冷却水强度和控制保护渣传热性能,有效地控制了结晶器液面周期性波动,提高了铸坯质量。     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 1 Mould heat transfer

Abstract

With the drive to cast higher quality, many minimills are adopting mould powder as a lubricant for the continous casting of steel billets. Over the past three decades considerable experience has been accumulated on the relationship between mould behaviour and billet quality for oil lubrication, but comparatively few studies have been conducted for mould powder lubrication. This study, conducted at a Canadian minimill, involved instrumenting four faces of a copper mould with thermocouples and monitoring mould temperatures during casting of 208 × 208 mm billets with mould flux lubrication. Billet samples were also taken to coincide with periods of measurements. Mould temperatures were monitored for two different mould powder compositions, for different mould oscillation frequencies, two mould cooling water velocities, and a range of steel compositions. An inverse heat conduction model was developed to calculate mould heat transfer from the measured temperatures. In this paper, which is the first part of a two part series, details of the inverse heat conduction model and mould heat transfer data are presented. The results obtained for mould flux lubrication have been compared with those for mould heat transfer for oil lubrication. For peritectic steels, with carbon content in the range 0·12–0·14%, it was found that lubricant type has little influence on the measured mould heat flux distribution at the centreline of a face. The peak mould heat flux was found to be approximately 2500 kW m^-2 . In contrast, for medium carbon steels, mould heat transfer with mould powder was significantly lower than when oil was employed as a lubricant. For instance, at the meniscus, the peak heat flux with mould powder was approximately 2500 kW m^-2 , which was half that recorded with oil as a lubricant. The influence of oscillation frequency, mould cooling water velocity, and mould powder type on mould heat flux has also been presented.     

Influence of nitrogen on hot ductility of steels and its relationship to problem of transverse cracking

Abstract

The present review examines the influence of nitrogen on the hot ductility of steels, with particular relevance to the problem of transverse cracking during continuous casting. Nitrogen itself is not detrimental to hot ductility, but when it is present with aluminium or microalloying additions, ductility can be adversely affected through the formation of nitrides or carbonitrides. The addition of aluminium to low nitrogen C–Mn steels (0·005%N)impairs ductility during casting at an acid soluble level as low as 0·02%Al. This arises because segregation of aluminium to the grain boundaries occurs on solidification, and the temperature cycling that takes place when the strand is cooled encourages AlN precipitation. However, for low nitrogen, high strength low alloy (HSLA) steels with carbon levels in the peritectic range 0·08–0·17%C, transverse cracking is not generally encountered until the aluminium level is >0·04%. Higher nitrogen levels are likely to cause problems even at very low aluminium levels, as precipitation of AlN is controlled by the product of the aluminium and nitrogen contents. The microalloying additions vanadium and niobium are detrimental to ductility but, of the two elements, niobium is more damaging, as it gives finer precipitation. Increasing the nitrogen level has a more pronounced influence on ductility in vanadium containing steels, since vanadium forms a nitride while niobium forms Nb (CN), which is mainly carbon based. Nevertheless, the product of vanadium and nitrogen contents has to approach 1·2 × 10^-3, for example 0·1%V and 0·012%N, before ductility deteriorates to that normally given by a niobium containing steel with 0·03%Nb and 0·005%N. When small titanium additions are made to low nitrogen C–Mn–Al steels (0·005%N), the best ductility is likely to be given by a high Ti/N ratio of 4–5 : 1; the excess titanium in solution encourages growth of the TiN particles. For high nitrogen steels (0·01%N), a low titanium level (0·01%)is recommended to limit the volume fraction of TiN particles. A low soluble aluminium level is also needed to prevent the excess nitrogen from combining to form AlN. For C–Mn–Nb–Al steels, similar recommendations can be made with regard to adding titanium. However, the presence of niobium and aluminium appears to have little influence on ductility, since these elements coarsen the titanium containing precipitates.     

Clogging in submerged entry nozzles

Casting high quality steel grades requires the use of a submerged entry nozzle (SEN) to prevent oxygen and nitrogen pick‐up and to achieve favourable steel flow conditions in the mould. Clogging of the SEN leads to instability of the casting operation as well as to a decrease in slab quality. To understand the interactions between liquid steel and the SEN, 156 nozzles from the Sidmar steel plant were investigated with respect to the amount of clogging. A linear relation between the clogging volume and the clogging weight was observed. This indicates that the clogging material's density remains constant during the clogging process for the different steel grades. A distinct relationship was also found between the amount of clogging and the cast steel weight. No relation between the amount of clogging and the superheating of the steel nor the steel flow rate was noticed, however. It was also concluded that for AI killed steels the high Mn (> 3000 ppm) and high C (> 500 ppm) grades cause less clogging than AI killed steel grades with low Mn content (< 3000 ppm) and that Al/Si killed steel grades cause less clogging when compared to AI killed steel grades. When comparing these different steel grades with respect to their total oxygen concentration in the ladle, it appears that steel grades, which have a low total oxygen concentration, cause less clogging in the SEN. Based on these observations three combinations SEN/steel grade were selected for further analysis with electron microscopy.     

Final solidification,centre segregation and precipitation phenomena in continuously cast round billets

Archimedes’ principle was applied to strand lengths of up to 14 m of round 177 mm diameter billets to determine cavity volume V (in cm³/m) for a series of tubemaking steels. Under steady state casting conditions, V depends primarily on the chemical composition and solidification behaviour of the steels. Averages ranged up to in an extreme case (bearing steel; v_C = 2.0 m/min). Stopping the strand and abrupt lowering of withdrawal speed made it possible to artificially induce individual shrinkage cavities with peak values of V = 45 cm³/m. In the round mould, subsequent necking of the liquid crater is initiated in this process. Whisker-like and icicle-like dendrites can be observed in horizontally formed macroscopic shrinkage cavities. The liquid crater is broken off by periodic bridging (the mini-lingot model). As permeability decreases, the bridges deflect downward in a U-shape under the ferrostatic load. The residual inter-dendritic melt is squeezed out in this plastic deformation process. Some bridges ultimately break. This interpretation is based on comprehensive CMA studies and concentration maps. Cores taken from the centre of a strand of bearing steel were submitted to various heat treatments between 1100 and 1350°C. Homogenization occurs below solidus (T < 1225°C). Carbides dissolve and sulfides become globular. Spatial segregation, on the other hand, increases, above solidus. In the extreme case at 1350°C/30 h, the residual melt at the grain boundaries migrates practically completely from the surface zone to the centre of the cylindrical sample (40 mm diameter). The propellent forces for this macro-heterogenization are provided by the activity gradient from the cylindrical surface to the centre and minimization of interface energy γ/I.     

Effect of austenitisation temperature on austenite transformations in 0·7%C Cr–Mo PM steel

Abstract

The effect of austenitisation temperature on austenite transformations on 0·7%C Astaloy CrL steel was studied by dilatometry. The steel has a good hardenability, forming martensite at most of the austenitisation temperatures and cooling rates investigated. Only on cooling from 1073 K, austenite transforms into bainite completely at 3 K s^?1 and partially at 12·5 K s^?1. The effect of austenitisation temperature on the prior austenitic grain size is quite poor because of the pinning effect of pores. The martensite start temperature M_s increases slightly with the austenitisation temperature up to 1173 K and decreases at 1523 K. This trend is due to the presence of nanometric carbides (Cr₂₃C₆), which were detected at TEM. They dissolve almost completely in austenite at 1523 K only, increasing the stability of austenite against the martensitic transformation. The effect of temperature in the range from 1073 K up to 1523 K is poor. As a consequence, the microstructural characteristics of hardened steels are very similar.     

Fatigue crack propagation in some PM steels

Abstract

Mechanisms of fatigue crack growth have been studied for a range of PM steels at relative densities of 0·90 and 1·0, for which strength, fracture toughness, and microstructural information was also available. It is shown that the Paris exponents for steady state crack growth are between 8 and 18 when ρ_r is ～0·9 but when ρ_r is ～1·0 the exponents are between 2·6 and 4·0, i.e in the range typical of wrought steels (2–4). At both densities, threshold stress intensities are between 5·5 and 10·8 MPa m_1/2 when R = 0·1. Combinations of these thresholds and yield strengths are comparable with those for wrought steels. When R = 0·8, reductions in threshold to between 2·7 and 5 MPa m_1/2 are attributed to crack closure effects. At ρ_r = 0·90, Fe–0·5C fails by progressive rupture of sinter necks. Astaloy A, with 0·2%C and 0·6%C, and Distaloy AB–0·6C have smaller plastic zone sizes and the cracks follow more difficult paths through particles as well as necks. When ρ_r is ～1·0, fracture is partially by true fatigue modes and partly by cleavage, the bursts of cleavage being more noticeable when K_maxis high.     

C-Mn-Si系超级贝氏体钢连续冷却转变（CCT）曲线

采用热膨胀法和金相法,通过Gleeble-1500热模拟试验机测定C-Mn-Si系低碳(/%:0.11C、1.15Si、1.85Mn、0.032Al、0.003 Ti、0.002 4N)和中碳(/%:0.35C、1.11Si、1.82Mn、0.041Al、0.002 Ti、0.004 2N)贝氏体钢在0.5～30℃/s的冷却速度下连续冷却时的膨胀曲线,确定相变点,并结合显微组织,借助Origin软件分别绘制出两种钢的连续冷却转变(CCT)曲线。结果表明,0.11%C钢当冷却速度≤1℃/s时获得铁素体+贝氏体+马氏体组织,冷却速度≥2℃/s时为贝氏体+马氏体组织,0.35%C钢冷却速度≥0.5℃/s即可获得贝氏体+马氏体组织;随碳含量增加,贝氏体和马氏体转变温度均降低。     

Φ500 mm大圆坯连铸机的生产实践

20钢(0.19%～0.23%C)Φ500 mm铸坯的生产流程为70 t转炉-LF-VD-Φ500 mm大圆坯连铸机从机械设备和二冷参数等方面进行研究和优化第1次生产试验连续浇铸8炉20钢,采用的优化工艺包括使用专用结晶器保护渣,控制过热度25～35℃,拉速0.30 m/min,比水量0.14 L/kg,结晶器电磁搅拌400 A/1.5 Hz等。检验结果表明,铸坯表面无可见冷疤、鼓肚等缺陷,中心缩孔0.5级,中心疏松1级,碳偏析≤1.12,钢中氧含量≤15×10^-6,氮含量≤65×10^-6,达到设计要求。     

Minimisation of melt spitting during continuous billet casting of low carbon steel

Abstract

Less critical grades of steel for long products are commonly produced from open cast billets. However, melt spitting from the mould is one of the potential safety hazards and may adversely affect the cast steel quality besides upsetting the caster productivity. In the present work, investigations have been carried out in an attempt to minimise the spitting problem during open pour casting. A typical fairly low carbon steel grade has been chosen for the study, as the problem of spitting was more severe for this steel grade as compared with other grades of steel. Influences of some of the relevant operating parameters were considered for investigation, and extent of spitting was quantified by determining the spitting for each case. Among the various variables considered, electromagnetic stirrer position in the mould was found to have the dominating influence. In addition, several other counter measures were adopted, and the spitting problem was minimised significantly.     

宝钢条钢产品的质量及品种开发

介绍了宝钢条钢产品的质量及品种开发。重点介绍了金属制品行业用的各类硬线钢、冷顶锻用碳素结构钢和合金结构钢、气保焊及埋弧焊用各类焊丝钢，采用转炉加ＲＨ真空脱气工艺生产的车轴坯，氧气瓶钢坯和齿轮，火箭弹用大规模圆钢等。