Advanced run-out table cooling technology based on ultra fast cooling and laminar cooling in hot strip mill

In order to meet the severe requirements of market and reduce production costs of high quality steels,advanced run-out table cooling based on ultra fast cooling(UFC) and laminar cooling(LC) was proposed and applied to industrial production.Cooling mechanism of UFC and LC was introduced first,and then the control system and control models were described.By using UFC and LC,low-cost Q345B strips had been produced in a large scale,and industrial trials of producing low-cost dual phase strips were completed successfully.Application results show that the ultra fast cooling is uniform along the strip width and length,and does not affect the flatness of strips.The run-out table cooling system runs stably with a high precision,and makes it possible for the user to develop more high quality steels with low costs.

Control and application of cooling path after rolling for hot strip based on ultra fast cooling

Hot rolled strip requires diverse and flexible control of cooling path in order to take full advantages of strengthening mechanisms such as fine grain strengthening, precipitation strengthening, and transformation strengthening, adapting to the development of advanced steel materials and the requirement of reduction-manufacturing. Ultra fast cooling can achieve a great range of cooling rate, which provides the means that the hardened austenite obtained in high temperature region can keep at different dynamic transformation temperatures. Meanwhile, through the rational allocation of the UFC （ultra fast cooling） and LFC （laminar flow cooling）, more flexible cooling path control and cooling strategy of hot rolled strip are obtained. Temperature distribution and control strategies under different cooling paths based on UFC are investigated. The process control temperature can be limited within 18 ℃, and the mechanical properties of the steels get a great leap forward due to the cooling paths and strategies, which can decrease costs and create great economic benefits for the iron and steel enterprises.

Precipitation Behavior of Nb in Steel under Ultra Fast Cooling Conditions

By measuring the expansion curves of a Nb bearing steel at different cooling rates by using Gleeble-3800 thermomechanical simulator, combining with metallographic analysis, different phase zones were determined. Also, precipitation behavior of Nb at different phase zones was investigated under ultra fast cooling conditions. The experimental results showed that adopting a proper deformation temperature, the ultra fast cooling process can restrain the precipitation of Nb at austenite phase zone. More quantities and smaller size precipitates of Nb can be found at the ferrite or bainite phase zone by controlling the ultra fast cooling ending temperature. With the increase of holding time at austenite, ferrite and bainite phase zones respectively, the volume fraction of precipitation, density, and average size of precipitates will increase obviously. With the decrease of early ultra fast cooling ending temperature, the density of Nb precipitates first increase（at ferrite phase zone） and then decrease（at bainite phase zone）, the volume fraction of Nb precipitation decreases and precipitates can be refined. The optimal early ultra fast cooling ending temperature is located at ferrite phase zone. The combination of high rolling temperature with early ultra fast cooling technology opens the way for new cooling schedules and makes the production of high strength steels easier and cheaper by making full use of Nb precipitation strengthening effect.

Ultra Fast Spray Cooling and Critical Droplet Daimeter Estimation from Cooling Rate

Spray cooling is an effective tool to dissipate high heat fluxes from hot surfaces. This paper thoroughly investigates the effects of spray parameters on the cooling time and cooling rate under varying inlet pressure using water as the coolant. Cylindrical samples of stainless steel with constant diameter, D = 25 mm, and thickness δ: 8.5 mm, 13 mm, 17.5 mm and 22 mm were investigated. Critical droplet diameter to achieve an ultrafast cooling rate of 300°C/s was estimated by using analytical model for samples of varying thickness. At an inlet pressure of 0.8 MPa, maximum cooling rates of 424.2°C/s, 502.81°C/s and 573.1°C/s were achieved for wall super heat ΔT = 600°C, 700°C and 800°C respectively.

Case-Based Reasoning(CBR) Model for Ultra-Fast Cooling in Plate Mill

New generation thermo-mechanical control process（TMCP） based on ultra-fast cooling is being widely adopted in plate mill to product high-performance steel material at low cost. Ultra-fast cooling system is complex because of optimizing the temperature control error generated by heat transfer mathematical model and process parameters. In order to simplify the system and improve the temperature control precision in ultra-fast cooling process, several existing models of case-based reasoning（CBR） model are reviewed. Combining with ultra-fast cooling process, a developed R5 CBR model is proposed, which mainly improves the case representation, similarity relation and retrieval module. Certainty factor is defined in semantics memory unit of plate case which provides not only internal data reliability but also product performance reliability. Similarity relation is improved by defined power index similarity membership function. Retrieval process is simplified and retrieval efficiency is improved apparently by windmill retrieval algorithm. The proposed CBR model is used for predicting the case of cooling strategy and its capability is superior to traditional process model. In order to perform comprehensive investigations on ultra-fast cooling process, different steel plates are considered for the experiment. The validation experiment and industrial production of proposed CBR model are carried out, which demonstrated that finish cooling temperature（FCT） error is controlled within±25℃ and quality rate of product is more than 97%. The proposed CBR model can simplify ultra-fast cooling system and give quality performance for steel product.

Microstructure and Mechanical Properties of Nb-B bearing Low Carbon Steel Plate: Ultrafast Cooling versus Accelerated Cooling

The microstructure and mechanical properties of low carbon bainite high strength steel plate were studied via different cooling paths at the pilot scale. There was a significant increase in mechanical properties, and notably, the yield strength, tensile strength, and toughness at-40 ℃ for the tested steel processed by ultra-fast cooling were 126 MPa, 98 MPa and 69 J, respectively, in relation to steel processed by accelerated cooling. The ultra-fast cooling rate not only refined the microstructure, precipitates, and martensiteaustenite（M/A） islands, but also contributed to the refinement of microstructure in thick plates. The large size M/A constituents formed at lower cooling rate experienced stress concentration and were potential sites for crack initiation, which led to deterioration of low-temperature impact toughness. In contrast, the acicular ferrite and lath bainite with high fraction of high-angle grain boundaries were formed in steel processed by ultra-fast cooling, which retarded cleavage crack propagation.

New research developments on ultra fast cooling technologies and new generation TMCP

Since the 21^(st) century,great attention has been paid to ultra fast cooling(UFC) technology in the whole world.The industries and the research institutions began to carry out investigations on basic theories and industrial applications.Since 2003,the RAL of Northeastern University has made some progresses on microstructure control theories,understanding of strengthening mechanisms and their industrial applications.In this paper,these achievements since the last Baosteel BAC in 2008 will be reported on the industrialization of UFC, strengthening mechanism,development of new steel grades,and so on.

The development and application of ultra fast cooling in hot-rolled

Ultra fast cooling is a new technology which used to control the hot-rolling strip cooling in recent years on the international developed.If suitably cooperated with a number of other new controlled rolling technologies,can achieve fast and accurate temperature control in the hot-rolled strip production process to obtain corresponding transformation microstructure and ideal mechanical properties.This article describes the technical principle and layout of ultra fast cooling in hot-rolled as well as application profiles in the major iron and steel enterprise in China and abroad.Carried out the layout of ultra fast cooling analysis on the adaptability of steel that install between the finishing mill and laminar cooling,on this basis,proposed the use of ultra fast cooling technology proposals.

Comparative Study of Tensile and Charpy Impact Properties of X70 and X80 Linepipe Steels After Ultra Fast Cooling Processing

Ultra fast cooling（UFC） processing after hot deformation was conducted on X70 and X80 linepipe steels. Tensile and charpy impact properties of both steels have been investigated in this work. The results have shown that the mechanical properties satisfy all the standard requirements of the X70 and X80 steels. UFC results in a presence of microstructure containing quasi polygonal（QF）, acicular ferrite（AF） and granular bainite（GB）. The alloying elements and UFC enhance the strengthening contribution caused by solid solution, dispersion, dislocation and precipitation strengthening. The size and distribution of precipitates in the linepipe steels are fine and dispersed. MA is also homogeneously dispersed due to UFC. Average grain size in the X80 steel is finer than that in the X70 steel. The volume fractions of secondary phases in the X80 steel are greater than those in the X70 steel. The X80 steel remains finer and more dispersed precipitates compared to the X70 steel. As a result, the tensile properties of X80 steel are higher than those of X70 steel. The Charpy absorbed energies of X70 and X80 steels at-10 ℃ reached 436 and 460 J, respectively. They reached 433 and 461 J at-15 ℃, respectively. This is mainly attributed to the presence of larger amounts of AFs in the X80 steel. A microstructure of AF for the X80 steel results in combining high strength and high toughness.

Nozzle Spray Diffusivity Changing Law for Ultra Fast Cooling in Hot Strip Mill

Slot nozzle and intensive nozzle can be used in ultra fast cooling equipment. The spray cooling method with higher water pressure can be taken in order to achieve ultra fast cooling for hot rolled strip. Water will be diffused after it is sprayed out from ultra fast cooling nozzle. Spray diffusivity will affect water velocity and penetrability of water into residual water layer on top of the strip,and then it will affect strip cooling effect. Water spraying process can be simulated by Fluent and some conclusions were obtained. Slot nozzle width and outlet velocity within setting range could not affect the length of potential core zone and the spray diffusivity. Intensive nozzle diameter and outlet velocity will affect the length of potential core zone and the spray diffusivity with different extent. These conclusions will provide referenced role for confirming ultra fast cooling nozzle size and distance between ultra fast cooling nozzle and hot rolled strip.

Mathematic modeling on flexible cooling system in hot strip mill

A novel cooling system combining ultra fast cooling rigs with laminar cooling devices was investigated.Based on the different cooling mechanisms,a serial of mathematic models were established to describe the relationship between water flow and spraying pressure and the relationship between water spraying heat flux and layout of nozzles installed on the top and bottom cooling headers.Model parameters were validated by measured data.Heat transfer models including air convection model,heat radiation model and water cooling capacity model were detailedly introduced.In addition,effects on cooling capacity by water temperature and different valve patterns were also presented.Finally,the comparison results from UFC used or not have been provided with respect to temperature evolution and mechanical properties of Q235B steel grade with thickness of 7.8 mm.Since online application of the sophisticated CTC process control system based on these models,run-out table cooling control system has been running stably and reliably to produce resource-saving,low-cost steels with smaller grain size.

中型H型钢生产线超快冷技术研究

通过对比国内H型钢超快冷技术的研发情况,结合某公司中型H型钢生产线制约因素及存在的问题,在综合考虑各方面影响因素的基础上,提出超快冷(气雾冷却)工艺的设计方案,旨在利用该技术解决冷床冷却能力不足带来的产品平直度超标、力学性能不稳定、锈蚀严重等问题。

中厚板UFC-ACC联动冷却控制系统的开发

在深入研究某中厚板厂新增超快速冷却系统的特点及功能的基础上,结合已掌握的德国西马克层流冷却控制技术,提出了两个冷却设备的基础自动化系统和过程自动化系统联动控制技术方案,为新增超快冷控制系统的嵌入找到了一个合适的切入点。基于UFC-ACC联动冷却控制系统,该厂已开发并批量生产了多个钢种(管线钢、高强钢、压力容器钢等)中厚板产品。

SUFC工艺下830MPa级高强精轧螺纹钢组织特征和碳氮化物析出

研究了表层超快冷工艺下(SUFC)830 MPa级高强精轧螺纹钢筋碳氮化物析出形态、强化方式以及组织特征。结果表明,高强精轧螺纹钢筋析出相有方形、球形和片状三种形态,析出相结构基本相同,均为(TixV1-x)(CN)。钢筋芯部析出物较多,细小、弥散,呈片状或细片状,尺寸50 nm左右;边部析出物相对较少,呈方形或球形,尺寸100 nm左右。该钢具有典型组织特征:边部回火索氏体,芯部铁素体+珠光体,在组织强化的基础上通过弥散析出、相间析出和铁素体基体中位错线上析出等析出强化方式,弥补了芯部软相和边部过冷强化相之间的强度差异,保证了轧材的性能稳定。

SUFC工艺在大规格棒材生产中的应用

首钢采用SUFC工艺试制了大规格(Φ40mm)棒材,本文通过实验确定了变形工艺参数,测定了试样的CCT曲线,并对棒材断面的径向温度场进行了模拟,分析了试样组织。工业生产表明,柔性控轧+SUFC+自回火工艺大幅提高了材料的强度,且生产成本较低。

Improvement of Hole-Expansion Property for Medium Carbon 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 440MPa.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.

Development and industrial application of ultra-fast cooling technology

A new-generation TMCP(NG-TMCP) with ultra fast cooling(UFC) as core technique was suggested instead of the conventional TMCP in which some problems are to be solved.And the ultra fast cooling technology has been successfully applied to the plate rolling production in Shouqin Metal Materials Co.Ltd.Industrial trials are reported for the production of X70 pipeline steels(about 70000 t) under UFC conditions.Pipeline steels with reasonably good properties have been produced on an industrial scale.UFC can improve the temperature uniformity,the mechanical properties and the plate profile of X70 pipeline steel.The performance features of the trial plates can meet the standard of X70 pipeline steel.Spot test results have shown that the qualification rate of shape control,the performance and charpy impact properties for the trial plates are 95%,97% and 98%,respectively.The impact values of X70 pipeline steel at UFC mode exceed the standard requirements of X80 pipeline steel.The productivity is enhanced because cutting quantity of head and tail of the plate is reduced.The enhancement of control accuracy of the red back temperatures is favorable for industrial production of X70 pipeline steel.The reduced production of the steel-making has been industrially achieved,since a new composition was designed by reducing the vanadium content to less than 0.045 wt.%.

Upgrade Rolling Based on Ultra Fast Cooling Technology for C-Mn Steel

By measuring the expansion curves of a C-Mn steel at different cooling rates by using an MMS-300 thermo- mechanical simulator, continuous cooling transformation curves were obtained. The new process ＂ultra fast cooling＋ laminar cooling＂ was simulated and the effects of ultra fast cooling ending temperature on microstructure had also been investigated. The hot rolling experiment was done by adopting ＂high temperature rolling-[-forepart ultra fast cooling＂ technologies at laboratory scale. The results revealed that ultra fast cooling can delay the decrease of disloca- tion density and refine ferrite grains. Diversity control of the microstructure and phase transformation strengthening can be realized by changing the ultra fast cooling ending temperature. With the decrease of ultra fast cooling ending temperature, the strength and toughness increase, but plasticity does not decrease obviously. The new technique can improve the yield strength by over 50 MPa. Therefore, the upgrade of mechanical properties of C-Mn steel can be realized by using ＂high temperature rolling＋ ultra fast cooling＋laminar cooling＂ technique. Compared with ＂low temperature rolling with large deformation degree＂ technique, this new technology can decrease the roiling force and in- crease the production efficiency.

Effect of Ultra-Fast Cooling on Microstructure of Large Section Bars of Bearing Steel

The ultra-fast cooling technology of large section bars and the microstrueture for different cooling patterns were studied by optical microscope, transmission electron microscope and energy spectrometer. The results indicated that the large section bars were passed through the zone of secondary carbide precipitation quickly by ultra-fast cooling technology （UFC） at instantaneous cooling rate of about 200 ℃/s and the finishing cooling temperature was higher than Ms. The lamellar spacing of pearlite decreased and the microhardness increased with decreasing the rereddening temperature. The precipitation of network carbide was restrained when re-reddening temperature was 690 ℃. And fine laminated pearlite was obtained through transformation of pseudopearlition that induced the reduction of the diameter of pearlite grain and refinement of the lamellar spacing of pearlite, so ideal microstructures of promoting spheroidizing annealing were obtained.

An On-line Finite Element Temperature Field Model for Plate Ultra Fast Cooling Process

Taking the element specific-heat interpolation function into account, a one-dimensional （l-D） finite ele- ment temperature field model for the on-line control of the ultra fast cooling process was developed based on the heat transfer theory. This 1-D model was successfully implemented in one 4 300 mm plate production line. To improve the calculation accuracy of this model, the temperature-dependent material properties inside an element were considered during the modeling process. Furthermore, in order to satisfy the real-time requirements of the on-line model, the variable bandwidth storage method and the Cholesky decomposition method were used in the programming to storage the data and carry out the numerical solution. The on-line application of the proposed model indicated that the devia- tion between the calculated cooling stop temperature and the measured one was less than ± 15 ℃.