Behavior of Transversal Crack on Slab Corner During V-H Rolling Process

The behavior of transversal cracks on the surface of the slab corner during vertical and horizontal （V-H） rolling process with flat vertical roll and groove vertical roll was simulated by explicit dynamic finite element method. The closure and growth of crack and the contact pressure on surfaces of the crack in contacting zone between slab and roll during rolling process were analyzed. The results showed that during vertical rolling process, when the groove vertical roll is used, the maximum contact pressure on surfaces of the crack is 115 MPa, and the closure of crack is stable when the flat vertical roll is used, the maximum contact pressure on surfaces of the crack is 70 MPa, and it fluctuates greatly. During horizontal rolling process, when groove vertical roll is used, the contact pressure becomes zero which may accelerate the growth of crack when flat vertical roll is used, there is still contact pressure. The cal culated results are in good agreement with the results of test.

Formation mechanism of internal cracks in continuously cast slab

In order to make clear the formation mechanism of centerline cracks incontinuously cast slabs, the form, distribution and other characteristics of the cracks wereanalyzed. The final solidification point, surface temperature of the slabs and strain in solidifyingshell were investigated. The results were that: (1) Five relatively low temperature zones exist onslab surface below the three water spraying nozzles and near the two edges, respectively, whichcorresponds to the places of centerline cracks and triangle-zone cracks. (2) Centerline cracks andtriangle-zone cracks occur because of weak secondary cooling, uneven cooling along slab width, andlarge variation of roll gap. (3) After minimizing the variation of roll gap and applying the newsecondary cooling pattern, the occurring frequency of centerline and triangle-zone cracks minimizesto zero.

Longitudinal surface cracks of thin slabs

Based on the production practice of medium carbon thin slabs in the CSP plant,the reasons and influencing factors for the formation of longitudinal cracks were investigated,and some industrial measures were taken to eliminate the cracks.The results show that the efficient solutions to reduce longitudinal cracks are improving the performance of the mold powder,stabilizing the mold heat flux,and maintaining a proper taper of the mold during casting.Proper pouring temperature and secondary cooling also play important roles in preventing longitudinal surface cracks.

Corner crack mechanism of boron-containing LCAK steel slabs

At Baoshan Iron ＆ Steel Co., Ltd., comer cracks of boron containing LCAK steel slabs had caused remarkable quality loss and mass flow disorder. With the help of fractography and thermodynamics analysis, the embrittlement mechanism of this steel grade was studied and the results are as follows： 1 The transformation from 3＇ to a starts at the austenite grain boundaries and a layer of thin ferrite film gradually forms around the austenite grains. Strain concentration will preferentially start inside the ferrite phase when the stress accumulates to a certain level. 2 The coarse BN particles acceleratedly precipitated at the γ/α interfaces further deteriorate the ductility of the ferrite film, and brittleness results in strain concentration and microvoid coalescence inside the ferrite film. Therefore the austenite grain boundaries are prone to intergranular failure. 3 The stoichiometry among Al, N and B is a very important factor influencing the hot ductility of this steel grade. By controling the B-to-N atomic ratio to above 1, all N can be fixed by B instead of A1. Thus coarsegrained steel is available and fewer grain boundaries and higher ductility can reduce the risk of comer cracks. （4） By adjusting the B-to-N atomic ratio,Baoshan Iron ＆ Steel Co.,Ltd. successfully reduced the number of cracks to nearly one tenth of that in the past and the hot tensile tests confirmed remarkable improvement in the hot ductility of this steel.

A Model for Diagnosing the Formation of Internal Cracks in Continuously CastSlabs

The internal cracks in continuously cast slabs are attributed to the excessive tensile strain occurring at the solidifying frontduring the continuous casting process. Based on the understanding, a model for diagnosing the formation of the internal cracks was established, in which the strains at the solidifying front caused by' bulging, straightening or unbending, and roll misalignment were calculated and compared with a critical strain value to estimate whether the internal cracks form. Moreover, the established model was appliedto a real slab caster to reveal the distribution of the strains in casting direction and its effect on the internal cracks. It was proved that themodel was reliable and useful for optimizing the operation of continuous casting.

Longitudinal surface cracks on continuous casting slabs of P and Cu containing container steel

The fact that the amount of the mold flux components differs at differentlocations on the cracking surface indicates that the longitudinal surface cracks are initiallyformed in the mold and are enlarged in the secondary cooling zone. Based on the hot ductilitymeasurement of two typical container used steels, it is known that the steels are in severeembrittlement state in the temperature range of 825-775 deg C. By means of increasing Cr/Ni platingthickness on the upper part of the mold, reducing mold heat flux, adopting new secondary coolingpattern, etc., the occurrence of the surface longitudinal cracks on the steel CC (continuouscasting) slabs has been significantly reduced.

COPLANAR CRACKS IN A FINITE TRANSVERSELY ISOTROPIC ELASTIC SLAB UNDER ANTIPLANE SHEARL STRESSES

An antiplane crack problem concerning a pair of coplanar cracks in a finite transversely isotropic elastic slab is considered. Using Fourier integral transform together with singular integral equation which can be solvel numerically by suing a collocation technique. Once the integral equation is solved, the relevant crack energy and stress intensity factors of the problem are given. The analysis present can be easily extended to include cases where there are two or more pairs of coplanar cracks in the slab.

TWO COPLANAR CRACKS IN A TRANSVERSELY ISOTROPIC ELASTIC SLAB SUBJECTED TO ANTIPLANE SHEAR STRESSES

The problem of a transversely isotropic elastic slab containing two coplanar cracks subjected to an antiplane deformation is considered. With the aid of an integral transform technique, we formulate the problem in terms of a finite-part singular integral equation which can be solved numerically, Once the integral equation is solved, relevant quantities such as the crack energy can be readily computed.

Preventing Occurrence of Transverse Corner Cracks on Nb, V and Ti Microalloying Steel CC Slabs

Based on their hot ductilities, Nb, V and Ti microalloying steels can be classified into two groups. The first group includes steels with lower carbon content (≤0.10%). Ductilities of steels of this group recover and rise with decreasing temperature when temperature lowers to below 825℃. Another group includes steels which contain more carbon (>0.12%) or contain more Nb and V. The low ductility temperature Region Ⅲ for steels of mis group extends to temperature as low as 725℃. The occurrence of the transverse corner cracks of the Nb, V and Ti microalloying steel CC slabs has be considerably decreased by stabilizing casting speed, increasing mold steel level automatic control ratio,enhancing caster segment radial alignment and adopting proper secondary cooling patterns to make slab corner temperature at straightening out off the low ductility temperature region.

Strain in solidifying shell of continuous casting slabs

Two-dimension unsteady heat transfer model was applied to obtain the surfacetemperature and the shell thickness of continuous casting slabs during the process ofsolidification. On the basis of which, the mathematical model of strain at the interface of solidand liquid steel was set up. Through which, the strain in the solidifying shell under normal andabnormal operation conditions was gained. The results indicate that the strain is small under thenormal operation conditions and the internal crack never happens. However, when the variation of theroll gap is above 2 mm, the strain caused by which is greater than that caused by bulging.Furthermore, the total strain exceeds the critical one and the internal crack is the result. So itis of great importance to maintain the fine state of continuous casting machine to avoid theappearance of internal crack.

Evaluation of the performance of mold powder for continuous casting of plate slabs

The No. 3 slab caster,which mainly provides slabs to the 5000 mm plate mill at Baosteel, was put into production in December,2004. The size of the biggest slab produced by this caster is 2300 mm in width and 300 mm in thickness. The designed output of the caster is 2.3 Mt/a. Slab surface longitudinal crack defects,which were related to the heat flux of the mold, frequently occurred in the early stage of the startup of the caster. As mild cooling powder is beneficial to the uniformity of the shell of initial slabs ,the concentration of stress is reduced, and the longitudinal cracking on the surface is avoided. This study evaluates the performance of several kinds of powder, and the results show that mold powder of high basicity, high crystallization proportion and low heat flux is to the benefit of the reduction of the longitudinal cracks on the surface and the defects of slabs.

板坯连铸机系统改造后边裂问题分析与改进

板坯连铸机改造后,铸坯连续出现南北两侧振痕不均匀和边裂问题,严重地影响了正常的生产流程,对产品质量控制也造成了很大影响。针对铸坯边裂,依据生产数据和生产跟踪情况,从工艺、设备工况和操作等方面分析,排除了生产工艺和操作因素所造成的影响,最终确定是由于结晶器起振中心位置不合适导致的。结晶器起振中心位置调整之后,边裂问题明显减少,铸坯质量得到改善,边裂发生率由10.5%降低到0.5%以下。

The optimization of mold fluxes used for casting the peritectic steel of heavy slabs at Baosteel

In order to solve the problem of the high surface longitudinal crack ratio of heavy peritectic steel slabs produced by the No. 3 continuous caster at Baosteel,the physical properties of the original mold flux and the optimized mold flux were compared in a comprehensive way by using analytical measures, such as a slag film heat-flow simulator, a thermowire molten flux crystallization tester and an X-ray diffractometer in the laboratory. The results reveal that one of the major reasons for the cracks is the poor heat transfer ability of the original mold flux. However, the optimized mold flux with a high basicity features a high crystallizing rate,low crystallization temperature and low heat-flow density. Therefore, the optimized mold flux is more suitable for casting peritectic steel by the heavy slab continuous caster. The test results show that the slabs produced by using the optimized mold flux had no surface longitudinal crack in four test casts, while the surface longitudinal crack ratio of the slabs produced by using the original mold flux was 5%. The optimized mold flux can effectively prevent slab surface longitudinal cracks from occurring.

高铁隧道内地下水作用下双块式无砟轨道道床板裂缝扩展研究

针对我国高速铁路隧道内CRTSⅠ型双块式无砟轨道施工阶段道床板开裂问题,运用有限元软件建立隧道-无砟轨道三维实体模型,研究地下水作用下隧道内无砟轨道各结构层位移隆起变化规律,并结合FRANC3D软件,在道床板结构受力最不利位置插入不同形状的预制三维裂缝,分析水压、裂缝长度对道床板裂缝扩展的影响。研究结果表明:(1)三角形荷载作用相较于均布荷载作用时,道床板位移隆起量虽有所减小,但地下水压的反复作用始终会影响轨道结构的平顺性,甚至导致道床板与仰拱填充间出现离缝;(2)裂缝长度是裂缝失稳扩展的控制因素,在排水系统完全堵塞时,建议将裂缝长度控制在200 mm以内;(3)水压为裂缝扩展的关键影响因素,同等条件下水压越大,道床板裂缝扩展越迅速,均布荷载下道床板裂缝尖端应力强度因子是对应三角形荷载下的1.2~1.4倍。

钢纤维细石混凝土-钢组合板抗弯性能试验研究

钢纤维细石混凝土是一种具有优异力学性能和良好工作性的高性能混凝土复合材料。本试验针对正交异性钢桥面铺装层裂缝病害问题,对4组采用不同钢筋配筋率和保护层厚度的横桥向钢纤维细石混凝土-钢组合桥面板进行了抗弯破坏试验,研究了组合板的荷载-挠度关系与裂缝开展特征;用等效截面法计算了开裂应力、裂缝宽度和钢筋应力,验证了钢纤维细石混凝土-钢组合板的抗裂性能。结果表明:60 mm厚钢纤维细石混凝土铺装层在小配筋率(2.6%)和低于规范要求的保护层厚度(15 mm)条件下呈现出多缝开裂的破坏形态,具备较高的延性和开裂应力,满足规范和工程应用的要求。

考虑混凝土面板裂缝的堆石坝三维渗流敏感性分析

为研究面板有裂缝时堆石坝渗透系数对其渗流特性的影响,以青海省某混凝土面板堆石坝为例,结合等效区块连续介质模型和Van Genuchten模型,提出考虑混凝土面板裂缝的饱和-非饱和渗流计算方法。通过设计正交试验开展堆石坝渗透系数敏感性分析,利用响应面法研究各部位渗透系数之间的交互作用对大坝总渗流量Q和下游坡脚渗透坡降J的影响。结果表明:坝体各分区渗透系数对Q、J的敏感程度不同,对大坝渗流特性影响显著的3个渗透系数为垫层渗透系数(K_(a))、主堆石区渗透系数(K_(c))、覆盖层Q_(3)^(a1)渗透系数(K_(e))。对于Q来说,各分区渗透系数的交互作用均不显著,而对于J来说,K_(a)与K_(c)、K_(a)与K_(e)的交互作用呈现出显著性。

带波形钢板的UHPC免模板湿接缝抗弯模型试验研究

针对预制装配式桥梁中桥面板湿接缝处的受力特点和现有接缝构造形式,提出一种以预埋波形钢板为底模的超高性能混凝土(Ultra-High Performance Concrete,UHPC)免模板湿接缝构造形式,解决接缝现场支模问题。为研究新型接缝的力学性能,设计了5片试验梁,以钢纤维含量为试验参数,开展四点弯曲全过程抗弯性能试验。结果表明:各试验梁主裂缝均位于预制段,发生弯曲破坏,接缝段未产生较大破坏;提高钢纤维含量可以增强UHPC-NC(Normal Concrete)界面黏结,提升接缝的抗裂性能,但对结构的抗弯承载力影响较小;钢纤维含量为1.0%的免模板接缝梁抗裂性能仅次于UHPC整体梁,同时具有结构简单、施工简化、造价经济等优点,说明该接缝构造形式运用于实际工程中是可行和可靠的;UHPC钢纤维含量建议取1.0%。

Q235连铸板坯角部裂纹形成机理及控制

为了解决Q235连铸坯角部裂纹缺陷,采用缺陷微观形貌分析和工艺参数影响规律统计分析相结合的方法研究了缺陷形成原因。结果表明:铸坯矫直前AlN大量析出是其产生角部裂纹的主要原因。基于AlN析出热力学分析,随着钢中铝和氮含量降低,AlN开始析出的温度下降,矫直温度范围内AlN析出量显著减少。考虑实际生产,确定了降铝措施作为改善铸坯角部裂纹缺陷的控制措施,现场应用后,Q235连铸板坯角部裂纹指数由3.59下降到0.19,钢中铝的质量分数平均下降126.64×10^(-4)%,合金使用成本降低。

装配式地铁车站叠合顶板受力性能试验研究

为减轻装配式地下车站结构中预制顶板构件的重量、增加结构整体性和防水性、提高施工现场的安装效率,提出一种由预制带肋底板和现浇混凝土叠合层组成的新型预应力混凝土带肋叠合顶板。针对无支撑和有支撑2种安装工况,设计制作4块长度为9 m的足尺叠合顶板试件。采用单调静力加载试验的方式分别研究叠合顶板构件在1阶段受力模式和2阶段受力模式下的受力性能。对预制带肋底板变形、叠合顶板试件的裂缝分布、跨中弯矩-位移曲线和钢筋应变等进行对比和分析,并结合试验结果修正叠合顶板最大裂缝宽度的理论计算方法。研究结果表明:叠合顶板整体受力性能良好,加载过程中叠合面未出现贯通裂缝和滑移破坏;2阶段受力叠合顶板在板底裂缝宽度达到控制限值时,对应的跨中弯矩较1阶段受力叠合顶板减少30.0%,在跨中位移达到变形限值时,对应的跨中弯矩较1阶段受力叠合顶板减少11.6%,说明其在设计时应更加关注变形和裂缝控制;2阶段受力叠合板试件板底和板顶纵筋分别存在着“拉应力超前”和“压应力滞后”现象,在配筋设计时可进一步优化配筋;修正的最大裂缝宽度计算方法可用于叠合顶板1阶段和2阶段受力工况的计算,并保证安全。研究结果为进一步丰富装配式地铁车站的构件形式以及新型叠合板试件的设计提供参考。

基于黏弹性近场动力学方法的素混凝土板冲击破坏形态分析

通过拉格朗日方程推导出一种基于单键双参数键型近场动力学的黏弹性模型,对冲击荷载下的素混凝土板的破坏过程进行模拟,采用单轴动态强度S准则得出物质点对间键的动态断裂准则。分别将低速落锤撞击下的素混凝土板的仿真结果与连续-不连续介质力学方法(CDEM)的模拟结果,高速子弹射击下的素混凝土板的仿真结果与试验结果进行对比。结果表明,黏弹性单键双参数近场动力学模型能合理地描述混凝土在冲击荷载下的破坏形态,且该模型能针对不同加载速率做出不同的反应,为冲击作用下的混凝土防护与混凝土的结构改造提供了一种有效的方法。