Microstructure and Mechanical Properties in Hot-Rolled Extra High-Yield-Strength Steel Plates for Offshore Structure and Shipbuilding

Key parameters for a thermomechanically controlled processing and accelerated cooling process (TMCP-AcC) were determined for integrated mass production to produce extra high-yield-strength microalloyed low carbon SiMnCrNiCu steel plates for offshore structure and bulk shipbuilding. Confocal scanning microscopy was used to make in-situ observations on the austenite grain growth during reheating. A Gleeble 3800 thermomechanical simulator was employed to investigate the flow stress behavior, static recrystallization (SRX) of austenite, and decomposition behavior of the TMCP conditioned austenite during continuous cooling. The Kocks–Mecking model was employed to describe the constitutive behavior, while the Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach was used to predict the SRX kinetics. The effects of hot rolling schedule and AcC on microstructure and properties were investigated by test-scale rolling trials. The bridging between the laboratory observations and the process parameter determination to optimize the mass production was made by integrated industrial production trials on a set of a 5-m heavy plate mill equipped with an accelerated cooling system. Successful production of 60- and 50-mm-thick plates with yield strength in excess of 460 MPa and excellent toughness at low temperature (213 K (–60 °C)) in the parent metal and the simulated coarse-grained heat affected zone (CGHAZ) provides a useful integrated database for developing advanced high-strength steel plates via TMCP-AcC.     

微铌处理550MPa热轧带钢的工艺研究

采用Gleeble3500热模拟试验机,研究了含铌Q345钢奥氏体静态再结晶行为、铌的碳氮化物在奥氏体和铁素体中的析出行为等实验,铌在钢中的强化作用机理。根据上述结论,邯钢2250热轧厂生产了以碳、锰为主要成分,并加入微量铌元素,将原热轧带钢Q345的屈服强度由345 MPa提高到550M Pa。对生产的微铌处理550 M Pa钢的组织和力学性能作了检验,在细晶强化、沉淀强化和相变强化等复合强化的综合作用下,热轧带钢屈服强度均达到550 MPa以上,塑性良好。     

L360管线钢热轧过程静态再结晶行为的研究

奥氏体再结晶行为是影响热轧钢带组织和力学性能的1个主要因素。在热模拟试验的基础上,采用应力松弛法和显微组织观察法,对含Nb和Ti的L360管线钢热轧过程奥氏体静态再结晶行为进行了研究,分析了应变、变形温度、应变速率、原始奥氏体晶粒尺寸对奥氏体静态再结晶的影响。利用线性回归分析,计算出试验钢静态再结晶激活能为203kJ;同时通过回归,得出试验钢热轧过程静态再结晶动力学方程。     

Thermomechanical processing of microalloyed powder forged steels and a cast vanadium steel

The effects of controlled rolling on transformation behavior of two powder forged (P/F) microalloyed vanadium steels and a cast microalloyed vanadium steel were investigated. Rolling was carried out in the austenitic range below the recrystallization temperature. Equiaxed grain structures were produced in specimens subjected to different reductions and different cooling rates. The ferrite grain size decreased with increasing deformation and cooling rate. Ferrite nucleated on second phase particles, deformation bands, and on elongated prior austenite grain boundaries; consequently a high fractional ferrite refinement was achieved. Deformation raised the ferrite transformation start temperature while the time to transformation from the roll finish temperature decreased. Cooling rates in the cast steel were higher than in P/F steels for all four cooling media used, and the transformation start temperatures of cast steels were lower than that of P/F steel. Intragranular ferrite nucleation, which played a vital role in grain refinement, increased with cooling rate. Fully bainitic microstructures were formed at higher cooling rates in the cast steel. In the P/F steels inclusions and incompletely closed pores served as sites for ferrite nucleation, often forming a ‘secondary’ ferrite. The rolling schedule reduced the size of large pores and particle surface inclusions and removed interconnected porosity in the P/F steels. Formerly Postgraduate Researcher in the Department of Metallurgy and Materials Science, UMIST/University of Manchester, United Kingdom     

Model for Microstructure Prediction in Hot Strip Rolled Steels

Phase transformations from work‐hardened austenite after hot rolling of steel are of paramount importance for achieving the required mechanical properties. Moreover, it influences the control of cooling process in the cooling table. This paper proposes semi‐empirical mathematical models for predicting such transformations in carbon‐manganese and microalloyed steels, subjected to transformations conditions leading to a microstructure constituted by ferrite and pearlite. To develop the models, dilatometric tests were carried out in a Gleeble 3500 machine, simulating the industrial processing conditions as close as possible, including the effect of coiling. Emphasis was placed on the application of a criterion for determining the start of pearlite formation, which was based on carbon enrichment of austenite during the ferrite formation. Based on comparisons between measured and calculated ferrite fractions in tested specimens, it was proposed to consider a transition of the thermodynamic conditions prevailing during ferrite formation, which depend on steel chemistry and transformation conditions. As a preliminary assessment of its predictability the model was applied to a typical industrial process condition for microalloyed steels in hot strip rolling. It was shown that the model was capable of predicting reasonably well the evolution of ferrite and pearlite during the cooling in and also during the subsequent coiling process.     

层流冷却过程低碳钢相变模型的适用性分析

选取了现有典型的C-Mn钢相变过程的物理冶金模型,包括5组孕育期模型、7组相变动力学方程模型、5组相变后铁素体晶粒尺寸模型.利用自行开发的组织性能预报系统软件模拟计算了在3组实际冷却工艺条件下各模型的奥氏体转变过程,并对各模型进行了评价.结果表明,对于所设定的成分和工艺条件,适用性较好的孕育期模型是Kwon所提出的模型...     

Microstructures developed during thermomechanical treatment of HSLA steels

One purpose of thermomechanical treatment of steels, for example the control rolling of plate, is to produce the finest uniform microstructure in the product to optimize its strength and toughness. To achieve this end requires control of the structual changes that occur during reheating, high-temperature (roughing) deformation, lower temperature (finishing) deformation, and austenite transformation. A study has been made of the effects of the deformation processing variables on the microstructural changes that occur in high-strength low-alloy (HSLA) steels in the temperature ranges in which complete, partial, or no recrystallization occurs. The experimental technique comprised a sequence of plane-strain compressions of specimens being cooled at rates controlled to simulate the rate of cooling of slabs being rolled to plates. The results show that in the complete recrystallization range a fixed pass schedule refines the initial grain sizes in steels of a wide range of compositions and initial grain sizes to about the same final size; the final recrystallized grain size decreases with deformation temperature (within the complete recrystallization range), increasing strain rate, and increasing draft,i.e., with any deformation parameter that increases flow stress; the solute content and initial grain size, which are fixed by reheat temperature, control the temperature at which complete recrystallization stops during hot rolling. Deformation in the “partial recrystallization” range causes duplexing in the austenite that cannot be removed by subsequent rolling, consequently rolling in this range should be avoided. During finishing, the height of the flattened austenite grains decreases with increasing finishing reduction, but at a rate less than the rate of decrease of specimen height, indicating that some recovery is occuring. The grain diameter of the ferrite formed from fine, elongated austenite is about half the austenite grain-boundary separation (measured in the throughthickness direction) indicating that the elongated boundaries are the primary nucleation sites for ferrite.     

Si-Mn系相变诱导塑性钢热轧过程中组织演变的预测

 建立了热轧相变诱导塑性(TRIP)钢奥氏体动态和静态再结晶、晶粒尺寸和流变应力预测模型,定量分析了不同硅含量对其再结晶的影响。结果表明,预测值与实测值符合较好;硅含量增加可抑制、减缓动态和静态再结晶发生,抑制奥氏体晶粒长大,提高流变应力、残余应变和位错密度,从而有利于铁素体相变的发生。     

TRIP钢在分段冷却过程中的相变行为

 建立了热轧低碳Si Mn系TRIP钢相变动力学模型。将该模型用于模拟热轧TRIP钢分段冷却过程中显微组织的演变,并定量分析了热变形和冷却速率对热变形奥氏体相变行为的影响。结果表明,预测值与实测值符合较好;降低终轧温度、提高终轧变形量,延长两相区缓冷时间都能促进铁素体相变,从而有利于提高残余奥氏体中的碳含量。     

Prediction of Microstructure and Mechanical Properties of Hot Rolled Steel Strip: Part I ‐ Description of Models

A metallurgical through‐process model is presented which describes the microstructural evolution and predicts the final mechanical properties of low carbon steel during hot strip rolling. Process models concern the thermal and deformation phenomena, which take into account the strain, strain rate and temperature distribution along the length of the strip. And the metallurgical models cover five modules, which are (i) austenitization of cast slab in reheating furnace, (ii) recrystallization of austenite in hot rolling, (iii) phase transformation of austenite‐ferrite in laminar cooling on the run‐out‐table, (iv) grain growth after coiling, and (v) final structure‐mechanical properties of products. Temperature is the main parameter and has dominant influence on the microstrutural evolution and the mechanical properties. The related temperature variation in hot strip rolling concerns air cooling, scaling, water cooling, heat transmission by roll contact, heat generation by deformation and friction. These complex factors are incorporated into the thermal models to simulate the temperature distribution along the length of the strip from the reheating furnace exit to the down‐coiler. A self‐learning algorithm is employed to improve the calculation accuracy and the computational temperatures are compared with the measured ones at typical locations. In the structure‐property relationships, two key process parameters (e.g., finishing exit temperature (FT7) and coiling temperature (CT)) are introduced in the model to consider the influence of morphology of microstructure on mechanical properties.     

Ferrite nucleation and growth during continuous cooling

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Recrystallization Behavior Design for Controlling Grain Size in Strip Rolling Process

To promote effectively dynamic recrystallization and obtain a homogeneous distribution of ultrafine grain size in strip finish rolling process,the behavior of static and dynamic recrystallization must be appropriately designed to provide an ultrafine austenite microstructure without mixed grain size.The design of rolling schedule was analyzed based on the control of the recrystallization behavior to achieve ultrafine grain size in the strip rolling process of niobium microalloyed steel.The experimental simulations were presented to validate the twice dynamic recrystallization design to achieve ultrafine grain size control.     

热轧低碳钢不同冷却模式下的相变模拟与验证

利用计算机模拟技术，研究了控轧控冷中输出辊道上冷却模式对低碳钢相变的影响。以低碳钢SS400为例，针对鞍钢热轧带钢厂1780生产线，计算了输出辊道上3种不同冷却模式下低碳钢的相变行为，包括铁素体转变开始温度、组成相体积分数随时间的变化以及铁素体体积分数沿带钢长度方向的分布。研究表明，冷却模式对输出辊道上带钢的温度一时间曲线、铁素体转变开始温度及组成相的演化过程影响较大，但对最终组成相的体积分数影响较小。     

The thermal and metallurgical state of steel strip during hot rolling: Part III. Microstructural evolution

A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill. The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening. The Institut de Recherches de la Sidérurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature. Formerly Graduate Student, The Centre for Metallurgical Process Engineering, The University of British Columbia Metallurgical transactions a     

400 MPa级C-Mn钢控轧控冷生产过程组织-性能的预测

建立了C－Mn钢在控制轧制和控制冷却生产中微观组织演变和力学性能预测的物理冶金模型，模型包括加热、再结晶、相变和力学性能四部分，分别描述了带热轧及冷却过程中的物理冶金现象，根据现场数据，计算了轧制过程奥氏体晶粒尺寸和再结晶分数的演变，预测了在不同工艺条件下连续冷却转变各相的体积分数和铁素体的晶粒尺寸等显微组织参数和相关的力学性能，预测结果和实测值吻合较好。     

Effect of Niobium on Transformations From Austenite to Ferrite in Low Carbon Steels

Niobium has an important effect on the transformation behaviour,grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels,with even a low content of niobium having a strong effect on the transformation rate from austenite to ferrite.However,the effects of niobium on transformation behaviour have not been fully characterised and understood to date.This paper examines in detail austenite grain growth as a function of austenitisation time in high strength low alloy (HSLA) steels with three different niobium contents,together with the effect of niobium on the isothermal transformation kinetics from austenite to ferrite as a function of temperature.It is shown that austenite has the slowest grain growth rate in the steel with the highest niobium content.When austenite grain sizes are consistent,the steel with the highest niobium content was found to have the slowest transformation rate from austenite to ferrite.     

Grain Size Dependence of Austenite Decomposition in Air-Cooled 16MnCr5 Steel

A hot-rolled and controlled rolled 16MnCr5 steel was analyzed after similar industrial cooling conditions. The hot rolled steel had a ferrite–bainite microstructure whereas the controlled rolled steel had a ferrite–pearlite microstructure. The prior austenite grain size was found to be the controlling factor based on a cooling analysis. The effect of prior austenite grain size on the bainite start temperature had to be considered in the transformation model.     

Grain Refinement of Nb Steels by Control of Recrystallization during Hot Rolling

Control rolling of Nb-bearing steels is employed to produce the uniform fine-grained product needed to meet the most stringent strength and toughness specifications. To produce such structures consistently requires a thorough understanding of the effects of compositional and process variables on austenite grain refinement at every stage of the hot rolling process. These effects were examined by means of a laboratory simulation of plate rolling. It was found that limiting the reheated grain size to about 100 μm (by lowering the reheat temperature or by adding Ti to the steel) is essential to assure complete recrystallization and refinement of the initial grain structure without an excessive amount of reduction at high temperatures. Because of the slow cooling rate at the slab center, this initial breakdown stage of rolling must be followed by a hold to allow the slab to cool to the lowest temperature at which recrystallization during rolling will be complete. This procedure will assure that the second roughing stage will produce the finest and most uniform recrystallized structure possible. This structure transforms to a very uniform ferrite-pearlite with a grain size of 10 to 15 μm. If, however, the fine recrystallized austenite is further rolled below the recrystallization temperature (control rolled) it will produce flattened austenite grains that transform to ultra-fine ferrite 4 to 6 μm in diameter. The uniformity of the final structure depends critically on the state of the austenite prior to grain flattening. The austenite must be completely recrystallized, since partially recrystallized regions produce duplex ferrite. Two attempts to change the control-rolling temperature range by modifying composition were unsuccessful. Increasing the Nb content from 0.05 to 0.10 to promote higher-temperature precipitation and boundary pinning did not raise the temperature at which grain flattening began. The addition of Ti to a Nb steel to remove soluble nitrogen and thereby prevent formation of NbCN in favor of the lower-temperature NbC did not promote recrystallization to lower temperatures.     

Nb-V-Ti微合金化高强度钢08MnCr连续冷却转变曲线和组织

利用ThermecMaster-Z热模拟实验机测定了一种Nb-V-Ti微合金化高强度钢08MnCr(S2)在910～1 200℃不变形(静态)和变形(动态)奥氏体0.05～30℃/s冷速下连续冷却转变(CCT)曲线,并分析和观察了对应的相变及组织。实验结果表明,提高轧后的冷却速度使Ar₃降低,导致钢的晶粒进一步细化;冷却速度大于10℃/s开始出现贝氏体转变。提高加热温度时相变温度降低,变形奥氏体相变温度较不变形奥氏体相变温度高。冷速较低时,铁素体晶粒呈多边形;冷速高时,铁素体晶粒多呈尖角形。