C. Isaac Garcia

The Microstructure Evolution of Nb, Ti Complex Microalloyed Steel during the CSP Process

The development of microstructure of Nb,Ti-bearing microalloyed steel during the CSP process was studied. Three samples were taken from the as-cast slab prior to tunnel furnace, intermediate bar after stand F2 and the hot band, respectively. In the as-cast slab, the average austenite grain size is 654 µm with a large size range from 150 to 2000 µm. In the intermediate bar after stand F2, the austenite grains are remarkably refined, but are heterogenous due to the incomplete recrystallization, which are in the size range of 23 to 116 µm. In the hot band is mainly non-polygonal ferrite. Microstructural heterogeneity exists in the hot band. It is attributed to the heterogeneous austenite grain size in the intermediate bar and the less rolling reduction after stand F2. With regards to precipitation, cubic TiN and fine precipitates less than 20nm are commonly observed in the as-cast slab and the intermediate bar. Some complex (Ti,Nb)(C,N) precipitates with a slightly larger size also exist. In the hot band, most particles are complex (Ti,Nb)(C,N) precipitates, in a shape of irregular or cruciform. The fine precipitates which can strengthen the ferrite matrix are seldom seen. These results are in good agreement with the size distribution of the precipitates determined using small angle X-ray scattering method. The chemical phase analysis reveals that 45%Nb of the total and 43%Ti of the total are still in solution in ferrite of the hot band.

A New Frontier in Microalloying: Advanced High Strength, Coated Sheet Steels

TRIP steels containing Mn, Si, Al, Mo, and Nb have been examined using a laboratory simulation of a continuous hot dipped galvanizing line. The evolution of microstructure has been studied as the steel passes through the various stages of CG line processing. Tensile strengths approaching 800 MPa and ductilities approaching 30% have been achieved in the 1.5Mn-0.5Si- 1.0Al-0.015Mo-0.03Nb system.

Optimization of Nb HSLA Microstructure Using Advanced Thermomechanical Processing in a CSP Plant

There are two obstacles to be overcome in the CSP production of HSLA heavy gauge strip and skelp, especially for API Pipe applications. First, the microalloying should be conserved by eliminating the high temperature precipitation of complex particles. Second, the heterogeneous microstructure that normally results from the 800 micron initial austenite in the 50mm slab as it is rolled to 12.5mm skelp must be eliminated to optimize the final microstructure and improve the final mechanical properties. Alteration in the hot rolling sequence can strongly homogenize the final austenite and resulting final ferritic microstructure. When coupled with a low coiling temperature near 550°C, the new rolling practice can result in Nb HSLA steels that can easily meet requirements for strength, toughness and ultrasonic testing in 12.5mm skelp gauges for X70 API pipe applications. The underlying physical metallurgy of these two breakthroughs will be presented and discussed in detail.

The Alloy Design and Thermomechanically Controlled Processing (TMCP) of Plate for High Pressure, Large Diameter Pipelines

Modern, cost-effective pipelines are moving beyond the API X70-X80 limits of the 1990s. Over the last few years, more interest has been placed on the X100-120 grades because they are potentially more economical to build and operate. To reach the impressive properties required by these new grades, the proper combination of alloy and rolling process design must be implemented, along with highly controlled interrupted accelerated cooling and hot leveling. This paper discusses some of the underlying physical metallurgy that is required and points out areas where further research and development would be useful.

The Thermomechanical Controlled Processing of High-Strength Steel Plate: A New View of Toughness Based on Modern Metallography

Modern thermomechanical controlled processing (TMCP) of advanced steels is now an important processing route in the production of engineering structures and products that are of value to society. The principles of TMCP are now practiced in the hot mill, cold mill and press forming shops around the world. Successful TMCP means that the proper metallurgical microstructure has been obtained in the required areas of the steel. The ideal microstructure is often defined by the correct phase balance and dimensions either of the parent austenite or final ferritic phase. Technological and economic demands have led to ever increasing levels of strength, especially for applications such as large diameter linepipe. The operative yield strengths in 18mm hot rolled plate have increased from X52(ferrite pearlite) in 1970 to X80(ferrite-bainite) today. The next frontier is the X100-X120 strength range, where bainitic or martensitic microstructures are required. It is clear that achieving a high-strength bainitic microstructure in heavy plate requires a high Carbon Equivalent value (C. E. II or Pcm), a rapid cooling rate, and a low water-end temperature. The requirement of high toughness and good weldability also means a low carbon content. This paper will describe the results of a research program where a steel of C. E. 0.56 and Pcm 0.23 was reheated, rough rolled for grain refinement, finish rolled for austenite pancaking, and direct quenched to below the Bs temperature. It was found that the strength and especially the toughness of the fully processed plates could not be explained using conventional metallographic techniques in conjunction with known structure-property relationships. However, the application of modern metallographic techniques based on FEG-SEM incorporating OIM led to microstructural characterization that more fully explained the observed mechanical properties. Of particular importance were the amount of MA micro-constituent, the crystallographic packet size of the bainite, and the high angle boundary character, especially the CSL boundaries, found in the microstructure. In the future, improved modeling of microstructural evolution and attendant mechanical properties will incorporate these important features.

Microstructural Analysis of Thermite Welds

Thermite welding is a simple and cost-effective process widely used in the field for rail repair and joining. Despite the well-accepted use of this technology, there is a major concern regarding the soundness of the weldments which are often found to be very sensitive to wear and cracking. In order to gain a better understanding of the structural factors that contribute to the performance behavior of thermite welds, systematic microstructural analyses of a series of welds was conducted. Of particular interest in this study was to carefully examine and compare the microstructure of the weld metal, heat affected zone (HAZ) and base metal of a series of thermite welded samples with different carbon content. The results of this work revealed the presence of proeutectoid cementite along the prior austenite grain boundaries at the three locations examined. In addition, microhardness evaluation of the welds revealed that substantial softening takes place in the HAZ, independent of the chemical composition of the rails or weld processing conditions. The presence of proeutectoid cementite along the prior austenite grain boundaries and the softening that takes place in the HAZ are two of the structural factors most likely responsible for the lower than expected wear behavior observed in welded or repaired rail steels. This paper will present and discuss the microstructural and processing factors associated with the formation of proeutectoid cementite and the causes leading to the observed softening.Copyright

Development of High Performance Steels for Rail Applications

Higher requirements of efficiency on railroad systems have set off (among other measures) higher axle load on rails. The increase in axle loads can contribute to a series of defects on perhaps the most unappreciated component of a railroad system. Higher axle loads can lead to excessive wear, fatigue and ultimately fracture of the steel rails. Therefore to answer the challenge demanded by the increase in axle loads the development of high performance steels for rail applications is of primary importance. A research program to study the microstructural aspects of near-eutectoid steels with improved mechanical properties and wear resistance was recently completed. The new high performance rail steels were developed through a combination of advanced alloy design-thermomechanical processing-and-controlled cooling. The mechanical properties exhibited by the new steels have exceeded the AREMA requirements for this type of rail steel application. The wear resistance of the newly developed steels was evaluated and the results obtained compared to commercial rails were superior under the testing conditions used in this study. The alloy design philosophy, thermomechanical processing and properties of the new steels will be presented and discussed in this paper.Copyright

Recrystallization Study of Nb-Bearing HSLA Steels After Laboratory Batch Annealing Using OIM-EBSD Techniques

The present work studied the recrystallization kinetics of a Nb-bearing high strength low alloy (HSLA) steel using a fully-computer controlled laboratory batch annealing (BA) process. This work was designed to study the effect of thermo-mechanical processing (TMP), transformation products, and the amount of cold deformation on the kinetics of recrystallization. The amount of deformation above and below the non-recrystallization (Tnr) temperature of the steel used in this study was investigated in terms of the grain boundary character distribution (GBCD) assessed in the hot band condition. The hot band condition was then subjected to 60 % cold deformation prior to the BA studies. The cold rolled samples were placed in the laboratory BA furnace using the simulated cold spot temperature (CST) annealing process. The selected annealing temperature was 650°C and the holding times varied between 15 minutes and 12 h. The central focus of this work was to understand how the TMP schedule affects the GBCD of the hot band; how the frequency of this initial GBCD changes with the amount of cold deformation; and how the steel composition, stored energy, GBCD, and annealing processing parameters influence the annealing behavior of the steel used in this study. Advanced microstructural techniques including orientation imaging microscopy and electron back-scattered diffraction (EBSD) provided the evolution of the GBCD, the changes in stored energy with annealing times. The results of this investigation clearly showed that when the TMP is conducted in a temperature range where the deformation of austenite favors twin formation, leading to higher levels of high angles grain boundaries (HAGB) and special coincidence site lattice (CSL) boundaries, the kinetics of recrystallization are increased. The results of this work were presented and discussed.

On the Microstructure of Plate Steels for API-5L X120 Applications

The very high strength now achievable in low carbon HSLA steel plates is caused by the formation of bainite or martensite during the post-hot rolling cooling in interrupted direct quenching. Modern electron optical examination, especially FEG-SEM, has allowed the microstructural features such as packet, block and lath dimensions and crystallography to be quantitatively determined. Several recent studies have attempted to relate the strength and toughness to these features, with limited success. However, one observation is clear, these microstructural features scale with the prior-austenite grain size and state of recrystallization. The role of microalloying, beyond grain refinement, remains inconclusive. This paper will discuss these microstructures and suggest possible ways of further refining them.

Simulation of Line Annealing of Type 430 Ferritic Stainless Steel

The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject of this paper. The steel was cold rolled 79%, then heated at 6 °C/s to the soaking temperature of 841 °C, which is just below the Ae1 temperature. During heating, specimens were quenched from selected temperatures between 650 and 841 °C and after various times at 841 °C. These quenched samples underwent metallographic examination and micro-hardness determination. The results indicated that under the prevailing experimental conditions, the hardness appeared to correlate strongly with the extent of recrystallization. The kinetics of recrystallization appeared to originate in the cold worked state, where three kinds of grain were found: (i) smooth elongated, featureless of α-fiber orientation {001}<100>; (ii) irregular fishbone grains of the γ-fiber orientations {111}<112> plus {111}<110>; and (iii) twisted grains of the η-fiber orientation {001}<100>. It was found that the twisted grains of the η-fiber were the first to recrystallize, with the fishbone grains of the γ-fiber second, and the smooth elongated, featureless grains of the α-fiber last. It was found that the grains of the α-fiber orientation {001}<100> and the η-fiber orientation {001}<100> were replaced with grains of the γ-fiber orientations as recrystallization progressed. These results are discussed in terms of recrystallization and texture development.