Anthony J. DeArdo

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.

Precipitation of NbC and Effect of Mn on the Strength Properties of Hot Strip HSLA Low Carbon Steel

Two low-carbon microalloyed steels were investigated: 0.07C-0.02Nb-0.33Mn and 0.07C-0.028Nb-1.1Mn. The two steels were hot rolled in a similar way, followed by either air cooling to room temperature (ACRT) or accelerated cooling (AC) to 680°C and held isothermally at 650°C for various times prior to ACRT. The mechanical properties of the steels in their fully-processed condition were evaluated. As expected, the steel with the higher Mn content showed the highest tensile (YS and UTS) properties for all processing conditions. The results of this study also indicated that the tensile properties of both steels remained unchanged, prior to and after the isothermal treatments. The central goal of this study was to conduct a detailed microstructural analysis which could help in the understanding of the observed mechanical properties. The results from the ACRT samples after hot rolling revealed two distinct precipitation sequences: (i) precipitation of fine NbC (5 nm in average size) in austenite, and (ii) interphase and multivariant precipitation of NbC in ferrite. The samples which were AC and then held isothermally also exhibited fine NbC precipitates. All the precipitates observed in these samples had formed in austenite. The absence of the Baker-Nutting (B-N) orientation relationship clearly confirmed that precipitation in ferrite did not take place. Atom Probe analysis of the precipitates revealed that the stoichiometry of the precipitates was close to NbC 0.9 . From the detailed TEM microstructural analysis, all the hardening increments contributing to the yield strength of the steels were computed the results showed a good agreement with the measured tensile properties.

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.

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.