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International Conference on Microalloyed Steels: Processing, Microstructure, Properties and Performance

Sponsored By:

AIST's Process and Product Metallurgy Operating Committee, Basic Metals Processing Research Institute (BAMPRI) ­ University of Pittsburgh, and TMS

Organizing Subcommittee:

A.J. DeArdo, University of Pittsburgh (conference co-chair) C.I. Garcia, University of Pittsburgh (conference co-chair) R. Asfahani, U. S. Steel M.G. Burke, Bechtel Bettis R.J. Glodowski, Stratcor D. Haezebrouck, U. S. Steel J. Fekete, General Motors L. Ruiz, Allegheny Technology

July 16­19, 2007 Soldiers and Sailors Hall Pittsburgh, Pennsylvania

About the Program

This symposium will cover recent applications of microalloying technology to a broad range of products including plate and pipes, strip, sheet, bar and forgings. Papers representing production, fabrication and end usage will be presented by national and international experts from industry, government and academia. The papers will cover a range from fundamental aspects through final applications. The goals of the technical program are to further educate experienced engineers and metallurgists while at the same time providing an introduction to new engineers and students.

Company Discount

Three or more individuals from the same facility attending any one seminar can receive a 10 percent discount per person. All registrations must be received together along with payment to qualify for the discount. Not applicable with any other discount.

Attention Nonmembers

Nonmember registrants will have the opportunity to join AIST on-site at the conference with 2007 membership dues waived. Membership is not automatic. A completed membership application must be returned to AIST.

Who Should Attend

Practicing engineers and metallurgists in production, fabrication and end usage; researchers and consultants; professors, graduate and undergraduate students.

Registration Confirmation

You will receive a confirmation letter in the mail once your registration has been paid in full. Please contact AIST at (724) 776-6040 ext. 642 if you have not received written confirmation two weeks prior to the conference.

Registration Fees

Advance registration by June 18, 2007: Member US$825, Nonmember US$975, Student US$200. Registration after June 18, 2007: Member US$925, Nonmember US$1,075, Student US$300. Registration fee includes Monday reception, Tuesday boat cruise, continental breakfasts and lunches Tuesday and Wednesday, continental breakfast on Thursday and a set of Conference Proceedings.

Cancellation/Substitution

If you must cancel, please fax a notice of cancellation to (724) 776-6619 and a refund will be issued. Cancellations received less than two weeks prior to the event will be nonrefundable. If you would like to send a substitute, a new registration form must be faxed for that person, indicating the replaced person on the form. Be certain that the membership status of each person is equivalent or note otherwise.

Hotel Accommodations

A block of rooms has been reserved at the Holiday Inn Select ­ University Center. Please call the Holiday Inn Select at (412) 682-6200 by June 25, 2007, to secure the AIST discount rate of US$99 per night for single/ double occupancy. To reserve your room online, visit www.aist.org.

Sponsorships

To sponsor a reception, lunch or break, please contact Jamie Furnival at (724) 776-6040, ext. 642.

Any audio and/or video recording of sessions is strictly prohibited.

Monday, July 16

4­6 p.m. Registration 5­6 p.m. Reception

Schedule of Events

Monday, July 16 4­6 p.m. 5­6 p.m. 7 a.m. 8 a.m. 8:15 a.m. Noon 1 p.m. 4:15 p.m. 5­9 p.m. 8 a.m. 9 a.m. 9:15 a.m. Noon 1 p.m. 5:45 p.m. 7:30 a.m. 8:30 a.m. 8:45 a.m. Noon 1:30 p.m. 3:30 p.m. Registration Reception Registration and Continental Breakfast Introductions and Opening Remarks Invited, Plate, Skelp and Pipe Steels Technical Session Lunch Plate Steels Technical Session Closing Remarks Gateway Clipper Reception Registration and Continental Breakfast Introductions and Opening Remarks Thermomechanical Processing/Physical/ Mechanical Metallurgy Technical Session Lunch Thermomechanical Processing/Physical/ Mechanical Metallurgy Technical Session Closing Remarks Continental Breakfast Introductions and Opening Remarks Sheet Steels Technical Session Lunch (on your own) Sheet Steels Technical Session Closing Remarks and Adjourn

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Tuesday, July 17

Wednesday, July 18

Tuesday, July 17 Invited and Plate, Skelp and Pipe Steels

7 a.m. Registration and Continental Breakfast 8 a.m. Introductions and Opening Remarks 8:15 a.m. AHS Steel for Automotive Applications Jim Fekete, General Motors Corp.

Implementation of advanced high-strength steels in light vehicles is accelerating, to achieve mass and cost reduction resulting from improved body and chassis structural efficiency. Experience with parts currently in production, and other parts still being developed, has cast light on issues that impact manufacturing performance and have yet to be solved. For example, DP steels fail in shear fracture

Thursday, July 19

during stamping operations. This failure mode is unusual for "typical" automotive sheet steels, and these failures are difficult to predict and don't correlate with traditional measures of formability (e.g., percent elongation and n-value). In addition, the consistency of material is not sufficient to support robust manufacturing processes. This includes coil/ blank shape as well as mechanical properties. Also, regional differences in material developments are not consistent with GM's global strategy. Applications of certain grades are blocked by lack of global availability. This paper will review the current status of these issues and their impact on usage of AHSS materials.

suitable combinations of high strength and adequate machinability, and in some cases very specific properties such as for fracture split connecting rods. Recent examples of steels developed for automotive forgings include air-cooled bainitic grades for crankshafts and high-yield-strength ferrite-pearlite grades for fracture splittable connecting rods, both of which feature higher-than-normal additions of vanadium. This paper will provide an overview of some of these recent developments in the context of automotive engine development trends.

8:45 a.m. Current Topics In Galvanizability of Advanced High-strength Steels Frank Goodwin, International Lead Zinc Research Organization

The availability of new grades of advanced highstrength steels has enabled automakers to continue improvement in vehicle weight, crash performance, fuel economy and other attributes while maintaining steel-based strategies. Hot-dip galvanizing continues to be the lowest-cost way of achieving a corrosion-resistant coating on these steels, but the requirement in many cases for specific multiphase microstructures together with evolving steel compositions has presented processing challenges. Current approaches in developing the understanding of how such steels can be processed to give a highquality coating, together with targeted mechanical properties, will be reviewed, along with an overview of concerns, considerations and opportunities for galvanizability of advanced high-strength steels.

9:45 a.m. Production Experience With New Heavyplate Grades for Bridges and Shipbuilding Using Microalloying Alexander Wilson, Mittal Steel USA

Over the past decade, product development activities have led to new steel grades for bridge and shipbuilding applications. A brief review of the development activities will be presented, particularly identifying the role of microalloy additions and the driving force behind the development effort. The production experience, properties and microstructures of the new grades will be detailed. Specific grades that will be discussed will include ASTM A709 grades HPS 70W and HPS 100W for bridges to 4-inch thickness and ASTM A945 grade 65 (HSLA-65) for U.S. Navy ships in thicknesses to 2.5 inches. Processing routes utilized include thermal-mechanical controlled rolling for HPS 70W plates to 2 inches and A945 plates to 1.25 inches, and quenching and tempering for thicker sizes and grades.

9:15 a.m. New Concepts in Microalloyed Bar and Forging Steels David Milbourn, Vanitec

The accelerating demand for engines with higher specific power outputs, reduced emissions and increased fuel efficiency has provided an opportunity for high-strength steel forgings, with a consequent requirement for the development of improved steels. These new steels must balance the needs of high fatigue resistance with acceptable through-process costs, and must therefore possess

10:15 a.m. Break 10:30 a.m. Niobium in Structural Steel and Long Product Applications Steven Jansto, Reference Metals Co.

New applications of niobium-microalloyed steels continue to be developed for advanced high-strength steel requirements in the structural plate and long products market. Customer demands are driving steel producers in the development of steels that exhibit better low-temperature toughness to resist brittle fracture in the structure, sustain higher loads per unit area, demonstrate improved fire-resistant properties and provide for improved weldability to assure defect-free welded joints. Niobium-bearing steel plate development and commercialization will be presented for a variety of large-scale structural applications such as bridges, ships, storage tanks and pressure vessels exhibiting these improved properties. Recently, Nb-bearing steel grades are also being applied at an accelerated pace in long products requiring these improved mechanical properties in more demanding value-added applications. Specific unique applications in reinforcing bar for earthquakeprone zones, bars and sections for construction, and rail steels will be described.

non-destructive test reflections. It is now clear that high strength, high performance HSLA steels can be produced in thick gauges by the EAF plus thin slab casting route. This paper will review both some recent advances as applied to skelp production for ERW pipe and their underlying physical and mechanical metallurgy.

Noon Lunch

Plate Steels

1 p.m. Quantitative Characterization of the Microstructure of Bainitic Linepipe Steels Doug Boyd, Queens University

11 a.m. Development of Higher-strength Ultrathin Strip Cast Products Produced via the CASTRIP® Process 11:30 a.m. Optimizing Microstructure and Properties of Microalloyed Steel Using the CSP Process Isaac Garcia, University of Pittsburgh

Heavy gauge X-70 skelp for high pressure ERW pipe for oil and gas transmission has been produced by the CSP process by Nucor Steel Berkeley (NSB). Niobium-bearing HSLA steel skelp 8-12 mm thick has been produced as hot band coils after coiling from 550°C. During the trials leading to this new product, a new intensified hot rolling practice practice was developed, where heavy gauge skelp of Nb HSLA steel for API pipe can be rolled resulting in a uniform ferrite microstructure, impressive improvement in strength and toughness, and the absence of aberrant

To obtain the unique combination of strength, toughness, weldability and cost required for highstrength linepipe steels (X90­X120), it is necessary to exploit all the advanced technologies of steelmaking, microalloying and thermomechanical processing (including on-line accelerated cooling). The microstructures of current X90­X120 steels are predominantly fine bainite. Two distinct types of bainite are identified: (1) conventional bainite (CB), which nucleates on prior-austenite boundaries and grows as packets of ferrite laths with carbon-rich interlath martensite/austenite phase, and (2) acicular ferrite (AF), which is intragranularly nucleated ferrite laths with dispersed particles of M/A. In the present study, an experimental heat of a commercial X-80 steel composition was prepared and rolled to 15-mm-thick plate. Two different accelerated cooling treatments were employed to vary the relative amounts of CB and AF in the final microstructure. The mechanical properties of the plates were determined by tensile and Charpy impact tests. Three microstructural

quantities were determined for the two steels: (1) texture components by XRD, (2) dislocation density by XRD and (3) crystal orientation domain size by EBSD. The results are discussed in terms of the contributions of individual microstructural features to yield strength and the ductile-brittle transition temperature.

1:30 p.m. The Effect of Composition on Phase Transformation Temperatures and Hardenability in Direct Quenching Mahesh Somani, University of Oulu

This research was concerned with the effect of steel composition on phase transformation temperatures and hardenability under direct quenching conditions. The main effects of seven alloying elements -- C, Mn, Cr, Ni, Mo, Nb and V -- were determined by using eight experimental steels based an orthogonal Taguchi L8 matrix. All steels contained constant additions of B, Ti and Si. Using this screening approach, only the main linear effects of the alloying elements are considered; interactions between the elements are ignored. CCT diagrams covering cooling rates 1.5­48°C/s were determined using Gleeble simulations with or without controlled deformation below Tnr. The effects of strain below Tnr and the alloying elements were clearly revealed. The start of the bainite transformation was modeled as a function of chemical composition and cooling rate. The model showed convergence of Ar3(bainite) with the martensite start temperature (Ms) at high cooling rates. The data in the CCT diagrams was used to derive hardenabilities expressed as equivalent ideal critical diameters, DI. These values of DI were, however, very different from those predicted using the methods given in Appendix X2 of the ASTM standard A 255. This led to the conclusion that the standard approach to predicting hardenability was inapplicable for boron-containing steels transformed from austenite strained at low temperatures prior to cooling.

Cu addition was investigated. The microstructure of the steel after water quenching and aging at 913 K during 0.6­100 hours was observed with optical, transmission and scanning electron microscopy. EDS combined with STEM and SEM technique was used to analyze chemical composition of the precipitates. The aim of the study was to describe the role of copper in the precipitation process, and to make use of copper in precipitation hardening. The quantitative determination of the average diameter of precipitates and interparticle spacing was studied to calculate the precipitation effect on yield strength according to a modified equation of Orowan-Ashby's type. Timetemperature parameters for the optimum mechanical properties of the investigated ultralow-carbon bainitic steel were established. That steel belongs to a group of advanced structural steels that will be used by the U. S. Navy.

2:30 p.m. Break 2:45 p.m. Metallurgical Aspects And Latest Process Technological Development in Accelerated Cooling and Direct Quenching of Heavy Plate Dirk Schmidt, SMS Demag AG

The first part of this paper reviews metallurgical main aspects of thermomechanical controlled processing. The differences between accelerated cooling (ACC) and direct quenching (DQ) are outlined. Effects of the most important cooling variable on mechanical properties are shown. Strengthening mechanism and underlying microstructures are covered. Typical plate applications of both cooling variants are brought up. The second part explains some physical aspects of plate cooling. Design, engineering and operational aspects of modern cooling facilities are explained. Of particular importance for homogeneous cooling are process control features in combination with the most modern process model.

2 p.m. Precipitation-Strengthening in HSLA Steel With Copper Addition Andrzej Lis, Czestochowa University of Technology

The effect of aging parameters on the microstructure evolution and the precipitation-strengthening yield strength increase of HN3M1.5Cu steel with 1.4 percent

3:15 p.m. Microstructure and Properties of HSLA Steels With/without Boron Addition for Thermomechanical Processing Sihai Jiao, Baoshan Iron & Steel Co. Ltd.

Two heats of very low-carbon steels microalloyed with niobium and titanium and with copper addition were melted in a laboratory. The microstructure

and properties of the steels were investigated in thermomechanical processing state and with/without boron addition. After being controlled rolled, tens of seconds relaxed and accelerated cooled, the steel without boron addition achieved yield strength of more than 520 MPa, tensile strength of 675 MPa and Charpy impact absorbed energy around 240 J at ­60°. Compared with the steels without copper addition reported in literature, the strengths are about 20 MPa higher. When the steel addition was 0.0013 percent boron, its yield strength increased to 710 MPa, tensile strength to 840 MPa and Charpy impact absorbed energy remained at 230 J at ­60°. The microstructure observation results show that the steel without boron addition contains mainly the acicular ferrite plus a small amount of bainite, while bainite is the major component of the microstructure of the steel with boron addition. Therefore, it has been proven by the experimental results that the boron effectively suppressed transformation under the condition of experimental process and the composition design of tested material. The precipitation state of niobium and titanium were also observed and discussed.

66 J at ­60°. It seems there is no positive effect on toughness when addition an extra 0.2 wt% Mo to the base steel, but lower yield ratios and somewhat higher tensile strengths can be observed in this case under both DQ and DQ/T conditions. After tempering, all three steels improved in toughness, especially in the steel with higher Mn content and in the steel with Mo addition. Microstructure and precipitation were also observed and discussed.

4:15 p.m. Closing Remarks 5­9 p.m. Gateway Clipper Reception

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3:45 p.m. Effect of Alloying Elements on the Microstructure and Properties of Copper and Boron-bearing Very Low-carbon Steels Treated with DQ/T Xiaojun Liang, University of Pittsburgh and Baoshan Iron & Steel Co. Ltd.

Based on the chemistry design of microalloyed with both Nb and Ti and with Cu and B addition, investigation has been carried out on the effects of variation of C, Mn, Mo contents on the microstructure and properties of the very low-carbon steels treated with control rolled, tens of seconds relaxed, direct quenched and tempered. More than 710 MPa yield strength, 840 MPa tensile strength and around 230 J Charpy impact absorbed energy at ­60°C were achieved on the referent steel, which contains 0.03 wt% C, 1.50 wt% Mn and no Mo under direct quenching state. Tempering at 500°C caused a 20 MPa increase in yield strength and a 60 MPa decrease in tensile strength and no significant change in toughness. A 1.78 wt% Mn addition leads to 50~70 MPa correspondent increases in strengths in both DQ and DQ/T states without obvious changes in toughness. A 0.069 wt% C addition results in a more than 320 MPa increase in tensile strength, which reaches to 1,165 MPa. Although decreasing obviously, the Charpy impact absorbed energy remains above

Wednesday, July 18 Thermomechanical Processing/ Physical/Mechanical Metallurgy

8 a.m. Registration and Continental Breakfast 9 a.m. Introductions and Opening Remarks 9:15 a.m. Influence of Strain-induced Precipitation of Nb(CN) on the Recrystallization Behavior of Austenite During Multipass Hot Deformation Eric Palmiere, The University of Sheffield

The precipitation of microalloying elements such as Nb plays a major role in controlling the final

microstructure and hence the properties of HSLA steel products. The pinning force exerted by fine precipitates on dislocation microbands suppresses the process of recrystallization, thus enabling the retention of the deformed austenite structure from one rolling pass to the next. As a result of particles inhibiting recrystallization, which subsequently leads to a higher temperature where austenite can exist and remain fully unrecrystallized (T5%), industrial controlled rolling practice can occur at higher temperatures and subsequently at smaller rolling loads. Besides increasing the T5%, the presence of precipitates also increases the grain-coarsening temperature, which plays an important role during controlled reheating practices. Considering the enormous practical significance of the precipitation process, it is essential to understand the mechanisms of precipitation during multipass hot rolling of microalloyed steels. In this research, the kinetics of precipitation was studied over multiple deformation sequences. Multipass deformation tests were performed at different conditions of temperature, time and strain. The precipitate evolution was studied using electron microscopy, and a mathematical model has been developed for the precipitation behavior during multipass deformation.

10:30 a.m. Increasing the Non-Recrystallization Temperature of Nb Microalloyed Steels by Mo Addition Jose Rodriguez-Ibabe, CEIT

Mo microaddition is a common practice to increase strength and toughness in low-C steels by its effect in promoting low-temperature transformation products. In addition to this benefit, Mo can also interact with the softening mechanisms during hot rolling. On the other hand, Nb is well recognized by its availability to retard recrystallization both by solute drag and strain- induced precipitation, leading to pancake austenite microstructures. The present work aims to investigate the effect of Mo addition to Nb microalloyed steels on austenite conditioning in terms of the non-recrystallization temperature (Tnr). Multipass torsion tests, performed under continuous cooling conditions (in the range of 1,150­1,770ºC), were used for thermomechanical simulation. A total of six steels (0.05%C) with two Nb contents (0.03 and 0.06%) and three different levels of Mo additions (0, 0.16 and 0.31%) were investigated. Initial situations corresponding to those present in TSDR route were selected (coarse austenite grain size and high supersaturation level of microalloying elements). For all the steels investigated, the Tnr value decreases with increasing the pass-strain. On the other hand, an increment of the values of the Tnr, between 40 and 50ºC, was found with the addition of Mo to the steel with 0.03%Nb. However, no effect of Mo was observed when the Nb increased to 0.06%. The analysis performed suggests that solute drag may be the main controlling mechanism in the conditions selected in the case of Nb-Mo steels, while strain induced precipitation defines the Tnr temperature in the majority of the tests done with Nb microalloyed steels.

9:45 a.m. The Influence of Grain Refinement and Precipitation Processes on Mechanical Properties of C-Mn-Nb Heavy-plate Steels Andrzej Lis, Czestochowa University of Technology

Thermomechanical rolling processes are designed to produce a desired microstructure via control rolling without subsequent heat treatment. Evaluation of the microstructure development during industrial rolling of 1-inch heavy plate of C-Mn-V-Nb steel was done with computer modeling. Observation of thin foils revealed NbCN and VCN precipitates in fine ferrite grains. The strengthening effect from grain size refinement and the precipitation process accounted for yield strength increase at low finish rolling deformation temperature.

11 a.m. Softening Kinetics of Microalloyed Highstrength Steel Grades During Hot Forming Carl-Peter Reip, SMS Demag AG

High-strength microalloyed steel grades have been successfully tried and tested for more than 30 years as high-quality feedstock for the production of tubes. As a rule, the market demands tube materials that feature sufficiently high ductility at low temperatures and good welding behavior while having adequately high strength. The resulting microstructure is largely

10:15 a.m. Break

determined by, among other things, the metallurgical phenomena taking place during hot forming, such as recrystallization, grain coarsening, precipitation and transformation. By analogy with the near-net-shape CSP® casting process, compression test specimens were partly melted in a Gleeble simulator and, after solidification and controlled cool-down, were directly formed as they came out of the casting heat. The investigations make it possible to characterize the softening kinetics of microalloyed higherstrength steel grades as a function of temperature, deformation rate and deformation history, and hence allow optimized definition of pass schedules for industrial hot rolling.

is applicable to slowly cooled or high-temperature coiled low-carbon steels. For the study, several final gauge thicknesses, as well as different Nb contents and rolling start temperatures, were considered. Based on the results, the comparison of final ferrite grain size distributions allows for the determination of optimum processing conditions, so as to exploit all the benefits of Nb microalloying, and to fix rolling start temperatures for each final gauge thickness. As the model works with grain size distributions instead of mean grain size values, microstructural heterogeneities can be also predicted.

Noon Lunch 1 p.m. Strain Rate Dependency of the Dislocation Substructure Formation in HSLA and IF Steels Monika Stefanska-Kadziela, AGH University of Science and Technology

In order to clarify the strain rate influence on the microstructure evolution and mechanical behavior of microalloyed steels, axisymmetrical compression tests were conducted in a strain rate range of 0.001­ 2,500s­1. Microalloyed steels and applied testing conditions allowed for the analysis of the evolution of dislocation structure in an inhomogeneous, dynamically strengthened matrix. In these conditions, the dislocation movement is difficult (both via precipitations and solid solution), and forming substructures are significantly different from those observed in statically deformed steel. The compression tests were performed at room temperature using static and dynamic testing machines, drop-weight and SHPB. Inverse analysis was successfully applied to identification of the dislocation-based flow stress model for investigated steels and tests. The analysis of microstructure of deformed materials was performed using transmission electron microscopy (TEM). It was found that the dislocation structure depends on initial microstructure and chemical composition of the studied steels. The constitutive description of investigated correlations is based on the generation and annihilation processes of dislocations, both in the cell interiors and the cell walls. On the basis of measurements and characteristics of dislocation structure, it is possible to determine the effect of strain rate on the dislocation cell size and the total dislocation density.

11:30 a.m. Analysis of the Influence of Rolling Conditions and Nb Microalloying on the Final Ferrite Microstructure in Steels Processed by TSDR Route Beatriz Lopez, CEIT

In several previous works, the evolution of austenite microstructure during thin slab direct rolling (TSDR) was modeled, taking into account all the microstructural mechanisms (dynamic, static and metadynamic recrystallization, strain-induced precipitation) that could take place during deformation. This model takes the as-cast austenite grain size distribution as input, and provides austenite grain size distribution and retained strain after any rolling pass. Making used of the model, the processing maps for optimum austenite conditioning were derived. This model has been implemented with a new module to predict the final ferrite microstructure obtained after transformation. The transformation model uses the austenite grain size distribution derived from the rolling module as input and provides the size distribution of ferrite grains after cooling. The model

1:30 p.m. High-temperature Processing and Accelerated Cooling of X65 Linepipe Steel on an Underpowered Plate Mill Kevin Banks, University of Pretoria

Thick-walled API X65 linepipe steel has been successfully manufactured on a plate mill with limited force capacity using high-temperature processing (HTP) and accelerated cooling (ACC). To achieve the necessary cooling rates, a low-cost watercooling facility was installed to increase the strength of thermomechanically controlled rolled microalloyed plates. For good shape and mechanical properties, the combination of rolling and ACC parameters included sufficient reduction below 950°C, small pass strains, finish rolling at relatively high temperatures, water cooling at reasonably slow rates and cooling stop above 600°C. The ferrite-pearlite microstructure was mixed but, together with precipitation of fine carbonitrides in ferrite, provided sufficient strength and excellent DWTT toughness. Maintaining a fairly constant force value contributed to producing plates up to 2.4 m wide with good shape after rolling. This work showed that, even in a relatively underpowered plate mill, with the addition of some basic ACC plus intelligent alloy and schedule design based on the fundamentals, one can produce relatively thick X65 linepipe steel with good DWTT toughness.

bars. Parameters of thermomechanical treatment were chosen on the basis of the DCCT diagrams. The experiments were carried out with the use the simulator of metallurgical processes Gleeble 3800, which makes it possible to apply the high strain and cooling operation, which can simulate the real manufacturing process. Samples from continuously cast slabs were heated to the temperature for the start of the rolling process and deformed according to the roll pass plan of the rolling operation.

2:30 p.m. Break 2:45 p.m. The Effect of Nb-V Combined Microalloying on Medium-carbon Steels Under Hot Deformed Conditions Hongtao Zhang, Central Iron & Steel Research Institute

Research and development was carried out on the effect of Nb-V combined microalloying on mediumcarbon steels under hot deformed conditions. The research concept was to search for physicometallurgical changes and changes in mechanical properties caused by Nb microadditions to a sole V-microalloyed medium-carbon steel based on the 49MnVS3 forging steel grade. A series of experimental steels V and/or V+Nb microalloyed with lower carbon content and increased Mn content was melted. A thermomechanical processing simulating the hot forging and air-cooling of 80-mm-diameter bar was conducted. After examining the microstructures by using optical microscope, SEM, TEM, and EDS analysis techniques, and after mechanical property testing, the effects of combined Nb-V microalloying on the microstructure, precipitation and mechanical properties were revealed in comparison with sole Vmicroalloyed steel. The results are summarized in this paper. New steel grades 35MnVNb and 35MnVNbS have been developed. The applications of these steels under hot forging or hot rolling conditions will be introduced.

2 p.m. The Physical Simulation of the Bar Rolling Process for Steel With Microalloyed Addition of Nb, Ti and V Marcin Knapinski, Czestochowa University of Technology

The result of the thermomechanical rolling process is an optimal structure with a special hardening phase and substructure that guarantee the growth of mechanical properties in the final product. The properly designed technology requires the knowledge of many factors that influence the material after the process. The factor that may have the strongest effect on microstructure is the addition of microalloys like Nb, Ti and V, which can accelerate ferrite formation and carbide precipitation, which favor the fine grain microstructure. This paper presents results of the investigation of thermomechanical treatment and the parameters that influence the structure and mechanical properties of steel with Nb, Ti and V microalloyed steel, destined for reinforcement

3:15 p.m. The Analysis of the Mechanical Properties of the Steel With Microalloyed Nb, Ti and V Additions After Thermomechanical Treatment Bartosz Koczurkiewicz, Czestochowa University of Technology

The mechanical properties of a final product are determined during the processes of plastic working and cooling. In industry, there are times when rolling conditions cannot be obeyed because of the technical environment on the production line. The required mechanical properties can be achieved only with the proper temperatures of the finishing rolling process and cooling rate. Changing the value of these parameters provides different mechanical properties of the same material. That is why knowledge of the structure during the various conditions of the hot working process allows one to control the mechanical properties of the final product. To properly select cooling parameters, the influence of cooling rate on the phase transformation temperature and carbides formation are essential information. In this work, the austenite decomposition during continuous cooling for S355J2-NbTi, S355J2-Nb and S355J2-V steels was determined. Those steels are used to produce reinforcement bars. To describe the strain history, DCCT diagrams were also constructed using the experiments on the DIL805A/D dilatometer machine with the plastometer attachment.

4:15 p.m. Complex Precipitates in the Hot Band of Microalloyed Steels Produced by Using the CSP Process Wang Ruizhen, CISRI 4:45 p.m. Effect of Ti Versus Al Alloying for Deoxidation on Microstructural Evolution in Low-carbon High-manganese Steel Naoki Kikuchi, Carnegie Mellon University

The effect of deoxidation inclusions resulting from Al and Ti additions on microstructure evolution due to solidification, phase transformations and austenite grain growth in low-carbon steels (0.07% C, 0.9% Mn) was investigated. The purpose is to find methods to refine the microstructure through inclusion control during casting in direct hot charge processes where TMPC is not possible. Deoxidation tests were carried out with 400 g-scale vacuum furnace. Either aluminum (0.05 wt%) or titanium (0.05, 0.03 or 0.015 wt%) was used as the deoxidation agent. The oxide inclusion sizes in all the Ti-killed steels were smaller, and inclusion densities were higher than those in the Al-killed steel. Solidification structure was finer with increasing inclusion density, and thus a finer structure was found for the Ti-killed steels. Confocal scanning laser microscopy (CSLM) and differential scanning calorimeter (DSC) were used to study differences in solid-state microstructural evolution. Austenite grain growth under isothermal conditions was in terms of boundary mobility, and final grain size was lower in the Ti-killed sample than for the others. With regard to austenite decomposition, the resulting austenite decomposition structure was finer for the Ti-killed sample due to a higher Widmanstätten lath density as a result of precipitation at particles.

3:45 p.m. Nb Precipitation Kinetics in the Austenitic Range in a Nb-Ti-V Microalloyed Linepipe Steel Martin Buhler, Tenaris Center for Industrial Research

Niobium is used in microalloyed steels to control the grain size during thermomechanical processing and to provide precipitation strengthening. Nb precipitation also plays a fundamental role in phase transformation kinetics. In this work, a study of the Nb precipitation kinetics in the austenitic range is performed by the stress relaxation technique. Tests were performed in a Gleeble thermomechanical simulator using different precipitation temperatures from 860 to 950°C. During the tests, the samples were heated at 1,170°C in order to dissolve all the Nb carbides, cooled down to testing temperature, compressed and held at this temperature for 30 minutes, and cooled down to room temperature. Different contents of Nb in solid solution prior to transformation were additionally obtained by reducing the holding time at temperature just after deformation. The phase transformation kinetics were derived from measurements of the electrical power needed to keep the sample cooling rate fixed. The results obtained were used to fit the parameters of a precipitation model based on classical nucleation and growth controlled by Nb diffusion.

5:15 p.m. Hot Deformability Behavior of Microalloyed Steels Aldo Mannucci, Tenaris Dalmine

High-strength low-alloy steels (HSLA) are steels containing small amounts, singly or in combination, of such elements as vanadium, titanium, niobium (called microalloying elements, MA), which have chemical affinity to carbon and nitrogen. These MA elements form carbides and nitrides, which effectively cause the austenite grain size to decrease, refining

the products formed during the decomposition of austenite, simultaneously causing the impact transition temperature (ITT) to decrease and the strength to increase. At the same time, these carbonitrides can markedly worsen the hot ductility behavior. The industrial production of microalloyed steels generally involves several steps in which the fabricating product undergoes severe plastic deformation at high temperatures: rolling, bending, piercing. The present work aimed at describing the common mechanism with which different HSLA steels start yielding during hot deformation, as well as the variables (chemical composition, level of inclusions, grain size) making the propensity to yielding more or less accentuated. Interrupted hot tensile tests followed by immediate quenching (i.e., to freeze the microstructure) and further metallurgical characterization (LM, SEM and TEM observations) were carried out through out a special device developed at PoliMi. As a result, it has been found that grain boundary sliding is the common mechanism of yielding, while the propensity of a given steel to present low deformability is principally driven by the presence of fine precipitates at austenite grain boundaries.

8:30 a.m. Introductions and Opening Remarks 8:45 a.m. Effect of Boron Addition on the Mechanical Properties of TRIP-assisted Steels Jai-Hyun Kwak, POSCO

TRIP-assisted steels containing ferrite, bainite, retained austenite and martensite exhibit favorable mechanical property combinations, high strengths and good ductility. The amount and stability of retained austenite is important because it governs the TRIP effect, which is obtained by the deformation-induced transformation of austenite to martensite. However, the TRIP-assisted steels containing high-carbon and high-alloying elements cannot be easily applied for automobile parts due to their low weldability and surface condition. Hence, the content of carbon and other alloying elements must be limited. As the limited contents of carbon and alloying elements alone cannot provide above 1 GPa TRIP steels, other microalloying elements -- such as Al, P, Nb, Mo, Cu, Ni and Cr -- were added. But boron alloying of TRIPassisted steels has not been sufficiently investigated yet. In this study, the effect of boron addition on the mechanical properties of TRIP-assisted steels, in conjunction with the other alloying systems, is investigated by taking into account the isothermal bainitic transformation treatment after rapid cooling. The influence of boron on the transformation behavior of austenite is presented and discussed.

5:45 p.m. Closing Remarks

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9:15 a.m. Influence of Chemical and Processing Variables on Annealing Response of Cold Rolled Microalloyed Steels Beth Blumhardt, Colorado School of Mines

Precipitation of AlN has been extensively studied in conventional sheet steel products, but is less understood in continuous cast sheet products. AlN formation in microalloyed steels may limit the nitrogen content available to form other microalloy carbonitrides, such as V(C, N) or Nb(C, N). Prior research reported an interesting competition between V and Al for N in 0.04 wt% V steels. In this paper, the competition for N will be extended to assess a wider range of HSLA steels, in terms of the annealing response and mechanical properties. The HSLA steels being studied include 0.038 wt% V and 0.032 wt% Nb microalloyed grades. In the V-HSLA steels,

Thursday, July 19 Sheet Steels

7:30 a.m. Continental Breakfast

both the Al and N content are varied at three levels; and in the Nb-HSLA steel, the Al content is varied to four levels. Mechanical properties and microstructure of the steels are examined in hot rolled, cold rolled, and simulated batch and continuous annealing conditions.

9:45 a.m. Break 10:00 a.m. The Effect of AL Additions on the Recrystallization Kinetics of Ferrite During Intercritical Annealing of Low-Si TRIP Steels Manuel Gomez, University of Pittsburgh

A series of model ferrous alloys with various Al additions and commercial high-Al, low-Si, Nbbearing TRIP steels were studied following intercritical annealing treatments. The major goal of this work was to gain a fundamental understanding of the effect of Al on the static recrystallization kinetics of ferrite during intercritical annealing. Quantitative metallography of isothermally treated samples allowed the measurement of the recrystallized fraction. By using different reheating times and temperatures, the kinetics of recrystallization and the activation energy for recrystallization were determined. The influence of previous cold rolling strain was also assessed, and the relationship between the rates of austenite formation and ferrite recrystallization during heating and isothermal intercritical annealing was evaluated. In addition, several experimental techniques (OM, SEM, EBSD-IQ, TEM, XRD, magnetometry) were used to describe the microstructural evolution (grain size, recrystallization, phase balance) of the steels during intercritical annealing and subsequent transformation during cooling. Computer-assisted thermodynamic calculations using JMatPro software helped to explain the results obtained and to interpret the influence of Al on the transformation lines of the multicomponent equilibrium diagram and carbon content of phases.

and 33 percent elongation, as well as ductile-brittle transition temperature lower than ­80° based on microstructures of 7.2 percent retained austenite volume fraction and around 5 m fine ferrite grain size. The experimental results showed that aluminum had a close relation to retained austenite volume fraction for experimental steels. The retained austenite was difficult to obtain for 1.33%Al steel, but the retained austenite dramatically increased to 7.2 percent when aluminum content increased to 1.74 percent. Retained austenite had a great role in improving plasticity and ductility for 1.74%Al steel. The elongation value increased from 28 to 33 percent, with tensile strength simultaneously increasing from 520 to 570 MPa when retained austenite volume fraction increased from zero to 7.2 percent.

11:00 a.m. Metallurgical Modeling of Recrystallization of Cold Rolled Nb-Ti Steel Strip for Structural Applications Miguel Vicente Alvarez, Center for Industrial Research ­ Tenaris

Cold rolled steel strips are annealed, either in batch or continuous furnaces, to obtain the required mechanical properties. During this process, several metallurgical phenomena occur, including recovery, recrystallization, grain growth and precipitation of elements in solid solution. The mechanical properties of the material after annealing depend on the final microstructure (especially grain size) and the state of precipitates. In this work, a metallurgical model whose output is the recrystallization fraction, the state of precipitates, the grain size and the mechanical properties (yield strength and tensile strength), is presented. The parameters of the model were fitted with experimental results obtained during laboratory simulation on Nb-Ti low-carbon steel. Finally, continuous and batch annealing cycles were simulated in a laboratory furnace. On the annealed probes, mechanical properties were determined and compared with the model predictions.

10:30 a.m. Effects of Aluminum on Microstructures and Mechanical Properties for Hot Rolling TRIP Steels Chen Yu, Wuhan Iron and Steel Group Corp.

The newly developed hot rolled TRIP steel was developed with well-balanced, comprehensive mechanical properties of 570 MPa tensile strength

11:30 a.m. Nanoscale Microstructural Characterization of Modern High-strength Steels for Automotive Industry Ilana Timokhina, Monash University

The microstructures of dual-phase (DP) and transformation-induced plasticity (TRIP) steels,

conventional and advanced high-strength low-alloy (HSLA) steels were characterized using transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). All these steels offered a unique combination of ultimate tensile strength and elongation. The microstructure of the TRIP steel contained ferrite and retained austenite as main phases with addition of martensite and bainite, while the microstructure of the DP steel consisted of polygonal ferrite and martensite. The average carbon content of ferrite, bainitic ferrite, martensite and retained austenite was determined using APT for both steels. The carbon content of ferrite was higher than expected from para-equilibrium between austenite and ferrite, while the average carbon content of retained austenite and martensite was lower than that determined by XRD. The distribution of substitutional elements within ferrite, martensite and retained austenite was also studied for both steels. The formation of nanoscale Ti0.98Mo0.02C0.6 carbides with an average radius of 2 ± 0.5 nm was observed by APT in the advanced HSLA steel. These were formed by repeated nucleation along the / interface. The conventional HSLA steel showed the formation of fine TiC with an average radius of 3 ± 1.2 nm. The distribution of elements in the matrix for both HSLA steels was also analyzed using APT.

for high-fatigue-restricted applications, microalloyed high-strength steels can also be employed for some structural parts subjected to cyclic loads, depending on the mechanical design, surface finishing and level of applied loads.

2:30 p.m. The Characterization and Quantification of Complex Ferritic Microstructures in Modern High Strength Steels Using EBSD-IQ Tony DeArdo, University of Pittsburgh

In the 1970s, steels with the strengths in the range 350-420 MPa exhibited ferrite-pearlite microstructures which were easy to characterize. The pursuit of higher strengths in steels either the hot rolled or CRA/CGL conditions has led to more complex microstructures. It is not uncommon today for a 490 MPa yield strength steel to exhibit several types of ferrite, plus bainite, martensite and perhaps retained austenite. Traditional metallographic techniques such as optical microscopy and SEM are no longer capable of analyzing these complex microstructures, especially in a quantitative fashion. The inability to characterize and quantify these complex microstructures means that the true origins of mechanical properties in these steels may be incorrectly understood and evaluated. The recent application of the quantitative analysis of the image quality of the Kikuchi pattern resulting from EBSD (EBSD-IQ), together with BAMPRI's Multi-peak Model, have led to an innovative way to quantitatively analyze complex microstructures in the higher strength steels. This paper will present the technique and show where it has been used successfully in research studies involving a broad range of microalloyed steels including HSLA, DP, TRIP, IF and heat treated steels.

Noon Lunch (on your own) 1:30 p.m. Producing Nb-bearing, High-strength DP Steels on CG Lines 2:00 p.m. Fatigue Resistance Comparison Between Microalloyed High-strength and Dual-phase Steels Silvio Alzari, Ternium Siderar

The goal of this work was to study and compare the fatigue resistance of some structural steels in relation to dual-phase steels. All the steels studied were industrially manufactured, and two different kinds of fatigue test were carried out: complete reversal constant strain fatigue test on flat polished specimens, and dynamic cornering fatigue tests on automobile wheels. Additionally, modeling of metallurgy evolution during hot rolling was performed and related to microstructure and fatigue phenomena. In spite of dual-phase steels being the right choice

3:00 p.m. Closing Remarks and Adjourn

International Conference on Microalloyed Steels: Processing, Microstructure, Properties and Performance

July 16­19, 2007 Soldiers and Sailors Hall · Pittsburgh, Pa

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