Applied Metallurgy of the MicroNiobium® Alloy Approach...

23. - 25. 5. 2012, Brno, Czech Republic, EU

APPLIED METALLURGY OF THE MICRONIOBIUM® ALLOY APPROACH IN LONG AND PLATE

PRODUCTS

Steven G. Jansto

CBMM – Reference Metals Company 1000 Old Pond Road Bridgeville, PA 15017

USA

[email protected]

Abstract

The application of the MicroNiobium Alloy Approach® in carbon steel long product and plate steels enhances

both the metallurgical properties and processability and reduces the operational cost per tonne. The process

and product metallurgical improvements relate to the Nb-pinning effect of the austenite grain boundaries. The

metallurgical mechanism of the MicroNiobium Alloy Approach is related to the retardation of austenite grain

coarsening during reheat furnace soaking of the billet, slabs or shapes before rolling. Also, in the case of heat

treatment or carburizing, higher processing temperatures can be applied to the finished components, thereby

reducing process time and increasing productivity. The MicroNiobium Alloy Approach has been applied in high

carbon (AISI1050 grade) automotive and long product steel applications, such as fasteners, seismic resistant

rebar and pre-stressed concrete wire rod. The Micro-Niobium Alloy Approach mechanism is described and

correlated to a variety of medium and high carbon steel grades and applications. Development opportunities and

applications include pressure vessels, automotive coil springs, eutectoid rail steels, alloy tool and die steels, and

tyre rod. This approach contributes to the achievement of desired ultra-fine grain, homogeneous higher carbon

steel microstructures that exhibit superior toughness, strength, fatigue performance, less mechanical property

variation in the final hot rolled product, reduced cost of quality and improved weldability. The improvement in the

reduced cost of production and internal quality far exceeds the additional alloy cost for the niobium (Nb).

Keywords: austenite grain size, fatigue, MicroNiobium Alloy Approach®, reheat furnace

1. INTRODUCTION

Although the Nb-solubility is limited when higher amounts of Nb are used in higher carbon steels compared to

low carbon steels, through empirical evidence and actual operating data, the Micro-Niobium Alloy Approach has

demonstrated very positive results on high carbon grades such as steel wire rods and bars, eutectoid steels,

and other medium carbon engineering alloy applications. This technology is being introduced at an accelerated

pace throughout the world. The important resultant effect of the MicroNiobium Alloy Approach is the prevention

of austenite grain coarsening during reheat furnace soaking of the billet, slabs or shapes before rolling. Also, in

the case of a carburizing or austenitizing heat treatment, higher carburizing or austenitizing temperatures of

finished components or parts are possible. [1]

The global steel market technological advancement known as the Micro-Niobium Alloy Approach may ultimately

be applied across all carbon levels to improve product homogeneity during the steelmaking and hot rolling

process resulting in improved rollability, finer austenite grain size and less product variation. Specifically, most of

the development activity has been between 0.20%C to 0.95%C steels. The concept involves the micro-addition

of .005 to .020%Nb across nearly all carbon grades. Product and industrial process development results

indicate that these micro-Nb alloy additions will significantly pin the austenite grain boundary and minimize the

heterogeneous abnormal austenite grain growth that occurs in actual reheat furnace mill operations. This micro-

approach offsets the inhomogeneous austenite grain coarsening that occurs in the slab during normal reheat

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furnace or heat treat furnace operation when temperature excursions occur that cause overheating of the steel

in normal operation.

Two Nb strategies are employed in practice depending upon the intended purpose; 1) the Micro-Niobium Alloy

Approach simply minimizes the austenite grain size coarsening during reheating through the addition of 0.005%

to 0.020%Nb and 2) the Thermomechanical Control Process (TMCP) approach at higher Nb levels for the dual

purpose of grain size stabilization, complex precipitation strengthening and thermomechanical processing.

Figure 1 below schematically illustrates the key elements of the MicroNiobium Alloy Approach.

Fig. 1 MicroNiobium Alloy Approach [2]

2. DISCUSSION

2.1 Reheat Furnace Operation for Nb-Modified Medium and High Carbon Steels

The initiation point for proper austenite grain size control is the effectiveness and consistency of the heating of

the slabs, billets or profiles prior to hot rolling. Homogeneous heating and soaking of slabs is vital in order to

minimize temperature gradients (ΔT) between the surface and center of the slab and the ΔT from the front end

to back end of the slab. Often during the rolling of C/Mn and microalloyed steels, variability of the ΔT from the

front end to the tail end and/or high ΔT’s from surface to center of the slab, billet or shape translate into variable

mechanical properties within a coil, bar or plate regardless of the mode of rolling. Variable prior austenite grain

size translates directly into variable final ferrite size in the hot rolled product. In addition, homogeneous heating

results in flatter and straighter hot rolled product (i.e. improved flatness and shape), more uniform and finer

austenite grain size, assured solubility of the microalloy carbon nitrides and improved rollability.

Depending upon the reheat furnace efficiency and heating schedules, reheat temperatures for medium carbon

and high carbon steels should generally range between 1125°C to as high as 1230 °C range. Inefficient

reheating is the reason some mills overheat Nb-bearing medium and high carbon slabs, billets and sections at

1230°C, whereas efficient heating can lower the soak temperature well below 1200°C. Generally, laboratory

derived Nb-solubility data varies depending on the researcher and should be incorporated into the furnace

model with caution. Actual operational experience indicates that over 75% Nb-solubility is achieved at


temperatures as much as 25 to 50°C lower than solubility model predictions based upon actual hot rolling mill

experience. [3] Consequently, the Nb is extremely effective in pinning the prior austenite grain boundary

compared to other grain pinning elements such as Al or Ti.

Actual operational experience and performance indicate the following root causes which promote the formation

of coarse grain austenite in these medium and high carbon steels:

Excessive soak zone temperatures exceeding 1230°C

Improper furnace control cutback on operational delays, thereby overheating slabs, resulting in

slab/billet sticking and lost production

Poor combustion fan efficiency (which directly correlates to surface quality) especially on high carbon

sheet production exceeding 0.50%C)

Same air-to-gas ratios for all steel grades (i.e. low, medium and high carbon steels)

Inefficient burner combustion at the orifice and maintenance considerations

Therefore, the role of the MicroNiobium Alloy Approach in the reheat furnace operation provides some

processing flexibility in retarding the austenite grain growth due to several of the aforementioned operational

and heating issues experienced in actual operating conditions.

2.2 Nb Microalloy Design Considerations

In some instances, Nb has not been the microalloy of choice or even considered for that matter in high carbon

equivalent steels because of the predicted lower solubility of the Nb carbonitrides in higher carbon steels.

Although there is lower solubility, current industrial applications validate the effectiveness of Nb in the grain

refinement and precipitation strengthening mechanism in Nb-only and Nb-modified V containing steels. Over the

past two decades, within this higher carbon steel segment, in microalloy metallurgical research studies where

Nb was added to high carbon grades, researchers incorporated higher Nb levels than necessary with

unfavorable results.

These higher Nb levels (exceeding 0.040%) were thought to be necessary in order to obtain proper grain

refinement, microstructural control and strength in higher carbon equivalent steels. Experience to-date has in

fact indicated that the higher Nb levels in high carbon steels certainly make the processing more challenging,

more costly and the resultant properties are not optimized. Recent developments have determined Nb levels of

0.005% to 0.020%Nb in high carbon steels optimize properties. It is important to also consider the synergistic

precipitation behavior effect between the Nb and V and, in some cases Mo, which may contribute to the

improved mechanical performance. This duplex or triplex microalloy complex precipitation behavior is under

further study.

Based upon actual recent commercial product applications, a richer understanding of the Nb-high carbon

technology mechanisms, metallurgy and processing parameters have been achieved. This information is

invaluable for the implementation process to successfully incorporate such low levels of Nb in existing high

carbon equivalent steels to improve fatigue, fracture toughness, ductility and overall product performance. The

development of the coil suspension spring is an excellent example of such a successful application and is

presented.

Other results from product applications reveal that in some cases more Nb is not always better. Based upon

operational experience, the optimization of the Nb content and the proper control of the reheating furnace are

critical. An optimum Nb concentration may be directly correlated to a given carbon level depending upon the


reheat furnace process metallurgy parameters, heating practices and combustion conditions at a given mill. For

example, the quality and consistency of reheating high carbon (>0.50%C) billets and slabs can be enhanced

through the incorporation of combustion practices resulting in an air to gas ratio less than 1.00.

Another evolving long product development trend involves micro additions of Nb for grain refinement in

carburized grades and other higher carbon heat treated grades for the purpose of shortening the heat treat

cycles on quench and tempered products. Also, as-forged microalloyed Nb steels may replace quench and

temper alloy products, thereby reducing both energy and production costs. [4]

3. MICRONIOBIUM IN MEDIUM AND HIGH CARBON APPLICATIONS

Some limited applications over the past several years have employed the TMCP approach in higher carbon

engineering tool steels and a few long products. Generally, the TMCP in some engineering steels will apply Nb

at levels of 0.030% to 0.045%. However, the application of Nb in these higher carbon steels was limited. With

the introduction of the MicroNiobium Alloy Approach to higher carbon steels ranging in composition from

0.20%C to 0.95%C steels in both long and plate products, this new technology has gained acceptance,

momentum and high interest globally. The MicroNiobium Alloy Approach applications and global product

development activities are highlighted (in red) with several industrial applications in Table 1. [1]

Tab. 1 Nb-bearing medium and high carbon end user steel application via Micro-Niobium Alloy Approach® or

TMCP Approach

Shapes Bar Wire Rod Structural

Pipe & Tube

Rebar

Power plants 9259 Spring

steels*

1080 High

carbon pre-

stressed*

Structural

scaffolding

Seismic

resistant*

Trailer

support rails

Forging

quality

Engineering Construction Fire

resistant*

Rails* 1050

Automotive

fasteners

Cold

headed*

Irrigation and

utilities

Bridges

High alloy

tool steels

Carburized

gears &

shafts

High

strength

bolts*

Boiler tubing Buildings

Quench &

Temper

Wire rope Utility power

plants

Tunnels

3.1 Retardation of Austenite Grain Growth

One of the most important effects of Nb in high carbon steel such as wire rods and bars is the prevention of

austenite grain coarsening during heat treatments such as carburizing or the reheating of slabs or billets prior to

hot rolling. The start temperature for grain coarsening increases with increasing Nb content.

From a practical operational perspective, soak zone temperatures exceeding 1250°C is deleterious to steel

quality and mechanical property performance. The Nb-micro strategy is to set the percent Nb concentration

based upon the solubility calculation at the point just above the mill’s maximum soak zone actual temperature,

thereby preventing grain coarsening and pinning the austenite grain boundary during soaking. These low


concentrations of Nb can compensate for several of the operational and combustion variances that occur during

reheating of the slabs, billets and/or shapes.

The fine precipitation of the niobium carbides and niobium carbonitrides play an important role during the

heating and hot rolling in achieving a fine grain structure. It is at the soaking temperature where the fine Nb-

precipitates are stable, such that the grain-growth process is retarded and the mode of grain growth is normal.

There appears to be an optimum Nb-concentration to ensure that austenite grain boundary pinning is effective.

To-date, based upon industrial trials, a 0.005% to 0.020%Nb concentration appears optimal based on the quality

performance for these high carbon steels. The following case example on 1035 steel illustrates this MicroNb

effect pinning the grain boundary.

3.2 AISI 1035 MicroNb and Effect on Properties

The influence of reheat furnace soak temperature is also important in terms of fracture toughness behavior. In

order to validate this effect, the micro addition of Nb to a 1035 steel grade (0.35%C-0.3%Si-1%Mn) enhances

the yield strength, tensile strength and toughness. The Charpy impact properties are markedly improved with a

billet reheat temperature of 1100°C and controlled rolling practice. The process metallurgy reheat furnace

control and consistency of the combustion assists greatly in achieving these excellent toughness properties.

Since part of the Nb remains as a precipitate at this temperature, both grain refinement and precipitation occur

and are complementary. Figure 2 illustrates this improvement in 1035 steel properties [5]

Fig. 2 Effects of Nb on tensile and Charpy V notch impact properties of 1035 steel

3.3 AISI9259 MicroNb Coil Spring Application

Mechanical properties improve with the addition of Nb in rebar, structural shapes and automotive structural

components, such as springs. For example, a North American vehicle front suspension coil spring composed of

0.51%C with Mo-V-Nb was developed and commercialized with improved mechanical properties compared to


conventional springs. A similar effect was observed when adopting 0.035%Nb in a 9259 engineering alloy spring

steel grade. The improved properties are attributed to the grain refinement, microstructure, microalloy

carbonitride precipitate morphology and precipitate strengthening provided by Nb. [6] The chemistry of the Nb-

modified spring steel is shown below in Table 2.

Tab. 2 Nb-Modified 9259 spring steel heat analysis

Grade C Mn P S Si Cu Ni Cr Mo V Nb N ppm

SAE 9259 .61 .86 .014 .021 .78 .008 .008 .51 .008 .005 .002 55

V-SAE 9259 .60 .81 .020 .017 .85 .007 .009 .51 .003 .100 .002 110

Nb-V-Mo 9259 .51 .69 .016 .020 1.31 .007 .012 .45 .040 .120 .035 120

The improvement in hardness at temper temperature has translated into increased strength, better fatigue

endurance limits and good fracture toughness, thereby allowing for a lighter weight design coil spring. This Nb-

V-Mo modified coil spring steel has resulted in the reduction in weight of a coil spring by approximately 15%,

improved fatigue resistance by 12% and improved fracture toughness by 27% over the conventional 5160 or

9259 and/or the V-modified 9259.

0

5

10

15

20

25

30

35

40

5160 9259 9259+V 9259+

Nb-V

KIC

Fig. 3 KIC Fracture toughness comparison of Nb-V bearing coil spring [6]

The resultant Nb-V modified grade exhibits improved yield and tensile strength which translates into better cyclic

fatigue life and improved fracture toughness. The adjusted steel chemistry, grain refinement, Nb-V(CN)

precipitation strengthening and overall lower volume fraction of hard oxide inclusions results in the improved

properties. This application illustrates the complimentary synergy between Nb and V in these higher carbon

engineering tools steels. In actual operation, the Nb to V stoichiometric ratio has been reduced thereby lowering

the cost of the V and Nb additions at 0.020% Nb.

K IC

Fracture Toughness [MPa-m

-1/2]


4. CONCLUSIONS

The application of the MicroNiobium Alloy Approach in over 0.20% carbon steels enhances the metallurgical

properties, consistency and processability of the hot rolled product. Such process and product metallurgical

improvements relate to the Nb-pinning effect of the austenite grain boundaries in Nb- microalloyed steels

exceeding 0.20%C steels. The key operational attribute is the micro-addition of Nb in higher carbon steels which

mechanistically pins the austenite grain boundary during the reheat furnace process, thereby minimizing

abnormal grain growth in the billet or slab prior to rolling. Typically, abnormal grain growth occurs when thermal

fluctuations and furnace abnormalities exist in actual reheat furnace operations. This abnormal grain growth

leads to inhomogeneous ferrite grains in the final hot rolled product and subsequent variations and reductions in

mechanical property performance such as fatigue, fracture toughness and yield-to-tensile properties.

LITERATURE

[1] JANSTO, S. “21st Century Niobium-Bearing Structural Steels,” HSLA2011International Microalloy

Conference, May 31-June 2, 2011, Beijing, China.

[2] JANSTO, S., “Current Development in Niobium High Carbon Applications,” MS&T Conference, October 16-20, 2011,

Columbus, Ohio.

[3] KLINKENBERG, C., and JANSTO S. “Niobium Microalloyed Steels for Long Products,” International Conference on

New Development in Ferrous and Forged Products, TMS, June 2006, Winter Park, CO.

[4] SPEER, J., MATLOCK, D. and KRAUSS, G. Materials Science Forum, 500-501 2005, p.87.

[5] SAMPEI, T., ABE, T., OSUZU, H., and KOZASU, I. HSLA Steels Technology & Applications, 1984, p 1063.

[6] HEAD, M., KING, T., and RADSULESCU, A. “Development of New Microalloy Steel Grades for Lightweight

Suspension Systems,” presented at AISI Great Designs in Steel Seminar, 2005, Livonia, Michigan

(www.autosteel.org).

http://www.autosteel.org/

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