1. INTRODUCTION
1.1. Background
The need of utilization of HSS grows continuously. Currently, HSSs are used more frequently and in a diverse number of industries. Primarily, HSS was just used in the car industry, but today the material is used in a more diverse assortment of industries and locations including the arms of cranes and the frames of lumber carriers, although this list is by no means extensive.
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To date, HSS has not been formally standardized. At the lower end, structure steels have a yield strength in the range of 235-355 MPa. Recent literature has stated that strong steels should have yield strength of at least 460 MPa, while steels with a yield strength of more than 550 MPa should be categorized as ultra HSSs. Today, the yield strength of some steel has increased to 1100 MPa, while in the commercial sector, steel with a rating of up to 1300 MPa (1500 MPa) is sold.
There are three different ways to make HSS. First, the oldest method is the QT method (quenched and tempered method), followed by the TMCP (thermomechanical controlled process) and finally, the last method is direct quenching (DQ). The common goal of all of these above mentioned production methods is to create a steel of high yield strength and good ductility. All the steels that are created using one of these three different methods (QT, TMCP or DQ) have a bainite and/or martensite small microstructure in the main structure. TMCP steel can also have a ferrite-bainite main structure. This small microstructure is created through the alloying of various microelements such as niobium, titanium, vanadium, and boron, which in turn make inclusions like carbides and nitrides. Together with fast cooling and tempering, the resulting microstructure is small and the hardness of structure is high despite the small content of carbon. Some manufacturers have developed DQ steel to replace QT steel using this new method (Porter 2006).
Additionally, chromium, nickel, molybdenum, aluminium, carbon, magnesium, silicon, phosphorus and other alloying elements are added (or are not taken away during the manufacturing process) to iron to make HSS. It is typical of HSSs to have a low carbon content which gives the steel a lower CEV (Carbon Equivalent Value) and good weldability.
Before starting to use HSS in old structures, the entire structure must be redesigned. Simply thinning the structures is not enough as buckling, springing, or bending can easily occur. In their publication from GMA-welded AHSS structure, Kaputska et al. (2008) explained that it is important for designers and
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manufacturing engineers to understand the factors that may be affected in these performances. As there are a large variety of manufacturers that make HSSs using different methods, it is important to clarify differences between these steels. Sampath (2006) explained that manufacturers must exercise extreme caution when transferring allowable limits of certified secondary construction practices from one type of HSS plate steel to another, even for same plate thickness.
2. STATE OF THE ART
A large number of scientific reports and design guidelines have been published regarding the welding of HSSs (Zeman 2009, Shi & Han 2008, Liu et al. 2007, Pacyna & Dabrowski 2007, Yayla et al. 2006, Juan et al. 2003, Keehan et al.
2003, Miki et al. 2002, Zaczek & Cwiek 1993). Special attention has been devoted to welding HSSs with matching filler material, however, only a limited number of publications consider welding HSSs with undermatching filler material (Rodriques et al 2004a). In the 1980s HSS was pioneered in Japan and organized so that individual manufacturers had their own research projects on specific steels. As a result of this rigorous research, today’s steels are of much better caliber and quality.
There are three different popular and widely available HSSs on the market including those manufactured through the QT, TMCP and DQ processes. QT has been available the longest and DQ HSS has only recently been developed and acquirable on the market. Consequently, most of the research has focused on QT steels, however DQ steel research has emerged in the 2000s and recently, comparing all three HSSs has been an emerging field of investigation.
17 2.1. What is HSS?
The term HSS is variable concept. Today, HSSs are steels with a yield strength greater than 550 MPa. Classifying steels according to their yield strength allows for the correct comparison between different types of steels. Fig. 1 (World Auto Steels 2009) depicts the classifications of different HSS types.
Conventional HSSs (HSS) have a yield strength lower than 550 MPa. Included in this group of steels are IF-HS (High Strength Interstitial Free) steels, BH (Bake Hardenable) steels, IS (Isotropic steels), CM (Carbon Magnanese) steels, and HSLA (High Strength Low Alloy) steels (World Auto Steel 2009).
Advanced HSSs (AHSS) have yield strengths greater than 550 MPa. Some steels that fit into this category are TRIP (Transformation-Induced Plasticity) steels, DP-CP (Dual Phase or Complex Phase) steels, and MS (Martensitic) steels. MS steels are used in many different industries and can be found in cranes, earth-movers, harvesters, and more.
Traditional HSSs, such as high-strength low-alloy (HSLA), have more than three decades of shop experience upon which to build a technology base. In contrast, users of AHSS demanded a fast track accumulation of knowledge and dissemination as they implemented these new steels. A considerable challenge arises along the total elongation and yield strength axes, as the trend shows that higher strengths steels have decreasing total elongation percentages.
Manufacturers are currently looking for ways to maintain the total elongation percentages with steels of increased yield strength.
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Figure 1. Relationship between yield strength and total elongation for various types of steels (World Auto Steel 2009).
Fig. 2 depicts the developmental history of HSS for commercial use. The first HSS, S355, was developed in the 1940s with a yield strength of 355 MPa. By the 1970s, HSSs with a yield strength of up to 690 MPa had been created. By 1990, the maximum MPa had been increased to 960 MPa, and currently, HSSs with a yield strength of up to 1300 MPa can be found (Kömi 2009).
History of Ultra High Strength steels
Yield Strength, MPa Hardness, HBW
Figure 2. The history of ultra HSS (modified from Jukka Kömi figure 2009, Rautaruukki Ltd).
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HSSs have been used in the war industry since 1946. The U.S. Navy has used high yield (HY) strength steel, including HY-80, HY-100, and HY-130 steels (Moon et al. 2000 according to Holsberg, P.W. et al. 1989). However, these steels were originally quite expensive to make and additionally, the knowledge of this new generation of steel was kept within the government and therefore the private sector was, for a time, excluded from this new industry. The HY-strength steel corresponds with the ISO system, where the tensile HY-strength of HY-70 (70 ksi) corresponds to 490 MPa, HY-80 (80 ksi) corresponds to 700 MPa, HY-100 (100 ksi) corresponds to 780 MPa, HY-120 (120 ksi) corresponds to 840 MPa and HY-130 (130 ksi) corresponds to 910 MPa.
2.2. Effects of alloying elements in HSS and in its weld
Alloying elements are used in HSSs to reduce the phase microstructure. There are many appropriate alloying elements that can be used when making HSSs, including Cr, W, Mo, V, B, Ti, Nb, Ta, Zr, Ni, Mn and Al. Every alloy or blend of alloys has a different effect on the steel. These elements compose inclusions and precipitations such as nitrides, carbides, carbonitrides and composites in the HSS and inhibit grain growth. In order to create a HSS with a small grain size an alloy or combination of alloys should be used, and additionally planned rolling can contribute to the creation of a steel with the above mentioned desired characteristics.
To prevent the growth of austenite grains, a maximum temperature, which is dependent on the alloying element, where carbides and nitrides will dissolve to austenite, must not be exceeded. Fig. 3 shows how carbide and nitride inclusions quickly dissolve into austenite once these temperatures have been exceeded.
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Figure 3. The effects of microalloying on Al, Zr and Ti to austenite grain growth starting temperature (modified from Harri Nevalainen figure 1984).
Titanium, niobium, zirconium, and vanadium are also effective grain growth in-hibitors during reheating. However, for steels that are heat treated (QT, TMCP and DQ steels) these four elements may have adverse effects on hardenability because their carbides are quite stable and difficult to dissolve in austenite prior to quenching (Metal Handbook 1990).
In many research projects alloying elements of HSSs and its welds have been under examination. For example, Kou (2003) reported that increasing the alloying content of weld metal increases its hardenability by pushing the nose of continuous cooling curves to longer times. Moon et al. (2000) noticed that the microhardness variations in the weld and HAZ areas can be examined to correspond with the microstructure of the weldment. At the same time they concluded that the HAZ of the base metal was the hardest region in each weldment examined, regardless of filler metal type, base metal, or heat input.
Maximum hardness was reached about midway through the HAZ of each
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weldment studied. Fig. 4 describes hardness areas with different heat inputs (4.33 kJ/mm, 2.17 kJ/mm and 1.18 kJ/mm) using HSSs HSLA100 and HY80.
Figure 4. Microhardness maps of welds made with three different filler metals and different welding parameters. The corresponding microhardness scale is included at the bottom of this figure (Moon et al. 2000).
Hamada (2003) reported that it is necessary to combine the values of the constituents in the steel material and the welding conditions after taking into account the necessary joint properties. In their research, they used five different HSSs, HT50, HT60, HT80 and two HT100. They concluded that the properties of the weld HAZ, especially those of the coarse grain HAZ and fine grain HAZ
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heated to more than the AC3 transformation point, are determined by the composition of the steel along welding conditions, as seen in fig 5.
Figure 5. Structural distribution within multi-layer welded joint HAZ (Hamada 2003 according to Shishida et al. 1987).
Toughness deterioration is one of worst things that can happen when welding HSSs. Caballero et al. (2009) investigated HS bainite steel and concluded that a high degree of microstructural banding, as a result of an intense segregation of manganese during dendritic solification, leads to a dramatic deterioration in toughness in these advanced bainitic steels. They concluded that the stress concentration associated with heterogeneous hardness distribution in the microstructure can be considered a possible factor contributing to premature crack nucleation.
2.2.1. Aluminium and Silicon
Aluminium (Al) is widely used as a deoxidizer and it was the first element used to control austenite grain growth during reheating. When Al or silicon (Si) reacts with oxygen, soft oxides are formed. These soft oxides do not create crack initiations of growth similar to what is seen in TiO precipitations (Vähäkainu 2003). However, in HSSs it has been noticed that niobium (Nb) and titanium (Ti) are more effective grain refiners than Al (Metal Handbook 1990). High Al
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content weakens the toughness of steel, as it promotes the formation of preferred orientation of ferrite and upper bainite. Free Al promotes forming local areas which contain high contents of carbon, which are known as M-A islands.
This mechanism prevents carbon diffusion and the formation of carbides (Matsuda et al. 1995).
With regard to Al, Kaputska et al. (2008) have also observed that while Al has many effects in steel making, the CEV does not consider Al in its calculation.
Si is one of the principal deoxidizers used in steel making. Killed steels may contain moderate amounts of Si, from 0 to a 0.6 % maximum (Metal Handbook 1990). Low-alloy steels are reinforced by Si, but Si does not affect the features of low carbon steels (Harrison & Wall 1996).
2.2.2. Niobium
As an alloying element, Nb has an important role in HSS. The effects of niobium on steel and HAZ are not solely derived from niobium. Niobium affects steel and HAZ when it is combined with other alloying elements, such as Ti and V, and precipitations. In the welded joints of HS steels, the effects of niobium depend upon the heat input. If welding and using a low heat input, this will increase impact toughness, while if a high heat input is used it will decrease the impact toughness in the HAZ. In these HSSs, as carbon content increases, there in an inverse relationship as the impact toughness decreases (Tian 1998; Hatting &
Pienaar 1998).
In certain amounts, Nb (0.02-0.05 wt.%) increases austenite recrystallization temperature, provides strengthening by forming thermally stable, Nb(C,N) and Nb,Ti(C,N) precipitates. During fusion welding, the precipitates limit austenite grain growth in the weld HAZ, and thereby limit hardenability or improve weldability. Excessive amounts of Nb (>0.05 wt.%) can potentially impair HAZ toughness in high heat input weldments (Sampath 2005). Small additions of Nb
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increase the yield strength of carbon steel. The addition of 0.02 % Nb can increase the yield strength of medium-carbon steel from 490 MPa to 700 MPa.
This increased strength may be accompanied by considerably impaired notch toughness unless special measures are used to refine grain size during rolling.
Grain refinement during rolling involves special thermomechanical processing techniques such as controlled rolling practices, low finishing temperatures for final reduction passes, and accelerated cooling after rolling is completed (Metal Handbook 1990).
In HSLA steel with niobium, granular bainite is dominant within a wider cooling rate range. In addition, martensite is observed at high cooling rates with Nb 0.026 %, but is not produced in the same steel without Nb (Zhang et al. 2009).
Zhang also reports that at lower cooling rates, under 32 °C/s, Nb addition suppresses grain boundary ferrite transformation and promotes the formation of granular bainite. Li et al. (2001) have reported that the addition of 0.031 % Nb to low carbon micro alloyed steel produced the largest size and greatest area of M-A phase.
2.2.3. Vanadium
Vanadium (V) increases the austenite recrystallization temperature in HS steels.
It provides room temperature strengthening by forming VN, V(C,N) and (V,Ti)N precipitates in ferrite (Sampath 2005). V also strengthens HSLA steels in two ways. First, the precipitation hardens the ferrite and secondly, the precipitation refines the ferrite grain size. The precipitation of V carbonitride in ferrite can develop a significant increase in strength that depends not only on the rolling process used, but also on the base composition. Carbon content above 0.13 to 0.15 % and Mn content of 1 % or more enhances the precipitation hardening, particularly when nitrogen content is at least 0.01 %. Grain size refinement depends on thermal processing (hot rolling) variables, as well as V content (Metal Handbook 1990).
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Chen et al. (2006) have reported that there is a correlation between V content and the size of M-A particles. This is a direct correlation as the size of M-A particles increase with increased V content from 0 % to 0.151 %. When increasing V content, there is a decrease in the impact toughness in HSS. The coarse austenite and ferrite grain and M-A constituent were thought to be the main factors resulting in impact toughness deterioration.
Both Chen et al. (2006) and Zhang et al. (2009) reported after their experiments on that the concentration of V should be limited to a low level, near 0.05 %. If the V content is 0.1 % or more, this results in a greater area fraction of the M-A phase, larger average and maximum sizes of M-A particles, and deterioration in toughness.
2.2.4. Titanium
When considering the welding of steel, Ti is most important micro alloying element. Stable Ti nitrides that form in high temperatures inhibit grain growth in the HAZ. Consequently, because of this grain size CGHAZ cannot grow destructively (Liu & Liao 1998).
Ti is unique among common alloying elements, because it provides both precipitation strengthening and sulfide shape control. Small amounts of Ti (<0.025 %) are also useful in limiting austenite grain growth in HSSs. However, it is only useful in fully killed (aluminium deoxidized) steels because of its strong deoxidizing effect. The versatility of Ti is limited because variations in O, N, and S affect the contribution of Ti as a carbide strengthener (Metal Handbook 1990).
In controlled amounts (0.01-0.02 wt.%) Ti acts as a grain refiner, increases rerystallization temperature, fixes solute nitrogen as TiN, and provides strengthening by forming thermally stable, complex Ti(C,N) precipitates. During fusion welding, TiN precipitates limit austenite grain growth in the weld HAZ, thereby limiting hardenability and improving the HAZ strength and toughness.
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Precipitation of TiN invariably reduces the HAZ toughness, especially at low temperatures (Sampath 2005).
Ti can react with nitrogen in liquid condition. Large TiN precipitates will grow in steel and their formation is easier when the Ti/N ratio is large. These kinds of precipitates cannot prevent grain growth as the precipitates which form in lower temperature. Precipitates which are big and angular can nucleate cracks and decrease fatigue durability (Lee & Pan 1995). The size of some inclusions are explained in fig. 6.
Figure 6. The nucleation ability of various inclusions (Lee & Pan 1995).
Ti improves HAZ microstructure and toughness of welded structure with three inter-related mechanism. Those mechanism are refining of ferrite grains by the pinning effect of thermally stable Ti-nitride and Ti-oxide particles which are distributed in austenite, by formation of pure Ti-nitride and Ti-oxide particles which disperse in austenite at high temperature and then this particles can be as nucleation sites for acicular ferrite during the ɣ-α transformation. Third mechanism is formation of fine nitrides which decrease the detrimental effect of soluble nitrogen in ferrite (Rak et al. 1997).
27 2.2.5. Zirconium
Zirconium can also be added to killed high-strength low-alloy steels to improve inclusion characteristics. This occurs with sulfide inclusions, where the changes in inclusion shape improve ductility in transverse bending (Metal Handbook 1990).
2.2.6. Boron and Copper
Boron (B) is added to fully killed steel to improve hardenability. The average B content in steels ranges from 0.0005 to 0.003 %. When B is substituted in part for other alloys, it should be done only to alter the hardenability. The lowered alloy content may be harmful for some applications; however B is most effective in lower carbon steels (Metal Handbook 1990).
According to Moon et al. (2008), the addition of B to high strength low alloy plate steel makes a fine martensite microstructure, which increases hardenability by making the prior austenite grain boundary more stable.
Vickers hardness of base steels and CGHAZ increasing Cu and B content, solid-solution hardening as uncovered by Moon et al. (2008) investigation. In the same investigation, it was also noticed that Charpy V-notch toughness showed an opposite tendency. This is mainly due to the formation of the hard phase by increasing hardenability with Cu and B addition and where toughness in the CGHAZ is decreased as compared to base steels.
The results published by Moon et al. (2008) indicate that Cu addition is not useful to improve the toughness of the HAZ in high strength low alloy plate steel. Hwang at al. (1998) studied that the structure of low-carbon (C 0.04 %) copper-bearing (Cu 1.8 %) alloy steel plate manufactured by the DQ&T process has been transformed into a fine structure with high dislocation density. During tempering, fine NbC and ɛ-Cu particles are precipitated in large amounts, which
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do not get coarsened even when the tempering temperatures rise, resulting in excellent mechanical properties. The results of Hwang et al. (1998) indicate that the addition of alloying elements and the application of the DQ&T process to low-carbon alloy steel plates contribute to the production of plates with excellent strength and toughness.
2.2.7. Manganese and Nickel
Manganese (Mn) improves the strength of steel without decreasing its impact toughness and is commonly used in steel making. Mn reacts with oxygen and sulphur quite easily and makes precipitations and is important because all non hopeless effects are outclosed. The use of Mn needs to limited to under 1.5 % as steel with over 1.5 % Mn content can be brittle (Vähäkainu 2003, Lindroos at el. 1986). Excessive amounts Mn increase hardenability and reduce weldability (Sampath 2005).
In his study, Keehan (2004) investigates the effects of Ni and Mn in weld metal.
TEM investigations in conjunction with APFIM (Atom Probe Field Ion Microscopy) concluded a mixed microstructure of martensite, bainitic and
TEM investigations in conjunction with APFIM (Atom Probe Field Ion Microscopy) concluded a mixed microstructure of martensite, bainitic and