1. MECHANICAL HEAT TREATMENT OF STRUCTURAL STEELS

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1 1. MECHANICAL HEAT TREATMENT OF STRUCTURAL STEELS Classification of chapter: 1. Mechanical heat treatment of structural steels 1.1. Basic stages of mechanical-heat treatment 1.2. Deformation in austenite area accompanied by recrystallization processes 1.3. Deformation in area of suppressed recrystallization 1.4. Deformation in austenite-ferrite (double) area 1.5. Controlled transformation of austenite matrix with accelerated cooling application Analysis of segregation banding formation Summarisation of capter terms and questions Literature Necessary time for study: 250 minutes Aim: After study of this capture you will be able to analyse forming operations in region of austenite, suppressed recrystallization and in double region; you will understand principles of accelerated cooling process following-up to controlled rolling; you will be able to suggest deformation processes leading to balanced strength and plastic properties without subsequent heat treatment; you will be partially able to predict final properties after forming operations Author: Eva Mazancová 6

2 Lecture 1.1.Basic stages of thermo-mechanical processing The aim of the modern methods of hot rolling is to obtain a homogenous, fine-grained structure with higher strength, good toughness at decreased temperatures and a high level of weldability without performing subsequent heat treatment, if possible. To obtain optimized conditions for the above mentioned, a certain balance needs to be maintained between chemical composition, heating conditions, thermo-mechanical conditions during forming, transformation characteristics, including maintaining appropriate parameters of cooling rate. This is a thermo-mechanical control process of forming, which does not include only controlled hot rolling. Fig. 1.1 Scheme of processes running in three temperature areas Author: Eva Mazancová 7

3 A description of this processing procedure is presented by three stages of the controlled rolling process see Fig , or more precisely the following phases: a) Forming in a region where recrystallization of an austenitic matrix occurs (high temperature forming); b) A follow-up forming operation is performed in a region where recrystallization growth is suppressed; c) Forming is performed in a heterogeneous region consisting of austenite and ferrite (i.e. in a dual-phase region) In the first stage ad a) refinement of originally coarse initial austenite occurs by effect of repeated deformation and recrystallization cycles, however, relatively coarse ferritic grains are obtained after the subsequent phase transformation, possibly even mixed microstructure, comprising also coarse bainite. In the stage ad b) deformation bands are formed in a volume of extended nonrecrystallized austenitic grains. Ferrite nucleates both on boundaries of primary austenitic grains and on deformation bands. The final effect is further refinement of the structure. In the stage ad c) a deformation process is performed in conditions of dual-phase austenitic-ferritic microstructure. The deformation occurs not only in the austenitic, nontransformed matrix, but also in ferrite at the same time (subgrains are formed). The microstructure formed in ad b) and ad c) stages consists of uniform ferritic grains (as a product of the austenite decomposition) and deformed (hard) ferritic grains which show specific substructure. One of the main positive characteristics of the controlled rolling process is formation of deformation bands which divide particular austenitic grains to number of blocks, or "pseudo grains (other potential sites for ferrite nucleation). Accelerated cooling (ACC), which follows the hot rolling process, influences a change in the obtained final structure significantly. The accelerated cooling effect is schematically depicted in frame of Fig If the rolling process was finished in the high-temperature austenitic region and the follow-up air cooling was performed, predominantly a mixed microstructure of ferrite and upper bainite occurs. If the accelerated cooling method was chosen after final rolling in the region of suppressed recrystallization of austenite (i.e. the region ad b), then the resulting microstructure of low-carbon steel comprises ferrite with a possibility of dispersively segregated islands of pearlite, bainite and also martensite. A type of the ferritic structure and a Author: Eva Mazancová 8

4 Temperature C Subject Modern methods of Heat Treatment possibility of formation of a multi-phase structure are then influenced by speed of cooling from the finishing temperature, which also affects a microstructure type in the final stage. The important fact is that the accelerated cooling in the given region suppresses banding of segregations, which results in unbalances between strength and plastic properties. austenite deformation bends polygonal ferrite ACC 10 C/s min. air cooling fine deformed ferrite Time F1 F3 F3 F3 Cooling rate 10 C/s Fig. 1.2 Effect of rolling processes on the final structures If the accelerated cooling method was chosen after final rolling in the region of existence of dual-phase microstructures (i.e. the region described ad c)), then the resulting microstructure in steel (low-carbon) comprises ferrite with occurrence of pearlite, or with a low proportion of bainite, occurring in areas of banded segregations. These segregation bands are very fine, but usually very thick, which also corresponds with refinement of the final matrix. In any case, after the accelerated cooling the microstructure is significantly finer and more homogenous than after the conventional controlled rolling (even if the final forming process was performed in the intercritical, dual-phase area). Here the banding of segregations represents inhomogeneity of final properties, particularly plastic, because each segregation band is a source of an obstacle to cleavage crack propagation in a transverse direction, however, not in the longitudinal direction. Author: Eva Mazancová 9

5 1.2. Deformation in austenitic area accompanied by recrystallization processes The aim of a deformation in an austenitic region, where recrystallization can occur, is to obtain an austenitic structure refined to maximum as a result of repeated deformation and recrystallization processes. In the controlled rolled steel, a "mixed ferritic microstructure with various sizes of ferritic grains occurs frequently, which reduces the effectiveness of controlled rolling and leads to a certain degradation of notch toughness values. As a matter of fact, a mixed ferritic microstructure is a result of formation of an initial heterogeneous austenitic microstructure, which can be often found for example in steels micro-alloyed with e.g. niobium. So an important requirement is to suppress the formation of the heterogeneous initial austenitic matrix and to reach the maximal homogeneity and minimal grain size (the initial state) prior to the subsequent transformation to ferrite. A heterogeneous austenitic matrix occurs as a result of a local coarsening, supported by development of a strain induced migration of austenite grain boundaries initiated by small passes (a low degree of deformation) in the initial stage of rolling. Aside from grain coalescence, an influence of other processes may be considered: a) Abnormal grain growth during a relatively long holding period after the deformation resulting in formation of a heterogeneous austenitic microstructure; b) An influence of an initial heterogeneous austenitic microstructure - resulting in formation of heterogeneously recrystallized austenite; c) Partial recrystallization either prior to a following deformation (in repeated deformation cycles) or prior to the very final phase transformation Deformation in suppressed recrystallization area The aim of deformation in a suppressed recrystallization region is to increase a number of potential sites for ferrite nucleation as a result of formation of deformation bands inside the volume of deformed austenitic grains. Figure 1.3 summarizes transformation characteristics and parameters of a microstructure formed in a hot deformed material subjected to a subsequent heat treatment. An essential difference between the conventionally hot rolled steel and controlled rolled steel is following: Author: Eva Mazancová 10

6 a) In the conventional processing, nucleation of ferritic grains occurs solely on grain boundaries of the initial austenite; b) In the controlled rolled material, ferrite nucleation occurs both inside the volume (intragranularly) and also on grain boundaries (intergranularly). Fig. 1.3 Comparison of influence of selected treatment types in final microstructure Ferritic transformation intensity is also higher on boundaries of deformed austenite grains than on boundaries of recrystallized austenite grains. At the same time, the nucleation process can occur (in a case of non-recrystallized, strain hardened austenite grains) intragranularly, too, as mentioned above. Then, formation of different ferrite microstructures is also a result of these different nucleation characteristics. Deformation bands with their nucleation effect are equivalent to grain boundaries of the deformed austenite, so austenitic grains distributed into blocks by deformation bands provide more surfaces potential sites for the following transformation. A difference in the ferrite nucleation in conventionally hot rolled steel and controlled rolled steel is similar, such as in normalized steel and quenched steel. In normalized steel the ferrite nucleation occurs solely on austenite grain boundaries, while in quenched steel the initial austenite grains are distributed into particular blocks as a result of martensitic phase transformation. In term of the ferrite nucleation, deformation bands play the same part as grain boundaries, because disorientation between neighbouring areas at a deformation band is high, such that deformation bands can be considered a certain type of a high-angle boundary. Author: Eva Mazancová 11

7 Inagoti and Umemoto dealt with deformation bands in details. A deformation band consists of two parallel lines of a very sharp contrast with regard to a matrix and its look is similar to an annealing twin. Its width is related to dissociation of a full dislocation to partial ones and is inversely proportional to the matrix stacking fault energy. During hot forming, twin boundaries lose their coherence and the matrix is then strongly deformed. In order to maintain the deformation compatibility between twin boundaries, the surrounding matrix must be strongly deformed, so that preferential nucleation of ferrite grains may occur here. The matrix areas around the grain boundary are strongly deformed, too, so that the deformation compatibility is maintained here as well. In principle, a positive effect of the controlled rolling on refinement of the ferrite grains relates to a higher nucleation efficiency of grain boundaries (related to a unit of a surface area of austenite grain boundaries), which are places of a strong local deformation. In this sense a different efficiency of grain boundaries of recrystallized and strain hardened austenite is exhibited and it can be assumed for recrystallized austenite grains that grain boundaries are not places of a localized deformation. The ferrite nucleation intensity level can be overviewed as follows: a) Austenite grain boundaries potential sites for ferrite nucleation (edges of austenite grains are preferential sites, then a more frequent ferrite nucleation occurs on grain boundaries of a deformed austenitic matrix); b) Twin boundaries regarding low energy, coherent twin boundaries cannot be considered as suitable nucleation sites. In the case of an intensive deformation in a temperature area corresponding to suppressing the austenitic matrix recrystallization development, strongly deformed areas occur around twin boundaries, where nucleation of ferrite grains occurs; c) Deformation bands can be considered effective sites for ferrite nucleation. Though, their nucleation effectiveness is not the same (probably due to a considerably variable energy potential). Deformation bands of higher energy relate to a higher degree of deformation of the austenitic matrix. It can be assumed that the development of recovery processes (recovery of a dislocation substructure) in areas situated between deformation bands hobbles the intensity of nucleation processes (formation of ferrite grains) in austenite intensely; Author: Eva Mazancová 12

8 d) Second-phase particles ferrite nucleation is observed on a phase interface of undissolved precipitates; e) Subgrain boundaries it is very difficult to obtain a definite microscopic evidence on ferrite nucleation on sub-grain boundaries originating during the matrix recovering, however, quite frequent origination of ferrite inside austenitic grains without a detectable presence of second-phase particles or deformation bands can be actually interpreted as a manifestation of this process, or of an effect of subgrain boundaries. The ferrite nucleation efficiency in a deformed austenitic matrix depends on the deformation degree before the austenite-ferrite phase transformation. This may imply a significant role of high localized energy at a grain boundary, as well as the very grain boundary, which play significant parts in the ferrite grain size control. The above mentioned analysis has shown the preferential nucleation on grain boundaries, twin boundaries and deformation bands in the deformed austenite. Out of these potential sites, grain boundaries represent the most effective positions for ferrite grain nucleation. The high nucleation intensity of grain boundaries relates to the high energy of grain boundaries; there is also a relating demand for grains of the initial austenitic matrix to be as fine as possible and thus, along with occurrence of numerous deformation bands, a very fine ferritic microstructure can be obtained. Fig. 1.4 Schematic image of deformation bands in suppressed recrystallization area Author: Eva Mazancová 13

9 In Fig. 1.4 deformation bands after a deformation in a region of suppressing recrystallization process development are depicted schematically. The following process sequence can be found in this figure: 1) The first one corresponds to fine-grain recrystallization and a follow-up strong deformation of a matrix; 2) The second one corresponds to the state after repeated heating of austenite and its deformation; 3) The third one corresponds to recrystallized grains of a mixed type originated as a result of grain coalescence and their subsequent deformation. Deformation bands are not always distributed evenly in a matrix, for example some grains not oriented favourably for formation of deformation bands do not contain any in their volume or only a limited number of deformation bands. A mean distance between the effective boundaries, which include both austenite grain boundaries and deformation bands, is substantially shorter in a fine-grained microstructure than in a microstructure consisting of coarse grains, e.g. formed by a coalescence process. Fig. 1.4 shows, that the most advisable procedure in order to reach a homogenous and fine-grained ferritic matrix is to initiate from a recrystallized fine-grained and homogenous austenitic matrix subsequently deformed in a region of suppressed (restricted) recrystallization Deformation in austenitic-ferritic area Enhancement of strength characteristics, mainly tensile strength, can be achieved through a deformation in the dual-phase austenitic-ferritic region. Steels deformed in the dualphase region have a microstructure consisting of equiaxed ferritic grains with a specific substructure. The character of the so-called hard grains is different, namely depending on a recovery degree, from recrystallized ones up to strongly deformed ferritic grains. Then this structure behaves as in a case of dual-phase steels, in which the strength level depends on a volume proportion of hard grains and on the hard to soft grain hardness ratio. Deformation in the dual-phase region influences the achieved toughness level through the following two ways: a) Texture formation Author: Eva Mazancová 14

10 b) Development of microstructure changes After hot rolling in the dual-phase region a crystallographic texture originates, which is the more noticeable, the higher the deformation level and the deformed ferrite content is. Two types of textures are formed: 1) 110 parallel to the rolling direction 2) 001 perpendicular to the rolling direction A texture of a type ad 1) induces anisotropy occurrence, the second type of a texture ad 2) contributes to the brittle failure susceptibility development, for instance along the thickness of a rolled product, which subsequently contributes to delamination effects (i.e. a decrease in cohesion strength along the thickness). The delamination development comes out as a decrease in the transient temperature in the longitudinal as well as transverse direction, while toughness decreases along the thickness at the same time. It can be stated that the delamination actually acts favourably on the achieved level of low-temperature toughness in the longitudinal as well as transverse direction, but it has a negative impact on toughness along the thickness of a product. The deformed ferrite changes into a recrystallized, finegrained structure or into a recovered ferritic matrix, or non-recovered ferrite, strain hardened, namely depending on a degree of development of recovery processes. Toughness (TG) is improved under decreased temperatures by formation of a recrystallized, fine-grained microstructure, but a change in sub-grain formation has not been found, as a result of the recovery process development. Summarization of effects of individual factors having an impact on notch toughness the transition temperature (T t ) level in longitudinal/transverse direction can be predicted as follows: T t = - d -1/2 + T text -.n s + k 1 ( ph + k 2 /k 1. dh + k 2 /k 1. sub ) (1) Where d represents ferritic grain size, T text is contribution to transition temperature thanks texture development, n s represents delamination number, ph is precipitation strengthening, dh is dislocation strengthening, sub expresses strengthening caused by sub-grains formation and,,, k 2, k 1 are constants. The 4 th term in equation (1).n s and the last term of mentioned equation strongly influence the transition temperature. In case of higher contribution of dh than the delamination contribution (.n s term), the transition temperature is going up and on the contrary. Author: Eva Mazancová 15

11 1.5. Controlled transformation of austenitic matrix with accelerated cooling application Further to the above discussed behaviour of steel and meeting the metallurgical principles, application of accelerated cooling (ACC) from the finishing temperature immediately following the controlled forming process (CR) can be considered effective. Through this processing variant a high degree of microstructure homogeneity can be achieved, which then exhibits not only uniform mechanical properties along all directions, but also e.g. high resistance to hydrogen embrittlement, leading to suppression of occurrence of the banding of segregations. During conventional processing (i.e. using only the controlled forming process with following air cooling) multiple heat treatment needs to be performed very often in order to reduce, or possibly to eliminate, the above mentioned banding of segregations. This does not represent an up-to-date approach to the issue and brings along only financial costs and uncertain results. Fig. 1.5 shows (in a general view and in a detail) a microstructure exhibiting reliable resistance to hydrogen embrittlement and, at the same time, a high level of tensile properties along with favourable toughness. The given microstructure predominantly comprises (approximately 80 %) fine intragranular acicular ferrite, which thanks to specific mutual blocking of laths features not only favourable toughness, but even hydrogen response. A matrix is pearlitic as a result of final air cooling. a) b) THE ANALYSIS OF MICROSTRUCTURE CHARACTERISTICS INCREASING THE RESITANCE OF SOUR GRADE STEEL TO HYDROGEN EMBRITTLEMENT Fig. 1.5 Microstructure image of micro-alloyed, low carbon steel after controlled rolling application and ACC (accelerated cooling process a) general view, b) detail of the a) case Author: Eva Mazancová 16

12 Conclusions from former studies dealing with issues of hydrogen embrittlement imply that in the ACC technology a number of strict principles arising from physical-metallurgical requirements on the controlled mastering of this process have to be heeded. Ensuing from this context is a need to maintain the temperature level not only at the beginning, but also at the end of ACC and, at the same time, also the cooling rate value between the beginning and the end of ACC. At the beginning of accelerated cooling from the temperature lower than Ar 3 an increase in toughness can be achieved compared to conventional conditions, however, in central parts of sheets a differently developed band microstructure is still maintained. Then, a potential possibility of susceptibility to hydrogen embrittlement relates to this. If the ACC beginning is shifted to the temperature region above Ar 3 (at least about 15 C), the harmful banding suppression can be achieved (of course at meeting basic metallurgical principles described above). Under these conditions a homogenous microstructure originates, which comprises a uniform distribution of ferrite and bainite regions in a matrix, thus creating more favourable conditions for a distribution of hydrogen atoms. Fig. 1.6 Schematic depiction of rolling processes resulting to different final microstructure types Author: Eva Mazancová 17

13 Figure 1.6 demonstrates various variants of cooling after forming. The most advantageous cooling method is F2 variant with a majority proportion of a chaotic microstructure of acicular ferrite without the harmful banding. The achieved toughness level as well as hydrogen response is influenced also by temperature of stopping ACC. A temperature ranging around 500 C is considered optimal. This temperature at which cooling begins is determined both by a requirement for suppressing pearlite origination, and has to prevent (after full cooling) formation of martensite or bainite, because both present hard components which contribute to less favourable toughness level. Existing experience imply that during ACC the chosen cooling rate affects the achieved toughness level, and thus also hydrogen response. At the cooling rate not exceeding 10 C/s a full suppression of formation of a band microstructure usually does not occur, therefore achieving the cooling rate about 15 C/s to 30 C/s is recommended, namely e.g. for sheets of 16 to 20 mm thickness. The basic principles, as described above and summarized into a scheme including also the ACC processes and air cooling, have been presented in papers before. However, more comprehensive outlined designs have occurred recently, taking advantage of repeated ACC processes within the given thermo-mechanical processing. In the given case the following fundamental stages can be characterized: a) Steel austenitization (slabs) including ensuring dissolving segregated carbides of micro-alloying additives in a matrix; b) Rough rolling leading to a fine-grained recrystallized austenitic structure; c) Rolling in a suppressed recrystallization region ensures hardening of a matrix; d) Final forming shown in Fig for F2 variant is finished closely above Ar 3 temperature, after which ACC down to a range of 500 C follows in order to prevent the unfavourable banding. The subsequent cooling can be performed in air. The use of AF (acicular ferrite) microstructure offers achieving enhanced strength at maintaining the demanded toughness level. AF origination is related to two transformation processes, as in bainite, namely a mixture of diffusion (carbon diffusion) and displacement transformation (through shear) of plates (or laths). A beginning of this transformation is on the upper bainite formation level, but a mechanism of the formation differs in principle. AF is nucleated intragranularly in a volume of grains on appropriate types of nonmetallic inclusions or precipitates. The given mechanism leads to nucleation of AF plates (or laths) Author: Eva Mazancová 18

14 oriented in various ways further to inclusions, or precipitates. As a result this means enhanced disorientation between AF plates (or laths), which is characterized by short unit free routes for cleavage cracks growth (determined by higher disorientation between plates, possibly laths), while in bainite a higher disorientation level is only reached between individual bainitic packets. These consist of a set of parallel plates (or laths) usually initiated on grain boundaries. A free route for cleavage cracks growth in AF is about 3 to 5 m, while in bainite free routes between packets reach approximately 15 to 20 m. Between AF plates the high-angle phase interface is prevailing, while in a case of bainite this type of interface has been only noticed between individual packets and between plates (or laths) of bainite. Then, the AF microstructure shows a significantly higher resistance to cleavage crack growth. This tendency is maintained also in term of a resistance to the harmful effect of hydrogen. The schematic depiction of austenite transformation to bainite, AF and other possible products is shown in Fig Fig. 1.7 Schematical depiction of austenite transformation products and their differences Analysis of segregation banding formation Formation of a microstructure consisting of pearlitic bands relates to a heterogeneous distribution of carbon in a matrix. This type of microstructure can be found in a case of air cooling of rolled products, to which also an average cooling rate ranging between 0.5 to 1.0 C / s corresponds. In the matrix initial state only Mn segregation development is assumed, while in the case of C only a homogenous distribution is assumed. Considering Mn high Author: Eva Mazancová 19

15 segregation ranging usually around 50% and in the extreme even 70%, in differently nonsegregated areas a difference in Ar 3 temperatures by as much as 50 C occurs. Then, this difference corresponds (at cooling rate 0.5 C/s) with a time difference at the beginning of austenite decomposition to ferrite t up to 100s. Providing that the austenite decomposition to ferrite is controlled by carbon diffusion in austenite, then a width of formed ferritic areas (F) in time -t- can be expressed as follows: F =.t 1/2 (2) where represents the so-called Zener parabolic growth constant which is determined by the following relation: = D 1/2 (C e C 0 ) / (C e C e ) 1/2. (C 0 C e ) 1/2 (3) where D stands for carbon diffusivity in austenite, C 0 stands for the initial C concentration in e steel and C and C e represent equilibrium concentrations of carbon in austenite, or in ferrite on the phase interface austenite ferrite. Solution of conditions for austenite decomposition in a segregated zone and in a matrix non-segregated area arises from the so-called paraequilibrium conditions. Redistribution of substitution dissolved elements in a specified phase transformation (in this case Mn) is not included. At the unidirectional diffusion flow of C and above mentioned difference of Ac 3 temperatures and time t = 100s, the ferritic area width corresponds to approximately 36 m. In the calculation, Ar 3 mean value on 700 C level was considered in a segregated as well as non-segregated zone. Carbon atoms are pushed out of ferritic regions of a width mentioned above, i.e. 36 m, into neighbouring segregation zones. These C redistribution processes occur during air cooling. When applying some ACC variant (of course on condition of meeting the requirement for an adequate cooling rate), a change in C distribution in the basic matrix does not occur. This results in suppressing martensite formation, (low bainite) in segregation areas, as mentioned above. Then the matrix consists of homogenous ferritic grains with small interferritic bainite regions. As the above discussed results for resistance to hydrogen embrittlement imply, AF formation in the basic matrix can also take a significant share. Both a proper selection of a sequence of individual deformation processes, related to recrystallization or not (at lower temperature), and an existence of appropriate nonmetallic inclusions (of a size about 0.5 to 3 m) participate in AF formation. Author: Eva Mazancová 20

16 At proper energy of the phase interface inclusion-af, these act as potential nucleation sites for AF formation. Summarization of chapter terms In the end of chapter main terms are recapitulated that you should master and understand their sense, resp. mutual connections and their impact on final properties of matrix. Deformation in recrystallization area, suppressed recrystallization and in two phase area in relation to austenite transformation products, acicular ferrite, the ACC process, segregation banding. Qustions: 1. What the basic mechanical-heat treatment stadiums do you know? 2. What does it go on in deformation process in recrystallization area? 3. What is under way in matrix in suppressed recrystallization area? 4. What the impact on structure has deformation process in two phase area? 5. What the impact have deformation bands on properties of deformed matrix? 6. What microstructures are formed after cooling from finishing rolling temperature in recrystallization area, suppressed recrystallization and in two phase area? 7. What the potential ferrite nucleation position do you know? 8. How is it possible to control austenite transformation so that optimised microstructure and properties after rolling process and subsequent cooling were reached? 9. What the microstructure type is connected with ACC process and what impact on final properties given process demonstrates? 10. What forming conditions and ACC result to optimisation of steel properties without subsequent heat treatment? 11. Could you explain differences between acicular ferrite and upper bainite? Author: Eva Mazancová 21

17 12. Do you know what is source of segregation banding and how it influences the final properties of matrix? Literature: TANAKA, T. Internat. Metallurgical Reviews, 26 (1981) 185. QU, S., ZHANG, P., WU, S.D., ZANG, Q.S., ZHANG, Z.F. Scripta Materialia, 59 (2008) INAGATI, M. Trans ISIJ, 23 (1983) 775. ÜMEMOTO, M., OTSUKA, H., TAMURA, I. Trans. ISIJ, 24 (1983) MAZANEC, K., MAZANCOVÁ, E. Physical metallurgy of thermomechanical treatment of structural steels. The 1st. Ed. Cambridge Int. Sci. Publishing, (1997) 143. ZHAO, M.C., SHAN, Y.Y., XIAO, F.R., YANG, K. Mater. Sci. Tech., 19 (2003), 355. GARCIA-MATRO, C., CORNIDE, J., CAPDEVILA, C., CABALLERO, F.G., GARCIA DE ANDR0AS, C. Acicular ferrite transformation under the influence of V precipitates. In proc. of New developments on metallurgy and applications of high strength steels. Asoc. Argentina de materiális. Buenos Aires, (2008) 1. MAZANCOVÁ, E. The thick sheet resistance against hydrogen induced cracking treated under various conditions. In. proc. of conf. METAL2010, Ed. Tanger, s.r.o. Ostrava, Rožnov pod Radhoštěm, (2010).471. Author: Eva Mazancová 22