Achievement of the required level of strength properties of contemporary steels for pipes may be realized by such methods as thermomechanical treatment (TMT) and addition of alloying/microalloying elements in a certain ratio [1, 2]. It is well known [3] that the main task of microalloying steel subjected to TMT includes action on structure formation processes in different stages of the production process with the aim of obtaining two other types of structure even before austenite transformation: recrystallized fine-grained or deformed with a high effective specific surface. In the first case the main task is expansion of the field for occurrence of complete austenite recrystallization and retardation of recrystallized grain growth, and in the second in contrast it is expansion of the region of recrystallization retardation [4]. In this case steel microalloying makes it possible to improve metal strength properties by a precipitation hardening mechanism [5].

The AO Vyksa Metallurgical Plant (AO VMZ) specializes in the production of straight-seam electrically welded pipes including those from in-house rolled product that is prepared under conditions of a casting and rolling unit (CRU) by contemporary technology: melting, thin slab casting, direct rolling followed by accelerated cooling, and winding on a coil [6]. The existing concept of chemical composition for rolled product manufacture under CRU conditions is microalloying steel solely with niobium in order to achieve the required set of mechanical properties [7]. In this case alternative versions of microalloying are also considered [8,9,10].

Price instability for niobium and vanadium in world markets requires metal product manufacturers to consider immediately several microalloying schemes making it possible to obtain the required strength properties in relation to the current situation. The effect of each of these elements on processes of structure formation is broadly known [2]. Niobium is used for grain refinement, increasing steel strength, ductility, and cold resistance [11]. Vanadium may be used in these steels for improving strength [12, 13]. It is used to a considerable extent in heat-treated steels, and also steels with increased strength but limited specifications for toughness and cold resistance, for example for building structures, including pipe steels. Use of vanadium is also possible in hot-rolled metal, within which due to implementing a precipitation hardening mechanism strength increases with a significant reduction in ductility properties [14].

Three different microalloying systems are considered in the present work in order to provide strength class K52 for rolled product:

  • basic with addition of niobium (Nb) alone;

  • with addition of niobium and vanadium (Nb–V) in order to compensate part of the manganese by vanadium without loss of strength properties and level of cold resistance;

  • with addition of vanadium and nitrogen (V–N) in order to reduce the manganese content and complete replacement of niobium by vanadium.

The Aim of Work is to study the effect of microalloying system on structural state, and also on the type and nature of dispersed phase particle precipitates (carbonitride phases) on the level of mechanical properties of lowcarbon steels produced under AO VMZ CRU conditions.

Research Material and Procedure

The material studied was rolled product of low-carbon steels of three microalloying systems (Nb, Nb–V, V–N) produced under CRU conditions by continuous casting technology for thin slabs combined with subsequent rolling and winding on a coil.

Steels were melted in an electric arc furnace, given extra-furnace treatment, cast in a CBCM into slabs 90 mm thick, the temperature was levelled out through the slab section in a tunnel furnace to 1150 °C, and rolled in a 1950 broad-strip continuous mill in two stages: the first in two roughing group stands, and the second in six finishing group stands. After completion of rolling strip 8 mm thick was cooled rapidly and wound on a coil. Test metal chemical composition is provided in Table 1.

Table 1 Test Steel Chemical Composition, wt.%
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It is well known that apart from chemical composition the final rolled product structure due to the TMT regime has a marked effect [2, 15]. Considering the microalloying features of the test steels, two main versions of TMT were used: controlled and recrystallization rolling. Controlled rolling was used for steel microalloyed with niobium (Nb, Nb–V). In the roughing stage regimes were implemented providing 100% occurrence of static recrystallization. In the finishing pass stages there was no recrystallization due to the retarding effect of niobium and selection of the appropriate temperature for the start of deformation.

A recrystallization rolling regime was used for steels with a V–N microalloying system, with which alongside complete recrystallization in the roughing stage, total recrystallization is also provided in finishing rolling stage passes, i.e., vanadium cannot retard recrystallization process as efficiently as niobium [16].

Metal structural state was evaluated in microsections prepared in the longitudinal direction with respect to the rolling direction after chemical etching in 4% alcoholic nitric acid solution, using an Axio Observer.D1m optical microscope with a the Thixomet Pro analysis system.

The fine structure was studied in a JEM200CX transmission electron microscope with an accelerating voltage of 120 kV. The TEM resolution capacity was ≈ 1 nm (without using direct resolution). Specimens in the form of thin foils were prepared by a standard procedure of electric polishing using an electrolyte based on orthophosphoric acid and chromic anhydride. Then numerous fields of view were examined in the foils prepared with aim of revealing carbonitride phase particles.

Determination of the test coiled rolled product mechanical properties for the three microalloying system was performed:

  • with static tensile tests according to GOST 1497 on flat total thickness specimens with determination of ultimate strength (σf, N/mm2), yield strength (σ0.2, N/mm2), and relative elongation (δ5, %);

  • with dynamic tests for impact bending according to GOST 9454 of specimens with a Charpy stress concentrator in the temperature range from 0 to –100 °C impact strength (KCV, MJ/m2) and the proportion of ductile component in fractures (BKCV, %) were determined. Ten specimens were tested at each temperature.

Research Results and Discussion

Steel microstructure of the three alloying systems studied using an optical microscope is represented by a ferrite-pearlite mixture with different morphology (Fig. 1, Table 2).

Fig. 1
figure 1

Test metal of three microalloying systems microstructure: (a) Nb; (b) Nb–V; (c) V–N.

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Table 2 Test Steel Microstructure Evaluation
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Within the metal structure with addition solely of niobium (Nb microalloying system) presence of two types of ferritic component was observed: polygonal (quasipolygonal) and acicular morphology in the ratio 20/80% (Fig. 1a). The microstructure had different grain size: relatively coarse grains of polygonal ferrite with a size of ≈ 5–10 μm, and fine grains with a size of ≈ 2.5 μm were observed. Presence is noted of disseminated cementite over grain boundaries.

Steel with niobium and vanadium (Nb–V microalloying system) mainly has a ferritic matrix of polygonal morphology with an average diameter of ≈ 8 μm, but acicular morphology ferrite is encountered, whose volume faction within the structure is ≈ 40% (Fig. 1b). Areas of high carbon-containing phases (pearlite/cementite inclusions over grain boundaries) with size of 2–3 μm are present in a small amount, i.e., up to 2%.

Steel structure with vanadium and nitrogen (V–N microalloying system) consists entirely of polygonal ferrite (Fig. 1c). Differences in grain size are observed: the main proportion is coarse grains of polygonal ferrite with a size of ≈ 10–15 μm. Alongside them are accumulations of finer grained ferrite with a size of 5 μm or smaller. Two types of high-carbon phase are encountered: a small proportion of pearlitic component with a size of ≈ 5–10 μm, and quite coarse precipitates (≈ 0.5 μm) of darkly etching phase (cementite) that are observed within the body of a ferrite grain.

By comparing results of studying the microstructure of low-carbon steel of three microalloying systems using optical microscopy it may be noted that addition of niobium not only refines the metal matrix structure, but also changes ferrite grain morphology, i.e., it increases the proportion of acicular structure.

A study of the fine structure in specimens of the Nb microalloying system showed that the matrix structure is predominantly bainitic ferrite (Fig. 2a) having the shape of blocks, i.e., from similar to equiaxed to variably extended, and presence of a small proportion of polyhedral ferrite. Apart from ferrite a small amount fine (as a rule up to 10 μm) areas of pearlite and degenerated pearlite is observed, and the amount of the latter of predominates (Fig. 2b). Over ferrite grain boundaries areas of cementite precipitates are encountered, and in this case extended inclusions (with a size of several microns) are hardly observed. The dislocation density within ferrite is variable, and the distribution is random (Fig. 2c).

Fig. 2
figure 2

Results of studying the fine structure of metal of the Nb microalloying system; (a) bainitic ferrite (light field), × 15,000; (b) pearlite light field) × 15,000; (c) dislocation density within ferrite (light field), × 30,000; nanoparticles formed within austenite (dark field in nanoparticle reflection), × 30,000.

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Interphase particles or particles of mixed type are not detected. All of the nano-particles revealed were formed within austenite. Their bulk density as a rule is low, and rarely moderate (Fig. 2d), and a typical size is 3–30 nm (individually up to ≈ 25 μm). Apart from nano-particles arranged within the body of ferrite grains areas are revealed of their precipitation over grain or block boundaries.

In specimens of the Nb–V microalloying system the matrix structure is polyhedral ferrite with a grain size of 10 μm with a small amount of grains with a size of more than 10 μm (Fig. 3a), and also bainitic ferrite, having the shape of blocks from close to equiaxed to variably extended (Fig. 3b). The volume fractions of polyhedral ferrite (PF) and bainitic ferrite (BF) are comparable. The dislocation density within ferrite is variable, and the distribution is random (see Fig. 3c).

Fig. 3
figure 3

Results of studying the fine structure of metal of the Nb–V microalloying system; (a) polygonal ferrite ((light field); (b) bainitic ferrite (light field), × 15,000; c) dislocation density within ferrite (light field), × 30,000; (d) nanoparticles (dark field), × 30,000.

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Disseminated cementite is observed in a small amount over ferrite grain boundaries, and also within the composition of pearlitic sections and regions occupied by degenerated pearlite (size the same as for other regions does not exceed 10 μm).

Nano-carbonitrides are rarely observed and in a small amount: often they are observed over ferrite grain boundaries (Fig. 3b) are rarely within the body of grains. For the reason of low cleanliness the volumetric density of particles is low, and interpretation of their type is difficult. The size of intragranular particles is ≈ 2–4 nm, and particles up to ≈ 6–7 nm are distributed over grain boundaries.

In specimens of the V–N microalloying system the matrix structure is mainly represented by polyhedral ferrite. Grains of different sizes are observed, i.e., within the limits of 10 μm and coarser. The proportion of grains with size of more than 10 μm is quite high. The dislocation density within ferrite is variable and low to moderate, and the distribution is random (Fig. 4a). Apart from ferrite a small of fine (up to 10 μm) areas of fine pearlite are present.

Fig. 4
figure 4

Results of studying the fine structure of metal of the V–N microalloying system; (a) dislocation density within ferrite (light field), × 30,000; (b) pearlite light field) × 15,000; (c) Nanoparticles of mixed type (dark field), × 30,000; (d) nanoparticles formed within austenite (dark field), × 30,000.

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The frequency of revealing nano-carbonitrides within the structure is high. Interphase nanoparticles (mixed type) are observed, whose main volume faction was formed within the ferritic region (Fig. 4c). The size of nano-particles varies from section to section, i.e., from 4–12 nm.

Nano-carbonitrides, formed within austenite, are observed systematically (Fig. 4d). The volumetric density of particles varies from low to moderate with a size of 3–10 nm. These particles during rolling may be centers for generation of new ferrite grains during polymorphic transformation and thereby refine the grain size [17].

As a result of studying the fine structure of specimens of the three microalloying systems three main types have been detected (with respect to generation and growth) of carbonitride particles of microalloying elements (MAE): particles formed within ferrite, separated into interphase and mixed, and particles precipitated within austenite (Table 3).

Table 3 Analysis of Specimen Fine Structure Parameters Studied by Transmission Electron Microscopy
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Particles of MAE formed within the ferritic region were observed in all specimens containing V within the composition. In this vase within steel of the V–N microalloying system precipitate density was higher with a maximum size of 15–20 nm. Within metal with a reduced vanadium and nitrogen content the density of particles and their size appeared to be significantly smaller, i.e., they were hardly seen in TEM images (less than 2 nm).

Nanoparticles formed within austenite are a unique from of particles detected by the TEM method in steel containing solely Nb, and they are very small, which is probably connected with the niobium content within steel (0.03%).

Results of static tensile tests showed that as a result if implementing different strengthening mechanism for coiled rolled product after controlled rolling and accelerated cooling the required level of strength properties is obtained in the steels corresponding to strength class K52 (Table 4). Due to solid solution strengthening on alloying with manganese in an amount of 1% and formation within the structure of finely dispersed ferrite grains of different morphology, on microalloying with niobium in an amount of 0.03% in steel of an Nb microalloying system values of ultimate strength of 530–550 MPa are very high. With a reduction in manganese content to 0.65%, niobium to 0.02%, and a little microalloying with vanadium in an amount of 0.04% the ultimate and yield strengths of the steel of the Nb microalloying system are at the lower acceptable boundary according to standard technical documentation for the range and comprise 510–520 MPa. In steel of the V–N microalloying system containing a very small amount of manganese 0.5% and also a very high amount of vanadium (0.10%) and nitrogen (0.022%) due to implementing precipitation hardening mechanism with vanadium particles results are obtained for ultimate strength similar to steel with niobium (520–540 MPa).

Table 4 Test Steel Mechanical Properties
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It should be noted that in all of the test steels high values of relative elongation (25–31%) were obtained outside the dependence on microalloying system.

Dynamic tests for impact bending of Charpy specimens showed a very low level of impact strength (Fig. 5a) and the proportion of ductile component in a fracture (Fig. 5b) in the temperature range from 0 to –80 °C exhibited by steel of the V–N microalloying system within whose structure after controlled rolling and accelerated cooling there was formation of a very coarse ferrite grain size of polygonal morphology. The ductile- brittle transition temperature (T50) with which in an impact specimen fracture after dynamic testing for impact bending 50% of ductile and 50% of brittle components are observed, is very high and comprises –65 °C. The greatest cold resistance was provided by a very fine steel structure with a large proportion of acicular morphology (Nb microalloying system). It seen clearly (Fig. 5a) that in the temperature range 0 to –60 °C within metal of the Nb microalloying system values of impact strength are observed on average higher by 100 J/cm2 compared with Nb–V and V–N steels. Such high impact strength values are explained by presence of acicular morphology ferrite within the structure providing crack propagation resistance compared with polygonal and quasi-polygonal ferrite. In this case the T50 temperature for steels with addition of niobium (Nb, Nb–V) is almost identical and corresponds to –75 to –80 °C. Steel with addition of niobium and vanadium showed intermediate results.

Fig. 5
figure 5

Dependence of impact strength (a) and proportion of ductile component in Charpy specimen fractures (b) on test temperature.

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As is well known, fineness and uniformity of the structure, and also final grain morphology, affect brittle failure resistance at negative temperatures [2]. It has been clearly demonstrated that addition of niobium not only leads to refinement of the final metal matrix structure, but also changes ferrite grain morphology from entirely polygonal to quasi-polygonal and acicular, which facilitates an increase in the level of rolled product cold resistance. The more acicular morphology within the metal structure correspondingly impact strength is greater.

Therefore, obtaining the strength class required may be provided by using different microalloy systems (Nb, Nb–V, V–N) under CRU conditions in relation to the current market situation and correspondingly the cost of niobium and vanadium.

On microalloying steel with niobium to a small extent metal strengthening is realized due to grain size refinement and forming an acicular ferrite structure. Combined microalloying with vanadium and nitrogen will make it possible to achieve the strength properties required by precipitation hardening. In this case the V–N microalloying system may save part of the manganese with provision of similar strength and ductility properties, but with limited cold resistance.

Conclusion

Achievement of the required level of user properties (strength, ductility, cold resistance) for low-carbon steels is possible by using different microalloying systems, implementing occurrence within metal of different strengthening mechanisms. Under casting and rolling unit conditions of the AO Vyksa Metallurgical Plant introduction into steel of additions of niobium and vanadium in different ratios has been checked in order to achieve strength class K52 for coiled rolled product. It has been demonstrated that use of the Nb, Nb–V, V–N microalloying systems provides the required strength level and in this case cold resistance is better than for steels microalloyed with niobium.