Influence of V and Ti on hot ductility of Nb containing steels of peritectic C contents

Abstract

The hot ductility of in situ melted tensile specimens of Ti–Nb containing steels having C contents in the peritectic C range 0·12–0·17% with and without V has been examined over the temperature range 700–1000°C. An improved testing regime for simulating the continuous casting process was used, which takes into account both primary and secondary cooling conditions. For the Nb containing steels, the ductility improved in the temperature range 750–850°C as the Ti/N ratio increased. However, ductility at 800°C was still below the 35–40% reduction in area values required to avoid transverse cracking. This was attributed to the copious precipitation of sub 40 nm NbTi(CN) precipitates along the grain boundaries and finer precipitates within the grains. Adding V to the Ti–Nb containing steels resulted in significantly improved ductility with reduction in area values at 800°C in excess of 45%. This improvement was due to a decrease in the fraction of fine particles, and in accord with this better ductility, transverse cracking of industrial slabs was avoided.     

Discussion

Abstract

The effects of sulphur on hot ductility of niobium steels, in which cracking susceptibility on the continuously cast slab surface is highest, have been studied by means of hot tensile testing with particular emphasis on the segregation of sulphur atoms to the matrix/grain boundary Nb(C,N) precipitate interfaces. When low manganese niobium steels are solution treated at high temperature and then deformed at temperatures ranging from the low temperature γ to the γ/α duplex phase regions, two troughs appear in the ductility versus strain rate curve, accompanied by intergranular fracture of γ, at strain rates of ~1 s^?1 and 10^?3?10^?4 s^?1. The loss of ductility at high and low strain rates is caused by dynamic precipitation of iron rich (Fe,Mn)S and Nb(C,N) particles, respectively, both within γ grains and on the γ grain boundaries. The dependence on sulphur content is obvious at high strain rates, but it is found that the loss of ductility owing to Nb(C,N) precipitation is also reduced by decreasing the sulphur content to less than 10 ppm. This can be explained by the reduced segregation of sulphur atoms to the grain boundary Nb(C,N) precipitate/matrix interfaces, leading to suppressed decohesion and consequent nucleation of microvoids which result in ductile intergranular fracture of γ.

MST/1425     

Influence of titanium and nitrogen on hot ductility of C–Mn–Nb–Al steels

Abstract

The influence of small additions of titanium on the hot ductility of C–Mn–Nb–Al steels has been examined. Titanium and nitrogen levels varied in the ranges 0·014–0·045 and 0·004–0·011 wt-%, respectively, so that a wide range of Ti/N ratios could be studied. The tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min^-1 to test temperatures in the range 1100–800°C and strained to failure at a strain rate of 2 × 10^-3 s^-1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the Ti/N ratio above the stoichiometric ratio for TiN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ～0·005 wt-%N ductility was very poor at the stoichiometric Ti/N ratio of 3·4 : 1. Ductility was better at the higher Ti/N ratios but only two of the titanium containing niobium steels gave better ductility than the titanium free niobium containing steels and then only at temperatures below about 950–900°C. One of these steels had the lowest titanium addition (0·014 wt-%), thus limiting the volume fraction of fine Ti containing particles and the other had the highest Ti/N ratio of 8 : 1. However, even for these two steels ductility was worse than for the titanium free steels in the higher temperature range. The commercial implications of these results are discussed.     

微合金元素Nb对低碳铸钢强度和冲击韧性的影响

采用中频炉冶炼制备不同Nb含量的微合金低碳铸钢,用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、液压万能强度试验机、半自动冲击试验机等手段研究了Nb微合金化对低碳铸钢显微组织、强度和冲击韧性的影响.结果表明,添加合适的微合金元素Nb可以使低碳铸钢的晶粒尺寸减小20.8％～34.6％,同时促进细小NbC析出相的形成,能有效提高低碳铸钢的强度和冲击韧性,晶粒细化和析出强化为其主要的强韧化机制.其中,含Nb量为0.044％的微合金铸钢屈服强度为350 MPa,抗拉强度为520 MPa,室温冲击功为119.7 J.与普通低碳铸钢相比,其塑性基本保持不变,但屈服强度、抗拉强度和室温冲击功分别提高了20.7％、7.2％和25.6％.     

Influence of peritectic phase transformation on hot ductility of high aluminium TRIP steels containing Nb

Abstract

Increasing Al from 0·05 to 1% in Nb containing transformation induced plasticity steel resulted in deepening and considerable widening of the hot ductility trough. Further increase in the Al level to 1·5% produced a trough similar to the low Al steel but having better ductility in the temperature range of 650–800°C. This improved ductility could be ascribed to its finer austenite grain size. Nb(CN) was able to precipitate readily in these steels and was important in influencing the hot ductility of the 0·05 and 1·5%Al steel in the temperature range of 750–1000°C, with ductility improving as the particle size increased with test temperature. No AlN was found in 0·05%Al containing steel, and there was no significant dendritic precipitation of AlN in 1·5%Al containing steel, although precipitation of AlN in plate form was readily observed. In 1%Al steel, copious dendritic precipitation of AlN was present at the γ grain boundaries, leading to rock candy fracture. The poor ductility shown in 1%Al containing steel is due to a combination of this dendritic precipitation, which took place only in a steel of peritectic carbon composition, and its coarse grain size. Both low and 1·5%Al containing steels had compositions outside the peritectic range. It is strongly advised that for this type of steel, the composition should be designed to fall outside the peritectic carbon range.     

The hot ductility of Nb/V containing high Al,TWIP steels

Abstract

The hot ductility of Nb/V containing high Al, twin induced plasticity (TWIP) steels has been examined over the temperature range 650–1150°C after melting and after ‘solution treatment’. Previous work had shown that the hot ductility is poor for the 1·5 mass-%Al, TWIP steel due to precipitation of AlN at the austenite grain boundaries, the depth of the trough being similar to that for an X65 grade pipeline steel but with the trough covering a much wider temperature range. Adding Nb and V made the ductility even worse due to the additional precipitation of NbCN and VN. Very low reduction of area values, 10–20% were obtained in the temperature range 700–900°C. Increasing the cooling rate to the test temperature resulted in even worse ductility. The ductility of these steels after ‘solution treatment’ is similar to that obtained after melting but when the cast was hot rolled followed by ‘solution treatment’ and cooling to the test temperature ductility improved due to grain refinement.     

Microstructure and mechanical properties of a rapid solidified boron- modified duplex stainless steel

Two duplex stainless steels 2205 and 2205 with 2.5?wt-% B addition prepared by a fast solidification technique were investigated. The samples were arc melted and cast in a cylindrical mould with varying diameters in a single cavity that provided different cooling rates. The hardness increased in both cases for smaller diameters, however, there was a different profile from the surface to the centre in case of 2205 with 2.5?wt-% B. The microstructural investigation indicated that boron addition led to the formation of hard borides and grain refinement. Different boride morphologies that varied with the cooling rates were identified. The compression strength at room temperature improved by a factor of 3.5 with boron addition without considerably decreasing the ductility.     

Influence of silicon,aluminium, phosphorus and boron on hot ductility of TRansformation Induced Plasticity assisted steels

Abstract

The hot ductility of steels having high aluminium or phosphorus contents, which are currently being considered as possible replacements for the conventional high silicon TRansformation Induced Plasticity (TRIP) steel, has been examined. Tensile specimens were cast in situ and tested in the temperature range 750 - 1000 ° C at a strain rate of 3 × 10^-3 s^-1. The ductility trough for the conventional high silicon TRIP steel was controlled by the austenite - ferrite transformation, intergranular failure occurring when a thin band of the softer ferrite phase formed around the austenite grains. Void formation at the sulphides situated in the soft ferrite at the boundaries then occurred, and the strain concentrated locally there. The thin bands of ferrite were deformation induced and, as such, formed at temperatures above Ar ₃ and could form at as high a temperature as Ae ₃. Adding ferrite formers such as silicon, phosphorus and aluminium increased the Ae ₃ temperature and thus widened the trough. The high aluminium (2%) TRIP steel exhibited good ductility throughout the temperature range examined, since large amounts of ferrite were always present, preventing strain concentration, and the AlN particles were too coarse to influence the hot ductility. In contrast, the 1%Al containing steel gave poor ductility below 850 ° C, the band of strain induced ferrite being extremely thin. The ductility trough in the titanium containing high phosphorus steel was poor, owing to fine precipitation of TiN. Adding boron to the steel and reducing the manganese content from 1.4 to 1% resulted in better ductility. Generally, the TRIP type steels had superior ductility to the conventional niobium containing high strength low alloy steel.     

Influence of grain size and precipitation on hot ductility of microalloyed steels

Abstract

The influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a fine grain-refining precipitate, AlN reduced the hot ductility in the C–Mn–Al steel and delayed the onset of dynamic recrystallization. Coarser precipitates produced by raising the reheating temperature allowing dynamic recrystallization to occur gave improved ductility. For the niobium- and vanadium-containing steels, precipitate distributions which were in a coarse randomly precipitated form gave the best hot ductility. These occurred with the niobium-containing steel when heated to 1100°C and more generally in the vanadium-containing steel throughout a wide temperature range. The worst precipitate distribution occurred in the niobium containing steel when the NbCN was taken into solution before testing and reprecipitated in a fine form at the γ grain boundaries and within the matrix during the test.

MST/490     

Effects of silicon and nitrogen on hot ductility of low carbon steels

The effects of N on the hot ductility of low carbon steels have been studied with particular emphasis on the relation with Si. The ductility of Si, Al-killed steels is largely reduced by slow strain rate (10_?3–10_?4S_?1) deformation at temperatures from low temperature γ to γ/α duplex phase region (from 750 to 950 °C in this case), accompanied by ductile intergranular fracture of austenite. The cause of the loss of ductility is found to be dynamic precipitation of hexagonal close packed (hep) (Si, Al)N both on the γ grain boundaries and within the grains, and the phenomenon is largely enhanced by either increasing Si or N content. Similar phenomena, i.e. precipitation hardening-like behaviour and dependencies both of deformation conditions and of Si and N contents, are also observed in Al-free Si-killed steels. The cause of this ductility loss should be ascribed to dynamic precipitation of some kind of silicon nitride, although the precipitation has not been detected directly in all the specimens examined.     

Influence of roughing rolling passes on kinetics of strain induced precipitation of Nb(C,N)

Abstract

The effects of reheating temperature and of the strain and temperature of roughing deformation passes on the kinetics of strain induced precipitation after a finishing deformation pass have been investigated for several niobium high strength low alloy (HSLA) steels. Strain induced precipitation was detected via its strengthening effect on a second finishing deformation carried out either by experimental rolling or by plane strain compression tests. Precipitates were also observed using thin foil electron microscopy. Decreasing reheating/roughing temperature and increasing roughing strain were found to significantly accelerate precipitation – this acceleration is attributed to clustering of niobium in solution.

MST/1409     

Structure and mechanical properties of unidirectionally solidified low alloy steels

Abstract

Two low alloy steels have been unidirectionally solidified in a liquid metal cooling Bridgman crystal grower. The dendrite morphology and dendrite arm spacing have been determined as a function of distance along the bars for solidification at various rates and under different temperature gradients. Microsegregation in the as solidified material was studied by electron probe microanalysis. The tensile properties of heat treated unidirectionally solidified material were determined and their values of elongation to failure found to be considerably greater than for the conventionally cast material. Similarly, the impact energy values of heat treated unidirectionally solidified material are higher than those of the conventionally cast alloy. The tensile and impact properties are discussed in terms of the strain incompatibility present during deformation both at the dendritic grain boundary and at the individual dendrite. Incompatibility of strain leads to a propensity for secondary cracking at these boundaries, the amount of which depends upon the detailed morphology of the dendrite which is determined by the solidification parameters.

MST/880     

Hot ductility of high Al TWIP steels containing Nb and Nb-V

The hot ductility of B-Ti-Nb-high Al (1.5%Al) containing TWIP steels having Ti/N ratios mainly in excess of 3.4/1 was obtained. After soaking at 1250°C, the tensile specimens were cooled at 12 or 60°C?min^?1 to the test temperature and then strained to failure at 3?×?10^?3?s^?1. Ductility was always good (reduction of area >40%), independent of Ti/N ratio or cooling rate. The good ductility is due to B segregation strengthening the grain boundaries and the low S level (0.005%S) limiting the volume fraction of MnS inclusions and restricting AlN precipitation to the matrix. Increasing the cooling rate, higher N levels and Nb resulted in a small improvement in ductility. An addition of V to the Nb-containing steels caused a slight deterioration in the hot ductility.     

Relative importance of transformation temperatures and sulphur content on hot ductility of steels

Abstract

Low (0·3%) and high manganese (1·4%) plain C – Mn steels with varying sulphur levels have had their hot ductility determined over the temperature range 700 – 1000°C, both after 'solution treatment' at 1330°C and directly after casting. It has been established that the width, depth and position of the hot ductility curves after solution treatment is more related to the transformation behaviour than either the sulphur in solution or the sulphide volume fraction or distribution. The growth of deformation induced ferrite at the austenite boundaries seems to be mainly diffusion controlled, and the higher is the transformation temperature for the γ – α phase change, the faster is the growth. Large amounts of ferrite can then form, giving good ductility. Thus, high transformation temperatures Ae ₃ or Ar ₃ are required to produce narrow ductility troughs. It is believed that any detrimental influence of the sulphides on these 'solution treated' steels is swamped by the rapid increase in ferrite volume fraction. For the as cast state, as more sulphides are able to precipitate at the interdendritic boundaries and austenite grain boundaries than in the solution treated condition, increasing the sulphur level causes a small deterioration in ductility at the high temperature end of the trough. In the present work, only narrow troughs have been found. This is in contrast to previous work on as cast C – Mn – Nb – Al steels, which exhibited wide troughs in the ductility curves, where it was shown that higher total sulphur levels lead to considerably worse ductility and that sulphur can be as detrimental to the ductility as niobium. It is recommended that, to avoid transverse cracking during continuous casting, in addition to keeping the sulphur level low, the carbon and manganese should also be as low as possible.     

Influence of calcium on hot ductility of steels

Abstract

The hot ductility of C–Mn–Al and C–Mn–Nb–Al steels with and without calcium additions have been examined over the temperature range 700–1000°C both after solution treating at 1330°C followed by cooling to the test temperature and directly after casting. Calcium additions invariably improved hot ductility. For hot rolled plate reheated to 1330°C and cooled to the test temperature, calcium is beneficial to hot ductility because it reduces the amount of sulphur able to redissolve and precipitate in a fine form at the new γ-grain boundaries produced on solution treating. For the C–Mn–Al steels, strain concentration occurred in the thin films of softer ferrite surrounding the γ-grains causing voiding around the sulphide inclusions that link up to cause intergranular failure. The removal of the sulphides by calcium addition therefore accounts for the improvement in hot ductility. Similar behaviour was observed for the C–Mn–Nb–Al steels, but for temperatures above the Ae₃ temperature calcium containing steels continued to give improved hot ductility over calcium free steels and this is believed to be due to the fewer sulphides present at the boundaries allowing an earlier onset of dynamic recrystallisation. For steels subjected to direct casting, interdendritic failure as well as intergranular failure by microvoid coalescence occurs. Calcium additions reduce the total amount of sulphur in the steel so that the volume fraction of sulphides precipitated at the interdendritic and γ-boundaries is low.

MST/962     

Effect of quench aging on drawability in low carbon steels

Abstract

The equilibrium solubilities of C and N decrease sharply between 721°C and room temperature, although equilibrium is rarely approached during the cooling cycles employed industrially. The resulting supersaturated ferrite is thus susceptible to the precipitation of C and N during storage and subsequent processing. Such ‘quench aging’ effects are evaluated for three typical post-hot rolling cooling rates employed in the processing of low C steel wire rod. The investigation was carried out on steels containing 32 ppm N (low N) and 102 ppm N (high N); these were heated to 900°C, cooled at 8,3, or 1·5 K S^?l, and then aged at room or freezer temperatures for periods of between 1 h and 532 days. The specimens subjected to the fastest cooling rate exhibited considerable quench aging effects, as did industrially processed specimens that had been cooled in the mill and aged for several weeks at room temperature. It is shown that the high N steel is more susceptible to quench aging than the low N material. The implications regarding the occurrence of breaks during wire drawing are discussed.

MST/3175     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Influence of B on hot ductility of high Al,TWIP steels

Abstract

The influence of B on the hot ductility of high Al, Ti containing twinning induced plasticity (TWIP) steels has been examined. It was established that provided the B was fully protected by adding sufficient Ti to combine with all the N, then B could segregate to the austenite grain boundaries and improve ductility. This improvement was particularly marked for the temperature range of 700–900°C, the range in which the straightening operation often takes place in continuous casting. Of most importance in the present work has been the detection of B at the boundaries using a secondary ion mass spectrometry technique. The cooling rate from the reheating temperature of 1250°C to the tensile testing temperature range of 700–1200°C was 60 K min^?1, but it is likely that slower cooling rates ≤25 K min^?1, more in keeping with the secondary cooling rate on continuous casting, will give even better ductility. Ti additions in themselves are beneficial to the hot ductility of these steels as precipitation of AlN at the austenite boundaries is avoided, but only if the cooling rate is sufficiently slow to allow the TiN particles to coarsen. However, to ensure freedom from cracking, an addition of B is also required.     

Effect of restoration on hot ductility of high alloy and duplex stainless steels

Abstract

The effect of restoration on the hot ductility of two high alloy austenitic stainless steels and one ferritic–austenitic stainless steel was investigated by means of hot rolling and stress relaxation testing. Cracking tendency was assessed on the basis of the length of the cracks formed. It was found that the recrystallisation kinetics of the high alloy steels is relatively slow, so only partial softening can occur between rolling passes. In the ferritic–austenitic steel the restoration is fairly fast, so softening can be completed between hot rolling passes. The cracking tendency of the steels in the as cast condition increases with increasing pass strain and temperature, but it is negligible in rolling of the steels in the as wrought condition and also minimal in rolling of the as cast steels when using a small strain of 0.1 in the first pass. On the basis of these results, it can be concluded that the cracking problems in these steels are present in the cast structure only. The hot ductility of even partially recrystallised material is perfectly adequate. Hot ductility improves nearly independently of the degree of static recrystallisation, which indicates that ductility is controlled mainly by the grain or phase sizes, not by recrystallisation itself.     

Influence of B and Ti on hot ductility of high Al and high Al,Nb containing TWIP steels

Abstract

The addition of ～0·002%B and ～0·04%Ti as microalloying additions to improve the poor hot ductility and high risk of cracking on continuous casting of high Al containing twinning induced plasticity (TWIP) steels has been examined. Tensile specimens were either cast in situ or heated to 1250°C before cooling at 60 K min^?1 to test temperatures in the range 700–1100°C and strained to failure at 3×10^?3 s^?1. For tensile specimens reheated to 1250°C, the presence of B with sufficient Ti to combine with all the N improved ductility over the temperature range of 700–950°C, the reduction in area (RA) values being >40%. For the higher strength more complex high Al, TWIP steels having Nb present, there was no improvement in ductility with a similar B and Ti addition, when the average cooling rate after melting to the test temperature was 60 K min^?1. Reducing the cooling rate to 12 K min^?1 resulted in the RA values being close to the minimum required to avoid transverse cracking throughout the temperature range 800–1000°C. Using these additions of B and Ti, transverse cracking was found not to be a problem when continuously casting these high Al containing TWIP steels.