Development of low carbon NbTiB micro-alloyed

Transcription

1 Development of low carbon NbTiB micro-alloyed steels for high-strength large-diameter linepipe C.J. Heckmann Salzgitter Mannesmann Forschung, Duisburg, Germany D. Ormston Arcelor Innovation R&D-IRSID, Maizières les Metz, France F. Grimpe Mannesmannröhren Mülheim, Mülheim, Germany H.-G. Hillenbrand EUROPIPE GmbH, Mülheim, Germany J.-P. Jansen EUROPIPE France S.A., Dunkerque, France 2 nd International Conference on Thermomechanical Processing of Steels June 15-17, 2004, Liege, Belgium TP60 EUROPIPE. The world trusts us.

2 DEVELOPMENT OF LOW CARBON NbTiB MICRO-ALLOYED STEELS FOR HIGH-STRENGTH LARGE-DIAMETER LINEPIPE C. J. Heckmann 1, D. Ormston 2, F. Grimpe 3, H-G. Hillenbrand 4, J-P. Jansen 5 1 Salzgitter Mannesmann Forschung, Duisburg, Germany, 2 Arcelor Innovation R&D-IRSID, Maizières les Metz, France, 3 Mannesmannröhren Mülheim, Mülheim, Germany, 4 Europipe GmbH, Mülheim, Germany, 5 Europipe France S.A., Dunkerque, France ABSTRACT This paper deals with the development of low carbon (< 0.04%) microalloyed steel plates for high-grade (>X80, yield strength >550MPa), longitudinally welded largediameter pipes for gas transmission. The interest in increasing the grade level of steels for pipeline is to improve transportation efficiency by an increase in operation pressure. In addition, increasing the grade allows thinner wall pipelines to be used, reducing costs and improving the rate of field joint welding. A general requirement for pipeline steels is a simultaneous increase in fracture toughness as the strength increases. However it is remarkably difficult to achieve a combination of high strength and good lowtemperature toughness. Bainitic microstructures have shown the capability to attain such requirements. INTRODUCTION In the developments which will be presented in this paper on high grade pipeline steels, bainitic microstructures have been promoted by the addition of small amounts of boron (< 40ppm) to the steel. Boron is an efficient promoter of bainite as it suppresses the nucleation of ferrite by segregating to the austenitic grain boundaries thus retarding the transformation to ferrite. Early studies of microalloyed boron steels did not show a good compromise of high tensile properties and toughness. This has been attributed to the high levels of carbon (above 0.06%) which were used in these early studies where the reductions in toughness were caused by the formation of a banded structure of polygonal ferrite, upper bainite and martensite 1. The effect of boron as a promoter of the bainitic transition is though to be obtained only by boron free in solution. Reducing the carbon level to below 0.06% reduces the amount of solute carbon unfixed by niobium, hence preventing the formation of coarse Fe 23 (CB) 6 precipitates. Titanium has an important role in the efficiency of the boron by preventing the formation of boro-carbon nitrides and therefore protecting the boron from precipitation. The level of titanium is controlled as a function of the nitrogen level in order to ensure a stoichiometric Ti/N ratio. Niobium also inhibits the nucleation and growth of ferrite but also aids in the homogenous distribution of boron to austenitic grain boundaries. This kind of metallurgy has allowed high tensile properties to be achieved in combination with high levels of toughness. Yield strengths of up to 760MPa and tensile strengths of up to 800MPa have been obtained. Such strength levels were achieved with a thermo-mechanically 311

3 controlled rolling process (TMCP) at slow and accelerated cooling rates in laboratory rolling experiments. The Charpy toughness of an X100 pipeline grade (270J at -40 C) to be operated under arctic conditions is satisfied. Even higher strength steel grades with yield strengths 820MPa are feasible with this new material. The results of experimental laboratory rolling trials on 12, 20 and 32mm plates will be presented. The transfer of the encouraging laboratory results to full-scale heavy plate mill production is in progress. EXPERIMENTAL Processing of NbTiB steel During the investigation the influence of different re-heating temperatures on the properties of the new steel have been investigated. The choice of an optimum reheating temperature is influenced by the contents of carbon and niobium in the chemical composition 2. Due to the low carbon content of these steels for the Mosteel variant a low re-heating temperature was chosen in addition to a temperature of 1130 C that is more common for the processing of simply NbTi microalloyed steel. This low re-heating temperature was 1060 C. The reheating temperature for the MoCr steel was estimated at 1120 C taking into account the slightly higher carbon and niobium contents. The grain coarsening of austenite during the re-heating process is influenced by the time of re-heating and the re-heating temperature itself. Lowering the re-heating temperature on the one hand reduces the initial grain size of austenite which positively influences the toughness properties of the product. On the other hand the re-heating temperature must be sufficiently high enough for a maximum disolution of Niobium which forms carbonitride precipitates during the thermomechanical rolling process. The higher the re-heating temperature is the more Niobium is in solution. The proper balance between lowering the re-heating temperature to avoid too high grain coarsening and increasing the re-heating temperature for the solution of the microalloying elements was one challenge during this investigation. Lowering the re-heating temperature significantly can lead to the problem that the temperature in the roughing phase of the rolling process drops down under the temperature which is necessary for a recrystallisation of the Austenite which is dependent on the specific chemical composition. This drastically affects the toughness properties, especially the results of the Battelle drop weight tear test, as the critical grain refinement of Austenite during roughing is not achieved when this part of the rolling process is performed at a too low a temperature. Therefore, the temperature of the roughing phase has to be controlled very carefully. The time between discharging the slab from the re-heating furnace and finishing the roughing or prerolling is relatively short. Due to the strong transformation retarding effect of Boron as a microalloying element, the finishing rolling temperature as well can be decreased. Microalloying with 20 ppm Boron reduces the Ar3 transition temperature by up to 50 C lower than conventional NbTi microalloyed steels. Lowering the finish rolling temperature to mark the possible process boundaries can drastically increase the time between the roughing and the finish rolling. The effect of this is twofold. On the one hand the productivity of a heavy plate mill is affected negatively and will go down. On the other hand and more important from the metallurgical point of view the increase of time between the individual rolling stages can lead to the softening of the material due to a possible recovery or re-crystallisation 312

4 of the material 3. To keep productivity on a reasonable level and to avoid the recovery and softening of the steel the introduction of an intermediate cooling to the rolling process appears to be one option. A very positive effect of the retarding of the Austenite to Ferrite transformation can be found in the widening of the production window for bainitic steels. This leads to a decreased sensitivity of the production process to possible uncertainties of the process parameters. A bainitic microstructure and therefore the best possible combination of strength and toughness properties can be produced even if the finishing rolling process is carried out at relatively low temperatures. The cooling rate for the accelerated cooling was also to be chosen very carefully. Especially with respect to an optimum combination of the mechanical-technological properties of the product, the formation of Martensite had to be avoided. Furthermore, in this case the transfer of the laboratory results to an industrial scale heavy plate rolling mill for mass production was of great importance. Therefore, a moderate cooling rate was chosen for the trials. During the investigation it became evident that even with very low cooling rates and air cooling the formation of bainitic microstructures was possible. This added more to the robustness of the production process due to the use of Boron as a microalloying element. As the avoidance of Martensite in the microstructure is very important to ensure the best possible toughness level in combination with a high strength grade, the cooling stop temperature had to be very carefully controlled. Too high a cooling stop temperature on the other hand leads to an upper bainitic microstructure that is also to be avoided. The testing was carried out between the laboratories of Salzgitter Mannesmann Forschung and Arcelor Innovation R&D. Laboratory rolling simulations of 12, 20 and 32mm thick plates were carried out on a two-high experimental rolling mill at a rolling speed of 1ms -1. The rolling schedule was temperature controlled during rolling and cooling by a thermocouple reading in the mid-length, mid-thickness position of the plate. For most of the experiments, the finish rolling temperature was fixed to about 20 C above Ar3 (+Ar3) so as to ensure a 100% austenitic microstructure at the start of cooling. However, since boron retards the formation of ferrite, rolling can be carried out at lower temperatures with minimal formation of ferrite. For this reason and especially to mark the process boundaries, some rolling simulations were carried out with a finish rolling temperature below Ar3 ( Ar3). This was also done to investigate the robustness of the steel with respect to variations of the process parameters. Accelerated cooling conditions were reproduced using specialised cooling simulation equipment. The cooling paths investigated for the different thickness plates are shown in Table 1. Table 1: Cooling paths studied Simulated FRT CR, CST, Cooling thickness, mm K/s C 12 Ar ACC 12 Ar DQ 12 + Ar ACC 20 + Ar ACC 32 + Ar ACC FRT:Finish Rolling Temperature, CR:Cooling Rate, CST:Cooling Stop Temperature, ACC:ACcelerated Cooling, DQ:Direct Quenching Ar3:FRT below Ar3, + Ar3:FRT 20 C above Ar3 Alloying concept of NbTiB steel / chemical compositions Two low carbon steel compositions were studied, a Mo-NbTiB and a MoCr-NbTiB (Table 2). The effect of boron on mechanical properties was investigated for 313

5 the CrMo steel in order to optimise the composition and to study its robustness. Synthetic melts were produced in the laboratory for each composition and cast into ingots from which samples were cut for rolling. Table 2: Compositions studied (weight %) C Si Mn Nb Ti others MoB * MoCrB * two compositions with and without Mo; CE IIW =0.38; Pcm = 0.18 two compositions with low and high boron content CE IIW =0.48; Pcm = 0.19 Due to the fact that Boron is a very reactive element which easily combines with other alloying elements such as carbon or nitrogen and therefore is not easy to handle, the advantages of Boron as a microalloying element have to be pointed out very clearly. Firstly, regarding the mechanicaltechnological properties, Boron affects the strength. Therefore, by adding small amounts of Boron of less than 40 ppm higher strength grades are feasible even with drastically reduced carbon contents in the chemical composition. In the investigation presented here the level of carbon was lower than 0.04 %. This low carbon content leads to a very low carbon equivalent for these kinds of high strength steel grades. Therefore, by lowering the carbon content as a result of the use of boron the weldability of this material will also be affected very positively. not occur. The nitrogen is fixed by titanium if and when the Ti/N ratio is and therefore at least stochiometric. The chemical composition which is presented in this paper contains amongst the above mentioned elements all other alloying components that ensure the safe production of a high strength line pipe grade such as API X80 or X100. The basic microalloying concept was NbTi with a Niobium content of about %. To investigate the influence of molybdenum on the mechanical properties a molybdenum free variant as well was processed. RESULTS Influence of B and Mo Figure 1 shows the influence both of boron and molybdenum on the strength properties of the Mo-NbTi steel variant. Aiming at grade X80 which was reached by this steel already due to the proper thermomechanical processing, the actual strength level was even improved by alloying with B or Mo. Secondly, toughness properties will strongly depend on the Boron content. Indeed, boron has, as mentioned above, a high affinity to carbon and nitrogen and will form Borocarbonitrides. These precipitates have a detrimental effect on the toughness properties. By fixing the nitrogen with titanium and lowering the level of the carbon content down to > 0.04 % the precipitation of Borocarbonitrides should Figure 1: Influence of boron and molybdenum on the strength properties of the Mo-NbTiB steel variant While boron was added to a Mo containing NbTi steel, an addition of molybdenum was made to a basic NbTiB alloy. As can be 314

6 seen from Figure 1, both the addition of B and Mo lead to a very clear improvement of the strength properties of the investigated steel. Independent from the wall thickness the strength level of a careful processed Mo-NbTiB steel can lead to API grades X90 or even X100. Keeping in mind the very low carbon content as well as the fact that apart from Mo no other alloying element was used here these results underline not only the importance of the thermomechanical rolling process but also the strong influence of boron as a microalloying element. The mechanical properties of these steels are very sensitive to the boron content. It is crucial that the boron level in the steel is tightly controlled since it has been shown in previous literature that coarse iron borocarbides can form above a certain level, about 20ppm boron for a simple NbTiB steel 4. Too high a boron content leads to a deterioration of toughness. Optimising the boron content in the MoCr steel has shown to be imperative in order to have the best strength-toughness compromise (Figure 2). Since boron has been shown to shift the ferrite nose in transformation diagrams, we should expect little change in the microstructure over a wide cooling rate. It has been shown from the results on the MoCr steel that the X100 grade is even achieved with air cooling conditions showing the high process window possible for B steels. Figure 2: The effect of B content in the MoCr steel on tensile and toughness properties (open symbols = low boron, full symbols = high boron). Influence of rolling parameters on the mechanical technological properties The reheating temperature very strongly affects the strength properties of the Mo- NbTiB steel variant, as can be seen from Figure 3. By increasing the reheating temperature from 1060 C to 1130 C the strength properties of all investigated wall thickness variants were improved. This may be a result of the higher amount of Nb(NC) in solution at the higher reheating temperature. 315

7 The influence of the reheating temperature on the Charpy impact toughness at a testing temperature of 20 C is shown in Figure 5. All wall thickness variants are positively affected by increasing the reheating temperature. Figure 3: Influence of reheating temperature on the strength properties of the Mo-NbTiB steels But even a reheating temperature of 1060 C which is quite low for conventional NbTi steels, it results in the safe fulfilment of the strength requirements of an API grade X80 even for a heavy wall thickness of 32 mm. As illustrated by Figure 4, lowering the finish rolling temperature only influences the level of the tensile strength. The yield strength stays on the same level for both finish rolling temperatures. Figure 5: Influence of reheating temperature on the toughness properties of the Mo-NbTiB steels Due to the lower deformation ratio the roughing phase of the rolling process in which the Austenite is forced to recrystallise is less pronounced for the heavy wall thickness of 32 mm. Therefore, an enlargement of the temperature range of this rolling phase does not lead to a clear increase in the impact toughness when compared to the increases observed for the thinner plates. Lowering the finish rolling temperature did not influence the toughness properties. This actually makes the Mo-NbTiB steel variant independent from variations in the process parameters. Figure 4: Influence of finish rolling temperature on the strength properties of the Mo-NbTiB steels Lowering the finish rolling temperature especially in the case of 12 mm wall thickness leads to a strength level which is on the edge of API grade X100. Considering the low carbon equivalent (CE IIW = 0.38) this is quite remarkable. Retarding of the ferrite transformation also gives a flexibility to the rolling processs parameters. Rolling at low temperatures is possible because very little ferrite forms below the Ar3 temperature and therefore the structure does not change. We have shown this on the MoCr steel that the finish rolling temperature (FRT) has little affect on the tensile properties and that X100 is achieved at a range of temperatures (Figure 316

8 6). In contradiction with the trend seen for the Mo-NbTiB steel, the tensile strength of the MoCr steel is virtually unaffected by decreasing the FRT but there is a significant increase in yield strength at a very low FRT. ferrite in a bainitic matrix that changes the cleavage crack propagation mechanism 5. It is possible with these boron steels that small amounts of fine ferrite forms during rolling below Ar3 before the bainitic transformation occurs. Additional investigations were done with this steel composition and a wall thickness of 20 mm. A high and a low re-heating and two different finish rolling temperatures, >Ar3 and <Ar3, were chosen. In this case the strength properties are strongly affected by this change in process parameters. Both the yield and the tensile strength decreased by 40 MPa for the low re-heating temperature. On the one hand the higher reheating temperature increases the amount of Nb in solution, thus leading to a higher strength level. On the other hand a relatively large amount of Martensite was found in the microstructure of the variant with the high re-heating temperature. This may also have added to the higher strength level. Figure 6: The influence of FRT on strength and toughness for the MoCr steel (open symbols = air cooling, full symbols = accelerated cooling). At 40 C almost all conditions with the exception of the air cooled high FRT, achieve the 270J level specified for X100. Reducing the FRT to below Ar3 (< Ar3) leads to a shift in the transition curve to a lower temperature. This trend continues when reducing further the FRT (<< Ar3) where toughness levels at 80 C remain above 270J. It is not clear why such a trend is seen but it has been suggested that gains in low temperature toughness can be achieved by the formation of very fine As a result of the change in strength levels and the martensite in the microstructure, changes in the toughness properties were also influenced, as can be seen in Figure 7. The variant with the lower re-heating and finish rolling temperature showed a transition curve that was significantly shifted down to lower temperatures, all in keeping the upper shelf level constant. Figure 7: Influence of TR and FRT and toughness for the MoCr steel with 20 mm wall thickness 317

9 Figure 8 gives an example for a typical microstructure of the steels presented in this paper. This mainly bainitic microstructure also contains small amounts of ferrite (<10 %) and martensite (<4 %). at low cooling rates. That means that the window of rolling and cooling conditions is opened up to wider ranges and gives the steel a high robustness. We have shown evidence of this robustness for both steels studied in this paper. As the strong influence of boron especially on the strength properties is documented for example by this investigation, intensive research is also being carried out on the development of an X120 linepipe grade using boron-based metallurgy 6,7. REFERENCES Figure 8: Typical microstructure of the investigated steels CONCLUSIONS In this paper, two low carbon steel grades have been studied, a Mo-NbTiB and a MoCr-NbTiB composition. It is clear that it is important to optimise the reheating parameters of these steels since we see that it is not sufficient to estimate this only on the dissolution of Nb precipitates. For the Mo-NbTiB steels there is an improvement in strength and toughness when the reheating temperature is increased which may not be explained by increased Nb dissolution alone. 1 Tamehiro et. al., "Optimum chemical composition and thermo-mechanical processing condition for Nb- B steel", Trans.ISIJ, Vol.27, 1987, Hulka et al., "High temperature processing of line pipe steels", Proceedings of the international symposium Niobium 2001, Orlando, USA, Dec. 2 nd 5 th, Bleck et al., "Softening behaviour of mild steels during hot deformation in the austenite and ferrite range", THERMEC 97, Wollongong, Australia July 7 th 11 th, Tamehiro et. al., "Optimum microalloying of Nb and B in HSLA steel for thermomechanical processing", Trans.ISIJ, 1987, Kim et al., "Effect of microstructure on the yield ratio and low temperature toughness of linepipe steels, ISIJ Int., Vol.42, 2002, Hillenbrand et.al., "Development of grade X120 pipe material for high pressure gas transportation lines", 4 th ICPT, Ostend-Belgium, 9 th -12 th May Schwinn et.al., "Bainitic steel plates for X100 and X120", 4 th ICPT, Ostend-Belgium, 9 th -12 th May 2004 For the CrMo steel, a compromise of finish rolling temperature and productivity of the heavy plate mill is possible since the X100 grade is achieved within a range of FRTs. In the case of a low FRT, an three phase rolling schedule would be necessary in order to reduce interpass recovery and softening of the steel. The shifting of the gamma-alpha transition curve by the addition of boron, allows the formation of bainitic microstructures even 318

10