- Email: [email protected]
High strength microalloyed CMn(V–Nb–Ti) and CMn(V–Nb) pipeline steels processed through CSP thin-slab technology: Microstructure, precipitation and mechanical properties
Materials Science and Engineering A 424 (2006) 307–317
High strength microalloyed CMn(V–Nb–Ti) and CMn(V–Nb) pipeline steels processed through CSP thin-slab technology: Microstructure, precipitation and mechanical properties C.P. Reip a , S. Shanmugam b , R.D.K. Misra b,∗ a
b
SMS Demag Aktiengesellschaft, Edward-Schloemann-Strasse 4, 40237 Dusseldorf, Germany Center for Structural and Functional Materials, University of Louisiana at Lafayette, Lafayette, LA 70504-4130, USA Received 7 February 2006; accepted 7 March 2006
Abstract Compact strip production (CSP) technology is an important upcoming processing route to produce low cost microalloyed high strength pipeline steels that meet the API standards. Hot strips of CMn(VNbTi) and CMn(VNb) steel grades with fine-grained ferrite–pearlite microstructure and small volume fraction of lower transformation product (non-polygonal ferrite: acicular ferrite/bainite) were produced using CSP technology with high strength and excellent low-temperature toughness up to −60 ◦ C. For strip thicknesses between 6 and 12.5 mm, yield strength levels of up to 590 MPa and tensile strength levels up to 680 MPa were achieved. The CMn(VNb) steel exhibited outstanding notch-toughness in the range of 200 and 400 J/cm2 , in spite of its higher yield strength (∼100 MPa or greater) over the CMn(VNbTi) steel. The precipitates present in CMn(VNbTi) and CMn(VNb) steels were characterized in terms of morphology, size and chemistry, and crystallography. The microalloying elements, Ti, Nb, and V form M4 C3 type of carbides in the ferrite matrix of both the steels. The multi-microalloying approaches of CMn(VNbTi) and CMn(VNb) results in the formation of duplex and triplex carbonitrides, respectively. The results of the development effort are described. © 2006 Elsevier B.V. All rights reserved. Keywords: Microalloyed pipeline steel; CSP processing technology; Precipitation; Microstructure
1. Introduction High strength microalloyed steels have been used for the production of welded pipes for more than 30 years. However, the alloy design of pipeline grades is being continuously modified and the process technology optimized because of increasing demand of high strength–toughness combination requirement of pipeline steels. Thermomechanically processed microalloyed steels are preferred because of their good strength–toughness combination at low temperatures and cost effectiveness [1]. Compact strip production (CSP) technology is an upcoming promising route for the production of high quality steel grades [2]. The CSP thin-slab technology is an integral chain process starting from the production of liquid steel to the refined hot and cold strip. The CSP can also be combined with the tradi-
∗
Corresponding author. Tel.: +1 337 642 4130; fax: +1 337 482 1220. E-mail address: [email protected] (R.D.K. Misra).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.03.026
tional methods that makes use of blast furnaces and converters or electric steel making. In the case of latter, scrap, direct reduced iron (DRI), as well as hot-briquetted (HBI) iron and pig iron are used as starting material. In traditional thin-slab technology, the casting and rolling processes are coupled, which minimizes energy consumption and operating costs. Since the introduction of CSP technology in 1989, the technology is being continuously developed and is poised to become an important process for the production of high quality steel grades. The American Petroleum Institute (API) provides standards for pipe that are suitable for use in conveying gas, water, and oil in both the oil and natural gas industries. The API 5L specification describes the requirements of chemistry, tensile test characteristics and toughness behavior, as depicted in Fig. 1. The property requirements of steel vary depending on the particular application and operating conditions. The basic requirements, however, are high strength together with superior toughness at low temperature and excellent weldability. It is also important that steels should exhibit superior corrosion resistance, especially when petroleum and natural gases in recent years have become more
308
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
Fig. 1. Property requirements and the processing strategy for producing pipeline steel according to the API specification 5L.
Fig. 2. Schematic diagram of basic concept of compact strip production.
of the type that contain wet H2 S (source of sulfur). This had necessitated strict control of sulfur and phosphorus and cleanliness of steel, in general. In casting, the parameters of concern are solidification microstructure, segregation, strand guiding system, casting temperature and the cooling rate. In the case of thin-slab casting, the higher solidification rate results in smaller dendrite arm spacing, significantly reduced micro- and macrosegregation and improved homogeneity [3]. Furthermore, the quality of the hot strips and its properties are determined by the rolling and cooling process parameters (pass schedule, cooling rate, recrystallization temperature), and the metallurgical events involved in recrystallization, grain coarsening, transformation, and the precipitation behavior. The schematic representation of the basic concept of CSP Technology adopted for the work described here is presented in Fig. 2. The caster supplies thin slabs of thickness ∼50–90 mm thickness, which are hot charged without intermediate cooling and rolled to hot strips in the finishing mill. This continuous process yields benefits in terms of capital investment, power consumption and operating cost [4]. It also allows the production of the whole range of flat-steel products with superior properties in terms of surface and inner quality of hot strip. Thus, CSP
technology symbolizes a highly productive and cost-effective process for making hot steel strips. In the work described here two material strategies were implemented, i.e. the development of microalloyed steels with mainly a ferrite microstructure and strength levels in the range of API X60/65 as well as ferrite microstructure with portions of lower temperature transformation products like acicular ferrite and bainite and strength levels in the range of API X70/80 [5,6]. Acicular ferrite and bainitic microstructures are preferred for best combination of strength and toughness. However, judicious selection of microalloying elements and optimization of process parameters is essential to obtain the required microstructure. It is demonstrated here that using the CSP process, the required good combination of strength and toughness can be achieved by grain refinement, lower transformation products, and precipitation strengthening through microalloying with Nb, Ti, and V individually or in combination, as in conventional integral steel making process [7]. However, multi-microalloying approach promotes formation of complex chemistries of carbonitrides, even at low concentrations [8,9] independent of the individual parameters during thermomechanical processing.
Table 1 Chemical composition (in wt.%) of microalloyed steels Material
C
Si
Mn
P
S
V
Nb
Ti
N
CMn(VNbTi) steel CMn (VNb) steel
0.056 0.059
0.25 0.22
1.39 (< 2) 1.31 (< 2)
0.014 0.011
0.0023 0.0021
0.074 (< 0.10) 0.073 (< 0.10)
0.037 (< 0.10) 0.043 (< 0.10)
0.01 (< 0.02) 0.002 (< 0.002)
0.0062 0.0065
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
309
Fig. 3. (a) Process parameters and production schedule for API X grades in a CSP plant. (b) Example of finishing and coiling temperature for 8 mm hot strip.
The objective of the present paper is to describe the results of the development effort of two high strength pipe steels grades processed using the CSP thin-slab technology that meet the API standards, in terms of tensile and impact properties, microstructure, and precipitation behavior. The precipitates present in CMn(VNbTi) and CMn(VNb) steels were characterized in terms of morphology, size and chemistry, and crystallography. Furthermore, the orientationship between precipitates and ferrite was established. 2. Steel composition and processing methods The chemical composition of the investigated CMn(VNbTi) and CMn(VNb) pipeline microalloyed steels is presented in Table 1. The approach and details of production for API X grades on a CSP plant is presented in Fig. 3. As a basis for initial development, a low carbon content of ∼0.05% was preferred from the viewpoint of segregation, toughness, and weldability. The sulfur, phosphorus and nitrogen content was controlled to
∼0.002%, 0.016% and 0.006%, respectively, through selection of feedstock (e.g. selected scrap, direct reduction iron DRI, hotbriquetted iron HBI) during steel making. The thickness of the near-net-shaped casting slabs was 60 mm for tundish temperatures in the range of 1540–1560 ◦ C (superheat of 20–40 ◦ C). The casting speed was approximately 4.5 m/min. Variations in speed during casting were avoided. The casting conditions with respect to superheat, casting speed, casting powder, and secondary cooling were controlled to achieve superior surface quality and minimize segregation. The 1220 mm wide thin slabs were first homogenized in the soaking furnace at temperatures of 1100–1130 ◦ C. Thermomechanical rolling was done on a 6-stand rolling mill. For 8 mm thick hot strip, total forming strain of ∼87% was achieved for the starting slab thickness of 60 mm. The finish rolling temperatures (Fig. 4) were between 770 and 860 ◦ C. In the laminar-cooling zone, cooling took place as a function of strip thickness at cooling rates varying between 14 and 25 K/s to coiling temperatures between 530 and 640 ◦ C. As outlined above, CMn(VNbTi) was used at process conditions, which suggest a ferrite–pearlite
310
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317 Table 2 Mechanical properties of microalloyed steels Properties
API X65–70 CMn(VNbTi) steel
API X75–80 CMn(VNb) steel
Yield strength (MPa) Tensile strength (MPa) Yield/tensile ratio % elongation
465–490 530–575 0.83–0.9 20–30
540–590 615–675 0.85–0.9 28–35
microstructure, whereas for CMn(VNb), a microstructure with portions of lower temperature transformation products were aimed. In addition to above processing of the microalloyed steels in a commercial caster, laboratory experiments with simplified conditions in comparison to CSP were carried out to reveal the influence of the alloying concepts and the deformation conditions on the final microstructure, i.e. the fraction of lower temperature transformation products. The experimental details of the laboratory dilatometric experiments were: • one-step: ◦ heating with 3 K/s–1250 ◦ C, holding for 600 s, ◦ cooling with 30 K/s–850 ◦ C, ◦ forming with ε = 0.5 and ε = 12 1/s, ◦ cooling with 25 K/s to room temperature. • two-step: ◦ heating with 3 ◦ C/s–1250 ◦ C, holding for 600 s, ◦ cooling with 30 ◦ C/s–850 ◦ C ◦ forming with ε = 0.5 and ε = 12 1/s, ◦ cooling with 30 K/s–800 ◦ C, ◦ forming with ε = 0.7 and ε = 12 1/s, ◦ cooling with 25 K/s to room temperature. Fig. 4. Representative (a) light (b) scanning electron micrographs of CMn(VNb) steel.
Fig. 5. Light micrographs of CMn(VNbTi) and CMn(VNb) steels after one-step and two-step deformation in a laboratory rolling-mill set up.
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
311
3. Experimental testing methods
Fig. 6. Plot of tensile strength vs. yield strength for CMn(VNbTi) and CMn(VNb) steels.
Mean grain size was estimated by the circular intercept method. Intercept lengths were determined and then converted to nominal grain size using the standard tables. At least 5 coils were examined and about 4000 grains were counted to determine the grain size. Tensile tests were done according to ASTM E8 specification. Charpy v-notch impact tests were carried out according to ASTM E 23 standard. From the impact tests, both the absorbed energies and % shear-fracture were measured. Drop weight tear tests were carried out according to API RP SL3 specification and % shear determined for notched, full-walled large test specimens as a function of the temperature. Transmission electron microscopy (TEM) studies were carried out on thin foils of CSP processed hot strips with a thickness of 8 mm. These foils were prepared by cutting thin wafers from the steel samples, and grinding them to 0.1 mm in thickness. Three millimeter discs were punched from the wafers, and electropolished using a solution of 5% perchloric acid/95% acetic
Fig. 7. Plot of Charpy V-notch impact energy at −40 ◦ C vs. yield strength for CMn(VNbTi) and CMn(VNb) steels for hot strips with thickness between 8 and 10 mm.
Fig. 8. (a) Plot of % shear as function of impact testing temperature of the 8 mm hot strips of CMn(VNbTi) and CMn(VNb) steels. Filled and unfilled symbols correspond to Charpy and DWTT, respectively. (b) Dependency of CVN 50% FATT on the degree of forming during hot rolling for slab thickness of 60 mm.
Fig. 9. Bright field TEM micrographs together with EDX analysis of precipitates of different morphology in CMn(VNbTi) steel: (i) spherical, (ii) cuboidal, and (iii) irregular and fine.
312
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
Fig. 10. Bright field TEM micrographs together with EDX analysis of precipitates of different morphology in CMn(VNbTi) steel: (i) cuboidal, (ii) irregular, and (iii) fine.
acid. Foils were examined by conventional transmission electron microscope operated at 120 kV using standard bright field and dark field imaging techniques. X-ray analysis of precipitates was performed with a JEOL 2010F FEG TEM/STEM at 200 kV. 4. Results and discussion 4.1. General microstructure The microstructure of CMn(VNbTi) steel was predominantly ferrite with small volume fraction of pearlite [6]. The mean grain size according to ASTM standard E 112 was 12, which corresponds to a mean grain diameter of ∼6 m. Fig. 4(a) and (b) shows representative light and scanning electron micrographs of CMn(VNb) steel. The mean grain size was 14 and the corresponding mean grain diameter was 2.6 m. Some elongation along the rolling direction is apparent. For the hot strip production of CMn(VNb), as a function of the parameters selected for thermomechanically processing and the cooling conditions on the exit roller table, the resulting volume fractions of lower tem-
perature transformation products are between 20 and 65%. The volume fractions can be increased by reducing the intensity of the thermomechanical processing, thus reducing the mechanical stability of the austenite. This statement is in agreement with the observations made on laboratory heats. In Fig. 5, the light micrographs of CMn(VNbTi) and CMn(VNb) steels after laboratory performed hot rolling and cooling with a fixed cooling rate of 25 K/s are compared. Two hot rolling strategies: onestep deformation as well as two-step deformation where second deformation occurred at lower temperatures were considered. Generally, an intensification of the thermomechanically processing (increases value of accumulated strain in the two-step) by the second-step deformation results in a reduction of the portions of lower temperature transformation products for both the microalloying concepts. It is known that an increased dislocation density opposes the formation of acicular ferrite and bainite through a mechanical stabilization effect [10]. Furthermore, the results suggest that the exclusion of titanium favors the formation of acicular ferrite and bainite. This may be due to the higher amount of dissolved niobium which supports the formation of bainite and acicular ferrite because Ti precipitated as TiN may
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
313
Fig. 11. (a) Bright field TEM micrograph of the ferrite matrix of CMn(VNbTi) steel showing pinning of carbide precipitates by dislocations and (b) selected area diffraction pattern analysis for region shown in the image (a).
provide nucleation site for the precipitation of Nb(C,N) during the final phase of rolling [11]. 4.2. Mechanical properties The tensile properties of the CMn(VNbTi) and CMn(VNb) steels is summarized in Table 2. Fig. 6 shows the yield and tensile strength data obtained for CMn(VNbTi) and CMn(VNb) steels. For strip thicknesses between 6 and 12.5 mm, the yield strength was between ∼530 and 680 MPa. In this range, no effect of strip thickness on tensile properties was observed. This range
allows minimum values necessary to meet the API specification for X65, X70 and X80 grades. In the case of CMn(VNb) steel grade, in spite of high yield and tensile strength with yield ratio between 0.85 and 0.90, the elongation was high (28–35%). For the CMn(VNbTi) steel, similar yield ratio of 0.83–0.90 was obtained. These properties correspond to the 1220 mm wide slabs fed from the caster to an equalization furnace set at 1100–1130 ◦ C. Fig. 7 shows the plot of impact toughness for hot strips with thickness between 8 and 10 mm, tested at a test temperature of −40 ◦ C versus yield strength. It may be noted that
314
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
Fig. 12. Bright field TEM micrographs of CMn(VNb) steel showing the coarse precipitates of different morphology; (a) spherical, (b and c) irregular in the ferrite matrix and on the grain boundaries.
the CMn(VNb) steel exhibits excellent notch-toughness with absorbed energy values of 200–400 J/cm2 in spite of higher yield strength of ∼100 MPa or greater compared to CMn(VNbTi) steel. Fig. 8a shows the plot of % shear as a function of Charpy V-notch impact and drop weight tear test temperature for the CMn(VNb) steel belonging to a strip thickness of 8 mm. As a function of the hot rolling and cooling parameters, even at temperatures of −60 ◦ C, % shear of up to 93% were still observed in the notched-bar impact bending test. In the more critical drop weight tear test, the dependence on temperature of the % shear part shifted to higher temperature levels. At test temperatures of down to −50 ◦ C, however, % shear of more than 75% were still attained. From the dependence of % shear-fracture on temperature established in the drop weight tear test, a transition temperature FATT (fracture appearance transition temperature) can be derived. For a 8 mm thick hot strip made from the CMn(VNb) steel, excellent FATT levels of −50 ◦ C were attained at % shear
of 85%. To produce hot strip with a thickness in excess of 8 mm, the total degree of forming in hot rolling must be taken into account. The dependency of the FATT with a shear fraction of 50% on the total forming degree in hot rolling is presented in Fig. 8b. It may be seen from Fig. 8b that in case of particularly stringent requirements in terms of low-temperature toughness, forming degree of more than 85% are needed. 4.3. Precipitation in CMn (VNbTi) and CMn (VNb) steel Figs. 9 and 10 are representative examples of bright field TEM micrographs with the EDS analysis of precipitates in CMn(VNbTi) steel. The precipitates analyzed are marked with arrows in the figure. The precipitates of three different morphology were observed and are characterized by irregular/spherical (100–150 nm), cuboidal (30–70 nm), and fine precipitates (10–20 nm). The presence of large size precipitates was
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
315
Fig. 13. Bright field TEM micrographs of coarse precipitates together with EDX analysis of CMn(VNb) steel.
occasional in ferrite grains with low dislocation density. They are believed to have formed during solidification of the liquid melt and are thermomechanically stable in austenite and ferrite. The fine precipitates (<20 nm) were randomly (or partially in rows) distributed and some of them pinning the dislocations. The EDS analysis indicated that all the precipitates were rich in niobium and also contained titanium and vanadium. Fig. 11 shows the bright field TEM micrograph illustrating pinning of fine pre-
cipitates by dislocations. A composite diffraction pattern of the ferrite matrix and the precipitates was obtained from the center region in Fig. 11a and the SAD pattern analysis is presented in Fig. 11b. The analysis indicated that the presence of four types of orientation relationships between precipitates and the ferrite matrix. The orientation relationships [1 2 3]␣ //[0 1 1]M4 C3 and [0 1 3]␣ //[0 1 1]M4 C3 were close to Kurdjumov-Sachs relationship, in addition two other orientations relationships
Table 3 Classification of different precipitates in microalloyed steels Steel
Type
Morphology
Size range (nm)
Spectral ratios
Precipitates
CMn(VNbTi)
Triplex
Fine Cuboidal Spherical/irregular
10–20 30–70 100–150
Nb0.62 Ti0.27 V0.11 Nb0.44 Ti0.32 V0.24 Nb0.33 Ti0.33 V0.33
(TiNbV)C (Ti,Nb,V)N (Ti,Nb,V)C,N
CMn(VNb)
Duplex
Fine Cuboidal Spherical/irregular
10–20 30–70 100–150
V0.62 Nb0.38 Nb0.76 V0.24 Nb0.5 V0.5
(Nb,V)C (Nb,V)N (Nb,V)C,N
316
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
[1 3 3]␣ //[0 1 1]M4 C3 and [110]␣ //[011]M4 C3 were also identified. Figs. 12 and 13 are the bright field TEM micrographs showing the nature and morphology of precipitates in CMn(VNb) steel. The precipitation behavior in both the steels was similar in spite of the difference in the microalloying content. Also, in CMn(VNb) steel, the precipitates were of different size and morphology notably, irregular/spherical (100–150 nm), cuboidal (30–70 nm), and fine precipitates (10–20 nm). In CMn(VNbTi) steel, the fine precipitates were randomly dispersed and also present in rows, where as in CMn(VNb) steel, majority of the coarse precipitates were individually dispersed. However, in both the steels the coarse precipitates were present in ferrite grains with low dislocation density. The EDS analysis obtained on the individual precipitates indicated that they are rich in niobium and vanadium. Fig. 14a is a bright field TEM micrograph of another ferrite region depicting pinning of fine precipitates by dislocations and the corresponding SADP analysis is presented in Fig. 14b. The orientation relationships between ferrite and the precipitates were [1 1 1]␣ //[0 1 1]M4 C3 and [1 1 0]␣ //[0 1 1]M4 C3 . The size and morphology of the precipitates observed here are similar to those observed in conventionally hot rolled microalloyed steels [7–9,12–16]. The cuboidal-type precipitates were earlier identified as titanium/niobium nitrides, [(Ti,Nb)N], spherical/irregular precipitates as titanium/niobium carbides [(Ti,Nb)C], and fine precipitates as carbides, [(Ti,Nb,V)C] [7,9,12]. The estimated precipitate size range and the spectral ratios of CMn(VNbTi) and CMn(VNb) steel are given in Table 3. However, comparison of precipitate size and spectral ratios of the conventional and CSP processed steels may suggest that in CSP process, the triplex compounds are richer in niobium. But this observation may also be a consequence of higher niobium content in microalloyed steels presented here. Multi-microalloying design approach such as Nb–Ti, Ti–V, and Nb–V generally results in the formation of duplex carbonitrides [14], while V–Nb–Ti microalloying approach led to formation of triplex carbonitrides [8]. An important conclusion that can be drawn from this work and from the previous work [8,9] is that as the number of microalloying element increases, the precipitates are multi-microalloying compounds. Based on recent solubility product calculations carried out in the temperature range of 700–1300 ◦ C [9], it was shown that the microalloying elements, Ti, Nb, V are interchangeable in the precipitate lattice because of similarity in crystal structures (FCC NaCl type) and lattice parameter [13,14]. This explains the observation of EDS analysis of triplex and duplex precipitates that indicated the presence of all the three or two microalloying elements in a single precipitate. Furthermore, we have estimated the orientation relationships outlined above that suggest that the precipitation in CMn(VNbTi) and CMn(VNb) steels presumably occurred during austenite to ferrite transformation [13]. It is also likely that they may have formed mainly in ferrite and partially at the interphase because of the high cooling rate and the limited evidence of rows of precipitates. The fine precipitates identified as carbides (MX or M4 C3 type where M is V or Ti or Nb) have a cubic crystal structure (NaCl crystal structure of B1 type) and
Fig. 14. (a) Bright field TEM micrograph of CMn(VNb) steel showing pinning of fine carbide precipitates by dislocations in the ferrite matrix and (b) SAD pattern analysis for the ferrite matrix and the carbide precipitates.
are precipitated in the ferrite matrix. Our recent work on conventionally processed V–Nb–Ti and V-steels exhibited similar kind of precipitation behavior and a detailed discussion on crystallography and ordering of the multi-microalloying precipitates has been previously discussed [8,9]. The above results demonstrate that the CSP technology can be successfully used to produce high strength API grades with superior toughness. The strength is derived from solid solution, precipitation, and dislocation hardening mechanisms and the superior toughness of steels is predominantly related to finegrained microstructure and near absence of large TiN or VN precipitates that promote cleavage fracture.
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
5. Conclusions The compact strip production (CSP) technology can be successfully used to process microalloyed high strength steels of desired strength and superior toughness. Hot strips of CMn(VNbTi) and CMn(VNb) steel grades were produced using CSP technology with high strength and low-temperature toughness up to −60 ◦ C. For strip thicknesses between 6 and 12.5 mm, the resulting yield strength levels were between ∼ 530 and 680 MPa. The yield ratios of the two steels were in the range of 0.85 and 0.90, and their elongations were 28–35%. The CMn(VNb) alloy exhibited very good notch-toughness of 200–400 J/cm2 in spite of its higher yield strength of ∼100 MPa or greater over CMn(VNbTi) steel. The superior toughness is attributed to the fine grain size of the steels. The precipitates in CMn(VNbTi) steel were of triplex-type containing Ti, Nb, and V even at low concentration of titanium, while in CMn(VNb) steels they were of duplex-type containing Nb and V. The precipitates were irregular/spherical, cuboidal, and fine with size range of ∼100–150, 30–70, and 10–20 nm, respectively. The microalloying elements (Ti, Nb, and V) form coherent M4 C3 type of carbides in the ferrite matrix of both the steels. References [1] A.J. DeArdo, The metallurgy of high strength line-pipe steels, in: 44th Annual Conference of Metallurgists of CIM, Proceedings of the International Symposium on Pipelines for the 21st Century, Calgary, Alta., Canada, August 21–24, 2005, pp. 85–100.
317
[2] C.P. Reip, W. Hennig, J. Kempken, R. Hagmann, Recent developments in integrated technology of compact strip production, Iron Steel Technol. 2 (2) (2005) 25–32. [3] R. Kaspar, Steel Res. 75 (2003) 318–326. [4] C.P. Reip, W. Hennig, J. Kempken, R. Hagmann, Development of CSP processed high strength pipe steels, Conference on Microalloying for New Steel Processes and Applications, San Sabastian, Spain, in press. [5] C.P. Reip, W. Hennig, R. Hagmann, Recent developments in integrated technology of compact strip production, in: Proceedings of the Materials Science and Technology Conference and Exhibition, New Orleans, 2004, pp. 781–791. [6] C.P. Reip, W. Hennig, J. Kempken, R. Hagmann, Production of high strength pipe steels by CSP thin slab technology, in: 44th Annual Conference of Metallurgists of CIM, Proceedings of the International Symposium on Pipelines for the 21st Century, Calgary, Alta., Canada, August 21–24, 2005, pp. 143–153. [7] R.D.K. Misra, G.C. Weatherly, J.E. Hartmann, A.J. Boucek, Mater. Sci. Technol. 17 (2001) 1119–1129. [8] S. Shanmugam, M. Tanniru, R.D.K. Misra, D. Panda, S. Jansto, Mater Sci. Technol. 21 (2005) 165–177. [9] S. Shanmugam, M. Tanniru, R.D.K. Misra, D. Panda, S. Jansto, Mater Sci. Technol. 21 (2005) 883–892. [10] C. Lee, H. Bhadeshia, H. Lee, Mater. Sci. Eng. A 36 (2003) 249–257. [11] C. Klinkenberg, K. Hulka, W. Bleck, Niobium Carbide Precipitation in Microalloyed Steel, vol. 75, Steel Research International, 2003, pp. 744–752. [12] R.D.K. Misra, K.K. Tenneti, G.C. Weatherly, G. Tither, Metall. Trans. 34A (2003) 2341–2351. [13] M. Charleux, W.J. Poole, M. Militizer, A. Deschamps, Metall. Trans. 32A (2001) 1635–1646. [14] T.N. Baker, Y. Li, J.A. Wilson, A.J. Craven, D.N. Crowther, Mater. Sci. Tech. 20 (2004) 720–730. [15] M.J. Crooks, A.J. Garratt-Reed, J.B. Vander Sande, W.S. Owen, Metall. Trans. 12A (1981) 1999–2013. [16] R.D.K. Misra, S.W. Thompson, T.A. Hylton, A.J. Boucek, Metall. Trans. 32A (2001) 745–760.
High strength microalloyed CMn(V–Nb–Ti) and CMn(V–Nb) pipeline steels processed through CSP thin-slab technology: Microstructure, precipitation and mechanical properties C.P. Reip a , S. Shanmugam b , R.D.K. Misra b,∗ a
b
SMS Demag Aktiengesellschaft, Edward-Schloemann-Strasse 4, 40237 Dusseldorf, Germany Center for Structural and Functional Materials, University of Louisiana at Lafayette, Lafayette, LA 70504-4130, USA Received 7 February 2006; accepted 7 March 2006
Abstract Compact strip production (CSP) technology is an important upcoming processing route to produce low cost microalloyed high strength pipeline steels that meet the API standards. Hot strips of CMn(VNbTi) and CMn(VNb) steel grades with fine-grained ferrite–pearlite microstructure and small volume fraction of lower transformation product (non-polygonal ferrite: acicular ferrite/bainite) were produced using CSP technology with high strength and excellent low-temperature toughness up to −60 ◦ C. For strip thicknesses between 6 and 12.5 mm, yield strength levels of up to 590 MPa and tensile strength levels up to 680 MPa were achieved. The CMn(VNb) steel exhibited outstanding notch-toughness in the range of 200 and 400 J/cm2 , in spite of its higher yield strength (∼100 MPa or greater) over the CMn(VNbTi) steel. The precipitates present in CMn(VNbTi) and CMn(VNb) steels were characterized in terms of morphology, size and chemistry, and crystallography. The microalloying elements, Ti, Nb, and V form M4 C3 type of carbides in the ferrite matrix of both the steels. The multi-microalloying approaches of CMn(VNbTi) and CMn(VNb) results in the formation of duplex and triplex carbonitrides, respectively. The results of the development effort are described. © 2006 Elsevier B.V. All rights reserved. Keywords: Microalloyed pipeline steel; CSP processing technology; Precipitation; Microstructure
1. Introduction High strength microalloyed steels have been used for the production of welded pipes for more than 30 years. However, the alloy design of pipeline grades is being continuously modified and the process technology optimized because of increasing demand of high strength–toughness combination requirement of pipeline steels. Thermomechanically processed microalloyed steels are preferred because of their good strength–toughness combination at low temperatures and cost effectiveness [1]. Compact strip production (CSP) technology is an upcoming promising route for the production of high quality steel grades [2]. The CSP thin-slab technology is an integral chain process starting from the production of liquid steel to the refined hot and cold strip. The CSP can also be combined with the tradi-
∗
Corresponding author. Tel.: +1 337 642 4130; fax: +1 337 482 1220. E-mail address: [email protected] (R.D.K. Misra).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.03.026
tional methods that makes use of blast furnaces and converters or electric steel making. In the case of latter, scrap, direct reduced iron (DRI), as well as hot-briquetted (HBI) iron and pig iron are used as starting material. In traditional thin-slab technology, the casting and rolling processes are coupled, which minimizes energy consumption and operating costs. Since the introduction of CSP technology in 1989, the technology is being continuously developed and is poised to become an important process for the production of high quality steel grades. The American Petroleum Institute (API) provides standards for pipe that are suitable for use in conveying gas, water, and oil in both the oil and natural gas industries. The API 5L specification describes the requirements of chemistry, tensile test characteristics and toughness behavior, as depicted in Fig. 1. The property requirements of steel vary depending on the particular application and operating conditions. The basic requirements, however, are high strength together with superior toughness at low temperature and excellent weldability. It is also important that steels should exhibit superior corrosion resistance, especially when petroleum and natural gases in recent years have become more
308
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
Fig. 1. Property requirements and the processing strategy for producing pipeline steel according to the API specification 5L.
Fig. 2. Schematic diagram of basic concept of compact strip production.
of the type that contain wet H2 S (source of sulfur). This had necessitated strict control of sulfur and phosphorus and cleanliness of steel, in general. In casting, the parameters of concern are solidification microstructure, segregation, strand guiding system, casting temperature and the cooling rate. In the case of thin-slab casting, the higher solidification rate results in smaller dendrite arm spacing, significantly reduced micro- and macrosegregation and improved homogeneity [3]. Furthermore, the quality of the hot strips and its properties are determined by the rolling and cooling process parameters (pass schedule, cooling rate, recrystallization temperature), and the metallurgical events involved in recrystallization, grain coarsening, transformation, and the precipitation behavior. The schematic representation of the basic concept of CSP Technology adopted for the work described here is presented in Fig. 2. The caster supplies thin slabs of thickness ∼50–90 mm thickness, which are hot charged without intermediate cooling and rolled to hot strips in the finishing mill. This continuous process yields benefits in terms of capital investment, power consumption and operating cost [4]. It also allows the production of the whole range of flat-steel products with superior properties in terms of surface and inner quality of hot strip. Thus, CSP
technology symbolizes a highly productive and cost-effective process for making hot steel strips. In the work described here two material strategies were implemented, i.e. the development of microalloyed steels with mainly a ferrite microstructure and strength levels in the range of API X60/65 as well as ferrite microstructure with portions of lower temperature transformation products like acicular ferrite and bainite and strength levels in the range of API X70/80 [5,6]. Acicular ferrite and bainitic microstructures are preferred for best combination of strength and toughness. However, judicious selection of microalloying elements and optimization of process parameters is essential to obtain the required microstructure. It is demonstrated here that using the CSP process, the required good combination of strength and toughness can be achieved by grain refinement, lower transformation products, and precipitation strengthening through microalloying with Nb, Ti, and V individually or in combination, as in conventional integral steel making process [7]. However, multi-microalloying approach promotes formation of complex chemistries of carbonitrides, even at low concentrations [8,9] independent of the individual parameters during thermomechanical processing.
Table 1 Chemical composition (in wt.%) of microalloyed steels Material
C
Si
Mn
P
S
V
Nb
Ti
N
CMn(VNbTi) steel CMn (VNb) steel
0.056 0.059
0.25 0.22
1.39 (< 2) 1.31 (< 2)
0.014 0.011
0.0023 0.0021
0.074 (< 0.10) 0.073 (< 0.10)
0.037 (< 0.10) 0.043 (< 0.10)
0.01 (< 0.02) 0.002 (< 0.002)
0.0062 0.0065
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
309
Fig. 3. (a) Process parameters and production schedule for API X grades in a CSP plant. (b) Example of finishing and coiling temperature for 8 mm hot strip.
The objective of the present paper is to describe the results of the development effort of two high strength pipe steels grades processed using the CSP thin-slab technology that meet the API standards, in terms of tensile and impact properties, microstructure, and precipitation behavior. The precipitates present in CMn(VNbTi) and CMn(VNb) steels were characterized in terms of morphology, size and chemistry, and crystallography. Furthermore, the orientationship between precipitates and ferrite was established. 2. Steel composition and processing methods The chemical composition of the investigated CMn(VNbTi) and CMn(VNb) pipeline microalloyed steels is presented in Table 1. The approach and details of production for API X grades on a CSP plant is presented in Fig. 3. As a basis for initial development, a low carbon content of ∼0.05% was preferred from the viewpoint of segregation, toughness, and weldability. The sulfur, phosphorus and nitrogen content was controlled to
∼0.002%, 0.016% and 0.006%, respectively, through selection of feedstock (e.g. selected scrap, direct reduction iron DRI, hotbriquetted iron HBI) during steel making. The thickness of the near-net-shaped casting slabs was 60 mm for tundish temperatures in the range of 1540–1560 ◦ C (superheat of 20–40 ◦ C). The casting speed was approximately 4.5 m/min. Variations in speed during casting were avoided. The casting conditions with respect to superheat, casting speed, casting powder, and secondary cooling were controlled to achieve superior surface quality and minimize segregation. The 1220 mm wide thin slabs were first homogenized in the soaking furnace at temperatures of 1100–1130 ◦ C. Thermomechanical rolling was done on a 6-stand rolling mill. For 8 mm thick hot strip, total forming strain of ∼87% was achieved for the starting slab thickness of 60 mm. The finish rolling temperatures (Fig. 4) were between 770 and 860 ◦ C. In the laminar-cooling zone, cooling took place as a function of strip thickness at cooling rates varying between 14 and 25 K/s to coiling temperatures between 530 and 640 ◦ C. As outlined above, CMn(VNbTi) was used at process conditions, which suggest a ferrite–pearlite
310
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317 Table 2 Mechanical properties of microalloyed steels Properties
API X65–70 CMn(VNbTi) steel
API X75–80 CMn(VNb) steel
Yield strength (MPa) Tensile strength (MPa) Yield/tensile ratio % elongation
465–490 530–575 0.83–0.9 20–30
540–590 615–675 0.85–0.9 28–35
microstructure, whereas for CMn(VNb), a microstructure with portions of lower temperature transformation products were aimed. In addition to above processing of the microalloyed steels in a commercial caster, laboratory experiments with simplified conditions in comparison to CSP were carried out to reveal the influence of the alloying concepts and the deformation conditions on the final microstructure, i.e. the fraction of lower temperature transformation products. The experimental details of the laboratory dilatometric experiments were: • one-step: ◦ heating with 3 K/s–1250 ◦ C, holding for 600 s, ◦ cooling with 30 K/s–850 ◦ C, ◦ forming with ε = 0.5 and ε = 12 1/s, ◦ cooling with 25 K/s to room temperature. • two-step: ◦ heating with 3 ◦ C/s–1250 ◦ C, holding for 600 s, ◦ cooling with 30 ◦ C/s–850 ◦ C ◦ forming with ε = 0.5 and ε = 12 1/s, ◦ cooling with 30 K/s–800 ◦ C, ◦ forming with ε = 0.7 and ε = 12 1/s, ◦ cooling with 25 K/s to room temperature. Fig. 4. Representative (a) light (b) scanning electron micrographs of CMn(VNb) steel.
Fig. 5. Light micrographs of CMn(VNbTi) and CMn(VNb) steels after one-step and two-step deformation in a laboratory rolling-mill set up.
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
311
3. Experimental testing methods
Fig. 6. Plot of tensile strength vs. yield strength for CMn(VNbTi) and CMn(VNb) steels.
Mean grain size was estimated by the circular intercept method. Intercept lengths were determined and then converted to nominal grain size using the standard tables. At least 5 coils were examined and about 4000 grains were counted to determine the grain size. Tensile tests were done according to ASTM E8 specification. Charpy v-notch impact tests were carried out according to ASTM E 23 standard. From the impact tests, both the absorbed energies and % shear-fracture were measured. Drop weight tear tests were carried out according to API RP SL3 specification and % shear determined for notched, full-walled large test specimens as a function of the temperature. Transmission electron microscopy (TEM) studies were carried out on thin foils of CSP processed hot strips with a thickness of 8 mm. These foils were prepared by cutting thin wafers from the steel samples, and grinding them to 0.1 mm in thickness. Three millimeter discs were punched from the wafers, and electropolished using a solution of 5% perchloric acid/95% acetic
Fig. 7. Plot of Charpy V-notch impact energy at −40 ◦ C vs. yield strength for CMn(VNbTi) and CMn(VNb) steels for hot strips with thickness between 8 and 10 mm.
Fig. 8. (a) Plot of % shear as function of impact testing temperature of the 8 mm hot strips of CMn(VNbTi) and CMn(VNb) steels. Filled and unfilled symbols correspond to Charpy and DWTT, respectively. (b) Dependency of CVN 50% FATT on the degree of forming during hot rolling for slab thickness of 60 mm.
Fig. 9. Bright field TEM micrographs together with EDX analysis of precipitates of different morphology in CMn(VNbTi) steel: (i) spherical, (ii) cuboidal, and (iii) irregular and fine.
312
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
Fig. 10. Bright field TEM micrographs together with EDX analysis of precipitates of different morphology in CMn(VNbTi) steel: (i) cuboidal, (ii) irregular, and (iii) fine.
acid. Foils were examined by conventional transmission electron microscope operated at 120 kV using standard bright field and dark field imaging techniques. X-ray analysis of precipitates was performed with a JEOL 2010F FEG TEM/STEM at 200 kV. 4. Results and discussion 4.1. General microstructure The microstructure of CMn(VNbTi) steel was predominantly ferrite with small volume fraction of pearlite [6]. The mean grain size according to ASTM standard E 112 was 12, which corresponds to a mean grain diameter of ∼6 m. Fig. 4(a) and (b) shows representative light and scanning electron micrographs of CMn(VNb) steel. The mean grain size was 14 and the corresponding mean grain diameter was 2.6 m. Some elongation along the rolling direction is apparent. For the hot strip production of CMn(VNb), as a function of the parameters selected for thermomechanically processing and the cooling conditions on the exit roller table, the resulting volume fractions of lower tem-
perature transformation products are between 20 and 65%. The volume fractions can be increased by reducing the intensity of the thermomechanical processing, thus reducing the mechanical stability of the austenite. This statement is in agreement with the observations made on laboratory heats. In Fig. 5, the light micrographs of CMn(VNbTi) and CMn(VNb) steels after laboratory performed hot rolling and cooling with a fixed cooling rate of 25 K/s are compared. Two hot rolling strategies: onestep deformation as well as two-step deformation where second deformation occurred at lower temperatures were considered. Generally, an intensification of the thermomechanically processing (increases value of accumulated strain in the two-step) by the second-step deformation results in a reduction of the portions of lower temperature transformation products for both the microalloying concepts. It is known that an increased dislocation density opposes the formation of acicular ferrite and bainite through a mechanical stabilization effect [10]. Furthermore, the results suggest that the exclusion of titanium favors the formation of acicular ferrite and bainite. This may be due to the higher amount of dissolved niobium which supports the formation of bainite and acicular ferrite because Ti precipitated as TiN may
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
313
Fig. 11. (a) Bright field TEM micrograph of the ferrite matrix of CMn(VNbTi) steel showing pinning of carbide precipitates by dislocations and (b) selected area diffraction pattern analysis for region shown in the image (a).
provide nucleation site for the precipitation of Nb(C,N) during the final phase of rolling [11]. 4.2. Mechanical properties The tensile properties of the CMn(VNbTi) and CMn(VNb) steels is summarized in Table 2. Fig. 6 shows the yield and tensile strength data obtained for CMn(VNbTi) and CMn(VNb) steels. For strip thicknesses between 6 and 12.5 mm, the yield strength was between ∼530 and 680 MPa. In this range, no effect of strip thickness on tensile properties was observed. This range
allows minimum values necessary to meet the API specification for X65, X70 and X80 grades. In the case of CMn(VNb) steel grade, in spite of high yield and tensile strength with yield ratio between 0.85 and 0.90, the elongation was high (28–35%). For the CMn(VNbTi) steel, similar yield ratio of 0.83–0.90 was obtained. These properties correspond to the 1220 mm wide slabs fed from the caster to an equalization furnace set at 1100–1130 ◦ C. Fig. 7 shows the plot of impact toughness for hot strips with thickness between 8 and 10 mm, tested at a test temperature of −40 ◦ C versus yield strength. It may be noted that
314
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
Fig. 12. Bright field TEM micrographs of CMn(VNb) steel showing the coarse precipitates of different morphology; (a) spherical, (b and c) irregular in the ferrite matrix and on the grain boundaries.
the CMn(VNb) steel exhibits excellent notch-toughness with absorbed energy values of 200–400 J/cm2 in spite of higher yield strength of ∼100 MPa or greater compared to CMn(VNbTi) steel. Fig. 8a shows the plot of % shear as a function of Charpy V-notch impact and drop weight tear test temperature for the CMn(VNb) steel belonging to a strip thickness of 8 mm. As a function of the hot rolling and cooling parameters, even at temperatures of −60 ◦ C, % shear of up to 93% were still observed in the notched-bar impact bending test. In the more critical drop weight tear test, the dependence on temperature of the % shear part shifted to higher temperature levels. At test temperatures of down to −50 ◦ C, however, % shear of more than 75% were still attained. From the dependence of % shear-fracture on temperature established in the drop weight tear test, a transition temperature FATT (fracture appearance transition temperature) can be derived. For a 8 mm thick hot strip made from the CMn(VNb) steel, excellent FATT levels of −50 ◦ C were attained at % shear
of 85%. To produce hot strip with a thickness in excess of 8 mm, the total degree of forming in hot rolling must be taken into account. The dependency of the FATT with a shear fraction of 50% on the total forming degree in hot rolling is presented in Fig. 8b. It may be seen from Fig. 8b that in case of particularly stringent requirements in terms of low-temperature toughness, forming degree of more than 85% are needed. 4.3. Precipitation in CMn (VNbTi) and CMn (VNb) steel Figs. 9 and 10 are representative examples of bright field TEM micrographs with the EDS analysis of precipitates in CMn(VNbTi) steel. The precipitates analyzed are marked with arrows in the figure. The precipitates of three different morphology were observed and are characterized by irregular/spherical (100–150 nm), cuboidal (30–70 nm), and fine precipitates (10–20 nm). The presence of large size precipitates was
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
315
Fig. 13. Bright field TEM micrographs of coarse precipitates together with EDX analysis of CMn(VNb) steel.
occasional in ferrite grains with low dislocation density. They are believed to have formed during solidification of the liquid melt and are thermomechanically stable in austenite and ferrite. The fine precipitates (<20 nm) were randomly (or partially in rows) distributed and some of them pinning the dislocations. The EDS analysis indicated that all the precipitates were rich in niobium and also contained titanium and vanadium. Fig. 11 shows the bright field TEM micrograph illustrating pinning of fine pre-
cipitates by dislocations. A composite diffraction pattern of the ferrite matrix and the precipitates was obtained from the center region in Fig. 11a and the SAD pattern analysis is presented in Fig. 11b. The analysis indicated that the presence of four types of orientation relationships between precipitates and the ferrite matrix. The orientation relationships [1 2 3]␣ //[0 1 1]M4 C3 and [0 1 3]␣ //[0 1 1]M4 C3 were close to Kurdjumov-Sachs relationship, in addition two other orientations relationships
Table 3 Classification of different precipitates in microalloyed steels Steel
Type
Morphology
Size range (nm)
Spectral ratios
Precipitates
CMn(VNbTi)
Triplex
Fine Cuboidal Spherical/irregular
10–20 30–70 100–150
Nb0.62 Ti0.27 V0.11 Nb0.44 Ti0.32 V0.24 Nb0.33 Ti0.33 V0.33
(TiNbV)C (Ti,Nb,V)N (Ti,Nb,V)C,N
CMn(VNb)
Duplex
Fine Cuboidal Spherical/irregular
10–20 30–70 100–150
V0.62 Nb0.38 Nb0.76 V0.24 Nb0.5 V0.5
(Nb,V)C (Nb,V)N (Nb,V)C,N
316
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
[1 3 3]␣ //[0 1 1]M4 C3 and [110]␣ //[011]M4 C3 were also identified. Figs. 12 and 13 are the bright field TEM micrographs showing the nature and morphology of precipitates in CMn(VNb) steel. The precipitation behavior in both the steels was similar in spite of the difference in the microalloying content. Also, in CMn(VNb) steel, the precipitates were of different size and morphology notably, irregular/spherical (100–150 nm), cuboidal (30–70 nm), and fine precipitates (10–20 nm). In CMn(VNbTi) steel, the fine precipitates were randomly dispersed and also present in rows, where as in CMn(VNb) steel, majority of the coarse precipitates were individually dispersed. However, in both the steels the coarse precipitates were present in ferrite grains with low dislocation density. The EDS analysis obtained on the individual precipitates indicated that they are rich in niobium and vanadium. Fig. 14a is a bright field TEM micrograph of another ferrite region depicting pinning of fine precipitates by dislocations and the corresponding SADP analysis is presented in Fig. 14b. The orientation relationships between ferrite and the precipitates were [1 1 1]␣ //[0 1 1]M4 C3 and [1 1 0]␣ //[0 1 1]M4 C3 . The size and morphology of the precipitates observed here are similar to those observed in conventionally hot rolled microalloyed steels [7–9,12–16]. The cuboidal-type precipitates were earlier identified as titanium/niobium nitrides, [(Ti,Nb)N], spherical/irregular precipitates as titanium/niobium carbides [(Ti,Nb)C], and fine precipitates as carbides, [(Ti,Nb,V)C] [7,9,12]. The estimated precipitate size range and the spectral ratios of CMn(VNbTi) and CMn(VNb) steel are given in Table 3. However, comparison of precipitate size and spectral ratios of the conventional and CSP processed steels may suggest that in CSP process, the triplex compounds are richer in niobium. But this observation may also be a consequence of higher niobium content in microalloyed steels presented here. Multi-microalloying design approach such as Nb–Ti, Ti–V, and Nb–V generally results in the formation of duplex carbonitrides [14], while V–Nb–Ti microalloying approach led to formation of triplex carbonitrides [8]. An important conclusion that can be drawn from this work and from the previous work [8,9] is that as the number of microalloying element increases, the precipitates are multi-microalloying compounds. Based on recent solubility product calculations carried out in the temperature range of 700–1300 ◦ C [9], it was shown that the microalloying elements, Ti, Nb, V are interchangeable in the precipitate lattice because of similarity in crystal structures (FCC NaCl type) and lattice parameter [13,14]. This explains the observation of EDS analysis of triplex and duplex precipitates that indicated the presence of all the three or two microalloying elements in a single precipitate. Furthermore, we have estimated the orientation relationships outlined above that suggest that the precipitation in CMn(VNbTi) and CMn(VNb) steels presumably occurred during austenite to ferrite transformation [13]. It is also likely that they may have formed mainly in ferrite and partially at the interphase because of the high cooling rate and the limited evidence of rows of precipitates. The fine precipitates identified as carbides (MX or M4 C3 type where M is V or Ti or Nb) have a cubic crystal structure (NaCl crystal structure of B1 type) and
Fig. 14. (a) Bright field TEM micrograph of CMn(VNb) steel showing pinning of fine carbide precipitates by dislocations in the ferrite matrix and (b) SAD pattern analysis for the ferrite matrix and the carbide precipitates.
are precipitated in the ferrite matrix. Our recent work on conventionally processed V–Nb–Ti and V-steels exhibited similar kind of precipitation behavior and a detailed discussion on crystallography and ordering of the multi-microalloying precipitates has been previously discussed [8,9]. The above results demonstrate that the CSP technology can be successfully used to produce high strength API grades with superior toughness. The strength is derived from solid solution, precipitation, and dislocation hardening mechanisms and the superior toughness of steels is predominantly related to finegrained microstructure and near absence of large TiN or VN precipitates that promote cleavage fracture.
C.P. Reip et al. / Materials Science and Engineering A 424 (2006) 307–317
5. Conclusions The compact strip production (CSP) technology can be successfully used to process microalloyed high strength steels of desired strength and superior toughness. Hot strips of CMn(VNbTi) and CMn(VNb) steel grades were produced using CSP technology with high strength and low-temperature toughness up to −60 ◦ C. For strip thicknesses between 6 and 12.5 mm, the resulting yield strength levels were between ∼ 530 and 680 MPa. The yield ratios of the two steels were in the range of 0.85 and 0.90, and their elongations were 28–35%. The CMn(VNb) alloy exhibited very good notch-toughness of 200–400 J/cm2 in spite of its higher yield strength of ∼100 MPa or greater over CMn(VNbTi) steel. The superior toughness is attributed to the fine grain size of the steels. The precipitates in CMn(VNbTi) steel were of triplex-type containing Ti, Nb, and V even at low concentration of titanium, while in CMn(VNb) steels they were of duplex-type containing Nb and V. The precipitates were irregular/spherical, cuboidal, and fine with size range of ∼100–150, 30–70, and 10–20 nm, respectively. The microalloying elements (Ti, Nb, and V) form coherent M4 C3 type of carbides in the ferrite matrix of both the steels. References [1] A.J. DeArdo, The metallurgy of high strength line-pipe steels, in: 44th Annual Conference of Metallurgists of CIM, Proceedings of the International Symposium on Pipelines for the 21st Century, Calgary, Alta., Canada, August 21–24, 2005, pp. 85–100.
317
[2] C.P. Reip, W. Hennig, J. Kempken, R. Hagmann, Recent developments in integrated technology of compact strip production, Iron Steel Technol. 2 (2) (2005) 25–32. [3] R. Kaspar, Steel Res. 75 (2003) 318–326. [4] C.P. Reip, W. Hennig, J. Kempken, R. Hagmann, Development of CSP processed high strength pipe steels, Conference on Microalloying for New Steel Processes and Applications, San Sabastian, Spain, in press. [5] C.P. Reip, W. Hennig, R. Hagmann, Recent developments in integrated technology of compact strip production, in: Proceedings of the Materials Science and Technology Conference and Exhibition, New Orleans, 2004, pp. 781–791. [6] C.P. Reip, W. Hennig, J. Kempken, R. Hagmann, Production of high strength pipe steels by CSP thin slab technology, in: 44th Annual Conference of Metallurgists of CIM, Proceedings of the International Symposium on Pipelines for the 21st Century, Calgary, Alta., Canada, August 21–24, 2005, pp. 143–153. [7] R.D.K. Misra, G.C. Weatherly, J.E. Hartmann, A.J. Boucek, Mater. Sci. Technol. 17 (2001) 1119–1129. [8] S. Shanmugam, M. Tanniru, R.D.K. Misra, D. Panda, S. Jansto, Mater Sci. Technol. 21 (2005) 165–177. [9] S. Shanmugam, M. Tanniru, R.D.K. Misra, D. Panda, S. Jansto, Mater Sci. Technol. 21 (2005) 883–892. [10] C. Lee, H. Bhadeshia, H. Lee, Mater. Sci. Eng. A 36 (2003) 249–257. [11] C. Klinkenberg, K. Hulka, W. Bleck, Niobium Carbide Precipitation in Microalloyed Steel, vol. 75, Steel Research International, 2003, pp. 744–752. [12] R.D.K. Misra, K.K. Tenneti, G.C. Weatherly, G. Tither, Metall. Trans. 34A (2003) 2341–2351. [13] M. Charleux, W.J. Poole, M. Militizer, A. Deschamps, Metall. Trans. 32A (2001) 1635–1646. [14] T.N. Baker, Y. Li, J.A. Wilson, A.J. Craven, D.N. Crowther, Mater. Sci. Tech. 20 (2004) 720–730. [15] M.J. Crooks, A.J. Garratt-Reed, J.B. Vander Sande, W.S. Owen, Metall. Trans. 12A (1981) 1999–2013. [16] R.D.K. Misra, S.W. Thompson, T.A. Hylton, A.J. Boucek, Metall. Trans. 32A (2001) 745–760.