Patent Application
20180327882
This invention relates to austenite grain size control by preventing grain coarsening of austenite in upstream processing of niobium microalloyed steel in order to produce thicker gage products with excellent drop weight tear test (DWTT) toughness (as measured in accordance with API RP 5L3 (Apr. 1, 1996)). In current technological practice, there is no intentional control measure to prevent grain coarsening of austenite before entry to finish rolling. As a consequence, heavy rolling reductions are applied to coarse grained austenite during finish rolling in order to increase the surface to volume ratio by geometric means through heavily pancaked austenite. Heavy rolling reductions applied in the finishing mill, often approaching limits of mill loading, inevitably limit thickness of the final product. In order to improve the safety and efficiency of the transport of natural gas and oil through the pipe lines, there is a growing demand for thicker gage pipes, particularly for deep offshore projects.
This invention targets austenite grain size control upstream i.e., at high temperatures in order to produce thicker gage product. This invention utilizes the formation of nano-scale TiN—NbC composite precipitates to pin austenite grain boundaries, and prevent them from coarsening at high temperatures (>980° C.) so that less rolling reduction and pancaking of austenite is required to obtain target properties of high strength and excellent toughness at low temperature as measured by ductile to brittle transition temperature and percentage shear in drop weight tear tests. There are additional benefits related to reduced texture related anisotropy of properties and improved ductile fracture arrestability of pipes in service resulting from engineering TiN—NbC composites by controlling the spacing of the precipitates by TiN independently from the size of precipitates by strain induced growth of NbC on pre-existing TiN to control the Zener pinning pressure of precipitates on austenite grain boundaries.
Although austenite grain size at the end of roughing is fine (<30 microns), significant grain coarsening of austenite occurs subsequent to the end of roughing, which must be prevented from occurring in order to produce a thicker gage product. If the austenite grain size is coarse, it is feasible to apply heavy rolling reductions to “pancake” the austenite and increase the surface to volume ratio of austenite, which increases the nucleation sites for ferrite upon transformation. To the extent that grain coarsening is controlled by a diffusion mechanism which is dependent on time and temperature, rapid thermal cooling should decrease the kinetics of diffusion for grain coarsening. However, it has not been found feasible to adequately cool the center of thick transfer bars to prevent grain coarsening. Nevertheless, accelerated cooling upstream is still beneficial to avoid rolling in the partial recrystallization regime and avoid depletion of solute niobium by excessive growth of niobium carbide precipitates upstream. Therefore, there is a need to develop alternative strategies to prevent grain coarsening. Although solute niobium retards grain coarsening by retarding boundary mobility, the magnitude of solute drag on boundary mobility is weak at high temperatures. As a consequence, even though most of the niobium is available as solute in conventional processing, it is not found to be effective in preventing grain coarsening in the described conditions, e.g. where the temperature is greater than 1100° C. during roughing. As shown in
In a modern Steckel mill engaged in producing 19 mm gage thickness, the cooling time between the end of roughing and the start of finish rolling is typically about 300 seconds. Thus, grain coarsening of austenite during the holding period is a significant problem.
One of the aims of embodiments of the present invention is to prevent grain coarsening between the end of roughing and the start of finish rolling. Thus, an object of embodiments of the present invention is to prevent grain coarsening of austenite grains by metallurgical means, through pinning austenite grain boundaries by second phase particles using the Zener pinning mechanism. The use of TiN particles to pin austenite grain boundaries by the Zener pinning mechanism is well established and has been disclosed in prior patents (see, for example, U.S. Pat. No. 6,899,773; U.S. Pat. No. 6,183,573; and U.S. Pat. No. 5,900,075). While these patents identify conditions under which high number density of TiN precipitates can be promoted, the limiting austenite size achievable by TiN alone is typically 60 to 80 microns in the high temperature window of processing. NbC is sluggish to nucleate by itself and is aided by dislocations generated by deformation to promote strain induced nucleation of NbC, which is associated with large undercooling. Strain induced precipitation of NbC is used in controlled rolling of microalloying technology, where strain induced precipitation of NbC is used to pin austenite grain boundaries during thermo-mechanical controlled rolling during the low temperature window of processing. However, by promoting epitaxial growth of NbC on pre-existing TiN in accordance with embodiments of the present invention, TiN—NbC composite precipitates are obtained with negligible undercooling in the high temperature window towards the end of roughing. These nano-scale TiN—NbC composite precipitates are available to pin austenite grains towards the end of roughing and limit austenite grain size to under about 30 microns on entry to finish rolling, which is essential to produce thicker gage in line pipe grades.
Accordingly, it is an object of the invention to reduce the need for large rolling reductions and heavy pancaking during finish rolling in order to obtain increased gages of finished product.
It is another object of the invention to produce uniform fine austenite grain size before pancaking and apply less pancaking to produce thicker gage product, which exhibits homogeneous properties without anisotropy due to unfavorable crystallographic texture development.
It is yet another object of the invention to obtain consistently low ductile to brittle transition temperature (DBTT) and good drop weight tear test (DWTT) performance. DWTT properties are empirically correlated with thickness of pancaked austenite grain. By refining the austenite grain size, the thickness of pancaked austenite grains can be decreased to meet target DWTT properties.
It has now been discovered that the addition of niobium to titanium-bearing super-martensitic stainless steel refines the austenite grain size due to the formation of titanium-niobium bearing composite precipitates. This led to the present invention's development of nano-scale precipitation engineering of TiN—NbC composite precipitates to prevent austenite grains from coarsening. According to embodiments of the present invention, TiN precipitates, which are formed just after solidification in the continuous cast slab, are used to control the interparticle spacing, while NbC precipitates growing on pre-existing TiN particles are used to control the size of the precipitates, both size and spacing of TiN—NbC composite precipitates are the key to pinning austenite grains of the required size to prevent them from coarsening. The driving force for grain coarsening is capillary force, which can be determined from the equation: capillary force=2 γ/R, where R is the radius of curvature of the grain boundary and γ is the surface energy of the boundary.
The driving force for grain coarsening tends to decrease as the grain size increases. In accordance with embodiments of the present invention, a 30 micron grain size is targeted instead of the conventional 60 microns. This driving force for boundary movement is counteracted if particles pin the boundary. The pinning force increases with the number density and size of the particles. Thus, the driving force for grain coarsening when the target grain size is 30 microns can be determined from the number density of particles [TiN], which sets up the interparticle spacing that can be measured, e.g., 200 nm. But the particle size of TiN is too small, about 15 nm. The limiting austenite grain size is 90 microns, which is rather coarse. By growing NbC, TiN—NbC composites can be formed, which are now large, about 25 nm. The limiting austenite grain size is 32 microns, which is close to target. By growing to 30 nm, as can be seen from Table 1 below, the limiting austenite grain size is decreased to 22 microns. It should be noted that the number density and the volume fraction of precipitates are controlled by the thermodynamics and kinetics of precipitation which, in turn, depend upon the chemical composition and processing parameters of the steel.
The validity of the mechanism underpinning the technology of nano-scale precipitation engineering for austenite grain size control in upstream processing is demonstrated by experimental results on line pipe grades processed under plate rolling and hot strip rolling conditions.
In conventional processes, roughing is carried out in a high temperature window where there is no significant thermodynamic potential for precipitation of NbC and Zener drag from TiN precipitates is inadequate to stop recrystallization. Austenite grains are refined in roughing by static recrystallization that follows each pass, and austenite grains are refined below 30 microns at the end of roughing. However, in conventional processes, austenite grains tend to coarsen during the cooling time before entry to finish rolling temperature.
In traditional processes, there is no deliberate step to prevent grain coarsening of austenite to 60 microns. According to the embodiments of present invention, there is a second stage of roughing, comprising one or two heavy passes towards the end of roughing after cooling the center of thick sections to a low temperature, where there is adequate thermodynamic potential for strain induced growth of NbC on pre-existing TiN to form TiN—NbC. This second stage of roughing is conducted when the center of the steel product is at about 1040° C., well below the dissolution temperature of NbC, so that NbC can grow epitaxially on pre-existing TiN upon deformation to promote strain induced growth of NbC on pre-existing TiN. This increases the Zener pinning pressure, e.g. drag, exerted by the TiN—NbC precipitates on austenite grain boundaries to counteract the driving force for recrystallization, and arrest recrystallization. Since the driving force for grain coarsening of 30 microns is less than that for recrystallization, austenite grain coarsening is limited to a maximum of 30 microns. Thus, temperature of no recrystallization (Tnr) is reached in the center of the transfer bars at the end of the second stage of roughing, which, as described below, acts to suppress rotated cube texture and prevents grain coarsening in the long cooling time between the end of roughing and the start of finish rolling.
The cubic texture of recrystallized austenite grains upon transformation promotes undesirable rotated cube texture, which promotes brittle cleavage fracture. In embodiments of the present invention, by work-hardening the recrystallized austenite grains, the texture is changed to brass or copper texture, which upon transformation gives desirable texture, shown in the bottom half of
By controlling the spacing and size of TiN—NbC, delayed strain induced precipitation of NbC is suppressed, thereby retaining adequate solute niobium in the matrix to suppress polygonal ferrite nucleation. In the prior art by Bai et al., the final microstructure is described as 30% polygonal ferrite and 70% acicular ferrite. It is the objective of the current invention to suppress undesirable delayed strain induced precipitation of NbC in austenite that promotes polygonal ferrite in the final microstructure. It is the objective of the embodiments of the present invention to promote acicular ferrite transformed at low temperature in order to obtain high number density of high angle boundaries that suppress brittle fracture.
This also acts to suppress rotated cube texture and prevents grain coarsening in the long cooling time between the end of roughing and the start of finish rolling. In the case of processing heavier gage in a conventional hot strip mills to produce coil, one heavy pass after cooling the center to Tnr by nano-scale precipitate engineering is found to be adequate for a gage thickness of 20 mm, see
Embodiments of the present invention provide a platform for austenite grain size control in upstream processing of austenite of niobium microalloyed steels to which downstream processing and final properties of the product are related.
A process for controlling austenite grain size in austenite processing of niobium microalloyed steel through nano-scale precipitation engineering of TiN—NbC composites to produce thicker gage product comprises controlling the base chemical composition of a steel product to include about 0.003-0.004 wt % nitrogen, 0.012-0.015 wt % titanium, 0.03-0.07 wt % carbon, and 0.07-0.15 wt % niobium; lowering the temperature of roughening to end the roughening operation in the temperature range of from about 940° C. to 1040° C., e.g. 960° C. at the surface; retaining greater than about 0.03 wt % niobium in solution in the matrix by rapid cooling of the product to enter the finish rolling operation below the temperature of no recrystallization, with an austenite grain size of about 30 microns; and applying reduced rolling reduction in the finish rolling operation. The temperature for the last roughening/roughing pass is usually based on surface temperature, but can also be based on the temperature at the center of the thickest section of the steel product, which is about 100° C. higher than the surface temperature. For example, the temperature for the last roughing deformation pass can be defined as 1040° C. in the center of the rolled section, but in the mill, it has to be inter-related to surface temperature. Lowering the temperature of roughening prevents grain refined austenite from coarsening above about 30 microns by formation of TiN—NbC composite precipitates. Applying reduced rolling reduction in the finish rolling operation acts to pancake the fine austenite grain size of about 30 microns to obtain a sufficient surface to volume ratio to produce thicker gage resulting steel product.
In accordance with some embodiments, a process for controlling austenite grain size in austenite processing through nano-scale precipitation engineering of TiN—NbC composites to produce thicker gage product of niobium microalloyed steel for line pipe application with good crack arrestability to ductile fracture propagation includes controlling the base chemical composition of a steel product to include about 0.003-0.004 wt % nitrogen, 0.012-0.015 wt % titanium (to control the interparticle spacing of TiN to be about 200 nm), 0.03-0.07 wt % carbon, and 0.07-0.15 wt % niobium (to control thermodynamic potential for precipitation of NbC). The process includes carrying out the following process steps in roughing of the steel product (e.g. a reheated slab) with austenite grain size of 200-300 microns and the chemistry designed to give TiN precipitates of 10-15 nm size with an interparticle spacing of 200-300 nm.
Process Step-1: Conducting the first stage of roughing passes in high temperature window (about 1080° C. to 1150° C.), in which there is no significant thermodynamic potential for precipitation of NbC, as found in traditional processes, in order to refine the austenite grains from 300 microns to less than 30 microns by repeated static recrystallization.
Process Step-2: Holding for 100-120 seconds for temperature homogenization and the center of the section to cool to a temperature typically about 1040° C. at which there is significant thermodynamic potential for precipitation of NbC to occur. The surface temperature of the thick section of the steel product (slab) will be significantly less than the center, by up to 100° C. less. This will increase thermodynamic potential for precipitation of NbC available in the surface than in the center.
Process Step-3: Conducting a second stage of roughing, e.g. rough rolling, that includes applying one or two heavy pass reductions (>25% reduction relative to steel slab after the first stage of roughing) when the center of the section is at about 1040° C., in order to promote strain induced growth of NbC on pre-existing TiN. This will be aided by accelerated diffusion by dislocations generated by deformation to form TiN—NbC composites of 25-30 nm with an interparticle spacing of 200-300 nm. These precipitates will exert Zener pinning pressure of 0.1 MPa to stop recrystallization in the center of thick section, thereby promoting Tnr at about 1040° C. This enables the center of thick transfer bars (e.g. the steel product after the second stage of rough rolling) at high temperature to be fully pancaked to suppress the occurrence of undesirable rotated cube texture from occurring in the center of thicker gage products. The Zener pinning pressure of 0.1 MPa is more than adequate to counteract the driving force for grain coarsening by capillarity of austenite grains of about 30 microns size.
Process step-4: Conducting rapid cooling to enter finish rolling so that the transfer bar has a center temperature below 920° C. so that adequate solute niobium (Nb>0.03 wt %) is retained in the matrix for transformation hardening. The typical cooling rate ranges from 1 to 0.5° C./second.
Process Step-5: Applying an adequate rolling reduction of about 70% to pancake the austenite grains of about 30 microns in finish rolling to obtain large surface to volume ratio (Sv factor) of the austenite grains with large strain accumulation and so that there is increased surface area of austenite that acts as nucleation sites for ferrite as well as high dislocation density to increase the number density of nucleation sites per unit area of austenite grain surface. The aim is to control thickness of pancaked austenite grains to be less than 10 microns. In the absence of Zener pinning pressure from TiN—NbC, austenite will coarsen to 60 microns, as exemplified by the conventional practices used in the prior art.
Process Step-6: Conducting accelerated cooling of austenite grains with large Sv factor, large strain accumulation and adequate solute niobium (0.03 wt % Nb) in the matrix to promote transformation of acicular ferrite at low temperature, e.g. laminar water cooling at a rate ranging from 8-15° C./s, or higher in modern mills (10-30° C./s), and to promote acicular ferrite grains of 1-3 microns with high density and dispersion of high angle boundaries to suppress brittle fracture to produce thicker gage with enhanced crack arrestability to ductile fracture propagation and/or in order to promote crystallographic variants that give high density and dispersion of high angle boundaries to suppress brittle fracture. Those skilled in the art will readily appreciate that ferrite grains nucleate on austenite grain surfaces and therefore, by pancaking the austenite, we increase the surface area per unit volume of austenite. This is a geometric parameter that increases the nucleation sites for ferrite. The kinetics of nucleation is aided by dislocation density, which increases the number density of acicular ferrite nucleating per unit area of austenite boundary (Rate of nucleation per unit area of austenite surface—Kinetics). I. Kozazu, G. Ouchi, T. Sampei and T. Okita, Proc. of International conf. on Microalloying 75, Union Carbide, New York, (1976), 100.
The increased Tnr by nano-scale precipitate engineering of TiN—NbC composite to 1040° C. instead of 940° C. for Tnr by NbC is adequate to pancake austenite grains occurring in the center of thicker sections before the transformation, thereby suppressing undesirable rotated cube cleavage texture from occurring in the center of thicker gage steel product. The control of interparticle spacing of TiN—NbC composite is effective in suppressing delayed strain induced precipitation of NbC. In conventional practice, delayed strain induced precipitation of NbC occurs, which renders it difficult to control residual solute niobium required to suppress polygonal ferrite formation in transformation product. Hardenability, in embodiments of the present disclosure, is controlled by adequate solute niobium to promote acicular ferrite transformation at low temperature by shear transformation to give high density and dispersion of high angle boundaries that suppress brittle fracture, thereby increasing percentage shear area in DWTT at low temperature. The control of interparticle spacing of TiN—NbC is effective in promoting austenite grain size to about 30 microns before pancaking thereby increasing the surface to volume ratio of austenite grains (Sv factor) that enables production of thicker gage.
The grain size can be controlled in the range of about 20-40 microns at entry to the finish rolling operation. TiN precipitates can be in the range of about 10-20 nm and the interparticle spacing can be about 200-300 nm. Thermodynamic potential for precipitation of NbC can occur towards the end of the roughing operation at temperatures ranging from about 940° C. to about 1040° C. TiN—NbC composites can be in the size range of about 20-50 nm. Accelerated cooling of the product can be applied to avoid rolling in the partial recrystallization regime. The process can include controlling nitrogen at or below about 40 ppm and making a titanium addition to meet the stoichiometric requirement to combine with all nitrogen to form high number density of TiN precipitate in about the 10-20 nm size range.
The process can include processing the steel product by conventional plate rolling, conventional hot strip rolling, steckel mill rolling, and/or near net shape processing. The steel product can be line pipe steel, infra-structure steel, and/or super-martensitic stainless steel. The crystallographic texture-related anisotropic properties of the resulting steel product can be minimized. The process can include substituting titanium partially or fully in the base chemistry with a member of the group consisting of Zr, Hf, Ta, W, V, Cr, Mo, Al and mixtures thereof, each with high affinity for nitrogen to form nano-scale precipitates on which NbC can grow epitaxially to give composite precipitates.
The process also can include partially substituting niobium in the base chemistry (with a minimum amount of 0.06 wt % niobium based on the available data) with other microalloying elements with high affinity for carbon selected from the group consisting of Zr, Hf, Ta, W, V, Cr, Mo and mixtures thereof, each to give composite precipitates. The process also can include substituting solute niobium on entry to finish rolling with other elements, which exhibit solute drag comparable to niobium. Still further, the process can include rapidly cooling the steel product to enter finish rolling with a center temperature at or below about 920° C. The steel product can exhibit a gage thickness of about 17-40 mm.
Embodiments of the invention are based on strain induced growth of NbC on pre-existing TiN with short interparticle spacing of about 200-300 nm to form TiN—NbC composites of adequate size of about 30 nm at negligible undercooling (<5° C.) so that the temperature of no recrystallization of austenite grains (Tnr) is promoted at about 100° C. higher than in conventional processing of niobium microalloyed steels in which temperature of no recrystallization occurs by strain induced nucleation of NbC precipitates at large undercoolings (>100° C.). By raising temperature of no recrystallization, the undesirable rotated cube texture that promotes brittle fracture is suppressed by pancaking the austenite grains before transformation. Delayed strain induced precipitation renders it difficult to control the transformation temperature in downstream processing and the resulting structure and properties. By controlling the interparticle spacing of TiN—NbC, the undesirable delayed strain induced precipitation of NbC in austenite is suppressed.
The steel without niobium addition but with titanium exhibits an austenite grain size of 80 microns, which shows that titanium addition alone is not effective in refining austenite grain size. But with the addition of 0.1 wt percent niobium, the austenite grain size is significantly decreased to 35 microns. The white lines in
The area occupied by particles on the boundary must be recreated before the boundary moves, and this is the energy preventing grain coarsening and is referred to as Zener drag. Solute atoms piling up at the interface exert a drag force on boundary mobility, which is referred to as solute drag. Solute drag of niobium is less pronounced in the high temperature window. Zener drag is increased as the interparticle distance is reduced and the particle size becomes bigger for a given volume fraction of precipitate, which is determined by the thermodynamic potential for precipitation for a given steel composition. TiN and TiN—NbC precipitates occur on a nano-scale and therefore engineering the size and dispersion of nano-scale precipitates is termed “engineering nano-scale precipitates for pinning grain boundaries.”
The breakthrough in austenite grain size control upstream arose out of observations in super-martensitic stainless steel, where a titanium addition by itself did not produce a fine austenite grain size. But when combined with niobium additions, a fine austenite grain size was obtained. This was caused by TiN—NbC composite precipitates, formed by NbC growing on pre-existing TiN. The number density of precipitates was controlled by the TiN. By increasing the size of the precipitates by growth of NbC on pre-existing TiN, it is possible to increase the pinning pressure of precipitates to arrest austenite grain boundary movement at the required austenite grain size. This discovery underlies the present invention of nano-scale precipitate engineering. The austenite grain size is controlled by interparticle spacing of TiN precipitates and the size of the precipitates, each of which can be independently controlled by design of the base steel composition. A high number density of TiN is promoted when the precipitation occurs in the matrix at low temperature, which calls for lowering the nitrogen content and adding titanium to the stoichiometric requirement to form TiN, providing one atom of titanium for every one atom of nitrogen. By lowering nitrogen to less than 40 ppm and adding titanium to the stoichiometric requirement of about 0.014 wt percent, the interparticle spacing was found to be around 200 nm and the TiN precipitate size was found to be in the 10-15 nm range. The Zener pinning pressure on the boundary is relatively small, capable of arresting austenite grains of about 80 microns from coarsening. The pinning pressure can be increased by growing NbC precipitates on TiN, thereby increasing the size of the composite precipitates of TiN—NbC. This requires lowering the temperature window of roughing so that the thermodynamic potential for growth of NbC on pre-existing TiN is obtained to form TiN—NbC composites. By increasing the particle size while retaining the same interparticle spacing, the pinning pressure is increased to arrest the finer austenite grains from coarsening. By increasing the mean size of precipitates to about 30 nm with an interparticle spacing of about 220 nm, the Zener pinning pressure is increased to prevent austenite grain size of 30 microns from coarsening.
The concept of nano-scale precipitation engineering to arrest grain coarsening is illustrated in the schematic diagram given in
Nitrogen is controlled to promote the formation of TiN precipitates at lower temperatures. The resulting finely-dispersed nano-precipitates of TiN then act as scaffolds for the epitaxial formation of NbC, thereby raising the volume fraction of dispersed composite precipitates by a factor of about 3×. This is sufficient to hold the austenite grain size to about 30 micrometer size in low nitrogen steel compared to 60 microns for higher nitrogen steel. The advantage here lies in the reduced austenite grain size, permitting the application of a reduced rolling reduction during final processing and the consequent ability to produce thicker gages of higher strength material (X-70, X-80, X-90, X-100) compared with high nitrogen steel, which requires heavy rolling reduction that limits final gage of the product.
By comparison,
The technology of nano-scale precipitation engineering of TiN—NbC composites involves two microstructural parameters. The first is the interparticle spacing. The second is the particle size. This invention is based on the discovery that TiN—NbC composites offer a window of opportunity to control interparticle spacing through optimum TiN distribution and the size of the particle by epitaxial growth of NbC on pre-existing TiN particles. The first step is to engineer a high number density and uniform dispersion of TiN particles. This is done by promoting nucleation of TiN in austenite at lower temperatures through control of the base steel chemical composition. Since the precipitates occur on a nano-scale, it is essential to characterize the precipitates by transmission electron microscope. The well-known carbon replica technique is used in this work to extract the precipitates occurring in benchmarked steels.
Steel-A with the highest nitrogen content of 0.0075 wt. percent exhibits a large mean interparticle spacing of about 550 nm compared with Steels-B and C with a low N content of 35-40 ppm, which exhibit a mean interparticle spacing of about 220 nm. Steel-D with intermediate nitrogen content of 55 ppm exhibits an intermediate interparticle spacing of about 400 nm. Clearly, the interparticle spacing of 220 nm can be achieved by lowering nitrogen content to or below 40 ppm and adding titanium to the stoichiometric requirement to tie up all the nitrogen. The precipitate size of TiN—NbC of the highest nitrogen Steel-A is 83 nm, which gives Zener limiting austenite size of 62 microns. By comparison, Steel-B and Steel-C with low nitrogen give Zener limiting austenite grain size of about 27 microns.
In conventional processing of conventional nitrogen-bearing niobium microalloyed steel (0.005-0.008 wt. percent nitrogen), roughing is carried out where there is no thermodynamic potential for precipitation of NbC. The loss of niobium by excessive growth of NbC on pre-existing TiN particles is reduced by minimizing the time of processing in the mill. In the case of nano-scale precipitation engineering of TiN—NbC composites with finer interparticle spacing, it is even more critical to prevent depletion of solute niobium in the matrix by accelerated cooling upstream between the end of roughing and the start of finish rolling. It is essential to control the finish rolling entry temperature below the temperature of no recrystallization in order to avoid rolling in the partial recrystallization regime, which requires accelerated cooling. Thus, accelerated cooling is required to prevent depletion of solute niobium by precipitate growth, subsequent to pinning the austenite grains of the required size in higher grade line pipe steel.
Table 6 is extracted from Kozazu's diagram in
As shown in
With reference now to
As shown in
Control of Rotated cube texture:
In traditional microalloyed steel, the center of heavy gage plate as well as strips of thermo-mechanically rolled steels is found to exhibit rotated cube texture, which promotes cleavage brittle fracture. This is caused by transformation of cubic texture associated with recrystallized austenite grains. By work-hardening the austenite before transformation, it is possible to suppress the occurrence of undesirable rotated cube texture. In order to work harden the austenite in the center of thick section, it is essential to suppress recrystallization by raising temperature of no recrystallization. In the prior art, the temperature of no recrystallization by strain induced nucleation of NbC occurs at large undercoolings, and hence, Tnr is typically lower than 960° C. Since the center of thick section is about 100° C. higher than the surface, it is difficult to suppress rotated cube texture from occurring in the center of thicker gage unless Tnr is raised substantially. Nano-scale precipitate engineering of TiN—NbC composite, in accordance with embodiments of the present invention, raises Tnr by about 100° C. by strain induced growth of NbC on pre-existing TiN at negligible undercoolings, see Table 4. Since nano-scale precipitate engineering of TiN—NbC raises temperature of no recrystallization significantly (by more than 100° C.), it can be used to advantage to arrest recrystallization of austenite occurring in the center of thick section which is about 100° C. higher than the surface temperature, thereby work-harden the austenite to modify the cubic structure of austenite to brass or copper before transformation in order to eliminate rotated cube texture.
The principal differences in the processing of higher niobium steels between the prior technology without austenite grain size control upstream and the technology of the present invention based on austenite grain size control are examined in further detail and their consequence on product in terms of gage thickness and properties are highlighted in the following examples.
Steel-A with High Nitrogen Content
Steel-A is representative of prior technological practice, where a higher nitrogen content of 75 ppm gives coarse TiN particles with large interparticle spacing of 550 nm. Roughing is carried out in the temperature window where there is no thermodynamic potential for precipitation of niobium carbide. Thus, the austenite grain size entering finish rolling is 60-80 microns. This then requires heavy pancaking below the temperature of no recrystallization. Thus, the final gage is generally limited to 16 mm. Typical property results obtained from 10 mm gage are reproduced below in Table 7:
Steel-E: Plate Rolling with Low Nitrogen Content:
Steel-E has a lower nitrogen content (40 ppm) with titanium and niobium addition comparable to Steel-A. The low nitrogen and stoichiometric addition of titanium to combine with nitrogen to form TiN has produced a high number density of TiN with a mean interparticle spacing of 220 nm. This steel was processed under two distinctly different conditions. The first set of conditions was where the rough rolling window was similar to Steel-A, that is where there is no thermodynamic potential for NbC precipitation to occur. Under these conditions, TiN particles alone are not able to develop pinning pressure adequate to pin a fine austenite grain size. Thus, the resulting coarse austenite grain size warrants heavy rolling reduction, which is not possible to achieve in 22 mm gage thickness. As a consequence, the final product fails as percentage shear area in the DWTT specimen is lowered to 55 percent at −15° C. (See Table 8).
Steel-E was also processed under a second set of conditions, more than one heavy pass of about 25% rolling reduction was carried out below the dissolution temperature of NbC. In this case, NbC grows on pre-existing TiN to increase the particle size so that the pinning pressure is increased to prevent austenite grain coarsening above 30 microns.
Once austenite grain size is refined at the entry to finish rolling, less rolling reduction is required in finish rolling in accordance with Kozazu's diagram in
22 mm thick gage—X80 Plate: (nitrogen 0.004, titanium 0.016, carbon 0.05, niobium 0.1)
Effect of processing temperature window on low nitrogen and high niobium steel.
Effect of rough rolling in the temperature window with and without thermodynamic potential for precipitation of NbC.
Steel-D with Intermediate Nitrogen Content
Steel-D presents a case, where nitrogen content is at an intermediate level of about 55 ppm and therefore the mean interparticle distance is 390 nm. Though the temperature of finish rolling promoted growth of NbC on pre-existing TiN, the TiN—NbC composite did not have adequate pinning pressure to arrest austenite grains finer than 59 microns, see Table 9. This is partly due to low niobium content, i.e. 0.067 wt. percent. Thus, the strip rolled to 20 mm gage thickness exhibited 100 percent shear only at −10° C. and above, see Table 10a and 10b.
Steel-D: 20 mm thick gage X80 Strip
High nitrogen and lower niobium with rough rolling in the temperature regime where there is thermodynamic potential for precipitation of NbC.
In-depth characterization of Steel-C has confirmed that the interparticle spacing of TiN is 220 nm. Steel-C represents low nitrogen content, with optimized addition of titanium to promote high number density and uniform dispersion of TiN with an interparticle spacing of 220 nm. The end of roughing is in the temperature window where thermodynamic potential for precipitation of NbC occurs.
The process steps involve two stages of roughing. The first stage of roughing is to refine the austenite grain size by static recrystallization in the first few passes of rough rolling in the high temperature window where there is no significant thermodynamic potential for precipitation of NbC. The second stage of roughing is at a much lower temperature, e.g. 1040° C. in the core and a corresponding surface temperature of 940° C. at the surface in order to obtain significant thermodynamic potential for strain induced growth of NbC on pre-existing TiN. This requires holding for about 100 seconds after the first stage of roughing for temperature homogenization through the section thickness. In this context, temperature homogenization refers to the temperature gradient across the section thickness decreasing. The target temperature is to promote Tnr in the center of the section, which is about 1040° C., the corresponding surface temperature will be considerably less (940° C.). Thus, a heavy rolling pass reduction (>25%) is applied as the last pass of the second stage of roughing in order to promote Tnr in the center of transfer bar by strain induced growth of NbC on pre-existing TiN to promote adequate pinning pressure due to TiN—NbC precipitates of about 0.1 MPa, on austenite grain boundaries to arrest recrystallization and prevent grain coarsening during subsequent cooling to enter finish rolling with a center temperature at about 920° C. Without the rolling reduction of the second stage of roughing at a low temperature to promote strain induced growth of NbC, austenite grains will coarsen from less than 30 microns to 60 microns and above. By Zener pinning of austenite grains, grain coarsening of austenite beyond 30 microns is prevented. Upon pancaking austenite grains with a large rolling reduction of 70%, the thickness of pancaked austenite grains is limited to 8 microns and below, which gives excellent toughness at low temperature.
TEM-EELS characterization of TiN—NbC precipitates shown in
Steel-C: 16.4 mm thick gage X90 Strip.
Low nitrogen and higher niobium with rough rolling in the temperature regime where there is thermodynamic potential for precipitation of NbC.
There are different mill designs available for compact strip processing. Nano-scale precipitate engineering of TiN—NbC composites offers a generic platform for preventing austenite grain coarsening by controlling interparticle distance by TiN, and particle size by NbC growing on the pre-existing TiN. In near net shape processing, in some cases, the transfer bar is reheated for the purpose of temperature homogenization, when the austenite grains inevitably coarsen in the absence of second phase particles. The technology of nano-scale precipitation engineering offers a sound basis for pinning austenite grain boundary with TiN—NbC composite precipitates at the end of roughing, and also during reheating. This process can be combined with accelerated cooling to prevent depletion of solute niobium by excessive growth of NbC, over and above the composite particle size required to prevent grain coarsening of austenite of a specific grain size. Trials of nano-scale precipitation engineering in a mill with two roughing stands and accelerated cooling at 4° C./s have given uniformity of microstructure, which is beneficial in achieving consistent strength and fracture properties. Delayed strain induced precipitation of NbC is reported to cause unpredictable mill loading in near net shape processing. The control of interparticle spacing of TiN—NbC offers a novel metallurgical solution to overcome this problem by suppressing delayed strain induced precipitation of NbC.
Experimental results are presented that confirm the benefits of nano-scale precipitate engineering of TiN—NbC composite by strain induced growth of NbC on pre-existing TiN over conventional processing by strain induced nucleation of NbC in processing 32 mm gage plate, using low nitrogen base chemistry in Example-6.
Trial-1 and 2 involved two alternative methods of processing respectively but using low nitrogen base chemistry. Steel chemistry of Trial-1 contained 0.04C, 0.0022N, 0.087Nb and 0.017Ti and that of Trial-2 contained 0.035C, 0.0029N, 0.083Nb and 0.016 Ti. Each trial was conducted with low nitrogen (<30 ppm), Ti (0.015 wt %) at the stoichiometric requirement to tie up all N, low carbon (0.04%) and high niobium (0.085 wt %). Plate rolling in Trial-1 was based on promoting strain induced nucleation of NbC, whereas rolling in Trial-2 was based on promoting strain induced growth of NbC on pre-existing TiN.
A comparison of the DWTT of Trial-1 (traditional strain induced precipitation) and Trial-2 (austenite grain refining by nano-scale precipitate engineering of TiN—NbC in accordance with the present invention) can be found in Table 13, below. The crack tip opening displacement (CTOD) results can be found in Table 15, below. Those skilled in the art will readily appreciate that CTOD can be measured in accordance with ISO/FDIS 3183:2006(E), May 17, 2006 or ANSI/API Specification 5L/ISO 3183. Table 13, below, shows percentage shear area measured in the DWTT as a function of test temperature for Trial-1 and Trial-2.
In Trial-1, roughing was carried out in high temperature window in which there is no thermodynamic potential for precipitation of NbC and hence strain induced precipitation of NbC was promoted during finish rolling by pancaking coarse grained austenite. The microstructure exhibited quasi-polygonal ferrite along with acicular ferrite. There is ultra-fine precipitation of NbC in the matrix and pronounced cleavage texture was obtained. The microstructure exhibited low CTOD toughness, characteristic of low crack arrestability.
In comparison, Trial-2 involved lowering the temperature window of roughing (<1040° C.) so that NbC grows on pre-existing TiN to give TiN—NbC composite that could limit grain coarsening of austenite by pinning pressure exerted by TiN—NbC composite. Even though the chemistry of plates processed in Trial 1 and 2 are comparable, the difference lies in time-temperature-deformation schedule in roughing. In Trial-1, roughing was carried out in high temperature window (>1100° C.) to refine the austenite grains below 30 microns. But the finish rolling was applied in low temperature window to promote Tnr by strain induced nucleation of NbC at large undercoolings (<920° C.). There was no protection against grain coarsening of austenite between the end of roughing (1060° C.) and the start of finish rolling (920° C.) and hence austenite grains coarsen to 60-80 microns. In Trial-2, after the first stage of roughing, a second stage of roughing including two heavy passes (>25% reduction) were applied after holding for temperature homogenization in roughing so that Tnr is promoted before the end of roughing by strain-induced growth of NbC on pre-existing TiN, which is well dispersed. By promoting Tnr at about 1040° C., the occurrence of undesirable rotated cube texture is suppressed in Trial-2, whereas high intensity of rotated cube texture is obtained in Trial-1. By promoting strain induced growth of NbC in upstream processing, delayed strain induced precipitation of fine NbC is suppressed in Trial-2. In contrast, the matrix from Trial-1 exhibits high number density of fine NbC precipitates in the matrix. Table 14, below, summarizes the differences in structure and properties between Trial-1 (processed according to the prior art) and Trial-2 (processed according to embodiments of the present invention).
By pancaking grain refined austenite in Trial-2, acicular ferrite microstructure was obtained. Both cleavage texture and tendency for ultra-fine precipitation of NbC in the matrix were suppressed. The sample from Trial-2 exhibited excellent DWTT performance and CTOD toughness. Table 15, below, shows the CTOD test results of K60-E2 plate of Trial-1 and Trial-2. Table 16, below, shows the tensile properties of the 32 mm thick gage K60-E2 plate for Trial-1 and Trial-2. This validates the concept of nano-scale precipitate engineering to obtain excellent strength and crack arrestability in thicker gage (32 mm) plate.
By refining the austenite grain size, 100% shear area at low temperature is obtained in 16.4 mm strip in X-90, as shown in
The application of nano-scale TiN—NbC composite precipitation engineering offers a generic platform for austenite grain size control in upstream processing. A potential application to in-line strip rolling involving three roughing stands to produce X-80 grade strip of 15 mm gage is illustrated in
The foregoing examples distinguish the principal differences in processing higher niobium steels between the prior art without intentional austenite grain size control upstream and the present invention based on austenite grain size control and the consequences thereof on product in terms of gage thickness and properties. The salient points are summarized in Table 12.
According to the embodiments of the present invention, the process steps for controlling austenite grain size upstream before entry to finish rolling to produce thicker gage product are given below:
(i) Lower the nitrogen content in the base chemistry to 30-40 ppm and add titanium to the stoichiometric requirement (0.012-0.015 wt percent titanium) to combine with all nitrogen to form in austenite high number density of TiN precipitates in the size range of 10-20 nm with an interparticle spacing of 200-300 nm, before the start of roughing;
(ii) Refine austenite grain size by static recrystallization in rough rolling to a target grain size of 10-30 microns but preferably 10-20 microns by roughing passes in the temperature window above 1080° C. in which there is no significant thermodynamic potential for precipitation of NbC in a base chemistry containing carbon of about 0.045 wt % and niobium content of about 0.09 wt %. This constitutes the first phase of rough rolling aimed at refining the austenite grain size by repeated static recrystallization occurring between pass reductions.
(iii) After the first stage of roughing to refine austenite grain size, hold for about 100-120 seconds so that temperature homogenization occurs through the section thickness and the temperature at the center of the section is lowered to 1040° C.; the corresponding surface temperature will be considerably lower, depending on the section thickness. The second phase of roughing, one or more heavy passes are given to promote strain induced growth of NbC on pre-existing TiN to achieve a Zener pinning pressure of about 0.1 MPa to stop recrystallization and grain coarsening. One heavy pass reduction was used in the second stage of rough rolling in producing 20 mm coil in conventional hot strip mill. More than one heavy pass reduction was used in the second stage of roughing in producing 32 mm plate by nano-scale precipitate engineering of TiN—NbC composite.
(iv) Apply one or more heavy pass reductions (>25%) when the temperature in the center of the section is cooled to about 1040° C. so that there is significant thermodynamic potential for precipitation of NbC to occur. Large strain rolling deformation is applied in a temperature window at which significant thermodynamic potential for precipitation of NbC occurs. This is a critical process step in order to promote strain induced growth of NbC on the pre-existing TiN to target TiN—NbC composites to achieve a Zener pinning pressure of about 0.1 MPa on austenite grain boundary to stop recrystallization during the second stage of roughing. This is achieved by strain induced growth of TiN—NbC composites having a mean size of 25-30 nm with a mean interparticle spacing of 200-300 nm, which can pin austenite of 30 microns grain size in the center of the transfer bar. Since the temperatures at the quarter point and on the surface are lower than the center, there is adequate thermodynamic potential for giving adequate Zener pinning pressure to stop the recrystallization and prevent grain coarsening at these regions. It should be noted that even if high number density of TiN particles with short interspacings are obtained, strain induced growth of NbC will not be promoted unless large strain deformation is coupled to temperature of deformation at which significant thermodynamic potential for precipitation of NbC occurs and therefore Tnr by TiN—NbC will not be promoted. Thus the temperature and pass reduction in second stage of roughing should be coupled to thermodynamic potential for precipitation of NbC.
(v) Apply rapid cooling between the end of roughing and the start of finish rolling so that the temperature of the center of the transfer bar on entry to finish rolling is below 920° C., and (ii) adequate solute niobium >0.03 wt percent, but preferably 0.04 to 0.05 wt percent is retained for strain accumulation during finish rolling and transformation hardening on subsequent accelerated cooling; and
(vi) Control fine austenite grain of about 30 micron size in the transfer bar enables thicker gage (17-40 mm) to be produced with less pancaking in finish rolling. By comparison, heavy pancaking will be required for coarse austenite grain sizes of about 60 microns in conventional thermo-mechanical rolling of higher niobium grades, which results in traditional thinner gage steel products.
The foregoing examples are given to demonstrate how nano-scale precipitation engineering of TiN—NbC composite precipitates can be used for austenite grain size control in upstream processing of austenite to derive benefits in (i) producing thicker gage product (>17 mm) with excellent strength and fracture toughness at low temperature as measured by DBTT and DWTT, (ii) obtaining more uniform microstructures, and (iii) minimizing unfavorable crystallographic texture related problems. Strain induced growth of NbC on pre-existing TiN to form TiN—NbC composites at low undercoolings is a paradigm shift from conventional technology based on strain induced nucleation of NbC at large undercoolings. The increase in Tnr by TiN—NbC composites is about 100° C. more than Tnr by NbC. This enables Tnr to be obtained in rough rolling, which prevents grain coarsening between the end of roughing and the start of finish rolling. By raising Tnr, the center of thick sections can be pancaked before transformation, thereby suppressing undesirable rotated cube texture from occurring in the center of thicker gage product. By controlling the interparticle spacing of TiN—NbC composites, it is shown that delayed strain induced precipitation of NbC can be suppressed, thereby retaining adequate solute niobium for hardenability control to produce fine acicular ferrite with crystallographic variant control to suppress brittle fracture. Nano-scale precipitate engineering enables control of undesirable rotated cube texture and promotes desirable crystallographic variants that suppress brittle fracture. Thus the target structure achieved by nano-scale precipitate engineering of TiN—NbC composites is acicular ferrite of 1-3 microns transformed at low temperature by shear mechanism, which is characterized by high density and dispersion of high angle boundaries that suppress brittle fracture. The target structure exhibits minimal rotated cube texture and minimal ultra-fine precipitates that contribute to strengthening. The increase in strength by precipitation hardening will increase the ductile to brittle transition temperature (DBTT) and hence it is not a desirable mechanism to obtain toughness at low temperature. Impact transition temperature will be raised by 0.26° C. per 1 MPa increase in yield strength. Nano-scale precipitate engineering of TiN—NbC composites is designed to improve crack arrestability of running crack to enhance public safety. The design is aimed at promoting (i) morphological microstructural parameters of fine acicular ferrite grain size of 1-3 microns with good matrix plasticity to increase resistance to ductile fracture propagation, and (ii) crystallographic parameters to prevent brittle fracture by (a) suppressing undesirable rotated cube texture that promotes brittle fracture, and (b) promoting crystallographic high angle boundaries to arrest micro-cracks before they grow to initiate brittle fracture. The roles of TiN—NbC and solute niobium in microstructural engineering of target structure for enhanced crack arrestability are summarized in
These examples are for illustrative purposes only and the invention is not intended to be limited to any of the specific examples. However, it will be understood by those skilled in the art that modifications and changes may be made to the present invention to combine other elements having a high affinity for nitrogen and carbon similar to titanium and niobium without departing from their scope of controlling particle interspacing and size independently to bring about adequate pinning pressure on the austenite boundary and prevent austenite grain coarsening upstream.
This application is a continuation-in-part of U.S. patent application Ser. No. 14/325,940 filed on Jul. 8, 2014, and published as US 2016/0010190 A1 on Jan. 1, 2016, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14325940 | Jul 2014 | US |
| Child | 16028800 | US |
