PROCESSES FOR PRODUCING THICKER GAGE PRODUCTS OF NIOBIUM MICROALLOYED STEEL

BACKGROUND OF THE INVENTION

1. Field of the Invention

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 less thermo-mechanical rolling reduction during finish rolling.

2. Description of Related Art

Although austenite grain size at the end of roughing is fine (<20 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. Thus, an object 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 the present invention, TiN—NbC composite precipitates are obtained with negligible undercooling in the high temperature window at the end of roughing. These nano-scale TiN—NbC composite precipitates are available to pin austenite grains at 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, less pancaking is required to meet target DWTT properties.

SUMMARY OF THE INVENTION

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 the present invention, TiN precipitates, which are formed just after solidification in the continuous cast slab, are used to control the inter-particle 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 y is the surface energy of the boundary.

The driving force for grain coarsening tends to decrease as the grain size increases. In accordance with 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.

TABLE 1

Zener limiting Austenite grain size in micrometers

Increasing N

concentration
Increasing base Nb concentration

Inter Particle
Particle Diameter, nm

Distance, nm
10
15
20
25
30
35
40
50
60
70
80

150
85
38
21
14
10
7
5
3
2
1.8
1.3

200
203
90
51
32
22
17
13
8
6
4
3

250
397
176
99
63
44
32
25
16
11
8
6

300
687
305
171
110
76
56
43
28
19
14
11

350
1091
485
273
174
121
89
68
44
30
22
17

400
1629
724
407
260
181
133
102
65
45
33
25

450
2320
1030
580
371
257
190
145
93
65
47
36

500
3183
1414
795
509
353
259
199
127
88
65
50

550
4236
1882
1059
678
471
346
265
170
118
86
66

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. The present invention provides 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 through nano-scale precipitation engineering of TiN—NbC composites to produce thicker gage product of niobium microalloyed steel 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 % nobium; lowering the temperature of roughening to end the roughening operation in the temperature range of from about 980° C. to 1030° C.; retaining greater than about 0.03% 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. 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.

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 inter-particle 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 980° C. to about 1030° C. TiN—NbC composites can be in the size range of about 20-50 nm. The process can include applying accelerated cooling upstream between the end of the roughing operation and the start of finish rolling to avoid depletion of solute niobium from the matrix to less than about 0.03 wt percent. 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 supermartensitic 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 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 at a temperature at or below about 920° C. The rolling reduction can be reduced substantially by more than about 15%. The steel product can exhibit a gage thickness of about 17-30 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show Electron Back-Scattered Diffraction (EBSD) Images, revealing austenite grain size of specimens of two 13% Cr-5% Ni-2% Mo supermartensitic stainless steels. FIG. 1A corresponds to the control steel without niobium additions, and FIG. 1B is the sample with 0.1 wt percent niobium addition, both steels contain titanium, as shown in Table 2. The addition of 0.1 wt percent niobium decreased the austenite grain size from 80 to 35 microns. i.e., titanium by itself could only produce an austenite grain size of 80 microns. But it is only with the addition of 0.1 wt. percent niobium, that austenite grain size can be decreased from 80 to 35 microns under identical processing conditions.

FIG. 2 is a TEM image of precipitates extracted on a carbon replica, showing nano-scale Ti—Nb bearing precipitates in the size range of 25-30 nm with a mean interparticle spacing of 230 nm. Energy dispersive analysis of the precipitates is shown alongside. The spectrum shows X-ray signals characteristic of titanium and niobium. The precipitates appear to be TiN—NbC composites similar to those found in line pipe steels, see FIGS. 4, 12 and 13.

FIG. 3 is a plot of driving force for grain coarsening of austenite as a function of austenite grain size. The coarsening is driven by reduction in surface energy of the grains. This is counteracted by particle pinning the boundary. The pinning pressure is governed by the particle size and number density. The number density determines interparticle spacing. Thus, small interparticle spacing and increased particle size are required to increase pinning pressure to counteract and prevent grain coarsening of fine grains. The particle limited grain size with and without niobium shows the effectiveness of TiN—NbC composites compared with TiN precipitates in pinning fine grains.

FIG. 4 is a high resolution TEM image of TiN—NbC composites obtained in low nitrogen line pipe steel microalloyed with titanium and niobium. Energy dispersive analysis shows X-ray signals characteristic of niobium and titanium in the composite precipitates. NbC precipitates appear to have grown on preexisting cuboidal TiN precipitates.

FIGS. 5A and 5B are salient results from my previous work on the microstructural evolution of TiN—NbC composites in low interstitial titanium-niobium microalloyed steels investigated by hot torsion simulation of rolling. Based on quantitative analysis of thermodynamic potential for precipitation, mole fraction of TiN—NbC is plotted as a function of temperature. FIG. 5A shows the precipitate evolution curve for the high niobium low interstitial steel-G, containing carbon 0.03, nitrogen 0.003, titanium 0.014 and niobium 0.095 wt percent. Thermodynamic potential for precipitation of NbC starts at 1060° C. FIG. 5B shows the mean flow stress from hot torsion simulation (shown as open circles) as a function of the inverse of the absolute pass temperature for Steel-G. The bold line is the flow stress pertaining to a fully recrystallised steel. The onset of recrystallization retardation starts at 1060° C. corresponding to the onset of the thermodynamic potential for precipitation of NbC. Growth of NbC on preexisting precipitates of TiN is confirmed in this work, which obviates the need for independent nucleation of NbC. Thus, the resulting TiN—NbC composite retards recrystallization, causing the increase in flow stress detected by hot torsion rolling simulation results.

FIG. 6 is a schematic diagram that inter-relates the increase in size of TiN—NbC composite to volume fraction of NbC, which is determined by the thermodynamic potential for precipitation of NbC. The interparticle spacing is fixed by TiN on which NbC grows. This diagram illustrates that the rough rolling temperature window has to be lowered so that thermodynamic potential for growth of NbC is obtained on pre-existing TiN precipitates to form TiN—NbC composites at the end of rough rolling.

FIG. 7 is a process flow diagram of prior technology in which there is no intentional control of austenite grain size in upstream processing of rough rolling, and the austenite grain size on entry to finish rolling may be coarse, generally ranging in size from 60-80 microns. Therefore, heavy rolling reduction is applied in finish rolling stands downstream to reduce the thickness of pancaked austenite in order to obtain good toughness at low temperature in the final product. This limits the thickness of the final product generally well below 16 mm, processed by conventional plate rolling or conventional hot strip rolling of niobium microalloyed steel. This is illustrated with the specific example of Steel-A of 10 mm gage, with a high nitrogen content of 75 ppm. Rough rolling is carried out in the temperature window above the equilibrium temperature for precipitation of NbC. TEM characterization shows coarse precipitate of mean size 83 nm with a large interparticle spacing of 550 nm, which gives a Zener limiting austenite grain size of 62 microns. This requires heavy rolling reduction for pancaking austenite grains, resulting in thinner gage (<16 mm).

FIG. 8 is a process flow diagram based on the present invention wherein austenite grain size upstream is controlled by the size and spacing of TiN—NbC composite precipitates, which is referred to herein as “nano-scale precipitation engineering.” The austenite grain size is intentionally controlled to be fine with a target grain size under 30 microns. This requires less rolling reduction to reduce the thickness of pancaked austenite grain size in order to obtain good toughness at low temperature as measured by percentage shear area in DWTT tests. As a result of applying less rolling reduction to the transfer bar, the thickness of the final product processed by conventional plate rolling or conventional hot strip rolling of niobium microalloyed steel can be increased well above 16 mm. This is demonstrated with the specific example of Steel-C. The steel contains a low nitrogen content of 0.004 wt percent and stoichiometric addition of Ti to combine with nitrogen. TEM characterization shows high number density of TiN precipitates with an interspacing of 220 nm. The end of rough rolling temperature is lowered to a temperature in the range from 980 to 1030° C., preferably 1000° C. to promote growth of NbC on pre-existing TiN to give TiN—NbC composites of 32 nm size. Electron energy loss spectroscopy has confirmed growth of NbC on pre-existing TiN. The limiting austenite grain size by TiN—NbC composite precipitates is less than 30 microns, which requires less pancaking in finish rolling, resulting in thicker gage (>16 mm).

FIG. 9 is a montage that relates interparticle spacing of nano-scale TiN—NbC composites to titanium and nitrogen content in the base chemical composition, which is mapped on the equilibrium solubility product for TiN precipitation as a function of temperature. The montage represents a comprehensive data base on inter-particle spacing of TiN obtained in line pipe steel in which nitrogen content is varied. The interparticle spacing of TiN is in the 200-250 nm range when nitrogen content is lowered to 40 ppm, titanium is added in the stoichiometric requirement to combine with all the nitrogen. High number density of TiN—NbC composite precipitates occurs in the size range of 25-35 nm. By contrast, when nitrogen content is raised to 75 ppm, the interparticle size is large at about 550 nm, and the particle size is coarse (80 nm size).

FIG. 10 is an optical micrograph showing austenite grain size in the transfer bar of Steel-D quenched after shearing. The austenite grain size is about 48-55 microns. This is in agreement with Zener limiting austenite grain size, based on measured values of precipitate size and interparticle spacing of Steel-D, shown in Figure-9.

FIG. 11 is Kozazu's diagram, inter-relating rolling reduction and austenite grain size with surface to volume ratio, Sv factor, of pancaked austenite grain size, to which the final structure and properties can be related. Kozazu's diagram shows that a large rolling reduction (70 percent) is required to pancake coarser austenite grain of 70 micron compared with lower rolling reduction (<50 percent) required to pancake finer austenite grain of 30 micron grain size to achieve the same surface to volume ratio, i.e., Sv factor.

FIG. 12 is a photograph compiled from Electron Energy Loss Spectroscopy (EELS) data of nano-scale TiN—NbC precipitates observed in nano-scale precipitation engineered high grade line pipe steel processed by conventional hot strip rolling. The epitaxial growth of NbC on faces of the TiN cubic precipitates can be clearly seen in Steel-C(X-90 grade).

FIG. 13 shows elemental mapping from EELS data of the TiN—NbC composite precipitates, shown in FIG. 12. These results show unambiguously epitaxial growth of NbC on pre-existing TiN.

FIG. 14 illustrates the application of nano-scale precipitation engineering of TiN—NbC composite precipitates for austenite grain size control in near net shape processing for a typical lay out of mill design with three roughing stands.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B are electron backscatter diffraction images of 13% Cr-5% Ni-2% Mo super-martensitic stainless steels without and with 0.1 wt percent niobium addition. These steel specimens were identically processed, solution treated at 1050° C. and air-cooled. The detailed chemical compositions of the two steels are given in Table 2.

TABLE 2

Chemical composition of two 13% Cr—5% Ni—2% Mo

steels without and with niobium in wt percent

Steel grade
C
Si
Mn
P
S
Cr
Ni
Mo
N
Nb
Ti

13Cr5Ni2Mo
0.020
0.42
0.51
0.016
0.004
12.59
5.01
1.90
0.013
—
0.0062

13Cr5Ni2MoNb
0.022
0.41
0.48
0.016
0.006
12.91
5.16
2.05
0.010
0.11
0.0043

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 FIG. 1 delineate the austenite grain boundaries.

FIG. 2 shows TEM images of composite precipitates of TiN—NbC observed in the steel with a niobium addition. The mean inter-particle spacing of composite precipitates is 231 nm and the mean particle size is increased from 15 nm for TiN to 30 nm for the TiN—NbC composite particles. The pinning pressure exerted by the particles of TiN—NbC is 0.08 MPa, which counteracts the driving force for grain coarsening of austenite of grain size 35 microns as shown in Table 3.

TABLE 3

Calculated limiting austenite grain size by TiN and TiN—NbC in

13Cr5Ni2Mo steel with 0.1 wt. percent niobium

Zener
Limiting

pinning
austenite

Size
Inter-particle
Volume
pressures
grain size

Precipitate
(nm)
Spacing (nm)
fraction
(MPa)
(μm)

TiN
15
231
0.00015
0.02088
134

TiN—NbC
30
231
0.00123
0.08028
35

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.”

FIG. 3 shows that TiN particles by themselves are not effective in pinning austenite grains. The epitaxial growth of NbC on pre-existing TiN effectively increases the particle size from 15 to 30 nm, with the corresponding increase in Zener pinning pressure nearly threefold (2.5 times) compared with TiN alone and thus decreases the limiting austenite grain size. This composite precipitate involves growth of NbC on pre-existing TiN, which is to be distinguished from MX type precipitates reported in the literature and previous U.S. Pat. No. 6,899,773.

FIG. 4 and FIG. 5 provide a summary of the prior work on titanium-niobium microalloying, which shows that if nitrogen is controlled under 40 ppm and titanium additions are made to the stoichiometric requirement of N to form TiN, a high number density of TiN can be promoted. On reaching the temperature where thermodynamic potential for precipitation of NbC occurs, NbC will start to grow on pre-existing TiN. This work was reported by S. V. Subramanian, F. Boratto, J. J. Jonas and C. M. Sellars and published in the Proceedings of International Symposium on “Microalloyed Bar and Forging Steels” edited by Mike Finn, CIM, held at Hamilton, Ontario, Canada, Aug. 26-29, 1990, pp. 120-136.

FIG. 4 shows a TEM image of TiN—NbC composite precipitates. The energy dispersive analysis and EELS (Electron energy loss spectrum) have confirmed that NbC precipitates grow epitaxially on the faces of cuboidal precipitates of TiN (with the NaCl crystal structure.)

FIG. 5 shows that growth of NbC on pre-existing TiN can be detected by an increase in flow stress during hot torsion simulation of rolling. This temperature is found to coincide with the equilibrium temperature of precipitation of NbC, as shown in Table 4. The implication is that the volume fraction of NbC growing on pre-existing TiN can be determined by the thermodynamic potential for precipitation of NbC.

TABLE 4

Effect of chemical composition on precipitation kinetics of Ti—Nb microalloyed

steel during hot torsion simulation of rolling; Growth of NbC on pre-existing

TiN occurs close to the equilibrium temperature for precipitation of NbC,

which can be detected by flow stress increase during hot torsion.

T_NbC
Sellars Model
T_nr

Base Chemistry
Equilibrium
RLT
RST
Hot Torsion

Steel
C
N
Ti
Nb
° C.
° C.
° C.
° C.

Fe—Nb—C
0.15
0.005
0
0.031
1130
955
922
960

Line pipe A
0.029
0.0023
0.013
0.055
1010
898
880
1010

Line pipe B
0.026
0.0027
0.020
0.057
1020
900
885
1016

Line pipe C
0.016
0.0018
0.013
0.049
960
850
836
994

Line pipe G
0.027
0.0035
0.014
0.099
1060
936
923
1060

Line pipe K
0.027
0.0019
0.014
0.093
1050
936
923
1040

RLT = Recrystallization Limit Temperature

RST = Recrystallization Stop Temperature

T_nr= Temperature of No Recrystallization

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 precipitation engineering. The austenite grain size is controlled by inter-particle 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 inter-particle 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 inter-particle spacing, the pinning pressure is increased to arrest the finer austenite grains from coarsening. By increasing the size of precipitates to 30 nm with an interparticle spacing of 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 FIG. 6. The upstream processing of austenite for austenite grain size control requires a high number density of precipitates with short interparticle spacing and adequate precipitate size with good dispersion to apply adequate pinning pressure to prevent coarsening of fine grains of austenite obtained at the end of roughing. This innovation relates to product-process integration, where refinement of austenite grain size in upstream processing by Zener pinning by TiN—NbC composite precipitates of grain refined austenite to prevent grain coarsening is used to reduce total rolling reduction in finish rolling downstream to produce thicker gage product.

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 3X. 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.

FIG. 7 is the flow diagram of product-process integration of the prior art technology without any intentional control of austenite grain size upstream and its consequence on heavy packing downstream resulting in thin gage product. A high nitrogen content in the base composition results in coarse precipitates of TiN with a large interparticle spacing of 550 nm. TEM characterization shows the coarse precipitate of TiN. Rough rolling is carried out in a temperature window above the equilibrium temperature for precipitation of NbC. Thus, Zener limiting austenite grain size is 62 microns, as shown in the Table in FIG. 7. Therefore, heavy pancaking is required to obtain consistently good DWTT performance.

By comparison, FIG. 8 is the flow diagram of product-process integration of the present invention based on austenite grain size control by engineering size and spacing of TiN—NbC composite precipitates and its consequence on reduced rolling reduction downstream, resulting in production of thicker gage product. A low nitrogen content of 40 ppm with stoichiometric addition of Ti to combine with all nitrogen promotes in high number density of TiN precipitates with an interparticle spacing of 220 nm. The temperature window of roughing is lowered below the equilibrium temperature for precipitation of NbC to promote growth of NbC on pre-existing TiN, which is confirmed by EELS characterization of TiN—NbC composite precipitates. The limiting austenite grain size is below 30 microns. Therefore, less rolling reduction is applied to produce thicker gage product.

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. FIG. 9 shows a comprehensive database of four bench marked steels in which nitrogen content is varied under different mill processing conditions. The chemical compositions of the four steels are given in Table 5.

TABLE 5

Effect of varying nitrogen content on thermodynamic potential for precipitation of TiN, and

its consequence on ppt size and Zener pinning pressure, Zener limiting austenite grain size

TiN Inter-
TiN—NbC
Volume fraction

N
Ti
C
Nb
particle
Ppt size
of TiN—NbC
Zener pinning
Limiting austenite

I.D.
(wt %)
(wt %)
(wt %)
(wt %)
spacing in (nm)
(nm)
at 1000° C.
Pressure (MPa)
grain size (μm)

A
0.0075
0.015
0.06
0.09
553
83
0.00176
0.044
62

B
0.0035
0.014
0.07
0.08
218
32
0.0016
0.108
26

C
0.0040
0.015
0.05
0.09
221
32
0.00159
0.104
27

D
0.0055
0.012
0.048
0.067
397
52
0.00117
0.047
59

Steel-A with the highest nitrogen content of 0.0075 wt. percent exhibits a large mean inter-particle spacing of about 550 nm compared with Steels-B and C with a low N content of 35-40 ppm, which exhibit a mean inter-particle 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 inter-particle 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. FIG. 10 shows the austenite grain size measured in the center of a thick transfer bar of 53 mm of Steel-D, quenched after rough rolling with an intermediate nitrogen content of 55 ppm and an inter-particle spacing of 397 nm. The predicted Zener limiting austenite grain size is 59 microns, which compares well with the measured value of 55 microns, which validates the approach. Thus, nano-scale precipitation engineering offers a sound metallurgical basis for controlling austenite grain size during upstream processing of austenite.

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 inter-particle 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

Effect of austenite grain size (GS) and percent reduction below

temperature of no recrystallization (T_NR) on Sv factor

and ferrite grain size.

Sv Factor
Austenite
% Reduction

Ferrite GS (um)
mm²/mm³
GS (um)
below T_NR

9
80
40
60

9
80
30
30

9.4
70
55
60

9.4
70
35
30

11
60
70
60

11
60
40
30

Table 6 is extracted from Kozazu's diagram in FIG. 11, which illustrates the benefit of austenite grain refinement before pancaking in reducing the rolling reduction below the temperature of no recrystallization to achieve the same surface to volume ratio. Thus, by reducing the austenite grain size from 40 to 30 microns, the rolling reduction can be decreased from 60 to 30 percent to attain the same Sv factor of 80 mm²/mm³in order to obtain ferrite grain size of 9 micrometers and consequently the gage (thickness of final product) can be significantly increased. It is well established that by refining the austenite grain size upstream, excellent strength and fracture properties can be obtained in thicker gage product. Wenjin Nie et al. have demonstrated the importance of austenite grain size control on final DWTT properties of heavy thick X-80 pipe line steels (Advanced Materials Research, Vols. 194-196, (2011), pp. 1183-1191).

EXAMPLES

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.

Example 1
Plate Rolling
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 inter-particle 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:

TABLE 7

End of
Limiting austenite

N
Ti
C
Nb
roughing
grain size by TiN

0.0075
0.015
0.06
0.088
1100° C.
90 microns

DWTT % SA at −7° C.: 100%;

CVN toughness at −7° C.: 140 Joules;

Yield Strength/Rp0.5: 610 MPa;

Ultimate Tensile Strength/Rm: 714 MPa

Example 2
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, where thermodynamic potential occurs for growth of NbC on pre-existing particles at the end of rough rolling. 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 FIG. 11 to obtain adequate surface to volume ratio to obtain fine grains in the final product. In this case, 100 percent shear area on the fracture surface in DWTT is obtained. This example shows that TiN by itself cannot grain refine austenite even though the interparticle spacing may be fine unless the particle size is increased by growth of NbC on pre-existing TiN particle. This example demonstrates the importance of lowering the temperature window of roughing to promote growth of NbC on pre-existing TiN to limit austenite grain coarsening at the end of roughing in order to produce 22 mm gage with excellent DWTT performance.

Steel-E

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.

TABLE 8

#2 Condition

#1 Condition (Roughing
(Roughing to

without NbC growth on
promote NbC

Heat
TiN)
growth on TiN)

A_k−20° C./Joules
328
372

DWTT (−15° C.) average
55
98

SA %

Example 3
Conventional Hot Strip Rolling
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 inter-particle 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.

TABLE 9

Limiting

Pancaking

End of
austenite
Pancaking
austenite

N
Ti
C
Nb
roughing
grain size
reduction
grain

0.0055
0.012
0.048
0.067
980° C.
59 microns
62.6
20 microns

TABLE 10a

Ultimate tensile

Yield strength/Rp0.2
Strength/Rm
Yield ratio
Total elongation/%

588 MPa
670 MPa
0.88
48

TABLE 10b

Test

DWTT Shear Area %

Temperature/
Charpy V-notched
T
L
45°

° C.
toughness/Joules
direction
direction
direction

0
—
95
95
95

−10
—
—
100
90

−20
485
70
95
100

Example 4
Conventional Hot Strip Rolling

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 inter-particle spacing of 220 nm. The end of roughing is in the temperature window where thermodynamic potential for precipitation of NbC occurs.

TEM-EELS characterization of TiN—NbC precipitates shown in FIGS. 12 and 13 confirms epitaxial growth of NbC on pre-existing TiN particles. This steel exhibits remarkable toughness at very low temperature (−40° C.), see Tables 11a and 11b. The steel exhibits uniformity of microstructure which is less prone to anisotropic properties due to unfavorable texture development.

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.

TABLE 11a

Yield strength/
Ultimate tensile

Total
Uniform

Rp0.2
Strength/Rm
Yield ratio
elongation/%
elongation/%

670 MPa
800 MPa
0.84
17
5.6

TABLE 11b

DWTT

Testing Temperature/
Charpy V-notched
(T direction)

° C.
toughness/Joules
Shear area %

10
313

0
302
100

−10
300
100

−20
315
100

−40
318
100

−60
329

Example 5
Compact Strip Processing and Thin Slab Processing

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, then 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.

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 FIG. 14 along with critical processing parameters.

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.

TABLE 12

ID
Prior Art
Present Invention

1
No specific nitrogen target
N control (nitrogen 0.003-0.004, titanium 0.012-0.015)

2
Coarse and non-uniformly dispersed TiN
Fine uniformly dispersed and high number

density of TiN

3
Roughing in temperature range where there is
Roughing in temperature range where there is

no thermodynamic potential for precipitation of
thermodynamic potential for precipitation of NbC

NbC

4
NbC growth on coarse TiN precipitates before
High number density of TiN and adequate

entry to finish rolling; Inadequate Zener drag to
volume fraction of TiN—NbC composite

prevent grain coarsening of fine austenite
precipitates nano scale engineered to give

grains
adequate Zener drag to pin fine austenite grains

from coarsening

5
Fast cooling between roughing and finish
Fast cooling (laminar cooling at 4° C./s) between

rolling is beneficial for retaining niobium in
roughing and finish rolling is essential to retain

solution
adequate niobium in solution

6
Coarsened austenite grains 50-70 μm at entry to
Zener limiting austenite grain size (30 μm) at

finish rolling
entry to finish rolling

7
Heavy pancaking is required (total reduction
Less pancaking (total reduction 50-66%) is

66-80%) to achieve target Sv factor for coarse
adequate to achieve target Sv factor for austenite

austenite grain size in 50-70 μm
grain size under 30 μm

8
Production limited to thinner gage
Production of thicker gage high grade product

product (10-17 mm); potential for
(17-30 mm)

unfavorable texture development
less texture related anisotropy

Process Steps for Controlling Austenite Grain Size in Upstream Processing of Austenite:

According to 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 at the end of roughing;

(iii) Adjust carbon content in the range of about 0.03 to 0.07 wt percent but preferably 0.04-0.05 wt percent and niobium in the range of about 0.07 to 0.15 but preferably 0.09 to 0.1 wt percent so that thermodynamic potential for growth of NbC on pre-existing TiN to form TiN—NbC composites occurs towards the end of roughing, i.e., between 980°-1030° C.;

(iv) Target TiN—NbC composites to grow to 25-50 nm but preferably 25-30 nm so that pinning pressure from TiN—NbC composites of 25-50 nm with an interparticle spacing of 200-300 nm can pin austenite of 30 microns grain size in the transfer bars;

(v) Apply rapid cooling between the end of roughing and the start of finish rolling so that (i) the temperature of the transfer bar on entry to finish rolling is below 920° C., the temperature of no recrystallization 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 to enable thicker strip (17-30 mm) to be produced with less pancaking in finish rolling compared with heavy pancaking in coarse austenite grain size of about 60 microns in conventional thermo-mechanical rolling of higher niobium grades that results in thinner gage.

Advantages of the Present Invention Based on Nano-Scale TiN—NbC Composite Precipitate Engineering for Austenite Grain Size Control:

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. 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.

PROCESSES FOR PRODUCING THICKER GAGE PRODUCTS OF NIOBIUM MICROALLOYED STEEL

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