Low Finish Rolling Temperature Research Articles

Evolution of microstructure and toughness of very tough ferritic stainless steels

In present work, the hot rolling with different finish rolling temperatures and annealing with different annealing temperature are employed with high toughness AISI444 ferritic stainless steel, and different microstructure are generated. It was indicated that low finish rolling temperature could lead to the significant refinement of the rolled and annealed microstructures and decrease the fraction of grains with {001} planes which are parallel to notch plane in the annealed microstructure. Therefore, the ductile-brittle transition temperature was lowered from -44.2°C to -48.8°C, resulting in the improvement of toughness after employing the low finish rolling temperature for ferritic stainless steel. When high annealing temperature can be applied, the coarsening of annealed microstructure occurred, but the fraction of grains with {001} planes which are parallel to notch plane in the recrystallized microstructure decreased. Thus, the ductile-brittle transition temperature has no significant change after annealing with different temperatures.

Influences of Thermomechanical Treatment and Nb Micro-alloying on the Hardenability of Ultra-High Strength Steels

For the construction of mobile crane booms, ultra-high strength steels produced via thermomechanical processing (TMP) have widely substituted steels fabricated through the conventional quenching and tempering (Q+T) route. A strong deformation of the austenite grain during hot rolling followed by direct quenching (DQ) offers benefits in terms of strength and toughness. To guarantee an optimal through-hardening, alloying elements retarding the γ to α transformation are used. To explore the influence of the processing route on the critical cooling rate and the hardenability, hot deformation tests were performed on a deformation dilatometer. Different cooling rates were applied after deformation corresponding to two different rolling cycles with varying finish rolling temperatures (FRTs). The obtained hardness values were compared to those received through conventional quenching after austenitization. These investigations conducted on three steels with varying micro-alloying contents showed that Nb in combination with TMP raises strength significantly, and promotes a bainitic and ferritic transformation in solid solution. When applying low FRTs and in combination with other micro-alloying elements, NbC coarsens and reduces the effect of precipitation hardening.

Correlation Between Structure and Properties of Low-Carbon Cu-Ni-Mo-Ti-Nb Ultrahigh-Strength Steel

Low-carbon Cu-Ni-Mo-Ti-Nb steel having more than 1700 MPa tensile strength and good low-temperature impact toughness was successfully designed and processed for automobile, defence and structural applications. The steel was thermomechanical controlled processed at a different finish rolling temperatures (750-850 °C) and cooled at two different conditions: air cooling and water quenching. Evolutions of microstructure and precipitates were thoroughly characterised to understand the effect of different factors, such as steel composition, finish rolling temperature and cooling rate on the mechanical properties. The results indicate that the steel processed at 750 °C finish rolling temperature (FRT) followed by water quenching has exhibited superior tensile properties with a yield strength of 1034-1449 MPa, the tensile strength of 1598-1726 MPa and total elongation of 10-13%. Such an ultrahigh-strength level is primarily attributed to the combined effect of lower bainite dominated microstructure along with precipitation hardening. The satisfactory low-temperature toughness is promoted by low finish rolling temperature and higher amount of misorientation offered by the boundaries within the lower bainite.

Study of Microstructure and Mechanical Properties of Hot-Rolled Ultra-High Strength Ferrite-Bainite Dual Phase Steel

Effects of thermo-mechanical control processing (TMCP) on microstructure and mechanical properties of hot-rolled ultra-high strength ferrite-bainite dual phase steel were investigated on a laboratory hot rolling mill. The results have shown that the microstructure containing ferrite and bainite can be obtained by TMCP. Ultimate tensile strength of all the specimens exceeded 1000MPa. Finish rolling temperatures affect the mechanical properties of ultra-high strength ferrite-bainite dual phase steel. Ultimate tensile strength reached 1078MPa at relatively low finish rolling temperature because of the ferrite grains refined. Fast cooling after low temperature rolling results in the ferrite grains refined and the formation of martensite islands. As a result, the product of ultimate tensile strength and total elongation (Rm×A50) of specimen 4 with fast cooling after low temperature rolling reaches the maximum value (18096MPa%).

The effect of finish rolling temperature and tempering on the microstructure, mechanical properties and dislocation density of direct-quenched steel

A unique batch tempering treatment for industrial scale direct-quenched steel coils has been studied using laboratory simulations. The tempering treatment was non-isothermal with slow heating to 570 °C and slow cooling to simulate the tempering of large steel coils. The paper presents the effect of finishing rolling temperature (FRT) relative to the non-recrystallization temperature (TNR) and the effect of long time tempering on the microstructure, dislocation density and mechanical properties of direct-quenched coiled strips. Conditioning austenite below the recrystallization stop temperature resulted in a finer effective grain size distribution, which correlated strongly with the impact toughness of the final product. Furthermore low finish rolling temperature resulted in partially ferritic microstructures while higher finishing rolling temperatures led to mixtures of bainite and martensite. Dislocation densities determined with TEM and XRD showed somewhat different trends regarding the effect of tempering: intra-lath dislocation density, as measured with TEM, showed a statistically significant drop in only one case, while XRD analysis indicated a drop in all cases. Furthermore, no significant correlation between finishing rolling temperature and dislocation density existed in XRD studies. The XRD results indicate that the decrease in dislocation density corresponds to about 100 MPa lower dislocation strengthening. However, precipitation hardening and potential internal micro stress relief compensates this as yield strength remains unchanged or even increases during tempering.

Effect of Rolling Temperature and Ultrafast Cooling Rate on Microstructure and Mechanical Properties of Steel Plate

Microstructure can vary significantly through thickness after ultrafast cooling of rolled steel plates, impacting their mechanical properties. This study examined the microstructure, microstructural banding at centerline, and mechanical properties through thickness for different ultrafast cooling conditions and rolling temperatures. One set of steels (UC1 and UC2) were ultrafast-cooled (UFC) at 40 K/s after finish rolling at 1223 K and 1193 K (950 °C and 910 °C), respectively, while the second set (LC) was cooled by laminar cooling at 17 K/s after finish rolling at 1238 K (965 °C). UFC produced microstructural variation through thickness; highly dislocated lath-type bainitic ferrite was formed near the surface, whereas the primary microstructure was acicular ferrite and irregular polygonal ferrite in the interior of UC1 and UC2 steels, respectively. However, UFC has the advantage of suppression of microstructural banding in centerline segregation regions. The ferrite grain size in both UFC-cooled steels was refined to ~5 μm, increasing strength and toughness. The optimum combination of properties was obtained in UC2 steel with appropriate low finish rolling temperature, being attributed to the distinct microstructure resulting from work-hardened austenite before UFC.

Analysis of the strengthening mechanisms in pipeline steels as a function of the hot rolling parameters

The yield strength of different pipeline steel grades, rolled under four different conditions, was correlated with calculated strengthening contributions. Slabs with the same composition were rolled under identical roughing conditions but varied finish rolling temperature (FRT). Two cooling routes, consisting of accelerated water cooling condition (ACC) followed by slow cooling in an oven to simulate coiling and air cooling were applied after the last rolling pass. The microstructures obtained after each thermo mechanical controlled process (TMCP) schedule, were characterized using Transmission Electron Microscopy (TEM), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Electron backscatter diffraction (EBSD). The mechanical properties of the plates were determined by means of tensile tests and Charpy V-notch impact test. It was confirmed that a combination of fast cooling rate and low finish rolling temperature produces higher strength than the slow cooling rate and high finish rolling temperature. Contributions to the strengthening arising from the various microstructural features like solid solution strengthening, grain size, dislocation density and precipitation hardening, were analyzed using Taylor, Hall–Petch and Ashby–Orowan approaches. The root of the sum of the squares method was applied to link the experimental with the model-predicted strength. It is believed that this approach provides a better understanding of the effect of TMCP parameters on the microstructure and strengthening mechanisms in pipeline steels.

Creep and Fatigue Behavior in Micro-alloyed Steels – A Review

AbstractThis is a study of microalloyed steels for power plants and reactors. Components operate at coal dust fire temperature or thermal states of reactors, prone to creep during its service. This is to assess remaining life after passage of valuable life by variation in microstructure, e.g. cavity formation. Precipitation at the sub-grain boundaries and grain interior has increased high temperature strength. Coarsening of these appears at the end of life. Variation of heat treatment like spheroidising in place of solutionizing has been responsive to deteriorate performance. Dislocation interplay with precipitate has been acceptable while interaction among dislocations to forest dislocation has been unacceptable. Dislocation assisted nucleation of precipitates of fine size has been found to strengthen steel by thermo-mechanical control process with in greater heating temperature and lower finish rolling temperature. High temperature performance of materials has been assessed by creep, accelerated creep, creep-fatigue and fatigue performances. Increasing temperature for increasing efficiency has correlated the phase transformation of steel. Fatigue performances have been included in creep properties of materials when intermittent shut down–shut up schedules are operated, e.g. peaking power plants.

Effects of finish rolling temperature on inverse fracture occurring during drop weight tear test of API X80 pipeline steels

In this study, drop-weight tear tests (DWTT) were conducted on API X80 pipeline steels fabricated with different finish rolling temperatures in order to analyze abnormal fracture appearance, i.e., inverse fracture, occurring in the region impacted by a hammer. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, DWTT absorbed energy, and strain hardening of the hammer-impacted region. As the finish rolling temperature decreased, the volume fraction of fine-grained acicular ferrite increased, while that of large-grained upper bainite or granular bainite decreased. According to the DWTT results, the absorbed energy tended to increase with increasing volume fraction of acicular ferrite (with decreasing finish rolling temperature). A large area of inverse fracture of cleavage type was found in the hammer-impacted region of the steels fabricated with high finish rolling temperatures, but the area fraction of inverse fracture was reduced in the steels fabricated with low finish rolling temperatures. Since the area fraction of inverse fracture was closely related with strain hardening of the hammer-impacted region, it could be successfully reduced by lowering strain hardening and by promoting the formation of acicular ferrite via low finish rolling temperatures.

Processing of Microalloyed Steels by CSP Process

Microalloyed steels containing Nb and V have been processed by CSP route. The process involved continuous casting of slabs into 50 mm thickness followed by their homogenization at 1100°C during continuous movement in a tunnel furnace. Subsequently, the slabs are hot rolled in a six stand rolling mill. The effects of deformation in the initial passes and finish rolling temperature on the microstructure and mechanical properties of the hot-rolled strips have been examined. The results indicate equiaxed grain morphology of the ferrite phase coexisting with globular carbides. The grain size of the ferrite phase was observed to vary from 4 to 8 µm both in the longitudinal and transverse directions of strips. The hot-rolled strips showed an average yield strength (YS) value of 495 MPa, UTS value of 596 MPa, YS/UTS ratio of 0.83, and percentage elongation of 38% with no surface cracks. However, relatively lower strength coupled with surface cracks were observed on the steel strips processed with low degree of deformation in the initial passes followed by low finish rolling temperature and accelerated cooling. The reasons for microstructural refinement and origin of surface cracks on the hot-rolled strips in CSP process are discussed.

Effect of controlled rolling and cooling on the microstructure and mechanical properties of 60Si2MnA spring steel rod

The effect of controlled rolling and cooling process parameters, such as finish rolling temperature, loop-laying temperature and the cooling rate in the process from the loop layer to the coil station, on the microstructures and mechanical properties of 60Si2MnA spring steel rod were carefully investigated by thermal simulation, tensile tests and quantitative metallography. Experimental results indicate that the microstructure is mainly determined by finish rolling temperature, especially the cooling rate in the process from the loop layer to the coil station. In a given range, the more rapid the cooling rate, the thinner the interlamellar pearlite spacing, the less the proeutectoid ferrite bulk volume and the smaller the average ferritic grain size. Low finish rolling temperature is in favor of decreasing pearlite colony size. Therefore, low finish rolling temperature followed by properly accelerated cooling is suggested so as to improve its cold workability.

Effect of thermomechanical processing and aging on microstructure and precipitation hardening in low carbon Cu–B steel

The effect of thermomechanical controlled processing (TMCP) on structural refinement and enhancement of properties of a low carbon Cu–B high strength low alloy steel has been investigated. Thermomechanical processing was carried out according to various schedules. A few hot rolled bars were solution treated, quenched and aged at 600°C to study the effect of precipitation of copper. From the study of microstructures by optical metallography and TEM, it has been found that a significant improvement of properties can be attained through a proper selection of TMCP schedule. Precipitation of copper and alloy carbides and dislocation pinning by these precipitates appear to be the principal strengthening mechanisms. Using a lower finish rolling temperature in the TMCP schedule, the precipitation of copper is increased and hence the aging response of the steel becomes poorer. Furthermore, the addition of boron to copper bearing high strength low alloy steels delays the precipitation of copper during post TMCP aging. A higher aging temperature of 600°C of the thermomechanically processed alloys improves the subambient impact strength.

Experimental and Computational Modelling of Temperature Gradients during Hot Rolling

During industrial production of hot strip, problems with cooled edges can occur. At low finish rolling temperatures, the microstructure of the material can be fully austenitic and the edges can display a partially transformed microstructure due to a temperature gradient over the strip width. So the edges could be rolled in the two-phase region of ferrite and austenite, whereas the centre is rolled in the austenitic region. For a proper prediction of the resulting microstructure and mechanical properties distribution, it is necessary to include the temperature gradient in the simulation of the transformation under a given deformation schedule. In this project the microstructure evolution in the final pass during hot rolling with cooled edges is described in a pilot-scale experiment. The transformation behaviour of the material is calculated by means of a semi-empirical transformation model with respect to the temperature gradient over the strip width.

The effects of small titanium additions on the mechanical properties and the microstructures of controlled rolled niobium-bearing HSLA plate steels

Abstract The effects of small titanium additions (0.010% and 0.022%) on the mechanical properties and the microstructures of niobium-bearing HSLA plate steels under two different rolling schedules have been investigated. For comparison, a Ti-free steel was controlled rolled to 814°C. A side effect of the small titanium additions was observed on the strength of the niobium-bearing steels. The lower yield stress values and hardness levels for all TiNb steels were reduced both in as-rolled and normalized conditions, depending on Ti/N ratio and the details of the thermomechanical process. However, the TiNb steels with a lower finish rolling temperature (FRT), 800°C, showed better toughness properties than the Ti-free control steel. For TiNb steels, the toughness was improved significantly by lowering the FRT from 940 to 800°C, owing mainly to a considerable refinement of the ferrite grain sizes and fewer fine carbides available for dispersion hardening. No advantages have been observed for an over-stoichiometric titanium addition (Ti/N = 4.4). The steel with an under-stoichiometric titanium addition (Ti/N = 2) and a lower FRT (800°C) showed the best overall mechanical and toughness properties among all the processed TiNb steels. The precipitation of niobium nitrides and carbides in the niobium steels was changed by titanium additions which led to the formation of complex TiNb nitrides and carbonitrides, i.e. Ti-rich plates and cuboids. Moreover, these plates and cuboids acted as nucleating cores on which pure carbides formed, i.e. niobium carbide in the 0.01% Ti steels whereas Nb-rich TiNb carbide in the 0.022% Ti steels. In the as-rolled samples, no aluminium was found in the complex particles or as individual AlN while in the normalized condition, fine AlN precipitates were detected frequently with the understoichiometric titanium addition (0.01% Ti), whereas the formation of AlN was suppressed by the over-stoichiometric titanium addition (in 0.022% Ti). The Ti/N ratio, therefore, has a strong influence on the type of precipitation and the size distribution of TiNb particles. With the higher Ti/N ratio, coarse Ti-rich plates and cuboids were observed at a much higher frequency. As a result, a greater amount of niobium was used in forming the TiNb compounds for a given Ti/N ratio, thus, the volume fraction of fine carbides precipitated in ferrite was reduced proportionally, which would be responsible for the loss of yield strength. However, titanium additions can produce a more uniform ferrite grain size and reduce dispersion hardening by fine niobium carbides, which led to excellent toughness properties when a lower FRT was utilized.

The influence of controlled rolling on the pulse height distribution of magnetic Barkhausen noise in steel

Magnetic Barkhausen noise (MBN) refers to the discontinuous changes in magnetization due to domain walls becoming pinned on, and subsequently overcoming, obstacles to their motion. Barriers to domain wall motion may be microstructural in nature, such as precipitates, grain boundaries and regions of localized strain; for this reason techniques based on MBN detection have the potential to characterize and to evaluate non-destructively ferromagnetic materials. The present study investigates the influence of furnace cooling, normalizing and controlled rolling on the MBN pulse height distribution of relatively low carbon steel. In general, randomly oriented undeformed grains result in a narrow symmetrical pulse height distribution which shifts to lower values as grain size is reduced. Controlled rolling to low finish rolling temperatures results in an asymmetrical pulse height distribution when the applied magnetic excitation field lies parallel to the steel rolling direction; this is attributed to domain walls “jumping” beyond individual grains as a result of the influence of texture and/or microstructural strain.

Effects of Composition and Processing Factors on the Mechanical Properties of As-hot-rolled Dual-phase Steels

As-hot-rolled, ferrite-martensite dual-phase steels of rather simple composition can be produced by the “Dual Phase Rolling (DPR) process” which involves a low finish rolling temperature and a very low coiling temperature. Laboratory DPR experiments have been carried out using C-Mn steels and those with Cr or Si additions, to examine the effects of alloying and processing factors on the structure and mechanical properties of the processed steels. Major results obtained are as follows:(1) To attain a sufficiently low yield-to-tensile strength ratio, the final finish pass temperature should be at about Ar3 point which varies depending on the composition, so as to bring about early separation of the alpha phase from the gamma phase before cooling starts. The coiling after a rapid cooling should be done at a temperature lower than 200°C, almost regardless of the steel composition, to suppress auto-tempering of the transformed martensite and aging of the ferrite.(2) Both Cr and Si additions enhance the hardenability of partitioned austenite, allowing a relaxed cooling rate to obtain the martensite phases. However, Cr addition is prone to hinder the early phase separation making the gamma-to-alpha transformation sluggish. Silicon addition accelerates the phase separation, so that a wide range of finishing temperature is available.

The Influence of Rolling Practice on Fracture Properties of Hot Rolled Plate

AbstractThe fracture behaviour of ferrite-pearlite steels finish-rolled in the (α + γ) region has been investigated and correlated with specific microstructural features. The effect of increasing the amount of deformation at lower temperatures in the (α + γ) region is to skew the ferrite grain-size distribution toward larger grain sizes at the same mean value of ferrite grain size, and to increase the dislocation density of the ferrite. In addition, the proportion of large ferrite grains oriented with (001) parallel to the rolling plane is increased. In conventional fracture tests it was observed that the lower finish rolling temperature did not alter either the impact transition temperature or the K1C value for cleavage (measured at – 196°C). However, the through-thickness fracture properties deteriorate with lower finish rolling temperature because of the increased ease of nucleating microcracks at arrays of inclusions as well as the preferential nucleation of cleavage cracks in the large ferrite grains...

Controlled rolling of plates with forced-water cooling during rolling

To reduce the long processing time in the controlled rolling of plate in one-stand reversible rolling mills, water cooling can be used. Experiments in both a pilot plant and a production mill showed no significant change in bulk mechanical properties if heavy water cooling was used during the holding period, but a thin surface layer with a very fine-grained structure was found. This surface effect is not believed to affect the bulk properties, except at very low finish-rolling temperatures, where there is a risk of tensile stresses occurring at the surface. Water cooling can also be used during the roughing stage. This method gave an even shorter processing time and no surface effects, but the toughness deteriorated somewhat. Controlled rolling with water cooling during rolling is believed to be suitable when rolling short runs of many different products.

The Effect of Low Finish Rolling Temperatures on the Structure and Properties of an HSLA Steel

The Effect of Low Finish Rolling Temperatures on the Structure and Properties of an HSLA Steel

The Effect of Low Finish Rolling Temperatures on the Structure and Properties of an HSLA Steel

Abstract A commercially produced HSLA steel was rolled at 980°C, 780°C and 660°C to study the effects of three different types of microstructure and texture on the tensile and impact properties of the product. The rolling reductions varied between 63 and 67%. The tensile and impact properties were measured in the longitudinal direction and compared with those of the original stock. Deformation at all three temperatures produced a textured product, although in the material deformed at 980°C it was very weak as partial recrystallisation had occurred prior to transformation. Rolling at 980°C and 780 °C produced little change in tensile properties although the microstructures in the two cases were very different. Rolling at 660°C produced a 66% increase in yield strength but with a loss in ductility. The impact toughness was improved in all three cases. After similar rolling reductions at 660°C the HSLA steel has a higher impact transition temperature than mild steel. This difference is partially attributable...