Warm-rolled Steel Research Articles

Excellent combination of strength and ductility in austenitic lightweight steel achieved by warm rolling process

In this study, warm rolling was applied to Fe–28Mn–11Al–1C–5Ni (wt.%) austenitic lightweight steel, which realized the higher dislocation density and finer dislocation-cell structures, achieving a better mechanical properties with ∼2.0 GPa of yield strength and ∼6.8 % of total elongation than cold rolling in the same conditions. In the subsequent partial recrystallization condition, the numerous dislocations and substructures existed in warm rolled steel provide more heterogeneous nucleation sites for grain refinement, resulting in the ultrafine-sized austenite and B2 with the size of 0.36 μm and 0.24 μm, respectively. Combined with grain refinement strengthening, solid solution strengthening, precipitation strengthening, and dislocation strengthening in the unrecrystallized austenite, the present steel shows an ultimate tensile strength of 1734 MPa, a uniform elongation of 21.7 % and a specific tensile strength of 267 MPa g−1 cm3. Such an excellent combination of strength and ductility induced by simple warm rolling and annealing is much better than previously reported for lightweight steels.

Effect of caliber rolling temperature on the deformation behavior and mechanical properties of dual-phase medium Mn steel

Dual-phase medium manganese steel (MMS) was caliber rolled at 800, 900 and 1000 °C, respectively, in order to clarify the deformation behavior and mechanical properties of warm deformed MMS. Herein, the microstructure evolution, tensile behavior and fracture characteristics exhibit obvious dependence on the caliber rolling temperature. The warm rolled steel contains martensite and austenite phases with heterogeneous grains including coarse fiber grains (CFG) and ultrafine equiaxed grains (UFG), which shows different microstructural stability. With the increase of rolling temperature, dynamic recrystallization (DRX) is more obvious, corresponding with the increasing percent of UFG, as well as the decreasing percent of CFG and martensite volume fraction. Meanwhile, high deformation temperature also leads to high austenite stability and the decreasing Transformation Induced Plasticity (TRIP) effect. The strength, strain hardening behavior and total elongation are gradually increased with the decrease of rolling temperature, which is attributed to severe strain partitioning and unique laminated structure in the caliber rolled MMS with low rolling temperature. Moreover, the warm rolled MMS reveals obvious ductile-ductile transition behavior in the temperature range from 25 °C to −60 °C. In addition, the low caliber rolling temperature also results into high impact energy, which is attributed to the high volume fraction of CFG.

A strong and ductile ferrite-based lightweight steel manufactured via high-temperature warm rolling process

A trade-off between strength and ductility is unavoidable in ferrite-based lightweight steels due to the presence of δ-ferrite, and improving the properties of such steels requires precise control of the retained austenite and the complete mobilization of multiple strengthening mechanisms. In this study, we obtain a ferrite-based lightweight steel with the chemical composition Fe-0.3C-2.8Mn-3.5Al-0.5Cr-0.2V by adopting a higher temperature warm rolling process, endowing the best comprehensive properties. The results demonstrate that multi-scale heterogeneous microstructures are built without additional recrystallization annealing under the higher temperature (>700 °C) warm rolling process. Meanwhile, a large amount of retained austenite, higher dislocation density and strain-induced vanadium carbide precipitation were obtained through the warm rolling process. As warm rolling refines austenite grains, Mn and Cr are further diffused and enriched, enhancing the chemical stability of the retained austenite. After low-temperature tempering, this warm-rolled steel shows excellent comprehensive properties, including a tensile strength of 1140 MPa and elongation of 30%. The enhanced strength-ductility is attributed to the higher stability of retained austenite, stable maintenance of dislocation density and the pinning effect of precipitation relative to dislocation lines. Furthermore, a stronger discontinuous yielding behavior of the warm-rolled and tempered steel was obtained, enhancing the overall strain.

Obvious different formability and mechanical properties of warm caliber rolling Q345 steel with normalized and tempered states

The formability and strengthening-toughening behaviors of the warm rolled steels are highly dependent on the raw heat treatment state. Herein, the normalized and tempered Q345 steels were both subjected to caliber rolling at 500 °C with different rolling reduction ratios (18%, 44%, and 68%) to obtain elongation grains and carbide particles with different distribution and size. With the increase of caliber rolling reduction ratio, the strength and fracture elongation of the rolled normalized Q345 steels are gradually decreased, whereas the rolled tempered Q345 steels exhibit obvious improvement in these properties. The warm rolled normalized steel contains severe uneven grain distribution and coarse carbides, which leads to high stress concentration and low mechanical properties, as well as poor formability. On the other hand, warm rolled tempered steel contains nanoscale carbides and homogeneous elongation grains that help to release high stress and strain concentration, resulting in improved formability and mechanical properties to a certain extent. Moreover, the warm rolled tempered steel (Tempformed/TF) exhibited a significant improvement in the impact absorb energy, about 3.1 and 47 times higher than those of the quenched and tempered (QT) state at 20 °C and −80 °C, respectively. This remarkable improvement in low-temperature impact toughness can be attributed to the delamination behavior and subsequent induced super-high plastic deformation energy. These findings have important implications for the design and development of high-performance materials with superior strength-toughness combinations.

Microstructure evolution and compressive properties of a low carbon-low alloy steel processed by warm rolling and subsequent annealing

A low carbon-low alloy steel was processed by warm rolling with reductions range from ~30% to ~70% followed by annealing at 450°C. Then, the microstructural evolution was characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Electron Backscatter Diffraction (EBSD), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and compressive testing under strain rates of 1.0 × 10−3–2.0 × 103 s−1 was carried out. Microscopy analyses showed that ultrafine-grained structures with high-density dislocations and more and finer M3C carbides by comparison with the tempered steel were achieved after warm rolling. Subsequent annealing promoted the further precipitation of finer carbides and led to dislocation recovery as well as a slight coarsening of grains. Compressive testing results indicated that the yield strengths of the warm rolled steels at different strain rates were significantly increased by ~40–70% compared with the as-received sample, which was mainly attributed to a combination of dislocation strengthening, grain boundary strengthening and precipitation strengthening. After annealing, the yield strength decreased slightly due to a dislocation recovery and a slight increment of the grain sizes. In addition, the influence of microstructure evolutions including dislocation densities, grain sizes and carbide precipitations during warm rolling and subsequent annealing on the strain rate dependence of strength for steels was also analyzed.

Cryogenic impact toughness of a work hardened austenitic stainless steel

Warm rolling at 473 K with total reductions up to 65% significantly strengthened a 304 L austenitic stainless steel. The tensile strength at room temperature increased from 600 MPa to 1040 MPa. Outstanding mechanical properties of the warm rolled steel were observed at 77 K, when the ultimate tensile strength of 1830 MPa and a fracture elongation of 39% were achieved. Moreover, the warm rolled steel exhibited high impact toughness, especially, at cryogenic temperature that was attributed to the crack branching crosswise to the principal crack direction.

Strengthening contributions of dislocations and twins in warm-rolled TWIP steels

A twinning-induced plasticity (TWIP) steel was processed by warm rolling with different reduction ratios to elevate yield strength while maintaining considerable ductility. To investigate the effect of prior warm rolling on the deformation mechanisms during subsequent tensile straining, the contributions of individual strengthening mechanisms to the tensile flow stress of the as-received (AR) and warm-rolled (WR) steels were decoupled, based on the measured dislocation density evolutions determined by synchrotron X-ray diffraction (XRD) experiments. Whereas dislocation multiplication invariably governs the maximum flow stress of the AR and WR steels, deformation twinning becomes increasingly important for the strain hardening of steels with larger warm rolling reductions. In addition, twinning kinetics is enhanced by the pre-existing high dislocation densities caused by warm rolling, leading to an enhancement of twinning-induced hardening. The present work provides further insight into the influence of warm rolling on the deformation mechanisms of TWIP steels.

Texture and microstructure evolution during different rolling methods in strip-cast grain-oriented 6.5% Si steel

Grain-oriented 6.5% Si steel was produced by strip casting and the effect of warm rolling and cold rolling at the latter stage of rolling was investigated. The result showed the rolling method had a minor influence on the rolling microstructure but significant on the rolling texture. Compared to the cold-rolled sheet, the warm-rolled steel showed strong α-fiber and γ-fiber texture at the surface but weak α-fiber and γ-fiber texture in the center layer. After primary annealing, the warm-rolled sheet showed inhomogeneous microstructure and strong {223}〈110〉 texture together with weak γ-fiber texture. By comparison, the cold-rolled sheet showed fine equiaxed grain with favorable {111}〈112〉 texture component. During secondary annealing, the warm-rolled sample displayed significantly normal grain growth at the initial stage and abnormal grain growth at ~1150 °C. The final microstructure was characterized by coarse matrix grains and abnormal Goss grain. In contrast, the cold-rolled sheet showed complete abnormal grain growth. The different secondary annealing behavior was attributed to different grain structure and texture.

Texture homogeneity and stability in severely warm-rolled and annealed ultrafine pearlite

ABSTRACTThe evolution of microstructure and texture was investigated in a severely warm-rolled ultrafine pearlitic steel. The steel was 95% warm-rolled at 600°C and annealed at 700°C for different time intervals. The spheroidisation of cementite initiated after 30% reduction and completed beyond 70% reduction. The 95% warm-rolled steel showed elongated as well as ultrafine equiaxed ferrite grains. Texture inhomogeneities were evidenced by the presence of Goss component ({011} <100>) on the surface originating due to surface shear and diffuse texture at the interior. Formation of equiaxed microstructure after annealing through continuous recrystallisation resulted in the retention of the surface Goss component. However, the Goss component was destroyed beyond annealing for 180 minutes due to the abnormal growth of other grains.

An investigation on the microstructure and mechanical properties in an ultrafine lamellar martensitic steel processed by heavy warm rolling and tempering

An ultrafine lamellar martensitic steel was fabricated by heavy warm rolling of undercooled austenite and subsequent quenching. The martensite transformed from warm rolled austenite was composed of ultrafine heterogeneous lamellar plates, which were further subdivided into laths and twins. The warm rolled steel after low temperature tempering exhibited excellent mechanical properties with a yield strength of 2343 MPa, ultimate tensile strength of 2586 MPa and total elongation of 9.4%. The combination of ultrahigh strength and high ductility was primarily attributed to the united effect of ultrafine lamellar plates, high density dislocations, twins and retained austenite. After high temperature tempering, the strength of warm rolled steel decreased dramatically, which was attributed to the severe recovery of martensite.

Deformation microstructures and tensile properties of an austenitic stainless steel subjected to multiple warm rolling

The deformation microstructures of a 316L-type austenitic stainless steel subjected to multiple bar rolling to a total strain of 2 at temperatures of 773–1173K and their effect on the mechanical properties at ambient and elevated temperatures were studied. The multiple warm rolling was accompanied by significant grain refinement. The finally evolved transverse grain size decreased from 3.4µm to 0.85µm with a decrease in the rolling temperature from 1173K to 773K. The warm rolled steel samples were characterized by significantly increased strength properties. The strengthening was studied by tensile tests at ambient and elevated temperatures. A decrease in the rolling temperature from 1173K to 773K increased the yield strength from 720MPa to 945MPa or from 395MPa to 470MPa at room temperature and 973K, respectively. The strengthening obeyed Hall-Petch relationship with a grain size strengthening coefficient decreased from 410MPaμm0.5 at room temperature to 140MPaμm0.5 at 973K.

An investigation into the effect of strain ageing on mechanical properties of low-carbon steels after warm rolling

In this study, the effect of strain ageing on mechanical properties of warm-rolled steel is investigated. Warm-rolling experiments together with mechanical testing are employed to investigate the effect of processing parameters on the ageing kinetics during and after warm rolling of a low-carbon steel. To do so, the occurrence of dynamic strain ageing is first determined by means of a two-dimensional thermo-mechanical model and then the effect of this phenomenon on the subsequent static strain ageing after warm rolling is studied employing mechanical testing. The results show that the occurrence of dynamic strain ageing during warm rolling may effectively alter the progress of the subsequent static strain ageing as well as the final mechanical properties of the warm-rolled steel.

Comparison of the annealing behaviour between cold and warm rolled ELC steels by thermoelectric power measurements

The variations of thermoelectric power (TEP) measurements during annealing of warm and cold rolled extra low carbon (ELC) steels were analysed. Where a prominent influence of recovery was detected in cold rolled steels during annealing, almost no variation of TEP due to both recovery and recrystallisation was observed during annealing of warm rolled steels. On the other hand, TEP measurements were proven to be a powerful technique to monitor dissolution of cementite and precipitation of aluminium nitrides. It was found that these two phenomena significantly affect TEP measurements and recrystallisation kinetics during annealing of warm rolled steels.

Effects of deformation parameters on the final microstructure and mechanical properties in warm rolling of a low-carbon steel

The effects of warm rolling parameters on the final microstructure and mechanical properties of a low-carbon steel were investigated. In doing so, the steel strips were pre-heated at various temperatures and rolled at different reductions and rolling speeds. Then, microstructural studies were performed and, also, the mechanical properties of the rolled samples were measured by tensile and hardness testing. An optimum rolling program was determined with the aids of experimental results and a mathematical model based on the finite element method (FEM) developed for the warm rolling conditions. The results show that, at high temperatures and low rolling speeds, the roll forces are reduced; however, the possibility of strain inhomogeneity and non-uniformity in the grain size distribution are increased. On the other hand, at very low temperatures, the roll force increases sharply. For the case of the employed steel, a temperature between 550–700°C and a rolling speed of 55–70 (rpm) are found to be the suitable rolling conditions for producing warm-rolled steels with uniform and fine-grain structures.

Effect of Hot Band Grain Size on the Anisotropy of Warm Rolled Low Carbon Steels

In warm rolled steels, the intensity of the &lt;111&gt;//ND annealing texture, which favours formability, has been related to the formation of shear bands during rolling. Coarse hot band grain sizes (HBGS’s) facilitate flow localization, the mechanism associated with the formation of shear bands.In this work, the effect of grain size after hot rolling was studied in a low carbon steel containing small additions of Cr and Mn. The formation of shear bands and their subsequent influence on the normal anisotropy rm and planar anisotropy Dr in the annealed steels were of particular interest. Two HBGS’s (18 and 30mm) were employed and the specimens were warm rolled to reductions of 65 and 80% at various temperatures between 640 and 700°C. The results show that the frequency of shear banding is slightly lower for the smaller grain size. The normal anisotropy was not affected by the HBGS; by contrast, much lower Dr values were associated with the finer grained steel.

Effect of Rolling Temperature, Reduction and Alloying Additions on the Texture of Warm Rolled Steels

The effect of warm and cold rolling parameters on the development of annealing textures was studied in two low carbon steels containing additions of chromium. Two warm rolling temperatures (640 and 700°C) were employed together with a reduction of 65%. The effects of an additional cold rolling reduction of 40% and of decreasing the heating rate during annealing were also studied. The ND fiber, &lt;111&gt;//ND, of the recrystallization texture was strengthened as the warm rolling temperature was decreased. However, all the warm rolled steels contained a retained RD fiber, &lt;110&gt;//RD. A noticeable improvement in both the continuity and intensity of the ND fiber was obtained when the sample was submitted to an additional 40% cold rolling reduction. The ND fiber was even more continuous and intense when a low heating rate was utilized, yielding r-values of 1.1 and 1.3 for the warm rolled and warm plus cold rolled samples, respectively.

Effect of Alloying Elements on the Microstructure and Texture of Warm Rolled Steels

The effect of Cr, B and P addition on microstructure and texture formation was studied in warm rolled at 640 and 710°C interstitial free (IF) and low carbon (LC) steels. Samples were characterised using Electron Back Scattering Diffraction analysis, optical and electron microscopy. The LC steel showed a decrease in the number of grains with shear bands compared with IF steel that deteriorated the deformation texture. The addition of chromium to the LC steel reversed this tendency. The increase in shear band frequency in the LC(Cr) steel appears to be due to the formation of coarse Cr 23 C 6 carbides and fine strain-induced Cr 23 C 6 and Cr 3 C 2 carbides. Chromium addition also led to an increase in grain thickness and thus in the tendency to form shear bands. The addition of phosphorus to the LC(Cr) steel did not improve the deformation texture significantly. The addition of boron to the LC(Cr) steel decreased the number of γ-fibre grains and is therefore expected to reduce the formability after annealing. Short, long, intense short and intense long shear bands were observed. Their formation depended on grain size, as well as on the amount of work hardening and precipitation hardening. The long and short (but not the intense) shear bands were linked to the presence of the γ-fibre after rolling. The main outcome of this work is that the addition of alloying elements affects the volume fraction of grains containing shear bands, the grain thickness and the shear band morphology; these, in turn, affect the texture of the steel, particularly after annealing.

Effect of Rolling Temperature on the Recrystallization Texture of Warm Rolled Steels

Effect of Rolling Temperature on the Recrystallization Texture of Warm Rolled Steels

Effect of rolling temperature on the deformation and recrystallization textures of warm-rolled steels

Warm-rolling trials were carried out on three interstitial-free (IF) steels (stabilized with either niobium or titanium), an extralow-carbon (ELC) steel, and an experimental low-carbon chromium (LC Cr) material at temperatures between 440 °C and 850 °C. The influence of rolling temperature on their as-rolled microstructures and deformation and recrystallization textures was investigated. Also, the effect of coiling simulation and degree of rolling reduction on the r values of some of these materials was examined. In-grain shear bands were evident in all as-rolled microstructures, but their sensitivity to deformation temperature varied between steels. Shear bands of moderate intensity were formed in the IF steels across all temperatures. In the ELC material, intense shear bands were formed at low rolling temperatures, but at higher temperatures, this intensity was drastically reduced. The development of shear bands at the higher rolling temperatures was significantly enhanced by alloying with chromium. The deformation textures produced were typical of rolled ferrite materials. The intensity of this texture increased markedly with temperature for the ELC grade. Conversely, the intensity of the recrystallization texture decreased with increasing temperature. The addition of chromium was found to strengthen the {111} component and, hence, the formability. The sharpness of both the deformation and recrystallization textures of the IF steels was relatively unaffected by rolling temperature. These differences are attributed to the intensity and frequency of shear-band formation and the dynamic strain-aging (DSA) behaviors of the various materials.

温間圧延によって製造した超微細フェライト-セメンタイト組織鋼板の特性

温間圧延によって製造した超微細フェライト-セメンタイト組織鋼板の特性