Rolling temperature and rolling reduction intensively influence the formation of Luder lines and fluting marks in mild steels. They govern these effects through control of strain aging. In order to enhance the strain aging resistance and the consequent reduction of yield point elongation and fluting intensity, warm rolling without using the skin pass process is applied. The development of microstructure and crystallographic textures during deformation process and the determination of fluting intensity and mechanical properties consisting of tensile and formability properties in terms of different thermomechanical conditions (RT and RR%) were investigated in this study. These properties are determined through the use of bending, tensile tests, optical microscope, and EBSD analysis.
The mild steel known for being very strong, has been used as the structural components in the part of building construction like gutters for water drainage, pipes, and channels. High formability is required to form these components, but the solute carbon and nitrogen, as the interstitial atoms in low-carbon steel, play a significant role in formability deterioration. The segregation of interstitial atoms around the dislocation and formation of the Cottrell atmosphere lead to blockage of dislocations motion. This phenomenon appears as wrinkled lines (fluting lines) in bended steels [
According to Cottrell and Bilby, strain aging and the subsequent phenomenon, that is, yield point elongation, could be controlled by the concentration of the interstitial solute atoms and of the mobile dislocation density. Based on previous investigations, the kinetics of the strain aging is modified and slowed in low-carbon steel by adding some alloying elements (Nb, V, and Ti), with the formation of the Ti, V, and Nb (C and N) precipitates. In addition, the introduction of a higher amount of mobile dislocations through temper rolling can remove the discontinuous yielding. However, the last action is often not considered as an invariable solution for eliminating the yield point elongation in mild steels because after aging, the interstitial atoms pine the dislocations and form the Cottrell atmosphere [
Numerous studies were carried out on the hot rolling of the microalloyed steels to produce steel with high mechanical performances, that is, a good strength-toughness combination obtained by precipitation strengthening of (Nb, V) (C, N) precipitates and by removing C, N from the matrix [
In our work, the mild steel was rolled in the intercritical region (
The chemical composition of tested steel is listed in Table
Elemental composition of mild Steel (wt.%).
C | Mn | Si | P | S | Cu | Al | Nb | V | Ti | B | N |
---|---|---|---|---|---|---|---|---|---|---|---|
0.034 | 0.103 | 0.008 | 0.0064 | 0.0064 | 0.197 | 0.045 | 0.000 | 0.002 | 0.0009 | 0.0002 | 0.0067 |
Differential scanning calorimetry was conducted to determine the intercritical region range of the steel. This test was performed by reheating the samples to 1060°C at a heating rate of 10°C/min and then cooling them to room temperature at the same rate. The achieved curve from the DSC test is shown in Figure
DSC curves of mild steel cooled and heated at 10°C/min.
The rolled strips were bent around the cylinder of diameter 50 mm. The fluting intensity is defined as the average length of the fluting segments that form on the bent sheet, as shown in Figure
Fluting lines and segments observed on bended sample.
The crystallographic textures were analyzed through electron back scattered diffraction (EBSD). The deformed samples, after being polished with a colloidal Silica suspension, were investigated using EBSD operating at an acceleration voltage of 20 Kev and a magnification of 500x.
The effects of the initial rolling temperature and the rolling reduction on the microstructure are shown in Figure
The microstructure of rolled mild steel affected by rolling temperatures of 790, 830, 870°C, and rolling reduction of 30% and 60%.
Mean equivalent diameter (D0) of ferrite grains as a function of the rolling temperature and rolling reduction %.
Steel | % rolling reduction | Ferrite grain size ( |
Ferrite grain size ( |
Ferrite grain size ( |
---|---|---|---|---|
Mild steel | 30 | 27.15 | 38.35 | 7.9 |
60 | 15.9 | 24.6 | 25.35 |
The ferrite grain size decreased as the rolling reduction increased to 60%, and the grains elongated along the rolling direction at lower rolling temperatures. The grain growth was obviously observed as the rolling temperature increased. Although deformation increased, the high reheating interpass temperature resulted in the easy mobility of the high angle grain boundaries of the recrystallized grains, and the grain growth was rapid according to the Arrhenius relationship.
The crystallographic texture is featured in Figure
Orientation distribution function of warm rolled steels at different thermomechanical conditions at
The histograms of grain boundary misorientation degrees versus the percentage of their numbers in different rolling temperatures and rolling reductions are depicted in Figure
Misorientation distribution in warm rolled steels. (a) RT = 790°C, RR = 30%, (b) RT = 790°C, RR = 60%, (c) RT = 830°C, RR = 30%, (d) RT = 830°C, RR = 60%, (e) RT = 870°C, RR = 30%, and (f) RT = 870°C, RR = 60%.
The correlation between the yield point elongation % and the fluting intensity is presented in Table
Effect of the rolling temperature and the rolling reduction on yield point elongation and fluting intensity in mild steel.
Rolling temperature of mild steel (°C) | YPE% (30% rolling reduction) | Fluting intensity (mm) (30% rolling reduction) | YPE% (60% rolling reduction) | Fluting intensity (mm) (60% rolling reduction) |
---|---|---|---|---|
790 | 2 ± 0.34 | 4.1 ± 1.8 | ∼0 | ∼0 |
830 | 0.9 ± 0.27 | 2.3 ± 0.3 | ∼0 | ∼0 |
870 | 0.42 ± 0.1 | 1.7 ± 0.1 | ∼0 | ∼0 |
The yield strength of the samples was influenced by the thermomechanical parameters employed in this work. Figure
(a) The trend of yield strength values. (b) The trend of tensile strength values. (c) The trend of elongation values for warm rolled steel as function of rolling temperature and rolling reduction.
The amount of imposed strain affects the yield strength of samples as well. The sheets subjected to the rolling reduction of 60% had a higher yield strength compared with the steels deformed by the rolling reduction of 30%. As depicted in Figure
The trend of the total elongation variation in different levels of the imposed strain is represented in Figure
Concerning the effect of the rolling temperature and the rolling reduction, the optical microstructure results show that the shear bands appeared sharply when the steel strip was deformed at the lowest rolling temperature. An increase in the rolling temperature led to a recovery phenomenon that was accompanied by annihilation of the shear bands. The formation of subgrains which subdivided some grains deformed at a rolling temperature of 830°C occurred after the recovery phenomenon [
In ODF maps representing the steels rolled at
Induction of the dislocations through rolling and the use of a rolling temperature near the recrystallization temperature form different textures including weak {001} <110>, weak {111} <011>, weak {110} <001>, and weak {110} <001> and {110} <110>. This effect can be due to dynamic recrystallization in which the diverse texture components with low intensity appeared.
Increasing the dislocation densities by applying more rolling reduction and a high reheating temperature in the interpass process causes the induction of high stored energy to form the intense textures of {110} <001> and {001} <110>. As the rolling temperature is high, the recrystallized grains grow and feature the new texture components intensely. The existence of a strong {111} <011> texture resulted in deformed steel sheets with a high normal anisotropy value. This property is desirable for applications in which high draw ability is required [
The formation of a large number of high angle grain boundaries at the highest rolling temperature and a rolling reduction of 30% indicates the occurrence of the recrystallization phenomenon, which is consistent with the equiaxed and refined grains observed in Figure
The highest value of YPE% related to the samples rolled with the lowest rolling temperature and the lower level of rolling reduction indicates that a low reheating temperature is not sufficient to reduce the dislocation density drastically, so the large amount of mobile dislocations was arrested by the solute atoms of N and C. In contrast to the steels deformed in a previous thermomechanical condition, the lowest value of YPE% in steels rolled at the highest rolling temperature can be attributed to the recrystallization phenomenon. Due to the softening mechanism, less solute atoms can segregate around the remaining dislocations, and less interaction happens between the dislocations and the solute atoms to form a Cottrell atmosphere (pipe diffusion) [
It is observed that the smooth yielding behavior is revealed in the rolled steels subjected to the higher level of rolling reduction. As the content of interstitial atoms (C, N) remained constant during the rolling process, and the same amount of atoms were diffused to the larger number of dislocations; consequently less dislocations were immobilized with the interstitial atoms. Moreover, the entrapped dislocations in the Cottrell atmosphere could be released by the generated dislocations.
A higher yield strength for the steel sheets rolled at
The use of warm rolling led to the formation of strips with a low total elongation % value. This result can be attributed to the formation of the oxide scale with low plastic deformation reducing the surface elongation of the strips [
The results show the best mechanical properties of mild steel obtained by controlling the thermomechanical parameters (rolling temperature and rolling reduction) in the intercritical region phase ( In this work, the highest elongation % values as the formability property and the lowest yield point elongation and fluting intensity, which are the desired results for our study, were obtained for the strip that was warm rolled at The high yield strength and tensile strength values in low temperature rolled strips with a rolling reduction of 30% and 60% are due to the formation of high dislocations. The improvement in the formability property and the lower yield point elongation and fluting intensity in steel deformed at
The authors declare that they have no conflicts of interest.