2020 Volume 60 Issue 2 Pages 324-329
A novel quenching and partitioning (Q&P) processing was applied to the ultra-high-strength tailor welded blanks (TWBs) with an equiaxed martensite, retained austensit and carbides in the weld. The Q&P processing consisted of a cooling step and a partition step at 450°C for 10 s to 30 s. The fraction of martensite after the processing was nearly 64%. During partitioning, carbides precipitates with an average size of 30 nm formed inside the martensite. Currently, the interstitial content of austenite was increased to an average of almost 1.2 wt.%. After Q&P processing, the TWB of partition joints exhibited outstanding mechanical properties including a yield strength of 450 MPa, a tensile elongation of 15% at room temperature, and a formability ratio of 108.72% and 81.66%, with respect to the BMs DP1180 and DP590. Furthermore, the tempered martensite formation and austenite ductile-effect were attributed to the formability improvement of ultra-strength-steel TWBs.
In the past few decades, tailor welded blanks (TWBs) have achieved popularity due to their various advantages over conventional sheet forming methods, such as reduced weight, high precision, and cost saving. However, due to the dissimilar thicknesses/strengths combinations of base materials (BMs), the ductility or formability of TWBs is reduced in comparison with that of BMs.1) Since 2010, numbers of studies have focused on the formation of TWBs, and most studies in literature ascribed the decrease of formability to two major aspects: one is the discrepancy in BMs, and the other is low ductility of welding seam.2,3) Cheng et al. studied the stainless steel TWBs with different dimensions, thickness combinations, and welding orientations. The results indicated that the thinner part of TWBs dominated the majority of deformation. In similar welded blanks, fracture would occurred in the thinner part, which withstanded higher stress and strain in the vertical direction on the weld line.4)
Effects of thickness and strength ration on formability of TWBs were investigated by researchers.5,6,7) The deformation inhomogeneity in the direction perpendicular to the weld rises with the increase of the thickness ratio, which decreases the formability of the TWB in this direction. Azuma pointed out that when thickness ratio researches TR1.5, the forming height longitudinal and transverse TWBs was dramatically decreased to approximately 60% of that of the thicker base material. Beside the geometry effect of BMs, the constitution of the weld in TWBs besides its constraint effect on forming and tensile behaviour was also particularly studied.3,8,9) The plasticity of TWBs was carefully studied by simulation models and DIC strain measurement system. It was proved that the constraint effect of weld, especially the fusion zone, resulted in the transformation of the slop of strain path from plain to a steeper curve, and finally declined the formability of TWBs.
Various methods were applied to improve the plasticity or formability of TWBs, such as decreasing thickness ratio, welding orientations or punch locations adjustment. But, in some special cases, especially when the base metal was in ultra-high-strength or weld suffers a large deformation, the forming process which neglect the harden effect of weld seam as traditional method may cause a loss in accuracy.10,11,12,13)
To improve the formability of TWBs, hot stamping or local thermal input method was studied in austenitic stainless steel and aluminum TWBs.14,15,16,17,18) Satya Suresh created specialized punch with selective heating to control the weld line movement, which achieved 7% reduction in thickness by heating on stronger ASS base metal. And in this study, a novel local quenching and partitioning (Q&P) processing was applied to the ultra-high-strength TWBs inspired by “Materials genetic engineering” to control the constitution of the TWBs system.
From the last decades, to further enhance the strength and ductility of the 3rd generation Advanced High Strength Steel (AHSS), Q&P processing of martensitic low-alloy was began to be investigated, and the excellent mechanical properties are achieved through TRIP/TWIP (transformation-induced plasticity/twinning-induced plasticity) effect. During the process, the partitioning step enables the diffusion of interstitial elements from supersaturated martensite to the untransformed austenite,19,20,21,22) improves the stability of the austenite and finally inhibits transformation to martensite in the following second quenching step. Because of its ductility-enhancing effect, the volume fraction of retained austenite plays an important role in Q&P steels.
In 2017 and 2018, Farnoosh et al., from Luleå University of Technology, Sweden and Saarland university, Germany, firstly used the two-step quenching and partitioning treatment to the 5.5 mm thickness Domex960 steel joint. They pointed out that the Q&P processing could improve tensile strength and impact toughness of the joint, which was considered as a promising method to give quick industrial application.23,24) So, the aim of the present study is to demonstrate how the TWBs can be made ductility improved by means of Q&P processing. And the main focus was placed on the formability, hardness measurement and the microstructure observation during the Q&P processing. Furthermore, the ductile-effect mechanism was also revealed.
The chemical composition of the dual steels DP1180 and DP590 used in the present study is shown in Table 1. Laser welding was conducted on 200 mm×100 mm sheets of 1.2 mm DP1180 and 1.5 mm DP590. Welding parameters were adjusted to 2 kW power, travel speed of 1.5 m/min, and argon was used as a shielding gas with flow rate of 25 L/min.
Sample B were fully austenitized at 850°C for 60 s, quenched to 350°C for 5 s and reheated to 450°C for 5 s–30 s. The isothermal holding time at the quenching temperature was kept short to avoid isothermal transformation products below Ms. Assuming that all alloying elements except carbon were homogeneously distributed during slowcooling, quenching and partitioning, it is possible to estimate the martensitic transformation start (Ms) temperature for both high and low carbon austenite by the following empirical equation,25,26)
The welding seam of laser welded specimens were subjected to post quenching and partition process to achieve tailor welded partition blanks after the laser welding process as shown in Fig. 1.
Schematic representation of heat treatment corresponding to samples B.
Microstructures of the weld joints before and after quenching and partition process were characterized by scanning electron microscopy (SEM, S-3400N, Hitachi). And the deformation of specimen after formability test was also detected with EBSD analysis.
Formability test and microhardness measurement were performed. The formability of TWBs/partition blanks were carried out using a 20 mm diameter hemispherical punch according to the standard GB/T 4156-2007. And a load of 0.3 kg was used for the welded samples in which the hardness was determined.
Biaxial stretch formability testing of welded blanks was performed by standard Erichsen cupping test. Figure 2 showed the punch force-displacement curves and the morphology of the specimen during cupping testing with different partition process in welded blanks. As shown in Fig. 2, peak punch load of sample A was the lowest, 27.6 kN, and little deformation was occurred at FZ according to the morphology and FEM analysis results in Figs. 1(c) and 1(d). With the quenching and partition process applied, peak load increased and much more deformation was occurred in the TWBs.
(a) Formability test results and (b) cupping depth and formability ratio of welded blanks. (Online version in color.)
Figure 2(b) shows Erichsen cupping depth and formability ratio of tailor welded partition blanks. In sample A, the Erichsen cupping depth was 8.5 mm, and the calculated formability ratio was only 65.49% of BM DP590. Compared with sample A, sample B had a higher cupping depth. When the partition time was ranging from 5 s to 20 s, the cupping depth was fluctuated around 10.0 mm a superior depth of 10.6 mm and formability ratio of 81.66% and 108.72%, with respect to the BM DP590 and DP1180.
Researchers pointed out that the thickness ratio(TR) and strength ratio (SR) would affect on the position of fracture in the TWBs. Chen studied correlation between thickness ratio and formability in DC56 TWBs and gave a concept of the critical thickness ratio (CTR) to predict the tendency of the formability and failure modes of TWBs.5) It was reported that when thickness ratio is more than CTR 1.065, the failure appears on the base metal. Bandyopadhyay pointed out that difference strength made the punch bottom experienced stretching perpendicular to the weld line and resulted in disproportionate strain accumulation at the weaker DP600 side in DP600-DP980 TWBs.27)
In DP590-DP1180 TWBs, the TR and SR value was 1.2 and 1.36, respectively. Compared with the CTR 1.065 and SR1.15 by Chen et al. and Bandyopadhyay et al., the TR and SR was much higher and the strain accumulation would be much more obvious, leading to thinning and ultimately premature failure at DP590 side of base metal, which was inconsistent with the observations by researchers.
Finite simulation images were also exhibited in Fig. 2. In consideration of the major strain direction, the symmetry line perpendicular to the weld line was selected for investigating the constraint. It was found that deformation first occurred on the DP590 base metals and the strain was concentrated at the region approximately ~3.2 mm away from the weld centerline. Constraint effect played an important role in the strain path, which was associated with the sharp gradient of strength or hardness in TWBs.13)
The tensile properties was also studied and it was showed that the engineering stress-strain curve of tailor welded partition blanks sample B have a lower strain hardening rate, higher total elongation and yield point compared to sample A.
According to the microstructure observation, it has been proved that the presence of hard blank martensite with little austenite results in locally higher stress levels. In these areas, the austenite grains transform at lower stress levels, decreasing the overall austenite stability. Thus in sample A, austenite transformed fast in the first deformation stages at low overall stress-levels resulting in a low yield point and high strain hardening exponent. However, the homogeneous strain distribution in sample B resulted in a higher yield point and the continuous TRIP effect contributed to a lower n-value. Besides, the slower transformation rate of the retained austenite could explain its higher total elongation.
Usually, the softening phenomenon in heat affected zones and the harden effect in fusion zone was an unavoidable phenomenon which deteriorated the mechanical properties of TWBs.28) Vickers hardness profile of laser welded blanks before and after partition process was shown in Fig. 3(b). It was found that by the quenching and partition process applied in sample B, the curve fluctuation decreased especially in DP1180 side. As shown in Fig. 3(b), the microhardness of FZ and HAZ in DP1180 side was around 332±5HV and 320HV±5HV. Compared with the microhardness (387HV±5HV) of fusion zone in sample A, the value was decreased by more than 55HV. The hardening and constrain effect was weaken because of the drop of hardness gradient of welded joint during the quenching and partition process.
(a) Engineering strss-strain curves in the Q&P condition, (b) Vickers hardness profile, (c) average mechanical properties in the Q&P condition and (d) constraint effect of welding seam during deformation in TWB.29) (Online version in color.)
The constraint effect of welding seam during deformation in TWB was presented in Fig. 3(d). The strain paths near the welding seam was shifted to the plane strain condition with much smaller limit strains, from line OA to line OB and from line OC to line OD in Equi-biaxial strain path and uniaxial tensile strain path conditions, which was just like the conditions in tensile and formability tests. It has been reported that the change of strain path and its history due to constraint applied by laser welding was important factors affected the formability of TWBs, especially the failure happened out of weld seam in base metals.29) Song et al. proposed a novel approach to characterize the inhomogeneous mechanical properties of weld materials. Corresponding to the newly-built relationship between the Vickers hardness and material parameters, it can be seen that for a given Vickers hardness, the strain hardening exponent increases with increasing strength coefficient.30) So it was confirmed that by dropping the microhardness, the constraint effect of weld seam in TWBs was decreased, which finally achieved much better ductility and formability in mechanical tests.
Microstructures and XRD analysis of specimens before and after partitioned at 450°C for 30 s are compared in Figs. 4 and 5. As shown in Fig. 4, the XRD results of specimens were similar, consisting mainly martensite and austenite formed. However, when the Q&P processing applied to the DP1180 base metal and TWBs, the amount of the austenite was a little increased. Retained austenite was distributed as lattice of around 1–2 μm thickness and 2–5 μm in length between martensite laths in Fig. 5.
Microstructure and XRD analysis of (a) (c)sample A and (b) (d))sample B. (Online version in color.)
EBSD results of fusion zone of TWBs (a) before and (b) after Q&P process. Specimen B were quenched to 350°C and partitioned at 450°C for 30 s. On the first line, phases map (blue is martensite, yellow is austenite) are shown. On the second line, the orientation maps for the BCC and FCC phases. On the third line, the corresponding orientation maps for the FCC phase. And the fouth line presented martensite by with higher Kernel average misorientation. (Online version in color.)
In samples, laths of martensite are clearly etched than others. However, the morphology of martensite was dramatically transformed from lath to, including darker and brighter area, with Q&P processing applied. This observation was also suggested that martensite with higher band contrast and larger size was formed during the first quench and then the carbon supersaturated in martensite was partitioned to the surrounding austenite, contributing to its thermal stabilization, whereas fresh martensite with lower band contrast was formed during final quenching, which had higher Kernel average misorientation (upper limit of 5°) than the rest of the microstructure, which was similar with the investigation by Lawrence Cho.31) Partitioning step in sample B formed tempered equixed martensite, and the slower transformation rate of the martensite-austenite structure would result in the continuous TRIP effect in deformation process. But the content of austenite and its morphology was not satisfied to further improved the ductility of the joint, and Farnoosh Forouzan et al. gave out the explanations as austenite stability deficiency and high content of fresh martensite in Q&P processing.32,33,34)
In this paper, Q&P processing was firstly applied to tailor welded partition blanks of DP1180-DP590, and the mechanical properties were carefully studied. The main conclusions are drawn as follows:
(1) Based on the microstructure observation, the hardened lath martensite in the fusion zone was transformed to equaxied tempered martensite, with much austenite and fresh martensite formed.
(2) The peak microhardness value was dropped from 387HV±5HV to 332±5HV. The harden effect in fusion zone was weaken.
(3) After Q&P processing, the TWB of partition joints exhibited outstanding mechanical properties including a yield strength of 450 MPa, a tensile elongation of 15% at room temperature, and a formability ratio of 108.72% and 81.66%, with respect to the BMs DP1180 and DP590.
This work was financially supported by Postdoctoral Science Foundation funded project of China under Grant No.20171M611317 and the Development of Science and Technology of Jilin province, China under Grant No. 20180520005JH.