Feasibility of 0.02% Nb-Based Microalloyed Steel for the Application of One-Step Quenching and Partitioning Heat Treatment

To attain an enhanced combination of mechanical properties for low alloyed steel, the current study has been made to fulfill that growing need in the industry. Its results are introduced within this paper. One step Quenching and Partitioning (Q&P) heat treatment has been applied on Niobium-based microalloyed steel alloy with 0.2 %C, in the form of 2 mm thickness sheets. The target of this study is to investigate the viability of applying that significantly recommended, results-wise, heat treatment on the highly well-suited alloy steel samples, to achieve the main target of enhanced properties. A single temperature of 275°C was used as quenching and Partitioning temperature. Four Partitioning periods (30, 200, 500, and 1000 Seconds) were used for soaking at the same temperature. The results were analyzed in the light of microstructural investigation and mechanical testing. All applied cycles did not enhance the strength but moderately improved the ductility and toughness, mainly caused by the slightly high soaking temperature used. Niobium impact of grain refining was apparent through all cycles. The cycle of 500 Seconds Partitioning time obtained optimum values at that particular temperature. The 1000 Seconds Cycle obtained the worst combination of properties. A set of recommendations are set. More research is required at this point, where a lower Partitioning temperature is advised. In the light of the applied combination of parameters, the Partitioning period at such temperature is advised to be between 500 and 1000 Seconds. A high probability that periods closer to 500 than 1000 Seconds will produce better results. More research is needed between those two values of Partitioning time to precisely determine the optimum time at that temperature on that specific alloy.


Introduction
The steel-based industry in general and the Automotive industry always aspired to continuously enhance mechanical properties, such as strength, ductility, toughness, and other relevant properties. That combination of properties is the interpretation of the persistent need for better fuel efficiency, and higher passenger safety, which are the significant targets concerning Automotive industry development. There have been many trials within different research paths to take another step forward towards that aspiration.
The Advanced High Strength Steels (AHSS) are a product of that research.
The use of AHSS has been progressively increasing, mainly in the Automotive industry, amongst others. The high fulfillment capability and promising potential of AHSS towards the industry's defined needs are the reason for that increase of use. More elaborately, using AHSS in the Automotive industry provided a combination of enhanced properties [1] [2], such as high strength, high formability (ductility), high capacity of energy absorption, lower vehicle weight, more economical fuel consumption, lower cost, enhanced crash resistance, and higher passenger safety. Based on that combination of privileges, AHSS are deployed within the Automotive industry in several places through the vehicle body [1] [2] [3], such as Body In White components (BIW), sills, reinforcements in the bumper, hood, doors, and other parts.
AHSS, as a family of steels, consists of three generations [2]. Each generation has its distinctive combination of characteristics. The first generation consists of several steel grades, such as Dual Phase (DP), Complex Phase (CP), and Transformation Induced Plasticity (TRIP) steels. The first generation is generally distinguished with low strength and ductility levels compared to the two other generations. The second generation of AHSS is featured by an enhanced combination of high strength and ductility with a downside of high cost and processing difficulty. Twinning induced plasticity (TWIP), and Austenitic stainless steel (AUST-SS), are two grades within the second generation AHSS.
On one hand, we have the first generation of AHSS with a good combination of mechanical properties; on the other, we have the second generation with a much superior combination, but with industrial limitations. The third generation of AHSS achieved the needed balance between acquiring enhanced mechanical properties and processing combination with more economical cost. Third-generation AHSS consists of several grades [3], such as medium-Mn steels and Quenched and Partitioned (Q&P) steel. Q&P steel is the main interest in the present work.
Attaining the desired enhanced combination of properties through Q&P is done by reaching a suitably ranged combination of Martensite phase, Ferrite For the first time, Q&P heat treatment was introduced through J. Speer et al. [5]. Through that work, the defined sequence of the treatment was set and explained. A thermodynamical model was also introduced to simulate the Q&P treatment on any suitable chemically alloyed steel to recommend the optimum conditions for performing the treatment. The model was built on the metastable equilibrium Constrained Paraequilibrium condition [6] and its relevant assumptions. The recommendation of the treatment conditions through the model is based on the optimum combination of phase fractions, which is determined at the endpoint of the treatment. The combination of inherited steel properties, and the microstructure, after the application of Q&P, was explained through earlier work [1] [7] [8]. Several research attempts have been made in the light of the proposed work of J. Speer et al. [5] and validated the proposed findings. A group of concepts was adopted to explain the results of applying the treatment. The chemical alloying role was highlighted [9] [10], especially silicon, Manganese, Aluminum, and several other elements. The focus was mainly on chemical elements' role in suppressing different reactions (carbides precipitation, mainly) competing against carbon enrichment from martensite into untransformed Austenite during the Partitioning stage. The role of Niobium (Nb) was highlighted in previous work [11] [12], and the focus was mainly on the grain refinement effect. Other concepts were highlighted, such as Partitioning kinetics [13], stability of Retained Austenite [14], effect of multiphase-microstructure and attained properties relationship [15], the effect of Partitioning temperature and time [

Material
The approximate chemical composition of the investigated alloy in the As Received condition is presented in Table 1. The chemical analysis was done using an optical emission spectrometer (Model: Spectro-Ametec, Germany, Technique: Hyper-PMT + CCD, Standard: ASTM A751-14a (ASTM E415-15)). This material is classified as hypo-eutectoid Micro-alloyed steel with low carbon alloying (0.192%) and other alloying elements, which in total follows the typical chemical composition ranges of Quenching and Partitioning steel, which grants this alloy the potential for attaining the characteristics of advanced high strength low alloyed steels and shows promise in resulting excellent enhanced combination of physical and mechanical properties after going through the novel heat treatment of Quenching and Partitioning. The material was received as rolled sheets of 2 mm thickness, 250 mm length, and 200 mm width. The used specimens to perform the applied One Step Quenching and Partitioning heat treatment cycles were cut from the basic received sheets dimensions to the standard sub-sized tensile specimen. Critical transformation temperatures determination was vital to proceed with performing the heat treatment. Ac 1 , Ac 3 , and M s , which are the main temperature of interest, were combinedly determined empirically through Equations [28], and Thermo-Calc Software, TCFE10 Steels/Fe-alloys database [29]. Eventually, the approximated values of Ac 1 , Ac 3 , and M s , were calculated to be 720˚C, 1000˚C, and 403˚C, respectively. 1 Ac 742 29C 14Mn 13Si 16Cr 17Ni 16Mo 45V 36Cu Ac 955 350C 25Mn 51Si 106Nb 100Ti 68Al 11Cr 33Ni 16Cu 67Mo  The quenching to room temperature in distilled water was the final step of all applied cycles. The used specimens to perform the applied One Step Quenching and Partitioning heat treatment cycles were cut from the basic received sheets dimensions to the standard sub-sized tensile specimen. The sub-sized specimen was cut from the As Received sheets using a computer numerically controlled water jet machine, which provides high accuracy cutting with negligible heat so that the specimen's microstructure cannot be affected or altered.

Metallographic Examination
Small samples were cut for microstructural investigation from a specific rectangular piece attached to the tensile specimens on which each cycle was applied.
Standard steps of microstructure samples preparation of grinding, polishing, and 2% Nital etching (5 -10 Seconds), were applied, followed by an inverted type of light optical microscope. The As Received sample went through the same metallographic preparation methodology.

Tensile Testing
Mechanical uniaxial tensile testing was performed on all sub-sized tensile specimens of all applied quenching, and Partitioning heat treatment cycles besides the As Received tensile specimen. As mentioned before, the tensile specimens were cut before the application of heat treatment cycles. They were cut to a profile, which follows the Standard (ASTM-E8/E8M) [30] for a plate-type test. Each specimen had the As Received thickness of 2 mm. The overall length is 110 mm.
Each grip section was 35 mm long and 10 mm wide. The radius of each fillet was 6.37 mm. The gauge length was 25 mm long and 6 mm wide (as shown in Figure   2). All tested tensile specimens were submitted to the same tensile testing conditions.   decrease Carbon and Nitrogen interstitial atoms that impede dislocation movement [32]. Consequently, the mobile dislocation density is increased [32]. Table 2

Summary and Conclusions
In this work, heat treatment cycles of 0. • Concerning the applied combination of parameters, the Partitioning period at such temperature is advised to be between 500 and 1000 Seconds, with the high probability that periods closer to 500 than 1000 Seconds will produce better results. • More research is needed between those two values of Partitioning time (500 and 1000 Seconds) to precisely determine the optimum time at that temperature on that specific alloy.