**1. Introduction**

The low carbon, low alloy ferrite-martensite dual-phase (DP) steels are characterized by a mixture of soft ferritic matrix phase in conjunction with dispersed hard martensite. Attractive engineering properties are nowadays attained in these steels by the use of innovative heat treatments [1], which allow significant improvement in continuous yielding behavior, resulting in superior strength–ductility combinations in association with rapid strain hardening at early stage of plastic deformation due to the formation of a favorable microstructure [2–6]. These engineering properties are imparted by some of the variable microstructural parameters, such as ferrite and martensite volume fractions, their morphology and distribution, ferrite grain size, and the complex interaction of microphases with each other causing a considerable variation in mechanical behavior of low alloy DP steels [7–10]. Bag et al. [11,12] studied the impact and tensile properties of high martensite containing low alloy, ferrite-martensite DP steels and reported that an equal amount of finely divided ferrite and martensite microphases facilitate an optimum combination of high ductility and strength with good impact toughness. Using a 0.11% C, 1.6% Mn, 0.73% Si steel, Cai et al. [13] measured room temperature tensile properties of various samples with different ferrite and martensite morphologies, but with almost the same martensite volume fraction, derived from different primary microstructures. They reported that relatively higher strengths were attained in the DP samples, which received an intermediate quenching heat treatment, whereby the martensite content after intercritical annealing was very fine and fibrous morphology in the microstructures. Chang et al. [14] studied the effect of ferrite

**Citation:** Khajesarvi, A.; Banadkouki, S.S.G.; Sajjadi, S.A.; Somani, M.C. Abnormal Trend of Ferrite Hardening in a Medium-Si Ferrite-Martensite Dual Phase Steel. *Metals* **2023**, *13*, 542. https://doi.org/10.3390/ met13030542

Academic Editor: Andrea Di Schino

Received: 18 January 2023 Revised: 25 February 2023 Accepted: 6 March 2023 Published: 8 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

grain size on the tensile properties of a low alloy DP steel and presented that the effect of ferrite grain size on the yield strength was much stronger than on the tensile strength.

A careful literature review of relevant articles regarding the structure-property relationships of low alloy DP steels indicate that the individual hardening behavior of ferrite and martensite microphases have been one of the noteworthy research topics in physical metallurgy of advanced high strength low alloy steels [15–17]. Kumar et al. [8] have reported that the ferrite hardness changed as a function of its volume fraction in a low alloy ferrite-bainite DP steel. They found that about 8% alteration in ferrite microhardness can be related with the ferrite-bainite DP samples consisting of 10 to 50% volume fraction of ferrite in conjunction with the remaining bainite regions. Another experimental work has also been conducted by Kumar et al. [8] showing that the mechanical behavior of low alloy ferrite-martensite DP steels was relevant to the size of ferrite grains. Also, the interaction between ferrite and martensite microconstituents has been believed to introduce unpinned dislocations generated within ferrite, which can affect the ferrite strain hardening in the low alloy ferrite-martensite DP steels [18,19]. Therefore, the ferrite hardening in ferrite– martensite DP steels has been presented to be variable depending on various parameters such as volume fraction, morphology and grain size of ferrite, and of course, the role of carbon and other alloying elements partitioning between ferrite and prior austenite (martensite) microphases are questionable and have been not followed significantly. Accordingly, it has been tried to find out the effect of carbon and other alloying elements partitioning on the ferrite hardening of different ferrite–martensite DP microstructures using a commercial grade of medium Si low alloy 35CHGSA steel by means of microhardness measurements and EDS analyses.
