Generation Advanced High Strength Steels Research Articles

The formation of a mixed martensitic/bainitic microstructure and the retainment of austenite in a medium-carbon steel during ultra-fast heating

The current work focuses on the microstructure evolution of a medium-carbon steel during an ultra-fast heating cycle. Electron microscopy methods such as Scanning Electron Microscopy, Electron BackScatter Diffraction and Transmission Electron Microscopy are utilized for the characterization of microstructural constituents of the studied steel samples. The applied heating rate during this treatment was 100 °C/s and the heating temperature was 930 °C, slightly above the Ac3 temperature, and subsequent quenching in water. It is found that the microstructure consists of martensite (60 %), lower bainite (37 %) and retained austenite (3%). The formation of bainite during this treatment is attributed to the heterogeneity of the chemical composition in the microstructure due to the lack of time for carbon to diffuse from cementite in long distances. Regions that are located away from the former pearlitic colonies are not enriched in carbon and manganese along the austenitization process and, according to the Controlled Cooling Transformation diagram, will form bainite upon cooling. The retainment of austenite in films is observed between the bainitic plates and on the martensitic lath boundaries, where carbon and manganese are segregated. Such microstructures are utilized by the current 3rd Generation Advanced High Strength Steels for the optimization of the mechanical properties.

Mechanisms of paint bake response in resistance spot-welded first and third generation AHSS

Implementation of 3rd generation advanced high strength steels (3GAHSS) in automotive structures requires careful consideration of joining process implications. Due to the presence of increased carbon equivalent in 3GAHSS such as 980 HF relative to commonly-applied 1st generation AHSS such as 980 DP, the resultant properties of the weld nugget and heat affected zones in resistance spot welded must be engineered to provide consistent and acceptable performance, specifically in cross-tension loading mode. Although the as-welded cross-tension strength (CTS) of 980 HF is less than 980 DP, there is a significant improvement of the CTS after paint baking in 980 HF welds. 980 DP steel welds do not exhibit such marked improvement upon baking. The mechanism of the strong and beneficial effect of paint baking on 980 HF in comparison to 980 DP is discussed in light of its effect on residual stress distributions and microstructures. Although residual stress (strain) reduction is observed via synchrotron x-ray diffraction for all tested steels, this effect is demonstrated as secondary to the performance restoration of the weld joint in cross tension. High-resolution electron channeling contrast imaging (ECCI) micrographs reveal that paint baking affects a more beneficial degree of martensite tempering in 980 HF as compared to 980 DP. The martensite tempering is also evidenced by both a reduction in nano-hardness of critical microstructural features and re-emergence of ductile fracture behavior following the paint baking process. The tempering of martensite is concluded to be the dominant mechanism in improving cross tension strength of 3GAHSS welds following paint baking.

Austenite formation and mechanical behavior of a novel TRIP- assisted steel with ferrite/martensite initial structure

Abstract A newly developed TRIP-assisted steel with ferrite/martensite initial structure was designed to investigate the microstructural evolution during overall annealing cycle. The results were compared with the traditional cold-rolled TRIP steel. The characteristic of austenite formation from different initial microstructure was clarified based on combining the experiments and modeling. Results indicated that the new austenite obtained during intercritical annealing was fine-grained and uniformly-distributed. It comprised of acicular structure formed in pre-existing martensite (Mpre) matrix and blocky structure occurred at phase interfaces or prior austenite grain boundaries. The kinetics of austenite formation in specimens involving Mpre are similar in the early heating stage, and then decreased with decreasing Mpre content at temperatures greater than 770 °C. Both experimental and DICTRA results indicated that the austenite fractions of specimens with 90% and 100% Mpre fraction attained almost the maximum at the end of heating, and decreased with isothermal holding time during the intercritical annealing. Moreover, the retained austenite at interfaces of ferrite and pre-existing phase followed the Kurdjumov-Sachs (K–S) relationship with adjacent tempered martensite, while only part of the retained austenite had identical relationship with neighboring ferrite. In addition, the amount of retained austenite increased with Mpre content, although their stability against martensite transformation during tensile testing was similar and greater than that of cold-rolled specimen. As a consequence, the yield strength was increased from 590 MPa to 753 MPa and the tensile strength increased slightly from 995 MPa to 1046 MPa with Mpre fraction increasing from 50% to 100%, and the total elongation was ~30%, similar to the third generation advanced high strength steels.

Experimental Characterization and Deterministic Prediction of In-Plane Formability of 3rd Generation Advanced High Strength Steels

The objective of the current study is to develop a practical, deterministic approach to the prediction of the in-plane formability of two third generation advanced high-strength steels (AHSS) of 980 and 1180 MPa ultimate tensile strength using only quasi-static mechanical property data. The hardening response to large strains was experimentally measured with the use of simple shear and tensile tests and validated in tensile simulations. The process-corrected limit strains in the Nakazima and Marciniak tests were compared to various analytical Forming Limit Curve (FLC) models for in-plane stretching. It was observed that the widely-used Marciniak–Kuczynski model can adequately predict the experimental FLC in biaxial stretching but significantly underestimated the limit strains in uniaxial stretching for both third generation AHSS. The observed through-thickness shear fracture mode in biaxial stretching was reasonably well-captured by the Bressan–Williams (BW) instability model for the 1180 MPa steel. A proposed extension of the BW model to uniaxial tension by adoption of the maximum in-plane shear stress criterion (BWx model) provided superior experimental correlation relative to the zero-extension model of Hill that was too conservative. Finally, a linearized version of the modified maximum force criterion (MMFC) was proposed that markedly improved the correlation with the process-corrected FLC for in-plane stretching of AHSS. The developed framework for FLC prediction was then applied to a DP980 AHSS and an AA5182 aluminum alloy from the literature. The DP980 corroborated the observed trend for the two third generation AHSS whereas the MK and the BWx models performed best for the AA5182 with its saturation-type hardening behavior and non-quadratic yield surface.

Bake Hardening Behavior of DP, TBF, and PHS Steels with Ultimate Tensile Strengths Exceeding 1 GPa

<div class="section abstract"><div class="htmlview paragraph">Third generation advanced high strength steels (AHSS) have been developed combining high strength and formability, allowing for lightweighting of vehicle structural components. These AHSS components are exposed to paint baking operations ranging in time and temperature to cure the applied paint. The paint baking treatment, combined with straining induced from part forming, may lead to increased in-service component performance due to a strengthening mechanism known as <i>bake hardening.</i> This study aims to quantify the bake hardening behavior of select AHSS grades. Materials investigated were press hardenable steels (PHS) 1500 and 2000; transformation induced plasticity (TRIP) aided bainitic ferrite (TBF) 1000 and 1200; and dual phase (DP) 1000. The number designations of these grades refer to minimum as-received ultimate tensile strengths in MPa. Paint baking was simulated using industrially relevant times and temperatures from 15 to 60 min and 120 to 200 °C, respectively. Samples were prestrained 0, 2, or 5 pct to replicate part forming. Bake hardening values ranging from 90 to 140 MPa were observed for DP and TBF grades that were prestrained 2 pct and baked at 170 °C for 20 min. However, ductility diminished for these steels when subjected to 5 pct prestrain with uniform elongations after baking decreasing to 1 pct in some instances. PHS steels, on the other hand, showed substantial increases in yield strength without prestrain. Increases of 122 and 175 MPa were recorded for PHS 1500 and PHS 2000, respectively, following baking at 160 °C for 60 min. However, ultimate tensile strengths decreased due to reduced strain hardening, while total elongations decreased slightly.</div></div>

Effect of Low-Temperature Austempering and Quenching and Partitioning Treatments on Adhesive Wear Resistance of High-Silicon Multiphase Steels

The quest for vehicle safety improvement with weight reduction and consequently lower fuel consumption led the automotive industry to begin research into the third generation of advanced high-strength steels. These steels present complex microstructures, composed of martensite, bainite and stable retained austenite. Two of the main treatments for obtaining these microstructures are the low-temperature austempering and quenching and partitioning (Q&P). The objective of this work is to evaluate the microhardness and adhesive wear performance of C-Si-Mn and C-Si-Mn-Nb steels submitted to the treatments mentioned above. Both austempering and the partitioning steps were conducted at Ms+50 °C. Treatment times were 1 and 3 h for austempering and 10, 30 and 60 min for Q&P. Results show that niobium addition promotes improvement in wear performance despite inferior levels of austenite stabilization and changes in the bainite morphology.

Multiple etchings methodology: a new approach in multiphase steel characterization.

Following the first and second generations, the challenge in obtaining a better balance between strength and elongation is still the main characteristic of the third generation of advanced high-strength steels (AHSS). With this, the use of multiphase microstructures has increased over the last few years. It can be difficult to characterize all the phases with only optical microscopy (OM), so the use of scanning electron microscopy (SEM) is essential for accuracy in cases where researchers lack experience. To expand the possibilities, this research proposed a new approach that would allow experienced researchers to characterize multiphase steels using only OM. A high silicon steel was austempered slightly below martensite start (Ms) temperature for three different time periods in order to obtain different quantities of martensite, bainite and retained austenite. X-ray diffraction was carried out in order to confirm and obtain retained austenite volume fractions, and the results indicated that shorter holding times were not enough to enrich and stabilize retained austenite. Then, each samples was etched with four different etchants. Results showed that the new multiple etchings methodology (MEM) allowed a better visualization of all the phases when viewed together. Beraha martensitc revealed nontempered martensitic microstructures. Sodium metabisulfite revealed retained austenite. LePera and Nital were the best at revealing the evolution of the microstructure over time, even with the changes which occurred due to martensite tempering. SEM images confirmed the results obtained via MEM. LAY DESCRIPTION: For improving safety, environmental protection, mechanical resistance and others issues, many different steel grades have been studied. These grades were named depending on their mechanical properties. The current generation is the third, which is still searching for one of the main antagonists in material science: the best balance between mechanical resistance (how strong it is) and elongation (how long it can be stretched to). In order to achieve this goal, many researchers are studying variations in the production processes. During production, the materials are able to internally change their basic microstructure, named phases. In the past, steels were usually produced to have only a few phases. Today's advanced high strength steels (AHSS) can have many. Each of these phases has its own characteristics. The main focus of this research was to give a new way of identify these phases using an optical microscope. For revealing these phases, etchants are normally used. The etchants used in this research are capable of tinting each of these phases with a different colour or tone. So the purpose of this work was to suggest a new approach in order to allow for more precise identification of the phases in the steel. The results were positive, showing that looking at the samples as a whole is better than the traditional methods. Also, different etchants' characteristics were observed during the changes obtained by this work's chosen processes.

Boro-Austempering Treatment of High-Strength Bainitic Steels

High-strength bainitic steels are considered potential candidates for the 3rd generation of advanced high-strength steels (AHSS). The main characteristic of silicon-alloyed steels is the presence of carbide-free bainite, obtained by low-temperature austempering. Salt bath boriding is an effective method for increasing wear resistance and provides high corrosion resistance. The combination of these two treatments is called boro-austempering and is a promising alternative to increase the wear resistance of AHSS. In the present work, samples were borided at 900 °C for 2 h, direct-cooled from that temperature and isothermally held in a salt bath at 360 °C for 1 and 3 h. The substrate and the layers produced were characterized by optical microscopy (OM), scanning electron microscopy (SEM), x-ray diffraction (XRD), Vickers microhardness (HRV) and microadhesive wear tests. The tribological characteristics of the layers were compared with those of the substrate. The microscopic analysis showed the effectiveness of boro-austempering treatment in the production of carbide-free bainite microstructure and the surface borided layers. As a result, there were increases in surface wear resistance up to 115% when compared to the substrate.

Low and Medium Carbon Advanced High-Strength Forging Steels for Automotive Applications

This paper presents the microstructural and mechanical properties of low and medium carbon advanced high-strength forging steels developed based on the third generation advanced high-strength sheet steels, in conjunction with those of conventional high-strength forging steels. Hot-forging followed by an isothermal transformation process considerably improved the mechanical properties of the forging steels. The improvement mechanisms of the mechanical properties were summarized by relating to the matrix structure, the strain-induced transformation of metastable retained austenite, and/or a mixture of martensite and austenite.

Effect of tempering time on microstructure, mechanical, and electrochemical properties of quenched–partitioned–tempered Advanced High Strength Steel (AHSS)

The quenched–partitioned–tempered (QPT) steel is one of the novel steel in third–generation advanced high strength steels. Herein, an attempt has been made to investigate the microstructure, mechanical, and electrochemical properties of QPT steel as a function of tempering time. Microstructural analysis reveals that a high volume fraction of martensite with low fractions of retained austenite and bainite were achieved after 30 s of tempering. Mechanical tests validated that this triplex composite microstructure resulted in an 18% improvement in Vickers hardness, 73% in tensile strength (Rm) with little reduction of 12% elongation (εT) in comparison to non–heat treated steel sample. Tempering for 90 s increased the volume fractions of stable retained austenite, bainite and decreased the fractions of martensite by carbon diffusion to austenite. Further increase in tempering time stabilized retained austenite at an expense of bainite and martensite phases, resulting in reduced hardness and improved strength and elongation. Electrochemical analysis of the QPT heat–treated samples performed in 3.5% NaCl solution showed that the high corrosion rate was yielded from the steel sample tempered for 30 s, while other samples tempered for 90 to 150 s showed much lower corrosion rate than non–heat–treated steel sample.

Microstructure evolution during thermomechanical processing in 3Mn-0.1C medium-Mn steel

ABSTRACTMedium-Mn steels are energetically investigated as a candidate of the third generation advanced high strength steels (AHSSs). However, their phase transformation and microstructaure evolution during various heat treatments and thermomechanical processing are still unclear. The present study first confirmed the kinetics of static phase transformation behaviour in a 3Mn-0.1C medium-Mn steel. Hot compression tests were also carried out to investigate the influence of high-temperature deformation of austenite on subsequent microstructure evolution. It was found that static ferrite transformation was quite slow in this steel, but ferrite transformation was greatly accelerated by the hot deformation in austenite and ferrite two-phase regions. Characteristic dual-phase microstructures composed of martensite and fine-grained ferrite were obtained, which exhibited superior mechanical properties.This paper is part of a Thematic Issue on Medium Manganese Steels.

Microstructure evolution during high-temperature partitioning of a medium-Mn quenching and partitioning steel

Medium-Mn Quenching & Partitioning (Q&P) steels have been recently considered as potential candidates for the 3rd generation advanced high-strength steels. The processing of these steels aims to induce the partitioning of substitutional alloying elements from martensite to austenite during an isothermal treatment at high temperature, where the diffusivity of substitutional alloying elements is sufficiently high. In this way, austenite increases its concentration of austenite-stabilising elements and thus its thermal stability. The present study aims to investigate the microstructural evolution during high temperature partitioning treatments in a medium-Mn steel and the possible occurrence of additional phase transformations that may compete with the process of atomic partitioning between martensite and austenite. Q&P routes in which the partitioning steps take place in the range of 400 °C–600 °C for times up to 3600 s were investigated. The final microstructures display an increased fraction of retained austenite with increasing holding times during partitioning at 400 °C, while at higher partitioning temperatures, 450 °C–600 °C, leads to cementite precipitation in austenite films and pearlite formation in blocky austenite, resulting in a decrease of the fraction of retained austenite with the holding time. This observation is supported with theoretical calculations of the volume change, suggesting that for maximising the fraction of retained austenite, short holding times are preferred during partitioning at high temperatures. Observations from the current study reveal that the successful application of high-temperature partitioning treatments in medium-Mn steels requires microstructure design strategies to minimize or suppress competitive reactions.

The Evaluation of Laser Weldability of the Third-Generation Advanced High Strength Steel

To meet the demands of vehicular safety and greenhouse gas emission reduction, the automotive industry is increasingly using advanced high strength steels (AHSS) in the production of the components. With the development of the new generation of AHSS, it is essential to study their behavior towards manufacturing processes used in the automotive industry. For this purpose, the welding capability of newly developed third-generation Gen3 980T steel was investigated using the Nd:YAG (Neodymium:Yittrium Aluminum Garnet) laser-welding with different parameter conditions. The analysis was made by uniaxial tensile tests, micro-hardness, Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The criteria used to evaluate the quality of the weld were the distance between the fracture and the weld bead and the surface finish. A relationship between the quality of the weld and the energy density was observed, expressed by a partial penetration for values below the optimal, and by irregularities in the weld bead and a high number of spatters for the values above the optimal.

Characterization of Inclusions in 3rd Generation Advanced High-Strength Steels

Samples taken from laboratory-produced 3rd generation advanced high-strength steels, solidified at a low cooling rate, have been investigated to study the characteristics of non-metallic inclusions. Two steels, containing 2 and 5 pct Mn content, were produced for this purpose. A higher number of total inclusions were observed in 5 pct Mn steel. The four main types of inclusions observed were Al2O3, AlN, MnS, and AlSiMn-oxide. These classes were divided into subclasses according to variations in their chemistry. The major subclasses of AlN inclusions are either plate-like or regular in shape and have different size distributions. Thermodynamic calculations suggest that plate-like AlN inclusions are formed at the initial stage of solidification, while faceted/regular-shaped inclusions are precipitated toward the end of solidification. Moreover, it was found that the size of nitride inclusions is related to their N content. This phenomenon is discussed from the viewpoint of nucleation theory.

Interactions of negative strain rate sensitivity, martensite transformation, and dynamic strain aging in 3rd generation advanced high-strength steels

Medium Mn transformation-induced plasticity (TRIP) steels, a subgroup of third generation advanced high-strength steels (AHSS), often present a combination of both dynamic strain aging (DSA) and TRIP. It has been previously shown that TRIP coincides with the passage of strain bands due to DSA, but their interdependence has not been well-established. This study seeks to explore the interactions between the two phenomena as well as the commonly-observed negative strain rate sensitivity. Digital image correlation (DIC) and in-situ magnetic measurements of the retained austenite fraction were employed to demonstrate the variations in mechanical behavior with increasing strain rate. Strain rate jump tests provided measurements of the strain rate sensitivity and were combined with either DIC to characterize PLC bands or with magnetic measurements to measure variations in the martensite fraction during the jumps. Finally, the micromechanics defining the peculiar bursts of martensite transformation in response to strain rate jumps are discussed in terms of a competition between dislocation mobility and austenite stability.

Microstructure Evolution During High-Temperature Partitioning of a Medium-Mn Quenching and Partitioning Steel

Medium-Mn Quenching & Partitioning (Q&P) steels have been recently considered as potential candidates for the 3rd generation advanced high-strength steels. The processing of these steels aims to induce the partitioning of substitutional alloying elements from martensite to austenite during an isothermal treatment at high temperature, where the diffusivity of substitutional alloying elements is sufficiently high. In this way, austenite increases its concentration of austenite-stabilising elements and thus its thermal stability. The present study aims to investigate the microstructural evolution during high temperature partitioning treatments in a medium-Mn steel and the possible occurrence of additional phase transformations that may compete with the process of atomic partitioning between martensite and austenite. Q&P routes in which the partitioning steps take place in the range of 400 °C - 600 °C for times up to 3600 s were investigated. The final microstructures display an increased fraction of retained austenite with increasing holding times during partitioning at 400 °C, while at higher partitioning temperatures, 450 °C - 600 °C, leads to cementite precipitation in austenite films and pearlite formation in blocky austenite, resulting in a decrease of the fraction of retained austenite with the holding time. This observation is supported with theoretical calculations of the volume change, suggesting that for maximising the fraction of retained austenite, short holding times are preferred during partitioning at high temperatures. Observations from the current study reveal that the successful application of high-temperature partitioning treatments in medium-Mn steels requires microstructure design strategies to minimize or suppress competitive reactions.

3<sup>rd</sup> Generation AHSS by Thin Slab Technology

Modern steel making and hot rolling processes like CSP® thin slab technology require precise data on casting and rolling behavior of the produced steel grades. Up today only few data is available for the latest generations of advanced high strength steel (AHSS) grades. AHSS have developed by 3 generations [1, 2]. 1st Generation AHSS as dual phase (DP), complex phase (CP), martensitic (MS) and transformation induced plasticity (TRIP) steel grades are currently applied in automotive industry. 2nd and 3rd Generation AHSS typically have elevated Mn-content as well as Al and Si content. High Mn-content of up to 30% seriously affects casting and forming properties of 2nd Generation AHSS. In particular, the large solidification range of more than 100 K prevents commercial production of these steel grades by continuous casting [3]. 3rd Generation AHSS with reduced Mn-content up to about 12% are currently under development [1-4]. Investigations have been carried out to assess the CSP® thin slab process for the production of such grades. To this purpose solidification and hot forming properties of different alloys having Mn-content up to 10% have been examined by thermodynamic calculations and laboratory testing by hot forming dilatometry. The achieved flow curves match figures achieved on a hot rolling mill.

Microstructure Evolution and Change in Mechanical Properties of Medium Mn Steels during Thermomechanical Processing

Medium manganese steels are nowadays energetically investigated as the third generation advanced high strength steels (AHSS) because of their excellent balance between material cost and mechanical properties. However, the phase transformation and microstructure evolution in medium manganese steels during various heat treatments and thermomechanical processing are still unclear. The present study firstly examined kinetics of static phase transformation behavior and microstructural change in a 3Mn-0.1C medium manganese steel. Hot compression tests were also carried out to investigate the influences of high-temperature thermomechanical processing on the microstructure evolution. It was found that ferrite transformation was quite slow in static conditions but greatly accelerated by hot compression in (austenite and ferrite) two phase region. Dual phase microstructures composed of martensite and ferrite with ferrite grain sizes of 1~2 μm were obtained, which exhibited superior mechanical properties.

Metallographic, Structural and Mechanical Characterization of REM-Containing Fe-30Mn-8Al-1.8C Low Density Steel in As-Cast Condition

ABSTRACTRecently, low-density steels have received increased attention as promising alternatives for automotive applications of the next generation of Advanced High-Strength Steels (AHSS), considering that vehicle´s weight decrease has been the subject of intense interest. It is well-known that the addition of rare earth metals (REM) has a remarkable effect on shape control and the modification of inclusions. Also, REM additions affect the grain size refinement as well as the tendency to form oxides and sulfides. The aim of this research work was to determine the effect of REM (Ce, La) addition on the microstructure and mechanical properties of the Fe-30Mn-8Al-1.8C low-density steel in as-cast condition. In order to clarify the REM effect on the Fe-Mn-Al-C system, non-microalloyed (LD-NM) and REM microalloyed (LD-REM) specimens were examined in detail by means of light optical and scanning electron microscopy for microstructural characterization. In the same way, the primary and secondary phases founded in the studied steels were identified by X-ray diffraction (XRD). Meanwhile, in order to evaluate the mechanical properties, ten microhardness measurements were carried out on the overall bulk by the Vickers hardness testing. In general, the results showed a dendritic refinement effect due to the addition of REM to low-density steel. REM acted as effective inoculants agents which reduced the primary and secondary arm spacing. Also, the strong segregation tendency at the grain boundaries in the liquid phase was limited. XRD profiles revealed the presence of austenite, ferrite, κ and DO3 phases. Low density steel microalloyed with REM showed a moderate increase in hardness compared to the non-microalloyed steel in the as-cast condition.

On the reactive wetting of a medium-Mn advanced high-strength steel during continuous galvanizing

The effects of process atmosphere oxygen partial pressure (pO2) and Sn-addition on the reactive wetting of a prototype medium manganese third generation advanced high strength steel by a 0.20 wt% Al (dissolved) zinc bath was determined through galvanizing simulations. External MnO layers with thicknesses ranging from 19 nm to 45 nm were formed after annealing for 120 s at 690 °C under a variety of experimental process atmospheres. Despite the presence of surface oxides prior to immersion, an integral zinc coating and good coating adherence was obtained on all steel substrates, with the exception of the reference steel annealed under the lowest pO2−50 °C dew point process atmosphere, which exhibited poor reactive wetting. Excellent reactive wetting was demonstrated by the Sn-containing steels annealed under the −30 °C and +5 °C atmospheres. It was determined that the Al uptake as well as the population of the Fe-Al intermetallics at the coating/steel interface increased as a result of annealing under the higher pO2 atmospheres and adding Sn to the steel. The significant improvement in reactive wetting of the Sn-added steel was attributed to the decreased surface enrichment of Mn and the discrete distribution of the fine external oxide particles formed on the surface of these steel prior to dipping; which allowed for progression of reactive wetting through mechanisms such as the aluminothermic reduction of MnO, oxide bridging and oxide lift-off by the molten galvanizing bath.